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Metal oxides as electrode materials for
electrochemical capacitors
Zhou Jin LaoUniversity of Wollongong
Lao, Zhou Jin, Metal oxides as electrode materials for electrochemical capacitors, MEng-Res,Institute for Superconducting and Electronic Materials, University of Wollongong, 2006.http://ro.uow.edu/au/theses/487
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i
Metal oxides as electrode materials for electrochemical capacitors
A thesis submitted in fulfilment of the requirements for the award of the degree
Master of Engineering - Research
From
University of Wollongong
By
Zhuo Jin Lao, B. Eng
Institute for Superconducting and Electronic Materials
Faculty of Engineering
2006
ii
Certification
I, Zhuo Jin Lao, declare that this thesis, submitted in fulfilment of the requirements
for the award of Master of Engineering - Research, at the Institute for
Superconducting and Electronic Materials, Faculty of Engineering, University of
Wollongong, is wholly my own work unless otherwise referenced or acknowledged.
The document has not been submitted for qualifications at any other academic
institution.
Zhuo Jin Lao
25 January 2006
iii
Acknowledgements
I would like to express my deep gratitude to my supervisors, Dr. Konstantin
Konstantinov and Prof. Shi Xue Dou for their academic guidance, financial support
and constant encouragement throughout the project.
Many thanks should be given to Prof. Hua Kun Liu, Dr. Guo Xiu Wang, Mr. Li
Yang, Mr. Yann Tournayre, Dr. Zai Ping Guo, Dr. Jia Zhao Wang and all the
members in the Institute for Superconducting and Electronic Materials, and to all
the technicians in the Faculty of Engineering. Thanks should also go to Dr. T. Silver
for helpful comments and advice on this thesis.
iv
Contents Certification ……………………………………………………………….….…… ii
Acknowledgements ...…………………….………………………………………..iii
Contents …………………………………………………………………...……… iv
Abstract ...………………………………………………………………..…...……vii
Chapter 1. Introduction ………………………………………………...………….. 1
Chapter 2. Literature review ………………………………….……………...….… 5
2.1. Introduction …………………………………………………...………….… 5
2.2. Principles of energy storage ……………………………...………………… 7
2.2.1. Electrical double-layer capacitors ……………………...……………… 7
2.2.2. Pseudocapacitance ………………………………….……..…………. 12
2.3. Electrode materials for electrochemical capacitors …………...………….. 17
2.3.1. Activated carbons as electrodes for electrochemical capacitors …...… 17
2.3.2. Carbon aerogels and xerogels as electrodes
for electrochemical capacitors ……………………………………………… 21
2.3.3. Carbon nanostructures as electrodes for electrochemical capacitors…. 23
2.3.4. Metal oxides as electrodes for electrochemical capacitors ………...… 27
2.3.5. Polymers as electrodes for electrochemical capacitors ………...…….. 29
2.4. Electrolytes for electrochemical capacitors …………………………...….. 30
2.4.1. Organic ………………………………………………………………...30
2.4.2. Aqueous ………………………………...…………………...……….. 31
2.5. Applications ……………………………………………………...……….. 32
2.6. Summary …………………………………………………...……………... 35
v
Chapter 3. Experimental …………………………………………...…………….. 38
3.1. Materials and chemicals …………………………………………...……... 38
3.2. Experimental procedures …………………………………………...…….. 39
3.3. Materials preparation …………………………………………………...… 41
3.3.1. Metal oxides prepared by spray pyrolysis ……………………...……. 41
3.3.2. Metal oxides prepared by co-precipitation and heat treatment …….… 42
3.4. Structural and physical characterization of oxide materials ………...……. 43
3.5. Electrode preparation and test cell fabrication ...………………...……….. 44
3.5.1. Electrode preparation ……..…………………..……………....……….44
3.5.2. Test cell fabrication ...…………...…………………………..……….. 44
Chapter 4. Nanocrystalline Co3O4 powders as electrode materials
for electrochemical capacitors ………………………………………...…………. 46
4.1. Introduction ………………………………………………..……………... 46
4.2. Experimental …………………………………………………....………… 48
4.3. Results and discussion ………………………………………...…………. 49
4.3.1. Materials characterization …………….…………………...…………. 49
4.3.2. Electrochemical properties ………………………………...…………. 53
4.4. Summary ………………………...………………………………...……… 56
Chapter 5. Nanocrystalline NiO powders as electrode materials
for electrochemical capacitors ……………………………………...……………. 57
5.1. Introduction ……………………………………………………..………... 57
5.2. Experimental …………………………………………………...…………. 59
5.3. Results and discussion …………………………………………...….……. 61
5.3.1. Materials characterization ………………………………...……….…. 61
5.3.2. Electrochemical properties …………………………………...………. 64
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5.4. Summary ………………………………………..……………...…...…….. 68
Chapter 6. Crystalline V2O5 powders as electrode materials
for electrochemical capacitors …………………………….……………...……... 69
6.1. Introduction …………………………………………………………...…... 69
6.2. Experimental ………………………………………………………...……. 70
6.3. Results and discussion ……………………………………………....……. 71
6.3.1. Materials characterization ……………………………………………. 71
6.3.2. Electrochemical properties …………………………………...………. 74
6.4. Summary ………………………………………………………...………... 79
Chapter 7. Amorphous and nanocrystalline MnO2 as electrode materials
for electrochemical capacitors ………………………………………...………… 80
7.1. Introduction …………………………………………………...…………... 80
7.2. Experimental ………………………………………………………...……. 82
7.3. Results and discussion …………………………………………...……….. 83
7.3.1. Materials characterization ………….……………………...…………. 83
7.3.2. Electrochemical properties ………………………………...…………. 87
7.4. Summary ………………………………………………………...………... 93
Chapter 8. General conclusions ……………………………………...…………... 94
References …………………………………………………………...…………… 96
List of symbols ……………………………………………………………...……107
vii
Abstract
Electrochemical capacitors are becoming attractive energy storage devices and fill
the gap between batteries and conventional capacitors because they have higher
energy density than conventional dielectric capacitors and have higher power
density and a longer cycling life than batteries. In this study, transition metal
oxides, such as Co3O4, NiO, V2O5 and MnO2, have been successfully synthesized
by different chemical-based solution methods. Their physical properties were
characterized by X-ray diffraction, SEM, and BET analysis. The as-prepared
Co3O4, NiO, V2O5 and MnO2 were investigated as electrode materials for
electrochemical capacitors and demonstrated very high specific capacitances, which
were 168 F/g, 203 F/g, 262 F/g, and 406 F/g, respectively. This may be due to their
large surface areas (Co3O4 (82 m2/g), NiO (90 m2/g), V2O5 (41 m2/g) and MnO2 (269
m2/g)) and pseudocapacitive behaviour. Compared with expensive RuO2, which has
been used extensively as electrode material for electrochemical capacitors, the as-
prepared Co3O4, NiO, V2O5, and MnO2 are much cheaper. This makes them very
promising candidates as electrode materials for electrochemical capacitors.
1
Chapter 1. Introduction Capacitors that store energy within the electrochemical double layer at the
electrode/electrolyte interface are known under various names, which are trade marks
or established colloquial names such as ‘double-layer capacitors’, ‘supercapacitors’,
‘ultracapacitors’, ‘power capacitors’, ‘gold capacitors’ or ‘power cache’. ‘Electrical
double-layer capacitor’ is the name that describes the fundamental charge storage
principle of such capacitors. However, due to the fact that there are, in general,
additional contributions to the capacitance other than double layer effects, we will
call these capacitors electrochemical capacitors (EC) throughout this paper.
High surface area activated carbon has been extensively chosen as an electrode
material for electrochemical capacitors. Theoretically, the higher the surface area of
the activated carbon, the higher the specific capacitance. However, the practical
situation is more complicated, and usually the capacitance measured does not have a
linear relationship with the specific surface area of the electrode material. The main
reason for this phenomenon is that nanopores with small diameter may not be
accessible to the electrolyte solution simply because the electrolyte ions, especially
big organic ions and ions with the solvation cell, are too big to enter into the
nanopores. Thus, the surface area of these non-accessible nanopores will not
contribute to the total double layer capacitance of the electrode material.
2
Several research teams have focused on the development of an alternative electrode
material for electrochemical capacitors. Many transition metal oxides have been
shown to be suitable as electrode materials for electrochemical capacitors. Among the
oxide materials for application in electrochemical capacitors, ruthenium and iridium
oxides have achieved much attention [1-6]. Ruthenium oxides are easy to prepare,
e.g. thermal decomposition of RuCl3·H2O onto titanium or tantalum foil [4], show
metallic conductivity, have a high double-layer and pseudo-capacitance and are stable
in aqueous acid and alkaline electrolytes. The capacitance sensitively depends on the
method of preparation. Up to 380 F/g [4] or 720 F/g [5] are reported for amorphous
water-containing ruthenium oxides. In these amorphous ruthenium oxides the
interaction of the proton of the hydroxide group with the constitutional water and the
electrolyte is the reason for the very high capacitance.
The disadvantage of RuO2 is the high cost of the raw material. Therefore, in recent
years great efforts have been undertaken to find new and cheaper materials. Several
metal oxides and hydroxides, for example, those of Ni, Co, V, and Mn, are being
studied extensively [7-9]. In our work, Co3O4, NiO, V2O5 and MnO2 were synthesized
by different chemical-based solution methods. Their physical and electrochemical
properties as electrode materials for electrochemical capacitors have been studied
systematically.
3
Chapter 2 covers the history and the energy storage principles of electrochemical
capacitors. The electrode materials, the electrolytes for electrochemical capacitors,
and the applications of these devices are systematically reviewed.
Chapter 3 describes the experimental methods and procedures used in this study, and
the materials and chemicals chosen to fulfil the research work.
Chapter 4 presents the synthesis of nanocrystalline Co3O4 powders by spray
pyrolysis, and the physical and electrochemical properties of the as-prepared Co3O4
powders as electrode materials for electrochemical capacitors in KOH electrolytes.
In Chapter 5, we describe how we used co-precipitation and a spray dry technique to
obtain the nickel hydroxide and then calcined it at 300 °C to obtain nanocrystalline
NiO powders. Then the prepared NiO powders were studied to determine their
suitability as electrode materials for electrochemical capacitors in different
concentrations of KOH.
The preparation of crystalline V2O5 powders by co-precipitation and calcination is
presented in Chapter 6. Their physical and electrochemical properties as electrode
materials for electrochemical capacitors with different electrolytes have been
investigated.
4
Chapter 7 describes a new method based on co-precipitation and the spray dry
technique for preparing amorphous or nanocrystalline MnO2 powders. The effects of
the spraying temperature and the electrolytes on as-prepared MnO2 electrodes for
electrochemical capacitors were systematically investigated.
Chapter 8 gives an overview and summary of the metal oxides studied and suggests
that they are promising electrode materials for electrochemical capacitors.
5
Chapter 2. Literature review
2.1. Introduction
Electrochemical capacitors have been known for many years. The first patents date
back to 1957 when Becker invented a capacitor based on high surface area carbon
[10]. Later, in 1969, the first attempts to market such devices were undertaken by
SOHIO [11]. However, only in the 1990s did electrochemical capacitors become
famous in the context of hybrid electric vehicles. A DOE ultracapacitor development
program was initiated in 1989. Then, short-term and long-term goals were defined for
1998–2003 and for after 2003, respectively [12].
The electrochemical capacitor was supposed to boost the battery or the fuel cell in the
hybrid electric vehicle to provide the necessary power for acceleration and
additionally allow for recuperation of brake energy. Today, several companies such
as Maxwell Technologies, Siemens Matsushita (now EPCOS), NEC, Panasonic,
ELNA, TOKIN, and several others are investing in electrochemical capacitor
development. The applications envisaged are principally boost components
supporting batteries or replacing batteries, primarily in electric vehicles. In addition,
alternative applications of electrochemical capacitors where they compete not with
batteries, but with conventional capacitors, are appearing up and show considerable
market potential.
6
The reason why electrochemical capacitors were able to raise considerable interest is
visualized in Fig. 1, where typical energy storage and conversion devices are
presented in a so called ‘Ragone plot’ in terms of their specific energy and specific
power [13]. Electrochemical capacitors fill in the gap between batteries and
conventional capacitors such as electrolytic capacitors or metallized film capacitors.
