MANGANESE OXIDE-BASED NANOCOMPOSITE

ELECTRODES FOR SUPERCAPACITORS

RUSI

THESIS SUBMITTED IN FULFILMENT OF THE

REQUIREMENT FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY

FACULTY SCIENCE

UNIVERSITY OF MALAYA

KUALA LUMPUR

2016

ii

UNIVERSITI MALAYA

ORIGINAL LITERARY WORK DECLARATION

Name of Candidate: RUSI (I.C/Passport No: A5809715)

Registration/Matric No: SHC120094

Name of Degree: DOCTOR OF PHILOSOPHY

Title of Project Paper/Research Report/Dissertation/Thesis (“this Work”):

MANGANESE OXIDE-BASED NANOCOMPOSITE ELECTRODES FOR

SUPERCAPACITORS

Field of Study: ADVANCED MATERIALS

I do solemnly and sincerely declare that:

(1) I am the sole author/writer of this Work

(2) This Work is original;

(3) Any use of any work in which copyright exists was done by way of fair

dealing and for permitted purposes and any excerpt or extract from, or

reference to or reproduction of any copyright work has been disclosed

expressly and sufficiently and the title of the Work and its authorship have

been acknowledged in this Work;

(4) I do not have any actual knowledge nor do I ought reasonably to know that

the making of this work constitutes an infringement of any copyright work;

(5) I hereby assign all and every rights in the copyright to this Work to the

University of Malaya (“UM”), who henceforth shall be owner of the

copyright in this Work and that any reproduction or use in any form or by

any means whatsoever is prohibited without the written consent of UM

having been first had and obtained;

(6) I am fully aware that if in the course of making this Work I have infringed

any copyright whether intentionally or otherwise, I may be subject to legal

action or any other action as may be determined by UM.

Candidate’s Signature: Date:

Subscribed and solemnly declared before,

Witness’s Signature: Date:

Name:

Designation:

iii

ABSTRACT

The increasing demands on energy have led to environmental issues and the

depletion of fossil fuels. The intense research on energy storage and conversion has

attracted much attention for future technology development. Batteries have become

a choice for energy storage devices in many applications. However, expanding

markets are pushing for alternative pulse batteries that offer high power and long

cycle life. Inspired by this, supercapacitors have attracted growing interest due to

their high power density, long cycle life, and fast charging rate, which display great

potential in complimenting or even replacing batteries in many applications. The

research work in this project concentrates on improving the specific capacitance or

energy density of nanostructure composite metal oxides electrodes, namely oxides

of manganese, nickel and cobalt (MnO2, MnO2/NiO and Mn3O4/NiO/Co3O4) with

porous texture morphology. First, MnO2 was deposited on top of stainless steel by

simple chronopotentiometry electrodeposition method with various manganese

acetate tetrahydrate (Mn(CH3COO)2·4H2O) concentrations in deposition solution.

The optimum performance was obtained using 0.01 M Mn(CH3COO)2·4H2O. In

order to enhance the specific capacitance of MnO2 electrode, the NiO was

incorporated into the MnO2 electrode by adding different concentrations of nickel

acetate tetrahydrate (Ni(CH3COO)2·4H2O) mixed with 0.01 M

Mn(CH3COO)2·4H2O. The optimum performance of MnO2-NiO electrode was

obtained using the deposition solution containing 0.25 M Ni(CH3COO)2·4H2O

mixed with 0.01 M Mn(CH3COO)2·4H2O. The optimum MnO2-NiO electrode was

further studied in different electrodeposition modes and different CV’s cycles. The

work proceeded by adding different concentrations of cobalt ion into 0.01 M

Mn(CH3COO)2·4H2O mixed with 0.25 M Ni(CH3COO)2·4H2O concentration. The

iv

effect of the addition of different cobalt ion concentrations was explored and the

obtained Mn3O4-NiO-Co3O4 ternary electrode exhibited optimum specific

capacitance of 7404 F g-1 with high energy and power density of 1028 Wh kg-1 and

99 kW kg-1 respectively at current density of 20 A g-1 in 0.5 M potassium hydroxide

(KOH)/0.04 M potassium ferrocyanide (K3Fe(CN)6) mixture electrolyte. Lastly, the

electrochemical performance of supercapacitors were enhanced by incorporating

MnO2 electrode with carbon based composite (reduce graphene oxide

(RGO)/glucose carbon). The incorporation of RGO/glucose carbon into MnO2

electrode increased the electrical conductivity of the electrode. The introduction of

D (+) glucose into the deposition solution slowed down the nucleation process of

MnO2 particles and led to uniform and ultrathin nanoflakes structure. The optimized

electrode exhibited low transfer resistance and resulted in excellent electrochemical

performance in three electrolyte systems viz. sodium sulfate (Na2SO4), KOH and

KOH/K3Fe(CN)6 electrolytes. The optimum specific capacitance obtained was

13,333 F g-1 with energy density and power density of 1851 Wh kg-1 and 68

kW kg-1 respectively at current density of 20 A g-1 in mixed 0.5 M KOH/0.04 M

K3Fe(CN)6 electrolyte. The preparation of electrodes in this work using

electrodeposition method is simple, low-cost, and environmental-friendly. It holds

great potential to produce cost-effective and high energy density supercapacitors.

v

ABSTRAK

Penggunaan tenaga yang semakin meningkat telah membawa kepada isu-isu

alam sekitar dan pengurangan bekalan bahan api fosil. Penyelidikan yang intensif

terhadap penyimpanan dan penukaran tenaga telah menarik banyak perhatian

penyelidik untuk pembangunan teknologi masa depan. Bateri telah menjadi pilihan

sebagai peranti penyimpanan tenaga dalam banyak aplikasi. Akan tetapi, pasaran

yang berkembang lebih cenderung memilih bateri yang memiliki kuasa yang tinggi

dan kitaran hidup panjang. Berinspirasikan isu ini, minat dalam penyelidikan

superkapasitor telah meningkat kerana ciri-ciri seperti ketumpatan kuasa tinggi,

kitaran hidup panjang, dan kadar pengisian cepat yang dimiliki oleh peranti ini

memainkan peranan penting dalam menggantikan bateri pada banyak aplikasi.

Kerja-kerja penyelidikan dalam projek ini tertumpu kepada peningkatan kapasitan

spesifik atau ketumpatan tenaga oleh elektrod oksida logam komposit yang

berstruktur nano, seperti oksida logam daripada mangan, nikel, kobalt (MnO2,

MnO2 / NiO dan Mn3O4 / NiO / Co3O4) dengan bertekstur poros. Pertama, MnO2

dideposit melapisi bahagian atas keluli tahan karat dengan kaedah

kronopotensimetri elektrodeposit. Pencapaian elektrod yang optimum telah

diperolehi dengan menggunakan larutan deposit mangan asetat tetrahidrat

(Mn(CH3COO)2·4H2O) berkepekatan 0.01 M. Untuk meningkatkan kapasitan

spesifik elektrod MnO2, NiO telah dikompositkan ke dalam elektrod MnO2 dengan

menambah kepekatan nikel asetat tetrahidrat (Ni(CH3COO)2·4H2O) yang berbeza

dalam 0.01 M Mn(CH3COO)2·4H2O sebagai larutan deposit. Pencapaian elektrod

yang optimum daripada MnO2-NiO telah diperolehi apabila larutan deposit

mengandungi 0.25 M Ni(CH3COO)2·4H2O bercampur dengan 0.01 M

Mn(CH3COO)2·4H2O. Elektrod MnO2-NiO ini dikaji dengan lebih lanjut

vi

menggunakan mod pengelektrodepositan yang berbeza dan kajian kitaran melalui

kaedah kitaran voltametri. Kajian ini diteruskan lagi dengan menambah ion kobalt

berbeza kepekatan ke dalam 0.01 M Mn(CH3COO)2·4H2O yang telah dicampurkan

0.25 M Ni(CH3COO)2·4H2O. Kesan penambahan ion kobalt yang berbeza

kepekatan telah dikaji dan elektrod komposit yang diperolehi daripada Mn3O4-NiO-

Co3O4 memiliki kapasitan spesifik optimum 7404 F g-1 dengan ketumpatan tenaga

dan ketumpatan kuasa yang tinggi iaitu 1028 Wh kg-1 dan 99 kW kg-1 pada

ketumpatan arus 20 A g-1 dalam elektrolit 0.5 M KOH/0.04 M K3Fe(CN)6. Akhir

sekali, teknik yang berbeza digunakan untuk meningkatkan pencapaian elektrokimia

superkapasitor dengan mengkomposit elektrod MnO2 dengan karbon komposit

(RGO / karbon glukosa). Komposit RGO/karbon glukosa dalam elektrod MnO2

boleh meningkatkan kekonduksian elektrik elektrod. Pencampuran glukosa D (+)

dalam larutan deposit boleh melambatkan proses penukleusan zarah MnO2 dan

membawa kepada struktur deposit yang teratur dan sangat tipis. Elektrod yang telah

dioptimum menghasilkan rintangan pemindahan yang rendah dan memberikan

aktiviti elektrokimia yang bagus dalam tiga sistem elektrolit iaitu, Na2SO4, KOH

dan KOH/K3Fe(CN)6. Elektrod MnO2/RGO memiliki kapasitan spesifik 13333

F g-1 dengan ketumpatan tenaga dan ketumpatan kuasa masing-masing 1851

Wh kg-1 dan 68 kW kg-1 pada ketumpatan arus 20 A g-1 dalam campuran elektrolit

0.5 M KOH/0.04 M K3Fe(CN)6. Penggunaan kaedah pengelektrodepositan bagi

penyediaan elektrod dalam projek ini merupakan kaedah yang mudah, murah, dan

mesra alam yang mempunyai potensi besar untuk menghasilkan superkapasitor

dengan kos efektif dan berketumpatan tenaga tinggi.

vii

LIST OF PUBLICATIONS

Articles published in ISI-cited journals which are related to this thesis:

1. Rusi, & Majid, S. R. (2013). Synthesis of MnO2 particles under slow cooling

process and their capacitive performances. Materials Letters, 108, 69-71.

2. Rusi, & Majid, S. R. (2014). Controllable synthesis of flowerlike α-MnO2

as electrode for pseudocapacitor application. Solid State Ionics, 262, 220-

225.

3. Rusi, & Majid, S. R. (2014). High performance super-capacitive behaviour

of deposited manganese oxide/nickel oxide binary electrode system.

Electrochimica Acta, 138,1-8.

4. Rusi, & Majid, S. R. (2015). Electrodeposited Mn3O4-NiO-Co3O4 as a

composite electrode material for electrochemical capacitor. Electrochimica

Acta, 175, 193-201.

5. Rusi, Chan, P. Y., & Majid, S. R. (2015). Layer by Layer Ex-Situ Deposited

Cobalt-Manganese Oxide as Composite Electrode Material for

Electrochemical Capacitor. PLoS ONE, 10(7), e0129780.

6. Rusi, & Majid, S.R. (2015). Green synthesis of in situ electrodeposited

rGO/MnO2 nanocomposite for high energy density supercapacitors.

Scientific Reports, 5, (16195).

7. Rusi, & Majid, S.R. (2016). Effects of electrodeposition mode and

deposition cycle on binary manganese-nickel oxide electrode for

electrochemical capacitor. PLoS ONE, 11(5), e0154566.

viii

Articles submitted and under review in ISI-cited journals:

1. Rusi, Sim, C.K., & Majid, S.R. (2016). Morphology-controlled PANI

electrode by deposition scan rate studies with H2SO4/PVA polymer

electrolyte for electrochemical capacitor. Submitted to Journal of Colloid

and Interface Science.

Papers presented:

1. Rusi, & Majid S.R. (27 May 2013). Poly(vynilpyrrolidone) / poly

(vinylidenefluoride-co hexafluoropropylene) – Magnesium

trifluoromethanesulfonate gel polymer electrolytes for electrochemical

double layer capacitor. Oral presented at the 2013 international Conference

on Advanced Capacitors (ICAC-2013), Osaka, Japan.

2. Rusi, & Majid, S.R. (7 June 2013). Controllable synthesis of flowerlike α-

MnO2 as electrode for pseudocapacitor application. Poster presented at the

19th International Conference on Solid State Ionics, Kyoto, Japan.

3. Rusi, & Majid, S.R. (8 January 2014). Controllable synthesis of flowerlike

α-MnO2 as electrode for pseudocapacitor application. Poster presented at

the 9th Mathematics and Physical Sciences Graduate Congress 2014 (9TH

MPSGC), University of Malaya, Kuala Lumpur.

4. Rusi, & Majid, S.R. (13 June 2015). Green synthesis of in situ

electrodeposited rGO/MnO2 nanocomposite for high energy density

supercapacitors. Poster presented at International Conference on

Nanotechnology, Nanomaterials & Thin Films for Energy Applications

(NANOENERGY), Manchester, UK.

ix

ACKNOWLEDGEMENTS

First of all, I would like to show my deepest respect and most sincere

gratitude to my supervisor Assoc. Prof Dr. Siti Rohana Majid for her continues

guidance, advice, patience and warm encouragement. She imparts her knowledge

and experience to me and gives me good suggestions in my experiments, manuscript

and thesis preparation.

I thank my fellow members of Center Ionics University of Malaya (CIUM)

Dr. Jun Hieng Kiat, Dr. Woo Haw Jiunn, Dr.Sim Lina, Dr. Liew Chiam Wen, Dr.

Teo Li phing, Auyong Chee meng, Chan Pei Yi, Sim Cheng Kim, Tey Jin Ping,

Shah and the rest for their laughter, advices, support and friendship through the

years. I also thank lab assistance, Mr. Ismail for helping me in purchase any research

materials.

My acknowledgments are also extended to all technicians and officers of

University of Malaya for their efforts in helping me to accomplish my research

works. I am also grateful to university for the Bright Sparks scheme scholarship

award and IPPP program (grant no. PG 010-2013A) for the financial support.

Lastly, my deepest gratitude goes towards my beloved my family, Tzu Chi

family and partner for their endless love, pray, support and motivation in numerous

way.

x

TABLE OF CONTENTS

Original literary work of declaration ii

Abstract iii

Abstrak v

List of Publications vii

Acknowledgements ix

Table of Contents x

List of Figures xiv

List of Tables xix

List of Abbreviations xx

Chapter 1: Introduction to the thesis

1.1 Background 1

1.2 Objectives of research 3

1.3 Organization of thesis 4

Chapter 2: Literature review

2.1 Introduction and historical prospective 6

2.2 Overview of supercapacitor 8

2.2.1 Electrical double layer capacitor (EDLC) 8

2.2.2 Pseudocapacitor 10

2.2.3 Hybrid capacitor 11

2.2.4 Parameters of supercapacitor 12

2.2.5 Comparison of battery and supercapacitor 13

2.3 Electrode Materials 14

2.3.1 Carbon materials 15

2.3.2 Faradic materials 17

2.3.2.1 Conducting polymers 17

2.3.2.2 Transition Metal oxides 20

2.3.3 Composite manganese oxide based electrode 21

2.3.3.1 Single manganese oxide electrode 21

2.3.3.2 Binary manganese oxide-nickel oxide electrode 31

2.3.3.3 Ternary manganese oxide-nickel oxide-cobalt oxide

electrode

2.3.4 Composite carbon-MnO2 electrode 37

35

xi

2.4 Summary 41

Chapter 3: Methodology

3.1 Introduction 42

3.2 Materials and preparation 43

3.2.1 Electrodes 43

3.2.2 Materials 44

3.3 Electrode preparation for manganese based electrode 44

3.3.1 Electrodeposition of MnO2 electrode 45

3.3.2 Electrodeposition of MnO2-NiO electrode 45

3.3.3 Electrodeposition of Mn3O4-NiO-Co3O4 electrode 46

3.3.4 Electrodeposition of RGO-MnO2 with glucose carbon electrode 47

3.4 Materials characterization and electrochemical test 48

3.4.1 Materials Characterization 48

3.4.1.1 X-ray diffraction (XRD) 48

3.4.1.2 Field-emission scanning electron microscopy

(FESEM) and energy dispersive X-ray spectroscopy

(EDX)

3.4.1.3 Transmission electron microscopy (TEM) 50

3.4.1.4 Thermogravimetric analysis (TGA) 51

3.4.1.5 Raman spectroscopy 51

3.4.2 Electrochemical test 52

3.5 Summary 54

Chapter 4: Manganese oxide electrode system

4.1 Introduction 55

4.2 Results and discussion 56

4.2.1 Optimization of Mn(CH3COO)2.4H2O concentration 56

4.2.1.1 Schematic illustration of electrodeposited MnO2

electrode

4.2.1.2 Characterization of composition and morphology of

MnO2 electrodes

4.2.1.3 Electrochemical performance of MnO2 electrodes

in KOH electrolyte

4.3 Summary 65

49

56

57

64

xii

Chapter 5: The studies of MnO2-NiO binary electrode system

5.1 Introduction 66

5.2 Results and discussion 67

5.2.1 Optimization of Ni(CH3COO)2.4H2O concentration mix with

0.01 M of Mn(CH3COO)2.4H2O for deposition electrolyte

solution

5.2.1.1 Characterization of composition and morphology of

MnO2-NiO electrode

5.2.1.2 Electrochemical performance of MnO2-NiO

electrode in Na2SO4 electrolyte

5.2.2 Effects of electrodeposition modes of MnO2-NiO electrode 78

5.2.2.1 Characterization of composition and morphology 78

5.2.2.2 Electrochemical performance of MnO2-NiO

electrode in Na2SO4 electrolyte

5.2.3 Effect of electrodeposition cycle of deposited MnO2-NiO

electrode by using cyclic voltammetry mode

5.2.3.1 The morphology studies of MnO2-NiO 82

5.2.3.2 Electrochemical performance of MnO2-NiO in

Na2SO4 electrolyte

5.2.3.3 Electrochemical performance of optimum MnO2-NiO

(CY4) electrode in various electrolytes

5.3 Summary 94

Chapter 6: The studies of Mn3O4-NiO-Co3O4 ternary electrode system

6.1 Introduction 96

6.2 Results and discussion 97

6.2.1 Optimization of CoSO4.7H2O concentration for Mn3O4-NiO-

Co3O4 electrode system

6.2.1.1 Characterization of composition and morphology of

Mn3O4-NiO-Co3O4 electrode

6.2.1.2 Electrochemical performance of Mn3O4-NiO-Co3O4

electrode in Na2SO4 electrolyte

6.2.1.3 Electrochemical performance of Mn3O4-NiO-Co3O4

electrode in various electrolytes

6.3 Summary 110

67

72

80

82

89

97

97

103

106

67

84

xiii

Chapter 7: The studies of RGO-MnO2 based nanocomposite electrode

system

7.1 Introduction 112

7.2 Results and discussion 113

7.2.1 Optimization of RGO-MnO2 electrode by varied the ratio of

GO:Mn(CH3COO)2.4H2O in deposition electrolyte solution

7.2.1.1 Schematic illustration of electrodeposited RGO-

MnO2 electrode

7.2.1.2 The composition and morphology studies of RGO-

MnO2 electrode

7.2.1.3 Electrochemical performance of RGO-MnO2

electrode in Na2SO4 electrolyte

7.2.2 Optimization of D(+)glucose content in deposition

electrolyte solution for RGO-MnO2-glucose carbon electrode

system

7.2.2.1 The composition and morphology studies of RGO-

MnO2-glucose carbon electrode

7.2.2.2 Electrochemical performance of RGO-MnO2-

glucose carbon electrode in Na2SO4 electrolyte

7.2.2.3 Electrochemical performance of RGO/MnO2-

glucose carbon electrode in various electrolytes

7.3 Summary 134

Chapter 8: Conclusions and suggestions for future work

8.1 Conclusions 135

8.2 Suggestions for future work 141

References 142

119

122

123

128

130

113

114

113

xiv

LIST OF FIGURES

Figure Caption Page

Figure 1.1 Ragone plot (specific power vs. specific energy) for

various electrochemical energy storage devices.

