CHAPTER I GENERAL INTRODUCTION AND LITERATURE SURVEY …shodhganga.inflibnet.ac.in/bitstream/10603/4057/8/08_chapter 1.pdf · CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY

CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY

1

CHAPTER I

GENERAL INTRODUCTION AND LITERATURE SURVEY

Sr. No. Title Page

No.

1.1 General 2

1.2 Literature Survey of Metal Oxide Supercapacitors 5

1.3 Literature Survey of MnO2 Thin Films 15

1.4 Orientation and Purpose of the Dissertation 21

References 25


2

1.1 General: Introduction

Recent increases in demand for oil, associated price increases, and

environmental issues are continuing to exert pressure on an already

stretched world energy infrastructure. Significant progress has been made

in the development of renewable energy technologies such as solar cells,

fuel cells, and biofuels. In the past, these types of energy sources have been

marginalized, but as new technology makes alternative energy more

practical and price competitive with fossil fuels, it is expected that the

coming decades will usher in a long-expected transition away from oil and

gasoline as our primary fuel. Although a variety of renewable energy

technologies as well as new storage devices have been developed, they

have not reached wide-spread use. Therefore there is a strong need of

development of improved methods for storing energy when it is available

and retrieving when it is needed. Electrical energy storage devices are

mandatory in myriad applications viz., telecommunication devices (cell

phones, remote communication, walkie-talkies etc), standby power

systems, and electric hybrid vehicles in the form of storage components

(batteries, supercapacitors and fuel cells). These prompted the need for

advanced power sources offering high power density [1]. The

electrochemical capacitors (ECs) or supercapacitors (SCs) represent a new

generation of electrochemical energy storage components with very high

capacitance for energy storage. Supercapacitors store energy in either

capacitive (double layer of electrostatic charges) or pseudocapacitive (a

faradic battery-like reaction) nature. Exploiting both the advantages of

battery (high energy density) and conventional capacitors (high power

density), supercapacitors easily offer higher specific capacitance values up

to several thousand Farad for applications requiring pulse power

(appliances requiring high power bursts in the seconds range). They can

also be cycled several hundred thousand times. Being an entity of

supercapacitors, hybrid capacitors (incorporating a battery-like anode (+)


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and a carbon based cathode (-) having non-faradic character) have more to

render in terms of power and energy [1]. This class of energy storage

device is commonly known in many names such as supercapacitor,

ultracapacitor (SC) or electrochemical double-layer capacitor (EDLC). It is

capable of condensing energy, by arraying electrical charges,

electrostatically at the electrode/electrolyte interface, known as the

Helmholtz layer, achieving capacitance in the order of Farads. The term

“supercapacitor” is referred commonly in this thesis. Penetrating into the

current market as a feasible alternative to batteries, supercapacitors are

paving ways for researchers to investigate all possible materials that could

deliver enhanced performances in terms of power and energy density,

charge-discharge characteristics, cycling stability and reversibility [1, 2].

New materials for electrodes such as activated carbons, nano sized

transition metal oxides, conducting polymers etc provides high specific

surface area with good electrical conductivity. Since electrical capacitance

of supercapacitors is quite dependent on the number of ions (anions or

cations) present at the electrode/electrolyte interface, highly increased

specific surface area of these new electrode materials is essential for the

supercapacitors to obtain remarkably increased number of ions adsorbed

on the surface of electrodes so as to realize the so-called

“supercapacitance”. The charge-discharge cycle life of supercapacitors can

be over 300,000 cycles (charge/discharge) and the turn around efficiency

is up to 96% without significant degradation between the operating

temperatures of –25 and +50°C. In addition, the charge time becomes very

rapid up to a few seconds and the specific power density is at least two

orders higher than the secondary or rechargeable batteries [1, 2]. These

are the most distinctive outstanding characteristics as a new type of

energy storage power source that any other types of electric storage

devices such as advanced lithium-ion and lithium polymer rechargeable

batteries cannot offer power density as high as what supercapacitors could


4

offer. However, the specific energy density of the supercapacitors is

hitherto one order of magnitude less than that of rechargeable lithium

batteries.

