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Ph.D. Dissertation
Study on Electrochemically Active
Supercapacitor and Applications
Yudanga Yasun Kannangara
Department of Electrical and Computer Engineering
The Graduate School
Sungkyunkwan University
Ph.D. Dissertation
Study on Electrochemically Active
Supercapacitor and Applications
Yudanga Yasun Kannangara
Department of Electrical and Computer Engineering
The Graduate School
Sungkyunkwan University
Study on Electrochemically Active
Supercapacitor and Applications
Yudanga Yasun Kannangara
A Ph.D. Dissertation Submitted to the Department of
Electrical and Computer Engineering and
the Graduate School of Sungkyunkwan University
in partial fulfillment of the requirements for the degree of
Ph.D. in Engineering
October 2019
Approved by
Jang-Kun Song
Major Advisor
This certifies that the Ph.D. dissertation
of Yudanga Yasun Kannangara is approved.
Thesis Supervisor:
Committee Chair:
Committee Member 1
Committee Member 2
Committee Member 3
The Graduate School
Sungkyunkwan University
December 2019
Contents
List of Tables ............................................................................................................... iv
List of Figures .............................................................................................................. iv
Nomenclature ............................................................................................................... xi
Abstract ........................................................................................................................... xii
Chapter 1. ......................................................................................................................... 1
1.1
Background ........................................................................................................ 1
1.2
Objective of this study ....................................................................................... 6
1.3
Content of the Thesis ......................................................................................... 7
1.4
Reference ........................................................................................................... 8
Chapter 2 ........................................................................................................................ 10
2.1 Introduction ........................................................................................................... 10
2.1.1 Background of the active composite material .............................................. 10
2.2
Experimental .................................................................................................... 12
2.2.1 Preparation of the CuS/CuSCN composite .................................................. 12
2.2.2 Material Characterization ............................................................................. 13
2.2.3 Electrochemical evolution measurements .................................................... 13
2.2.4 Asymmetric supercapacitors ........................................................................ 14
2.3 Results ................................................................................................................... 15
2.3.1 In-situ Synthesis of CuS/CuSCN nanocomposite ........................................ 15
2.3.2 Material characterisation ................................................................................ 17
i
2.4
Conclusion ....................................................................................................... 36
2.5
Reference ......................................................................................................... 37
Chapter 3 ........................................................................................................................ 41
3.1
Introduction ...................................................................................................... 41
3.1.1 Overview of the MOF .................................................................................. 41
3.1.2 p-PDA based MOFs for supercapacitor applications ................................... 43
3.2
Experimental .................................................................................................... 44
3.2.1 Synthesis Methodologies.............................................................................. 44
3.2.2 Material Characterisation ............................................................................. 47
3.2.3 Electrochemical Characterisation ................................................................. 47
3.2.4 Fabricating Asymmetric supercapacitor (ASC) ........................................... 48
3.3
Results and Discussion .................................................................................... 50
3.4
Conclusion ....................................................................................................... 70
3.5
Reference ......................................................................................................... 71
Chapter 4 ........................................................................................................................ 76
4.1
Introduction ...................................................................................................... 76
4.1.1 Overview ...................................................................................................... 76
4.1.2 Our work ...................................................................................................... 77
4.2
Experimental .................................................................................................... 79
4.2.1 Synthesis of Zn-pPDA MOF structure ......................................................... 79
4.2.2 Material Characterization ............................................................................. 79
4.2.3 Electrochemical measurements .................................................................... 80
ii
4.3
Results and Discussion .................................................................................... 82
4.4
Conclusion ..................................................................................................... 106
4.5
Reference ....................................................................................................... 107
Chapter 5 ...................................................................................................................... 113
5.1
Introduction .................................................................................................... 113
5.1.1 Overview .................................................................................................... 113
5.1.2 Our Study ................................................................................................... 115
5.2
Experimental .................................................................................................. 115
5.2.1 Preparation of MSMA cross-linker coated cotton fabric (S-cotton) .......... 115
5.2.2 Preparation of Polyaniline cross-linked flexible electrodes (P/S-cotton) .. 115
5.2.3 Preparation of f-Supercapacitor electrodes (Ag-P/S-cotton) ..................... 116
5.2.4 Fabrication of f-Supercapacitors: ............................................................... 116
5.2.5 Characterization of materials: .................................................................... 117
5.2.6 Electrochemical performance of f-Supercapacitor:.................................... 118
5.3
Results and discussion ................................................................................... 119
5.4
Conclusion ..................................................................................................... 134
5.5
Reference ....................................................................................................... 135
Chapter 6 ...................................................................................................................... 140
iii
List of Tables
Table 3.1
The Electrochemical performance parameters and obtained values of NipPDA MOF and Mn-pPDA MOFs.
65
Table 4.1
Assignment of common ATR-FTIR spectral features to their related
functional groups of Zn-pPDA MOF materials.
87
Table 4.2
The Electrochemical performance parameters and obtained values of
supramolecular complex (Zn-pPDA MOF) electrodes.
96
List of Figures
Fig.1.1
Ragone plot of the entire energy storages and energy conversion
devices.
3
Fig 2.1
Schematic diagram of the synthesis process of CuS, CuSCN, and
CuS/CuSCN nanocomposites.
17
Fig.2.2
XRD patterns of CuS, CuSCN and CuS/CuSCN composite.
18
Fig.2.3
SEM images of (a) CuS, (b) CuSCN, and (c) the nanocomposite of
CuS/CuSCN (The bottom images are the magnified ones.)
19
Fig.2.4
Survey spectrum for XPS data for (a) CuS, nanocomposite of
CuS/CuSCN (b) 3:1, (c) 1:1, (d) 1:3 and (e) CuSCN
20
Fig.2.5
High-resolution XPS spectra for nanocomposites of ratios (a) 3: 1, (b)
1: 1, (c), 1: 3 and (d) β-CuSCN nanosheets in the N 1s region.
22
Fig.2.6
Survey spectrum for XPS data for (a) CuS, nanocomposite of
CuS/CuSCN (b) 3:1, (c) 1:1, (d) 1:3 and (e) CuSCN
23
Fig 2.7
Cyclic voltammogram of (a) pure CuS and (b) CuSCN electrodes with
different scan rates and Galvanostatic charge-discharge curve of pure
iv
25
CuS electrode (c) and CuSCN electrode (d) with different current
densities.
Fig.2.8
Comparison of pure CuS, CuSCN, and 1:1 nanocomposite with CV
curves at a scan rate of 100 mV s-1 in a 3 M KOH electrolyte, and (d)
GCD curves at a current density of 1.11 Ag-1. Comparison of
nanocomposites with (e) CV curves at a scan rate of 100 mVs-1 in a 3 M
KOH electrolyte, and (f) GCD curves at a current density of 1.11 A g-1.
26
Fig.2.9
(a–c) CV curves of 3:1, 1:1, and 3:1 nanocomposite at different scan
rates, and (d–f) GCD curves of the same materials at different current
densities.
28
Fig.2.10
(a) GCD curve of the 1:1 nanocomposite at a current density of 1.11
A/g and (b) the cyclic performance of nanocomposites at a current
density of 10.5 A/g within the potential window of 0–0.7 V (vs. Ag/
AgCl) in a 3 M KOH electrolyte. (c) Ragone plot of specific power vs.
specific energy of the CuS/CuSCN nanocomposites within the full
range of all the positive electrodes.
30
Fig.2.11
Nyquist plots for the 3:1, 1:1, and 1:3 nanocomposites from left to
right.
32
Fig.2.12
(a) CV curves for AC and CuS-CuSCN 1:1 NC electrodes at a scan rate
of 500 mV s-1. (b) CV curves for the CuS-CuSCN 1:1 NC//AC hybrid
supercapacitor at various scan rates. (c) CV curves in different potential
windows for the hybrid supercapacitor at 400 mV s-1. (d) GCD curves
for the hybrid supercapacitor device at various current densities. (e)
Power densities as a function of current densities for the hybrid
supercapacitor and a CuS-CuSCN single electrode. (f) A photograph of
two CuS-CuSCN 1:1 NC//AC hybrid supercapacitors connected to 247
LEDs.
34
Fig.3.1
chemical Structure of (a) 1,3,5-benzenetricarboxylic acid, (b)
2,3,6,7,10,11-Hexaiminotriphenylene and (C) 2,3,6,7,10,11Triphenylenehexol.
44
v
Fig.3.2
Schematic diagram of liquid-liquid interfacial reaction, synthesis of NipPDA MOF at room temperature.
47
Fig.3.3
Powder X-ray Diffraction (pXRD) patterns of synthesised Ni-MOF and
Mn-MOF with their starting materials.
53
Fig.3.4
Characterisation results of the two synthesised MOF samples (a) FTIR
spectrums, (b) survey XPS spectrums.
54
Fig.3.5
Chemical structure of the synthesised Ni and Mn MOFs. (M represent
Ni or Mn).
55
Fig.3.6
(a–b) SEM images for Ni-pPDA MOF at magnifications of (a) ×75,000
and (b) ×30,000, and (c-d) its HRTEM image. (e–f) SEM images for
Mn-pPDA MOF at (e) ×10,000 and (f) ×20,000, and (g-h) its HRTEM
image. The insets in (c) and (g) are the corresponding selected area
electron diffraction (SAED) patterns and the inverse fast Fourier
transform (IFFT) patterns.
56
Fig.3.7
(a,c) are the CV curves of Ni-pPDA and Mn-pPDA MOFs in different
scan rates, (b,d) are the GCD curves of Ni-pPDA and Mn-pPDA MOFs
in different current densities.
58
Fig.3.8
Electrochemical Impedance Spectroscopy data, (a) the Nyquist and (b)
Bode plot of Ni-pPDA, Mn-pPDA MOFs
59
Fig.3.9
(a) CV curve and (B) GCD curves of GC electrode measure in three
electrode system. These electrodes (as negative electrode) are
assembled with MOF electrodes for prepare the hybrid supercapacitors.
61
Fig.3.10
The CV curves of Ni-pPDA, Mn-pPDA MOFs and GC electrodes
measured in three electrode system at scan rate of 500 mV/s.
61
Fig.3.11
(a) and (b) are the CV curve of Ni-pPDA MOF//GC hybrid
supercapacitor in different scan rates and different potential windows,
respectively, (c) and (d) are the GCD curve of Ni-pPDA MOF//GC
hybrid supercapacitor in different current densities and different
vi
63
operated potential windows, respectively.
Fig.3.12
(a) Ragone plot of Ni-pPDA// GC hybrid device, and its rate capability
(inset) (b) corresponding relationship between square root of the scan
rate vs peak current.
64
Fig.3.13
(a) and (b) are the CV curve of Mn-pPDA MOF//GC hybrid
supercapacitor in different scan rates and different potential windows,
respectively, (c) and (d) are the GCD curve of Mn-pPDA MOF// GC
hybrid supercapacitor in different current densities and different
operated potential windows, respectively.
66
Fig.3.14
(a) Ragone plot of Mn-pPDA// GC hybrid device, and its rate capability
(inset) (b) corresponding relationship between square root of the scan
rate vs peak current.
67
Fig.3.15
The cycling stability and Coulombic efficiency of (a) Ni-pPDA//GC
and (b) Mn-pPDA//GC hybrid supercapacitors, (c) the Ragone plot of
our two assembled MOF hybrid devices comparing with the reported
values based on various substrates and electrolytes, (d) the digital
photograph of the light emitting diode (LED) powered by two NipPDA//GC hybrid supercapacitors connected as a series.
68
Fig.3.16
Ragone plot of specific power vs. specific energy of the Ni-MOF and
Mn-MOF within the ranges of Energy storages.
70
Fig.4.1
Synthesis of Zn-MOFs (Top) and Schematic illustration of the structure
of Zn-pPDA MOF (tube model) and photograph of the separated layers
in reaction vessel (Bottom).
80
Fig.4.2
FTIR spectrum of Zn-5MOF, Zn-8MOF and Zn-12MOF.
86
Fig.4.3
Powder X-ray diffractograms of pPDA, Zn(acac)2 (precursors) and ZnpPDA MOF (product).
88
Fig.4.4
XPS survey and high-resolution spectra of Zn-pPDA MOFs (a) Zn 2p,
(b) N 1s, (c) C 1s and (d) survey spectrum.
90
vii
Fig.4.5
FE-SEM images of (a, b) Zn-5MOF (d, e) Zn-8MOF and (g, h) Zn12MOF, and their EDEX spectrums (c, f, i).
91
Fig.4.6
Electrochemical performance of all Zn-5MOF, Zn-8MOF and Zn12MOF single electrodes. (a) CV curves comparing the electrochemical
properties of the positive and negative electrodes in their stable
operating voltage windows, obtained at 500 mV/s (b) GCD curves (c)
Nyquist plot with a magnified high-frequency region and (d) Bode plots
over a frequency range from 105 Hz to 1 Hz.
92
Fig.4.7
Electrochemical performance and evaluation of single electrodes for
assembling the ASC device (a) the CV curve of the GC negative
electrode (left) and Zn-pPDA MOF-5 positive electrode (right) at
various scan rated and GCD curves of individual (b) GC negative
electrode and (c) Zn-pPDA MOF-5 positive electrode.
95
Fig.4.8
Electrochemical performance, Cyclic Voltammetry and Galvanostatic
charge discharge of Zn-5MOF (a, b), Zn-8MOF (c, d) and, Zn-12MOF
(e, f).
97
Fig.4.9
Electrochemical performance of asymmetric devices. (a) CV curves of
Zn-pPDA MOF-5//GC ASC at various scan rates in a potential window
range of 1.5 to 0.0 V, (b) CV curves of Zn-pPDA MOF-5//GC ASC at
different operational potential gaps, (c) GCD curves of Zn-pPDA MOF5//GC ASC at various current densities, (d) GCD curves of Zn-pPDA
101
MOF-5//GC ASC at different operational potential gaps, (e,f) CV
curves of three as-assembled ASCs at scan rate of 400 mV/s and GCD
curves at current density of 1.0 A/g. (‘MOF-#’s in legends represent
‘Zn-pPDA MOF-#’s.
Fig.4.10
Further analysis of the three ASCs. (a) Linear fitting curves of peak
current vs. scan rate, (b) cycling stability of the first 70 cycles at
different current densities, (c) specific capacitance (S. capacitance) at
different current densities (inset: Ragone plot), and (d) cycling
performance.
102
Fig.4.11
Electrochemical performance of the hybrid device, Cyclic Voltammetry
and Galvanostatic charge discharge of Zn-5MOF//GC (a, b), Zn-
103
viii
8MOF//GC (c, d) and, Zn-12MOF//GC (e, f), respectively.
Fig.4.12
Ragone plots of our two assembled MOF hybrid devices, (a) compared
to the data reported to date. (‘MOF-#’s in legends represent ‘Zn-pPDA
MOF-#’s.) and (b) Range of various energy-storing devices.
107
Fig.5.1
Schematic illustration physical appearance and mechanical behavior of
smart fabric. (a) Schematic layered structure of the Ag-P/S-cotton with
bonding nature. (b) Digital microscopic images of the fabric convert
into Smart garment. (c) Photographs of the smart fabric under various
mechanical deformations; looped, wrapped, crumpled, bending and
twisting. (d) Schematic layered structure of the P-cotton with bonding
nature.
124
Fig.5.2
Material characterization data of the smart fabric. (a) the comparison of
the FTIR spectrum of the fabrics and deep comparison of the regions of
(b) carbonyl –C=O and (c) aliphatic C=C peaks in FTIR spectrum. (d)
comparison of the X-ray diffraction pattern of the fabric samples. (e)
BET surface area plot of the fabrics and, (f) Change in the sheet
resistance (Ω/sq) and the variation of the electrical conductivity (S/cm)
of the fabric electrodes.
128
Fig.5.3
SEM images of Pure cotton (a, b), P/S-cotton (c, d) and, Ag-P/S-cotton
(e, f).
129
Fig.5.4
Electrochemical performance of the of the negative and positive P/Scotton electrodes; CV curve of the P/S-cotton (a) negative and (b)
positive electrodes under various scan rates and, charge discharge
curves of P/S-cotton (c) negative and (d) positive electrodes under
various current densities.
130
Fig.5.5
Electrochemical performance of the of the negative and positive AgP/S-cotton electrodes; CV curve of the Ag-P/S-cotton (a) negative and
(b) positive electrodes under various scan rates and, charge discharge
curves of Ag-P/S-cotton (c) negative and (d) positive electrodes under
various current densities.
131
ix
Fig.5.6
Electrochemical performance of the f-Supercapacitor single electrode
application of the smart fabric. The comparision of (a) CV curve at scan
rate of 400 mV/s and (b) GCD curves at current density of 1 mA/cm2,
132
and the comparison of Electrochemical Impedance spectroscopic data;
(c) Nyquist plot (d) Bode plot of Ag-P/S-cotton and P/S-cotton.
Fig.5.7
Performance of the f-Supercapacitor assembled device (a) CV curves,
(b) GCD curves and (c) Ragone plots of f-Supercapacitor devices,
compared to the data reported to date (d) cycling performance.
x
134
Nomenclature
EDLC
Electric Double Layer Capacitor
REDOX
Reduction and Oxidation
CuS
Copper Sulphides
CuSCN
Copper Thiocyanate
MOF
Metal Organic Framework
pPDA
para-Phenylenediamine
FTIR
Furrier Transfer Infrared Spectroscopy
PXRD
Powder X-ray Diffraction
SEM
Scanning Electron Microscopy
EDEX
Energy-Dispersive X-Ray
TEM
Tunneling electron Microscopy
SAED
Selected Area Electron Diffraction
IFFT
Inverse Fast Fourier Transform
MSMA
3-trimethoxylsilylpropyl methacrylate
PANI
Polyaniline
AA
Ascorbic acid
KOH
Potassium Hydroxide
NF
Nickel Foam
xi
Abstract
Study on Electrochemically Active Supercapacitor and Applications
Supercapacitors have attracted great attention because of their high power density,
long life cycle and high efficiency. They can be generally classified into two types:
electrical double-layer capacitors (EDLCs) and pseudocapacitors. Compared with EDLC,
Pseudocapacitors have a higher energy density, higher specific capacitance. On the other
hand, compared with Pseudocapacitors, EDLC have a very fast charge/discharge rate,
higher power density, higher Coulombic efficiency and longer cycle life. Recently, in
order to meet the requirements of energy storage, supercapacitor development is moving
towards higher energy and higher power densities. For achieving that conditions, the
asymmetric or hybrid supercapacitors are using, due to hybrid supercapacitors have
advantages from both EDLC and pseudocapacitors. This thesis presents the synthesis,
fabrication, performance testing and optimisation novel energy material for asymmetric/
hybrid supercapacitors for provide the solution for ever-worsening energy crisis. In this
research, dual semiconductor CuS/CuSCN nanocomposite which includes both
components have redox nature was synthesised and fabricated. The redox nature of the
nanocomposite has analysed with compared to its pure starting materials. The next novel
energy material that we introduced is the new Metal Organic Framework (MOF) that
using p-phenylenediamine as organic ligand. MOFs are introduced to the energy storage
xii
research field in recently. The key limitations in developing MOF supercapacitors are
limited number of ligands and extensive laboratory protocols for synthesis. To solve these
problems, we introduced the new organic ligand (p-phenylenediamine) with facile
synthesis methodology called modified liquid-liquid interfacial reactions. On the other
hand, we develop new strategy for fabric based electrodes to produce the flexible
supercapacitor using conductive polymer and Ag metal nano particles. This also
promising for wearable energy storages.
Keywords:
Supercapacitors, Novel Material, Pseudocapacitors,
Energy Material Characterization
xiii
Chapter 1.
Introduction
1.1
Background
Most of the current energy supply is based on fossil fuels resources. However, their reserves
are limited and will be depleted in a near future. Meanwhile, the environmental pollution caused
by the consumption of fossil-fuels becomes worse with the consumption of fossil-fuels increasing.
There is a high requirement to use energy effectively and to develop renewable and clean energy
sources to enable sustainable development [1]. The fast-growing market of portable electronics
and electric vehicles stimulates the development of environmental friendly energy storage devices
with high energy and power density, such as batteries and supercapacitors [2-4].
Supercapacitors, also named electrochemical capacitors and ultracapacitors, as energy storage
devices with a high power capability, excellent reversibility (90–95% or higher), and a long cycle
life (>105 cycles) have been studied over the past few decades [5, 6]. Categorised by the working
mechanism, supercapacitors can be generally classified into two types:
(1) Electrical Double-Layer Capacitors (EDLCs), in which capacitance arises from the charge
separation at the electrode/electrolyte interface, such as carbon based supercapacitors
and,
(2) Pseudocapacitors, in which capacitance arises from reversible faradic reactions occurring
at the electrode surface, such as transition metal oxide based supercapacitors.
1
Figure 1.1 shows the Ragone plot of typical energy storage and conversion devices, in which,
their specific energy and specific power are presented [4]. It is found that batteries and fuel cells
have a low power density whereas conventional capacitors have a high power density (more than
106 W/kg), with a low energy density. Supercapacitors also have a larger energy density than
conventional capacitors [2]. The reason is that conventional capacitors store charge on low surface
area plates, but supercapacitors store charge in an electric double layer set up by ions at the
interface between a high-surface-area carbon electrode and an electrolyte [7].
Compared to batteries, supercapacitors can deliver hundreds to many thousands of times more
than the power of a similar sized battery [8]. In addition, the charge is stored in EDLCs by an
electrostatic way which is very fast and highly reversible. The charge is stored in pseudocapacitors
by highly reversible redox reactions in addition to the electric double-layer storage, and this gives
supercapacitors a much longer cycle life than batteries. However, the charge is stored by redox
reactions of the active masses in batteries which need longer time for charging/and discharging
and also limit cycle life of batteries [2, 3]. In a word, supercapacitors occupy an important position
in terms of the energy density as well as power density, and can fill in the gap between batteries
and conventional capacitors as shown in Figure 1.1 [4].