In terms of specific energy as well as in terms of specific power this gap covers
several orders of magnitude.
Fig. 2-1. Sketch of Ragone plot for various energy storage and conversion devices. The indicated areas are rough guide lines [13].
Batteries and low temperature fuel cells are typical low power devices, whereas
conventional capacitors may have a power density of > 106 watts per dm3 at very low
7
energy density. Thus, electrochemical capacitors may improve battery performance in
terms of power density or may improve capacitor performance in terms of energy
density when combined with the respective devices. In addition, electrochemical
capacitors are expected to have a much longer cycle life than batteries because no or
negligibly small chemical charge transfer reactions are involved.
In the following sections, the basic principles of electrochemical capacitors, the
different types of electrochemical capacitors, and some applications will be discussed.
2.2. Principles of energy storage
The performance of an electrochemical capacitor combines simultaneously two kinds
of energy storage, i.e. electrostatic attraction as in electrical double-layer capacitors
and faradaic reactions similar to the processes proceeding in accumulators.
2.2.1. Electrical double-layer capacitors
In an electrical double-layer capacitor (EDLC), the electrical charge is accumulated in
the double layer mainly by electrostatic forces without phase transformation in the
electrode materials. The stored electrical energy is based on the separation of charged
species in an electrical double layer across the electrode/solution interface (Fig. 2-2)
[14]. The maximal charge density is accumulated at the distance of the outer
8
Helmholtz plane, i.e. at the centre of the electrostatically attracted solvated ions. The
electrochemical capacitor contains one positive electrode with electron deficiency and
a second negative one with electron excess, both electrodes being built from the same
material (Fig. 2-3) [3]. The amount of electrical energy W accumulated in such
capacitors is proportional to the capacitance C and the voltage U according to the
formula:
W = 1/2CU2 (2-1)
Fig. 2-2. Schematic of the electrical double layer [14].
9
Fig. 2-3. Schematic of an electrochemical double layer capacitor [3].
At all the working voltages of the capacitor, the electrode materials should be
electrochemically inert, as is the case with non-functionalized carbon. The stability of
the electrolytic medium must be also carefully considered, especially in the aqueous
solutions for which the maximum voltage is restricted to ~1 V due to the
thermodynamic electrochemical window of water (1.23 V). The operating voltage of
the capacitor is determined by the decomposition voltage of the electrolyte [3].
Hence, the electrical energy accumulated in an electrochemical capacitor can be
significantly enhanced by the selection of an aprotic medium where the
decomposition potential of the electrolyte varies from 3 V to 5 V. Unfortunately, due
to the low conductivity of such a solution (20 mS/cm against 1 S/cm for water
medium), this advantage can be quite doubtful in the case of the high specific power
demands. Additionally, for practical applications (e.g. power supply of electrical car),
the use of an aprotic medium must meet certain technological, economical and safety
requirements. However, the possibility of reaching 3 V or more is still very attractive
10
and a lot of research is being performed, especially for applications with a low
specific power. Finally, the choice of the electrolyte depends on the specific power
and energy values demanded.
Presently, the development of electrochemical capacitors is largely connected with
searching for optimal electrode materials capable of a high, efficient accumulation of
electrical energy with simultaneous long durability. In a simple model, an
electrochemical capacitor is formed by two polarizable electrodes, a separator and an
electrolyte (Fig. 2-3). The overall capacitance C is determined by the series
equivalent circuit, consisting of the anode capacitance Ca and the cathode capacitance
Cc according to the equation:
1/C = 1/Ca + 1/Cc (2-2)
In the case of capacitors built from materials with significantly different surfaces, the
component of smaller capacitance will contribute more to the total capacitance due to
the reciprocal dependence [3].
The amount of electrical charge accumulated by pure electrostatic forces that is
typical for electrical double-layer capacitor depends on the surface of the
electrode/electrolyte interface and on the ease of access of the charge carriers to this
interface. The capacitance is proportional to the surface area S of the material and to
11
the relative permittivity of the solution ε, and reciprocally dependent on the thickness
d of the double layer:
C = Sε/d (2-3)
In concentrated electrolytic solutions, the charge separation is of the order of a few Å.
(For diluted solutions the diffusive part of the double layer is ~1000 Å.)
Theoretically, the higher surface area and the concentration of electrolyte, the higher
the value of the capacitance. In the case of carbon, the double-layer capacitance is
associated with the electrode/solution interface and has a value of 15–50 µF/cm2.
Taking an average value of 25 µF/cm2 and a specific area of 1000 m2/g for carbon,
the ideal attainable capacitance would be 250 F/g. The practically obtained values are
of a few tens of F/g in electrical double-layer capacitors due to the limited
accessibility of the carbon surface to the electrolyte. The developed surface area of
the carbon essentially consists of micropores (< 2 nm) often hardly accessible or non-
accessible to ions [15-17]. In practice, the real surface area estimated by gas
adsorption differs significantly from the electrochemically active surface available for
charged species.
Another way to increase the capacitance values is the usage of the pseudocapacitance
(see section 2.2.2), which depends on the surface functionality of the carbon and/or
on the presence of electroactive species, e.g. oxides of transition metals such as Ru,
Ir, W, Mo, Mn, Ni, Co or conducting polymers deposited on the carbon surface
12
[2,3,17-21]. In electrochemical capacitors, electrosorption or redox processes can
enhance the value of the capacitance for the carbon material by about 10–100 times.
2.2.2. Pseudocapacitance
Pseudocapacitance arises when, for thermodynamic reasons, the charge q required for
the progression of an electrode process is a continuously changing function of
potential U [2,3]. Then, the derivative C = dq/dU corresponds to a faradaic kind of
capacitance. The term ‘pseudo’ originates from the fact that the double-layer
capacitance arises from quick faradaic charge transfer reactions and not only from
electrostatic charging. Pseudocapacitance effects (electrosorption of H or metal ad-
atoms, redox reactions of electroactive species) strongly depend on the chemical
affinity of carbon materials to the ions sorbed on the electrode surface.
Good examples of materials giving pseudocapacitance properties are conducting
polymers [22,23]. They can be doped and dedoped rapidly to high charge densities.
Hence, they can be applied as active materials for electrochemical capacitors. Higher
energy densities can be achieved because charging occurs throughout the volume of
the material. Comparison of the charge density for conducting polymers, e.g.
polyaniline, with a high surface area carbon electrode gives values of 500 C/g and 50
C/g, respectively. Taking into account the cost and compatibility of these two
materials, the modification of carbon by conducting polymers for capacitor
application seems to be a very attractive method [19-21].
13
Generally, the enhancement of specific capacitance for the carbon materials by quick
faradaic reactions can be realized by the following modifications:
1. A special oxidation of carbon for increasing the surface functionality (through
chemical treatment [20], electrochemical polarization [24], or
plasma treatment [25]).
2. The formation of carbon/conducting polymer composites by electropolymerization
of a suitable monomer (aniline, pyrrole) on the carbon surface [19-21] or by using a
chemical method for polymerization.
3. Insertion of electroactive particles of transition metals oxides, such as RuO2, TiO2,
Cr2O3, MnO2, Co2O3, into the carbon material [3, 26-29].
Chemical treatment of carbons, e.g. by hot nitric acid, significantly enriches the
surface functionality often by enhancing the surface area, but in some cases the
resistivity can also be simultaneously increased, excluding such a material from
practical usage for capacitors. Electrochemical polarization also provides the
possibility for surface modification, however, these changes are reversible, and they
disappear with capacitor cycling (Fig. 2-4) [20].
14
Fig. 2-4. Voltammetry characteristics of a capacitor made from carbon fabric with 10 M H2SO4, scan rate 2 mV/s; —— non-modified, - - - - - - electrochem. modified, – – – electrochem. modified after cycling [20].
Modification of carbon materials by electroconducting polymers are responsible for
an interesting feature of such composites [19-21]. Electroconducting polymers are
capable of storing of charges, and this process depends not only on the preparation
conditions and the state of oxidation, but also on the solvent [30]. The values of the
specific capacitance of carbon fabrics modified by polyaniline can be significantly
enhanced from 30 F/g to 150 F/g, however, a gradual degradation of such composites
takes place during cycling, which degrades the capacitor performance. The charge
storage in the electroconducting polymer depends on many parameters. Hence, the
shape of voltammograms is not stable. Fig. 2-5 represents the voltammetry
characteristics of carbon fabrics modified by electrodeposition of polyaniline [20]. It
15
shows separately the various redox processes taking place on the anode and the
cathode. Reversible reactions connected with the electrochemical behaviour of the
polyaniline are especially remarkable on the anode of capacitor.
Fig. 2-5. Voltammetry characteristics of both electrodes of a capacitor constructed from carbon fabric modified by polyaniline; with 1 M H2SO4, scan rate 2 mV/s [20].
Capacitance enhancement of carbon materials by electroactive species is extremely
attractive, but not always from an economic point of view. For example, carbon after
modification by hydrous ruthenium oxides shows a higher value of specific
capacitance [26-29] through the pseudocapacitance effect. However, the increase in
capacitance is proportional to the amount of very expensive oxide.
16
Pure RuOxHy is a mixed electron-proton conductor with a high specific capacitance
ranging from 720 F/g to 900 F/g [27-31]. It is noteworthy that the faradaic nature of
RuOx solids is very sensitive to their degree of hydration and crystallinity [31]. For
example, the capacitance of amorphous RuO2 · 0.5H2O is equal to 900 F/g, whereas
the highly crystalline anhydrous RuO2 presents only the low value of 0.75 F/g. Upon
insertion of electroactive RuOxHy particles into the carbon capacitor electrodes, the
rate of electrochemical protonation becomes limited by the diffusion process of the
proton donating species to the electroactive sites. It is important to mention that the
BET surface area of such pseudofaradaic active RuOxHy particles does not exceed
100 m2/g. After the deposition of oxide particles into the carbon matrix the total
surface area of the material will probably diminish. Fig. 2-6 presents the effect of
modification of carbon aerogel material by ruthenium oxide particles, where the
untreated sample presents 95 F/g and after RuOx treatment reaches 206 F/g [28]. A
unique increase of capacitive current is observed in all the scan range of potential.
Fig. 2-6. Voltammetry of Ru/carbon aerogel composite electrodes for electrochemical capacitor; with 1 M H2SO4, scan rate 2 mV/s [28].
17
2.3. Electrode materials for electrochemical capacitors
2.3.1. Activated carbons as electrodes for electrochemical capacitors Among the different carbon materials, activated carbons are especially attractive as
electrodes for capacitors from the economic point of view. In this case, a very highly
developed surface area on the order of 2000 m2/g, with a controlled distribution of
pores during the activation process, can be reached. Theoretically, the higher the
specific surface area of the activated carbon, the higher the specific capacitance.
Practically, the situation is more complicated. Some activated carbons with smaller
surface area have a larger specific capacitance than those with a larger surface area
(Table 2-1). The relationships between the BET surface area, the total pore volume,
the average pore size and the pore size distribution of activated carbons and their
electrochemical performance as electrodes for electrochemical capacitors have been
discussed in detail by Shi et al. [32-34].
There are several reasons for the absence of proportionality between specific
capacitance and surface area: the double layer capacitance (µF/cm2) varies with
different types of carbons prepared from various precursors through different
processes and subsequent treatments (Table 2-1); an important factor is also the
accessibility of the micropores to aqueous solutions. Therefore, it has been concluded
that since the size of a single nitrogen molecule is similar to that of hydrated OH− or
K+ ions, those micropores that can adsorb nitrogen molecules at 77 K are also
18
available for the electro-adsorption of simple hydrated ions at a low concentration
dependent rate [32,33]. In principle, the pores larger than 0.5 nm should be accessible
electrochemically to aqueous solutions. On the other hand, in an aprotic medium,
taking into account the size of the bigger solvated ions (e.g. on the order of 2 nm for
BF4− in propylene carbonate or 5 nm for (C2H5)4N+), the smaller and non-accessible
pores will not contribute to the total double-layer capacitance of the material.
Depending on the electrolytic medium, a convenient porous carbon material should
be selected for capacitor electrode.
Table 2-1. Comparison of specific capacity, surface area, pore volume and average pore size of activated carbons [33].
