Structural Polymer Composites for Energy Storage Devices ... · Structural Polymer Composites for...
Transcript of Structural Polymer Composites for Energy Storage Devices ... · Structural Polymer Composites for...
StructuralPolymerCompositesforEnergyStorageDevices
PhDThesis
AtifJavaid
December2011
DepartmentofChemicalEngineering&ChemicalTechnologyImperialCollegeLondon,SouthKensingtonCampus,London,SW72AZ,UnitedKingdom
StructuralPolymerCompositesforEnergyStorageDevices
Adissertationby
ATIFJAVAID
SubmittedtotheImperialCollegeLondoninpartialfulfilmentoftherequirementsforthe
degreeof
DOCTOROFPHILOSOPHY
Andthe
DIPLOMAOFIMPERIALCOLLEGELONDON
DepartmentofChemicalEngineering&ChemicalTechnologyImperialCollegeLondon,SouthKensingtonCampus,London,SW72AZ,UnitedKingdom
Supervisors
ProfAlexanderBismarckDrEmileSGreenhalghProfMiloSPShafferDrJoachimHGSteinke
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“In the name of Allah, Most Gracious, Most Merciful”
Dedication
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Dedicated to my supervisor Professor Alexander Bismarck.
Thank you for all your help during my PhD.
I could not have done it without your help.
Declaration
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Declaration
This dissertation is a description of the work carried out by the author in the Department of Chemical Engineering and Chemical Technology, Imperial College London between October 2007 and September 2010 under the supervision of Prof Alexander Bismarck, Dr Joachim Steinke, Dr Emile Greenhalgh and Prof Milo Shaffer. Except where acknowledged, the material is the original work of the author and includes nothing, which is the outcome of work in collaboration, and no part of it, has been submitted for a degree at this or any other university. Keywords: Multifunctional, Composites, Polymer electrolytes, Structural, Supercapacitors
Abstract
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Abstract Multifunctional composites have attracted a great deal of attention as they offer a way to cut
down the parasitic weight in vehicles which not only reduces the operational costs but also
reduces the fuel consumption in vehicles. Current engineering design is increasingly
sophisticated, requiring more efficient material utilisation; sub-system mass and volume are
crucial application determinants. This dissertation contributes to the fabrication of composites
that can store electrical energy and are known as structural supercapacitors. The key in the
fabrication of structural supercapacitors was not simply to bind two disparate components
together, but to produce a single coherent material that inherently performed both roles of a
structural composite and a supercapacitor. This design approach is at a relatively early stage,
and faces significant design and material synthesis challenges. Disparate material
requirements, such as structural and electrochemical properties, have to be engineered and
optimised simultaneously.
This study investigates on structural supercapacitors fabricated by using as-received as well
as activated carbon fibre cloths as reinforcement and electrodes; multifunctional resin as
electrolyte and matrix; and glass fibre cloths, filter papers or polymer membranes as
insulators. Such a system should deliver electrical energy storage capacity as well as bear
mechanical loads. Different liquid electrolytes, such as ionic liquids and salts based on Li+
and NH4+, were studied in order to optimise the multifunctionality of polymer electrolyte.
Mesoporous silica particles were also introduced into polymer electrolytes in order to
enhance the mechanical and electrochemical performance of polymer electrolytes. Nano-
structured/multifunctional resin blends were cured in cylindrical form and were examined by
compression testing as well as impedance spectroscopy. An ionic conductivity of 0.8 mS/cm
and a compression modulus of 62 MPa have been synthesised for the polymer electrolyte in
the current study. By varying the separators, multifunctional resins and the electrodes,
different structural supercapacitor configurations were manufactured using a resin infusion
under flexible tooling (RIFT) method and were characterised to study the electrochemical
performance by using charge/discharge method and mechanical performance by using ±45°
laminate shear testing. The improved structural supercapacitors showed an energy density of
0.1 Wh/kg, a power density of 36 W/kg and a shear modulus of 1.7 GPa.
Acknowledgements
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Acknowledgements This expedition would not have been as fascinating without the interesting people I met along
the way. I would first like to thank my supervisors, Prof Alexander Bismarck, Dr Emile
Greenhalgh, Dr Joachim Steinke and Prof Milo Shaffer for giving me the opportunity to work
on this extraordinary project. I am very thankful for the time, support and the liberty that my
supervisors provided during this course of research work. They also helped me to improve
my working skills and clarify my fundamentals. I am also grateful for the many
brainstorming sessions as well as the leisurely chats we had. I would especially like to thank
Alexander for his kind support and encouragement throughout my PhD work. He always had
a time to listen to my problems with patience and is always a source of inspiration for me.
He always made me smile no matter how tense the situation was. I really enjoyed the
conversations that we had either on the project or on the social matters including the political
system in Pakistan. His jokes on Taliban always brought a smile on my face. I am indeed
fortunate to have him as one of my supervisors.
My sincere appreciation also goes to the University of Engineering and Technology (UET),
Lahore, Pakistan for providing me the financial support during my PhD. It was not possible
for me to carry out my research work in a world renowned institution without the grant that
UET had provided me. I would also like to thank Prof Ghulam Mustafa Mamoor and Prof
Mehmood Ahmad for all their support and help during my undergraduate studies as well
during my lectureship in Polymer Engineering department of UET, Lahore. Special thanks to
Prof Bismarck for supporting me for my 3rd year tuition fees. I would also like to thank Dr
Greenhalgh (MAST Project Coordinator), Ministry of Defence, UK and BAE Systems for
their financial support during the last year of my research work. Thank you for becoming
wind under my wings.
My heartfelt gratitude goes out to my parents, Mr and Mrs Javaid Iqbal, who provided love,
affection and encouragement throughout my PhD. This dissertation would not have been
possible without the support, guidance and sacrifices that they made from all the hard times
growing up to this very day. They were always there when I needed them. Thank you very
much for everything. I would also like to thank my sisters, uncles, aunties and cousins for all
their support and encouragement. Thank you Annie, Shanza, Mishal, Aimen, Kashan, Umair,
Zaid, Ahmar, Farzeen, Shaza, Zimal, Kinza, Minahil, Ryan, Linta, Meerab, Mahad, Maham,
Acknowledgements
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Laiba, Anosh, Wamiq, Haider, Zaini. Thanks Asifa Khala for your support and
encouragement. Thanks Ammi for all your wishes and support. Unfortunately, you did not
have a chance to see the completion of my PhD today but I am sure, wherever you are, you
must be very proud of me.
I am also grateful to Dr Kingsley Ho for always being there when I needed him. He always
helped me whenever I had problems. He taught me a lot during the past four years and for
which I am sincerely thankful for him. I am most thankful for his support in preparing my
first conference presentation. The succeeding nerve-wracking experience of presenting in
front of a large audience at CSCST Conference, Oxford was somewhat decreased by knowing
that Dr Ho was also there to back me up. A special thank you goes to Dr Hui Qian (Sherry)
for providing the activated carbon fibres. I really enjoyed the fruitful discussions about the
project that we had after she joined the project during my 3rd year. I wish to also thank Prof
Anthony Kucernak and Dr John Hodgkinson for their many insightful recommendations and
their continuous support throughout my research.
At the end, I would like to continue by expressing my sincere obligations to the many friends
and colleagues who made my stay at Imperial unforgettable. I would like to thank Sheema,
Humera, Rose, Ali, Muddassir, Faisal, Ghiyas, Ammar, Ilyas and many more. I would also
like to thank other members of PaCE, Steinke and Nano groups notably Dr Charnwit (Jo)
Tridech, Dr Steven Lamoniere, Dr Johny Blaker, Dr Angelika Menner, Dr Natasha
Shirshova, Dr Emilia Kot, Nadine Graeber, KoonYang Lee, Dr Ivan Zadrazil, Dr Anthony
Abbott, Henry Maples, Su Bai, Jing Li, Hele Diao, Edyta Lam, Dan Cegla, Sally Ewen, Bryn
Monnery, Wei Yuan, Stephen Hodge and many more. I am sure that I have forgotten many
others and for that I apologise. You all have made my research work exciting and fun. My
sincere appreciation is also for the tremendous support provided by the technical staff of
Chemical and Aeronautical engineering departments. A special thanks to Sarah Payne,
Patricia Carry, Richard Wallington, Gary Senior, Joseph Meggyesi, Keith Walker, Susi
Underwood, Rayner Simpson, Jon Cole and other technical staff.
List of Publications and Presentations
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List of Publications and Presentations Current and Future Journal Publications
[1] K. K. C. Ho, S. Shamsuddin, S. Riaz, S. Lamorinere, M. Q. Tran, A. Javaid, A.
Bismarck, Wet impregnation as route to unidirectional carbon fibre reinforced
thermoplastic composites manufacturing, in 2011, 100.
[2] A. Javaid, E. S. Greenhalgh, M. S. P. Shaffer, J. H. G. Steinke, A. Bismarck, Ionically
conductive and mechanically robust crosslinked polymer electrolytes for
multifunctional structural supercapacitor applications, (in preparation).
[3] A. Javaid, A. Bismarck, E. S. Greenhalgh, M. S. P. Shaffer, J. H. G. Steinke,
Improving the ionic conductivity and compression properties of crosslinked polymer
electrolytes through mesoporous silica particle reinforcements for use in structural
supercapacitors, (in preparation).
[4] A. Javaid, K. K. C. Ho, A. Bismarck, J. H. G. Steinke, M. S. P. Shaffer, E. S.
Greenhalgh, Exploring the design parameters of multifunctional structural
supercapacitors with improved mechanical and electrochemical performance for
energy storage applications, (in preparation).
[5] A. Javaid, K. K. C. Ho, H. Qian, E. S. Greenhalgh, J. H. G. Steinke, M. S. P. Shaffer,
A. Bismarck, Multifunctional Structural supercapacitors for energy storage devices,
(in preparation).
[6] A. Javaid, K. K. C. Ho, A. Bayley, E. S. Greenhalgh, J. H. G. Steinke, M. S. P. Shaffer,
A. Bismarck, Improving the multifunctionality of structural supercapacitor through
addition of high surface area carbon black, (in preparation).
Conference Proceedings
[1] A. Bismarck, E. S. Greenhalgh, K. K. C. Ho, A. Javaid, A. Kucernak, M. S. P. Shaffer,
N. Shirshova, J. H. G. Steinke, "Structural power composites as energy storage devices",
List of Publications and Presentations
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presented at 18th International Conference on Composite Materials, Jeju Island, Korea,
2011.
[2] A. Bismarck, P. T. Curtis, E. S. Greenhalgh, K. K. C. Ho, A. Javaid, A. Kucernak, H.
Qian, M. S. P. Shaffer, N. Shirshova, J. H. G. Steinke, "Structural power composites for
energy storage devices", presented at 14th European Conference on Composite materials,
Budapest, Hungary, 2010.
Poster Presentation
[1] A. Bismarck, E.S. Greenhalgh, K.K.C. Ho, A. Javaid, M.S.P. Shaffer, J.H.G. Steinke,
"Structural polymer composite for power storage", presented at Macro Group Young
Researchers Meeting, University of Nottingham, 2010.
1st prize won in poster presentation.
Conference Presentations
[1] A. Bismarck, E. S. Greenhalgh, K. K. C. Ho, A. Javaid, A. Kucernak, M. S. P. Shaffer,
N. Shirshova, J. H. G. Steinke, "Structural power composites as energy storage devices",
presented at The 18th International Conference on Composite Materials, Jeju Island,
Korea, 2011.
[2] A. Bismarck, E. S. Greenhalgh, K. K. C. Ho, A. Javaid, A. Kucernak, M. S. P. Shaffer,
N. Shirshova, J. H. G. Steinke, "Structural power composites as energy storage devices",
presented at The 17th Joint Annual Conference of CSCST and SCI, Oxford, United
Kingdom, 2010.
Table of Contents
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Table of Contents
Abstract ..................................................................................................................................... 6
Acknowledgements .................................................................................................................. 7
List of Publications and Presentations ................................................................................... 9
List of Figures ......................................................................................................................... 18
List of Tables .......................................................................................................................... 26
List of Abbreviations ............................................................................................................. 31
List of Notations ..................................................................................................................... 34
Chapter 1 Introduction.......................................................................................................... 38
1.1 Motivation ............................................................................................................. 39
1.2 Methodology ......................................................................................................... 41
1.3 Aims and objectives .............................................................................................. 43
1.4 Thesis Outline ....................................................................................................... 44
Chapter 2 Literature Review ...................................................................................... 46
2.1 Traditional carbon fibre reinforced thermoset composites ................................... 47
2.2 Energy storage devices .......................................................................................... 48
2.3 Multifunctional composites................................................................................... 51
2.3.1 Structural batteries .................................................................................. 52
2.3.1.1 What are batteries? ........................................................................ 52
2.3.1.2 Research trends in structural batteries .......................................... 54
2.3.1.3 Other multifunctional energy storage materials ............................ 59
2.3.1.4 Challenges in structural batteries .................................................. 60
2.3.2 Structural fuel cells .................................................................................. 60
2.3.2.1 What are fuel cells? ....................................................................... 60
2.3.2.2 Research trends in structural fuel cells ......................................... 61
2.3.2.3 Challenges in structural fuel cells ................................................. 63
2.3.3 Structural capacitors ............................................................................... 64
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2.3.3.1 What are capacitors? ..................................................................... 64
2.3.3.2 Research trends in structural capacitors ........................................ 65
2.3.3.3 Challenges in structural capacitors ............................................... 67
2.4 Structural Supercapacitors .................................................................................... 67
2.4.1 Historical background of supercapacitors .............................................. 68
2.4.2 Working principle of supercapacitor ....................................................... 70
2.4.3 Types of supercapacitors ......................................................................... 72
2.4.4 Research trends in supercapacitors ......................................................... 73
2.4.5 Structural polymer electrolytes ................................................................ 75
2.4.6 Activated carbon fibre electrodes ............................................................ 79
2.4.7 Challenges in structural supercapacitors ................................................ 81
Chapter 3 Experimental Section ................................................................................ 83
3.1 Materials................................................................................................................ 84
3.1.1 Uncured epoxy materials ......................................................................... 84
3.1.2 Crosslinker ............................................................................................... 84
3.1.3 Electrolyte salt ......................................................................................... 85
3.1.4 Solvents .................................................................................................... 85
3.1.5 Silica precursor ........................................................................................ 86
3.1.6 Block copolymer surfactant ..................................................................... 86
3.1.7 Woven fibre mats...................................................................................... 86
3.1.8 Paraffin oil ............................................................................................... 87
3.1.9 Separators ................................................................................................ 87
3.2 Mesoporous silica ................................................................................................. 87
3.2.1 Preparation of mesoporous silica monoliths [161] ................................. 87
3.2.2 Preparation of mesoporous silica particles [15] ..................................... 88
3.2.3 Surface area analysis-BET method .......................................................... 88
3.2.4 Particle size analyses- Light scattering method ...................................... 90
3.2.5 Scanning electron microscopy (SEM) ...................................................... 91
3.3 Polymer electrolytes .............................................................................................. 91
3.3.1 Preparation of crosslinked PEGDGE polymer electrolytes .................... 91
3.3.1.1 Preparation of crosslinked PEGDGE electrolytes using TBAPF6
salt…….. ...................................................................................................... 91
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3.3.1.2 Preparation of crosslinked PEGDGE electrolytes using LiTFSI
salt…….. ...................................................................................................... 92
3.3.1.3 Preparation of crosslinked PEGDGE electrolytes using EMITFSI
ionic liquid ................................................................................................... 92
3.3.2 Preparation of crosslinked DGEBA electrolytes ..................................... 93
3.3.2.1 Preparation of crosslinked DGEBA electrolytes using LiTFSI
salt…….. ...................................................................................................... 93
3.3.2.2 Preparation of crosslinked DGEBA electrolytes using EMITFSI
ionic liquid ................................................................................................... 94
3.3.3 Preparation of PAN gel based polymer electrolytes................................ 94
3.3.4 Preparation of crosslinked PEGDGE/DGEBA electrolytes .................... 95
3.3.4.1 Preparation of crosslinked PEGDGE/DGEBA electrolytes using
10 wt% LiTFSI salt ..................................................................................... 95
3.3.4.2 Preparation of crosslinked PEGDGE/DGEBA electrolytes using
10 wt% EMITFSI ionic liquid ..................................................................... 95
3.3.4.3 Preparation of crosslinked PEGDGE/DGEBA electrolytes using
50 wt% EMITFSI ionic liquid ..................................................................... 96
3.4 Composite polymer electrolytes............................................................................ 97
3.4.1 Preparation of MSP/PEGDGE composite polymer electrolytes ............. 97
3.4.1.1 Preparation of crosslinked MSP/PEGDGE composite polymer
electrolytes using LiTFSI salt ...................................................................... 97
3.4.1.2 Preparation of crosslinked MSP/PEGDGE composite polymer
electrolytes using EMITFSI ionic liquid ..................................................... 98
3.4.2 Preparation of crosslinked MSP/DGEBA composite polymer
electrolytes.. .......................................................................................................... 99
3.4.3 Preparation of crosslinked MSP/PEGDGE/DGEBA composite polymer
electrolytes .......................................................................................................... 100
3.4.3.1 Preparation of crosslinked MSP/PEGDGE/DGEBA composite
polymer electrolytes using LiTFSI salt ..................................................... 100
3.4.3.2 Preparation of crosslinked MSP/PEGDGE composite polymer
electrolytes using EMITFSI ionic liquid ................................................... 100
3.5 Chemical Activation of carbon fibre mats .......................................................... 101
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3.6 Electrochemical impedance spectroscopy of polymer electrolytes .................... 102
3.7 Mechanical characterisation of polymer electrolytes .......................................... 103
3.7.1 Rheological characterisation of PAN gel polymer electrolytes ............. 103
3.7.2 Mechanical characterisation of solid polymer electrolytes (Compression
testing).. ............................................................................................................... 104
3.8 Composite fabrication using Resin Infusion under Flexible Tooling (RIFT) ..... 105
3.9 Electrochemical characterisation of structural supercapacitors .......................... 108
3.9.1 Cyclic voltammetry ................................................................................ 108
3.9.2 Potential square-wave voltammetry (Charge/discharge) ...................... 109
3.9.3 Electrochemical impedance spectroscopy ............................................. 109
3.10 Mechanical characterisation of composites (±45° laminate tensile test) ... 110
3.11 Fibre volume fraction of structural supercapacitors by acid digestion ...... 112
Chapter 4 Polymer Electrolytes ............................................................................... 114
4.1 Selection of salts for inclusion into polymers ..................................................... 115
4.2 Polyacrylonitrile gel polymer electrolytes .......................................................... 116
4.3 Crosslinked PEGDGE polymer electrolytes ....................................................... 121
4.3.1 Effect of different ionic salts on ionic conductivity and compression
properties of crosslinked PEGDGE electrolytes ................................................ 121
4.3.2 Effect of increasing EMITFSI concentration on ionic conductivity and
compression properties of crosslinked PEGDGE electrolytes ........................... 123
4.4 Crosslinked DGEBA polymer electrolytes ......................................................... 125
4.5 Crosslinked PEGDGE/DGEBA polymer electrolytes ........................................ 126
4.5.1 Crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% [LiTFSI]
in PC…. ............................................................................................................... 126
4.5.2 Crosslinked PEGDGE/DGEBA electrolytes containing 10wt%
EMITFSI… .......................................................................................................... 127
4.5.3 Crosslinked PEGDGE/DGEBA electrolytes containing 50wt%
EMITFSI… .......................................................................................................... 129
4.6 Multifunctionality of polymer electrolytes ......................................................... 131
Table of Contents
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Chapter 5 Polymer Composite Electrolytes ............................................................ 135
5.1 Mesoporous silica ............................................................................................... 136
5.1.1 Surface characterisation of mesoporous silica monoliths (MSMs) and
mesoporous silica particles (MSP) ..................................................................... 137
5.2 Effect of mesoporous silica on the mechanical and electrochemical properties of
polymer composite electrolytes .................................................................................... 145
5.2.1 Ionic conductivity and compression properties of crosslinked
PEGDGE/MSM composite electrolytes containing TBAPF6/PC ........................ 146
5.2.2 Ionic conductivity and compression properties of crosslinked
MSP/PEGDGE composite electrolytes containing TBAPF6/PC ........................ 147
5.2.3 Ionic conductivity and compression properties of crosslinked
MSP/PEGDGE composite electrolytes containing LiTFSI/PC .......................... 148
5.2.4 Ionic conductivity and compression properties of crosslinked
MSP/PEGDGE composite electrolytes containing EMITFSI ............................. 151
5.2.5 Ionic conductivity and compression properties of crosslinked
DGEBA/MSP composite electrolytes containing LiTFSI/PC ............................. 153
5.2.6 Ionic conductivity and compression properties of crosslinked PEGDGE/
DGEBA/MSP composite electrolytes containing LiTFSI/PC ............................. 153
5.2.7 Ionic conductivity and compression properties of crosslinked PEGDGE/
DGEBA/MSP composite electrolytes containing 50 wt% EMITFSI ................... 155
5.3 Multifunctionality of polymer composite electrolytes ........................................ 156
Chapter 6 Structural Supercapacitors ..................................................................... 160
6.1 Influence of glass fibre separators on the specific capacitance of structural
supercapacitors ............................................................................................................. 161
6.2 Influence of varying charging time on the specific capacitance of structural
supercapacitors ............................................................................................................. 164
6.3 Influence of different types of electrolyte salts on the electrochemical and
mechanical performance of structural supercapacitors ................................................ 166
6.4 Influence of separator type on the specific capacitance and shear properties of
structural supercapacitors ............................................................................................. 169
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6.5 Influence of the polymer electrolyte composition on the electrochemical and
mechanical performance of structural supercapacitors ................................................ 172
6.6 Influence of EMITFSI concentration on electrochemical and mechanical
performance of structural supercapacitors .................................................................... 175
6.7 Influence of the connectivity of copper tape and copper wire on the
electrochemical performance of structural supercapacitors ......................................... 179
6.8 Influence of charge-discharge cycles on the specific capacitance of structural
supercapacitors ............................................................................................................. 181
6.9 Influence of applied potential difference on the energy density of structural
supercapacitor ............................................................................................................... 183
6.10 Influence of addition of MSP on the electrochemical and mechanical
performance of structural supercapacitors .................................................................... 184
6.11 Configuration of structural supercapacitors .............................................. 187
6.12 Influence of CF activation on the electrochemical and mechanical
performance of structural supercapacitors .................................................................... 190
6.12.1 Structural supercapacitors with a crosslinked PEGDGE matrix
containing 10 wt% EMITFSI .............................................................................. 190
6.12.2 Structural supercapacitors with a crosslinked 40:60 PEGDGE/DGEBA
blend matrix containing different EMITFSI concentrations ............................... 195
6.12.3 Structural supercapacitors with a crosslinked MSP/PEGDGE matrix
containing 10 wt% EMITFSI .............................................................................. 198
6.13 Multifunctionality of structural supercapacitors........................................ 202
Chapter 7 Conclusions and Suggestions for Future Work .................................... 205
7.1 Conclusions ......................................................................................................... 206
7.1.1 Developments of the polymer electrolytes ............................................. 206
7.1.2 Developments of the polymer composite electrolytes ............................ 208
7.1.3 Developments of the structural supercapacitors ................................... 210
7.2 Suggestion for future work ................................................................................. 213
7.2.1 Improvements in the multifunctionality of polymer electrolytes ............ 213
7.2.2 Improvements in the energy density of structural supercapacitors ....... 214
Table of Contents
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7.2.3 Improvements in the power density of structural supercapacitors ........ 215
7.2.4 Improvements in the mechanical performance of structural
supercapacitors ................................................................................................... 215
Appendix A Result tables of polymer electrolytes and polymer composite electrolytes217
Appendix B Instructions of measuring the ionic conductivity of polymer electrolytes and
composite polymer electrolytes ........................................................................................... 225
Appendix C Instructions of measuring the machine compliance for determining the
compression modulus of polymer electrolytes ................................................................... 227
Appendix D Microscopic Evaluation on the MSP reinforced polymer electrolytes ...... 228
Appendix E Shear stress and straincurves for the ±45º laminated structural
supercapacitor specimens .................................................................................................... 233
Appendix F Nuclear magnetic resonance spectroscopy (NMR) of diglycidylether of
bisphenol-A epoxy and 4,4’ methylene bis(cyclo hexyl amine) crosslinker .................... 234
Reference List ....................................................................................................................... 236
List of Figures
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List of Figures Chapter 1
Figure 1.1 BAE systems Mantis UAV that will employ structural energy composites
(Courtesy of BAE systems). .................................................................................................... 40
Figure 1.2 Spare-wheel floor [11] of a Volvo car replaced with a multifunctional composite
to be developed in the STORAGE project. .............................................................................. 41
Figure 1.3 Cross sectional view of proposed multifunctional structural supercapacitors. ...... 42
Chapter 2
Figure 2.1 Schematic of different electrical energy storage devices by Sels et al. [28]. ......... 49
Figure 2.2 Ragone plot showing energy storage delivery performance for different storage
devices by Kotz et al. [31]. ...................................................................................................... 50
Figure 2.3 Schematic diagram of battery by Goodenough et al. [27]. ..................................... 53
Figure 2.4 Cross sectional view of structural lithium ion battery fabricated by Thomas et al.
[50] ........................................................................................................................................... 54
Figure 2.5 Layup schematic of an embedded thin film lithium energy cells on CF reinforced
epoxy composites by Pereira et al. [51] ................................................................................... 55
Figure 2.6 Schematic (a) and geometry (b) of PowerFibre invented by Neudecker et al. [53]
.................................................................................................................................................. 56
Figure 2.7 Schematic (a) and cross-sectional view (b) of the integrated battery on CF
reinforced epoxy composites by Kim et al. [54] ...................................................................... 56
Figure 2.8 Schematic of a model geometry of a Li-ion battery cell by Kim et al. [41] ........... 57
Figure 2.9 Schematic of the cross-section of structural battery described by Wong. [59] ...... 58
Figure 2.10 Schematic of a structural battery developed by and taken from Liu et al. [60] ... 59
Figure 2.11 Schematic of an autophagous structure-power system for an unmanned air
vehicle by Thomas et al. [61] ................................................................................................... 59
Figure 2.12 Schematic diagram of a fuel cell by Goodenough et al. [27]. .............................. 61
List of Figures
19
Figure 2.13 Pultruded fuel cell panel developed by Peairs et al. [14] ..................................... 62
Figure 2.14. Schematic of a structural fuel cell by South et al. [62]. ...................................... 63
Figure 2.15 Schematic of a capacitor. ...................................................................................... 65
Figure 2.16 Schematic of a structural capacitor by O’Brien et al. [46] ................................... 66
Figure 2.17 Schematic of piezoelectric fibre also acting as structural capacitor by Lin et al.
[69] ........................................................................................................................................... 67
Figure 2.18 Electrolytic capacitor patented by General Electric Company, New York [76]. . 68
Figure 2.19 Electrical energy storage apparatus patented by the Standard Oil Company,
Cleveland, Ohio [77]. ............................................................................................................... 69
Figure 2.20 Schematic of a supercapacitor by Halper et al. [95]. ............................................ 71
Figure 2.21 Schematic of the types of supercapacitors by Haler et al. [95]. ........................... 73
Figure 2.22 Schematic of supercapacitor assembly by Tien et al. [100] ................................. 74
Figure 2.23 History of improvements in ionic conductivity of the polymer electrolytes by
Murata et al. [114]. ................................................................................................................... 77
Chapter 3
Figure 3.1 Chemical structures of PEGDGE (a), DGEBA (b) and PAN (c). .......................... 84
Figure 3.2 Chemical structures of TETA (a) and MCHA (b). ................................................. 85
Figure 3.3 Chemical structures of LiTFSI (a), TBAPF6 (b) and EMITFSI (c). ...................... 85
Figure 3.4 Chemical structure of PC........................................................................................ 86
Figure 3.5 Chemical structure of TEOS. ................................................................................. 86
Figure 3.6 Chemical structure of Pluronic P123 (x = 20, y = 70, z = 20). ............................. 86
Figure 3.7 Adsorption isotherms I to VI classified after IUPAC 1984 (image taken from P.
Somasundaran, 2006). .............................................................................................................. 89
Figure 3.8 Schematic of light scattering through laser diffraction by Malvern [166]. ............ 90
Figure 3.9 An oscillating shear strain and the stress response for viscoelastic materials [171].
................................................................................................................................................ 104
Figure 3.10 Schematic of a RIFT process. ............................................................................. 106
List of Figures
20
Figure 3.11 Vacuum bag during RIFT process (a) Rift setup, (b) sandwiched CF and GF mats
before RIFT process, (c) Structural supercapacitors after RIFT process. .............................. 107
Figure 3.12 Schematic of a ±45° laminated structural supercapacitor during tensile test in
accordance with ASTM D 3518. ........................................................................................... 111
Figure 3.13 Tensile testing of a structural supercapacitor specimen (a) pre tensile test
specimen, (b) post tensile test specimen. ............................................................................... 112
Chapter 4
Figure 4.1 Temperature (a) and frequency (b) sweep tests of PAN1-3D, PAN1-6M and
PAN2 gel based polymer electrolytes (G/ = storage modulus and G// = loss modulus). ....... 117
Figure 4.2 Cyclic voltamograms (a) and impedance spectroscopy plots (b) of PAN1-3D,
PAN1-6M and PAN2 gel polymer electrolytes at room temperature. ................................... 120
Figure 4.3 Ionic conductivity ҡ as function of storage modulus G/ (peak maximum) of three
PAN gel polymer electrolytes by varying PAN/plasticiser concentration at 25°C. .............. 121
Figure 4.4 Effect of increasing EMITFSI concentration on crosslinked PEGDGE electrolyte,
(a) EMITFSI concentration vs. ionic conductivity ҡ and compression modulus E; and ....... 124
Figure 4.5 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and
compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in
crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% LiTFSI/PC. ....................... 127
Figure 4.6 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and
compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in
crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% EMITFSI. ......................... 128
Figure 4.7 Stress strain curves of crosslinked PEGDGE/DGEBA blend polymer electrolytes
containing 10 wt% EMITFSI ................................................................................................. 129
Figure 4.8 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and
compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in
crosslinked PEGDGE/DGEBA electrolytes containing 50 wt% EMITFSI. ......................... 130
Figure 4.9 Photograph of what on tissue paper showing phase separation of EMITFSI from
crosslinked PEGDGE/DGEBA electrolyte with xDGEBA of 0.8 containing 50 wt% EMITFSI.
................................................................................................................................................ 131
List of Figures
21
Figure 4.10 Compression modulus E (a) and compression strength σ (b) of different
crosslinked PEGDGE (P) and crosslinked DGEBA (B) electrolytes containing either
LiTFSI/PC (Li) or EMMITFSI (E) as a function of ionic conductivity ҡ at room temperature.
................................................................................................................................................ 133
Chapter 5
Figure 5.1 Schematics of the proposed structural evolution of silica network showing
micelles with PEO-co-PPO corona (thin lines), TEOS molecules (thick lines)and embedded
PEO-co-PPO chains in silica, adopted from Rodriguez-Abreu et al. [191]. .......................... 136
Figure 5.2 BET nitrogen adsorption/desorption isotherms (a) and BJH pore size distribution
(b) of mesoporous silica from samples with varying reaction layer thickness. ..................... 137
Figure 5.3 BET nitrogen adsorption/desorption isotherm (a) and BJH pore size distribution
(b) of mesoporous silica monoliths by varying the curing temperature. ............................... 139
Figure 5.4 Mesoporous silica monolith having internal cracks after ethanol washing. ......... 141
Figure 5.5 Particle size distributions of MSP and NSP obtained by dynamic light scattering.
................................................................................................................................................ 142
Figure 5.6 BET nitrogen adsorption/desorption isotherm (a) and BJH pore size distribution
(b) of mesoporous (MSP) and non-porous (NSP) silica particles. ......................................... 143
Figure 5.7 SEM images of crushed mesoporous silica monoliths (a) and mesoporous silica
particles (b and c). .................................................................................................................. 145
Figure 5.8 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked MSP/PEGDGE composite electrolytes, containing 0.8 wt% TBAPF6/PC, as a
function of increasing MSP concentration. ............................................................................ 148
Figure 5.9 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked MSP/PEGDGE composite electrolytes containing 0.8 wt% LiTFSI/PC as a
function of increasing MSP concentration. ............................................................................ 149
Figure 5.10 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked PEGDGE/NSP composite electrolytes, containing 0.8 wt% LiTFSI/PC, as a
function of increasing non-porous silica particles NSP concentration. ................................. 150
List of Figures
22
Figure 5.11 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked MSP/PEGDGE composite electrolytes containing 10 wt% EMITFSI as a
function of increasing MSP content. ...................................................................................... 152
Figure 5.12 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of 40/60 weight ratio of PEGDGE/DGEBA composite electrolytes containing 50 wt%
EMITFSI, as function of increasing MSP concentration. ...................................................... 156
Figure 5.13 Compression modulus E (a) and compression strength σ (b) of different MSP (M)
or NSP (N) reinforced crosslinked PEGDGE (P) and crosslinked DGEBA (B) composite
electrolytes containing TBAPF6/PC (A), LiTFSI/PC (Li) or EMITFSI (E) as a function of
ionic conductivity ҡ at room temperature. ............................................................................. 158
Chapter 6
Figure 6.1 Optical micrographs of various commercially available glass fibre fabrics, (a)
ACG 1, (b) ACG 2, (c) Tissa 1, (d) Tissa 2 and (e) Tissa 3 (microscopic images taken by Dr.
Hui Qian). .............................................................................................................................. 162
Figure 6.2 Charge discharge curves of investigated structural supercapacitors with various
charging times of (a) 10s showing high charge loss and incomplete discharge, (b) 100 s, (c)
250 s and (d) 500 s. ................................................................................................................ 164
Figure 6.3 Specific capacitance Cg as a function of charging time during charge-discharge
experiment.............................................................................................................................. 166
Figure 6.4 Charge-discharge curves of as-received CF reinforced crosslinked PEGDGE
composites containing (a) 0.8 wt% LiTFSI/PC and (b) 10 wt% EMITFSI and two layers of
glass fibre mats as separator. Charging time = 600 s. ............................................................ 167
Figure 6.5 Charge-discharge curves for the as-received CF reinforced crosslinked PEGDGE
composites containing 10 wt% EMITFSI electrolyte with (a) filter paper, (b) polypropylene
(PP) membrane and (c) two layers of glass fibre mat as separators. ..................................... 170
Figure 6.6 Charge-discharge curves for the as-received CF and GF reinforced crosslinked
PEGDGE/DGEBA composites containing 10 wt% EMITFSI as a function of the PEGDGE to
DGEBA weight ratio.............................................................................................................. 173
Figure 6.7 Photographs of post-test in-plane shear specimens of CF and GF reinforced
polymer electrolytes containing 10 wt% EMITFSI with varying content of PEGDGE and
List of Figures
23
DGEBA in the crosslinked matrix (a) Pure DGEBA, (b) 20P:80B, (c) 40P:60B, (d) 60P:40B,
(e) 80P:20B and (f) Pure PEGDGE. ...................................................................................... 175
Figure 6.8 Charge-discharge curves for supercapacitors made using as-received CF and GF
with: (a) 100 wt% EMITFSI, (b) 50 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix,
or (c) 10 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix as electrolyte. .............. 176
Figure 6.10 Nyquist plots for the as-received CF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix containing increasing amounts of EMITFSI. .................... 177
Figure 6.11 Photographs of supercapacitor specimens made from CF and GF reinforced
crosslinked 40:60 PEGDGE/DGEBA blend matrix with (a) 10 wt% EMITFSI and (b) 50
wt% EMITFSI after in-plane shear testing. ........................................................................... 179
Figure 6.12 Different configurations of CF based electrodes during fabrication of structural
supercapacitors. ...................................................................................................................... 180
Figure 6.13 Charge-discharge curves of different CF electrode configurations in as-received
CF and GF reinforced crosslinked PEGDGE supercapacitors with 10wt% EMITFSI. ........ 180
Figure 6.14 Evolution of specific capacitance measured at 150 s of charging time for as-
received CF and GF reinforced crosslinked PEGDGE supercapacitors containing 10 wt%
EMITFSI as function of number of charge/discharge cycles. ............................................... 182
Figure 6.15 Charge-discharge curves for the as-received CF and GF reinforced crosslinked
PEGDGE supercapacitors containing 10wt% EMITFSI at cycle number (a) 1, (b) 500 and (c)
1000 in charge-discharge experiment. ................................................................................... 183
Figure 6.16 Complex impedance plots for structural supercapacitors made from as-received
CF and GF reinforced crosslinked PEGDGE matrix (a) or crosslinked PEGDGE matrix with
7.5 wt% MSP (b) containing 10 wt% EMITFSI. ................................................................... 185
Figure 6.17 Charge-discharge curves for structural supercapacitors made from as-received CF
and GF reinforced crosslinked PEGDGE matrix (a) or crosslinked PEGDGE matrix with 7.5
wt% MSP (b) containing 10 wt% EMITFSI. Charging time = 600 s .................................... 185
Figure 6.18 Photographs of CF and GF reinforced crosslinked PEGDGE containing 10 wt%
EMITFSI composites after in-plane shear testing with (a) crosslinked PEGDGE and (b)
crosslinked PEGDGE/7.5 wt% MSP. .................................................................................... 187
List of Figures
24
Figure 6.19 Schematic of (a) series, or (b) parallel lay-up combination of two structural
supercapacitors. ...................................................................................................................... 188
Figure 6.20 Charge/ discharge curves for the as-received CF and GF reinforced PEGDGE
containing 10 wt% EMITFSI (a) baseline, (b) three supercapacitors laid-up and tested in
series, or (c) three supercapacitors laid-up and tested in parallel. ......................................... 189
Figure 6.21 Impedance plot for CF and GF reinforced crosslinked PEGDGE composites with
10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes. ............................. 191
Figure 6.22 Charge/ discharge curves for CF and GF reinforced crosslinked PEGDGE
composites with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.
Charging time = 600 s. ........................................................................................................... 192
Figure 6.23 Charge/ discharge curves for CF and GF reinforced crosslinked PEGDGE
composites with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.
Charging time = 1500 s. ......................................................................................................... 192
Figure 6.24 Photographs of CF and GF reinforced crosslinked PEGDGE composites
containing 10 wt% EMITFSI after in-plane shear testing with (a) as-received carbon fibre, or
(b) activated carbon fibre (ACF) reinforcements. .................................................................. 194
Figure 6.25 Nyquist plot for the ACF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix composites containing various concentrations of EMITFSI.
................................................................................................................................................ 195
Figure 6.26 Charge-discharge curves for the ACF and GF based supercapacitors with (a) 100
wt% EMITFSI, (b) 50 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix,
and (c) 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix. ............... 196
Figure 6.27 Photographs of ACF and GF reinforced crosslinked 40:60 PEGDGE/DGEBA
blend matrix with increasing amounts of EMITFSI after shear testing; (a) 10 wt% EMITFSI
and (b) 50 wt% EMITFSI. ..................................................................................................... 198
Figure 6.28 Complex impedance plots for CF and GF reinforced crosslinked MSP/PEGDGE
matrix containing 10 wt% EMITFSI with (a) as-received CF electrode, or (b) ACF
electrodes. .............................................................................................................................. 199
Figure 6.29 Charge/discharge curves for (a) as-received CF, or (b) ACF electrodes and GF
reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI. .............. 200
List of Figures
25
Figure 6.30 Charge/discharge curves for (a) as-received CF, or (b) ACF electrodes and GF
reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI. .............. 200
Figure 6.31 Photographs of structural supercapacitors consisting of crosslinked
MSP/PEGDGE matrix containing 10 wt% EMITFSI, GF separator and (a) as-received CF or
(b) ACF reinforcements after in-plane shear testing. ............................................................. 202
Figure 6.32 Ragone plot relating energy density E to the power density P of studied structural
supercapacitors in comparison to other energy storage devices. ........................................... 203
Figure 6.33 Multifunctional plot of studied structural supercapacitors relating shear modulus
G12 to the specific capacitance Cg. ......................................................................................... 204
List of Tables
26
List of Tables Chapter 1
Table 1.1 Contributions of the individual components in the proposed multifunctional
composites................................................................................................................................ 42
Chapter 2
Table 2.1 Comparison of battery, capacitor and supercapacitor (values taken from NuinTEK
[36]).......................................................................................................................................... 51
Table 2.2 Capacitance range C, working potential range V and estimated specific energy Eest
of commercially available supercapacitors. ............................................................................. 70
Chapter 3
Table 3.1 Summary of relevant properties of fibre mats. ........................................................ 87
Table 3.2 Composition of PEGDGE polymer electrolytes by increasing the concentrations of
EMITFSI. ................................................................................................................................. 93
Table 3.3 Composition of DGEBA polymer electrolytes by increasing the concentrations of 1
M LiTFSI/PC. .......................................................................................................................... 93
Table 3.4 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of 1 M
LiTFSI/PC by varying the PEGDGE and DGEBA concentrations. ........................................ 95
Table 3.5 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of
EMITFSI by varying the PEGDGE and DGEBA concentrations. .......................................... 96
Table 3.6 Composition of PEGDGE:DGEBA blend polymer electrolytes with 50 wt% of
EMITFSI by varying the PEGDGE and DGEBA concentrations. .......................................... 97
Table 3.7 Composition of MSP/PEGDGE composite polymer electrolytes with 0.8wt% of 1
M LiTFSI/PC by varying the MSP concentrations. ................................................................. 98
Table 3.8 Composition of MSP/PEGDGE composite polymer electrolytes with 10wt% of
EMITFSI by varying the MSP concentrations......................................................................... 99
Table 3.9 Composition of DGEBA/MSP composite polymer electrolytes with 20wt% of 1 M
LiTFSI/PC by varying the MSP concentrations. ..................................................................... 99
List of Tables
27
Table 3.10 Composition of PEGDGE/DGEBA/MSP composite polymer electrolytes with
20wt% of 1 M LiTFSI/PC by varying the MSP concentrations. ........................................... 100
Table 3.11 Composition of PEGDGE/DGEBA/MSP composite polymer electrolytes with
50wt% of EMITFSI by varying the MSP concentrations. ..................................................... 101
Table 3.12 Single fibre diameter df, BET surface area As, specific capacitance Cg, tensile
modulus ET and tensile strength σT of as-received and activated carbon fibre mats (results
courtesy of Dr. Hui Qian). ..................................................................................................... 102
Chapter 4
Table 4.1 Ionic conductivity of different salts. ...................................................................... 115
Table 4.2 Specific capacitance Cg and ionic conductivity ҡ of the PAN gel polymer
electrolytes. ............................................................................................................................ 119
Table 4.3 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked PEGDGE electrolytes by varying different salts (A-0.1 M TBAPF6/PC, Li-1.0 M
LiTFSI/PC, Na-1.0 M NaClO4/PC, E- EMITFSI). ................................................................ 122
Table 4.4 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked DGEBA electrolytes as function of increasing LiTFSI/PC concentrations. ....... 125
Chapter 5
Table 5.1 Surface areas, SBET and SLangmuir, pore volume VP, pore width dP, full width at the
half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous
silica monoliths MSM synthesised at 90°C with increasing monolith thickness hsample. ....... 138
Table 5.2 Surface areas, SBET and SLangmuir, pore volume VP, pore width dP, full width at the
half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous
silica monoliths MSM cured at different temperatures TCuring. .............................................. 140
Table 5.3 Surface areas, SBET and SLangmuir, pore volume VP, pore width dP, mass-median
particle diameter d50 and bulk density of mesoporous (MSP) and non-porous silica particles.
................................................................................................................................................ 144
List of Tables
28
Table 5.4 Ionic conductivity ҡ compression modulus E and compression strength σ of
PEGDGE (0.1 M TBAPF6/PC) polymer electrolyte with increasing crushed MSM
concentration. ......................................................................................................................... 146
Table 5.5 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked DGEBA/MSP composite electrolytes containing 20 wt% LiTFSI/PC as a function
of increasing MSP concentration. .......................................................................................... 153
Table 5.6 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked PEGDGE/DGEBA/MSP composite electrolytes containing 10 wt% LiTFSI/PC as
a function of increasing MSP concentration. ......................................................................... 154
Table 5.7 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked PEGDGE/DGEBA electrolytes containing 20 wt% LiTFSI/PC as a function of
increasing MSP concentration. .............................................................................................. 155
Chapter 6
Table 6.1 Thickness and areal weight of various commercially available glass fibre woven
mats studied. .......................................................................................................................... 161
Table 6.2 Charging and discharging capacity, charge loss and specific capacitance Cg of
structural supercapacitors manufactured using as-received carbon fibre mat, crosslinked
PEGDGE containing 0.8 wt% LiTFSI/PC and various glass fabric separators. .................... 163
Table 6.3 The specific capacitance Cg as determined by charge-discharge experiment of
structural supercapacitor as function of varying charging time. ............................................ 165
Table 6.4 Discharge capacity, charge loss ∆ and specific capacitance Cg of CF and GF
reinforced crosslinked PEGDGE composites containing LiTFSI or EMITFSI as electrolyte.
Charging time = 600 s. ........................................................................................................... 168
Table 6.5 CF volume fraction Vf, maximum shear strength τ12m, shear strength at 5000 µε and
shear modulus G12 of structural supercapacitors with crosslinked PEGDGE containing
LiTFSI/PC or EMITFSI. ........................................................................................................ 169
List of Tables
29
Table 6.6 Discharge capacity, charge loss ∆, specific capacitance Cg and density ρ of as-
received CF reinforced crosslinked PEGDGE or DGEBA composites containing 10 wt%
EMITFSI and various separators. Charging time = 600 s. .................................................... 171
Table 6.7 Carbon fibre volume fraction Vf, maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and
crosslinked DGEBA containing 10wt% EMITFSI electrolyte and filter paper (FP), glass fibre
mats (GF) or PP membrane separators. ................................................................................. 172
Table 6.8 Discharge capacity, charge loss Δ, specific capacitance Cg and bulk density ρ of as-
received CF and GF reinforced crosslinked PEGDGE/DGEBA composites as function of
PEGDGE to DGEBA ratio. Charging time = 600 s. .............................................................. 174
Table 6.9 CF volume fraction Vf, maximum shear strength τ12m, shear strength at 5000 µε and
shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and DGEBA
polymer electrolytes containing 10wt% EMITFSI as function of PEGDGE to DGEBA ratio.