2
Power Density (W/ kg)
1M
Capacitors
100k
10k
1k
100
10
Supercapacitors
1
100m
10m 100m
Batteries
Fuel Cells
1
10
100
1k
10k 100k
Energy Density (Wh/ kg)
Fuel Cells
Figure 1.1: Ragone plot of the entire energy storages and energy conversion devices [4].
The indicated areas are rough guide lines and the direction that researchers trying to deviate their
research to high energy and power shown by the arrow.
Based on typical properties, supercapacitors are widely used in the following ways: firstly,
the largest proportion of commercial supercapacitors are used in consumer electronics, in which
they mainly serve as backup sources for memories, system boards, clocks and microcomputers;
secondly, supercapacitors are used as the main power source, such as power source for toys, failsafe positioning devices and starter applications; thirdly, supercapacitors are used as alternative
3
power sources, such as an application for solar watch and solar lanterns, road marking lanterns and
traffic warning signals. Supercapacitors are also used as a short time energy storage device in
electric vehicles, hybrid electric vehicles, fuel cell vehicles and industrial equipment, such as
emergency power supplies in hospitals or factories, airport buses, railway systems, trams and for
seaport rubber-tired gantry cranes and digital forklifts [6, 9, 10]
Batteries and supercapacitors have long been compared and contrasted by performance
criteria. Batteries offer a superior energy density and possess a higher breakdown voltage, while
supercapacitors are lighter, have more robust operating limits, possess a longer life expectancy,
and have an unparalleled power density. Supercapacitors fall somewhere between traditional
electrolytic capacitors and rechargeable batteries in lifespan, energy storage, and efficient
operating temperature. They effectively bridge the functional gap between these two technologies
and are gaining traction as we develop new ways to use their unique combination of energy
exchange and storage abilities. Pairing supercapacitors with batteries in hybrid arrays offers the
possibility to get the best of both worlds. We should expect to see supercapacitors more often in
the future [4].
Supercapacitors are particularly well matched to any application that expects frequent
charge and discharge cycles, extreme operating temperatures, or rapid discharge of high amounts
of energy. Here are some exciting applications on the horizon:
4
1. Public Transportation: Hybrid buses and other vehicles (such as small electric cars for ridesharing) can benefit from supercapacitors’ wide operating temperature. Supercapacitors may
help ensure that vehicles will work well even in the dead of winter or the dog days of summer.
Some hybrid buses already use supercapacitors to boost acceleration, and supercapacitors help
trams travel from one stop to the next, recharging at the stations.
2. Hybrid supercapacitor-battery: This arrangement would combine the supercapacitors rapid
energy intake with the battery’s long-term storage abilities, offering the best of both worlds. A
successful merging of these technologies would enhance the balance between charge time and
range. We would also see exciting possibilities to improve regenerative braking efficiency in
everything from electric cars to hybrid trains and construction equipment.
3. Extending run times: Run times may seem minor compared to the other applications. But
consider the benefits of extending the life of consumer electronics (such as laptops and mobile
devices) and stabilizing the power supply in devices that have fluctuating loads. Power tools
like electric drills have considerably shorter run times when they employ supercapacitors rather
than batteries, but you can recharge them rapidly (in about 90 seconds), making them efficient
for on-site job use.
4. Power stabilization: Supercapacitors are useful for a variety of power-stabilizing applications
like backup systems and power buffers. They provide significant cost savings in uninterruptible
power supplies when they replace electrolytic capacitors.
5
1.2
Objective of this study
Pseudocapacitors mainly show some drawbacks such as low power density and poor
cyclebility. Therefore, this situation led to the research problems based on possibility to develop
supercapacitors with high power and high energy densities [4]. In this work I aim to develop novel
material for supercapacitor applications which consist of low cost synthesis protocols. I further
aim to contribute to the development of novel fabrication routes for the formation of supercapacitor
electrodes with enhanced performance characteristics [11]. Most of synthesis protocols are
extensively consuming energy. For instance, high temperature, high vacuum, radiation of UV and
electric arcs are used in those methodologies. On the other hand, the high material cost also made
a huge impact [1, 3]. These drawbacks trigger the scientists to search low cost material which give
high power and high energy density, including low energy consuming synthesis protocols. This
thesis work depicted these points that agreed with the main objective;
•
Synthesis of novel pseudocapacitive materials which starting from low cost raw materials.
•
Introducing the novel methodologies which can easily synthesis active materials with low
energy consuming pathways.
•
Investigation of synthesised pseudocapacitive materials’ electrochemical performance,
which overcome the main drawbacks of conventional pseudocapacitive material.
6
1.3
Content of the Thesis
The rest of the thesis is organized as follows: Beginning from an introduction of the
research background of this thesis project in Chapter 1, Chapter 2 presents a literature review of
previous studies of the working principle, electrode materials, types and applications of
supercapacitors, as well as the development trends of the supercapacitors with different energy
applications, such as merging supercapacitors with the energy harvesting devices. Chapter 3
explains the experimental methodologies and the materials, including the fabrication apparatus and
chemical reagents. Chapter 4 is focusing about the dual semiconductor nanocomposite
CuS/CuSCN and their synthesis, fabrication of electrodes and electrochemical performance. The
final application of this material is asymmetric supercapacitor. In Chapter 5, the novel metal
organic frameworks based on organic ligand p-phenylenediamine are demonstrated. The novelty
and the solutions for the key problems in MOF based supercapacitors are directly addressed. IN
Chapter 6, the effect of starting materials with the different concentrations, to the electrochemical
performance are investigated. The conjugated structure of the MOFs is studied with the material
characterisation techniques. Chapter 7 is totally based on to explaining the new strategy of
synthesising flexible supercapacitor electrodes with facile low cost materials. This study is
evaluating up to smart devices by combining the different energy applications. In this chapter the
flexible device, that able to harvesting the energy and can store the energy in the same device.
Finally, the main conclusions of the study and suggestions for future work are outlined in Chapter
8 and Chapter 9.
7
1.4
1.
Reference
Zamarayeva, A.M., et al., Flexible and stretchable power sources for wearable electronics.
Science advances, 2017. 3(6): p. e1602051.
2.
Kannangara, Y.Y., U.A. Rathnayake, and J.-K. Song, Hybrid supercapacitors based on metal
organic frameworks using p-phenylenediamine building block. Chemical Engineering Journal,
2019. 361: p. 1235-1244.
3.
Kannangara, Y.Y., U.A. Rathnayake, and J.-K. Song, Redox active multi-layered Zn-pPDA
MOFs as high-performance supercapacitor electrode material. Electrochimica Acta, 2019.
297: p. 145-154.
4.
Kannangara, Y.Y., P. Prabunathan, and J.-K. Song, Facile synthesis of a hierarchical
CuS/CuSCN nanocomposite with advanced energy storage properties. New Journal of
Chemistry, 2018. 42(18): p. 15387-15396.
5.
Kötz, R. and M. Carlen, Principles and applications of electrochemical capacitors.
Electrochimica acta, 2000. 45(15-16): p. 2483-2498.
6.
Miller, J.R. and P. Simon, Electrochemical capacitors for energy management. Science, 2008.
321(5889): p. 651-652.
7.
Seo, D.H., et al., Single-step, plasma-enabled reforming of natural precursors into vertical
graphene electrodes with high areal capacitance. ACS Sustainable Chemistry & Engineering,
2015. 3(3): p. 544-551.
8.
Shin, D., et al., Constant-current regulator-based battery-supercapacitor hybrid architecture
for high-rate pulsed load applications. Journal of Power Sources, 2012. 205: p. 516-524.
9.
Shen, C., et al., Wearable woven supercapacitor fabrics with high energy density and loadbearing capability. Scientific reports, 2017. 7(1): p. 14324.
10. Dudem, B., et al., Wearable and durable triboelectric nanogenerators via polyaniline coated
cotton textiles as a movement sensor and self-powered system. Nano Energy, 2019. 55: p.
305-315.
8
11. Lerf, A., Intercalation compounds in layered host lattices: Supramolecular chemistry in
nanodimensions, in Handbook of nanostructured materials and nanotechnology. 2000,
Elsevier. p. 1-166.
9
Chapter 2
CuS/CuSCN dual redox nanocomposite for Supercapacitor
application
2.1 Introduction
Supercapacitors, combined with secondary batteries and conventional dielectric capacitors,
constitute the basic rechargeable electric storage device system to meet different demands in power
density, energy density and lifetime. Integrating pseudocapacitive materials with other active
materials is an effective strategy, in which composite materials greatly enhance the electrical
conductivity for the electrode [1]. Recently, nanostructured transition-metal sulphides such as
V2O5/ Ni3S2 [2], Ni3S2/ CdS [3], SnS2 / RGO [4], CuS/ CNT [5], Co2CuS4 [6], and Ni(OH)2 [7]
have attracted increasing attention as new electrode materials for pseudocapacitors, owing to their
high electrical conductivity and stronger redox properties compared to transition metal oxides.
Nanomaterials have been widely investigated, especially in energy storage fields [1, 7]. Thus, there
is a strong demand for a better composite material with improved conductivity and high
capacitance.
2.1.1
Background of the active composite material
Compared with these transition metal oxides, metal sulfides are abundant and cheap due to
the existence of minerals in nature [8]. CuS is one of the most promising candidates, and it is
10
characterised by its low cost, environmentally-friendly nature, metal-like electronic conductivity
(103 S cm-1), and high theoretical capacity [1].
CuSCN is a remarkable inorganic p-type semiconductor [9, 10] and has good charge
mobility and high chemical stability [11, 12]. The combination of these two compounds of the
same transition metal is not a usual approach to prepare composite materials, and nanocomposites
of CuS/CuSCN for supercapacitors applications. Here, CuS/CuSCN nanocomposite are
synthesised via an in situ ratio-tunable synthesis methodology, in which the compositional ratio of
the two compounds is arbitrarily controllable. The CuS/CuSCN nanostructure provides an ultrahigh specific capacitance by increasing the surface area of the active material exposed to the
electrolyte and increasing the conductivity. We clarify the importance of the boosted charge
conductivity for significantly enhancing the specific capacitance. The morphology, thermal
stability, and energy storage behaviour of the developed nanocomposite are characterised.
11
2.2
Experimental
2.2.1
Preparation of the CuS/CuSCN composite
All the chemicals were of analytical grade and used without further purification. We used
non-toxic Na2S2O3 as a reducing agent to prepare the CuS/CuSCN nanocomposite as presented in
Fig. 1. Initially, 100 ml of a 0.1 M aqueous CuSO4 solution was mixed with 100 ml of a 0.1 M
aqueous Na2S2O3 solution, followed by stirring for 30 min; subsequently, the colour of the solution
started to change from blue to light green. Subsequently, varying amounts of 0.1 M NH4SCN were
slowly added in a drop-wise manner for several hours, and the solution turned slightly whitish
owing to the growth of CuSCN crystals. Thereafter, the mixture was stirred for 24 h at room
temperature, and subsequently, dark green CuS crystals grew on the CuSCN crystals. In this step,
the amount of added 0.1 M NH4SCN determined the composition ratio of CuSCN. When NH4SCN
was not added, pure CuS crystals of green colour were obtained. By adding 25 ml, 50 ml, and 75
ml of NH4SCN in this step, the composition ratios of CuS: CuSCN were controlled to be 3:1, 1:1,
and 1:3, respectively. When 100 ml of NH4SCN was added, we could obtain pure CuSCN crystals.
Thus, the compositional ratio could be easily tuned by adjusting the amount of NH4SCN in this
step. Afterwards, the dispersion was filtered and washed with deionized water and subsequently
with anhydrous ethanol several times each to remove the residual reactants. The CuS/CuSCN
powder was dried in a vacuum oven at 60 oC for 6 h.
12
2.2.2
Material Characterization
The crystalline structures of the materials were determined by powder X-ray diffraction
(XRD) analysis using an automated X-ray diffractometer system (D8 Focus; Bruker, Germany)
with Cu-Kα radiation (l = 1.5418 Å) from 15o to 65o. The morphologies of the nanostructures of
the CuS nanorods, CuSCN nanosheets, and CuS/CuSCN heterostructure were analysed using field
emission scanning electron microscopy (FE-SEM, JEM-7500F; JEOL Company, Japan) and their
atomic compositions were mapped using energy-dispersive X-ray spectroscopy (EDX). The
compositions of the powder materials were analysed using X-ray photoelectron spectroscopy (XPS)
and thermo-gravimetric analysis (TGA) techniques from 30 oC to 1000 oC by supplying N2 (100
ml min-1) to ascertain the stoichiometry and chemical state of the species. The XPS data were
further processed using the Casa-XPS software (Casa Software Ltd, UK) to determine the chemical
compositions.
2.2.3
Electrochemical evolution measurements
The working electrode was fabricated by mixing the CuS/CuSCN composite and a
polytetrafluoroethylene (PTFE) binder at a mass ratio of 90:10. Subsequently, the mixture was
dispersed in ethanol and loaded into nickel foam (MTI, Korea) as a current collector using the drop
and drying method. The prepared electrode was dried at 160 oC for 2 h. A platinum electrode and
Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively, and a 3 M
KOH solution was used as the electrolyte. The redox capacitive performances of the electrodes
13
made of the CuS/CuSCN composites were investigated using a three-electrode system. CV and
GCD analyses were performed in the potential window of 0–0.7 V at varying scan rates, and EIS
was performed under the application of 10 mV AC voltage in the frequency range of 0.01 Hz to
100 kHz.
The specific capacitance, Cs (F/g), was calculated according to the following equation [1]:
𝐼
∆𝑡
𝐶𝑆 = 𝑚 × ∆𝑉
------------------ (1)
where I (A) represents the discharge current, and m (g), ΔV (V), and Δt (s) indicate the mass of the
active material, potential drop during discharge, and total discharge time, respectively. Other
important parameters of the performance of a supercapacitors are the specific energy density (ED)
in Wh kg-1 and power density (PD) in W kg-1, which were calculated as follows [1]:
𝐸𝐷 =
𝐶𝑆 × 𝑉 2
𝑃𝐷 =
7.2
--------------- (2) and,
𝐸𝐷 ×3600
𝑡
------------- (3), where V (V) is the voltage change during the discharge process
after IR drop and t (s) is the discharge time.
2.2.4
Asymmetric supercapacitors
The CuS–CuSCN 1:1//activated carbon (AC) asymmetric supercapacitor was fabricated to
analyses the capacitive behaviour of an asymmetric full cell configuration. The CuS–CuSCN 1:1
composite coated Ni foam, 3 M KOH, grade-1 Whatmann cellulose filter paper, and AC coated Ni
14
foam were used as the positive electrode, electrolyte, separator, and negative electrode,
respectively. The negative electrode was prepared by following the method in the literature [13].
For capacitance of the Asymmetric device is calculate using the same equation (Equation: 01) and,
at that situation the mass is m (g) is represented by the total mass of the active material embedded
in both negative and positive electrodes.
2.3 Results
This section, mainly discussed the material analysis data of the nanocomposite that
characterised by the advanced analytical instruments and electrochemical behaviour of the single
electrodes and the assembled device that analysed by the WiZecm 1200-premium electrochemical
workstation.
2.3.1
In-situ Synthesis of CuS/CuSCN nanocomposite
The synthesis of the nanocomposites has different stages of the formation such as
nucleation, aggregation and growth. The fully dissolved the 0.1 M CuSO4 solution, supply the
Cu2+ ions in equation (2.1) to react with the HS- ions (equation 2.2 & 2.3) under 333 K heated
round bottom flask. The solvent mixture was continuously stirred for preventing additional
aggregation and coagulation. Finally, the Cu2+ ions and HS- ions react in slightly acidic medium
and then produced the dark indigo blue colour CuS flakes (equation 2.4). Meanwhile, Na2S2O3
plays a major roll to produce both CuS and CuSCN. According to the Girish P. Pati et al, producing
CuSCN, the S2O32- ions reduce Cu2+ ions into Cu1+ ions (equation 2.5 & 2.6). The SCN- ions were
15
reacting with the Cu1+ ions produced thiosulphatocuprate (I) complex (equation 2.7) and finally
produced the white colour CuSCN precipitation (equation 2.8). the total reactions were taking 48
hours with continuous stirring at 333 K temperature. This methodology provides the composite
material enriched with dual components included nanocomposite in different ratios (3:1, 1:1, 1:3)
as well as individual components (CuS or CuSCN) at once. Therefore, this proposed methodology
has tunable properties.
CuSO4 .5H 2 O  H 2 O  Cu 2  SO42  6H 2 O
(2.1)
Na2 S 2 O3  H 2 O  H 2 S  Na2 SO4
(2.2)
H 2 S  H   HS 
(2.3)
Cu 2  H   HS   CuS  2H 
(2.4)
S 2 O32  2SO32  S
(2.5)
2Cu 2  2SO32  2Cu   S 2 O32
(2.6)
Cu   S 2 O32  CuS 2 O3 
(2.7)
CuS 2O3   SCN   CuSCN  S 2O32
(2.8)

16
Figure 2.1: Schematic diagram of the synthesis process of CuS, CuSCN, and CuS/CuSCN
nanocomposites.
2.3.2 Material characterisation
2.3.2.1 Crystallinity
The nanocomposites are well explained by the crystallite sizes from the XRD patterns
shown in Fig. 2.2. The sharp peaks of pure CuS, CuSCN, and CuS/CuSCN in the XRD patterns
clearly demonstrate their good crystallinity. The pure CuS and CuSCN crystalline patterns are
compared with the reported crystalline patterns from the database files. The crystalline peak values
of CuS and CuSCN at 2θ are consistently match with the peaks from the corresponding crystal
planes shown in JCPDS cards (card no. 06-0464 for CuS and 29-0581 for CuSCN). The sharp
XRD peaks of the CuS/CuSCN 1:1 nanocomposite shown in Fig. 2.2 indicate that the composite
17
is also perfectly crystalline. The diffraction peaks of CuS are almost the same as the combination
of peaks from CuS and CuSCN, indicating no change in the phase structure.
Figure 2.2: XRD patterns of CuS, CuSCN and CuS/CuSCN composite
2.3.2.2 Surface morphology
The Fig. 2.3 reveals the structures of CuS, CuSCN, and the nanocomposite of CuS/CuSCN.
All the samples were synthesised under the same conditions except for the concentration of
NH4SCN. On the one hand, the CuS crystals have a petal-like shape (Fig. 2.3a), and their surface
morphology is rough owing to the protruding nanorods but the core is a large lump with no pores,
as indicated in the bottom image of Fig. 2.3a. On the other hand, the fully grown micro-sized
CuSCN crystals are prism-shaped as shown in Fig. 2.3b, but the small crystals are round and flat.
18
In the case of the nanocomposite of CuS/CuSCN (Fig. 2.3c), larger flat CuSCN particles are
covered by smaller CuS nanorods. Owing to the lower concentration of NH4SCN, the size of the
CuSCN crystals is mostly smaller than that of the crystals of the neat CuSCN sample. Remarkably,
the CuS nanorods do not form a petal-shaped lump crystal, but have a distinguished rod shape.
The CuS nanorods cover the CuSCN particles, and many pores exist.
CuS
CuSCN
CuS/CuSCN
CuS
CuS
Figure 2.3: SEM images of (a) CuS, (b) CuSCN, and (c) the nanocomposite of CuS/CuSCN (The
bottom images are the magnified ones.)
19
2.3.2.3 Composition
X Axis Title
100
TG %
a
14.2 %
75
100
6.2 %
6.1 %
75
100
11.8 %
5.1 %
75
100
13.7 % 3.2 %
75
100
2.8 %
9.9 %
b
c
11 %
d
16 %
e
19.9 %
75
150
300 450 600 750
o
Temperature ( C)
900
Figure 2.4 - Survey spectrum for XPS data for (a) CuS, nanocomposite of CuS/CuSCN (b) 3:1,
(c) 1:1, (d) 1:3 and (e) CuSCN
The TGA analyses of the composites were performed to study the thermal behaviour and
compositions of their nanostructures. All the composites and CuS except CuSCN indicate that the
first weight loss starts in the temperature range of 200–320 oC, caused by the dehydration of water
content. The CuS nanorods decompose to Cu2S and Sulphur at approximately 400–450 oC (Fig.
2.4a) [14]. As shown in Fig. 2.4e, CuSCN undergoes a decomposition of approximately 20% at
433–450 oC, producing CuCN and Sulphur.
20
The second decomposition with approximately 16% of mass loss occurs in the range of
580–720 oC, which corresponds to the generation of CS2 gas [15]. The TGA curves for the three
nano-composites (CuS:CuSCN = 3:1, 1:1, and 1:3 in Fig. 2.4b-d, respectively) show multiple
decomposition drops with varying mass loss depending on the chemical structures. The mass loss
ratios corresponding to the generation of CS2 gas increase as the composition ratio of CuSCN
increases, which is consistent with the expected results. At approximately 350–500 oC, the nanocomposites (Fig. 2.4b-d) show two types of phase decompositions of the two compounds. We
believe that the first drop occurs owing to the decomposition of CuSCN into CuCN and S and the
second drop corresponds to the decomposition of CuS to Cu2S and S [14, 15]. As the composition
of CuSCN increases, the first drop also increases; this indicates that the compositional ratios of
CuS and CuSCN in the nanocomposites are well controlled as expected.
21
(a)
(b)
800
Intensity (a.u.)
Intensity (a.u.)
2000
N 1S, -SCN
400
N 1S, -NCS
1000
N 1S, -NCS
0
404
Intensity (a.u.)
3000
0
402
400
398
396
Binding Energy (eV)
394
N 1S, -SCN
2000
1000
404
N 1S, -NCS
0
404
402
400
398
396
394
Binding Energy (eV)
(d)
Intensity (a.u.)
(c)
N 1S, -SCN
6000
N 1S, -SCN
4000
2000
N 1S, -NCS
0
402
400
398
396
394
Binding Energy (eV)
404
402
400
398
396
394
Binding Energy (eV)
Figure 2.5: High-resolution XPS spectra for nanocomposites of ratios (a) 3: 1, (b) 1: 1, (c), 1: 3
and (d) β-CuSCN nanosheets in the N 1s region.
