38 

 

CHAPTER - II

LITERATURE SURVEY AND SCOPE OF THE

PRESENT INVESTIGATION

In this chapter a summary of earlier work carried out on supercapacitors and

scope for the present investigation are presented.

2.1. INTRODUCTION

Electrochemical energy generation and storage has become extremely

important with increasing environmental pollution as it can decrease our dependence

on limited fossil fuel reserves. The recent needs for electrical energy storage as a

couple of combining energy and power have stimulated research into a new class of

charge storage materials for electrochemical redox supercapacitors. These have

higher capacitance than the double-layer capacitance. Electrochemical

supercapacitors have higher energy density than dielectric capacitors and higher

power density than batteries. Based on the different operation mechanisms,

supercapacitors are classified into two different categories (i) double-layer

capacitors, which are based on the non-faradaic charge separation at the

electrode/electrolyte interface and (ii) pseudocapacitors, which are based on the

Faradaic redox reaction of electroactive materials [1]. These electrochemical devices

occupy the area between batteries and conventional capacitors in the specific power

vs specific energy plot (Ragone plot).

Active electrode materials for supercapacitors are broadly classified into three

categories as (1) Activated high surface area carbon (2) Conducting polymers and

(3) Transition metal oxides [2]. Nanocrystalline metal oxides, which usually possess

39 

 

the characteristics of large surface area, high conductivity, electrochemical stability

and pseudocapacitive behaviour. Among the various metal oxides, RuO2 has

metallic conductivity, high chemical and thermal stability, catalytic activities,

electrochemical redox properties etc., in both crystalline and amorphous form. Hence

it gained importance with a large capacitance exceeding 700 F/g. Despite, the

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

metal, cost and its probable toxicity have prompted to identify compounds to replace

RuO2 by cheaper materials [3]. Hence there is a need to have cheaper electrode

materials for asymmetric supercapacitor applications.

2.2. EARLIER WORK

Cobalt oxide film was developed by B.E.Conway et al [4]. In irreversibly

chargeable state it behaves analogous to RuO2 or IrO2, with multiple recyclability

without degradation. It has limited/unfavourable performance features: (a) a non-

constant capacitance over its operable potential range; (b) possessing short (0.7V)

range for reversible charge admission and withdrawal.

The application of a thin film process such as sputtering for the fabrication of

RuO2 electrodes for supercapacitors can also exclude the drawback of high cost.

However, the poor reversibility during the charge-discharge process results in the

capacitance fade which is larger than that of bulk supercapacitors [5].

Lead ruthenate pyrochlore (Pb2Ru2O6.5) synthesized by the method of

Horowitz et al, as electrodes for the fabrication of electrochemical capacitors

exhibited high pseudo capacitance with improved specific energy, under constant-

power discharge of more than 5Wh/kg at 750 W/kg power level. The significant

40 

 

saving in material cost compared to pure noble metal oxides was studied by Fei Cao

et al [6].

Nanocrystalline TixFeyRuzOn materials were prepared by mechanical alloying

using high energy ball milling and its electrochemical properties were investigated in

1M NaOH and 1M H2SO4 aqueous solutions [3]. The materials were tested for two

types of materials as O/Ti ratio larger than 1 in which Ru atom were found in a

hexagonal phase and O/Ti ratio smaller than 1, where Ru atoms were found in a

cubic phase, along with Ti and Fe atoms. The first type of materials in H2SO4 or

NaOH upon cycling exhibited a significant increase of capacitance from 5 to 50 F/g

due to the progressive growth of ruthenium oxide layer at the surface. The growth of

stable oxide phase at the surface of the material occurred only when it was cycled

in the basic electrolyte showing a maximum attainable capacitance of 50-60 F/g.

The individual crystallites of as-milled nanocrystalline materials suffered from a

strong tendency to agglomerate. The best performances was obtained with leached

Ti2FeRuO2 with a maximum capacitance of 110 F/g.

Nae-Lih Wu [7] extensively studied the two sets of SnO2 based

supercapacitors: one set comprised of Sb-doped SnO2 nanocrystalltes synthesized

by sol-gel process with a specific capacitance of 16 F/g or 64 F/cm3 in 1M KOH and

the other included composite electrodes of nanocrystalline SnO2 and RuO2 or Fe3O4.

Fe3O4-SnO2 composite electrode exhibited a capacitance of 33 F/g/130 F/cm3 in 1M

Na2SO4 was believed to have a great potential for large scale application.

The improvement of SnO2 electrodes with a maximum specific

capacitance of 10 F/g in 1M KOH at 200 mV/s after firing at 800°C was studied

41 

 

by N.L.Wu et al [8]. The enhancement became increasingly pronounced with

increasing charge-discharge rate due to improved oxide conductivity.

RuO2 thin film electrodes have been prepared by heat treatment of metal

alkoxide adsorbed onto carbon paper. This new composite material was prepared

via solution dip-coating of a Ru-ethoxide precursor and heat conversion. RuO2

coated carbon paper had a relatively high specific capacitance of 620 F/g. This was

due to the porous graphite fiber substrate that acted to increase the contact area and

allowed complete reaction between the RuO2 and the electrolyte. The composite

material also possessed good power characteristics [9].

E.Frackowiak etal [10] reported the effect of polypyrrole on the capacitance

properties of A/Co700 (i.e. MWNTS prepared along with cobalt particles supported

on silica) by galvanostatic dc discharge. Linear galvanostatic discharge proved a

perfect capacitive behaviour and the capacitance value reached to 140 F/g and the

same material without polypyrrole incorporation supplied 65 F/g.

Electrochemical characteristics of electrodes for supercapacitors built from

RuO2 /multiwalled carbon nanotube (CNT) nanocomposite was investigated by Jong

Hyeok Park et al [11]. The specific capacitance of RuO2/pristine CNT

nanocomposites based on the combined mass was about 70 F/g and the specific

capacitance based on the mass of RuO2 was 500 F/g. The specific capacitance of

RuO2/hydrophilic CNT nanocomposite based on the combined mass was about

120 F/g and the specific capacitance based on the mass of RuO2 was about 900 F/g.

The electrochemical characterization of a hybrid supercapacitor using a

composite Fe3O4 negative electrode and a composite MnO2 positive electrode in

42 

 

0.1 M K2SO4 was reported by Thierry Brousse et al [12]. The specific capacitances

of MnO2 and Fe3O4 were found to be 150 ± 10 and 75 ± 8 F/g, respectively whereas

the specific capacitance of the Fe3O4/MnO2 capacitor was equal to about 20 F/g of

the active material. The hybrid electrochemical capacitor was cycled between 0 and

1.8 V for over 5000 constant current charge/discharge cycles. A real energy density

of 7 Wh/kg was reproducibly measured with a real power density upto 820 W/kg.

W.Sugimoto et al [13] reported the electrochemical behaviour of a hydrous

ruthenic acid consisting of an electroconducting crystalline network of ruthenium

oxide layers nanometrically interleaved with a hydrous layer. The specific

capacitance at 2 mV/s was 390 F/g and at a fast sweep rate of 500 mV/s, the

specific capacitance was 330 F/g, showing excellent fast charge/discharge property.

C.Lin et al [14] developed a new sol-gel synthesis route to make a high

surface area Ru-carbon xerogel composite for use as an electrode material in a

supercapacitor. The capacitance was found to increase with an increase in the Ru

content and reached 197 F/g for the xerogel with Ru/R = 0.1 (resorcinol=R). The

galvanostatic experimental results showed a capacitance as high as 300 F/g with a

discharge at a current density of 0.2 mA/mg but this decreased to about 75 F/g at a

current density of 5 mA/mg.

A potentiodynamically deposited nanostructured SnO2 onto an inexpensive

stainless steel (SS) substrate for redox supercapacitor application in 1M Na2SO4

was reported by N.Miura et al [15] which exhibited a specific capacitance as

high as 285 F/g and 101 F/g at the scan rates of 10 mV/s and 200 mV/s,

respectively. These values of capacitance were much higher for SnO2 with high

stability of the material and also with high power density.