As was already mentioned, the electrical conductivity of carbon materials is closely
related to their morphology. The higher the surface area, the smaller the particle size
and the poorer the conductivity. The electrical conductivity, which depends on the
19
flow of electrical carriers, is another limiting factor for the power density of a
capacitor. However, it will not drastically influence the energy density. Among all the
physical properties, it has also been proved that the electronic properties of activated
carbons very strongly affect the electrical double layer of the material [35,36].
For practical applications, activated carbons with a large percentage of big pores are
found to be more convenient as capacitor electrodes for high power electrochemical
capacitors because they can deliver high energy at a high rate, even though they can
store less total energy. The selection of activated carbon materials for capacitor
applications can be helped by the impedance spectroscopy technique combined with
pore size analysis. The electrochemical accessibility time for pores of various sizes
has been obtained from the fitting of impedance spectroscopy data using a
transmission line equivalent circuit model [33].
A typical cyclic voltammogram (CV) for electrochemical capacitors based on
activated carbon in aqueous electrolyte is shown in Fig. 2-7 [37]. A voltammogram
close to the ideal rectangular shape is observed. The value of specific capacitance in
this case is 90 F g−1 of activated carbon.
20
Fig. 2-7. Cyclic voltammogram of a capacitor with 6 mol l−1 liquid KOH electrolyte. Scan rate 5 mV s−1 [37].
Impedance spectroscopy is a very useful technique for the measurement of
capacitance, giving complementary results, e.g. the frequency dependence C = f(v).
Fig. 2-8 presents the impedance spectrum between 2 kHz and 8 mHz of a real 20
electrode capacitor made from activated carbon [38]. At 100 mHz, the so-called knee
frequency [3] appears, which separates two different behaviour regimes of the
electrochemical capacitor, i.e. above the knee frequency, the real part of the
impedance is frequency dependent, while below this value, the resistance changes
weakly with frequency, and the capacitor behaviour tends to approach that of a pure
capacitance. The character of the impedance spectrum changes significantly with the
number of electrodes and their thickness.
21
Fig. 2-8. Impedance spectrum of a real 20 electrode capacitor [38].
2.3.2. Carbon aerogels and xerogels as electrodes for electrochemical
capacitors
Carbon aerogels, i.e. monolithic three-dimensional mesoporous networks of carbon
nanoparticles, are considered as promising materials for electrochemical capacitors.
They are obtained by the pyrolysis of organic aerogels based on resorcinol-
formaldehyde (RF) or phenol-furfural (PF) precursors via a sol–gel process. The gel
composition (catalyst, precursor, solid ratio) and the pyrolysis temperature determine
the microtexture of the final product, especially the particle size and the pore
distribution. In order to simplify their production, a supercritical drying of the RF gels
is favoured with a very low catalyst concentration; this means with high molar
22
resorcinol to catalyst (R/C) ratios. The catalyst concentration controls the particle
sizes, and the degree of dilution determines the density of the material.
The advantages of carbon aerogels for capacitor applications are their high surface
area, low density, good electrical conductivity, and the possibility of their usage
without binding substances [39-45]. The special porosity of aerogels is based on the
interconnection of carbon nanoparticles of the same size that is at the origin of an
uniform mesoporous microtexture with a specific surface area between 500 and 900
m2/g and a high pore volume (0.4–2.6 cm3/g). The pore size distribution of the
material strongly affects the nitrogen adsorption data and the electrochemical
behaviour. It was proved [41] that carbon aerogels with a pore diameter in the range
of 3 to 13 nm showed the best voltammetry characteristics and the highest
capacitance values (70–150 F/g). The carbon aerogels obtained at temperatures over
900 °C showed some degradation of specific capacitance. On the other hand, the
functionalization of the carbon surface by a heat treatment at 500 °C in air caused an
improvement in the specific capacitance through the pseudocapacitance effects [41].
After this oxidative treatment, symmetric peaks appeared on the cyclic voltammetry
plots that revealed the existence of faradaic type reactions taking place on the surface.
In this case, the charge stored in the electrode/electrolyte interface depends on the
potential of the electrodes.
Carbon aerogels obtained from a precursor prepared by conventional drying, i.e. not
by the supercritical method in CO2, are called xerogels [44,45]. It has been proved
23
that the elaboration method and the final temperature of pyrolysis affect the pore
structure of carbon aerogels and xerogels. These investigations have demonstrated the
competing effects of particle size and bulk density on the specific capacitance.
Capacitance increases almost linearly with the surface area. However, for pore
volumes of aerogels/xerogels over the value of 0.5 cm3/g, capacitance maintains
constant. The obtained values of capacitance varied from 60 F/g to 180 F/g (per
single electrode of capacitor).
2.3.3. Carbon nanostructures as electrodes for electrochemical capacitors
The application of different types of nanotubes for building capacitors established the
high affinity of this material for the accumulation of charges [46,47]. Very different
nanotubes with open and closes central canals, as well as entangled and stiff, were
intentionally selected for investigating capacitor electrodes [47]. Multiwalled carbon
nanotubes (MWNTs) with an open central hollow canal were obtained by the
decomposition of acetylene at 700 °C, using cobalt supported on silica as the catalyst.
A general Transmission Electron Microscopy (TEM) view of purified carbon
nanotubes obtained after elimination of the catalyst is presented in Fig. 2-9 [48]. A
sample obtained by the same method but at 900 °C was characterized by a fishbone
morphology with an ill-defined central canal. Both types of catalytic MWNTs have a
sinuous shape and are extremely entangled; their internal diameter varied from 4 to 6
nm, whereas the external diameter was from 15 to 30 nm. It is important to mention
that for the purification hydrofluoric and nitric acids were used for removing silica
24
and cobalt particles. Nitric acid treatment caused a modification of the carbon
nanotubes, i.e. the formation of oxygenated surface groups, with the amount of
oxygen varying from 2 to 10 wt%. Diametrically different, i.e. straight and rigid
nanotubes, were obtained by chemical vapour deposition (CVD) of propylene at 800
°C within the pores of an alumina template [49]. For these nanotubes, a wide central
canal was remarkable, with a size on the order of 10 nm, but only a few concentric,
non-continuous graphitic layers formed the nanotube walls.
Fig. 2-9. General view of purified multiwalled carbon nanotubes obtained by catalytic decomposition of acetylene at 700 °C [48].
25
The capacitance properties of MWNTs were studied in two-electrode carbon/carbon
cells. The electrodes were prepared in the form of pellets of ~10 mg from a mixture
of carbon MWNTs (85%), acetylene black (10%), and a binding substance (5% of
polyvinylidene fluoride, PVDF). The accumulation of charges in the electrical double
layer was investigated by the voltammetry technique. An example is given in Fig. 2-
10 for nanotubes prepared at 700 °C with cobalt supported on silica [50]. A regular,
almost box-like shape of the curve can be observed from which the specific
capacitance has been estimated of 70 F/g.
Fig. 2-10. Voltammetry characteristics of a capacitor built from carbon nanotubes obtained by decomposition of acetylene at 700 °C on Co/SiO2; with 6 M KOH, scan rate 1 mV/s [50].
The value of the capacitance could be enhanced from 70 to 120 F/g through an
additional treatment of the carbon nanotubes with nitric acid (69.5%) at 80 °C for 1 h.
In this case the voltammetry characteristics of the capacitor are definitively changed.
26
Instead of a typical rectangular shape, a quite remarkable region of reversible
pseudofaradaic reactions is observed at ~0.2 V (Fig. 2-11) [50].
Fig. 2-11. Voltammetry characteristics of a electrochemical capacitor constructed from carbon nanotubes obtained at 700 °C and modified by 69% nitric acid; 6 M KOH, 10 mV/s [50]. Another form of nanostructured material that has been considered for capacitor
electrodes is a carbon film grown at room temperature by supersonic cluster-beam
deposition [51]. The low-density granular structure of the material, with a grain size
of a few tens of nanometers, was established by atomic force microscopy (AFM).
Aggregated clusters were responsible for porosity on two different scales, i.e. on the
single grain and grain network levels. The porosity of the grain network could be
favourable for the formation of the electrical double layer. The nanostructured
electrodes, with a density of 1 g/cm3, deposited on an aluminium substrate as a
27
current collector, were impregnated by a quaternary ammonium salt dissolved in
propylene carbonate (PC). Hence, the nominal voltage of the capacitor in the dc
regime was 2.7 V. Due to the highly accessible surface area of the film, the specific
capacitance per electrode was 75 F/g.
2.3.4. Metal oxides as electrodes for electrochemical capacitors The cyclic voltammogram of RuO2 (and also IrO2) electrodes have an almost
rectangular shape and exhibit good capacitor behaviour [52,53]. However, the shape
of the CV is not a consequence of pure double-layer charging, but of a sequence of
redox reactions occurring in the metallic oxide. The valence state of Ru may change
from III to VI within a potential window of slightly > 1 V. The ratio of surface
charging to bulk processes is nicely described by Trasatti [52]. In aqueous acid
electrolytes the fundamental charge storage process is proton insertion into the bulk
material.
A very high specific capacitance of up to 750 F/g was reported for RuO2 prepared at
relatively low temperatures [54]. Conducting metal oxides like RuO2 or IrO2 were the
favoured electrode materials in early electrochemical capacitors used for space or
military applications [55]. The high specific capacitance in combination with low
resistance resulted in very high specific powers. These capacitors, however, turned
out to be too expensive. A rough calculation of the capacitor cost showed that 90% of
28
the cost resides in the electrode material. In addition, these capacitor materials are
only suitable for aqueous electrolytes, thus limiting the nominal cell voltage to 1 V.
Several attempts were undertaken to obtain the advantages of the material properties
of such metal oxides at reduced cost. The dilution of the costly noble metal by
forming perovskites was investigated by Guther et al. [56]. Other forms of metal
compounds such as nitrides were investigated by Liu et al. [57].
Many researchers have focused on searching for other, cheaper, materials to take the
place of ruthenium-oxides, but the selection has traditionally been limited by the use
of concentrated sulfuric acid as an electrolyte. It was believed that high capacitance
and fast charging were largely a result of H sorption, so a strong acid was therefore
necessary to provide good proton conductivity. This resulted in a narrow range of
possible electrode materials, however, since most metal oxides break down quickly in
acidic solutions. Milder aqueous solutions such as potassium chloride have therefore
been considered for use with metal oxides such as manganese oxides, and Fig. 2-13
shows the charging profile of prototypes produced by Jiang et al. [58]. Although
manganese oxide electrodes currently appear to possess lower specific capacitances
than ruthenium oxides, the lower cost and milder electrolyte may be enough of an
advantage to make them a viable alternative [59].
29
Fig. 2-12. Cyclic voltammogram for MnO film with KCl electrolyte [58].
2.3.5. Polymers as electrodes for electrochemical capacitors Polymeric materials, such as p- and n-dopable poly(3-arylthiopene), p-doped
poly(pyrrole), poly(3-methylthiophene), or poly(1,5-diaminoanthraquinone) have
been suggested by several authors [60-62] as electrodes for electrochemical
capacitors. The typical cyclic voltammogram of a polymer, however, is in general not
of rectangular shape, as is expected for a typical capacitor, but exhibits a current peak
at the respective redox potential of the polymer. In order to be able to use one and the
same electrode material on both capacitor electrodes polymers with a cathodic and an
anodic redox process were utilized recently [62].
Using a polymeric material for electrochemical capacitor electrodes gives rise to a
debate as to whether such devices should still be called capacitors or whether they are
better described as batteries. In terms of the voltage transient during charge and
discharge and with respect to the CV they are batteries. Compared to metallic oxides,
30
however, the term capacitor is justified. The difference is only that the metallic oxides
exhibit a series of redox potentials, giving rise to an almost rectangular CV, while the
polymer typically has only one redox peak.
For such capacitors rather high energy density and power density have been reported
[62]. The long-term stability during cycling, however, may be a problem. Swelling
and shrinking of electroactive polymers is well known and may lead to degradation
during cycling.