We also performed a comprehensive XPS study to confirm the compositions of the
nanocomposites. The XPS survey spectra confirm the presence of Copper, Sulphur, Carbon, and
Nitrogen, and the results are shown in Fig.2.6. The accurate quantitative and qualitative analyses
for CuSCN and its composites were performed with their N1s peaks using XPS, as shown in Fig.
2.5a-d. The N1s spectrum shows a major peak at 398.2 eV, [16] which corresponds to the N in the
nitrile form (NRC) with a small shoulder. The intensities of these peaks are proportional to the
22
amount of –SCN groups, providing the information of the amount of CuSCN. The N1s peak
Cu 3p
S 2p
S 2s
O 1s
Cu LMM
Cu 2p 1/2
Intensity (a.u.)
C 1s
CuS
N 1s
Cu 2p 3/2
intensities continue to increase and accurately match the concentration of CuSCN in the materials.
3:1
1:1
1:3
CuSCN
1000
950
400
600
200
0
Binding energy (eV)
Figure 2.6: Survey spectrum for XPS data for (a) CuS, nanocomposite of CuS/CuSCN (b) 3:1, (c)
1:1, (d) 1:3 and (e) CuSCN
Recently, it was reported that the CuS/CuSCN nanocomposite has significantly improved
conductivity compared to either CuS or CuSCN [17]. The creation of copper vacancies during the
deposition of CuS on the CuSCN particles is one of main reasons for the increase in the
conductivity. Smooth interconnection between the CuSCN and CuS particles in the
nanocomposites also reduces the resistivity [17].
23
2.3.3 Electrochemical characterisation
For the purpose of comparison of electrochemical performance, we also prepared a device
with a pure nickel metal form electrode (NF) without any active material, and another device with
a manual mixture prepared by grinding both CuS and CuSCN materials in the composition ratio
of 1:1. As shown in Fig. 2.7a, the CV curves of all the samples measured at a scan rate of 100 mV
s-1 in 3 M KOH solution confirm that the mechanism and charge storage behaviour are completely
different from those of electrical double-layer capacitors with an approximately rectangular shape.
The curves show that the supercapacitors using the CuS, CuS/CuSCN, and CuSCN electrodes are
all pseudocapacitors having redox peaks. The corresponding electrochemical reactions for the
CuS/CuSCN electrode redox pair can be explained as follows:
CuS + OHCuSCN + OH-
→ CuSOH + e→ CuSCNOH + e-
(3.9)
(3.10)
As shown in the green CV curve with a negligible CV integrated area in Fig. 2.7a, the NF
itself does not contribute capacitance. Hence, the CV curves of the other electrodes are attributed
to the capacitance contribution of the added active materials. The electrode containing either pure
CuS or pure CuSCN has a relatively low specific capacitance, as indicated by the small integrated
areas of the corresponding CV curves in Fig. 2.7a. Notably, the electrode with the manual mixture
(not composite) exhibits a higher capacitance than those with either CuS or CuSCN. Further, the
integrated area in the CV curve of the 1:1 nanocomposite cell exhibits a dramatic enhancement
24
compared to those using other materials. Although the manual mixture and 1:1 nanocomposite
have an identical composition ratio of CuS: CuSCN, the nanocomposite electrode exhibits a much
higher capacitance than the manual mixture electrode.
(a)
(b)
(c)
(d)
Figure 2.7: Cyclic voltammogram of (a) pure CuS and (b) CuSCN electrodes with different scan
rates and Galvanostatic charge-discharge curve of pure CuS electrode (c) and CuSCN electrode (d)
with different current densities.
Pure CuS has its cathodic peak (O1) and anodic peak (R1) at approximately 0.4– 0.6 V and
25
0.2– 0.3 V, respectively, and CuSCN has its cathodic (O2) and anodic peak (R2) at 0.2– 0.4 V and
0.45– 0.55 V, respectively (Fig 3.7 a and b). The GCD curves of the Pure CuS and CuSCN (Fig
3.7 c and d) has flat plateau in their charge and discharges curves corresponding to the oxidation
at cathodic peak (O1) and reduction at the anodic peak (R1).
(a)
(b)
A1
A2
B1
B2
(d)
(c)
IR
Figure 2.8: Comparison of pure CuS, CuSCN, and 1:1 nanocomposite with CV curves at a scan
rate of 100 mV s-1 in a 3 M KOH electrolyte, and (d) GCD curves at a current density of 1.11 Ag1
. Comparison of nanocomposites with (e) CV curves at a scan rate of 100 mVs-1 in a 3 M KOH
electrolyte, and (f) GCD curves at a current density of 1.11 A g-1.
26
In order to optimise the compositional ratio in terms of capacitive capability, the
electrochemical behaviour of the three nanocomposites with varying ratios are also evaluated, as
shown in Fig. 2.8. The 1:1 compositional ratio exhibits the largest integrated area compared to the
other two compositional ratios of 3:1 and 1:3 (Fig. 2.8a). Fig. 2.8d shows the GCD test of the
nanocomposites within the potential window of 0– 0.7 V, and the 1:1 nanocomposite exhibits the
longest discharging time Compared to the 3:1 and 1:3 composites. The specific capacitance of the
1:1 nanocomposite is 1787.3 Fg-1 at 1.11 Ag-1. Thus, the specific capacitance of the 1:1
nanocomposite is larger than those of the composites with different mixing ratios. This indicates
that the 1:1 nanocomposite has an optimum structure with a large specific surface area and an
excellent electrical interconnection of CuS nanorods with CuSCN nanosheets. The IR drop was
0.16 V, as shown in Fig. 2.7c, and all the other GCD curves in Fig. 2.8d and Fig. 2.9e–f shows a
similar level of IR drop. Furthermore, the GCD curves (Fig. 2.9d-f) exhibited an intercalation
pseudocapacitive behaviour in the KOH electrolyte, which has kinetics similar to a typical
pseudocapacitive behaviour [18]. The intercalation pseudocapacitive behaviour has the properties
of both pseudocapacitors and batteries, and its electrochemistry is similar to that of batteries [19].
A similar intercalation behaviour with an electrolyte has been reported in supercapacitors using
CuSbS2 and MnO2 [20].
27
(a)
(b)
(c)
(d)
(e)
(f)
Figure 2.9: (a–c) CV curves of 3:1, 1:1, and 3:1 nanocomposite at different scan rates, and (d–f)
GCD curves of the same materials at different current densities.
Fig. 2.9a–c show the CV curves of the nanocomposite electrodes measured at various scan
rates ranging from 50 mV s-1 to 500 mV s-1 in the range of 0– 0.7 V in 3 M KOH. These curves
demonstrate the high rate capability and reversibility of the CuS/CuSCN nanocomposite electrode
even at high scan rates. In particular, the CV curves of the 1:1 nanocomposite (Fig. 2.9b) have the
largest integrated area for all the scan rates than those of the other two composites (Fig. 2.9a and
c). Their electrochemical behaviour was further evaluated using GCD measurements over a wide
range of current densities. As shown in Fig. 2.2d –f, the potential was in the range of 0–0.7 V with
the current densities ranging from 1.11 to 10.5 A g-1. The multiple flat stages in the discharge part
28
of the GCD curves (Fig. 2.9d–f) of the CuS/CuSCN NCs appear due to the pseudocapacitive nature,
indicating the redox reactions of both CuS and CuSCN [21]. These discharge curves have two
independent flat stages at approximately 0.2–0.3 V and 0.45–0.55 V, representing the flat stages
of the pure CuS (Fig. 2.7c) and CuSCN curves (Fig. 2.7d), respectively. The specific capacitances
of the optimum material, i.e. the 1:1 composite, are 1787.3 F g-1, 1267.5 F g-1, 634.07 F g-1, 318.2
F g-1, 296.82 F g-1, and 259.64 F g-1 at current densities of 1.11 A g-1, 1.48 A g-1, 2.00 A g-1, 3.32 A
g-1, 6.18 A g-1, and 10.5 A g-1, respectively, as shown in Fig. 2.8e.
Reversibility and cyclic stability are essential factors for supercapacitor performance. For
further understanding, the 1:1 nanocomposite was evaluated using GCD cycles at different current
densities. As shown in Fig. 2.10a, the 1:1 nanocomposite at 1.11 A g-1 in 3 M KOH shows a perfect
cycle reversibility. The stability of the 1:1 nanocomposite at a current density of 10.5 A g-1 was
measured using GCD (Fig. 2.10b). The 1:1 nanocomposite shows a specific capacitance retention
of 93.7% after 2000 cycles, representing the exceptional cycling stability of the CuS/CuSCN
nanocomposite. In the first 70 cycles, the capacitance retention was larger than 100%, which may
be attributed to the gradual activation of the electrode [22].
29
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
50
100
Time (s)
150
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90
93.7 %
80
70
60
50
0
500
1000
1500
Number of Cycles
2000
Specific Power (W/ kg)
(b)
Capacitance retention (%)
Potential (V) vs. Ag/AgCl
(a)
1M
100k
Capasitors
10k
NCs 3:1
NCs 1:1
NCs 1:3
High Energy
& Power
1k
100
10
Supercapacitors
Batteries
1
100m
10m 100m 1
Fuel Cells
10
100
1k
10k 100k
Energy Density (Wh/ kg)
Figure 2.10: (a) GCD curve of the 1:1 nanocomposite at a current density of 1.11 A g -1, and (b)
the cyclic performance of nanocomposites at a current density of 10.5 A g-1 within the potential
window of 0–0.7 V (vs. Ag/AgCl) in a 3 M KOH electrolyte. (c) Ragone plot of specific power vs.
specific energy of the CuS/CuSCN nanocomposites within the full range of all the positive
electrodes [1, 23].
Ragone plot (power density vs. energy density) of the nanocomposites is shown in Fig.
2.10c. The Ragone plot shows that the properties of the pseudocapacitors using our
nanocomposites are positioned between those of the conventional supercapacitor and battery
applications [23]. Researchers in this field aim to achieve a higher power density and a high energy
density simultaneously, as indicated by the arrow in Fig. 2.9c. The devices using our
nanocomposites exhibit a slightly higher energy density than conventional supercapacitors and a
slightly higher power density than conventional batteries. Notably, the energy density of the
pseudocapacitor using the 1:1 nanocomposite reaches approximately 62.0 W h kg-1 at a power
30
density of 198.4 W kg-1, which is superior to that of the other CuS/CuSCN nanocomposites (40.1
W h kg-1 at a power density of 182.7 W kg-1 in the case of the 3:1 nanocomposite, and 43.3 W h
kg-1 at a power density of 182.6 W kg-1 in the case of the 1:3 nanocomposite). The high energy
density in the 1:1 nanocomposite is close to the value of a usual battery, as indicated in Fig. 3.10c,
and this may be attributed to the effect of intercalation pseudocapacitive behaviour. The 1:1
nanocomposite exhibits an outstanding power density of 1873.5 W kg-1 at a low energy density of
9.0 W h kg-1, indicating a greater power capability. According to the literature, the energy density
of the CuS nano-hollow spheres is 47.4 W h kg-1 at 0.3 W kg-1 [24] and that of the CuS
microspheres is 15.06 W h kg-1 at 392.9 W kg-1 [25]. Thus, the result obtained using the 1:1
CuS/CuSCN nanocomposite is much greater than that obtained using any of these materials
including CuS. In particular, our material is superior to other typical supercapacitor materials; for
example, the energy density of pomelo peel activated carbon is 12.8 W h kg-1 at 3854 W kg-1, [26]
that of Ni(OH)2 nanospheres is 35.7 W h kg-1 at 490 W kg-1, that of Ni(OH)2 /graphene composition
is 48 W h kg-1 at 0.23 kW kg-1, and that of RuO2/ graphene composition is 14 W h kg-1 at 21 kW
kg-1 [27].
31
NC 1:1
NC 1:3
6
5
5
5
4
4
4
3
2
1
0
RS=2
3
2
RS=3
1
Rct= 2
0 2 4 6 8 10
Z re. ()
Z im. ()
6
Z im. ()
Z im. ()
NC 3:1
6
0
4
6
8
Z re. ()
RS=3.6
2
1
Rct= 0.5 
2
3
10
0
Rct= 7.2
0
5
10
15
20
Z re. ()
Figure 2.11: Nyquist plots for the 3:1, 1:1, and 1:3 nanocomposites from left to right.
The Nyquist plot (Fig. 2.11) intercepts the X-axis at RS (solution resistance) in the highfrequency area; RS includes the sum of the electrolyte solution resistance, [28] intrinsic resistance
of the active material, and contact resistance of the electrode–electrolyte interface [29]. The values
of RS of the 3:1, 1:1, and 1:3 nanocomposites are 2 Ω, 3 Ω, and 3.6 Ω, respectively. RS gradually
increases with the increase in the CuS content of the nanocomposite from 1:3 to 3:1, which
suggests that CuS acts as the interconnection agent between the composite nanoparticles. The 3:1
nanocomposite has the lowest solution resistance among the nanocomposites. The charge transfer
resistance, Rct, is represented by the diameter of the semicircle in the high frequency region [29].
It is evident that the 1:1 nanocomposite has the lowest Rct value (0.5 Ω) as compared to the other
nanocomposites (2 Ω for 3:1 and 7.2 Ω for 1:3 composites). This indicates that the 1:1 composite
possesses a lower charge transfer resistance. The straight line in the low-frequency area appears
32
owing to the ion diffusion behaviour [28]. When the concentration of CuS is gradually increased,
the steepness of the slope of the curves increases, indicating faster ion diffusion and charge
adsorption into the electrode surface at a high concentration of CuS [30].Although the 1:1
nanocomposite has a higher RS than the 3:1 nanocomposite, Rct dominantly influences the
electrochemical performance. Hence, the 1:1 nanocomposite with the lowest Rct exhibits the
highest electrochemical performance and the highest capacitance [31]. These results reveal that
the good electrical conductivity and high ion diffusion behaviour of the 1:1 nanocomposite result
in its high performance as an active material for a supercapacitor. Even with a very low internal
resistance, the voltage drop can be significant when the current is high, as shown in Fig. 2.8c.
(a)
(b)
(c)
0.03
AC
CuS/CuSCN 1:1 NC
0.4
0.0
0.01
0.00
-0.01
-0.4
-0.02
-0.8
-0.4
0.0
0.4
-0.03
0.8
Potential (V) vs Ag/AgCl
(d)
0.0
(e)
Potential (V)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
400
800
1200
Time (s)
1600
Power density (W/ kg)
1.11 A/g
1.48 A/g
2.00 A/g
3.32 A/g
6.18 A/g
10.5 A/g
0.4
0.8
400 mv/s
300 mv/s
200 mv/s
100 mv/s
50 mv/s
1.2
1.6
Potential (V)
1.6
1.4
0.03
0.02
8.0x10
3
6.0x10
3
4.0x10
3
2.0x10
3
0.0
0
Current (A)
0.02
Current (A)
Current (A)
0.8
0.01
0.00
-0.02
-0.03
(f)
1.0 V
1.2 V
1.4 V
1.5 V
-0.01
0.0
0.4
0.8
1.2
1.6
Potential (V)
CuS-CuSCN 1:1 NC // AC
CuS-CuSCN 1:1 NC
2
4
6
8
10
Current density (A/g)
Figure 2.12: (a) CV curves for AC and CuS-CuSCN 1:1 NC electrodes at a scan rate of 500 mV
33
s-1. (b) CV curves for the CuS-CuSCN 1:1 NC//AC hybrid supercapacitor at various scan rates. (c)
CV curves in different potential windows for the hybrid supercapacitor at 400 mV s -1. (d) GCD
curves for the hybrid supercapacitor device at various current densities. (e) Power densities as a
function of current densities for the hybrid supercapacitor and a CuS–CuSCN single electrode. (f)
A photograph of two CuS–CuSCN 1:1 NC//AC hybrid supercapacitors connected to 247 LEDs.
To further validate the practicability of CuS–CuSCN nanocomposites for supercapacitor
application, we prepared an asymmetric hybrid supercapacitor using the CuS–CuSCN cathode and
an AC anode. The CuS–CuSCN 1:1 NC electrodes at 500 mV s-1 tested in a three-electrode system
are shown in Fig. 2.12a, where the AC and 1:1 NC operated at –0.8 to 0.0 V and 0.0– 0.7 V,
respectively, revealing the capability of these two electrodes. The assembled hybrid supercapacitor
can be operated at 1.5 V, as confirmed by Fig. 2.12b. Degradation of aqueous KOH and gas
generation can occur when the applied voltage exceeds 1.5 V, and so, 1.5 V was selected to
investigate the electrochemical performance of the device. Fig. 2.12c shows that the supercapacitor
can be operated from 0.0 V–1.0 V to 0.0 V–1.5 V at 400 mV s-1. The GCD curves shown in Fig.
2.12d clearly show the perfect charge/discharge mechanism. The specific capacitance calculated
from the GCD curves at different current densities showed the highest capacitance of 657.86 F g1
at a current density of 1.11 A g-1. Under other conditions, the capacitances were 538 F g-1, 298.66
F g-1, 274.5 F g-1, 273.9 F g-1 and 238 F g-1 at current densities of 1.48 A g-1, 2.0 A g-1, 3.32 A g-1,
6.18 A g-1 and 10.5 A g-1, respectively. The power density as a function of current density is
34
presented in Fig. 2.12e.
The maximum power density of 7.87 kW kg-1 was recorded for our hybrid device at a
current density of 10.5 Ag-1. It clearly shows that the power density increases by roughly 4 times
upon assembling the CuS–CuSCN 1:1 NC cathode and the AC anode, compared to the single
electrode measurement. It is known that a carbon based material is suitable for use as an anode in
a supercapacitor with a high power density [32]. In addition, the use of a hybrid device increases
the operating potential window, which is essential to expand its applicability. Using two hybrid
supercapacitors connected in series, a panel of 247 LEDs composed of 134 red LEDs (L813SRDC, Kingbright Electronics Co. Ltd, Taiwan) and 113 green LEDs (L813GD) was easily lighted up,
as shown in Fig. 2.12f. The operating voltages of the green and red LEDs are around 2.2 V and 1.8
V, respectively. The 247 LEDs were arranged in a parallel connection, and so, the minimum
operational voltage to light the LED panel is 2.2 V. The CuS/CuSCN 1:1 NC//AC hybrid
supercapacitor device has the maximum operating voltage of 1.5 V. Therefore, we connected two
hybrid supercapacitor devices in series to double the maximum operating voltage, which can easily
light the LED panel.
35
2.4
Conclusion
We introduced a dual pseudocapacitive CuS/CuSCN nanocomposite using in-situ
methodology, in which the compositional ratio between the CuS and CuSCN in the
nanocomposites could be controlled in a facile manner, and mesoporous CuS-covered-CuSCN
nanocrystals with a large surface area could be obtained. The novel hetero-nanostructure could be
characterised as CuSCN nanosheets wrapped by CuS nanorods. We used the CuS/CuSCN
nanocomposites as an active material for pseudocapacitors. Both CuS and CuSCN demonstrated
pseudocapacitive behaviour independently and their nanocomposites showed high electrochemical
performances. Consequently, the fabricated CuS/CuSCN 1:1 nanocomposite electrode exhibited a
high specific capacitance of 1787.3 F g-1 at 1.11 A g-1 and a high energy density (approximately
63 Wh kg-1) at an operating voltage of 0.7 V in 3 M KOH in the half-cell test. In the asymmetric
hybrid cell combined with an AC electrode, the specific capacitance decreased to 657.86 F g -1 at
1.11 A g-1, but the power density increased by approximately four times. The high energy density
may be partially attributed to the effect of intercalation pseudocapacitive behaviour. The superior
long GCD cycles demonstrated that it can be applied in both supercapacitors and batteries. Such a
superior capacitive behaviour is attributed to the hierarchical novel nanostructure assembled from
the CuS nanorods with the CuSCN nanosheets.
36
2.5
Reference
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40
Chapter 3
Synthesis of the novel Metal Organic Framework (MOF) using
p-Phenylenediamine organic ligand for energy storage applications.
3.1
Introduction
Metal organic frameworks (MOFs) are a variety of complex molecules that have been
developed recently and consist of metal ions bonded with organic linkers called “Ligands” [1].
These MOFs are innovative materials that widely used in the interdisciplinary field combining
molecular coordination chemistry and materials science [2, 3]. The study of MOFs is growing
rapidly because of their unique chemical and structural diversity, because they are using in a wide
range of research fields such as medicine [4], organic chemistry [5], polymer science [6], optics
[7], energy harvesting [8], and energy storage [2]. Several advantages of MOFs such as quantum
size effect, conjugated π bonds, remarkable electrolyte penetrability and low steric hindrance result
in rapid electron transfer and electrolyte diffusion; these are the main reasons for the high
popularity of MOFs in energy storage devices [2, 3, 9].
3.1.1
Overview of the MOF
Metal-organic frameworks (MOFs) are a class of crystalline materials that consist of
coordination bonds between transition-metal cations and multi-dentate organic linkers [2]. The
structure of MOFs is characterized by an open framework that can be porous (porous materials).
Therefore, MOFs are used for storage of gas/ energy, purification and separation, as well catalysis
41
and sensing applications.
3.1.1.1 Characteristics of the MOFs

High surface area: MOFs are organic-inorganic hybrid crystalline porous materials that
consist of a regular array of positively charged metal ions surrounded by organic ligand
molecules. The metal ions form nodes that bind the arms of the ligand together to form a
repeating, cage-like structure. Due to this hollow structure, MOFs have an extraordinarily large
internal surface area.

Crystallinity: MOFs are porous materials that composed of both organic and inorganic
components in a rigid periodic networked structure. This coordination of the structure provides
the crystallinity of the MOFs [10].

Electrical conductivity: Electrical conductivity most probably occurred by the π electron
conjugation of the organic ligand. Most of ligands are not includes the π electron conjugation.
Therefore, for energy storage applications, the electrical conductivity is more important. There
are few well known ligands which enriched the π electron conjugated structure, has extensively
used in energy storage applications by the researchers [2, 11, 12].
