thesis_avireddy hemesh_web
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Thesis Submitted in the partial fulfilment for the award of degree of
Master of Technology (5 Years Integrated)
in
Power Systems
Issued by SASTRA University, India
Thesis location - ICGM – CNRS - UMR 5253 - Equipe AIME
Université Montpellier
2 Place Eugène Bataillon
CC 1502 Montpellier CEDEX 5 – France – 34095
Thesis Title – Development of nanocomposite electrode
materials for supercapacitors based on reduced graphene
oxide/metal-oxide
Presented by – AVIREDDY Hemesh
(Reg no. 115104006)
Department of Electrical & Electronics Engineering
SASTRA University, India
Supervision (External)
Dr. Frédéric Favier, Research Director - CNRS, UMR 5253, Equipe AIME,
Université Montpellier, France
Prof. Patrice Simon, Professor, FR, CNRS – 3459- CIRIMAT, UMR 5085,
Université Paul Sabatier, Toulouse, France
Supervision (Internal)
Prof. R John Bosco Balaguru, Associate Dean, Research and Professor,
Centre for Nanotechnology & Advanced Biomaterials (CeNTAB), SASTRA
University, India
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Development of nanocomposite electrode materials for
supercapacitors based on reduced graphene
oxide/metal-oxide
2
Thesis location - ICGM – CNRS - UMR 5253 - Equipe AIME
Université Montpellier
2 Place Eugène Bataillon
CC 1502 Montpellier CEDEX 5 – France – 34095
Cover Page (logos)
SASTRA University, India
Institut Charles Gerhardt Montpellier, France
Université Montpellier, France
Centre national de la recherche scientifique, France
Research network on electrochemical energy storage, France
Pole chime balard - Foundation balard, France
Back Page
1 Figure: 1.2. Specific capacitances due to MnO2 and overall content of all the binder based
electrodes at various to scan rates in 1 M Na2SO4 aqueous electrolyte.
Pg. 55
2 Table: 2.1. Overview of the choice of binder Pg. 41
3 Figure: 1.1. Specific capacitances due to MnO2 and overall content of all the binder based
electrodes at various to scan rates in 1 M Na2SO4 aqueous electrolyte.
Pg. 55
4 Figure: 2.3. Contact angle measurement between binder and binder electrode composite. Pg. 37
5 Figure: 3.4. (A) Raman spectroscopy of aTiO2-GO and GO (B) FTIR of GO and aTiO2-rGO Pg. 48
6 Figure: 3.3. (A) P-XRD of aTiO2-rGO, TiO2-rGO, TiO2, GO and graphite (B) Raman
spectroscopy of aTiO2-rGO, TiO2-rGO, TiO2.
Pg. 47
7 Figure: 3.6. (A) Cumulative pore area and pore volume distribution to pore diameter (B) Pore
area and pore volume distribution to pore diameter and (C) N2 adsorption-desorption
isotherms of aTiO2-rGO and TiO2-rGO samples.
Pg. 51
8 Figure: 3.5. (A) TEM image (B) P-XRD pattern of TiO2-GO (0.1 M – 1 h) and (C) Variation
of molar concentration used during the experiments
Pg. 61
9 Figure: 3.8. TEM images of TiO2-CNF Pg. 62
10 Figure: 3.10. (A) SEM image of TiO2-CNF (0.1 M – third layer) (B) SEM images of TiO2-
CNF (0.3 M- Second layer) (C) SEM images of TiO2-CNF (0.3 M – third layer) (D) P-XRD
pattern of TiO2-CNF at various molar concentrations of precursors and layers.
Pg. 63
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Declaration
I submit this thesis work entitled “Development of nanocomposite electrode materials for
supercapacitors based on reduced graphene oxide/metal-oxide” to SASTRA University,
Thanjavur in partial fulfillment of the requirements for the award of the degree of “Master of
Technology (5 Years Integrated)” in “Power Systems”. I declare that this work was carried
out independently by me under the supervision of Dr. Frédéric Favier, Research Director –
CNRS, UMR 5253, Equipe AIME, Université Montpellier, France and also under the co-
supervision of Prof. Patrice Simon, CNRS – 3459- CIRIMAT, UMR 5085, Université Paul
Sabatier, and Toulouse, France
(AVIREDDY Hemesh)
Montpellier, France
Date : 18th
June, 2015
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Abstract
Efficient and sustainable economic energy accessibility to consumers (both industrial
and domestic) is one of the major issues in the present power systems. Supercapacitor (SC)
based Energy Storage Systems (ESS) can be some of the potential solution to solve this issue
because of their high specific capacitance and power availability without deterioration for
longer operation time periods. However, their energy density compared to battery ESS is low.
The energy density of SCs can be enhanced by improving their electrode material and
operating them in wider voltage windows. Electrodes materials can be improved by using
new nanocomposites or optimising the electrode formulation. Therefore, in the present thesis,
the study is composed into two sections: (1) optimizing the electrode formulation and (2) one
pot synthesis of new carbon-metal oxides nanocomposites as electrode materials. In the first
section, electrode optimization was done by studying the effect of commonly used binders
such as PTFE, PVDF and PVOH in 1 M Na2SO4 and 1 M KOH aqueous electrolytes on the
electrochemical performances of SCs. The study concludes by providing an overview of
suitable binder depending upon the electrochemical experimental parameters. In the second
section, various carbon-metal oxides nanocomposites (TiO2-rGO and TiO2-CNF) were
synthesized by one pot route. The present thesis briefly discusses the scalable and facile one
pot sol-gel assisted synthesis route to form amorphous TiO2-reduced graphene oxide
nanocomposites (aTiO2-rGO). The electrochemical characterization studies were also
preceded between aTiO2-rGO and crystalline TiO2-rGO (TiO2-rGO) electrode materials to
understand the effect of amorphous materials on the electrochemical performances of SCs.
Based on the results and discussions, thicker electrodes of aTiO2-rGO have shown higher
specific capacitances as compared to that of TiO2-rGO due to their high BET surface area,
larger pore volume and diameter which can provide efficient ion transportation. However, the
specific capacitances of these thicker electrodes are low for present required application
which can be increased by optimizing the electrode formulation.
Keywords: Energy Storage Systems; Electrochemical Supercapacitors; Electrode materials;
Carbon- Metal oxide nanocomposities; Aqueous electrolytes; Binders; One pot synthesis;
Reduced graphene oxide; Graphene oxide hybrids
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Acknowledgements
Foremost, I would like to express my sincere gratitude to my supervisor Dr. Frédéric
Favier for his invaluable advice, encouragement and constant support during my research
visit at Université Montpellier. I would also like to express my gratitude to my colleagues in
the research group – Dr. Khalil Rajoua, Mr. Paul Aricdiacono, Ms. Linda Baklouti, Ms.
Laura Coustan and Mr. Anthony Battaglia for helping me out during the experiments and my
stay in Montpellier. I am grateful to the entire research group for me giving the happiest and
most memorial time of my life.
I sincerely thank Prof. Patrice Simon (Université Paul Sabatier) for giving me the
opportunity to study and enhance my research skills in France and his long term invaluable
suggestions during my research stay. I would also wish to thank our collaborator Dr. Nicolas
Louvain (Université Montpellier) for his guidance and giving me the opportunity to work
with Lithium ion and Sodium ion batteries.
Furthermore, I would also like to express my sincere thanks to the officials of
SASTRA University for giving me the opportunity to work on my thesis in France through
SASTRA Desh-Videsh Scholarship (Semester Abroad program). I would also like to thank
Prof. John Bosco Balaguru (SASTRA University) for his continuous advice and support
during my thesis. I would also like to express my thanks to the supportive colleagues both at
ICGM-AIME and SASTRA University. I would also like to extend my thanks to Dr. Bernard
Fraisse and Dr. Julien Fullenwarth (XRD), Mr. Frank Godiard (TEM) and Mr. Yanng
Nedellec for their technical assistances; as well I would also like to deeply thank Chaire Total
- Fondation Balard for fellowship
Finally, I would like to thank to my friends and family (members) for the memorable
time both in Montpellier and India, especially to my father who has always been a great
support in for decisions. Special thanks to Ms. Julia Schmitt (Germany) for her comments on
my thesis.
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Table of Contents
Contents
Certificate
Pg.
3
Declaration
Abstract
5
7
Acknowledgements 9
Chapter 1 Introduction 13
1.1. Supercapacitors as energy storage systems 13
1.2. Historical hints in the development of supercapacitors 16
1.3. Supercapacitors : Market trends and Applications 17
1.4. Electrochemical supercapacitors 19
1.4.1. Electrochemical Supercapacitors: Energy storage mechanisms 20
1.4.1.1. Electochemical double layer capacitors 20
1.4.1.2. Electrochemical pseudocapacitors 20
1.4.2. Components of electrochemical supercapacitors 22
1.4.2.1. Configuration of electrode test cell 22
1.4.2.2. MnO2 and carbon composites as electrode materials for supercapacitors 22
1.5. Conclusions - present work 25
1.6. References 25
Chapter 2 Effect of various binders on the electrochemical performances of MnO2 electrode
based supercapacitors
33
2.1. Introduction 33
2.2. Experimental section 33
2.3. Results & discussions 34
2.4. Conclusions & further work 41
2.5. References 41
13
Chapter 3 Amorphous TiO2-reduced graphene oxide as electrode for supercapacitors 45
3.1. Introduction 45
3.2. Experimental section 46
3.3. Results & discussions 47
3.4. Conclusions 52
3.5. References 52
Chapter 4 Conclusions 53
Appendix 1 Effect of various binders on the electrochemical performances of MnO2 electrode
based supercapacitors
55
Appendix 2 Amorphous TiO2-reduced graphene oxide as electrode for supercapacitors 58
Appendix 3 One pot synthesis of TiO2-carbon nanocomposites 60
Appendix 4 Characterization techniques
List of figures
64
65
List of Tables 68
Abbreviations 69
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Chapter 1 - Introduction
Summary: The present chapter highlights the importance and fundamentals of
supercapacitors as energy storage systems and also discuss the scope of improvement in their
electrochemical performance by using potential electrode materials such as MnO2 and
carbon hybrids.
1.1. Supercapacitors as Electrochemical Storage Systems
The efficient and sustainable economic energy accessibility to consumers (both
industrial and domestic) is one of the major issues in the present energy systems. Some of the
potential candidates to resolve this issue are Energy Storage Systems (ESS). ESS can be used
to defer transmission and distribution networks and upgrades, curtailment and compensates
intermittency of renewable energy with constant frequency operations. ESS can also be a
turnkey for residential and commercial solutions (1-10 kWh), base transceiver station
solutions (1-20 kWh), UPS solutions (10-50 kWh) and other higher demand utility (> MWh)
solutions at on-grid and off-grid locations [1]. Figure: 1.1 shows an overview of ESS
application.
Figure: 1.1. ESS Overview- residential and commercial solutions (1-10 kWh), base
transceiver station solutions (1-20 kWh), UPS solutions (10-50 kWh) and utility (> MWh)
solutions [1].
