11

CHAPTER – I

General Introduction and Literature Survey

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1.1 General: Need of supercapacitor, nanomaterials for supercapacitors

1.1.1 Need of supercapacitor

Now days, due to use of fossil fuels, environment pollution, global

warming, and rapid resource depletion, the technology advances in modern

society have driven the development of variety of electrochemical power

sources, which offer “clean energy”. The exponential growth in various

portable electronic devices as well as interest in electric vehicle for clean air has

created intense worldwide activity in electrochemical power sources. There are

several power types of electrochemical power systems. These are batteries, fuel

cell and supercapacitors [1]. The electrochemical power systems offer several

advantages compared to fuel combustion systems. First of all, they provide

clean energy and attractive from an environmental point of view. Secondly,

they are potentially more efficient than fuel combustion systems. In the case of

a fuel combustion system, efficiency of conversion is limited by the 2nd law of

thermodynamics (Carnot cycle) and the converted thermal energy needs another

conversion step such as the generation procedure to get electrical energy. They

converted directly the Gibbs free energy of chemical reaction to electrical

energy. Thirdly, the electrochemical systems are appealing in terms of world

energy security and utilizing the available energy resources in a more

economical way [2].

Easy accessible source now present is the electrical energy which is not

available at all time. Consequently, there is need of development of improved

methods for storing electrical energy when it is available and retrieving when it

is needed. Stored electrical energy is required in many house hold as well as

industrial applications e.g. cell phones, pagers, solar cells, computers, satellites,

stand by power supply systems, hybrid electric vehicles, etc [3]. Electrical

energy can be stored in two fundamental ways; (a) Batteries (Indirect way): In

batteries as potentially available chemical energy required faradic oxidation and

13

reduction of the electroactive reagents to release charges that can perform

electrical work when they flow between two electrodes having different

electrode potentials. During the storage of electrochemical energy in a battery,

chemical inter-conversions of electrode material occurs usually with

concomitant phase changes. Although the overall energy changes can be

conducted in a relatively reversible thermodynamic route, the charge and

discharge processes in a storage battery often involve irreversibility in inter-

conversions of the chemical electrode-reagents. Accordingly, the cycle life of

storage batteries is usually limited, and varies with battery type [4]. (b)

Capacitor (Direct way): in capacitors, the electrostatic way as negative and

positive electrical charges on the plates of a capacitor by a process as non-

faradic electrical energy storage. A battery has two different types of active

materials entrapped in a suitable conductive matrix as anodes and cathodes to

sustain the net cell reactions, while a capacitor comprises a dielectric

sandwiched between two identical electrodes. Energy stored by capacitor, only

an excess and a deficiency of electron charges on the capacitor plates have to be

established on charge and reverse on discharge, and no chemical changes

involved. Accordingly, a capacitor has an almost unlimited recyclablity,

typically between 105 to 106 times [3, 5].

At relatively low rates of discharge, say five hours, a battery can totally

discharged and 100 percent of the stored energy can be obtained. However, at

very high rates of discharge, say just a few seconds, only few percent of energy

stored capacity used for the discharge. So, although the battery can perform

duty, and regular does, it requires and electrical an electrical capacity, and

therefore weight and volume, which is much higher than electrical output

required. Lead-acid batteries have additional problem that, when optimized for

higher-rate discharges (as an automotive batteries), their operating life in

industrial applications is very short [6]. Capacitors exist at the opposite end of

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the scale. They can deliver all their stored energy virtually instantaneously

within few thousands of second. But the amount available is very tiny, just

enough to pop a flash bulb.

Now, with the recent emergence of supercapacitor, there seem to

possibility of an ideal compromise, which combines some storage capabilities

of batteries and some of the power discharge characteristic of capacitors in a

device capable of storing useful quantities of electricity which can discharge

very quickly. To understand exactly why supercapacitor so special it is useful to

consider the classic capacitor. Capacitance is measured in Farads (F) - a 1 F

capacitor charged to 1V can supply 1A of current for 1 Second. Still, highest

value capacitors available do not even approach one Farad and are actually

measured in microfarads, which of course required for most electronic

application. In contrast to ordinary capacitors supercapcitor’s capacitance

measured in whole Farads (F), and now even kiloFarads (kF), which means that

they can store million times the electrical charge.

Fig. 1.1 Ragone chart showing logarithmic plot of energy density Vs. power

density for various energy-storage devices [7].

Fig. 1.1 shows Ragone chart, which is a bubble chart used for

performance comparison of various energy storing devices [Both axes are

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logarithmic, which allows comparing performance of very different devices.].

On such a chart the values of energy density (in Wh.kg-1) are plotted versus

power density (in W.kg-1). Conceptually, the vertical axis describes how much

energy is available, while the horizontal axis shows how quickly that energy

can be delivered. Powering a small light-bulb may require low amounts of

power, but the power should be delivered slowly enough to operate a flashlight

for minutes or hours of use (batteries). Conversely, a high speed electronic

switch inside a computer may require very little energy to activate; yet it must

be delivered rapidly enough to complete the transaction in mere microseconds

(supercapacitors). These two devices would find themselves at opposite corners

of the Ragone chart. Supercapacitors offer high power density when compared

to battery systems and also have relatively large energy density compared to

conventional capacitors [8, 9].

Supercapacitor offers advantages over the conventional capacitor and

battery [10].

1 Specific capacitance in several Farads to several hundred Farads (105-106

times ordinary capacitors).

2 Virtually unlimited cycle life - can be cycled millions of time.

3 Low impedance – enhances load handling when put in paralleled with a

battery.

4 Rapid charging – discharging, supercapacitors charge in seconds.

5 High power density.

6 Simple charging methods. No special charging or voltage detection

circuits required like batteries.