2

Figure 2.1 Chronology of capacitor history. 6

Figure 2.2 Schematic of Leyden Jar. 7

Figure 2.3 Charging and discharging electrical double-layer

capacitor.

8

Figure 2.4 Different types of redox mechanism in pseudocapacitor:

(a) underpotential deposition, (b) redox

pseudocapacitance, (c) intercalation pseudocapacitance.

10

Figure 2.5 The specific capacitance performance of different

electrode materials for both EDLC electrodes and

pseudocapacitor electrodes.

15

Figure 2.6 (a) CV and (b) CDC for carbon aerogels electrode. 16

Figure 2.7 Schematic visualization of Ppy’s conduction mechanism. 18

Figure 2.8 Schematic illustration of the crystal structure of

manganese oxide. (a) Rock salt, (b) spinel Mn3O4, (c)

bixbyite Mn2O3, (d) pyrolusite β-MnO2 (rutile-type), (e)

ramsdellite (diaspore-type) ([MnO6] octahedra form

infinite double layers) , (f) phyllomanganate (birnessite-

buserite family layered MnO2).

22

Figure 2.9 The specific capacitance and cyclability of different MnO2

allotropes in 0.5 M K2SO4 electrolyte at 5 mV s-1.

23

Figure 2.10 SEM images of low magnification (a,c and e) and high

magnification (b,d and f) of urchin-like MnO2 powders

synthesized at different temperatures: (1) a and b, 80°C;

(2) c and d, 110 °C and (3) e and f, 140 °C.

25

Figure 2.11 SEM images of low magnification (a and c) and high

magnification (b and d) of hierarchical MnO2 nanoflowers

synthesized at different concentrate: (1)

n(KMnO4):n(KCl) = 1:3; (2) n(KMnO4):n(KCl) = 3:1.

26

Figure 2.12 (a) Lower and (b) higher magnification SEM images of α-

MnO2 nanowire coated on CFP, c) TEM and (d) HRTEM

images of prepared α-MnO2 nanowire, (e) SAED pattern.

26

Figure 2.13 CV of α-MnO2/CEP electrode and bare CFP in 1 M

Na2SO4 electrolyte at scan rate of 10 mV s-1.

27

Figure 2.14 SEM images of: (a-c) as-prepared silkworm cocoon-like

MnO2 micropowders; (d) MnO2 nanosilks from MnO2

micropowders.

27

xv

Figure 2.15 (a) SEM images of MnO2, (b) CV curve of MnO2 electrode

in 2 M (NH4)2SO4 aqueous solution at scan rate of 10 mV

s-1.

28

Figure 2.16 SEM and cross-section images of deposited manganese

oxide prepared from: (a) 0.003 M Mn(CH3COO)2 solution

at 0.25 mA cm−2; (b) 0.005 M; (c) 0.007 M; (d) 0.01 M;

(e) 0.02 M; (f) 0.03 M Mn(CH3COO)2 solution at 5 mA

cm−2 for 10 min (T = 60 C and pH = 7.5).

29

Figure 2.17 FESEM images of 0.02 M, 0.05 M, 0.1 M, 0.2 M and 0.5

M MnSO4·H2O.

30

Figure 2.18 CV of different composite electrodes, scan rate: 10mV s-1,

(a) MnO2; (b) NiO; (c) 5% NiO; (d) 10% NiO; (e) 25%

NiO; (f) 20% NiO; and (g) 15% NiO.

32

Figure 2.19 Scanning electron micrographs of (a) MnO2, (b) Ni-Mn-O

and transmission electron micrograph of (c) MnO2, (d) Ni-

Mn-O synthesized via the hydrothermal process at 125 °C

for 3 h.

33

Figure 2.20 (a) AFM image of NMO, (b) TEM image of NMO

prepared by a potentiodynamic method at a scan rate of

200 mV s-1.

34

Figure 2.21 SEM images of: (a) Ni1/3Co1/3Mn1/3(OH)2 and (b)

Ni0.37Co0.63(OH)2.

35

Figure 2.22 Cyclability of the MNCO nanowire array at a current

density of 2 A g−1 (vs Ag/AgCl).

36

Figure 2.23 SEM images of (a, b) the hydroxide precursor and (c, d)

the MNCO nanowire array.

36

Figure 2.24 Illustrative fabrication process of the composite electrode

and SEM image of as coated MnO2-graphene.

38

Figure 2.25 FESEM images using deposition duration of: (a) 10

minutes, (b) 13 minutes and (c) 18 minutes.

39

Figure 2.26 (a) Schematics for MnOx/CNT/RGO nanohybrid

fabrication via Routes I and II. Route II includes a plasma

functionalization step prior to MnOx deposition. (b) SEM

image of the CNT/RGO supporting layer, with RGO

highlighted by dotted lines. (c) Low- and (d) high-

magnification SEM images of MnOx nanoparticles

deposited on a CNT/RGO layer.

40

Figure 3.1 Electrodeposition set up. 44

Figure 3.2 Deposited electrode of: (a) 0.01 M of section 3.2.1, (b)

NiO-15 of section 3.2.2 (c) 0.15 M of section 3.2.3.

47

Figure 3.3 Bragg diffraction by crystal plane. The path different

between beams 1 and 2 is BD+DC= 2AD sin θ.

49

Figure 4.1 Illustration of electrodeposited MnO2. 56

xvi

Figure 4.2 (a) XRD pattern of all electrodes on top of SS, (b) XRD

pattern of scrapped off deposits from 0.01 M electrode, (c)

EDX of 0.01 M electrode, (d) EDX of plain SS.

58

Figure 4.3 FESEM images of: (a) 0.0025 M, (b) 0.005 M, (c) 0.01 M,

(d) thickness of 0.01 M, (e) 0.02 M, (f) TEM image of 0.01

M electrode and (g) lattice spacing of deposited MnO2.

60

Figure 4.4 (a) CV curves of all deposited electrode at 0.5 M KOH

electrolyte, (b) CV of scan rate dependent of 0.01 M

electrode, (c) the specific capacitance against scan rate

plot, (d) CDC curve at applied constant current of 1 mA.

63

Figure 4.5 Cycling performance of 0.01 M electrode in 0.5 M KOH

electrolyte at current density of 1 A g-1.

64

Figure 5.1 XRD patterns of (a) empty stainless steel and all deposited

electrode on top of SS, (b) powder of NiO-25 which was

scraped off from stainless steel (c) EDX of the NiO-25

deposited powder.

68

Figure 5.2 FESEM images of: (a) NiO-15, (b) NiO-20, (c) NiO-25,

(d) NiO-30 and (e) NiO-40.

70

Figure 5.3 TEM images of (a) NiO-15, (b) NiO-25, (c) NiO-30, and

(d) lattice fringes of NiO-25 deposited film.

72

Figure 5.4 (a) Nyquist plots of the all deposited electrodes, (b) CDC

profiles of all deposited electrodes at a current density of

1 A g–1, (c) CV curves of all deposited at scan rate of 1 mV

s-1, (d) CV curves of NiO-25 at wide operation potential

range of -1 V until 1 V at different scan rates, (e) specific

capacitance of NiO-25 at wide operation potential range of

-1 until 1 V at different scan rates.

75

Figure 5.5 Specific capacitance retention at scan rate of 10 mV s–1 in

0.5 M Na2SO4 electrolyte with potential range of -1 to 1 V.

77

Figure 5.6 (a) XRD pattern of the deposited powder using different

electrodeposition modes.

78

Figure 5.7 FESEM (left) and TEM (right) images from: (a) CP, (b)

CA, (c) CY7.

79

Figure 5.8 CV profiles of deposited electrodes using different

electrodeposition modes in Na2SO4 electrolyte.

81

Figure 5.9 FESEM images of : (a) CY4, (b) CY7, (c) CY10, (d) CY13

(e) TEM images of CY4 electrode (inset: lattice spacing).

83

Figure 5.10 (a) CV curves at a scan rate of 5 mV s-1, (b) CDC profile

at a current density of 1 A g-1 in a voltage range of -

0.6 V to 1 V, (c) Specific capacitance calculated from

discharge curve and deposit mass over deposition cycle.

85

Figure 5.11 (a) Nyquist plot of all electrodes in frequency range from

10 mHz to 100 kHz at Na2SO4 electrolyte, (b) Bode plots

of frequency dependence on the impedance magnitude

(Z); (c) Bode plots of frequency dependence on phase

88

xvii

angle (), (d) Specific capacitance retention until the

1000th cycle in 0.5 M Na2SO4 electrolyte with potential

range of -1 to 1 V.

Figure 5.12 CV curve of CY4 at scan rate of 5 mV s-1 within potential

range of -0.5 V to 0.5 V at : (a) 0.5 M Na2SO4, (b) 0.5 M

KOH, (c) mix 0.5 M KOH/0.04 M K3Fe(CN)6 and (d) 0.04

M K3Fe(CN)6 electrolyte.

91

Figure 5.13 (Schematic of the role of hexacyanoferrate (II) and (III) in

the process of: (a) charge and (b) discharge of CY4

electrode.

92

Figure 5.14 CDC profiles of CY4 electrode at: (a) current density of

20 A g-1 in three different electrolytes and (b) different

applied current densities in mix KOH/K3Fe(CN)6

electrolyte.

93

Figure 5.15 Specific capacitance retention after 1500th cycle at scan

rate of 10 mV s-1 in potential range of -0.5 V to 0.5 V.

93

Figure 6.1 (XRD pattern for (a) all deposited electrode on the SS, (b)

deposited powder of 0.15 M scraped off from SS, (c) EDX

of deposited powder.

99

Figure 6.2 FESEM images of: (a) 0.05 M, (b) 0.1 M, (c) 0.15 M, (d)

0.15 M (cross-section), (e) 0.2 M and (f) 0.3 M.

101

Figure 6.3 TEM images of: (a) 0.1 M, (b) 0.15 M and (c) the lattice

fringes of 0.15 M deposited film.

103

Figure 6.4 (a) CV curves of all electrodes at scan rate of 5 mV s-1

(inset: graph of calculated specific capacitance), (b) CDC

profiles for electrodes at current density of 1 A g-1 (inset:

graph of calculated specific capacitance), (c) impedance

spectra for all electrodes.

105

Figure 6.5 CV curve of 0.15 M electrode in voltage range of -0.5 V

to 0.5 V at scan rate of 5 mV s-1 in: (a) 0.5 M Na2SO4

electrolyte, (b) 0.5 M KOH electrolyte, (c) 0.04 M

K3Fe(CN)6 electrolyte, (e) 0.5 M KOH/0.04 M K3Fe(CN)6

electrolyte.

108

Figure 6.6 CDC profiles of 0.15 M electrode at different current

densities in: (a) 0.5 M KOH electrolyte, (b) 0.5 M

KOH/0.04 M K3Fe(CN)6 electrolyte, (c) electrochemical

stability of electrode in three different electrolytes, (d)

0.15 M electrode after 1800 cycles in 0.5 M KOH/0.04 M

K3Fe(CN)6 electrolyte.

110

Figure 7.1 A schematic illustration of RGO-MnO2 mechanism via

electrodeposition.

114

Figure 7.2 XRD pattern of: (a) M30, M60, and M90 on top of SS, (b)

scraped-off deposits powder of M30, M60, and M90; (c)

Raman spectroscopy of selected electrodes.

116

xviii

Figure 7.3 EDX spectrum of: (a) empty SS, (b) MnO2 without GO,

(c) M30.

116

Figure 7.4 FESEM morphology images and cross section thickness

of: (a1, a2) pristine MnO2, (b1, b2) M30, (c1, c2) M60, and

(d1, d2) M90.

118

Figure 7.5 TEM images of: (e) M30 and (f) M60. 119

Figure 7.6 (a) CV curve in 0.5 M Na2SO4 electrolyte at a scan rate of

5 mV s-1 of: pristine MnO2, M30, M60, and M90, and (b)

Nyquist plot of: (c) M30, M60, and M90.

121

Figure 7.7 A schematic illustration of RGO-MnO2 mechanism via

electrodeposition: (a) without glucose and (b) with

glucose.

122

Figure 7.8 (a) XRD pattern of G01, G03 and G06 electrode, (b) The

Raman spectroscopy of G01, G03 and G06 electrode, (c)

deconvolution of M30 G03 in the range of 1000 to 1800

cm-1, (d) deconvolution of G03 in the range of 1000 to

1800 cm-1, (e) EDX result of G03 electrode, (f) TGA of D-

(+)-glucose.

125

Figure 7.9 FESEM morphology images and cross section of: (a1, a2)

G01, (b1, b2) G03, (c1, c2) G06.

126

Figure 7.10 TEM images of G03 M in: (a) low-magnification, (b) high-

magnification.

128

Figure 7.11 (a) CV curve in 0.5 M Na2SO4 electrolyte at a scan rate of

5 mV s-1 of G01, G03, and G06, and (b) Nyquist plot of

G01, G03, and G06.

129

Figure 7.12 CV curve of G03 at a scan rate of 5 mV s-1 within potential

scan of -0.5 V to 0.5 V in : (a) 0.5 M Na2SO4, (b) 0.5 M

KOH, (c) 0.04 M K3Fe(CN)6, and (d) 0.5 M KOH/0.04 M

K3Fe(CN)6 electrolyte solution.

132

Figure 7.13 (a) CDC curve of G03, and (b) the cyclability test of G03

electrode in three different electrolytes.

133

xix

LIST OF TABLES

Table Caption Page

Table 2.1 Overall comparisons of supercapacitor and battery

characteristics.

14

Table 2.2 Theoretical and experimental specific capacitance of

conducting polymers.

18

Table 2.3 Specific capacitance of composite and treated materials. 19

Table 2.4 Specific capacitance dependence on MnO2 phase structure

and specific surface area (SSA).

24

Table 5.1 Equivalent circuit parameters deducted by fitting Nyquist

plots and frequency at ɸ=-45° for all electrodes.

87

Table 7.1 Equivalent circuit parameters deducted by fitting Nyquist

plots of M30, M60 and M90.

120

Table 7.2 Raman bands, IDx/IG ratio and vibration modes of M30 and

G03 electrode.

124

Table 7.3 Equivalent circuit parameters deducted by fitting Nyquist

plots of G01, G03 and G06.

129

Table 8.1 The specific capacitance of all deposited MnO2 electrode

in 0.5 M KOH solution in at scan rate of 5 mV s-1

136

Table 8.2 The specific capacitance of all deposited MnO2-NiO

electrode in 0.5 M Na2SO4 electrolyte in potential range of

0 to 1 V at scan rate of 5 mV s-1.

137

Table 8.3 The specific capacitance of Mn3O4-NiO-Co3O4 electrode

in 0.5 M Na2SO4 electrolyte in potential range of 0 to 1 V

at scan rate of 5 mV s-1.

138

Table 8.4 The specific capacitance of RGO-MnO2 electrode in 0.5

M Na2SO4 electrolyte in potential range of -1 to 1 V at

scan rate of 5 mV s-1.

139

xx

LIST OF ABBREVIATIONS

AC Activated carbon

Ag Silver

AgCl Silver chloride

CA Chronoamperometry

CDC Charge-discharge

CFP Carbon filter paper

CNT Carbon nanotube

CoSO4·7H2O Cobalt sulphate

Co0.5Mn0.4Ni0.1C2O4*nH2O Cobalt manganese nickel oxalates

micropolyhedrons

Co3O4 Cobalt oxide

CP Chronopotentiometry

CPE Constant Phase element

CV Cyclic voltammetry

C6H12O6 (+) glucose anhydrous

ECs Electrochemical capacitors

EDLC Electrical double layer capacitor

EDX Energy dispersive X-ray spectroscopy

Ef Final potential

Ei Initial potential

EIS Electrochemical impedance spectroscopy

EVs Electric vehicles

FESEM Field-emission scanning electron microscopy

Fe3O4 Iron oxide

FRGO Functionalized RGO

GO Graphene oxide

H2 Hydrogen

H2O Water

H2SO4 Sulphuric acid

KCl Potassium cloride

KOH Potassium hydroxide

xxi

KMnO4 Potassium permanganate

K3Fe(CN)6 / K4Fe(CN)6 Potassium ferricyanide

MnSO4 Manganese (II) Sulfate

MnxOx Manganese oxides

MnSO4·H2O Manganous sulfate monohydrate

MnO2/ Mn3O4 /MnO6 Manganese oxide

MWCNTs Multiwalled carbon nanotubes

Mn(OH)2 Manganese hydroxide

Mn(CH3COO)2 Manganese acetate

Mn(CH3COO)2·4H2O Manganese acetate tetrahydrate

NaCl Sodium Chloride

Na2SO4 Sodium sulphate

NiO Nickel oxide

NiMn2O4 Nickel Manganese Oxide

Ni1/3Co1/3Mn1/3(OH)2 Nickel-cobalt-manganese oxide

Ni(CH3COO)2·4H2O Nickel acetate tetrahydrate

O2 Oxygen

PAni Polyaniline

PEDOT Poly (3,4-ethylenedioxythiophene)

PEDOT-PSS poly(3,4-ethylenedioxythiophene) polystyrene

sulfonate

PPy Polypyrrole

PTh Derivatives polythiophene

PVDF Polyvinylidene fluoride

PVP Polyvinylpyrrolidone

Rct Charge transfer resistance

RGO Reduced graphene oxide

Rs Series resistance

RuOx Ruthenium oxide

RuO2 Ruthenium dioxide

SC Specific dependence

SDS Sodium dodecyl sulfate

SnO2 Tin oxide

SS Stainless steel

SSA Specific surface area

xxii

SWCNTs Single-walled carbon nanotubes

TEM Transmission electron microscopy

TGA Thermogravimetric analysis

VO/V2O5 Vanadium oxide

W Warburg impedance

XRD X-ray diffraction

ǀZǀ Impedance magnitude

ɸ Phase angle

1

CHAPTER 1: INTRODUCTION

1.1 Background

Today world population is increasing and reaches its 7 billion mark, the

energy usage will also increase. The increasing demand on energy will definitely

lead to environmental issues and depletion of fossil fuels oil. Thus, the urgent need

for efficient, clean and sustainable energy source with new energy conversion and

storage devices are needed (Wang et al., 2012a).