Research into supercapacitors is presently classified into two main

areas that are based primarily on their mode of energy storage, namely: (i)

the electrochemical double layer capacitor also referred to as

pseudocapacitors. The former stores energy (electricity) in the form of

electrostatic means that is typically the same way as a traditional capacitor

and secondly (ii), the redox supercapacitor exhibits reversible Faradaic-

type charge transfer and the resulting capacitance is not electrostatic in

origin and hence the name pseudo capacitors [1, 2].

Invoking the developmental pace of advanced materials such as

nanostructured transition metal oxides, carbons and electro-conductive

porous polymers, the supercapacitors and the battery (lithium battery)

will soon be rolled in the same area of energy storage in which energy is

paramount in the so-called hybrid energy storage device. It is with the

above-astounded advantages and applications in mind, the present work

was embarked at developing supercapacitors using novel nanostructured

and inexpensive metal oxides as potential electrodes focusing on high

power supercapacitors in general and pulse power applications in

particular.

Nanostructured materials are found to demonstrate unique

properties in terms of electrode conductivity and particle to particle

contact due to their nanometer sizes where electron tunneling is quicker

than micron-sized particles. The latter aspect is vital for supercapacitor as

it directly influence the equivalent series resistant (ESR) of the cell.

Therefore, employing nanosized particles as electrode for such application

would certainly help to achieving such high rate capability within the cell.

It is in this context, the present work has thus been justified as timely and

important to develop such high rate power sources namely


5

supercapacitors. Obtaining uniform nanograin sizes in metal oxide thin

films is a crucial challenge for material scientists. This requires an

appropriate synthesis approach. Therefore, the present work has been

aimed at developing uniform sized Fe: MnO2 nano-sized particles with a

view to enhancing the electrode properties and characterizing its electrode

active qualities in electrochemical supercapacitors.

To prepare such nanostructured materials having large surface area

within the limited weight is the important aspect while developing

supercapacitors. Electrochemical synthesis of oxide film is economical and

suitable for large-scale applications. It enables us to deposit relatively

uniform thin films onto large area substrates of complex shape. Also by

adjusting electrochemical parameters, one can control the thickness and

morphology. The deposition rate is higher and the process does not

require too high temperature, sophisticated instrumentation, high purity

salt and extreme cleaning of substrates. The electrochemical reaction is a

unit process occurring on the working electrode where either oxidation or

reduction takes place without any chemical agent being required for the

reaction. Electricity accomplishes the oxidation and reduction, so that

there are no by-product species. Today, this is a very important feature of

electrochemical processes from the viewpoint of environmental protection

and materials conservation.

1.2 Literature Survey of Metal Oxide Supercapacitors

In recent years, supercapacitors based on metal oxide thin films are

attracting great attention as energy storage systems due to their potential

applications in micro-electronic devices. Various transition metal oxides,

such as RuO2, Co3O4, NiO, Fe2O3, Ir2O3, SnO2, MnO2 etc., are being studied

for the supercapacitor applications with their charge storage mechanisms

based on pseudocapacitance. Among these metal oxides for supercapacitor

electrodes, amorphous hydrous ruthenium oxide is the most promising


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material for supercapacitors because of its high specific capacitance,

excellent reversibility and long cycle-life [3, 4]. Powder form of amorphous

and hydrous ruthenium oxide (RuO2.xH2O) have been formed by the sol-

gel method and found to be promising material for electrochemical

capacitor with high power density and energy density [5]. However, RuO2

is expensive, toxic and naturally less abundant which has limited their

commercial use. Also RuO2 requires the use of a strong acidic electrolyte

such as sulfuric acid. The acidic media can dissolve the metal oxide over

extended cycling leading to fade in the specific capacitance with cycle-life.

Of course, the requirement of concentrated acid does not render the RuO2

technology obsolete, as the success and wide spread use of Pb-acid

batteries illustrates, however, a low cost technology employing non

corrosive electrolytes would certainly find numerous applications. As a

result, numerous metal oxides have also been tested as possible candidates

for electrochemical supercapacitor devices. Candidate systems include

IrO2 [6] or CoOx, [7] but they suffer from limitations similar to RuO2, that

is, they are expensive and require strong acidic or alkaline electrolyte. In

addition the potential window over which they operate reversibly is

significantly smaller than that for RuO2. On the other hand, MoO3 [8], V2O5

[9] and MnO2 [10] systems seem promising primarily due to their lower

cost.