Ex: 1,3,5-benzenetricarboxylic acid,
2,3,6,7,10,11-Hexaiminotriphenylene
or
42
2,3,6,7,10,11-Triphenylenehexol.
(A)
(C)
(B)
Figure 3.1: chemical Structure of (A) 1,3,5-benzenetricarboxylic acid, (B) 2,3,6,7,10,11Hexaiminotriphenylene and (C) 2,3,6,7,10,11-Triphenylenehexol.

Chemical functionality in massive area: Most of organic ligands (tridentate, bidentate) has
a functional group at the end of the node, which is highly reactive with the metal groups and
other chemical components. Particular MOFs are showing redox reactions with alkaline
solutions [12] and other biological fluids contained amino acids and proteins.
3.1.2
p-PDA based MOFs for supercapacitor applications
In the present study, we introduce a new MOF, a p-phenylenediamine (pPDA) coordinating ligand
with Ni2+ and Mn3+ metal ions, and also apply the liquid–liquid interfacial reaction under a neutral
N2 environment [13]. This type of pPDA ligand based MOFs has introduced by the literature with
different synthesis protocol [1]. Our proposed method enables a straightforward synthesis of
nanostructure MOFs, reacting metal salts and organic ligands from two immiscible liquid phases
[3]. The liquid–liquid interfacial reaction follows the bottom-up approach for nanofabrication [14,
15] and can be an effective route for synthesising crystalline nanostructures [16]. The MOFs
43
exhibit enhanced redox behaviour from metal ion and amine groups and reasonable internal
conductivity owing to the π-conjugated structure. We fabricate hybrid supercapacitors by
assembling the Ni-pPDA and Mn-pPDA electrodes with a graphite carbon (GC) electrode, which
yields remarkable performance.
3.2
Experimental
3.2.1
Synthesis Methodologies
3.2.1.1
Limitations of Conventional Methods
Some methodologies are used extreme conditions such as high temperature, high pressure
or radiation of UV or microwave. Sometimes several methodologies need vacuumed conditions as
well [11, 12]. The most abundant methodology is conventional solvothermal method [17]. Mostly,
this solvothermal method has used high temperature around 100 – 200 oC with different solvents
such as Ammonia, DMF, THF and also aqueous solutions. Some solvothermal methods are
additionally used microwave /UV irradiation or Ultrasonic waves for react the ligand and the metal
group [17]. Selected particular methodologies used electrochemical reactions for emit the metal
ions into the ligand dissolved solution and some are used mechanical energy such as ball milling
for break the intramolecular bonds [17]. All of these methodologies used external energy for make
the bond in-between the Metal and Ligand group.
44
3.2.1.2
Limited number of Ligands
The few types of available ligand materials have been reported mainly due to their
nonconductive properties [18]. The MOFs based on benzenedicarboxylic acid have lack of
conductivity, suffering from poor cycle rate [19-21]. Hence, conductive ligand materials such as
benzenetricarboxylic acid and hexaiminotriphenylene are in high demand, and are intensively
studied as an organic unit of MOFs [22, 23]. These ligands are more abundantly used in research
studies and need of the new ligands are majorly detained.
These are the main key limitations that need to be solved in the research field and
synthesised novel MOFs and the introduced synthesis methodology in this chapter overcome those
for these key-limitations.
3.2.1.3
Liquid-Liquid Interfacial Reaction
This introduced liquid–liquid interfacial reaction is the bottom-up synthesis, which does
not used any external energy sources. Therefore, this method can be categorized into green bottom
–up synthesis methodology. Ni-pPDA MOF was synthesised using the liquid–liquid interfacial
reaction, as illustrated in Fig. 3.2. The aqueous and ethyl acetate (EtOAc) organic phases are the
two main layers used inside the reaction container under a neutral N2 environment. 0.15 g of pPDA
was dissolved in 100 ml of deionized (DI) water with 5 mM of Na2CO3 and NaBr for each. The
aqueous layer was bubbled with N2 gas for deoxygenating the solution. Then, 100 ml of ethyl
acetate layer was added into the reaction container; finally, as a source of metal ions, 0.5 g of
45
Ni(acac)2 dissolved in 20 ml of ethyl acetate was added drop-wise and very gradually into the ethyl
acetate layer without disturbing the ethyl acetate and aqueous interfacial layer. Then, the chamber
was filled with N2 to remove the oxygen. The sealed chamber was kept undisturbed for 48 h. The
synthesised MOF was dark brown and turbid in the aqueous layer. The MOF clusters were
collected using a separation flask and were washed four times using DI water and ethyl acetate.
The final product, a black powder, was dried overnight at 60 °C in an oven under an N2
environment. The procedure described above was repeated for synthesising the Mn-pPDA MOFs
by replacing Ni(acac)2 with Mn(acac)3 as a source of Mn3+ metal ions, respectively. All chemicals
were purchased from Sigma Aldrich (Germany) and used without further purification except when
mentioned specifically.
EtOAc
Organic-aqueous
layer interface
n
Aqueous
layer
Figure 3.2: Schematic diagram of liquid-liquid interfacial reaction, synthesis of Ni-pPDA MOF
at room temperature.
46
3.2.2
Material Characterisation
The crystallinity of the synthesised materials was examined using an automated powder Xray diffractometer system (PXRD, D8 Focus, Bruker, Germany) with Cu Kα radiation (λ = 1.5418
Å) from 15° to 65°. The nanoscopic morphology of the synthesised Ni-pPDA and Mn-pPDA
MOFs was analysed using field emission scanning electron microscopy (FESEM, JEM-7500F,
JEOL, Japan) and high resolution tunneling electron microscope (HRTEM, JEM-2100F, JOEL,
Japan) with Gatan digital microscope software. The X-ray photoelectron spectroscopy (XPS)
measurement system (ESCALAB 250Xi, Thermo-Scientific Company, UK) was used to ascertain
the chemical state of the MOFs, and the obtained spectroscopic data were processed using
CasaXPS software for analysing the chemical composition of the composite materials. The surface
area and pore distribution were analysed using Brunauer Emmett-Teller (BET) and Barret-JoynerHalenda (BJH) methods (BELLSORP Mini-X, UK). The Fourier transform infrared (FTIR)
analysis was performed using MCT Mid-IR (Bruker, Germany) in the range of 400 – 4000 cm−1.
3.2.3
Electrochemical Characterisation
The redox capacitive performances of the MOF electrodes were investigated using three
electrode systems. The auxiliary Pt electrode and Ag/AgCl reference electrode were used with 1M
KOH solution as an electrolyte at room temperature. Electrochemical impedance spectroscopy
(EIS) measurement was performed using an electrochemical work station, VSP (Bio Logic Science
47
instrument, France); moreover, cyclic voltammetry (CV) and galvanic charge discharge (GCD)
measurements were carried out using an electrochemical workstation, WizECM premium-1200
(WizMAC, Korea). The CV and GCD analyses were performed in the potential windows of 0.7–
0.3 V and 0.7– 0.2 V for Ni-pPDA and Mn-pPDA, respectively, with varying scan rates. EIS was
performed under 10 mV of AC voltage with the frequency range from 0.01 Hz to 100 kHz.
3.2.4
Fabricating Asymmetric supercapacitor (ASC)
The Ni-pPDA or Mn-pPDA electrode, GC electrode and Whatmann cellulose paper
(grade:1) were taken as positive electrode, negative electrode and separator, respectively. The ASC
were operated under maximum potential 1.5 V in 1 M KOH. where C− and C+ are the capacitances
(in F/g) measured at the same scan rate, using the three electrode system, for negative and positive
electrodes, respectively while ΔV+ and ΔV− denote the working potential window for the positive
and negative electrodes, respectively. The required mass ratio for positive and negative electrode
material (m+/m−) was thus estimated as 0.33 for Ni-MOF// GC device and 0.27 for Mn-MOF// GC
device. [11];
𝑚+
𝐶 ∆𝑉
− −
=
−
𝑚
𝐶 ∆𝑉
+
(3.1)
+
Where, m is the mass of the active material, Cs is the specific capacitance, and ΔV is the
voltage range for positive and negative electrodes where, CT is the total capacitance of the cell, C+
is the capacitance of the positive electrode (Ni-pPDA and Mn-pPDA MOFs, here 594 F/g and
48
421.1 F/g, respectively), C─ is the capacitance of the negative electrode (two graphite carbon
electrodes were fabricated for assemble with both Ni-MOF and Mn-MOF electrodes, here 259.5
F/g, 200 F/g, respectively.) These three ASCs devices are symbolized as Ni-pPDA//GC and MnpPDA//GC. The specific capacitance, Cdevice (F/g), was calculated using GCD curves according to
the following equation:[11, 12]
𝐼 × ∆𝑡
𝐶𝑑𝑒𝑣𝑖𝑐𝑒 =
(3.2)
𝑚𝑇𝑂𝑇𝐴𝐿 × ∆𝑉
Where, I (A) represents the discharge current, and mTOTAL (g), ΔV (V) and Δt (s) represent
as the mass of active material from both positive and negative electrode, ∆V is the operating
potential of the asymmetric device during discharge and total discharge time, correspondingly. The
energy density/ ED (Wh/ kg) and power density/ PD (W/ kg) of the asymmetric supercapacitor
device were calculated according to the following equations [11, 12];
𝐸𝐷 =
1
𝑃𝐷 =
𝐸𝐷 ×3600
2
𝐶𝑑𝑒𝑣𝑖𝑐𝑒 ∆𝑉 2
(3.3)
(3.4)
𝑡
Where V (V) is the voltage change during the discharge process and t (s) is the discharge time.
49
3.2.4.1 Theoretical capacitance calculation
The theoretical capacitance of ASCs are calculated using following equation [12];
1
𝐶𝑇
=
1
𝐶+
+
1
(3.5)
𝐶−
Where, CT is the total capacitance of the device, C+ is the capacitance of the positive electrode and
C- is the capacitance of the negative electrode.
3.3
Results and Discussion
The introduced liquid-liquid interfacial reaction method of the pPDA based MOF (Fig. 3.2)
is unique and facile, and it is also effective for obtaining MOF without supplying external energy
[13]. The p-PDA is water soluble and Metal-acetylacetonate (M-acac) is soluble in organic
solvents (hydrophobic). The interface between the hydrophilic and hydrophobic liquids was used
as immiscible reaction partners to synthesised MOFs, as shown in Fig. 3.2. Therefore, pPDA
aqueous solution and M-acac dissolved ethyl acetate solution is used as two immiscible solvents.
The MOF synthesis occurs at the interfacial layer in-between these two solvents (Fig.4.2). The
synthesised MOFs exhibit the π-conjugated polymeric nature. After the reaction for 48 h, the
aqueous phase exhibited a distinctive colour change from orange to dark brown, which indicates
the synthesis of MOF nanoparticles. This is belongs to bottom up approach synthesis methodology
[11]. This type of bottom-up approach offers several advantages such as the reduction of organic
50
waste, efficient heat transfer of the synthesised materials [13]. Using Ni(acac)2 and Mn(acac)3 the
Ni-pPDA and Mn-pPDA MOFs are synthesised, respectively.
The Powder X-ray Diffraction (PXRD) was employed to characterize the crystallinity of
obtained MOF samples (Fig. 3.3). The obtained two MOFs were assumed to be crystalline
materials and isostructural on the basis of their pXRD. Moreover, the synthesis of MOFs from
starting materials was clearly depicted the different crystalline structure of Ni and Mn based MOF,
from their starting material.
In FTIR analysis (Fig. 3.4a), the two prominent bands in 3462 and 3418 cm-1 , respectively
evidenced the primary aromatic amine [1], and the strong peaks in the range of 1600- 1500 cm-1
prove the aromatic carbon [24] can be presented regarding the scanned MOFs. The peaks at 1240
cm-1 were representing the C-N stretching [25] in pPDA ligand and especially, coordination bond
between metal and amine group, presented Ni-N stretching [26] as well as Mn-N stretching [27]
was clearly exhibited in the spectrum. The XPS survey spectrum in Fig. 3.4b, shown the elemental
analysis of the regarding MOFs, the C1s, N 1s and O1s at binding energy 288.9, 399.1 and 533 eV,
respectively, [28] are presented in all the MOFs and the two metals Ni 2p 3/2, 1/2 and Mn 2p 3/2,
1/2 at binding energy 855.3, 872.5 [29] and 642, 653.8 eV [30] on each spectrums, respectively,
confirmed the described structure in Fig 2 d. Additionally the individual peaks of Ni 2p, C 1s, N
1s from Ni-pPDA and N1s from the Mn-pPDA are provide additional support for the confirmation
of material characterization. In Fig.3.4b, the SP2 hybridized aromatic ring, C-C in aliphatic groups
from acetylacetonate anions and C-NH from the primary amines at 283.2, 284.6 and 285.9 eV
51
sequentially shown [31]. The binding energy shown at 287.6 eV can be attributed to the C=O from
carbonyl group and shakeup peaks at 289.8─299.2 eV due to aromatic ring of carbon [32]. Most
importantly the N1s peaks from both Ni-pPDA and Mn-pPDA MOFs shown the data that
supporting the bridging mechanism of Metal with p-phenelendiamine to make conjugated structure.
The coordination bond from amine to metal, at 398.7 [33] contribute the electrons bound to the
metal strongly agreed with the presented MOF structure (Fig. 3.5).
(a)
(b)
+
d =4.19 oA
+ +
+
d =3.89 oA
Ni-MOF
#
+ *+
*
+
pPDA
+
+
+
+
+
#
Intensities (a.u.)
Intensities (a.u.)
+
+ +
+
#
*#*
+
++
+
++
Mn-MOF
#
#
+
+
#
+
#
#
#
+
#
#
#
+
+
+
+
+
pPDA
+
+ +
+
+
+
++
*
+
*
Mn(acac)3
40
50
Ni(acac)2
10
* * **
*
*
20
30
40
50
60
70
10
20
30
60
70
2(degree)
2(degree)
Figure 3.3: Powder X-ray Diffraction (pXRD) patterns of synthesised Ni-MOF and Mn-MOF with
their starting materials
52
Intensity (a.u.)
(a)
Ni-pPDA MOF
Mn-pPDA MOF
 (Ni-N)
N-H stretch
asymmetric
symmetric
C-N
N-H
 (Mn-N)
N-H
4000
C-N
3000
2000
1000
Wave number (cm-1)
(b)
Intensity (a.u.)
Ni-pPDA MOF
Mn-pPDA MOF
O 1s
O KLL
Ni 2p 3/2
C 1s
N 1s
Mn 2p 3/2
Ni 2p1/3
1000
Mn 2p 1/2
800
600
400
200
0
Binding energy (eV)
Figure 3.4: Characterisation results of the two synthesised MOF samples (a) FTIR spectrums, (b)
survey XPS spectrums
53
Figure 3.5: Chemical structure of the synthesised Ni and Mn MOFs. (M represent Ni or Mn)
The synthesised MOF structure was examined by SEM, as shown in Fig. 3.6. For Ni-pPDA,
disjointed layered structure or small flat particle clusters were observed (Fig. 3.6a), which creates
a large surface area around the particles. Moreover, Ni-pPDA exhibits a porous surface owing to
the profoundly broken sheet structures, as shown in Fig. 3.6b. This improves the charge exchange
between the active material and electrolyte. The HRTEM images for Ni-pPDA particles in Fig.
3.6c and Fig. 3.6d clearly show the flat sheet-like structures. The measured interatomic spaces vary
from 4.2 Å to 3.1 Å, and these values agree well with the d-space from the XRD patterns in Fig.
3.3. The SAED and IFFT patterns in the inset of Fig. 3.6c reveal the polycrystalline phase with
4.19 Å of interline distance. The SEM images of Mn-pPDA exhibit a well-defined layered
structure, as shown in Fig. 3.6e and f. Additionally, multi-layered cakes with rough surfaces are
observed (Fig. 3.6d). The HRTEM image in Fig. 3.6g and h, also reveals the sheet-like layered
structure of Mn-MOF and the periodic structure with the interline distance varying from 3.8 Å to
4.4 Å. The layered structure is likely to permit the penetration of liquid electrolyte, which is
necessary for high specific capacitance. The SAED and IFFT patterns in the inset also confirm the
crystalline nature with 3.8 Å interline space, which accords well with the highest peak in XRD
pattern Fig. 3.6.
54
(c)
(d)
55
Figure 3.6: (a–b) SEM images for Ni-pPDA MOF at magnifications of (a) ×75,000 and (b)
×30,000, and (c-d) its HRTEM image. (e–f) SEM images for Mn-pPDA MOF at (e) ×10,000 and
(f) ×20,000, and (g-h) its HRTEM image. The insets in (c) and (g) are the corresponding selected
area electron diffraction (SAED) patterns and the inverse fast Fourier transform (IFFT) patterns.
To assessment the electrochemical performance, the as- synthesised electrodes were tested
in a three- electrode system in 1 M KOH aqueous solution. The cyclic voltammogram (CV) curves
of as synthesised two MOFs: Ni-pPDA and Mn-pPDA measured in -0.3 V ─ 0.7 V and -0.2 V ─
0.7 V potential windows, respectively within different scan rates indicate strong pseudocapacitive
behavior in Fig. 3.7 a, b respectively. Furthermore, these CV curves indicating the good rate
capability of the Ni-pPDA and Mn-pPDA MOFs as an active material. Ni-pPDA has shown similar
shape of CV curve with NiO/ RGO[34]. However, the current density has been larger than
NiO/RGO because of the high conductivity enhanced by the conjugated chemical structure of
MOFs. These similar phenomena can be seen in MnO2/RGO [35] composite. The Galvanostatic
charge-discharge (GCD) curves are carried out at various current densities (0.25- 2.00 A/g) shown
in Fig. 3.7 b and d corresponding to Ni-pPDA and Mn-pPDA, displayed plateaus between 0.0 V
─ 0.2 V (in both Fig. 3.7 b and d), which totally agreed with its CV curves, respectively. This is
further demonstrating the strong redox reactivity of both Ni-pPDA and Mn-pPDA MOFs.
56
100
80
60
40
20
(b)
500 mV/s
400 mV/s
300 mV/s
200 mV/s
100 mV/s
50 mV/s
10 mV/s
Potential (V) vs Ag/AgCl
Current density (A/g)
(a)
0
-20
-40
-60
-80
-0.2
0.0
0.2
0.4
0.6
0.8
8.0 A/g
5.0 A/g
4.0 A/g
2.5 A/g
2.2 A/g
1.5 A/g
1.0 A/g
0.6
0.4
0.2
0.0
-0.2
0
200
400
Potential (V) vs Ag/AgCl
(d)
40
20
0
500 mV/s
400 mV/s
300 mV/s
200 mV/s
100 mV/s
50 mV/s
10 mV/s
Potential (V) vs Ag/AgCl
Current density (A/g)
(c)
-20
-40
-0.2
0.0
0.2
0.4
0.6
0.8
800 1000 1200 1400
Time (s)
0.8
8.0 A/g
5.0 A/g
4.0 A/g
2.5 A/g
2.2 A/g
1.5 A/g
1.0 A/g
0.6
0.4
0.2
0.0
-0.2
0
Potential (V) vs Ag/AgCl
600
200
400
600
800
Time (s)
Figure 3.7: (a,c) are the CV curves of Ni-pPDA and Mn-pPDA MOFs in different scan rates, (b,d)
are the GCD curves of Ni-pPDA and Mn-pPDA MOFs in different current densities.
57
250
200
-50
Mn-MOF
=0.026 s
o
Ni-MOF
=0.041 s
o
-40
10
150
Z" (ohm)
Z" (ohm)
(b)
Ni-MOF
Mn-MOF
100
5
(degree)
(a)
Ni-MOF
Mn-MOF
-30
-20
Mn-MOF
Ni-MOF
-10
50
0
0
5
10
15
20
Z' (ohm)
0
0
100
200
300
400
0
500
600
0
1
2
3
4
Z'(ohm)
log f (Hz)
Figure 3.8: Electrochemical Impedance Spectroscopy data, (a) the Nyquist and (b) Bode plot of
Ni-pPDA, Mn-pPDA MOFs.
The Electrochemical impedance spectroscopy (EIS) test of single MOF electrodes in Fig
3.8a, as shown in the Nyquist plot supply the evidences about electrical conductivity of MOFs.
Specifically, a semicircle at high frequencies with diameter symbolizes charge transfer resistance
[36] shown 0.9 Ω and 1.1 Ω, corresponding to the Ni-pPDA and Mn-pPDA respectively indicate
low charge transfer resistance (Rct) due to conjugation of the MOFs chemical structure. The low
values of equivalent series resistance (Rs) in high-frequency region sequentially shown as 3.1 and
6.4 Ω in Ni-pPDA and Mn-pPDA, denotes high electrochemical conducting properties in between
active materials and electrolyte. Besides, the exposure of numerous active sites of MOFs connected
each other by conjugated chemical structure, enhance the conductivity overall in the electrode,
presence of aqueous KOH electrolyte. Fig. 3.8b, depicted the Bode plot of the Ni and Mn MOFs,
represent that the Mn-MOF has higher relaxation time constant (τo). This is measured using the
58
5
following equation τo= 1/fo, where τo is relation time constant and fo is the peak frequency value at
highest phase angle (ɸ). According to Fig. 3.8b, the highest relaxation time constants have been
reported by the Mn-MOF (information available in Table 1 in below). It is well known that power
delivery and better responsiveness has improved corresponds to lower τo values [37, 38]. This has
been proved by the Mn-pPDA MOF given higher power density, 6 kW/ kg than power density,
5.98 kW/ kg of Ni-pPDA MOF.
59
(b)
40
20
50 mV/s
100 mV/s
200 mV/s
0
-20
300 mV/s
400 mV/s
500 mV/s
-40
-1.2
-1.0
-0.8
-0.6
-0.4
Potential (V) vs Ag/AgCl
Current density (A/g)
(a)
8.0 A/g
5.0 A/g
4.0 A/g
2.5 A/g
2.2 A/g
1.5 A/g
1.0 A/g
-0.2
-0.4
-0.6
-0.8
-1.0
0
-0.2
100
200
300
400
500
Time (s)
Potential (V) vs Ag/AgCl
Figure 3.9: (a) CV curve and (B) GCD curves of GC electrode measure in three electrode system.