Renewable energy production from sun and wind have rapidly growth as

well as the development of electric vehicles (EVs) with low carbon. Since the

renewable source from sun and wind are both intermittent, electrochemical storage

devices such as rechargeable batteries and supercapacitors (which also known as

electrochemical capacitors (ECs)) are very important to store energy during off-peak

which make the electricity of technologies can smooth out when it operates. Energy

storage devices have attracted much attention for future technologies (Wei et al.,

2011).

Lithium batteries have been widely used in many electronic devices such as

mobile phones, laptops and hybrid vehicles because of their high energy density

with sustainable power supply (Figure 1.1). Supercapacitors / ECs may not have

energy density as high as lithium ion batteries but the development of

supercapacitors with combination of high power and reasonable energy density may

create another landmark in industries and become a versatile solutions to a variety

2

of emerging energy applications, such as power backup, pacemakers, air bags and

electric vehicles (Mai et al., 2011; Wei et al., 2011).

Supercapacitors can operate at higher operating voltage and their

performance do not depend on temperature. They have extremely low maintenance

with lifetimes up to 20 years. However, the low energy density and high production

cost are still some of the major challenges need to be overcome for commercialized

supercapacitor.

Figure 1.1 Ragone plot (specific power vs. specific energy) for various electrochemical energy

storage devices. Taken from Wei et al., 2011.

The research on the improvement of supercapacitor’s electrode has attracted

a lot of attention among researchers. To date, activated carbon (AC), carbon

nanotubes (CNT), reduced graphene oxide (RGO)) (Yang et al., 2012; Zhu et al.,

2012), transition metal oxides (ruthenium dioxide (RuO2), manganese dioxide

3

(MnO2), nickel oxide (NiO), cobalt oxide (Co3O4) (Wang et al., 2010b; Wei et al.,

2011) and conducting polymers (polypyrrole (PPy), polyanaline (PAni), poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS) and polythiophene)

(Alvi et al., 2011; Snook et al., 2011) have been recognized as the most promising

materials for supercapacitors. In recent years, many researchers have focused on

transition metal oxides materials due to its fast faradic reaction. Ruthenium oxide

(RuOx) has been found to have a good electrochemical performance. However, its

high cost and high toxicity have restricted its further applications. Therefore, the

research spotlight has turned towards inexpensive metal oxides such as MnO2, NiO,

Co3O4 and vanadium oxide (VO) (Wei et al., 2011). The electrochemical

performance of those inexpensive metal oxide electrodes are significantly affected

by their morphologies and porous texture. Hence, the design of morphology with

great porous texture is one of the great interests to develop high performance of

supercapacitor.

1.2 Objectives of research

MnO2 is one of the promising materials which can replace ruthenium oxide

due to its average specific capacitance (~1370 F g-1), natural abundance,

environmental compatibility and low cost (Hu et al., 2011a; Xiong et al., 2013).

However, it is still hampered by its poor electrical conductivity, lack specific

capacitance and material dissolution during electrochemical cycling. Thus, making

nanocomposite MnO2 with secondary metal oxide or carbon mixing is an important

approach in enhancing the electrochemical performance (Li et al., 2012a). In this

work, the electrochemical performance of MnO2 electrode is enhanced by using

secondary metal oxide (NiO and Co3O4) and nanocomposite MnO2 with carbon

4

based (RGO-glucose carbon) electrodes. The correlation between electrode

morphologies and their electrochemical performances are systematically studied.

Our results could pave way for the future development of high performance

supercapacitor.

The objectives of this work can be listed as follows:

a) To produce high performance manganese oxide based electrode by

optimizing the concentration of metal ions in deposition electrolyte.

b) To enhance the specific capacitance and energy density of the manganese

oxide based electrode by using secondary metal oxide (NiO and Co3O4) and

compositing with carbon from glucose molecule.

c) To study the influence of various electrolytes of Na2SO4, KOH and mix

KOH/K3Fe(CN)6 on the performance of optimized electrode.

1.3 Organization of thesis

This thesis is divided into 8 main chapters. Chapter 1 begins with an

overview of supercapacitor followed by the objectives of this thesis. Chapter 2

serves to provide some background on supercapacitor and different types of metal

oxides used in this work. Chapter 3 presents a detail description on the synthesis

and material characterization methods used in this research.

Chapter 4 discussed the optimization of MnO2 electrode. The optimum

concentration of manganese acetate in deposition solution as single metal oxide

source to produce the best MnO2 electrode is reported and this work has been

published in “Rusi, Majid SR, synthesis of MnO2 particles under slow cooling

process and their capacitive performances, Materials Letters 108, 69-71 (2013)”.

5

In chapter 5, the optimization of nickel acetate concentration mixed with

0.01 M manganese acetate in deposition solution to produce best MnO2-NiO

electrode is studied and the paper has been published in “Rusi, Majid SR, high

performance super-capacitive behaviour of deposited manganese oxide/nickel oxide

binary electrode system, Electrochimica Acta 138, 1-8 (2014)”. The various

electrodeposition modes in best MnO2-NiO electrode is further investigated, the

paper has been published in “Rusi, Majid SR, effects of electrodeposition mode and

deposition cycle on binary manganese-nickel oxide electrode for electrochemical

capacitor, PLoS ONE 11(5), e0154566 (2016).

In Chapter 6, the different concentrations of cobalt ion have been added into

0.01 M manganese acetate mixed 0.25 M nickel acetate. The optimization of Mn3O4-

NiO-Co3O4 ternary electrode has been studied. This work has been published as

“Rusi, Majid SR, electrodeposited Mn3O4-NiO-Co3O4 as a composite electrode

material for electrochemical capacitor, Electrochimica Acta175, 193-201 (2015)”.

The study of MnO2 electrode composite with RGO/glucose carbon is

reported in Chapter 7. In this work, the different approaches were used to improve

the electrochemical performance of MnO2 electrode. The investigation of additional

different concentrations D (+) glucose in deposition electrolyte in MnO2/RGO

electrode is included. Chapter 7 has been published in “Rusi, Majid SR, green

synthesis of in situ electrodeposited RGO/MnO2 nanocomposite for high energy

density supercapacitors, Scientific Reports 5, 16195 (2015)”. Chapter 8 concludes

the thesis with some suggestions for future work.

6

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction

2.1 Introduction and historical prospective

Energy storage devices in the form of batteries and supercapacitors are

widely used for powering the portable electronics in our society as well as in

transport sector (Augustyn et al., 2014). The usage of energy is expected to be

increased year after year. Storage devices like supercapacitor is highly expected to

reduce the over energy demand problem. It has higher power density and longer

cycle life than batteries (Feng et al., 2014; Wang et al., 2012a). Capacitor was first

made in mid of the 18th century during the period when the phenomena associated

with “static electric” were studied (Conway et al., 1997). The chronology of

capacitor history is shown in Figure 2.1.

Figure 2.1 Chronology of capacitor history.

In 1746, a Leyden jar was discovered by either Kleist at Leyden or almost

simultaneously by Musschenbroek at Kamin, Pomerania. In later terminology, the

device was referred as capacitor. Leyden jar consist of a glass plate as electrolyte

which was contacted with electrodes of metal foil on each surface of glass, or rigid

metal plates separated by vacuum or air (Figure 2.2)

Layden jar/

capacitor

1746 EDLC1957 Psudo-capacitor1975

Hybrid capacitor1994

7

Figure 2.2 Schematic of Leyden Jar, taken from Conway, 1999.

During the mid-18th century the full significance of charge-discharging (CDC)

mechanism could not have been fully understood and remind poorly understood for

140 years (Conway, 1999).

In 1957, Backer has patented his work on electrical energy storage by means

of charge held in the interfacial double layer porous carbon material in an aqueous

electrolyte, which known as electrical double layer capacitor (EDLC). In order to

increase the energy density, the different principles of supercapacitor has been

developed by Conway (Conway, 1999). Conway in Ottawa has developed new type

of supercapacitor so-called “pseudocapacitor”. The pseudocapacitor was firstly

developed using ruthenium dioxide (RuO2) electrode in H2SO4 electrolyte. The

pseudocapacitance is highly depending on the oxide redox reaction based on

absorption of H+. This system has ideal capacitive behaviour with large degree of

reversibility and stable cyclability over some 105 cycles (Conway et al., 1997). Later

on, hybrid capacitor has been discovered by David A. Evan in the year of 1994 by

utilizing both electrodes of EDLC and pseudocapacitor.

8

2.2 Overview of supercapacitor

2.2.1 Electrical double layer capacitor (EDLC)

EDLC has similar design and manufacturing to batteries which consists of

two carbon based electrodes, electrolyte and a separator that electrically isolates two

electrodes (Kötz & Carlen, 2000; Wang et al., 2012a). The charge storage

mechanism in EDLC is based on the pure electrostatic charge accumulation at

electrode-electrolyte interface, Figure 2.3.

Figure 2.3 Charging and discharging electrical double-layer capacitor.

During charging process, the electrons travel from negative electrode to positive

electrode through external load. The cations/anions in electrolyte will accumulate at

the electrodes surface and form double layers in order to provide electro neutrality

of the system (Kötz & Carlen, 2000; Wang et al., 2012a). The size of electrolyte

ions and concentration of electrolyte will influence the thickness of formed double

layers; normally the thickness is in order of 5–10 Å for concentrated electrolytes.

The mechanisms of surface electrodes include the surface dissociation as well as

ions adsorption from both electrolyte and crystal defects, however there is no

Discharge

Recharge

Anode

Separator Electrolyte Cathode

9

charges transfer or ion exchanges occur between electrolyte and electrode (Wang et

al., 2012a). In this way, the energy is stored in the double layer interface.

There are few important factors which affect the overall performance of the

devices, such as electrode, current collector, separator as well as the electrolyte.

Thus, the suitable materials are needed to use in order to produce good

supercapacitors. The carbon based materials such as activated carbon (AC), porous

carbon, carbon nanotube (CNT) and graphene oxide (GO) have been widely used in

EDLC (Obreja, 2008; Yu et al., 2013). Up to now, the best activated carbon can

achieve specific capacitance of EDLC at around 15 F g-1 , using carbon with

optimum pore size in ionic liquid electrolytes (Simon & Gogotsi, 2008). The high

surface area of graphene electrodes have gave specific capacitance in the range of

100-250 F g-1 (Augustyn et al., 2014; Tsai et al., 2013).

In summary, the use of the high surface area of carbon for electrodes will

lead to higher capacitance and energy density stored compared to conventional

capacitors (Kötz & Carlen, 2000). However the energy density is still much lower

than batteries. Most commercially available EDLC devices can store energy density

between 3 to 6 Wh kg-1 (Augustyn et al., 2014; Obreja, 2008). Thus, the

involvement of redox reaction mechanism in electrode is used to overcome the low

specific capacitance problem (Conway,1999).

10

2.2.2 Pseudocapacitor

Pseudocapacitor is another type of supercapacitor which the charge storage

mechanism is based on faradic mechanism of electroactive materials (Zhang et al.,

2009). Conway (1999) identified that several faradic mechanisms which possible to

occur and result in capacitive electrochemical performance, include: (1) under

potential deposition, (2) redox pseudocapacitance, (3) intercalation of

pseudocapacitance (illustrated in Figure 2.4 by specific example of electroactive

electrode and electrolyte materials) (Augustyn et al., 2014).

Figure 2.4 Different types of redox mechanism in pseudocapacitor: (a) underpotential deposition, (b)

redox pseudocapacitance, (c) intercalation pseudocapacitance, taken from Augustyn et al., 2014.

Underpotential deposition (adsorption) mechanism occurs when monolayer

from electrolyte metal ions is adsorbed on the electrode metal’s surface as illustrated

in Figure 2.4 (a). The second mechanism of redox pseudocapacitance will occur

when the ions are electrochemically adsorbed on the surface or near surface of

electrode active material with a concomitant faradic charge transfer. While for

intercalation pseudocapacitance mechanism, it occurs when the ions intercalate into

Underpotential

Deposition Redox

Pseudocapacitance

Intercalation

Pseudocapacitance (a) (b)

(c)

Au + xPb2++ 2xe- Au-xPbads RuOx(OH)y + δH+ +δe- RuOx-δ(OH)y+δ Nb2O5 + xLi+ + xe- LixNb2O5

Pb2+ in

electrolyte

Pb

Monolayer Au

electrode

RuO2

nanocluster Hydrous grain

boundary H+ in

electrolyte

Insertion host

material Li+ in

electrolyte

Cu

rren

t co

llec

tor

Cu

rren

t co

llec

tor

Cu

rren

t co

llec

tor

11

the tunnels of active materials accompanied by a faradic charge transfer with no

crystallographic phase change. The different physical processes, morphologies and

different type of materials will influence the occurrence of these three mechanisms.

The high rate capability to store the energy is limited by the surface and not limited

by solid state diffusions, while the power capability is limited by the solid-state

diffusion within the cathode and anode active materials (Augustyn et al., 2014).

Commonly, conducting polymers and metal oxides are used for electrode

materials for pseudocapacitor. The adsorption mechanism and redox reactions will

occur in transition metal oxides electrodes and reversible electrochemical doping–

dedoping is presented in conductive polymer electrodes. Pseudocapacitor has higher

energy density but lower power density than EDLC and often lacks of cycle stability.

Thus, this field has attracted a great attention for researchers to study and to find out

the suitable electrode materials for improving the performance of pseudocapacitor.

2.2.3 Hybrid capacitor

Other than EDLC and pseudocapacitor, there is another type of

supercapacitors which called hybrid capacitors. Hybrid capacitor is an asymmetric

capacitor which builds from combination of faradic electrode combined with

another capacitive electrode (Simon & Gogotsi, 2008; Yu et al, 2013). This system

has utilized the combination of battery-like electrode which provide high energy

density and EDLC electrode which enables for high power capability in one system

as cathode and anode electrode. This type of supercapacitor generally has

advantages in improving overall cell potential, energy and power densities in

compared with EDLC (Wang et al., 2012a; Yu et al., 2013; Zhang et al., 2009). In

12

this hybrid system, the novelty of electrode materials with rational design of

composite materials, morphology particle size and electrolyte are the key to achieve

high electrochemical performance (Yu et al., 2013).

2.2.4 Parameters of supercapacitor

In electrochemical storage device, energy density and power density have

become the most important parameters for determining the electrochemical

performance. There are also several main parameters which affect their

performance. The theoretical (maximum) energy density (E) and power density (P)

can be expressed as following Equation 2.1 and Equation 2.2 (Obreja, 2008; Zhang

& Zhao, 2009) :

𝐸 =1

2 𝐶𝑉2 (Equation 2.1)

𝑃 =𝑉2

4 𝑚𝑅𝑠 (Equation 2.2)

Where Q is the storage total charge, C, V, m and Rs are specific capacitance, range

of operating potential, mass of electroactive materials and the equivalent resistance

of supercapacitor.

From the equation, the parameters of C, V and Rs are the most important

parameters which influence the performance of the devices. In order to increase the

energy and power density, the values of both V and C have to be increased and the

value of Rs need to be reduced (Obreja, 2008). The operating potential (V) is

determining by electrolyte window stability which means the operating potential can

be increased by finding the suitable electrolyte for the system. For example aqueous

13

electrolyte normally can operate about 1 V, while organic electrolytes have cell

potential in the range of 3 to 3.5 V. The high operating potential also can be achieved

by finding the suitable electrode materials and optimizing the electrode structure.

The specific capacitance has been known as the most important and effective

parameter in increasing the energy density. Based on equation 2.1, higher

capacitance will results in higher energy density. The capacitance is strongly

dependent on its electrode layer structure and the electron/ion transfer within the

electrode layer. Thus, optimizing the electrode structure become an important

research topic for enhancing the electrochemical performance (Wang et al., 2012a).

Also, the power density can be increased by lowering the cell internal

resistance. Internal resistance is caused by the electrode-electrolyte transfer. In

pseudocapacitor, the power density is still higher than battery which is due to the

very fast electron and ion transfer within electrode-electrolyte (Wang et al., 2012a).

2.2.5 Comparison of battery and supercapacitor

In energy storage by EDLC, there is only excess of electrons accumulation

on capacitor plate when charging and discharging. In the case of pseudocapacitor,

there is a faradic charges transfer without any crystallographic phase change. For

electrochemical energy storage in battery, the cell will go through faradic reaction

and chemical interconversion usually results in phase changes of anode and cathode.

Battery usually has irreversibility redox reaction. Furthermore the interconversion

of chemical electrode reagents in the battery can lead to poor cycle life up to one

thousand to several thousand of charge discharge cycles (Conway et al., 1997). The

14

overall comparison of supercapacitor and battery (Conway, 1999) are summarized

in Table 2.1.

Table 2.1 Overall comparisons of supercapacitor and battery characteristics, taken from Conway,

1999.

No Capacitor Battery

1 Relative low energy density Moderate or good energy density

2 Good power density Relative low power density

3 Excellent cycle life (EDLC type) Less cycle life due to irreversibility

of redox and phase change process

in three dimensional

4 Intrinsically sloping CDC curve Ideally has constant discharge or

recharge potential, except Li

intercalation system

5 Good intrinsic stage of charge

indication

Does not have good intrinsic stage

of charge indication except for Li

intercalation system

6 Little or no activation polarization

by capacitance may be

temperature dependent

Significant T dependent activation

polarization

7 Long life time except for current

collector corrosion

Poor life time caused by degradation

or reconstruction of active materials

8 Electrolyte conductivity can

diminish on charging due to ion

adsorption

Electrolyte conductivity can

decrease and increase on charging,

depending on chemistry reaction of

cell

2.3 Electrode Materials

In earlier discussion, energy and power density are strongly depend on the

electrode, thus electrode materials plays an important role to produce a good

supercapacitor. The specific capacitance value of electrode materials for carbon,

polymers and metal oxide materials is displayed in Figure 2.5 (Yu et al., 2013).