2.4. Electrolytes for electrochemical capacitors
2.4.1. Organic The advantage of an organic electrolyte is the higher achievable voltage. According
to Eq. (2-1) the square of the unit-cell voltage determines the maximum stored
energy. Organic electrolytes allow for a unit cell voltage above 2 V. Typically the cell
float voltage is 2.3 V with the possibility of increasing the voltage for a short time to
2.7 V. The cell voltage is most probably limited by the water content of the
electrolyte. In order to achieve higher voltage, and some companies plan to go up to a
float voltage of 3.2 V, extreme purification procedures involving special electrolyte
have to be applied, and the corrosion of the carbon electrodes has to be reduced by
special protective coatings [63]. However, similar problems concerning the potential
window of organic electrolytes are known from Li-ion battery production and can be
overcome.
31
On the other hand, organic electrolytes have a significantly higher specific resistance.
Compared to a concentrated aqueous electrolyte the resistance increases by a factor of
at least 20, typically by a factor of 50. The higher electrolyte resistance also affects
the equivalent distributed resistance of the porous layer and consequently reduces the
maximum usable power, which is calculated according to
P = U2/4R (2-4)
where R represents the total effective series resistance (ESR). A listing of potential
organic electrolytes for electrochemical capacitors is provided in [64].
2.4.2. Aqueous Aqueous electrolytes limit the unit cell voltage of the electrochemical capacitors to
typically 1 V, thus reducing the available energy significantly compared to organic
electrolytes. Advantages of the aqueous electrolyte are the higher conductance (0.8
S/cm for H2SO4) and the fact that purification and drying processes during production
are less stringent. In addition the cost of aqueous electrolytes is usually much lower
than for suitable organic electrolytes. Capacitors built by NEC [65] and ECOND use
aqueous electrolyte. It should be pointed out that a capacitor has to be developed for
one or the other type of electrolyte, not only because of the material aspects, but also
because the porous structure of the electrode has to be tailored to the size and the
properties of the respective electrolyte.
32
In order to avoid electrolyte depletion problems during charging of electrochemical
capacitors, the electrolyte concentration has to be high. If the electrolyte reservoir is
too small compared to the huge surface area of the electrodes, performance of the
capacitor is reduced. This problem is particularly important for organic electrolytes
where the solubility of the salts may be low. Zheng and Jow found, however, that
concentrations higher than 0.2 molar are sufficient [66].
2.5. Applications
Many applications are demanding local storage or local generation of electric energy.
This may be required since they are in portable or remote equipment, since the supply
of power may be interrupted, or since the main power supply is not able to deliver the
peak power needed. Local generation of energy (diesel generator, fuel cell, gas
turbine, photovoltaics, etc.) normally means a more complex system than a storage
system, but it is most appropriate if a large amount of energy is needed for a long
time. Storage of electric energy can be done in electric fields (capacitors), by means
of chemical reactions (batteries), in magnetic fields (SMES: superconducting
magnetic energy storage), or by converting the electrical energy to mechanical
(flywheel). The choice of the energy storage device should be adequate for the
application. Similarities and differences between batteries and electrochemical
capacitors can be found in Ref. [3].
33
The ideal applications for electrochemical capacitors are all those demanding energy
in the time range 10−2 ≤ t ≤ 102 (s). For those applications, for batteries as much as for
conventional capacitors, the ratio of stored energy to available power is unfavorable,
and the devices have to be over-dimensioned due to either the power or energy
demands. The needs for long lifetime, for many charge-discharge cycles (e.g., in
combination with photovoltaics) or for fast recharging rates may increase the time
range to days and weeks. The poor energy density of low voltage capacitors makes
electrochemical capacitors also attractive for pulsed power applications in the ms
range.
The basic technology of electrochemical capacitors with carbon electrodes is
independent of polarity. Nevertheless, present electrochemical capacitors are not
suitable for AC applications and for applications involving a high ripple current.
Their internal resistance is higher than that of conventional capacitors, and thermal
degradation may occur. In addition, some manufacturers use asymmetric electrode
systems or have special treatments of one of the two electrodes, causing a polarity of
the devices.
Most electrochemical capacitors are short circuit proven [3]. On the one hand, the
larger internal resistance in comparison to conventional capacitors limits the peak
power. On the other hand, the smaller amount of energy stored in comparison to
batteries allows only a limited heating of the electrochemical capacitors, so that self-
ignition does not occur. Another important advantage of electrochemical capacitors is
34
that in general, they do not contain hazardous or toxic materials and disposal of them
is easy. They do not need any servicing during their life time and can withstand a
huge number of charge-discharge cycles [67,68]. In a properly designed system,
cycling efficiency is 95% and higher. They are usable over in a large temperature
range. Particularly at low temperature, they substantially outperform conventional
batteries. In the short term (ms–s), over-voltage is in general not critical to the
devices. If the applied voltage exceeds the nominal voltage for longer duration, the
lifetime of the electrochemical capacitor will be shortened. Gas may be produced,
which can cause leakage or rupture of the device. The characteristic time for self-
discharge is on the order of days to months. The low voltage of the unit cells allows
easy adaption to the desired voltage level by connecting cells into series and a
modular construction of large banks.
The first electrochemical capacitors appeared on the market in 1978 (Gold Capacitors
from Panasonic/Matsushita) and in 1980 (Supercap from NEC/Tokin). Two other
Japanese companies entered into the markets with products of comparable ratings at
the end of the 1980s(Dynacap from ELNA, Polyacene Capacitor/Battery from Seiko
Instruments). All those manufacturers have products with nominal voltages in the
range of 2.3–6 V and capacitance values of 10−2 F up to several Farads. Tokin also
offers capacitors at 11 V. The costs of those electrochemical capacitors are on the
order of a few cents to a few ten cents per Joule. The RC-time constant (defined as the
low frequency capacitance times the 1 kHz resistance) is several s. They are most
suitable for consumer electronic applications. Several hundred million
electrochemical capacitors are manufactured and shipped each year.
35
Since the beginning of the1990s, two Russian companies have been selling
electrochemical capacitors (PSCap from Econd, SC from ELIT). They offer
capacitors with nominal voltages in the range of 12–350 V and capacitance values of
1 F to several hundred Farads. These capacitors are most suitable for starter and
actuator applications.
Panasonic for several years has sold cylindrical single cell capacitors with
capacitances up to 1500 F (Power Capacitor, 2.3 V). Maxwell has prism-shaped
electrochemical capacitors (PowerCache Ultracapacitors, 2.3 V) with capacitance
values between 8 and 2700 F. Recently Siemens Matsushita (now EPCOS) has started
to offer identical products. Manufacturing capabilities for those types of
electrochemical capacitors are presently being strongly increased.
At present, the electrochemical capacitors take up < 1% of the world market for
electrical energy storage (batteries, capacitors) [69]. They show nicely growing
market numbers. The improving performance, the drop in prices, and new
applications all lead to the prediction of an exciting future for electrochemical
capacitors.
2.6. Summary
Electrochemical capacitors fill an important and otherwise vacant niche in the current
set of energy storage devices, bridging the gap between batteries and conventional
36
capacitors. They offer greater energy densities than electrostatic capacitors, making
them a better choice for backup applications. They also possess higher power
densities than batteries, allowing them to perform a role in load levelling of pulsed
currents. They can help to improve battery performance when combined in a hybrid
power source, or can provide an efficient and long-lasting means of energy storage
when used on their own.
High surface area activated carbon has been chosen as an electrode material for
electrochemical capacitors. Theoretically, the higher the surface area of the activated
carbon, the higher the specific capacitance. Howerver, the capacitance measured does
not have a linear relationship with the specific surface area of the electrode material.
The main reason for this phenomenon is that nanopores with small diameter may not
be accessible to the electrolyte solution, simply because the electrolyte ions,
especially big organic ions and ions with the solvation cell, are too big to enter the
nanopores.
Considering their relatively moderate surface area, multiwall carbon nanotubes are
quite efficient for the accumulation of charge. The best materials are those which
possess accessible mesopores formed by entanglement and by the central canal.
Activation of multiwalled carbon nanotubes may be of benefit to get higher values of
capacitance through the development of micropores.
37
The modification of carbon materials by conducting polymers is a promising way to
improve electrochemical capacitance. However, this improvement of capacitance by
electroactive species often decreases the total surface area and the access to the bulk
of the electrode.
Conducting metal oxides like RuO2 or IrO2 were the favored electrode materials in
early electrochemical capacitors used for space or military applications. However,
these capacitors turned out to be too expensive. A rough calculation of the capacitor
cost showed that 90% of the cost resides in the electrode material. Therefore, in
recent years great efforts have been undertaken in order to find new and cheaper
electrode materials. Several metal oxides, such as Co3O4, NiO, V2O5 and MnO2, seem
to be promising electrode materials for electrochemical capacitors.
38
Chapter 3. Experimental
3.1. Materials and chemicals Several chemical companies supplied the materials and chemicals. Most of them were
from Aldrich Chemical Company Pty. Limited. The details are given in Table 3-1:
Table 3-1. Materials and chemicals used in this study
Materials or Chemicals Formula Purity Supplier
Cobalt (II,III) oxide Co3O4 99 % Aldrich
Cobalt nitrate hexahydrate Co(NO3)2 · 6H2O 98 % Aldrich
Nickel (II) chloride NiCl2 98 % Aldrich
Vanadium (III) chloride VCl3 99 % Aldrich
Ammonium hydroxide
solution
NH4OH 28% in H2O,
99.99 %
Aldrich
Manganese (II) acetate
tetrahydrate
(CH3COO)2Mn · 4H2O 99 % Aldrich
Manganese (II) nitrate
tetrahydrate
Mn(NO3)2 · 4H2O > 97 % Fluka
Carbon black C Lexel
39
Polyvinylidene difluoride
(PVdF)
99% Aldrich
N-methyl-2-pyrrolidone
(NMP)
C5H9NO Aldrich
Potassium hydroxide KOH ≥ 85% Aldrich
Potassium chloride KCl ≥ 99.0% Aldrich
Sodium chloride NaCl ≥ 99.5% Aldrich
Lithium chloride LiCl ≥ 99.0% Aldrich
3.2. Experimental procedures Metal oxides, such as Co3O4, NiO, V2O5 and MnO2, were synthesized by different
methods in the present work. The synthesized materials were characterised by using a
Philips PW1730 X-ray diffractometer. The morphologies of the synthesized metal
oxides were observed by scanning electron microscopy (SEM). The specific surface
areas of the synthesized materials were calculated using the Brunauer-Emmett-Teller
(BET) multipoint method. Then, cyclic voltammetry (CV) was carried out to measure
the specific capacitances of the as-prepared metal oxides as electrode materials for
electrochemical capacitors. The overall experimental procedure is schematically
illustrated in Fig. 3-1.
40
Fig. 3-1. Schematic diagram of experimental procedure.
41
3.3. Materials Preparation
3.3.1. Metal oxides prepared by spray pyrolysis A schematic of the spray pyrolysis apparatus used to produce and collect the particles
is shown in Fig. 3-2. The main equipment consists of a two-fluid nozzle that converts
the starting solution into droplets, the carrier gas, a tubular furnace and a vacuum
pump. Liquid is sprayed through the double-nozzle with the aid of a carrier gas into
the tubular furnace, which is a quartz tube with an inner diameter of 20 mm and about
200 mm long. The furnace consists of 3 controlled heating zones that allow accurate
control of the experimental temperature distributions. The spray precursor solution is
first prepared. Distilled water is used for the preparation of a well-dissolved precursor
solution. The synthesis starts with aerosol generation of the liquid precursor. This
aerosol is subsequently directed into a pyrolysis chamber where the powders are
formed. After the spray process, the powders are collected in a stainless container.
It was found that the size of the particles produced by spray pyrolysis depends on
many parameters, such as the concentration and viscosity of the precursor solution,
the feed rate of the solution, the chamber temperature, and the flow rate of the carrier
gas. In order to obtain materials that give good reproducible results, it is necessary to
optimize all these parameters [70].
42
Fig. 3-2. Experimental setup for preparing metal oxides by the spray pyrolysis technique.