43 

 

Nanostructured and nanorod-shaped three-dimensional In2O3 prepared by

potentiodynamic method at high scan rates showed specific capacitance as high as

190 F/g and 80 F/g at 10 mV/s and 100 mV/s scan rates respectively. These higher

values of specific capacitance was reported by N.Miura et al than other inexpensive

oxides of much lower scan rates indicating the very high power characteristics of the

material than RuO2 [16].

Matheiu Toupin et al [17] synthesized MnO2 powders by co-precipitation and

adopted both thick and thin film electrodes for the supercapacitor applications. The

specific capacitance of 1380 F/g was obtained for the thin film electrode which was

close to the theoretical value of 1370 F/g expected for a redox process involving one

electron per manganese atom. The coulombic efficiency was about 100% indicating

that all the MnO2 material was taking part in the electrochemical process.

N.Miura et al [18] studied the potentiodynamic deposition of mixed oxides

based on MnO2. Nickel-manganese oxide (NMO) and cobalt-manganese oxide

(CMO) were synthesized and characterized for supercapacitor applications.

Maximum specific capacitance values of 621 and 498 F/g were obtained with NMO

and CMO electrodes, respectively at a cyclic voltammetric scan rate of 10 mV/s.

Specific capacitance values of 377 and 307 F/g were obtained with NMO and CMO

electrodes, respectively, at a high scan rate of 200 mV/s indicating high power

characteristics of the binary oxides. The results were more interesting with NMO and

CMO electrodes, respectively at a current density of 2 mA/cm2. These values of

electrical parameters were much higher than those obtained with just MnO2 and

these materials showed long cycle-life and excellent stability.

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Thin film ruthenium oxide electrodes were prepared by cathodic

electrodeposition on a titanium substrate. The electrodes were used to form a

supercapacitor with 0.5M H2SO4 electrolyte. The specific capacitance and charge-

discharge periods were found to be dependent on the electrode thickness. A

maximum specific capacitance of 788 F/g was achieved with an electrode thickness

of 0.0014 g/cm2 [19].

A hybrid electrochemical capacitor using MnO2 and activated carbon (AC) as

positive and negative electrodes, respectively has been designed and individually

tested in a mild aqueous electrolyte of 0.65 M K2SO4. This was cycled between 0

and 2.2V for over 10,000 constant charge/discharge cycles exhibiting a real and

reproducible energy density of 10 Wh/kg and a real power density reaching

3600 W/kg [20].

Chen-Ching Wang et al [21] studied the capacitive behaviour of activated

carbon (AC) and AC-ruthenium (AC-Ru) composites fabricated by wet impregnation

method and investigated in 0.1M H2SO4 by cyclic voltammetry (CV) and

chronopotentiometry (CP). The total specific capacitance of this AC-Ru(OH)x

composite was about 117.6 F/g with a maximum specific capacitance of RuO2

equal to 1170 F/g.

RuO2 /TiO2 nanotubes composites was reported by Wang yong - gang etal

[22] by loading various amounts of RuO2 on TiO2 nanotubes .The three dimensional

nanotube network of TiO2 offered a solid support for the RuO2 as active material. A

maximum specific capacitance of 1236 F/g was obtained for the RuO2 with loading

on TiO2 nanotube.

45 

 

Fang Xiao et al [23] adopted the simple calcined method to prepare the

brown-millerite type SrCoO2.5 which was characterized by X-ray diffraction (XRD)

and scanning electron microscopy (SEM). The maximal capacitance obtained by

cyclic voltammogram and charge-discarge studies were 170 F/g and 168.5 F/g

respectively exhibiting good characteristic of the electrochemical capacitor. The

perovskite and perovskite-related materials existed the characteristics of

supercapacitors.

Yunhong Zhou et al [24] reported the capacitive characteristics of manganese

oxides and polyaniline (M-P) composite thin film deposited on porous carbon in 0.1M

Na2SO4 solution. The capacitance of M-P film materials reached a value of 500 F/g

and it maintained 60 % of the initial capacitance after 5000 cycles.

The characteristics of RuO2-SnO2 nanocrystalline-embedded amorphous

electrode, grown by DC reactive sputtering was widely studied by Han-ki Kim et al

[25]. The cyclic voltammogram result of the RuO2-SnO2 nanocrystalline-embedded

amorphous film in 0.5M H2SO4 liquid electrolyte was similar to a bulk-type

supercapacitor behaviour with a specific capacitance of 62.2 mF/cm2 µm. This result

suggested that the RuO2-SnO2 nanocrystalline-embedded amorphous film showed

larger capacitance than that of the amorphous RuO2 film.

M.S.Dandekar et al [26] reported the impregnation of RuOx(OH)y as an

effective method for improving the energy-storage capabilities of activated (high

surface area) carbon. The impregnation process resulted in a composite material

with homogeneously dispersed RuOx(OH)y colloidal particles throughout the porous

network of the activated carbon. Cyclic voltammetric studies revealed an increase in

46 

 

the specific capacitance of the activated carbon from 100 to 250 F/g for the

composite with 9 wt% ruthenium. Electrochemical impedance analysis suggested

that the energy-storage process of these composite materials as well as that of pure

activated carbon, was limited by the mass transport of electrolyte ions in the pores of

the activated carbon. This type of composite material containing a small amount of

amorphous ruthenium oxide is a promising candidate for supercapacitor electodes.

The fabrication of supercapacitor electrodes with nickel oxide (NiO)/carbon

nanotubes (CNTs) nanocomposite was formed by a simple chemical precipitation

method. The presence of CNT network in the NiO significantly improved (i) the

electrical conductivity of the host NiO by the formation of conducting network of

CNTs and (ii) the active sites for the redox reaction of the metal oxide by increasing

its specific surface area. This increased the specific capacitance by 34% at a

percolation limit of 10 wt% of CNTs. The specific capacitance of the bare NiO and

NiO/CNT (10%) nanocomposite were 122 and 160 F/g, respectively at a discharge

current density of 10 mA/g. In addition, the power density and cycle life were also

improved [27].

CNTs-MnO2 nanocomposite prepared by coating MnO2 on the surface of

CNTs was studied by Meigen Deng et al [28]. The deposition of MnO2 made the

specific capacitance of the CNTs increased from 49 to 146 F/g. It also showed an

outstanding power performance and an energy density of 5.1 Wh/kg.

Manganese and molybdenum mixed oxides in a thin film form was used as

electrode for supercapacitors. It exhibited a specific capacitance of 190.9 F/g and

better reversibility compared to that of pure Mn oxide reported by M.Nakayama etal

[29].

47 

 

A new material of symmetric electrochemical supercapacitor in which zinc

oxide (ZnO) with carbon aerogel (CA) was used as active material was synthesized

by a simple co-precipitation process. ZnO had the hexagonal structure and the

average grain size was 8 nm. The result of cyclic voltammetry indicated that the

specific capacitance was approximately 25 F/g at 10 mV/s in 6M KOH

electrolyte. AC impedance analysis revealed that ZnO with carbon aerogel powder

enhanced the conductivity by reducing the internal resistance. Galvanostatic

charge/discharge measurements were done at various current densities, namely, 25,

50, 75 and 100 mA/cm2 exhibiting a maximum specific capacitance value of 500

F/g at 100 mA/cm2. It was also observed that specific capacitance was constant

upto 500 cycles at all current densities [30].

V.Subramanian et al [31] employed the hydrothermal technique to prepare

the interesting nanostructures of MnO2 under mild conditions. The SEM and TEM

studies revealed a mixture of nanostructured surface with a distinct plate-like

morphology and nanorods of MnO2. The synthesized nanostructured MnO2 with a

specific surface area of 132 m2/g showed a good pseudocapacitive behaviour with a

specific capacitance of 168 F/g with a cycling efficiency of 83% for 100 cycles at

fairly high current of 200 mA/g.