15

Figure 2.5 The specific capacitance performance of different electrode materials for both EDLC

electrodes and pseudocapacitor electrodes, taken from Yu et al., 2013.

As shown in the Figure 2.5, the ruthenium oxide (RuOx) and its composite

materials electrodes have exhibited high specific capacitance due to RuOx has high

reversible charge-discharge (CDC) feature and good electrical conductivity (Kim et

al., 2005; Simon & Gogotsi, 2008). However, it is still not applicable for commercial

use due to its high cost and toxicity. Hence, the studies of inexpensive and nontoxic

materials have been explored for supercapacitor’s electrodes. In following section,

we will discuss the supercapacitve behaviour of some of the inexpensive electrode

materials.

2.3.1 Carbon materials

Carbon based electrodes with various modification morphologies are most

frequently used for electrochemical electrodes (Kötz & Carlen, 2000). The

advantages of carbon materials include high specific surface area, low cost, good

electronic conductivity, high stability, non-toxicity and easy processing has attracted

many researchers for using it especially for EDLC electrode materials.

16

Generally, carbon based electrode has good rectangular shape of Cyclic

voltammetry (CV) curves, shown in Figure 2.6 (a) and triangular symmetrical

distribution obtained for the galvanostatic CDC profiles (Figure 2.6 (b)) (Liu et al.,

2013). This profiles reveal that the system has ideal capacitor behaviour and good

capacitive properties (Li et al., 2010).

Figure 2.6 (a) CV and (b) CDC for carbon aerogels electrode, taken from Liu et al., 2013.

According to Conway, the good materials for EDLC must have good inter-

particle conductivity in porous matrices and good electrolyte accessibility. In order

to fulfill these requirements, high and accessible surface area of carbons is needed

(Conway, 1999). High surface area of carbon materials with suitable pores structure

is consider as an ideal materials for supercapacitor in term of good specific

capacitance, specific power and cycle life. Activated carbon, carbon aerogels,

carbon nanotubes (CNTs), porous carbons and carbon nanofibres are categorized as

high surface area carbons (Wang et al., 2012a). As reported, both activated carbon

and carbon aerogels can only exhibit limited specific capacitance of 40-160 F g-1

(Wang et al., 2012a). Multi walled carbon nanotubes (MWCNTs) electrodes have

specific capacitance in the range of 4-135 F g-1 (Frackowiak et al., 2001; Frackowiak

et al., 2000; Obreja, 2008) and single-walled carbon nanotubes (SWCNTs)

(a) (b)

17

electrodes have been known to achieve a maximum specific capacitance of 180

F g-1 in potassium hydroxide (KOH) electrolyte (Obreja, 2008).

Based on previous studies, although carbon with high surface area and good

electrolyte accessibility is used, the device has also exhibited limited specific

capacitance. This is caused by their high resistivity arises from high internal series

resistance and contact resistance. The development of high performance and low

cost carbon electrodes remains a major challenge.

2.3.2 Faradic materials

Others than carbon based materials, faradic materials are also widely used

as electrode materials for supercapacitors which the storage charge mechanisms are

based on the fast and reversible surface redox reactions (faradic reactions). Faradic

materials are generally classified into two types: Conducting polymers and transition

metal oxides. Recently a lot of research works have been carried out to modify and

develop the suitable faradic materials for pseudocapacitor.

2.3.2.1 Conducting polymers

Conducting polymers are organic polymers that have ability to conduct

electricity through a conjugated bond system along the polymer chain (Shown et al.,

2015). It is widely used as supercapacitor electrode materials due to their reversible

faradic redox nature. In addition to this, compared to metal oxides, conducting

polymer is cheaper and can be easily integrated into flexible electronics system

(Rudge et al., 1994; Snook et al., 2011). Some of the commonly used conducting

18

polymers are polypyrrole (PPy), polyaniline (PAni) derivatives polythiophene (PTh)

and poly (3,4-ethylenedioxythiophene) (PEDOT). Their typical dopant level, their

typical specific capacitance and operation potential range are listed in Table 2.2.

Table 2.2 Theoretical and experimental specific capacitance of conducting polymers, taken from

snook et al., 2011.

Conducting

polymer

Mw

(g mol-1)

Dopant

level

Potential

range (V)

Theoretical

specific capacitance

(F g-1)

Measured

Specific capacitance

(F g-1)

PAni 93 0.5 0.7 750 240

PPy 67 0.33 0.8 620 530

PTh 84 0.33 0.8 485 -

PEDOT 142 0.33 1.2 210 92

The storage mechanism of conducting polymer electrode is originated from

a doping process. For PAni and PPy, the conductivity arises from p-doping (known

as oxidative doping) wherein the net positive charge will occurs when the pi-

electrons from conjugation is removed (Ramya et al., 2013). It leads to existence of

“polarons” and “bipolarons” as to imply the creation of charge carriers in the

polymer chain. A schematic illustration of the charge storage mechanism of PPy is

displayed in Figure 2.7.

Figure 2.7 Schematic visualization of Ppy’s conduction mechanism, taken from Ramya et al., 2013.

Neutral polymer

Bipolaron

Polaron

19

Other than p-doping process, the n-doping may also occur wherein the net

negative charge arise, this happen in the case of polyacetylene (PTh). The charge

storage mechanisms in conducting polymers are not only involved with p-doping

and n-doping, but may also involve the intercalation of cations which can help in

increasing specific capacitance (Conway, 1999). The choices of dopants and

morphology of conducting polymers are become important factor to enhance the

specific capacitance of the system (Ramya et al., 2013; Shown et al., 2015).

The conducting polymers are believed can improve the storage ability and

reduce self-discharge, however one significant drawback of these materials is

relative low power (low rate of CDC) which cause by sluggish rate of ions diffusion

the redox (Snook et al., 2011). This weakness can be obviated by employing

nanostructure with suitable morphology and composite with other conducting

carbon based/metal oxide materials. Table 2.3 is the comparisons of specific

capacitance of composite conducting polymer electrodes in previous studies.

Although power density is lower than EDLC, but conducting polymer’s

supercapacitors are believe can bridge the gap between batteries and EDLC (Du

Pasquier et al., 2002; Snook et al., 2011).

Table 2.3 Specific capacitance of composite and treated materials, take from Snook et al., 2011.

Electrode Material Specific

Capacitance (F g-1) Electrolyte

PPy-SWNTs 144 Aqueous

PPy-funct-SWNTs 200 Aqueous

PEDOT-on-PPy 230 1 M LiClO4 (aq)

PEDOT-on-PPy 290 1 M KCl (aq)

PPy-Fe2O3 420 LiClO4 (aq)

PPy 78-137 PVDF-HFP gel electrolyte

PEDOT-MoO3 300 Non-aqueous Li+

Non-irradiated HCl doped

PAni 259 Gel polymer electrolyte

Non-irradiated HCl doped

PAni 210 (10,000 cycles) Gel polymer electrolyte

20

Irradiated HCl doped PAni 243 Gel polymer electrolyte

Irradiated HCl doped PAni 220 ( 10,000

cycles) Gel polymer electrolyte

RuOx-PEDOT-PSS 1409

PPy-fast CV deposited 480 1 M KCl (aq)

ACP-PAni 273 1 M H2SO4

Non-treated PEDOT 72 1 M H2SO4

Ultrasonicated synthesis of

PEDOT 100 1 M H2SO4

MWNT/PANI 20/80 wt % 360

MWNT/PPy 20/80 wt % 190

PAni coated CNF (20 nm) 264

PEDOT/MSP-20 56 (1000 cycles) Et4NBF4 in PC, LiPF6 in

EC/DMF

2.3.2.2 Transition Metal oxides

There are several requirements for transition metal oxides which can be

considered as electrode materials for supercapacitor. The oxide should be

electronically conductive. The metal can exist in two or more oxidation states with

no phase change, and the protons can freely intercalate into oxide lattice (Wang et

al., 2012a). Transition metal oxides such as RuOx, MnO2, NiO, Co3O4, tin oxide

(SnO2), iron oxide (Fe3O4) and vanadium oxide (V2O5) are widely used as electrode

materials of pseudocapacitor. As mention previously, the remarkable performance

of RuOx in supercapacitor (theoretical specific capacitance of 1358 F g-1) has

attracted many researchers, however the high cost and toxic nature has restricted its

applications. Other inexpensive metal oxides (MnO2, NiO, Co3O4, etc) have their

own limitations such as poor electrical conductivity, poor stability and low specific

capacitance. Thus, many efforts have been made to optimize the performance of

inexpensive metal oxide electrodes such as compositing the metal oxide with carbon

materials, conducting polymers and secondary transition metal oxides (Wang et al.,

2012a). The electronic conducting carbon can help to shorten the transport path

21

length for both electrons and cations. The use of conducting polymers can improve

the conductivity, structure stability and flexibility of electrodes. A composite with

secondary metal oxide can provide more active sides for fast redox reaction which

results in good conductivity, excellent energy and power density (Rusi & Majid).

2.3.3 Composite manganese oxide based electrode

MnO2 is one of the most attractive candidates for pseudocapacitor electrode

materials because of its environmental friendliness and low cost. However, in

comparing with RuOx electrode, MnO2 electrode exhibit much lower

electrochemical capacitance performance (Wang et al., 2012a; Zhang et al., 2007).

In order to overcome this problem, the composite manganese oxide based electrodes

have been widely studied. The mixed binary, ternary metal oxides and carbon

composite manganese oxide based electrodes systems (such as MnO2-NiO, MnO2-

Co3O4, Co3O4-NiO, MnxOx-NiO-Co3O4, RGO-MnO2) have been studied and their

supercapacitive performances were discussed and compared in the following

sections.

2.3.3.1 Single manganese oxide electrode

Manganese oxides (MnxOy) as electroactive material of supercapacitor

electrode is used because of relative low cost, low toxicity as well has high

theoretical capacitance ranging from 1100 to 1300 F g-1 (Augustyn et al., 2014).

There are varieties of stable manganese oxides such as MnO, Mn3O4, Mn2O3 and

MnO2 (Brousse et al., 2006; Wei et al., 2011). The various crystal structures of

MnxOy are displayed in Figure 2.8.

22

Figure 2.8 Schematic illustration of the crystal structure of manganese oxide. (a) Rock salt, (b) spinel

Mn3O4, (c) bixbyite Mn2O3, (d) pyrolusite β-MnO2 (rutile-type), (e) ramsdellite (diaspore-type)

([MnO6] octahedra form infinite double layers) , (f) phyllomanganate (birnessite-buserite family

layered MnO2), taken from Brousse et al. (2006).

The first study on capacitive behaviour of manganese dioxide by Lee and

Goodenough in 1999 has gained major attention as starting point to develop

manganese oxide electrodes for supercapacitor devices (Augustyn et al., 2014;

Wang et al., 2012a; Wei et al., 2011). The storage mechanism of manganese oxide

electrode is based on reversible redox transition which involve the exchange of

cations within electrode-electrolyte that change the electrode oxidation state of

Mn(III)/Mn(II), Mn(IV)/Mn(V) (Wang et al., 2012a). The possible reaction that

occurs during CDC in MnO2 electrode is shown in Equation 2.3.

(MnO2)surface/bulk + C+ + e- (MnOOC)surface/bulk (Equation 2.3)

Based on previous studies, the electrochemical performances of manganese

oxides electrodes are strongly depend on the morphology, crystal forms, defect

O2+ Mn2+

Oxygen

A-atoms

Tetrahedral side

B-atoms

Octahedral side

(a) (b) (c)

(d) (e) (f)

23

chemistry, porosity and texture. Thus, the structural parameters have played an

important role in determining the electrochemical properties (Wei et al., 2011).

Extensive efforts have been paid to adjust the synthesis parameters in order to obtain

optimum capacitance and power density of manganese oxide with suitable

morphology and crystal structure (Wang et al., 2012a).

The effect of crystal structure on capacitance MnO2 in aqueous electrolyte

has been reported. The specific capacitance and stability performance of MnO2

allotropes is shown in Figure 2.9.

Figure 2.9 The specific capacitance and cyclability of different MnO2 allotropes in 0.5 M potassium

sulphate (K2SO4) electrolyte at 5 mV s-1, taken from Ghodbane et al., 2009.

The spinel form of MnO2 has found to exhibit optimum specific capacitance of 245

F g-1 by Ghodbane et al. (2009), while Devaraj & Munichandraiah (2008) found

that hollandite MnO2 can achieve capacitance as high as 297 F g-1. The difference

of these results were caused by use of different synthesis and electrochemical

characterization methods (Devaraj & Munichandraiah, 2008; Ghodbane et al.,

2009). The recent updated of specific capacitance dependence on MnO2 structure

phase and surface area studies are shown in Table 2.4 (Wang et al., 2015).

Sp

ecif

ic C

ap

aci

tan

ce (

F g

-1)

Cycle number

spinel

birnessite

OMS-5

cryptomelance

ramsdellite

Ni-todorokite

pyrolusite

24

There are several techniques can be used to synthesize MnO2 electrodes.

Some of the methods are hydrothermal/solvothermal method, sol gel method,

electrodeposition method, etc. Different type of nanostructures can be obtained

using hydrothermal method by controlling the reaction temperature, time and

solvent (Wei et al., 2011).

Table 2.4 Specific capacitance dependence on MnO2 phase structure and specific surface area (SSA),

taken from Wang et al., 2015.

MnO2

Phase Tunnel Size (Å)

SSA

(m2/g) SC (F g-1) Electrolyte

α(m) 2 x 2 (1D) 4.6 x 4.6 200 150

0.1 M K2SO4

(Hydrated size

of K+ : 3Å)

α(m) (H2SO4) 208 150

α(m) (H2O) 8 125

δ (H2O) 1 x ∞ (2D) 7.0 (interlayer) 17 110

δ (H2SO4) 89 105

Δ 3 80

λ (spinel) 3D - 35 70

Γ 1 x 2 (1D) 2.3 x 4.6 41 30

Β 1 x 1 (1D) 1.89 x 1.89 1 5

Α 2 x 2 (1D) 4.6 x 4.6 19.29 241

0.1 M Na2SO4

(Hydrated size

of Na+ : 4 Å)

α(m) 2 x 2 (1D) 4.6 x 4.6 123.39 297

Δ 1 x ∞ (2D) 7.0 20.93 236

Γ 1 x 2 (1D) 2.3 x 4.6 31.56 107

λ (spinel) 3D - 5.21 21

Β 1 x 1 (1D) 1.89 x 1.89 - 9

λ (spinel) 3D - 156 241

0.5 M K2SO4

Δ 1 x ∞ (2D) 7.0 45 225

OMS-5 4 x 2 (1D) 9.2 x 4.6 19 217

Α 2 x 2 (1D) 4.6 x 4.6 29 125

Γ 1 x 2 (1D) 2.3 x 4.6 85 87

Ni-todorokite 3 x 3 (1D) 6.9 x 6.9 33 42

Β 1 x 1 (1D) 1.89 x 1.89 35 28

From the studies of Zhao et al. (2015) a hydrothermal route has been used

to synthesis urchin-like MnO2 electrode based on mixture of aqueous manganese

sulphate (MnSO4) and (NH4)2S2O8 (Zhao et al., 2015b). By varying the

hydrothermal temperature, different type of morphologies were obtained, as shown

in Figure 2.10.

25

Figure 2.10 SEM images of low magnification (a,c and e) and high magnification (b,d and f) of

urchin-like MnO2 powders synthesized at different temperatures: (1) a and b, 80°C; (2) c and d, 110

°C and (3) e and f, 140 °C, taken from Zhao et al., 2015b.

The urchin-like structure exhibited optimum specific capacitance of 151.5

F g-1 at current density of 1 A g-1 in 1 M Na2SO4 electrolyte. By controlling the ratio

of potassium permanganate to potassium chloride, Zhao et al. (2015a) obtained

MnO2 nanoflowers (Figure 2.11) with specific capacitance exhibited of 197.3 F g-1

at current density of 1 A g−1 in 1 M Na2SO4 electrolyte (Zhao et al., 2015a).

(a) (b)

(c) (d)

(e) (f)

26

Figure 2.11 SEM images of low magnification (a and c) and high magnification (b and d) of

hierarchical MnO2 nanoflowers synthesized at different concentrate: (1) n(KMnO4):n(KCl) = 1:3; (2)

n(KMnO4):n(KCl) = 3:1, taken from Zhao et al., 2015a.

Su et al. (2015) reported another simple hydrothermal process of MnO2 on

the carbon filter paper (CFP) electrode. The α-MnO2 nanowire was obtained with

specific capacitance of 251 F g-1 at current density of 1 A g-1 in 1 M Na2SO4

electrolyte. The morphologies structure and CV performances are shown in Figure

2.12 and Figure 2.13.

Figure 2.12 (a) Lower and (b) higher magnification SEM images of α-MnO2 nanowire coated on

CFP, c) TEM and (d) HRTEM images of prepared α-MnO2 nanowire, (e) SAED pattern, taken from

Su et al., 2015.

(a) (b)

(c) (d)

27

Figure 2.13 CV of α-MnO2/CEP electrode and bare CFP in 1 M Na2SO4 electrolyte at scan rate of 10

mV s-1, taken from Su et al., 2015.

The CV curve of the porous α-MnO2/CFP electrode showed approximately

rectangular mirror image, representing an ideal capacitive behaviour. The CV curve

revealed that the specific capacitance was mainly contributed from MnO2 instead of

CFP (Su et al., 2015).

Recently, MnO2 nanosilks self-assembled micro powders have been

synthesized by Li et al. (2014) as shown in Figure 2.14.

Figure 2.14 SEM images of: (a-c) as-prepared silkworm cocoon-like MnO2 micropowders; (d) MnO2

nanosilks from MnO2 micropowders, taken from Li et al., 2014.

C

urr

ent

Den

sity

(A

g-1

) Potential (V Vs. SCE)

28

The results of nanosilks with inter-wined network distribution with diameter of 30-

50 nm and length up to 2 mm exhibited a desirable specific capacitance of 135

Fg-1 at current density 0.15 A g-1 in 1 M Na2SO4 electrolyte (Li et al., 2014d).