Table 1.1 shows the specific capacitance, specific energy and

specific power, values exhibited by virgin metal oxides and/or their

composites using preparation methods with the electrolyte used. Since the

power requirements for many applications have increased noticeably, the

development of high energy density capacitors or electrochemical

capacitors has been undertaken by various groups around the world.


7

Table 1.1: Supercapacitors based on metal oxides

Sr.

No.

Material Method of

preparation

Electrolyte Specific

capacitance

(F.g-1)

Specific

energy

(Wh.kg-1)

Specific

power

(kW.kg-1)

Reference

1 Ruthenium

oxide

Templeting

method

1.0 M

H2SO4

954 32.7 - 11

2 Ruthenium

oxide

Electrodeposition 0.1 M

H2SO4

788 - - 12

3 Ruthenium

oxide

Electrophoretic

deposition

1.0 M

H2SO4

734 - - 13

4 Ruthenium

oxide

Sol-gel 0.5 M

H2SO4

720 26.7 - 14

5 Ruthenium

oxide

Electrostatic spray

deposition

0.5 M

H2SO4

650 17.6 4 15

6 Ruthenium

oxide


H2SO4

650 - - 16

7 Ruthenium Electrodeposition 0.5 M 599 17.6 - 17


8

oxide H2SO4

8 Ruthenium

oxide

Thermal

decomposition

0.5 M

H2SO4

593 - - 18

9 Ruthenium

oxide

Spray pyrolysis 0.5 M

H2SO4

551 - - 19

10 Ruthenium

oxide


H2SO4

534 - - 20

11 Ruthenium

oxide

Chemical

oxidation

0.1 M

H2SO4

500 66 4.7 21

12 Ruthenium

oxide

Cyclic

Voltammetry

0.5 M

H2SO4

100 - - 22

13

Ruthenium

oxide

Non-ionic

surfactant

templeting

method

0.5 M

H2SO4

58 - - 23

14 Ruthenium

oxide

M-CBD 0.5 M

H2SO4

50 - - 24


9

15 Pb-RuO2 Solid state

reaction

0.5 M

H2SO4

160 - - 25

16 RuO2 – TiO2 Loading of

nanotube

PVA H3PO4 1263 - - 26

17 RuO2 – TiO2 chemical 1.0 M KOH 46 5.7 1.207 27

18 C - RuO2 Wet impregnation 0.1 M

H2SO4

760 - - 28

19 RuO2-SnO2 Sol-gel 0.1 M

H2SO4

690 - - 29

20 RuO2 - C Chemical method 1.0 M

H2SO4

650 - - 30

21 RuO2 - C Colloidal method 1.0 M

H2SO4

407 - - 31

22 LixRuO2+

0.5x·nH2O

chemical 1.0 M

Li2SO4

391 65.7 - 32

23 RuO2 - C

composite

Colloidal solution

method

1.0 M

H2SO4

250 - - 33


10

24 RuO2-SnO2 DC sputtering 0.5 M

H2SO4

88 - - 34

25 Ni(OH)2 CBD 2.0 M KOH 398 - - 35

26 NiO CBD 2.0 M KOH 167 - - 36

27 Ni(OH)2 SILAR 2.0 M KOH 350 - - 37

28 Nickel

oxide(NiO)

