University of Wollongong Thesis Collections

University of Wollongong Thesis Collection

University of Wollongong Year

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

This paper is posted at Research Online.

http://ro.uow.edu.au/theses/487

NOTE

This online version of the thesis may have different page formatting and pagination from the paper copy held in the University of Wollongong Library.

UNIVERSITY OF WOLLONGONG

COPYRIGHT WARNING

You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: Copyright owners are entitled to take legal action against persons who infringe their copyright. A reproduction of material that is protected by copyright may be a copyright infringement. A court may impose penalties and award damages in relation to offences and infringements relating to copyright material. Higher penalties may apply, and higher damages may be awarded, for offences and infringements involving the conversion of material into digital or electronic form.

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

vi

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

References

[1] D. Galizioli, F. Tantardini and S. Trasatti. J. Appl. Electrochem. 4 (1974), p. 57.

[2] B.E. Conway. J. Electrochem. Soc. 138 (1991), p. 1539.

[3] B.E. Conway, Electrochemical Supercapacitors, Plenum Press, New York, 1999.

[4] J. Melsheimer and D. Ziegler, Thin Solid Film, 163 (1988), p.301.

[5] S. Trasatti and P. Kurzweil. Plat. Met. Rev. 38 (1994), p. 46.

[6] J.E. Oxley, in: Proceedings of 34th International Power Sources Symposium,

Cherry Hill, NJ, USA, 25–28 June 1990.

[7] Y. Chabre and J. Pannetier, Prog. Solid State Chem. 23 (1995), p. 1.

[8] M.M. Thackeray, Prog. Solid State Chem. 25 (1997), p. 1.

[9] M. Winter, J.O. Besenhard, M.E. Spahr and P. Novák, Adv. Mater. 10 (1998), p.

725.

[10] H.E. Becker, U.S. Patent 2 800 616 (to General Electric) (1957).

[11] D.I. Boos, U.S. Patent 3 536 963 (to Standard Oil, SOHIO) (1970).

[12] T.C. Murphy, R.B. Wright and R.A. Sutula. In: F.M. Delnick, D. Ingersoll, X.

Andrieu and K. Naoi, Editors, Electrochemical Capacitors II, Proceedings 96–25, The

Electrochemical Society, Pennington, NJ (1997), p. 258.

[13] R. Kotz and M.Carlen Electrochimica Acta. 45 (1999), p. 2483.

[14] M. Matsumoto, Electrical phenomena at interfaces, Marcel Dekker Press, New

York. 76 (1998), p.87.

97

[15] H. Shi, Electrochim. Acta 41 (1996), p. 1633.

[16] J. Gamby, P.L. Taberna, P. Simon, J.F. Fauvarque and M. Chesneau. J. Power

Sources 101 (2001), p. 109.

[17] C. Lin, B.N. Popov and H.J. Ploehn. J. Electrochem. Soc. 149 (2002), p. 167.

[18] S. Sarangapani, B.V. Tilak and C.P. Chen. J. Electrochem. Soc. 143 11 (1996),

p. 3791.

[19] E. Jurewicz and K. Frackowiak. In: Carbon fibres for electrochemical capacitors.

Extended abstracts, 50th ISE Meeting, Pavia (Italy), (1999), p. 924.

[20] K. Jurewicz and E. Frackowiak, Modified carbon materials for electrochemical

capacitors. Mol. Phys. Reports. 27 (2000), p. 36–43.

[21] M.D. Ingram, A.J. Pappin, F. Delalande, D. Poupard and G. Terzulli.

Electrochim. Acta. 43 (1998), p. 1601.

[22] A. Rudge, I. Raistrick, S. Gottesfeld and J.P. Ferraris. Electrochim. Acta. 39

(1994), p. 273.

[23] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld and J.P. Ferrais. J. Power Sourc.

47 (1994), p. 89.

[24] T. Momma, X. Liu, T. Osaka, Y. Ushio and Y. Sawada. J. Power Sourc. 60

(1996), p. 249.

[25] M. Ishikawa, A. Sakamoto, M. Morita, Y. Matsuda and K. Ishida. J Power Sourc

60 (1996), p. 233.