The sol gel template synthesis is another effective way to prepare MnO2

powders with different morphologies. Wang et al. (2005) obtained the nanowires or

nano rods of α-MnO2. The morphology and the CV curve are shown in Figure 2.15

(Wang et al., 2005). The optimum specific capacitance was 165 F g-1 in 2 M

(NH4)2SO4 aqueous solution at scan rate of 10 mV s-1.

Figure 2.15 (a) SEM images of MnO2, (b) CV curve of MnO2 electrode in 2 M (NH4)2SO4 aqueous

solution at scan rate of 10 mV s-1, taken from Wang et al., 2005.

Generally, the thickness of electroactive materials using hydrothermal or

sol-gel method is hard to control. The thickness of MnO2 electrode is ultimately

limited by poor electrical conductivity of MnO2 which resulted in low specific

capacitance (Wang et al., 2012a). Here, the electrodeposition method is the best

method to control the thickness of electroactive material and also a feasible method

for controlling well-ordered MnO2 nanostructures on various conducting substrates

(Wang et al., 2015). Babakhani & Ivey (2011) have studied different parameters of

deposited MnO2 using galvanostatic electrodeposition techniques. By changing the

(a) (b)

1 µm

Potential (V Vs. SCE)

Cu

rren

t (A

)

29

parameters of current density, deposition electrolyte concentration, solution

temperature and pH, a series of nanocrystalline manganese oxide electrode with

various morphologies were obtained. Morphology images of MnO2 in various

concentration of deposition electrolyte of aqueous manganese acetate

(Mn(CH3COO)2) solution is shown in Figure 2.16. The optimum specific

capacitance was ∼230 F g−1 with capacitance retention rates of ∼88 % after 250

cycles in 0.5 M Na2SO4 at scan rate of 20 mV s−1 (Babakhani & Ivey, 2011).

Figure 2.16 SEM and cross-section images of deposited manganese oxide prepared from: (a) 0.003

M Mn(CH3COO)2 solution at 0.25 mA cm−2; (b) 0.005 M; (c) 0.007 M; (d) 0.01 M; (e) 0.02 M; (f)

0.03 M Mn(CH3COO)2 solution at 5 mA cm−2 for 10 min (T = 60 C and pH = 7.5), taken from

Babakhani & Ivey, 2011.

Dongale et al. (2005) has carried out systematic studies on MnO2 thin films

deposited using electrodeposition technique. The concentration of deposition

electrolyte was varied and different film morphologies were obtained as shown in

10 µm

10 µm 30 µm

2µm

3µm

5 µm 5µm

5µm

10 µm 30 µm

30 µm

5 µm 5 µm

5 µm

(a)

(b) (c)

(d) (e) (f)

30

Figure 2.17. The maximum specific capacitance was estimated to be 392 F g-1 in

1 M Na2SO4 electrolyte at scan rate of 10 mV s-1 (Dongale et al., 2015).

Figure 2.17 FESEM images of 0.02 M, 0.05 M, 0.1 M, 0.2 M and 0.5 M MnSO4·H2O, taken from

Dongale et al., 2015.

Based on literatures studies, electrodeposition method always gives high

specific capacitance. However, the specific capacitance of manganese oxides still

exhibited within 100-250 F g-1 which are much lower than its theoretical capacitance

(Liu et al., 2009; Wei et al., 2011). This lower specific capacitance is believed due

to poor electrical conductivity (~10-7 to 10-3 S cm-1) (Augustyn et al., 2014). In

additional to their poor electrical conductivity, another limitation of MnO2 electrode

which become important issues are the electrochemical cyclability and dissolution

of active material during electrochemical cycling (Wei et al., 2011).

(a) (b)

(c) (d)

(e)

31

In our work, different secondary metal oxide and carbon based materials

have been added in order to improve the electrical conductivity. In following

section, the literature of manganese oxide composite with others metal oxides or

carbon electrodes which related to this thesis is compared and studied.

2.3.3.2 Binary manganese oxide-nickel oxide electrode

Nickel oxide is another good candidate material for supercapacitor due to its

high theoretical specific capacitance (3750 F g-1), good electrochemical, good

thermal stability and high specific surface area (Kim et al., 2013b). Porous NiO can

provide a short diffusion pathway for electrolyte cations to electroactive sites on the

electrode and increase the rate of faradic redox reactions (Vijayakumar et al., 2013).

Passed researches on the incorporation of manganese dioxide and nickel oxide have

shown an enhancement in energy, power density and improvements in cycle stability

(Kim & Popov, 2003; Liu et al., 2009; Ramesh & Kamath, 2008; Wu et al., 2012).

The lack porosity and surface area of manganese oxide electrode can be improved

by adding nickel oxide which is good for electrolyte penetration and increase the

electrode conductivity (Ramesh & Kamath, 2008; Wu et al., 2008b). From Kim &

Popov (2003) work, it was found that the specific capacitance of MnO2 electrode

have increased from 166 F g-1 to 210 F g-1 when NiO was added in Mn-Ni oxide

electrode. The findings also highlighted the contribution of NiO in enhancement of

storage ability and surface area of mixed metal oxides electrode with the formation

of micropores (Kim & Popov, 2003).

In the work of Liu et al. (2009) sol gel has been adopted to prepare

nanostructure NiO-MnO2 electrode. The NiO content in MnO2 based binary

32

electrode preparation was varied from 0 to 25 wt % and the CV curve of electrodes

is displayed in Figure 2.18.

Figure 2.18 CV of different composite electrodes, scan rate: 10 mV s-1; (a) MnO2; (b) NiO; (c) 5 %

NiO; (d) 10 % NiO; (e) 25 % NiO; (f) 20 % NiO; and (g) 15 % NiO, taken from Liu et al., 2009.

The figure revealed almost mirror image with nearly rectangular-like/symmetric of

I–V curves response. Sample with 20 wt % content NiO exhibited broad CV curve

with specific capacitance of 453 F g-1, while for pure NiO and MnO2 were 209

F g-1 and 330 F g-1 in 6 M KOH electrolyte at scan rate of 10 mV s-1. They have

proved that the composite electrode showed much larger specific capacitance, high

power density and stable electrochemical properties than each pristine component

(Liu et al., 2009).

In the work of Wu et al. (2012) hydrothermal has been used to prepare

manganese nickel oxide electrode. They found that the additional of nickel ions

significantly affect the morphology. Figure 2.19 shows the evaluation of rod-like of

MnO2 to plate-like electrode when nickel ions were added. The optimum specific

capacitance was 284 F g-1 at scan rate of 5 mV s-1 in 2 M sodium chloride (NaCl)

electrolyte (Wu et al., 2012).

C

urr

ent

Den

sity

(A

g-1

)

Potential (V Vs. SCE)

33

Figure 2.19 Scanning electron micrographs of (a) MnO2, (b) Ni-Mn-O and transmission electron

micrograph of (c) MnO2, (d) Ni-Mn-O synthesized via the hydrothermal process at 125 °C for 3

hours, taken from Wu et al., 2012.

Other than sol gel and hydrothermal method, the electrodeposition method

has been used to synthesize binary manganese nickel oxide electrode. In the work

of Chen and Hu, Mn-Ni oxide was successfully deposited using the anodic

electrodeposition method with a solution containing a high Ni2+/Mn2+ ratio of 10:1,

the high ratio differences is due to high differences in the reduction potential of Mn2+

(-1.18 V) and Ni2+ (-0.25 V) (Chen & Hu, 2003). Other studies on electrodeposition

techniques have been done by Parasad and Munira (2004) where nanostructured and

microporous manganese nickel oxide has been successfully deposited on SS. The

(a) (b)

(c) (d)

34

AFM and TEM of manganese nickel oxide (NMO) electrode are shown in Figure

2.20.

Figure 2.20 (a) AFM image of NMO, (b) TEM image of NMO prepared by a potentiodynamic

method at a scan rate of 200 mV s-1, taken from Prasad & Munira., 2004.

They claimed that nanostructured with highly porous morphology of NMO is

expected to have a high specific surface area and led to high specific capacitance.

They have obtained a high specific capacitance of 621 F g-1 and 377 F g−1 at a scan

rate of 10 mV s−1 and 200 mV s−1 in 1 M Na2SO4 electrolyte (Rajendra Prasad &

Miura, 2004).

In summary, the electrochemical performance of pristine MnO2 electrode

can be enhanced by additional of NiO into MnO2 electrode. However, the suitable

parameters of deposition method for binary MnO2-NiO electrode have not clearly

studied. Thus, in our work, the suitable concentration, variation of electrodeposition

modes and suitable electrolytes of binary MnO2-NiO electrode have been

systematically investigated.

(a) (b) 20 nm

500.00 x 500.00 [nm] z 0.00-50.14 [nm]

35

2.3.3.3 Ternary manganese oxide-nickel oxide-cobalt oxide electrode

Cobalt oxide has been added into manganese nickel oxide electrode due to

increase the redox activity and good reversibility of the electrode (Hsu et al., 2013).

Based on literature, the specific capacitance of manganese nickel oxide is still lower

than RuOx electrodes, thus utilization of cobalt oxide in manganese nickel oxide

electrode is needed. Recently the utilization of ternary metal oxides electrode has

been widely investigated.

Before year 2014, there was only one report found in utilization of composite

cobalt manganese nickel oxide as supercapacitor’s electrode. Wang et al. (2013)

first studied the nickel-cobalt-manganese oxide (Ni1/3Co1/3Mn1/3(OH)2) electrode for

supercapacitor using chemical precipitation method. The morphology gave an

amorphous structure (Figure 2.21) which exhibited highest specific capacitance of

1403 F g-1 in the potential window of 0–1.5 V at scan rate of 1 mV s-1 in mix 1 M

NaOH + 0.5 M Na2SO4 electrolyte (Wang et al., 2013). The specific capacitance

showed a higher value than RuOx electrodes but the stability was still not stable in

comparison to RuOx electrodes.

Figure 2.21 SEM images of: (a) Ni1/3Co1/3Mn1/3(OH)2 and (b) Ni0.37Co0.63(OH)2, taken from Wang et

al., 2013.

(a) (b)

36

Recent publication on ternary Mn-Ni-Co oxide has been reported by Li et al.

(2014). Aligned spinel Mn-Ni-Co nanowires oxide was synthesized using facile

hydrothermal method on top of nickel foam. The nanowires array electrode

exhibited specific capacitance of 638 F g-1 at 1 A g-1 with excellent stability in 6 M

KOH electrolyte as shown in Figure 2.22.

Figure 2.22 Cyclability of the MNCO nanowire array at a current density of 2 A g−1 (vs Ag/AgCl),

taken from Li et al., 2014.

They have claimed that the nanowires (Figure 2.23) structure can provide large

reaction surface area, fast ion and electron transfer. At the same time, Mn-Ni-Co

oxide electrode is also greatly reduces the cost and have a better performance (Li et

al., 2014a).

Sp

ecif

ic C

ap

aci

tan

ce (

F g

-1)

Number of cycles

37

(Figure 2.23, continued)

Figure 2.23 SEM images of (a, b) the hydroxide precursor and (c, d) the MNCO nanowire array,

taken from Li et al., 2014a.

Another studies of cobalt manganese nickel oxalates micro polyhedrons

(Co0.5Mn0.4Ni0.1C2O4*nH2O) using chemical co-precipitation method was reported

by Zhang et al. (2015). They fabricated the asymmetric electrode with

Co0.5Mn0.4Ni0.1C2O4*nH2O as positive electrode and graphene nanosheets as

negative electrode. A flexible supercapacitor devices were fabricated and devices

gave an excellent electrochemical performance and good stability (Zhang et al.,

2015).

2.3.4 Composite carbon-MnO2 electrode

Another method to increase the conductivity of metal oxides electrode (e.g

MnO2) is by incorporating highly conductive carbon as composite electrode

materials (Yu et al., 2013). Graphite layers or graphene is known as 3D or 2D

monolayer of carbon atoms which has high conductivity, outstanding mechanical

strength, anomalous quantum hall effects and high surface area (Geim & Novoselov,

2007; Wintterlin & Bocquet, 2009). Oxidized graphite to graphene oxide (GO) or

reduced graphene oxide (RGO) has overshadowed the AC and CNT due to offers

38

great potential for energy storage and could produce supercapacitors with ultrahigh

power (Cheng et al., 2011; El-Kady & Kaner, 2013). GO has reported to have

specific capacitance of 10-40 F g-1, while RGO has higher specific capacitance

around 205 F g-1 (Wang et al., 2009).

In order to increase the low conductivity of MnO2 electrode, RGO is mixed

with MnO2 to form composite materials. Zhang et al. (2011) reported composite

materials of functionalized RGO (FRGO) with MnO2 nanosheet. A composite of

FRGO and MnO2 nanosheets was synthesized by anchoring negatively charged

MnO2 nanosheets on the positively charged FRGO via an electrostatic. Layer-like

structure of composite has been obtained. They observed the MnO2 nanosheets

disperse on functionalized RGO and exhibited optimum specific capacitance of 188

F g-1 at current density of 0.25 A g-1 in 1 M Na2SO4 electrolyte with 89 % of

capacitance retention after 1000 cycles (Zhang et al., 2011).

Graphene decorated with flower-like MnO2 nanostructures was prepared via

electrodeposition method by Cheng et al. (2011). As prepared graphene film was

used as working electrode to deposited MnO2 flowerlike (Figure 2.24).

Figure 2.24 Illustrative fabrication process of the composite electrode and SEM image of as coated

MnO2-graphene, taken from Cheng et al., 2011.

Graphene suspension Graphene

Filtration

Dyndall effect

Two electrode testing

Filter paper Cut

CE RE WE

Assembly

γ- MnO2

electrodeposition 5 µm

2 µm

Laser pen

39

The optimum specific capacitance of pristine graphene electrode was around 245

F g-1 and increased to 328 F g-1 for binder free MnO2-coated graphene electrode at

applied current of 1 mA in 1 M KCl electrolyte (Cheng et al., 2011).

Chan et al. (2014) reported the electrodeposited RGO-MnO2 nanostructures

electrode fabricated using in-situ electrodeposition method where the manganese

cations and RGO are mixed as deposition electrolyte. The galvanostatic mode with

variation of deposit time was studied. They found that MnO2 particle size increased

when the electrodeposition duration increased. The FESEM images revealed that

the MnO2 nanoparticles were being wrapped by RGO nanosheet as shown in Figure

2.25. The optimum specific capacitance obtained was 378 F g-1 at scan rate of 1

mV s-1 for 13 minutes of electrodeposition duration time in 1 M Na2SO4 electrolyte

(Chan et al., 2014).

Figure 2.25 FESEM images using deposition duration of: (a) 10 minutes, (b) 13 minutes and (c) 18

minutes, taken from Chan et al., 2013.

1 µm 100 nm

100 nm

40

Ternary of MnOx/CNT/RGO studies have been reported recently by Han et

al. (2014) without any conductive additive and binder. Ultrathin layer of CNT/RGO

was spray coated on top of graphite foil and used for working electrode for

depositing MnOx. Figure 2.26 shows the systematic procedure of their work together

with morphology images.

Figure 2.26 (a) Schematics for MnOx/CNT/RGO nanohybrid fabrication via Routes I and II. Route

II includes a plasma functionalization step prior to MnOx deposition. (b) SEM image of the

CNT/RGO supporting layer, with RGO highlighted by dotted lines. (c) Low- and (d) high-

magnification SEM images of MnOx nanoparticles deposited on a CNT/RGO layer, taken from Han

et al., 2014.

In this work, they utilized the atmospheric-pressure DBD plasma to

functionalize the ultrathin CNT/RGO layer prior MnOx deposition. The adhesion

of MnOx nanoparticle on CNT/RGO has been improved using the atmospheric-

pressure treatment as well as improved the electrode cycling stability. These results

proved that the utilization nano hybrids and plasma-related effect can use to

synergistically enhance the electrochemical performance. The reported

MnOx/CNT/RGO nano hybrids electrode has a highest specific capacitance values

in compare with other works. The optimum specific capacitance was 1070 F g-1 in

1 M Na2SO4 electrolyte at scan rate of 10 mV s-1 which is close to the theoretical

capacitance of MnOx (Han et al., 2014).

(a)

(b) (c) (d)

41

2.4 Summary

Supercapacitor is believed to overcome the problem of increasing energy

demands due to their higher power density and longer cycle life than

batteries.

The electrode of supercapacitor is one of the important aspects to achieve

high energy and power density devices. Various electrode materials are used

for supercapacitor including carbon based materials, transition metal oxides

and conducting polymers. However, the pristine materials exhibited poor

electrochemical performance. Therefore composite electrode materials are

one of the effective alternatives to improve the performance.

Different composite materials and synthesis techniques will results in

different film morphologies which will affect the electrochemical

performance.

The drawbacks of MnO2 pristine electrode is effectively overcome by

composite it with several transition metal oxides or carbon based materials.

42

CHAPTER 3: METHODOLOGY

3.1 Introduction

There are several method that can be used for preparing supercapacitor’s

electrodes, e.g. hydrothermal, sol-gel method, co-precipitation method,

electrodeposition, and chemical vapour deposition (Wei et al., 2011). Most of the

MnO2 thin films have been synthesized in powder form, therefore the addition of

additives and binders is necessary to prepare the active materials as an electrode.

This procedure requires several preparation steps, starting from the preparation of a

slurry mixture containing the active materials, the binder (e.g. polyvinylidene

fluoride (PVDF)/polyvinylpyrrolidone (PVP)) and a conductivity enhancer (e.g.

carbon black), followed by slurry coating on a current collector (spin/dip coating,

doctor blade techniques) (Rusi & Majid, 2014b). The electrodes are ready for use

after heat treatment and pressurization process to eliminate the solvent and prevent

irregularities on the surface. The addition of insulating binders can cause blocking

effect on some parts of the active surface and reduce the specific capacitance of

cells. This also contributes to the additional resistance of the contact area between

the electrical conductor (i.e. the current collector and/or conductivity enhancer) and

the active materials, thus resulting in reduced power density (Park & Choi, 2010;

Yan & Cui, 1999). Therefore a more effective method to synthesize manganese

oxide based electrodes for electrochemical capacitor applications such as

electrodeposition is important to explore in which the oxide films are directly

deposited on the current collector. Electrodeposition is a simple, easy set up and able

to cater for large area deposition. In addition to this, it only requires low deposition

temperature and the film thickness can be controlled easily (Jagadale et al., 2013).