Electrodeposition 1.0 M KOH 277 - - 38

29 Nickel oxide Electrochemical

precipitation

1.0 M KOH 146 - - 39

30 Nickel oxide Sol-gel 1.0 M

2.0 KOH

125 - - 40

31 Nickel oxide Calcinations 2.0 M KOH 120 - - 41

32 Ni - Co CVD 1.0 M KOH 569 - - 42

33 Ni(OH)2 Electrodeposition 3.0 M KOH 578 - - 43

34 Ni-C Chemical BMIM-PF6

RTIL

357 50 0.458 44

35 Mn-Ni - Co Co-precipitation 6.0 M KOH 1260 - - 45


11

36 NiO - RuO2 Co-precipitation 1.0 M KOH 210 - - 46

37 NiFe2O4 CBD 1.0 M

Na2SO3

223 - - 47

38 NiFe2O4 SILAR 1.0 M

Na2SO3

369 - - 47

39 α-Co(OH)2 Electrodeposition 1.0 M

KOH

860 - - 48

40 Co3O4 Template-free

growth method

6.0 M KOH 746 - - 49

41 Cobalt

Oxide

SILAR 1.0 M KOH 165 - - 50

42 Co(OH)2 Electrodeposition 6.0 M KOH 280

23.7 8.1 51

43 Co- MnO2 Electrodeposition 0.5 M

Na2SO4

396 - - 52

44 Co – MnO2 Electrodeposition 1.0 M

Na2SO4

498 - - 53


12

45 Co(OH)2/

TiO2

Precipitation

method

6.0 M

KOH

229 - - 54

46 Cobalt oxide

(Co3O4)

Spray Pyrolysis 2.0 M KOH 74 - - 55

47 V2O5 Chemical method 0.1M

K2SO4

170 - - 56

48 V2O5 - C Melt quenching 2.0 M

NaNO3

32.5

- - 57


KNO3

31.5 - - 57


LiNO3

29.9 - - 57


Na2SO4

29.3 - - 57


K2SO4

28 - - 57

53 V2O5 - C Melt quenching 1.0 M 25.1 - - 57


13

Li2SO4

54 Fe2O3 Electrosynthesis 0.25 M

Na2SO3

210 - - 58

55 Fe3O4 Chemical method 0.1M

K2SO4

75 8.1 10 59

56 Fe3O4-C Microwave

method

6.0 M KOH 37.9 - - 60

57

MoO2-C Electrochemically

induced

deposition

method

0.1 M

Na2SO4

597 - - 61

58

MoO2 Thermal

decomposition

method

1.0 M

H2SO4

140 - - 62

59 Bismuth

oxide


NaOH

98 - - 63

60 Bismuth Electrodeposition 1.0 M 81 - - 64


14

iron oxide NaOH

61 SnO2 Electrodeposition 0.1 M

Na2SO4

101 - - 65

62 SnO2 Electrodeposition 0.1 M

NaOH

43 - - 66

63 SnO2 Sol- gel 1.0 M KOH 16 - - 67

64 Copper

oxide


Na2SO4

36 - - 68

65 Copper

oxide

CBD 1.0 M

Na2SO4

43 - - 69

66 In2O3 Electrodeposition 1.0 M

Na2SO3

190 - - 70

67 IrO2 – MnO2 Thermal

decomposition

0.5 M

H2SO4

550 - - 71


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1.3 Literature Survey of MnO2 Thin Films

In recent years, manganese oxides have generated considerable

scientific and technological interest because of their electronic and

magnetic properties [72]. There are four crystalline phases of manganese

oxides (MnO, Mn2O3, MnO2, and Mn3O4) which have different structural

and compositional properties and serve in various applications.

Manganese dioxide (MnO2) can be used as a catalyst in oxidation–

reduction reactions, as electrode materials in batteries, and in energy

storage devices such as ultracapacitors [73]. The dimanganese trioxide

phase (Mn2O3) is quite attractive owing to its applications to produce soft

magnetic materials [74] to catalyze the removal carbon monoxide and

nitrogen oxide from waste gas [75, 76] and in the catalytic combustion of

methane [77]. The hausmannite phase, Mn3O4, has also been shown to

possess electrochromic properties [78]. Furthermore, manganese oxide

thin films serve as the substrate in the synthesis of magnetic oxide

perovskite materials, which have important electrical and magnetic

properties such as giant magnetoresistance, and metal-insulator

transitions [79, 80]. There are several oxidation states, including Mn(0),

Mn(II), Mn(III), Mn(IV), Mn(V), Mn(VI), and Mn(VII) for manganese oxides.