In terms of specific energy as well as specific power this gap covers

several orders of magnitude. The supercapacitors 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

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(schematic diagram shown in fig. 1.2), also particularly associated with cellular

phones and pace-maker for reduction of the size of batteries [11,12]. Today

several companies such as Maxwell Technologies, Siemens Matsushita, HEC,

Panasonic, TOKIN and several others are investing in supercapacitor

development. The applications envisaged are to principally boost components

supporting batteries or replacing batteries, primarily in electric vehicles. In

addition, alternative applications of supercapacitors where they compete not

with batteries, but with conventional capacitors are appearing up and show

considerable market potential.

Fig. 1.2 Schematic diagram of hybrid electrical vehicle.

Now a day’s considerable interest has been placed on developing high

energy and power density electrochemical energy source, due to small size and

shape as well as long term reliability and environmental compliance. Hence

main goal is to increase the energy density of an electrochemical device namely

as ‘supercapacitor’ furthermore to develop electrode materials with high

specific surface area, low cost and high reversible redox reaction [13].

1.1.2 Nanomaterials for supercapacitors

Technology in the present century requires the miniaturization of the

devices in to nanometer size with dramatically enhanced ultimate performance.

Nano-phase or nanostructured materials, a new branch of materials research, is

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attracting in great areas such as solar cells, sensors, electronics, optics, etc [14-

16]. Nanocrystalline materials are not only significantly interesting but also

hold great potential for various industrial applications. The properties of the

material in nanocrystalline form are quite different and often superior to those

of conventional coarse grained crystalline materials. Because of extremely

small dimensions, a large fraction of atoms in these materials are located in the

grain boundaries and confirms special attributes. Nanocrystalline materials are

of great interest because; the function and performance of the material are

strongly dependent on its morphology, size and shape. Nanometersized

materials have recently gained a considerable amount of attention of their

unique physical and chemical properties, because of their large surface to

volume ratio and quantum size effect. A nanocrystalline structure offers a

comparatively large internal surface area, which is advantageous for many

applications where a good accessibility to the film surface becomes necessary.

Nanocrystalline and porous materials as electrode material exhibit good

electrochemical performance because these materials possess both a high

surface area and pores which are adapted to the size of ions [17-19].

Assemblies of low-dimensional building blocks (nanodots, nanowires,

nanobelts, nanotubes, etc.) into hierarchical architectures on various substrates

have great interest because of the demands for many practical applications in

functional devices. To synthesize nanomaterials various methods are under

consideration, which are divide in two processes as follows: the particles can

either be built from separate atoms (an approach from the “bottom-up”) or by

various dispersion and aggregation procedures (an approach from the “top-

down”). The approach from the “bottom-up” largely pertains to chemical

methods of preparation of nanosize particles, whereas the approach from the

“top-down” is typical of physical methods. Naturally, the proposed division is

rough and fig. 1.3 illustrates both schematic approaches. Preparation of

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nanoparticles from atoms allows individual atoms to be considered as the lower

limit of nanochemistry. Its upper boundary corresponds to atomic clusters,

whose properties no longer undergo qualitative changes with an increase in the

number of constituent atoms, thus resembling the properties of compact

material. It is also of paramount importance that the structures of equal-size

nanoparticles can differ if they were obtained by using different approaches. As

a rule, dispersion of compact materials into nanosize particles retains the

original structure in resulting nanoparticles. In particles formed by aggregation

of atoms, the positions of atoms can be different, which affects their electronic

structure [20].

Fig. 1.3 Two approaches to the synthesis of nanoparticles (a) “Bottom-up”

(b) “Top-down” approaches [20].

The key components of supercapacitors are the electrodes most

commonly made of activated carbon, transition metal oxides and conducting

polymers [21-22]. Nanotechnology allows the tailoring of the electrode

structure at the nanometer scale. Since the amount of energy storage is

proportional to the surface area, optimization of the pore structure is the key for

maximum energy storage and uniform pores in nanometer size the most desired

for this application [23].

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1.1.3 Thin Film

Any solid system possesses at most two dimensional orders or

periodically is called as a “thin film”. The third dimension, being the

thickness is very small compared to other two dimensions. Properties of

thin film often differ significantly from those of bulk due to surface and

interface effects, which may dominate the overall behavior of the films [24].

Thin films are used for number of applications in various fields such as, A. R.

coating, interference filters, polarizer, narrow band filters, solar cell, photo-

detectors, wave guide coating, photo-thermal solar coatings, magnetic films,

superconductivity, high temperature wear resistance films, hard coatings,

battery electrodes, electrochemical capacitor electrodes, etc [25].

Considerable progress is being made in surface physics and surface

characterization, which is naturally of great importance in the study of materials

in a configuration with a large surface to volume ratio, (i.e. thin film). These

developments have encouraged many laboratories; begin to examine solid-state

surface phenomena, as distinct from bulk, while simultaneously observing the

surface structure [24-26]. The significance of such studies to solid thin films

and their application is obvious, as thin-film properties are often dominated by

surface phenomena.

1.2 Literature survey on TiO2, RuO2 and TiO2-RuO2 thin films

1.2.1 Literature survey on synthesis of TiO2 and RuO2 thin films.

1.2.1. (a) Literature survey on nanocrystalline TiO2 thin films

Nanometer sized TiO2 has been one of the most extensively studied

oxides because of its remarkable optical and electrical properties and its

potential utilization in as low cost material for photocatalysis [27-29],

photovoltaic cells [30], dye sensitized solar cells (DSSC) [31] and gas sensors

[32]. A recent interest has been focused on an amphiphilic TiO2 surface induced

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by UV irradiation, which is expected to be applicable to windshield and mirrors

for vehicles [33]. Due to the wide band-gap (Eg = 3.2 eV), TiO2 is a very

attractive material for the development of optoelectronic devices, such as light

emitting diodes [34], and photodetectors [35]. Many researchers [36-38] are

actively involved in synthesis of porous/mesoporous TiO2 thin film working

electrodes in solar cells due to its high surface area, which provides absorption