................................................................................................................................................ 174
Table 6.10 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and Power density P of as-received CF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix composites as function of decreasing EMITFSI
concentration. Charging time = 600 s. ................................................................................... 178
Table 6.11 Maximum shear strength τ12m, shear strength at 5000 µε and shear modulus G12 of
structural supercapacitors made using as-received CF and GF with crosslinked 40:60
PEGDGE/DGEBA blend matrix as function of decreasing EMITFSI concentration. .......... 178
Table 6.12 Discharge capacity, charge loss ∆ and specific capacitance Cg for different CF
electrode configurations of as-received CF and GF reinforced crosslinked PEGDGE
supercapacitors containing 10 wt% EMITFSI. ...................................................................... 181
Table 6.13 Influence of applied potential difference on the discharge capacity, charge loss Δ,
specific capacitance Cg and energy density E of structural supercapacitors made from CF and
GF reinforced crosslinked PEGDGE containing 10wt% EMITFSI. ..................................... 184
Table 6.14 Influence of MSP addition on the charge loss Δ, specific capacitance Cg,
equivalent series resistance ESR, energy density E and power density P of structural
supercapacitors made using as-received CF and GF reinforced crosslinked PEGDGE
composites containing 10 wt% EMITFSI. ............................................................................. 186
List of Tables
30
Table 6.15 Effect of MSP additions on the maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of as-received CF and GF reinforced crosslinked PEGDGE
matrix composites. ................................................................................................................. 186
Table 6.16 Discharge capacity, charge loss Δ, specific capacitance Cg and theoretical specific
capacitance of structural supercapacitor assembly made using as-received CF and GF
reinforced crosslinked PEGDGE containing 10 wt% EMITFSI connected either series or
parallel combinations. Charging time= 600 s ........................................................................ 189
Table 6.17 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and power density P of structural supercapacitors made from as-received CF or
ACF and GF reinforced crosslinked PEGDGE matrix containing 10 wt% EMITFSI at a
charging time of 600 s or 1500 s. ........................................................................................... 193
Table 6.18 Influence of CF activation on the maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of CF and GF reinforced crosslinked PEGDGE matrix
composites containing 10 wt% EMITFSI. ............................................................................. 194
Table 6.19 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and power density P of structural supercapacitors made from ACF and GF
reinforced crosslinked 40:60 PEGDGE/DGEBA blend matrix composites containing various
concentrations of EMITFSI. .................................................................................................. 197
Table 6.20 Maximum shear strength τ12m, shear strength at 5000 µε and shear modulus G12 of
ACF and GF based supercapacitors with crosslinked 40:60 PEGDGE/DGEBA blend matrix
containing increasing amounts of EMITFSI. ......................................................................... 197
Table 6.21 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and power density P of structural supercapacitors made using CF or ACF and GF
reinforced crosslinked MSP/PEGDGE matrix containing 10 wt% EMITFSI at a charging
time of 600 s or 1500 s. .......................................................................................................... 201
Table 6.22 Influence of CF activation on the maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of CF and GF reinforced crosslinked MSP/PEGDGE matrix
composites containing 10 wt% EMITFSI. ............................................................................. 201
List of abbreviations
31
List of Abbreviations ACN acetonitrile
ACF activated carbon fibre
AFC alkaline fuel cell
AC alternating current
Al2O3 aluminium oxide
Aq. HCl aqueous hydrochloric acid
ASTM American society for testing and materials
ARL army research laboratory
BT/BaTiO3 barium titanate
BJH Barrett-Joyner-Halenda
BET Brunauer Emmett Teller
CF carbon fibre
DEC diethyl carbonate
DGEBA diglycidylether of Bisphenol-A
DMC dimethyl carbonate
DC direct current
DMFC direct methanol fuel cell
EDL electrochemical double layer
EDLC electrochemical double layer capacitors
EIS electrochemical impedance spectroscopy
EDR equivalent distributive resistance
ESR equivalent series resistance
EtOH ethanol
EC ethylene carbonate
EMITFSI 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
FP filter paper
FWHM full width at the half maximum
GPE gel polymer electrolyte
GF glass fibre
ILSS interlaminar shear strength
List of abbreviations
32
ISO international standards organisation
PMn-PT lead magnesium niobate-lead titanate
LVE linear viscoelastic
LiTFSI lithium bis(trifluoromethylsulfonyl)imide
LiCoO2 lithium cobalt oxide
LiPF6 lithium hexafluorophosphate
LiFePO4 lithium iron phosphate
LiPON lithium phosphorus oxynitride glass
MEA membrane electrode assembly
MS mesoporous silica
MSM mesoporous silica monoliths
MSP mesoporous silica particles
MCHA 4,4’-methylenebis(cyclohexylamine)
MW molecular weight
MCFC molten carbonate fuel cell
NiMH nickel metal hydride
NSP non-porous silica particle
PDA personal digital assistant
PAFC phosphoric acid fuel cell
PAN polyacrylonitrile
PEEK poly(etheretherketone)
PEGDGE polyethylene-glycol-diglycidylether
PEGDMA poly(ethylene glycol) dimethacrylate
PEO poly(ethylene oxide)
PPO poly(propylene oxide)
PMMA poly(methyl methacrylate)
PPy polypyrrole
PETF poly(ethylene terephthalate) film
PVDF-HFP poly(vinyldifluoride-co-hexafluoro propene)
PaCE polymer and composite Engineering
PSD pore size distribution
KOH potassium hydroxide
PC propylene carbonate
List of abbreviations
33
PEMFC proton exchange membrane fuel cell
RIFT resin infusion under flexible tooling
RT room temperature
SEM scanning electron microscopy
SiO2 silica/silicon dioxide
SiC silicon carbide
SWCNT single wall carbon nanotube
SDS sodium dodecyl sulphate
SOFC solid oxide fuel cell
Surfactant surface active agent
TBAPF6 tetrabutylammoniumhexafluorophosphate
TEATFB tetraethyl ammonium tetrafluoroborate
TEOS tetraethyl orthosilicate
TiO2 titanium dioxide
TETA triethylenetetramine
UAV unmanned air vehicles
VARTM vacuum assisted resin transfer moulding
20P:80B weight ratio of 20% PEGDGE and 80% DGEBA
40P:60B weight ratio of 40% PEGDGE and 60% DGEBA
60P:40B weight ratio of 60% PEGDGE and 40% DGEBA
80P:20B weight ratio of 80% PEGDGE and 20% DGEBA
List of Notations
34
List of Notations Notation Description Unit
° degree of angle -
% per cent -
vol% per cent by volume -
wt% per cent by weight -
L Length meter [m]
centimetre [cm]
milli meter [mm]
micro metre [µm]
nano metre [nm]
m mass kilogram [kg]
grams [g]
milli gram [mg]
t time seconds [s]
minutes [min]
hours [h]
milli seconds [ms]
ҡ ionic conductivity Siemens/centimetre [S/cm]
E compression modulus mega Pascal [MPa]
ET tensile modulus mega Pascal [MPa]
σ compression strength mega Pascal [MPa]
σT tensile strength mega Pascal [MPa]
τ12m maximum in-plane shear stress mega Pascal [MPa]
τ120.5 in-plane shear stress at 0.5% strain mega Pascal [MPa]
γ12 i shear strain at i-th data point -
ϵχi longitudinal normal strain at i -
ϵyi transverse normal strain at i -
G12 unidirectional shear modulus mega Pascal [MPa]
R resistance ohm [Ω]
E specific energy watt hours/kilogram [Wh/kg]
List of Notations
35
P specific power watt/kilogram [W/kg]
π pi =3.1416 -
I current ampere [A]
T temperature degree Celsius [°C]
V voltage volt [V]
cm2 unit of area square centimetre [cm2]
cm3 unit of volume cubic centimetre [cm3]
df fibre diameter micrometre [µm]
ε0 permittivity of vacuum -
εr dielectric constant of electrolyte -
C capacitance farads [F]
Eest estimated specific energy watts/kilogram [W/kg]
F force newton [N]
Fmax maximum force newton [N]
GPa unit of stress Giga Pascal [GPa]
h thickness of specimen millimetre [mm]
I electrical current amperes [A]
J unit of energy Joule [J]
kN unit of force kilo Newton [kN]
MPa unit of stress mega Pascal [MPa]
MHz unit of frequency mega hertz [MHz]
ml unit of volume milli liter [mL]
mmol milli moles - [mmol]
mV unit of voltage milli volts [mV]
μA current micro ampere [µA]
N Newton - [N]
η viscosity Pascal second [Pa.s]
P power watt [W]
Pa Pascal - [Pa]
rpm revolutions per minute -
ρ density gram/cubic centimetre [g/cm3]
v specific volume cubic centimetre/gram [cm3/g]
Tg glass transition temperature degree Celsius [°C]
List of Notations
36
T Temperature Kelvin [K]
t Time seconds [s]
τ Shear stress mega Pascal [MPa]
ω Frequency hertz [Hz]
Tm Melting temperature degree Celsius [°C]
tan δ loss or dissipation factor -
δ Phase lag angle degrees [°]
ε Strain -
C BET parameter -
€ European currency euros
Fmax maximum force newton [[N]
Q charge coulombs [C]
E electric field newton/coulomb [N/C]
d plate separation centimetre [cm]
A surface area square centimetre [cm2]
AS specific surface area square centimetre/gram [cm2/g]
AS BET BET surface area square centimetre/gram [cm2/g]
Cg specific capacitance farads/cubic centimetre [F/cm3]
MW molecular weight gram/mole [g/mol]
ρa areal density gram/square metre [g/m2]
P/PO relative pressure -
na nitrogen gas molecules adsorbed cubic centimetre / gram [cm3/g]
nm specific monolayer amount of adsorbate cubic centimetre / gram [cm3/g]
L Avogadro’s number (= 6.022×1023) -
Am molecular cross sectional area squared metre/moles [m2/mol]
d0.5 mean diameter micrometre [µm]
Ф phase angle - [°]
E potential difference volts [V]
Z impedance ohm [Ω]
Z’ real part of impedance ohm [Ω]
Z” imaginary part of impedance ohm [Ω]
pH potential of hydrogen, measure of
acidity/basicity -
List of Notations
37
strain rate -
τ0 maximum stress mega Pascal [MPa]
γ0 maximum strain -
G* complex modulus Pascal [Pa]
G/ elastic or shear modulus Pascal [Pa]
G// loss or viscous modulus Pascal [Pa]
FCmax maximum compression force newton [N]
dE/dt voltage sweep rate volts/second [V/s]
Vf fibre volume fraction -
Mi initial mass of specimen grams [g]
Mf final mass of fibres after acid digestion grams [g]
ρc density of specimen grams/cubic centimetre [g/cm3]
ρf density of fibres grams/cubic centimetre [g/cm3]
∆ charge loss per cent [%]
Chapter 1 Introduction
38
Chapter 1 Introduction
This chapter provides an introduction to research into structural power composites for energy
storage devices. The chapter starts with the motivation and focuses on the importance of
multifunctionality in the design of an engineering material. This is followed by the
methodology used in the current research. The aims and objectives are discussed afterwards
followed by the brief description of the structure of the thesis.
Chapter 1 Introduction
39
1.1 Motivation
The motivation of the current research is the premise that the weight and volume is
considered to be a primary concern in engineering design of a load-carrying product [1].
Any material that does not contribute to load-carrying is structurally parasitic. Conventional
design practices pursue optimisation by maximising the efficiency of individual
subcomponents by using advanced materials with higher specific properties or by utilising
new performance technologies. Increasingly complex requirements for numerous applications
require a corresponding increase in the efficiency with which these systems utilise their mass
and volume [2]. Another approach is to design multifunctional materials that can perform two
or more functions simultaneously. Multifunctional designs not only improve the system
efficiency through weight and volume savings but also reduce the complexity of the system.
However, on the other hand, the functions of multifunctional materials are conflicting i.e. the
improvement in one of the properties of a multifunctional material results in the reduction of
another [3]. Therefore, optimisation is required between different properties of the
multifunctional material. There is numerous research underway in the field of multifunctional
materials [4]. One such multifunctional design concept that has attracted a great deal of
attention is energy storage. Structural power composites which exhibit simultaneously
structural and electrical energy storage functions are one such example.
Structural power composites have many possible military as well as civilian applications. For
example, a substantial volume of a laptop or mobile phone is the battery. However, by using
multifunctional composites, the casing of the laptop or mobile phone could also store
electrical energy along with the battery and thus reduce the final system weight and volume.
The range and speed of current unmanned aerial vehicles (UAVs) as well as ground electric
vehicles is limited by the performance of batteries [5]. The outer body of the UAVs and
ground electric vehicles does not contribute to energy generation/storage and therefore, the
replacement of the outer body by multifunctional composites could further improve the
performance of UAVs and ground electric vehicles. Soldiers are required to wear protective
clothing, sensors, communication tools and power sources which are becoming more
complex with the advancement in defence technology [2]. If a soldier will carry heavy items
then this may cause considerable decrease in his/her performance. Therefore, the
development of multifunctional systems having certain levels of mechanical integrity as well
as an electrical energy storage capacity could prove valuable for an extensive variety of
applications.
Chapter 1 Introduction
40
Figure 1.1 BAE systems Mantis UAV that will employ structural energy composites (Courtesy
of BAE systems).
The current study will focus on composite multifunctionality and potential for the
development of structural supercapacitor technology. A structural supercapacitor, a
supercapacitor which in addition to storing electrical energy can carry structural loads, may
offer substantial benefits in many systems including UAVs, ground electric vehicles, mobile
phones, laptops as mentioned above. In the fabrication of structural supercapacitors, two
different roles, including the structural and energy storage, are bound together in a single
coherent material. Different material requirements, such as structural and electrochemical
properties, need to be engineered and optimised simultaneously. The focus of this study is to
manufacture materials that can simultaneously carry mechanical loads whilst storing (and
delivering) electrical energy. The versatility of composite materials means that they provide
an ideal opportunity to develop novel multifunctional materials which can store electrical
energy required to power systems, whilst meeting the demands of the mechanical loading [6].
Although the current study is in its embryotic stage, the concept has received huge attention
in the scientific world. BAE Systems have worked with Imperial College to demonstrate the
concept on the development of a component from Mantis UAV (Figure 1.1). Imperial
Chapter 1 Introduction
41
College is also leading a € 3.3m EU 7th Framework research programme (STORAGE) for
the development of structural power composites to be used in hybrid cars [6]. These
multifunctional composites will be employed in Volvo cars by replacing the spare-wheel
floor of the car saving 15% of its system weight. Different newspapers and science
magazines including Daily Mail [7], CNN [8], Daily Mirror [9], The Economist [10] and
Materials World [11] etc. have published articles on the development of structural energy
storing composites.
Figure 1.2 Spare-wheel floor [11] of a Volvo car replaced with a multifunctional composite
to be developed in the STORAGE project.
1.2 Methodology
In the recent past, research efforts have targeted the development of other multifunctional
storage systems including structural batteries [12], structural capacitors [13] and structural
fuel cells [14] (Chapter 2). The paradigm of a novel multifunctional structural supercapacitor
was adopted in this work in order to develop a low weight multifunctional composite
possessing specified electrical and mechanical properties. Carbon fibre reinforced polymer
composite, having a laminated architecture as shown in Figure 1.3, was the main focus of this
work. Glass fibres, along with filter papers and polymer membranes, were used as separators.
Different polymer matrices were used as polymer electrolytes, including polyacrylonitrile
(PAN), diglycidylether of bisphenol-A (DGEBA) and polyethylene glycol diglycidylether
(PEGDGE). Mesoporous silica particles (Section 5.1) were used as reinforcements embedded
into the polymer matrix (Section 5.2). Table 1.1 shows the contribution of each component of
Chapter 1 Introduction
42
a structural supercapacitor to the multifunctionality of the final composite and its
requirements.
Mechanical Electrical
conductivity
Ionic
conductivity Requirements
Polymer
matrix
Light weight, decent
mechanical properties and
ionic conductivity
Carbon fibre
mats
High surface area, Excellent
mechanical properties
Glass fibre
mats
Good mechanical properties,
porosity, Electronic insulator
and sufficiently dense
Mesoporous
silica
High surface area, narrow
particle size distribution, high
porosity, mechanical properties
Table 1.1 Contributions of the individual components in the proposed multifunctional
composites.
Figure 1.3 Cross sectional view of proposed multifunctional structural supercapacitors.
Chapter 1 Introduction
43
The research into the optimisation of mechanical and electrochemical functionalities of
structural supercapacitors is a challenge because the requirements tend to be contradictory to
each other. The improvement in one functionality leads to the loss in the other functionality.
The interactions between these two functionalities are not immediately understandable
without further exploration of the performance of individual components of structural
supercapacitors. This involves the implementation of a holistic research approach, embracing
the optimisation of the mechanical and electrochemical performance of the individual
subcomponents of structural supercapacitors, including the CF based electrodes, separator
and solid polymer electrolytes.
1.3 Aims and objectives
The central objective of this research is to develop carbon fibre reinforced composites that
simultaneously act as a structural component as well as an energy-storing supercapacitor. The
goal led to the following objectives for this work;
Formulate solid polymer electrolytes [Chapter 4] to achieve both electrical and
mechanical performance (targets: Young’s modulus in compression around 1 GPa and
ionic conductivity around 10-3 S/cm);
Characterise the mechanical and electrochemical properties of solid polymer
electrolytes;
Synthesise mesoporous silica (MS) (with a surface area > 500 m2/g and an average
pore size of 6-7 nm [15]) and characterise the mechanical as well as electrochemical
properties of mesoporous silica filled solid polymer electrolytes (Chapter 5);
Develop composite materials that can be used for energy storage device and structural
performance simultaneously;
Characterise the mechanical and electrochemical performance of the resulting
composites. Electrochemical characterisation involved measuring the specific
capacitance, charging/discharging, internal resistance, equivalent series resistance and
energy and power density characteristics using methods, such as impedance
spectroscopy and charge-discharge cycling methods. Mechanical characterisation
involved shear properties of structural supercapacitors.
Thus, in conclusion, structural supercapacitors should,
o Be light in weight;
Chapter 1 Introduction
44
o Have high mechanical properties, strength (e.g. shear strength of around 100
MPa) and stiffness (e.g. shear modulus of around 2 GPa), to meet the structural
requirements;
o Have high specific capacitance (around 1 F/cm3) and ionic conductivity
(approximately 10-3 S/cm), to deliver sufficient electrical energy when required;
o Be low in cost;
o Perform at operational temperatures (-30°C to 80°C);
o Have long cycle life (at least 15 years [16]);
o Amenable to system integration.
1.4 Thesis Outline
The dissertation is divided into seven chapters. Chapter 1 describes the motivation,
methodology, aims and objectives of the research project. Chapter 2 consists of a review of
the relevant literature providing the background for the following chapters. It starts with the
background on the carbon-fibre reinforced thermoset composites and different energy storage
devices followed by the concept of multifunctional composites focussing on the mechanical
and electrochemical functionalities of the composites. The concept, research trends, and the
current challenges of different structural energy storage devices including structural batteries,
structural fuel cells and structural capacitors are discussed in detail. Particular focus is given
to the concept of structural supercapacitors and the research trends devoted to the
development of the individual subcomponents, including CF based electrodes, polymer
electrolytes and separators.
Chapter 3 contains the materials and experimental methods which were used throughout this
study. It begins by listing all raw materials followed by the formulation of mesoporous silica
reinforcements, polymer electrolytes, composite polymer electrolytes as well as the
fabrication of structural supercapacitors by resin infusion under flexible tooling (RIFT)
process. Characterisation techniques for the evaluation of mechanical and electrochemical
performance of polymer electrolytes as well as structural supercapacitors are also explained
in this chapter. Chapter 4 presents the results and discussion on the characterisation of
polymer electrolytes. Different matrices for polymer electrolytes studied in this work are
polyacrylonitrile (PAN) gel, polyethylene glycol diglycidylether (PEGDGE) and
Chapter 1 Introduction
45
diglycidylether of bisphenol-A (DGEBA). The compression modulus, compression strength
and ionic conductivity measurements of PEGDGE and DGEBA polymer electrolytes are also
discussed in Chapter4.
Mesoporous silica is characterised and discussed in Chapter 5. Different forms of
mesoporous silica including mesoporous silica monoliths (MSMs) and mesoporous silica
particles (MSP) were added as reinforcement to polymer electrolytes and the results of the
structural and electrochemical characterisation of composite polymer electrolytes are
presented. Chapter 6 discusses the characterisation and analysis of various structural
supercapacitors containing different separators, polymer electrolytes, and CF based
electrodes. Chapter 7 summarises the major findings and presents an outlook for future work.
Chapter 2 Literature Review
46
Chapter 2 Literature Review
In this chapter, a literature survey, on carbon fibre reinforced thermoset composites as well as
energy storage devices, is first presented followed by a review of the specific topic of
multifunctional composites and supercapacitors. Subsequently, an introduction of a very
promising multifunctional composite technology identified as “structural supercapacitors” is
presented. The development of other energy storage devices having structural properties is
also discussed. Focussing on the prospects of augmenting composite systems with
supercapacitor technology in military and civil applications, the survey then highlights the
literature on the development of individual components of structural supercapacitors, i.e. the
electrodes and electrolytes as well as supercapacitors in order to draw attention to the
research objectives and challenges of this work.
Chapter 2 Literature Review
47
2.1 Traditional carbon fibre reinforced thermoset composites
A composite is a structural product fabricated from two or more distinctive materials that are
disparate in nature with widely differing properties (fibre and matrix usually) but when
combined possess an engineering performance exceeding that of the individual components
[17]. Polymer composites play an important role in wide variety of civil and military
applications. In particular, carbon fibre (CF) reinforced epoxy based composites have gained
prime interest because of their outstanding properties, ease of fabrication, low shrinkage after
curing and good thermal resistance [18]. In CF reinforced epoxy based composites, the epoxy
matrix binds the reinforcing carbon fibres together resulting in a coherent structure in which
applied stresses are transferred from matrix to the fibres via the interface [18].
Usually, carbon fibres have the largest volume fraction in CF reinforced epoxy composites.
Most widely used volume fractions of carbon fibre content in CF reinforced composites range
from 50-60 vol% [18, 19]. The main factors affecting the performance of CF reinforced
composites are volume content of all constituents (usually carbon fibres, polymer matrix and
voids), fibre orientation, fibre aspect ratio as well as the strength-moduli characteristics of
both fibres and matrix [20]. Carbon fibres have excellent structural properties and good
electrical conductivity. The electrical resistivity of CFs is very low [21] (of the order of
µΩm) and, therefore, when these fibres, protected by the matrix, are used in fabricating CF
reinforced composites, there exists a good fibre to fibre contact resulting in a composite
having good electrical conduction. The longitudinal electrical resistivity of carbon fibres is
reported to be about 80 µΩm and the transverse electrical resistivity is approximately 4000
µΩm in a CF reinforced composite having CF volume fraction of 55% [21].
The matrix of CF reinforced composites also plays an important role in the overall
performance of CF reinforced composites. The most important issues in selection of matrices
for composites include reinforcement-matrix compatibility in terms of bonding, processing
temperature and processing time. Epoxy resins are the most widely used class of polymer
matrix in the fabrication of CF reinforced composites because of their superior mechanical
properties, thermal resistance, and high tolerance to alkaline conditions [22]. Although
significant interest is now growing in the utilisation of some other thermosets (e.g. polyester,
polyimides, phenolics), as well as thermoplastic (e.g. nylon, PEEK) polymer matrix systems,
in composite fabrication, epoxy resins [23] heavily dominate as matrices in high performance
CF reinforced composite applications.
Chapter 2 Literature Review
48
CF reinforced thermoset composites are now widely used in sectors such as wind energy,
aerospace, marine and automotive industries. CF reinforced thermoset composites are used to
manufacture products such as surgical and sports equipment, civil infrastructure and dentistry
equipment. CF reinforced composites are superior as compared to traditional metallic
materials due to their strength-weight ratio, low thermal expansion and high fatigue
resistance [18]. Although composite materials are now used to make a significant part of
aircraft structure (e.g. Boeing 787 Dreamliner aircraft [24, 25]), one of the main focus now in
composite technology is that these materials should serve multiple functions, behave
intelligently, and be greener [26]. Conventional materials such as epoxy resins are now taking
on a new life with the development of more focused and effective processing methods.
Merging conventional materials in unconventional and novel ways is opening a new era of
opportunities.
2.2 Energy storage devices
Development of energy storage devices is another promising field of interest. The availability
of inexpensive energy has become a primary focus of modern economy. Globally, it is
accepted that in the coming years, electrical energy related problems will become of
increasing interest [27]. Electrical energy related problems influence both technical and
economic aspects of modern society. One of the biggest problems regarding electrical energy
is the voltage variation, such as voltage sags and momentary interruptions [27]. Industrial
machinery is mainly affected by these voltage sags. This may result in potential damage or
complete loss of automated production units. Not only factories but residential electricity
consumers are also affected by these voltage sags. Domestic electrical appliances and
personal computers are also very sensitive to these momentary interruptions of electrical
energy. Therefore, a certain energy reservoir is required that can inject electrical energy into
the electrical grid during voltage sags. Great interest has been developed in making and
refining more efficient energy storage devices. A wide variety of energy storage devices is
schematically shown in Figure 2.1.
Chapter 2 Literature Review
49
Figure 2.1 Schematic of different electrical energy storage devices by Sels et al. [28].
The selection of the correct device for energy storage depends on various factors such as total
cost, environmental conditions and restrictions and, most important, power and energy
density of the energy storing devices. Different electrical energy storage devices, such as
batteries, supercapacitors, capacitors or fuel cells have different power-energy capabilities.
These electrical energy storage devices are usually performance rated on the basis of the
“Ragone plot” [29, 30]. The Ragone plot (Figure 2.2) shows the discharge rate of specific
power as a function of the specific energy being available for a load. Ideally, the storage
device should have a high energy density as well as high power density, but in reality
compromises have to be made between energy density and power density. Generally, if the
energy is discharged quickly then the energy delivery will be low. Thus, the energy storage
device which provides the most energy at the maximum power discharge rates will be
considered the best in terms of electrical performance [30].
Energy Storage
Indirect Storage Direct Storage
Natural
Reservoir
Artificial
ReservoirMagnetically Electrically
Capacitors
Supercapacitors
Super Magnetic Storage
Systems (SMES)
Batteries
Fuel cells
Flywheels
Pumped hydro
Heat
Compressed air
Hydrogen
Chapter 2 Literature Review
50
Figure 2.2 Ragone plot showing energy storage delivery performance for different storage
devices by Kotz et al. [31].
The Ragone plot (Figure 2.2) discloses the current status of the energy storage performance
in which batteries have a high specific energy (approx. 250Wh/kg) but low specific power
(below 1000 W/kg), capacitors have rather high specific power (approximately 107 W/kg) but
low specific energy (below 0.06 Wh/kg) and fuel cells have high energy density (above 1000
Wh/kg) but low power density (below 200 W/kg). Supercapacitors possess intermediate
power density and energy density and also have long life cycles due to the absence of
chemical reactions. Conway [32] has given a comprehensive review on properties and
principles of supercapacitors. Supercapacitors exhibit several advantages over
electrochemical batteries and fuel cells including rapid charge-discharge processes (within
seconds), longer cycle life, and longer shelf life. Large-scale supercapacitors can even
perform power quality regulation of the electrical grid that can avoid costly industrial power
shutdowns [27].
In comparison to batteries that store electrical energy as a result of chemical reactions,
supercapacitors store energy in a different way. In batteries, ions move from one electrode to
the other through an electrolyte and a chemical reaction takes place at the electrodes (or at
least at one of the electrodes). Supercapacitors (Section 2.4), however, store energy
physically without any chemical reaction (Figure 2.20). That is why the process is reversible
Chapter 2 Literature Review
51
and the charge-discharge cycle can be repeated almost without limit (hundreds of thousands
of cycles). Supercapacitors store charge in an electrochemical double layer at the electrolyte-
electrode interface. Due to the high surface area of electrodes and extremely low double layer
thickness, supercapacitors have exceptionally high specific and volumetric capacitances. The
only limitation of supercapacitors is their low energy density as compared to batteries and
fuel cells [32]. Due to intermediate power and energy densities of supercapacitors as
compared to batteries, capacitors and fuel cells, applications of supercapacitors have
increased from vehicles and cell phones to large industrial drive systems [31-33]. A
comparison of properties and performance of capacitors, batteries and supercapacitors is
shown in Table 2.1 [34]. A brief overview of the research trends and working of
supercapacitor is presented in the following section 2.4.
Parameters Capacitor Supercapacitor Battery
Charge time 10-6 ~ 10-3 s 1 ~ 30 s 0.3 ~ 3 h
Discharge time 10-6 ~ 10-3 s 1 ~ 30 s 1 ~ 5 h
Energy Density (Wh/kg) < 0.1 1~10 20 ~ 100
Power Density (W/kg) >1,000 1,000 ~ 2,000 50 ~ 200
Cycle life >500,000 > 100,000 500~ 2,000
Charge/Discharge efficiency ~1.0 0.90~0.95 0.70~0.85
Table 2.1 Comparison of battery, capacitor and supercapacitor (values taken from NuinTEK
[34]).
2.3 Multifunctional composites
In recent years, multifunctional composites have attracted a great deal of attention [4, 12, 35-
38]. Multifunctional composite systems are used for material development and thus, act as a
structural material and also exhibit at least one additional performance-linked function such
as electrical, thermal, optical, chemical or electromagnetic functions. Composite systems are
ideally suited for multifunctional performance as the best features of different materials can
be integrated to form a new material that behaves as a homogeneous entity.
Chapter 2 Literature Review
52
Generally, composite materials are optimised for improvement in structural performance
[35]. However, in multifunctional materials, two or more different, but useful, functionalities
are inherently available. Ideally, multifunctional materials should have low weight with
desirable mechanical, chemical, thermal, electrical, magnetic and optical properties. The
feasibility of a multifunctional composite depends on the physical/chemical compatibility of
the individual constituents as well as the internal/external interfacial capability of the desired
combination of the constituents’ functions.
In recent years, several multifunctional composites have been designed for various
applications. Self-healing composite materials are one such example in which a
microencapsulated healing agent is embedded into the structural composite matrix containing
a catalyst that polymerises the healing agent upon contact, resulting in healing/repairing of
the damaged region [39]. Structural batteries, for the skin of micro air vehicle, are another
example in which flight time is increased while maintaining the total aircraft weight [35].
Another example of a multifunctional composite is a structural supercapacitor (Section 2.4)
that carries, by nature of its application, mechanical loads as well as storing electrochemical
energy and thus could replace the traditional static load bearing components to reduce the
volume and mass of the overall system. Previous studies have utilized different approaches
for optimising the electrical and mechanical properties in a composite system, including
structural batteries [12, 40-43], structural capacitors [13, 44, 45] and structural fuel cells [14,
46].
2.3.1 Structural batteries
2.3.1.1 What are batteries?
Electrical energy storage, in the form of chemical energy in batteries, is the most
conventional and oldest approach [47]. A battery contains one or more electrochemical cells
connected in series or parallel to give a required power and voltage. Electrons are generated
from the anode as a result of chemical reaction and migrate through an external electrical
circuit to the cathode delivering electrical energy to the load en route. In a lithium-ion battery
cell, the anode contains lithium ions, commonly intercalated within graphite (Figure 2.3).
Positive ions, e.g. lithium ions, migrate inside the cell through an electrolyte from one
electrode to another. The electrolyte does not allow the flow of electrons (only ions) and
should be compatible with both electrodes. The current collectors (anode and cathode),
typically metals, allow electron flow to and from electrodes. Typically, copper and
Chapter 2 Literature Review
53
aluminium are used for the anode and cathode, respectively, as the lighter weight aluminium
cannot be used for lithium-based anodes due to its reactivity with lithium. Cell voltage is
defined by the chemical reaction energy occurring in the cell. Usually, the anode and cathode
of batteries are complex composites as they contain polymeric binders, besides the active
material, to hold the powder structure [27]. The anode and cathode also contain conductive
diluents (e.g. carbon black) to give the whole structure electronic conductivity so that
electrons can be transported to the active material [27]. The anode and cathode components
are combined in order to allow a liquid electrolyte to penetrate the structure and the ions to
reach the reacting sites.
Figure 2.3 Schematic diagram of battery by Goodenough et al. [27].
Different battery energy storage applications consist of lead acid, lithium ions, nickel,
cadmium, sodium sulphur, and sodium nickel chloride as electrolytes [27]. Generally, lithium
ion batteries are considered the most powerful batteries offering the same energy as nickel
metal hydride (NiMH) batteries at 20-30% less mass [27]. Li-ion batteries are more
expensive than older technology batteries but are valued for high power portable applications
such as laptops, cell phones and PDAs (personal digital assistant).
Chapter 2 Literature Review
54
2.3.1.2 Research trends in structural batteries
At present, the major on-going research in the development of structural batteries is to reduce
the volume and weight of a battery and to enhance its energy density. In order to develop a
multifunctional structure, the battery should be an integral part of a load bearing structure.
Different approaches have been adopted to fabricate multifunctional structural batteries,
including the embedding of batteries on composites (multifunctional structures) as well as the
fabrication of composites acting as a battery itself (multifunctional materials). Pioneering
work in developing such multifunctional structural batteries was done by Thomas et al. [48]
who fabricated a structural battery using electrodes made from active particles bonded by
lithium ion containing poly(vinyl difluoride) and hexafluoropropene (PVDF-HFP) polymer
matrix, current collectors made from metal meshes, and a separator made of micro porous
polyolefin. Structural batteries were manufactured using hot press lamination as shown in
Figure 2.4. They highlighted plastic lithium-ion structure battery materials and examined the
multifunctional potential of commercial cells acting as shear panels and spar caps (a load
bearing structure) [5]. They studied three different configurations of structural batteries and
were able to obtain a specific energy of 95.2 Wh/kg and a stiffness index of 56
(MPa)1/2/(g/cm3) from a carbon-epoxy reinforced structural battery.
Figure 2.4 Cross sectional view of structural lithium ion battery fabricated by Thomas et al.
[48]
Pereira et al. [49, 50] physically embedded solid-state thin film lithium energy cells into a CF
reinforced epoxy based composite without deteriorating the structural properties of composite
and electrochemical properties of lithium energy cells as shown in Figure 2.5.
Charge/discharge baseline performance remained unchanged when the composite was
exposed to uniaxial tensile loading up to 450 MPa. The maximum tensile stress applied to the
Chapter 2 Literature Review
55
composite without affecting the performance of lithium energy cells was recorded to be
approximately 50% of the ultimate tensile strength of the composites.
Figure 2.5 Layup schematic of an embedded thin film lithium energy cells on CF reinforced
epoxy composites by Pereira et al. [49]
Another novel approach of developing thin film batteries around fibre substrates was
introduced by Neudecker et al. [51] They fabricated a thin layered structure of battery around
a fibre substrate and used a Lipon (lithium phosphorus oxynitride glass) based electrolyte.
The schematic of a thin layered fibre battery termed “PowerFibre” is shown in Figure 2.6.
The PowerFibre combined the energy storing capability with structural properties and has a
diameter of 33-150 µm. A patch consisting of 1000 PowerFibres delivered a 9 W of power at
3V and 3 A while supplying 0.1 Wh of energy.
(a)
Chapter 2 Literature Review
56
(b)
Figure 2.6 Schematic (a) and geometry (b) of PowerFibre by Neudecker et al.[51]
Kim et al. [52] also described another approach by amalgamating a thin film lithium ion
battery and an amorphous silicon solar cell on a printed circuit board and then integrating this
conducting circuit board into a CF reinforced epoxy composite by inkjet printing technique.
A cross-sectional image of the integrated battery cells and composites is shown in Figure
2.7b. The integrated battery composite was fabricated using a vacuum bag moulding process
in an autoclave. The integrated energy storing composite was able to perform as a power
laminate until 0.45% applied static strain after which the battery stopped working within the
integrated structure.
(a) (b)
Figure 2.7 Schematic (a) and cross-sectional view (b) of the integrated battery on CF
reinforced epoxy composites by Kim et al. [52]
Chapter 2 Literature Review
57
Hossain et al. [53] suggested carbon-carbon composite anode materials for batteries showing
good mechanical performance and achieving a specific energy of more than 200 Wh/kg.
Some of the most popular work of this type was performed by Kim et al. [40] They
developed three dimensional Li-ion battery cells with networked electrodes that had
improved power delivery capabilities. A schematic of a three-dimensional Li-ion battery cell
is shown in Figure 2.8.
Figure 2.8 Schematic of a model geometry of a Li-ion battery cell by Kim et al. [40]
Although structural batteries were fabricated by embedding batteries into composites, the
above mentioned studies were multifunctional structures instead of multifunctional materials.
A truly multifunctional material was fabricated by a research team at U.S. ARL (Army
Research Laboratory) led by Wetzel. They designed a load bearing battery i.e. a
multifunctional structural battery [54]. If designed with sufficient structural and energy
efficiency, these structural batteries exhibited a significant decrease in system level weight
and volume. Snyder et al. [12] have explored new structural resin electrolytes [55], structural
anode and structural cathode layered metal meshes [56]. They proposed that multifunctional
structural materials can be realized through the focussed development of new materials,
material architectures and low cost scalable fabrication routes [12]. They chose a carbon
fabric based anode, metal mesh cathode (LiFePO4 with acetylene black for electrical
conduction and poly(ethylene oxide) as a binder) and a vinyl ester random copolymer as
polymer electrolytes for their multifunctional Li ion composite batteries [57]. The polymer
electrolyte, used in the fabrication of a structural battery, had a compression modulus of 25
Chapter 2 Literature Review
58
MPa and an ionic conductivity of 4 µS/cm at a density of 1.1 g/cm3. The schematic of the
structural battery proposed by Wong et al. is shown in Figure 2.9. The structural battery was
fabricated using VARTM (Vacuum Assisted Resin Transfer Moulding) process. The
structural battery had an electrical resistance of 4.5Ω and a tensile specific stiffness of 3.6
GPa/(g/cm3).
Glass fabric separator
Glass fabric separator
Carbon fabric anode
Carbon fabric anode
Stainless steel cathode substrate
Figure 2.9 Schematic of the cross-section of structural battery described by Wong. [57]
More promising work in the field of structural batteries was reported by Liu et al. [58] who
developed structural batteries with tuneable mechanical properties. These structural batteries
had elastic or potential structural load bearing capabilities. The 100 µm thick cathode of the
structural battery was manufactured by dissolving N-methyl pyrrolidone, carbon black,
carbon nanofibres and LiCoO2 in a high molecular weight PVDF. The anodes were also
manufactured in a similar way to the cathode, with the difference that LiCoO2 was replaced
with coke. The separator used in the structural battery was a polymer blend of poly(vinyl
difluoride) hexafluoropropene (PVDF-HFP) and poly(ethylene glycol) dimethacrylate
(PEGDMA) with 1 M LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), diethyl
carbonate (DEC) and dimethyl carbonate (DMC). Aluminium and copper grids were used as
current collectors. The schematic of the structural battery is shown in Figure 2.10. A specific
capacity of 90 Ah/kg and a modulus of 650 MPa were measured for the electrodes. A flexural
modulus of 3.1 GPa and an energy density of 35 Wh/kg was achieved for the structural
battery.
Chapter 2 Literature Review
59
Figure 2.10 Schematic of a structural battery developed by and taken from Liu et al. [58]
2.3.1.3 Other multifunctional energy storage materials
Another novel approach in the structural-battery field was adopted by Thomas et al. [59] at
Naval Research Laboratory, USA who developed an autophagous (“self-consuming”)
GasSpar system that used the vapour pressure of a two-phase liquid-gaseous butane or
propane fuel for strengthening an inflatable composite beam. Autophagous GasSpar system
prototype showed a specific energy of 20 Wh/kg, specific power of 2.9 W/kg, burning time of
7 h and a total usable electrical energy of 8.4 Wh. An autophagous GasSpar system for UAVs
is shown in Figure 2.11 in which GasSpar constitutes the main structural element of the
aircraft wing and combustion thermoelectric conversion process converts the two-phase
hydrocarbon fuel into electrical energy.
Figure 2.11 Schematic of an autophagous structure-power system for an unmanned air
vehicle by Thomas et al. [59]
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2.3.1.4 Challenges in structural batteries
The current state of the art structural batteries are facing many challenges. The biggest
challenge is in the fabrication of structural batteries. A structural battery having an energy
density equivalent to conventional battery cells will become quite expensive. Also, relative to
the currently available batteries, structural batteries offer very small energy and power
densities [12]. Therefore, the main goal in the development of structural batteries is to
improve the energy and power densities and to reduce the manufacturing costs. Structurally,
multifunctional batteries should allow good load transfer without affecting their performance
and long term durability. Electrochemically, these structural batteries should attain high
power density while maintaining high energy density. Therefore, a major effort is required to
tailor and optimise the multifunctionality of individual constituents of structural batteries, i.e.
anode, cathode, separator and polymer electrolyte.
2.3.2 Structural fuel cells
2.3.2.1 What are fuel cells?
A fuel cell is an electrochemical energy conversion device that converts chemicals, for
example hydrogen and oxygen, into water and in the process produces electricity. Chemicals
constantly flow into the cell and as a consequence, electrical energy is produced (Figure
2.12). The conversion of the fuel (e.g. hydrogen) to energy takes place without combustion
and therefore, the process is efficient, clean and quiet.
A fuel cell consists of two electrodes separated by electrolyte. Usually, hydrogen and oxygen
(air) are fed into the anode and cathode of the fuel cell, respectively. The hydrogen splits into
protons and electrons in the presence of a catalyst. The protons pass through the electrolyte
but the electrons must take the long way around, creating a separate current that can be
utilized before they return to the cathode, to be combined with the hydrogen and oxygen to
form a water molecule. Individual cells are “piled” together to generate useful quantities of
energy.
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Figure 2.12 Schematic diagram of a fuel cell by Goodenough et al. [27].
The five major types of fuel cells, depending on the type of electrolytes used, are: Alkaline
(AFC), Proton Exchange Membrane (PEMFC), Molten Carbonate (MCFC), Phosphoric Acid
(PAFC), and Solid Oxide (SOFC) fuel cells. Direct Methanol Fuel Cells (DMFCs) are a type
of PEMFC that directly uses methanol as the fuel. The use of expensive catalyst materials,
such as platinum, makes fuel cells costly. Fuel cells are usually classified on the basis of
operating temperature and the type of electrolyte used. Fuel cells are different from batteries
as batteries store chemical energy in a closed system but fuel cells consume reactants. Also,
electrodes within a battery react and change as a battery is charged or recharged. Fuel cells
range in size from hand-held systems to megawatt power stations and operate most efficiently
over a narrow range of performance parameters and at elevated temperature (approximately
100°C - 1000°C). However, fuel cells are not suitable for high power demands.
2.3.2.2 Research trends in structural fuel cells
Peairs et al. [14] described the manufacturing of a structural methanol fuel cell, through
pultrusion process, which serves both to provide an electrical power source as well as to
oppose mechanical loads that the system may experience during its operation. A conceptual
diagram of a pultruded fuel cell is shown in Figure 2.13. The pultruded structural methanol
fuel cell achieved a current density of 4 mA / cm2 at 0.1 V while the VARTM structural
methanol fuel cell achieved a current density of approximately 5.5 mA /cm2 at 0.1V.
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Figure 2.13 Pultruded fuel cell panel developed by Peairs et al. [14]
The research team at ARL led by Wetzel is also working on structural multifunctional fuel
cells. In ARL, South et al. [60] developed a structural fuel cell by using aluminium foam of
very high porosity as the anode and cathode current collectors, carbon cloth as a gas diffusion
layer, a standard membrane electrode assembly (MEA) and a CF-reinforced epoxy composite
skin for structural support. A schematic of this structural fuel cell is shown in Figure 2.14.
The highly porous aluminium foam, served as an electron conductor, improved the structural
performance of the multifunctional fuel cell and also acted as a distributing medium by
permitting fuel and air reactants to reach the MEA. The structural fuel cell was fabricated
using a hand layup process in a continuous 0°/90° configuration. The ARL structural fuel cell
showed a power density of 12.5 mW/cm2 and an average bending stiffness of 2.3 GPa. High
porosity and high density aluminium foam had the best overall multifunctional performance
in the structural fuel cell.
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Figure 2.14. Schematic of a structural fuel cell by South et al. [60].
Leading more towards the practical implementation of fabricating structural fuel cells,
Kaempgen et al. [61] introduced a highly conductive single wall carbon nanotube (SWCNT)
based multifunctional electrode which showed a similar performance as amorphous carbon
based regular electrodes. SWCNT based electrodes offered required functions for fuel cell
operation such as current collection, catalyst support, gas diffusion and electrolyte contact.
Commonly used amorphous carbon based electrodes in fuel cells require binders as well as a
structural support (e.g. carbon cloth or metal mesh) at the back side within the fuel cell.
SWCNT based electrodes form a self-supporting film without any binder or structural
support. However, SWCNT based fuel cell turned out to be more expensive than amorphous
carbon based fuel cell.
2.3.2.3 Challenges in structural fuel cells
The development of structural fuel cells faces many challenges. Structurally, the cores of the
fuel cell assembly should be engineered in order to reduce the effect of the midplane
membrane on the shear properties, as the midplane membrane provides poor shear stiffness
and strength. Electrochemically, structural fuel cells need to attain a high power density (at
least an order of magnitude increase) while maintaining high energy density. Also, the cost of
electrodes, catalyst and fuel required for the fuel cell operation is very high. Fuel and waste
product/ stream management is another challenge. Therefore, new potential materials should
be explored for the advancement of structural fuel cells. In addition, further consideration is
required on material architectures as well as fabrication techniques.
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2.3.3 Structural capacitors
2.3.3.1 What are capacitors?
A capacitor stores energy in the form of an electrostatic field on electrically conducting plates
that are placed very close together but separated with an insulator (called dielectric), shown
in Figure 2.15. Usually a capacitor has more than two plates depending on the capacitance or
dielectric type. The dielectric materials are ceramic, paper, polymer or other insulating
materials and the conducting plates are metals in foil, thick film or thin film form [44]. In
general, the energy density E and power density P can be calculated from the following
equations 2.1 and 2.2,
12
(2.1)
4 (2.2)
Where C is the capacitance, V is the applied potential difference and R is the equivalent
series resistance (ESR). ESR is the undesired internal resistance of a capacitor that appears
with the desired capacitance in series at a specified frequency. Usually, the ESR of a
capacitor is just a small fraction of an Ohm for a low voltage, high capacitance capacitor (e.g.
1000µF, 16V), and can be as high as 2-3 Ohm for a high voltage, low capacitance capacitor
(e.g. 1uF, 450V).
The bulkiness and high frequency performance of capacitors remains an issue. The bulkiness
is specially a major concern when high capacitance is required. Capacitors have very low
energy density (equation 2.1) in comparison to batteries and supercapacitors but the power
density (equation 2.2) is very high as shown in Ragone plot (Figure 2.2). This means that
capacitors are able to deliver or accept high currents, but only for extremely short periods.
For achieving high capacitance in capacitors, the dielectric should be very thin, the
conducting plates should be large in area and several dielectric layers and conducting plates
should be alternatively stacked.
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Figure 2.15 Schematic of a capacitor.
2.3.3.2 Research trends in structural capacitors
Small, et al. [62] have developed gas dielectric capacitors of 5 and 10 pF with Zerodur (a
glass-ceramic made by Schott AG) as the structural material which are claimed to have small
temperature and voltage coefficients and are stable with time [63].
Windlass et al. [64] have demonstrated that the dielectric properties of polymer matrices can
be improved by the addition of ceramic fillers such as lead magnesium niobate-lead titanate
(PMN-PT) and barium titanate (BT). Colloidal processing of nanoparticle filled epoxy was
used to obtain up to 2 µm thin dielectric films. The results showed that a dielectric constant
of more than 135 was achieved in a PMN-PT filler based epoxy system. A capacitance of 35
nF/cm2 was achieved for the structural capacitor having the thinnest films (2.5-3.0 µm) of
PMN-PT/epoxy dielectric.
Research is being carried out in the US army research laboratory on structural capacitors that
can carry mechanical loads by intercalating glass fibre reinforced polymer dielectric layers
with metalized polymer film electrodes [45]. Researchers are trying to maintain the high
dielectric strength and high capacitance in polymer composites. A simple structural capacitor
can be constructed by placing electrically conductive electrode layers between composite
dielectric plies (Figure 2.16). The dielectric strength and the energy density of a structural
capacitor, with the thinnest woven glass fibre (40 µm) as a separator, was in the range of 100-
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150 V/µm and 0.2-0.5 J/cm3 respectively. The authors have also devised a multifunctional
metric based on the electrical and structural properties of the structural capacitor through
which they were able to determine the overall multifunctional efficiency of each capacitor
design.
Figure 2.16 Schematic of a structural capacitor by O’Brien et al. [45]
Luo et al.[13] have proposed a continuous carbon fibre / epoxy-matrix composites, with a
paper interlayer (0.04 mm thickness after composite fabrication), as structural capacitors. The
prototype had a capacitance of 1.2×10-7 mF /cm2 and a resistance of 1.89×106 Ω at 2 MHz.
The authors also showed that the composite was conducting in the through-the-thickness
direction without an interlayer or with a more porous paper interlayer. The carbon fibre
epoxy matrix composite having an epoxy impregnated paper of thickness 0.1 mm showed the
capacitance of 0.21×10-7 mF /cm2 due to increase in dielectric thickness.
Carlson et al. [65] developed structural capacitors from CF-reinforced pre-pregs woven
lamina separated by a paper or polymer film dielectric. Different polymer films including
polyamide, polyester and polycarbonate were used as dielectric. A maximum capacitance of
2.5×10-7 mF/cm2 and an inter-laminar shear strength (ILSS) of 21.8 MPa was obtained by
using 80 g/m2 paper as dielectric in structural capacitors. The ILSS of the structural capacitor
with 80 g/m2 paper as dielectric was comparable to ILSS of a standard composite material
(23 MPa).
Another previously developed SiC/BaTiO3 piezoelectric structural fibre [66] was also
reported by Lin et al. [67] to act as a structural capacitor, The schematic of this structural
capacitor is shown in Figure 2.17. This structural capacitor benefited from the dielectric
nature of the BaTiO3 coating applied to the silicon carbide (SiC) core fibre as a cylindrical
capacitor and mechanical reinforcement. The best fibres for energy storage were found to
have an aspect ratio of 0.23 and showed an energy density in the range between 0.0253 and
0.0325 mWh/cm3.
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Figure 2.17 Schematic of piezoelectric fibre also acting as structural capacitor by Lin et al.