Various methods have been reported for the preparation of

manganese oxide materials to serve as electrodes for supercapacitors,

such as the sol–gel technique [81], solution-based chemical routes [82],

electrochemical deposition [83, 84], hydrothermal method [85],

electrostatic spray deposition (ESD) [86] and sonochemistry [87]. Most of

these studies have focused on the synthesis methods with the goal of

achieving enhanced electrochemical performance, e.g., a high specific

capacitance, long-term cycling behaviour, and fast charging/discharging

rate. Prasad and Miura [88] prepared manganese oxide by

potentiodynamic deposition on a stainless steel substrate and obtained

specific capacitance of 480 F.g-1 at scan rate of 10 mVs-1. Broughton and


16

Brett [89] reported the supercapacitance of 700 F.g-1 for MnO2 films

prepared by anodic oxidation of sputtered manganese films. Pang et al.

[81] prepared MnO2 films by sol–gel synthesis with subsequent annealing

at 573 K and reported the supercapacitance of 698 F.g-1. Toupin et al. [82]

reported manganese oxide synthesised by an easy method based on

chemical reaction of potassium permanganate with manganese sulphate in

aqueous solution. They reported specific capacitance of 180 F g-1 from the

untreated powder and, a little different value of 160 F.g-1 from the powder

heated for 3 h between 373 and 473 K, that was a 12% lower of the initial

value. More recently, Brousse et al. [90] reported that long-term cycling

behaviour with stable performance (>1, 00,000) was realized in a carbon–

MnO2 hybrid electrochemical supercapacitor cells. Hsieh et al. [91]

reported a wide range of capacity fading for thick MnO2 electrodes, ranging

from 5 to 30% in 1000 cycles, which is rather sensitive to the scan rate and

binder content. West et al. [92] synthesized manganese oxide array by

deposition of the manganese oxide sol–gel within the porous template.

Sugantha et al. [93] deposited the manganese oxide in a porous template to

form array electrode and demonstrated a much improved high-rate

performance. Subramanian et al. [94] reported the synthesis of MnO2 by a

hydrothermal route under mild condition. Hu et al. [83] reported on the

hydrated MnOx nanostructured films via a galvanostatic electrodeposition,

with specific capacitance of 230 F.g-1 at 25 mVs-1. Wu and Wu et al. [95, 96]

prepared the MnO2 nanowire films using CV electrodeposition, with

specific capacitance of 350 F.g-1 under 0.1 mAcm-2 discharge rate. Ghaemi

et al. [97] used γ-MnO2 nanowires made by employing a galvanostatic

technique in the presence of surfactant for rechargeability in alkaline

Zn/MnO2 batteries. Chang and Tsai [98] have reported hydrous

manganese oxide synthesized by potentiostatic method at anodic

potentials of +0.5–+0.95V versus SCE. A specific capacitance value of 240

F.g−1 was reported at a scan rate of 5 mV.s-1, at +0.5 V/SCE. Yang et al. [99]


17

prepared MnO2 by reduction of sodium permanganate with disodium

fumaric acid. Hu et al. [83] prepared amorphous hydrous manganese oxide

by anodic deposition onto a graphite substrate from a MnSO4.5H2O

solution and reported specific capacitance of 320 F.g-1 in 1.0 V potential

window. Xiao et al. [100] synthesized single crystal α-MnO2 nanotubes by a

facile hydrothermal method without the assistance of a template, a

surfactant and heat-treatment. This single crystal α-MnO2 nanotubes with

specific capacitance = 220 F.g-1 can be a promising candidate as

supercapacitor material. Tang et al. [101] prepared hierarchical hollow

manganese oxide nanospheres with both a large surface area and a layered

structure by a templating assisted hydrothermal process exhibited an ideal

capacitive behaviour and good cycling stability in a neutral electrolyte

system with 299 F.g-1. Yuan et al. [102] reported MnO2-pillared layered

manganese oxide via delamination/reassembling process followed by

oxidation reaction with specific capacitance value of 206 F.g-1. Nam et al.