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ESS can store electrical energy of a power network and coverts back to electrical
energy during high energy demand. The electrical energy can be stored during off-peak
period and later can be used during on-peak period which can decrease electricity cost and
improve the ability of energy accessibility [2]. Depending upon the energy demand and grid
operating conditions, EES can be a source or can be coupled with a generating unit.
Various simulations and experiments have been reported in the literature regarding
ESS and ESS hybrid systems. For example, Abbey et al. reported an integration of a short
term energy storage device in a doubly fed induction generator design to smooth the fast
wind induced power variations [3]. Figure 1.2 shows the energy flow diagram in the doubly
fed induction generator during normal operation and also during low-voltage conditions.
Figure: 1.2. Energy flow diagram in the doubly fed induction generator during (a) normal
operation and (b) low – voltage conditions [3].
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Chakraborty et al. have also reported the dynamics of real and reactive power
responses of a STATCOM integrated ESS to improve the transient which plays an important
role in improving the power system operations (Figure: 1.3, Figure: 1.4 and Figure: 1.5) [4].
With recent advances, no ideal ESS can be available to meet the economic and technical
requirements of the growing spectrum of applications. Electrochemical batteries and
supercapacitors, magnetic and superconducting storage systems, flywheels, pumped hydro,
hydrogen storage and compressed air energy storage are competitive solutions for ESS [5].
Among these, electrochemical supercapacitors (SCs) have received significant attraction due
to their high specific capacitance and power availability without deterioration for longer
operation time periods [5] [6].
Figure: 1.3. STATCOM and ESS connected in a power utility system [4].
Figure: 1.4. Control interface of Battery ESS connected FACTS for maintenance stability [4].
17
1.2. Historical hints in the development of supercapacitors
Figure: 1.5. Timeline for the evolution of SCs [5].
In 1853, Helmholtz recognized that the electrical charge can not only be stored on the
surface of a conductor but also at the double layer interface between an electrode and an
electrolyte [5] [7] [8]. Around a century later, the first potential and patented products were
shown by General Electric and Sohio which were by-products of research & development of
the electrodes for novel batteries and fuel cells. One of the most important inferences during
this period was the implementation of high specific capacitances using high surface area
carbons in aqueous and molten salt electrolytes (5-50 µF/cm2 up to 1500 m
2/g) which were
six times higher in magnitude to the conventional capacitors. In the 1970s, companies like
Sohio, NEC and Corning Glass developed the initial SCs prototypes as commercial products.
However, the final commercial transformation was achieved in 1979 by NEC with the release
of the carbon electrode SC which was branded as “Supercap”. Since then, various Japanese
companies have continued to make the progress in the market. Untill the beginning of the
1980s, numerious investigations were performed to improve the performances of SCs.
However, during these investigations, researchers discovered that the charge was partially
stored in the double layer as it was also partially stored by charge transfer of electron and
proton (Faradic reactions) between an electrode and an electrolyte [9]. These SCs started to
be termed as pseudocapacitors which were fabricated using ceramic electrodes composed of
transition metal oxides. Other turnkey SCs were developed and also marketed by Gould
Ionics which were based on solid-state electrolytes (sodium-silver). However, they were
withdrawn from the market due to the high cost of cathode solid electrolyte material. Since
the 1980s to the present scenario, various companies have commercialized and marketed SCs
as energy storage systems. Table: 1.1 shows the list of several companies and their products
in the field of supercapacitors.
Device
or
producer
Rated voltage VR
(V)
C
(F)
ESR
(m Ω) Specific
energy
(Wh kg–
1)
Specific
power
(W kg–1
)
Specific
power
(W kg–1
)
Weig
ht
(kg)
Vol (L)
Maxwell 2.7 2,885 0.375 4.2 994 8,836 0.55 0.414
Maxwell 2.7 605 0.90 2.35 1,139 9,597 0.20 0.211
ApowerCap 2.7 55 4 5.5 5,695 50,625 0.009 –
ApowerCap 2.7 450 1.4 5.89 2,574 24,595 0.057 0.045
Ness 2.7 1,800 0.55 3.6 975 8,674 0.38 0.277
Ness 2.7 3,640 0.30 4.2 928 8,010 0.65 0.514
Ness (cyl.) 2.7 3,160 0.4 4.4 982 8,728 0.522 0.38
Asahi Glass
(propylene
2.7
1,375
2.5
4.9
390
3,471
0.210
0.151
18
carbonate)
Panasonic
(propylene
carbonate)
2.5 1,200 1.0 2.3 514 4,596 0.34 0.245
LS Cable 2.8 3,200 0.25 3.7 1,400 12,400 0.63 0.47
BatScap 2.7 2,680 0.20 4.2 2,050 18,225 0.50 0.572
Power Sys.
(activated
carbon,
propylene
carbonate)
2.7
1,350
1.5
4.9
650
5,785
0.21
0.151
Power Sys.
(graphitic
carbon,
propylene
carbonate)
3.3
1,800
3.0
8.0
486
4,320
0.21
0.15
3.3 1,500 1.7 6.0 776 6,903 0.23 0.15
Fuji Heavy
Industry-
hybrid
(AC/graphiti
c Carbon)
3.8
1,800
1.5
9.2
1,025
10,375
0.232
0.143
JSR Micro
(AC/graphiti
c carbon)
3.8 1,000 4 11.2 900 7,987 0.113 0.073
2,000 1.9 12.1 1,038 9,223 0.206 0.132
Table: 1.1. Overview of the present companies in the field of SCs and description of their
products [5].
1.3. Supercapacitors : Market trends and Applications
In the last few decades, SCs have been significantly proliferated and improved using
high voltage electrolytes (such as ionic and organic electrolytes), high surface area materials,
activated carbons and their composite due to market demand on energy storage. Initially,
these SCs were proposed for small scale applications (like micro solar power generators
electric vehicles, mobile phones, digital cameras, pulse layer techniques, UPS) and now also
being proposed for high power applications like power quality systems, electric and hybrid
electric vehicles and smart grids [5], [7], [8], [6], [65]–[74]. Several simulations and
experiments have been proposed with supercapacitors based hybrid energy storage systems.
For example, Wei Li et al proposed a flow battery supercapacitors hybrid ESS in which flow
battery is directly coupled to wind turbine generator DC bus while supercapacitor has a
DC/DC IGBT convertor interface which absorbs high frequency power surges leading to
highly efficient, low cost and high longevity of the system [75]. Several supercapacitor based
energy storage projects have also been carried out over the globe, of which the highest rated
one’s are the Gigacapacitor based projects (15000 kW). Table: 1.2 shows the list of
companies and their products in the field of SCs.
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Project Name Rated Power in
kW
Status Country
LIRR Malverne WESS: Maxwell Technologies 1000 Operational United
States
Palmdale Micro Grid Energy Storage Demonstration 0 Offline/Under
Repair
United
States
Endesa STORE: La Palma Project 4000 Operational Spain
GigaCapacitor Rosh Pinna Test Project 15000 Under
Construction
Israel
GigaCapacitor Putrajaya Test Project 15000 Under
Construction
Malaysia
GigaCapacitor Hyperadad Test Project 15000 Under
Construction
India
UC San Diego CPV Firming - Maxwell Technologies
28kW Ultracapacitor
28 Under
Construction
United
States
SEPTA Wayside Energy Storage System - Griscom
Ultracapacitors
70 Under
Construction
United
States
Win Inertia Ferrolinera WESS: Ultracapacitors 300 Operational Spain
LIRR Malverne WESS: Ioxus 1000 Operational United
States
Terna Storage Lab 1, Sardinia 1000 Announced Italy
Terna Storage Lab 2, Sicily 920 Announced Italy
Woojin/Maxwell Gyeongsan Test Line 525 Operational Korea,
South
Woojin/Maxwell Daejeon Line 1 - Daedong Station 1870 Operational Korea,
South
Woojin/Maxwell Daejeon Line 1 - Cityhall Station 1400 Operational Korea,
South
Woojin/Maxwell Daejeon Line 1 - Gapcheon Station 1400 Operational Korea,
South
Woojin/Maxwell Seoul Line 7 - Sang-dong Station 1870 Operational Korea,
South
Woojin/Maxwell Seoul Line 2 - Seocho Station 2340 Operational Korea,
South
Woojin/Maxwell Incheon Line 1 - Technopark Station 2340 Operational Korea,
South
Woojin/Maxwell Daegu Line 2 - Jukjeon Station 1050 Operational Korea,
South
Woojin/Maxwell Seoul Line 9 - COEX Station 1870 Under
Construction
Korea,
South
Woojin/Maxwell Seoul Line 4 - Ssangmun Station 2340 Operational Korea,
South
Woojin/Maxwell Incheon Line 2 - Depot S/S 1 1200 Under
Construction
Korea,
South
Woojin/Maxwell Incheon Line 2 - Guwol Station 1200 Under
Construction
Korea,
South
Woojin/Maxwell Incheon Line 2 - Namdong gucheong
Station
1200 Under
Construction
Korea,
South
Woojin/Maxwell Incheon Line 2 - Wanggil Station 1200 Under
Construction
Korea,
South
Woojin/Maxwell Incheon Line 2 - Wanjeong Station 1200 Under
Construction
Korea,
South
Woojin/Maxwell Incheon Line 2 - Depot S/S 2 1200 Under
Construction
Korea,
South
Table: 1.2. List of supercapacitors based energy storage projects – DOE Global Energy
storage database [76].
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With battery powered electric vehicles, SCs can be integrated to meet the highest load
demand during an accelerating or climbing process, as SCs can provide high power density
[77]. In addition to this, the poor life cycles of memory back-up batteries make the regular
replacement expensive, which is generally 20% of the price of the battery operated
applicances [6]. Furthermore, SCs integrated with batteries can improve braking energy
recovery, increased battery life, powerful acceleration and excellent cold weather starting.
Thounthong et al. proposed a control strategy for a voltage regulated DC hybrid power source
with fuel cell as the main source and SCs as an auxiliary power source for a distributed
system to improve the transient performance [78]. Therefore, SCs can either be
complemented or replaced with batteries in the energy conversion and storage fields with
applications to hybrid electric vehicles and metro trains [74], [77]. Figure 1.6 shows some of
the application of SCs.
Figure: 1.6. Applications of supercapacitors – (Left) Honda FCX prototype (Right) SC unit in
the Vossloh trolleybus [5].
At present times, the SC market is steeply growing with new efficient electrode
materials and electrolytes. In 1989, around 40 million dollars revenue was generated with
about 75 million units. In 1999, about 115 million dollars revenue was generated with about
201 million units, while in 2004 the revenue increased to 276 million dollars with about 660
million units. The global market value was projected to be around 900 million dollars by the
end of 2014 and it is estimated to be growing at double digits rate by 2019 which highlights
the potential future of SCs [15] [79] [5].
1.4. Electrochemical Supercapacitors
SCs differ from traditional capacitors because they have lower charge storage
densities, greater power densities and different material requirements. SCs consist of an
electrolyte that allows charge ions to assemble on the porous electrode surface or charge
transfer through faradic reaction or - in certain instances - both kind of mechanisms which
have much higher capacitance than traditional capacitors. The combination of small charge
separations and high surface area enables high energy density compared to traditional static
capacitors. In addition to this, SCs consist of electrolyte which forms a conductive connection
between two electrodes whereas conventional capacitors consist of solid a dielectric medium.