of large number of dye molecules result in better performance in

photoelectrochemical cells. There are seven known polymorphs of TiO2

observed over the lifetime of the electrode; six of which have distinct

structures, such as anatase (β-TiO2), rutile (α-TiO2), brookite type (γ- TiO2),

pyrite (Pa3), α-PbO2, baddeleyite (ZrO2) type, fluorite, etc [39]. All of them

have the same fundamental octahedral structural units except for their three-

dimensional arrangements. Each phase, however, has different physical

properties such as refractive index, dielectric constant, photochemical activity

etc [40]. Among these crystalline phases, the anatase phase exhibits a high

photocatalytic activity and thus has recently attracted much attention in the field

of photocatalysts for decomposition of environmental pollutants and dye

sensitized solar cells (DSSC) [41]. As far catalysis, photocatalysis and dye

sensitized solar cells concerned, a rutile TiO2 has attracted less attention [42]

due to the expectation that the rutile phase may exhibit lower electrochemical

performance than the anatase phase [43]. However, Rutile TiO2 has some

advantages over anatase such as high refractive index, higher chemical stability,

cheaper production cost etc.

Consequently, a low cost preparation and fixation of the TiO2 with nano-

sized particle is necessary for practical application. Recently, much emphasis

has been put on the soft solution chemical processes for the preparation of

advance inorganic materials such as pervoskite-type-oxide, spinel type oxides,

and nanodots with quantum size effects. Therefore, these soft chemical

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processes are important for the preparation and fixation of TiO2 particles.

Performance of TiO2 for technical applications is strongly influenced by its

morphology, crystallite size, crystalline phase and impurity type concentration.

Porous, nanocrystalline TiO2 is of great interest because; the function and

performance of the material are strongly dependent on its morphology [44].

Accordingly, the advance of synthetic methods, in which the crystalline phase,

size and shape of the TiO2 nanocrystals can be controlled, is of importance. In

recent years, many studies related to the synthesis of anatase TiO2 have been

focused on the control of morphology as well as particle size.

Since this work is concerned with chemical methods for synthesis, the

literature survey is limited to the chemical methods only. Although, the

commercial methods of TiO2 thin film fabrication used today are the gas phase

techniques such as sputtering [45], and MOCVD [20], many workers have

employed low cost soft solution chemical methods for preparation TiO2 thin

films. Such processes include thermal and anodic oxidation of titanium [46],

electron beam evaporation [47], chemical vapor deposition [48], sol–gel [49,

50], chemical bath deposition (CBD) [51-61], successive ionic layer adsorption

and reaction (SILAR) method [62-65], spray paralysis [66], electrochemical

deposition [67], etc. Many workers have attracted towards the CBD method for

preparation of TiO2 thin films due to its simplicity and low cost, besides the

capability to achieve large area coating. Mane et al. [51] have grown compact

TiO2 film and used for 3-D solar cells by using CBD method at room

temperature from TiCl3 as precursor solution and pH maintained at 1 to 2.

Lokhande et al. [52] obtained amorphous TiO2 thin films on ITO coated

substrate by slow hydrolysis of TiCl3 solution at room temperature. It was

found that with increasing pH, rate of hydrolysis was increased resulting into

TiO2 precipitate formation in bulk of solution without film formation on the

substrates, and suitable pH range was found to be 3-3.5. Sankapal et al. [54, 55]

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have obtained anatase TiO2 thin films by CBD method using peroxo-titanium

complex as a single precursor maintained at 298 K and annealed at 773 K for 1

h for large-area photon conversion. Pathan et al. [56] have studied structural

and morphological properties of the TiO2 thin films by CBD method at room

temperature. The effect film thickness on the properties of the TiO2-anatase

films prepared on Si (100) wafers using CBD method for dielectric application

has been studied by Souni et al. [57]. Crystal growth of anatase-type TiO2 in

aqueous solutions of titanium tetrafluoride and titanyl sulfate at 333 K by CBD

method was applied for low-temperature preparation of dye-sensitized solar

cells by Watanabe et al. [58]. Crystalline TiO2 thin films were obtained on glass

and various kinds of organic substrates at 313 K by CBD method from aqueous

solutions of titanium tetrafluoride by Shimzu et al. [59]. Gao et al. [60]

prepared TiO2 thin films onto the glass substrate from aqueous peroxotitanate

solution by CBD method at room temperature. However, anatase TiO2 films

obtained after heating the films at 773 K. Rutile and anatase TiO2 films were

successfully fabricated in TiOSO4 aqueous solution containing urea at near

room temperature (333 K) by Yamabi et al. [61]. They studied molar ratio of

urea to TiOSO4 in the precursor solutions which determined the crystal phase of

the TiO2 films by CBD method. In order to avoid material consumption in CBD

method, due to bulk precipitation in the solution, many workers adopted SILAR

method. Kale et al. [62] obtained amorphous TiO2 thin films on glass substrate

by titanium isopropoxide solution using SILAR method. Photoelectrochemical

properties of the TiO2 thin films by SILAR method onto ITO coated substrate

have been studied by Pathan et al. [63]. Kim et al. [64] have fabricated TiO2

thin films by SILAR method assembled with TiO2 nanoparticles and oppositely

charged polyelectrolytes or titanium (IV) bis (ammonium lactato) dihydroxide

(TALH) and characterized for structural, morphological and optical properties.

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Photocatalytic activity of TiO2 nanoparticles thin films on silicon wafers have

been studied by Park et al. using the SILAR method [65].

These processes have been used environmentally benign conditions.

Therefore, these soft chemical processes are important for the preparation and

fixation of TiO2 particles.

1.2.1 (b) Literature survey on RuO2 thin films

A polyvalent hard metal, ruthenium is a member of the platinum group.