These electrodes (as negative electrode) are assembled with MOF electrodes for prepare the hybrid
supercapacitors.
Current density (A/g)
100
Ni-pPDA
@500 mV/s
Mn-pPDA
GC electrode
80
60
40
20
0
-20
-40
-60
-80
-1.2
-0.8
-0.4
0.0
0.4
0.8
Potential (V) vs Ag/AgCl
Figure 3.10: The CV curves of Ni-pPDA, Mn-pPDA MOFs and GC electrodes measured in three
electrode system at scan rate of 500 mV/s.
60
A hybrid supercapacitor was fabricated by using Ni-pPDA and Mn-pPDA MOFs as
positive electrodes, which assembled with GC negative electrodes (Fig. 3.9), denoted as NipPDA//GC and Mn-pPDA//GC devices, respectively. Both of pseudocapacitive and EDLC
performance in practical application of these two devices has been observed. The CV curves of
GC and synthesised two MOFs (Fig. 3.10) has shown their operational potential windows in three
electrode system. This is establishing the feasibility of these two MOFs can assemble with GC
electrodes to make Hybrid supercapacitor in 1.5 V of operational potential. We observed 1.5 V is
the optimum potential window for operate these two hybrid devices without degradation of
aqueous 1M KOH electrolyte.
Fig. 3.11a shows CV curves of the Ni-pPDA// GC hybrid device, operated in scan rate 10
─ 400 mV/s, has a good rate capability by exhibiting similar shapes approximately around all scan
rates. Furthermore, this Ni-pPDA// GC hybrid device was measured with various working
potential windows from 0.0 ─ 1.0 V to 0.0 ─ 1.5 V at scan rate 400 mV/s reveal a stable potential
window up to 1.5 V in Fig. 3.11b. The typical GCD curves of optimal potential window (1.5 V) in
Fig. 3.11c, has been measured under various current densities (A/g) exhibited the symmetric
charge-discharge curves, attributed to the good capacitive properties. The specific capacitance
calculation done by using equation 3.2 in the GCD curves (Fig. 3.11c) were 184.7, 107.5, 90.9,
83.3, 66.6, 40 and 6.9 F/g at current densities of 1.0, 1.5, 2.2, 2.5, 4.0, 5.0 and 8.0 A/g, respectively,
based on total mass of active materials. The Fig. 3.11d shows the GCD curves which operated in
different potential gaps under 1.0 A/g current density to demonstrate the maximum stable potential
61
window that can be operated in 1 M KOH aqueous electrolyte. These GCD curves (Fig. 3.11c)
were agreed consistently with the described CV curves (Fig 3.11a).
(a)
(b)
Potential (V)
Current (A)
0.01
0.00
-0.01
100 mV/s
50 mV/s
10 mV/s
-0.02
(c)
1.0 V
1.2 V
1.3 V
1.4 V
1.5 V
0.02
400 mV/s
300 mV/s
200 mV/s
0.01
0.00
-0.01
@ 400 mV/s
-0.02
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential (V)
Potential (V)
1.6
8.0 A/g
5.0 A/g
4.0 A/g
2.5 A/g
2.2 A/g
1.5 A/g
1.0 A/g
1.2
0.8
0.4
(d)
1.6
0.8
1.0 V
1.2 V
1.3 V
1.4 V
1.5 V
0.6
@1.0 A/g
1.4
Potential (V)
Current (A)
0.02
1.2
1.0
0.4
0.2
0.0
0
100
200
300
400
500
0.0
600
0
Time (s)
100
200
300
400
500
600
Time (s)
Figure 3.11: (a) and (b) are the CV curve of Ni-pPDA MOF // GC hybrid supercapacitor in
different scan rates and different potential windows, respectively, (c) and (d) are the GCD curve
of Ni-pPDA MOF//GC hybrid supercapacitor in different current densities and different operated
potential windows, respectively.
62
The Ragone plot (Fig. 3.12a) shows the gravimetric energy density and power density
values of the Ni-pPDA//GC hybrid supercapacitor device reported the maximum value of energy
density is 57.5 Wh/ kg and power density is 5.98 kW/ kg. These energy densities and power
densities were calculated using equation 3.3 and 3.4. The specific capacitance of the Ni based
MOF hybrid device were measured using equation 3.2, as a function of current densities, shown
in inset of Fig 3.12a, the obtained highest value of specific capacitance of Ni-pPDA//GC hybrid
device is 184.66 F/g at current density 1.0 A/g. In Fig. 3.12b, the linear relationship between redox
peak current and square root of scan rate, indicate dominant pseudocapacitive behaviour and
intercalation reactions of active material especially from the positive electrode and capacitive
behaviour from the entire device.
(b)
100
Ni-pPDA // GC
10
1
0.1
Peak current (A)
0.02
240
Capacitance (F/g)
Energy density (Wh/kg)
(a)
Ni-pPDA
200
160
120
80
40
0
0
0.01
100
1
2
3
4
5
6
7
8
0.01
Anodic scan
Cathodic scan
0.00
-0.01
9
Current density (A/g)
1000
10000
-0.02
4
8
12
16
20
(Scan rate)1/2 (mV/s)1/2
Power density (W/kg)
Figure 3.12: (a) Ragone plot of Ni-pPDA// GC hybrid device, and its rate capability (inset) (b)
corresponding relationship between square root of the scan rate vs peak current.
63
Tested Parameters
Ni-MOF
Mn-MOF
RCT (Charge transfer resistance)
0.9 Ω
1.1 Ω
RS (solution resistance)
3.1 Ω
6.4 Ω
Relaxation time constant (τo)
0.041 s
0.026 s
Capacitance of ASC (from GCD curves)*
184.7 F/g
109.3 F/g
Capacitance of ASC (Theoretical calculation)*
180.6 F/g
135.5 F/g
Energy density (ASC device)*
57.5 Wh/kg
34.16 Wh/kg
Power density (ASC device)*
5981.33 W/kg
6000.01 W/kg
90.5 %
103.1 %
Capacitive retention (under 2000 cycles)*
 The measurements were carried out on asymmetric supercapacitor device (ASC).
Table 3.1: The Electrochemical performance parameters and obtained values of Ni-pPDA MOF
and Mn-pPDA MOFs.
Theoretically, the Ni-MOF// GC and Mn-MOF//GC devices are achieved 180.6 F/g and
135.5 F/g. The originally the specific capacitance calculate by using GCD curves (equation 3.2)
obtained 184 .7 F/g and 109.3 F/g for Ni-pPDA// GC and Mn-pPDA// GC Hybrid supercapacitors.
This indicate that, our study has high accuracy.
64
(a)
(b)
0.06
0.02
0.00
-0.02
-0.04
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.00
-0.02
@ 400 mV/s
0.0
1.6
Potential (V)
(d)
1.6
1.0 A/g
1.5 A/g
2.2 A/g
2.5 A/g
4.0 A/g
5.0 A/g
8.0 A/g
0.8
0.2
0.4
0.6
0.8
1.0
1.2
1.6
Time (s)
0.8
1.5 V
1.4 V
1.3 V
1.2 V
1.0 V
0.6
@1.0 A/g
1.2
0.4
1.4
1.6
1.4
Potential (V)
1.2
Potential (V)
0.02
-0.04
0.0
(c)
1.5 V
1.4 V
1.3 V
1.2 V
1.0 V
0.04
Potential (V)
0.04
Current (A)
0.06
400 mv/s
300 mv/s
200 mv/s
100 mv/s
50 mv/s
10 mv/s
1.0
0.4
0.2
0.0
0.0
0
50
100
150
200
250
300
350
0
Time (s)
50
100
150
200
250
300
350
Time (s)
Figure 3.13: (a) and (b) are the CV curve of Mn-pPDA MOF// GC hybrid supercapacitor in
different scan rates and different potential windows, respectively, (c) and (d) are the GCD curve
of Mn-pPDA MOF// GC hybrid supercapacitor in different current densities and different operated
potential windows, respectively.
65
The Mn based MOF assembled hybrid device (Mn-pPDA// GC) has good rate capability,
indicating by (Fig. 3.13a) similar shapes of CV curves with 10─400 mV/s, operated under potential
window 1.5 V. Same as Ni based MOF were tested, the Mn-pPDA// GC hybrid device was
measured with similar different working potential windows at scan rate 400 mV/s in Fig. 3.13b
and current density 1.0 A/g in Fig. 3.13d, that can be operated in 1M KOH aqueous electrolyte,
also revealed 1.5 V as a maximum consistent potential window. Depend on this obtained value,
the GCD curves also been measured within various current densities at Fig. 3.13c.
(b)
Peak current (A)
100
Mn-pPDA // AC
10
120
1
0.1
Capacitance (F/g)
Energy density (Wh/kg)
(a)
Mn-pPDA
100
80
60
40
20
0.02
Anodic scan
Cathodic scan
0.00
-0.02
0
0
0.01
100
0.04
1
2
3
4
5
6
7
8
9
Current density (A/g)
1000
10000
Power density (W/kg)
0
4
8
12
16
20
Scan rate1/2 (mV/s)1/2
Figure 3.14: (a) Ragone plot of Mn-pPDA// GC hybrid device, and its rate capability (inset) (b)
corresponding relationship between square root of the scan rate vs peak current.
From the Ragone plot shown in Fig. 3.14a, the maximum energy and power densities of
the Mn-pPDA//GC device were obtained as 34.2 Wh/kg and 6kW/kg, respectively. The inset of
Fig. 3.14a exhibits the rate capability, i.e., the specific capacitance as a function of the current
66
densities; the maximum specific capacitance was 109.3 F/g at the current density of 1.0 A/g. The
Mn-MOF device also exhibits the dominant pseudocapacitive property as shown in Fig. 3.14b.
100
120
Ni-pPDA MOF// GC
80
100
60
80
40
60
20
0
0
500
40
2000
1500
100
140
100
120
Mn-pPDA MOF// GC
80
100
60
80
40
60
20
0
0
Cycle number
(c)
Energy density (Wh/kg)
1000
120
(d)
500
1000
1500
40
2000
Coulombic efficiency (%)
140
Retention (%)
(b)
120
Coulombic efficiency (%)
Retention (%)
(a)
Cycle number
This work Mn-MOF// GC
This work Ni-MOF// GC
Ref. 51
Ref. 48
Ref. 14
Ref. 19
Ref. 50
Ref. 49
Ref. 47
Ref. 46
Ref. 45
10
1
1
10
100
1000
10000
Power density (W/kg)
Figure 3.15: The cycling stability and Coulombic efficiency of (a) Ni-pPDA//GC and (b) MnpPDA//GC hybrid supercapacitors, (c) the Ragone plot of our two assembled MOF hybrid devices
comparing with the reported values based on various substrates and electrolytes, (d) the digital
photograph of the light emitting diode (LED) powered by two Ni-pPDA//GC hybrid
supercapacitors connected as a series.
67
The stability of the two hybrid devices can be measured by continuous cycling at high
current density[39] and Coulombic efficiency can provide clear picture of evaluation and
chemistry of electrode active material [40]. The capacitive retention and Coulombic efficiency was
monitored around 2000 cycles in of Ni-pPDA//GC and Mn-pPDA//GC hybrid devices at current
density (8.0 A/g), clearly depicted on Fig. 3.15 a, and b, respectively. The Ni based MOF hybrid
device was shown (Fig. 3.15a) high capacitance, 10 % of capacitance fade in 2000 cycles, signified
that excellent stability of the active material in Ni-pPDA//GC hybrid device. The Coulombic
efficiency was increased up to 100 % during the 2000 cycles, indicate the reversibility of the redox
reactions has been improved and stabilizing. The Mn-pPDA//GC hybrid device (Fig. 3.15b)
exhibited the retention value was increased beyond the 100 % value at 2000 cycles, indicating the
entire device was still activating with the 1M KOH electrolyte. Even though, its coulombic
efficiency was decrease from 98 % to 93 %, the overall stability and reliability about the active
material Mn-MOF was in higher range. When consider the Ni-pPDA and Mn-pPDA MOFs, NiMOFs has shown superior electrochemical performance as an active material (table 3.1).
To put the results in a perspective with current technologies, we characterized number of
supercapacitors publications studied on Carbone materials, conductive polymers and their
composites with our results, are presented in the same Ragone plot (Fig. 3.15c). Our Ni-pPDA//GC
hybrid capacitor shown energy density 57.5 Wh/ kg at 0.75 kW/kg and maximum power 5.98 kW/
kg at energy density 6.93 Wh/ kg. And also Mn-pPDA//GC hybrid capacitor shown energy density
34.2 Wh/ kg at 0.75 kW/kg and maximum power 6.0 kW/ kg at energy density 13.3 Wh/ kg. The
68
asymmetric supercapacitors has shown its maximum power and energy densities such as
Graphene/ CNT/
and energy densities, respectively, reported in literature. The digital
photograph (Fig. 3.15d) were provided evidence for the real life application to test these assembled
MOF devices, powered one green LED (1.8 V) by using two Ni-pPDA// GC hybrid devices
connected in a series. The overall electrochemical performance exhibited in Table 3.1. This results
reveals the potential application of these MOF hybrid supercapacitors in the real life as energy
Power Density (W/ kg)
storage devices.
1M
Capacitors
100k
10k
1k
100
10
Supercapacitors
1
Batteries
100m
10m 100m
Fuel Cells
1
10
100
1k
10k
100k
Energy Density (Wh/ kg)
Figure 3.16: Ragone plot of specific power vs. specific energy of the Ni-MOF and Mn-MOF
within the ranges of Energy storages.
69
The Ragone plot shows that the properties of the pseudocapacitors using our Ni-pPDA and
Mn-pPDA MOFs are positioned between those of the conventional supercapacitor and battery
applications (Fig. 3.16). The energy density and power density were calculated using equation 3.3
and 3.4. The devices using our two MOFs exhibit a slightly higher energy density than
conventional supercapacitors and a slightly higher power density than conventional batteries.
3.4
Conclusion
In summary, we demonstrated that the novel layered MOF synthesised using p-
phenelendiamine, has analysed entirely as Ni-pPDA and Mn-pPDA are promising as an active
material for pseudocapacitor. Two asymmetric hybrid devices made with neat Ni-pPDA and MnpPDA taken as positive electrode, which combine with GC electrode. The active mass normalized
capacitance was obtained in Ni-pPDA//GC and Mn-pPDA//GC hybrid devices are 184.7 F/g and
109.3 F/g, sequentially. Furthermore, higher comparision of these synthesised new MOF powder
gives increase to maximum power density 5.98 kW/ kg, 6.0 kW/ kg and maximum energy density
57.5 Wh/ kg, 34.2 Wh/ kg in Ni-pPDA//GC, Mn-pPDA//GC hybrid devices, respectively, that
ascertain some of the best high energy and power capacitive materials. To our knowledge, this is
the first time to use p-phenelendiamine organic ligand based MOF in supercapacitor based
applications. This work thus added as a new member in to family of MOFs in energy storages.
70
3.5
1.
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75
Chapter 4
Supramolecular multilayer Zn-pPDA MOFs for Supercapacitor
application
4.1
Introduction
4.1.1
Overview
Metal organic frameworks (MOFs) are newly developed complex molecule consisting of
metal ions bond with organic linkers [1, 2]. It has exponential growth and popularity in large
variety of research fields such as medical field [3], organic chemistry [4], polymer science [5],
optics [6], energy harvesting [7] and energy storage material [8], because of their unique chemical
and structural diversity. To imitate such popular energy storage applications; MOFs supercapacitor
material is introduced [8, 9]. The MOFs in supercapacitors, mainly shown both EDLC [8, 10] and
redox [11] behavior. Each of these classes have their strengths and weakness: carbon based
material, which shown EDLC behavior, has long life cycles with high power but low capacitance,
and the redox material have high capacitance but low life cycle [12]. Therefore, these MOF type
compounds are mainly used as a composites with polymers[13, 14], metal oxides[15] and other
porous carbon materials like graphene [16, 17], polysaccharides [18] and CNT [15, 19]. However,
there are few organic linkers extensively used in MOFs, such as Benzenedicarboxylic acid (BDC),
Benzenetricarboxylic acid (BTC) and 2,3,6,7,10,11-hexaminotriphenylene (HITP) [8]. According
to the reports, some of the MOFs with these organic ligands has lack of conductivity and poor
76
cycle rate. [20-23]
4.1.2
Our work
In the present study, we synthesised new variety of MOF using bottom up approach [24],
can be integrated into supercapacitor devices to uncover their long life cycle and high performance
(high capacitance and high power); desirable features in supercapacitor studies. As bottom-up
approach, the liquid-liquid interfacial reaction method [25], attributed to easily synthesis of the
crystalline 2D multilayered nanostructures regardless the crystallinity of starting /bulk materials
[26], are used for synthesis the MOF. The metal triad salt: Zn(acac)2 and organic ligand:
p─phenelendiamine (pPDA) has used as the precursors. Facile, can be deployed in small space
and no oblige extensive reaction setups are the main advantages of this synthesis methodology.
Interestingly this MOF is carbon based material which shown redox behavior. Therefore, the ZnpPDA (the present MOF) has shown both advantages of EDLC and redox material: high
capacitance and high energy and power density with long life cycles. No investigation of pPDA
ligand based crystalline MOFs as energy storage material has been reported up to now, but
synthesis and characterization at room temperature of hybrid material using few metals with pphenelendiamine has been reported [1]. Further evidences, we assembled hybrid supercapacitor
device with synthesised Zn-pPDA MOF and graphite carbon (GC) as positive and negative
electrode. Herein, the excellent performance: high capacitance and long life cycles has shown in
as-assembled hybrid supercapacitor device and provided the momentous information establish the
novelty and importance of the new developed MOF as capacitive material. All these excellent
77
results proved that the Zn-pPDA MOFs are promising and innovative materials for high
performance supercapacitor.
Zn(acac)2 5,8 and 12 g/dm3
in EtOAc
p-Phenylenediamine
in di-water
H
C
N
Zn(acac)2 in EtOAc
Zn
Zn-MOF
pPDA (aq)
Figure 4.1: Synthesis of Zn-MOFs (Top) and Schematic illustration of the structure of Zn-pPDA
MOF (tube model) and photograph of the separated layers in reaction vessel (Bottom)
78
4.2
Experimental
4.2.1
Synthesis of Zn-pPDA MOF structure
The p-phenelendiamine (pPDA) 0.15 g was dissolved in 100 ml of deionized water and
transferred into bottom of the reaction vessel. The 100 ml of Zn(acac)2 5 g/dm3, 8 g/dm3 and 12
g/dm3 in ethyl acetate solutions were prepared and each of Zn(acac)2 solvents were gently added
on the top of the pPDA aqueous phase in three separate trials such that it covered approximately
half of the reaction vessel. The reaction vessel was filled with the neutral gas (N2) and sealed and
kept 48 h to settle the solvent layers allow slow release of Zn(acac)2 in the ethyl acetate layer into
interfacial area and react with pPDA (Fig. 4.1). After synthesised the Zn-MOF, the colour of
aqueous layer was turn from light orange to black and yield was given the turbid appearance. The
yield has been filtered and washed with DI water several times. After washing, the obtained ZnMOF was air dried in N2 environment. The synthesised MOFs has been denoted as Zn-5MOF, Zn8MOF and Zn-12MOF corresponding to their starting solution of 5 g/dm3, 8 g/dm3 and 12 g/dm3
of Zn(acac)2, respectively.
4.2.2
Material Characterization
The Crystallinity of the materials were examined by powder X-ray diffraction (PXRD)
analysis on D8 Focus (Bruker, Germany) automated X-ray diff ractometer system with Cu-Kα
radiation (λ = 1.5418 Å) from 15o to 65o. The morphology of the nanostructures of the synthesised
Ni-pPDA and Mn-pPDA MOFs were analysed using the field emission scanning electron
79
microscopy (FESEM, JEOL JEM-7500F, Japan). The X-ray photoelectron spectroscopy-XPS
measurements (Thermo-Scientific, UK -Model: ESCALAB 250Xi) were to ascertain the chemical
state of the MOFs and the obtained spectroscopic data were processed using CasaXPS software
for understand the chemical composition of different composite materials. The organic compounds
functional analysis has proceeded with FTIR analysis on MCT Mid-IR (Bruker, Germany) at range
of 400─ 4000 cm-1.
4.2.3
Electrochemical measurements
The electrochemical performance of pure Zn-MOFs was investigated with 1 M KOH
solution. To evaluate the electrochemical property, the three-electrode system used as synthesised
material on NF, Pt wire and Ag/AgCl electrode represents: working, counter and reference
electrodes. CV and GCD measurements were collected using electrochemical workstation,
WizECM premium-1200 (S. Korea) and EIS measurements was measured using electrochemical
work station (Bio Logic Science instrument-VSP). As-synthesised black coloured MOF powder
was grinding with Carbon black and nafion binder according to mass ratio of 85: 10: 5. The mixture
of powder was sonicated with ethanol in ultrasonic bath (40 kHz) in 15 min to keep the dispersion.