[67]
2.3.3.3 Challenges in structural capacitors
There is an increasing demand for lightweight and compact multifunctional structural
capacitors that are easily adaptable for varying electrical requirements. Unlike structural
batteries and structural fuel cells, structural capacitors provide a quick discharge for short
time periods. The mechanical performance of conventional capacitors is improved by using
composites as dielectrics [68-70] but there has been very little research on structural
capacitors. The requirement of thin separators in a capacitor also limits the structural
properties of the system. Electrically, structural capacitors should attain high energy density
(equation 2.1) while maintaining high power density (equation 2.2). Therefore, new potential
dielectric materials, with high dielectric strength and good structural properties, along with
improved fabrication techniques, are being explored in order to improve the performance of
structural capacitors.
2.4 Structural Supercapacitors
Supercapacitors have matured significantly over the last decade with the potential to facilitate
major advances in energy storage. This dissertation will focus on multifunctional composites
and its potential in supercapacitor technology. A structural supercapacitor is a supercapacitor
which, in addition of storing electrical energy, is also capable of bearing mechanical loads.
Supercapacitors, also called ultra-capacitors or electrochemical double layer capacitors,
utilize high surface area electrodes and thin electrolytic dielectrics (i.e. thin electrical double
layer) to achieve very high capacitance [32]. The performance of a supercapacitor is
dependent on charge accumulation from an electrolyte solution at the electrode/electrolyte
interface through electrostatic attraction by polarized electrodes [71]. Supercapacitors have
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been used as energy storing devices for nearly a century [32]. Recent developments in
supercapacitor technology have allowed supercapacitor to store greater amounts of energy
(e.g. 30 Wh/kg [72]) as compared to conventional capacitors (e.g. 0.06 Wh/kg [30]) and
greater amounts of power delivery ( e.g. 3200 W/kg [72, 73]) as compared to conventional
batteries (e.g. 1000 Wh/kg [30]). In addition of filling the gap of energy and power densities
between capacitors and batteries, supercapacitors also offer several other promising
advantages including a large number of charge/discharge cycles and the ability of operating
over a wide temperature range.
2.4.1 Historical background of supercapacitors
Although chemists studied the electrical charge storage at the interface between a metal and
an electrolytic solution since the 19th century [32], Becker [74] from General Electric
Company, Inc. New York was the first to patent a low voltage electrolytic capacitor
comprising of porous carbon electrode and electrolytic solution in 1954, as shown in Figure
2.18. The electrolytic capacitor stored electric charge in the pores of carbon and showed a
capacitance of 6 F at 1.5 V. Later, another device was patented in 1966 by Rightmire from
The Standard Oil Company, Cleveland, Ohio and was named as “Electrical Energy Storage
Apparatus” [75]. The device (Figure 2.19) stored energy in the double layer interface and had
a maximum storage capacity of 22 Wh/kg. The Standard Oil Company, Cleveland, UK also
filed another patent [76] on a disc shaped capacitor in 1970 which was comprised of carbon
paste soaked in an electrolyte. Due to subsequent lack of sales, The Standard Oil Company
gave the licence of the capacitor technology to NEC in 1971 who developed its first
commercial double layer capacitor under the name of “supercapacitor” which was primarily
used for memory backup [77].
Figure 2.18 Electrolytic capacitor patented by General Electric Company, New York [74].
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Figure 2.19 Electrical energy storage apparatus patented by the Standard Oil Company,
Cleveland, Ohio [75].
In 1978, Matsushita Electric Industrial Co. (also known as Panasonic) developed a
supercapacitor under the trade name of “Gold Capacitor” primarily for backup applications.
ELNA Corporation Ltd. manufactured supercapacitors under the trade name of “Dynacap”.
Various other companies including Maxwell Technologies and PRI also started
manufacturing supercapacitors under different trade names in late 1980s. Initially
supercapacitors were designed for military applications, i.e. for missile guidance systems;
laser guided weapons, arms, power conditioners and electromagnetic launchers. More than a
decade later, supercapacitor devices were used in vehicle application [78]. Currently,
supercapacitors, with high power densities, are being manufactured by a number of
companies around the world. A comparison of the performance of various currently available
commercial supercapacitors is presented in Table 2.2.
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Device Name Company C (F) V (volt) Eest (Wh/kg‡)
Supercapacitor [77] NEC-TOKIN, Japan 0.013-100 2.7-12 0.04-4.22
Supercapacitor [79] CAP-XX, Australia 0.075-2.40 2.3-5.5 0.26-1.40
Boostcap [80] Maxwell Tech. Inc. USA 5-3000 2.5-130 1.38-5.96
Gold Cap [81] Panasonic, Japan 0.015-10 2.1-5.5 0.23-2.80
Dynacap [82] ELNA Co. Ltd., Japan 0.047-300 2.5-6.3 ---**
Powercap [82] ELNA Co. Ltd., Japan 500-1500 2.5 ---**
Bestcap [83] AVX Corporation, USA 0.015-1.0 3.6-16 0.03-0.18
Capacitor modules[84] ESMA, Russia 107-8000 16-52 1.74-7.30
PowerStor [85] Cooper Bussman Electronics, USA 0.47-110 2.3-5.0 0.68-4.34
ESCap [86] Tavrima Canada Ltd., Canada 2.0-160 14.0-300 0.06-0.66
Ultracapacitor [87] LS Mtron Ltd., Korea 3-5400 2.5-84 2.14-6.14
EDLC [88] Nesscap Co., Ltd., Korea 5000 2.7 5.80
Capattery [89] Evans Capacitor Company, USA 0.033-1.5 5.5-25 0.01-0.83
KAPower [90] Kold Ban Int’l, USA 1000 3.0-14.5 4.29
Superfarad [33] Superfarad, Sweden 250 50 5.4
Saft (Gen 3) [33] Saft, France 132 3.0 6.8
Premlis [72] Advanced Capacitor Tech., 2000 4.0 15
Table 2.2 Capacitance range C, working potential range V and estimated specific energy Eest
of commercially available supercapacitors.
‡ Eest ; ** Mass of device not available in literature.
2.4.2 Working principle of supercapacitor
The key components of supercapacitors are electrodes, electrolyte and separator [32] [Figure
2.20]. Energy storage is associated with the build-up and separation of electric charge
accumulated in the electric double layer at the interface between the surface of an electrode
and an electrolyte. When an electrode (electrical conductor) is immersed into an electrolyte,
there is a spontaneous organisation of charges which creates an electrochemical double layer
at the electrode/electrolyte interface with one layer at the surface inside the electrode and the
other layer in the electrolyte as shown in Figure 2.20. This electrochemical double layer
(EDL) behaves as a physical capacitor with the charges in the electrode and electrolyte
separated by a distance of the order of nanometres. The formation of EDL is dependent on
the structure of electrode surface, composition of electrolyte and the potential difference
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applied between the charges at the electrode/electrolyte interface [91]. At the
electrode/electrolyte interface, the EDL forms and relaxes almost instantaneously (time of
formation/relaxation is ~10-8 s [91]). Thus, the double layer responds rapidly to the potential
changes. There is only a charge rearrangement (no chemical reaction) taking place in the
process. The working voltage of the supercapacitor is determined by the electrolyte
decomposition voltage and is dependent on the current density, operational temperature and
the required lifetime [92].
The characteristics of the electrode materials for supercapacitors include long term stability,
high surface area, high cyclability, and resistance to electrochemical redox reactions [56].
The separator acts as a spacer to prevent the opposing electrodes from touching one another
and causing a short circuit. Thus, a separator should be ionically conducting but
electronically insulating. The voltage window of a supercapacitor is dictated by the operating
pH and the thermodynamic stability of various species in the electrolyte [93]. Energy density
of the supercapacitor (E = ½ CV2) mainly depends on the voltage applied and therefore
voltage window is very important. The electrolyte can be either aqueous or organic (although
non-aqueous are better because of high voltage applications).
Figure 2.20 Schematic of a supercapacitor by Halper et al. [94].
This dissertation will focus on multifunctional composites and their potential in
supercapacitor technology. There are a number of reasons for choosing a supercapacitor
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approach in this work. Supercapacitors consist of two electrodes, a separator and an
electrolyte. Thus, a relatively straightforward embodiment can be imagined by using
activated carbon fibres (Section 2.4.6) as the electrodes, possessing high surface area and
suitable pore size, combined with an electrolyte (Section 2.4.4) that is low in resistivity but
high in stability. Since high surface area electrodes are required, carbon fibres were activated
in order to introduce mesopores on fibres without compromising their mechanical properties.
Glass fibre was chosen as a separator between the electrodes as it is electronically insulating
and also possesses mechanical properties as compared to other commonly used separator e.g.
glass fibre, porous polypropylene or filter paper [32].
2.4.3 Types of supercapacitors
Supercapacitors can be divided into three general types: pseudo-capacitors, electrochemical
double layer supercapacitors and hybrid capacitors [91]. Each type has its unique charge
storage mechanism which are Faradic, non-Faradic and the combination of the two,
respectively. Faradic processes (e.g. redox process) involve the charge transfer between
electrolyte and electrode. Non-Faradic processes do not involve the chemical mechanism.
However, charges are distributed on surface by physical mechanism without making or
breaking any chemical bond. Types of supercapacitors are schematically shown in Figure
2.21.
In pseudo-capacitors, charge transfer between electrode and electrolyte is accomplished
through electro-sorption, redox-reactions and intercalation processes [32]. Thus, pseudo-
capacitors have greater capacitances and energy densities than electrochemical double layer
capacitors. Electrode materials, used for pseudo-capacitors are metal oxides and conducting
polymers [31]. Conducting polymers have higher conductivity and capacitance than carbon
electrode materials but their reduced cycling stability has hindered the development of
conducting polymer pseudo-capacitors [33]. The most commonly-used metal oxide in
pseudo-capacitors is ruthenium oxide [33] as it incorporates higher conductivity, higher
energy and power densities than similar electrochemical double layer capacitors; the main
limitation is its high cost.
Hybrid capacitors store charge by utilizing both Faradic and non-Faradic processes in order
to get better performance characteristics. Composite hybrid capacitors combine carbon-based
materials with conducting polymer or metal oxide materials in a single electrode [31].
Asymmetric hybrid-capacitors integrate activated carbon electrodes with a conducting
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polymer anode [31]. Battery type hybrid capacitors couple a supercapacitor electrode with a
battery electrode [32]. Enormous increases in the performance characteristics have been
observed by employing these hybrid capacitors, delivering higher average specific power (0.5
kW/kg) and maximum specific power (9 kW/kg) at 5mA/cm2 in the potential region of 1.0-
3.0 V than the individual double layer carbon supercapacitors that deliver 0.335 kW/kg
average specific power and 5 kW/kg maximum specific power in the potential range of 0.0-
2.8 V [95].
.
Figure 2.21 Schematic of the types of supercapacitors by Haler et al. [94].
2.4.4 Research trends in supercapacitors
Although supercapacitors have been fabricated in the form of composites by various
researchers, the structural properties of the fabricated supercapacitors have not been reported
yet. One such supercapacitor was developed by Zhang et al. [96]. The authors employed
ZnO-CNT (zinc oxide and carbon nanotube) as electrodes and PVA-PMA (poly vinyl alcohol
and phosphomolybdic acid) as a gel polymer electrolyte. The resultant supercapacitor showed
a specific capacitance of 126.3 F/g. However, the specific capacitance was reduced by
increasing the amount of deposited ZnO in CNT due to the decrease in the specific area of
electrodes through ZnO agglomeration.
Supercapacitors
Electric double layer capacitors
Activated Carbons
Carbon Nanotubes
Carbon Aerogels
Hybrid capacitors
Composite Hybrids
Asymmetric hybrids
Battery type hybrids
Pseudocapacitors
Conducting polymers
Metal Oxides
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Another promising supercapacitor, having electrodes made of polyacrylonitrile (PAN) based
carbon nano-fibre paper, was fabricated by Ra et al. [97]. The PAN based nano-fibre paper
was activated in order to increase the surface area up to 705m2/g. A capacitance of 100 F/g
was obtained when the activated nano-fibres and organic electrolyte was used to prepare a
supercapacitor.
Hu et al. [98] developed a supercapacitor based on polyaniline (PANI) and tin oxide (SnO2)
nano-composite. The nanostructured SnO2 particles were embedded within the netlike PANI
and thus, increase the overall specific surface area of the nano-composite. The authors
reported a specific capacitance of 305.3 F/g and an energy density of 42.4 Wh/kg for the
PANI/SnO2 nano-composite based supercapacitor. However, the supercapacitor showed a
4.5% decrease in the available capacity after 500 cycles.
Copolymer of poly (ethylene oxide) and poly (propylene oxide) and graphite oxide based
composites were used as high performance electrodes for supercapacitor fabrication by Tien
et al. [99]. The graphite oxide was well dispersed in polymer resulting in high accessibility of
graphene oxide sheets to the electrolyte ions. The supercapacitor showed a double layer
specific capacitance of 130 F/g. The schematic of the supercapacitor assembly is shown in
Figure 2.22.
Figure 2.22 Schematic of supercapacitor assembly by Tien et al. [99]
Lewandowski et al. [100] used activated carbon cloth as electrodes and three different
electrolytes (aqueous, organic and ionic liquids based) to manufacture their supercapacitors.
The authors showed that the supercapacitor filled with ionic liquid as electrolyte worked at a
potential difference of 3.5 V and thus showed the highest energy density of 215 kJ/kg.
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Ionic liquid based electrolyte and mesoporous activated carbon fibre based electrodes were
also used by Xu et al. [101]. The activation of carbon fibre increased the surface area as high
as 3291 m2/g with a pore volume of 2.162 cm3/g. At room temperature, the specific
capacitance of 187 F/g was obtained and was further increased to 196 F/g by increasing the
operating temperature to 60°C.
A team of researchers at U.S. Army Research Laboratory led by Wetzel [102] developed
structural supercapacitors, using different electrodes, which can store electrochemical energy
as well as can carry structural loads. The structural supercapacitors showed a specific
capacitance of 35 mF/g, a tensile modulus of 10 GPa and a lap shear strength of 0.75 MPa.
The authors also suggested of using PPy (polypyrrole) coatings on structural fabric electrodes
in order to further improve the multifunctionality of structural supercapacitors.
2.4.5 Structural polymer electrolytes
The electrolyte can be solid state, aqueous or organic. Aqueous electrolytes are usually
H2SO4 or KOH possessing a dissociation voltage of 1.23 V. Organic electrolytes are prepared
by dissolving alkali metal salts in organic solvents possessing a dissociation voltage greater
than 3 V and therefore resulted in high energy density (E = ½ CV2) when employed in a
supercapacitor. Polymer electrolytes are the most commonly used ionic conductors as they
combine the advantages of solid state electrochemistry with the ease of processing inherent to
plastic materials. Initially, crystalline domains in polymer electrolytes were assumed to be
responsible for the ionic transport; however, it was soon established that the amorphous phase
is solely responsible for the ionic transport in polymer electrolytes [103]. In polymer
electrolyte (not to be confused with the polyelectrolyte in which polymer repeating unit is
covalently bonded with either cation or anion), both the cations and anions contribute to the
ionic conductivity. In polymer electrolytes, the main conductivity controlling parameter is the
segmental motion of polymer host matrix. Therefore, plasticisers are used as internal
lubricants in order to improve the conductivity [104]. By the addition of plasticisers
(excellent solvents for Li salts themselves), polymer electrolyte networks, possessing low to
moderate crosslink density, swell to form gel polymer electrolytes (GPEs). Therefore, the
addition of plasticisers negatively affects the structural properties of polymer electrolytes.
Depending on molarity, Mori et al. [105] found an ionic conductivity of 60 mS/cm from
TEATFB (tetra-ethyl-ammonium-tetra-fluoro-borate) in acetonitrile. In the literature,
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optimisation of the electrolyte has always been highlighted as the key step towards improving
supercapacitors. The ideal electrolyte should:
a) enter the pores of porous electrodes, i.e. wet the electrodes
b) show a broad electrochemical stability range at the electrode interface (> 5V) [42]
c) produce high ionic mobility for a very fast charge/discharge rate (<1 s) [27]
d) be stable over a broad temperature range (-30°C to 100°C) [27]
e) possess high ionic conductivity (at the level of ≥10-4 S/cm) [42]
f) emit no vapours
The major limitations of aqueous solutions as electrolytes are their narrower operating
temperature window, higher corrosion activity, and lower discharge voltage. Non aqueous
liquid electrolytes have a broader operating temperature, lower corrosion activity and higher
decomposition voltage (> 2.3 V) but they have some limitations including lower electro-
conduction [32] and higher cost [31]. Therefore, very significant research effort has now
been directed to solid or gel polymer electrolytes which possess higher decomposition
voltages, reduced current leakage, lower cost, broader operating temperatures, low
flammability as well as the possibility for thin layer applications. Moreover, their limitations
include relatively low ionic conductivity and poor penetration into pores [31]. But these
limitations can be overcome by utilizing organic plasticisers [104]. Solid or gel polymer
electrolytes not only prevent liquid leakage but also provide vibration shock resistance.
Typically, a solid polymer electrolyte is composed of a salt complex and a polymer having
electron-donor atoms (e.g. nitrogen, oxygen or phosphorous etc). Different polymer
electrolytes have been proposed due to variations in their chemistries and compatibilities with
electrodes that can affect both the mechanical and electrochemical properties of the polymer
electrolytes. These polymer electrolytes include poly(vinyl-sulfone) (ionic conductivity of
374 µS/cm [106]), polyacrylonitrile (ionic conductivity of 2880 µS/cm [107]), poly(vinyl-
chloride) [108], poly(methyl-methacrylate) (gel, ionic conductivity of 1200 µS/cm [109]),
poly(vinylidene-fluoride) [41], poly(ethylene-oxide) (ionic conductivity of 0.1 µS/cm [110]),
polyaniline [98], and diglycidylether-of-bisphenol-A (ionic conductivity of 1.7 µS/cm [55]).
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Figure 2.23 History of improvements in ionic conductivity of the polymer electrolytes by
Murata et al.[111].
Engineering mechanical properties of polymer electrolytes can be achieved by improving the
robustness of polymer electrolytes as a separator layer between electrodes. Different
approaches have been employed for augmenting structural properties in polymer electrolytes
by taking either a rigid polymer matrix (high mechanical but low electrochemical properties)
and then incorporating salt/solvent and thus making it conductive (compromising
electrochemical properties) [55] or relatively soft polymer matrix (low mechanical but high
electrochemical properties) and then adding ion conducting fillers (electrically insulating to
avoid short circuit) to enhance mechanical properties (approach followed here). The
mechanical properties can be improved by cross linking [112], clay fillers [113], fibre fillers
[114], block copolymers [115], hybrid organic/inorganic polymers [116] and ceramic particle
fillers [98, 117-120].
Among gel polymer electrolytes, polyacrylonitrile possess a high dipole moment and high
ionic conductivity (10-3 S/cm) as compared to PMMA (ionic conductivity of 10-4 S/cm) [92].
Vinyl ester derivatives of polyethylene glycol are another promising class of polymers that
are currently employed in structural solid polymer electrolytes. PEGDGE (Section 4.3) and
DGEBA (Section 4.4) belong to the group of vinyl ester derivatives and possess high
Chapter 2 Literature Review
78
compression stiffness (~ 0.1 GPa to 1.2 GPa) and also show reasonable ionic conductivity (~
10-5 S/cm) when a suitable amount of ionic salt is added. These structural polymer
electrolytes are mixed with lithium bis (trifluoro methyl sulfonyl) imide (LiTFSI), tetra butyl
ammonium hexafluorophosphate (TBAPF6) or 1-ethyl-3-methylimidazolium bis (trifluoro-
methyl-sulfonyl) imide (EMITFSI). A lithium salt has been selected because of the
compatibility with polymer electrolytes [55]. Compression stiffness is used as a metric of
mechanical performance as it can be quantified easily for small amounts of materials.
Currently investigated polymer gel electrolytes possess ionic conductivities as high as 10-3
S/cm (e.g. PAN gel containing plasticisers [107]) and currently investigated structural epoxy
resins possess a compression stiffness as high as 4 GPa [55]. However, these investigated
polymer gel electrolytes and structural epoxy resins are not multifunctional. Even though
these results are valuable targets, it is not necessary to achieve them in order to attain overall
weight reduction within multifunctional materials systems and can be attained only through
collaborative increase in conductivity and stiffness.
Reinforcement of nano and micron size inorganic fillers in polymer electrolytes is the most
extensively used recent approach to reduce the natural tendency of crystalline spherulite
formation in polymer host matrix. Reinforcement by inorganic fillers (SiO2 [121], TiO2 [118],
Al2O3 [122], K-LiAlO2 [123], BaTiO3 [124], MgO [125]) in polymer electrolytes is claimed
to be responsible for the enhancement of ambient temperature ionic conductivity and
mechanical properties by about 2-3 orders of magnitude. Filler size (nano better than micron
[121]), surface area and surface nature (type of surface functionality, acidic/basic) are the
main parameters that have been shown to affect the ionic conductivity and morphology of
composite polymer electrolytes [126]. Among inorganic fillers, mesoporous silica (particles
or monoliths) has the potential to provide the desired electrical and mechanical properties to
the structural polymer electrolyte. Mesoporous silica allows the conduction of ions within the
polymer electrolyte. Generally, a significant increase (one to two orders of magnitude) in
ionic conductivity as well as mechanical performance is obtained by the addition of
mesoporous silica in polymer electrolytes [41, 117, 121]. Enhancement of ambient
temperature ionic conductivity is attributed to the stable amorphous phase formation due to
high surface area inorganic filler interactions with the ethylene oxide units which prevent
chain reorganisation and maintain disorder [126]. In addition to crystallization suppression, a
phenomenological model based on effective medium theory has been developed in which
ionic conductivity is assumed to be associated with the existence of a highly conducting layer
Chapter 2 Literature Review
79
at or near the filler/polymer interface [127]. Another explanation of increased ionic
conductivity by filler reinforcement in polymer electrolytes is the suppressed ion coupling
and the formation of lithium ion conducting pathways at the filler surface due to Lewis acid-
base interactions between the filler/ion and polymer/filler in the interfacial region. Thus, there
is a great potential of using mesoporous silica, not only from the multifunctional aspect, but
also from the ability to tune the mechanical and electrical performance of a composite by
optimising the filler content in the structural electrolyte.
Poly(ethylene glycol) diglycidylether (PEGDGE), polyacrylonitrile (PAN) and bisphenol-A-
diglycidylether (DGEBA) based polymer electrolytes are employed in this work [Section
4.2]. DGEBA has a clear structural advantage over PEGDGE and PAN but possesses low
ionic conductivity (Section 4.2). PAN possesses high ionic conductivity (~ 10-3 S/cm) but at
the same time possesses negligible mechanical properties as compared to crosslinked
DGEBA and crosslinked PEGDGE. Since PEGDGE has no structural properties (liquid at
room temperature), crosslinking and the incorporation of mesoporous silica particles are
employed to enhance the mechanical properties. Since the ionic conductivity of polymer
electrolytes is mainly dependent on the interactions between monomer chains of the polymer
electrolyte and organic solvent and/or ions. There exists a little interaction in acrylonitrile but
in PEGDGE and DGEBA, ions are almost held by ether-oxygen due to strong interaction
between ions and ether-oxygen resulting in decreased ionic conductivity at ambient
temperature. Thus, long range ionic transport is supported by the ether oxygen in the
amorphous domains [55], coupled to the segmental mobility of the chains, with ions hopping
between coordination sites, whereas structural properties are enhanced by the crosslinking
and inorganic filler (mesoporous silica particles and monoliths).
2.4.6 Activated carbon fibre electrodes
Usually three types of carbon based electrode materials are used for electrochemical double
layer capacitors, (i) high surface area activated carbons, (ii) carbon aerogels and (iii) carbon
nanotubes. Activated carbons are very porous and can exhibit surface areas of up to 3000m2/g
[128]. The porous structure is composed of differently sized nano pores (< 20 Å wide),
mesopores (20-500 Å) and macropores (> 500 Å). In modern power-generating/energy-
storing technologies, carbon is the most widely used material for electrodes. Metal oxides and
conducting polymers are also used as electrodes but high surface area carbon materials
optimise the double layer effect and thus enhance the electrochemical performance of
Chapter 2 Literature Review
80
supercapacitors [56, 129]. There are numerous reasons for using carbon as electrode,
including low cost, high surface area, low weight, high efficiency, availability and established
electrode production technologies [31]. The capacitance of carbon based electrodes increases
linearly with the surface area and may reach the capacitance of 250 F/g [31]. Carbon based
supercapacitors come close to what one would call the pure electrochemical double layer
capacitor but the presence of surface functional groups on activated carbons can give rise to
pseudo-capacitance [32]. Pseudo-capacitance can be further enhanced by the application of
conducting polymer coatings on activated carbon fibres that result in improved energy
density at the expense of lower mechanical properties (as conductive polymers are not known
for their mechanical properties), slower response times, and creating a more complex system.
Energy density is improved as the electrodes undergo redox reactions whilst electrolyte
counter ions accumulate [56]. Carbon nanotubes, as an electrode material for supercapacitors,
have received enormous interest [96, 130]. Due to lower mechanical properties than carbon
fibres and difficulty in organising CNTs, it cannot be solely used as electrode in
multifunctional composites for supercapacitor applications. However, Baughman et al. [131].
have developed a coagulation-based CNT spinning method to prepare CNT fibres with a
Young’s modulus of 80 GPa and a tensile strength of 1.2 GPa and used these spun CNT
fibres to fabricate a supercapacitor.
Considerable effort has been directed towards the electrochemical properties of different
forms of carbon e.g. fibres [132], carbon black [133], carbon aerogel [134], skeleton carbon
[135], microbeads [136], nano-tubes [137] and nano-foams [138] (carbonised product of
polymer aerogels containing chopped carbon fibres). The problem with the black carbon is
the slow oxidation as well as high equivalent series resistance. Carbon fibres are the most
important component in high performance structural composites (five times stronger than
grade 1020 steel for structural parts, yet still five times lighter) [139]. Since the ultimate
objective of this work is the preparation of multifunctional composites, so carbon fibres are
used as electrodes in this work. Commercial carbon fibres typically have a disordered core
surrounded by a graphitic sheath and are often not available without sizing [56]. Carbon
fibres are sized by applying an uncured epoxy resin on the surface in order to enhance
fibre/matrix interaction and to facilitate handling. Due to low graphitic content and low
surface area (approx. 0.4-1 m2/g), carbon fibres are not, generally, used as electrodes without
activation [56, 132]. Activation of carbon fibre increases the specific surface area by
introducing mesopores at the surface of fibres. Introduction of mesopores enhances the
Chapter 2 Literature Review
81
electrochemical performance of supercapacitors by increasing the capacity of ions to be
stored at the electrode/electrolyte interface. However, it is unfortunate that a significant part
of the surface area contains nanometre-sized pores (<2 nm) which are not accessible by the
electrolyte ions and, thus, do not take part in overall capacitance [56]. Thus, in order to
achieve better results, it is important to consider electrolyte as well as electrode
characteristics at the same time. Two different general activation methods, physical and
chemical activation, are usually applied to carbon fibres for achieving high surface area and
good microporosity. In physical activation, fibres are, initially, carbonised in nitrogen
environment and then, finally, controlled high temperature (600-1000°C) gasification of these
fibres is performed in an oxidizing gas (e.g. steam, carbon dioxide, air, gas mixture) [140]
atmosphere. Various other physical activation methods are under research including cold
plasma treatment [141], copper electrodeposition [142], laser treatment [143] and oxygen
treatment [144]. In chemical activation, carbon fibres are, initially, impregnated with reactive
reagent (e.g. NaOH [145], KOH [146], HNO3 [147], and H2SO4 [148], H3PO4 [149]) and then,
finally, heat treated (<800OC) in a nitrogen environment. These modifications enhance the
surface area and microporosity but significantly reduce the structural performance of the
original carbon sources and thus an optimisation is required in enhancing surface area so that
mechanical properties are not compromised.
2.4.7 Challenges in structural supercapacitors
The current state of the art supercapacitors are facing many challenges. Although research
and development of supercapacitors is showing significant progress, there is very little
research yet available on structural supercapacitors. For vehicle applications, the structural
supercapacitor should have high energy density (greater than 5 Wh/kg [33]), high power
density, long cycle life as well as good structural performance. Electrically, structural
supercapacitors are facing different challenges, including poor ionic conductivity of the
polymer electrolyte, thicker and lower surface area electrodes, high contact resistance
between electrode and electrolyte, poor bonding of current collectors and electrodes,
fabrication techniques and thickness/porosity of separator. Mechanically, structural
supercapacitors should employ electrolytes with good structural performance, high surface
area electrodes with good mechanical properties and good adhesion of electrode/electrolyte
by enhanced mechanical interlocking and/or chemical bonding. In comparison to structural
batteries, capacitors and fuel cells, structural supercapacitors have distinctive benefits
including high energy to power ratio, large modularity with respect to capacitance and
Chapter 2 Literature Review
82
voltage, long cycle life, low self-discharge, environmental friendly (do not utilise hazardous
materials), do not require any servicing and do not require any cooling or other auxiliary
installations [31].
Chapter 3 Experimental Section
83
Chapter 3 Experimental Section
This chapter describes the chemicals utilised as well as the experimental methods adopted in
the preparation of structural supercapacitors. Starting from the preparation of mesoporous
silica and polymer electrolytes (using different electrolyte systems), a nanostructured
polymer electrolyte system was then developed by incorporating mesoporous silica in
polymer electrolytes. This chapter then describes the fabrication of structural supercapacitors
using the RIFT process. The different electrochemical as well as mechanical techniques
employed to characterise the polymer electrolytes and structural supercapacitors are also
discussed in brief.
Chapter 3 Experimental Section
84
3.1 Materials
3.1.1 UncuredepoxymaterialsPoly(ethylene glycol) diglycidylether (PEGDGE, Lot MKBG7915V, Sigma Aldrich, UK,
[150]) and the diglycidylether of bisphenol-A (DGEBA, Lot MKBD2770V, Sigma Aldrich,
UK, [151]) were used as the uncured polymer epoxy resins and are shown in Figure 3.1. The
number averaged molecular weight (Mn) of PEGDGE was 526 with a density of 1.14 g/mL
(at 25°C) and a flash point of 197°C (data from [150]). The degree of polymerisation of
PEGDGE on this basis has been calculated to be ~ DP = 9. The molecular weight of DGEBA
was 340.41 g/mol with an epoxide equivalent weight of DGEBA between 172 to176 (see
NMR in Appendix F) and a density of 1.16 g/mL (at 25°C) (data from [151]).
Polyacrylonitrile (PAN) having a molecular weight of 150,000 was purchased from
Polysciences Inc. (UK) (data from [152]).
Figure 3.1 Chemical structures of PEGDGE (a), DGEBA (b) and PAN (c).
3.1.2 CrosslinkerTriethylenetetramine (TETA, technical grade, purity ≥ 70%, [153]) was used as crosslinker
for PEGDGE having a molecular weight of 146.2 g/mol and a density of 0.98 g/mL (at
25°C). TETA has a melting point of 12°C and a closed cup flash point of 129°C (data from
[153]). 4,4'-Methylenebis(cyclohexylamine) (MCHA, technical grade, 95%, [154]) was used
as a curing agent for DGEBA having a molecular weight of 210.4 g/mol and a density of 0.95
g/mL (25°C) (Please see Appendix F for the NMR of MCHA). MCHA (Figure 3.2b) has a
melting point of 45°C and a closed cup flash point of 159°C (data from [154]). Both
crosslinkers were purchased from Sigma Aldrich (UK).
Chapter 3 Experimental Section
85
Figure 3.2 Chemical structures of TETA (a) and MCHA (b).
3.1.3 ElectrolytesaltLithium bis(trifluoromethyl sulfonimide (LiTFSI, purity ≥ 99.0 wt%, [155]), tetrabutyl
ammonium hexafluorophosphate (TBAPF6) (battery grade, ≥ 99.0 wt%, [156]) and 1-ethyl-3-
methylimidazolium bis(trifluoromethyl sulfonyl)imide (EMITFSI, purity ≥ 98%, [157]) were
used as ionic salts (Figure 3.3) and were purchased from Sigma Aldrich (UK). LiTFSI has a
molecular weight of 287.1 g/mol, a density of 1.334 g/mL and a melting point of 234°C (data
from [155]). TBAPF6 has a molecular weight of 387.4 g/mL and a melting point of 244°C
(data from [156]). EMITFSI is an ionic liquid having a molecular weight of 391.3 g/mol, a
density of 1.524 g/mL (at 20°C) and a melting point of -9°C (data from [157]).
Figure 3.3 Chemical structures of LiTFSI (a), TBAPF6 (b) and EMITFSI (c).
3.1.4 SolventsPropylene carbonate (PC, HPLC grade, 99.7%, [158]) was used as a solvent for ionic salts
(LiTFSI and TBAPF6) and was purchased from Sigma Aldrich (UK). PC (Figure 3.4) has a
molecular weight of 102.1 g/mL, a density of 1.20 g/mL and a boiling point of 240°C (data
from [158]).
Chapter 3 Experimental Section
86
Figure 3.4 Chemical structure of PC.
3.1.5 Silica precursor Tetraethyl orthosilicate (TEOS, >99%, [159]) was used as a silica source and was purchased
from Sigma Aldrich (UK). TEOS (Figure 3.5) has a molecular weight of 208.3 g/mol, a
density of 0.93 g/mL (at 25°C) and a boiling point of 168°C (data from [159]).
Figure 3.5 Chemical structure of TEOS.
3.1.6 Blockcopolymersurfactant Pluronic P123 (Figure 3.6) was kindly supplied by BASF Corporation, USA (Batch number
WPDA622B, [160]) and was used as a block copolymer surfactant in mesoporous silica
preparation. Pluronic P123 has an average molecular weight of 5750 g/mol and a density of
1.01 g/mL (at 25°C). It has a pour point of 31°C (data from [160]).
Figure 3.6 Chemical structure of Pluronic P123 (x = 20, y = 70, z = 20).
3.1.7 Woven fibre mats Polyacrylonitrile (PAN) based woven carbon fibre (CF) mat, with a fibre diameter of 7 µm,
was used as an electrode in the structural supercapacitors. The glass fibre (GF) mat was used
as a separator in the fabrication of structural supercapacitors. Summary of the relevant
properties of the CF and GF mats is shown in Table 3.1.
Chapter 3 Experimental Section
87
Fibre mat
Characteristics HTA Carbon fibre mat Glass fibre mat
Supplier Tissa Glasweberei AG (Oberkulm, Switzerland)
Fibre mat code 862.0200.01 842.0200.01
Weave plain weave plain weave
Areal density (g/m2) 202 200
Mat thickness (mm) 0.21 0.16
Warp Thread count (Fd/cm) 4.90 17.0
Weft Thread count (Fd/cm) 5.00 12.0
Conductivity (S/cm) 656 N/A
Table 3.1 Summary of relevant properties of fibre mats.
3.1.8 Paraffin oil Paraffin oil was used in the preparation of mesoporous silica monoliths and was purchased
from Sigma Aldrich, UK. Paraffin oil has a density of 0.83 g/mL (at 20°C) and a closed cup
flash point of 215°C.
3.1.9 Separators In addition to glass fibre mats, two other separators were also used in the fabrication of
structural supercapacitors including the filter paper and polypropylene membrane. Filter
paper (1001-917, Grade 1) was purchased from Whatman, UK. Polypropylene membrane
(3500, 0.064 µm pore size, 55% porosity, 25 µm thickness) was provided by Celgard, UK.
All other chemicals were used as received.
3.2 Mesoporous silica
3.2.1 Preparationofmesoporoussilicamonoliths[161]Pluronic P123 (2.0 g, 0.020 mmol) was dissolved in a mixture of EtOH (10 g, 11 mmol) and
1.0 M aq. HCl (0.40 g, 0.55 mmol) and stirred for 45 min to prepare a homogeneous solution.
While still stirring, TEOS (4.16 g, 1.00 mmol), weighed in a separate beaker, was then added
immediately to the solution under stirring and the mixture was stirred for 10 min in open air.
The homogeneous solution was then transferred into a porcelain dish (75mm rim diameter,
33mm height and 100 ml capacity) and the dish was covered with perforated aluminium foil.
The dish was kept in a refrigerator for 5 days at 14°C. The silica mixture was then covered
with liq. paraffin oil (Section 3.1.9) layer (3-4 mm). The dish was then placed into an oven at
Chapter 3 Experimental Section
88
70°C for 12 h for complete ethanol removal. The paraffin oil was then removed from the
surface using filter paper (Whatman, Qualitative grade 1). For the removal of the templating
surfactant (Pluronic), the product was washed with EtOH (20 mL) for 24 h. For this purpose,
the product was slowly (to avoid breaking) placed in a beaker containing ethanol for 24 h.
The silica monolith was then dried in an oven at 50°C to constant weight. Once weight
constant, the BET surface area measurements of mesoporous silica monoliths were carried
out on the following day (see section 5.1.1 for measurement details and BET data).
3.2.2 Preparationofmesoporoussilicaparticles[15]Pluronic P123 (4.0 g, 0.020 mmol) was added to 2.0 M HCl (120 g, 81 mmol) and distilled
water (30 g, 41 mmol) and the mixture was stirred for 1 h at 35°C in an oil bath until a clear
solution was formed. TEOS (8.50 g, 1.00 mmol) was then added and the mixture was left for
14 h at 35°C. The mixture was then sonicated at 60˚C for 1 h using a Transonic T570/H
sonicator (Camlab, UK). Silica particles were separated through gravity filtration (sintered
glass funnel of porosity grade 5) and then washed with ethanol (97 wt%, 50g) in order to
remove the Pluronic. Silica particles were dried in a vacuum oven at 1 bar and 80°C for 24 h
and were stored afterwards in a sealed plastic container. Different characterisation techniques
including BET analysis (Section 5.1.2), particle sizing through light scattering technique
(Section 5.1.2) were also conducted. SEM images (Section 5.1.2) were also recorded for the
mesoporous silica particles.
3.2.3 Surfaceareaanalysis‐BETmethodFor the determination of the specific surface area (AS) and pore size distribution of
mesoporous silica from nitrogen adsorption isotherm at 77.35K (BET method), a
Micromeritics ASAP 2010 analyser (Micromeritics Ltd. UK) was used and the specific
surface area, AS, determined according to the industrial standard ISO 9277 [162].
The BET method is named after Brunauer, Emmet and Teller [163] who developed it in 1938
while working on ammonia catalysts. This was the first method to measure the specific
surface of finely divided and porous solids. Applications of the BET method range from
pharmaceuticals to catalysts, projectile propellants to medical implants, filters to cement etc
[162]. The BET method is a cheap, fast and reliable method and is very well understood and
applicable in many fields [162]. During the measurement, nitrogen molecules are weakly
adsorbed at a given pressure onto the mesoporous silica until saturation. The amounts
adsorbed are measured in equilibrium with the adsorptive gas pressure, p, and plotted against
Chapter 3 Experimental Section
89
relative pressure, P/PO, to give an adsorption isotherm (Figure 3.7). An isotherm, in this case,
is the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or
concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always
normalized by the mass of the adsorbent to allow comparison of different materials (Section
5.1).
The specific surface area, AS, is related to the total number of nitrogen gas molecules
adsorbed to mesoporous silica surface at a given pressure. Specific surface area, AS, is
calculated from the following equation,
1 1∙ (3.1)
where P/PO is the relative pressure, C is the BET constant, na is the number of nitrogen gas
molecules adsorbed, and nm is the specific monolayer amount of adsorbate.
Figure 3.7 Adsorption isotherms I to VI classified after IUPAC 1984 (image taken from P.
Somasundaran, 2006).
0.50 g of mesoporous silica was filled into a glass vessel and was degassed at 110°C for 12 h
in order to remove moisture. The specific surface area, AS, was determined by plotting
vs. . The intercept ( ) and slope ( ) of the resultant curve was used to
derive the value of nm and C. The specific surface area, AS, of mesoporous silica (Section 5.1)
can then be calculated from the equation,
(3.2)
Chapter 3 Experimental Section
90
where Am is is molecular cross sectional area (for nitrogen Am=0.162 nm2 at 77.3 K) and L is
the Avogadro’s number (= 6.022×1023) [162]. The specific surface area of mesoporous silica
is discussed in Section 5.1 and the tabulated data for the mesoporous silica samples can be
found in table 5.3.
3.2.4 Particlesizeanalyses‐Lightscatteringmethod
The size distribution of mesoporous silica particles was confirmed by light scattering
measurements using a Malvern Mastersizer 2000 particle size analyser (Malvern Instruments
Ltd., UK) that determined the average particle size (d0.5) according to the industrial standard
ISO 13320 [164].
During particle size distribution (PSD) measurements employing a laser diffraction
technique, particles scatter light in different directions with an intensity pattern. The light
scattering pattern is dependent on particle size, particle shape and the optical properties of the
particulate material [165]. In a light scattering apparatus, a light source, capable of emitting
laser light (or other narrow-wavelength source of light), generate a monochromatic consistent
parallel beam of light that is scattered by the particles at different angles. The scattered light
is then traced by focal plane detectors as shown in Figure 3.8.
Figure 3.8 Schematic of light scattering through laser diffraction by Malvern [166].
A silica suspension was prepared by dissolving dodecyl trimethyl ammonium bromide (0.020
g, 0.20 wt% ) in deionised water (10 g, 95 wt%) followed by the addition of mesoporous
silica particles (see Section 3.2.2 for their preparation/or characterisation data) (0.5 g, 4.8
wt%). The mixture was stirred at room temperature using a magnetic stirrer overnight to
ensure that a homogenous silica suspension was obtained. For a statistical average value, all
Chapter 3 Experimental Section
91
measurements were repeated 5 times per condition and are discussed in Section 5.1.2. Particle
size distribution experiments were repeated five times and the mean diameter of the
maximum volume of MSP in the suspension is reported as d0.5 with an accuracy of ± 1% in
Section 5.1.2.
3.2.5 Scanningelectronmicroscopy(SEM)Scanning electron microscopy (SEM) is a method to obtain high resolution pictures of
surfaces. The SEM uses a beam of energetic electrons as compared to a typical optical
microscope that uses visible light. An SEM can provide much higher magnification
(>100,000 times) and much larger depth of field (>100 times) [167] than an optical
microscope.
Scanning electron microscopy was used as a qualitative tool for the mesoporous silica
particles. The surface morphology of mesoporous silica particles was observed using a LEO
Gemini 1525 FEG-Scanning Electron Microscope (Oberkochen, Germany). Samples were
attached directly to the SEM stubs with double sided carbon tape and were sputter coated
with gold (15 nm thickness, 1.5 min, 20 mA, Edwards, UK) as silica particles are electrically
insulating. The SEM micrographs of the MSP are shown in Figure 5.7.
3.3 Polymer electrolytes All polymer electrolyte systems were tested for mechanical compression performance and
ionic conductivity. PEGDGE and DGEBA polymer electrolytes were cut using Lathe
machine (Colchester Student, England) into cylinders with a 13 mm diameter and 25 mm
height for mechanical characterisation and 4 mm height for ionic conductivity measurements.
All the polymer electrolyte specimens were stored in air seal bags and characterisation
measurements were carried out within 2 days of preparation of the polymer electrolytes. The
results for the compression and ionic conductivity measurements of the polymer electrolyte
systems (reported in Chapter 4) were averaged from five individual measurements and the
error was reported as standard deviation. The ionic conductivity and compression properties
of polymer electrolyte systems can be found in Appendix A.
3.3.1 PreparationofcrosslinkedPEGDGEpolymerelectrolytes
3.3.1.1 PreparationofcrosslinkedPEGDGEelectrolytesusing TBAPF6 salt The synthesis of PEGDGE based polymer electrolyte involves PC as a solvent, PEGDGE and
TBAPF6 as conducting electrolyte and TETA as a hardener (crosslinker). TBAPF6 (0.020 g
Chapter 3 Experimental Section
92
of 0.050 wt%, 0.040 mmol, 0.010 cm3) was dissolved in PC (0.36 g of 0.73 wt%, 3.6 mmol,
0.30 mL) and while still stirring, PEGDGE (46 g, 91 wt%, 86 mmol, 40 mL) was added and
stirred continuously until a homogeneous solution was obtained (approx. 5 min). TETA (4.2
g, 8.3 wt%, 28 mmol, 4.2 mL) was then added to the solution and the mixture was further
stirred for 10 min. The solution was then transferred to four 10 ml syringes (BD Plastipak, 13
mm inner diameter, no needle). The filled syringes were finally placed in an oven for 24 h at
80°C.
3.3.1.2 PreparationofcrosslinkedPEGDGEelectrolytesusing LiTFSI salt The synthesis of PEGDGE based polymer electrolyte involves PC as a solvent, PEGDGE,
LiTFSI as conducting electrolyte and TETA as a hardener (crosslinker). LiTFSI (0.090 g,
0.18 wt%, 0.32 mmol, 0.070 cm3) was dissolved in PC (0.30 g, 0.60 wt%, 3.0 mmol, 0.25
mL) and while still stirring, PEGDGE (46 g, 93 wt%, 88 mmol, 41 mL) was added and
stirring continued until a homogeneous solution was obtained (approx. 5 min). TETA (3.3 g,
6.6 wt%, 23 mmol, 3.4 mL) was then added to the solution and the mixture was further
stirred for 10 min. The solution was then transferred to four 10 ml syringes. The filled
syringes were finally placed in an oven for 24 h at 80°C.
3.3.1.3 PreparationofcrosslinkedPEGDGEelectrolytesusing EMITFSI ionic liquid The synthesis of PEGDGE based polymer electrolyte involves PEGDGE, EMITFSI as
conducting electrolyte and TETA as a hardener (crosslinker). EMITFSI (5.0 g, 10 wt%, 13
mmol, 3.3 mL) was mixed in PEGDGE (41 g, 83 wt%, 78 mmol, 36 mL) and was stirred on
magnetic stirrer until a homogeneous solution was obtained (approx. 5 min). TETA (3.7 g,
7.4 wt%, 26 mmol, 3.8 mL) was then added to the solution and the mixture was further
stirred for 10 min. The solution was then transferred to four 10 ml syringes. The filled
syringes were finally placed in an oven for 24 h at 80°C. PEGDGE based polymer electrolyte
samples were also prepared by increasing the wt% of EMITFSI from 10 wt% to 20 wt%, 30
wt%, 40 wt%, 50 wt% and 60 wt% (Section 4.3.2). The compositions of the PEGDGE
polymer electrolyte samples by increasing the EMITFSI concentrations are listed in Table
3.2.
Chapter 3 Experimental Section
93
Sample code EMITFSI EMITFSI PEGDGE TETA
wt% (g) / (mmol) (g) / (mmol) (g) / (mmol)
E20P80 20% 10.0 / 25.6 36.6 / 69.6 3.40 / 23.2
E30P70 30% 15.0 / 38.3 32.1 / 61.0 2.90 / 19.8
E40P60 40% 20.0 / 51.1 27.5 / 52.3 2.50 / 17.1
E50P50 50% 25.0 / 63.9 22.9 / 43.5 2.10 / 14.4
Table 3.2 Composition of PEGDGE polymer electrolytes by increasing the concentrations of
EMITFSI.
3.3.2 PreparationofcrosslinkedDGEBAelectrolytes
3.3.2.1 PreparationofcrosslinkedDGEBAelectrolytesusing LiTFSI salt The synthesis of DGEBA based polymer electrolyte involves DGEBA, PC as a solvent,
LiTFSI as conducting electrolyte and MCHA as a hardener (Section 3.1.2). LiTFSI (1.16 g,
2.33 wt%, 4.06 mmol, 0.870 mL) was dissolved in PC (3.84g 7.67 wt%, 37.6 mmol, 3.20
mL) and the mixture was stirred for 15 min. DGEBA (35.3 g, 70.7 wt%, 104 mmol, 30.5 mL)
was added and the mixture was further stirred with a magnetic stirrer (300 rpm) in open air at
room temperature for 1 h. MCHA (9.66 g, 19.3 wt%, 45.9 mmol, 10.2 mL) was added to
mixture and was further stirred for 2-3 minutes and was then immediately shifted to four 10
mL syringes. The mixture was left for crosslinking for 2 days at room temperature inside the
syringes. DGEBA based polymer electrolyte samples were also prepared by increasing the
wt% of 1.0 M LiTFSI/PC from 10 wt% to 20 wt%, 40 wt%, 60 wt% and 80 wt% and the
compositions are reported in Table 3.3.
Sample code 1 M LiTFSI/PC LiTFSI PC DGEBA MCHA
wt% (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)
Li20B80 20% 2.33 / 8.12 7.67 / 75.1 32.3 / 94.9 7.70 / 36.6
Li40B60 40% 4.66 / 16.2 15.3 / 150 24.2 / 71.1 5.84 / 27.8
Li60B40 60% 6.99 / 24.3 23.0 / 225 16.2 / 47.6 3.81 / 18.1
Li80B20 80% 9.32 / 32.5 30.7 / 301 8.08 / 23.7 1.90 / 9.03
Table 3.3 Composition of DGEBA polymer electrolytes by increasing the concentrations of 1
M LiTFSI/PC.