[103] prepared manganese oxide on three dimensional carbon nanotube

substrate by electrodeposition method. Staiti et al. [104] synthesised

manganese oxide material by precipitation method based on reduction of

potassium permanganate (VII) with manganese (II) salt for which highest

specific capacitance of 267 F.g-1 was obtained. Ma et al. [105] reported

specific capacitance of 580 F.g-1 for MnO2/CNT nanocomposite prepared

spontaneous direct redox reaction between the CNTs and permanganate

ions (MnO4⁻). Reddy and Reddy [106] prepared MnO2 thin films by sol-gel

method and obtained maximum capacitance of 130 F.g-1 at a scan rate of 5

mV.s-1. Reddy and Reddy [107] reported sol-gel method for the

preparation of amorphous MnO2 thin films by the reduction of NaMnO4

with solid fumaric acid. A maximum capacitance of 110 F.g-1 was obtained

in 2 M NaCl solution. The specific capacitance of MnO2 remained constant

up to 800 cycles at 5 mV.s-1 scan rate. Wu and Lee [108] prepared

nanostructured manganese oxide electrodes directly by electrochemical


18

deposition and reported that electrode with thinner nanowires deposited

at high-current density. Sharma et al. [109] prepared carbon-supported

MnO2 nanorods using a microemulsion process and a manganese

oxide/carbon (MnO2/C) composite is investigated for use in a

supercapacitor and obtained specific capacitance of 458 F.g-1. Kuratani et

al. [110] prepared sodium manganese oxide nanorods with 2×4 tunnel

structure using layered manganese oxide as starting material by

surfactant-assisted hydrothermal method and reported specific

capacitance of 140 F.g-1 with 57 % capacitance retained when scan rate

increased to 100 mV.s-1. Nakayama et al. [111] have electrodeposited

layered manganese oxide conducted in a colloidal crystal template formed

by self-assembly of polystyrene particles on an indium tin oxide substrate.

The resulting macroporous film exhibited good pseudocapacitive behavior

in neutral electrolyte. Chang et al. [112] prepared amorphous, hydrous

manganese oxide by anodic deposition in manganese acetate solution. The

result indicated that the pseudocapacitive characteristics, reversibility,

and cyclic stability of the deposited manganese oxide were improved by

introducing the proper heat-treatment. Nagarajan et al. [113] reported

specific capacitance of 425 F.g-1 using cathodically electrodeposited

manganese oxide thin films. The specific capacitance decreased by ∼20%

after 1000 cycles. Wu et al. [114] prepared thick composites composed of

crystalline manganese dioxide (MnO2) and multiwall carbon nanotubes

(MWCNTs) co-deposited onto a graphite substrate. The capacitive

performance of these thick MnO2 deposits in 0.1 M Na2SO4 is significantly

improved by the application of electrochemical activation and the

introduction of MWCNTs, revealing the promising improvement in the

capacity of electrodes. Devaraj et al. [115] prepared MnO2 by

electrodeposition method and effect of surfactant has been studied.

Specific capacitance of 310 F.g-1 obtained for the oxide prepared in the

presence of surfactant over an extended charge-discharge cycling is higher


19

by about 25% in relation to the oxide prepared in the absence of

surfactant. Haung et al. [116] deposited hydrous MnO2 thin films by

electrodeposition method and effect of different electrolyte has been

studied. These hydrous MnO2 thin films showed specific capacitance of 275

F.g-1 in KCl and 310 F.g-1 in (NH4)2SO4 electrolytes. Devaraj et al. [117]

prepared MnO2 thin films by microemulsion method in presence of

surfactant (sodium dodecyl sulphate) and reported specific capacitance of

240 F.g-1. Chen et al. [118] prepared MnO2 thin films by anodic deposition

and the influence of different precursors on deposition rate of hydrous

manganese oxide and the effect of oxide thickness on the electrochemical

properties of MnO2 was investigated. Yang et al. [119] prepared the porous

manganese dioxide (MnO2) by an interfacial reaction of potassium

permanganate in water/ferrocene in chloroform. Electrochemical results

indicated that the sample with a large pore size shows a better rate

capability, while the sample with a small pore size but large surface area

delivers a large capacitance at low current rate.

Recently, it is reported that compounds of mixed oxides composites

have superior capacitive performance to single transition metal oxide as

electrode. Li et al. [120] reported orchid-like Cr-doped MnO2

nanostructures via a hydrothermal method, using KClO3 as the oxidant.