3.3.2. Metal oxides prepared by co-precipitation and heat treatment
Co-precipitation is a classical method for synthesizing several types of crystalline and
amorphous oxides. The procedure consists of solubilizing inorganic or organometallic
salts of metals in an aqueous or non-aqueous solvent followed by hydrolysis using
strong hydrolysing agents such as NH4OH or NaOH. In most cases, the precipitated
hydroxides are heat-treated to yield the oxides. In some cases, such as amorphous
MnO2, is directly produced by the reaction between a Mn(II) salt aqueous solution
and a KMnO4 aqueous solution according to the following equation [71]:
2MnO4− + 3Mn2+ + 2H2O = 5MnO2 + 4H+ (3-1)
43
Amorphous MnO2 produced by co-precipitation was then suspended in distilled water
and spray dried at different temperatures by using a vertical spray pyrolysis apparatus
to obtain high surface area amorphous or crystalline MnO2.
3.4. Structural and physical characterization of oxide materials
The metal oxides produced were characterized by using a Philips PW1730 X-ray
diffractometer with monochromatised Cu Kα radiation (λ = 1.5418 Å). An estimate
of the crystallite sizes was calculated using the Scherrer equation: ecrystallitD =
),cos(/9.0 Bθβλ where λ represents the x-ray wavelength, β is the observed full
width at half maximum (FWHM), and Bθ is the Bragg angle [72].
The morphology of the powders was observed by a JEOL JSM-6460A scanning
electron microscope (SEM).
The specific surface area of the powders was calculated using the Brunauer-Emmett-
Teller (BET) multipoint method with N2 adsorbate at 77 K using a Quantachrome
Nova 1000 Autosorb-1 Gas Sorption system [73]. Powders were weighed, placed in a
PyrexTM chamber and outgassed at 140 ºC under inert gas flow for 1 hour prior to
measurement.
44
3.5. Electrode preparation and test cell fabrication
3.5.1. Electrode preparation
The electrodes were formed by mixing active materials, carbon black (Lexel, 99%)
and polyvinylidene difluoride (PVdF Aldrich, 99%) binders for 30 min and then N-
methyl-2-pyrrolidone (NMP) was dropped into the above mixture, which was ground
to form the coating slurry. This slurry was smeared onto a substrate and then dried in
a vacuum oven at 110oC overnight. The weight of the electrode material in every
experiment was approximately 1mg.
3.5.2. Test cell fabrication Beaker-type three-electrode test cells consist of the sample electrode to act as the
working electrode, a saturated calomel electrode (SCE) or Ag|AgCl electrode as
reference electrode, a platinum foil as counter electrode, and an aqueous solution of
KOH, KCl, NaCl or LiCl as electrolyte. In order to examine the electrochemical
properties of the prepared electrode materials, cyclic voltammetry (CV) was carried
out using a CH Instruments Electrochemical Workstation (CHI 660A). The specific
capacitance of the material was estimated from the CV by integrating the area under
the current–potential curve and then dividing by the sweep rate, the mass of the
electrode and the potential window according to the equation:
45
(3-2)
where (Va−Vc) represents the potential window.
46
Chapter 4. Nanocrystalline Co3O4 powders as electrode materials for electrochemical capacitors
4.1. Introduction Cobalt oxide is one of the most studied transition metal oxides for numerous
scientific technologies. Cobalt oxide has many industrial applications, such as solar
selective absorber, catalyst in the hydrocracking processing of crude fuels, pigment
for glasses and ceramics [74], and catalyst for oxygen evolution and oxygen reduction
reactions [75]. It is also widely used as an electrochromic material [76], in sensors, in
electrochemical anodes [77,78], and in newly invented application in electrochemical
capacitors [79].
Cobalt has three polymorphs; the monoxide or cobaltous oxide (CoO), the cobaltic
oxide (Co2O3) and the cobaltosic oxide or cobalt cobaltite (Co3O4). CoO is the final
product formed when the cobalt compound or other oxides are calcined at a high
temperature (1173 K). Pure CoO is difficult to obtain, since it takes up oxygen even
at room temperature and reforms to a higher valence oxide. Cobaltic oxide (Co2O3)
can be formed when cobalt compounds are heated at a low temperature in the
condition of an excess of air. Co2O3 can be completely converted into Co3O4 at
temperatures > 538 K [80].
In comparison with expensive RuO2, which has been used extensively as an electrode
material for electrochemical capacitors, Co3O4 as an electrode material has been
47
found to have good efficiency, good corrosion stability, good long-term performance,
and low cost. These qualities make it a very promising candidate for use in
electrochemical capacitors. The possibility of cobalt oxide being a candidate for
capacitor applications was explored by Lin et al. [81], who prepared cobalt oxide
xerogel powders by using a sol–gel technique, followed by a heating step to different
temperatures. The capacitance of the material was estimated to be 291 F/g for a single
electrode by using charge and discharge measurements. However, the material did not
have the box-shaped characteristic typical of capacitors, but rather battery-like
behaviour was observed. No capacitance was seen at potentials less than 0.0 V versus
the saturated calomel reference electrode.
Liu et al. [82], investigating the redox behaviour and charge-storage mechanism of
thick cobalt oxide films, which were grown on Co metal electrodes in aqueous NaOH
under conditions of potential cycling in cyclic voltammetry, showed that cobalt oxide
films exhibit pseudocapacitance behaviour through about 2800 cycles over the
potential range of −0.2 to 1.56 V at a sweep rate of 20 mV s−1. However, the author
did not mention the capacitance of those thick cobalt oxide films.
Cobalt oxide (Co3O4) films were deposited at different sputtering gas-ratios of
O2/(Ar+O2) by Kim et al. [79]. Room temperature charge–discharge measurements of
Co3O4/LiPON/Co3O4 thin-film electrochemical capacitors demonstrated that the
Co3O4-based thin-film electrochemical capacitors exhibited bulk-type capacitor
behaviour. However, no capacitance values were mentioned by the author.
48
Spray pyrolysis is probably the easiest, lowest cost, fastest and most convenient
technique to prepare cobalt oxide. Cobalt oxide (Co3O4) thin films were prepared on
glass substrates by spray pyrolysis technique from an aqueous cobalt chloride
solution by Shinde et al. [83]. The Co3O4 electrode exhibited a specific capacitance of
74 F/g. The relatively low capacitance may be due to the high temperature
preparation technique and the resistance of the current collector, i.e. fluorine-doped
thin oxide coated glass substrate.
In our work, the spray pyrolysis technique was applied to synthesize nanocrystalline
Co3O4 powders. The spray pyrolysis in situ process ensures that the chemical reaction
is completed during a very short time, preventing the crystals from growing larger at
500 °C. Their physical and electrochemical properties as electrode materials for
electrochemical capacitors have been tested systematically.
4.2. Experimental
The spray precursor was prepared from a 0.2 M aqueous solution of cobalt nitrate
hexahydrate (Co(NO3)2 · 6H2O, 98%, Aldrich Chemical company Pty, Limited). The
solution was then fed into a vertical spray pyrolysis reactor. An atomizing nozzle was
used in combination with compressed air. The liquids were fed at a rate of 3 ml/min,
and the spraying was carried out at a pressure of 2.0 MPa and an atmospheric
temperature of 500 °C to produce Co3O4.
49
The as-prepared Co3O4 was characterized by X-ray diffraction. The morphology of
the cobalt oxide powders was observed by SEM. The specific surface area of the
powders was determined by the gas sorption technique using the BET method.
Beaker-type three-electrode testing cells were fabricated to test the electrochemical
properties of cobalt oxide powders. The working electrode was made by dispersing
70 wt% Co3O4, 22 wt% carbon black, and 8 wt% polyvinylidene fluoride (PVDF)
binder in dimethyl phthalate solvent to form a slurry, which was then spread onto a
nickel foil. A platinum foil was used as counter electrode. All potentials were
referenced to saturated calomel reference electrode (SCE). The electrolytes used in
this study were KOH solutions. Cyclic voltammetry scans were recorded from –0.2 V
to 0.32 V at different scan rates.
4.3. Results and discussion
4.3.1. Materials characterization
The XRD pattern of Co3O4 is shown in Fig. 4.1. The Co3O4 powders prepared by
spraying nitrate solutions at 500 °C show very broad diffraction lines, indicating good
crystallinity. Using the (311) diffraction peak and the Scherrer formula: d = kλ/βcos θ,
the crystal size was calculated to be about 5 nm.
50
20 30 40 50 60 70
0
20
40
60
80
100
(440)(511)
(400)
(311)
(220)
(111)In
tens
ity (a
.u.)
2 theta (o)
Fig. 4-1. XRD pattern of Co3O4 prepared by spraying nitrate solutions at 500 °C.
SEM images with different magnifications of the as-prepared Co3O4 are shown in
Fig. 4-2 (a and b). Agglomerated particles with spherical or ‘doughnut’ structures of
Co3O4 can be seen from the SEM images. The “doughnut” structure is typical for
spray pyrolysis of nitrate solutions where the precursor decomposition and release of
nitric oxides takes place at high temperature, which leads to breaks and the
appearance of significant holes in the agglomerates.
51
a
b
Fig. 4-2. SEM images of the as-prepared Co3O4 at different magnifications: (a) 2000×, (b) 50,000×.
52
BET analysis was performed and the results are shown in Table 4-1. The BET results
show that Co3O4 prepared by spraying nitrate solution has a high specific surface area
of 82 m2/g, which is much higher than that of commercial Co3O4. This is due to the
almost instant chemical decomposition and short reaction time used.
Table 4-1. Results from BET analysis of cobalt oxides.
Sample Specific surface area determined by
multipoint BET method (m2/g)
Commercial Co3O4 (Aldrich) 33
Co3O4 powder from Spraying
cobalt nitrate at 500 °C 82
53
4.3.2. Electrochemical properties In order to study the application of Co3O4 in electrochemical capacitors, the
electrochemical properties of Co3O4 relevant to electrochemical capacitors were
studied from C–V curves in aqueous 2M KOH electrolytes. Fig. 4-3 shows the cyclic
voltammertric (CV) behaviour of the prepared Co3O4 electrode in 2 M KOH solution
over a potential range from −200 to +320 mV versus SCE at various scan rates. The
shape of the CV curves in Fig. 4-3 is similar to that previously reported by Popov et
al. [81]. The CV curves of Co3O4 show that this material does not exibit pure double-
layer capacitance, but also faradaic pseudocapacitance. A redox insertion reaction
originates from the three-dimensional absorption of electroactive species into the bulk
solid electrode material [84]. The redox peaks seen in Fig. 4-3 can be attributed to
[81,85]:
(4-1)
As revealed in Fig. 4-3, the shapes of CV curves were significantly influenced by
changes in the scan rate. In addition, the specific capacitance gradually decreased as
the potential scan rate was increased from 5 to 50 mV s−1 for cobalt oxide electrodes.
For instance, the specific capacitance of the as-prepared cobalt oxide was as high as
168 F g−1 at a sweep rate of 5 mV s−1 but decreased to112 F g−1 as the sweep rate was
raised to 50 mV s−1. Fig. 4-4 shows a plot of specific capacitance as a function of
scan rate in 2 M KOH electrolyte. Similar results were obtained by Cao et al. [86].
54
-0.2 -0.1 0.0 0.1 0.2 0.3 0.4
-12
-10
-8
-6
-4
-2
0
2
4
6
8 50 mV/s 20 mV/s 10 mV/s 5 mV/s
I (A
)
U (V)
Fig. 4-3. Cyclic voltammograms of Co3O4 synthesized by spray pyrolysis in 2 M KOH solution at various scan rates.
020
406080
100120140
160180
0 10 20 30 40 50 60
Scan rate (mV/s)
Spec
ific
capa
cita
nce
(F/g
)
Fig. 4-4. Specific capacitance vs. scan rate in 2 M KOH electrolyte.
55
Two more pieces of evidence can clarify the charge-storage mechanism. First,
although electrical double-layer capacitance contributes to the measured capacitance
due to the surface area of the material, it is only a minor part of the overall measured
capacitance for the as-prepared Co3O4 electrode. Calculation of the pure electrical
double-layer capacitance using the BET specific surface area implies an average
value of 20 µF/cm2, thus giving the electrical double-layer capacitance Cd has 16.4
F/g for Co3O4 prepared by spraying nitrate solutions. These values are about ten times
lower than the corresponding measured capacitance of 168 F/g. Therefore, it is further
demonstrated that the main component of the measured capacitance comes from the
pseudocapacitive surface redox process. Second, a theoretical capacitance CT is
estimated on the basic of the redox mechanism [81]. The calculated theoretical
capacitance CT is 144.5 F/g corresponding to a specific surface area of 82 m2/g for
Co3O4. As listed in Table 4-2, considering either the present Co3O4 or the Co3O4 from
the literature, the measured C is very close to Cd + CT, implying that the measured C
consists of both the electrical double-layer capacitance and the pseudocapacitance
based on the redox process.