[26] J.M. Miller and B. Dunn. Langmuir. 15 3 (1999), p. 799.

98

[27] J.W. Long, K.E. Swider, C.I. Merzbacher and D.R. Rolison. Langmuir. 15

(1999), p. 780.

[28] J.M. Miller, B. Dunn, T.D. Tran and R.W. Pekala. J. Electrochem. Soc. 144

(1997), p. L309.

[29] B.E. Conway, V. Birss and J. Wojtowicz. J. Power Sourc. 66 (1997), p. 1.

[30] T.F. Otero, I. Cantero and H. Grande. Electrochim. Acta. 44 (1999), p. 2053.

[31] D.A. McKeown, P.L. Hagans, L.P.L. Carette, A.E. Russell, K.E. Swider and

D.R. Rolison. J. Phys. Chem. B 103 (1999), p. 4825.

[32] H. Shi. Electrochim. Acta. 41 (1996), p. 1633.

[33] D. Qu and H. Shi. J. Power Sourc. 74 (1998), p. 99.

[34] H. Shi. Carbon '97, Penn State, USA, (1997), p. 826.

[35] B. Kastening, M. Hahn, B. Rabanus, M. Heins and U. Felde. Electrochim. Acta.

42 (1997), p. 2789.

[36] B. Kastening. Ber Bunsenges Phys. Chem. 102 (1998), p. 229.

[37] A. Lewandowsk, M. Zajder, and F. Béguin. Electrochimi. Acta. 46 (2001), P.

2777.

[38] L. Bonnefoi, P. Simon, J.F. Fauvarque, C. Sarrazin, J.F. Sarrau and P. Lailler. J.

Power Sourc. 83 (1999), p. 162.

[39] S.T. Mayer, R.W. Pekala and J.L. Kaschmitter. J. Electrochem. Soc. 140 (1993),

p. 446.

99

[40] R.W. Pekala, J.C. Farmer, C.T. Alviso. J. Non-Cryst. Solids. 225 (1998), p. 74.

[41] S. Escribano, S. Berthon, J.L. Ginoux and P. Achard. Eurocarbon '98,

Strasbourg, France, (1998), p. 841.

[42] R. Saliger, U. Fischer, C. Herta and J. Fricke. J. Non-Cryst. Solids. 225 (1998),

p. 81.

[43] P. Gouerec, D. Miousse, F. Tran-Van and L.H. Dao. Eurocarbon '98, Strasbourg,

France, (1999), p. 203.

[44] C. Lin, J.A. Ritter, B.N. Popov. 23rd Biennial Conf. on Carbon, Penn State,

USA. American Carbon Society. 2 (1997), p.160.

[45] J.A. Ritter, E.J. Zanto, C.E. Holland and B.N. Popov. 24th Biennial Conf. on

Carbon Charleston, South Carolina, USA. American Carbon Society. 1 (1999), p.10.

[46] E.Frackowiak, K. Méténier, T. Kyotani, S. Bonnamy and F. Béguin. 24th

Biennial Conf. on Carbon, Charleston, South Carolina, USA. American Carbon

Society. 2 (1999), p. 544.

[47] E. Frackowiak, K. Méténier, R. Pellenq, S. Bonnamy and F. Béguin. H.

Kutzmany, Editor, Electronic properties of novel materials (1999), p. 429.

[48] F. Leroux, K. Méténier, S. Gautier, E. Frackowiak, S. Bonnamy and F. Béguin.

J. Power Sources. 81 (1999), p. 317.

[49] T. Kyotani, L. Tsai and A. Tomita. Chem. Mater. 8 (1996), p. 2109.

[50] E. Frackowiak, K. Méténier, V. Bertagna and F. Béguin. Appl. Phys. Lett. 77

(2000), p. 2421.

100

[51] L. Diederich, E. Barborini, P. Piseri, A. Podesta and P. Milani. Appl. Phys. Lett.

75 (1999), p. 2662.

[52] S. Ardizzone, G. Fregonara and S. Trasatti. Electrochim. Acta. 35 (1990), p. 263.

[53] R. Kötz and S. Stucki. Electrochim. Acta. 31 (1986), p. 1311.