An advanced electrode material with a 3D, arrayed, nanotubular

architecture-annealed RuO2.xH2O NTs which can achieve the ultrahigh power

characteristics and high capacity performances required for the next generation

supercapacitors by using a very simple, one-step, reliable, cost-effective, anodic

deposition technique was studied by Chi-Chang Hu et al [32]. This unique structured

electrode material not only reduced the diffusion resistance of electrolytes but also

48 

 

enhanced the facility of ion transportation and maintained the very smooth electron

pathways for the extremely rapid charge/discharge reactions. The specific power and

specific energy of RuO2.xH2O nanotubular arrayed electrodes (with annealing in air

at 200°C for 2h) measured at 0.8 V and 4 kHz was equal to 4320 kW/kg and

7.5 Wh/kg, respectively achieving the perfect performance for next generation

supercapacitors.

In order to demonstrate that pseudocapacitance could occur by a coupled

proton-electron transfer in non-aqueous electrolytes, D.Rochefort et al [33] studied

the electrochemical behaviour of a thermally prepared RuO2 in a proton exchange

room temperature molten salt composed of 2-methyl pyridine and trifluoroacetic acid.

The shape of the cyclic voltammograms obtained for the RuO2 electrode in the

protic ionic liquid was similar to that obtained in H2SO4 and showing distinct redox

peaks attributed to Faradaic reactions across the electrolyte/electrode interface,

giving rise rise to pseudocapacitance. The specific capacitance of the electrode in

the protic ionic liquid (83 F/g) was in the same order of magnitude to that obtained in

water-based electrolyte but was ten times higher than that in an aprotic ionic liquid

composed of 1-ethyl-3-methyl imidazolium tetrafluoroborate.

A new composite film, (PEDOT-PSS-RuO2 film) was prepared by depositing

hydrous ruthenium oxide (RuO2) particles into a polymer matrix comprising of

poly(3,4-ethylene dioxythiophene) (PEDOT) and poly(styrene sulfonic acid) (PSS).

Specific capacitance of the PEDOT-PSS-RuO2 composite was found to be 1409 F/g.

The enhancement of capacitance was due to uniform distribution of hydrous RuO2

into a three-dimensional conducting PEDOT-PSS matrix [34].

49 

 

MnFe2O4-CB (carbon black) composite powders was synthesized by a co-

precipitation method and characterized for their electrochemical properties. The

composite showed pseudocapacitance in chloride, sulphate and sulphite salts of

alkali and alkaline cations, with NaCl giving the highest capacitance of 56 F/g and

the smallest value of 43.5 F/g in aqueous CsCl. The pseudocapacitance was

associated with the charge transfer at both the Mn and Fe sites for the chlorides and

sulphates electrolytes. Since, the specific capacitance and self-discharge behaviour

of the composite was dependent on the ferrite content, the optimum capacitance

occurs at ferrite:CB weight ratio of 7:3 [35].

Man-Jong Lee [36] et al reported the various possible applications of the

solid oxide Ta2O5 thin film and also presented a prototype TFSCs based on the

RuO2/Ta2O5/RuO2/Pt structure, with typical supercapacitor behaviour judged form

the charge-discharge measurements with constant current density at room

temperature. Owing to the fast intercalation-deintercalation of small H+ ions within

the Ta2O5 lattice, the capacitance per volume of RuO2/Ta2O5/RuO2/Pt TFSCs was

maintained to be constant during 800 cycles (9.4 x 10-4 F/cm2 µm). These findings

suggested that a high performance TFSCs could be made with a solid electrolyte

thin film with high ionic mobility. It was also found that the concentration of highly

mobile proton ions should be effectively increased for the practical application.

Highly porous NiO was prepared via a combination of sol-gel process with

supercritical drying method. Cyclic voltammetric measurements indicated that the

optimal heating temperature of 300°C was found to favour the formation of

mesopores, accounting for the large specific capacitance of as high as 125 F/g. The

average specific capacitance of the aerogel like NiO was observed to be about 75-

50 

 

125 F/g between a potential window of 0-0.35 V vs SCE. The presence of ordered

mesopores favoured the electrolyte diffusion into the electrode material leading to a

complete redox reaction [37].

T.C.Girija et al [38] reported the analysis of underpotential deposition (UPD)

based systems for the development of electrochemical supercapacitors employing

the UPD of Tl on Ag in the presence of bromide ions. At a current density of

10µA/cm2, the initial capacitance was 240 F/g, while the capacitance after 100 cycles

became ~235 F/g suggesting that there was no appreciable decrease with the

number of cycles. But, when the current density was increased up to 75µA/cm2,

the specific capacitance decreases from ~127 F/g to ~108 F/g. The

charge/discharge experiments indicated the reversibility was maintained for ~102

cycles.

The amorphous MnO2 and single walled carbon nanotubes (SWNTs)

composites, at a high charge-discharge current of 2 A/g was studied by

V.Subramanian et al [39]. The cyclic voltammetric studies showed typical capacitive

behaviour for the pure MnO2, the pure SWNTs and the composites with the

capacitance values ranging from 22 to 200 F/g. The EIS measurements showed a

decrease in resistance with respect to the increase of SWNT content in the

composites. All the composites with different SWNT loads showed excellent cycling

capability, even at the high current of 2 A/g, with the MnO2:20 wt% SWNT composite

showing the best combination of coloumbic efficiency of 75% and specific

capacitance of 110 F/g after 750 cycles. And the composites with 5 wt% SWNTs

showed the highest specific capacitance during initial cycles.

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A novel 3V sandwich-type asymmetrical supercapacitor which consisted of a

positive NMRO (nickel-based mixed rare-earth oxide) electrode, a negative AC

(activated carbon) electrode and a hydrophobic (BMIM-PF6) RTIL electrolyte [(1-

butyl-3-methylimidazolium hexafluorophosphate) at room temperature ionic liquid],

was extensively studied by Hongtao Liu et al [40]. The specific capacitance of the

KOH-treated Ac in BMIM-PF6 RTIL electrolyte was calculated from the cyclic

voltammogram was 174 F/g, while that of NMRO material was 357 F/g. The

asymmetrical supercapacitor delivered a real power density of 458 W/kg as well as a

real energy density of 50 Wh/kg with no capacity decay after 500 cycles.

T.Cottineau etal [41] studied different transition metal oxide, namely MnO2 ,

Fe3O4 and V2O5 as possible electrodes for electrochemical supercapacitors in

netural aqueous media of 0.1 M K2SO4. The MnO2 and V2O5 hybrid devices exhibit a

specific capacitance of 150 F/g and 170 F/g respectively and Fe3O4 showed only

75F/g of specific capacitance. Both MnO2 and Fe3O4 maintained its specific

capacitance values even after 1000 cycles. The MnO2/Carbon device exhibits

15 Wh/kg of power density in K2SO4 neutral electrolyte. V2O5 electrode drastically

fades after five hundred cycles.

The composite electrode consisting of carbon nanotubes and spherical

Ni[OH] was reported by Wang xiao – feng etal [42] by mixing nickel hydroxide,

carbon nanotubes and carbonyl nickel powder in the ratio of 8:1:1. A maximum

capacitance of 311 F/g was reported with specific energy and specific power of

25.8 Wh/kg and 2.8 Wh/kg, respectively.

A new hybrid capacitor [HC] cell assembled using activated carbon [Ac]

negative electrode, an Ni[OH]2 positive electrode and polymer hydrogel electrolyte

52 

 

prepared from cross linked potassium poly(acrylate) [PAAK] and KOH aqueous

solution was extensively studied by Shinji Nohara etal [43]. The HC cell worked in

the large voltage range of 1.2 V with high columbic efficiency compared to EDLC

cell. The discharge capacitance was also maintained over 90% of its initial value

after 20,000 cycles.

Jianguo Wen et al [44] reported the hydrous ruthenium oxide (RuOx.nH2O)

deposited on titanium substrate by cyclic voltammetric deposition in the range of -

100 to 1000 mV in 1 M H2SO4 electrolyte. The average specific capacitance was

measured as 105 F/g. Their SEM morphologies were uniform and compact after 60

growth cycles.

Hydrous ruthenium oxide, RuOx(OH)y acts as a mixed electronic-protonic

conductor represented in the following potential-dependent equilibrium as [45],

RuOx(OH)y + δH+ + δe- → RuOx - δ(OH)y+δ

Its electrochemical properties depend on the amount of water incorporated in

its structure and change in oxidation state (Ru4+/Ru3+) of ruthenium, which was

actually responsible for the capacitance of 770 F/g for high performance ultra-

capacitors.