Chapter 3 Experimental Section
94
3.3.2.2 PreparationofcrosslinkedDGEBAelectrolytesusing EMITFSI ionic liquid The synthesis of DGEBA based polymer electrolyte involves DGEBA, EMITFSI as a
conducting electrolyte and MCHA as a hardener (crosslinker). EMITFSI (5.00 g, 10.0 wt%,
12.8 mmol, 3.28 mL) was mixed in DGEBA (35.3 g, 70.7 wt%, 104 mmol, 30.5 mL) and the
mixture was stirred through magnetic stirrer (300 rpm) in open air at room temperature for 1
h. MCHA (9.66 g, 19.3 wt%, 45.9 mmol, 10.2 mL) was added in mixture and was further
stirred for 2-3 minutes and was then shifted to four 10 mL syringes and was left for
crosslinking for 2 days at room temperature.
3.3.3 PreparationofPANgelbasedpolymerelectrolytes
The preparation of PAN gel electrolyte involves PAN as polymer matrix, EC as a plasticiser,
TBAPF6 as conducting electrolyte, and PC as a solvent. PAN (3.60 g, 7.20 wt%) was dried in
a vacuum oven (1 bar) for 24 h at 75°C. TBAPF6 (1.60 g, 3.27 wt%, 4.22 mmol) was
dissolved at room temperature in a mixture of EC (26.8 g, 53.5 wt%, 304 mmol, 20.3 mL)
and PC (18.0 g, 36.0 wt%, 176 mmol, 2.2 mL) and the solution was stirred for 30 min to
make it homogeneous. PAN was then added to this homogeneous solution. This mixture was
then placed on a preheated hot plate (110°C); attached with thermocouple, under slow
stirring. After about 3-4 min, a very viscous solution was formed. This PAN gel was named
as PAN1. PAN1 was stored in bottle at room temperature before using it for rheological as
well as electrochemical characterisation measurements (Section 4.2). Oscillatory rheological
tests were conducted for these PAN gel based polymer electrolyte (Figure 4.2).
For the preparation of a second PAN gel electrolyte (PAN2, rheological characterisation after
3 days of preparation), PAN (13.0 g, 26.0 wt%) was dried in a vacuum oven (1 bar) for 24 h
at 75°C. TBAPF6 (10.2 g, 20.4 wt%, 26.3 mmol) was dissolved at room temperature in a
mixture of EC (18.3 g, 36.6 wt%, 208 mmol, 13.9 mL) and PC (8.50 g, 17 wt%, 83.2 mmol,
7.08 mL) and the solution was stirred for 30 min to make it homogeneous. PAN was then
added slowly to this homogeneous solution in order for complete dissolution. This mixture
was then placed on a preheated hot plate (110°C), attached with thermocouple, under slow
stirring. After about 3-4 min, a very viscous solution was formed. This PAN gel was named
as PAN2. PAN2 was stored in bottle at room temperature before using it in rheological as
well as electrochemical characterisation measurements (Table 4.2). The reported results for
all the characterisation measurements for PAN gel polymer electrolytes (Section 4.2) were
averaged from five individual measurements and the error was reported as standard deviation.
Chapter 3 Experimental Section
95
3.3.4 PreparationofcrosslinkedPEGDGE/DGEBAelectrolytes
3.3.4.1 Preparationofcrosslinked PEGDGE/DGEBA electrolytes using 10 wt% LiTFSI salt The synthesis of crosslinked PEGDGE/DGEBA electrolyte involves PEGDGE and DGEBA
as polymer matrix, PC as a solvent, LiTFSI as a conducting electrolyte, combined TETA and
MCHA as hardeners (crosslinkers). The preparation was identical for all polymer electrolytes
(other than quantities) and the following procedures describe the preparation of the
PEDGE/DGEBA 80:20 polymer electrolyte.
LiTFSI (1.17 g, 2.33 wt%, 4.07 mmol, 0.870 mL) was dissolved in PC (3.83 g, 7.67 wt%,
37.5 mmol, 3.20 mL) and the mixture was stirred for 15 min and then DGEBA (7.07 g, 14.1
wt%, 20.8 mmol, 6.09 mL) and PEGDGE (33.0 g, 66.0 wt%, 62.7 mmol, 28.9 mL) was
added and the mixture was further stirred in magnetic stirrer (300 rpm) in open air at room
temperature for 1 h. MCHA (1.93 g, 3.86 wt%, 9.18 mmol, 2.03 mL) and TETA (3.02 g, 6.05
wt%, 20.7 mmol, 3.08 mL) were added in mixture and further stirred for 2-3 minutes. The
mixture was transferred quickly to a 10 mL syringe and left to crosslink for 2 days at room
temperature. The crosslinked polymer was then named as Li10:80P:20B polymer electrolytes
blend. Other weight ratios of crosslinked PEGDGE to crosslinked DGEBA were also
prepared by varying the PEGDGE to DGEBA weight ratio i.e. 60P:40B, 40P:60B, 20P:80B
with P referred to PEGDGE and B referred to DGEBA respectively; the compositions of the
remaining polymer electrolytes are tabulated in Table 3.4.
Sample code
Ionic Electrolyte Polymer 1 (P) Polymer 2 (B)
LiTFSI PC PEGDGE TETA DGEBA MCHA
(g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)
Li10:60P:40B 1.17 / 4.08 3.83 / 37.5 24.7 / 47.0 2.30 / 15.7 14.1 / 41.4 3.90 / 18.5
Li10:40P:60B 1.17 / 4.08 3.83 / 37.5 16.5 / 31.4 1.50 / 10.2 21.2 / 62.3 5.80 / 27.6
Li10:20P:80B 1.17 / 4.08 3.83 / 37.5 8.25 / 15.7 0.75 / 5.13 28.3 / 83.1 7.70 / 36.6
Table 3.4 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of 1 M
LiTFSI/PC by varying the PEGDGE and DGEBA concentrations.
3.3.4.2 Preparationofcrosslinked PEGDGE/DGEBA electrolytes using 10 wt% EMITFSI ionic liquid The synthesis of crosslinked PEGDGE/DGEBA electrolyte involves PEGDGE and DGEBA
as polymer matrices, EMITFSI as a conducting electrolyte, combined TETA and MCHA as
Chapter 3 Experimental Section
96
hardeners (crosslinkers). EMITFSI (5.00 g, 10.0 wt%, 12.8 mmol, 3.28 mL) was mixed in
DGEBA (7.07 g, 14.1 wt%, 20.8 mmol, 6.09 mL) and PEGDGE (33.0 g, 66.0 wt%, 62.7
mmol, 28.9 mL) and the mixture was stirred on magnetic stirrer (300 rpm) in open air at
room temperature for 1 h. MCHA (1.93 g, 3.86 wt%, 9.18 mmol, 2.03 mL) and TETA (3.02
g, 6.05 wt%, 20.7 mmol, 3.08 mL) were added in mixture, further stirred for 2-3 minutes, and
then immediately transferred to the 10 mL syringe. The mixture was left for crosslinking for
2 days at room temperature. The crosslinked polymer was then named as E10:80P:20B
polymer electrolytes blend. The specimens were stored in air seal bags before all the
characterisation measurements. Other weight ratios of PEGDGE to DGEBA were also
prepared, using the identical procedure described above, by varying the PEGDGE to DGEBA
weight ratio i.e. 60P:40B, 40P:60B, 20P:80B with P referred to PEGDGE and B referred to
DGEBA respectively and the compositions of polymer electrolytes are tabulated in Table 3.5.
Sample code
Ionic Electrolyte Polymer 1 (P) Polymer 2 (B)
EMITFSI PEGDGE TETA DGEBA MCHA
(g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)
E10:60P:40B 5.00 / 12.8 24.7 / 47.0 2.30 / 15.7 14.1 / 41.4 3.90 / 18.5
E10:40P:60B 5.00 / 12.8 16.5 / 31.4 1.50 / 10.3 21.2 / 62.3 5.80 27.6
E10:20P:80B 5.00 / 12.8 8.35 / 15.9 0.85 / 5.81 28.0 / 82.2 7.80 / 37.1
Table 3.5 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of
EMITFSI by varying the PEGDGE and DGEBA concentrations.
3.3.4.3 Preparationofcrosslinked PEGDGE/DGEBA electrolytes using 50 wt% EMITFSI ionic liquid The synthesis of crosslinked PEGDGE/DGEBA electrolyte involves PEGDGE and DGEBA
as polymer matrix, EMITFSI as a conducting electrolyte, combined TETA and MCHA as
hardeners (crosslinkers). EMITFSI (25.0 g, 50.0 wt%, 63.9 mmol, 16.4 mL) was mixed in
DGEBA (3.93 g, 7.85 wt%, 11.5 mmol, 3.38 mL) and PEGDGE (18.3 g, 36.6 wt%, 34.8
mmol, 16.1 mL) and the mixture was stirred on magnetic stirrer (300 rpm) in open air at
room temperature for 1 h. MCHA (1.07 g, 2.15 wt%, 5.10 mmol, 1.13 mL) and TETA (1.67
g, 3.34 wt%, 11.4 mmol, 1.71 mL) were added in mixture, further stirred for 2-3 minutes and
then immediately transferred to the 10 mL syringe. The mixture was left for crosslinking for
2 days at room temperature. The crosslinked polymer was then named as E50:80P:20B
polymer electrolytes blend. Other weight ratios of crosslinked PEGDGE to crosslinked
Chapter 3 Experimental Section
97
DGEBA were also prepared, using the identical procedure described above, by varying the
PEGDGE to DGEBA weight ratio i.e. 60P:40B, 40P:60B, 20P:80B with P referred to
PEGDGE and B referred to DGEBA respectively and the compositions of polymer
electrolytes are tabulated in Table 3.6.
Sample code
Ionic Electrolyte Polymer 1 (P) Polymer 2 (B)
EMITFSI PEGDGE TETA DGEBA MCHA
(g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)
E50:60P:40B 25.0 / 63.9 13.8 / 26.2 1.20 / 8.21 7.85 / 23.1 2.15 / 10.2
E50:40P:60B 25.0 / 63.9 9.15 / 17.4 0.83 / 5.68 11.8 / 34.7 3.22 / 15.3
E50:20P:80B 25.0 / 63.9 4.58 / 8.70 0.43 / 2.94 15.7 / 46.1 4.29 / 20.4
E50:0P:100B 25.0 / 63.9 N/A N/A 19.6 / 57.6 5.40 / 25.7
Table 3.6 Composition of PEGDGE:DGEBA blend polymer electrolytes with 50 wt% of
EMITFSI by varying the PEGDGE and DGEBA concentrations.
3.4 Composite polymer electrolytes All composite polymer electrolyte systems were tested for mechanical compression
performance and ionic conductivity. The samples were produced as cylinders having flat and
parallel ends by cutting the composite polymer electrolyte systems (24 mm height and 13 mm
diameter for the mechanical characterisation measurements and 4 mm height and 13 mm
diameter for the electrochemical measurements) on Lathe machine (Colchester Student,
England). The samples were stored in a sealed plastic bag at room temperature and
characterisation measurements were carried out within 2 days of preparation of composite
polymer electrolytes. The results for the compression and ionic conductivity measurements of
the composite polymer electrolyte systems (reported in Chapter 5) were averaged from five
individual measurements and the error was reported as standard deviation. The
electrochemical and mechanical characterisation results of the composite polymer electrolyte
systems can be found in Appendix A.
3.4.1 PreparationofMSP/PEGDGEcompositepolymerelectrolytes
3.4.1.1 Preparationofcrosslinked MSP/PEGDGE composite polymer electrolytes using
LiTFSI salt
LiTFSI (0.100 g, 0.180 wt%, 0.348 mmol, 0.0610 mL) was dissolved in PC (0.300 g, 0.590
wt%, 2.94 mmol, 0.240 mL) and MSP (Section 3.2.2) (1.25g, 2.50 wt%) were added as a
Chapter 3 Experimental Section
98
powder. While still stirring on magnetic stirrer (500 rpm), PEGDGE (45.1 g, 90.2 wt%, 85.7
mmol, 39.3 mL) was added and stirring continued for 5 h at room temperature at which point
a homogeneous solution was obtained. TETA (3.25 g, 6.51 wt%, 22.2 mmol, 3.30 mL) was
added to the solution and stirring continued for 15 min. The solution was transferred into a
mould of 10 ml syringe and the syringes were finally placed in an oven for 24 h at 80°C. The
reported results for the electrochemical (Section 3.5) and mechanical (Section 3.6.2)
characterisation measurements of MSP/PEGDGE based polymer electrolytes with 0.80 wt%
of 1 M LiTFSI/PC (see Table A.7 of Appendix A) were averaged from five individual
measurements. An identical procedure was adopted by increasing the wt% of MSP from 2.50
wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt% in composite PEGDGE based polymer
electrolytes and were characterised mechanically as well as electrochemically (Table A.7 of
Appendix A). The compositions of other PEGDGE polymer electrolytes by increasing the
MSP concentration are reported in Table 3.7.
Sample code MSP LiTFSI PC MSP PEGDGE TETA
wt% (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)
Li0.80P100M5.0 5.00 0.10 / 0.35 0.30 / 2.94 2.50 / 41.7 43.9 / 83.5 3.20 / 21.9
Li0.80P100M7.5 7.50 0.10 / 0.35 0.30 / 2.94 3.75 / 62.5 42.8 / 81.4 3.05 / 20.9
Li0.80P100M10 10.0 0.10 / 0.35 0.30 / 2.94 5.00 / 83.4 41.6 / 79.1 3.00 / 20.5
Li0.80P100M12.5 12.5 0.10 / 0.35 0.30 / 2.94 6.25 / 104 40.4 / 76.8 2.95 / 20.2
Table 3.7 Composition of MSP/PEGDGE composite polymer electrolytes with 0.8wt% of 1 M
LiTFSI/PC by varying the MSP concentrations.
3.4.1.2 Preparationofcrosslinked MSP/PEGDGE composite polymer electrolytes using
EMITFSI ionic liquid
EMITFSI (5.00 g, 10.0 wt%, 12.8 mmol, 3.28 mL) was mixed in mesoporous silica particles
(1.25 g, 2.50 wt%, 20.8 mmol, 0.690 mL) and the mixture was stirred for 30 min followed by
the addition of PEGDGE (40.1 g, 80.2 wt%, 76.2 mmol, 35.2 mL). The mixture was stirred
again on magnetic stirrer (300 rpm) in open air at room temperature for 5 h. TETA (3.67 g,
7.35 wt%, 25.1 mmol, 3.75 mL) was added in mixture and was further stirred for 2-3
minutes. The solution was then transferred into 10 ml syringes and was finally placed in an
oven for 24 h at 80°C. Similar procedure was adapted by increasing the wt% of MSP from
2.50 wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt% in composite PEGDGE based
polymer electrolytes and were characterised mechanically as well as electrochemically (see
Chapter 3 Experimental Section
99
Table A.9 of Appendix A). The compositions of other PEGDGE polymer electrolytes by
increasing the MSP concentration are reported in Table 3.8.
Sample code MSP EMITFSI MSP PEGDGE TETA
wt% (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)
E10P100M5.0 5.00 5.00 / 12.8 2.50 / 41.7 38.9 / 74.0 3.60 / 24.6
E10P100M7.5 7.50 5.00 / 12.8 3.75 / 62.5 37.8 / 71.9 3.45 / 23.6
E10P100M10 10.0 5.00 / 12.8 5.00 / 83.4 36.6 / 69.6 3.40 / 23.3
E10P100M12.5 12.5 5.00 / 12.8 6.25 / 104 35.5 / 67.5 3.25 / 22.2
Table 3.8 Composition of MSP/PEGDGE composite polymer electrolytes with 10wt% of
EMITFSI by varying the MSP concentrations.
3.4.2 PreparationofcrosslinkedMSP/DGEBAcompositepolymerelectrolytesLiTFSI (2.33g, 4.66 wt%, 8.12 mmol, 1.75 mL) was dissolved in PC (7.67 g, 15.3 wt%, 75.1
mmol, 6.39 mL) and after that, MSP (Section 3.2.2) (1.25 g, 2.50 wt%) were added. While
still stirring, DGEBA (30.4 g, 60.9 wt%, 89.4 mmol, 26.2 mL) was added and stirring
continued for 5 h at room temperature at which point a homogeneous solution was obtained.
MCHA (8.31 g, 16.6 wt%, 39.5 mmol, 8.75 mL) was then added to the solution and was
further stirred for 15 min. The solution was then transferred into 10 ml syringes and was
finally placed in an oven for 24 h at 80°C. Identical procedure was adopted by increasing the
wt% of MSP from 2.50 wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt% in composite
DGEBA based polymer electrolytes and were characterised mechanically (Section 3.6.2) as
well as electrochemically (Section 3.5) and are tabulated in Table A.10 of Appendix A. The
composition of the remaining composite polymer electrolytes are reported in Table 3.9.
Sample code
MSP MSP LiTFSI PC DGEBA MCHA
wt% (g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
Li20B100M5.0 5.00 2.50 / 41.7 2.33 / 8.12 7.67 / 75.1 29.4 / 86.4 8.10 / 38.5
Li20B100M7.5 7.50 3.75 / 62.5 2.33 / 8.12 7.67 / 75.1 28.5 / 83.7 7.75 / 36.8
Li20B100M10 10.0 5.00 / 83.3 2.33 / 8.12 7.67 / 75.1 27.5 / 80.8 7.50 / 35.6
Li20B100M12.5 12.5 6.25 / 104 2.33 / 8.12 7.67 / 75.1 26.5 / 77.8 7.25 / 34.4
Table 3.9 Composition of DGEBA/MSP composite polymer electrolytes with 20wt% of 1 M
LiTFSI/PC by varying the MSP concentrations.
Chapter 3 Experimental Section
100
3.4.3 PreparationofcrosslinkedMSP/PEGDGE/DGEBAcompositepolymerelectrolytes
3.4.3.1 Preparationofcrosslinked MSP/PEGDGE/DGEBA composite polymer electrolytes using LiTFSI salt LiTFSI (2.33 g, 4.66 wt%, 8.12 mmol, 1.75 mL) was dissolved in PC (7.67 g, 15.3 wt%, 75.1
mmol, 6.39 mL) and after that, mesoporous silica particles (1.25 g, 2.50 wt%) were added
(Section 3.2.2) under stirring followed by PEGDGE (14.2 g, 28.4 wt%, 27.0 mmol, 12.4 mL)
and DGEBA (18.3 g, 36.5 wt%, 53.6 mmol, 15.7 mL). The mixture was stirred again on
magnetic stirrer (300 rpm) in open air at room temperature for 5 h. TETA (1.30 g, 2.60 wt%,
8.90 mmol, 1.33 mL) and MCHA (4.99 g, 9.97 wt%, 23.7 mmol, 5.25 mL) was added in
mixture and was further stirred for 2-3 minutes. The solution was then transferred into 10 ml
syringes and was finally placed in an oven for 24 h at 80°C. Similar procedure was adapted
by increasing the wt% of MSP from 2.5 wt% to 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt% in
composite PEGDGE based polymer electrolytes and were characterised mechanically as well
as electrochemically (Table A.11). The composition of the remaining composite polymer
electrolytes are reported in Table 3.10.
Sample code
Samples Electrolyte MSP
Polymer 1 (P) Polymer 2 (B)
MSP LiTFSI PC PEGDGE TETA DGEBA MCHA
wt% (g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
Li20:40P:60B:M5.0 5.00 2.33 /
8.12
7.67 /
75.1
2.50 /
41.7
13.7 /
26.0
1.26 /
8.62
17.7 /
52.0
4.84 /
23.0
Li20:40P:60B:M7.5 7.50 2.33 /
8.12
7.67 /
75.1
3.75 /
62.5
13.3 /
25.3
1.21 /
8.27
17.1 /
50.2
4.64 /
22.1
Li20:40P:60B:M10 10.0 2.33 /
8.12
7.67 /
75.1
5.00 /
83.4
12.8 /
24.3
1.19 /
8.14
16.5 /
48.5
4.51 /
21.4
Table 3.10 Composition of PEGDGE/DGEBA/MSP composite polymer electrolytes with
20wt% of 1 M LiTFSI/PC by varying the MSP concentrations.
3.4.3.2 Preparationofcrosslinked MSP/PEGDGE composite polymer electrolytes using EMITFSI ionic liquid EMITFSI (25.0 g, 50.0 wt%, 63.9 mmol, 16.4 mL) was mixed in mesoporous silica particles
(Section 3.2.2) (1.25 g, 2.50 wt%, 20.8 mmol, 0.690 mL) and the mixture was stirred for 30
min followed by the addition of PEGDGE (8.71 g, 17.4 wt%, 16.6 mmol, 7.64 mL) and
Chapter 3 Experimental Section
101
DGEBA (11.2 g, 22.4 wt%, 32.9 mmol, 9.65 mL). The mixture was stirred again on magnetic
stirrer (300 rpm) in open air at room temperature for 5 h. TETA (0.790 g, 1.59 wt%, 5.43
mmol, 0.810 mL) and MCHA (3.06 g, 6.12 wt%, 14.5 mmol, 3.22 mL) was added in mixture
and was further stirred for 2-3 minutes. The solution was then transferred into 10 ml syringes
and was finally placed in an oven for 24 h at 80°C. Similar procedure was adapted by
increasing the wt% of MSP from 2.50 wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt%
in composite PEGDGE based polymer electrolytes and were characterised mechanically as
well as electrochemically (Table A.13). The composition of the remaining composite
polymer electrolytes are reported in Table 3.11.
Sample code
MSP EMITFSI MSP Polymer 1 (P) Polymer 2 (B)
PEGDGE TETA DGEBA MCHA
wt% (g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
(g) /
(mmol)
E50:40P:60B:M5.0 5.00 25.0 / 63.9 2.50 / 41.7 8.25 / 15.7 0.75 / 5.13 10.6 / 31.1 2.90 / 13.8
E50:40P:60B:M7.5 7.50 25.0 / 63.9 3.75 / 62.5 7.79 / 14.8 0.71 / 4.86 10.0 / 29.4 2.75 / 13.1
E50:40P:60B:M10 10.0 25.0 / 63.9 5.00 / 83.4 7.33 / 13.9 0.67 / 4.58 9.42 / 27.7 2.58 / 12.3
E50:40P:60B:M12.5 12.5 25.0 / 63.9 6.25 / 104 6.87 / 13.1 0.63 / 4.31 8.83 / 25.9 2.42 / 11.5
Table 3.11 Composition of PEGDGE/DGEBA/MSP composite polymer electrolytes with
50wt% of EMITFSI by varying the MSP concentrations.
3.5 Chemical Activation of carbon fibre mats
Activated carbon fibre mats were provided by Dr Hui Qian and were used as received.
Chemical activation of carbon fibre involves impregnation of carbon fibres with thick
potassium hydroxide (KOH) slurry followed by heat treatment. A 30×21 cm sized carbon
fibre mat was placed in Pyrex® tray while slurry of 1.85 M KOH in water (typical KOH
loading was approximately 6.5 wt% after drying) was transferred onto the fibre mat. The
sample was left inside a fume cupboard for 3 hours before the excess KOH was dispensed
away and the cloth was placed on a PET (PolyEthylene Terephthalate) film to dry overnight.
Subsequently, the dried carbon fibre fabric was transferred onto a perforated stainless steel
frame and positioned into the retort (Lenton ECF 12/30) where it was heated at 5˚C/min to
800˚C under nitrogen (flow rate of 0.5 L/min) and held at temperature for 30 min. The system
was allowed to cool to room temperature while in inert nitrogen atmosphere over
approximately 14 h. The resultant activated carbon fibres mat was then washed extensively
Chapter 3 Experimental Section
102
with deionised water to eliminate any residual alkali until pH7, and was finally dried in a
vacuum oven at 80 ˚C. The typical mass loss was around 6 wt%. The characteristics of
activated carbon fibre are shown in Table 3.12.
CF mats df
(μm)
As
(m2/g)
Cg
(F/g)
ET
(GPa)
σT
(MPa)
As received 7 0.21 0.06 3.29 ± 0.09 204 ± 4
Activated 6.9 21.4 2.63 3.96 ± 0.13 207 ± 4
Table 3.12 Single fibre diameter df, BET surface area As, specific capacitance Cg, tensile
modulus ET and tensile strength σT of as-received and activated carbon fibre mats (results
courtesy of Dr. Hui Qian).
3.6 Electrochemical impedance spectroscopy of polymer electrolytes
Electrochemical impedance spectroscopy (EIS) is a useful technique for the investigation of a
wide range of bulk and interfacial electrical properties associated with materials where ionic
conduction is in prime consideration e.g. ionic salts, liquid electrolytes, solid and gel polymer
electrolytes and ionic conductive glasses [168]. EIS is widely employed in the study of
rechargeable batteries, supercapacitors and fuel cells. EIS is usually determined by applying
an AC potential to an electrochemical cell and measuring the current through the cell [169].
The AC potential penetrates into the bulk pores of the electrochemical cell and shows the
amount of solvated ions at a specific frequency that reach the pore surface [170].
Electrochemical impedance is defined as,
exp Φ Φ Φ (3.3)
where Z0 is the magnitude of Z ( / and Ф is the phase angle of current I
versus potential difference E.
The real part of impedance is usually plotted against the imaginary part of impedance to get a
complex plot called Nyquist plot.
For the electrochemical characterisation of polymer electrolytes (Section 3.4), an Ivium-n-
Stat Multichannel potentiostat (Ivium Technologies, The Netherlands) was used. Ionic
conductivity for liquid electrolytes was measured using JenWay 4330 conductivity and pH
metre (UK). The impedance measurements were conducted in frequency window of 10-1 Hz
to 105 Hz and the amplitude of sinusoidal voltage was 0.5 V. Detailed procedures for the
Chapter 3 Experimental Section
103
calculation of ionic conductivity of polymer electrolytes (Section 4.3) are given in Appendix
B. Ionic conductivity of all polymer electrolytes (Section 3.3 and Section 3.4) can be found in
Appendix A.
3.7 Mechanical characterisation of polymer electrolytes
3.7.1 RheologicalcharacterisationofPANgelpolymerelectrolytes
In order to mechanically characterise the polyacrylonitrile gel based polymer electrolyte
(Section 3.3.3), oscillatory rheological tests were conducted at room temperature (PAAR
Physica UDS200 Universal Dynamic Spectrometer (Germany), by using a concentric,
cylinder type (Z3 DIN with bob and cup gap of 1.06 mm) measuring system. Frequency
sweep tests of PAN gel polymer electrolytes (see Section 3.3.3 for preparation) were
conducted in a frequency window of 0.01 to 100 Hz at a strain rate of 0.1% and temperatures
of 25°C, 40°C and 90°C. Temperature sweep tests on these PAN gel polymer electrolytes
were conducted in a temperature window of 25°C to 120°C at a frequency of 0.1 Hz and a
strain rate of 0.1%.
At constant frequency, ω, and strain amplitude, γ0, the polyacrylonitrile gel based polymer
electrolyte was deformed sinusoidally (γ = γ0sinωt). As a result, the stress, τ, started
oscillating sinusoidally at the same frequency, ω, (radial frequency, ω= 2πf) after few start-up
cycles (generally it is shifted by a phase angle, δ, with respect to strain wave [171]). This can
be represented by two wave forms as shown in Figure 3.9. Figure 3.9 shows that the
oscillating stress, τ, is produced as a result of oscillating strain, γ, but is shifted by phase
angle, δ, as at lower frequencies, response is of viscous liquid [172]. The stress wave is
broken down into two waves. τ/ is the wave in-phase with the strain wave and τ// is the wave
90° out-of-phase of the strain wave. However, τ// wave is in-phase with the strain rate wave,
, (i.e.γ dγ ⁄ dt). The maximum stress, τ0, divided by the maximum strain, γ0, is a constant
for a given frequency ω and is called complex modulus, G*.
∗ / (3.4)
where G* is a complex number and has a real part G/ and imaginary part G//.
Chapter 3 Experimental Section
104
Figure 3.9 An oscillating shear strain and the stress response for viscoelastic materials
[171].
The stress wave can be analyzed by decomposing it into two waves of same frequency. Thus,
the resulting stress has following two dynamic moduli:
τ =γ0 [G/sinωt+ G//cosωt], where G/ is the elastic or shear modulus and is in-phase with the
deformation. G// is the loss or viscous modulus and is 90° out-of-phase with deformation. G//
is measure of energy dissipated per cycle of deformation per unit volume. Loss or dissipation
factor tan δ is the ratio of the loss component (G//) to the storage component (G/). The value
of the loss or dissipation factor (tan δ) characterises the ratio of viscous-to-elastic properties
in viscoelastic materials. By decreasing δ, and consequently decreasing viscous losses,
material transits from pure viscous to pure elastic.
(3.5)
3.7.2 Mechanicalcharacterisationofsolidpolymerelectrolytes(Compressiontesting)
Compression properties are important in engineering practise to determine the compression
modulus and compression strength while the material is in service. Compression testing of
polymer electrolytes was conducted because the testing is relatively simple and require small
amount of specimen material with relatively simple geometry [173]. Unlike tensile testing,
compression testing does not require expensive grips [173]. Compression tests were
conducted between the plates of a compression testing machine (Easy 50, Lloyds
Instruments, UK) having 50 kN load cell and 50 kN frame in accordance to the ASTM
standard D695 [173]. The samples were prepared as cylinders (see section 3.3) of
Chapter 3 Experimental Section
105
approximately 13 mm in diameter and 25 mm in length having parallel flat ends. Specimens
were cut using scalpel and ends were made parallel flat by using abrasive sheet. Specimens
were placed between two PTFE plates (50 mm diameter, 15 mm height, RS Components,
UK) in order to avoid slippage and compression force was applied in an axial direction to the
faces of the specimen. The compression strength σ, maximum stress supported by the
samples, was calculated from the following equation 3.6.
(3.6)
where is the maximum compression force carried by a polymer electrolyte sample
during the test and A is the original minimum cross-sectional area of polymer electrolyte
sample. Cross-head deflection speed of testing was 1 mm/min. Compression stiffness was
determined by recording the force-displacement graph and linearly fitting tangent to the
steepest part of the plot (0.02 %-0.25 % of the strain applied). Five tests were conducted on
nominally identical polymer electrolyte samples (Specimens prepared from the same polymer
electrolyte preparation but cut out from a different mould (syringe) (containing cured
polymer electrolyte) prepared in a same batch (Section 3.3) and the results were reported as
an average value with errors as standard deviation. The test machine compliance was
accounted for when determining the compressive modulus values (see Appendix C for
details).
3.8 Composite fabrication using Resin Infusion under Flexible Tooling (RIFT)
Composites were fabricated using a resin infusion under flexible tooling (RIFT) process.
During a RIFT process, resin is allowed to impregnate into a dry fibre pack loaded into a
vacuumed bagging flexible film. The flow of resin was due to vacuum drawn under the film.
The vacuum bag had two ends connected with tubing in which one of the tubing delivered the
resin while the other maintained a negative pressure in the vacuum bag. The negative
pressure removed the air from the stack of dry laminates and thus resulted in a minimise
voids. A schematic of a RIFT process is shown in Figure 3.10.
Chapter 3 Experimental Section
106
Hot PlateResin
Inlet OutletLaminate 1 Laminate 2
VacuumPump
Vacuum bag
Resin Diffusion membrane
Release fabric
Melinex film
Flashtape 1
Vacuum bagging sealent
FEP tube
Figure 3.10 Schematic of a RIFT process.
During the composite fabrication by RIFT process, 800 mm×430 mm of polyester based
Melinex® film (50 µm thickness, PW 122-50-RL, PSG group, UK) was first attached on a
heating plate (920mm×460 mm, Wenesco, Inc., USA) using a high temperature resistant
polyester based adhesive tape (Silicon adhesive, Flashtape 1, Aerovac, UK). The heating
plate was coupled to a temperature controller (Wenesco, Inc., USA). 700 mm×330mm PTFE
coated glass release fabric (FF03PM, Aerovac, UK) was placed on top of the Melinex® film
followed by a 500mm×330mm polyester flow media based resin diffusion membrane (warp
knitted diffusion knap, 15087B, Newbury engineer textile, UK). Another PTFE coated glass
release fabric (700 mm×330 mm) was placed on top of the resin diffusion membrane. Two
carbon fibre mats (180 mm×140 mm) were cut at ±45° through a 45°angled square set
protractor (RS number 663-768, RS Components, UK) and copper tape (copper foil coated
with an electrically conductive acrylic adhesive, 0.035 mm thickness, 25 mm width, 542-
5511, RS components, UK) was applied around the corners of the carbon fibre mats
(specifications of the carbon fibre mats can be found in Section 3.1.8). Carbon fibre mats
were then placed in hot press (Carver, UK) at 100°C for 6 h under pressure of 1 ton. Two
glass fibre mats (220 mm×180 mm) were cut at ± 45° through a 45° angled square set
protractor and were sandwiched between two ± 45° cut carbon fibre mats (180 mm×140
mm). The laminate was laid up by hand ensuring that the lay-up was balanced (i.e. the crimp
Chapter 3 Experimental Section
107
lines of the CF plies were mirrored around the midplane). Two copper wires (RS No. 177-
0621, 7 strands/0.1 mm, RS components, UK) were also placed in laminate assembly such
that each copper wire was placed between carbon fibre and glass fibre mats (see Section 6.7
for the effect of copper tape). Another release fabric and resin diffusion membranes were
placed on top of the laminate assembly. Two 700mm long Legris fluoropolymer FEP tubes
(RS components, UK) possessing an inner diameter of 6 mm were connected on each end of
the setup using a vacuum bag sealant tape (SM5127, Aerovac, UK). One of the tubes was
used as resin inlet and the other one connected to the vacuum pump (Island Scientific, UK).
The whole setup was then covered with a 600 mm×1000 mm Capran-518 heat stabilised
Nylon 6 blown tubular vacuum bagging film (Aerovac, UK) which was sealed using a
vacuum bag sealant tape. Composite fabrication by RIFT process is shown in Figure 3.11.
Figure 3.11 Vacuum bag during RIFT process (a) Rift setup, (b) sandwiched CF and GF
mats before RIFT process, (c) Structural supercapacitors after RIFT process.
Chapter 3 Experimental Section
108
The resin inlet tube was blocked by bending it several times and then wrapping it with
Flashtape-1. Vacuum was applied from the second tube connected to a vacuum pump (Rotary
pump, Model EDM6, Island Scientific, UK). Laminates were left under vacuum for 20 min at
1 bar (pressure determined using pressure gauge attached to RIFT setup). After 20 min of
vacuum, the resin inlet tube was unblocked and was dipped in polymer electrolyte container
so that polymer electrolyte started flowing through the laminate assembly. The resin inlet
tube was blocked again when the polymer electrolyte (approximately 350 g in total was
needed for the fabrication of 4 supercapacitors) completely went through the laminate
assembly. The laminates were left to cure at 80°C (temperature reading from controller of the
RIFT setup) for 24 h in the RIFT setup. The structural supercapacitors were removed from
the RIFT setup after 24 hr and were used for electrochemical and mechanical testing. The
structural supercapacitors were stored in air-tight plastic bags. The electrochemical (see
Section 3.8) and mechanical (see Section 3.9) testing was conducted within 7 days of
composite fabrication.
3.9 Electrochemical characterisation of structural supercapacitors
For the electrochemical characterisation of structural supercapacitors, an Ivium-n-Stat
Multichannel potentiostat (Ivium Technologies, The Netherlands) was used. The most widely
used electrochemical analyses for the characterisation of energy storage devices are:
1) Cyclic voltammetry method
2) Charge-discharge method
3) Electrochemical impedance spectroscopy
3.9.1 Cyclicvoltammetry
Cyclic voltammetry is a relatively quick and simple method as it can give directly the
accessible capacitance (C) as a response current (I), given by,
(3.7)
where dE/dt is the voltage sweep rate (s). Typical sweep rates are of the order of 3mV s-1 to 1
V s-1 but the values can be larger. The maximal energy density of structural supercapacitors
can be obtained from the capacitance as shown in equation 3.8,
12
(3.8)
Chapter 3 Experimental Section
109
where E is the maximal energy density (J/kg), C the capacitance (F), V the potential
difference applied in (V) and m the mass of structural supercapacitor. Cyclic voltammetry
tests for the structural supercapacitors were conducted using sweep rates of 3 mV s-1, 20 mV
s-1 and 50 mV s-1 in the voltage range of -0.5 V to 0.5 V as in this voltage range water
decomposition can be avoided. The capacitance [Chapter 6] was calculated from the
following equation 3.9,
(3.9)
where I is the discharge current density for the discharge time t.
3.9.2 Potentialsquare‐wavevoltammetry(Charge/discharge)
Potential square-wave voltammetry test of structural supercapacitors [Chapter 6] was
performed by applying step voltage of 0.1 V for 10 min. The capacitance (C) value (in F)
was obtained from the equation,
∆
∆ (3.10)
where I was the discharge current and ∆E the change in voltage in time ∆t. The specific
capacitance was calculated as capacitance per volume unit (F/cm3).
3.9.3 Electrochemicalimpedancespectroscopy
Impedance spectroscopy is a useful technique for the investigation of wide range of bulk and
interfacial electrical properties associated with the supercapacitors. Electrochemical
impedance spectroscopy (EIS) is usually determined by applying AC potential to
supercapacitor cell and measuring the current through the cell [169]. The AC potential
penetrates into the bulk pores of the electrochemical cell and shows the amount of solvated
ions at a specific frequency that reach the pore surface of the electrodes. Electrochemical
impedance is defined as,
exp Φ Φ Φ (3.11)
where Z0 is the magnitude of Z ( / and Ф is the phase angle of current I
versus potential difference E.
The real part of impedance is usually plotted against the imaginary part of impedance to get a
complex plot called Nyquist plot. The two copper wires of the structural supercapacitors were
Chapter 3 Experimental Section
110
connected to the two electrodes of the Potentiostat. The impedance measurements were
conducted in frequency window of 1 Hz to 105 Hz and the amplitude of sinusoidal voltage
was 500 mV. Impedance method was used to calculate the equivalent series resistance (ESR)
of structural supercapacitors (Chapter 6). ESR was measured (measurement was identical to
the procedure described in Appendix B) from the x-intercept of high frequency curve in the
Nyquist plot.
3.10 Mechanical characterisation of composites (±45° laminate tensile test)
In-plane shear response of ±45° laminated structural supercapacitors was investigated by
tensile test in accordance with ASTM D3518 [174]. The tensile test of a ±45° laminate is a
matrix dominated property as during application of a tensile force on ±45° laminate, the
carbon fibres tried to align themselves resulting in the damage of polymer electrolyte that is
supporting the fibre ±45° alignment, as shown in Figure 3.12. In the tensile test of a ±45°
laminate, transverse and longitudinal strains were recorded by applying a uniaxial tension.
The ±45° laminated structural supercapacitor was made symmetric about the mid plane by
using two glass fibre mat based separators. Therefore, the shear strain relationship was
developed by calculating the maximum in-plane shear stress for the ±45° laminated structural
supercapacitor from the equation 3.12,
2
(3.12)
where is the maximum in-plane shear stress in MPa, is the maximum load at or
below 5% shear strain in N, and A is the cross sectional area in mm2.
The shear strain was also calculated at each required data point using Equation 3.13,
(3.13)
where is the shear strain at i-th data point, is the longitudinal normal strain at i-th data
point and is the transverse normal strain at i-th data point
Chapter 3 Experimental Section
111
y
x
tensile force
fibre direction
Figure 3.12 Schematic of a ±45° laminated structural supercapacitor during tensile test in
accordance with ASTM D 3518.
The unidirectional shear strength (translaminar i.e. through thickness), G12, was obtained by
taking the maximum load, in plane shear strain, taken from the initial linear portion of the
unidirectional shear stress and shear strain curve by using the equation 3.14:
∆∆
(3.14)
An Instron 4505 machine (Bucks, UK) was used for the tensile test of ±45° laminated
structural supercapacitors. For PEGDGE based structural supercapacitors, a 5kN load cell
was used and for the DGEBA based structural supercapacitors, a 10 kN load cell was used.
The test specimen was in a rectangular shape with a length of 160 ± 2.50 mm, a width of 25 ±
0.50 mm and a thickness of 0.80 ± 0.020 mm. The ends of each specimen were grit blasted
and adhesively bonded with fibre glass composites end tabs, with a gauge length of 100 mm.
Two strain gauges (Type FLA-10-11, Tokyo Sokki Kenkyujo Co. Ltd, Japan) were attached
to the each face of the bar to measure longitudinal strain as well as transverse strain. The
crosshead speed was 2 mm/min. The tensile force applied to the specimen was recorded
every 0.5 s through the test until the specimen failed as shown in Figure 3.13.
Chapter 3 Experimental Section
112
Figure 3.13 Tensile testing of a structural supercapacitor specimen (a) pre tensile test
specimen, (b) post tensile test specimen.
3.11 Fibre volume fraction of structural supercapacitors by acid digestion
The fibre volume fraction of all types of structural supercapacitors was also determined using
ASTM D3171 [175]. Acid digestion of structural supercapacitors was carried out using a
mixture of sulphuric acid (95-97% purity, Sigma Aldrich) and hydrogen peroxide (50 wt%
solution in water, Sigma Aldrich) as mentioned in Procedure B of ASTM D 3171 [175].
Procedure B was selected because the method is suitable for epoxy resin based composites
and does not require any reflux condenser as compared to other procedures mentioned in
ASTM D 3171. Each specimen (3cm×2cm) from different structural supercapacitors was
weighed and pre-digestion weight was recorded to the nearest 0.0001 g. The density of each
pre-digested specimen was also measured through helium pycnometry using Accupyc 1330
(Micromeritics, USA). The specimen was then placed in a 100-mL conical flask and 25mL of
sulphuric acid (95-97% purity) was added. The beaker was placed on hot plate at 120°C for
10 min until the mixture started to fume and the solution changed its colour to dark brown. 35
mL of hydrogen peroxide (50 wt% in H2O) was added slowly in the mixture in order to
Chapter 3 Experimental Section
113
oxidise the polymer electrolyte until the solution changed to transparent. The carbon fibre and
the glass fibre started floating to the top of the solution. The conical flask was removed from
the hot plate and the solution was allowed to cool. Carbon fibre and glass fibre were then
separated through gravity filtration (sintered glass funnel of porosity grade 4) and were then
washed with excess deionised water for 30 min followed by acetone (99.5% purity, GPR
RECTAPUR, VWR, UK) washing (20 mL for 5 min) in order to improve drying times. The
specimens were placed in oven for 24 h at 100°C. The mass and density of the dried fibres
were measured using the balance and Accupyc 1330 (Micromeritics, USA) respectively. The
fibre volume fraction was calculated using equation 3.15,
100 (3.15)
where is fibre volume fraction, is the initial mass of specimen, is the final mass of
fibres after acid digestion, is the density of specimen and is the density of fibres.
Chapter 4 Polymer Electrolytes
114
Chapter 4 Polymer Electrolytes
This chapter covers the results and discussion of polymer electrolytes used for the preparation
of structural supercapacitors. The ionic conductivity results of different salts in propylene
carbonate are first presented followed by the results as well as discussion of the
multifunctionality of PEGDGE matrix containing different salts. Subsequently, LiTFSI/PC
and EMITFSI (ionic liquid) were selected for various other PEGDGE, DGEBA and
PEGDGE/DGEBA formulations. A PAN gel based polymer electrolyte was also prepared
and characterised mechanically and electrochemically. The ionic conductivity and
compression properties of different polymer electrolytes are discussed to draw attention to the
research goals and challenges of this work.
Chapter 4 Polymer Electrolytes
115
4.1 Selection of salts for inclusion into polymers
In electrochemical devices, operating at ambient temperatures, the electrolyte salt should,
ideally, dissolve and dissociate in the solvent and the solvated ions should be able to move in
polymer medium with high mobility. Ions should be inert to electrolyte solvent and should be
non-toxic and thermally stable. The degree of dissociation of salts dissolved in the polymer
host depends on the total concentration of salt in matrix. Generally, the degree of dissociation
decreases with increasing salt concentration and thus, at optimal salt concentration, the
electrolyte possesses maximum total concentration of free ions. In the present work, the ionic
conductivity of six different salts were measured. Lithium ions and sodium ions based salts
had been selected because of the high energy density and compatibility with polymer
electrolytes [55]. Tetrabutyl ammonium hexafluorophosphate (TBAPF6) was selected as it is
less hygroscopic as compared to lithium and sodium based salts. Since lithium, sodium and
ammonium based salts are solid at room temperature, propylene carbonate (PC) was used as
solvent. PC was selected as a solvent for salts because of its low melting point [176], high
dielectric constant [176], low vapour pressure [176], low toxicity [176] and low
decomposition due to oxidation [177]. Ionic liquid from the imidazolium family i.e. 1-ethyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) was also selected
because of its high ionic conductivity, large electrochemical window and non-flammability
[178]. EMITFSI is also less hygroscopic as compared to lithium bis(trifluoromethyl
sulfonyl)imide (LiTFSI) and sodium perchlorate (NaClO4) which made it an ideal candidate
to be used as a salt for the study of the selected polymer electrolytes. Table 4.1 shows the
ionic conductivity of different salts in PC and of the ionic liquid studied in this work.
No. Salt/PC ҡM† ҡL‡
mS/cm mS/cm
1 1.0 M LiClO4/PC 5.65 ± 0.05 5.45 [93]
2 1.0 M LiPF6/PC 5.91 ± 0.09 5.80 [93]
3 1.0 M LiTFSI/PC 7.82 ± 0.03 8.50 [179]
4 0.1 M TBAPF6/PC 2.49 ± 0.04 ---*
5 1.0 M NaClO4/PC 6.46 ± 0.02 ---*
6 EMITFSI 9.73 ± 0.06 10.0 [178]
Table 4.1 Ionic conductivity of different salts.
† ҡM is the ionic conductivity measured using a conductivity meter
‡ ҡL is the literature value for the ionic conductivity; *Value not available.
Chapter 4 Polymer Electrolytes
116
The ionic conductivity of LiClO4 and LiPF6 in PC were lower as compared to the LiTFSI/PC
but higher than TBAPF6/PC. However, LiClO4 was not used in polymer electrolytes due to its
hazardous nature (can intensify fire as it is an oxidiser [93]) and LiPF6 was not used as it is
thermally unstable at elevated temperatures [93]. LiTFSI was proved to be safe, thermally
and hydrolytically stable, highly conducting and melts at 23°C (decomposition up to 380°C
[93]). However, the only problem with LiTFSI was its hygroscopic nature. LiTFSI had low
chemical stability towards ambient moisture as it crystallised resulting in low ionic
conductivity. Therefore, TBAPF6 was also studied. Although TBAPF6 had two times lower
ionic conductivity than lithium based salts and possesses a high melting point (387.4°C),
however, it is stable at ambient moisture and thus could be used in open air. LiTFSI had to be
mixed with solvent before adding into the polymer matrix. However, the solvent (PC)
negatively affects the mechanical properties of the polymer, since PC acts as a plasticiser as
will be discussed in the following sections (Section 4.3.1). Hence, the ionic liquid EMITFSI
was also studied. EMITFSI has a high ionic conductivity and also does not require any
solvent as it is liquid at room temperature.
4.2 Polyacrylonitrile gel polymer electrolytes
The PAN based polymer electrolyte was a gel at room temperature because of high
concentration of plasticiser. In order to determine the mechanical properties of a PAN gel
polymer electrolyte, oscillatory rheological characterisation was carried out. Although the
mechanical properties of PAN gel based polymer electrolyte will be low (as PAN is a gel), it
does possess higher ionic conductivity than PEGDGE (Table 4.3). Initially, shear strain
amplitude sweep (or for short strain sweep) tests were conducted. Strain sweep tests are
oscillatory tests performed at variable amplitude sweeps, keeping the frequency as well as the
measuring temperature constant. At low amplitude sweep values, the shear moduli (G/ and
G//) were constant. This plateau called the linear viscoelastic (LVE) range. Strain sweep tests
are usually carried out to determine the limit of the LVE range. The dynamic moduli
remained constant as long as the strain rates were below 1.5%. Thus, from the strain sweep
tests, a strain rate of 1.5 % was chosen at which frequency sweep tests were conducted in the
frequency window of 0.1-10 Hz. Frequency sweep tests were carried out for investigating the
time dependent shear behaviour.