56
Table 4-2. The BET specific surface area and the double-layer (Cd) and theoretical (CT) components of the measured capacitance for the Co3O4 studied here compared with Co3O4 values from the literature.
4.4. Summary
Nanocrystalline Co3O4 powders were successfully prepared by spraying nitrate
solutions at 500 °C. The XRD pattern shows that single-phase and pure crystalline
Co3O4 with a crystal size of 5 nm was obtained. Agglomerated particles with
spherical or ‘doughnut’ structures of Co3O4 can be seen from the SEM images. The
BET result shows that Co3O4 prepared by spraying nitrate solution has a very high
specific surface area of 82 m2/g. The specific capacitance of this material in 2 M
KOH solution at a sweep rate of 5 mV/s was 168 F/g, which has persuaded us to
propose Co3O4 as a promising material for electrochemical capcitors.
57
Chapter 5. Nanocrystalline NiO powders as electrode materials for electrochemical capacitors
5.1. Introduction Nickel oxide has received a considerable amount of attention over the last few years
due to its large surface area, high conductivity, and pseudocapacitive behavior. It is
applied in diverse fields, such as smart windows, active optical fibers [87], catalysis
[88], electrochromic films [89], fuel cell electrodes [90,91], gas sensors [92,93], and
others [94,95].
Nam et al. [96] and Yang et al. [97] have prepared NiOx thin film electrodes by an
electrodeposition method, however, the conditions are rigid and the deposition
quantity restricted the dimensions of the electrode. Other methods have been used to
obtain NiO electrodes for capacitors, for example, the sol–gel dip-coating method and
the cathodic precipitation method. Liu and Anderson [98] used the sol–gel method to
fabricate a porous NiO electrode. In this process, nickel foil was withdrawn from the
prepared Ni(OH)2 sol and then heated at 300 °C to convert Ni(OH)2 to NiO. The
prepared NiO layer was 0.4 µm thick. The structure of this type of NiO is, however,
uncontrollable and disordered. In addition, only a small amount of Ni(OH)2 can be
attached to the current collector. In order to control the coating mass of the active
material, Srinivasan and Weidner [99] applied an electrochemical precipitation
method to fabricate NiO electrodes. The prepared NiO film was 0.1–1 µm thick and
7.0–70.0 µg in weight. The average capacitance of a 35 µg nickel oxide film was
58
168 F g−1. In more recent work [100], these authors reported that the specific capacity
had reached 155 F g−1 when cycling a 350 µg NiO film over a 0.5 V range. Although
the precipitated amount of Ni(OH)2 can be controlled by using this method, the
structure of the precipitated Ni(OH)2 is less porous than that obtained by the sol–gel
method, and the mass of precipitated materials is still much less than 1 mg. From the
results of these groups of workers, it can be concluded that the capacitance decreases
dramatically with an increased mass of nickel oxide on the electrode, even though the
total mass of active material is still much less than 1 mg. This means that the sol–gel
dip-coating method and the cathodic precipitation method will inevitably encounter a
serious fall in capacitance during scaling-up.
Zhang et al. [101] synthesized nanocrystalline nickel oxide (NiO) by a liquid-phase
process to obtain the hydroxide and then calcined at different temperatures. The NiO
powders calcined were examined by cyclic voltammograms (CV), and it was found
that the nickel oxide calcined at 300 °C had the largest specific capacitance, which
was 300 F g−1 in 6 M KOH at a sweep rate 5 mV s−1. Xing et al. [102] prepared
Ni(OH)2 by using sodium dodecyl sulfate as a template and urea as a hydrolysis-
controlling agent. NiO with a centralized pore-size distribution was obtained by
calcining Ni(OH)2 at different temperatures. The specific capacity reached 124 F g−1
in 3 wt.% KOH at a sweep rate of 10 mV s−1.
In recent years, nanostructured electrode materials have attracted great interest since
the nanostructured electrodes show better rate capabilities than conventional
59
electrodes composed of the same materials. The surface area of the nanostructured
electrode is much larger, leading to an effective current density smaller than that of a
conventional electrode at the same current density during charge and discharge. The
high specific surface area of these materials has significant implications with respect
to energy storage devices based on electrochemically active sites (batteries,
supercapacitors) and energy conversion devices depending on catalytic sites in defect
structures (fuel cells and thermoelectric devices) [103-107].
Therefore, in this work, nanostructured Ni(OH)2 powders were produced by a
modified method including co-precipitation of nickel hydroxide and further spray
drying of the precipitate. Then the Ni(OH)2 powders were heat-treated at 300 °C in
air for 1 hour in order to obtain nanocrystalline NiO powders. Their physical and
electrochemical properties as electrode materials for electrochemical capacitors have
been tested systematically.
5.2. Experimental
Generally, to produce pure spherical nickel hydroxide, an aqueous Ni salt solution is
supplied to a reaction vessel together with an alkali metal hydroxide solution and an
ammonium ion donor, with the system maintained at a constant stirring rate and
temperature (in the range of 20-80 °C), and a constant pH value in the range of 9-12.
Under these conditions spherical Ni(OH)2 agglomerates will grow. In our work, we
used a modified process, based on the spray dry method, for the formation of
60
Ni(OH)2. The first step of the process is co-atomization of the alkali solution to obtain
a nanostructured precipitate of pure Ni(OH)2. There are no spherical agglomerates
grown at this stage as the co-precipitation is done quickly. The precipitate consists of
particles with irregular shapes, and it is easily and quickly obtained and washed. The
next step is spray drying of the washed slurry. During this process spherical, dried
agglomerates are instantly obtained with the desired diameter, which is mainly
controlled by the diameter of the spray nozzle. The preparation process is described in
detail in Ref. [108]. The prepared Ni(OH)2 powders were then calcined at 300 °C in a
box furnace for 1 hour in order to obtain nanocrystalline NiO powders.
The as-prepared NiO was characterized by X-ray diffraction. The morphology of the
oxide powders was observed by SEM. The specific surface area of the powders was
determined by the gas sorption technique using the BET method.
Beaker-type three-electrode test cells were fabricated to test the electrochemical
properties of the nickel oxide powders. The working electrode was made by
dispersing 75 wt% NiO, 15wt% carbon black, and 10 wt% polyvinylidene difluoride
(PVdF) binder in dimethyl phthalate solvent to form a slurry, which was then spread
onto a nickel foil. A platinum foil was used as counter electrode. All potentials were
referenced to Ag|AgCl reference electrode. The electrolytes used in this study were
KOH solutions. Cyclic voltammetry scans were recorded from –0.2 V to 0.4 V at
different scan rates.
61
5.3. Results and discussion
5.3.1. Materials characterization The XRD pattern of the as-synthesized nickel oxide is shown in Fig. 5-1. All these
diffraction peaks, including not only the peak positions, but also their relative
intensities, can be perfectly indexed into the cubic crystalline structure of NiO. The
result is in accordance with the standard spectrum (JCPDS, card no 04-0835). The
crystal size was about 12 nm, calculated from the major diffraction peak (2 0 0) using
the Scherrer’s formula. Ni(OH)2 is converted to NiO at 300°C via the following
reaction:
Ni(OH)2 → NiO + H2O (5-1)
62
10 20 30 40 50 60 70 80
0
200
400
600
800
1000
(222)(311)
(220)
(200)
(111)
Inte
nsity
(a.u
.)
2 theta (o)
Fig. 5-1. XRD pattern of as- prepared NiO.
The morphological features of the Ni(OH)2 produced are shown in Fig. 5-2 (a). SEM
images of the as-prepared NiO with different magnifications are shown in Fig. 5-2 (b
and c). It can be clearly seen that almost all of the agglomerates have spherical
shapes. The agglomerates consist of small particles with sizes of 1-5 µm. Their
appearance suggests a highly developed surface area, which has been confirmed by
BET analysis.
63
a
b
64
c Fig. 5-2. SEM images of the as-prepared Ni(OH)2 (a) and SEM images of the as-prepared NiO at different magnifications: (b) 2000×, (c) 20,000×. BET analysis was performed and the results show that the prepared NiO has a very
high specific surface area, which is 90 m2/g.
5.3.2. Electrochemical properties Cyclic voltammetry at different sweep rates was used to determine the
electrochemical properties of NiO electrode in 1 M KOH and thus to quantify the
specific capacitance of NiO electrode. The mass of the NiO in each electrode was
65
1 mg. Fig. 5-3 shows the cyclic voltammetry of the as-prepared NiO as electrode for
electrochemical capacitors. The electrode potential was scanned between –0.2 and
0.4 V at the different sweep rates of 5, 10 and 20 mV s−1, and the current response
was measured. Fig. 5-3 shows that crystalline NiO calcined at 300 °C exhibits
capacitive behaviour; the current–potential response is potential dependent, in
contrast to the potential-independent current response of an ideal capacitor. It should
be noted that the shape of the CV changed as the sweep rate increased. The shape of
the cyclic voltammogram at 5 mV s−1 is almost symmetrical. The maximal
capacitance of the as-synthesized crystalline NiO is 203 F g−1 at 5 mV s−1 in 1 M
KOH.
-0.2 -0.1 0.0 0.1 0.2 0.3 0.4
-6
-4
-2
0
2
4
I (A
)
U (V)
20mv/s 10mv/s 5mv/s
Fig. 5-3. Cyclic voltammograms of NiO in 1 M KOH solution at various scan rates.
66
The following Eq. (5-2) shows that the reaction of NiO to NiOOH occurs at the
surface of the NiO, and has been determined that these surface redox reactions may
contribute to the measured capacitance over a certain potential range [101].
(5-2)
Many studies of the nickel oxides, such as Nagai [109] have examined the coloration
reaction of NiOx prepared by e-beam evaporation in LiOH solution and reported that
the coloration reaction of NiOx was due to pure OH− insertion (extraction) during
oxidation (reduction). Torresi et al. [110] and Faria et al. [111] found that when
anodic polarization of NiO(OH)x films prepared by e-beam evaporation in KOH
solution began, the mass decreased, and they reported that it was caused by an
expulsion of OH− from the film. Then, it was reported that a small mass decrease at
the initial stage of oxidation and a large increase in the latter stage of oxidation of the
NiO(OH)x and then OH− insertion in the latter stage of oxidation [112].
The reason that the as-prepared NiO has a very high capacitance may be because the
powders are nanosized (~12 nm) and crystalline. Nanostructured electrode materials
exhibit more attractive properties compared with conventional electrode materials,
such as very small particle size, large exposed surface areas, and high surface energy.
These properties can enlarge the contact area, make the most of any electro-active
materials, and enhance the electrochemical reaction rate. In view of the low-cost and
67
environmentally benign nature of the material, this electrode is believed to be very
promising for large-scale applications.
Fig. 5-4 indicates that the specific capacitance of NiO increases, but the anodic
potential limit decreases, when the KOH concentration is increased. Lower KOH
concentrations result in an increase in the anodic potential limit of the electrode, since
oxygen evolution occurs at more positive potentials.
-0.2 -0.1 0.0 0.1 0.2 0.3 0.4
-8
-6
-4
-2
0
2
4
I(A)
U(V)
1M KOH 2M KOH 3M KOH
Fig. 5-4. Cyclic voltammograms of NiO in different KOH concentrations.
68
5.4. Summary Nanostructured Ni(OH)2 powders were produced by a modified method including
co-precipitation of nickel hydroxide and further spray drying of the precipitate. The
as-prepared Ni(OH)2 powders were calcined at 300 °C in a box furnace for 1 hour in
order to yield nanocrystalline NiO powders. The XRD pattern shows that single-
phase and pure crystalline NiO with a crystal size of 12 nm was obtained.
Agglomerated particles with spherical NiO structures can be seen from the SEM
images. The BET result shows that the as-prepared NiO has a very high specific
surface area of 90 m2/g. The specific capacitance of this material in 1 M KOH
solution at a sweep rate of 5 mV/s was 203 F/g, which is very high.