The optimization of the performance of SCs devices depends upon the selection of
appropriate electrodes, separators, electrolytes and sealants. In the following sections of this
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chapter, components and electrochemical storage mechanisms of electrochemical
supercapacitors will be discussed.
1.4.1. Electrochemical supercapacitors : Energy Storage Mechanisms
Based on the charge storage mechanisms, electrochemical supercapacitors can be
classified as electrochemical double layer supercapacitors and electrochemical
pseudocapacitors. We will now discuss these mechanisms in detail in the following sections
of this chapter.
1.4.1.1. Electrochemical double layer Capacitors (EDLC)
A typical EDLC, consists of two electrodes connected by an electrolyte which can be
solid or solution. The positive or negative charge is developed along the interface of the
electrode and electrolyte which can be balanced by induced accumulation of oppositely
charged ions near the electrode surface. The induced accumulation is forming an electric
double layer through Coulomb’s force. The negative ions are scattered with a higher
concentration near the electrode surface and with a lower concentration in the electrolyte.
This scattered layer and electrode charge array is known as a diffuse double layer which
depends upon the temperature, concentration of the electrolyte, the dielectric constant of the
electrolyte and the charge number carried by ion [10]. Figure 1.7 illustrates an EDLC.
Figure: 1.7. (Left) Electric double-layer supercapacitor (Right) Porous carbon electrode for
electrochemical double-layer supercapacitor [10].
1.4.1.2. Electrochemical pseudocapacitors
In an electrochemical pseudocapacitor, charge storage can fundamentally differ from
the electrostatic mechanism which governs capacitance by double layer. In a typical
electrochemical pseudocapacitor, a faradic charge transfer occurs in the electrode layer
through a kinetically and thermodynamically favoured electrochemical reduction- oxidation
reaction [11], so the charge is stored on the surface near it or in the bulk of an electrode and
an electrolyte. Figure: 1.8 illustrates the mechanism of pseudocapacitance. Figure: 1.9
illustrates the basic mechanisms in a double layer capacitor, pseudocapacitor and Li-ion
battery. Therefore, in certain cases, electrochemical pseudocapacitors have much higher
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capacitances compared to EDLC. In this case, the redox reaction is strongly dependent on the
charge quantity arising from the reaction and also on the electrode potential. For
electrochemical redox reactions, reactant site in the bulk phase contributes to one or more
charges towards the energy storage, unlike the double layer mechanism where charges can
physically accumulate on the material particle’s surface. Figure: 1.7 (Right) represents a
porous carbon electrode EDLC where charges are actually accumulated in the double layer.
Figure: 1.8. Mechanism of pseudocapacitance with specifically absorbed ions [12].
Figure: 1.9. Basic schematics for (a) all carbon EDLS (left), (b) a pseudocapacitor (MnO2
depicted center) and (c) a lithium ion battery (right). All devices have an active material (e.g.,
carbon, MnO2, LiCoO2), a current collector, a separating membrane and electrolyte (e.g.,
Na2SO4, or LiPF6 solutions) [13].
Theoretically, the coupling of both capacitive effects is possible [14] [11]. In carbon
composite or oxide composite electrodes, it can be difficult to quantify the coupling effects
because of overlapping of potential regions. The higher conductivity of carbon contributes to
the boosting of an active pseudocapacitive material. For example, the pure PANI was not able
to be charged at 10 mV/s scan rate, but when a conductive CNT-graphene was introduced,
PANI was able to charge effectively [15]. In certain cases, the functionality of carbon
materials induces the pseudocapacitive component that can boost the capacitance even for the
carbons with limited surface areas. In this regard, heteroatom like nitrogen are often infused
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into the carbon based systems to boost their performance via pseudocapacitance and
wettability [16][17].
1.4.2. Components of electrochemical supercapacitors
The electrode materials used in SCs must be highly porous and conductive to enhance
the charge storage ability. The collectors used in SCs must remain stable towards corrosion
during the charge and discharge cycles, must be strongly connected with the electrode layer
and should be highly conductive in nature to enhance the electron transport. The electrolyte
materials require a high ion mobility to provide ions. The electrolyte performance also
depends on the level of optimization of toxicity, corrosion, operating voltages and safety. The
separators used in SCs must have an highly ionic mobility from the electrolyte to the
electrode surface and must be electronically insulating to avoid short circuits between the
electrodes. In the following section, the configuration of an electrode test cell and discussions
on the potential electrode materials will be highlighted.
1.4.2.1. Configuration of electrode test cell
Electrochemical test cells can be classified into conventional three electrode test cell
and two electrode test cells. The three electrode test cells consist of a working electrode
(synthesised composites), a counter electrode (Platinum foil or net) and a reference electrode
(example- Ag/AgCl and Cu/Cu (II)). This kind of configuration gives the electrochemical
information only from the working electrode which can be used as a fast screening technique
associated with the electrode structure and optimization. This technique can be suitable for
asymmetrical supercapacitor systems because the separation of electrochemical information
from individual cells is difficult and may also not reflect the real time operating conditions.
On the other hand, the two electrode test cells consist of two active electrode surfaces
separated by an electrolyte emerged membrane which is placed between two metal plates.
These metal plates act as current collectors during the electrochemical experimental
procedure. This type of configuration gives electrochemical information of the entire cell
assembly which is closer to real time operating conditions. The capacitance of the individual
electrode can be separated as the capacitance of the entire cell and is equivalent to the
capacitance of the two electrodes connected in series. This technique is suitable for
symmetrical supercapacitor systems, as both the electrodes are identical and the
electrochemical information obtained from both of the electrodes can easily be separated
[10]. In the present studies, we have used both types of the electrode test cell configurations
in aqueous and organic electrolyte mediums.
1.4.2.2. MnO2 and carbon composites as electrode materials for supercapacitors
RuO2 based electrode materials are some of the potential candidates among the transition
metal oxides due their high ability of reversible redox reactions, a wide potential window, a
high proton conductivity, a remarkable specific capacitance, three distinct oxidation states
accessible within a 1.2 V voltage window, a good thermal stability, a long cycle life, a high
rate capability and the metallic type conductivity [19] [20]. Although amorphous hydrous
RuO2 is a suitable candidate because of the above mentioned properties, the relative cost and
24
its environmental harmfulness prevents it from being used for commercial application [21].
In this regard, researchers have put significant efforts to develop cheaper and
environmentally friendlier materials which can exhibit electrochemical performances similar
to the ones of RuO2. Some of the potential replacement oxides to RuO2 are MnO2, NiO,
Fe3O4 and V2O5, of which manganese oxides (MnOx) have shown a low toxicity, low cost
and an high environmental safety as well as high theoretical capacities (1100 to 1300 F/g)
[22]–[26]. This makes MnOx as a promising alternative [27]–[32]. There are reversible redox
transitions involving the exchange of protons and/or cations with the electrolyte. In addition
to this, there are also redox transitions between Mn(IV)/Mn(III) within the electrode potential
[33], [34]. These transition mechanisms are one of the main reasons of higher capacitances of
MnOx based materials. The proposed mechanisms can be expressed as equation 1 [23], [33],
[35]–[37], where MnOα(OC) and MnOα- β(OC) β+δ represent the MnO2.nH2O in high and low
oxidation states respectively, whereas C+ represents the alkali metal cations and protons (Li+,
Na+, K
+) in the electrolyte.
MnOα(OC)β + δC+
+ δe+ MnOα- β(OC) β+δ (1)
By equation 1, we can infer that both protons and the alkali cations are involved in the
redox process because of which the MnOx material must have an high electronic and ionic
conductivity [24]–[26]. Furthermore, despite the redox nature of MnOx, MnOx based
electrodes can show rectangular-shaped cyclic voltammetry curves which are similar to non-
faradic mechanism [38], [39].
The crystallinity, crystal structure, morphology, thickness of the electrode layer, specific
surface area and pore structure are the important factors affecting the pseudocapacitance of
MnOx [6]. In general, the specific capacitance decreases with increasing thickness of the
electrode layer due to low conductivity of MnO2 (MnO2 – specific capacitance decreased
from 400 to 177 F/g with the increase in MnO2 loading from 50 to 200 µg/cm2 [40]). Some
of the researchers have also reported the difference in using thick layers and thin layers (220
to 50 F/g as MnO2 loading was increased from 100 µg/cm2 to 4 mg/cm
2 ) [41]. The major
benefits of thin layers are (i) higher electronic conductivity, (ii) lower series resistance due to
shorter transport paths for the diffusion of protons and (iii) ease of access of electrolyte and
ions to the active surface of MnO2. The unfavourable capacitive thickness dependence is an
issue for low conducting electrode materials, which can be associated with the limitations in
the transport of both electrons and ion [42] [43], [44] leading to a dramatical capacitance fall.
This issue can be resolved by using solution processing techniques and nanocondutors such
as acetylene black, carbon nanotubes and carbon black or graphene to form composite
electrodes which leads to an increase in the electrical conductivity [36], [45]–[47]. These
nanoconducting materials can be mixed with MnO2 and binder materials to form a composite
electrode. The binders depend upon the nature of the ionic conductivity and stability in the
electrolyte and may also play an important role in the ion percolation on the electrode
surface. However, no studies were reported on the effect on electrochemical performances of
SCs by using different binders of active material in aqueous electrolytes. Therefore, we will
discuss the effect of electrochemical performances using different binders (PVDF, PTFE and
PVOH) in 1 M Na2SO4 and 1 M KOH in the following chapters.
25
Carbon and carbon based metal oxide electrodes have been in great attention due to their
higher surface areas and electrochemical performances [48]. Carbon nanofibers (CNF) can be
massively fabricated by vapour growth or electrospining techniques. The specific surface area
of 940 m2/g can be achieved with optimized pore distribution [49]. CNF are also attractive as
a support for metal oxides in CNF based metal oxide composites. In general, the metal oxides
are coated on the CNF surface to form a core-shell structure where CNF not only acts as a
physical backbone but also offers a path for the charge transport. The metal oxide in the
composites acts as a redox component which contributes to the specific energy density and
the specific capacitance.
Two Dimensional (2-D) carbon materials such as graphene oxide (GO), graphene or
reduced graphene oxide (rGO) have a high conductivity and surface areas which make them
potential candidates for the SCs electrode materials. These 2-D carbon materials are also
attractive because of their hydrophilic nature which is not only suitable for wettability in
aqueous electrolytes but also favourable to the composition with metal oxides. In addition to
this, rGO and GO materials are also rich in surface functional groups such as carboxylic,
carbonyl, alcohol and epoxides. These materials can also be subjected to additional functional
groups by further modifications [50] which can serve as a redox reaction center, contributing
pseudocapacitance leading to higher specific capacitances [51]. For example, Ruoff et al.
reported a specific capacitance of 166 F/g of activated rGO at a current density of 5.7 A/g
(Surface area – 3100 m2/g) [52]. The 2-D nanomaterials are easier and more flexible to
integrate with metal oxides. The graphene and graphene like materials can be a good support
for metal oxides because of their unique planar structure. Several studies have been reported
on the synthesis of rGO based metal oxides composites [53]–[60]. Among these, TiO2 is a
low cost transition metal oxide which is non-toxic and greatly available in abundance [61].