Chapter 4 Polymer Electrolytes
117
40 80 120 160
10-3
10-1
101
103
105
G/ , G
// (P
a)
Temperature (C)
PAN1-3D G/
PAN1-3D G//
PAN1-6M G/
PAN1-6M G
PAN2 G/
PAN2 G//
//
(a)
10-2 10-1 100 101 10210-2
100
102
104
106
G/ , G
// (P
a)
Frequency (Hz)
PAN1-3D G/
PAN1-3D G//
PAN1-6M G/
PAN1-6M G//
PAN2 G/
PAN2 G//
(b)
Figure 4.1 Temperature (a) and frequency (b) sweep tests of PAN1-3D, PAN1-6M and PAN2
gel based polymer electrolytes (G/ = storage modulus and G// = loss modulus).
Inspection of Figure 4.1b reveals that both the dynamic moduli G/ and G// of PAN1-6M,
remained constant as long as the frequency was below the limiting value (0.8-1 Hz). The
PAN1-6M gel was stable under this condition. At higher frequencies (> 1 Hz), the limit of the
linear viscoelastic range was exceeded and the structure of the sample had been reformed
already or even completely destroyed molecularly [180]. Figure 4.1b also shows that the
PAN1-3D polymer electrolyte has a liquid character up to the frequency 1 Hz and after 1 Hz,
polymer electrolyte has a gel character. However, PAN2 gel performance was unaffected
over a wide range of frequencies (0.01 to 100 Hz). Figure 4.1a shows the evolution of
dynamic moduli during a temperature scan from 25°C to 150°C performed on different PAN
gel samples. At low temperature (20 to 60°C), the storage modulus G/ and loss modulus G//
Chapter 4 Polymer Electrolytes
118
decreased slowly as the temperature increased and (i.e., at 60°C, G//G//≈10) indicating the
elastic behaviour of a gel and the microstructure remained unchanged over this temperature
range. Such behaviour is characteristic of a strong gel with a three dimensional network
structure [180]. The network responded elastically at small deformations. The magnitude of
G/ ( ≈ 105 Pa) for PAN2 was an indication of a high density of network forming bonds.
Above 60°C, the storage modulus decreased rapidly as temperature increased up to 150°C
indicating a structural change that corresponded to a decreasing elastic response which was
also consistent with visual observation of the gel transforming into a soft viscoelastic state
(weak gel). The storage modulus was greater than the loss modulus which was entirely
different from PAN1-3D showing the typical viscoelastic response for a strong gel network
and did not change the dependence of G/ on frequency gradually.
In PAN1-3D, the storage modulus G/ was greater than the loss modulus G// indicating a gel
character but the magnitude of G/ was lower (~ 103 Pa) as compared to PAN2 gel based
polymer electrolyte (~105 Pa), the PAN/solvent ratio was of 26/74 by mass and in case of
PAN1-3D the PAN/solvent ratio was 7/93 by mass (section 3.3.3). It is clearly evident from
Figure 4.1a that as the temperature increased the storage modulus of the gel decreased. PAN
gel polymer electrolytes behaved more elastically at 25°C than at 60°C which is in
accordance with the observation by Nicotera et al. [180]. PAN based polymer electrolyte
(PAN1-3D and PAN1-6M) exhibited low dynamic moduli G/ and G// (Figure 4.1) at low
frequency (0.1 Hz) which was possibly due to weaker gel interactions and low concentration
of polymer (7 wt% by weight) in the polymer electrolyte.
The PAN gel polymer electrolyte showed an ionic conductivity of 3.8 mS/cm (Table 4.2).
The results were little higher than the room temperature ionic conductivity of 3.20 mS/cm
reported by Nicotera et al. [180] for a similar polymer electrolyte but of slightly different
composition. The ionic conductivity of the studied polymer electrolyte (PAN2) was also little
higher than the room temperature ionic conductivity of 3.16 mS/cm reported by Perera et al.
[181] for a PAN gel polymer electrolyte containing Mg (ClO4)2. PAN1-3D (prepared as
described in Section 3.3.3) had a low polymer concentration (7.7 wt%) as compared to PAN2
and, therefore, had poor rheological properties. The PAN based polymer electrolyte was a
viscous liquid and was not completely transformed into gel which is completely evident from
Figure 4.1; the loss modulus was greater than the storage modulus. However, after 6 months,
the PAN based polymer electrolyte had gelled completely and thus rheological properties
Chapter 4 Polymer Electrolytes
119
were slightly changed as shown in Figure 4.1. However, an optimisation of polymer and
electrolyte concentrations was required in order to get high rheological properties without
compromising the ionic conductivity. Therefore, the polymer concentration was increased
from 7.7 wt% to 26 wt%. 26 wt% of PAN polymer was the maximum concentration limit in
the electrolyte (0.1 M TBAPF6/PC) where the PAN dissolved fully in the solvent. A further
increase of the polymer concentration resulted in incomplete dissolution, polymer aggregates
were observed. The storage modulus of the PAN gel polymer electrolyte increased two orders
of magnitude from 1kPa to 100 kPa.
Gel polymer
electrolyte
Cyclic Voltammetry* Charge/Discharge Impedance spectroscopy
Cg C
g ҡ Cg
(µF/g) (µF/g) (mS/cm) (µF/g)
PAN1-3D 18.71 15.98 1.350 31.17
PAN1-6M 22.38 41.01 1.324 32.51
PAN2 20.78 18.34 3.791 40.37
Table 4.2 Specific capacitance Cg and ionic conductivity ҡ of the PAN gel polymer
electrolytes.
* Cyclic voltammetry test was conducted at a sweep rate of 50 mV/s.
The electrochemical properties of PAN gel polymer electrolytes were studied using cyclic
voltammetry, charge-discharge and electrochemical impedance spectroscopy measurements
to show the possibility of using them as electrolytes in a supercapacitor. The ionic
conductivity was improved three times in PAN2 even though the TBAPF6/PC concentration
was decreased from 92.3 wt% to 74 wt%. The salt TBAPF6 in EC/PC concentration was
increased from 0.1 M to 1.0 M. The increased salt concentration in a given polymer resulted
in an increased number of charge carriers which led to a rise in ionic conductivity.
Chapter 4 Polymer Electrolytes
120
-0.5 0.0 0.5
-2x10-6
0
2x10-6
I (A
)
E (V)
PAN1-3D PAN1-6M PAN 2
(a)
0 2x104 4x104
0
2x104
4x104
-Z//
Z/ (
PAN1-3D PAN1-6M PAN 2
(b)
0 200 400 6000
80
160
240
Figure 4.2 Cyclic voltamograms (a) and impedance spectroscopy plots (b) of PAN1-3D,
PAN1-6M and PAN2 gel polymer electrolytes at room temperature.
Cyclic voltamograms and impedance plots of PAN gel polymer electrolytes are presented in
Figure 4.2a. The impedance curve (Figure 4.2b) can be clearly divided into two parts. At
higher frequencies, the capacitive impedance Z’’ is small, but increases with decreasing
frequency. There is a semicircular shape associated with a parallel combination of capacitive
and resistive components, relating to the parallel plate geometry of a capacitor and net
leakage/ionic resistance, respectively [31]. At lower frequencies, the electrochemical double
layer formation during charging process becomes significant as ions had enough time to
move to the electrode surface which is clear at a critical “knee frequency” [32]. The knee
frequency is the maximum operating frequency at which the majority of the electrochemical
capacitance can be obtained. The capacitance is dependent on the frequency and thus
decreases at higher frequencies. It follows that the device should be operated below the knee
Chapter 4 Polymer Electrolytes
121
frequency. The voltamogram (Figure 4.2a) deviated from the ideal rectangular shape. Figure
4.3 attempts to capture the multifunctional character of the PAN based gel polymer
electrolytes by plotting conductivity as a function of storage modulus (peak maximum). PAN
2 gel polymer electrolyte outperforms the other two PAN gels by three orders of magnitude
in terms of storage modulus and approximately three times in terms of ionic conductivity.
100 101 102 103 104 1051
2
3
4
(m
S/cm
)
G/ (Pa)
PAN1-3D PAN 2 PAN1-6M
Figure 4.3 Ionic conductivity ҡ as function of storage modulus G/ (peak maximum) of three
PAN gel polymer electrolytes by varying PAN/plasticiser concentration at 25°C.
4.3 Crosslinked PEGDGE polymer electrolytes
4.3.1 Effect of different ionic salts on ionic conductivity and compression
propertiesofcrosslinkedPEGDGEelectrolytes
Although PAN gel polymer electrolytes have a high ionic conductivity they do not possess
any significant mechanical properties rendering them a rather poor choice for structural
supercapacitor applications. Although the mechanical properties of PAN gel polymer
electrolyte were improved by increasing the polymer concentration (Figure 4.3) but still it
was gel and thus, not suitable. Thus, possible alternatives to PAN gel polymer electrolyte
were sought. Since PEGDGE is liquid at room temperature and has no structural properties,
crosslinking of PEGDGE was done by adding the amine hardener triethylenetetramine
(TETA) in order to enhance the mechanical properties of polymer. Amorphous PEG unit is
the carrier for ionic species in PEGDGE polymer electrolytes. Thus, a reduction in ionic
Chapter 4 Polymer Electrolytes
122
conductivity by adding the hardener concentration was attributed to the decreased flexibility
of the polymer chains resulting in reduced ion coordination sites.
Different salts in PC as well as ionic liquid were used as electrolyte in crosslinked PEGDGE
polymer electrolyte to optimise the electrochemical and mechanical performance. 0.8 wt% of
electrolyte to the crosslinked PEGDGE was fixed because, in case of TBAPF6/PC electrolyte,
a small increase in concentration of electrolyte to crosslinked PEGDGE led to a very soft
polymer electrolyte. 0.1 M concentration of TBAPF6/PC was used as it was the saturation
concentration of TBAPF6 in PC. Since PC is acting as a solvent for the salt as well as a
plasticiser for the polymer, the increased concentration of PC negatively affected the
mechanical properties (Table 4.3).
Sample code 0.8 wt% of Electrolyte
in PEGDGE
ҡ E σ
(µS/cm) (MPa) (MPa)
A0.80P99.2 0.1 M TBAPF6/PC 12.3 ± 1.23 5.46 ± 0.200 1.86 ± 0.210
Li0.80P99.2 1.0 M LiTFSI/PC 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350
Na0.80P99.2 1.0 M NaClO4/PC 18.3 ± 3.53 11.3 ± 0.230 5.11 ± 0.401
E0.80P99.2 EMITFSI 19.5 ± 3.20 12.2 ± 0.250 5.96 ± 0.241
Table 4.3 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked PEGDGE electrolytes by varying different salts (A-0.1 M TBAPF6/PC, Li-1.0 M
LiTFSI/PC, Na-1.0 M NaClO4/PC, E- EMITFSI).
The ionic conductivity of PEGDGE electrolytes, containing different salts in PC or ionic
liquid, were slightly different as 0.1M TBAPF6/PC in crosslinked PEGDGE showed the
lowest ionic conductivity (12.3 ± 1.23 µS/cm) but EMITFSI in crosslinked PEGDGE showed
the highest ionic conductivity (19.9 ± 2.24 µS/cm) and did not require a solvent. A room
temperature ionic conductivity of 36 µS/cm has been reported previously by Liang et al.
[182] for PEGDGE films containing 22 wt% lithium perchlorate in PC cured with α-ω-
diamino-poly(propylene oxide). Overall, the compression modulus of crosslinked PEGDGE
electrolytes increased from 5.46 MPa (using 0.1 M TBAPF6) to 12.2 MPa (using EMITFSI),
but for the creation of structural supercapacitor, the polymer matrix is required to exhibit both
ionic conductivity and appropriate mechanical properties. Currently investigated structural
epoxy resins possess a compression stiffness as high as 4 GPa [55]. Similarly, the ionic
conductivity of ionic liquids have reached values of 0.01 S/cm [178]. Therefore, further
Chapter 4 Polymer Electrolytes
123
increase in compression properties as well as ionic conductivity of polymer matrix is required
in order to use it as a matrix for structural supercapacitors.
4.3.2 Effect of increasing EMITFSI concentration on ionic conductivity and
compressionpropertiesofcrosslinkedPEGDGEelectrolytes
The effect of increasing EMITFSI concentrations from 0.8 wt% to 60 wt% on the ionic
conductivity and compression properties of crosslinked PEGDGE electrolytes were also
investigated. The results (Figure 4.4) showed that ionic conductivity increased with
increasing EMITFSI concentration from 0.8 wt% to 50 wt%. The increasing EMITFSI
concentration generally resulted in an increase of the ionic conductivity and this behaviour
was in accordance with other results which were reported earlier in the literature for other
polymer electrolytes with ionic liquids [183, 184]. The highest ionic conductivity of 176
µS/cm was achieved for a 50 wt% EMITFSI concentration in crosslinked PEGDGE (Figure
4.4a). When the concentration of EMITFSI in crosslinked PEGDGE electrolytes was
increased beyond 50 wt%, the conductivity decreased slightly from the maximum value
possibly due to the formation of ion aggregates within the polymer electrolyte. A similar
trend of ionic conductivity as function of electrolyte concentrations in PEO polymer was
previously reported by Kim et al. [120] who showed that the ionic conductivity increased
from 50 µS/cm to 325 µS/cm and then decreased to 250 µS/cm as the ionic liquid (1-ethyl-3-
methylimidazolium tetrafluoroborate) concentration was increased from 0 to 0.3 mol.
The increase in the concentration of EMITFSI up to 50 wt% resulted in increasing the ionic
strength and, therefore, enhanced ionic mobility within the crosslinked PEGDGE. The ionic
conductivity of crosslinked PEGDGE electrolyte increased to 176 µS/cm at 50 wt%
EMITFSI concentration (Figure 4.4a). However, the decrease in ionic conductivity at higher
EMITFSI concentration (60 wt%) was possibly due to the excessive ions starting aggregate
within the polymer electrolyte [185, 186]. On the other hand, the compression properties
decreased with increasing EMITFSI concentration. It is well known that the stiffness of the
polymer electrolyte drops with increasing ionic conductivity [55, 93, 182]. The compression
modulus gradually decreased from 12.2 MPa to 4.53 MPa with increasing EMITFSI
concentration from 0.8 wt% to 30 wt% (Figure 4.4a). Matsumoto et al. [187] observed a
similar trend of decreasing tensile modulus of a crosslinked DGEBA electrolyte; it decreased
690 MPa to 45 MPa as the EMITFSI concentration was increased from 34 wt% to 50 wt%.
The compression modulus remained almost constant with further increasing EMITFSI
Chapter 4 Polymer Electrolytes
124
concentration up to 60 wt%. Compression strength also gradually decreased with increasing
concentration of EMITFSI (Figure 4.4b). By increasing the EMITFSI concentration, the
crosslinked structure of polymer electrolyte weakened as there was less crosslinked structure
and more liquid phase (i.e. ionic liquid) which decreased the compression strength. Overall,
variations in the EMITFSI concentration showed a large impact on the ionic conductivity and
compression properties of crosslinked PEGDGE electrolytes.
0 20 40 60 80 1000
250
500
750
1000 E
EMITFSI (wt%)
S/c
m)
0
3
6
9
12
E (M
Pa)
0 20 40 60 80 1000
250
500
750
1000
EMITFSI (wt%)
S/cm
)
0
2
4
6
MPa)
Figure 4.4 Effect of increasing EMITFSI concentration on crosslinked PEGDGE electrolyte,
(a) EMITFSI concentration vs. ionic conductivity ҡ and compression modulus E; and
(b) EMITFSI concentration vs. ionic conductivity ҡ and compression strength σ.
Chapter 4 Polymer Electrolytes
125
4.4 Crosslinked DGEBA polymer electrolytes
Another promising polymer resin is the crosslinked diglycidylether of bisphenol-A which is
currently under investigation at USA Army Research Laboratory [55]. Crosslinked DGEBA
was also studied by Matsumoto et al. [187] who reported a tensile modulus of 690 MPa and
an ionic conductivity of 10 µS/cm for crosslinked DGEBA electrolyte samples containing 34
wt% EMITFSI. The effect of increasing LiTFSI/PC concentration on the ionic conductivity
and compression properties was also investigated. Crosslinked DGEBA electrolytes
containing LiTFSI/PC had a higher compression modulus but lower ionic conductivity as
compared to PEGDGE based polymer electrolytes.
Sample code 1.0M LiTFSI/PC ҡ E σ
LixBy (wt%) † (wt%) (µS/cm) (MPa) (MPa)
Li0B100 0 N/A* 3044 ± 155 45.6 ± 1.68
Li10B90 10 1.92 ± 0.240 1583 ± 14.14 113 ± 17.2
Li20B80 20 6.10 ± 0.0301 905 ± 73.4 81.1 ± 0.270
Li40B60 40 11.9 ± 1.06 25.1 ± 0.58 68.5 ± 0.701
Li60B40 60 138 ± 3.58 0.922 ± 0.401 0.631 ± 0.0801
Li80B20 80 1580 ± 13.8 0.211 ± 0.0102 0.120 ± 0.0401
Li100B20 100 7820 ± 30.2 N/A**
Table 4.4 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked DGEBA electrolytes as function of increasing LiTFSI/PC concentrations.
† LixBy with x and y being the weight percentages of 1 M LiTFSI/PC (Li) and crosslinked diglycidylether of
bisphenol-A (B) respectively; * No ionic conductivity due to pure resin; ** No compression properties.
The compression modulus and ionic conductivity of crosslinked DGEBA electrolytes with
increasing amounts of 1 M LiTFSI/PC is summarised in Table 4.4. To impart ionic
conductivity, LiTFSI/PC was added into the polymer resin. The compression modulus of
crosslinked DGEBA electrolyte gradually decreased and the ionic conductivity gradually
increased with increasing LiTFSI/PC concentration. The possible explanation of increase in
ionic conductivity of crosslinked DGEBA electrolytes with increasing LiTFSI/PC
concentration is the increased number of ion coordination sites within the polymer matrix. It
is clearly evident from Table 4.4 that Li10B90 polymer electrolyte had excellent mechanical
properties (a compressive modulus of ~1.6 GPa), which was about two orders of magnitude
higher as compared to the crosslinked PEGDGE electrolyte (6.4 MPa). However, the ionic
Chapter 4 Polymer Electrolytes
126
conductivity of Li10B90 electrolyte (1.9 µS/cm) was only about an order of magnitude lower
as compared to the crosslinked PEGDGE electrolyte containing 10 wt% LiTFSI/PC (20.3
µS/cm). Since samples were characterised electrochemically in air and the LiTFSI is very
hygroscopic, it is possible that the hydrated ions forms at the surface of the sample resulting
in increased ionic conductivity. In order to determine the actual ionic conductivity of these
polymer electrolytes, it is required to prepare and characterise the samples in a moisture free
environment (e.g. glove box).
4.5 Crosslinked PEGDGE/DGEBA polymer electrolytes
4.5.1 CrosslinkedPEGDGE/DGEBAelectrolytescontaining10wt%[LiTFSI]inPC
In order to enhance the mechanical properties of PEGDGE based polymer electrolytes,
various stoichiometric amounts 80/20, 60/40, 40/60 and 20/80 of DGEBA were added to
PEGDGE. The addition of DGEBA to PEGDGE provided a material which was significantly
stiffer than crosslinked PEGDGE electrolytes (Figure 4.5). Crosslinked DGEBA/PEGDGE
electrolytes were also prepared and characterised. Compression modulus, compression
strength and ionic conductivity of crosslinked DGEBA/PEGDGE electrolytes with varying
PEGDGE to DGEBA concentrations are summarised in Figure 4.5.
The compression modulus of crosslinked PEGDGE electrolyte increased from 6.42 MPa to
932 MPa as the concentration of DGEBA increased in PEGDGE (Figure 4.5). A similar trend
of increasing compression strength from 2 MPa to 113 MPa was also observed with
increasing DGEBA concentration. The increase in modulus was attributed to the reduced
polymer mobility as DGEBA concentration was increased in PEGDGE. However, the ionic
conductivity of polymer electrolytes reduced by an order of magnitude with the addition of
DGEBA. Polymer electrolytes samples containing PEGDGE concentration xPEGDGE of 0.2 and
0.4, in Figure 4.5, were the points of interest as these samples showed high compression
properties with reasonable ionic conductivity than other combinations of the
PEGDGE/DGEBA blends.
Chapter 4 Polymer Electrolytes
127
0.0 0.2 0.4 0.6 0.8 1.01
10
100 E
xDGEBA
S/c
m)
(a)
1
10
100
1000
E (M
Pa)
0.0 0.2 0.4 0.6 0.8 1.01
10
100
xDGEBA
S/c
m)
(b)
1
10
100
MP
a)
Figure 4.5 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and
compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in
crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% LiTFSI/PC.
4.5.2 CrosslinkedPEGDGE/DGEBAelectrolytescontaining10wt%EMITFSI
In order to improve the ionic conductivity and compression properties of crosslinked
PEGDGE/DGEBA electrolytes over those of previous polymer electrolyte systems, various
stoichiometric amounts of PEGDGE/DGEBA containing 10 wt% EMITFSI were also
prepared. As evident from Figure 4.6, the incorporation of DGEBA to PEGDGE provides a
material which was significantly stiffer than the crosslinked PEGDGE electrolytes.
Crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI was selected as baseline
material for structural supercapacitors because of intermediate ionic conductivity (28 µS/cm),
compression strength (4.9 MPa) and compression modulus (9 MPa) as mentioned in Figure
4.4. Compression modulus and ionic conductivity of crosslinked PEGDGE/DGEBA
Chapter 4 Polymer Electrolytes
128
electrolytes with varying PEGDGE and DGEBA concentration but constant the EMITFSI
concentration of 10 wt% are shown in Figure 4.6a. The compression modulus of the polymer
electrolytes increased when adding proportionally more DGEBA segments into the
crosslinked PEGDGE network (Figure 4.6a). Since the length between the two crosslinking
epoxy groups in DGEBA is smaller as compared to that in the PEGDGE, the increased
concentration of DGEBA in PEGDGE enhanced the degree of crosslinking and thus reduced
the free volume of polymer [182].
0.0 0.2 0.4 0.6 0.8 1.01
10
100
Conductivity Compressive Modulus E
xDGEBA
S/c
m)
(a)
1
10
100
1000
E (M
Pa)
0.0 0.2 0.4 0.6 0.8 1.01
10
100
Conductivity Compressive strength
xDGEBA
S/c
m)
(b)
1
10
100 (MP
a)
Figure 4.6 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and
compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in
crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% EMITFSI.
The ionic conductivity of crosslinked DGEBA/PEGDGE electrolyte decreased from 28
µS/cm for pure crosslinked PEGDGE electrolytes to 3.6 µS/cm for pure crosslinked DGEBA
Chapter 4 Polymer Electrolytes
129
electrolytes containing 10 wt% EMITFSI as shown in Figure 4.6. The compression strength
of polymer electrolyte has a different trend to the ionic conductivity and compression
modulus as shown in Figure 4.6b. The compression strength increased up to 213 MPa in
45/55 weight ratio of PEGDGE/DGEBA in the blend and then started decreasing gradually.
The compression modulus, on the other hand, increased gradually with increasing DGEBA
concentration. This trend was due to the structural change in the polymer electrolyte when
DGEBA segments were added in PEGDGE. The stress strain curves (Figure 4.7) of the
PEGDGE/DGEBA blend electrolytes containing 10 wt% EMITFSI also confirmed the trend.
It can be observed that the slopes of the stress strain graphs clearly increased with the
increasing concentration of DGEBA in PEGDGE. The polymer electrolyte changed from
being a very soft and rubbery matrix (crosslinked PEGDGE electrolyte) to a very plastic
matrix (50/50 weight ratio of PEGDGE/DGEBA in the blend) and then to a very brittle
glassy matrix (crosslinked DGEBA electrolyte).
0.00 0.15 0.30 0.45 0.60 0.750
75
150
225
0.00 0.02 0.04 0.060
1
2
3
4(h)
(g)
(f)
(d)
(c)
(b) (a)
(e)
(h)
(g)
(f)
(e)
(d)
Str
ess
(MP
a)
Strain ()
100P:0B (a) 80P:20B (b) 60P:40B (c) 50P:50B (d) 40P:60B (e) 30P:70B (f) 20P:80B (g) 0P:100B (h)(c)
Figure 4.7 Stress strain curves of crosslinked PEGDGE/DGEBA blend polymer electrolytes
containing 10 wt% EMITFSI
4.5.3 CrosslinkedPEGDGE/DGEBAelectrolytescontaining50wt%EMITFSI
Crosslinked PEGDGE electrolytes containing 50 wt% EMITFSI showed highest ionic
conductivity (176 µS/cm) but low compression modulus (3.8 MPa) among various other
EMITFSI concentrations in crosslinked PEGDGE as discussed previously (Section 4.3.2).
Also, it was demonstrated previously in sections 4.5.1 and 4.5.2 that the mechanical
Chapter 4 Polymer Electrolytes
130
performance of crosslinked PEGDGE electrolytes was improved with the introduction of
DGEBA segments. Therefore, crosslinked PEGDGE electrolyte containing 50 wt% EMITFSI
with increasing DGEBA concentration was also investigated.
The compression modulus of the polymer electrolyte increased from 4 MPa to 32 MPa with
increasing DGEBA weight concentration xDGEBA from 0 to 0.6 respectively in crosslinked
PEGDGE/DGEBA electrolytes (Figure 4.8). However, further increasing the amount of
DGEBA in crosslinked PEGDGE network (xDGEBA = 0.8) resulted in a phase separation of
EMITFSI from the polymer blend as shown in Figure 4.9. This phase separation was even
clearer in the pure DGEBA polymer electrolyte containing 50 wt% EMITFSI.
0.0 0.2 0.4 0.6 0.8 1.010
100
1000
Conductivity Compressive Modulus E
xDGEBA
S/c
m)
(a)
1
10
100
E (M
PA
)
0.0 0.2 0.4 0.6 0.8 1.0
10
100
1000 Conductivity Compressive strength
xDGEBA
S/c
m)
(b)
1
10
100
MPa)
Figure 4.8 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and
compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in
crosslinked PEGDGE/DGEBA electrolytes containing 50 wt% EMITFSI.
Chapter 4 Polymer Electrolytes
131
The compression strength increased from 1.2 MPa of a crosslinked PEGDGE electrolyte
(xDGEBA = 0) to 11 MPa for a crosslinked PEGDGE/DGEBA blend electrolyte (xDGEBA = 0.4)
containing 50 wt% EMITFSI and then remained almost constant (as shown in Figure 4.8b).
This is because of the change in matrix structure from a very rubbery and soft to a very
ductile but plastic material. On the other hand, the ionic conductivity tended to increase five
times by the addition of DGEBA segments in PEGDGE network from DGEBA concentration
of 0 to 0.6 in crosslinked PEGDGE containing 50 wt% EMITFSI (Figure 4.8). However, the
decrease in ionic conductivity in 100% crosslinked DGEBA electrolyte containing 50 wt%
EMITFSI can not only be attributed to the phase separation of polymer matrix and ionic
liquid but also to the formation of ion aggregates and hence affecting the ionic mobility.
Figure 4.9 Photograph of what on tissue paper showing phase separation of EMITFSI from
crosslinked PEGDGE/DGEBA electrolyte with xDGEBA of 0.8 containing 50 wt% EMITFSI.
4.6 Multifunctionality of polymer electrolytes
Different structural polymer electrolytes were prepared using crosslinked PEGDGE and
crosslinked DGEBA electrolytes containing different electrolytes (1.0 M LiTFSI/PC and
EMITFSI). EMITFSI concentration was increased in crosslinked PEGDGE electrolytes.
Crosslinked PEGDGE electrolytes had high ionic conductivity (176 µS/cm) but poor
compression modulus (4 MPa) at an EMITFSI concentration of 50 wt%. Crosslinked
PEGDGE/DGEBA blends with varying weight ratios of PEGDGE and DGEBA were also
prepared. The structural performance of the crosslinked PEGDGE electrolytes was
remarkably increased by the incorporation of DGEBA but at the cost of a reduced ionic
conductivity. EMITFSI was selected as electrolyte for further studies because of its high ionic
conductivity among other salts studied and also, to avoid using the propylene carbonate (PC)
solvent in polymer electrolytes. The addition of PC solvent in polymer electrolyte negatively
affected their compression properties.
Chapter 4 Polymer Electrolytes
132
The multifunctional character of the polymer electrolytes, described in previous sections of
Chapter 4, can be truly captured by plotting the compression modulus or compression
strength, measures of the mechanical performance, as a function of ionic conductivity, a
measure of the electrochemical performance, for the full range of structural polymer
electrolytes (Figure 4.10). Ionic conductivity, compression modulus and compression
strength of various polymer electrolytes were plotted on logarithmic axes as the properties
span over several orders of magnitude. Structural resins, such as crosslinked DGEBA, are
positioned on the y axis of the multifunctional curve due to high compression properties but
negligible ionic conductivity. Similarly, liquid electrolytes, such as EMITFSI or LiTFSI/PC,
are positioned on the x-axis of the multifunctionality curve. Two reference lines were drawn
between the properties of crosslinked DGEBA or crosslinked PEGDGE (structural resins)
and EMITFSI, the ionic liquid electrolyte, in order to create a baseline for the full range of
polymer electrolytes. Another reference line for polymer electrolytes containing LiTFSI/PC
was also drawn between the properties of LiTFSI/PC (liquid electrolyte) and of crosslinked
DGEBA. The polymer electrolytes, positioned well above the reference line in Figure 4.10,
are multifunctional.
Figure 4.10a established a trend for compression modulus versus ionic conductivity of
polymer electrolytes. Figure 4.10a showed that crosslinked PEGDGE/DGEBA blend
electrolytes containing 50 wt% EMITFSI had the best multifunctionality in terms of
compression modulus and ionic conductivity. Some of the crosslinked PEGDGE/DGEBA
electrolytes containing 10 wt% of EMITFSI or LiTFSI/PC were also positioned above the
reference line but had poor ionic conductivity as were more close to the y-axis. All the
remaining polymer electrolytes were either positioned on or below the reference line.
Figure 4.10b detailed the trend in compression strength relative to the ionic conductivity of
polymer electrolytes. The reference lines, chosen here, were also drawn between the
compression strength of crosslinked DGEBA or crosslinked PEGDGE and ionic conductivity
of EMITFSI or LiTFSI/PC. Due to the brittle glassy nature, the compression strength of
reference crosslinked DGEBA was low (45 MPa). Therefore, most of the polymer
electrolytes positioned themselves well above the reference line in Figure 4.10b.
Chapter 4 Polymer Electrolytes
133
100 101 102 103 10410-1
100
101
102
103
104
Increasing multifunctionality
E (
MP
a)
(S/cm)
LiyB
b
Li10
:aP:bB
ExP
a
E10
:aP:bB
E50
:aP:bB
pure crosslinkedPEGDGE
pure LiTFSI/PC
pure EMITFSI
(a)
pure crosslinkedDGEBA
100 101 102 103 10410-1
100
101
102
103
pure crosslinkedPEGDGE
pure LiTFSI/PC
pure EMITFSI
Increasing multifunctionality
LiyB
b
Li10
:aP:bB
ExP
a
E10
:aP:bB
E50
:aP:bB
(M
Pa)
(S/cm)
pure crosslinkedDGEBA
(b)
Figure 4.10 Compression modulus E (a) and compression strength σ (b) of different
crosslinked PEGDGE (P) and crosslinked DGEBA (B) electrolytes containing either
LiTFSI/PC (Li) or EMMITFSI (E) as a function of ionic conductivity ҡ at room temperature.
A structural polymer electrolyte with a crosslinked 20:80 PEGDGE/DGEBA blend
containing 10 wt% EMITFSI (E10:20P:80B) had a compression modulus of 1.7 GPa and an
ionic conductivity of 4µS/cm. The ionic conductivity of E10:20P:80B polymer electrolyte was
however 250 times lower as compared to the target value of 1 mS/cm (Section 1.3). Another
promising structural polymer electrolyte with a crosslinked 40:60 PEGDGE/DGEBA blend
containing 50 wt% EMITFSI (E50:40P:60B) exhibited an ionic conductivity of 550 µS/cm
Chapter 4 Polymer Electrolytes
134
and a compression modulus of 35 MPa. The ionic conductivity of E50:40P:60B polymer
electrolyte was just 1.8 times lower as compared to the target ionic conductivity and its
compression modulus was 29 times lower than the target compression modulus of 1 GPa
(Section 1.3). In order to further improve the ionic conductivity and compression modulus of
polymer electrolytes, mesoporous silica particles could be used as reinforcements as
discussed in the following chapter (Chapter 5).
It is clearly evident from the multifunctionality plots (Figure 4.10) that different weight ratios
of PEGDGE/DGEBA blend containing 10 wt% and 50 wt% EMITSFI were clearly
multifunctional and were therefore selected as matrices for structural supercapacitors.
Overall, the improvements in ionic conductivity and compression properties of studied
polymer electrolytes indicate that these structural electrolytes have potential for
multifunctional structural supercapacitor applications.
Chapter 5 Composite Polymer Electrolytes
135
Chapter 5 Polymer Composite
Electrolytes
Results on the compression properties and ionic conductivity of crosslinked polymer
composite electrolytes are discussed in this chapter. As discussed in the literature review
(Section 2.4.5), among different fillers, mesoporous silica was selected as reinforcements in
polymer electrolytes. Two different forms of mesoporous silica i.e. mesoporous silica
monoliths (MSM) and mesoporous silica particles (MSP) were used to prepare polymer
composite electrolytes. Characterisation of the MSM as well as MSP shall be presented first.
Following this, results on the mechanical and electrochemical properties of MSP and crushed
MSM reinforcements in crosslinked PEGDGE will be discussed. MSP were also used as
reinforcements in different crosslinked PEGDGE and crosslinked DGEBA electrolyte
formulations in order to prepare a mechanically and electrochemically optimised polymer
composite electrolyte for structural supercapacitors.
Chapter 5 Composite Polymer Electrolytes
136
5.1 Mesoporous silica
Among inorganic fillers, mesoporous silica, with pore diameters between 2 and 50 nm, has
attracted considerable interest in the last decade from both the technological and scientific
point of view [188]. Surfactant templating in aqueous solutions is employed in the presence
of inorganic precursors so that as-synthesized material contains embedded hydrophobic
domains. These hydrophobic domains are later removed by either calcinations or solvent
extraction. In the present work, high surface area mesoporous silica monoliths and particles
were explored as fillers in polymer electrolytes with the aim to improve their ionic
conductivity and mechanical properties. Mesoporous silica monoliths and particles were
selected due to their high surface area, large pore volume, excellent mechanical and thermal
stability and ability to maximise the adsorption of liquid electrolyte by preserving a porous
structure in polymer electrolytes. The proposed scheme for the structural evolution of a
mesoporous silica network during the sol-gel process is shown in Figure 5.1. Initially, a
reverse hexagonal phase of surfactant was formed by solubilising aqueous HCl (1 M) in the
Pluronic P123 [(EO)20(PO)70(EO)20] [189]. TEOS and PEO-co-PPO copolymer chains were
mutually stable forming a continuous self-assembled medium. After hydrolysis and
condensation reactions, cores containing PEO and non-reacted ethanol form a regular array.
The deposition of silica around the micelles formed walls. During ethanol washing of the
monolith and particles, the PEO-co-PPO chains were extracted from the mesopores. In case
of mesoporous silica particles, the self-assembly of the silica network was continuously
disturbed during the sol-gel process by continuous stirring of the reaction mixture which
impeded the formation of thick silica walls resulting in the formation of silica particles.
Figure 5.1 Schematics of the proposed structural evolution of silica network showing
micelles with PEO-co-PPO corona (thin lines), TEOS molecules (thick lines)and embedded
PEO-co-PPO chains in silica, adopted from Rodriguez-Abreu et al. [189].
Chapter 5 Composite Polymer Electrolytes
137
5.1.1 Surface characterisation of mesoporous silica monoliths (MSMs) and
mesoporoussilicaparticles(MSP)
Nitrogen adsorption-desorption isotherms measured for mesoporous silica monoliths were
typical of a Type IV isotherm with type H2 hysteresis (formerly termed type E) [190]
showing obvious capillary condensation (hysteresis loop) at medium relative pressure which
indicated the presence of mesopores. The relative pressure range corresponding to capillary
condensation became wider with the decrease in thickness and curing temperature of MSM,
which showed the presence of pores and pore-widening in the channel-like mesopores. Large
hysteresis loops, between the adsorption and desorption curves, were typical of mesoporous
silica. Such strong hysteresis was believed to be related either to the capillary condensation
associated with large pore channels or to the modulation of the channel structure of MSM.
0.0 0.5 1.0
0
6
12
18
24
Qua
ntit
y A
dsor
bed
(cm
3 /g)
Relative Pressure (P/P0)
1.45 mm 1.87 mm 2.48 mm 3.11 mm 4.50 mm 5.52 mm
(a)
1 10 100
0.000
0.005
0.010
0.015
Pore
vol
ume
(cm
3 /g)
Pore size (nm)
1.45 mm 1.87 mm 2.48 mm 3.11 mm 4.50 mm 5.52 mm
Figure 5.2 BET nitrogen adsorption/desorption isotherms (a) and BJH pore size distribution
(b) of mesoporous silica from samples with varying reaction layer thickness.
Chapter 5 Composite Polymer Electrolytes
138
In the adsorption/desorption isotherm for mesoporous silica monoliths (Figure 5.2), three
well distinguished regions were evident: (i) monolayer/multilayer adsorption, (ii) capillary
condensation taking place in the mesopores and (iii) multilayer adsorption on the outer
surface of monoliths. Capillary condensation occurred at a higher relative pressure (p/p0 =
0.4-0.7) for the mesoporous silica with pore sizes ranging from 3.78 to 7.23 nm. A delay in
isotherm (i.e. lag between adsorption and desorption isotherms) was seen which could be due
to the interconnectivity of the real pore network or due to phase transition in an isolated pore.
It was found that the variation in MSM thickness by increasing the volume of the reaction
mixture affected the surface area and pore size distribution of the silica monoliths as shown
in Table 5.1.
Sample
code
hsample AS, BET AS, Langmuir VP dP FWHM†† ρ*
(mm) (m2/g) (m2/g) (cm3/g) (nm) (nm) (g/cm3)
MSM1 1.45 14.7 21.5 0.022 5.96 1.53
1.45 ±
0.06
MSM2 1.87 16.0 23.3 0.024 6.07 2.14
MSM3 2.48 31.2 45.6 0.040 5.14 1.48
MSM4 3.11 38.6 54.0 0.036 3.78 2.05
MSM5 4.50 2.73 3.99 0.004 6.27 1.60
Table 5.1 Surface areas, AS, BET and AS, Langmuir, pore volume VP, pore width dP, full width at
the half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous
silica monoliths MSM synthesised at 90°C with increasing monolith thickness hsample.
* bulk density of silica measured through Accupyc.
Figure 5.2b showed Barrett-Joyner-Halenda (BJH) pore size distributions for the mesoporous
silica samples with varying reaction layer thickness, calculated from the adsorption branch of
the isotherm. Around 90 % of the pore volume was made up of mesopores in the 2-10 nm
range. The pore size distribution (PSD) revealed that the silica monoliths had mesopores of
sizes centred at around 4.5 nm. The narrow pore size distribution (full width at the half
maximum FWHM of approximately 1.9 nm) indicated the uniformity in pore structure. There
were also some large mesopores with an average size of approximately 25 nm in the silica
network.
Chapter 5 Composite Polymer Electrolytes
139
0.0 0.5 1.00
60
120
180
Qua
ntit
y A
dsor
bed
(cm
3 /g)
Relative Pressure (P/P0)
90 C 70 C 60 C
(a)
1 10 1000.00
0.04
0.08
Por
e vo
lum
e (c
m3 /g
)
Pore size (nm)
90 C 70 C 60 C
(b)
Figure 5.3 BET nitrogen adsorption/desorption isotherm (a) and BJH pore size distribution
(b) of mesoporous silica monoliths by varying the curing temperature.
The surface area of the monoliths was low (38 m2/g) as compared to typical mesoporous
silica monoliths with surface areas up to 450 m2/g [161] that could be due to thick pore walls
compared to conventional materials (e.g. SBA-15 [191], MCM-41 [192]). Another possibility
of the low surface area of MSM was the very low fraction of microporosity in the samples
which for conventional mesoporous materials, contributes to most of the specific surface area
[189]. Microporosity in monoliths was related to the PEO-co-PPO chains of Pluronic P123
that were distributed molecularly in the silica network during the sol-gel process and left
micro-pores when taken away during washing with ethanol. However, portions of Pluronic
P123 remained in the core of the templating reverse aggregates [193] and thus, contribute to
the silica network with micro-pores. In addition to the low surface area, another problem with
the silica monoliths was the cracking which occurred during curing. One possible explanation
Chapter 5 Composite Polymer Electrolytes
140
of the cracking of the surface of monoliths was the high temperature (90°C) curing process as
given in the mesoporous silica monolith recipe [161]. Ethanol (boiling point 78.4°C) resided
in the bi-continuous phase of silica network and therefore, vaporised instantly at 90°C.
During evaporation, the ethanol vapour caused the silica walls to rupture and thus changed
the pore structure.
Sample
code
TCuring hsample AS, BET AS, Langmuir VP† dP
‡ FWHM †† ρ*
(°C) (mm) (m²/g) (m²/g) (cm³/g) (nm) (nm) (g/cm³)
MSM2 90
1.88 ±
0.02
16.0 23.3 0.02 6.07 2.84 1.45 ± 0.06
MSM6 70 159 229 0.22 5.49 2.30 1.51 ± 0.03
MSM7 60 226 325 0.30 5.29 1.66 1.53 ± 0.04
Table 5.2 Surface areas, AS, BET and AS, Langmuir, pore volume VP, pore width dP, full width at
the half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous
silica monoliths MSM cured at different temperatures TCuring.
In order to avoid cracking of mesoporous silica monoliths, the silica network was cured at
lower temperatures [161], i.e. 70°C and 60°C (Figure 5.3). Surface area and pore structure
parameters of mesoporous silica monoliths produced at various curing temperatures are listed
in Table 4.2. The BET surface area and total pore volume increased with decreasing curing
temperature. At the curing temperature of 60°C, the monoliths possessed the highest BET
surface area (226 m²/g) and total pore volume (0.3 cm³/g). The pores could not be templated
in the presence of either increased or decreased quantity of ethanol in mixture. The problem
of cracking of mesoporous silica monoliths during curing was also solved as a result of using
low curing temperature of 60°C. A monolith with few internal cracks was obtained (Figure
5.4).
Chapter 5 Composite Polymer Electrolytes
141
Figure 5.4 Mesoporous silica monolith having internal cracks after ethanol washing.
In addition to mesoporous silica monoliths, mesoporous silica particles were also prepared to
be used as reinforcements for polymer electrolytes. These silica particles contained
mesopores within the silica network which was confirmed by BET surface area analysis
(Figure 5.6). Particle size was determined for the mesoporous silica particles as well as non-
porous similar sized silica particles as MSP using light scattering technique (Section 3.2.4).
The mass-median particle diameter (d50) of the non-porous silica particles NSP (d50 of 0.41
µm) was calculated to be approximately four times smaller than for the mesoporous silica
particles MSP (d50 of 1.74 µm). Particle size distribution curves for the MSP and NSP
obtained using dynamic light scattering (DLS) analysis technique are shown in Figure 5.5.
The size distribution of the MSP and NSP are found to be unimodal. NSP had a narrower
particle size distribution with a FWHM of 0.64 µm. Conversely, MSP also showed a narrow
particle size distribution curve with a FWHM of 2.85 µm (Table 5.3).
Chapter 5 Composite Polymer Electrolytes
142
10-2 10-1 100 101 1020
2
4
6
8
10
Vol
ume
(%)
Particle Size (m)
MSP NSP
Figure 5.5 Particle size distributions of MSP and NSP obtained by dynamic light scattering.
Nitrogen adsorption-desorption isotherms of MSP and NSP are shown in Figure 5.6. MSP
showed a typical Type IV isotherm with Type H2 hysteresis and showed a capillary
condensation step at a relative pressure (P/Po) from 0.60 to 0.78. Such strong hysteresis was
believed to be related either to the capillary condensation associated with large pore channels
or to the variations in the MSP channel structure. In the adsorption/desorption isotherm for
the MSP (Figure 5.6), three well distinguished regions were also observed as discussed in
Section 5.1.1. A delay in isotherm of MSP (i.e. lag between adsorption and desorption
isotherms) was observed which could be due to the interconnectivity of the pore network or
due to phase transition in an isolated pore.
Chapter 5 Composite Polymer Electrolytes
143
0.00 0.25 0.50 0.75 1.00
0
200
400
600
Qua
ntit
y A
dsor
bed
(cm
3 /g)
Relative Pressure (P/Po)
MSP NSP
(a)
100 101 102 103
0.0
0.2
0.4
Por
e vo
lum
e (c
m3 /g
)
Pore size (nm)
MSP NSP
(b)
Figure 5.6 BET nitrogen adsorption/desorption isotherm (a) and BJH pore size distribution
(b) of mesoporous (MSP) and non-porous (NSP) silica particles.
The isotherm for the NSP did not have any hysteresis loop like MSP (Figure 5.6a). NSP
showed a typical reversible type II isotherm which is characteristic of non-porous materials
[190]. From a relative pressure (P/Po) of 0.1, the isotherm started forming an almost linear
section indicating the completion stage of monolayer coverage and the beginning of
multilayer adsorption. Barrett-Joyner-Halenda (BJH) pore size distributions for the MSP and
NSP, calculated from the adsorption branch of the isotherm, are shown in Figure 5.6b. No
porous structure was observed in the NSP as shown in Figure 5.6b. In the MSP, around 90 %
of the pore volume was made up of mesopores in the 6-20 nm range. From the pore size
distribution (PSD) curves, it was clear that the porosity of the MSP was made up of
mesopores of sizes centred at around 8 nm. The narrow pore size distribution indicated the
Chapter 5 Composite Polymer Electrolytes
144
uniformity in pore structure, as expected for surfactant template materials, with the full width
at the half maximum (FWHM) of approximately 2.6 nm (Figure 5.6). There were small
mesopores constituting 0.4 cm3/g of the MSP and also some larger mesopores with an
average size of approx. 40 nm.
Silica
Particles
AS, BET AS, Langmuir VP† dP d50
†† ρ*
(m²/g) (m²/g) (cm³/g) (nm) (µm) (g/cm³)
MSP 523 728 0.76 5.80 1.74 1.77 ± 0.02
NSP 50.6 70.4 0.11 87.9 0.41 2.19 ± 0.03
Table 5.3 Surface areas, AS, BET and AS, Langmuir, pore volume VP, pore width dP, mass-median
particle diameter d50 and bulk density of mesoporous (MSP) and non-porous silica particles.
SEM images of the MSP and crushed MSM are shown in Figure 5.7. The particle size of
MSP was also confirmed from the images. SEMs of crushed MSMs showed that the particle
size was very irregular. SEM image of the crushed silica monolith (Figure 5.7a) showed the
non-uniform crushing of the monoliths which was necessary to preserve the mesopores of
MSM. This resulted in irregular particle size distribution of MSM. Micrometre sized
mesoporous silica particles provided favourable ion transport properties when these silica
particles were used to reinforce polymer electrolytes as discussed in following Section 5.2.
SEM images of MSP showed that particles were spherical in shape.
Chapter 5 Composite Polymer Electrolytes
145
Figure 5.7 SEM images of crushed mesoporous silica monoliths (a) and mesoporous silica
particles (b and c).
5.2 Effect of mesoporous silica on the mechanical and electrochemical
properties of polymer composite electrolytes
In order to increase the mechanical as well as electrochemical performance of polymer
electrolytes (Chapter 4), mesoporous silica was used as filler. The interaction between silica
and polymer chains leads to the formation of the three-dimensional network and thus
provides mechanical stability to the polymer electrolyte. This exceptional microstructure is
expected to provide the silica with effective interactions towards other components of the
polymer electrolyte. Also, the addition of mesoporous silica preserves the porous structure
(i.e. free volume) of polymer electrolyte resulting in the reduced risk of leakage [194] and
maximum adsorption of liquid electrolyte [195]. Mesoporous silica monoliths (MSM),
Chapter 5 Composite Polymer Electrolytes
146
mesoporous silica particles (MSP) and non-porous silica particles (NSP) were incorporated
into various crosslinked polymer electrolytes and the polymer composite electrolytes were
characterised mechanically and electrochemically as discussed in following sections.