Ruthenium is a chemical element in the periodic table that has the symbol Ru

and atomic number 44. The oxidation state of ruthenium range from +1 to +8,

though +2, +3 and +4 are more common. The ruthenium (IV) oxide (RuO2),

oxidation states +4 is the stable oxide at room temperature and in a wide

temperature range. RuO3 is unstable at room temperature and radially

decomposes to give RuO2 and O2. Ruthenium oxide is generally used as a

catalyst in various industrial applications or an electrode in electrochemical

process. RuO2 is highly reactive with reducing agents due to its oxidizing

properties. Ruthenium dioxide exhibits a rare combination of material

properties; including a relatively low resistivity (~35 µΩ.cm at room

temperature), good thermal stability and high resistance to chemical corrosion

[68]. These desirable characteristics have attracted attention for its application

in diverse fields both in the electronics and chemical industry. In the area of

microelectronics, RuO2 has been proposed for use as an interdiffusion barrier

and also as a precision resistor element [69–71]. Recent studies have also

demonstrated its utility as a contact electrode material in ferroelectric random

access memory devices that offers superior polarization fatigue properties with

very low leakage current [72–74]. Ruthenium dioxide was investigated by a

number of authors, because of its extremely high corrosion resistance in

concentrated acids and high dielectric constant required for capacitors in very-

large-scale integrated circuits (VLSIs). Recent work on ruthenia shows that it is

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applicable in VLSI as thick film resistor. Also it is used in Y-Ba-Cu-O (YBCO)

superconductor as a buffer layer. Bulk RuO2 has a tetragonal structure

(a=b=0.4499 nm, c=0.3107 nm), and is closely lattice matched with

isostructural rutile oxides, such as TiO2 (a=b=0.4594 nm, c=0.2958 nm).

1.2.2 Literature survey on metal oxide, composite thin film based

supercapacitors

1.2.2 (a) Literature survey on RuO2 and metal oxide thin film based

supercapacitors

The concept and use of RuO2 as an supercapacitor material can be traced

on the paper by Trasatti and Buzzanca in 1971 [75-76]. Galizzioli et al. [77]

first recognized that the current response of thermally prepared anhydrous

ruthenium oxide film was similar to that of an ideal capacitor. In recent years,

[78, 79] the use of hydrous ruthenium oxide as an electrode material was

investigated. It was found that powder form of amorphous and hydrous

ruthenium oxide formed by the sol–gel method was a promising for

supercapacitor with high power density and energy density [80, 81]. A specific

capacitance of 768 Fg-1 has been obtained from an amorphous hydrous

ruthenium oxide prepared by sol-gel method [82]. A chemical oxidation of

RuCl3:xH2O with H2O2, is reported by Chang et al. [83], for the synthesis of

hydrous ruthenium oxide denoted as RuOx:nH2O. The annealed RuOx:nH2O

with the non-stiochoimetric amorphous structure exhibits the ideal capacitive

performance i.e., highly electrochemical reversibility, high-power property, and

excellent stability with specific capacitance >500 Fg-1.

In thin film form, Hu et al. [84–86], and Park et al. [87] have been

successfully employed electrochemically deposited hydrous ruthenium oxide

for supercapacitors (specific capacitance = 788 Fg-1). Fang et al. [88] have

prepared ruthenium oxide film electrode by organic precursor method and

obtained maximum specific capacitance of 593 Fg-1 and interfacial capacitance

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of 4 Fcm-2. Kim et al. [89] have prepared ruthenium oxide film electrode with

an average specific capacitance of 650 Fg-1 and good high rate capability by

electrostatic spray deposition. It is also seen that, performance of supercapacitor

depends upon the thickness of the film deposited material, i.e. it increases with

increase in thickness. In sol–gel process, aqueous RuCl3:xH2O powder and

NaOH solutions were used and maintained at pH~7.The film was annealed at

temperature of 373 K. The maximum energy density of RuO2.xH2O based

supercapacitor was 96 J.g-1. It is possible to prepare hydrous ruthenium oxide

by using cyclic voltametry (CV). The deposition of oxide films depends upon

several factors like temperature, pH of the deposition bath, potential ranges,

scan rates of CV and also concentration of RuCl3 solution used [90, 91].

In sputtering technique Ru target was presputtered by Ar ion plasma to

avoid surface oxide contamination. Ruthenium oxide film was grown on the

substrate by reactive sputtering of Ar/O2 at room temperature. The capacitance

per volume of the RuO2/Lipon/RuO2/Pt TFSC (Thin film Supercapacitor) at the

first cycle was 38 mF.cm-2 [92]. Ruthenium oxide films electrode was prepared

by spraying, a mole of RuCl3 in ethanol solvent with 3 mole Na (OC2H5) in

ethanol [93]. Ruthenium ethoxide solution was spread onto heated substrate by

an airbrush and annealed at about 15 min and then dipped into boiling water for

about 1 min. The highest specific capacitance of 593 F.g-1 was obtained from

the film electrode grown at 473 K [94]. In electrostatic spray deposition (ESD),

0.01-0.3 M RuCl3:XH2O solution is spread on the heated substrate of Pt, coated

silicon. The average specific capacitance was found to be 650 Fg-1 [95]. In sol-

gel process specific capacitance 768 Fg-1 was obtained. A NaOH solution added

to RuCl3:xH2O solution to maintain the pH~7 [96]. Colloidal method was also

adopted to prepare ruthenium oxide [97]. Specific capacitance 551 Fg-1 was

found for unhydrous amorphous RuO2 prepared by spray pyrolisis technique

(SPT) at 573 K by Gujar et al. [98]. Lee et al. [99] adopted simple chemical

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bath deposition method to synthesize hydrous RuO2 having specific capacitance

500 Fg-1 and Patake [100] et al. reported 50 Fg-1 specific capacitance for RuO2

electrode synthesized by M-CBD method. Although ruthenium oxide allows the

attainability of higher capacitance than all other oxide materials, the rarity of

the metal and its very high cost have prompted researchers to identify

compounds in which ruthenium could be replaced by cheaper transition metal

elements like Ni, Co, Mn etc. Table 1.1 shows the specific capacitance values

exhibited by virgin metal oxides using preparation methods with the electrolyte

used.