The reason may be because the powders have a nanosized crystal-like structure.
69
Chapter 6. Crystalline V2O5 powders as electrode materials for electrochemical capacitors
6.1. Introduction
Oxides of vanadium are of considerable interest because of their phase
transformations as well as their uses in energy-related device applications [113-115].
V2O5 has become of particular interest in recent years, because of its potential
applications in optical switching devices [116], in electrochromic devices [117-119],
and as a reversible cathode material for lithium batteries [120-121].
As specific capacitance value of 720 F/g was obtained by using amorphous ruthenium
oxide as electrode at a 2 mV/s scan rate in H2SO4 electrolyte [5]. However, hydrous
ruthenium oxide is very expensive. Efforts are being made to find a suitable material
to replace RuO2. V2O5 seems to be a viable electrode material for an electrochemical
capacitor because of its low cost, and vanadium exists in different oxidation states.
Very few investigations have been performed on V2O5 as an electrode material for an
electrochemical capacitor [122-124]. Lee et al. prepared V2O5 by quenching V2O5
fine powders at 950 °C in a bath of deionized water [122]. They studied this electrode
material in an aqueous KCl electrolyte. The material showed an ideal capacitance
curve under cyclic voltammetric conditions. They reported a specific capacitance of
346 F g−1 at a pH of 2.32. Kudo et al. synthesized a V2O5 sol by reacting metallic
vanadium with 30% H2O2 [124]. They studied V2O5 and carbon composite electrodes
in non-aqueous electrolytes. This material did not show ideal capacitance, and the
70
authors did not mention the specific capacitance in their paper. Reddy et al. [125]
prepared nanoporous layer structured V2O5 by using the sol–gel method. The V2O5
showed the highest capacitance in 2 M KCl electrolyte when compared to other
electrolytes such as NaCl and LiCl. It yielded a maximum specific capacitance of
214 F g−1 in 2 M KCl electrolyte.
In our work, we used a new method based on a co-precipitation and calcination
technique to prepare crystalline V2O5 powders. Their physical and electrochemical
properties as electrode materials for electrochemical capacitors were systematically
tested.
6.2. Experimental Commercial vanadium trichloride (VCl3) and ammonium hydroxide (NH4OH) were
purchased from Aldrich and used as starting materials. A 0.2 M vanadium trichloride
aqueous solution and a 0.2 M ammonium hydroxide aqueous solution were prepared
for co-precipitation. The ammonium hydroxide (NH4OH) aqueous solution was added
dropwise to the vanadium trichloride (VCl3) aqueous solution while stirring the
solution. The solution was continuously stirred to make the reaction proceed
thoroughly. The precipitate was filtered and washed several times with distilled water
to remove any soluble products. Then the precipitate was finally heat-treated at
300 °C in air for 1 hour to obtain the final V2O5 powders.
71
The as-prepared V2O5 was characterized by X-ray diffraction. The morphology of the
oxide powders was observed by SEM. The specific surface area of the powders was
determined by the gas sorption technique using the BET method.
Beaker-type three-electrode testing cells were fabricated to test the electrochemical
properties of V2O5 powders. The working electrode was made by dispersing 67 wt%
V2O5 material, 25 wt% carbon black, and 8 wt% polyvinylidene difluoride (PVdF)
binder in dimethyl phthalate solvent to form a slurry, which was then spread onto a
platinum foil. A platinum foil was used as counter electrode. All potentials were
referenced to saturated calomel reference electrode (SCE). The electrolytes used in
this study were NaCl, KCl and LiCl. Cyclic voltammetry (CV) was conducted in a
voltage range of −0.2 to 0.7 V and at varying scan rates.
6.3. Results and discussion
6.3.1. Materials characterization The XRD patterns of V2O5 powders are shown in Fig. 6-1. The V2O5 powders
prepared by co-precipitation and calcined at 300 °C show very sharp diffraction
peaks, indicating good crystallinity. The crystal size of V2O5 was calculated to be
about 70 nm by using the Scherrer formula.
72
10 20 30 40 50 60 70-50
0
50
100
150
200
250
300
350
400
Inte
nsity
(a.u
.)
2 theta (o)
Fig. 6-1. XRD pattern of the as- prepared V2O5.
The morphological features of the V2O5 powders with different magnifications are
shown in Figure 6-2. It can be clearly seen that the powders consist of agglomerates
with different shapes (Fig. 4-2a). The agglomerates consist of very fine particles (Fig.
4-2 (b and c)). Their appearance suggests a highly developed surface area, which has
been confirmed by BET.
73
a
b
74
c
Fig. 6-2. SEM images of the as-prepared V2O5 at different magnifications: (a) l000×, (b) 20,000× and (c) 50,000×.
A surface area of 41m2 g−1 was derived from a multi point BET measurement.
6.3.2. Electrochemical properties Fig. 6-3 shows cyclic voltammetric curves of V2O5 in 2 M KCl at different scan rates.
As revealed in Fig. 6-3, the shapes of the CV curves were significantly influenced by
the scan rate. At a low scan rate (5 mV s−1), the CV curve shows a near-ideal
rectangular shape, which indicates that charging and discharging took place at a
75
constant rate over the applied voltage range [126]. A specific capacitance of 262 F g−1
was obtained for V2O5 powders at a 5 mV s−1 scan rate. Fig. 6-4 shows a plot of
specific capacitance as a function of scan rate in 2 M KCl electrolyte. It can be seen
that the specific capacitance gradually decreases as the potential scan rate is increased
from 5 to 50 mV s−1 for V2O5 electrodes. This may be because at high scan rates,
diffusion limits the movement of K+ ions by the time constraint, and only the outer
active surface is utilized for the charge storage. However, at lower scan rates, all the
active surface area can be utilized for charge storage [125].
-0.2 0.0 0.2 0.4 0.6 0.8-14
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
I (A
)
U (V)
50 mV/s 20 mV/s 10 mV/s 5 mV/s
Fig. 6-3. Cyclic voltammograms of V2O5 in 2 M KCl at various scan rates.
76
0
50
100
150
200
250
300
0 10 20 30 40 50 60
Scan rate (mV/s)
Spec
ific
capa
cita
nce
(F/g
)
Fig. 6-4. Specific capacitance vs. scan rate
Fig. 6-5 shows cyclic voltammetric curves of V2O5 in 2 M KCl, 2 M NaCl and 2 M
LiCl at a 5 mV s−1 scan rate. Table 6-1 shows the specific capacitance of V2O5 in
different electrolytes. As evident from Fig. 6, V2O5 yielded the highest specific
capacitance in 2 M KCl electrolyte. It is interesting to note that V2O5 yielded similar
specific capacitance values of 166 and 160 F g−1 in 2 M NaCl and 2 M LiCl
electrolytes, respectively, despite the difference in the size of the sphere of hydration
of Na+ and Li+ ions. Table 6-1 shows the specific capacitance of V2O5 in different
77
electrolytes compared with the literature on V2O5 electrode materials. A higher
specific capacitance was achieved for the present V2O5 when compared to the
literature, possibly because the present V2O5 has a higher specific surface area
(41m2 g−1) than that (7 m2 g−1) reported in the literature.
-0.2 0.0 0.2 0.4 0.6 0.8-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.41.61.8
I (A
)
U (V)
2 M LiCl 2 M KCl 2 M NaCl
Fig. 6-5. Cyclic voltammograms of V2O5 in different electrolytes.
78
Table 6-1. Comparison of specific capacitance of V2O5 with previously studied V2O5 from the literature in different electrolytes at a 5 mV s−1 scan rate.
Fig. 6-6 shows cyclic voltammetric curves of V2O5 in 2 M KCl and 1 M KCl at a
5 mV/s scan rate. The specific capacitance of V2O5 increases when the concentration
of KCl is increased. This may be because a lower concentration of KCl has a higher
electrolyte resistance when compared to 2 M KCl.
79
-0.2 0.0 0.2 0.4 0.6 0.8-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
I (A
)
U (V)
1 M KCl 2 M KCl
Fig. 6-6. Cyclic voltammetric curves of V2O5 in 2 M KCl and 1 M KCl at a 5 mV/s scan rate.
6.4. Summary
V2O5 powders were prepared by co-precipitation and calcined at 300 °C. The XRD
pattern shows that single-phase and pure crystalline V2O5 was obtained.
Agglomerated particles of V2O5 can be seen from the SEM. The BET result shows
that the as-prepared V2O5 has a very high specific surface area of 41m2/g. V2O5
showed the highest capacitance in 2 M KCl electrolyte when compared to other
electrolytes such as 2 M NaCl and 2 M LiCl. It yielded a maximum specific
capacitance of 262 F g−1 in 2 M KCl electrolyte. The higher specific capacitance of
the present V2O5 when compared with the previously studied V2O5 from the literature
may be because it has a higher specific surface area.
80
Chapter 7. Amorphous and nanocrystalline MnO2 as electrode materials for electrochemical capacitors
7.1. Introduction
The natural abundance of manganese oxide and its environmental compatibility make
it the subject of increasing research interest. There are several oxidation states,
including Mn(0), Mn(II), Mn(III), Mn(IV), Mn(V), Mn(VI), and Mn(VII), for
manganese oxides [127]. They have often been employed as the cathode materials in
rechargeable batteries [128,129].
Recently, manganese oxides have been considered as potential candidates for the
electrode material of electrochemical capacitors [130-133]. Although there are several
oxidation states for manganese oxides, it has been widely observed that MnO2
exhibits a rather high performance in comparison to the other oxides such as
Mn(OH)2, Mn2O3, and Mn3O4 [132,134,135]. In addition, several methods have been
developed for preparing manganese oxides, including co-precipitation [130], thermal
decomposition [131], anodic deposition [132,136] and the sol–gel process
[133,135,137].
Studies of Lee and Goodenough [130] have shown that amorphous MnO2·nH2O is a
promising electrode material for electrochemical capacitors. A composite electrode
material containing amorphous MnO2·nH2O and acetylene black showed a specific
81
capacitance of 200 F/g. Lee et al. [131] prepared MnO2 by thermal decomposition of
KMnO4 at different temperatures, and a sample decomposed at 550 °C gave a specific
capacitance of 240 F/g. Amorphous hydrous manganese oxide (a-MnO2·nH2O)
produced by Hu and Tsou from a MnSO4·5H2O solution via anodic deposition
yielded a specific capacitance in the range of 265–320 F/g between 0 and 1.0 V in
0.1 M Na2SO4 solution [132]. Pang et al. [133,135] have shown that sol-gel-derived
MnO2 thin films exhibited a specific capacitance as high as 698 F/g in a potential
window of 0–0.9 V. However, the specific capacitance decreased with increasing film
thickness due to the low conductivity of MnO2. The potential use of such sol-gel-
derived MnO2 films for fabricating practical devices is limited by the very dilute
concentration (10–3 M) of the MnO2 colloidal suspension employed in coatings.
The solid phase reaction of KMnO4 with manganese (II) acetate tetrahydrate at room
temperature or at a low heating temperature was proved to be effective in preparing
nanosized MnO2 powders by Li and Luo [71]. However, the amount of water in the
solid reactant system and the length of grinding time would influence the extent of
aggregation and the morphology of the product particles. In our work, we used the
same chemicals but another new method based on co-precipitation and the spray dry
technique to prepare amorphous or nanocrystalline MnO2 powders. Their physical
and electrochemical properties as electrode materials for electrochemical capacitors
were systematically tested.
82
7.2. Experimental
Commercial purity manganese (II) acetate tetrahydrate (CH3COO)2Mn · 4H2O and
potassium permanganate (KMnO4) were purchased from Aldrich and used as starting
materials. A 0.3 M manganese (II) acetate aqueous solution and a 0.2 M KMnO4
aqueous solution were prepared for co-precipitation. The KMnO4 aqueous solution
was added dropwise to the manganese acetate aqueous solution while stirring. The
solution was continuously stirred and heated for 3 hours at 80 oC to make the reaction
proceed thoroughly. The precipitate was filtered and washed several times with
distilled water to remove any soluble products. Then the precipitate was resuspended
in distilled water and spray dried at the different temperatures of 200 °C, 300 °C and
400 °C by using a vertical spray pyrolysis apparatus to obtain amorphous or
crystalline MnO2 powders. The reaction between Mn(II) salt aqueous solution and
KMnO4 aqueous solution occurred according to the following equation [71]:
2MnO4− + 3Mn2+ + 2H2O = 5MnO2 + 4H+ (7-1)
The as-prepared MnO2 was characterized by X-ray diffraction. The morphology of
the oxide powders was observed by SEM. The specific surface area of the powders
was determined by the gas sorption technique using the BET method.