[54] J.P. Zheng, P.J. Cygan and T.R. Jow. J. Electrochem. Soc. 142 (1995), p. 2699.

[55] S. Trasatti and P. Kurzweil. Platinum Met. Rev. 38 (1994), p. 46.

[56] T.J. Guther, R. Oesten and J. Garche. In: F.M. Delnick, D. Ingersoll, X. Andrieu

and K. Naoi, Editors, Electrochemical Capacitors II, Proceedings 96–25, The

Electrochemical Society, Pennington, NJ (1997), p. 16.

[57] T.C. Liu, W.G. Pell, B.E. Conway and S.L. Roberson. J. Electrochem. Soc. 145

(1998), p. 1882.

[58] J.Jiang and A. Kucernak. Electrochimica. Acta. 47 (2002), p. 2381.

[59] H.Y. Lee, V. Manivannan and J.B.Goodenough. Comptes rendus de l'Academie

des sciences - Series IIC – Chemistry. 2 (1999), p. 565.

[60] X. Ren, S. Gottesfeld and J.P. Ferraris. In: F.M. Delnick and M. Tomkiewicz,

Editors, Electrochemical Capacitors, Proceedings 95–29, The Electrochemical

Society, Pennington, NJ (1996), p. 138.

[61] C. Arbizzani, M. Mastragostino and L. Meneghello. Electrochim. Acta. 41

(1996), p. 21.

[62] K. Naoi, Extended Abstracts, 49th Annual Meeting of the International Society

of Electrochemistry, Kitakyushu, Japan, September 1998, p.647.

101

[63] A. Yoshida, K. Nishida, S. Nonaka, S. Nomoto, M. Ikeda, S. Ikuta, Japanese

Patent 9-266143, (to Matsushita Electric), 1997.

[64] M. Ue, K. Ida and S. Mori. J. Electrochem. Soc. 141 (1994), p. 2989.

[65] Y. Kibi, T. Saito, M. Kurata, J. Tabuchi and A. Ochi. J. Power Sources. 60

(1996), p. 219.

[66] J.P. Zheng and T.R. Jow. J. Electrochem. Soc. 144 (1997), p. 2417.

[67] T.C. Murphy, W.E. Kramer, in: Proceedings of The 4th International Seminar on

Double Layer Capacitors and Similar Energy Storage Devices, Florida Educational

Seminar, December 1994.

[68] M. Bärtsch, A. Braun, B. Schnyder, R. Kötz, O. Haas, J. New Mater.

Electrochem. Syst. 2 (1999), p.273.

[69] D.M. Zogbi, Proceedings of the 6th International Seminar on Double Layer

Capacitors and Similar Energy Storage Devices, Florida Educational Seminar,

December 1996.

[70] I.W. Lenggoro, Y. Itoh, N. Iida and K. Okuyama, Mater. Res. Bull. 38 (2003), p.

1819.

[71] Q.w. Li, G.A. Luo, J. Li and X.Xia, J. Mater. Processing Technology. 137

(2003), p. 25.

[72] P. Scherrer, Nachr. Gotting, Gesell. 2 (1918), p. 98

[73] S. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. Soc. 60 (1938), p. 309

[74] T. Seike and J. Nagai, Sol. Energy Mater. 22 (1991), p. 107

102

[75] L.D. Kadam and P.S. Patil, Mater. Chem. Phys. 68 (2001), p. 225.

[76] C.N.P. Fonseca, M.A. Depaoli and A. Gorenstein, Sol. Energy Mater. Sol. Cells

33 (1994), p. 73.

[77] L. Yan, X.M. Zhang, T. Ren, H.P. Zhang, X.L. Wang and J.S. Suo, Chem.

Commun. 8 (2002), p. 860.

[78] T. Kobayashi and M. Ando, Electrochemistry 69 (2001), p. 872.

[79] H.K. Kim, T.Y. Seong, J.H. Lim, W.L. Cho and Y.S. Yoon, J. Power Source.

102 (2001), p. 167.

[80] In: R.S. Young, Editor, Cobalt, Its Chemistry Metallurgy and Users, Reinhold

Publ. Corp., New York (1960), p.23.

[81] C. Lin, J.A. Ritter and B.N. Popov J. Electrochem. Soc. 145 (1998), p. 4097.