5.2.1 IonicconductivityandcompressionpropertiesofcrosslinkedPEGDGE/MSM
compositeelectrolytescontainingTBAPF6/PC
Mesoporous silica monoliths were crushed into smaller particles and were used as filler for
crosslinked PEGDGE containing 0.1 M TBAPF6/PC as electrolyte. The resulting polymer
composite electrolyte showed very poor ionic conductivity and compression properties. The
ionic conductivity of polymer composite electrolyte dropped to half from 12.3 to 6.12 µS/cm
and the compression modulus decreased by an order of magnitude from 3.51 to 0.42 MPa at
15 wt% loading of crushed MSMs. This was attributed to monoliths having been crushed into
very irregular sized particles forming mesoporous micron-dust. Fine crushing of monoliths
was avoided in order to preserve the mesopores of the monoliths. The ionic conductivity and
compression properties of crosslinked PEGDGE containing TBAPF6/PC as a function of
increasing concentration of crushed MSM loading are shown in Table 5.4.
Sample codes† MSM ҡ E σ
wt% (µS/cm) (MPa) (MPa)
A0.80P99.2S0 0.0 12.3 ± 1.2 3.51 ± 0.04 1.89 ± 0.42
A0.80P96.7S2.5 2.5 9.21 ± 0.6 1.65 ± 0.14 1.88 ± 0.14
A0.80P94.2S5.0 5.0 10.2 ± 0.6 1.42 ± 0.24 1.75 ± 0.24
A0.80P91.7S7.5 7.5 7.40 ± 0.1 1.07 ± 0.22 1.70 ± 0.16
A0.80P88.2S10 10.0 7.32 ± 0.4 0.91 ± 0.02 1.67 ± 0.66
A0.80P86.7S12.5 12.5 13.0 ± 1.0 0.53 ± 0.31 1.57 ± 0.31
A0.80P84.2S15 15.0 6.71 ± 0.1 0.56 ± 0.07 1.52 ± 0.07
A0.80P81.7S17.5 17.5 6.12 ± 0.5 0.42 ± 0.09 0.45 ± 0.11
Table 5.4 Ionic conductivity ҡ, compression modulus E and compression strength σ of
PEGDGE (0.1 M TBAPF6/PC) polymer electrolytes with increasing crushed MSM
concentration.
† AxPaSb – x wt% of TBAPF6/PC (A), a wt% of crosslinked PEGDGE (P) and b wt% of crushed MSM.
Chapter 5 Composite Polymer Electrolytes
147
5.2.2 IonicconductivityandcompressionpropertiesofcrosslinkedMSP/PEGDGE
compositeelectrolytescontainingTBAPF6/PC
MSP were also used as filler for crosslinked PEGDGE containing 0.8 wt% 0.1 M
TBAPF6/PC as electrolyte. In comparison to crushed MSMs, MSP incorporation into
crosslinked PEGDGE electrolyte resulted in an increase of the ionic conductivity and the
compression properties (Figure 5.8). The addition of MSP to crosslinked PEGDGE
electrolyte may improve the conduction of ions at the polymer-silica interface resulting in an
increased ionic conductivity. The ionic conductivity of the crosslinked PEGDGE composite
electrolyte increased from 12.3 µS/cm to 175 µS/cm following the addition of 7.5 wt% MSP
into crosslinked PEGDGE electrolyte. However, further increasing the MSP loading resulted
in a decreased ionic conductivity which was possibly due to the silica aggregation within the
polymer network resulting in formation of local regions near MSP aggregates having reduced
ionic mobility (please see Appendix D for microscopic evaluation of 12.5 wt% MSP addition
in crosslinked PEGDGE polymer electrolytes).
The results obtained (Figure 5.8) are in accordance with the trends reported elsewhere [118].
Aravindan et al. [118] reported an increase in the ionic conductivity of PVDF-co-HFP
polymer electrolyte for 5 wt% addition of titanium dioxide particles followed by a decrease in
the ionic conductivity with increasing particle concentration. The incorporation of up to 7.5
wt% MSP into crosslinked PEGDGE also improved the compression modulus from 3.5 MPa
to 9.5 MPa (Figure 5.8a). The compression strength of crosslinked MSP/PEGDGE composite
electrolytes improved from 1.9 to 3.3 MPa (Figure 5.8b). Increase in compression properties
of polymer composite electrolytes may be attributed to the effective interactions among the
mesoporous silica particles, polymer and the liquid electrolyte as well as to the addition of
hard filler. MSP may hold a strong coordination with the polymer chains at the amount as
low as 7.5 wt% resulting in the enhancement of compression strength of polymer composite
electrolyte.
Chapter 5 Composite Polymer Electrolytes
148
0 5 10 15 200
50
100
150
200
E
MSP (wt.%)
(
S/c
m)
0
4
8
12
E (M
Pa)
(a)
0 5 10 15 200
50
100
150
200
MSP (wt.%)
(
S/c
m)
0
2
4
(M
Pa)
(b)
Figure 5.8 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked MSP/PEGDGE composite electrolytes, containing 0.8 wt% TBAPF6/PC, as a
function of increasing MSP concentration.
5.2.3 IonicconductivityandcompressionpropertiesofcrosslinkedMSP/PEGDGE
compositeelectrolytescontainingLiTFSI/PC
MSP were also used as filler for crosslinked PEGDGE containing 0.8 wt% LiTFSI/PC as
electrolyte. Ionic conductivity and compression properties of crosslinked MSP/PEGDGE
composite electrolytes are shown in Figure 5.9. The mechanical performance of crosslinked
PEGDGE containing 0.8 wt% 0.1 M TBAPF6/PC was poor as compared to the PEGDGE
containing 0.8 wt% 1.0 M LiTFSI/PC due to increased concentration of PC (see also Section
4.3.1). PC is a necessary solvent for both LiTFSI and TBAPF6 as it imparts ion mobility but
it also plasticises the matrix. Therefore, the effect of varying MSP concentration in
crosslinked PEGDGE electrolyte containing 1 M LiTFSI/PC was also studied. The ionic
Chapter 5 Composite Polymer Electrolytes
149
conductivity of the polymer composite electrolytes increased from 17.3 µS/cm to 246 µS/cm
upon the addition of 7.5 wt% MSP into crosslinked PEGDGE (Figure 5.9). However, ionic
conductivity decreased again with further increasing MSP loading which was possibly due to
the silica aggregation as discussed previously (Section 5.2.2).
0 5 10 150
90
180
270
360 E
MSP (wt.%)
(
S/c
m)
0
6
12
18
E (M
Pa)
(a)
0 5 10 150
80
160
240
320
MSP (wt.%)
(
S/c
m)
0
3
6
9
(M
Pa)
(b)
Figure 5.9 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked MSP/PEGDGE composite electrolytes containing 0.8 wt% LiTFSI/PC as a
function of increasing MSP concentration.
The increase in the ionic conductivity of polymer composite electrolyte by the addition of
MSP may be attributed to the dissociation of ion-pairs in polymer electrolyte. Li+ cations or
TFSI- anions are adsorbed on the silica surface leading to high counter ion concentration in
the vicinity of the silica (space charge layer) [196]. Similar to the previously reported trends
(Section 5.2.2), the compression properties of polymer composite electrolyte also improved
Chapter 5 Composite Polymer Electrolytes
150
with the addition of MSP. The compression modulus of polymer electrolyte was improved by
51% for the polymer composite electrolyte containing 7.5 wt% MSP loading (Figure 5.9a). A
60% improvement in the compression strength of polymer composite electrolyte (Figure
5.9b), from 5.1 MPa to 8.2 MPa, was also seen for crosslinked PEGDGE containing 7.5 wt%
MSP.
Nonporous silica particles (NSP) were also used as control inorganic fillers to separate the
effect of silica addition and mesoporosity. NSP were added to crosslinked PEGDGE
containing 0.8 wt% 1 M LiTFSI/PC as electrolyte. Just like increasing the MSP concentration
in crosslinked PEGDGE (Figure 5.9), increasing the concentration of NSP also improved the
ionic conductivity as well as compression properties (Figure 5.10).
0 5 10 150
40
80
120 E
NSP (wt.%)
(
S/cm
)
0
5
10
15
20
E (M
Pa)
(a)
0 5 10 150
30
60
90
120
NSP (wt.%)
(
S/cm
)
0
4
8
12
(M
Pa)
(b)
Figure 5.10 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked PEGDGE/NSP composite electrolytes, containing 0.8 wt% LiTFSI/PC, as a
function of increasing non-porous silica particles NSP concentration.
Chapter 5 Composite Polymer Electrolytes
151
However, only a 500% improvement in ionic conductivity of NSP polymer composite
electrolyte was seen as compared to the 1300% improvement of the ionic conductivity of
MSP polymer composite electrolytes at 7.5 wt% silica particles loading. The much larger
improvement in ionic conductivity obtained by MSP addition revealed that the mesopores of
the MSP (Table 5.3) did help to improve the ionic mobility by possibly allowing the liquid
electrolyte to pass through the silica pores. The compression properties of NSP containing
crosslinked PEGDGE were similar to those of MSP containing crosslinked PEGDGE. The
compression modulus improved from 10.2 MPa to 14.5 MPa at 10% NSP loading in
crosslinked PEGDGE composite electrolyte (Figure 5.10a). The compression strength also
improved from 5.06 MPa to 8.59 MPa at 10 wt% NSP addition. Similar compression
properties of NSP and MSP polymer composite electrolytes suggested that the porous nature
of MSP did not negatively affected the compression properties which were possibly because
of the filling of MSP with the electrolyte.
5.2.4 IonicconductivityandcompressionpropertiesofcrosslinkedMSP/PEGDGE
compositeelectrolytescontainingEMITFSI
In order to further improve the ionic conductivity without negatively affecting compression
properties, crosslinked MSP/PEGDGE composite electrolytes were also prepared using
EMITFSI as liquid electrolyte. The results of ionic conductivity and compression properties
of the polymer composite electrolytes with increasing concentration of MSP are summarised
in Figure 5.11. The ionic conductivity as well as compression modulus increased significantly
by one to two orders of magnitude upon incorporation of MSP into crosslinked PEGDGE
containing 10 wt% EMITFSI as electrolyte. It can be seen that the electrochemical as well as
the mechanical properties of the polymer electrolyte containing EMITFSI outperform those
containing Li+ salt due to the absence of propylene carbonate (see also Section 4.3.1). The
ionic conductivity and compression modulus of the polymer electrolyte containing MSP
increased by 1100% and 50%, respectively, when compared to crosslinked PEGDGE
containing 10 wt% EMITFSI. In addition to the explanations given for the increased ionic
conductivity discussed previously (Section 5.2.3), Wieczorek et al. [127] have proposed
another explanation for the enhancement of the ionic conductivity to be due to Lewis acid
base type interactions among surface centres, ions and ether-oxygen base groups of the
polymer electrolyte and, therefore, the filler/polymer interactions influence the cationic and
anionic transport within the polymer electrolyte.
Chapter 5 Composite Polymer Electrolytes
152
0 5 10 150
90
180
270
360
E
MSP (wt.%)
S/cm
)
(a)
0
5
10
15
20
25
E (M
Pa)
0 5 10 150
90
180
270
360
MSP (wt.%)
S/c
m)
(b)
0
3
6
9
MP
a)
Figure 5.11 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of crosslinked MSP/PEGDGE composite electrolytes containing 10 wt% EMITFSI as a
function of increasing MSP content.
Increasing the MSP concentration to 10 wt% and above may have led to the formation of
aggregates within the polymer matrix resulting in a reduction of compression modulus, which
decreased from 21.9 MPa for 7.5 wt% MSP to 9.24 MPa for 15 wt% MSP (Figure 5.11a), and
the compression strength, which decreased from 6.95 MPa for 7.5 wt% MSP to 3.62 MPa for
15 wt% MSP (Figure 5.11b). Nevertheless, this study has shown that the incorporation of
MSP into the crosslinked PEGDGE containing 10 wt% EMITFSI allowed developing a resin
with improved ionic conductivity and mechanical properties. However, for structural
supercapacitors, further enhancement of the mechanical performance without deteriorating
the electrochemical performance of the polymer composite electrolytes was required.
Therefore, another structural resin (DGEBA) was studied.
Chapter 5 Composite Polymer Electrolytes
153
5.2.5 Ionic conductivity and compressionpropertiesof crosslinkedDGEBA/MSP
compositeelectrolytescontainingLiTFSI/PC
Table 5.5 summarises the ionic conductivity and compression properties of crosslinked
DGEBA with increasing MSP concentration. A similar trend to that observed for the
crosslinked MSP/PEGDGE electrolytes (Section 5.2.3) was observed. Compression moduli
increased by 47% upon the incorporation of 7.5 wt% MSP into crosslinked DGEBA. The
ionic conductivity improved by almost 65% when 7.5 wt% MSP were added to crosslinked
DGEBA electrolytes. Compression strength was also improved by 2% with the incorporation
of 7.5 wt% MSP in crosslinked DGEBA electrolytes. As mesoporous silica particles were
filled with electrolyte by mixing them with the LiTFSI/PC first and then adding to the
monomer, Li+ and TFSI- ions remained in the mesopores of MSP resulting in improved ionic
mobility within the polymer electrolyte.
Sample code MSP ҡ E σ
% (µS/cm) (MPa) (MPa)
Li20B100M0 0 6.10 ± 0.03 905 ± 73.4 81.1 ± 0.27
Li20B100M2.5 2.50 6.54 ± 0.67 1232 ± 54.1 111 ± 1.02
Li20B100M5.0 5.00 8.42 ± 4.10 1278 ± 72.7 101 ± 5.43
Li20B100M7.5 7.50 10.1± 2.11 1328 ± 7.40 82.5 ± 6.31
Li20B100M10 10.0 7.85 ± 1.87 401.4 ± 63.1 85.1 ± 1.26
Li20B100M12.5 12.5 4.22 ± 0.64 859.6 ± 25.4 53.4 ± 4.02
Li20B100M15 15.0 2.14 ± 0.15 496.7 ± 39.1 41.8 ± 2.85
Table 5.5 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked DGEBA/MSP composite electrolytes containing 20 wt% LiTFSI/PC as a function
of increasing MSP concentration.
5.2.6 Ionic conductivity and compression properties of crosslinked PEGDGE/
DGEBA/MSPcompositeelectrolytescontainingLiTFSI/PC
The results of MSP reinforcement of crosslinked PEGDGE as well as crosslinked DGEBA
electrolyte clearly showed that both mechanical properties as well as ionic conductivity of the
polymer electrolytes can be improved. Therefore, MSP were also used to reinforce
crosslinked PEGDGE/DGEBA. The results of the ionic conductivity and compression
properties for the composite polymer electrolytes with 10 wt% LiTFSI/PC are summarised in
Table 5.6. A blend of 20% PEGDGE and 80% DGEBA was chosen for MSP incorporation as
Chapter 5 Composite Polymer Electrolytes
154
the blend had high compression properties and intermediate ionic conductivity as discussed in
previous section 4.5.1. A 150% increase in ionic conductivity was observed in the crosslinked
PEGDGE/DGEBA/MSP composite electrolyte containing 10 wt% LiTFSI/PC with the
addition 10 wt% of MSP. Compression modulus and compression strength of the polymer
composite electrolyte, containing 20/80 weight ratio of PEGDGE/DGEBA and 10 wt%
LiTFSI/PC, also improved by 6% and 38%, respectively, upon addition of 10 wt% MSP
(Table 5.6).
Sample code PEGDGE:
DGEBA
MSP ҡ E σ
wt% (µS/cm) (MPa) (MPa)
Li10:20P:80B:M0
20:80
0 3.10 ± 0.70 932 ± 14.9 105 ± 5.86
Li10:20P:80B:M7.5 7.5 7.57 ± 0.54 975 ± 10.4 135 ± 3.41
Li10:20P:80B:M10 10 7.81 ± 0.74 987 ± 11.7 145 ± 0.50
Li10:40P:60B:M0
40:60
0 5.44 ± 0.90 628 ± 34.2 89.5 ± 7.11
Li10:40P:60B:M7.5 7.5 9.75 ± 1.14 642 ± 24.3 81.0 ± 1.14
Li10:40P:60B:M10 10 10.5 ± 0.21 616 ± 33.4 84.5 ± 2.01
Table 5.6 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked PEGDGE/DGEBA/MSP composite electrolytes containing 10 wt% LiTFSI/PC as
a function of increasing MSP concentration.
Table 5.6 and Table 5.7 summarise the ionic conductivity and compression properties of
polymer composite electrolytes by the addition of 7.5 wt% and 10 wt% MSP reinforcements
into the crosslinked PEGDGE/DGEBA. DGEBA addition to PEGDGE resulted in a decrease
of the ionic conductivity but increase the compression modulus. Incorporation of 10 w%
MSP into crosslinked PEGDGE/DGEBA containing 20 wt% LiTFSI/PC resulted in a 57%
improvement in ionic conductivity, a 13% improvement in compression modulus and a 27%
improvement in compression strength.
Chapter 5 Composite Polymer Electrolytes
155
Sample code PEGDGE:
DGEBA
MSP ҡ E σ
wt% (µS/cm) (MPa) (MPa)
Li20:20P:80B:M0
20:80
0 6.28 ± 0.73 598 ± 13.7 59.4 ± 2.11
Li20:20P:80B:M7.5 7.5 6.69 ± 0.41 604 ± 14.40 64.4 ± 2.70
Li20:20P:80B:M10 10 9.87 ± 1.21 673 ± 18.70 75.6 ± 5.11
Li20:40P:60B:M0
40:60
0 11.8 ± 0.51 42.4 ± 4.31 6.84 ± 0.44
Li20:40P:60B:M7.5 7.5 27.7 ± 2.71 53.1 ± 2.19 10.2 ± 3.21
Li20:40P:60B:M10 10 17.5 ± 0.24 25.9 ± 1.53 5.72 ± 3.24
Table 5.7 Ionic conductivity ҡ, compression modulus E and compression strength σ of
crosslinked PEGDGE/DGEBA electrolytes containing 20 wt% LiTFSI/PC as a function of
increasing MSP concentration.
5.2.7 Ionic conductivity and compression properties of crosslinked PEGDGE/
DGEBA/MSPcompositeelectrolytescontaining50wt%EMITFSI
From the previous results of varying crosslinked PEGDGE/DGEBA concentration containing
50 wt% EMITFSI, the 40/60 weight ratio of PEGDGE/DGEBA, respectively, in polymer
electrolyte showed the best performance in terms of ionic conductivity as well as
compression properties (Section 4.5.3). Therefore, this formulation of crosslinked
PEGDGE/DGEBA was selected as a matrix for MSP incorporation. Similar to the previous
results of polymer composite electrolytes containing MSP, the 40/60 weight ratio of
crosslinked PEGDGE/DGEBA composite electrolytes also had two times higher ionic
conductivity and compression properties. The compression modulus increased by 94% upon
7.5 wt% MSP incorporation into polymer composite electrolyte (Figure 5.12). Compression
strength was also increased by 122% for a matrix containing 7.5 wt% MSP. A 93% increase
in ionic conductivity was observed for 10wt% MSP addition.
Increasing the concentration of MSP to 10 wt% and above caused the compression modulus
of the composite polymer electrolyte to decrease by 60% (Figure 5.12) which was possibly
due to the MSP aggregation. Also, a drop of 69% in compression strength was observed
when the MSP loading was increased beyond 7.5 wt%. Overall, this study showed that
incorporation of MSP had a positive impact on the both ionic conductivity and compression
properties of polymer electrolytes.
Chapter 5 Composite Polymer Electrolytes
156
0 5 10 150
300
600
900 E
MSP (wt.%)
S/c
m)
(a)
0
20
40
60
80
E (M
Pa)
0 5 10 150
300
600
900
MSP (wt.%)
S/c
m)
(b)
0
7
14
21
MPa)
Figure 5.12 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)
of 40/60 weight ratio of PEGDGE/DGEBA composite electrolytes containing 50 wt%
EMITFSI, as function of increasing MSP concentration.
5.3 Multifunctionality of polymer composite electrolytes
In this chapter, three different forms of silica, including the mesoporous silica monoliths
(MSM), mesoporous silica particles (MSP) and nonporous silica particles (NSP) were
investigated as reinforcements for structural polymer electrolytes. MSP reinforcement of
crosslinked PEGDGE electrolytes improved the ionic conductivity much more as compared
to the reinforcement of MSM and NSP. However, incorporation of either MSP or NSP into
crosslinked PEGDGE did not affect the compression properties. Different structural polymer
composite electrolytes were also prepared by incorporating MSP into crosslinked
PEGDGE/DGEBA electrolytes containing LiTFSI/PC or EMITFSI as electrolytes. Figure
5.13 attempts to capture the multifunctional behaviour of the structural polymer electrolytes.
Chapter 5 Composite Polymer Electrolytes
157
The ionic conductivity was plotted as a function of compression modulus and compression
strength. Pure structural resins and pure electrolytes lie upon either the x-axis (structural
resins) or y-axis (liquid electrolytes). Figure 5.13 summarises multifunctionality of the
structural polymer electrolytes containing different concentrations of MSP and NSP. The
incorporation of MSP to crosslinked PEGDGE/DGEBA electrolytes resulted in improved
ionic conductivity and compression properties as discussed in previous sections.
A reference line was drawn between the properties of crosslinked DGEBA or crosslinked
PEGDGE (structural resins) and EMITFSI (ionic liquid) or LiTFSI/PC (liquid electrolyte) in
order to create a baseline for the full range of polymer composite electrolytes. The polymer
composite electrolytes, positioned above the reference line in Figure 5.13, exhibited true
multifunctionality. Figure 5.13a detailed the trend in compression modulus as a function of
ionic conductivity of polymer electrolytes. Also, Figure 5.13b established a trend for
compression strength versus ionic conductivity of polymer electrolytes. It is clearly evident
from Figure 5.13 that most of the polymer composite electrolytes provide multifunctional
benefits. The MSP reinforced copolymer consisting of a 40/60 weight ratio of
PEGDGE/DGEBA in the crosslinked resin and 50 wt% EMITFSI outperforms the other
polymer composite electrolytes, in terms of multifunctionality, as they were positioned well
above the reference line at the centre of the plot (Figure 5.13).
A structural polymer electrolyte with a crosslinked PEGDGE containing 10 wt% EMITFSI
and 7.5 wt% MSP concentration (E10P100M7.5) had an ionic conductivity of 291 µS/cm and a
compression modulus of 21.9 MPa. The ionic conductivity of E10P100M7.5 polymer composite
electrolyte was 3.4 times lower as compared to the target ionic conductivity of 1 mS/cm
(Section 1.3) and its compression modulus was 46 times lower as compared to the target
compression modulus of 1 GPa (Section 1.3). Whereas the structural polymer composite
electrolyte made by curing a 40:60 PEGDGE/DGEBA blend containing 50wt% EMITFSI
and 7.5 wt% MSP exhibited an ionic conductivity of 849 µS/cm and a compression modulus
of 62.0 MPa. Although compression modulus was still 16 times lower as compared to the
target but its ionic conductivity was exceptional. It was also observed that all the polymer
composite electrolytes showed the maximum improvements in ionic conductivity and
compression properties at either 7.5 wt% or 10 wt% MSP additions. A further increase of
MSP concentrations in polymer composite electrolytes resulted in a decrease of their ionic
Chapter 5 Composite Polymer Electrolytes
158
conductivity and compression properties which was possibly due to the silica aggregation as
discussed in previous sections.
10-1 100 101 102 103 104 10510-1
100
101
102
103
104
pure crosslinkedPEGDGE
pure crosslinkedDGEBA
pure LiTFSI/PC
Increasing multifunctionality
A0.80
PaM
y
Li0.80
PaM
y
Li0.80
PaN
z
E10
PaM
y
Li20
BaM
y
Li10
:40P:60B:My
Li10
:20P:80B:My
Li20
:40P:60B:My
Li20
:20P:80B:My
E50
:40P:60B:My
E (
MP
a)
(S/cm)
(a)
pure EMITFSI
10-1 100 101 102 103 104 10510-1
100
101
102
103
Increasing multifunctionality
(M
Pa)
(S/cm)
A0.80
PaM
y
Li0.80
PaM
y
Li0.80
PaN
z
E10
PaM
y
Li20
BaM
y
Li10
:40P:60B:My
Li10
:20P:80B:My
Li20
:40P:60B:My
Li20
:20P:80B:My
E50
:40P:60B:My
(b)
pure crosslinkedPEGDGE
pure crosslinkedDGEBA
pure LiTFSI/PC
pure EMITFSI
Figure 5.13 Compression modulus E (a) and compression strength σ (b) of different MSP (M)
or NSP (N) reinforced crosslinked PEGDGE (P) and crosslinked DGEBA (B) composite
electrolytes containing TBAPF6/PC (A), LiTFSI/PC (Li) or EMITFSI (E) as a function of
ionic conductivity ҡ at room temperature.
It is clearly evident from the multifunctionality plots (Figure 5.13) that most of the composite
polymer electrolytes including the crosslinked MSP/PEGDGE electrolyte containing 10 wt%
EMITFSI and a crosslinked 40:60 PEGDGE/DGEBA blend containing 50 wt% EMITFSI
showed a clear multifunctionality. Overall, this study had shown that incorporation of MSP
Chapter 5 Composite Polymer Electrolytes
159
has a positive impact in developing a multifunctional resin. Both mechanical and
electrochemical properties of polymer composite electrolytes were improved simultaneously
by the addition of MSP which played a vital role in the enhancement for the
multifunctionality of structural supercapacitors.
Chapter 6 Structural Supercapacitors
160
Chapter 6 Structural
Supercapacitors
A significant effort of this research project has been devoted towards the development of a
low weight structural supercapacitor possessing energy storage capabilities and mechanical
properties. This chapter reports the results of the specific capacitance and shear properties of
structural supercapacitors fabricated using various separators, electrolyte salts at various
concentrations and various solid polymer electrolytes. The influence of varying the charging
time on the specific capacitance of structural supercapacitors is also analysed. Finally, the
effect of reinforcing the polymer electrolytes with MSP as well as carbon fibre activation on
the overall performance of the structural supercapacitors is presented.
Chapter 6 Structural Supercapacitors
161
6.1 Influence of glass fibre separators on the specific capacitance of
structural supercapacitors
The internal resistance of supercapacitors depends on the distance between two electrodes
and, therefore, on the thickness of the separator [32]. The electrochemical performance of the
supercapacitor could be improved by using thinner separators. A separator should be thin
enough to keep the electrodes apart and it should also contribute towards the overall
mechanical performance of the device. Woven glass fibre mats are potential separators as
they are electrical insulators, preventing the electrical contact between the carbon fibre
electrodes, and are available as thin mats (as thin as 35 µm thick) and possess good
mechanical properties.
Glass fibre mat Thickness (µm) Areal weight (g/m2)
ACG 1 30 22
ACG 2 60 49
Tissa 1 90 110
Tissa 2 160 200
Tissa 3 250 300
Table 6.1 Thickness and areal weight of various commercially available glass fibre woven
mats studied.
Different grades of woven glass fibre mats were studied as separators. These glass fabrics
ranged from a lightweight 49 g/m2 fabric with a thickness of 30 µm (ACG 1) to 300 g/m2
fabric with a thickness of 250 µm (Tissa 3). The weight and thickness of the woven glass
fabrics studied are summarised in Table 6.1. It would be expected that the supercapacitor
with the thinnest separator between electrodes would outperform all others in terms of
specific capacitance. However, it was found that the supercapacitor manufactured using the
thinnest available glass fibre mats (ACG 1 (30 µm) and ACG 2 (60 µm)) did not show any
discharge capacity during charge-discharge experiments. As can be seen on the optical
microscopic images (Figure 6.1) of the glass fibre mats, single mats of ACG1, ACG 2 and
Tissa 1 were not densely woven enough to prevent contact of the CF electrodes, which led to
shorting of the electrical circuit. However, both Tissa 2 and Tissa 3 glass fabrics were
completely closed and exhibited the tightest weave packing (Figure 6.1).
Chapter 6 Structural Supercapacitors
162
Figure 6.1 Optical micrographs of various commercially available glass fibre fabrics, (a)
ACG 1, (b) ACG 2, (c) Tissa 1, (d) Tissa 2 and (e) Tissa 3 (microscopic images taken by Dr.
Hui Qian).
The mechanical characterisation of structural supercapacitors requires that the number of
separator layers should always be in an even number in order to fabricate a balanced
composite with balanced crimp lines (i.e. have a symmetric layup). The number of layers of
glass fibre separators strongly influenced the apparent specific capacitance of supercapacitors
at a defined charging time. The specific capacitance of structural supercapacitors decreased
with increasing number of glass fabric layers. However, at the same time, the supercapacitors
Chapter 6 Structural Supercapacitors
163
made with two layers of ACG 1, ACG 2 or Tissa 1 exhibited very high short circuiting which
is possibly due to open fabric windows (Figure 6.1). By increasing the number of glass fibre
layers, the charge loss reduced dramatically which is possibly due to the closing of the tow
gap of glass fibre fabrics. The supercapacitors containing two layers of Tissa 2 glass fabric as
a separator showed high charge loss (87.2%) and intermediate specific capacitance (0.33
mF/cm3). The supercapacitor with two layers of Tissa 3 glass fabric as a separator had a very
small specific capacitance (0.02 mF/cm3) at a charging time of 10 s. Similarly, high charge
loss was also observed for the supercapacitors with six layers of ACG 2 (77.6%) and Tissa 1
(68.2%) but at the same time showed poor specific capacitance (0.06 and 0.14 mF/cm3,
respectively) at 10 s of charging time. Therefore, the study was continued by exploring Tissa
2 glass fabric as a separator in structural supercapacitors.
Separator Charge Discharge Method
GF Type No. of layers Charge (mC) Discharge (mC) Ơ (%) Cg (mF/cm3)
ACG 1
2 No discharging ability
4 42.0 1.25 97.0 1.021
6 18.6 1.40 92.5 0.978
ACG 2
2 47.6 0.96 98.0 0.681
4 15.7 0.43 97.3 0.253
6 0.49 0.11 77.6 0.063
Tissa 1
2 No discharging ability
4 1.79 0.17 90.5 0.159
6 0.63 0.20 68.2 0.135
Tissa 2 2 3.97 0.51 87.2 0.331
Tissa 3 2 0.82 0.05 94.0 0.023
Table 6.2 Charging and discharging capacity, charge loss and specific capacitance Cg of
structural supercapacitors manufactured using as-received carbon fibre mat, crosslinked
PEGDGE containing 0.8 wt% LiTFSI/PC and various glass fabric separators.
Charging time during charge discharge experiments = 10 s;
† ∆ is the percentage difference between the charging and discharging capacity of the composite divided by the
charging capacity.
Chapter 6 Structural Supercapacitors
164
6.2 Influence of varying charging time on the specific capacitance of
structural supercapacitors
Filter paper was also used as separator in the fabrication of structural supercapacitors.
However, from the charge/discharge curves of structural supercapacitors manufactured from
as-received carbon fibres as electrodes, crosslinked PEGDGE containing 0.8 wt% of 1 M
LiTFSI/PC as polymer electrolyte and filter paper as separator, it was seen that the
supercapacitor charged for 10 s exhibited a high charge loss (69.9%) and, therefore, not much
energy was stored within the supercapacitor. A huge charge loss was also observed in the
structural supercapacitors with different GF separators as discussed in previous section 6.1.
The charge loss was estimated from the difference between the charging and discharging
capacity of the structural supercapacitors. In addition to high charge loss, structural
supercapacitor also showed an incomplete discharge (Figure 6.2).
-4x10-5
0
4x10-5
8x10-5
Incomplete discharge
I (A
)
t (s)
10s High charge loss (69.9%)
0 10 20 30 40
(a)
-4x10-5
0
4x10-5
8x10-5
(b)
I (A
)
t (s)
250 s
0 250 500 750 1000
-4x10-5
0
4x10-5
8x10-5 (c) 600 s
I (A
)
0 600 1200 1800 2400t (s)
-4x10-5
0
4x10-5
8x10-5 1000 s
I (A
)
(d)
0 1000 2000 3000 4000t (s)
Figure 6.2 Charge discharge curves of investigated structural supercapacitors with various
charging times of (a) 10s showing high charge loss and incomplete discharge, (b) 250 s, (c)
600 s and (d) 1000 s.
Chapter 6 Structural Supercapacitors
165
It was anticipated that if the charging and discharging time were to increase, then perhaps
more energy could be stored in the structural supercapacitor. It was also observed that the
energy storage capacity was better utilised when the supercapacitors were charged longer
(Figure 6.2). It was shown that the discharge capacity, measured from the area under the
discharging curve in the charge/discharge cycle, of the structural supercapacitor also
increased 600% with charging time from 10 s to 1000 s (Table 6.3). Thus, when increasing
the discharge capacity, the resultant specific capacitance of supercapacitor also increased
with increasing charging time (Figure 6.3). The charge lost was expressed as a percentage of
total charge stored for the studied conditions and decreased with increasing the charging time
(Table 6.3). The summary of the data, described above including the specific capacitance
calculated using charge-discharge method is presented in Table 6.3.
Charge time
(s)
Charge
(mC)
Discharge
(mC)
Ơ
(%)
Cg
(mF/cm3)
10 0.331 0.0101 69.9 0.0804
50 1.15 0.602 47.5 0.502
100 1.84 1.25 31.8 1.03
150 2.37 1.80 23.8 1.49
200 2.87 2.30 20.1 1.90
250 3.73 3.26 12.8 2.69
500 4.36 3.75 13.9 3.10
600 5.75 5.01 12.9 3.52
750 5.01 4.41 12.0 3.64
1000 6.68 6.01 9.93 4.97
Table 6.3 Charge capacity, discharge capacity, charge loss ∆ and specific capacitance Cg as
determined by charge-discharge experiment of structural supercapacitor as function of
varying charging time.
† ∆ is the percentage difference between the charging and discharging capacity of the composite divided by the
charging capacity.
The specific capacitance of structural supercapacitors improved 60 times (from 0.08 mF/cm3
to 4.97 mF/cm3) when the charging time was increased from 10 s to 1000 s. A charging time
of 600 s was selected for further experiments due to a reduced charge loss (12.9%) and high
specific capacitance (3.5 mF/cm3). The high charge loss of structural supercapacitor which
Chapter 6 Structural Supercapacitors
166
resulted in low power density was a serious problem during electrochemical characterisation.
The high charge loss could have a number of reasons including the increase in thickness of
separator (details in Section 6.1), polymer electrolyte resistance (details in Sections 6.5 and
6.6) and poor layup configurations (details in Section 6.11). Another possible reason of the
high charge loss could be the poor electrical connection between the current collectors and
the electrodes due to the formation of a polymer layer around the edges of electrodes during
composite fabrication (details in Section 6.7) as the direct connection of the supercapacitor
electrodes (CF mats) to the potentiostat for electrochemical characterisation could also
introduce an additional contact resistance. All these possible reasons of high charge loss in
structural supercapacitors will be explored further in the following sections.
10 100 10000
1
2
3
4
5
Cg (
mF
/cm
3 )
Charging time (s)
Figure 6.3 Specific capacitance Cg as a function of charging time during charge-discharge
experiment.
6.3 Influence of different types of electrolyte salts on the electrochemical
and mechanical performance of structural supercapacitors
Two different salts, LiTFSI and EMITFSI (Section 3.1.3), were used in order to study their
effect on the electrochemical and mechanical performance of as-received CF reinforced
crosslinked PEGDGE matrix composites using glass fibre mats as an insulator. As discussed
in Section 4.3.1, the LiTFSI salt, when added to a polymer matrix, required a solvent (e.g.
propylene carbonate) in order to dissociate into ions. However, the addition of propylene
carbonate affected the mechanical performance of the polymer electrolytes. EMITFSI, an
ionic liquid, does not require any solvent. For that reason, LiTFSI or EMITFSI were used as
Chapter 6 Structural Supercapacitors
167
an ion source and blended into the resin with the aim to identify, which was the more suitable
to be used in the PEGDGE, DGEBA or PEGDGE/DGEBA resins to be used as matrix for
structural supercapacitors to enhance the energy storage capability. The results of charge-
discharge experiments for the structural supercapacitors using 600 s of charging time are
presented in Figure 6.4. The incorporation of EMITFSI into the crosslinked PEGDGE
resulted in a better specific capacitance of structural supercapacitors.
-5x10-4
0
5x10-4
1x10-3
2x10-3
(b)
I (A
)
t (s)
0.8 wt% LiTFSI/PC (a) 10wt% EMITFSI (b)
(a)
0 600 1200 1800 2400
Figure 6.4 Charge-discharge curves of as-received CF reinforced crosslinked PEGDGE
composites containing (a) 0.8 wt% LiTFSI/PC and (b) 10 wt% EMITFSI and two layers of
glass fibre mats as separator. Charging time = 600 s.
Different concentrations of LiTFSI (0.8 wt%) and EMITFSI (10 wt%) were chosen for this
study as the mechanical performance of crosslinked PEGDGE electrolyte with 0.8 wt%
LiTFSI or 10 wt% EMITFSI was very similar (Section 4.3). The aim of this study was to
improve the specific capacitance of supercapacitors without affecting their mechanical
properties. Therefore as expected, the composites containing 10 wt% EMITFSI had twice the
specific capacitance as the composite containing 0.8 wt% LiTFSI/PC. The specific
capacitance and total discharge capacity calculated from these charge discharge experiments
(Figure 6.4) are presented in Table 6.4. The composites containing EMITFSI (8.8 mF/cm3)
showed a higher specific capacitance compared to the other composite (5.8 mF/cm3).
However, in the case of structural supercapacitor containing EMITFSI; the composite lost
more than 80% of the charge (Figure 6.4 b). This high internal leakage was possibly due to
Chapter 6 Structural Supercapacitors
168
loose carbon fibres forced through the separator during the RIFTing process (Section 6.1) or
poor current collection during characterisation (Section 6.7).
Salt Discharge (mC) Ơ(%) Cg(mF/cm3)
0.8 wt% LiTFSI/PC 9.39 ± 0.97 21.2 ± 4.12 5.807 ± 0.43
10 wt% EMITFSI 15.6 ± 1.02 81.5 ± 6.58 8.808 ± 0.75
Table 6.4 Discharge capacity, charge loss ∆ and specific capacitance Cg of CF and GF
reinforced crosslinked PEGDGE composites containing LiTFSI or EMITFSI as electrolyte.
Charging time = 600 s.
† ∆ is the percentage difference between the charging and discharging capacity of the composite divided by the
charging capacity; Density of composites was 1.78 ± 0.11 g/cm3.
The original, as well as normalised, in-plane shear properties of structural supercapacitors are
presented in Table 6.5. The shear modulus (353 MPa) of as-received CF/GF reinforced
crosslinked PEGDGE composite containing 10 wt% EMITFSI was similar to one of the
supercapacitor with a crosslinked PEGDGE matrix containing 0.8 wt% LiTFSI/PC (334
MPa). The in-plane shear properties of the structural supercapacitors containing glass fibre
mats as separator were not affected by using a high concentration of EMITFSI (10 wt%).
Fibre volume fractions of structural supercapacitors were measured using acid digestion and
the shear properties normalised to the carbon content. Other separators were also used in the
fabrication of structural supercapacitors including the filter paper and polymer membrane
(Section 6.4). During acid digestion, the filter paper or polymer membrane separators in the
composites were also digested along with the epoxy. However, the glass fibres remained
unaffected during the acid digestion experiment. Therefore, in order to compare the results,
the carbon content was selected for the normalisation of the data as compared to the
reinforcement content.
Chapter 6 Structural Supercapacitors
169
Salt CF Vf
(%)
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
0.8 wt% LiTFSI/PC 36.8± 1.11 5.44 ± 0.37 1.67 ± 0.20 334 ±20.81 8.13 ± 0.55 499 ± 36.02
10 wt% EMITFSI 38.1± 0.82 6.12 ± 0.24 1.54 ± 0.30 353 ±27.80 8.83 ± 0.35 509 ± 40.11
Table 6.5 CF volume fraction Vf, maximum shear strength τ12m, shear strength at 5000 µε and
shear modulus G12 of structural supercapacitors with crosslinked PEGDGE containing
LiTFSI/PC or EMITFSI.
Normalised shear properties to 55 volume % carbon content original shear properties
carbon fibre vf×55; The thickness and density
of composites containing glass fibres were 0.78 ± 0.11 mm and 1.78 ± 0.11 g/cm3 respectively.
The data showed that 10wt% EMITFSI in as-received CF and GF reinforced crosslinked
PEGDGE electrolyte performed best electrochemically as well as mechanically. Therefore,
EMITFSI was used as an electrolyte salt in the matrices for further studies.
6.4 Influence of separator type on the specific capacitance and shear
properties of structural supercapacitors
One of the ways to improve the specific capacitance of structural supercapacitors should be
reducing the distance between the electrodes. Since the requirements for improving the
mechanical and electrochemical performance of a structural supercapacitor were conflicting,
an optimisation of electrochemical and mechanical properties was required. In order to
achieve this aim of improving the internal resistance and specific capacitance, different types
of separators were investigated in this study, glass fibre mats (GF), a filter paper (FP) and a
polypropylene (PP) membrane.
Chapter 6 Structural Supercapacitors
170
-1x10-3
-7x10-4
0
7x10-4
1x10-3
(c)
(a)
I (A
)
t (s)
Filter paper (a) PP membrane (b) Glass Fabric (c)
(b)
600 610 620
-1x10-3
0
1x10-3
1200 1210 1220
-1x10-3
0
1x10-3
0 600 1200 1800 2400
Figure 6.5 Charge-discharge curves for the as-received CF reinforced crosslinked PEGDGE
composites containing 10 wt% EMITFSI electrolyte with (a) filter paper, (b) polypropylene
(PP) membrane and (c) two layers of glass fibre mat as separators.
The composite with PP membrane as a separator showed the best electrochemical
performance of all composites (Figure 6.5). The filter paper containing composite showed the
lowest specific capacitance but least charge loss amongst the other composite
supercapacitors. The glass fibre containing composites had an intermediate specific
capacitance and the charge loss was highest among all the composites studied. The discharge
and specific capacitance data for the structural supercapacitors containing various separators
in as-received CF reinforced crosslinked PEGDGE or DGEBA composites is summarised in
Table 6.6. The charge-discharge curves were very noisy due to the low ion mobility in the
very brittle crosslinked DGEBA matrix.
Chapter 6 Structural Supercapacitors
171
Matrix Separator Discharge (mC) ∆† (%) Cg (mF/cm3) ρ (g/cm3)
PEGDGE
(10wt%
EMITFSI)
FP 10.2 ± 0.42 10.4 ± 0.34 7.06 ± 0.45 1.74 ± 0.01
GF 15.6 ± 1.02 81.5 ± 6.58 8.81 ± 0.75 1.82 ± 0.04
PP membrane 10.4 ± 0.04 14.5 ± 1.04 9.90 ± 0.22 1.69 ± 0.04
DGEBA
(10wt%
EMITFSI)
FP 3.9E-4 ± 3.5E-5 7.37± 1.78 2.7E-4 ± 2.0E-5 1.37 ± 0.01
GF 5.7E-4 ± 1.2E-4 28.2 ± 8.40 3.3E-4 ± 5.0E-5 1.53 ± 0.01
PP membrane 3.7E-4 ± 1.2E-5 6.15 ± 1.18 3.5E-4 ± 2.0E-5 1.32 ± 0.01
Table 6.6 Discharge capacity, charge loss ∆, specific capacitance Cg and density ρ of as-
received CF reinforced crosslinked PEGDGE or DGEBA composites containing 10 wt%
EMITFSI and various separators. Charging time = 600 s.
† ∆ is the percentage difference between the charging and discharging capacity of the composite divided by the
charging capacity.
As-received CF reinforced crosslinked PEGDGE composites containing 10wt% EMITFSI as
electrolyte and filter paper as separator had the highest shear modulus (420 MPa) as
compared to composites containing PP membrane (306 MPa) or glass fibre (353 MPa). This
was possibly due to the good adhesion between the crosslinked matrix and filter paper in the
structural supercapacitor as no delamination was observed in the structural supercapacitors
with filter paper as separator. Structural supercapacitors, fabricated using PP membrane as a
separator, had the highest electrochemical performance which was due to the lowest separator
thickness among others. It was also observed that the specimens containing PP membrane as
a separator underwent significant delamination upon mechanical testing and also exhibited
the lowest mechanical performance. However, a glass fibre mat containing structural
supercapacitors had an intermediate electrochemical (Table 6.6) as well as mechanical
performance (Table 6.7). Therefore, glass fibre mat was used as a separator in structural
supercapacitors for further studies. It should also be noted that the thickness of the specimens
varied due to the intrinsic thickness of the separators. Shear properties were normalised to a
carbon fibre volume fraction of 55% in order to allow the comparison of shear properties
among the various structural supercapacitors. The carbon fibre volume fraction was used as
standard for normalisation because of the degradation of filter paper as well as PP membrane
during the acid digestion.
Chapter 6 Structural Supercapacitors
172
Polymer
electrolyte Separator
CF Vf
(%)
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
PEGDGE
(10 wt%
EMITFSI)
FP 45.1±0.77 8.01±0.66 2.64±0.20 420±38.1 9.77±0.80 512±46.5
GF 38.1±0.82 6.12±0.24 1.54±0.30 353±27.8 8.83±0.35 510±40.1
PP 57.4±1.46 4.83±0.32 N/A* 306±26.4 4.63±0.31 293±25.3
DGEBA
(10 wt%
EMITFSI)
FP 42.4±0.82 44.2±7.47 14.9±2.02 2846±381 57.3±9.69 3692±494
GF 32.9±0.80 65.7±5.13 20.1±1.24 3668±229 110±8.58 6132±383
PP 55.4±0.94 45.2±1.38 19.6±2.22 3305±202 44.9±1.37 3281±201
Table 6.7 Carbon fibre volume fraction Vf, maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and
crosslinked DGEBA containing 10wt% EMITFSI electrolyte and filter paper (FP), glass fibre
mats (GF) or PP membrane separators.
†Normalised shear properties at 55 volume % carbon content original shear properties
carbon fibre vf×55;
* Composite failed before 5000 µϵ; the thickness of composites containing glass fibre mats, filter paper and PP
membrane was 0.78 ± 0.02 mm, 0.72 ± 0.03 mm and 0.51 ± 0.03 mm, respectively.
6.5 Influence of the polymer electrolyte composition on the
electrochemical and mechanical performance of structural supercapacitors
Pure crosslinked PEGDGE containing EMITFSI or LiTFSI/PC as electrolyte was used as
matrix in the composite laminates (Section 6.3). However, the mechanical performance of
crosslinked PEGDGE was very poor because of its soft and rubbery nature (Section 4.3).
Therefore, PEGDGE resin was blended with DGEBA (as discussed in Section 4.5) at a fixed
content of EMITFSI (10 wt%) with the aim to improve the mechanical properties while
maintaining the electrochemical properties of the supercapacitors.
Figure 6.6 shows the charge-discharge plots for the studied structural supercapacitors as
function of the PEGDGE to DGEBA ratio. All the studied composites contained 2 layers of
glass fibre mats as the separator and 10 wt% EMITFSI as the electrolyte. The composite with
pure crosslinked PEGDGE containing 10 wt% EMITFSI as polymer electrolyte had the best
charge-discharge performance amongst all the composites while the composite with a pure
crosslinked DGEBA matrix containing 10 wt% EMITFSI had the lowest specific capacitance.
The specific capacitance gradually decreased with increasing concentration of DGEBA in the
crosslinked PEGDGE matrix.
Chapter 6 Structural Supercapacitors
173
0 600 1200 1800 2400-4x10-4
-2x10-4
0
2x10-4
4x10-4
(f)
(e) (d)
(c)
(b)
I (A
)
t (s)
100P:0B (a) 80P:20B (b) 60P:40B (c) 40P:60B (d) 20P:80B (e) 0P:100B (f)
(a)
750 1000 1250
0
5x10-7
1x10-6
1250 1500 1750
-4.0x10-7
-2.0x10-7
0.0
Figure 6.6 Charge-discharge curves for the as-received CF and GF reinforced crosslinked
PEGDGE/DGEBA composites containing 10 wt% EMITFSI as a function of the PEGDGE to
DGEBA weight ratio.
Discharge capacity and specific capacitance of these systems after 600 s of charging time in
charge-discharge experiments is given in Table 6.8. It can be seen that the PEGDGE/DGEBA
ratio strongly influenced the specific capacitance of the supercapacitors. That is, as the
DGEBA content in PEGDGE increased, the specific capacitance decreased. The charge-
discharge curves for the structural supercapacitors with either pure crosslinked DGEBA
matrix or crosslinked 20:80 PEGDGE/DGEBA blend containing 10 wt% EMITFSI were very
noisy because of the low ion mobility associated with the very stiff crosslinked matrix and
also had a very small specific capacitance with high charge loss (Table 6.8). The pure
PEGDGE based composite had the highest charge loss which was possibly due to loose
carbon fibres passing through the separator during RIFTing. The high charge loss observed
for the structural supercapacitors with 40:60 and 20:80 PEGDGE/DGEBA blend matrix
composites was possible due to increased mechanical stiffness of the respective matrices
hindering the ionic mobility.