The electrochemical behaviours and pseudo capacitive effects of Cu2+ and

Fe2+ ions in porous carbons were investigated by cyclic voltammetry and constant

charge/discharge tests at constant current. The results indicated that the capacity of

an electrochemical capacitor (EC) was indeed significantly improved with the

introduction of Cu2+ to the electrolyte of H2SO4 (H2SO4 + Cu2+ system), appearing

unstable because of the relatively irreversible redox process. However, a stable

electrochemical process was obtained when Fe2+ ions were added to the H2SO4 +

53 

 

Cu2+ electrolyte (H2SO4 + Cu2+/Fe2+ system) with its capacity even reaching

223 Ah/g at a current density of 0.1A/g, which was higher than that of the H2SO4 +

Cu2+ system. This observation strongly suggested that a synergistic electrochemical

interaction occurred between Cu2+/Fe2+ ions during the charge/discharge process,

giving rise to the much improved reversibility of the electrochemical process of

Cu2+/Cu on the electrode surface [46].

A novel nanocomposite of Co(OH)2-Ni(OH)2 and ultrastable Y molecular

sieves was synthesized by an improved chemical precipitation method for

electrochemical capacitors. The electrochemical characterization displayed superior

capacitive performance. Upon annealing at 250°C, the maximum specific

capacitance was up to 479 F/g. Annealing temperatures higher than 250°C caused

the hydroxide to form oxide phase and decrease the surface activity of the oxide,

thereby leading to a decline of the specific capacitance [47].

A simple solution method was adopted to grow the single crystal α-MnO2

nanorods on MWNTs in the presence of KMnO4 in H2SO4 aqueous solutions. The

oxygen containing functional groups on the surface of MWNTs played an important

role in acting as nucleating centres, from which the single crystal α-MnO2 nanorods

grow. A high specific capacitance of 146 F/g was achieved at the first cycle for the

MWNTs/MnO2 composites compared to the pure MWNTs and MnO2 mechanically

mixed with MWNTs. The capacitance remained stable at 78 F/g after 20 cycles

which reduces to 16 F/g after 100 cycles [48].

Single crystalline α-MnO2 nanorods have been successfully synthesized via a

facile hydrothermal approach involving no templates and surfactants. Single-phase

α-MnO2 products can be obtained in a wide temperature range from 100 to 150°C.

54 

 

Electrochemical studies of the MnO2 materials prepared at 150°C for different

hydrothermal times indicated fine capacitive behaviours with specific capacitance of

83.9 and 79.3 F/g for 5 and 8 h respectively, which were higher than that prepared

for 1.5 and 12 h, respectively [49].

Chin-yi Chen et al [50] reported the nanocrystalline manganese oxide

powders prepared by spray pyrolysis (SP) from manganese acetate solution at

400°C. The as-obtained powders were subsequently coated onto graphite substrates

via an electrophoretic deposition technique (EPD). The EPD powder electrode thus

showed the quite reversible electrochemical behaviour in aqueous electrolyte with a

high specific capacitance of 275 F/g with a cycling efficiency of 85% after 300 cycles.

The specific capacitance was found to decrease to 234 F/g after hundreds of CV

cycles which was comparable to the values reported in the literature.

E.C.Rios et al [51] extensively studied the preparation of composite

electrodes by electrodeposition of manganese oxide on titanium substrate modified

with poly(3-methylthiophene) (PMeT) and compared it with Ti/MnO2 electrodes. The

electrodes were characterized by cyclic voltammetry in 1 mol/L Na2SO4. The specific

capacitance of Ti/MnO2 was 122 F/g, while for Ti/PMeT250/MnO2 and

Ti/PMeT1500/MnO2 the values were 218 and 66 F/g respectively. If only oxide mass is

considered, then the capacitances of the composite electrodes increases to 381 and

153 F/g respectively.

Amorphous RuO2.xH2O and a VGCF/RuO2.xH2O nanocomposite (VGCF=

vapour grown carbon fiber) were prepared by thermal decomposition. The specific

capacitance of RuO2.xH2O and VGCF/RuO2.xH2O nanocomposite electrodes at a

scan rate of 10 mV/s was 410 and 1017 F/g respectively, and at 1000 mV/s are

55 

 

258 and 824 F/g respectively. Long-term cycle life tests for 104 cycles showed that

the RuO2.xH2O and VGCF/RuO2.xH2O electrodes retain 90 and 97% capacity,

respectively [52].

RuO2.xH2O nanoparticulates in different crystal sizes with various water

contents were prepared via a hydrothermal synthesis route, successfully

demonstrating the independent control of crystal size and water content of

RuO2.xH2O. The specific capacitance of RuO2.xH2O reached a maximum of 540 F/g

when the hydrothermal time was equal to 1.5 h, it then decayed monotonously to

390 F/g with prolonging the hydrothermal time from 1.5 to 24 h [53].

Template-assisted synthesis of ruthenium oxide nanoneedles were

extensively studied by Mahima Subhramania et al [54]. The formation of bundles of

RuO2 nanoneedles occurred by the template-assisted electrodeposition from

aqueous RuCl3 solution under potentiostatic conditions at room temperature. The

specific capacitance of these nanoneedles showed a higher value compared to that

of the bulk material (3 F/g instead of 0.4F/g for the bulk), which is explained on the

basis of enhanced reactivity.

A simple, one-step, cost-effective method (i.e.) anodic deposition of

RuO2.xH2O onto Ti from a simple chloride precursor solution was successfully

demonstrated. The cycling stability, specific capacitance (increasing from 460 to

552 F/g), and power performances of as-deposited RuO2.xH2O were significantly

improved by annealing in air at 150°C for 2h. The high onset frequencies of 660 and

1650 Hz obtained from EIS spectra for the as-deposited and annealed RuO2.xH2O

films, respectively, definitely illustrated the high-power merits of both oxide films

prepared by means of the anodic deposition [55].

56 

 

A novel composite of Co(OH)2 and TiO2 nanotubes was synthesized by a

chemical precipitation method. A maximum specific capacitance of 229 F/g for a

weight ratio of 1:3 was obtained owing to its microstructure, which was much better

than the pure Co(OH)2. TiO2 nanotubes could make the Co(OH)2 more dispersed

and be the active sites for electrochemical reactions, which enhanced the utilization

of the Co(OH)2 [56].

C.-C. Hu et al [57] systematically compared the electrochemical behaviour of

mixed Ru-Sn oxides fabricated by a modified sol-gel process and a co-annealing

method. The power performance of RTOCCo-200-2 with 80 wt % RuO2, is better than

that of Ru0.8Sn0.2O2.xH2O annealed at 200°C for 2hours although the specific

capacitance of the latter oxide was higher. The co-annealing method showed an

additional advantage of simple, precise composition control, resulting in a minor

variation in the total capacitance of every oxide-coated electrode, which promotes

the practical reliability of the supercapacitor for stacking up the electrodes in the

bipolar design.

Nanosheets of α- Co(OH)2 were electrochemically synthesized on SS in 0.1 M

Co(NO3)2 electrolyte at -1.0 V vs Ag/AgCl. A specific capacitance of 860 F/g was

obtained for 0.8 mg/cm2 due to its α- Co(OH)2 deposit, and this did not decrease

significantly for the active mass loading range of 0.1-0.8 mg/cm2 due to its layered

structure, which allowed easy penetration of electrolyte and effective utilization of the

electrode material even at a higher mass [58].

C.D.Lokhande et al [59] extensively reported the supercapacitor performance

of electrochemically deposited perovskite nanocrystalline porous bismuth iron oxide

(BiFeO3) thin film electrode from alkaline bath. This showed comparable specific

57 

 

capacitance of 81 F/g and specific energy and specific power of 6.6832 J/g and

3.2958 W/g compared to other commonly used ruthenium based oxide perovskites.

The perovskite BiFeO3 electrodes were nanocrystalline with rhombohedral crystal

structure showing porous surface morphology.