1.2.3 (b) Literature survey on composite and TiO2-RuO2 thin film based

supercapacitors

Moreover, a relatively high-frequency response is an essential

requirement for supercapacitors delivering pulse power which should be

achieved by reducing the equivalent series resistance (ESR) employing suitable

electrode materials [101] with a proper utilization of the electroactive species.

Accordingly, developing and designing active materials as well as electrodes

meeting the above requirements becomes an interesting subject for many

electrochemists. The main drawback of hydrous ruthenium oxide is its high

cost. To overcome this, a strategy is to prepare composites in which particles of

hydrous ruthenium oxide are deposited on other materials of lower price, e.g.

carbons with different surface textures, or cheap oxides such as nickel oxide,

titanium oxide etc. The supports are also electrochemically active electrode

materials but they show lower specific capacitances compared to hydrous

ruthenium oxide. The idea of the composite is try to combine the high specific

capacitance of hydrous ruthenium oxide with a good dispersion over other

oxide, in order to get electrode materials with high capacitance and low or

moderate price.

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Arabale et al. [102] reported the supercapacitive behavior of ruthenium

oxide functionalized multiwalled carbon nanotubes (Ru/MWNT), for the first

time. By using cyclic voltammetry and impedance measurements, they have

found that the capacitance of MWNTs can be increased from 30 to 80 Fg-1 after

ruthenium oxide functionalization. Wang et al. [103] reported hydrous

ruthenium–tin oxides (denoted as (Ru–Sn)O2·nH2O)were synthesized under a

mild hydrothermal condition. The maximum specific capacitance 820 Fg-1 was

achieved. Preparation of RuOxHy /carbon black nanocomposite material was

performed by Panic et al. [104] by using impregnation method starting from

RuOxHy sol as a precursor. Black pearls 2000(BP) and Vulcan XC-72 R (XC)

were used as supporting materials. Samples of the composite were calcined in

nitrogen atmosphere at temperatures from 403 to 723 K. The highest specific

capacitance of electrode is about 700 Fg-1 was registered for RuOxHy supported

on BP and calcined at 573 K while four times lower values was obtained for

RuOxHy supported on XC. Mixtures of RhOx + Co3O4 have been

electrochemically studied by Souza et al. [105] in acid solution as a function of

composition. The electrodes were prepared by thermal decomposition at 673 K

of mixtures of nitrate precursors. Electrodes of this kind have been found to

perform as good materials in supercapacitor applications, exhibiting specific

capacitances of 500–800 Fg-1 over to 20–60 mol.% RhOx composition range.

Hu et al. [106] studied the electrochemical energy storage and delivery on the

electrodes composed of hydrous ruthenium oxide or activated carbon–hydrous

ruthenium oxide (AC–RuOx) composites are found to strongly depend on the

substrate employed. The maximum specific capacitance (CS,RuOx) of

RuOx:nH2O, 1580 Fg-1 (measured at 1 mV.s-1), very close to the theoretical

value, was obtained from an AC–RuOx/RuOx/Au/SS electrode with 10 wt.%

sol–gel-derived RuOx:nH2O annealed in air at 473 K for 2 h. A two-step

hydrothermal process was reported by Hu et al. [107] to synthesize hydrous

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30% RuO2 –70% SnO2 composites with much better capacitive performances

than those fabricated through the normal hydrothermal process, co-annealing

method, or modified sol–gel procedure. A very high specific capacitance of

RuO2 (CS, Ru), ca. 1150 Fg-1, was obtained. Fang et al. [108] reported

significant enhancement in supercapacitor performance has been achieved via a

new RuO2 nanocomposite materials prepared by direct ruthenium sputtering on

arrayed multi-walled carbon nanotubes supported by Ti-buffered Si wafer.

Well-dispersed and strongly adhered RuO2 NPs have been densely populated on

CNxNTs to obtain the overall specific capacitance (1380 Fg-1-RuO2), charging–

discharging rate (up to 600 mV.s-1) and operation stability (5000 cycles).

Amorphous Ru1−yCryO2/TiO2 nanotube composites were synthesized by loading

different amount of Ru1−yCryO2 on TiO2 nanotubes via a reduction reaction of

K2Cr2O7 with RuCl3:nH2O at pH 8, followed by drying in air at 423 K. A

maximum specific capacitance 1272 Fg-1 was reported with the proper amount

of Ru1−yCryO2 loaded on the TiO2 nanotubes by Bo et al. [109]. Electrochemical

characteristics of lithium ruthenate (LixRuO2+0.5x·nH2O) for electrochemical

capacitors electrode material were prepared by Zhao et al. [110]. The specific

capacitance of 391 F/g can be delivered at 1 mA charge–discharge current for

LixRuO2+0.5x·nH2O electrode with an energy density of 65.7 Wh.kg-1.

Morishita et al. [111] invented new electrode materials for

supercapacitor, tungsten carbide (WC) and molybdenum carbide (Mo2C) coated

by porous carbon, were prepared through a simple heat treatment of the mixture

of K2WO4 and K2MoO4, respectively, with hydroxy propyl cellulose. The

carbon-coated carbide gave a high capacitance in 1 mol./L H2SO4 electrolyte, as

about 350 Fg-1 for carbon-coated WC and 550–750 Fg−1 for carbon coated

Mo2C.