43

The composite metal oxide films also can be deposited from mixed precursors of

deposition electrolyte (Pauporté et al., 2003).

3.2 Materials and preparation

3.2.1 Electrodes

There are three type of electrodes that have been used in this work: working

electrode, counter electrode and reference electrode. Working electrode is inert.

Thus during the electrolysis process, it is stable against corrosion and does not

support the completing reaction of hydrogen (H2) evolution and oxygen (O2)

evolution/reduction. Counter electrode is used for controlling the current pass

through the working electrode. It provides the current of equal magnitude but

opposite sign to the working electrode. The reference electrode is used for

maintaining a constant potential throughout the experiment (Pletcher, 2009).

The working electrode used in this work was 4 cm2 of 304 stainless steel

(SS) substrate and it was purchased from Magna Value Sdn Bhd. Prior to

electrodeposition, the SS substrate was sonicated for 10 minutes and rinsed with

acetone for 3 times before dried in room temperature. Graphite rods of spectro-

Grade was used as counter electrode for electrodeposition step. Silver/silver chloride

(Ag/AgCl) and platinum electrodes were used as the reference electrode and counter

electrode for electrochemical test, respectively.

44

3.2.2 Materials

All the reagents used for synthesis and preparation were of analytic grade

and used as received without further purification. Manganese acetate tetrahydrate

(Mn(CH3COO)2·4H2O) (Fluka), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O)

(Fluka), cobalt sulphate (CoSO4·7H2O) (Unilab) and sulphuric acid (H2SO4)

(Friendeman Schmidt), sulphuric acid, graphene oxide (GO) (ACS materials), D (+)

glucose anhydrous (C6H12O6) (Friendeman Schmidt), sodium sulfate (Na2SO4)

(Fluka), potassium hydroxide (KOH) (Friendeman Schmidt) and potassium

ferricyanide (K3Fe(CN)6) (Friendeman Schmidt) were used in the experiment.

3.3 Electrode preparation for manganese based electrode

All manganese based electrodes were deposited via electrodeposition

technique by using Autolab PGSTAT 30. The electrodeposition set up consisted of

SS substrate as working electrode, graphite carbon rod as the counter electrode and

Ag/AgCl as the reference electrode. The electrodeposition set up is shown in Figure

3.1. After deposition, all obtained manganese based electrodes in our work were

rinsed and heated to 300 °C for 6 hours and cooled to room temperature.

Figure 3.1 Electrodeposition set up

45

3.3.1 Electrodeposition of MnO2 electrode

Mn(CH3COO)2.4H2O were first dissolved in distilled water and stirred for

30 minutes to get a homogeneous deposition electrolyte solution at room

temperature. MnO2 was electrodeposited onto 4 cm2 SS by chronopotentiometry

(CP) mode in 10 minutes at a constant current of 8 mA. MnO2 electrode was

deposited in various deposition electrolyte concentration, i.e 0.0025 M , 0.005 M,

0.01 M and 0.02 M of Mn(CH3COO)2. 4H2O aqueous solutions and the deposited

electrode denoted as 0.00025 M, 0.005 M, 0.01 M and 0.02 M, respectively. All

electrochemical tests were done using two electrodes system in 0.5 M KOH

electrolyte.

3.3.2 Electrodeposition of MnO2-NiO electrode

The binary MnO2-NiO electrodes were also electrodeposited onto SS using

the CP mode for 10 minutes at applied current of 8 mA. The deposition electrolyte

consist of 20 ml of aqueous 0.01 M Mn(CH3COO)2.4H2O, 4 ml of 0.8 M H2SO4 and

20 ml of different concentrations (0.15 M, 0.2 M, 0.25 M, 0.3 M, 0.35 M, and 0.4

M) of Ni(CH3COO)2.4H2O. These deposition electrolytes were then sonicated for

10 min. The deposited electrodes were denoted as NiO-15 NiO-20, NiO-25, NiO-

30, NiO-35, and NiO-40. For example, NiO-15 denotes the MnO2-NiO binary

deposited from a solution containing 0.8 M H2SO4, 0.01 M Mn(CH3COO)2.4H2O

and 0.15 M Ni(CH3COO)2.4H2O. The electrochemical tests of deposited electrodes

were studied using three electrodes systems in Na2SO4 electrolyte.

46

The optimum electrode from above methodology was further studied by

varying the electrodeposition modes. Three electrodeposition modes used in this

study were CP, chronoamperometry (CA), and cyclic voltammetry (CV). The CP

mode is electrodeposited using aforementioned procedure where the current was

maintained at 8 mA for 10 minutes of deposition time, while a constant voltage of

1.5 V was applied to deposit the manganese-nickel hydroxide using CA mode. In

CV mode, the electrodeposition was conducted in the voltage ranges from 0 to 1 V

at a scan rate of 20 mV s-1 with 7 deposition cycle. The electrochemical

performances of the optimum electrode were studied using three electrodes system

in three different electrolytes i.e. 0.5 M Na2SO4, 0.5 M KOH and mix 0.5 M

KOH/0.04 M K3Fe(CN)6 electrolytes. The results and discussion are provided in

Chapter 5.

3.3.3 Electrodeposition of Mn3O4-NiO-Co3O4 electrode

The electrodeposition of ternary composite on top of SS substrate was

carried out using the same method which used chronopotentiometry mode for 10

minutes at applied current of 8 mA. The deposition electrolyte contained 6 ml of 0.8

M H2SO4, 20 ml of 0.01 M Mn(CH3COO)2·4H2O, 20 ml of 0.25 M

Ni(CH3COO)2·4H2O and 20 ml of different concentrations (0.05 M, 0.1 M, 0.15 M,

0.2 M, 0.3 M) of CoSO4·7H2O. Before electrodeposition, the deposition electrolyte

sonicated for 10 minutes. The electrochemical performances of the best electrode

were further investigated in three different electrolytes i.e. 0.5 M Na2SO4, 0.5 M

KOH and mix 0.5 M KOH/0.04 M K3Fe(CN)6 electrolytes using three electrodes

system. The results and discussion for this section are provided in Chapter 6. Figure

3.2 shows the as prepared of selected electrodes from section 3.2.1, 3.2.2 and 3.2.3.

47

Figure 3.2 Deposited electrode of : (a) 0.01 M of section 3.2.1, (b) NiO-15 of section 3.2.2 (c) 0.15

M of section 3.2.3.

3.3.4 Electrodeposition of RGO-MnO2 with glucose carbon electrode

The MnO2-RGO composite on a 4 cm2 of stainless steel (SS) was

synthesized by potentiodynamic method in the voltage range of 0 to 2 V. The scan

rate was 50 mV s-1. Prior to deposition, 50 mg of GO was dispersed in 50 ml of

distilled water and then sonicated for 30 minutes. The deposition electrolyte

contained a fixed volume of GO (10 ml) solution and different volumes (30 ml, 60

ml and 90 ml) of 0.01 M Mn(CH3COO)2·4H2O aqueous solution. The deposited

electrodes were denoted as M30, M60 and M90, respectively.

The MnO2-RGO-glucose carbon composite was prepared by the same

aforementioned procedure. In a typical experiment, the deposition electrolytes were

prepared by dissolving 0.01 M, 0.03 M, 0.06 M of glucose in 30 ml of distilled

water. These solutions were then separately mixed with 30 ml manganese acetate

(0.01 M) aqueous solution. Subsequently, 10 ml of GO solution was drop into the

solution and the mixture was sonicated for 30 minutes at room temperature. The

prepared samples after heating treatment are denoted as G01, G03 and G06. The

electrochemical performances of the best electrode were also studied in three

different electrolytes i.e. 0.5 M Na2SO4, 0.5 M KOH and mix 0.5 M KOH/0.04 M

(a) (b) (c)

48

K3Fe(CN)6 electrolytes. The results and discussion of this section are provided in

Chapter 7.

3.4 Materials characterization and electrochemical test

3.4.1 Materials characterization

The compositions, morphologies and electrochemical performances of

obtained electrodes were investigated using following characterizations:

3.4.1.1 X-ray diffraction (XRD)

XRD is a non-destructive material analytical technique used to identify the

chemical compositions, crystal structure, and particle size of the samples. The X-

ray beam is applied in the sample experiment with wavelength, λ at different 2θ

angles on the sample. The process is based on the scattering of X-rays electrons

surround the crystal atom of the sample interest.

Figure 3.3 illustrates the X-ray beam incident on crystalline solid. There are

two in phase incident waves. Beam 1 and beam 2 are reflected in phase. The

deflected wave satisfied the Bragg’s Law by this following Equation 3.1.

nλ = 2d sinθ Equation 3.1

Two light wave travel together with the same wavelength and same direction can

either reinforce or cancel each other. When their phase difference is nλ (n is an

integer), constructive interface occur and when they have a phase difference of nλ/2,

49

the destructive interface occurs. This will result in diffraction patterns according to

arrangement of the sample’s atomic layer (Yang, 2008).

Figure 3.3 Bragg diffraction by crystal plane. The path different between beams 1 and 2 is BD+DC=

2AD sin θ.

In our work, the X-ray diffractograms were recorded using XRD (D8

Advance X-ray Diffractometer- Bruker AXS) with CuKα monochromatised

radiation at 40 kV and 40 mA at ambient temperature with a step size of 0.06° and

step time of 6 s. The XRD patterns of deposited electrode on top of SS in Chapter 7

were obtained by using PAN Analytical Empyrean.

3.4.1.2 Field-emission scanning electron microscopy (FESEM) and energy

dispersive X-ray spectroscopy (EDX)

FESEM is an analytical instrument which provides the information about

surface morphology and structure of the sample. FESEM consists of a field

emission gun and a series of electromagnetic lenses and apertures. First, high energy

electrons strike a specimen and they produce either elastic or inelastic scattering.

Elastic scattering happens when the incident electrons scattered by atoms in the

2

1 Incident

beam

Diffracted

beam

2”

1’

λ

50

specimen while for inelastic scattering, the electrons is ejected from the atoms in the

specimen. During inelastic scattering process, the incident electrons transfer the

kinetic energy to the electrons in the specimen’s atoms. The detected signal

electrons from the specimen are collected by detector and it is used to reconstruct

images (Yang, 2008).

EDX is used to investigate the elemental compositions of the specimen. A

high energy electrons beam is focused into the interested sample. The incident beam

may excite an electron in an inner shell of sample and ejecting it from the shell while

creating an electron-hole. An electron from its outer shells fills the hole and results

in the difference in energy between higher energy shell and lower energy shell which

may be released in the form of X-rays. The energy of X-rays emitted from the

specimen is then measured by energy dispersive spectrometer.

In this thesis, the FESEM equipped with EDX of Jeol JSM-7600F was used

to examine our sample morphologies and elemental compositions.

3.4.1.3 Transmission electron microscopy (TEM)

TEM is a microscopy instrument which use to investigate the morphology

of a sample with significantly high resolution. It has following component along its

optical path i.e. light source (electron gun), condenser lens (electromagnetic lens),

specimen stage, and objective lens and projector lens. The basic principle is based

on a high energy beam of electrons is transmitted through an ultra-thin specimen,

interacting with the specimen as it passed thought. An image is resulted from the

interaction of the electrons transmit through the specimen (Yang, 2008).

51

Jeol JEM-2100F TEM was used to investigate the composite electrode

structure. Prior to the TEM characterization, the scraped off film were dispersed in

distilled water and sonicated for 5 minutes. Then, the solution was dropped on a

copper grid and allowed to dry for 72 hours at room temperature.

3.4.1.4 Thermogravimetric analysis (TGA)

TGA is an analytic method that used to measure the mass changes of a

sample with the temperature increase at a constant heating rate. Samples are placed

in the furnace and its mass change is monitored by a thermo balance. The main

purpose of this TGA is to analyze the material decomposition and thermal stability

through mass change as a function of temperature.

In the work of Chapter 7, the thermal behaviours of D (+) glucose was

examined by Thermal Gravimentric Analyser, TGA Q500 under nitrogen gas at a

heating rate of 14° C min-1.

3.4.1.5 Raman spectroscopy

Raman spectroscopy is a spectroscopic technique that used to examine the

molecular vibrational, rotational, and other low-frequency modes of the sample.

When a monochromatic light source (i.e laser) illuminates on the sample, the laser

light interacts with molecular vibrations, phonons or other excitations in the system.

Energy of laser photons will be shifted up or down, this sifted energy gives

information of molecular vibrational of the system.

52

In Chapter 7, the Raman spectra were obtained using Renishaw in Via

Raman microscope with a green laser beam. The deconvolution of Raman spectra

band was done by using WiRE 3.3 software.

3.4.2 Electrochemical test

The electrochemical performances were studied by means of CV,

galvanostatic charge-discharge (CDC) and electrochemical impedance spectroscopy

(EIS) on an Autolab PGSTAT30.

CV is a potential sweep technique to study the reaction of working electrode

and determine the energy storage capacitance of supercapacitor. The working

electrode is swept between two selected potential limits (Ei and Ef) at a particular

rate in which current is monitored. The potential will be scanned initially in a

positive direction to study the oxidation and negative direction to investigate the

reductions. The presence of oxidation and reduction peaks indicates a redox reaction

has occurred in the cell. The peak weight and height of the CV profile may depend

on the applied sweep rate. The specific capacitance can be calculated using CV

curve by Equation 3.2 (Jena et al., 2013; Li et al., 2009).

𝐶 =∫ 𝑖 𝑑𝑡

∆𝑉×𝑚 (Equation 3.2)

Where I is the oxidation/reduction current, dt is time differential, m is the mass of

active material and ∆V is the operating potential.

CDC characterization is based on the voltage-composition relationship of an

electrochemical cell. In CDC study, the cell is charged-discharged at a constant

53

current between maximum potential until cut-off discharge potential is applied. The

CDC measurement of potential against time is obtained and the specific capacitance

can be calculated from CDC discharge curve by using the following Equation 3.3

(Li et al., 2009).

𝐶 =𝐼

𝑑𝑉

𝑑𝑡 ×𝑚

(Equation 3.3)

Where I is the discharge current, dV/dt is the change of discharge potential with

respect to discharge time and m is mass of the active materials.

In this thesis, most of CV and CDC studies were conducted on a three-

electrode system, except for the works done in Chapter 4. The deposited electrode

was use as working electrode. Platinum and Ag/AgCl were used as counter

electrode and reference electrode respectively. In Chapter 4, symmetric MnO2

electrodes were used for the two-electrode system to examine the CV and CDC

profile in 0.5 M KOH electrolyte.

EIS is used to study the impedance resistance or conductivity of the electrode

materials. The analysis of impedance study for various electrodes structure and

electrolyte are very important to design and optimize the device performance. In this

work, all of the EIS tests were performed within the frequency from 0.01 Hz to 100

KHz at applied potential of 0 V in Na2SO4 electrolyte.

54

3.5 Summary

This chapter discussed the methodology to prepare the manganese based

electrodes systems namely MnO2, MnO2-NiO, Mn3O4-NiO-Co3O4 and RGO-MnO2-

glucose carbon. All obtained electrodes were characterized using XRD, EDX,

FESEM, TEM, CV, CDC and EIS. Results and discussions from these

characterization techniques will be given in the following chapters.

55

CHAPTER 4: MANGANESE OXIDE ELECTRODE SYSTEM

4.1 Introduction

In the past decades, transition metal oxides have been widely used in many

applications such as sensors (Sun et al., 2014), optical (Buchholz et al., 2009) and

magnetic material (Cordente et al., 2001), catalyst (Lin et al., 2003) and storage

energy devices (Hasa et al., 2015; Shen et al., 2015; Wei et al., 2012). For

supercapacitor applications, RuO2 is well known as the best material to produce high

energy density and high stability devices. However its disadvantages in toxicity

environmental and high cost material limits its application, hence research interest

have fall into inexpensive and non-toxic transition metal oxides for the future

supercapacitors (Rusi & Majid, 2013, 2014a). MnO2 is one of the promising material

to replace ruthenium dioxide (Babakhani & Ivey, 2010, 2011; Reddy & Reddy,

2003). The crystal structure, size and morphology of MnO2 electrode materials are

the important factor which influences the capacitor performance (Babakhani & Ivey,

2010; Devaraj & Munichandraiah, 2008). As a result, developing metal oxide

electrodes with new morphologies for enhanced properties have attracted great

research interests. Many efforts have been focus to synthesis MnO2 electrode with

different morphologies and different effective approaches method (Li et al., 2012a).

In this work, the simple and easy galvanostatic of electrodeposition method

has been used to synthesis flowerlike structure MnO2 without using any physical

template and additional of any surfactant. Electrodeposition method is relative easy

and accurate control of the surface morphology of deposited film by changing

deposition parameters (Babakhani & Ivey, 2011).

56

4.2 Results and discussion

4.2.1 Optimization of Mn(CH3COO)2.4H2O concentration

4.2.1.1 Schematic illustration of electrodeposited MnO2 electrode

Figure 4.1 shows the deposited MnO2 on top of SS by using

electrodeposition method.

Figure 4.1 Illustration of electrodeposited MnO2.

In general, the formation of MnO2 is started when the electric current passed through

the electrolyte. The water molecules are separated into hydrogen and hydroxide ions

on SS surface, as described by Equation 4.1. The cations of Mn2+ in deposition

solution commonly have high binding affinity with hydroxide ions, forming a

nucleation of manganese hydroxide. The MnO2 particles settle onto SS substrate

during annealing process (Rusi & Majid, 2014a, 2014b; Yousefi et al., 2012).

57

2H2O + 2 e H2+2OH- (Equation 4.1)

Mn2+ + OH- Mn(OH)2 MnO2 (Equation 4.2)

The electro-crystallization process of metal oxides using electrodeposition

method can be modified by adjusting the deposition parameters. Hence the

nucleation and growth of crystal grains of MnO2 during electrodeposition can be

controlled using bath composition, pH of electrolyte, applied current and solution

temperature (Babakhani & Ivey, 2011; Nouzu et al., 2010; Shirale et al., 2006). The

detail of the compositions, morphologies, and electrochemical performances are

discussed in the following section.