Amorphous manganese oxide prepared by mechanochemical method was

tested with cyclic voltammetry and constant current charge-discharge for the

electrochemical supercapacitor performances. The positive electrode (PE) and

negative electrode (NE) were investigated to find out their different performances in

charge-discharge process. The capacitances of the NE were highly rate-dependent,

decreasing from 12.3 to 53.1 F/g by 56.2 % over the range 5-25 mV/s. The PE

appears to be weakly dependent and its capacitance dropped only by 22% [60].

The capacitive behaviour of CNT, V2O5, V2O5-CNT and SnO2-V2O5-CNT were

studied by M.Jayalakshmi et al [61] by cyclic voltammetry. The V2O5 and CNT

together gave a specific capacitance of 45-47 F/g on the 100th scan and the SnO2-

V2O5-CNT electrode provided the best performance value of 121.4 F/g, due to the

increase in the electronic properties of vanadium oxide by the presence of SnO2 in

the composite. This also included the capacitive behaviour of the composite.

Rak Young song et al [62] studied extensively the electrochemical capacitive

behaviour of ternary composite (Polyaniline/Nafion/hydrous RuO2) in an aqueous

electrolyte of 1M H2SO4. The ternary composite electrode displayed good cycleability

and initial specific capacitance value of 325 F/g and 260 F/g after 104 cycles (80%

capacitance retention) with 50 wt% loading of hydrous RuO2. At 100 mV/s the

ternary composite showed specific capacitance of 475 F/g and at 1000 mV/s, it

showed a specific capacitance of 375 F/g versus Ag/AgCl.

58 

 

RuO2/MWNT, TiO2/MWNT and SnO2/MWNT nanocrystalline composites for

supercapacitor electrodes prepared by chemical reduction method using

functionalized MWNT and respective salts was studied by A.Leela Mohana Reddy et

al [63]. Electrochemical measurement of functionalized MWNT gave a specific

capacitance of 67 F/g, and the nanocrystalline RuO2/MWNT, TiO2/MWNT and

SnO2/MWNT composites with specific capacitance of 138, 160, 93 F/g, respectively.

The enhancement of specific capacitance of metal oxide dispersed MWNT from that

of pure MWNT was due to the progressive redox reactions occurring at the surface

and bulk of transition metal oxides through Faradaic charge transfer reaction.

S.R.Sivakumar et al [64] reported the specific capacitance values of ternary

composite (CNT/hydrous MnO2/ polypyrrole), CNT/hydrous MnO2 and polypyrrole/

hydrous MnO2 in 1 M Na2SO4 electrolyte and were 281, 150, 32 F/g at 20 mV/s and

209, 75, 7 F/g at 200 mV/s for a fair MnO2 loading of 0.64 mg/cm2. The ternary

composite showed much better electrochemical performances compared to binary

composites. This was believed to be due to the good electrical conductivity of the

composite and effective dispersion of hydrous MnO2 in the composite electrode. The

specific capacitance value of full cell of ternary composite showed 146 F/g which

decreased rapidly during 500 cycles and ended with 35 F/g at 2000th cycle.

Composites consisting of RuO2.xH2O particles deposited on carbon

nanofibres had been prepared by accumulative treatments consisting of

impregnation of carbon nanotubes with RuCl3.0.5 H2O solution, filtering to remove

RuCl3.0.5 H2O excess and neutralization with NaOH solution to get RuO2.xH2O. The

specific capacitance decreased from 840 to 440 F/g as the particle size increased

59 

 

from 2 to 4 nm. The contribution of RuO2.xH2O to the specific capacitance of the

composite decreased because RuO2.xH2O particles grew [65].

Binary Mn-Co oxide with pseudocapacitive characteristics was successfully

prepared by anodic deposition and by controlling the composition of the plating

solution, the Co content ratio of the deposited oxide could be adjusted. The added

Co existed in three different forms in the oxide – Co2O3, CoOOH and Co(OH)2 [66].

The Co addition could suppress the anodic dissolution of Mn during charge-

discharge cycling, and therefore it greatly improved the electrochemical stability of

the oxide electrode. However, a large amount of Co addition should be avoided,

since it can cause a significant reduction in the specific capacitance of the oxide.

Hydrous ruthenium oxides with the addition of carbon nanotubes (CNT)

(0.0125 - 0.1 wt %) co-deposited on Ti substrate by cathodic deposition method was

studied by H.-S.Hwang et al [67]. The experimental results showed a maximum

capacitance of about 719 F/g with the addition of 0.05 wt% of CNT and for a

deposition period of 60 minutes.

An efficient way to decorate multiwalled carbon nanotubes with Ru was

developed, in which Ru nanoparticles were prepared by water-in-oil reverse

microemulsion and the produced Ru was anchored on MWCNTs. Cyclic voltammetry

results demonstrated that the specific capacitance of deposited ruthenium oxide

electrode i.e. upto 457.63 F/g at 100 mV/s was significantly greater than that of the

pristine MWCNTs electrode in the same medium [68].

Cobalt-nickel oxide/VGCF (vapour grown carbon fiber) composites were

prepared by thermally decomposing cobalt and nickel nitrates directly onto the pore

60 

 

of the 3-dimensional nickel foam substrate. Cyclic voltammetric studies exhibited the

specific capacitance values of 1271 F/g at 5 mV/s and 955 F/g at 100 mV/s. The

VGCF also played the role in reducing the interfacial resistance in the cobalt-nickel

oxide to improve the capacitance performance [69].

Z.Fan et al [70] studied the hydrothermal method for synthesizing

MnO2/graphite nanoplatelet (GNP) composites. The MnO2 nanorods dispersed

homogeneously on both surfaces of the GNPs and formed a coating structure

indicating the excellent electrochemical properties. The specific capacitance of

MnO2/GNPs (based on MnO2) increased monotonously with increasing GNP

content, and the maximum specific capacitance was 276.3 F/g. Additionally, the

coating structure also decreased the amount of conductive filler needed to meet the

requirement of the electrode material. The specific capacitance based on the

composite was 158 F/g by addition of only 10 wt% GNPs.

Hui-Ming Cheng [71] et al reported the mesoporous carbon with

homogeneous boron dopant prepared by the co-impregnation and carbonization of

sucrose and boric acid confined in mesopores of SBE-15 silica template. These

born-doped mesoporous carbon material possessed the characteristics of aligned

mesopores, catalytically rich-oxygen functional groups and improved electronic

structure in space charge-layer which paved the way of exhibiting interfacial

capacitance as high as 1.6 times of that with the boron-free carbon.

Nanocrystalline iron-added manganese oxide powders were prepared by

spray pyrolysis (SP) method from manganese acetate and iron nitrate solutions at

400°C. The as-obtained powders were subsequently deposited onto graphite

substrates via an electrophoretic deposition technique (EPD). The iron addition

61 

 

significantly improved the electrochemical properties of the Mn-oxide powder

coatings. The specific capacitance of as-deposited Mn-oxide coating was increased

from 202 F/g to 232 F/g when 2 wt% of iron was added. Even after 1200 cycles of

life test, the iron-added coatings exhibited higher cycling efficiency of (~ 78% of the

initial maximum capacitance) than that of the unadded one (~60% of its maximum

value) [72].

Wei-Chuan Fang [73] studied the capacitive properties of vanadium oxide on

carbon nanotubes. The carbon nanotubes deposited with uniformly dispersed oxides

gave rise to significant improvement in capacitive performance as compared with

V2O5 film. Hence, the V2O5/CNT nanocomposites showed promise as an auxiliary

power backup of hybrid electrical systems or consumer electronics.

Menqiang Wu et al [74] reported the electrochemical performance of

cathodically deposited amorphous tin oxide (SnO2) onto graphite electrode. A

maximum specific capacitance of 298 and 125 F/g was achieved from cyclic

voltammetry at a scan rate of 10 and 200 mV/s, respectively. It also showed good

cycle-life and stability. The specific capacitance of 265 F/g decreased to 95 F/g with

increasing deposition rate from 0.2 to 6 C/cm2.

