Nanostructured Metal Oxides as Electrode Materials for Electrochemical Capacitors

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Metal oxides as electrode materials for electrochemical capacitors Zhou Jin Lao University 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

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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

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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

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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.

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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 iv

Chapter 3. Experimental …………………………………………...…………….. 38 3.1. Materials and chemicals …………………………………………...……... 38 3.2. Experimental procedures …………………………………………...…….. 39 3.3. Materials preparation …………………………………………………...… 41 3.3.1. Metal oxides prepared by spray pyrolysis ……………………...……. 41 3.3.2. Metal oxides prepared by co-precipitation and heat treatment …….… 42 3.4. Structural and physical characterization of oxide materials ………...……. 43 3.5. Electrode preparation and test cell fabrication ...………………...……….. 44 3.5.1. Electrode preparation ……..…………………..……………....……….44 3.5.2. Test cell fabrication ...…………...…………………………..……….. 44 Chapter 4. Nanocrystalline Co3O4 powders as electrode materials for electrochemical capacitors ………………………………………...…………. 46 4.1. Introduction ………………………………………………..……………... 46 4.2. Experimental …………………………………………………....………… 48 4.3. Results and discussion ………………………………………...…………. 49 4.3.1. Materials characterization …………….…………………...…………. 49 4.3.2. Electrochemical properties ………………………………...…………. 53 4.4. Summary ………………………...………………………………...……… 56 Chapter 5. Nanocrystalline NiO powders as electrode materials for electrochemical capacitors ……………………………………...……………. 57 5.1. Introduction ……………………………………………………..………... 57 5.2. Experimental …………………………………………………...…………. 59 5.3. Results and discussion …………………………………………...….……. 61 5.3.1. Materials characterization ………………………………...……….…. 61 5.3.2. Electrochemical properties …………………………………...………. 64

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5.4. Summary ………………………………………..……………...…...…….. 68 Chapter 6. Crystalline V2O5 powders as electrode materials for electrochemical capacitors …………………………….……………...……... 69 6.1. Introduction …………………………………………………………...…... 69 6.2. Experimental ………………………………………………………...……. 70 6.3. Results and discussion ……………………………………………....……. 71 6.3.1. Materials characterization ……………………………………………. 71 6.3.2. Electrochemical properties …………………………………...………. 74 6.4. Summary ………………………………………………………...………... 79 Chapter 7. Amorphous and nanocrystalline MnO2 as electrode materials for electrochemical capacitors ………………………………………...………… 80 7.1. Introduction …………………………………………………...…………... 80 7.2. Experimental ………………………………………………………...……. 82 7.3. Results and discussion …………………………………………...……….. 83 7.3.1. Materials characterization ………….……………………...…………. 83 7.3.2. Electrochemical properties ………………………………...…………. 87 7.4. Summary ………………………………………………………...………... 93 Chapter 8. General conclusions ……………………………………...…………... 94 References …………………………………………………………...…………… 96 List of symbols ……………………………………………………………...……107

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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 asprepared Co3O4, NiO, V2O5, and MnO2 are much cheaper. This makes them very promising candidates as electrode materials for electrochemical capacitors.

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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.

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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.

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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.

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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.

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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.

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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.

Please see print copy for Table 2.1

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

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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

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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)

Please see print copy for Figure 2.2

Fig. 2-2. Schematic of the electrical double layer [14].

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Please see print copy for Figure 2.3

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

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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

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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 nonaccessible 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

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[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 adatoms, 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].

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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].

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Please see print copy for Figure 2.4

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

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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.

Please see print copy for Figure 2.5

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.

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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.

Please see print copy for Figure 2.6

Fig. 2-6. Voltammetry of Ru/carbon aerogel composite electrodes for electrochemical capacitor; with 1 M H2SO4, scan rate 2 mV/s [28].

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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

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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].

Please see print copy for Table 2.1

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

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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.

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Please see print copy for Figure 2.7

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.

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Please see print copy for Figure 2.8

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 resorcinolformaldehyde (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

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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

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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

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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.

Please see print copy for Figure 2.9

Fig. 2-9. General view of purified multiwalled carbon nanotubes obtained by catalytic decomposition of acetylene at 700 °C [48].

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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. 210 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.

Please see print copy for Figure 2.10

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.

25

Instead of a typical rectangular shape, a quite remarkable region of reversible pseudofaradaic reactions is observed at ~0.2 V (Fig. 2-11) [50].

Please see print copy for Figure 2.11

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

26

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

27

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].

28

Please see print copy for Figure 2.12

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,

29

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.

30

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.

31

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].

32

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 selfignition does not occur. Another important advantage of electrochemical capacitors is

33

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 selfdischarge 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.

34

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

35

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.

36

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.

37

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 NH4OH

28% in H2O, Aldrich

solution Manganese

99.99 % (II)

acetate (CH3COO)2Mn · 4H2O

99 %

Aldrich

(II)

nitrate Mn(NO3)2 · 4H2O

> 97 %

Fluka

tetrahydrate Manganese tetrahydrate Carbon black

C

Lexel

38

Polyvinylidene difluoride

99%

Aldrich

(PVdF) N-methyl-2-pyrrolidone

C5H9NO

Aldrich

(NMP) 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.

39

Fig. 3-1. Schematic diagram of experimental procedure.

40

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].

41

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)

42

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: Dcrystallite = 0.9λ / β cos(θ B ), where λ represents the x-ray wavelength, β is the observed full

width at half maximum (FWHM), and θ B 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-EmmettTeller (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.

43

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 Nmethyl-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:

44

(3-2)

where (Va−Vc) represents the potential window.

45

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

46

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.

47

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.

48

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.

