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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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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 (+)
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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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.
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
Electrodeposition 0.5 M
H2SO4
650 - - 16
7 Ruthenium Electrodeposition 0.5 M 599 17.6 - 17
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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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
Electrodeposition 0.5 M
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
49 V2O5 - C Melt quenching 2.0 M
KNO3
31.5 - - 57
50 V2O5 - C Melt quenching 2.0 M
LiNO3
29.9 - - 57
51 V2O5 - C Melt quenching 1.0 M
Na2SO4
29.3 - - 57
52 V2O5 - C Melt quenching 1.0 M
K2SO4
28 - - 57
53 V2O5 - C Melt quenching 1.0 M 25.1 - - 57
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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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
Electrodeposition 1.0 M
NaOH
98 - - 63
60 Bismuth Electrodeposition 1.0 M 81 - - 64
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
Electrodeposition 1.0 M
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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]
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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.
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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.
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
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.
CHAPTER I: GENERAL INTRODUCTION AND LITERATURE SURVEY
25
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