The energy storage of activated carbon modified with a semiconducting

oxide TiO2 is studied by Liang et al. [112]. The composite was prepared by

29

mixing nanosize TiO2 and activated carbon through a means of ultrasonic

vibration in ethanol solution for 30 min. It was found that with modification of

TiO2, the specific capacitance of activated carbon measured at 0.65 mA.cm-2

was increased from 47 to 63 Fg-1. Nanostructured and microporous nickel–

manganese oxide (NMO) and cobalt–manganese oxide (CMO) were deposited

by potentiodynamic method onto inexpensive stainless steel substrate at scan

rate 200 mV.s-1. Maximum specific capacitance (SC /Cs) values of 621 and 498

Fg-1 were obtained with NMO and CMO electrodes, respectively reported by

K.R. Prasad et al. [113]. Nickel–cobalt oxides/carbon nanotube (CNT)

composites were prepared by adding and thermally decomposing nickel and

cobalt nitrates directly onto the surface of carbon nanotube/graphite electrode to

form nickel and cobalt oxides. The effect of Ni/Co molar ratio on specific

capacitance of the composite electrode was investigated by Fan et al. [114] and

the highest specific capacitance 569 Fg-1 (at 10mA.cm-2) is obtained at Ni/Co

molar ratio = 1:1.

One of the most semiconducting materials for composite electrode is

TiO2 because of its high stability towards photocorrosion and its favorable band

gap energy and flat band potential, shown in fig.1.4 [115].

30

Fig. 1.4 Band positions of several semiconductors in contact with aqueous

electrolyte at pH 1.

Many synthesis of TiO2-RuO2 have reported by researchers for many

applications. RuO2 has been developed earlier as an excellent electrocatalytic

surface in combination with TiO2 on Ti substrate, through patent on electrodes

for anodic Cl2 evolution in the chloro-alkali process. The so called

dimensionally stable anode (DSA) now provide the ideal, extensively anode

materials for electrolytic production of Cl2. The DSA was prepared by thermo

chemical decomposition of RuCl3 or (NH4)3RuCl6 in addition with titanium

isopropoxide or TiCl3 in the temperature between 623 to 823 K.

Zhitomirsky [116] reported cathodic electrosynthesis of TiO2, RuO2

and composite RuO2-TiO2 films on Pt substrates was performed via

hydrolysis of TiCl3 and RuCl3:nH2O salts dissolved in water or mixed

methyl alcohol-water solvent. Faria and Trasatti have been prepared

RuO2+TiO2 mixed oxides by thermal decomposition of RuCl3+TiCl3 at

31

723 K [117]. Kristof et al. [118] reported the processes for formation of

RuO2/TiO2 coatings from precursor salts dissolved in isopropanol were

followed by combined thermo analytical and mass-spectrometric methods.

Miao et al. [119] reported epitaxial ruthenium oxide thin films have been

grown on TiO2 substrates by chemical vapor deposition at temperatures 573 K

as low as using tris(2,2,6,6-tetramethyl-3,5-heptanedionato) ruthenium

[Ru(TMHD)3] as a precursor with oxygen carrier gas. Song et al. [120] studied

effect of glass frit on corrosion resistance of Ti/TiO2/IrO2–RuO2 thin films. The

films were deposited on plasma sprayed TiO2 buffer layer above Ti metal by the

sol–gel and dip-coating method. Panic et al. [121] reported synthesis and

characterization of RuO2 and TiO2 sols of different aging times, obtained by

forced hydrolysis of appropriate chloride salts, was performed by transmission

electron microscopy (TEM). The aging time of TiO2 sols was observed to affect

the size of particles as well as the crystallinity of the solid phase of the sols. An

RuO2/TiO2(110) ultrathin film, epitaxially grown in ultrahigh vacuum by

reactive deposition of Ru3(CO)12 in an oxygen atmosphere (2×10−6 mbar) at 573

K, has been prepared by Rizzi et al. [122]. The influence of the aging time of

RuO2 and TiO2 sols used for the preparation of (40% RuO2+60% TiO2):Ti

anodes by the sol-gel procedure on the electrochemical properties and behavior

for the chlorine evolution reaction of obtained anodes was studied by V. Panic

et al. [123, 124]. Faria et al. [125] are reported the effect of mixing Ru and Ce

has been investigated by systematically substituting Ce for Ti in 30 mol% RuO2

+ 70 mol% TiO2 by thermal decomposition of aqueous acid solutions of the

chlorides as precursors using Ti as a support. Phvulescu et al. studied the

activity of the RuO2-TiO2 and RuO2-SiO2 membrane catalysts in oxidation with

air of the isopropylic alcohol was determined at temperatures ranging from 373

to 393 K. The RuO2-TiO2 and RuO2-SiO2 membranes were prepared by the sol-

gel process and supported on microporous glass membranes [126]. Yong-gang

32

et al. [127] reported a synthesis of RuO2/TiO2 composite by loading various

amounts of RuO2 on TiO2 nanotubes. The symmetric supercapacitors based on

these nanocomposites were fabricated by using gel polymer PVA–H3PO4–H2O

as electrolyte, a maximum specific capacitance of 1263 Fg-1 was obtained for

the RuO2 which was loading on TiO2 nanotubes. Table 1.2 shows the specific

capacitance values exhibited by composite and mixed metal oxides using

preparation methods with the electrolyte used.

In all, RuO2 and RuO2-TiO2 have provided their good ability in

supercapacitor application.

33

Table 1.1: Virgin metal oxide materials for supercapacitor with specific capacitance, specific energy and specific

power.

Sr.

No.