K.Karthikeyan et al introduced ZnCo2O4/CNF as electrode material for

supercapacitors [75]. This exhibited high specific capacitance and low impedance

compared to pure metal oxide nanocomposites. The introduction of CNF in metal

oxide increased the specific capacitance value by reducing the internal resistance of

the electrode and it showed a high capacitance of 77 F/g with long cycle life, high

coulombic efficiency with minimum IR.

62 

 

X.Liu et al [76] constructed the hydrous ruthenium oxide electrodes for

supercapacitor by sol-gel method. A high specific capacitance of 977 F/g was

obtained for Ru oxide loading of 5 mg/cm2 in 1M H2SO4 aqueous electrolyte. Their

average power densities and energy densities exceeded 30 W/g and 30 Wh/kg,

respectively during full discharge. This outstanding performances were due to the

primary use of carbon fiber paper as a support for Ru oxide .

Hybrid films of polyaniline (PANI)and manganese oxide (MnOX) obtained by

potentiodynamic deposition showed 44% increase in specific capacitance from that

of PANI (408 F/g) to 588 F/g, measured at 1 mA/cm2 in 1M KNO3 at pH 1. It also

retained 90% of its capacitance after 1000 cycles with a coulombic efficiency of

98% [77].

Single crystalline Mn3O4 and MnOOH, successfully synthesized by

hydrothermal process was reported by Chi–Chang Hu et al [78]. The

potentiodynamic activities for 200 cycles between 0 and 1V in 1M Na2SO4 at 25

mV/s showed relatively high capacitance of 170F/g with high power density and

excellent stability for supercapacitor application.

Chi–Chang Hu et al [79] extensively studied the effect of complex agents on

the anodic deposition and electrochemical characteristics of cobalt oxide. The Co

oxide deposited from the precursor solution with AcO- exhibited a specific

capacitance of 230 F/g which revealed the most promising applicability in

supercapacitor of asymmetric type.

The formation of RuO2.nH2O films with 0.17 mg by cathodic deposition

showed a specific capacitance of 786 F/g and highest energy efficiency of 99.58%

from charge-discharge curves. This was mainly due to the network like morphology

63 

 

with highly porous structure and small ESR as reported by Yan–Zhen Zheng et al

[80].

R.K.Selvan et al [81] reported the synthesis of hexagonal shaped SnO2

nanocrystals and SnO2/C nanocomposites for electrochemical redox

supercapacitors. The material synthesized at 700°C has a specific capacitance

of 37.8 F/g which is a suitable candidate for supercapacitor application.

B.C.Kim et al [82] reported the capacitive properties of RuO2 and Ru-Co

mixed oxides prepared by potentiodynamic deposition on the surface of SWNT.

Electrodes coated with the Ru-Co mixed oxide [Ru (13.13 wt%) and Co (2.89 wt%)]

or RuO2, exhibited a similar specific capacitance (~620 F/g) at low potential scan

rates (10 mV/s). However, at higher scan rates (500 mV/s) the mixed metal oxide

electrode showed superior performance (570 F/g) when compared to the RuO2

electrode (475 F/g). This increase in capacitance at high scan rates is attributed to

the role of Co in providing enhanced electronic conduction.

RuO2 electrodes with mesoporous structure were prepared using KIT-6, a

mesoporous silica, as the hard template. A specific capacitance of 200 F/g for the

high surface area electrode in aqueous H2SO4 at the scan rate of 1 mV/s was seen.

The introduction of the mesoporous structure proved the effectiveness for increasing

the capacitance per mass of the RuO2 [83].

Cobalt oxide aerogels of excellent supercapacitive properties, including high

specific capacitances and onset frequencies and excellent reversibility and cycle

stability, were successfully synthesized with an epoxide addition procedure by using

cobalt nitrate on the precursor [84]. Cobalt oxide was a promising low cost transition

64 

 

metal oxide for supercapacitors of the asymmetric type, and the epoxide synthetic

route enables preparation of transition metal oxide aerogels from low cost and stable

metal salts instead of much more expensive, moisture and heat sensitive alkoxides.

The specific capacitance of 623 F/g of cobalt oxide calcined at 200°C was much

higher than those achieved by other mesoporous cobalt oxides, such as loose-

packed cobalt oxide nanocrystals [85] and the cobalt oxide xerogels [86].

Low-cost layered manganese oxides with the ranciete structural type were

prepared by reduction of KMnO4 or NaMnO4 in acidic aqueous medium, followed or

not by successive proton and alkali-ion-exchange reactions [87]. Capacitances

measured at 2 mV/s, for the different samples with capacitances ranging from 7 F/g

to 112 F/g in K2SO4 electrolyte was extensively studied.

The synthesis and pseudocapacitive characteristics of block co-polymer

template anatase TiO2 thin films synthesized using either sol-gel reagents or

preformed nanocrystals as building blocks was reported by Torsten Brezesinski et al

[88]. The specific charge capacity was 168 mAh/g. Both a mesoporous morphology

and the use of nanocrystals as the basic building blocks were very promising for the

rational development of metal oxide pseudo/supercapacitors.

Qunting Qu et al [89] reported the electrochemical performance of MnO2

nanorods prepared by a precipitation reaction and was investigated in 0.5 mol/L

Li2SO4, Na2SO4 and K2SO4 aqueous electrolyte solutions. The nanorods showed the

largest capacitance of 201 F/g in Li2SO4 electrolyte at slow scan rates since the

reversible intercalation/deintercalation of Li+ in the solid phase produces an

additional capacitance besides the capacitance based on the absorption/desorption

reaction. An asymmetric activated carbon (AC/K2SO4/MnO2) supercapacitor could be

65 

 

cycled reversibly between 0 and 1.8 V with an energy density of 17 Wh/kg at 2

kW/kg and also it exhibited an excellent cycling behaviour with no more than 6%

capacitance loss after 23,000 cycles at 10 C rate even when the dissolved oxygen is

not removed.

A novel nanostructured mesoporous CoxNi1-x layered double hydroxides

(CoxNi1-x LDHs), was syntheiszed by Zhong-Ai Hu et al via a chemical precipitation

route using polyethylene glycol as the structure-directing reagent. The

Co0.41Ni0.59LDHs had an optimum capacitance property with specific capacitance

values of upto 1809 F/g at a charge-discharge current density of 1 A/g, and remains

at about 90.2% of the initial value after 1000 cycles at a current density of 10 A/g

[90].

The use of low-cost electrochemically synthesized hydrophilic and

nanocrystalline tin oxide film electrodes at room temperature in dye-sensitized solar

cells and electrochemical supercapacitors was studied by B.W. Cho et al [91]. The

interfacial and specific capacitances of 118.4 µF/cm2 and 43.07 F/g respectively in

0.1 M NaOH electrolyte were confirmed from cyclic voltammetry measurement.

Nanostructured MnO2 was synthesized by co-precipitation in the presence of

pluronic P123 surfactant. The electrochemical performance of the sample without

surfactant exhibited relatively low specific capacitance of 77 F/g, whereas the

nanostructured MnO2 prepared with 0.02% (wt%) P123 exhibited excellent

pseudocapacitive behaviour, with a maximum specific capacitance of 176 F/g [92].

A novel hybrid system consisting of KS6 graphite and Nb2O5 was tested as a

hybrid supercapacitor by Gum-Jae Park et al [93]. This system was found to exhibit

66 

 

a sloping voltage profile from 2.7 to 3.5 V during charging and presented a high

operating voltage plateau between 1.5 and 3.5V during discharging. The system also

delivered a highest initial discharge capacity of 55 mAh/g and a good cycle

performance.

G.Wang et al [94] synthesized nanoporous Co3O4 nanorods by a

hydrothermal method. Supercapacitance of the Co3O4 nanorod shaped electrodes

was measured by cyclic voltammetry which showed similar shapes with reduction

and oxidation peaks rather than ideally rectangular shapes indicating the

capacitance obtained was not pure double-layer capacitance but mainly originated

from Faradaic pseudocapacitance. The maximum specific capacitance of 281 F/g

was attributed to the nanoporous structure of nanorods with high specific surface

area, compared to commercial microcrystalline Co3O4 powders giving only 43 F/g of

specific capacitance.