Prasad and Miura [121] reported that the thin films of nickel–manganese

oxides synthesized by electrochemical method have fairly high specific

capacitance (621 F.g-1), excellent stability and long cycle life. Such thin

films could have shown higher SC values, but they would be suffering from

poor energy density values. In addition, the nanostructured electrodes

have demonstrated better rate capabilities than traditional materials. This

viewpoint was also confirmed by Cao et al. and Zhou et al. [122, 123].

Conversely, Chuang et al. [124] found that although adjusting the pH of the

plating solution vary the Co/Mn content ratio in the deposited oxide. The

electrode surface morphology and the specific capacitance of these oxides


20

essentially remained unchanged. Lee et al. [125] prepared Fe doped

manganese oxide thin films by anodic deposition and reported the specific

capacitance of 212 F.g-1 which is 21% higher than that for plain manganese

oxide. Patrice et al. [126] reported the effect of Fe doping on

electrochemical behaviour of MnO2 by using low temperature process.

Pasero et al. [127] prepared Co doped manganese oxide by solid state

synthesis and reported that limited substitution of Mn by Co in Mn3O4

leads to a dramatic increase in electrochemical performance. Liu et al

[128] prepared NiO/MnO2 mixed oxide by sol-gel route and reported the

specific capacitance as high as 453 F.g-1. Recently, some research groups

[129–133] prepared Cr, Al, Ni, and Co-substituted manganese oxide

nanowires through the redox reactions of solid-state precursors or ion-

adduct precursors under hydrothermal or non-hydrothermal condition

and reported that the partial replacement of Mn with transition metal ions

improve the electrode performance of nanostructured manganese oxide.

Yoo et al. [134] prepared vanadium and iron doped manganese oxide thin

films via one-pot hydrothermal reactions. According to electrochemical

measurements, doping with Fe and V can improve the electrode

performance of 1D nanostructured manganese oxide and such a positive

effect is much more prominent for the iron dopant. Lee et al. [135]

prepared Fe doped MnO2 thin films by anodic deposition method and

reported that the Fe addition improves specific capacitance of MnO2 from

205 to 255 F.g-1. Luo et al. [136] prepared Mn-Ni-Co oxide composite by

thermal decomposition of precursor obtained by chemical co-precipitation

of Mn-Ni and Co salts. A maximum specific capacitance of 1260 F.g-1 was

obtained within the potential range of -0.1 to 0.4 V in 6 M KOH electrolyte.

Chang et al. [137] prepared viologen doped manganese oxide thin films by

electrodeposition method and reported that due to viologen doping the

supercapacitance of MnO2 was increased by five times than that of plain

MnO2.


21

The literature survey shows that the MnO2 and Fe: MnO2 thin films

have been prepared by physical as well as chemical methods. Physical

methods are relatively expensive as compared to chemical methods. Our

intention is to prepare MnO2 and Fe: MnO2 using simple and low cost

electrochemical deposition method. It is one of the promising methods for

the production of metal oxide films. This is probably the easiest, low cost,

non-vacuum and suitable method to prepare large area thin films, which

has been also used for deposition of ferrite films.

1.4 Orientation and Purpose of the Dissertation

Growing demands for power sources of transient high-power

density have stimulated a great interest in electrochemical

supercapacitors in recent years with project applications in digital

communications, electric vehicles, burst power generation, memory back-

up devices and other related devices which require high power pulses.