4.2.1.2 Characterization of composition and morphology of MnO2 electrodes

In order to confirm the mechanism of electrodeposited MnO2, XRD has been

used to confirm the elemental compositions of the deposited electrodes. The XRD

pattern of deposited MnO2 on top of SS substrate is shown in Figure 4.2 (a). There

is no distinct peak other than the SS substrate peaks, this probably indicates that the

deposited MnO2 colloidal particles is in amorphous phase (Dubal et al., 2011) and

the all deposited MnO2 films are too thin (Takahashi et al., 2005). The XRD results

of scraped off deposited MnO2 from its substrate is shown in Figure 4.2 (b). The

peaks at 2θ= 29.1, 37.3, 42.5 and 56.6 confirms the formation of α-MnO2, indexed

by JCPDS NO. 44-0141 (Beaudrouet et al., 2009; Jiang et al., 2009). The diffraction

peak is broad indicating a poor crystallinity and small grain size of α-MnO2

(Aghazadeh et al., 2011; Ho & Wu, 2011). The EDX studies of selected electrodes

have been used to confirm the formation of MnO2 on SS substrate. There are

300o C

58

presence of Mn and O elements in Figure 4.2 (c) which belongs to MnO2 products,

while others elements such as small amount of Si, C, Fe, Cr, Ni, and Mo are

originated from SS, Figure 4.2 (d).

10 20 30 40 50 60

Inte

nsi

ty (

a.u

.)

2θ (°)

0.01 M

0.005 M

0.0025 M

SS

5 15 25 35 45 55

Inte

nsi

ty (

a.u

.)

2θ ( °)

(31

0)

(21

1)

(60

0)

(30

1)

(a)

(b)

59

(Figure 4.2, continued)

Figure 4.2 (a) XRD pattern of all electrodes on top of SS, (b) XRD pattern of scrapped off deposits

from 0.01 M electrode, (c) EDX of 0.01 M electrode, (d) EDX of plain SS.

The deposited MnO2 electrodes were prepared from manganese acetate

solution, with different concentrations at 2 mA cm-2 for 10 minutes. As the

concentrations of the Mn2+ ions are varied from 0.0025 M to 0.02 M, various

morphologies are observed. Figure 4.3 displays FESEM images for all the deposited

MnO2 film on top of SS. At lowest manganese acetate concentration (0.025 M),

there are only some manganese oxide nucleus formed (Figure 4.3 (a)). This is due

to low electrodeposition rate of MnO2 (Babakhani & Ivey, 2011). As the

concentration is increased to 0.005 M, the oriented spherical-like MnO2 particles are

KeV

KeV

(c)

(d)

Elements Weight (%)

C 5.78

O 15.96

Si 0.53

Cr 12.69

Mn 10.83

Fe 45.25

Ni 7.66

Mo 1.75

Elements Weight (%)

C 2.20

Si 0.58

Cr 18.15

Fe 62.75

Ni 15.52

Mo 1.79

60

realized (Figure 4.3 (b)). Within 10 minutes of deposition time, the nuclei still have

not enough time to grow over the substrate. This is attributed to slow deposition

rate. Further increases of manganese oxide concentration to 0.01 M, leading to

spherical-flowerlike of MnO2 particles are formed. The particle size of in the range

of 200 to 400 nm with thickness around 300 to 400 nm (Figure 4.3 (c-d)). However,

as the concentration increased to 0.02 M, compact and thick deposits are formed

(Figure 4.3 (e)). The higher mass loading of MnO2 leads to poor contact with

stainless steel. The increase of deposition electrolyte concentration will influence

the morphology of the deposits, indicating the different deposition rates is presented.

The flowerlike structure of 0.01 M electrode is obviously observed from TEM

images as shown in Figure 4.3 (f). The flowerlike structure with aligned spikes of

10 to 15 nm in diameter and around 30 to 40 nm of lengths are formed. High

resolution TEM of deposited MnO2 is displayed in Figure 4.3 (g), revealing the

measured lattice fringes of 0.31 nm, 0.24 nm and 0.21 nm are corresponding to

interplanar spacing of (310), (211) and (310) planes of α-MnO2 (Benhaddad et al.,

2011; Wang et al., 2012). This result is consistent with XRD analysis.

1 µm 100 nm

(a) (b)

61

(Figure 4.3, continued)

Figure 4.3 FESEM images of: (a) 0.0025 M, (b) 0.005 M, (c) 0.01 M, (d) thickness of 0.01 M, (e)

0.02 M, (f) TEM image of 0.01 M electrode and (g) lattice spacing of deposited MnO2.

4.2.1.3 Electrochemical performance of MnO2 electrodes in KOH electrolyte

MnO2 has been widely and commonly studied as the electrode for

supercapacitors. The electrochemical properties of as prepared MnO2 electrode have

been evaluated at room temperature using CV and CDC studies. The CV tests for

all deposited electrodes are performed in 0.5 M KOH electrolyte in potential range

5 µm 1 µm

5 µm

379.9 nm

321.1 nm

(c) (d)

(e) (f)

(g)

62

of -0.35 to 0.65 V at scan rate of 5 mV s-1. The CV curves are shown in Figure 4.4

(a), exhibiting rectangular shape. The CV results also reveal the electrical double

layer behaviour that dominate this system (Inamdar et al., 2011). The most

rectangular shape of CV curve is found in sample 0.01 M with high current pass

through than others sample, indicate a strong capacitive performance (Hu et al.,

2011).

The calculated specific capacitance values are 116 F g-1, 128 F g-1 and 143

F g-1 with mass loading of 4 x 10-5 g, 1.28 x 10-4 g and 9.4 x 10-4 g for 0.0025 M,

0.005 M and 0.01 M electrode, respectively. The optimum specific capacitance in

0.01 M electrode may due to the uniform structure of MnO2 on SS. The best

electrode 0.01 M is further investigated with applied different scan rate studies,

Figure 4.4 (b). The rectangular shape is still retained although high scan rate is

applied. However, the specific capacitance of device has decrease from 180 F g-1 at

scan rate of 1 mV s-1 to 71.4 F g-1 at scan rate of 10 mV s-1, Figure 4.4 (c). This is

due to high utilization of electrode active material during slow charge scan, hence

resulting in high specific capacitance (Rusi & Majid, 2014b).

The CDC performances of all electrodes is tested in potential range of -0.35

to 0.65 V at constant applied current of 1 mA. All CDC curves exhibits symmetric

and linear CDC profile for all electrodes (Figure 4.4 (d)), implying good capacitive

characteristics of the solid-state supercapacitor device (Yuan et al., 2012). The

specific capacitance calculated from discharge curve of 0.01 M electrode is 135

F g-1 with energy density of 18.8 Wh kg-1 and power density of 9.5 kW kg-1.

63

-1.4

-0.4

0.6

1.6

2.6

-0.4 -0.2 0 0.2 0.4 0.6

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential ( V )

0.005 M

0.0025 M

0.01 M

-2.5

-1.0

0.5

2.0

-0.4 -0.2 0 0.2 0.4 0.6

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential (V)

1 mV/s 3 mV/s

5 mV/s 7 mV/s

10 mV/s

0

40

80

120

160

200

0 2 4 6 8 10

Sp

ecif

ic c

ap

aci

tan

ce (

Fg

-1)

Scan rate (mVs-1)

(c)

(a)

(b)

64

(Figure 4.4, continued)

Figure 4.4 (a) CV curves of all deposited electrode at 0.5 M KOH electrolyte, (b) CV of scan rate

dependent of 0.01 M electrode, (c) the specific capacitance against scan rate plot, (d) CDC curve at

applied constant current of 1 mA.

As the cycle lifetime is one of the limitations in supercapacitor applications,

a cyclic stability test for the electrode within 1000 cycles CDC at current density of

1 A g-1 is performed (Figure 4.5). The specific capacitance slightly decreases when

reach 300 cycles and remain stable until 1000 cycles. The electrode exhibited good

cycle life, the specific capacitance retain 98 % after 1000 cycles.

Figure 4.5 Cycling performance of 0.01 M electrode in 0.5 M KOH electrolyte at current density of

1 A g-1.

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 50 100 150 200 250 300

Po

ten

tia

l (V

)

Time (s)

0.01M

0.005M

0.0025M

0

40

80

120

160

0 200 400 600 800 1000

Sp

ecif

ic c

ap

aci

tan

ce (

Fg

-1)

Cycle

(d)

65

4.3 Summary

The summary of this work is as listed below:

The concentration of manganese acetate in deposition solution is influenced

the growth mechanism of deposited MnO2, hence resulted on different

morphologies of MnO2.

0.01 M manganese acetate as deposition electrolyte is the suitable

concentration to produce the flower-like MnO2 structure with particle size is

in the range of 200 to 400 nm and thickness around 300 to 400 nm.

The specific capacitance of 0.01 M electrode is 180 F g-1 at scan rate of 1

mV s-1 in 0.5 M KOH solution.

The optimum energy density and power density of 0.01 M electrode are 18.8

Wh kg-1 and 9.5 kW kg-1 at applied constant current of 1 mA in KOH

electrolyte.

The specific capacitance of 0.01 M electrode retained 98 % after 1000 cycles

in 0.5 M KOH electrolyte at scan rate of 10 mV s-1.

The 0.01 M electrode exhibited good capacitance with good cycling stability

which can be considered as the promising electrode for nanostructure

materials in energy storage and others area.

66

CHAPTER 5: THE STUDIES OF MnO2-NiO BINARY

ELECTRODE SYSTEM

5.1 Introduction

Recently manganese oxide (MnO2) electrode has received attention because

of its physical and chemical properties with low cost, non-toxic and high theoretical

specific capacitance value (Hu et al., 2011a; Wei et al., 2011). However as a

supercapacitor’s electrode material, pristine MnO2 electrode is still hampered by its

poor electrical conductivity (Li et al., 2012a). To mitigate these problems, the

primary MnO2 can be incorporated with secondary/ternary metal oxide materials

(Wei et al., 2011; Wu et al., 2012). In this case, an electrode containing mixed metal

oxides performs better than a single transition metal oxide when it is used as the

electrode in a supercapacitor (Rajendra Prasad & Miura, 2004; Wu et al., 2012).

In order to optimize the performance of MnO2 electrode, incorporating

nickel oxide (NiO) in MnO2 electrode is one of the promising approaches for

nanocomposite materials in supercapacitor. With high surface area, NiO porous able

to provide pathway for electrolyte cations to electroactive sites on the electrode and

increase the rate of faradaic redox reactions (Vijayakumar et al., 2013; Wu et al.,

2008b). Thus, the lack of porosity and surface area of a manganese oxide electrode

can be improved.

Here, compositing MnO2-NiO has been done using binder free

electrodeposition method. The suitable concentration ratio between manganese and

nickel ion in the deposition electrolyte is determined by CP method. And the

67

optimum electrode has being further investigated by CA and CV methods. These

different electrodeposition modes significantly affect the surface morphology of

films, crystal structure, and performance in corresponding application (Dubal et al.,

2011; Jagadale et al., 2013). Finally, the variations of deposition cycles in CV

method will influence the deposit thickness and electrochemical performances.

5.2 Results and discussions

5.2.1 Optimization of Ni(CH3COO)2.4H2O concentration with 0.01 M of

Mn(CH3COO)2.4H2O for deposition electrolyte solution

5.2.1.1 Characterization of composition and morphology of MnO2-NiO electrode

The XRD patterns of pure SS and all binary MnO2-NiO deposited on top of

SS substrate is displayed in Figure 5.1 (a). The diffractogram reveals that there is

only SS peaks or no additional NiO or MnO2 peaks in all deposited films, suggesting

that a thin layer of the nickel-manganese oxide has homogenously adhered on the

SS surface (Rusi & Majid, 2013; Takahashi et al., 2005). Prior to determining the

deposited element of the electrode, thin layer of deposits was scraped off from SS

surface and the XRD pattern of deposited powder is shown in Figure 5.1 (b). The

diffraction peaks located at 2θ= 27.24° and 50.52° are corresponds to the (310) and

(411) diffraction planes of MnO2. While, the diffraction peaks at 2θ= 36.26° and

43.67° is belonged to the (111) and (200) planes of NiO. The diffraction peaks

confirms the formation of NiO and MnO2 on top of SS which is in accordance with

JCPDS no. 04-0835 and JCPDS no. 44-0141, respectively (Dharmaraj et al., 2006;

Li et al., 2011; Zhang et al., 2012b). Furthermore, no impurity peaks in the

diffraction pattern are observed, implying only pure MnO2-NiO has been deposited

68

on SS. The broad and low intensity diffraction peaks suggest that the deposited are

in amorphous with nano-scale dimensionality which will be confirmed by FESEM

and TEM studies (Dharmaraj et al., 2006; Ho & Wu, 2011). The EDX spectrum of

binary MnO2-NiO deposited powder is shown in Figure 5.1 (c), confirming there are

presence of Ni, Mn and O in the electrode sample.

5 15 25 35 45 55 65

Inte

nsi

ty (

a.u

.)

2θ(°)

SS

NiO-20

NiO-25

NiO-30

NiO-40

(a)

20 30 40 50 60

Inte

nsi

ty (

a.u

.)

2θ (°C)

NiO

α-MnO2

(b)

69

(Figure 5.1, continued)

Figure 5.1 XRD patterns of (a) empty stainless steel and all deposited electrode on top of SS, (b)

powder of NiO-25 which was scraped off from stainless steel (c) EDX of the NiO-25 deposited

powder.

The nature of binary oxides has been well-studied and the formation of

MnO2-NiO in this work is speculated began with the electric current pass through

the electrolyte. The water molecule is then separated into hydrogen gas and

hydroxide ions. The cations in deposition electrolyte solution commonly have high

binding affinity with hydroxide groups, leading to the nucleation of manganese or

nickel hydroxide particle. The manganese-nickel oxide nuclei settle down onto the

substrate during annealing process (Rusi & Majid, 2014b; Yousefi et al., 2012). The

formation of MnO2-NiO has been confirmed with XRD.

The morphologies of all deposited MnO2-NiO films were studied using

FESEM as shown in Figure 5.2. The change of morphology is observed as the

concentration of Ni2+ ion in deposition electrolyte solution is varied from 0.15 M to

0.4 M. The uniform layer with open structure of interconnected of nanoflakes started

to form at the deposited electrode NiO-15, as shown in Figure 5.2 (a). The

KeV

(c)

Elements Weight (%)

C 14.36

O 31.76

Mn 47.76

Ni 6.12

70

arrangement with an open structure lead to porous network structure which is

beneficial for ion transport to the electrode matrix (Li et al., 2008). A clear

interconnected nanoflakes is observed in NiO-20 electrode (Figure 5.2 (b)). The

flakes are denser and generate more porous network structure when the

concentration of Ni2+ increased to 0.25 M. The porous network in the NiO-25

electrode is made up of thin layer of flakes and the pore size is in the range of 10

nm to 15 nm, Figure 5.2 (c). The structure is less apparent as the concentration of

Ni2+ is increased to 0.3 M and 0.4 M, Figure 5.2 (d-e). As the concentration increase,

the deposition rate is believed to increase; hence lead to the stacking of dense metal

oxide layer on top of SS i.e. sample NiO-30 and NiO-40.

(a) (b)

(d) (c)

71

(Figure 5.2, continued)

Figure 5.2 FESEM images of: (a) NiO-15, (b) NiO-20, (c) NiO-25, (d) NiO-30 and (e) NiO-40.

In order to further investigate the structure of the deposited MnO2-NiO

electrode, TEM studies of selected samples are carried out and the results are

displayed in Figure 5.3. TEM images of deposited MnO2-NiO with a low

concentration of Ni2+, NiO-15 do not have noticeable flakes structure (Figure.5.3

(a)), whereas in the NiO-25 sample, the nanoflakes structure is clearly observed. As

seen in the Figure 5.3 (b), the dark strip corresponds to the flakes and bright region

are an open structure made up of flakes to form interconnected porous structure.

This unique structure plays a basic role in the morphological requirements for

electrochemical fast accessibility/diffusions of electrolyte ions to the active material

(Cross et al., 2011; Wu et al., 2008b). Generally, an increase in the electrolyte

concentration leads to aggregation and the loss of specific morphology, Figure 5.3

(c) (Babakhani & Ivey, 2011; Li et al., 2012b). This type of structure in NiO-30 is

probably due to an increase of growth rate of metal oxide deposits. This can also

cause by low electrostatic repulsive barriers during the nucleation stage which

prevents the flake structure from growing and produces denser NiO-MnO2. The high

resolution TEM lattice images are shown in Figure 5.3 (d). The periodic lattice

fringe of 0.31 nm is correspond to d-spacing of the (310) planes of MnO2 (Liu et al.,

(e)

72

2006), while lattice spacing of 0.24 nm and 0.21 nm is refer to (111) and (200)

planes of NiO (Li et al., 2006). The ineterplanar spacing is in good agreement with

the XRD results.

Figure 5.3 TEM images of (a) NiO-15, (b) NiO-25, (c) NiO-30, and (d) lattice fringes of NiO-25

deposited film.

5.2.1.2 Electrochemical performance of MnO2-NiO electrode in Na2SO4 electrolyte

The electrochemical performances of the obtained electrodes have been

investigated using EIS, CV and CDC. The EIS test is used to analysis the resistance

of MnO2-NiO electrode in 0.5 M Na2SO4 electrolyte. This technique can provide

information on the electrochemical frequency behaviour of the supercapacitors. The

equivalent circuit in accordance with Nyquist plots is displayed in Figure 5.4 (a).

The high frequency region is on the left part of the plot. In high frequency region,

Interconnected flake

Porous

(a) (b)

(c) (d)

73

the diameter of semicircle is used to determine the charge transfer resistance (Rct)

of the electrolyte ion to electrode. The intercept of initial point of semicircle in x-

axis represents the series resistance (Rs), correspond to the combination of ionic

resistance of electrolyte, contact resistance and internal resistance of materials (Rusi

& Majid, 2014b; Zhang et al., 2012a). The value of Rs for NiO-20, NiO-25, NiO-30

and NiO-40 are 1.3 Ω, 1.4 Ω, 2.1 Ω and 1.3 Ω respectively, while the values of Rct

for NiO-20, NiO-25, NiO-30 and NiO-40 are found to be 2.5 Ω, 0.7 Ω, 0.8 Ω and

1.3 Ω respectively. The smallest Rct is found in NiO-25. The constant phase element,

CPE1 and CPE2 in this system are used to replace the double layer capacity and

Warburg impedance resistance for semi-infinite linear diffusion, respectively (Yang

et al., 2014b). Meanwhile, the linear portion in the low frequency regions reflects

the frequency dependence of ion diffusion and transport in the electrolyte (Cai et al.,

2014; Chen et al., 2013). As can be seen in Figure 5.4 (a), the angle between the

spikes and the real axis of the sample inversely followed the Rct trend, i.e., higher

resistance lead to a smaller angle. The bigger angle is found in NiO-25 electrode,

exhibiting the characteristic of ideally polarize electrode (Sugimoto et al., 2005).