[82] T.C. Liu, W.G. Pell and B.E. Conway. Electrochim. Acta. 44 (1999), p. 2829.

[83] V.R. Shinde, S.B. Mahadik, T.P. Gujar and C.D. Lokhande, Applied Surface

Science, In Press, Corrected Proof, Available online 17 October 2005.

[84] R.A. Huggins, Solid state Ionics. 134 (2000), p. 179.

[85] F. Svegl, B. Orel, M.G. Hutchins and K. Kalcher. J. Electrochem. Soc. 143

(1996), p. 1532.

[86] L. Cao, M. Lu and H.L. Li. J. Electrochem. Soc. 152 (2005), p. A871.

[87] P. Lunkenheimer and A. Loidl. Physica. B. 44 (1991), p. 5927.

[88] B. Sheela, H. Gomathi and G.P. Rao. J. Electroanal. Chem. 394 (1995), p. 267.

[89] M. Chigane and M. Ishikawa. J. Chem. Soc., Faraday Trans. 88 (1992), p. 2203.

103

[90] P. Tomczyk, G. Mordarski and J. Oblakowski. J. Electroanal. Chem. 353 (1993),

p. 177.

[91] R.C. Makkus, K. Hemmes and J.H.W.D. Wir. J. Electrochem. Soc. 141 (1994),

p. 3429.

[92] C.B. Alcock, B.Z. Li, J.W. Fergus and L. Wang. Solid State Ion. 53 (1992), p.

39.

[93] H. Kumagai, M. Matsumoto, K. Toyoda and M. Obara. Mater. Sci. Lett. 15

(1996), p. 1081.

[94] M. Kitao, K. Izawa, K. Urabe, T. Komatsu, S. Kuwano and S. Yamado. Jpn. J.

Appl. Phys. 33 (1994), p. 6656.

[95] K. Yoshimura, T. Miki and S. Tanemura. Jpn. J. Appl. Phys. 34 (1995), p. 2440.

[96] K.W. Nam and K.W.B. Kim. J. Electrochem. Soc. 149 (2002), p. A346.

[97] M.C. Yang, C.H. Lin and C.L. Su. J. Electrochem. Soc. 142 (1995), p. 1189.

[98] K.C. Liu and M.A. Anderson. J. Electrochem. Soc. 143 (1996), p. 124.

[99] V. Srinivasan and J.W. Weidner. J. Electrochem. Soc. 144 (1997), p. 1210.

[100] V. Srinivasan and J.W. Weidner. J. Electrochem. Soc. 147 (2000), p. 880.

[101] F.B. Zhang, Y.K. Zhou and H.L. Li. Mater. Chem. Phys. 83 (2004), P. 260.

[102] W. Xing, F. Li, Z.F. Yan and G.Q. Lu. J. Power Sources. 134 (2004), P. 324.

[103] N. Li, C.J. Patrissi, G. Che and C.R. Martin. J. Electrochem. Soc. 147 (2000), p.

2044.

104

[104] M.E. Spahr, P.S. Bitterli, R. Nesper, O. Haas and P. Novák. J. Electrochem.

Soc. 146 (1999), p. 2780.

[105] N. Li, C.R. Martin and B. Scrosati. Electrochem. Solid State Lett. 3 (2000), p.

316.

[106] C.M. Shen, X.G. Zhang, Y.K. Zhou and H.L. Li. Mater. Chem. Phys. 78

(2003), p. 437.

[107] Y.K. Zhou and H.L. Li. J. Mater. Chem. 12 (2002), p. 681.

[108] S. Zhong, K.Konstantinov and C.Y.Wang, Australian Innovation Patent

2002100001.

[109] J. Nagai. Sol. Energy Mater. Sol. Cells. 31 (1993), p. 291.

[110] S.I. Cordoba-Torresi, C. Gabriell, A.H.L. Goff and R. Torresi. J. Electrochem.

Soc. 138 (1991), p. 1548.

[111] I.C. Faria, R. Torresi and A. Gorenstein. Electrochim. Acta. 38 (1993), p. 2765.

[112] A. Nemetz, A. Temmink, K. Bange, S.I. Cordoba-Torresi, C. Gabriell, R.