After sonication the dispersed solution was casted on Nickel foam (MTI, Korea) as a current
collector using drop and dried method. The Cyclic voltammetry (CV) and galvanic charge
discharge (GCD) analysis was performed in 0.7 V─ (-0.3) V potential window for Zn-pPDA with
different scan rates and different current densities, respectively. The electrochemical impedance
spectroscopy (EIS) was performed by under 10 mV of AC voltage with the frequency range from
80
0.01Hz to 100kHz. The GC electrode was prepared as explained in the literature.[27]
4.2.3.1 Fabricating Asymmetric supercapacitor (ASC)
The Zn-pPDA MOF electrode, GC electrode and Whatmann cellulose paper (grade:1) were
taken as positive electrode, negative electrode and separator, respectively. The ASC were operated
under maximum potential 1.5 V in 1 M KOH. The optimal mass ratio tuned between positive and
negative electrodes (m+ / m─) is 0.31 given the highest specific capacitance and best performance,
which calculate by the following equation[28]:
𝑚+
𝑚−
=
𝐶𝑠− ∆𝑉−
(4.1)
𝐶𝑆+ ∆𝑉+
Where, m is the mass of the active material, Cs is the specific capacitance, and ΔV is the
voltage range for positive and negative electrodes. Freshly prepared Zn-5MOF, Zn-8MOF and Zn12MOF positive electrodes (represented commonly as Zn-pPDA MOFs) were has equal active
mass and they were assembled with three GC identical electrodes for ASCs. These three ASCs
devices are symbolized as Zn-5MOF//GC, Zn-8MOF//GC and Zn-12MOF//GC. The specific
capacitance, Cdevice (F/g), was calculated using GCD curves according to the following
equation:[28, 29]
𝐶𝑑𝑒𝑣𝑖𝑐𝑒 =
𝐼 × ∆𝑡
(4.2)
𝑚𝑇𝑂𝑇𝐴𝐿 × ∆𝑉
Where, I (A) represents the discharge current, and mTOTAL (g), ΔV (V) and Δt (s) represent
as the mass of active material from both positive and negative electrode, ∆V is the operating
81
potential of the asymmetric device during discharge and total discharge time, correspondingly. The
energy density/ ED (Wh/ kg) and power density/ PD (W/ kg) of the asymmetric supercapacitor
device were calculated according to the following equations:[28, 30]
𝐸𝐷 =
1
𝑃𝐷 =
𝐸𝐷 ×3600
2
𝐶𝑑𝑒𝑣𝑖𝑐𝑒 ∆𝑉 2
(4.3)
(4.4)
𝑡
Where V (V) is the voltage change during the discharge process and t (s) is the discharge time.
4.2.3.2 Theoretical capacitance calculation
The theoretical capacitance of ASCs are calculated using following equation [31];
1
𝐶𝑇
=
1
𝐶+
+
1
(4.5)
𝐶−
where CT is the total capacitance of the device, C+ is the capacitance of the positive electrode and
C- is the capacitance of the negative electrode. The Theoretically and practically obtained specific
capacitance (CT and Cdevice) were compared for find the clear idea of the accuracy of the study.
4.3
Results and Discussion
The synthesis of the Zn-pPDA MOF has been unique and facile as well as very effective
for obtained the nanostructures [25]. The interface of the non-miscible aqueous and organic solvent
phases became dark black/purple colour after 48 h, because of the yield of MOFs. These bottomup approaches offer multiple advantages such as reduce organic wastes and high efficiency of heat
transferring [25], quite facile for synthesis 2D like layered structures [32], and high possibility to
82
synthesis the crystalline end-product with less defects done by oxidizing [33].
In FTIR analysis (Fig. 4.2), all the Zn based MOFs are shown asymmetric and symmetric
N-H vibrations as two prominent bands between 3500 and 3300 cm-1, clearly evidenced the
primary aromatic amine [1]. However, with increasing the concentration of Zn(acac)2 (from 5
g/dm3 to 12 g/dm3) the peaks around the 3600-3200 cm-1 has lose their sharp nature and getting
broad due to O─H stretch from embedded water molecules [34] in the Zn(acac)2/ Zn triad salts.
The very weak signal of C─H stretch (from alkene group in acetylacetonate anion) at 3000 cm -1
[35] in Zn-5MOF evidenced the small amount of
Zn(acac)2 in MOF conjugated structure, and
intensity of the C─H peak was gradually increased with increment of Zn(acac)2 concentration
indicated that, there are free Zn(acac)2 particles located in the complex conjugated polymer
structure. At the fingerprint region the aliphatic C=C stretch at 1685- 1605 cm-1 [36] also shown
the peak intensity increment with the increasing of the concentration of Zn(acac)2. The medium
peaks in the range of 1600- 1400 cm-1 prove the aromatic carbon [37] from pPDA ligands and
peaks at 1240 cm-1 were representing the C─N stretching [13] in pPDA ligand and especially,
coordination bond between metal and amine group, presented Zn─N stretching [38]. The C─O
stretching at 1005 cm-1 [39] also increased with the increment of Zn(acac)2 concentration,
demonstrated presence of the free Zn(acac)2. But bending vibration from the 830 cm-1 assign to
out of the plane folding of aromatic C-H group [40] exhibited the similar transmittance intensity
in all three Zn-pPDA MOFs because of its synthesised amount was mostly similar.
83
Zn-5MOF
Zn-8MOF
Zn-12MOF
3420
3330
1230
1275
1606
Transmitance
1515
3000
3200
520
1606
2970
3500
3000
1515
1515
1625
4000
830
520
463
2500
2000
1005
1400
1500
1000
500
-1
wavenumber (cm )
Figure 4.2: FTIR spectrum of Zn-5MOF, Zn-8MOF and Zn-12MOF
Overall, with the increment of Zn triad salt concentration the signals generating from the
aliphatic carbon from acetylacetonate/ triad salt anion has been increased, provided the clear idea
about the composition of Zn-5MOF, Zn-8MOF and Zn-12MOF, respectively. Interestingly, in Zn5MOF, we have seen the signals generate from aliphatic stretching was very weak and low,
confirmed the Zn-5MOF sample has pure conjugated MOF structure without excess Zn(acac)2.
(Further details provided at Table 4.1).
84
Nominal
frequency
of the Band
(cm-1)
3600- 3200*
Type of vibration
Assignment
Intensity
Reference
Stretching
O-H vibration from
water molecules
Broad, strong
[34]
3400─ 3100
Symmetric and
asymmetric bending
N-H vibration of the
Primary aromatic amine
Medium, multiple
peaks
[1]
1685-1605*
1600-1400
Stretching
Stretching
strong
medium
[36]
[37]
3000*
1262
1005*
830
Stretching
Stretching
Stretching
bending
weak
medium
variable
strong
[35]
[13]
[39]
[40]
463

Stretching
C=C alkene
C=C stretching of
benzene ring
C-H aliphatic stretching
C-N aromatic stretching
C-O aliphatic stretching
out of the plane folding
aromatic C-H
Zn-N vibration
weak
[38]
The increment of transmittance intensity has seen because of the Zn(acac) 2 concentration increased.
Table 4.1: Assignment of common ATR-FTIR spectral features to their related functional groups of ZnpPDA MOF materials.
85
Figure 4.3: Powder X-ray diffractograms of pPDA, Zn(acac)2 (precursors) and Zn-pPDA MOF
(product)
PXRD was employed to characterize the crystallinity of obtained Zn─MOF samples (Fig.
4.3). The obtained MOF were assumed to be crystalline materials and isostructural on the basis
of their PXRD in Fig. 4.3 Moreover, the synthesis of MOFs from starting materials was clearly
depicted the different crystalline nature of Zn-MOFs, from their starting material: pPDA and
Zn(acac)2. The XPS measurements were performed to confirm the composition and elemental
chemical stated of the Zn-pPDA MOF in Fig. 4.4. In the Zn 2P spectrum shown in Fig. 4.4a, the
major signal of 2P 3/2 peak observed as two de-convoluted peaks of Zn─N and Zn─O bonding at
1022.1 eV [41], 1021.2 eV [42], respectively. This is attributed to the coordination bonding of –
86
NH2 group and Zn2+ ion. In addition, the N1s spectrum (Fig. 4.4b) also shown the Zn─N [43]
coordination bond at 398.6 eV. Furthermore, pPDA organic ligand as a primary amine, –NH2 group
and C─N bonding has shown at 399.5 eV and 400.5 eV, respectively [44]. The C1s spectrum (Fig.
4.4c) is shown SP2 hybridized carbon at 284 eV and C─NH bonding at 285.9 eV which attributed
to benzene ring and primary amine[45]. Moreover, in Fig. 4.4c, From the Zn(acac)2, the C─C
(aliphatic) and C─O/C=O bonding group at 284.8 eV and 288.5 eV is shown, respectively [45].
The full XPS survey spectrum shown in Fig. 4.4d, further proves the presence of Zn, N, C and O,
which is also confirmed by the EDEX spectrums in Fig. 4.5c, 4.5f and 4.5i.
The Morphological study of as synthesised 3 types of MOFs were shown in Fig. 4.5. in
SEM images, the 2D like morphology of Zn-5MOF, Zn-8MOF and Zn-12MOF has been studied:
Zn-5MOF is composed of 2D sheets smoothly formed with inter-space among the multi layers is
high (Fig. 4.5 a, b), comparison to Zn-8MOF and Zn-12MOF. The absence or very less amount of
Zn(acac)2 probably leads to more smooth layered structure of Zn-5MOF. In contrast, the
concentration of Zn(acac)2 presence in Zn-12MOF has utilized into synthesised Zn-pPDA MOF
and rest were among the matrix of MOF on the thin 2D layers. This has violated the smooth surface,
lead the layered structure because of Zn-pPDA but layers are conjoint tightly with rough surface
due to high amount of Zn(acac)2 coagulations. The composition of these material in three positive
electrodes clearly exhibited by the EDEX spectrums and the inset: high atomic ratios depicted the
excess amount of Zn(acac)2 in the Multilayered Zn-pPDA MOF structures. This is confirmed by
87
the FTIR spectrums (Fig. 4.2) as well.
(a)
Zn─N
1022.1 eV
Zn─O
1021.2 eV
1032
1028
1024
(b)
N 1s
Intensity (a.u.)
Intensity (a.u.)
Zn 2p3/2
1020
-NH2
399.5 eV
408
Binding Energy (eV)
Zn─N
398.6 eV
C─N
400.5 eV
404
400
396
Binding Energy (eV)
Printed using UNLICENSED CasaXPS software
(c)
C─C aliphatic
284.8 eV
SP2─C
284 eV
C─NH
285.9 eV
C=O
288.5 eV
Printed using UNLICENSED CasaXPS softwa
Zn 2p
C 1s (d)
O 1s
Intensity (a.u.)
Intensity (a.u.)
C 1s
N 1s
Survey
294
291
288
285
1200
282
800
400
0
Binding Energy (eV)
Binding Energy (eV)
Printed using UNLICENSED CasaXPS software
Figure 4.4: XPS survey and high-resolution spectra of Zn-pPDA MOFs (a) Zn 2p, (b) N 1s, (c) C
1s and (d) survey spectrum.
88
Figure 4.5: FE-SEM images of (a, b) Zn-5MOF (d, e) Zn-8MOF and (g, h) Zn-12MOF, and their
EDEX spectrums (c, f, i).
89
Zn-12MOF
GC
Zn-8MOF
Zn-5MOF
20
0
(a)
-20
@500 mV/s
-40
-1.2
-0.8
-0.4
0.0
0.4
Potential (V) vs Ag/AgCl
Current density (A/g)
40
0.8
Zn-12MOF
Zn-8MOF
Zn-5MOF
0.6
0.4
0.2
(b)
0.0
-0.2
-0.4
0.8
0
500
Potential (V) vs Ag/AgCl
-60
2.0
10
Zn-12MOF
Zn-8MOF
Zn-5MOF
1.5
5
-75 60.76
(c)
Z" ()
Z" ()
15
Zn-12 MOF
Zn-8 MOF
Zn-5 MOF
1.0
0.5
0.0
0
5
10
15
4.5
Z' ()
20
o
2000
49.2o
0.71 s
0.96 s
(d)
-45
-30
Zn-12 MOF
Zn-8 MOF
Zn-5 MOF
45.9o
0.64 s
-15
4.0
1500
Time (s)
 (degree)
20
1000
5.0
0
25
Z' ()
0
1
2
3
log f (Hz)
4
5
Figure 4.6: Electrochemical performance of all Zn-5MOF, Zn-8MOF and Zn-12MOF single
electrodes. (a) CV curves comparing the electrochemical properties of the positive and negative
electrodes in their stable operating voltage windows, obtained at 500 mV/s (b) GCD curves (c)
Nyquist plot with a magnified high-frequency region and (d) Bode plots over a frequency range
from 105 Hz to 1 Hz.
90
The right hand side of the Fig. 4.6a, shows the CV curves of pure Zn-5MOF, Zn-8MOF
and Zn-12MOF at 500 mV/s scan rate in voltage range 0.7 V ─ (-0.3) V. Evidently, these CV
curves of exhibited a couple of redox peaks, indicating the strong pseudocapacitive property of the
Zn-pPDA MOF material. Redox peaks can be attributed to the conversion between the different
oxidation states of Zn and primary amine: pPDA (Fig. 4.6a and Fig 4.7). Besides, the left hand
side pseudo square shaped EDLC CV curve represented the Graphite carbon electrode on NF,
which used for fabricated asymmetric supercapacitor (ASC). Furthermore, the full figure of Fig.
4.7 shown the optimized potential range of as-prepared GC and Zn-5MOF electrodes,
demonstrating that the maximum operational potential window of the assembled hybrid device
could reach 1.5 V in various scan rates as well. Even though the assembled asymmetric
supercapacitor (ASC) go beyond 1.5 V voltage gap, we operated these ASCs in 1.5 V in 1M KOH
to prevent gas evaluation and degradation of the electrolyte [46]. The GCD curves of Zn-5MOF,
Zn-8MOF and Zn-12MOF was exhibited in Fig 4.6b demonstrated their charge discharge nature
at current density 1.0 A/g. The Zn-5MOF positive electrode has superior performance by shown
the longest charging and discharging curve at lowest current density 1.0 A/g. For further
clarifications, the individual CV and GCD curves of Zn-5MOF, Zn-8MOF and Zn-12MOF are
shown in Fig. 4.8.
91
The Electrochemical impedance spectroscopic techniques (EIS) was used to investigate the
electrochemical performance of the Zn -5MOF, Zn-8MOF and Zn-12 MOF single electrodes. The
EIS were measured in a frequency range between 0.01 and 105 Hz. The Nyquist plot from The EIS
study shown in Inset of Fig. 4.6c, Exhibited the lowest Rs (solution resistance) and lowest Rct
(charge transfer resistance) exhibited by Zn-5MOF, are 4.03 0.43 Ω, respectively, because of the
high electrical conductivity. The high conductivity generated by the conjugated MOF structure.
Zn(acac)2 concentration attributed to low conductivity, representing lower values of Rs and Rct
values in Zn-12MOF. To further analysis the EIS study the bode plot has deployed in Fig. 4.6d.
The relaxation time constant (τo) was measured using the following equation τo= 1/fo, where τo is
relation time constant and fo is the peak frequency value. The shortest and longest relaxation time
constants have been reported by the Zn-5MOF and Zn-12MOF electrodes, respectively, indicating
that the high conductivity of Zn-5MOF because of the higher proportion of conjugated Zn-pPDA
MOF structure that Zn(acac)2. (refer the Table 4.2). It is well known that power delivery and better
responsiveness has improved corresponds to lower τo values [47, 48]. Therefore, the Zn-5MOF
shown the excellent performance than other electrodes because of the Zn-MOF conjugated system.
The lower electrical conductivity provided the low value of electrochemical performance in Zn12MOF. (refer the Table 4.2)
92
Current density (A/g)
40
(a)
Zn-5MOF electrode
GC electrode
20
0
-20
300 mV/s
200 mV/s
500 mV/s
400 mV/s
-40
-1.2
-0.8
-0.4
0.0
100 mV/s
50 mV/s
10 mV/s
0.4
0.8
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
(b)
-0.2
-0.4
-0.6
-0.8
-1.0
0
100
200
300
400
500
Potential (V) vs Ag/AgCl
Potential (V) vs Ag/AgCl
Potential (V) vs Ag/AgCl
0.8
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
(c)
0.6
0.4
0.2
0.0
-0.2
-0.4
0
500
1000
1500
2000
Time (s)
Time (s)
Figure 4.7: Electrochemical performance and evaluation of single electrodes for assembling the
ASC device (a) the CV curve of the GC negative electrode (left) and Zn-pPDA MOF-5 positive
electrode (right) at various scan rated and GCD curves of individual (b) GC negative electrode and
(c) Zn-pPDA MOF-5 positive electrode.
93
Obtained Parameters
Zn-5MOF
Zn-8MOF
Zn-12MOF
Relaxation time constant (τo)
0.64 s
0.71 s
0.96 s
ɸ (phase angle)
45.90
49.20
60.760
RCT (Charge transfer resistance)
0.43 Ω
0.54 Ω
0.60 Ω
RS (solution resistance)
4.03 Ω
4.15 Ω
4.36 Ω
Capacitance of Single MOF electrode
(from GCD curves)
Capacitance of ASC (from GCD curves) *
861.6 F/g
628.9 F/g
551.7 F/g
200.9 F/g
184.1 F/g
167.3 F/g
Capacitance of ASC (Theoretical calculation)
198.5 F/g
182.7 F/g
176.5 F/g
Energy density (ASC device) *
62.76 Wh/kg
57.52 Wh/kg
52.25 Wh/kg
Power density (ASC device) *
4501.1 W/kg
4499.03 W/k
4498.92 W/k
g
g
*

The measurements were carried out on asymmetric supercapacitor device (ASC).
Table 4.2: The Electrochemical performance parameters and obtained values of supramolecular
complex (Zn-pPDA MOF) electrodes.
94
(b)
Zn-5MOF
500 mV/s
400 mV/s
300 mV/s
200 mV/s
40
20
0
-20
100 mV/s
50 mV/s
10 mV/s
-40
-0.4 -0.2 0.0 0.2 0.4
0.6 0.8
Potential (V) vs Ag/AgCl
Current density (A/g)
0.8
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
0.6
0.4
0.2
0.0
-0.2
-0.4
0
500
Potential (V) vs Ag/AgCl
Current density (A/g)
(c)
40
20
0
Zn-8MOF
500 mV/s
400 mV/s
300 mV/s
200 mV/s
-20
100 mV/s
50 mV/s
10 mV/s
-40
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
20
0
Zn-12MOF
500 mV/s
400 mV/s
300 mV/s
200 mV/s
100 mV/s
50 mV/s
10 mV/s
-20
-40
-0.4
(f)
-0.2
0.0
0.2
0.4
0.6
0.8
Potential (V) vs Ag/AgCl
Potential (V) vs Ag/AgCl
Current density (A/g)
40
1500
0.8
2000
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
0.6
0.4
0.2
0.0
-0.2
-0.4
Potential (V) vs Ag/AgCl
(e)
1000
Time (s)
(d)
Potential (V) vs Ag/AgCl
(a)
0
250
500
750
Time (s)
0.8
1000 1250
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
0.6
0.4
0.2
0.0
-0.2
-0.4
0
200
400
600
800
1000
Time (s)
Figure 4.8: Electrochemical performance, Cyclic Voltammetry and Galvanostatic charge
discharge of Zn-5MOF (a, b), Zn-8MOF (c, d) and, Zn-12MOF (e, f).
95
To further evaluate the electrochemical properties and practical application, ACSs were
assembled by using as-prepared Zn-5MOF, Zn-8MOF and Zn-12MOF on NF as positive
electrodes (Fig. 4.8) and GC on NF as the negative electrode. The assembled ASCs were denoted
as Zn-5MOF//GC, Zn-8MOF//GC and Zn-12MOF//GC (Fig. 4.9). To further illustrate the
electrochemical performance, the Zn-5MOF//GC device was performed with CV and GCD curves
in various scan rates in various potential windows. In Fig. 4.9 a and b, the CV curves of Zn5MOF//GC hybrid device, clearly shown its electrochemical performance indicating that dominant
pseudocapacitive nature, because of the redox peaks. The Fig. 5c, the GCD curves operated in
various current densities shown its charge discharge mechanism, also evidenced its
pseudocapacitive nature by shown the GCD curve which has plateaus.
Furthermore, Zn-5MOF//GC device was performed in various working potential windows
from 0.0 ─ 1.0 V to 0.0 ─ 1.5 V at scan rate 400 mV/s, reveal its stability of working in potential
up to 1.5 V. The fig. X1 d shows the GCD curves which operated in different potential gaps under
current density 1.0 A/g, point out the maximum operational potential gap and their stability.
Among the three ASC devices, the Zn-5MOF//GC ASC device shown its CV curves and GCD
curves in Fig. 4.9 a, c respectively, exhibited the largest enclosed area of CV curves and longest
discharge time of GCD curves, indicating superior capacitance performance. In addition, in Fig
4.11, shows the CV curves and GCD curves of Zn-5MOF//GC, Zn-8MOF//GC and Zn12MOF//GC devices at different scan rates and different current densities. The specific capacitance
of these three ASC devices were calculate using this GCD curves with equation (4.2).
96
The enhanced performance of Zn-5MOF//GC ASC device can be attributed to the
improved electrical conductivity and excellent ion transport and diffusion rate. It can be explained
in Fig. 4.10a accordingly, which illustrate the relationship between anodic peak current and the
square root of scan rate. The Cottrell equation [49, 50], i= nFACD1/2 /(πt)1/2 where i is peak current,
n is the number of electrons transferred in redox reactions, F is faradic constant and A is area of
active working electrode, C is the concentration of the electrolyte and D is the diffusion coefficient.
From this equation the simplified Cottrell equation, i=av1/2 that the anodic peak current (i) increase
linearly with square root of scan rates (v1/2), which satisfies Cottrell equation and indicates a
diffusion-controlled process. According to the Fig. 4.10a, for compassion, the Zn-5MOF//GC ASC
device has shown represent the fastest diffusion velocity [49] and Zn-12MOF//GC device has
slower diffusion velocity comparison to other ACS devices and Zn-8MOF//GC device rely
moderate diffusion velocity of ions in-between electrode and electrolyte. The rate stability was
also carried out as shown in Fig. 4.10b. Overall three ASC devices shows excellent stability when
tested at rate ranged from current density 1.0 to 6.0 A/g. After 60th charge discharge cycle, when
the current rate was decreased back to 1.0 A/g, the capacitance (F/g) was coming back to the
previous values recorded from 1st ten cycles, indicated the capability of recovering the capacitance
of these three ASC devices. The corresponding specific capacitance (S. capacitance) of the three
Zn-MOF//GC ASC devices is further calculated as demonstrated in Fig. 4.10c. From the
comparison of these three ASC devices the Zn-5MOF//GC exhibited the highest capacitance
values corresponding to the various current densities. Zn-8MOF//GC and Zn-12MOF//GC devices
97
are show their s. capacitance in descending order corresponding to the multiple current densities,
respectively. The S. capacitance drop was decreased when the scan rate is increased confirms the
developing excellent rate performance of the three ACS device.