The graphene based TiO2 nanocomposite electrodes can show higher specific capacitances
which may be originated because of the presence of high surface area and redox reaction
center, favouring pseudocapacitance. For example, A.K. Mishra et al. reported a higher
specific capacitance of 265 F/g at a scan rate of 100 mV/s for TiO2 decorated on a
functionalised graphene [62]. Several synthesis techniques for Graphene or rGO based TiO2
composites have been reported in the literature [63], [64]. However, these techniques involve
several synthesis steps to obtain the final products which may lead to complications and also
consumption of huge amount of experimental time. Therefore, we will also discuss the facile
and faster one-pot synthesis route in the following chapters. As discussed above, CNF and
rGO not only favours and physically supports metal oxides but also provides a charge transfer
path which may contribute to high specific capacitances. To grow or attach or decorate TiO2
nanoparticles by one-pot hydrothermal and microwave assisted synthesis route, we have used
CNF and rGO as a carbon template.
1.5. Conclusions - Present Work
As discussed in the previous sections of this chapter, the electrochemical performance
of SCs can be enhanced by improving the electrode materials. In the upcoming chapters, as
26
part of the present study, we will discuss the effect of different binders on electrochemical
performances of MnO2 electrode based SCs. In addition to this, we will also discuss the facile
one pot synthesis of rGO and CNF based TiO2 nanocomposites and their electrochemical
behaviour towards aqueous electrolyte based SCs. As aqueous electrolytes are more
environmental friendly compared to organic electrolyte.
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33
34
Chapter 2 – Effect of various binders on the
electrochemical performances of MnO2
electrode based supercapacitors
Summary: The following chapter presents the results and discussions about the effect of
various commonly used binders such as PTFE, PVDF and PVOH on the electrochemical
performances of MnO2 electrodes based supercapacitors. The electrode formulation of active
and conducting material to binder material was in the ratio of 65:35 and mass loading of the
electrodes was around 8 mg/cm2. The electrochemical characterization was done in 1 M
KOH and 1 M Na2SO4 aqueous electrolyte mediums. The chapter concludes by emphasizing a
concise choice of binders at various electrochemical experimental parameters.
2.1. Introduction
Manganese dioxide (MnO2) material is a low cost transition metal oxide which has
shown low toxicity and high environmental safety. This material has also shown high
theoretical capacities (1100 to 1300 F/g) [1], [2], [3]–[5] which makes them one of the
potential electrode materials for supercapacitors [4], [6]–[10]. As mentioned in section
1.4.2.2, MnO2 shows reversible redox transition mechanisms (pseudocapacitance). The
morphology, the thickness of the electrode, crystallinity, crystal structure, specific surface
area and pore structure are some of the most important factors affecting the electrochemical
performance of MnO2 [11]. Several studies have been reported in the literature on controlling
these factors [12]. In order to bind the active material (MnO2) into the compact electrodes,
binders are generally mixed with active and conductive materials. Several studies have shown
that the addition of binder increases the resistance of the electrode and lowers the specific
capacitance which might be due to the decrease of electrolyte accessible surface of active
materials [12]. However, studies on the suitable binder among commonly used binders for
supercapacitor electrodes have not been reported so far. Therefore, in the present study, we
will discuss the effect of commonly used binders, such as PTFE, PVDF and PVOH on
electrochemical performances of supercapacitors electrodes in 1 M KOH and Na2SO4
aqueous electrolytes. These binders show various characteristics in terms of chemical and
mechnical stability, ionic conductivity and wettability in 1 M KOH and 1 Na2SO4 aqueous
electrolytes.
2.2. Experimental Section
Synthesis of MnO2 nanomaterials: MnO2 nanomaterials for the present study were
synthesized by a previously reported co precipitation method of our group [13].
35
Electrode preparation: The working electrodes were prepared by mixing MnO2
nanomaterials, carbon black and binder material. The weight ratio for MnO2 and carbon black
to the binder material was in 65:35, whereas the weight ratio for MnO2 to carbon black was in
85:15. The loading of the working electrodes in the present study was around 8 mg/cm2. The
methodologies for making working electrode from different binders are the following: (1).
PTFE binder based electrodes - MnO2, carbon black and PTFE binder were mixed in the
ratio as mentioned above. The mixed composition was turned into slurry by adding an
appropriate volume of absolute ethanol. The electrode slurry was rolled to obtain an electrode
film. (2). PVDF binder based electrode - MnO2 and carbon black were mixed in the ratio as
mentioned above. The mixed composition was turned into slurry by adding PVDF solution
(100g/L). The electrode slurry was sonicated for 1 h and made into films by the doctor blade
technique (3). PVOH binder based electrode - MnO2 and carbon black were mixed in the
ratio as mentioned above. The mixed composition was turned into slurry by adding PVOH
solution (200g/L). Later, 8 μL of Poly(ethylene glycol)diglycidyl ether and 25 μL of 6 M
KOH were mixed with the slurry for cross-linking. The electrode slurry was sonicated for 1 h
and made into films by the doctor blade technique. The mentioned films were cut into 1 cm2
pieces and were pasted on the nickel foam and a steel grid to form as a working electrode.
Electrochemical Analysis: The three electrode test cell, as mentioned in section 1.4.2.1, was
used in the present study to evaluate the effect of binders on the electrochemical
performances of supercapacitor electrodes. The electrochemical performances of all the
binder based electrodes were evaluated by cyclic voltammetry (CV) at various scan rates (5,
10, 20, 50, 100 and 200 mV/s), electrochemical impedance spectroscopy (EIS) and
galvanostatic static charge-discharge analysis (GCPL) (current density of 1.2 A/g).
Electrochemical Impedance Spectroscopy (EIS) measurements using PTFE, PVDF and
PVOH binder based electrodes were performed at rest potentials, using sinusoidal signal of
±5 mV from 50 kHz to 10 mHz
2.3. Results and Discussions
CVs at various scan rates (5, 10, 20, 50, 100 and 200 mV/s) were performed on PTFE,
PVDF and PVOH binder based electrodes in 1 M KOH and 1 M Na2SO4 aqueous
electrolytes. Figure: 2.1 shows the CV curves of PTFE, PVDF and PVOH in 1 M KOH and 1
M Na2SO4 aqueous electrolytes. The potential windows for PTFE, PVDF and PVOH were
0.85 V in 1 M Na2SO4 aqueous electrolyte and 0.6 V in 1 M KOH aqueous electrolyte. The
potential windows for all the binders used in the present were wider in 1 M Na2SO4 aqueous
electrolyte as compared to that of 1 M KOH aqueous electrolyte. The wider potential
windows in 1 M Na2SO4 aqueous electrolyte shows that the wider potential window can be
obtained in a neutral electrolyte medium for PTFE, PVOH and PVDF binder based
electrodes.
36
Figure: 2.1. Cyclic voltammetry curves of PTFE, PVDF and PVOH in 1 M KOH and 1 M
Na2SO4 aqueous electrolytes.
Figure: 2. 2. A shows the specific capacitances at various scan rates for PTFE binder
based electrodes. The specific capacitance at low scan rates shown by PTFE binder based
electrode was higher in 1 M KOH compared to that 1 M Na2SO4 (1 M KOH: 154 F/g – 5
mV/s, 128 F/g – 10 mV/s and 98 F/g – 20 mV/s; 1 M Na2SO4: 114 F/g – 5 mV/s, 103 F/g –
10 mV/s and 90 F/g – 20 mV/s). The specific capacitance relative to the MnO2 content was
also noted to be higher in 1 M KOH compared to that of 1 M Na2SO4 at lower scan rates (1
M KOH: 285 F/g – 5 mV/s, 236 F/g – 10 mV/s and 182 F/g – 20 mV/s; 1 M Na2SO4: 212 F/g
– 5 mV/s; 191 F/g - 10 mV/s and 166 F/g – 20 mV/s). However, the specific capacitance was
noted to be higher in 1M Na2SO4 at higher scan rates (50, 100, 200 mV/s) either relative to
both MnO2 and the overall content of the electrode material as compared to that of PTFE
based electrodes in 1 M KOH. Appendix 1 - Table 1.1 shows the variation of specific
capacitances at lower and higher scan rates for PTFE, PVDF and PVOH binder based
electrodes in 1 M KOH and 1 M Na2SO4 aqueous electrolytes. From Appendix 1 - Table: 1.1,
it can be noted that the difference of specific capacitance relative to MnO2 and overall content
in 1 M KOH was higher as compared to that in 1 M Na2SO4 (1 M KOH: 131 F/g – 5 mV/s; 1
37
M Na2SO4: 98 F/g – 5mV/s). The decrease in specific capacitance may indicate the lower
electrolyte accessibility to the active material (MnO2 in our case).
Figure: 2.2. Specific capacitance at various scan rates for (A) PTFE (B) PVDF and (C)
PVOH binder based electrode
Figure: 2. 2. B shows the specific capacitances at various scan rates for PVDF binder
based electrodes. The specific capacitances of PVDF binder based electrode were higher in 1
M KOH as compared to that of 1 M Na2SO4. From Appendix 1 - Table 1.1, similar variation
of specific capacitances were also noted for PVDF binder based electrodes as for PTFE
binder based electrodes at low and high scan rates. In addition, the large difference of specific
capacitances when relative to MnO2 or the overall content was also noted for PVDF binder
electrodes which may indicate the low electrolyte accessibility to the active material.
Figure: 2. 2. C shows the specific capacitances at various scan rates for PVOH binder
based electrodes. The specific capacitances of PVOH binder based electrode were higher in 1
M Na2SO4 aqueous electrolyte as compared to that of 1 M KOH aqueous electrolyte. From
Appendix 1 - Table 1.1, lower differences of specific capacitances were noted for PVOH
binder electrodes as compared to both PTFE and PVDF binder based electrodes. To
understand these lower differences content in the case of PVOH binder based electrodes,
contact angle measurements were conducted.
38
The contact angle measurements were done on pure binder and binder composite
electrodes. Figure: 2.3. illustrates the contact angle measurements of pure binder and binder
composite electrodes. The contact angle for pure PTFE and PTFE binder based electrode was
around 1200 and 129
0 respectively, whereas the contact angle for pure PVDF and PVDF
binder based electrodes was around 850
and 1200
respectively. The contact angle increases
from pure state of binder to binder composite electrode state for both PTFE and PVDF
binders. This increase in contact angle implies that the hydrophobic nature further increases
from the pure binder to binder composite electrode. The contact angle for pure PVOH and
PVOH binder based electrodes were around 450
and 420
respectively. However, the contact
angle decreases from pure PVOH to PVOH binder based electrodes. The decrease in contact
angle implies the increase of hydrophilic nature from pure binder state to binder composite
electrode state. This increase in hydrophilic nature may facilitate the ease in electrolyte
accessibility to the active material. This ease of electrolyte accessibility may explain the
lower differences of specific capacitances when relative to MnO2 or overall content for
PVOH binder based electrodes.