H.N.Vasan et al [2] successfully enhanced the electrochemical properties of

nano MnO2 synthesized through one-pot reaction under ambient conditions from

KMnO4 and ethylene glycol. The nano-MnO2 possess a specific capacitance of

250 F/g with a very high electrochemical stability even after 1200 cycles with good

capacity retention. It was related to the physicochemical properties of nano-MnO2

with high surface area mostly owing to the existence of secondary mesopores and

proper water content. It was also very effective to use ethylene glycol as a reducing

agent in synthesizing nano-MnO2 as an electrode material for supercapacitors.

A network of CoXNiyAly layered triple hydroxides (LTHs) nanosheets was

prepared by the potentiodynamic deposition process at -1.0V onto stainless steel

electrodes. Cyclic voltammetry and charge-discharge measurements in the potential

67 

 

range of -0.1V to 0.5V and 0.0 – 0.4V, repectively vs. Ag/AgCl in 1M KOH

electrolyte indicated that CoxNiyAlzLTH have excellent supercapacitive

characteristics. The maximum specific capacitance of ~1263 F/g was obtained

Co0.59Ni0.21Al0.20 LTH. The impedance studies indicated highly conducting nature of

the CoxNiyAlz LTH [1].

A composite of activated carbon with cobalt-alumina layered double

hydroxide (LDH) with extended surface area was prepared by co-precipitation form

solution containing nitrates, carbon and urea. LDH/carbon composites were found to

be promising material of the electrode for the supercapacitor working at high current

density. The symmetric capacitor based on carbon/LDH composite electrodes

retained the value of specific capacitance after long cycling at high current density.

A combination of the redox reactive property from LDH and good electronic

conductivity of the carbon host could be at the origin of the improvement of

electrochemical performance [95].

The cathodic EPD method was developed by Wang et al [96] for the

deposition of composite manganese dioxide – MWCNT films. The films

electrochemically tested in 0.5M Na2SO4 solutions showed the pseudocapacitive

behaviour in a potential window of 0-0.9 V vs SCE. The highest specific capacitance

of ~650 F/g was obtained at a scan rate of 2 mV/s. The capacitance decreased with

an increase in scan rate and the films pepared by cathodic EPP method can be

considered as possible electrode materials for electrochemical supercapacitors

Feng – Jiin Liu et al [97] demonstrated a novel and simple route for preparing

a composite comprising of manganese oxide (MnO2) nanoparticles and polyaniline

(PANI) doped poly(4-styrene sulfonic acid-co-maleic acid) (PSSMA) by

68 

 

‘’electrochemical doping-deposition”. Cyclic voltammetry and chronopotentiometry

were employed in 0.5M Na2SO4 to evaluate the capacitor properties. An

improvement in the specific capacitance of the composite electrode was seen. The

specific capacitance of PANI-PSSMA-MnO2 (50.4 F/g) had improvement values of

172 % compared to that of PANI (18.5 F/g). When only the MnO2 mass was

considered, the composite had a specific capacitance of 556 F/g.

Xuan Du et al [98] reported the activated carbon (AC)-Fe3O4 nanoparticles for

the first time for asymmetric supercapacitors. The cells were assembled and

characterized in 6M KOH aqueous electrolyte. The electrochemical performances of

the hybrid AC-Fe3O4 supercapacitor were tested by cyclic voltammetry,

electrochemical impedance spectroscopy and galvanostatic charge-discharge tests.

The asymmetric supercapacitor delivered a specific capacitance of 37.9 F/g at a

current density of 0.5 mA/cm2 and it also retained 82 % of initial capacity over

500 cycles.

A prototype of flexible and transparent supercapacitors built on In2O3

nanowires/CNT heterogeneous films showed enhanced specific capacitance of

64 F/g from CV measurement, good power density and energy density and long

operation cycles with the incorporation of In2O3 nanowires. After 100 cycles, there is

a little specific capacitance decrease from 64 to 53 F/g, attributed to the dissociation

of In2O3 nanowires during the redox process in GV measurements reported by Chen

et al [99].

A novel mesostructred NiO/C composite with NiO nanoparticles studded in

the wall of mesoporous carbon synthesized by a facile co-casting process was

extensively studied by Hongfang Li et al [100]. A large specific capacitance of

243 F/g for NiO/C composites and 456 F/g for the NiO nanoparticle were obtained.

69 

 

Li et al [101] developed a simple and facile electrochemical deposition

method for the preparation of mesoporous structures with a high density of pores.

Novel mesoporous MnO2/carbon aerogel composites was synthesized in aqueous

electrolytes by electrochemical deposition accompanying hydrogen evolution, which

is crucial for the growth of mesoporous structures. The prepared mesoporous

MnO2/carbon aerogel composites was employed as supercapacitor electrodes and

give a highest specific capacitance (515.5 F/g). The high specific capacitance and

good cycle ability coupled with the low cost and environmentally benign nature of the

MnO2/aerogel composite made this material attractive for large applications.

Hideyuki Nakanishi et al [102] extensively studied the films comprising Au and

Ag nanoparticles that were transformed into porous metal electrodes by desorption

of weak organic ligands followed by wet chemical etching of silver. These prepared

electrodes provided the specific capacitances of about 70 F/g and a high power

density of ~12 kW/kg, comparable to that of the state-of-the-art carbon based

devices.

The preparation and characterization of [Ru3.8+O2]0.2- nanosheets with well-

defined hexagonal symmetry via the soft-chemical delamination of an α-NaFeO2

related layered ruthenium oxide was reported by W.T Sugimoto et al [103]. The

restacked RuO2 nanosheet electrode exhibited a large enhancement in the redox

related charge leading to a specific capacitance of ~700 F/g of RuO2 in an acidic

electrolyte, which was almost double the value of the non-exfoliated layered

protonated ruthenate.

70 

 

Rong-Rong Bi et al [104] demonstrated the direct growth of RuO2

nanoparticles on the CNTs surface with a simple and convenient solution-based

method. The RuO2 nanoparticles in the composite with a RuO2/CNT mass ratio of

6:7 achieved a specific capacitance of as high as 953 F/g. And at a power density

of 5000 W/kg, the RuO2/CNT composite delivered a energy density of 16.8 Wh/kg,

which is about 5.8 times larger than that of the bare RuO2 (2.9 Wh/kg).

Ruthenium oxide as a model pseudocapacitive material with good electronic

and protonic conduction was found to achieve very high gravimetric capacitances of

400-500 F/g. However, the capacitance of thermally prepared ruthenium oxide was

generally low because of low protonic conductivity resulting from dehydration of the

oxide upon annealing [105].

Multisegmented Au-MnO2 /carbon nanotube hybrid coaxial arrays for high

power supercapacitors was extensively studied by A.Leela Mohana reddy et al [106].

The Au-MnO2/CNT hybrid coaxial nanotube electrodes showed excellent

electrochemical performance with a maximum specific capacitance of 68 F/g, a

power density of 33 kW/kg and an energy density of 4.5 Wh/kg. The coaxial

combination of CNTS and metal oxide nanowires leads to a dual storage mechanism

of EDLC and pseudocapacitance.

Sheng Chen et al [107] demonstrated the graphene oxide (GO) –manganese

dioxide (MnO2) nanocomposite prepared by a simple solution method of low

temperature. The integration of GO and the needle like MnO2 crystals enable good

electrochemical behaviour to use as electrode materials for supercapacitors. It

showed a discharge specific capacitance of 216 F/g and retained about 84.1% of

initial capacitance over 1000 cycles than that of MnO2 (69%).

71 

 

Electrochemically prepared poly(methylthiophene) used as a cathode and

Li4Ti5O12 anode in asymmetric hybrid supercapacitor was reported by Aurelien Du

Pasquier et al [108]. This hybrid device exhibited a higher specific energy of

10 Wh/kg and better cycle life than a Li-ion battery.

Ransdellite, Li2Ti3O7 was used an anode material for asymmetric

supercapacitors by Fang Chen et al [109]. A reversible specific capacity of

150 mAh/g in aqueous electrolyte and good cycleability was observed that was

confirmed by its structure retention and capacity stability within 500 cycles. It also

exhibited good rate capability.