Chapter 6 Structural Supercapacitors
174
Matrix
PEGDGE:DGEBA Discharge (mC) Δ† (%) Cg (mF/cm3) ρ (g/cm3)
100:0 15.6 ± 1.02 81.5 ± 6.58 8.81 ± 0.75
1.784 ± 0.11
80:20 8.57 ± 1.13 20.1 ± 1.72 4.84 ± 0.59
60:40 6.12 ± 1.32 20.2 ± 3.65 3.41 ± 0.75
40:60 0.19 ± 0.09 72.3 ± 10.3 0.10 ± 0.04
20:80 0.10 ± 0.01 57.5 ± 3.54 0.06 ± 0.004
0:100 5.7E-4 ± 1.2E-4 28.2 ± 8.40 3.3E-4 ± 5.0E-5
Table 6.8 Discharge capacity, charge loss Δ, specific capacitance Cg and bulk density ρ of
as-received CF and GF reinforced crosslinked PEGDGE/DGEBA composites as function of
PEGDGE to DGEBA ratio. Charging time = 600 s.
The addition of DGEBA to PEGDGE improved the structural performance of the
supercapacitors. The supercapacitor with a crosslinked matrix consisting of 20% PEGDGE
and 80% DGEBA had 15 times higher shear modulus and strength as compared to pure
crosslinked PEGDGE matrix composites (See shear stress strain curves in Appendix E). The
improvement in the shear moduli and shear strengths of the composites with increasing
DGEBA content in the matrix was indicative of the increased matrix resin modulus (Section
4.5), which in turn led to changes in failure mechanism. Structural supercapacitors with a
pure crosslinked PEGDGE matrix had the smallest shear modulus (353 MPa) among all
structural supercapacitors because of the soft nature of crosslinked PEGDGE matrix.
Matrix
(wt. ratio)
CF Vf
(vol%)
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
100P:0B 38.1 ± 0.82 6.12 ± 0.24 1.54 ± 0.30 353 ± 27.8 8.83 ± 0.35 510 ± 40.1
80P:20B 36.4 ± 0.74 9.50 ± 0.44 2.08 ± 0.22 438 ± 47.4 14.4 ± 0.66 662 ± 71.6
60P:40B 35.1 ± 0.65 33.1 ± 4.22 2.58 ± 0.14 506 ± 31.1 51.9 ± 6.61 793 ± 48.7
40P:60B 37.1 ± 0.22 82.7 ± 2.23 11.4 ± 0.76 2307 ± 121 123 ± 3.31 3420 ± 179
20P:80B 37.7 ± 0.95 103 ± 6.94 17.2 ± 1.23 3605 ± 195 150 ± 10.1 5259 ± 284
0P:100B 32.9 ± 0.80 65.7 ± 5.13 20.1 ± 1.24 3668±229 110 ± 8.58 6132 ± 383
Table 6.9 CF volume fraction Vf, maximum shear strength τ12m, shear strength at 5000 µε and
shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and DGEBA
polymer electrolytes containing 10wt% EMITFSI as function of PEGDGE to DGEBA ratio.
†Normalised shear properties at 55 % Vf original shear properties
carbon fibre vf×55; Composite thickness = 0.78 ± 0.02 mm.
Chapter 6 Structural Supercapacitors
175
Photographs of post-in-plane shear tested specimens of structural supercapacitor specimens
made using as-received CF and GF as reinforcements with increasing concentration of
DGEBA in crosslinked PEGDGE containing 10 wt% EMITFSI are shown in Figure 6.7.
Stress whitening was observed in all the failed specimens. The other major feature, observed
in specimens composed of structural supercapacitors with 60:40, 80:20 and 100:0
PEGDGE/DGEBA blend matrices, was delamination between the CF and GF layers.
Delamination was possibly due to the fibre reorientation (scissoring) of the ±45° CF and GF
mats during testing in the direction of the applied load inducing interlaminar stresses at the
ply interfaces. However, delamination was not observed in the specimens, composed of
structural supercapacitors with 0:100, 20:80 and 40:60 PEGDGE/DGEBA blend matrices,
possibly due to the stiff resin-rich layer present between the CF and GF plies. Matrix
cracking was also observed in specimens with a pure crosslinked DGEBA matrix containing
10 wt% EMITFSI resulting in the detachment of strain gauge. Matrix cracking was due to the
brittle nature of crosslinked DGEBA containing 10 wt% EMITFSI (Figure 4.7).
Figure 6.7 Photographs of post-test in-plane shear specimens of CF and GF reinforced
polymer electrolytes containing 10 wt% EMITFSI with varying content of PEGDGE and
DGEBA in the crosslinked matrix (a) Pure DGEBA, (b) 20P:80B, (c) 40P:60B, (d) 60P:40B,
(e) 80P:20B and (f) Pure PEGDGE.
6.6 Influence of EMITFSI concentration on electrochemical and
mechanical performance of structural supercapacitors
EMITFSI concentration was also varied in structural supercapacitors made from as-received
CF and GF reinforced 40:60 PEGDGE/DGEBA blend matrix to study the influence on the
electrochemical and mechanical performance of the composites. 40:60 PEGDGE/DGEBA
blend matrix containing 50 wt% EMITFSI was selected because the blend showed the best
ionic conductivity and compression properties amongst others (Section 4.5.3). The charge-
discharge curves for the as-received CF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix composites as function of EMITFSI concentration are
Chapter 6 Structural Supercapacitors
176
presented in Figure 6.8. It was clear that the supercapacitor containing 10 wt% EMITFSI had
the lowest specific capacitance (0.1 mF/cm3) due to the high stiffness of matrix (Section
4.5.2). There was also an obvious charge loss (72%) underlying the exponential capacitive
charging current of this supercapacitor. The supercapacitor, with the pure EMITFSI
electrolyte, had a highest specific capacitance (12 mF/cm3) and a lowest charge loss (8.8%).
-8x10-4
-4x10-4
0
4x10-4
8x10-4
(c)
(b)
(a)
I (A
)
t (s)
100wt% EMITFSI (a) 50wt% EMITFSI+40P60B (b) 10wt% EMITFSI+40P60B (c)
(c)
0 600 1200 1800 2400
600 610 6200
1x10-6
2x10-6
1200 1210 1220-1x10-6
-5x10-7
0
Figure 6.8 Charge-discharge curves for supercapacitors made using as-received CF and GF
with: (a) 100 wt% EMITFSI, (b) 50 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix,
or (c) 10 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix as electrolyte.
Figure 6.9 shows the complex impedance plot (also called Nyquist plot) of the
supercapacitors made using as-received CF and GF as function of EMITFSI in the blend
matrix. The plot can be broadly divided into two regions in all cases; at high frequencies, the
capacitive impedance Z’’ (= -1/ωC) is small, but increases with decreasing frequency. The
semi-circular shape of the plot was associated with a parallel combination of a resistive
component, due to resistance in ionic mobility, and capacitive component, due to parallel
plate geometry of supercapacitor. This semicircular shape (also called Warburg region [31])
is a consequence of the distributed resistance/capacitance in a porous electrode. At high
frequencies, the resistance and capacitance of the supercapacitor decreases as the accessible
part of the active porous electrode is small. The equivalent series resistance (ESR) was
calculated from the x-intercept of the high frequency curve (see Section 3.9.3). The
supercapacitors made using as received CF and GF with 100 wt% EMITFSI, 50 wt%
EMITFSI and 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix had an
Chapter 6 Structural Supercapacitors
177
ESR of 0.6 Ω, 5.5 Ω and 280 Ω respectively (Table 6.10). In the low frequency region, the
electrochemical charging of the double layer surface becomes significant as the ions of the
electrolyte have enough time to settle at the surface of the CF electrodes. The impedance
response approached vertical in the low frequency region which is the purely capacitive
response of the electrodes.
0 100000 200000 3000000
30000
60000
90000
100 Hz100 kHz
100 kHz
(-1)
Z''
()
Z' ()
100wt% EMITFSI (a) 50wt% EMITFSI+40P60B (b) 10wt% EMITFSI+40P60B (c)
(a)
1 Hz
(b)
(c)
0 15 30 450
50
100
150
Figure 6.9 Nyquist plots for the as-received CF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix containing increasing amounts of EMITFSI.
Frequency range of 105 Hz to 1 Hz. Applied potential of 0.5 V.
The electrochemical performance for all supercapacitors made with increasing EMITFSI
concentration in the matrix determined from charge-discharge and impedance spectroscopy is
shown in Table 6.10. The energy density E and the power density P of the supercapacitors
decreased with decreasing concentration of EMITFSI in the matrix. The decrease in power
density with decreasing EMITFSI concentration was attributed to the decrease in the amount
of ions settling at the surface of the CF electrodes.
Chapter 6 Structural Supercapacitors
178
EMITFSI
(wt%) Matrix Δ (%) Cg (mF/cm3)
ESR
(Ω)
E
(Wh/kg)
P
(W/kg)
100 N/A 8.76 ± 0.65 12.1 ± 0.20 0.63 0.012 838
50 40P:60B
9.55 ± 1.28 10.1 ± 0.94 3.47 0.009 47.4
10 72.3 ± 10.3 0.10 ± 0.04 280 1.2E-4 0.37
Table 6.10 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and Power density P of as-received CF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix composites as function of decreasing EMITFSI
concentration. Charging time = 600 s.
E = energy density in Wh/kg .
, U = 3.5 V; P = power density in W/kg.
;
Density measured using helium pyncrometry = 1.78 ±0.11 g/cm3; composite thickness = 0.78±0.03 mm.
Although decreasing concentration of EMITFSI to crosslinked 40:60 PEGDGE/DGEBA
blend matrix deteriorated the electrochemical performance of the supercapacitors (Table
6.10), their structural performance improved (Table 6.11). The shear moduli of the laminates
were improved approximately three times and strength around ten times when EMITFSI
concentration in the supercapacitor matrix was decreased from 50 wt% to 10 wt%. The
increase in the shear moduli and strength of the composites with decreasing EMITFSI
concentration was indicative of the increased matrix resin modulus (Section 4.5.3).
EMITFSI
(wt%) Matrix
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
100 N/A*
50 40P:60B
7.48 ± 0.34 3.67 ± 0.56 802 ± 24.1 11.1 ± 0.50 1189 ± 35.7
10 82.7 ± 2.23 11.4 ± 0.76 2307 ± 121 123 ± 3.31 3420 ± 179
Table 6.11 Maximum shear strength τ12m, shear strength at 5000 µε and shear modulus G12 of
structural supercapacitors made using as-received CF and GF with crosslinked 40:60
PEGDGE/DGEBA blend matrix as function of decreasing EMITFSI concentration.
Thickness of composites = 0.78 ± 0.03 mm; CF volume fraction in composites = 37.1% ± 0.7 %;
†Normalised shear properties at carbon fibre volume fraction of 55% original shear properties
carbon fibre vf×55;
* No shear properties measured as the electrolyte was liquid.
Failed structural supercapacitor specimens showed stress whitening on the surface which
indicated matrix cracking during in-plane shear loading (Figure 6.10). The other feature
Chapter 6 Structural Supercapacitors
179
observed in the structural supercapacitors containing 50 wt% EMITFSI was delamination of
the CF and GF layers. However delamination was not observed in structural supercapacitor
containing only 10wt% EMITFSI in the matrix possibly due to the relatively high stiffness of
the polymer electrolyte (Section 4.5.2).
Figure 6.10 Photographs of supercapacitor specimens made from CF and GF reinforced
crosslinked 40:60 PEGDGE/DGEBA blend matrix with (a) 10 wt% EMITFSI and (b) 50 wt%
EMITFSI after in-plane shear testing.
6.7 Influence of the connectivity of copper tape and copper wire on the
electrochemical performance of structural supercapacitors
A huge charge loss of around 80% of the charging capacity was observed during the charge-
discharge experiments of the structural supercapacitors with a crosslinked PEGDGE matrix
containing 10 wt% EMITFSI. It was thought that the charge loss was possibly due to a
number of factors including poor ionic conductivity of the resin (Section 6.5), thickness of
separators (Section 6.4) or the open tow gap in separators (Section 6.1) but also poor
electrical contact between the electrochemical testing device and the CF electrodes. In order
to address the problem of high charge loss, CF based electrodes were connected with copper
wire and copper tape.
(a) Baseline (b) Copper tape electrodes (c) Copper wire connected
electrodes
Chapter 6 Structural Supercapacitors
180
(d) Copper tape and copper wire
connected electrodes (e) Copper tape on half electrodes (f) Overlapping of CF electrodes
Figure 6.11 Different configurations of CF based electrodes during fabrication of structural
supercapacitors.
Six different CF electrodes configurations were studied (Figure 6.11). Different CF electrode
configurations were prepared by applying copper tape around edges of the CF electrodes or
connecting silver plated copper wires to the end of each CF electrode during composite
manufacturing using RIFTing process or both. An overlapping configuration was also studied
in which both CF electrodes overlapped by placing GF, half the length of the CF mat, in the
middle. The edges of the CF electrodes were sealed with copper tape and were then
connected with copper wire (Figure 6.11 f). This overlapping configuration is also being used
by a team of researchers at the Army Research Laboratories (ARL), Aberdeen, USA [102].
600 1200 1800 2400-8x10-4
-4x10-4
0
4x10-4
8x10-4
I (A
)
t (s)
Baseline (a) Copper tape (b) Copper Wire (c) Copper tape and Wire (d) Copper tape on half mat (e) Overlapping (f)
600 650 700 7500
4x10-4
8x10-4
1200 1250 1300 1350-8x10-4
-4x10-4
0
Figure 6.12 Charge-discharge curves of different CF electrode configurations in as-received
CF and GF reinforced crosslinked PEGDGE supercapacitors with 10wt% EMITFSI.
Chapter 6 Structural Supercapacitors
181
The charge-discharge curves (Figure 6.12) showed that the structural supercapacitors were
charged more effectively by applying copper tape (Figure 6.11b) or copper wire (Figure
6.11c) or both (Figure 6.11d) and structural supercapacitor with copper wire/copper tape
(Figure 6.11d) showed the best electrochemical performance (Table 6.12). It was also
observed that the charging and discharging capacity of structural supercapacitors were also
affected by the change in CF electrode configurations possibly due to the different available
active area of CF electrodes for charge storage. The specific capacitances from the charge-
discharge experiments are summarised in Table 6.12. The results demonstrated that the
charge loss was small in the overlapping configuration (10% of charging capacity) and the
copper tape/wire configuration (8% of charging capacity) as compared to other CF electrode
configurations. Copper wire/copper tape configuration over CF electrodes was selected for
further studies because of the low charge loss. The specific capacitance of the composites
however was not significantly affected by the change of the electrode connection as the
change in specific capacitance is mainly associated with the change in surface area of the CF
electrodes (Section 6.12).
Configurations Discharge (mC) ∆ (%) Cg (mF/cm3)
Baseline 15.6 81.5 8.81
Copper tape 10.9 23.3 9.84
Copper wire 11.7 17.7 7.75
Copper tape / wire 10.7 8.14 10.3
Copper tape on half mat 9.56 36.8 9.26
Overlapping 7.41 9.95 7.76
Table 6.12 Discharge capacity, charge loss ∆ and specific capacitance Cg for different CF
electrode configurations of as-received CF and GF reinforced crosslinked PEGDGE
supercapacitors containing 10 wt% EMITFSI.
6.8 Influence of charge-discharge cycles on the specific capacitance of
structural supercapacitors
It is well known that energy storage devices (batteries and fuel cells) age faster when exposed
to repeated charge and discharge cycles [91]. The major drawback of energy storage devices,
such as batteries or fuel cells, is their relatively small cycle life due to redox chemical
reaction taking place at the electrode surface [32]. Supercapacitors are claimed to have long
cycle life of tens of thousands of charge-discharge cycles [32]. Therefore, it was necessary to
Chapter 6 Structural Supercapacitors
182
investigate the specific capacitance of the structural supercapacitors over a number of charge-
discharge cycles. The as-received CF and GF reinforced crosslinked PEGDGE
supercapacitors containing 10 wt% EMITFSI was subjected to repeated charge-discharge
cycling. The composite was copper taped around edges and two copper wires were connected
to each CF electrode (Figure 6.11d). The data obtained over about 1000 cycles are shown in
Figure 6.13. A charging time of 150 s was chosen for studying the evolution of specific
capacitance as function of repeated charging/discharging of structural supercapacitors during
a charge discharge experiment. At a 150 s of charging time, the charge loss (15%) was
relatively low . It is clearly evident that even after 1000 cycles of charging and discharging of
the structural supercapacitor, there was only a small change in specific capacitance (15% of
original value of specific capacitance).
0 200 400 600 800 10000
3
6
9
Cg (
mF
/cm
3 )
Cycle number
Figure 6.13 Evolution of specific capacitance measured at 150 s of charging time for as-
received CF and GF reinforced crosslinked PEGDGE supercapacitors containing 10 wt%
EMITFSI as function of number of charge/discharge cycles.
The charge-discharge curves for cycle number 1, 500 and 1000 for the structural
supercapacitor are shown in Figure 6.14. The structural supercapacitor showed best
discharging capacity during the first cycle and the worst during the 500th cycle. However,
after 1000 cycles, the specific capacitance (4.6 mF/cm3) nearly approached the original value
(5.0 mF/cm3). Although, there was a small change, a drop of 15% of its original value, in the
specific capacitance of the structural supercapacitor from cycle number 400 to cycle number
700 (Figure 6.13) but the slight decrease in specific capacitance could be attributed to the
Chapter 6 Structural Supercapacitors
183
number of reasons including the fact that the charge-discharge experiment lasted for 7 days.
However, this small variation in specific capacitance is common and reported elsewhere [97,
197-199]. The results indicated the high reversibility of the charge storage process taking
place on the active surface of CF based electrodes.
-2x10-4
-1x10-4
0
1x10-4
2x10-4
(c)
(b)
I (A
)
t (s)
Cycle 1 (a) Cycle 500 (b) Cycle 1000 (c)
(a)
0 200 400 600
Figure 6.14 Charge-discharge curves for the as-received CF and GF reinforced crosslinked
PEGDGE supercapacitors containing 10wt% EMITFSI at cycle number (a) 1, (b) 500 and (c)
1000 in charge-discharge experiment.
6.9 Influence of applied potential difference on the energy density of
structural supercapacitor
The electrochemical characterisation of structural supercapacitors was conducted at 0.1 V of
applied potential difference in the charge-discharge experiments. The influence of applied
potential difference on the electrochemical performance of structural supercapacitors was
further investigated by varying the potential difference from 0.1 V to 3.5 V. The structural
supercapacitors were dried before charge/discharge experiments. The charging and
discharging capacity as well as energy density increased with increasing applied potential
difference but the capacitance (discharge capacity divided by the applied potential, equation
3.10, Section 3.9.2) remained almost constant. The results are reported in Table 6.13 and
suggested using 3.5 V for the calculation of energy densities of the other studied composites.
The potential difference was not increased further above 3.5 V in the charge-discharge
experiment as the EMITFSI was stable only up to 4 V [200].
Chapter 6 Structural Supercapacitors
184
Potential Difference (V) Discharge (mC) Δ† (%) Cg (mF/cm3) E (mWh/kg)
0.1 10.7 8.14 10.3 0.01
0.5 56.3 16.8 10.8 0.22
1.5 175 29.6 11.2 2.02
2.5 283 22.4 10.9 5.42
3.5 357 23.0 9.81 9.59
Table 6.13 Influence of applied potential difference on the discharge capacity, charge loss Δ,
specific capacitance Cg and energy density E of structural supercapacitors made from CF
and GF reinforced crosslinked PEGDGE containing 10wt% EMITFSI.
† ∆ is the percentage difference between the charging and discharging capacity of the composite divided by the
charging capacity; density of supercapacitor = 1.78 ± 0.11 g/cm3
6.10 Influence of addition of MSP on the electrochemical and mechanical
performance of structural supercapacitors
The addition of DGEBA into the PEGDGE resin of structural supercapacitors resulted in
increased shear properties at the cost of reduced specific capacitance (Section 6.5). Similarly,
increasing the concentration of EMITFSI in structural supercapacitors improved the specific
capacitance but caused a drop in shear properties. Therefore, a different approach was
selected to enhance the mechanical as well as electrochemical performance of composites by
introducing MSP into the polymer electrolyte of structural supercapacitors. The addition of
7.5 wt % MSP into the crosslinked PEGDGE matrix containing 10 wt% EMITFSI resulted in
improved compression properties as well as ionic conductivity (as discussed in Section
5.2.4).
The electrochemical properties of the obtained as-received CF and GF reinforced crosslinked
MSP/PEGDGE composites containing 10 wt% EMITFSI were studied using charge-
discharge experiment and impedance spectroscopy. Figure 6.15 shows the Nyquist plot of the
composites with and without MSP incorporated in the matrix. The equivalent distributed
resistance (EDR), the x-intercept of the low frequency curve, for the composites containing
MSP was around 4 times lower as compared to the EDR for crosslinked PEGDGE matrix
supercapacitors. The equivalent series resistance (ESR), the x-intercept of high frequency
curve, was also reduced to half by the addition of 7.5 wt% MSP to crosslinked PEGDGE
matrix supercapacitors (Table 6.14). The decrease in ESR with MSP addition can be
attributed to the improved ionic conductivity of polymer electrolyte (Section 5.2.4).
Chapter 6 Structural Supercapacitors
185
0 200 400 6000
80
160
1 Hz
100 kHz
(b)
(a)
(-1)
Z''
()
Z' ()
PEGDGE (a) PEGDGE+7.5wt% MSP (b)
100 Hz
Figure 6.15 Complex impedance plots for structural supercapacitors made from as-received
CF and GF reinforced crosslinked PEGDGE matrix (a) or crosslinked PEGDGE matrix with
7.5 wt% MSP (b) containing 10 wt% EMITFSI.
Frequency range = 105 Hz to 1 Hz. Applied potential = 0.5 V
The results for the charge-discharge experiments for the same composites are presented in
Figure 6.16. It is clear that the composites containing MSP had a higher discharge capacity as
compared to pure crosslinked PEGDGE composites and thus had a higher specific
capacitance. The charge loss (8%) of the structural supercapacitor with a crosslinked
PEGDGE matrix containing 10 wt% EMITFSI was same as compared to the charge loss
(12%) of the supercapacitor with a crosslinked MSP/PEGDGE containing 10 wt% EMITFSI.
-6x10-4
-3x10-4
0
3x10-4
6x10-4
(b)
I (A
)
t (s)
PEGDGE (a) PEGDGE+7.5wt% MSP (b)
(a)
0 600 1200 1800 2400
Figure 6.16 Charge-discharge curves for structural supercapacitors made from as-received
CF and GF reinforced crosslinked PEGDGE matrix (a) or crosslinked PEGDGE matrix with
7.5 wt% MSP (b) containing 10 wt% EMITFSI. Charging time = 600 s
Chapter 6 Structural Supercapacitors
186
The electrochemical properties of the structural supercapacitors are presented in Table 6.14.
Both the energy density and the power density were doubled by the addition of MSP into the
crosslinked PEGDGE matrix. The increase in energy density was possibly due to the porous
structure of MSP which helped more ions settling in the electrochemical double layer of the
supercapacitor. MSP addition into matrix of structural supercapacitors may had limited the
ionic aggregation at the CF electrode surface and therefore, resulted in a 48% improvement in
specific capacitance.
MSP
(wt%) Matrix Δ (%)
Cg
(mF/cm3)
ESR
(Ω)
E
(Wh/kg)
P
(W/kg)
0 PEGDGE /
10wt% EMITFSI
8.14 10.3 8.63 0.010 18.0
7.5 11.7 15.2 5.14 0.019 34.7
Table 6.14 Influence of MSP addition on the charge loss Δ, specific capacitance Cg,
equivalent series resistance ESR, energy density E and power density P of structural
supercapacitors made using as-received CF and GF reinforced crosslinked PEGDGE
composites containing 10 wt% EMITFSI.
Density of supercapacitor = 1.78 ± 0.11 g/cm3.
The shear properties of the same composites are presented in Table 6.15. The addition of
MSP into the crosslinked PEGDGE matrix of the composites resulted in a four times
improvement of shear modulus and six fold improvements in shear strength. It was also
observed during testing that the failure was associated with delamination of the CF and GF
layers in both composites.
MSP
(wt%) Matrix
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
0 PEGDGE
10wt% EMITFSI
6.12 ± 0.24 1.54 ± 0.30 353 ± 27.8 8.83 ± 0.35 510 ± 40.1
7.5 39.4 ± 3.09 7.12 ± 0.98 1470± 253 56.9 ± 4.46 2122 ± 365
Table 6.15 Effect of MSP additions on the maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of as-received CF and GF reinforced crosslinked PEGDGE
matrix composites.
Thickness of composites was 0.78 ± 0.03 mm and the CF volume content of the composites was 38.1%.
Chapter 6 Structural Supercapacitors
187
Photographs of the structural supercapacitor specimens after in-plane shear testing are shown
in Figure 6.17. Whitening was observed in the middle of the specimens indicating matrix
cracking during shear testing. Delamination of CF and GF layers was also observed in both
structural supercapacitor specimens.
Figure 6.17 Photographs of CF and GF reinforced crosslinked PEGDGE containing 10 wt%
EMITFSI composites after in-plane shear testing with (a) crosslinked PEGDGE and (b)
crosslinked PEGDGE/7.5 wt% MSP.
6.11 Configuration of structural supercapacitors
In order to further improve the specific capacitance of structural supercapacitors, thicker
laminates were also fabricated by laying-up three structural supercapacitors together to form
a supercapacitor “bank”. Copper wires were used to connect each CF mat of the laminate in
such a way that the conjugated supercapacitors were either connected in series or parallel as
shown in Figure 6.18. Connecting supercapacitors in series or parallel has its pros and cons;
the major advantage of connecting supercapacitors in series is that the overall potential
difference is increased (VTotal = V1 + V2 + V3 +…) [32]. However on the other hand, the
overall capacitance is reduced in the series combination of supercapacitors (1/C total = 1/ C1 +
1/ C2 + 1/ C3 …..) [32]. For example, if three supercapacitors having 1 F/cm3 capacitance are
charged at 3.5 V they would produce an output of 10.5 V (approximately) with an overall
capacitance of 0.33 F/cm3 when connected in series. The number of panels connected in
series must be kept low in order to keep the Equivalent Series Resistance (ESR)1 to a
minimum. The increase in ESR, due to increase in number of panels connected in series,
reduces the power density by hindering the movement of ions towards the electrical double
layer on each electrode.
1 The ESR rating of a capacitor is a rating of quality. A theoretically perfect capacitor would be lossless and have an ESR of zero.
Chapter 6 Structural Supercapacitors
188
Series combination Parallel combination
Figure 6.18 Schematic of (a) series, or (b) parallel lay-up combination of two structural
supercapacitors.
When supercapacitors are arranged in parallel the voltage that can be applied is equal to the
lowest voltage rating of an individual supercapacitor. The capacitance equals the sum of all
the individual cells (C total = C1+C2+C3…). The structural supercapacitors tend to have a very
low supply voltage rating. A poor connection between supercapacitors can cause an even
higher charge loss than the internal resistance of the structural supercapacitors themselves.
Three structural supercapacitors were connected together prior to RIFTing and the charge-
discharge curves for different combinations are shown in Figure 6.19. The same conjugated
supercapacitor performed differently when connected in series or parallel combinations. The
charge as well as discharge capacities of series combination of conjugated structural
supercapacitors was around an order of magnitude less as compared to the parallel
combination of conjugated structural supercapacitors.
CF electrodes
GF separators
Chapter 6 Structural Supercapacitors
189
0 600 1200 1800 2400
-2x10-3
-1x10-3
0
1x10-3
2x10-3
(c)
(b)
(a)I
(A)
t (s)
Baseline (a) Series (b) Parallel (c)
600 1200 1800
-2x10-5
0
2x10-5
Figure 6.19 Charge/ discharge curves for the as-received CF and GF reinforced PEGDGE
containing 10 wt% EMITFSI (a) baseline, (b) three supercapacitors laid-up and tested in
series, or (c) three supercapacitors laid-up and tested in parallel.
The specific capacitances of different combinations of conjugated structural supercapacitor
are presented in Table 6.16. The results showed that the capacitance was doubled when
supercapacitors were joined together in parallel. However, when the same supercapacitors
were connected in series, the capacitance was reduced seven times. In the ideal case, the
overall capacitance of series combination should be 2.6 mF/cm3 and of the parallel
combination should be 23 mF/cm3 assuming each of the three conjugated supercapacitors had
equal specific capacitance.
Combinations Discharge
(mC)
Δ†
(%)
Cg
(mF/cm3)
Theoretical
Cg (mF/cm3)
One supercapacitor 11.7 17.7 7.75 N/A
Series 10.9 50.6 1.56 2.58
Parallel 129 57.6 18.6 23.2
Table 6.16 Discharge capacity, charge loss Δ, specific capacitance Cg and theoretical
specific capacitance of structural supercapacitor assembly made using as-received CF and
GF reinforced crosslinked PEGDGE containing 10 wt% EMITFSI connected either series or
parallel combinations. Charging time= 600 s
Density of supercapacitor is 1.78 ± 0.11 g/cm3
Chapter 6 Structural Supercapacitors
190
6.12 Influence of CF activation on the electrochemical and mechanical
performance of structural supercapacitors
Structural supercapacitors were also fabricated using activated carbon fibre mats as
electrodes (Section 3.5). Chemically activated CF mats were provided by Dr H. Qian and
were used as electrodes in supercapacitors because of their high surface area [102]. The
mechanical properties of the activated CF mats were similar to those of as-received CF mats
(Table 3.12). Copper tape and copper wire were used to connect each CF based electrode to
enable in-situ testing of electrochemical properties. The activation of CF mats was necessary
due to the dependence of specific capacitance mainly on the surface area of CF electrode. The
activation of CF mats formed nano-sized pits on fibre surface (as discussed in Section 2.4.6)
and therefore, increased the overall surface area of the electrode from 0.210 m2/g to 21.4 m2/g
(Table 3.12). This led to an increase in the number of sites for charge storage and thus,
increased the specific capacitance of the electrode [146].
6.12.1 Structural supercapacitors with a crosslinked PEGDGE matrix
containing10wt%EMITFSI
Activated CF and GF reinforced PEGDGE polymer electrolyte composites were fabricated
and were tested both electrochemically and mechanically. Figure 6.20 shows the impedance
plot for the structural supercapacitors fabricated by the impregnation of as-received CF and
ACF with PEGDGE polymer electrolyte containing 10wt% EMITFSI. It is evident that the
as-received CF based supercapacitor exhibited a smaller semicircle with low equivalent
distributed resistance (EDR), as compared to the ACF based supercapacitor. EDR arise from
the resistance offered by the ionic diffusion through the pores of the ACF electrodes and
therefore contributes to the overall resistance of the supercapacitor. Increased EDR of the
ACF based supercapacitor also confirmed the increase in the number of pores at the surface
of electrodes as compared to the CF based supercapacitor.
Chapter 6 Structural Supercapacitors
191
0 400 800 12000
80
160
240
(b)
(a)
-Z''
()
Z' ()
as-received CF (a) ACF (b)
100 Hz
1 Hz
100 kHz
Figure 6.20 Impedance plots for CF and GF reinforced crosslinked PEGDGE composites
with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.
Frequency range = 105 Hz to 1 Hz; Applied potential = 0.5 V.
The results of the charge-discharge experiment are shown in Figure 6.21. It was evident that
the as-received CF based supercapacitors showed lower charge loss (8% of the charging
capacity) as compared to the ACF based supercapacitors (47% of the charging capacity). The
ACF based supercapacitor had a high charge loss and incomplete charging and discharging
capacity after 600 s. The internal resistance was estimated from the final charging current
after the capacitive component died away. ACF based supercapacitors also showed high
internal resistance as compared to the as-received CF based electrodes which was in
comparison to the impedance study. The increase in the charging and discharging capacity of
the ACF based supercapacitors can be attributed to the high surface area of electrodes due to
nano-sized pit formation during CF activation. This resulted in an increased number of ions
stored at the double layer formed at the electrode/electrolyte interface.
Chapter 6 Structural Supercapacitors
192
-4x10-4
-2x10-4
0
2x10-4
4x10-4
I (A
)
t (s)
as-received CF (a) ACF (b)
600 s Charging time
0 600 1200 1800 2400
(b)
(a)
Figure 6.21 Charge/ discharge curves for CF and GF reinforced crosslinked PEGDGE
composites with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.
Charging time = 600 s.
Figure 6.22 Charge/ discharge curves for CF and GF reinforced crosslinked PEGDGE
composites with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.
Charging time = 1500 s.
Although, Figure 6.21 showed a clear advantage of ACF over CF based supercapacitors in
terms of electrochemical performance, the discharging part of the ACF based supercapacitors
was incomplete. Therefore, the charging time of supercapacitors was increased from 600 s to
1500 s in order to allow for a complete discharge and to reduce the charge loss during
-4x10-4
-2x10-4
0
2x10-4
4x10-4
(a)
(b)
I (A
)
CF/PEGDGE/GF (a) ACF/PEGDGE/GF (b)
0 1000 2000 3000 4000
t (s)
1500 s of Charging time
Chapter 6 Structural Supercapacitors
193
charging of ACF based supercapacitor. At 1500 s of charging time, the ACF based structural
supercapacitor exhibited a decreased charge loss from 47% to 8% of the charging capacity
(Figure 6.22).
The specific capacitance data obtained from charge-discharge experiments as well as
impedance spectroscopy for the as-received CF and ACF based supercapacitors at 600 s as
well as 1500 s of charging time is presented in Table 6.17. Irrespectively of the charging
time, the specific capacitance of the ACF based supercapacitor was around eight times higher
than that of the CF based supercapacitor. In comparison to the two orders of magnitude
increase in the BET surface area of ACF electrodes (Table 3.12), the increase in specific
capacitance of structural supercapacitors when using ACF electrodes was low. This may be
attributed to the lower ion content of the polymer electrolyte. However, similar behaviour
was also observed by Snyder et al. [102] who observed 7 times increase in specific
capacitance of structural supercapacitors when using HTA-modified ACF. Not only the
specific capacitance but also the energy density showed a remarkable increase when using
ACF (Table 6.17). However, the power density remained almost the same. Since power
density is dependent on how fast the ions moves to form the double layer at the
electrode/electrolyte interface and thus is mainly dependent on the ionic conductivity of the
polymer electrolyte. On the other hand, the energy density is dependent on number of ions
stored at the electrode/electrolyte interface. The increased number of ions stored at the
electrode surface led to increased charge storage and thus, increased capacitance and energy
density.
Electrodes Charging time (s) Δ† (%) Cg (mF/cm3) ESR (Ω) E (Wh/kg) P (W/kg)
as-received
CF
600 8.14 10.3 8.63
0.010 18.0
1500 4.12 13.1 0.013
ACF 600 47.1 86.1
10.3 0.081
16.1 1500 8.51 96.3 0.091
Table 6.17 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and power density P of structural supercapacitors made from as-received CF or
ACF and GF reinforced crosslinked PEGDGE matrix containing 10 wt% EMITFSI at a
charging time of 600 s or 1500 s.
Density of supercapacitors = 1.78 ± 0.11 g/cm3.
Chapter 6 Structural Supercapacitors
194
Table 6.18 shows the shear properties of the composites reinforced by as-received CF or ACF
and GF separators. The shear modulus as well as shear strength remained almost same
following the activation of CF. The results were normalised to a fibre volume fraction of 55%
in order to make the shear results comparable with the rest of the shear properties of the
studied structural supercapacitors as shown in Table 6.18.
Electrodes
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
as-received CF 6.12 ± 0.24 1.54 ± 0.30 353 ± 27.8 8.83 ± 0.35 510 ± 40.1
ACF 6.15 ± 0.74 1.34 ± 0.10 292 ± 16.0 8.88 ± 1.07 422 ± 23.1
Table 6.18 Influence of CF activation on the maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of CF and GF reinforced crosslinked PEGDGE matrix
composites containing 10 wt% EMITFSI.
†Normalised shear properties at carbon fibre volume fraction of 55%original shear properties
carbon fibre vf×55; The thickness of
composites was 0.78 ± 0.02 mm and the carbon fibre volume content of the composites was 38.1%.
Photographs of the failed in-plane shear specimens with CF and ACF reinforcements are
shown in Figure 6.23. Delamination was observed in both specimens possibly due to the soft
rubbery resin-nature of the matrix between the CF and GF plies. Stress whitening was also
observed in the failed specimens due to the cracking of matrix.
Figure 6.23 Photographs of CF and GF reinforced crosslinked PEGDGE composites
containing 10 wt% EMITFSI after in-plane shear testing with (a) as-received carbon fibre, or
(b) activated carbon fibre (ACF) reinforcements.
Chapter 6 Structural Supercapacitors
195
6.12.2 Structural supercapacitorswith a crosslinked 40:60 PEGDGE/DGEBA
blendmatrixcontainingdifferentEMITFSIconcentrations
The ACF based electrodes were also used as reinforcement to fabricate a supercapacitor with
crosslinked 40:60 PEGDGE/DGEBA blend matrix containing 10 wt% EMITFSI. The
EMITFSI concentration in the matrix of structural supercapacitors was raised to study the
influence of CF activation on their electrochemical and shear properties. The polymer
electrolyte based on a crosslinked 40 to 60 weight ratio of PEGDGE to DGEBA containing
50 wt% of EMITFSI performed best in terms of ionic conductivity and compression
properties (Section 4.5.3). The results for the as-received CF reinforced structural
supercapacitor were already presented in the previous section (Section 6.6). The semicircular
shape of the Nyquist plots (Figure 6.25) for all three composites were similar to those
previously studied. An equivalent series resistance (ESR) of 0.7 Ω, 3.6 Ω and 304 Ω was
found for the supercapacitors with 100 wt% EMITFSI, 50 wt% EMITFSI in crosslinked
40:60 PEGDGE/DGEBA blend matrix and 10 wt% EMITFSI in crosslinked 40:60
PEGDGE/DGEBA blend matrix (Table 6.19).
0 100000 200000 3000000
30000
60000
90000
120000
100 kHz
(c)
(b)
-Z''
()
Z' ()
100wt% EMITFSI (a) 50wt% EMITFSI+40P60B (b) 10wt% EMITFSI+40P60B (c)
(a)1 Hz
0 20 400
50
100
150
Figure 6.24 Nyquist plot for the ACF and GF reinforced crosslinked 40:60
PEGDGE/DGEBA blend matrix composites containing various concentrations of EMITFSI.
Frequency range = 105 Hz to 1 Hz; Applied potential = 0.5 V.
The results of the charge-discharge experiments for the ACF and GF reinforced crosslinked
40:60 PEGDGE/DGEBA blend matrix composites containing various concentrations of
EMITFSI are presented in Figure 6.25. It is clear that the supercapacitor containing 10 wt%
EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix had the poorest charge
Chapter 6 Structural Supercapacitors
196
discharge capacity. There was also an obvious internal resistance underlying the exponential
capacitive charging current of the supercapacitor with 10 wt% EMITFSI in crosslinked 40:60
PEGDGE/DGEBA blend matrix. Structural supercapacitor with crosslinked 40:60
PEGDGE/DGEBA blend matrix containing 10wt% EMITFSI had the highest charge loss
(75% of the charging capacity) whilst as expected the pure EMITFSI based supercapacitor
had the lowest charge loss (4% of charging capacity).
-4x10-3
-2x10-3
0
2x10-3
(c)
(b)
(a)
I (A
)
t (s)
100wt% EMITFSI (a) 50wt% EMITFSI+40P60B (b) 10wt% EMITFSI+40P60B (c)
Charging
0 600 1200 1800 2400
600 610 6200
3x10-6
6x10-6
1200 1210 1220-2x10-6
-1x10-6
0
Discharging
Figure 6.25 Charge-discharge curves for the ACF and GF based supercapacitors with (a)
100 wt% EMITFSI, (b) 50 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend
matrix, and (c) 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix.
The energy density E and the power density P of the supercapacitor (Table 6.19) decreased
with the decreasing concentration of EMITFSI in the crosslinked 40:60 PEGDGE/DGEBA
blend matrix. The decrease in power density was attributed to the low ionic conductivity of
the polymer electrolyte that resulted in a decrease in the number of ions accessing the pores
of the CF electrodes.
Chapter 6 Structural Supercapacitors
197
EMITFSI (wt%) Matrix Δ (%) Cg (mF/cm3) ESR (Ω) E (Wh/kg) P (W/kg)
100 N/A 4.28 189 ± 3.65 0.69 0.181 765
50 40P:60
B
12.2 76.4 ± 6.22 3.61 0.072 42.4
10 75.1 0.49 ± 0.09 304 4.8E-4 0.52
Table 6.19 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and power density P of structural supercapacitors made from ACF and GF
reinforced crosslinked 40:60 PEGDGE/DGEBA blend matrix composites containing various
concentrations of EMITFSI.
Density and thickness of laminates were 1.78 ± 0.11 g/cm3 and 0.78 ± 0.03 mm, respectively.
The decrease in concentration of EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend
matrix deteriorated the electrochemical performance of supercapacitors (Table 6.10) but at
the same time, the structural performance of the laminates was improved (Table 6.20). The
composites with 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix had
a three time larger shear moduli and around nine times higher shear strength as compared to
composites containing the same fibres but 50 wt% EMITFSI in crosslinked 40:60
PEGDGE/DGEBA blend matrix. The shear properties for the supercapacitor having pure
EMITFSI as electrolyte were not measured since the liquid electrolyte has no structural
performance. The reduction in the moduli and strength of the composites with increasing
EMITFSI concentration was indicative of the decreased matrix resin modulus (Section 6.6).
EMITFSI
(wt%) Matrix
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
100 No shear properties measured due to liquid electrolyte
50 40P:60B
7.52 ± 0.66 4.41 ± 0.48 762 ± 21.9 10.9 ± 0.95 1100 ± 31.6
10 64.5 ± 4.24 8.97 ± 0.92 1870 ± 194 93.1 ± 6.12 2699 ± 280
Table 6.20 Maximum shear strength τ12m, shear strength at 5000 µε and shear modulus G12 of
ACF and GF based supercapacitors with crosslinked 40:60 PEGDGE/DGEBA blend matrix
containing increasing amounts of EMITFSI.
Composite thickness = 0.78 ± 0.02 mm; CF Vf = 37.1 vol%
Photographs of the post-in-plane shear tested structural supercapacitor specimens containing
ACF and GF as reinforcements in a crosslinked 40:60 PEGDGE/DGEBA blend matrix with
Chapter 6 Structural Supercapacitors
198
increasing EMITFSI concentrations are shown in Figure 6.26. Delamination was observed in
structural supercapacitor specimens containing 50 wt% EMITFSI. Fibre reorientation
(scissoring) was also observed in both specimens in the direction of applied load. Stretching
of the fibres resulted in a necking behaviour at the middle of the structural supercapacitor
specimens containing 10 wt% EMITFSI was observed. Matrix cracking was also observed in
both specimens.
Figure 6.26 Photographs of ACF and GF reinforced crosslinked 40:60 PEGDGE/DGEBA
blend matrix with increasing amounts of EMITFSI after shear testing; (a) 10 wt% EMITFSI
and (b) 50 wt% EMITFSI.
6.12.3 Structural supercapacitors with a crosslinked MSP/PEGDGE matrix
containing10wt%EMITFSI
In order to further enhance the electrochemical as well as mechanical performance of
structural supercapacitors, the MSP reinforced crosslinked PEGDGE matrix containing 10
wt% EMITFSI was further reinforced with activated carbon fibres. The structural
supercapacitors were copper taped around the edges and were connected with copper wire.
The incorporation of 7.5 wt% MSP into crosslinked PEGDGE matrix containing 10 wt%
EMITFSI and as-received CF resulted in a remarkable improvement in the energy as well as
power densities and shear properties (Section 6.10). Therefore, it was decided to investigate
the effect of CF activation on the performance of MSP reinforced crosslinked PEGDGE
matrix structural supercapacitors with the aim to further improve their energy density.
The Nyquist plots for the as-received CF or ACF and GF reinforced MSP/PEGDGE polymer
electrolyte containing 10 wt% EMITFSI are shown in Figure 6.27. An equivalent series
resistance of 5.07 Ω and 5.14 Ω was measured for the supercapacitors containing as-received
CF and ACF based structural supercapacitors, respectively (Table 6.21). It is worth noting
that the ESR remained unaffected by the activation of CF electrodes. However, the EDR in
Chapter 6 Structural Supercapacitors
199
ACF based structural supercapacitors was increased due to increase in number of pores as
compared to the CF based supercapacitors.
0 50 100 150 2000
15
30
45
60
100 kHz
- Z
'' (
)
Z' ()
as-received CF (a) ACF (b)
(a)
(b)
1 Hz
100 Hz
Figure 6.27 Complex impedance plots for CF and GF reinforced crosslinked MSP/PEGDGE
matrix containing 10 wt% EMITFSI with (a) as-received CF electrode, or (b) ACF
electrodes.
Frequency range = 105 Hz to 1 Hz; Applied potential = 0.5 V.
Figure 6.28 shows the charge-discharge curves for the supercapacitors made from activated
CF or as-received CF electrodes, GF separators and MSP/PEGDGE containing 10 wt%
EMITFSI as matrix. The charge-discharge curves showed that the charge loss increased as
the as-received CF electrodes were replaced with ACF based electrodes in the fabrication of
structural supercapacitors. This high charge loss (46% of charging capacity) in ACF based
structural supercapacitor having MSP/PEGDGE matrix was further reduced by increasing the
charging time from 600 s to 1500 s. Figure 6.29 shows the charge-discharge plot for the as-
received CF and ACF based structural supercapacitors characterised at 1500 s of charging
time. The increase in charging time reduced the charge loss of the system from 46% to 12%
of charging capacity (Table 6.21). However, ideally, a supercapacitor should charge fully in
fraction of seconds but due to the low surface of CF electrodes as compared to the
commercially available electrodes and the high internal resistance of the structural
supercapacitors, high charging time was required.
Chapter 6 Structural Supercapacitors
200
-6x10-4
-3x10-4
0
3x10-4
6x10-4
600 s of charging time
(b)I
(A)
t (s)
as-received CF (a) ACF (b)
(a)
0 600 1200 1800 2400
Figure 6.28 Charge/discharge curves for (a) as-received CF, or (b) ACF electrodes and GF
reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI.
Charging time = 600 s.
Figure 6.29 Charge/discharge curves for (a) as-received CF, or (b) ACF electrodes and GF
reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI.
Charging time = 1500s.
The electrochemical data from the charge-discharge experiments and impedance
spectroscopy are summarised in Table 6.21. The composites showed a significant
improvement in specific capacitance as well as energy density by introduction of activated
CFs as electrodes during fabrication of structural supercapacitors. However, power density of
-8x10-4
-4x10-4
0
4x10-4
8x10-4
(a)
(b)
I (A
)
CF/MSP/PEGDGE/GF (a) ACF/MSP/PEGDGE/GF (b)
0 1000 2000 3000 4000
t (s)
1500 s of Charging time
Chapter 6 Structural Supercapacitors
201
structural supercapacitors remained unaffected as it is dependent on the ionic conductivity of
polymer electrolyte.
Electrodes Charging time (s) Δ† (%) Cg (mF/cm3) ESR (Ω) E (Wh/kg) P (W/kg)
as-received
CF
600 11.7 15.2 5.07
0.016 35.7
1500 1.02 20.0 0.020
ACF 600 46.1 94.0
5.14 0.092
35.5 1500 11.9 112 0.110
Table 6.21 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy
density E and power density P of structural supercapacitors made using CF or ACF and GF
reinforced crosslinked MSP/PEGDGE matrix containing 10 wt% EMITFSI at a charging
time of 600 s or 1500 s.
Density of supercapacitor is 1.78 ± 0.11 g/cm3.
The shear properties of the composites are presented in Table 6.22. As seen before the shear
properties remained unaffected by using the ACF in the structural supercapacitors (Table
6.22). There was minor improvement in the shear modulus from 1.5 GPa to 1.8 GPa as the
as-received CF electrodes were replaced with ACF electrodes in structural supercapacitors.
This minor improvement could be attributed to the improvement of the fibre/matrix interface.
Since the activation of CF introduced nano-sized pits at the fibre surface, the crosslinked
polymer electrolyte formed a stronger bond with rough surfaced CF and thus improved the
mechanical performance of composites.