Figure: 2.3. Contact angle measurement between binder and binder electrode composite.
Appendix 1- Figures: 1.1 and 1.2 show the specific capacitances due to MnO2 and
over all content of the binder composite electrodes in 1 M Na2SO4 and 1 M KOH electrolyte
respectively. From Appendix 1 - Figure: 1.1, we can infer that the PVOH binder based
electrode have shown lower specific capacitances in 1 M KOH aqueous electrolyte (29 F/g -
5 mV/s). To understand the reason behind the lower specific capacitances of PVOH binder in
1 M KOH aqueous electrolyte, EIS measurements was conducted.
EIS measurements using PTFE, PVDF and PVOH binder based electrodes were
performed at rest potentials, using sinusoidal signal of ±5 mV from 50 kHz to 10 mHz. Both
Figures: 2.4.A and 2.4.B show the Nyquist plots of all the binders based electrodes in both 1
M Na2SO4 and 1 M KOH aqueous electrolytes. From figure: 2.4.A, the ESR for PVOH
binder based electrodes in 1 M KOH aqueous electrolyte after CVs (i.e before conducting
GCPL) was noted to be 50 Ω cm2, whereas the ESR for both PTFE and PVDF binder based
electrodes in 1 M Na2SO4 was 4.4 Ω cm2 and 7.5 Ω cm
2 respectively. The ESR of PVOH
binder based electrodes as compared to both PTFE and PVDF binder based electrodes were
noted to be higher. This may explain the lower specific capacitances of PVOH binder based
39
electrodes in 1 M KOH aqueous electrolytes.
Figure: 2.4. (A) Nyquist plots from EIS for all the binder based electrodes in 1 M KOH
aqueous electrolyte.
Figure: 2.4. (B) Nyquist plots from EIS for all the binder based electrodes in 1 M Na2SO4
aqueous electrolyte.
40
Figure: 2.5. (A) Specific capacitance at a current density of 1.2 A/g for PTFE, PVDF and
PVOH binder based electrodes in 1 M KOH aqueous electrolyte.
Figure: 2.5. (B) Specific capacitance at a current density of 1.2 A/g for PTFE, PVDF and
PVOH binder based electrodes in 1 M Na2SO4 aqueous electrolyte.
In order to understand the stability of PTFE, PVDF and PVOH binder based
electrodes in aqueous electrolytes, GCPL measurement were conducted for 1500 cycles. The
GCPL measurement was conducted at a current density of 1.2 A/g. The specific capacitances
and their retention were noted by the end of the measurements. Figure: 2.5.A shows the
specific capacitances of PTFE, PVDF and PVOH binder based electrodes over 1500 GCPL
41
cycles at 1.2 A/g in 1 M KOH aqueous electrolyte. Figure: 2.5.B shows the specific
capacitances of PTFE, PVDF and PVOH binder based electrodes over 1500 GCPL cycles at
1.2 A/g in 1 M Na2SO4 aqueous electrolyte. The highest specific capacitance in M KOH was
noted for PVDF binder based electrode (105 F/g), whereas the highest specific capacitance
was noted for PTFE binder based electrode (70 F/g). However, during the GCPL
measurements, lower specific capacitances were noted for PVOH binder based electrode at
1.2 A/g in 1 M KOH. Appendix 1 - Figure: 1.3 shows that the specific capacitance at the
initial GCPL cycle was 0.77 F/g, which was very low. In order to compare the specific
capacitance retention of PVOH binder based electrodes with both PTFE and PVDF binder
based electrodes, 1500 GCPL cycles were conducted at 0.24 A/g.
Figures: 2.6.A and 2.6.B show the specific capacitance retention in both 1 M Na2SO4
and 1 M KOH aqueous electrolyte respectively. The specific capacitance retention for PVDF
binder based electrodes in 1 M KOH and 1 M Na2SO4 was noted to be the same (97%). The
specific capacitance retention for PTFE binder based electrodes in 1 M KOH and 1 M
Na2SO4 were 90% and 94% respectively, whereas the specific capacitance retention for
PVOH binder based electrodes in 1 M KOH and 1 M Na2SO4 were 46% and 94%
respectively. In comparison, the specific capacitance retention was noted to be higher for
PVDF binder based electrodes in both 1 M KOH and 1 M Na2SO4 aqueous electrolyte as
compared to that of PTFE and PVOH binder based electrodes. In general, PVDF binder
degrades in KOH medium due to dehyrdoflorination but in the present study, the PVDF
binder based electrode in 1 M KOH has not only shown higher specific capacitances (179 F/g
– 5 mV/s and 105 F/g – 1.2 A/g) but also higher specific capacitance retention (97 %) even
after 1500 GCPL cycles.
Figure: 2.6. Specific capacitance retention of PTFE, PVDF and PVOH in (A) 1 M Na2SO4
and (B) 1 M KOH aqueous based electrolyte.
42
2.4. Conclusions & Further Work
The effect of various commonly used binders such as PTFE, PVDF and PVOH on the
electrochemical performances of supercapacitors electrodes was examined out. The voltage
window was found to be wider in 1 M Na2SO4 electrolyte medium for all the binders used in
the present study. The choice of binders can be concluded depending upon the parameters
used in the experimental procedure. Table: 2.1 illustrates an overview on the choice of
binders and Appendix 1 – Table: 1.2 further presents an overall quantitative choice on the
binders. The present study can be further extended by varying the mass loading and the ratio
of binder to active and conducting materials to understand the effect of mass loading to the
electrochemical performances of supercapacitor electrodes.
Parameter 1 M KOH 1 M Na2SO4
Specific capacitance at lower scan rates – 5 mV/s PVDF PTFE
Specific capacitance at higher scan rates – 200 mV/s PVDF PVDF
Specific capacitance retention PVDF PVDF
Table: 2.1. Overview of the choice of binder
2.5. References
[1] S. Devaraj and N. Munichandraiah, “High Capacitance of Electrodeposited MnO2 by
the Effect of a Surface-Active Agent,” Electrochem. Solid-State Lett. , vol. 8 , no. 7 ,
pp. A373–A377, Jul. 2005.
[2] S. Pang, M. A. Anderson, and T. W. Chapman, “Novel Electrode Materials for Thin-
Film Ultracapacitors: Comparison of Electrochemical Properties of Sol-Gel-Derived
and Electrodeposited Manganese Dioxide,” J. Electrochem. Soc. , vol. 147 , no. 2 , pp.
444–450, Feb. 2000.
[3] M. Toupin, T. Brousse, and D. Bélanger, “Charge Storage Mechanism of MnO2
Electrode Used in Aqueous Electrochemical Capacitor,” Chem. Mater., vol. 16, no. 16,
pp. 3184–3190, Aug. 2004.
[4] J.-K. Chang, M.-T. Lee, and W.-T. Tsai, “In situ Mn K-edge X-ray absorption
spectroscopic studies of anodically deposited manganese oxide with relevance to
supercapacitor applications,” J. Power Sources, vol. 166, no. 2, pp. 590–594, Apr.
2007.
[5] M. Toupin, T. Brousse, and D. Bélanger, “Influence of Microstucture on the Charge
Storage Properties of Chemically Synthesized Manganese Dioxide,” Chem. Mater.,
vol. 14, no. 9, pp. 3946–3952, Sep. 2002.
[6] B. Babakhani and D. G. Ivey, “Anodic deposition of manganese oxide electrodes with
rod-like structures for application as electrochemical capacitors,” J. Power Sources,
vol. 195, no. 7, pp. 2110–2117, Apr. 2010.
43
[7] C.-C. Hu, K.-H. Chang, M.-C. Lin, and Y.-T. Wu, “Design and Tailoring of the
Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation
Supercapacitors,” Nano Lett., vol. 6, no. 12, pp. 2690–2695, Dec. 2006.
[8] S.-E. Chun, S.-I. Pyun, and G.-J. Lee, “A study on mechanism of charging/discharging
at amorphous manganese oxide electrode in 0.1 M Na2SO4 solution,” Electrochim.
Acta, vol. 51, no. 28, pp. 6479–6486, Sep. 2006.
[9] L. Li, Z.-Y. Qin, L.-F. Wang, H.-J. Liu, and M.-F. Zhu, “Anchoring alpha-manganese
oxide nanocrystallites on multi-walled carbon nanotubes as electrode materials for
supercapacitor,” J. Nanoparticle Res., vol. 12, no. 7, pp. 2349–2353, 2010.
[10] V. Subramanian, H. Zhu, and B. Wei, “Alcohol-assisted room temperature synthesis of
different nanostructured manganese oxides and their pseudocapacitance properties in
neutral electrolyte,” Chem. Phys. Lett., vol. 453, no. 4–6, pp. 242–249, Mar. 2008.
[11] G. Wang, L. Zhang, and J. Zhang, “A review of electrode materials for
electrochemical supercapacitors,” Chem. Soc. Rev., vol. 41, no. 2, pp. 797–828, 2012.
[12] V. Ruiz, C. Blanco, M. Granda, R. Menéndez, and R. Santamaría, “Effect of the
thermal treatment of carbon-based electrodes on the electrochemical performance of
supercapacitors,” J. Electroanal. Chem., vol. 618, no. 1–2, pp. 17–23, Jul. 2008.
[13] O. Ghodbane, M. Louro, L. Coustan, A. Patru, and F. Favier, “Microstructural and
Morphological Effects on Charge Storage Properties in MnO2-Carbon Nanofibers
Based Supercapacitors,” J. Electrochem. Soc. , vol. 160 , no. 11 , pp. A2315–A2321,
Jan. 2013.
44
45
46
Chapter 3- Amorphous TiO2-reduced
graphene oxide as electrode for
supercapacitors
Summary: In this present chapter, we will discuss the effect of amorphous TiO2-rGO on the
electrochemical performances of supercapacitor. The amorphous TiO2-rGO nanocomposites
were formed by one pot sol-gel assisted synthesis. Various material characterization
techniques were done to support the amorphous nature of TiO2 and the reduction of GO
during the one pot synthesis. The chapter concludes by discussing the various
electrochemical characterizations results which were done to compare the performance of
amorphous TiO2-rGO and crystalline TiO2-rGO in 1 M H2SO4, 1 M KOH and 1 M Na2SO4
aqueous electrolytes.
3.1. Introduction
TiO2 metal oxide has been noted as a low cost transition metal oxide which is non-
toxic and greatly available in abundance [1]. Several studies have been reported on the
nanocomposites made up of TiO2 and layered carbon nanostructures. As discussed previously
in section 1.4.2.2, Two Dimensional (2-D) carbon materials such as graphene oxide (GO),
graphene and reduced graphene oxide (rGO) have a high conductivity, hydrophilic nature and
surface area which make them potential electrode materials for SCs. In addition, the presence
of functional groups have shown higher specific capacitance due to their pseudocapacitance.