Material Method of

preparation

Electrolyte Specific

capacitance

(Fg-1

)

Specific

energy

(Wh.kg-1

)

Specific

power

(kW-1

kg)

Reference

1 Ruthenium

oxide (RuO2)

Electrodeposition 0.1 M H2SO4 788 - - 93

2 Ruthenium

oxide

Electrophoretic

deposition

1.0 M H2SO4 734 - - 128

3 Ruthenium

oxide

Sol-gel 0.5 M H2SO4 720 26.7 - 96

4 Ruthenium

oxide

Electrostatic spray

deposition

0.5 M H2SO4 650 17.6 4 129

5 Ruthenium

oxide

Electrodeposition 0.5M H2SO4 599 17.6 - 130

6 Ruthenium

oxide

Thermal

decomposition

0.5 M H2SO4 593 - - 131

34

7 Ruthenium

oxide

Spray pyrolysis 0.5 M H2SO4 551 - - 98

8 Ruthenium

oxide

Chemical

oxidation

0.1 M H2SO4 500 66 4.7 132

9 Ruthenium

oxide

Cyclic

Voltammetry

0.5 M H2SO4 100 - - 84

10

Ruthenium

oxide

Non-ionic

surfactant

templeting

0.5 M H2SO4 58 - - 133

11 Manganese

oxide

Chemical method 0.1 M K2SO4 150 3.3 3 134

12 Manganese

oxide (MnO)

Sputtering 1 M Na2SO4 700 - - 135

13 Manganese

oxide

Sonochemistry

method

0.5M

Na2SO4

344 - - 136

14 Manganese

oxide

Electrostatic spray

deposition (ESD)

0.1M

Na2SO4

330 - - 137

35

15 Manganese

oxide

Electrodeposition 1.0 M

Na2SO4

320 - - 138

16 Manganese

oxide

Chemical

reduction method

2.0 M NaCl 250 - - 139

17 Manganese

oxide

Electrodeposition 2.0 M KCl 202 - - 140

18 Manganese

oxide

Electrodeposition 0.5 M KCl 163 - - 141

19 Manganese

oxide

Sputtering Organic

electrolyte

145 - - 135

20 MnO2 Electrodeposition KCl 52 28.8 0.5 142

21 MnO2 Chemical method K2SO4 36 3.3 3.08 134

22 MnO2 Chemical method K2SO4 21.5 8.1 10.2 134

23 MnO2 Chemical method K2SO4 31 17.3 19 134

24 MnO2 Electrodeposition K2SO4 - 7.0 10 143

25 MnO2 Electrodeposition KNO3 160 1.9 3.8 144

26 MnO2 Electrodeposition KNO3 140 21 123 144

36

27 MnO2 Chemical method KNO3 - 5.86 42.1 145

28 MnO2 Chemical method H2SO4 - 7.37 62.8 145

29 MnO2 Chemical method KNO3 - 13.5 120.1 145

30 MnO2 Sputtering LiOH 62.4 19.5 - 146

31 MnO2 Chemical method K2SO4 21 11.7 - 147

32 LiMn2O4 Sol-gel Li2SO4 56 10 2 148

33 Nickel oxide

(NiO)

Electrodeposition 1.0 M KOH 277 - - 149

34 Nickel oxide Electrochemical

precipitation

1.0 M KOH 146 - - 150

35 Nickel oxide Sol-gel 1.0 M

2.0 KOH

125 - - 151

36 Nickel oxide Calcination 2.0 M KOH 120 - - 152

37 Ni(OH)2 Electrodeposition 3.0 M KOH 578 - - 153

38

Ni-C Chemical bath

deposition method

(CBD)

BMIM-PF6

RTIL

357 50 0.458 154

37

39 NiFe2O4 CBD 1.0 M

Na2SO3

223 - - 155

40 NiFe2O4 SILAR 1.0 M

Na2SO3

369 - - 155

41 Bismuth

oxide

Electrodeposition 1.0 M NaOH 98 - - 156

42 Bismuth iron

oxide

Electrodeposition 1.0 M NaOH 81

- - 157

43 Fe2O3 Electrosynthesis 0.25 M

Na2SO3

210 - - 158

43 Tin oxide Sol- gel 1.0 M KOH 16 - - 159

45

MnO2 Supramolecular

templeting

method

2 M KOH 299 - - 160

46 Co(OH)2 Electrodeposition 6.0 M KOH 280

23.7 8.1 161

47 Cobalt oxide SILAR 1.0 M KOH 165 6.4 0.47 162

38

48 Cobalt oxide

(Co3O4)

Spray pyrolysis 2.0 M KOH 74 - - 163

49 NiO Chemical method 2 M KOH 168 - - 164

50 β-Ni(OH)2 Chemical method 2M KOH 398 - - 165

Table 1.2: Mixed metal oxide, composites materials for supercapacitor with specific capacitance, specific energy

and specific power.