The Ragone plot was fitted using the energy density vs. power density as shown in inset of
Fig. 4.10c. The energy density and power density were calculated using equation 4.3 and 4.4. The
maximum power and energy densities were 4501.1 W/kg, 62.75 Wh/kg for Zn-5MOF//GC device,
4499.03 W/g ,57.52 Wh/kg for Zn-8MOF//GC device and 4498.92 W/kg, 52.25 Wh/kg for Zn12MOF//GC device, individually, indicated that Zn-5MOF//GC ASC device has superior
electrochemical performance (also refer Table 4.2). The S. Capacitance around 2000 cycles has
been calculate in Fig. 4.10d, presented the Zn-5MOF//GC, Zn-8MOF//GC and Zn-12MOF//GC
devices are shown 96.2 %, 96.81 % and 96.79 % of capacitance retention, respectively. This
indicated the overall high stability of the Zn based MOF ASC devices around 2000 cycles.
98
Current (mA)
10
(a)
0
200 mV/s
300 mV/s
400 mV/s
-10
-20
0.0
0.5
1.0
10
1.3 V
1.4 V
1.5 V
-10
-20
1.5
0.0
0.5
Potential (V)
1.0
0.5
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
1.5
0.0
1.5
@1.0 A/g
(d)
1.0 V
1.1 V
1.2 V
1.4 V
1.5 V
450
600
1.0
0.5
0.0
0
150
300
450
600
0
150
Time (s)
20
(e)
0
-10
@400 mV/s
0.0
0.5
1.0
(f)
1.5
10
-20
300
Time (s)
Potential (V)
Zn-12MOF//GC
Zn-8MOF//GC
Zn-5MOF//GC
30
Current (mA)
1.0
Potential (V)
Potential (V)
(c)
@400 mV/s
0
Potential (V)
1.5
(b)
1.0 V
1.1 V
1.2 V
20
Current (mA)
10 mV/s
20 mV/s
100 mV/s
20
1.0
0.5
@1.0 A/g
0.0
1.5
0
Potential (V)
Zn-12MOF//GC
Zn-8MOF//GC
Zn-5MOF//GC
150
300
450
600
Time (s)
Figure 4.9: Electrochemical performance of asymmetric devices. (a) CV curves of Zn-pPDA
MOF-5//GC ASC at various scan rates in a potential window range of 1.5 to 0.0 V, (b) CV curves
of Zn-pPDA MOF-5//GC ASC at different operational potential gaps, (c) GCD curves of Zn-pPDA
99
MOF-5//GC ASC at various current densities, (d) GCD curves of Zn-pPDA MOF-5//GC ASC at
different operational potential gaps, (e,f) CV curves of three as-assembled ASCs at scan rate of
400 mV/s and GCD curves at current density of 1.0 A/g. (‘MOF-#’s in legends represent ‘ZnpPDA MOF-#’s.
(b)
9
Zn-5MOF//GC
Zn-8MOF//GC
Zn-12MOF//GC
6
3
0
4
(c)
8
12
Scan rate
1/2
16
1.0 A/g
1.0 A/g
1.5 A/g
100
2.0 A/g
50
2.5 A/g
0
20
1/2
(mV/s)
0
200
150
1k
2k
3k 4k 5k
Power density (W/kg)
Zn-5MOF//GC
Zn-8MOF//GC
Zn-12MOF//GC
2
30
40
50
60
70
Cycle number
220
10
100
1
20
Zn-12MOF//GC
Zn-8MOF//GC
Zn-5MOF//GC
Zn-5MOF//GC
Zn-8MOF//GC
Zn-12MOF//GC
S. capacitance (F/g)
250
10
(d)
100
Energy density (Wh/kg)
S. capacitance (F/g)
300
0
150
3.0 A/g 6.0 A/g
0
50
Zn-5MOF//GC
Zn-8MOF//GC
Zn-12MOF//GC
200
S. capacitance (F/g)
Peak current (mA)
(a)
200
193.24 F/g
180
178.2 F/g
160
161.9 F/g
3
4
5
6
0
Current density (A/g)
500
1000
1500
2000
Cycle number
Figure 4.10: Further analysis of the three ASCs. (a) Linear fitting curves of peak current vs. scan
rate, (b) cycling stability of the first 70 cycles at different current densities, (c) specific capacitance
(S. capacitance) at different current densities (inset: Ragone plot), and (d) cycling performance
100
(a)
(b)
10
0
200 mV/s
300 mV/s
400 mV/s
-10
-20
0.0
(c)
1.0
10
5
600
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
1.0
0.5
0
1.5
150
300
20 mV/s
10 mV/s
-5
600
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
1.5
0
450
Time (s)
(f)
400 mV/s
300 mV/s
200 mV/s
100 mV/s
10
450
0.0
Potential (V)
Current (mA)
1.0
Potential (V)
15
300
Time (s)
(d)
50 mV/s
10 mV/s
0.5
150
1.5
-10
0.0
0.5
0
0
(e)
1.0
0.0
400 mV/s
300 mV/s
200 mV/s
100 mV/s
20
6.0 A/g
3.0 A/g
2.5 A/g
2.0 A/g
1.5 A/g
1.0 A/g
1.5
1.5
Potential (V)
30
Current (mA)
0.5
Potential (V)
Current (mA)
20
Potential (V)
10 mV/s
20 mV/s
100 mV/s
1.0
0.5
0.0
0.0
0.5
1.0
1.5
0
Potential (V)
150
300
450
Time (s)
Figure 4.11: Electrochemical performance of the hybrid device, Cyclic Voltammetry and
Galvanostatic charge discharge of Zn-5MOF//GC (a, b), Zn-8MOF//GC (c, d) and, Zn12MOF//GC (e, f), respectively.
101
For comparision of the Zn-pPDA MOFs with the current technologies, we characterized
number of supercapacitors publications studied on Zn based materials, carbon materials,
conductive polymers and their composites with our results, are presented in the same Ragone plot
(Fig. 4.12a). Mostly ZnO nanostructures were very popular researches in energy storage devices,
such as ZnO nanowires//AC (activated carbon) ASC has energy density of 24.2 Wh/ kg at the
power density of 896.44 W/ kg [51]. ZnO@MnO2// AC ASC device shown its maximum energy
density and power density 17 Wh/ kg and 6.5 kW/ kg [52]. The ZnO/α-Fe2O3//ZnO/C ASC has 41
Wh/kg and 7 kW/kg of energy and power densities [53] and Zn-Co-S// AC ASC has 31.9 Wh/kg
and 8.5 kW/ kg of energy and power densities [54], respectively. And also HPNCs/rGO800//HPNCs/ rGO-800
shown 12 kw/ kg and,18 Wh/ kg [55], ZIF-8/CNT// N-Carbon/CNT
shown 16.9 kW/ kg and 23.6 Wh/ kg [15], Ionic liquid functioned Chemically modified graphene
(IL-CMG) and RuO2–IL-CMG
shown 6.8 kW/ kg and 19.7 Wh/ kg [16]. Combination of
polypyrrole and Oxides of graphene and Zinc (PPy/GO/ZnO) 1.48 kW/ kg and 10.65 Wh/ kg
[56] and Boron and Nitrogen doped Carbon nanosheet (B/N–C) shown 6 kW/ kg and 8 Wh/ kg
[57] as their Power and energy densities, respectively, reported in literature.
The Ragone plot in Fig. 4.12b shows that the properties of the pseudocapacitors using our
Zn-MOFs are positioned between those of the conventional supercapacitor and battery applications.
Researchers in this field aim to achieve a higher power density and a high energy density
simultaneously, as indicated by the arrow in Fig. 6c. The devices using our nanocomposites exhibit
102
a slightly higher energy density than conventional supercapacitors and a slightly higher power
density than conventional batteries.
Theoretical capacitance calculation done according to the equation 4.5. where, CT is the
total capacitance of the cell, C+ is the capacitance of the positive electrode Zn-pPDA MOF (Zn5MOF, Zn-8MOF and Zn-12MOF, individually), C─ is the capacitance of the negative electrode
(three identical graphite carbon electrodes were fabricated for assemble with three Zn-pPDA MOF
electrodes, here 259.5 F/g). Theoretically calculated capacitance (CT) of the Zn-5MOF// GC, Zn8MOF// GC and Zn-12MOF// GC devices are given 198.5 F/g and 182.7 F/g and 176.5 F/g,
respectively. The originally the specific capacitance using GCD curves (Cdevice) obtained 200.86
F/g, 184.06 F/g, 167.26 F/g for Zn-5MOF// GC, Zn-8MOF// GC and Zn-12MOF// GC asymmetric
supercapacitors (ASC). We think the difference of the CT and Cdevice values occurring by the
practically and theoretically nature was difference when electrodes packing with separator. But the
difference was very slight, indicate that the high accuracy of the present study.
103
The electrochemical performance of the synthesised Ni-MOF, Mn-MOF and Zn-MOFs are shown
slight difference from each other because of the following reasons:

Surface morphology is different in each MOFs. Zn-MOF has shown 2D layered structure
in their SEM images (Fig.4.5 a,b) and Ni-MOF has shown fused layered structure (Fig. 4.6
a,b) but the broken clusters like particles and Mn-MOF has depicted entirely fused layered
structure (Fig.4.6 e,f). The surface area need to exchange the charges with the electrolyte
and according to the morphology the Zn-MOF shows higher surface area than other two
MOFs. Therefore, highest value of specific capacitance, 861.6 F/g shown in Zn-MOF and
lowest value, 421.1 F/g shown in Mn-MOF. The Ni-MOF has obtained 594 F/g of specific
capacitance for single electrode.

Regardless the metal ion, whole MOF structures, has shown conjugated structure, although
the oxidation and reduction of these metal ions are different, therefore, the shape of the CV
and GCD (charge-discharge) curves are not identical. Consequently, overall shape of the
CV and GCD curves are different from each other. This phenomenon indicates their
different electrochemical behavior.
104
Energy density (Wh/ kg)
(a)
Zn-5MOF//GC
Zn-8MOF//GC
Zn-12MOF//GC
Ref. 92
Ref. 148
Ref. 113
Ref. 112
Ref. 118
Ref. 151
Ref. 117
Ref. 149
Ref. 150
100k
1M
100
10
1
100m
100
1k
10k
Power density (W/ kg)
Power Density (W/ kg)
(b)
1M
Capacitors
Zn-5MOF
Zn-8MOF
Zn-12MOF
100k
10k
1k
100
10
Supercapacitors
1
100m
10m 100m
Batteries
Fuel Cells
1
10
100
1k
10k 100k
Energy Density (Wh/ kg)
Fuel Cells
Figure 4.12: Ragone plots of our two assembled MOF hybrid devices, (a) compared to the data
reported to date. (‘MOF-#’s in legends represent ‘Zn-pPDA MOF-#’s.) and (b) Range of various
energy-storing devices.
105
4.4
Conclusion
In summary, we introduced three types of 2D like multilayered Zn-pPDA MOFs (Zn-5MOF,
Zn-8MOF and Zn-12MOF) are synthesised by the liquid-liquid interfacial reaction from pphenelendiamine monomer as an organic ligand. Interestingly, this methodology provided the 2D
like multilayered nanostructures. Furthermore, it should be mentioned that the concentration of
Zn(acac)2 plays a main role of bridging mechanism using Zn bimetallic ions with organic ligand
enhancing the synthesis of conjugated structure of Zn-pPDA MOF. The as-synthesised Zn-5MOF
has shown the superior electrochemical performance than other two MOFs and overall the ZnpPDA MOF// GC ASCs are exhibited the high energy density 62 Wh/ kg and 4.5 kW/ kg of high
power density and also excellent capacitive retention of 96 % in 2000 cycles.
106
4.5
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112
Chapter 5
Advanced energy storing of flexible smart fabric
5.1
Introduction
5.1.1
Overview
Supercapacitors becomes more popular when they are flexible and wearable [1, 2]. For
consumer electronic devices, batteries and supercapacitors are the two main commercial power
sources [3, 4]. Supercapacitors play an increasing role in wearable devices in smart garments due
to their high power, rapid charge discharge and longer lifetime [5, 6]. But typical smart textiles
comprise bulky battery packs and inconvenient power units which need to plugged into wall,
indicating the key challenge that the demand for the flexible electrodes of the energy storages [7]
with recharge mechanism. For that, researchers use flexible solar cells and other piezoelectric
applications are high cost and low efficiency [8]. The carbon scaffold of cellulose in cotton fabrics
are used to synthesis flexible electronic applications, because their flexibility and the large surface
area which provided by the hierarchical structure of cellulose microfibers [9, 10]. However, these
substrates are naturally insulators and they need to be electrically conductive for use in
supercapacitive application [2, 10]. For overcome such a problem, producing conductive flexible
substrates by embedding electro-conductive nanomaterials on the textiles is a smart movements
for fabricate the wearable electronics [11]. The Polyaniline (PANI), polypyrrole (Ppy) like
conductive polymers are used in fabricating the conductive flexible surface [10, 12]. Among these
113
polymers PANI will be promising material for the energy storages [13], because of the low cost
[14], capability to optimized their conductivity under different pH values [10], electrochemically
active in wide range of pH [15] and can be used as combination of graphene , MnO2 , Fe3O4 ,
MnFe2O4
and MWCNT [16].
Several groups are reported that the fabricating electrodes from PANI coated on cotton
fabrics by in-situ polymerisation [17-20]. However, these traditional methodologies have shown
the active material and cellulose bonded each other via H-bonding [10, 21]. This type of
interactions has no hindrance for water molecules [22], thus lead the detaching the active material
from the cellulose microfibers due to poor adhesion, which will happens similarly when contact
with the aqueous electrolytes in long time. And also metal ink deposition of metal particles on the
dielectric separator results electrical short-circuit. This will lead rapid discharge of the energy
storage.[23]. These limitations become more critical when such materials are incorporated into
smart garments [24].
We comprehend this fully integrated device will also require sealing of the active surface
to enclose and contain the electrolyte. However, this is not part of the study. For further interesting
for continue this study we suggest that the solid-polymer electrolyte such as PVA- Na2SO4 mixture
is appropriate.
114
5.1.2
Our Study
The objective of this study is to demonstrate the facile strategy and fabricating prototype
of flexible low cost smart fabric. Majorly, here we delivered the smart fabric which can store the
energy as and electrical charges. We address the one of the key challenge: fibrous detaching by the
active materials and this strategy could be applied to a number of material and its composites. We
understand this is not first, but very vital step comprises the major technology of Supercapacitor:
energy storages embrace in application called smart textile [25, 26].
5.2
Experimental
5.2.1
Preparation of MSMA cross-linker coated cotton fabric (S-cotton)
Dried cotton fabric pieces (4 × 4 cm2) were mixed with anhydrous toluene (50 ml) in
conical flask. The 5 ml of 3-trimethoxylsilylpropyl methacrylate (MSMA) was added by the
syringe. The reaction was carried under 70 °C for 24 hours refluxing with continues stirring. The
fabrics were filtered and washed several times of fresh toluene and DI water, respectively. After
the washing the MSMA coated cotton fabrics were kept 65 °C in vacuum Owen for 6 h [27].
5.2.2
Preparation of Polyaniline cross-linked flexible electrodes (P/S-cotton)
In a typical run, the 5 pieces of dried SAM coated cotton fabrics are soaked in Polyaniline
(PANI) solution in 4 hours. The PANI solution is made with 1 g of PANI powder with 50 ml of
dodecyl benzene sulphonic acid (DBSA) as a solvent. The soaked fabrics were washed with
ethanol and deionized water several times and dried in room temperature. This PANI powder has
115
produced according to the literature [28]. These fabric pieces were pasted on Aluminium Film
Electrical Tape (3M, United States) as a current collector and directly used as electrodes. For the
control test we fabricate PANI in same type of cotton fabric without MSMA cross-linker by in-situ
polymerisation [19] denoted as P-cotton.
5.2.3
Preparation of f-Supercapacitor electrodes (Ag-P/S-cotton)
The dried bare P/S-cotton electrodes were treated with saturated Ascorbic acid (AA)
solution, continuously stirred with 60 °C at 1 h. Then those AA doped P/S-cotton electrodes were
padded to remove excess AA solutions and 1.0 M of AgNO3 were added drop-wisely each on the
P/S-cotton electrode and dried at room temperature. Finally, the electrodes were washed with
NaOH solution (pH 10) to remove dehydroascorbic acids and unreacted AgNO3 and AA molecules.
5.2.4
Fabrication of f-Supercapacitors:
As prepared flexible supercapacitor (f-Supercapacitor) electrodes were assembled each
other with 6 M Na2SO4 electrolyte with pure cotton fabric separator. The fully integrated fSupercapacitor device will also require sealing of the active surface to enclose and contain the
electrolyte. However, this is not part of the study. For further clarification, cotton fabric, MSMA
cross linker coated cotton fabric, the fabric that PANI cross-linked into cotton via MSMA, and Ag
NP decorated PANI cross-linked cotton fabrics are denoted as: cotton, S-cotton, P/S-cotton and
Ag-P/S-cotton, respectively.
116
5.2.5
Characterization of materials:
Analysis of functional groups of the flexible electrodes was performed using Fourier
transform infrared spectroscopy (FTIR, MCT Mid-IR, Bruker, Germany) in the range of 400 –
4000 cm−1. The crystallinity of the materials was examined by X-ray diffraction (XRD) analysis
on a D8 Focus (Bruker, Germany), an automated X-ray diffractometer system, with Cu-Kα
radiation (λ = 1.5418 Å) from 10° to 70°. The morphology of the synthesised flexible electrodes
was analysed using field emission scanning electron microscopy (FE-SEM, JEOL JEM-7500F,
Japan) and the Thermogravimetric analysis (TGA) techniques from 30 oC to 1000 oC by supplying
N2 (100 mL min-1) to ascertain the stoichiometry and chemical state of the species. The sheet
resistance of the flexible electrodes was measured using a digital multi-meter (DMM 7510,
Keithley, USA) based on 4-point probe method. The surface area analysed using BrunauerEmmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods (BELLSORP Mini-X, UK). The
compositions of the powder materials were analysed using thermos-gravimetric analysis (TGA)
techniques from 30 °C to 1000 °C by (SEIKO Inst, Japan) under N2 environment (100 ml min−1)
to ascertain the stoichiometry and chemical state of the species.
117
5.2.6
Electrochemical performance of f-Supercapacitor:
The electrochemical performance of the as prepared Ag-P/S-cotton flexible single
electrodes was investigated with 6 M Na2SO4 aqueous electrolyte. To evaluate the electrochemical
property, the three-electrode system used as synthesised Ag-P/S-cotton flexible electrode, Pt wire
and Ag/AgCl electrode represents: working, counter and reference electrodes. Electrochemical
impedance spectroscopy (EIS) measurement was performed using an electrochemical work station,
VSP (Bio Logic Science instrument, France); moreover, cyclic voltammetry (CV) and galvanic
charge discharge (GCD) measurements were carried out using an electrochemical workstation,
WizECM premium-1200 (WizMAC, Korea). The Cyclic voltammetry (CV) and galvanic charge
discharge (GCD) analysis was performed in 0.8 – 0.0 V potential window for positive electrode
and 0.0 – (-0.7) V for negative electrode with different scan rates and different current densities,
respectively. The electrochemical impedance spectroscopy (EIS) was performed by under 10 mV
of AC voltage with the frequency range from 0.01Hz to 100 kHz. For obtaining electrochemical
performance as a device Ag-P/S-cotton two electrode was assembled as a positive and negative
electrode. The electrolytes soaked pure cotton fabric has used as separator in-between the two
asymmetric electrodes.
118
5.3
Results and discussion
The structure of our proposed self-charging smart textile is included within flexible
supercapacitor (f-Supercapacitor). The introduced f-Supercapacitor based on fabric that included
PANI decorated with Ag nanoparticles bonded into the cotton surface. In our study, the commercial
cotton fabrics are first changed into a conductive flexible electrode by bonding PANI into the
cellulose substrate in fiber level by molecule ligand mediated self-assembled cross-linker; 3trimethoxysilylpropyl methacrylate (MSMA) [29]. The PANI cross-linked with MSMA via
intramolecular H-bonding and π-π interactions (Figure 5.1a). Typically, the conductive cotton
fabrics made by coating the PANI on cotton by oxidative polymerisation of aniline with immerse
cotton fabric (Figure 5.1b). In this situation, H-bonding create in-between PANI and cellulose [10]
in the cotton (P-cotton) shown in Figure 5.1d. Our methodology has changed from this
phenomenon due to the PANI has bonded to MSMA which, tightly bonded to cotton, by using H
bonded as well as π-π stack interactions shown in Figure 5.1a. Because of the PANI’s amphiphilic
nature [30] strong H bonds and additional π-π interactions lead the PANI bonded to cotton fabric
(P/S-cotton) is stronger than in conventional PANI coated cotton (P-cotton). As a result of this
additional bonding, the durability of the active material in cotton (P/S-cotton) with the liquid
electrolyte during the long-cycling process has increased.
119
The AA doped in PANI, deposits Ag NP on the fabric surface by reducing the AgNO3 [31],
respectively. These 3D metal network improve the conductivity and surface area of the PANI
coated fabric surface (Figure 5.1b). The low-cost and f-supercapacitor electrode (P/S-cotton),
decorated with either Ag 3D nanostructure using ascorbic acid (AA) and became Ag-P/S-cotton.
The AA converts Ag+ into Ag0 nanoparticles (Ag NP) on the surface of the fabric while reducing
itself into Dehydroascorbic acid. The AA doped PANI [32] produce Ag NP on its surface and
treated AAs are encapsulating the Ag NP, [33, 34]. This was significantly improved the deposition
of metal network and its porosity [34]. On the other hand, typically the fabrics can adhere to the
extra amount of solution because of the capillary action nature, also provides extra adhesion of AA
molecules to the fabric surface. Interestingly, this has increased the metal NP deposition on the
fabric surface. The overall adsorbed AA reduced the Ag+ solution into Ag NPs on the fabric surface.