The graphene-like materials have been shown as good support for metal oxides. The metal
oxide-graphene (for example – TiO2) nanocomposites have also shown high specific
capacitances (265 F/g – 100 mV/s) [2].
Numerous TiO2-rGO nanocomposites have been
reported in the literature with several numbers of synthesis steps which lead to complexity
[3], [4]. Therefore in the present study, we report facile one pot sol-gel synthesis of TiO2-rGO
nanocomposites. Furthermore, amorphous material has shown increased concentration of
interfacial regions which may enhance the ion diffusion pathways [5]–[7]. Several
performance studies have been reported with amorphous TiO2 based carbon composites.
Xiang Sun et al. have reported high specific capacitances for amorphous thin films anchoring
to graphene using atomic layer deposition technique [8]. However, the mass loading of the
electrode materials present by Xiang Sun et al. were around 1-2 mg/cm2
which may not be
suitable for real time devices. Moreover, the study effect of amorphous TiO2-rGO (aTiO2-
rGO) nanocomposite and crystalline TiO2-rGO (TiO2-rGO) nanocomposites in aqueous
electrolyte mediums have not been reported so far. In the present study, we will therefore also
compare the electrochemical performance of thicker aTiO2-rGO and TiO2-rGO (~ 10
mg/cm2) electrodes in 1 M Na2SO4, 1 M KOH and 1 M H2SO4 aqueous electrolytes.
47
3.2. Experimental Section
Synthesis of aTiO2-rGO nanocomposties: GO was synthesised by use of the modified
Hummers method which was previously reported by our group [9]. The synthesis procedure
for the formation of aTiO2-rGO was modified from the previous works reported by Dinh
Huong Nguyen et al. by using titanium isopropoxide (TTIP) (Sigma Aldrich) as TiO2
precursor and controlling the excess water addition which is an essential step to decorate on
the surface of GO with TiO2. In a typical synthesis, 0.4 g of GO was suspended in 400 ml of
N, N-dimethylformamide (DMF) by sonication for 4 h. The medium was stirred to form a
homogenous suspension. Later, 5.85 ml of TTIP precursor was added during stirring and the
resulting suspension was further stirred for an hour. Excess water was added drop by drop for
1 h and the final product (aTiO2-rGO) was filtered and later dried overnight in a hot air oven
at 500C (Figure: 3.1).
Figure: 3.1. Procedure for the synthesis of aTiO2-rGO nanocomposites.
Synthesis of TiO2-rGO nanocomposities: TiO2-rGO nanocomposites were formed by
thermal reduction of aTiO2-rGO under N2 atmosphere at 6000C for 6 h with heating rate of
50C/min.
Synthesis of TiO2 – Microwave method: TiO2 nanocomposites were prepared by using TTIP
as a precursor in a laboratory microwave equipement. In a typical synthesis, 7 ml of HCl (2.3
M) was added to 7 ml of TTIP precursor to obtain a milky solution. This milky solution was
subjected to stirring for 3.5 h to process the hydrolysis. The transparent solution was
irradiated by microwave for 10 min at 1000C. The final product was dried overnight in a hot
air oven at 1200C. This material was used as a reference for comparison with the carbon-TiO2
composites.
Electrode preparation: The active materials, aTiO2-rGO and TiO2-GO, were mixed with
carbon black and PTFE binder in the ratio of 75:15:10. The mixed materials were suspended
in an appropriate volume of absolute ethanol to form a slurry, which was later rolled to form
homogenous electrode films. These electrode films were cut uniformly in dimensions of 1 cm
x 1 cm and pasted on the nickel form and a steel grid to form as a working electrode.
Electrochemical characterization: The electrochemical characterization was done in a three
electrode cell where Ag/AgCl was used as the reference electrode and Pt foil as the counter
electrode. The cyclic voltammetry was preceded at various scan rates for 200 cycles and EIS
measurements were performed at rest potentials, using sinusoidal signal of ±5 mV from 50
kHz to 10 mHz.
48
3.3. Results & Discussions
Figure: 3.2. SEM image of uniformly growth of amorphous TiO2 on the surface of GO.
Figure: 3.2 shows the SEM image of uniformly decorated amorphous TiO2 on the
surface of GO. The amorphous nature of TiO2 can be confirmed by P-XRD and raman
spectroscopy. The Figure: 3.3.A shows the XRD pattern of aTiO2-rGO, TiO2-rGO, TiO2, GO
and graphite. The low intensity peak of aTiO2-rGO in the P-XRD confirms the amorphous
phase of TiO2. The intensity of peaks increases with the thermal treatment of aTiO2-rGO as a
proof of the crystallization of TiO2 in TiO2-rGO. This increase in the intensity of the peaks
can be compared to TiO2 nanoparticles which were synthesized by the microwave route.
Figure: 3.3.B shows the raman spectroscopy of aTiO2-rGO, TiO2-rGO, TiO2 samples. For
aTiO2-rGO, the raman spectroscopy shows the absence of peaks, confirming amorphous
nature of TiO2. Similar absence of peaks were also obtained by G. Bertoni et al. while
verifying the purity content of amorphous and crystalline phases in TiO2 samples through
raman spectroscopy [10].
49
Figure: 3.3. (A) P-XRD of aTiO2-rGO, TiO2-rGO, TiO2, GO and graphite (B) Raman
spectroscopy of aTiO2-rGO, TiO2-rGO, TiO2.
Figure: 3.4. (A) Raman spectroscopy of aTiO2-GO and GO (B) FTIR of GO and aTiO2-rGO
In the present synthesis, hydrolysis of TTIP was facilitated by the O-H group present
on the surface of GO. The DMF solvent not only provides stable GO suspensions [11] but
also an environment free of O-H groups. In these cases, the initial hydrolysis occurs due to O-
H group present on the surface of GO and later can be accelerated by adding excess water.
Raman spectroscopy and FTIR of aTiO2-rGO and GO were conducted to support this
mechanism. Figure: 3.4.A shows the raman spectra of aTiO2-GO and GO samples. The
raman spectroscopy of both aTiO2-rGO and GO exhibited strong D-band (~1340 cm-1
) and
G-band (~1600 cm-1
) which are usually assigned to a structural disorder and the graphitized
structures respectively [12]. The ID/IG ratios for GO and aTiO2-rGO were 1.042 and 1.106
respectively. The slight variation of ID/IG ratios were observed from GO to aTiO2-rGO which
may be related to the removal of functional group upon reduction. It can be proposed that the
O-H groups may have reacted with the TTIP precursor to form TiO2 and since the reaction
was carried out at room temperature, amorphous TiO2 was formed. To further support this
theory, FTIR measurements were conducted. FTIR measurements from Figure: 3.4.B shows a
slight decrease of the stretching of O-H groups in the aTiO2-rGO sample compared to the
ones in the TiO2-rGO sample. This decrease of stretching of O-H group may indicate that the
O-H groups were involved in the formation of TiO2 on the surface of GO and also lead to the
reduction of GO.
50
Cyclic voltammetry was conducted at various scan rates to compare the performances
of aTiO2-rGO and TiO2-rGO in 1 M H2SO4, 1 M KOH and 1 M Na2SO4 aqueous electrolytes.
Figure: 3.5 shows the CV curves of aTiO2-rGO and TiO2-rGO at various scan rates in 1 M
H2SO4, 1 M KOH and 1 M Na2SO4 aqueous electrolytes. Appendix 2- Table: 2.1 shows an
overview of specific capacitances of aTiO2-rGO and TiO2-rGO at low (5 mV/s) and high
(200 mV/s) scan rates. The specific capacitances in 1 M Na2SO4 aqueous electrolyte for
aTiO2-rGO and TiO2-rGO were 16 F/g (5 mV/s) and 7 F/g (5 mV/s) respectively. The
specific capacitances in 1 M H2SO4 aqueous electrolyte for aTiO2-rGO and TiO2-rGO were
12 F/g (5 mV/s) and 2.3 F/g (5 mV/s) respectively, whereas the specific capacitances in 1 M
KOH aqueous electrolyte for aTiO2-rGO and TiO2-rGO were 17 F/g (5 mV/s) and 1.04 F/g (5
mV/s) respectively. From the above mentioned specific capacitances of aTiO2-rGO and TiO2-
rGO, we can infer that higher specific capacitances were noted for aTiO2-rGO based
electrodes compared to TiO2-rGO based electrodes in all the aqueous electrolytes. To
understand the reason behind their higher specific capacitances, specific surface area
measurements were conducted.
Figure: 3.6.C shows the N2 adsorption-desorption isotherms of aTiO2-rGO and TiO2-
rGO samples. The N2 adsorption-desorption isotherms of aTiO2-rGO sample depicts the
mesoporous material whereas microporous material for TiO2-rGO sample. The BET specific
surface area of both aTiO2-rGO and TiO2-rGO were around 396 m2/g and 58 m
2/g
respectively. The BET specific surface area of aTiO2-rGO was noted to be higher than that of
TiO2-rGO.
51
Figure: 3.5. Cyclic voltammetry curves of aTiO2-rGO and TiO2-rGO at various scan rates in
1 M H2SO4, 1 M KOH and 1 M Na2SO4 aqueous electrolytes.
Figure: 3.6.A shows the cumulative pore area and pore volume distribution to pore
diameter of aTiO2-rGO and TiO2-rGO samples, whereas Figure: 3.6.B shows the pore area
and pore volume distribution to pore diameter aTiO2-rGO and TiO2-rGO samples. From both
figures: 3.6.A and 3.6.B, it was noted that the high cumulative pore and pore volume as well
as the high pore volume and area were at the pore diameter of 5.9 nm for aTiO2-rGO.
Additionally, the high cumulative pore area and volume as well as high pore volume and area
were at the pore diameter of 2.8 nm for TiO2-rGO. The presence of high cumulative
mesoporous area and volume expected to facilitate efficient ion transportation. The presence
of efficient ion transportation in case of aTiO2-rGO may have enhanced specific
capacitances.
From figure: 3.5, the higher specific capacitances were observed for aTiO2-rGO
electrode based supercapacitors in 1 M KOH and 1 M Na2SO4 aqueous electrolytes (1 M
KOH:17 F/g - 5 mV/s; 1 M Na2SO4: 16 F/g - 5 mV/s). To understand the reason behind these
high specific capacitances, EIS measurements were conducted.
EIS measurements using aTiO2-rGO and TiO2-rGO electrodes were performed at rest
potentials, using sinusoidal signal of ±5 mV from 50 kHz to 10 mHz. Both Figures: 3.7.A and
3.7.B show the Nyquist plots of all the binders based electrodes in 1 M H2SO4,1 M Na2SO4
and 1 M KOH aqueous electrolytes. The ESR of aTiO2-rGO in 1 M Na2SO4 and 1 M KOH
aqueous electrolytes can be depicted from the Nyquist plots. The ESR of aTiO2-rGO in both
1 M Na2SO4 and 1 M KOH aqueous electrolytes were 3 Ω.cm2 and 2.6 Ω.cm
2. These low
ionic resistances may explain the high specific capacitances of aTiO2-rGO electrodes in both
1 M KOH and 1 M Na2SO4 aqueous electrolytes.
52
Figure: 3.6. (A) Cumulative pore area and pore volume distribution to pore diameter (B) Pore
area and pore volume distribution to pore diameter and (C) N2 adsorption-desorption
isotherms of aTiO2-rGO and TiO2-rGO samples.