49

100

(311)

Intensity (a.u.)

80

60

(111) (220) (440)

(400)

40

(511)

20

0

20

30

40

50

60

70

o

2 theta ( )

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.

50

a

b Fig. 4-2. SEM images of the as-prepared Co3O4 at different magnifications: (a) 2000×, (b) 50,000×.

51

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.

Specific surface area determined by Sample

Commercial Co3O4 (Aldrich)

Co3O4 powder from Spraying

multipoint BET method (m2/g)

33

82

cobalt nitrate at 500 °C

52

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 doublelayer 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].

53

50 mV/s 20 mV/s 10 mV/s 5 mV/s

8 6 4 2

I (A)

0 -2 -4 -6 -8 -10 -12 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

U (V)

Fig. 4-3. Cyclic voltammograms of Co3O4 synthesized by spray pyrolysis in 2 M KOH solution at various scan rates.

Specific capacitance (F/g)

180 160 140 120 100 80 60 40 20 0 0

10

20

30

40

50

60

Scan rate (mV/s)

Fig. 4-4. Specific capacitance vs. scan rate in 2 M KOH electrolyte.

54

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.

55

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.

Please see print copy for Table 4.2

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.

56

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

57

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 hydrolysiscontrolling 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

58

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

59

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.

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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)

61

1000

Intensity (a.u.)

800

(200)

600

(111) 400

(220)

200

(311) (222)

0 10

20

30

40

50

60

70

80

o

2 theta ( )

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.

62

a

b

63

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

64

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.

20mv/s 10mv/s 5mv/s

4

2

I (A)

0

-2

-4

-6 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

U (V)

Fig. 5-3. Cyclic voltammograms of NiO in 1 M KOH solution at various scan rates.

65

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

66

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.

1M KOH 2M KOH 3M KOH

4 2

I(A)

0 -2 -4 -6 -8 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

U(V)

Fig. 5-4. Cyclic voltammograms of NiO in different KOH concentrations.

67

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 singlephase 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.

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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

69

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.

70

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.

71

400 350 300

Intensity (a.u.)

250 200 150 100 50 0 -50 10

20

30

40

50

60

70

o

2 theta ( )

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.

72

a

b

73

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

74

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].

50 mV/s 20 mV/s 10 mV/s 5 mV/s

12 10 8 6 4 2

I (A)

0 -2 -4 -6 -8 -10 -12 -14 -0.2

0.0

0.2

0.4

0.6

0.8

U (V)

Fig. 6-3. Cyclic voltammograms of V2O5 in 2 M KCl at various scan rates.

75

Specific capacitance (F/g)

300 250 200 150 100 50 0 0

10

20

30

40

50

60

Scan rate (mV/s)

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

76

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.

2 M LiCl 2 M KCl 2 M NaCl

1.8 1.6 1.4 1.2 1.0 0.8 0.6

I (A)

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -0.2

0.0

0.2

0.4

0.6

0.8

U (V)

Fig. 6-5. Cyclic voltammograms of V2O5 in different electrolytes.

77

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.

Please see print copy for Table 6.1

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.

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1 M KCl 2 M KCl

2.0 1.5 1.0

I (A)

0.5 0.0 -0.5 -1.0 -1.5 -0.2

0.0

0.2

0.4

0.6

0.8

U (V)

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.

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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

80

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-gelderived 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.

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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.

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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.

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500

400

Intensity (a.u.)

0

400 C

300

200

0

300 C 100

0

200 C

0

10

20

30

40

50

60

70

o

2 theta ( )

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.

84

a

b

85

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.

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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.

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200°C 300°C 400°C

3 2 1

I (A)

0 -1 -2 -3 -4 -5 0.0

0.2

0.4

0.6

0.8

1.0

U (V)

Fig. 7-3. Cyclic voltammograms (CVs) of MnO2 spray dried at 200 °C, 300 °C, and 400 °C.

specific capacitance (F/g)

450 400 350 300 250 200 150 100 50 0 0

100

200

300

400

500

temperature (°C)

Fig. 7-4. Specific capacitance vs. temperature.

88

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 zerocurrent 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

89

in the order: Cond K + > Cond Na + > Cond Li + , 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:

SC 2 MKCl > SC 2 MNaCl > SC 2 MLiCl . 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.

2 M KCl 2 M NaCl 2 M LiCl

3 2 1

I (A)

0 -1 -2 -3 -4 -5 0.0

0.2

0.4

0.6

0.8

1.0

U (V)

Fig. 7-5. shows cyclic voltammetric (CV) curves of MnO2 spray dried at 200 °C in different electrolytes.

90

Table 7-2. Comparison of specific capacitance of MnO2 in the present study with values reported by other researchers using different electrolytes.

Please see print copy for Table 7.2

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.

91

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.

50 mV/s 20 mV/s 10 mV/s 5 mV/s

15 10 5

I (A)

0 -5 -10 -15 -20 -25 0.0

0.2

0.4

0.6

0.8

1.0

U (V)

Fig. 7-6. CV curves of MnO2 at various scan rates in 2 M KCl electrolyte.

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Specific capacitance (F/g)

450 400 350 300 250 200 150 100 50 0 0

10

20

30

40

50

60

Scan rate (mV/s)

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.

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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

94

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.

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List of symbols

EC

Electrochemical capacitors

EDLC

Electrical double-layer capacitor

XRD

X-ray diffraction

SEM

Scanning electron microscope

BET

Brunauer-Emmett-Teller gas sorption technique

CV

Cyclic voltammetry

SCE

Saturated calomel reference electrode

SSA

Specific surface area

SC

Specific capacitance

nm

Nanometer

Wt%

Weight percent

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