Electrodes
Shear properties Normalised shear properties to CF Vf = 55%
12m/MPa 12
0.5/MPa G12/MPa 12m/MPa 12/MPa
as-received CF 39.4 ± 3.09 7.12 ± 0.98 1470 ± 253 56.9 ± 4.46 2122 ± 365
ACF 38.6 ± 2.07 8.44 ± 0.55 1763 ± 342 55.7 ± 2.99 2545 ± 494
Table 6.22 Influence of CF activation on the maximum shear strength τ12m, shear strength at
5000 µε and shear modulus G12 of CF and GF reinforced crosslinked MSP/PEGDGE matrix
composites containing 10 wt% EMITFSI.
Thickness of composites was 0.78 ± 0.02 mm and the carbon fibre volume content of the composites 38.1 vol%.
Chapter 6 Structural Supercapacitors
202
Photographs of the post-in-plane shear tested structural supercapacitor specimens containing
as-received CF or ACF and GF reinforced crosslinked MSP/PEGDGE matrix are shown in
Figure 6.30. As discussed in previous sections (Section 6.10), whitening due to matrix
cracking and delamination of CF and GF layers were also observed in the failed specimens.
The fibres in both CF and GF mats were reoriented in the direction of the applied load
inducing interlaminar stresses resulting in delamination at the ply interfaces.
Figure 6.30 Photographs of structural supercapacitors consisting of crosslinked
MSP/PEGDGE matrix containing 10 wt% EMITFSI, GF separator and (a) as-received CF or
(b) ACF reinforcements after in-plane shear testing.
6.13 Multifunctionality of structural supercapacitors
The Ragone plot (Figure 6.31) correlates the specific energy (energy density) with the
specific power (power density) of structural supercapacitors and is similar to the one plotted
elsewhere [91]. Figure 6.31 demonstrates that all the structural supercapacitors with ACF
electrodes lie in the supercapacitor range. However, all structural supercapacitors made with
CF electrodes were positioned in the capacitor range of the Ragone plot. This is possibly due
to the low surface area of CF electrodes (0.21 m2/g) in comparison to that of the ACF
electrodes (21 m2/g). The supercapacitor fabricated using ACF electrodes, GF separator and
pure EMITFSI ionic liquid outperforms the other supercapacitors but it has no structural
properties. The energy density and power density of the ACF based structural supercapacitors
is about two orders of magnitude below that of commercial supercapacitors. However, the
fabricated structural supercapacitors have an edge in possessing mechanical properties as
compared to the commercial storage devices.
Chapter 6 Structural Supercapacitors
203
0.01 0.1 1 10 100 1000100
102
104
106
CF/Ea:40P:60B/GF
CF/E10
:xP:zM/GF
ACF/E10
:P90
/GF
ACF/Ea:40P:60B/GF
ACF/E10
:xP:zM/GF
P (W
/kg)
E (Wh/kg)
Supercapacitors
Capacitors
Batteries
Fuel cells
Figure 6.31 Ragone plot relating energy density E to the power density P of studied
structural supercapacitors in comparison to other energy storage devices.
CF-as-received carbon fibre mat; ACF- activated carbon fibre mat; GF-glass fibre mat; P-crosslinked PEGDGE;
B- crosslinked DGEBA; E-EMITFSI; M-mesoporous silica particles.
Figure 6.32 summarises the multifunctional performance of the structural supercapacitors
fabricated and characterised, mechanically as well as electrochemically, as discussed in
previous sections. The plot correlates the specific capacitance with an in-plane shear modulus
of the structural supercapacitors. Materials with multifunctional performance in terms of
specific capacitance and shear modulus will lie in the upper right quadrant of the plot.
Reference lines were also drawn in Figure 6.32 between the in-plane shear modulus (24.2
GPa [201]) of commercial structural composite with ±45° fibre orientation and the specific
capacitance of commercial supercapacitors (8 F/cm3 [202]). Another reference line was also
drawn between the in-plane shear modulus of crosslinked DGEBA composites (4.8 GPa) and
the specific capacitance of ACF/pure EMITFSI/GF supercapacitor in order to compare the
results within the limits of the current study. ACF and GF reinforced crosslinked
PEGDGE/DGEBA composites containing 50 wt% EMITFSI outperformed the ones
containing only 10 wt% EMITFSI electrochemically. However, ACF and GF reinforced
crosslinked MSP/PEGDGE electrolyte composites showed a unique combination of specific
capacitance and shear modulus (Figure 6.32) indicating that these composites do deliver
useful multifunctional performance.
Chapter 6 Structural Supercapacitors
204
10-4 10-2 100 102 10410-1
100
101
102
CF/E10
:P90
/S
CF/E10
:B90
/S
CF/E10
:xP:yB/GF
CF/Ea:40P:60B/GF
CF/E10
:xP:zM/GF
ACF/E10
:P90
/GF
ACF/Ea:40P:60B/GF
ACF/E10
:xP:zM/GF
G12
(G
Pa)
Cg (mF/cm3)
Increasingmultifunctionality
Commercial supercapacitor
Structuralcomposite
ACF/E100
/GF
CF/B100
/GF
Figure 6.32 Multifunctional plot of studied structural supercapacitors relating shear modulus
G12 to the specific capacitance Cg.
CF- as received carbon fibre mat; ACF- activated carbon fibre mat; S- different separators including GF mat,
polypropylene membrane and filter paper; GF-glass fibre mat; P-crosslinked PEGDGE; B- crosslinked DGEBA;
E-EMITFSI; M-mesoporous silica particles.
Chapter 7 Conclusion and Future Works
205
Chapter 7 Conclusions and
Suggestions for Future Work
This chapter concludes the dissertation by summarising the major findings of the current
study and presents suggestions to further extend the research. In this dissertation, an effort
has been made to provide a new perspective to the multifunctionality of structural
composites. The current study has defined a broad framework to fabricate novel structural
composites that can store electrical energy and bear mechanical loads simultaneously.
Chapter 7 Conclusion and Future Works
206
7.1 Conclusions
The research presented a novel holistic concept of multifunctional materials based on carbon
and glass fibre reinforced composite systems that have dual functionalities; store electrical
energy and carry mechanical load. The carbon and glass fibre reinforced composites were
fabricated into a supercapacitor. Structural supercapacitors could be conceivably applied to
the load-carrying structures that require electrical energy, such as unmanned aerial vehicles
and ground electrical vehicles, mobile phones and laptops. The research into the optimisation
of mechanical and electrochemical functionalities of structural supercapacitors was
challenging because both functionalities have conflicting requirements; the improvement in
one functionality leads to the loss in other functionalities. Therefore, a thorough study was
conducted to explore the performance of individual subcomponents of structural
supercapacitors. This involved the implementation of a holistic research approach by
embracing the optimisation of the mechanical and electrochemical performance of the
individual subcomponents of structural supercapacitors including the carbon fibre based
electrodes, separator and polymer matrix. The research work is in its early stages and requires
considerable further development. This dissertation demonstrated the basic concepts. The
research demonstrated in this dissertation can be broadly divided into three main aspects
which are summarised in the following sections.
7.1.1 Developmentsofthepolymerelectrolytes
Three different polymer electrolytes, including, poly (ethylene glycol) diglycidylether
(PEGDGE), diglycidylether of bisphenol-A (DGEBA) and polyacrylonitrile (PAN) gel
polymer electrolytes, were formulated using different salts, as the ion source, and the
characterisation of the mechanical and electrochemical performance of these structural
polymer electrolytes was discussed in Chapter 4. Since the PAN based polymer electrolyte
was a gel, oscillatory rheological characterisation of PAN was carried out in order to
characterise its mechanical properties. The mechanical properties of crosslinked PEGDGE
and crosslinked DGEBA electrolytes were tested in compression because the testing required
a small amount of material. Structural supercapacitors require a very ion conductive but
mechanically strong polymer matrix. It was demonstrated that the pure gel electrolyte (PAN)
was clearly unsuitable for the role of polymer matrix in the development of structural
supercapacitors because of poor mechanical performance and therefore, was considered as
the control sample/matrix for a purely electrical device.
Chapter 7 Conclusion and Future Works
207
Crosslinked PEGDGE electrolytes were soft rubber-like matrices (compression modulus of 9
MPa) but showed reasonable electrochemical performance (ionic conductivity of 27.6
µS/cm). Crosslinked DGEBA electrolytes were brittle glass-like matrices with high
mechanical performance (compression modulus of 3068 MPa) but showed very poor
electrochemical performance (ionic conductivity of 3.6 µS/cm). The polymer electrolytes
under consideration contained a complex interlay of variables affecting both ionic
conductivity and compression modulus, including the amount and type of crosslinkers, length
and concentration of polyether, concentration and type of salt as well as degree of
crosslinking of the polymer matrix. Overall, it was found that increase in ion conductivity
resulted in a decreased mechanical performance. This suggested the formulation of polymer
electrolytes with a broad spectrum of mechanical and electrochemical behaviour spanning
from a highly ion conductive but structurally weak polymer electrolyte (PAN) to a highly
structural but poorly ion conductive (DGEBA) polymer electrolyte.
The electrochemical and mechanical performance of PAN and crosslinked PEGDGE polymer
electrolytes were improved by changing different parameters including the type of salt and
salt/polymer weight ratios. The storage modulus of PAN gel polymer electrolyte was
increased from 102 Pa to 105 Pa and the ionic conductivity of PAN based polymer electrolyte
was increased from 1.4 mS/cm to 3.8 mS/cm by using Li+ salt and increasing the polymer
concentration (as discussed in Section 4.2). The compression modulus and ionic conductivity
of crosslinked PEGDGE polymer electrolyte was increased by using different salts (as
discussed in Section 4.3.1). It was also demonstrated that amongst different salts studied for
polymer electrolytes, EMITFSI resulted in the best mechanical and electrochemical
performance. Different other salts studied in this work including LiTFSI and TBAPF6 were
solid and therefore required a plasticiser for the matrix (propylene carbonate). The addition of
propylene carbonate into the PEGDGE matrix, containing LiTFSI salt, resulted in improved
ion conductivity (from 7.9 µS/cm to 17 µS/cm) but negatively affected the compression
modulus (decreased from 14.8 MPa to 10.2 MPa). EMITFSI was an ionic liquid and therefore
did not require propylene carbonate. The addition of 0.8 wt% EMITFSI into crosslinked
PEGDGE matrix resulted in an ionic conductivity of 19.5 µS/cm and a compression modulus
of 12.2 MPa.
In order to further increase the mechanical properties without affecting the ionic conductivity
of polymer electrolytes, DGEBA was added to crosslinked PEGDGE polymer electrolytes
Chapter 7 Conclusion and Future Works
208
with 10 wt% EMITFSI and the compression modulus was increased by 2-4 orders of
magnitude (Section 4.5). An increase in compression modulus and ionic conductivity was
also observed by the increasing the EMITFSI concentration from 10 wt% to 50 wt% in
crosslinked PEGDGE/DGEBA electrolyte. A polymer electrolyte with an ionic conductivity
of 500 µS/cm and a compression modulus of 30 MPa was formulated by optimising the ratio
of PEGDGE, DGEBA and EMITFSI (Section 4.5.3). Overall, the results suggested that a neat
homo-polymer electrolyte will be unlikely to meet the structural as well as electrochemical
needs of the polymer electrolyte. The addition of bisphenol-A functional groups and
reduction of PEG content in the polymer electrolyte result in decreased ion conductivity but,
at the same time, increased compression modulus.
7.1.2 Developmentsofthepolymercompositeelectrolytes
One of the two different approaches used in the development of multifunctional polymer
electrolytes was the addition of high strength brittle matrix (crosslinked DGEBA) into the
low strength soft matrix with reasonable electrochemical performance (crosslinked
PEGDGE) for optimising the multifunctionality of structural polymer electrolytes (as
discussed in previous section 7.1.1). Another approach followed was the addition of
inorganic fillers into the polymer electrolytes with the aim to further improve the
electrochemical and mechanical performance. Mesoporous silica was considered best for the
improvement of the multifunctionality of polymer electrolyte due to the insulating nature of
silica inhibiting the electronic movement but at the same time facilitating the ionic movement
due to the presence of mesopores throughout the silica particles. The introduction of nano-
structured mesoporous silica particle reinforcement into the polymer electrolyte allowed the
matrix to successfully perform dual roles of mechanical and electrochemical functionalities.
Undoubtedly, there is further optimisation required in the pore size distribution and the
particle size in order to further improve the interfacial interactions of polymer electrolyte and
silica particles which is considered to be the controlling factor of stiffness and interfacial
performance.
Mesoporous silica particles and monoliths were prepared (Chapter 3) and characterised
(Chapter 5). Mesoporous silica monoliths had a poor mechanical performance when used as
reinforcements in crosslinked PEGDGE polymer electrolyte due to a poor monolith/polymer
interface. A clear phase separation between the monolith and PEGDGE matrix was observed
(Section 5.2.1.1). Therefore, another approach was chosen i.e. the addition of mesoporous
Chapter 7 Conclusion and Future Works
209
silica particles to crosslinked PEGDGE and crosslinked DGEBA polymer electrolytes as a
means to achieve high ionic conductivity as well as compression modulus. The introduction
of MSP into the polymer electrolyte resulted in an order of magnitude improvement in the
modulus and ionic conductivity. The increase of the ambient temperature ionic conductivity
was attributed to the dissociation of ion-pairs in the polymer matrix and therefore, based on
the surface acid-base property of such particles, these dissociated anions or cations were then
adsorbed on the surface leading to higher counter ion concentration in the vicinity of the
oxide (space charge layer) [196]. Wieczorek et al. [127] had also previously explained that
the enhancement of ionic conductivity of silica reinforced polymer electrolytes was due to the
Lewis acid base type interactions among surface centres, ions and ether-oxygen base groups
of the polymer electrolyte which were indicative of the filler/polymer interactions influence
on the cationic transport within the polymer electrolyte. The increase in compression
modulus was attributed to the addition of hard inclusions (MSP).
The presence of mesopores in the silica particles also facilitated the improvements in
electrochemical and mechanical properties of structural polymer electrolytes. This was
further confirmed by introducing comparable sized non-porous silica particles (NSP) into the
polymer electrolyte. The NSP addition into the structural polymer electrolytes also resulted in
improved ionic conductivity but the MSP incorporation showed much better ionic
conductivity (three times improvement) as compared to the NSP reinforcements without
affecting the mechanical performance (Section 5.2.1.4). A polymer composite electrolyte
with an ionic conductivity of 0.8 mS/cm and a compression modulus of 62 MPa was
formulated by optimising the PEGDGE, DGEBA, EMITFSI and MSP concentrations
(Section 5.2.4).
The major hindrance in the utilisation of MSP for the use of reinforcement in structural
polymer electrolytes and later for the structural supercapacitors was the slow production of
MSP. Currently, a maximum of 10g - 15 g silica particles could be produced in 5 days and a
minimum of 45-50 g MSP was required for the fabrication of a single supercapacitor
composite using a RIFT. Both the mechanical and electrochemical performance of
crosslinked PEGDGE polymer electrolytes were improved by the addition of MSP which
played a vital role in the augmentation of improved electrochemical and shear properties in
structural supercapacitors.
Chapter 7 Conclusion and Future Works
210
7.1.3 Developmentsofthestructuralsupercapacitors
The characterisation of the mechanical and electrochemical performance of structural
supercapacitors was discussed in Chapter 6. The effects of separator type and thickness,
polymer electrolyte and activation of CF electrodes on the mechanical and electrochemical
performance of structural supercapacitors were investigated. A structural supercapacitor with
as-received CF electrodes, crosslinked PEGDGE containing 0.8 wt% LiTFSI/PC as polymer
electrolyte and filter paper as separator was fabricated. The major problem observed during
the electrochemical characterisation of this structural supercapacitor was the huge charge loss
(70% of charging capacity) and incomplete charging (Section 6.1). The huge charge loss was
attributed to the poor ionic conductivity of the polymer electrolytes and the poor current
collection from the CF based electrodes during charging.
Different separators, including woven glass fibre mats and polypropylene membrane, were
also studied to optimise the mechanical and electrochemical properties of structural
supercapacitors. Glass fibres were selected because they are good insulators and have good
mechanical properties, adding to the mechanical performance of supercapacitors. Five
different thicknesses of glass fibre mats were studied (Section 6.1). The specific capacitance
was dependent on the distance between the electrodes [32]. Therefore, the electrochemical
performance of a supercapacitor could be improved by employing a thin separator. However,
a huge charge loss was observed in all the cases studied (more than 70% of charging
capacity). This was due to the relatively open weave of the glass fibre mat allowing the short
circuiting. Among the different separators studied (Section 6.4), the specific capacitance of
structural supercapacitors with a polypropylene membrane as a separator (9.9 mF/cm3) was
best as compared to the ones with glass fibre mats (8.8 mF/cm3) and filter paper (7.0
mF/cm3). However, supercapacitors with PP membrane separators had poor shear properties
resulting in premature delamination and early mechanical failure due to poor
membrane/polymer electrolyte adhesion. Glass fibre mat was best due to the reduced charge
loss as well as reasonable specific capacitance. In shear testing of the structural
supercapacitor specimens, scissoring of the carbon fibres of the ±45° laminates was observed
and therefore, the shear testing was stopped before the start of scissoring.
The effect of increasing charging time on the electrochemical performance of the structural
supercapacitors during charge-discharge experiment was also studied. It was observed that
the charge storage was directly proportional to the charging time. Surprisingly, the charge
Chapter 7 Conclusion and Future Works
211
loss was also reduced with an increase in charging time. The charge loss estimated from the
difference between the charging and discharging capacity during the charge-discharge
experiment of structural supercapacitors, was reduced from 70% of charging capacity to 13%
of charging capacity by increasing the charging time from 10 s to 10 min (Section 6.2).
The mechanical and electrochemical performance of structural supercapacitors with different
salts including LiTFSI and EMITFSI were also studied. It was observed that the composites
containing EMITFSI performed best mechanically as well as electrochemically (Section 6.3).
The influence of the polymer electrolyte composition on the mechanical and electrochemical
performance of structural supercapacitors was also studied. It was observed that the specific
capacitance decreased with an increasing concentration of DGEBA in crosslinked PEGDGE
polymer electrolyte due to reduced ion mobility. However, the shear properties were
improved. A composite based on crosslinked 40:60 PEGDGE/DGEBA blend matrix with
10wt% EMITFSI concentration showed an optimised shear modulus (83 MPa) and specific
capacitance (0.1 mF/cm3) as discussed in section 6.5. In order to further optimise the specific
capacitance, it was tried to increase the EMITFSI concentration from 10 to 50 wt% in the
crosslinked 40:60 PEGDGE/DGEBA blend matrix. The structural supercapacitors showed an
improvement in the power density but the specific capacitance as well as mechanical
performance of the composite remained almost same as the PEGDGE matrix containing
10wt% EMITFSI based structural supercapacitors. The influence of MSP addition to
crosslinked PEGDGE electrolyte based structural supercapacitors was also studied (Section
6.10). The energy density, power density and the shear modulus increased from 0.010 Wh/kg,
16.1 W/kg and 0.35 GPa to 0.016 Wh/kg, 35.5 W/kg and 1.5 GPa respectively.
The specific capacitance of structural supercapacitors was also improved by improving the
electrical connectivity of CF electrodes. This was achieved by attaching copper wires to the
CF mats and by sealing the edges of the CF mats with copper tape. It was observed that the
use of copper wire and copper tape in structural supercapacitors slightly reduced the charge
loss but the problem of charge loss was not completely eliminated (Section 6.7). The
performance of structural supercapacitors during 1000 charge-discharge cycles was also
studied and it was demonstrated that the specific capacitance remained almost constant
throughout the 1000 cycles of charging and discharging (Section 6.8). Charge-discharge
experiments were also conducted at the working potentials of up to 3.5 V (Section 6.9). It
was observed that the specific capacitance of the structural supercapacitors remained almost
Chapter 7 Conclusion and Future Works
212
constant by increasing the working potential of the experiment. In order to further enhance
the performance of structural supercapacitors, thicker laminates were also fabricated by
laying up three structural supercapacitors together to form a supercapacitor “bank”. These
thick laminates were characterised electrochemically by connecting them in series or parallel.
The specific capacitance decreased in series combination and increased in parallel
combination of same laid-up structural supercapacitor.
The influence of the activation of CF based electrodes was also studied. It was observed that,
upon CF activation, the specific capacitance of the structural supercapacitors was improved
by an order of magnitude. The activation of CF based electrodes was necessary due to the
dependence of specific capacitance mainly on the structure of fibre electrode. The activation
of CF mats formed nano-sized pits on fibre surface and therefore, increased the overall
surface area of the electrode from 0.21 to 21 m2/g. This led to an increase in number of sites
for charge storage and thus, increased the specific capacitance of the electrode [146].
However, introduction of increased number of nano-sized pits on the surface of the fibre
electrode during activation could lead to a substantial reduction in mechanical properties.
Therefore, CF mats were activated only up to the extent at which the mats retained their
mechanical strength (Table 3.12). The ACF based composites showed improved charging and
discharging capacity as compared to the as-received CF based composites which was due to
the high surface area of electrodes due to nano-sized pit formation during CF activation. The
energy density also showed a remarkable increase by the CF activation from 0.01 to 0.08
Wh/kg in PEGDGE based composites. However, the activation of CF did not affect the
power density as the power density is dependent on the mobility of ions in the polymer
electrolyte. The shear modulus as well as shear strength remained almost unaffected by the
activation of CF in PEGDGE based composites.
The ACF based electrodes were also used as reinforcement for crosslinked 40:60
PEGDGE/DGEBA blend matrix containing 50 wt% EMITFSI (Section 6.12.2). A
supercapacitor using pure EMITFSI (no polymer) was also fabricated. A supercapacitor with
a crosslinked 40:60 PEGDGE/DGEBA blend matrix containing 10wt% EMITFSI led to the
highest charge loss value (75% of the charging capacity) whilst the pure EMITFSI based
supercapacitor performed best (4% of charging capacity). The laminate containing 10 wt%
EMITFSI in a supercapacitor with a crosslinked 40:60 PEGDGE/DGEBA blend matrix
resulted in an around three times improvement in the shear moduli and an around nine times
Chapter 7 Conclusion and Future Works
213
improvement in strength as compared to 50 wt% EMITFSI in the supercapacitor with a
crosslinked 40:60 PEGDGE/DGEBA blend matrix. The shear modulus as well as shear
strength remained almost unaffected by the activation of CF in a supercapacitor with
crosslinked 40:60 PEGDGE/DGEBA blend matrix.
Electrochemical and mechanical performance of structural supercapacitors was further
enhanced by using MSP reinforced PEGDGE as a polymer electrolyte along with activated
carbon fibre mat to fabricate a structural supercapacitor (Section 6.12.3). The crosslinked
MSP/PEGDGE composites containing ACF electrodes showed a significant improvement in
specific capacitance and energy density because of increase in surface area of electrodes. The
improved structural supercapacitor, having crosslinked MSP/PEGDGE, ACF, GF as polymer
electrolyte, electrodes and separator respectively, had an energy density of 0.1 Wh/kg, a
power density 36 W/kg and a shear modulus of 1.7 GPa.
7.2 Suggestion for future work
This dissertation details the capacity and benefit of using structural supercapacitors in
military and civilian applications. Although the performance of structural supercapacitors
could be improved considerably, e.g. the specific capacitance had been improved from 0.08
mF/cm3 (as-received CF and filter paper reinforced crosslinked PEGDGE polymer
composites containing 0.8 wt% LiTFSI/PC) to 112 mF/cm3 (ACF and GF reinforced
PEGDE/MSP polymer composites containing 10 wt% EMITFSI), there are still many aspects
of structural supercapacitors that require further research work. In order to understand the
relationship between the ionic conductivity and compression properties of the polymer
electrolytes or specific capacitance and the shear properties of structural supercapacitors, a
simple Group Interaction Modelling (GIM) [203, 204] could be a way forward. Other aspects
that require further research work include the following challenges:
7.2.1 Improvementsinthemultifunctionalityofpolymerelectrolytes
An ionic conductivity of 0.8 mS/cm and a compression modulus of 62 MPa for the polymer
electrolyte were achieved in the current study. However further improvement is required in
order to increase the electrochemical performance of structural supercapacitors. Recently,
number of ionic liquids [205, 206] was synthesised exhibiting high ionic conductivity (20
mS/cm). Therefore, other improved ionic liquids should be explored in polymer electrolytes.
The tensile and shear properties of structural polymer electrolytes should also be explored in
future. Further optimisation of ionic liquid and polymer matrix concentration is required.
Chapter 7 Conclusion and Future Works
214
Addition of mesoporous silica particles to polymer electrolytes leads to an increase in ionic
conductivity and compression properties of polymer composite electrolytes. A further
optimisation of mesoporous silica particles is required. A decrease in particle size and
improvement in porosity as well as surface area of the MSP can further improve the
multifunctional performance of the polymer electrolyte.
7.2.2 Improvementsintheenergydensityofstructuralsupercapacitors
The improvement in energy density is mainly dependent on the specific capacitance of the
structural supercapacitors. The specific capacitance of the structural supercapacitors was
improved from 0.08 mF/cm3 to 112 mF /cm3. At a working voltage of 3.5 V, this will be
equivalent to an energy density of around 0.1 Wh/kg. Current state of the art supercapacitors
have energy densities of up to 10 Wh/kg and modern batteries have energy densities of up to
250 Wh/kg. To compare structural supercapacitor technology against these competitors
(current state of the art supercapacitors), the energy density needs to be increased to atleast
two orders of magnitude. A major improvement in energy storage can be achieved by
increasing the surface area of the CF mat based electrodes. The activation of CF mats has
increased the surface area from 0.2 to 22 m2/g (Section 3.5) but a further increase in surface
area is required. This could be achieved by grafting carbon nanotubes [207] or by coating the
carbon fibre electrodes with activated carbon black powder having high surface area.
Structural supercapacitors with activated carbon black coated ACF electrodes were later
studied by MSc student J. Tu [208] and mentioned an improvement in specific capacitance
from 70 to 126 mF/cm3, without any significant change in shear properties, when carbon
black powder was coated at the surface of ACF electrodes.
A small improvement in electrochemical performance of structural supercapacitors was
achieved by reducing the distance between an electrode and insulator (i.e. using thin
insulators). This can be done by switching from either glass fibre mats to polymer
membranes or by using a thinner glass fibre mat which still has to avoid the CF/CF contact
through the separator. Another approach to improving the energy storage will be to move to
hybrid storage device having a combination of supercapacitor and battery or more precisely
pseudo supercapacitors in which charge-transfer pseudo capacitance is generated from
reversible Faradic reactions occurring on the surface of CF mat based electrodes. However,
there are number of hurdles involved in pursuing this idea of hybrid energy storage devices
e.g. the consumption of materials involved in the battery component during the charging and
Chapter 7 Conclusion and Future Works
215
discharging of the devices resulting in an early mechanical degradation or the corrosion of
structural carbon fibres or glass fibres due to the use of alkalis or acids involved in battery
chemistry.
7.2.3 Improvementsinthepowerdensityofstructuralsupercapacitors
At this stage, the structural supercapacitors showed a power density of around 42 W/kg at 3.5
V. Current state of the art supercapacitors exhibit a power density of up to 10,000 W/kg. This
challenge is considered to be the dominant one. The power density can be improved by either
decreasing the equivalent series resistance of the structural supercapacitors or increasing the
applied voltage. Electrical tests of carbon fibre mats suggest that the high resistance values in
structural supercapacitors are not associated with the in-plane weave resistivity (around 0.6
Ω). The most probable cause of high resistance is the poor ionic conductivity of the polymer
electrolyte matrix. The addition of MSP to polymer electrolytes and the optimisation of salt
concentration in the polymer matrix may have improved the ionic conductivity but further
tailoring of the matrix microstructure is required to improve the paths for the ion migration
within the polymer electrolyte matrix. Another way of lowering the internal resistance is to
decrease the polymer electrolyte thickness between two electrodes [32]. However, a portion
of ions are removed from the bulk of polymer electrolyte during the charging of structural
supercapacitor and form a double layer at the electrode/electrolyte interface. This resulted in
a decrease of ionic concentration and eventually the ionic conductivity of the polymer
electrolyte. If the thickness of the polymer electrolyte is reduced then the concentration of
ions will be decreased and, therefore, will limit the power and energy density of the system.
The charge loss of the structural supercapacitors can also be improved by grafting ionic
species (e.g. sulfophenyl groups [209]) to the surface of the CF mat based electrodes. A
simple approach of increasing the power density can be to increase the applied voltage which
can be achieved by simply preparing and then characterising the structural supercapacitors in
an inert atmosphere as well as using ionic liquids with broader voltage window.
7.2.4 Improvementsinthemechanicalperformanceofstructuralsupercapacitors
Delamination and poor shear performance has been seen to be a critical failure mode for
these structural supercapacitors because of the unusual resin, non-optimised fibre surface
chemistry and non-optimised fabrication conditions. The use of thin polymer membranes in
the structural supercapacitor lead to an improved electrochemical performance but resulted in
poor mechanical performance due to early delamination at the electrode/insulator interface.
Chapter 7 Conclusion and Future Works
216
Therefore, further exploration of improving the electrode/insulator interface will be beneficial
by surface modifications of polymer membrane based insulators e.g. grafting or plasma
treatments. Increasing the laminate thickness by stacking multiple structural supercapacitor
laminates is another solution but this layup configuration of structural supercapacitor will
deteriorate the specific capacitance of the system. Different other mechanical properties of
structural supercapacitors including the compression and tensile properties could also be
determined. Although, HTA carbon fibre woven fabrics were studied as potential electrodes
of the structural supercapacitors, it is a possibility to use carbon fibre woven fabrics with
varying thickness, in future work, to see the effect of thickness of the electrodes on the
mechanical performance of structural supercapacitors. Different other structural geometries
of carbon based electrodes, e.g. micro-braided carbon fibres [210, 211], could also be studied
as potential structural supercapacitor electrodes as they may offer additional mechanical
performance without increasing the product cost.
In summary, these materials have enormous potential and, once mature, will have a
significant impact on a wide range of civilian and military applications. However, this field is
still very immature and there are still considerable hurdles to be addressed, most notably the
power density of these materials.
Appendix A
217
Appendix A Result tables of polymer electrolytes and polymer
composite electrolytes
Key
A stands for 0.1MTBAPF6/PC,
Li.stands for 1.0 M LiTFSI/PC,
Na stands for 1.0 M NaClO4/PC,
E stands for EMITFSI,
P stands for crosslinked PEGDGE with TETA
B stands for crosslinked DGEBA with MCHA
M stands for MSP
N stands for NSP
All electrochemical (Section 3.5) and mechanical (Section 3.6.2) characterisation
measurements were repeated 5 times and the error was reported as standard deviation
Table A.1 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE
polymer electrolytes containing 0.8 wt% of different electrolytes
Sample code 0.8 wt% of Electrolyte in
PEGDGE
ҡ E σ
(µS/cm) (MPa) (MPa)
A0.80P99.2 0.1 M TBAPF6/PC 12.3 ± 1.23 5.46 ± 0.200 1.86 ± 0.210
Li0.80P99.2 1.0 M LiTFSI/PC 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350
Na0.80P99.2 1.0 M NaClO4/PC 18.3 ± 3.53 11.3 ± 0.230 5.11 ± 0.401
E0.80P99.2 EMITFSI 19.5 ± 3.20 12.2 ± 0.250 5.96 ± 0.241
Appendix A
218
Table A.2 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE
polymer electrolytes by varying EMITFSI concentration
Sample code EMITFSI ҡ E σ
(wt %) (µS/cm) (MPa) (MPa)
E0.8P99.2 0.8 19.5 ± 3.20 12.2 ± 0.250 5.96 ± 0.241
E10P90 10 27.6 ± 2.63 9.00 ± 0.330 4.88 ± 0.452
E20P80 20 29.0 ± 2.03 6.34 ± 0.281 4.67 ± 0.304
E30P70 30 38.9 ± 2.97 4.53 ± 0.340 3.58 ± 0.164
E40P60 40 80.2 ± 2.58 4.01 ± 0.302 2.41 ± 0.337
E50P50 50 176 ± 2.95 3.83 ± 0.410 1.16 ± 0.101
E60P40 60 162 ± 2.15 3.91 ± 0.511 1.21 ± 0.100
Table A.3 Ionic conductivity and mechanical characterisation data for crosslinked DGEBA
polymer electrolytes by varying 1.0 M LiTFSI/PC
Sample code 1.0M
LiTFSI/PC ҡ E σ
nE-mB (wt%) (wt%) (µS/cm) (MPa) (MPa)
Li0B100 0 N/A 3044 ± 155 45.6 ± 1.68
Li10B90 10 1.92 ± 0.240 1583 ± 14.14 113 ± 17.2
Li20B80 20 6.10 ± 0.0301 905 ± 73.4 81.1 ± 0.270
Li40B60 40 11.9 ± 1.06 25.1 ± 0.58 68.5 ± 0.701
Li60B40 60 138 ± 3.58 0.922 ± 0.401 0.631 ± 0.0801
Li80B20 80 1580 ± 13.8 0.211 ± 0.0102 0.120 ± 0.0401
Appendix A
219
Table A.4 Ionic conductivity and mechanical characterisation data for crosslinked
PEGDGE/DGEBA electrolytes with 10 wt% of 1.0 M LiTFSI/PC
Sample code ҡ E σ
(µS/cm) (MPa) (MPa)
Li10:100P:0B 20.3 ± 2.42 6.42 ± 0.311 2.18 ± 0.180
Li10:80P:20B 10.7 ± 0.401 15.4 ± 0.302 6.19 ± 0.311
Li10:60P:40B 8.10 ± 0.414 60.7 ± 3.30 9.04 ± 0.351
Li10:40P:60B 5.44 ± 0.902 628 ± 34.2 89.5 ± 7.11
Li10:20P:80B 3.10 ± 0.701 932 ± 14.9 105 ± 5.86
Li10:0P:100B 1.92 ± 0.240 1583 ± 14.14 113 ± 17.2
Table A.5 Ionic conductivity and mechanical characterisation data for crosslinked
PEGDGE/DGEBA electrolytes with 10 wt% of EMITFSI.
Sample
code ҡ E σ
(µS/cm) (MPa) (MPa)
E10:100P:0B 27.6 ± 2.63 9.00 ± 0.330 4.88 ± 0.452
E10:80P:20B 19.1 ± 1.28 16.3 ± 0.781 6.06 ± 0.590
E10:60P:40B 7.41 ± 1.07 34.2 ± 2.56 21.1 ± 0.601
E10:50P:50B 5.13 ± 0.460 53.3 ± 0.901 192 ± 29.8
E10:45P:55B 4.93 ± 0.481 68.1 ± 1.54 213 ± 26.0
E10:40P:60B 4.67 ± 0.391 305 ± 7.55 179 ± 7.32
E10:35P:65B 4.59 ± 0.270 740 ± 39.2 180 ± 15.1
E10:30P:70B 4.21 ± 0.331 932 ± 20.9 162 ± 21.2
E10:25P:75B 4.11 ± 0.201 1253 ± 24.4 145 ± 28.1
E10:20P:80B 3.98 ± 0.142 1693 ± 24.3 97.2 ± 7.68
E10:0P:100B 3.58 ± 0.130 3068 ± 40.6 39.2 ± 1.19
Appendix A
220
Table A.6 Ionic conductivity and mechanical characterisation data for crosslinked
PEGDGE/DGEBA electrolytes with 50 wt% of EMITFSI.
Sample code ҡ E σ
(µS/cm) (MPa) (MPa)
E50:100P:0B 176 ± 2.95 3.83 ± 0.403 1.16 ± 0.0803
E50:80P:20B 106 ± 3.55 5.15 ± 0.620 2.68 ± 0.342
E50:60P:40B 158 ± 4.89 6.89 ± 0.311 11.4 ± 1.02
E50:40P:60B 538 ± 15.6 32.0 ± 3.04 9.04 ± 0.382
E50:20P:80B 1057 ± 25.4 28.8 ± 2.38 8.05 ± 0.242
E50:0P:100B 253 ± 17.5 91.4 ± 4.65 9.07 ± 0.580
Table A.7 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE
composite electrolytes containing 0.8 wt% of 0.1 M TBAPF6/PC as function of increasing
MSP concentration
Sample code MSP ҡ E σ
(wt%) (µS/cm) (MPa) (MPa)
A0.80P99.2M0 0.0 12.3 ± 1.23 3.51 ± 0.04 1.89 ± 0.22
A0.80P96.7M2.5 2.5 30.9 ± 0.90 3.68 ± 0.37 2.22 ± 0.18
A0.80P94.2M5.0 5.0 61.9 ± 7.60 4.68 ± 0.94 2.75 ± 0.24
A0.80P91.7M7.5 7.5 175 ± 13.8 9.54 ± 0.16 3.29 ± 0.12
A0.80P88.2M10 10 127 ± 12.1 4.52 ± 0.62 1.94 ± 0.19
A0.80P86.7M12.5 12.5 57.7 ± 4.74 4.37 ± 0.18 1.78 ± 0.12
A0.80P84.2M15 15 23.0 ± 3.20 3.53 ± 0.20 1.57 ± 0.13
A0.80P81.7M17.5 17.5 12.9 ± 1.80 1.89 ± 0.42 5.50 ± 0.15
Appendix A
221
Table A.8 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE
composite electrolytes containing 0.8 wt% of 1.0 M LiTFSI/PC as function of increasing
MSP concentration.
Sample code MSP ҡ E σ
(wt%) (µS/cm) (MPa) (MPa)
Li0.80P99.2M0 0 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350
Li0.80P96.7M2.5 2.50 60.9 ± 9.01 12.0 ± 0.370 7.11 ±0.681
Li0.80P94.2M5.0 5.00 81.9 ± 7.60 13.9 ± 0.940 7.68 ± 0.870
Li0.80P91.7M7.5 7.50 246 ± 22.9 15.4 ± 0.660 8.17 ± 0.570
Li0.80P88.2M10 10.0 187 ± 26.1 11.6 ± 0.621 6.84 ± 0.390
Li0.80P86.7M12.5 12.5 27.0 ± 0.741 9.87 ± 0.180 6.55 ± 0.621
Li0.80P84.2M15 15.0 10.4 ± 0.202 6.53 ± 0.201 5.42 ± 0.322
Table A.9 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE
composite electrolytes containing 0.8 wt% of 1.0 M LiTFSI/PC as function of increasing
NSP concentration.
Sample code MSP ҡ E σ
wt% (µS/cm) (MPa) (MPa)
Li0.80P99.2N0 0 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350
Li0.80P96.7N2.5 2.50 57.1 ± 3.56 11.9 ± 0.730 7.22 ± 1.32
Li0.80P94.2N5.0 5.00 73.5 ± 12.5 13.1 ± 0.321 7.14 ± 1.17
Li0.80P91.7N7.5 7.50 96.7 ± 11.8 13.9 ± 0.902 6.78 ± 1.67
Li0.80P88.2N10 10.0 47.3 ± 8.45 14.5 ± 0.411 8.59 ± 1.31
Li0.80P86.7N12.5 12.5 21.6 ± 7.86 11.2 ± 0.354 6.88 ±0.362
Appendix A
222
Table A.10 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE
composite electrolytes containing 10 wt% of EMITFSI as function of increasing MSP
concentration.
Sample code MSP ҡ E σ
% (µS/cm) (MPa) (MPa)
E10P100M0 0 27.6 ± 2.63 9.00 ± 0.330 4.88 ± 0.452
E10P100M2.5 2.50 57.9 ± 9.95 15.2 ± 2.24 4.58 ± 1.01
E10P100M5.0 5.00 121 ± 10.8 20.1 ± 1.95 5.14 ± 0.444
E10P100M7.5 7.50 291 ± 54.1 21.9 ± 0.841 6.95 ± 0.640
E10P100M10 10.0 217 ± 48.1 21.6 ± 0.712 6.47 ± 0.360
E10P100M12.5 12.5 42.5 ± 5.65 9.85 ± 0.642 2.14 ± 0.181
E10P100M15 15.0 12.4 ± 1.54 9.24 ± 0.780 3.62 ± 0.0921
Table A.11 Ionic conductivity and mechanical characterisation data for crosslinked DGEBA
composite electrolytes containing 20 wt% of 1.0 M LiTFSI/PC as function of increasing MSP
concentration.
Sample code MSP ҡ E σ
% (µS/cm) (MPa) (MPa)
Li20B100M0 0 6.10 ± 0.0301 905 ± 73.4 81.1 ± 0.270
Li20B100M2.5 2.50 6.54 ± 0.670 1232 ± 54.1 111 ± 1.02
Li20B100M5.0 5.00 8.42 ± 4.10 1278 ± 72.7 101 ± 5.43
Li20B100M7.5 7.50 10.1± 2.11 1328 ± 7.40 82.5 ± 6.31
Li20B100M10 10.0 7.85 ± 1.87 401.4 ± 63.1 85.1 ± 1.26
Li20B100M12.5 12.5 4.22 ± 0.641 859.6 ± 25.4 53.4 ± 4.02
Li20B100M15 15.0 2.14 ± 0.150 496.7 ± 39.1 41.8 ± 2.85
Appendix A
223
Table A.12 Ionic conductivity and mechanical characterisation data for crosslinked
PEGDGE/DGEBA composite electrolytes containing 10 wt% of 1.0 M LiTFSI/PC as
function of increasing MSP concentration.
Sample code MSP ҡ E σ
% (µS/cm) (MPa) (MPa)
Li10:20P:80B:M0 0 3.10 ± 0.701 932 ± 14.9 105 ± 5.86
Li10:20P:80B:M7.5 7.50 7.57 ± 0.541 975 ± 10.4 135 ± 3.41
Li10:20P:80B:M10 10.0 7.81 ± 0.740 987 ± 11.7 145 ± 0.502
Li10:40P:60B:M0 0 5.44 ± 0.902 628 ± 34.2 89.5 ± 7.11
Li10:40P:60B:M7.5 7.50 9.75 ± 1.14 642 ± 24.3 81.0 ± 1.14
Li10:40P:60B:M10 10.0 10.5 ± 0.210 616 ± 33.4 84.5 ± 2.01
Table A.13 Ionic conductivity and mechanical characterisation data for crosslinked
PEGDGE/DGEBA composite electrolytes containing 20 wt% of 1.0 M LiTFSI/PC as
function of increasing MSP concentration.
Sample code MSP ҡ E σ
% (µS/cm) (MPa) (MPa)
Li20:20P:80B:M0 0.0 6.28 ± 0.730 598 ± 13.7 59.4 ± 2.11
Li20:20P:80B:M7.5 7.5 6.69 ± 0.410 604 ± 14.4 64.4 ± 2.70
Li20:20P:80B:M10 10.0 9.87 ± 1.21 673 ± 18.7 75.6 ± 5.11
Li20:40P:60B:M0 0.0 11.8 ± 0.510 42.4 ± 4.31 6.84 ± 0.440
Li20:40P:60B:M7.5 7.5 27.7 ± 2.71 53.1 ± 2.19 10.2 ± 3.21
Li20:40P:60B:M10 10.0 17.5 ± 0.240 25.9 ± 1.53 5.72 ± 3.24
Appendix A
224
Table A.14 Ionic conductivity and mechanical characterisation data for crosslinked 40:60
PEGDGE/DGEBA blend composite electrolytes containing 50 wt% of EMITFSI as function
of increasing MSP concentration.
Sample code MSP ҡ E σ
% (µS/cm) (MPa) (MPa)
E50:40P:60B:M0 0 538± 15.6 32.0 ± 3.04 9.04 ± 0.382
E50:40P:60B:M2.5 2.50 598 ± 91.0 35.6 ± 1.17 9.07 ± 0.140
E50:40P:60B:M5.0 5.00 697 ± 28.8 38.8 ± 1.32 9.99 ± 0.271
E50:40P:60B:M7.5 7.50 849 ± 50.1 62.0 ± 2.69 19.6 ± 1.08
E50:40P:60B:M10 10.0 1038 ± 65.4 26.4 ± 0.721 6.91 ± 0.431
E50:40P:60B:M12.5 12.5 842 ± 45.2 25.4 ± 0.840 6.15 ± 0.940
Appendix B
225
Appendix B Instructions of measuring the ionic conductivity of
polymer electrolytes and composite polymer electrolytes
Required materials:
Polymer electrolyte (cylindrical disc of 13 mm diameter and 4mm height)
Stainless steel electrodes (cylindrical disc of 13 mm diameter and 2mm height)
Required equipment:
Sample holder
Ivium-n-Stat Multichannel Potentiostat (Ivium Technologies, The Netherlands)
Measurement:
1. Sandwich the cylindrical disk shaped polymer electrolyte between two stainless steel
electrodes to make a cell;
2. Place the sandwiched cell in the sample holder and connect the sample holder with the
Ivium Potentiostat;
3. Run the impedance spectroscopic test at a potential of 0.5 V and a frequency range of
100,000 Hz to 0.1 Hz;
4. Import and open the raw data from the impedance spectroscopic test in ZView software
(Scribner Associates, http://www.scribner.com/zplot-and-zview-for-windows.html).
5. Select the first semi-circular region (high frequency curve) using the moveable two
points on the Nyquist plot;
6. Open Instant Fit option in the Tools menu and fit the semi-circular curve using
different available models. Select the best fit of the curve and note down the RS value
which is the x-intercept of the high frequency curve;
7. The ionic conductivity ҡ can be measured using the following equation
κ
Note: Equivalent series resistance (ESR) of structural supercapacitors was also calculated
using the method described above (Steps 3 to 6). ESR was the x-intercept of high frequency
curve. Equivalent distributed resistance (EDR) was the x-intercept of low frequency curve.
Appendix B
226
Figure A.1 ZView software showing the x-intercept of the Nyquist plot for the 40:60
PEGDGE/DGEBA blend matrix with 50wt% EMITFSI and 7.5 wt% MSP.
Appendix C
227
Appendix C Instructions of measuring the machine compliance for
determining the compression modulus of polymer electrolytes
Measurement:
Compressive force was applied in the axial direction of the two platens of compression
testing machine (Easy 50, Lloyds Instruments, UK) without placing any sample in between.
The compression force against machine extension was recorded during the test (Figure B.1).
The test was stopped as soon as the compression force reached to 12 kN. The compliance test
was repeated four times. The compression force versus machine extension was divided into
following three regions.
1) Compression force (load) between 0 N and 800 N;
2) Compression force (load) between 800 N to 1200 N;
3) Compression force (load) between 1200 N to 12 kN.
0 4000 8000 120000.0
0.1
0.2
0.3
0.4
0.5
Mac
hine
Ext
ensi
on (
mm
)
Load (N)
Figure B.1 Load against machine extension during the compression of plates of test machine
Constants (a0, a1, a2, …., a7) of the polynomial curve (y = a0 + a1x + a2x2 + a3x
3 +….+a7x7) of
three different regions were measured using curve fitting technique in origin software. The
compliance error was calculated using the constants in the polynomial equation and was then
subtracted from the machine extension data recorded during the compression testing of the
polymer electrolytes.
Appendix D
228
Appendix D Microscopic Evaluation on the MSP reinforced polymer
electrolytes
Uniform distribution of 7.5 wt% MSP in crosslinked PEGDGE containing 10 wt%
EMITFSI
Figure D.1 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
7.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 200 µm.
Figure D.2 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
7.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 20 µm.
Appendix D
229
Figure D.3 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
7.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 10 µm.
Figure D.4 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
7.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 2 µm.
Appendix D
230
Figure D.5 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
7.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 1 µm.
12.5 wt% addition in crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI
showing MSP agglomeration in polymer composite electrolytes
Figure D.6 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
12.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 100 µm.
Appendix D
231
Figure D.7 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
12.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 100 µm.
Figure D.8 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
12.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 50 µm.
Appendix D
232
Figure D.9 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and
12.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 10 µm.
Figure D.10 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI
and 12.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 10
µm.
Appendix E
233
Appendix E Shear stress and straincurves for the ±45º laminated
structural supercapacitor specimens
0 5000 10000 150000
12
24
36
She
ar S
tres
s (M
Pa)
Shear Strain (
100P:0B (a) 80P:20B (b) 60P:40B (c) 40P:60B (d) 20P:80B (e) 0P:100B (f)
(a)
(b)
(c)
(d)
(f)
(e)
Figure E.1 Shear stress-strain curves of CF and GF reinforced crosslinked
PEGDGE/DGEBA blend polymer electrolyte based structural supercapacitor specimens
containing 10 wt% EMITFSI with varying PEGDGE to DGEBA ratio.
Appendix E
234
Appendix F Nuclear magnetic resonance spectroscopy (NMR) of
diglycidylether of bisphenol-A epoxy and 4,4’ methylene bis(cyclo
hexyl amine) crosslinker
Figure E. 1 NMR of diglycidylether of bisphenol A epoxy
Appendix E
235
Figure E. 2 NMR of 4,4’ methylene bis(cyclo hexylamine) crosslinker
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