Figure: 3.7. Nyquist plots of aTiO2-rGO and TiO2-rGO 1 M H2SO4, 1 M KOH and 1 M
Na2SO4 aqueous electrolytes at (A) low and (B) high frequencies.
53
3.4. Conclusions
In the present study, a one pot sol-gel synthesis of aTiO2-rGO was proceeded. The
amorphous nature of TiO2 decorated on the surface of GO was supported by both P-XRD and
raman spectroscopy. The reduction of GO during the one pot sol-gel synthesis were
supported by raman spectroscopy due to a slight increase of ID/IG ratio. The involvement of
O-H group during the one-pot sol-gel synthesis was supported by FTIR due to a decrease in
the stretching of the O-H group. The SEM imaging confirms the uniform growth of
amorphous TiO2-rGO on the surface of GO. The higher specific capacitances of aTiO2-rGO
based electrodes compared to the one of TiO2-rGO in all the aqueous electrolytes were due to
their higher BET surface area, presence of mesoporous, larger pore diameter, higher
cumulative pore area and volume which may support the efficient ion transportation. The
higher specific capacitances of aTiO2-rGO were noted in 1 M KOH which might be due to
their lower ESR. However, in the present study, the specific capacitances of aTiO2-rGO in all
the aqueous electrolytes (Table 3.1) were low compared to reported data by Xiang Sun et al.
for amorphous TiO2 on graphene by atomic layer deposition (97.5 F/g – 5 mV/s; 1 M KOH)
[8]. The higher specific capacitances by Xiang Sun et al. were due to thinner electrode films
(1-2 mg/cm2) which may be not suitable for real time devices. In addition, we have used
thicker electrode films (~10 mg/cm2) which can be used in real time supercapacitor devices.
Furthermore, the low specific capacitances of thicker aTiO2-rGO based electrodes can be
improved by varying the electrode formulation, binder material or binder less material which
can be the extended works of the present study.
3.5. References
[1] S. K. and C. Das D. Ghosh, S. Giri, “Synthesis and Characterisations of TiO2 Coated
Multiwalled Carbon Nanotubes/Graphene/Polyaniline Nanocomposite for
Supercapacitor Applications,” Open J. Appl. Sci., vol. 2, no. 2, pp. 70–77, 2012.
[2] A. K. Mishra and S. Ramaprabhu, “Functionalized Graphene-Based Nanocomposites
for Supercapacitor Application,” J. Phys. Chem. C, vol. 115, no. 29, pp. 14006–14013,
Jul. 2011.
[3] Y. Liang, H. Wang, H. Sanchez Casalongue, Z. Chen, and H. Dai, “TiO2 nanocrystals
grown on graphene as advanced photocatalytic hybrid materials,” Nano Res., vol. 3,
no. 10, pp. 701–705, 2010.
[4] Y. Yang, E. Liu, J. Fan, X. Hu, W. Hou, F. Wu, and Y. Ma, “Green and facile
microwave-assisted synthesis of TiO2/graphene nanocomposite and their
photocatalytic activity for methylene blue degradation,” Russ. J. Phys. Chem. A, vol.
88, no. 3, pp. 478–483, 2014.
[5] P. H. and S. Indris, “Diffusion and ionic conduction in nanocrystalline ceramics,” J.
Phys. Condens. Matter, vol. 15, no. 30, p. R1257, 2003.
54
[6] H. Xiong, M. D. Slater, M. Balasubramanian, C. S. Johnson, and T. Rajh, “Amorphous
TiO2 Nanotube Anode for Rechargeable Sodium Ion Batteries,” J. Phys. Chem. Lett.,
vol. 2, no. 20, pp. 2560–2565, Oct. 2011.
[7] P. Heitjans, E. Tobschall, and M. Wilkening, “Ion transport and diffusion in
nanocrystalline and glassy ceramics,” Eur. Phys. J. Spec. Top., vol. 161, no. 1, pp. 97–
108, 2008.
[8] X. Sun, M. Xie, G. Wang, H. Sun, A. S. Cavanagh, J. J. Travis, S. M. George, and J.
Lian, “Atomic Layer Deposition of TiO2 on Graphene for Supercapacitors,” J.
Electrochem. Soc. , vol. 159 , no. 4 , pp. A364–A369, Jan. 2012.
[9] O. Ghodbane, M. Louro, L. Coustan, A. Patru, and F. Favier, “Microstructural and
Morphological Effects on Charge Storage Properties in MnO2-Carbon Nanofibers
Based Supercapacitors,” J. Electrochem. Soc. , vol. 160 , no. 11 , pp. A2315–A2321,
Jan. 2013.
[10] G. Bertoni, E. Beyers, J. Verbeeck, M. Mertens, P. Cool, E. F. Vansant, and G. Van
Tendeloo, “Quantification of crystalline and amorphous content in porous samples
from electron energy loss spectroscopy,” Ultramicroscopy, vol. 106, no. 7, pp. 630–
635, May 2006.
[11] X. Li, G. Zhang, X. Bai, X. Sun, X. Wang, E. Wang, and H. Dai, “Highly conducting
graphene sheets and Langmuir-Blodgett films,” Nat Nano, vol. 3, no. 9, pp. 538–542,
Sep. 2008.
[12] M. B. and A. J. and D. A. and J. A. and A. B. and Y. T. and R. T. and H. O. and T. V.
T. and J. P. K. and A. Sandhu, “Characterization of graphene oxide reduced through
chemical and biological processes,” J. Phys. Conf. Ser., vol. 433, no. 1, p. 12001,
2013.
55
Chapter 4- Conclusions
Efficient and sustainable economic energy accessibility to consumers (both industrial
and domestic) is one of the major issues in the present energy systems. SCs are some of the
potential solution due to their high capacitance and specific power availability without
deterioration for longer operation time periods. However, their energy density compared to
battery ESS is low. The energy density of SCs can be enhanced by improving their electrode
material and operating them in wider voltage windows. Electrodes materials can be improved
by using new nanocomposites or optimising the electrode formulation. The appendix 3 and
chapter 3 present some one pot synthesis of nanomaterials which can enhance the
performance of SCs. In chapter 3, we have also presented the facile one pot sol-gel assisted
amorphous TiO2-rGO which can be easily reproduced for a large scale application. However,
the thicker electrode made up of amorphous TiO2-rGO have shown lower specific
capacitances. These can be improved by optimizing the electrode formulation by varying the
binder materials and their ratio or by using binderless material. The chapter 2 describes one
such optimisation pathway for MnO2 based electrode material. The study also provides an
overview of suitable binder among commonly used binders such as PTFE, PVDF and PVOH.
Therefore, by using newly developed nanocomposites with optimized electrode formulation,
we can improve the energy density of SCs which can eventually improve the power systems
operations and accessibility.
56
Appendix 1- Effect of various binders on
the electrochemical performances of MnO2
electrode based supercapacitors
Figure: 1.1. Specific capacitances due to MnO2 and overall content of all the binder based
electrodes at various scan rates in 1 M KOH aqueous electrolyte.
Figure: 1.2. Specific capacitances due to MnO2 and overall content of all the binder based
electrodes at various to scan rates in 1 M Na2SO4 aqueous electrolyte.
57
Scan rate
(mV/s)
PTFE
(1 M
KOH)
PTFE
(1 M
Na2SO4)
PVDF
(1 M
KOH)
PVDF
(1 M
Na2SO4)
PVOH
(1 M
KOH)
PVOH
(1 M
Na2SO4)
5 154 F/g 114 F/g 179 F/g 99 F/g 29 F/g 71 F/g
200 18 F/g 23 F/g 18 F/g 24 F/g 2 F/g 17 F/g
Table: 1.1. Variation of specific capacitances at lower and higher scan rates for PTFE, PVDF
and PVOH in 1 M KOH and 1 M Na2SO4.
Binder Ionic
resistance
(after
GCPL)
Specific
Capacitance
(5 mV/s)
Specific
Capacitance
(200 mV/s)
Specific
Capacitance
Retention
PTFE-
MnO2 –
CB
- + 0 -
PVDF-
MnO2 –
CB
1 M
Na2SO4 + 0 + +
PVOH-
MnO2 –
CB
0 - - -
PTFE-
MnO2 –
CB
- 0 + 0
PVDF-
MnO2 -
CB
1 M
KOH 0 + + +
PVOH-
MnO2 –
CB
+ - - -
Table: 1.2. Quantitative representation towards the choice of the binder in various aqueous
electrolyte mediums.
58
Figure: 1.3. Specific capacitance of PVOH at 1.2 A/g (brown – over all content and blue
MnO2 content)
59
Appendix 2- Amorphous TiO2-reduced
graphene oxide as electrode for
supercapacitors
Figure: 2.1. SEM image of aTiO2 decorated on the surface of GO.
Figure: 2.2. Raman spectroscopy of TiO2-rGO, aTiO2-rGO, TiO2, GO and graphite.
60
Scan
Rate
(mV/s)
aTiO2-rGO
(1 M
Na2SO4)
TiO2-rGO
(1 M
Na2SO4)
aTiO2-rGO
(1 M
H2SO4)
TiO2-rGO
(1 M
H2SO4)
aTiO2-
rGO
(1 M
KOH)
TiO2-
rGO
(1 M
KOH)
5 16 F/g 7 F/g 12 F/g 2.3 F/g 17 F/g 1.04 F/g
200 16 F/g 4 F/g 4 F/g 0.9 F/g 14 F/g 1.23 F/g
Table: 2.1. Overview of specific capacitance of aTiO2-rGO and TiO2-rGO at low and high
scan rates
61
Appendix 3- One pot synthesis of TiO2-
carbon nanocomposites
One pot microwave assisted synthesis of TiO2-GO
Synthesis:
Figure: 3.1. Procedure for the one pot synthesis of TiO2-GO
SEM Imaging:
Figure: 3.2. SEM imaging of uniform decoration of TiO2 on the surface of GO
P-XRD & TEM:
Figure: 3.3. TEM imaging of TiO2-GO and FTIR of TiO2-GO (thermal reduction), TiO2-rGO,
TiO2, GO and Graphite.
62
One pot hydrothermal assisted synthesis of TiO2-GO
Synthesis:
Figure: 3.4. Procedure for the one pot hydrothermal assisted synthesis of TiO2-GO
Figure: 3.5. (A) TEM image (B) P-XRD pattern of TiO2-GO (0.1 M – 1 h) and (C) Variation
of molar concentration used during the experiments
One pot microwave synthesis of TiO2-CNF
Figure: 3.6. Procedure for the one pot microwave assisted synthesis of TiO2-CNF
63
Figure: 3.7. (A) SEM image and (B) P-XRD of TiO2-CNF
Figure: 3.8. TEM images of TiO2-CNF
64
One pot filtration assisted synthesis of TiO2-CNF
Figure: 3.9. Procedure for one pot filtration assisted synthesis of TiO2-CNF
Figure: 3.10. (A) SEM image of TiO2-CNF (0.1 M – third layer) (B) SEM images of TiO2-
CNF (0.3 M- Second layer) (C) SEM images of TiO2-CNF (0.3 M – third layer) (D) P-XRD
pattern of TiO2-CNF at various molar concentrations of precursors and layers.