1 RuO2 – TiO2 Wet chemical

method

PVA H3PO4 1263 - - 127

2 RuO2 – TiO2 Chemical method 1.0 M KOH 46 5.7 1.207 166

3 C - RuO2 Wet impregnation 0.1M H2SO4 760 - - 128

4 RuO2-SnO2 Sol-gel 0.1 M H2SO4 690 - - 130

5 RuO2 - C Chemical method 1.0 M H2SO4 650 - - 131

6 RuO2 - C Colloidal method 1.0 M H2SO4 407 - - 132

7 LixRuO2+

0.5x·nH2O

Chemical method 1.0 M H2SO4 391 65.7 - 110

8 RuO2 - C

composite

Colloidal solution

method

1.0 M H2SO4 250 - - 133

39

9 RuO2-SnO2 DC sputtering 0.5 M H2SO4 88 - - 96

10 Ni - Co CVD 1.0 M KOH 569 - - 114

11 Ni-MnO2 Electrodeposition 1.0 M

Na2SO4

621 - - 113

12 Mn-Ni - Co Co-precipitation 6.0 M KOH 1260 - - 167

13 NiO - RuO2 Co-precipitation 1.0 M KOH 210 - - 168

14 IrO2 – MnO2 Thermal

decomposition

0.5 M H2SO4 550 - - 169

15

Pb-RuO2 Solid state

reaction

0.5 M H2SO4 160 - - 170

16

Co- MnO2 Electrodeposition 0.5 M

Na2SO4

396 - - 171

17 Co – MnO2 Electrodeposition 1.0M

Na2SO4

98 - - 113

18 Co(OH)2/

TiO2

Precipitation

method

6.0 M KOH 229 - - 172

40

19 V2O5 - C Melt quenching 2.0 M

NaNO3

32.5

- - 173

20

V2O5 - C Melt quenching 2.0 M

KNO3

31.5 - - 173

21 V2O5 - C Melt quenching 2.0 M

LiNO3

29.9 - - 173

22 V2O5 - C Melt quenching 1.0 M

Na2SO4

29.3 - - 173

23 V2O5 - C Melt quenching 1.0 M

K2SO4

28 - - 173

24 V2O5 - C Melt quenching 1.0 M

Li2SO4

25.1 - - 173

41

1.3 Orientation and purpose of dissertation

Supercapacitors have been recognized as a unique device exhibiting

high-power characteristics with an acceptable capacity and long cycle life for

energy storage/delivery and management in future power systems. The novel

performance of this device usually resulted from the high specific surface area

as well as the highly reversible redox reactions of the electrode materials. After

scanning through the literature porous activated carbon (AC), hydrous transition

metal oxides, conducting polymers, mixed metal oxides or their composites are

the main components in the electrode materials of supercapacitors. Their ESR

prevents supercapacitors from achieving power densities near to the theoretical

limits. Thus, determining how to lower the ESR of supercapacitors is becoming

an important area of research. Several methods for reducing the ESR already

have been developed, including polishing the surface of the current collector,

chemical bonding the electrode to the current collector and using colloidal thin

film suspensions. In addition, research is going on defining the relationship

between pore size and ESR in electrode materials and determining the intrinsic

ESR of various electrolytes. Accordingly, developing and designing active

materials as well as electrodes meeting the above requirements becomes an

interesting subject for many electrochemists. It is possible to obtain the high

working voltage of the supercapacitors by choosing a proper electrode material.

Both increase of the working voltage and high energy density of the metal

oxide electrode result in a significant increase of the overall energy density of

the supercapacitors. Recently, Wang et al. [127] is reported that the three-

dimensional nanotubes network of TiO2 nanotubes can increase greatly the

utilization of RuO2, and the capacitance profile of the symmetric capacitors

using of RuO2/TiO2 nanotubes as both positive and negative electrodes were

investigated. A maximum specific capacitance of 1263 Fg-1 was obtained for

42

the symmetric supercapacitor fabricated by using gel polymer PVA–H3PO4–

H2O as electrolyte.

Though the amorphous, hydrous ruthenium oxide exhibits excellent

pseudocapacitive behavior with large specific capacitance and good

reversibility, the low abundance and high cost of the precious metal are the

major limitations to commercial application. This has caused the researchers to

find new materials like transition metal oxides, or loading of small amount of

RuO2 in other transition metal oxides or in carbon electrode. Alternatively one

can fabricate the RuO2 electrode by a method having high yield, i.e. by a

method that can deposit material of large area at expense of small quantity of

initial ingredients. Recently there has been an increase interest in

nanocrystalline materials, where the physical properties are changing with size.

Part of this interest is fundamental, due to the great potential offered by group

of materials which are novel by virtue of their physical, rather than their

chemical structure. There are two approaches to produce nanocrystalline (or

amorphous) dimensionally stable anodes (DSA) for supercapacitor application,

“Top-down” (Physical methods) and “bottom-up” (Chemical methods)

approach. The electrochemical behavior of metal oxides results directly from

their morphology, which on the other hand, depends on the preparation

procedure and conditions. In order to improve the electrochemical properties of

the material, the size and distribution of the particles have to be adjusted. A

useful “Bottom-up” approach to coating preparation is based on the chemical

methods.

This work is concerned with the development of supercapacitor

electrodes of RuO2 and TiO2-RuO2 films prepared by “Bottom-up” approach

i.e. using simple chemical method, which can offer the fulfillment of the

requirements of the supercapacitor. Among various other deposition methods,

CBD and SILAR methods have many advantages over physical method. These

43

deposition methods result in pinhole free, uniform and well stoichoimetric

films. Since the basic building blocks are ions instead of atoms, also the

preparative parameters are easily controllable. These methods can be used for

the large area deposition.

Using these methods, it is possible to deposit amorphous, hydrous TiO2,

RuO2 and TiO2-RuO2 thin films by varying different preparative parameters

such as suitable metal ion sources, pH, deposition time and temperature, etc.

The X-ray diffraction (XRD) technique will be used for the phase identification

and crystallite size determination. The chemical bonding in the present material

will be studied by Fourier Transform Infrared Analysis (FT-IR). Surface

morphology of the films will be studied using scanning electron microscopy

(SEM). Optical study of the films will be studied by taking optical absorption

spectrum in the UV-VIS-NIR region. The electrical resistivity will be measured

at different temperatures and activation energy will be calculated. Wettablity

properties of the film surface will be studied my measuring the water contact

angle.

The supercapacitor properties of the RuO2 and TiO2-RuO2 films will be

studied by cyclic voltametry (CV) using Potentiostat, forming a electrochemical

cell comprising platinum as a counter electrode, saturated calomel electrode

(SCE) as a reference electrode in a suitable electrolyte. The effect of electrolyte,

thickness of electrode, scan rate and number of cycles on the performance of

supercapacitor electrode will be studied. The electrochemical impedance

spectroscopic (EIS) study will be carried out to measure ESR, relaxation time

of the formed electrode.

So keeping in mind the valuability of material, present study is

performed to reduce the cost and to increase the specific capacitance. Therefore,

the purpose of research work is to develop the RuO2 and TiO2-RuO2 electrodes,

44

by simple chemical methods that have the great advantage due to low cost and

check their supercapacitive behaviour for commercial application.

45

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