Yong – Gang Wang et al [110] reported a new concept hybrid electrochemical

super capacitor technology in which activated carbon was used as a negative

electrode and a lithium-ion intercalated compound LiMn2O4 as a positive electrode

in a mild Li2SO4 aqueous electrolyte. The charge/discharge process was associated

with the transfer of Li-ion between two electrodes. The hybrid cell exhibited a

sloping voltage profile from 0.8 to 1.8 V, and delivers an estimated specific energy of

35 Wh/kg based on the total weight of active electrode materials. The cell exhibited

excellent cycling performance with less than 5 % capacity loss over 20,000 cycles at

10C charge/discharge rate.

Guixin Wang et al [111] reported the LiNi0.8Co0.2O2 prepared by gel-like

method to use as electrode material for supercapacitors. This was mixed with

MWCNT to form composite electrodes, showing good capacitive and energy

characteristics. It showed a maximum specific capacitance of 271.6 F/g with

317.2 Wh/kg energy density and 96.7% coulombic efficiency.

A hybrid electrochemical capacitor was fabricated with LiCoO2 as a positive

electrode and activated carbon (AC) as a negative electrode in various aqueous

72 

 

electrolytes was studied by Lian-Mei Chen et al [112]. Pseudo-capacitive properties

of the LiCoO2/AC electrochemical capacitor were determined by cyclic voltammetry,

charge-discharge test, and electrochemical impedance measurement. The results

showed that the potential range, scan rate, species of aqueous electrolyte and

current density had great effect on capacitive properties of the hybrid capacitor. In

the potential range of 0-1.4 V, it delivered a discharge specific capacitance of

45.9 F/g (based on the active mass of the two electrodes) at a current density of

100 mA/g in 1 mol/L Li2So4 aqueous electrolyte. The specific capacitance remained

41.7 F/g after 600 cycles.

J.-H.yoon et al [113] studied the various methods of synthesis of layered

Li[Ni1/3Co1/3Mn1/3]O2 cathode materials and used as a positive electrode for

asymmetric electrochemical capacitor with activated carbon as negative electrode.

Li[Ni1/3Co1/3Mn1/3]O2 prepared by the carbonate co-precipitation exhibited higher

capacitance and better rate capability with stable cycling retention over 500 cycles

than Li[Ni1/3Co1/3Mn1/3]O2 prepared by the hydroxide-co-precipitation. The

asymmetric electrochemical capacitor (AEC) cell (AC/ Li[Ni1/3Co1/3Mn1/3]O2) had a

voltage slope from 0.2 to 2.2 V and delivered a capacity of 60 F/g with a capacity

retention of 88.4 % during 500 cycles based on the overall active materials weight.

The leakage current was largely decreased for the asymmetric electrochemical

capacitor and the maintained voltage was 84.4 % during 3 days.

LiNi1/3Co1/3Mn1/3O2 used as positive electrode and activated carbon as

negative electrode for asymmetric supercapacitor application was reported by

Y.Zhao et al [114]. Their electrochemical properties indicated that the species of

aqueous electrolyte, current density, scan rate and potential limit, etc. had

influenced on the capacitance properties of AC/ LiNi1/3Co1/3Mn1/3O2 supercapacitor.

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The initial discharge specific capacitance of 298 F/g was obtained in 1 mol/L Li2SO4

solution within potential range 0-1.4 V at the current density of 100 mA/g and was

cut down less than 0.058 F/g per cycling period in 1000 cycles. The asymmetric

supercapacitor exhibited a good cycling performance.

The supercapacitive performances of LiAlxMn2-xO4 synthesized by high

temperature solid-state reaction was investigated by means of galvanostatic charge-

discharge, cyclic voltammetry and AC impedance in 2 mol/L (NH4)2SO4 aqueous

solution was extensively studied by Yun Xue et al [115]. LiAlxMn2-xO4 represented

rectangular shape performance in the potential range of 0-1 V. The capacity and

cycle performance can be improved by doping Al. The composition of x = 0.1 was

only about 14 % after 100 cycles.

The electrochemical performance of a hybrid asymmetric supercapacitor with

activated carbon (AC) as anode and a lithium-ion intercalated compound

LiNi0.5Mn1.5O4 as cathode was studied by Huiming Wu et al [116]. The fabricated

hybrid supercapacitor, AC/LiNi0.5Mn1.5O4 in a non-aqueous electrolyte 1M LiPF6/EC-

DMC exhibited a sloping voltage profile from 1.0-3.0 V and delivered a specific

energy of ca. 56 Wh/kg. Moreover, it exhibited excellent cycling performance with

less than 5 % capacity loss over 1000 cycles.

A novel hybrid supercapacitor with Li2MnSiO4 as the negative electrode and

activated carbon as the positive electrode in 1.0 M LiPF6/EC:DMC electrolyte was

developed by K.Karthikeyan et al [117]. The initial specific capacitance obtained

from the charge-discharge curve was 43.2 F/g at a 1 mA/cm2 current density. The

specific capacitance decreased slightly with increasing current density and thereby

showed that the LMS/AC cell has a high charge-discharge rate capability. The

hybrid supercapacitor retained approximately 85% of its initial capacitance and

74 

 

exhibited a good cycleability with a high coulombic efficiency of greater than 99%,

even after 1000 cycles.

A review on the research progress in high voltage spinel LiNi0.5Mn1.5O4

material was reported by R.Santhanam et al [118] because of its high voltage (4.7

V), acceptable stability and good cycling performance in the case of Li-ion batteries.

Developments in synthesis, effect of doping, effect of coating are also given along

with the application of spinel LiNi0.5Mn1.5O4 material in asymmetric supercapacitors.

This system exhibited excellent capacity retention even after 1000 cycles with high

power density and high energy density.

2.3. SCOPE OF THE PRESENT INVESTIGATION

It is proposed to synthesize LiCoO2 and doped LiCoO2 nanofibrous cathode

materials such as LiAl0.2Co0.8O2, LiCr0.15Co0.85O2, LiFe0.1Co0.9O2 and LiNi0.4Co0.6O2

by electrospinning technique under optimized preparative conditions to use as

cathode materials for asymmetric supercapacitor applications.

The thermal analysis (TG/DTA) to be used to understand the thermochemical

properties such as decomposition temperature and phase formation temperature

etc., of the prepared nanofibrous precursor samples.

The crystallite properties of the LiCoO2 based nanofibrous cathode materials are

to be confirmed by XRD studies.

The molecular conformation of the prepared LiCoO2 based nanofibrous cathode

materials are to be carried out by FT-IR spectroscopy.

The morphology and the average diameter of the prepared LiCoO2 and doped

LiCoO2 nanofibrous cathode materials are to be observed using scanning

75 

 

electron microscope (SEM) studies.

Finally, the asymmetric supercapacitors will be fabricated by using the

synthesized LiCoO2 based nanofibers as the cathode material and the activated

carbon as an anode and a porous polypropylene separator in 1M LiPF6 - EC:

DMC (1:1 v/v) electrolyte.

Cyclic voltammetric studies will be carried out in the potential range of 0.0 to

3.0 V at different scan rates of 5 mV/s, 10 mV/s, 20 mV/s and 50 mV/s to find out

the specific capacitance (Cm) of the capacitor cell.

AC impedance studies will be carried out on the open circuit potential (OCP) in

the frequency range of 100 kHz to 1 mHz at the a.c. amplitude of 10 mV to find

out the internal resistance (Ri) of the capacitor cells.

The charge-discharge studies will be carried out at a constant current density of

1 mA/cm2 to find out the discharge specific capacitance (Cm), coulombic

efficiency (η), internal resistance (Ri), energy density (Ed) and power density (Pd)

of the capacitor cells.

Finally, the synthesized LiCoO2 doped LiCoO2 nanofibrous cathode materials are

to be graded based on the performance of the asymmetric supercapacitor

applications.

76 

 

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