65
Appendix 4- Characterization Techniques
TEM images were obtained with transmission JEOL 1200 II EXII microscopy using a
field emission gun operating at 120 kV. Sampling was performed by depositing a drop
containing the assembled particles on copper grids (Agar Scientific). SEM imaging was
carried out with a Hitachi S4800 microscopy equipped with a detector of secondary and
backscattered electrons. The acceleration voltage ranges from 0.1 kV to 30 kV. X-ray
diffraction (XRD) measurements were performed on a Philips X’Pert diffractometer using
CuKα1 radiation (λ=1.5405 Å). Raman spectra of the raw powders were recorded by using a
LabRAM ARAMIS IR2 spectrometer with a He/Ne laser (λ=633 nm). Porosity
characteristics were calculated from nitrogen sorption isotherms measured at 77 K, with a
Micromeritics ASAP 2020 porosimeter. The specific surface area was estimated by using
BET for nitrogen while the pore volumes and the pore size distributions were shown from
adsorption isotherms by using the Halsey - Faas Correction method. Electrochemical tests
were performed using a multichannel VMP3 potentiostat/galvanostat (Bio-Logic, France).
CVs were recorded in a potential window depending upon the behaviour of the electrode
materials at various scan rates. EIS measurements were performed at the rest potential, using
a sinusoidal signal of ± 5 mV from 50 kHz to 10 mHz.
66
List of Figures
Figure: 1.1.
Chapters
ESS Overview- residential and commercial solutions (1-10 kWh),
base transceiver station solutions (1-20 kWh), UPS solutions (10-50
kWh) and utility (> MWh) solutions.
Pg
13
Figure: 1.2. Energy flow diagram in the doubly fed induction generator during
(a) normal operation and (b) low – voltage conditions.
14
Figure: 1.3. STATCOM and ESS connected in a power utility system. 15
Figure: 1.4. Control interface of Battery ESS connected FACTS for maintenance
stability
15
Figure: 1.5. Timeline for the evolution of SCs 16
Figure:
1.10.
Applications of supercapacitors – (Left) Honda FCX prototype
(Right) SC unit in the Vossloh trolleybus
19
Figure: 1.6. (Left) Electric double-layer supercapacitor (Right) Porous carbon
electrode for electrochemical double-layer supercapacitor
20
Figure: 1.7. Mechanism of pseudocapacitance with specifically absorbed ions 21
Figure: 1.8. Basic schematics for (a) all carbon EDLS (left), (b) a
pseudocapacitor (MnO2 depicted center) and (c) a lithium ion
battery (right). All devices have an active material (e.g., carbon,
MnO2, LiCoO2), a current collector, a separating membrane and
electrolyte, (e.g., Na2SO4, or LiPF6 solutions)
21
Figure: 2.1. Cyclic voltammetry of PTFE, PVDF and PVOH in 1 M KOH and 1
M Na2SO4 aqueous electrolytes.
35
Figure: 2.2. Specific capacitance at various scan rates for (A) PTFE (B) PVDF
and (C) PVOH binder based electrode.
36
Figure: 2.3. Contact angle measurement between binder and binder electrode
composite.
37
Figure: 2.4. (A) Nyquist plots from EIS for all the binder based electrodes in 1
M KOH aqueous electrolyte.
38
Figure: 2.4. (B) Nyquist plots from EIS for all the binder based electrodes in 1
M Na2SO4 aqueous electrolyte.
38
Figure: 2.5. (A) Specific capacitance at a current density of 1.2 A/g for PTFE,
PVDF and PVOH binder based electrodes in 1 M KOH aqueous
electrolyte.
39
67
Figure: 2.5. (B) Specific capacitance at a current density of 1.2 A/g for PTFE,
PVDF and PVOH binder based electrodes in 1 M Na2SO4 aqueous
electrolyte.
39
Figure: 2.6. Specific capacitance retention of PTFE, PVDF and PVOH in (A) 1
M Na2SO4 and (B) 1 M KOH aqueous based electrolyte.
40
Figure: 3.1. Procedure for the synthesis of aTiO2-rGO nanocomposites. 46
Figure: 3.2. SEM image of uniformly growth of amorphous TiO2 on the surface
of GO.
47
Figure: 3.3. (A) P-XRD of aTiO2-rGO, TiO2-rGO, TiO2, GO and Graphite (B)
Raman spectroscopy of aTiO2-rGO, TiO2-rGO, TiO2.
47
Figure: 3.4. (A) Raman spectroscopy of aTiO2-GO and GO (B) FTIR of GO and
aTiO2-rGO.
48
Figure: 3.5. Cyclic voltammetry of aTiO2-rGO and TiO2-rGO at various scan
rates in 1 M H2SO4, 1 M KOH and 1 M Na2SO4 aqueous
electrolytes.
50
Figure: 3.6. (A) Cumulative pore area and pore volume distribution to pore
diameter (B) Pore area and pore volume distribution to pore
diameter and (C) N2 adsorption-desorption isotherms of aTiO2-rGO
and TiO2-rGO samples.
51
Figure: 3.7. Nyquist plots of aTiO2-rGO and TiO2-rGO 1 M H2SO4, 1 M KOH
and 1 M Na2SO4 aqueous electrolytes at (A) low and (B) high
frequencies.
51
Appendix
Figure: 1.1. Specific capacitances due to MnO2 and overall content of all the
binder based electrodes at various scan rates in 1 M KOH aqueous
electrolyte.
55
Figure: 1.2. Specific capacitances due to MnO2 and overall content of all the
binder based electrodes at various to scan rates in 1 M Na2SO4
aqueous electrolyte
55
Figure: 1.3. Specific capacitance of PVOH at 1.2 A/g (brown – over all content
and blue MnO2 content)
57
Figure: 2.1. SEM image of aTiO2 decorated on the surface of GO. 58
Figure: 2.2. Raman spectroscopy of TiO2-rGO, aTiO2-rGO, TiO2, GO and 58
68
graphite.
Figure: 3.1. Procedure for the one pot synthesis of TiO2-GO. 60
Figure: 3.2. SEM imaging of uniform decoration of TiO2 on the surface of GO. 61
Figure: 3.3. TEM imaging of TiO2-GO and FTIR of TiO2-GO (thermal
reduction), TiO2-rGO, TiO2, GO and Graphite.
60
Figure: 3.4. Procedure for the one pot hydrothermal assisted synthesis of TiO2-
GO.
61
Figure: 3.5. (A) TEM image (B) P-XRD pattern of TiO2-GO (0.1 M – 1 h) and
(C) Variation of molar concentration used during the experiments.
61
Figure: 3.6. Procedure for the one pot microwave assisted synthesis of TiO2-
CNF.
61
Figure: 3.7. (A) SEM image and (B) P-XRD of TiO2-CNF. 62
Figure: 3.8. TEM images of TiO2-CNF. 62
Figure: 3.9. Procedure for one pot filtration assisted synthesis of TiO2-CNF . 63
Figure:
3.10.
(A) SEM image of TiO2-CNF (0.1 M – third layer) (B) SEM
images of TiO2-CNF (0.3 M- Second layer) (C) SEM images of
TiO2-CNF (0.3 M – third layer) (D) P-XRD pattern of TiO2-CNF at
various molar concentrations of precursors and layers.
63
69
List of Tables
Table: 1.1.
Chapters
Overview of the present companies in the field of SCs and
description of their products.
Pg
17
Table: 1.2. List of supercapacitors based energy storage projects – DOE Global
Energy storage database.
18
Table: 2.1. Overview of the choice of binder. 41
Appendix
Table: 1.1. Variation of specific capacitances at lower and higher scan rates for
PTFE, PVDF and PVOH in 1 M KOH and 1 M Na2SO4.
56
Table: 2.2. Quantitative representation towards the choice of the binder in
various aqueous electrolyte mediums.
56
Table: 2.1. Overview of specific capacitance of aTiO2-rGO and TiO2-rGO at
low and high scan rates.
59
70
Abbreviations
SCs
CB
Supercapacitors
Carbon black
BET Brunauer Emmett Teller
PVOH Poly(vinyl alcohol)
CNF Carbon nanofibres
CV Cyclic voltammogram
EDLC Electrochemical double layer capacitor
ESR Equivalent series resistance
EIS Electrochemical Impedance spectroscopy
FTIR Fourier Transformed Infrared
GCPL Galvanostatic charge-discharge cycles
TEM Transmission electron microscopy
SEM Scanning electron microscopy
GO Graphene Oxide
rGO Reduced graphene Oxide
TiO2 Titanium dioxide
MnO2 Manganese dioxide
aTiO2-rGO Amorphous TiO2-rGO
TiO2-rGO Crystalline TiO2-rGO
TTIP Titanium(IV) isopropoxide
P-XRD Powder X-ray diffraction
PTFE Polytetrafluoroethylene
PVDF Polyvinylidene fluoride
71
Abstract: Efficient and sustainable economic energy accessibility to consumers (both industrial and
domestic) is one of the major issues in the present power systems. Supercapacitor (SC) based Energy Storage
Systems (ESS) can be some of the potential solution to solve this issue because of their high specific
capacitance and power availability without deterioration for longer operation time periods. However, their
energy density compared to battery ESS is low. The energy density of SCs can be enhanced by improving
their electrode material and operating them in wider voltage windows. Electrodes materials can be improved
by using new nanocomposites or optimising the electrode formulation. Therefore, in the present thesis, the
study is composed into two sections: (1) optimizing the electrode formulation and (2) one pot synthesis of
new carbon-metal oxides nanocomposites as electrode materials. In the first section, electrode optimization
was done by studying the effect of commonly used binders such as PTFE, PVDF and PVOH in 1 M Na2SO4
and 1 M KOH aqueous electrolytes on the electrochemical performances of SCs. The study concludes by
providing an overview of suitable binder depending upon the electrochemical experimental parameters. In the
second section, various carbon-metal oxides nanocomposites (TiO2-rGO and TiO2-CNF) were synthesized by
one pot route. The present thesis briefly discusses the scalable and facile one pot sol-gel assisted synthesis
route to form amorphous TiO2-reduced graphene oxide nanocomposites (aTiO2-rGO). The electrochemical
characterization studies were also preceded between aTiO2-rGO and crystalline TiO2-rGO (TiO2-rGO)
electrode materials to understand the effect of amorphous materials on the electrochemical performances of
SCs. Based on the results and discussions, thicker electrodes of aTiO2-rGO have shown higher specific
capacitances as compared to that of TiO2-rGO due to their high BET surface area, larger pore volume and
diameter which can provide efficient ion transportation. However, the specific capacitances of these thicker
electrodes are low for present required application which can be increased by optimizing the electrode
formulation.