83
Beaker-type three-electrode test cells were fabricated to test the electrochemical
properties of manganese oxide powders. The working electrode was made by
dispersing 68 wt% MnO2, 24 wt% carbon black, and 8 wt% polyvinylidene difluoride
(PVdF) binder in dimethyl phthalate solvent to form a slurry, which was then spread
onto a platinum foil. A platinum foil was used as counter electrode. All potentials
were referenced to saturated calomel reference electrode (SCE). The electrolytes used
in this study were NaCl, KCl and LiCl electrolytes with varying concentrations.
Cyclic voltammetry (CV) was conducted in the voltage range of 0.0 to 1.0 V and at
varying scan rates.
7.3. Results and discussion
7.3.1. Materials characterization
Fig. 7-1 illustrates the XRD patterns of MnO2 spray dried at different temperatures
from 200 °C to 400 °C. Broadening of peaks at 200 °C and 300 °C indicates the
amorphous nature of MnO2. The amorphous MnO2 transformed to crystalline MnO2
at 400 °C. From the X-ray pattern at 400 °C, the crystal size of MnO2 was calculated
to be about 6 nm by using the Scherrer formula.
84
10 20 30 40 50 60 70
0
100
200
300
400
500
4000C
3000C
200 0C
Inte
nsity
(a.u
.)
2 theta (o)
Fig. 7-1. XRD patterns of MnO2 spray dried at different temperatures.
The morphological features of the MnO2 prepared at 200 °C, 300 °C, and 400 °C are
shown in Fig. 7-2 (a, b, and c), respectively. It can be seen that the agglomerates have
different shapes and that the density increases with increasing temperature.
85
a
b
86
c
Fig. 7-2 SEM image of MnO2 prepared at 200 °C (a), 300 °C (b), and 400 °C (c)
BET analysis was performed and the results are shown in Table 7-1. It can be seen
that specific surface areas of MnO2 powders decrease significantly as the spray drying
temperature increases from 200 °C to 400 °C. This may be because amorphous MnO2
is transformed to crystalline MnO2 at high temperature.
87
Table 7-1. BET results on the as-prepared MnO2.
MnO2 powders Specific surface area determined by
multipoint BET method (m2/g)
MnO2 spray dried at 200 °C 269
MnO2 spray dried at 300 °C 194
MnO2 spray dried at 400 °C 183
7.3.2. Electrochemical properties
Cyclic voltammograms (CVs) of MnO2 spray dried at the different temperatures of
200 °C, 300 °C, and 400 °C, measured in 2 M KC1 solution at 25 °C at a potential
scan rate of 5 mV s−1, are shown in Fig. 7-3. The CV curves of all MnO2 samples are
close to rectangular shapes and exhibit mirror-image characteristics. The results
demonstrate the excellent reversibility and ideal pseudo-capacitive behaviour of the
as-prepared MnO2. As revealed in Fig. 7-3, the specific capacitances of MnO2 are
406, 334, and 297 F g−1 processed at the temperatures of 200 °C, 300 °C and 400 °C,
respectively. The reason may be that the specific surface area of MnO2 decreases
significantly as the spray dry temperature increases. Fig. 7-4 shows a plot of specific
capacitance as a function of temperature in 2 M KCl electrolyte.
88
0.0 0.2 0.4 0.6 0.8 1.0-5
-4
-3
-2
-1
0
1
2
3I (
A)
U (V)
200°C 300°C 400°C
Fig. 7-3. Cyclic voltammograms (CVs) of MnO2 spray dried at 200 °C, 300 °C, and 400 °C.
0
50
100
150
200
250
300
350
400
450
0 100 200 300 400 500
temperature (°C)
spec
ific
capa
cita
nce
(F/g
)
Fig. 7-4. Specific capacitance vs. temperature.
89
For electrochemical capacitors, electrolyte is another important factor. In this work,
some neutral aqueous solutions, such as 2 M KCl, 2 M NaCl and 2 M LiCl, were
chosen as electrolytes for supercapacitors. Fig. 7-5 shows cyclic voltammetric (CV)
curves of MnO2 spray dried at 200 °C in different electrolytes at a 5 mV/s scan rate.
The CV for the 2 M KCl solution shows a mirror image with respect to the zero-
current line and a rapid current response on voltage reversal at each end potential.
The CV for the 2 M NaCl solution shows that the current response on reversing the
potential at the two end potentials is not fast enough to maintain a mirror-image
shape. The CV for 2 M LiCl solution indicates an even slower response. A
comparison of the specific capacitance of MnO2 in the present study in different
electrolytes with values reported by other researchers is presented in Table 7-2. In the
present study MnO2 yielded the highest specific capacitance in 2 M KCl electrolyte.
The capacitance arises due to the intercalation of alkali ions into the MnO2 structure
causing redox transitions. Since water is a solvent, alkali ions are surrounded by the
water of hydration. Table 7-3 gives the ionic radius, the radius of the hydration
sphere, the free energy formation of water of hydration, and the conductivity of Li,
Na, and K ions [3,138,139]. The radius of the hydration sphere decreases in the order:
Li+ > Na+ > K+. Li+ and Na+ ions have larger hydration spheres when compared to the
K+ ion because of the Liδ+–H2Oδ− and Naδ+–H2Oδ− strong interactions. Howerver, the
overall radius of the hydration sphere of all ions is of the same order, ranging from
3.3 to 3.8 Å. So, the size of the hydration sphere may not be a deciding factor.
Potassium ions have distinctly higher conductivity when compared to sodium ions,
and sodium ions have a higher conductivity than lithium ions. Conductivity decreases
90
in the order: +KCond > +NaCond > +LiCond , due to a decrease in the mobility of the
ions. Conductivity and mobility of cations may be the determining factor for specific
capacitances of amorphous MnO2 in different electrolytes, decreasing in the order:
MKClSC2 > MNaClSC2 > MLiClSC2 . Lee and Goodenough [130] obtained similar results,
and they reported that the highest capacitance was obtained in 2 M KCl when
compared to 2 M NaCl and 2 M LiCl for amorphous MnO2. They attributed this result
to the higher mobility of the K+ ion due to its smaller hydration sphere size.
0.0 0.2 0.4 0.6 0.8 1.0-5
-4
-3
-2
-1
0
1
2
3
I (A
)
U (V)
2 M KCl 2 M NaCl 2 M LiCl
Fig. 7-5. shows cyclic voltammetric (CV) curves of MnO2 spray dried at 200 °C in different electrolytes.
91
Table 7-2. Comparison of specific capacitance of MnO2 in the present study with values reported by other researchers using different electrolytes.
Table 7-3. Crystal radius, radius of hydration sphere, free energy of hydration, and conductivity of alkali ions [3,138,139].
Free energy data is relative to H+ ion hydration; conductivity data corresponds to molar ionic conductivity in water solution at 25 °C.
92
As revealed in Fig. 7-6, the shapes of CV curves were significantly influenced by the
scan rate. In addition, the specific capacitance gradually decreased as the potential
scan rate was increased from 5 to 50 mV s−1 for manganese oxide electrode. For
instance, the specific capacitance of the as-prepared manganese oxide is as high as
406 F g−1 at a sweep rate of 5 mV s−1, but decreases to 235 F g−1 as the sweep rate is
raised to 50 mV s−1. Fig. 7-7 shows a plot of specific capacitance as a function of
scan rate in 2 M KCl electrolyte. This can be understood from the slow intercalation
of K+ ions into the MnO2 structure.
0.0 0.2 0.4 0.6 0.8 1.0
-25
-20
-15
-10
-5
0
5
10
15
I (A
)
U (V)
50 mV/s 20 mV/s 10 mV/s 5 mV/s
Fig. 7-6. CV curves of MnO2 at various scan rates in 2 M KCl electrolyte.
93
0
50
100
150
200
250
300
350
400
450
0 10 20 30 40 50 60
Scan rate (mV/s)
Spec
ific
capa
cita
nce
(F/g
)
Fig. 7-7. Specific capacitance vs. scan rate in 2 M KCl electrolyte.
7.4 . Summary A new method based on co-precipitation and the spray dry technique was used to
prepare MnO2. The XRD pattern shows that amorphous MnO2 was obtained at 200
°C and 300 °C. The amorphous MnO2 transformed to crystalline MnO2 at 400 °C.
Agglomerated particles of MnO2 can be seen in the SEM images. The BET result
shows that specific surface areas of MnO2 powders decrease significantly with
increasing spray dry temperature from 200 °C to 400 °C. Amorphous MnO2 prepared
at 200 °C showed the highest capacitance in 2 M KCl electrolyte when compared to
other electrolytes such as 2 M NaCl and 2 M LiCl. It yielded the maximum specific
capacitance of 406 F g−1 in 2 M KCl electrolyte.
94
Chapter 8. General conclusions
Nanocrystalline Co3O4 powders have been successfully prepared by spraying nitrate
solutions at 500 °C. The XRD pattern shows that single-phase and pure crystalline
Co3O4 with a crystal size of 5 nm was obtained. Agglomerated particles with
spherical or ‘doughnut’ structures of Co3O4 can be seen from the SEM images. The
BET results show that Co3O4 prepared by spraying nitrate solutions has a very high
specific surface area of 82 m2/g. The specific capacitance of the Co3O4 powders as
electrode materials for electrochemical capacitors in 2 M KOH solution at a sweep
rate of 5 mV/s was 168 F/g.
We used co-precipitation and a spray dry technique to obtain nickel hydroxide and
then calcined it at 300 °C to obtain the nanocrystalline NiO powders discussed in this
study. The as-prepared NiO has a very high specific surface area of 90 m2/g when
measured by BET. A specific capacitance of 203 F/g was obtained in 1 M KOH
solution at a sweep rate of 5 mV/s.
Crystalline V2O5 powders were prepared by co-precipitation and calcined at 300 °C in
this work. The as-prepared V2O5 showed the highest capacitance in 2 M KCl
electrolyte when compared to other electrolytes such as 2 M NaCl and 2 M LiCl. It
yielded a maximum specific capacitance of 262 F/g in 2 M KCl electrolyte. The
higher specific capacitance of the present V2O5 when compared with previously
95
studied V2O5 from literature may be because it has a higher specific surface area of
41 m2/g.
In this study, we used a new method based on co-precipitation and a spray dry
technique to prepare MnO2. The XRD pattern shows that amorphous MnO2 was
obtained at 200 °C and 300 °C. The amorphous MnO2 transformed to nanocrystalline
MnO2 at 400 °C. The BET results show that specific surface areas of MnO2 powders
decrease significantly with increasing spray dry temperature from 200 °C to 400 °C.
Amorphous MnO2 prepared at 200°C showed the highest capacitance of 406 F/g in
2 M KCl electrolyte when compared to other electrolytes such as 2 M NaCl and 2 M
LiCl.
In general, transition metal oxides, such as Co3O4, NiO, V2O5 and MnO2, were
successfully synthesized by spray pyrolysis or co-precipitation techniques. Their
crystal structure have been characterized by X-ray diffraction, SEM and BET
analysis. When the as-prepared metal oxides were used as electrode materials for
electrochemical capacitors, they demonstrated very high specific capacitances due to
their large surface areas and pseudocapacitive behaviours. In comparison with
expensive RuO2, the as-prepared Co3O4, NiO, V2O5, and MnO2 powders are much
cheaper. This makes them very promising candidates as electrode materials for
electrochemical capacitors.
96
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List of symbols EC Electrochemical capacitors
EDLC Electrical double-layer capacitor
XRD X-ray diffraction
SEM Scanning electron microscope
BET Brunauer-Emmett-Teller gas sorption technique
CV Cyclic voltammetry
SCE Saturated calomel reference electrode
SSA Specific surface area
SC Specific capacitance
nm Nanometer
Wt% Weight percent