Torresi and A.H.L. Goff. Sol. Energy Mater. Sol. Cells. 25 (1992), p. 93.

[113] C.G. Granqvist. Physica. Scripta. 32 (1985), p. 401.

[114] I.A. Serbinov, S.M. Babulanam, G.A. Niklasson and C.G. Grangvist. J. Mater.

Sci. 23 (1988), p. 2076.

[115] J.B. Goodenough. J. Solid State Chemistry. 3 (1971), p. 490.

[116] R.M. Wightman. Science. 204 (1988), p. 415.

[117] C.G. Granqvist. Solid State Ion. 70 (1994), p. 678.

105

[118] Y. Fujita, K. Miyazaki and T. Tatsuyama. Jpn. J. Appl. Phys. 24 (1985), p.

1082.

[119] A. Talledo and C.G. Granqvist. J. Appl. Phys. 77 (1995), p. 4655.

[120] J.G. Zhang, J.M. McGraw, J. Turner and D. Ginley. J. Electrochem. Soc. 144

(1997), p. 1630.

[121] C. Julien, B. Yebka and J.P. Guesdon. Ionics. 1 (1995), p. 316.

[122] H.Y. Lee and J.B. Goodenough, J. Solid State Chem. 148 (1999), p. 81.

[123] Ravinder N. Reddy, Ramana G. Reddy, Power sources for transportation

applications, in: A.R. Landgrebe, T.Q. Duong, A.J. Salking, K. Zaghib, B.S.

Battaglia, Z. Ogumi, I.B. Weinstock, eds. Proceedings of the International

Symposium, PV 2003-24, The Electrochemical Society Proceedings Series,

Pennington, NJ, 2004, p. 213.

[124] T. Kudo, Y. Ikeda, T. Watanabe, M. Hibino, M. Miyayama, H. Abe and K.

Kajita, Solid State Ionics. 152 (2002), p. 833.

[125] Ravinder N. Reddy and Ramana G. Reddy. J. Power Sources. In Press,

Corrected Proof, Available online 1 August 2005.

[126] Ravinder N. Reddy and Ramana G. Reddy, J. Power Sources. 124 (2003), p.

330.

[127] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions,

National Association of Corrosion Engineers, Houston, TX, 1966, p. 286.

[128] M.M. Doeff, A. Anapolsky, L. Edman, T.J. Richardson and L.C. De Jonghe. J.

Electrochem. Soc. 148 (2001), p. A230.

106

[129] L.I. Hill, R. Portal, A. Le Gal La Salle, A. Verbaere and D. Guyomard.

Electrochem. Solid State Lett. 4 (2001), p. D1.

[130] H.Y. Lee and J.B. Goodenough. J. Solid State Chem. 144 (1999), p. 220.

[131] H.Y. Lee, V. Manivannan and J.B. Goodenough. Comptes Rendus. Chimie. 2

(1999), p. 565.

[132] C.C. Hu and T.W. Tsou, Electrochem. Commun. 4 (2002), p. 105.

[133] S. C. Pang and M. A. Anderson, J. Mater. Res. 15 (2000), p. 2096

[134] R. Ma, Y. Bando, L. Zhang and T. Sasaki, Adv. Mater. 16 (2004), p. 918.

[135] S.C. Pang, M.A. Anderson and T.W. Chapman, J. Electrochem. Soc. 147

(2000), p. 444.

[136] J.K. Chang and W.T. Tsai. J. Electrochem. Soc. 150 (2003), p. A1333

[137] S.F. Chin, S.C. Pang and M.A. Anderson. J. Electrochem. Soc. 149 (2002), p.

A379.

[138] B.E. Conway, Ionic hydration in chemistry and biophysics, Elsevier,

Amsterdam, 1981, p. 73.

[139] P.W. Atkins, Physical Chemistry, third ed., W.H. Freeman and Company, New

York, 1986.

[140] H.Y. Lee, S.W. Kim and H.Y. Lee. Electrochem. Solid-State Lett. 4 (2001), p.

A19.

[141] Y.U. Jeong and A. Manthiram. J. Electrochem. Soc. 149 (2002), p. A1419.

107

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