At the end the super flexible conductive electrodes (P/S-cotton, Ag-P/S-cotton) in Figure 5.1c has
directly used the f-Supercapacitor.
120
Figure 5.1: Schematic illustration physical appearance and mechanical behavior of smart fabric.
(a) Schematic layered structure of the Ag-P/S-cotton with bonding nature. (b) Digital microscopic
images of the fabric convert into Smart garment. (c) Photographs of the smart fabric under various
mechanical deformations; looped, wrapped, crumpled, bending and twisting. (d) Schematic
layered structure of the P-cotton with bonding nature.
121
FTIR spectra of flexible electrodes are shown in Figure 5.2a depicts IR pattern of pure
cotton characteristically agreed with the literature [10, 35]. The pure cotton fabric attributed to the
peaks at 3346, 2924, 1740, 1056 and 914 which corresponding to their O−H, CH2, C=O, C−O and
β-linkage of cellulose, respectively. These peaks are belonging to the D-glucose units condense
through β(1→4) glycosidic bonds [36], shows their typical cellulose structure. The MSMA
attached to cotton by Si−OH bonds represent broad stretching at 3404 cm-1. Additionally, the
C=C−H, C=O and C=C stretching exhibits in 2954, 1718 and 1631 cm-1, representing the
methacrylate group of MSMA structure, respectively. In the fingerprint region the Si−O−Si
asymmetric and symmetric stretching exhibit at 1196 and 817 cm-1 also Si−OH stretching at 906
cm-1 indicating self-assembling of the MSMA layer on the cellulose structure. The self-assemble
of the MSMA molecules by Si−O−Si bond provides excellent coating around the cellulose
microfibers. Surprisingly, the reaction resulted a strong bonding interaction between PANI and
SAM layers to make it a perfect structure for the proposed application (Figure 5.1a). The
amphiphilic nature in-between PANI and MSMA representing by the hydrogen bonding
interactions between free -NH groups from PANI with the polarized carbonyl oxygen from acrylate
group and, π-π stacking/ interaction between conjugated p-orbitals from PANI layer with enone
group from acrylate region of MSMA. It is seen that the absorption band of the carbonyl group in
the acrylate of MSMA is shifted from 1730 cm-1 to 1715 cm-1 with adding PANI [21] assumed to
be interaction of H-bonding between oxygen atoms of the carbonyl group from MSMA and
hydrogen atoms in the amide groups from PANI (Figure 5.2c). Likewise, the similar red shifting
122
in aliphatic C=C group from 1646 to 1635 cm-1 (Figure 5.2d) representing the π-π stacking
interaction between C=C and aromatic ring from PANI [37]. In contrast, P-cotton (Figure 5.1b)
only shows H-bonds [10, 18-20]. Therefore, in P/S-cotton, the PANI attached to cotton surface via
stronger interactions than P-cotton.
The XRD spectrum (Figure 5.2b) shows that the crystalline nature of the materials that
has been used for fabricate the flexible electrode. Cotton displays typical diffraction peaks of
cellulose,
crystalline lattice plane 110 , 110, 200 and 004 at 14.7, 16.4, 22.7 and 34.5,
respectively, similar to the literature [38]. MSMA coated cotton fabric also shows the cellulose
crystal lattices. The XRD patterns of the Ag(0) powder and PANI displays in Figure 5.2b also
identical to the reported studies [39]. The Ag-P/S-cotton electrode depict the combination of the
crystal lattice patterns from cellulose, PANI and Ag (0). This indicates the composite nature of this
Ag-P/S-cotton electrode. Examination of the surface area was measured by BET method, which
obtained relative pressure in rage 0.0 – 0.5 due to the relative low surface area of the cotton fabrics.
All the surface areas shown in Figure 5.2e demonstrate the typical linear characteristics of the BET
curves. The surface area corresponding to the cotton, S-cotton, P/S-cotton and Ag-P/S-cotton is
0.526, 1.039, 2.233 and 2.367 m2/g, indicating the surface coatings of the PANI and Ag NP further
improve the surface area of the Ag-P/S-cotton electrode. We examined the electrical conductivity
of the electrode in different fabricating stages. Both cotton and S-cotton shows dielectric nature
123
(10-4 S/cm) and relatively high sheet resistance but when S-cotton bonding with the PANI (P/Scotton), its stating to show moderate electrical conductivity (10-1 S/cm) and, Ag-P/S-cotton shows
metal like electrical conductivity (2×103 S/cm) enhancing by the Ag NPs on the fabric surface.
The findings ascertain that high electrical conductivity of our smart fabric and indicated that the
layered nature and Ag NPs were bond to the outermost layer of the fabric layer.
In particular, in the case of bare cotton fabric with fibrous porous nature, it was confirmed
by the FE-SEM images (Figure 5.3). As shown in Figure 5.3a, the microporous and mesoporous
nature in-between the fibers are clearly depicted. This is mainly agreed with the porosity
measurement in Figure 5.2e. The porosity is improved when the S-cotton bound with PANI
particles as shown in Figure 5.3c and 5.3d. As displayed in Figure 5.3d, the PANI coating has
bounded on the cotton surface and additional larger PANI particles developed the surface
roughness on the microfibers [1, 40]. On the top of the PANI layer the Ag metal nanoparticles also
deposited and further improves the mesoporous structure. This will increase the surface area of the
fabric. Although some of the Ag particles are growing rapidly and construct as dense and
disordered structures in large amount on the fabrics outermost surface as shown in Figure 5.3f. As
a result, it was considered that this structural uniqueness of the Ag NP on the smart fabric surface
aids the fast charge transfer, which is mainly related to the power performance of this smart fabric
application.
124
200
110
Intensity (a.u.)
1196
cotton
110
696
665
1740
1620
1435
1359
1320
1230
1155
1103
1056
1029
983
914
(b)
906
817
Absorbance (a.u.)
3346
3290
2924
2882
(a)
S-cotton
P/S-cotton
004
S-cotton
PANI
200
200
10
20
220
111
200
220
Ag-P/S-cotton
30 40 50 60 70
2 (degree)
(d)
(c)
cotton
1643
cotton
Absorbance (a.u.)
1732
S-cotton
1715
P/S-cotton
4
3
0.5266 m /g
2
1.0389 m /g
2
2
2.2328 m /g
1
2
2.3656 m /g
0
0.0
0.1
0.2
0.3
p/p0
0.4
70k
68k
-1
2
Ag-P/S-cotton
P/S-cotton
S-cotton
cotton
P/S-cotton
(f)
-1
5
1635
Ag-P/S-cotton
1660
1640
1620
1600
Wavenumber (cm-1)
Sheet resistance ( sq )
(e)
S-cotton
1635
1715
Ag-P/S-cotton
1770 1740 1710 1680 1650
Wavenumber (cm-1)
1646
thikness = 0.07 cm
10
2
10
0
10
-2
66k
10
100m
0.5
Electrical conductivity (S cm )
Absorbance (a.u.)
1729
p/Va(p0-p)
Ag(0)
111
004
Ag-P/S-cotton
4000 3000
1500
1000
Wavenumber (cm-1)
cotton
-4
1m n
n
n
n 10
o
o
o
o
t
t
t
t
t
t
t
t
co S-co /S-co /S-co
P
P
Ag-
Figure 5.2: Material characterization data of the smart fabric. (a) the comparison of the FTIR
spectrum of the fabrics and deep comparison of the regions of (b) carbonyl –C=O and (c) aliphatic
C=C peaks in FTIR spectrum. (d) comparison of the X-ray diffraction pattern of the fabric samples.
125
(e) BET surface area plot of the fabrics and, (f) Change in the sheet resistance (Ω sq-1) and the
variation of the electrical conductivity (S cm-1) of the fabric electrodes.
Figure 5.3: SEM images of Pure cotton (a, b), P/S-cotton (c, d) and, Ag-P/S-cotton (e, f).
126
(a)
(b)
0.002
P/S-cotton (−)
P/S-cotton (+)
Current (A)
Current (A)
0.004
0.000
100 mV/s
200 mV/s
300 mV/s
400 mV/s
-0.002
-1.0
-0.8
-0.6
-0.4
-0.2
0.002
100 mV/s
200 mV/s
300 mV/s
400 mV/s
0.000
-0.002
0.0
0.0
Potential (V) vs Ag/AgCl
0.2
0.3
0.4
0.5
0.6
Potential (V) vs Ag/AgCl
(c)
(d)
-1.0
0.5 mA/cm2
Potential (V) vs Ag/AgCl
Potential (V) vs Ag/AgCl
0.1
1.0 mA/cm2
-0.8
2.0 mA/cm2
-0.6
3.0 mA/cm
2
-0.4
-0.2
P/S-cotton (-)
0.0
0
100
200
300
Time (s)
0.6
0.5 mA/cm2
0.5
1.0 mA/cm2
0.4
2.0 mA/cm2
0.3
3.0 mA/cm2
0.2
0.1
P/S-cotton (+)
0.0
0
150
300
450
Time (s)
Figure 5.4: Electrochemical performance of the of the negative and positive P/S-cotton electrodes;
CV curve of the P/S-cotton (a) negative and (b) positive electrodes under various scan rates and,
charge discharge curves of P/S-cotton (c) negative and (d) positive electrodes under various current
densities.
127
(a)
(b)
Ag-P/S-cotton (+)
Current (A)
0.004
0.000
100 mV/s
200 mV/s
300 mV/s
400 mV/s
-0.002
-0.004
-1.0
-0.8
-0.6
-0.4
-0.2
-1.0
0.0
0
500
1000
1500
2000
2500
Time (s)
0.1
0.2
0.3
0.4
0.5
0.6
Potential (V) vs Ag/AgCl
(d)
1 mA
2 mA
4 mA
6 mA
-0.2
100 mV/s
200 mV/s
300 mV/s
400 mV/s
-0.002
0.0
-0.8
-0.4
0.000
-0.006
0.0
Ag-P/S-cotton (-)
-0.6
0.002
-0.004
Potential (V) vs Ag/AgCl
(c)
Potential (V) vs Ag/AgCl
0.006
Potential (V) vs Ag/AgCl
Current (A)
0.002
Ag-P/S-cotton (-)
Ag-P/S-cotton (+)
0.6
0.5
0.4
1 mA
2 mA
4 mA
6 mA
0.3
0.2
0.1
0.0
0
500
1000
1500
2000
Time (s)
Figure 5.5: Electrochemical performance of the of the negative and positive Ag-P/S-cotton
electrodes; CV curve of the Ag-P/S-cotton (a) negative and (b) positive electrodes under various
scan rates and, charge discharge curves of Ag-P/S-cotton (c) negative and (d) positive electrodes
under various current densities.
128
(a)
(b)
0.006
Current (A)
0.004
0.002
Potential (V) vs Ag/AgCl
Ag-P/S-cotton (+)
Ag-P/S-cotton (-)
P/S-cotton (+)
P/S-cotton (-)
@ 400 mV/s
0.000
-0.002
-0.004
-0.9
-0.6
-0.3
0.0
0.3
P/S-cotton (+)
Ag-P/S-cotton (+)
0.6
0.3
@ 0.5 mA/cm2
0.0
0
500
-0.3
1000
1500
Time (s)
2000
-0.6
P/S-cotton (-)
Ag-P/S-cotton (-)
-0.9
0.6
Potential (V) vs Ag/AgCl
(c)
(d)
-40
(degree)
-Zim()
2
'= 0.0407 s
-50
Ag-P/S-cotton
P/S-cotton
1
Ag-P/S-cotton
P/S-cotton
-30
-20
"= 0.0603 s
-10
0
0
2
3
4
Zre()
5
6
0
1
2
3
log f (Hz)
4
5
Figure 5.6: Electrochemical performance of the f-Supercapacitor single electrode application of
the smart fabric. The comparision of (a) CV curve at scan rate of 400 mV/s and (b) GCD curves
at current density of 1 mA/cm2, and the comparison of Electrochemical Impedance spectroscopic
data; (c) Nyquist plot (d) Bode plot of Ag-P/S-cotton and P/S-cotton.
129
Using Ag-P/S-cotton and P/S-cotton for both negative and positive electrodes, the CV
curves of Ag-P/S-cotton presents a rectangular shape, indicating good EDLC type capacitive
behavior and P/S-cotton shows good EDLC behavior with slight redox reactions given the minor
peaks (Figure 5.6a). The GCD curves in Figure 5.6b, shows that the Ag-P/S-cotton electrodes have
quite larger charging and discharging times than P/S-cotton electrodes, indicating the capacitance
improvement by the Ag NPs on P/S-cotton surface. The areal capacitance calculated according to
the literature[11]. It is encouraging that optimised Ag-P/S-cotton electrodes obtained high areal
capacitance of 0.8665 F/cm2 and 0.535 F/cm2 at current density of 0.5 mA/cm2 as the positive and
negative electrodes in 6 M Na2SO4 aqueous electrolyte, respectively. The individual CV and GCD
curves corresponding to P/S-cotton and Ag-P/S-cotton is shown in Fig 5.4 and 5.5. Using the
electrochemical impedance spectroscopy, the Nyquist (Figure 5.6c) and Bode plots (Figure 5.6f)
are obtained. Fast kinetics of the charge exchange of the Ag-P/S-cotton electrode is shown by
obtained low equivalent series resistance ESR and charge transfer resistance RCT [4, 14] in Nyquist
plot. The ESR values of Ag-P/S-cotton is 3.1 Ω, lower than the P/S-cotton of 4.14 Ω because of the
Ag metal NPs at the outer surface provides the high electrical conductivity with improve substrate
intrinsic conductivity and electrolyte ionic conductivity[11, 41]. No obvious semicircle is observed
from the electrodes exhibit in Fig. 5.6c, however the RCT values are observed, 1.29 Ω and 0.92 Ω
for corresponding to Ag-P/S-cotton and P/S-cotton, indicating the lower RCT values of P/S-cotton
influenced by both EDLC and minor redox nature. The Bode plot demonstrates the relaxation time
constant (τo) of the Ag-P/S-cotton and P/S-cotton is 0.04 s and 0.06 s, respectively, indicating the
130
superior power delivery and fast responsiveness [4, 42] of the Ag-P/S-cotton than the P/S-cotton.
The τo was measured using the relationship τo = 1/ fo, where fo is the peak frequency at the highest
phase angle (ϕ) [4, 43].
(b)
0.003
500 mV/s
400 mV/s
300 mV/s
200 mV/s
100 mV/s
Current (A)
0.002
0.001
1.5
Potential (V)
(a)
0.000
-0.001
IR
drop
1.2
0.9
3.0 mA/cm2
2.0 mA/cm2
1.0 mA/cm2
0.5 mA/cm2
0.6
0.3
-0.002
0.4
0.8
1.2
Potential (V)
2
Energy density (Wh/ cm )
(c)
10
2
(d)
This Work
1
10 RGO+CNT
10
MnO2+CNT
0
10
-1
10
-2
PANI+ SS
CNT+OMC
ZnO+MnO2
GFCN
-3
10 -3
10
10
-2
10
-1
10
0
0.0
0
1.6
10
1
10
Power density (mW/ cm2)
2
Areal Capacitance (F/cm2)
0.0
300
600
900
1200 1500
Time (s)
0.3
91.8 %
0.2
0.1
0.0
0
Ag-P/S-cotton
P/S-cotton
P-cotton
92.0 %
62.1 %
1000 2000 3000
Cycle number
4000
Figure 5.7: Performance of the f-Supercapacitor assembled device (a) CV curves, (b) GCD curves
and (c) Ragone plots of f-Supercapacitor devices, compared to the data reported to date (d) cycling
performance.
131
The assembled f-Supercapacitor can be operated at 1.5 V, as confirmed by Fig. 5.7a.
Degradation of aqueous Na2SO4 and gas generation can occur when the applied voltage exceeds
1.5 V, and therefore, 1.5 V was selected to investigate the electrochemical performance of the
device. The GCD curves shown in Fig. 5.7b clearly show the perfect charge/discharge mechanism.
The specific capacitance calculated from the GCD curves at different areal current densities
showed the highest capacitance of 0.274 F/cm2 at a current density of 0.5 mA/cm2. Under other
conditions, the capacitances were 0.134 F/cm2, 0.126 F/cm2, and 0.042 F/cm2 at current densities
of 1.0 mA/cm2, 2.0 mA/cm2 and 3.0 mA/cm2, respectively. To compare the f-Supercapacitor with
current modern devices, we plotted the Ragone plots (Fig. 5.7c) for a number of supercapacitors
reported in recent publications, which include studies of areal supercapacitor material:
carbonaceous and redox based materials. The obtained maximum energy and power densities for
the f-Supercapacitor devices is 85.6 μWh/cm2 and 2.38 mW/cm2, respectively quite higher than
the stated literature below. The CNT based supercapacitor mixed with RGO energy and power
densities of 3.84 μWh/cm2 and 0.02 mW/cm2, respectively [44] and CNT combined with MnO2
supercapacitor had energy and power densities of 2.6 μWh/cm2 and 0.0669 mW/cm2 and CNT
with ordered mesoporous carbon (OMC) supercapacitor 1.77 µWh /cm2, 0.089 mW /cm2 [45]
respectively [46]. The graphene based areal capacitors such as porous electrochemical reduce
graphene (pErGO) supercapacitor reported 39.3 µW h /cm2, 17.6 mW /cm2 [47] and conducting
network of graphite felt (GFCN) supercapacitor reported 0.00197 µW h /cm2, 0.00133 mW /cm2
[48] of energy and power densities, respectively.
132
The areal capacitance during 4000 cycles is plotted for the three symmetric devices of AgP/S cotton, P/S-cotton and P-cotton in Fig. 5.7d, in which the Ag-P/S cotton// Ag-P/S cotton, P/S
cotton// P/S cotton, and P-cotton// P-cotton devices retain 91.8%, 92.0%, and 62.3% of their
capacitance, respectively. Using this amphiphilic nature developed by MSMA cross-linker, we are
successfully providing the solution to fibrous detachment of active materials [11, 40]. This is an
important issue for the future research. The areal capacitances remain almost unchanged in the
case of Ag-P/S-cotton and P/S-cotton, and are maintained around 92 % after 4000 cycles but Pcotton could hold the retention around 63 %. This is signifying that the MSMA self-assembly cross
linker tightly bonded the conductive polymer PANI into cotton surface (Fig. 5.1a) and this lead
more hindrance to aqueous electrolyte to penetrate in-between active material and the cotton fabric.
Therefore, longevity is higher than normal conductive fabric.
133
5.4
Conclusion
In summary, the concept of the autonomous smart fabric which leads to higher demand of
energy, requiring storage devices with high energy densities and comprising new strategy of
flexible conductive fabric are reported by this paper. The f-Supercapacitor reported 0.867 F/cm2
and 0.535 F/cm2 for highest positive and negative electrodes. The f-Supercapacitor devices
obtained the high areal energy and power densities; 85.6 μWh/cm2 and 2.38 mW/cm2, respectively,
while its holding 92 % of capacitive retention around 4000 cycles. The MSMA layer bond the
conductive polymer into the cellulose layer of the cotton fabric. This has provided extra stability
than normal polymer coated fabric (p-cotton).
134
5.5
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Chapter 6
Thesis Conclusion
The main conclusions of this research are summarized as follows,
1. We have proposed a dual pseudocapacitive CuS/CuSCN nano composite using an in-situ
methodology, in which the compositional ratio between the CuS and CuSCN in the
nanocomposites could be controlled in a facile manner, and mesoporous CuS-coveredCuSCN nanocrystals with a large surface area could be obtained. The novel heteronanostructure could be characterised as CuSCN nanosheets wrapped by CuS nanorods. We
used the CuS/CuSCN nanocomposites as an active material for pseudocapacitors. Both
CuS and CuSCN demonstrated pseudocapacitive behaviour independently and their
nanocomposites showed high electrochemical performances.
2. We revealed that the novel Ni-pPDA and Mn-pPDA MOFs are materials with potential for
application as active materials for pseudocapacitors. We have several reasons to choose
pPDA for the ligand material. Firstly, the strong coordination bond between metal and
pPDA is expected. The amine groups in pPDA are easily bonded with a metal group by
donating their lone-pair electron. Secondly, pPDA has π-conjugated electron cloud, which
may enhance the electrical conductivity. Thirdly, the green liquid–liquid interfacial
reaction is easily achieved in pPDA and metal acetylacetonate. Thus, pPDA is a promising
candidate for the energy storage devices.
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3. We have synthesised three supercapacitor electrodes (Zn-5MOF-5, Zn-8MOF-8, and Zn12MOF) using the modified liquid-liquid interfacial reaction and used the MOF materials
in energy storage devices for the first time. The modified liquid-liquid interfacial reaction
method does not use of external power source; hence, it is a green synthesis for multilayered Zn-pPDA MOFs. Residual Zn(acac)2 slowly leaches into the Zn-pPDA MOF
compound, producing supramolecular complexes, and can decrease the electrical
conductivity by deteriorating the π–electron cloud in conjugated structure. Overall, the ZnpPDA MOF ASCs exhibited a high energy density of approximately 62.8Wh/kg and a
power density of approximately 4500 W/kg, with an excellent capacitive retention of 96%
over 2000 cycles. We believe the findings presented in our paper will overcome the major
limitations of MOF-based energy storage devices, such as limited number of ligands and
extensive laboratory protocols for synthesis.
4. We have demonstrated, the concept of the smart fabric which leads to higher demand of
energy storage devices with high energy densities and comprising new strategy of flexible
conductive fabric are reported by this paper. The Supercapacitor we fabricated here
obtained the areal capacitance: 0.867 F/cm2 and 0.535 F/cm2 for highest positive and
negative electrodes. With good flexibility, ultrahigh areal capacitance, and excellent rate
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ability, the Ag-P/S-cotton fabric shows the maximum areal energy and power densities
around 85.6 μWh/cm2 and 2.39 mW/cm2, respectively. We have provided the streatergy to
develop the polymer coated fabric that has high durability with the aquoes electrolytes.
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