Supercapacitors possess high power density, excellent reversibility and

have long cycle-life compared to batteries [138, 139]. Recently, conducting

polymers, activated carbon and transition metal oxides are widely used for

supercapacitor electrode material. Recent research is focused on

increasing the specific capacitance of the oxides by introducing other

oxides technology [140]. The capacitance of MnO2 electrode is believed to

be predominant due to pseudocapacitance, which is attributed to

reversible redox transitions involving exchange of protons and/or cations

with the electrolyte [141]. However, the resistivity and the equivalent

series resistance (ESR) of MnO2 electrode are very large. Therefore, its

capacity is limited. In order to overcome this disadvantage, the composite

electrode materials of the manganese oxide were prepared with a

conducting additive such as carbon material (graphite, carbon nanotube,

porous carbon, activated carbon, and carbon aerogel, etc.) [142-144],

conducting polymers [145, 146], metal oxides [52, 65, 147] etc


22

Among various physical and chemical processes for thin film

preparation, electrodeposition possesses several advantageous points. The

interesting feature of electrochemical deposition is that the deposition

could be employed as one of the step in the preparation of oxides or

semiconductors. To deposit the film at high alkaline pH and over potential

to form the metal oxide thin film is a promising way to form metal oxide

thin films. Electrodeposition is one of the attractive methods for the

preparation of elementary, binary and ternary compound thin films.

Doping of suitable material in metal oxides is easy in electrodeposition

method relative to other deposition methods. Keeping this view in mind,

the emphasis will be given on the electrodeposition and characterization

of MnO2 and Fe: MnO2 from aqueous medium and finally their use in

electrochemical supercapacitors. The addition of divalent/trivalent Fe in

the manganese oxide alters the chemical state and surface morphology of

manganese oxide electrode which further improves the pseudo-capacitive

property of manganese oxide electrode. In order to dope Fe in manganese

oxide thin films, four different concentrations (0.5, 1, 2 and 4 at %) were

selected.

Electrodeposition is an isothermal process mainly controlled by

electrical parameters, which are easily adjusted to control film thickness,

morphology, composition etc. Hence, in the present work, Fe: MnO2 films

will be deposited by three modes of electrodeposition i.e. potentiodynamic

(cyclic voltammetry), potentiostatic (constant voltage) and galvanostatic

(constant current) mode from simple aqueous alkaline baths. Cyclic

voltammetric curves will be plotted to determine deposition potentials for

MnO2 and Fe: MnO2. Effect of various preparative parameters such as

deposition potential, concentration of the solution, bath temperature, pH

of the bath, deposition time etc. will be optimized to get uniform and well

adherent films.


23

In the past years the advancement in science has taken place mainly

with the discovery of novel materials. Characterization is an important

step in the development of exotic materials. The complete characterization

of any material consists of phase analysis, compositional characterization,

structural elucidation, micro-structural analysis and surface

characterization, which have strong bearing on the properties of materials.

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

identification. The surface morphology of the films will be studied using

scanning electron microscopy (SEM). The nanocrystalline nature will be

confirmed from transmission electron microscopy (TEM). The

compositional study will be carried out by energy-dispersive X-ray

analysis (EDAX) technique. The Fourier transform infrared (FTIR) spectra

of the samples were carried out in order to study the chemical bonding. In

order to study the interaction between electrode and electrolyte the

surface wettability test is carried out using contact angle meter.

The electrochemical supercapacitor properties of the MnO2 and Fe:

MnO2 films will be studied by cyclic voltametry (CV) on Potentiostat,

forming a electrochemical cell comprising platinum as a counter electrode,

saturated calomel electrode (SCE) as a reference electrode in a suitable

electrolyte. To obtain the different morphologies (High surface

area/porous) the Fe doped MnO2 thin film given different post deposition

heat treatments. Hence, Fe: MnO2 thin film electrode showing best

performance will be given the surface treatments like air annealing.

Performance of Fe: MnO2 thin films in supercapacitor will be evaluated

with respect to various parameters such as scan rate, specific capacitance,

stability cycles and potential range. The supercapacitive performance of

Fe: MnO2 thin films will be tested for symmetric and asymmetric modes,

for achieving high power density. The charge-discharge mechanism will be

studied using chronopotentiometry and the parameters such as average

capacitance, coulomb efficiency, specific energy, and specific power will be


24

calculated. Finally, supercapacitive properties of Fe: MnO2 thin films

obtained from three modes i.e. potentiodynamic, potentiostatic and

galvanostatic modes are compared and suitable mode of electrodeposition

of Fe: MnO2 thin films preparation will be proposed.

The purpose of research work is to improve pseudo-capacitive

properties such as specific power, specific energy and coulomb efficiency

of MnO2 thin films by iron (Fe) addition using simple and cost effective

electrodeposition method.


25

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