To evaluate the storage capability of the deposited electrode, the CDC

performances of all electrodes are tested at current density of 1 A g-1 in potential

range of 0 to 1 V, as shown in Figure 5.4 (b). The specific capacitance of the NiO-

15, NiO-20, NiO-25, NiO-30, and NiO-40 electrodes in 0.5 M Na2SO4 are

calculated as 278 Fg–1, 303 F g–1, 357 F g–1, 323 F g–1, and 303 F g–1 respectively.

The highest specific capacitance is found in sample NiO-25 with power density of

15 kW kg–1at a current density of 1 A g–1. The CV curves for all electrodes at

potential range of 0 until 1 V is displayed in Figure 5.4 (c). Compared to other

electrodes, NiO-25 electrode exhibits the high current passing through. The

74

calculated specific capacitance from CV studies at scan rate 5 mV s-1 in 0.5 M

Na2SO4 electrolyte are 135 F g-1, 152 F g-1, 173 F g-1, 166 F g-1 and 154 F g-1 for

NiO-15, NiO-20, NiO-25, NiO-30, and NiO-40 electrodes, respectively. The

highest specific capacitance is still found in sample NiO-25. This is due to the low

resistance and the highly porous electrode structure among others, leading to greater

ion diffusion and enhances faradaic reaction through the intercalation of cations

from the electrolyte into the electrode matrix (Dharmaraj et al., 2006; Yuan et al.,

2011). The decrease in the specific capacitance of electrodes NiO-30 and NiO-40

can be attributed to aggregation effects in MnO2-NiO and obstructed ion

movements. The presence of a small amount of nickel is adequate to stabilize the

MnO2 structure. The CV responses in wide potential operation range from -1 V to 1

V of NiO-25 in Na2SO4 electrolyte at different scan rates and its corresponding

specific capacitance is displayed in Figure 5.4 (d-e). The shapes of the CV curves at

all scan rates are identified and a pair of redox peaks is observed, implying that the

faradic reaction had occurred in the electrode. The possible reaction in NiO-25

electrode can be expressed by following equations (Inamdar et al., 2011; Li et al.,

2012a):

NiO + OH– ↔ NiOOH + e- (Equation 5.1)

MnO2 + H+ + e– ↔ MnOOH (Equation 5.2)

Or

MnO2 + K+ + e– ↔ MnOOK (Equation 5.3)

(K: cations in electrolyte (i.e., Na+ or K+))

The specific capacitance in lowest scan rate of 1 mV s-1 is found to be 435

F g-1 with energy density of 242 Wh kg-1. The decrease in the specific capacitance

Charge/discharge

75

of NiO-25 when subjected to 20 mV s-1 is 61 %, indicating that the faradic reaction

is greatly affected by the ion diffusion to the MnO2-NiO electrode matrix. At low

scan rate, cation diffusion could access almost all available pores and the active

materials are fully utilized, resulting in high specific capacitance. Meanwhile, at a

high scan rate, the diffusion of cations have the tendency to diminish, and cations

are capable of reaching only the outer surface of the active materials (Rusi & Majid,

2014b).

0

2

4

6

8

0 2 4 6 8

-Z"

(Ω

)

Z' (Ω)

NiO-20

NiO-25

NiO-30

NiO-40

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600

Po

ten

tia

l v

s A

g/A

gC

l (V

)

Time (s)

NiO-15

NiO-20

NiO-25

NiO-30

NiO-40

(b)

(a)

76

(Figure 5.4, continued)

Figure 5.4 (a) Nyquist plots of the all deposited electrodes, (b) CDC profiles of all deposited

electrodes at a current density of 1 A g-1, (c) CV curves of all deposited at scan rate of 1 mV s-1, (d)

CV curves of NiO-25 at wide operation potential range of -1 until 1 V at different scan rates, (e)

specific capacitance of NiO-25 at wide operation potential range of -1 until 1 V at different scan

rates.

-1.2

-0.8

-0.4

0

0.4

0.8

0 0.2 0.4 0.6 0.8 1

Cu

rren

t d

ensi

ty (

A g

-1)

Potential vs Ag/AgCl

NiO-15

NiO-20

NiO-25

NiO-30

NiO-40

-7

-3

1

5

9

-1 -0.6 -0.2 0.2 0.6 1

Cu

rren

t d

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

1 mV/s

5 mV/s

10 mV/s

15 mV/s

20 mV/s

(d)

20 mVs-1

15 mVs-1

10 mVs-1

5 mVs-1

1 mVs-1

0

100

200

300

400

500

0 5 10 15 20

Sp

ecif

ic c

ap

aci

tan

ce (

F g

-1)

Scan rate (mV s-1)

(e)

(c)

77

The cyclability of the electrode are important for practical applications

because give a high power demand from portable electronic power tool and EVs.

The cyclability test of NiO-25 is performed in 0.5 M Na2SO4 electrolyte at scan rate

of 10 mV s-1 until 900 cycles in potential range of -1 V to 1 V, as shown in Figure

5.5.

Figure 5.5 Specific capacitance retention at scan rate of 10 mVs–1 in 0.5 M Na2SO4 electrolyte with

potential range of -1 to 1 V.

The specific capacitance decreases during the initial cycle and slowly increase

before it reached 100 % retention after 900 cycles. A similar trend is also observed

in MnO2, NiO, nickel manganese oxide (NiMn2O4) and nickel cobalt oxide

(NiCo2O4) electrodes (Pang et al., 2000; Zhang et al., 2013b) . This increase is

probably due the increase in active sites in the materials during the electrochemical

process which lead to the incomplete diffusions of cations at a relative high scan

rate. Within this potential range, the diffusion of cations do not seems to induce

significant electrode structure change (Zhang et al., 2013b).

Although the stability of the electrode is good, the energy density and power

density of the electrode requires further improvements. In order to improve the

0

20

40

60

80

100

120

0 200 400 600 800 1000

Ret

enti

on

(%

)

Cycle

78

performance of NiO-25 electrode, the parameters of electrodepositon modes are

further varied and studied.

5.2.2 Effects of electrodeposition modes of MnO2-NiO electrode

5.2.2.1 Characterization of composition and morphology

The optimum electrode NiO-25 from previous studies which deposited from

a solution containing 0.8 M H2SO4, 0.01 M Mn(CH3COO)2.4H2O and 0.15 M

Ni(CH3COO)2.4H2O has been further investigated using CA and CV method,

denoted as CA and CY7 respectively. NiO-25 electrode is denoted as CP electrode

in the following section. The XRD pattern of scraped-off deposits of manganese-

nickel oxide powder of CP, CA, and CY7 electrode is shown in Figure 5.6.

Figure 5.6 (a) XRD pattern of the deposited powder using different electrodeposition modes.

All the diffraction peaks of the XRD for all deposited powder can be indexed to α-

MnO2 [JCPDS card no. 44-0141] and NiO [JCPDS card no. 04-0835], respectively

(Dharmaraj et al., 2006; Zhang et al., 2012b). This figure reveals that the crystalline

nature of the electrodes can be influenced by deposition modes, which can change

15 20 25 30 35 40 45 50 55 60

Inte

nsi

ty (

a.u

.)

2θ (°)

CP

CA

7 cy

MnO2

NiO

CY7

79

the crystal size and amorphousness of the deposits. The synthesized CY7 electrode

has relatively weak intensity of peaks, which suggests that the deposited MnO2-NiO

particles are in a poor crystalline state, which can contribute to a larger specific area

than a highly crystalline structure film (Wu et al., 2007). In this XRD studies, there

is no change on chemical compositions of MnO2 and NiO although the electrode has

deposited using different modes.

The CA, CP, and CY7 electrodes have the same chemical compositions, but

might exhibit different morphologies. The morphology of the deposited sample

using different electrodeposition modes has been investigated using FESEM and

TEM as shown in Figure 5.7. The FESEM images reveal that all of the MnO2-NiO

electrodes contain homogeneously distributed porous structures. Less number of

particle agglomerations can be found in the CY7 electrode, Figure 5.7 (c). On closer

inspection of TEM images, the CA and CY7 electrodes have displayed smaller pore

size (~ 2-4 nm) than the electrode prepared from CP mode (~10-15 nm). These

results indicate that the CA and CY7 samples are mesoporous type of materials

which can allow the electrolyte ion to easily diffuse through the electrode bulk

causing more faradic reactions to take place during the electrochemical performance

(Cross et al., 2011; Prasad et al., 2013; Wang et al., 2010b).

(a)

80

(Figure 5.7, continued)

Figure 5.7 FESEM (left) and TEM (right) images from: (a) CP, (b) CA, (c) CY7.

5.2.2.2 Electrochemical performance of MnO2-NiO electrode in Na2SO4 electrolyte

In order to determine the supercapacitive behaviour of the electrodes, the CV

in 0.5 M Na2SO4 was carried out in the wide potential window from -1 V to 1 V at

a scan rate of 5 mV s-1 as shown in Figure 5.8.

(b)

(c)

81

Figure 5.8 CV profiles of deposited electrodes using different electrodeposition modes in Na2SO4

electrolyte.

A pair of symmetric anodic and cathodic peaks can be easily observed on each

profile of the CV curve, implying the existence of redox reactions of the electrode.

The charge storage in MnO2 electrode can be described by two mechanisms. These

mechanisms are adsorption/desorption of Na+ ions on the surface of electrode

materials and diffusions of H+ or Na+ ions in the electrode during the

oxidation/reduction process, which can be described by Equation 5.2 and Equation

5.3 (Cross et al., 2011; Li et al., 2012a). Base on Equation 5.1, the OH- ion plays an

important role for NiO charge storage. The lower concentration of OH- in Na2SO4

aqueous electrolyte solution might lead to less-significant contribution of NiO redox

reaction and NiO is mainly contributes to the capacitance that is from electrical

double-layer storage behaviour (Inamdar et al., 2011; Wu et al., 2007). The largest

area under the curve is exhibited by CY7 electrode, indicating a higher

electrochemical activity and capacitance. The calculated specific capacitance of CP,

CA, and CY7 electrodes at scan rate of 1 mV s-1 are 435 F g-1, 458 F g-1, and 500

F g-1 respectively. The high specific capacitance of CY7 electrode can be due to the

lower agglomeration, well-distributed mesoporous structure, and low crystallinity

-3.5

-1.5

0.5

2.5

4.5

-1 -0.5 0 0.5 1

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

7 cy CV

CA

CP

CY7

82

(high amorphous) of the sample, which can be observed in the XRD, FESEM, and

TEM results. This suggests that phase structure and morphology, along with the

porous nanostructure of MnO2-NiO, have a significant influence on its

electrochemical properties.

The optimum electrode from this study is CY7 electrode which deposited

using CV method with 7 times of deposition cycle. In order to get the optimum

deposition cycle, the number of deposition cycles is varied i.e 4, 7, 10 and 13 to

obtain the optimum deposition cycle.

5.2.3 Effect of electrodeposition cycle on MnO2-NiO electrode

5.2.3.1 The morphology studies of MnO2-NiO

The morphologies of various deposition cycles electrodes are investigated

using FESEM. The FESEM images are displayed in Figure 5.9. The images indicate

that different deposition cycles give different morphologies. At the lowest

deposition cycle (CY4 electrode, Figure 5.9 (a)), clear interconnected metal oxide

particles with the formation of a porous structure is observed, yet the flakes structure

is not able to grow during this short deposition process. In Figure 5.8 (b-d), the clear

flakes structure is formed and get denser over each increment of the deposition cycle

number. These compact/denser flakes lead to increment of deposit mass load. The

possible mechanism that occurs during the deposition process is explained below.

The initial formation of stable interconnected MnO2-NiO is believed to occur at the

early stage of deposition (around the forth cycle of deposition) due to an

instantaneous nucleation process. The process will then be continued with the

progressive nucleation process in which the flakes’ structure will grow on the top of

83

previously-formed nuclei. As the cycle deposition time increases, the growing rate

of progressive nucleation enhances, which lead to denser flakes with the presence

of compact grains (Babakhani & Ivey, 2011; Hwang et al., 2000). The TEM image

of CY4 electrode is shown in Figure 5.9 (e). There is no flake structures found in

the deposits and it has more highly distributed pores than CY7 (Figure 5.7 (c)). The

elemental confirmation of MnO2-NiO in the CY4 electrode is analysed using high

resolution TEM as shown in the inset of Figure 5.9 (e). It is confirmed that the

measured interplanar spacing belongs to MnO2 and NiO (Li et al., 2006; Liu et al.,

2006).

(a) (b)

(c) (d)

89 nm 106 nm

93 nm

188 nm

189 nm

257 nm

242nm 227nm 212nm

311 nm 333 nm 307 nm

84

(Figure 5.9, continued)

Figure 5.9 FESEM images of : (a) CY4, (b) CY7, (c) CY10, (d) CY13 (e) TEM images of CY4

electrode (inset: lattice spacing).

5.2.3.2 Electrochemical performance of MnO2-NiO in Na2SO4 electrolyte

Figure 5.10 (a) shows the CV responses of the CY4, CY7, CY10, and CY13

electrodes in 0.5 M Na2SO4 electrolyte. The peak current is decreased as the

deposition cycle time increased, which infers to lower diffusion of cation into the

MnO2-NiO electrode. The structure of the denser and more compact flakes’ may

tend to prevent the cation from migrating into the electrode materials, lead to a

decrement of Na+ or OH- ion adsorption. The specific capacitance of CY4, CY7,

CY10 and CY13 electrodes at scan rate of 1 mV s-1 are 1060 F g-1, 500 F g-1, 479

F g-1 and 480 F g-1 respectively.

Figure 5.10 (b) displays the CDC curve of all electrodes at a current density

of 1 A g-1 in voltage ranging from -0.6 V to 1 V. The non-linearity of the curve is

due to redox reaction of the electrode active materials occur in this voltage range

(Dubal et al., 2013). The longer discharge time is achieved in the CY4 electrode,

indicating more efficient charge storage behaviour than other electrodes. The

specific capacitance for CY4, CY7, CY10 and CY13, based on the mass of deposited

(e)

85

MnO2-NiO calculated from discharge curve are 769 F g-1, 557 F g-1, 556 F g-1 and

227 F g-1 respectively. The trend of the specific capacitance is inversely proportional

to the mass of the electrode (Broughton & Brett, 2005) as shown in Figure 5.10 (c).

The observed downtrend in capacitance with the mass is due to increase in

resistances which influences the ion conduction, as shown in the Nyquist plot in

Figure 5.11.

-6

-4

-2

0

2

4

6

-1 -0.5 0 0.5 1

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

4 cy

7 cy

10 cy

13 cy

-0.6

-0.2

0.2

0.6

1

0 400 800 1200

Po

ten

tia

l v

s A

g/A

gC

l (

V)

Time (s)

4 Cy

7 Cy

10 Cy

13 Cy

(a)

(b)

CY4

CY7

CY10

CY13

CY4

CY7

CY10

CY13

86

(Figure 5.10, continued)

Figure 5.10 (a) CV curves at a scan rate of 5 mV s-1, (b) CDC profile at a current density of 1 A g-1

in a voltage range of -0.6 V to 1 V, (c) Specific capacitance calculated from discharge curve and

deposit mass over deposition cycle.

The characteristic of ion transport resistance for all CY electrodes are

investigated by EIS. The equivalent circuit in accordance with Nyquist plots (Figure

5.11 (a)) were fitted using Nova software and fitting parameters is listed in Table

5.1. It is found that the Rs and Rct values of CY4 electrode are smaller than those of

the CY7, CY10, and CY13 electrodes. This finding indicates good wettability of

electrode/electrolyte interfaces and suggests that the cation diffusion activity into or

from the CY4 electrode is easier than that of the other electrodes (Rusi & Majid,

2014b). Smaller Rs also indicates lower resistivity of the CY4 electrode, which may

be due to the increased surface area of the deposited composite electrode structure

(Hughes et al., 2002). Thus the CY4 electrode exhibits higher specific capacitance,

as it possesses better electrochemical performance than CY7, CY10 and CY13

electrodes.

Bode plots studies are carried out to get more informations on contribution

to the capacitance of the electrode. The Bode plots in Figure 5.11 (b–c) can be

0

0.0002

0.0004

0.0006

0.0008

0

200

400

600

800

1000

2 4 6 8 10 12 14

ma

ss (

g)

Sp

ecif

ic c

ap

aci

tan

ce (

F g

-1)

deposition cycle

Specific capacitance

Deposit mass

(c)

87

divided into three frequency regions: high (f>10Hz), medium, and low frequency

regions (f<1Hz). The typical capacitive behaviour is exhibited for both plots in the

whole frequency range. In the low frequency region, all deposited films present a

slope of ~ -1, in the f- |Z| plots. The phase angle in the f-ɸ plots fall between -70°

and -55°. The intermediate value of impedance magnitude ǀZǀ and phase angle (ɸ) is

obtained in the medium frequency region. In the high frequency region, the

capacitance is nearly zero, as the Z is independent and the phase angle start to

decrease to almost zero when the frequency is further increased. The frequency at

phase angle = -45° in f-ɸ plots (Figure 5.11 (c)) is known as the frequency response

region. The higher frequency at ɸ= -45° represents the better capacitive response.

The highest frequency response is 2.22 Hz for the CY4 electrode, indicating a

reasonably fast electron/proton transport response and lead to high capacitance

(Ding et al., 2013).

Table 5.1 Equivalent circuit parameters deducted by fitting Nyquist plots and frequency at ɸ=-45°

for all electrodes.

Deposition cycle Rs (Ω) Rct (Ω) fɸ=-45º (Hz)

4 1.10 0.75 2.22

7 1.29 1.78 1.26

10 1.30 1.8 0.72

13 1.31 1.8 0.63

The cycling performance of the MnO2-NiO, CY4 electrode is displayed in

Figure 5.11 (d). The capacitance retention experienced drastic drops during the first

50 cycles in the Na2SO4 electrolyte, which could be attributed to the partial

dissolution of active materials in the electrolyte. This partial dissolution is generated

by a high current passed through the materials and causes a loss of volume in the

active materials (Engstrom & Doyle, 2013). However, the capacitance retention

increased and stabilized at the two-hundredth cycle as a consequence of more

88

effective porous surfaces being accessible to the electrolyte after a particular

numbers of cycles are achieved, thus improving the capacitance.

0

2

4

6

8

1 3 5 7 9

-Z"

(Ω)

Z' (Ω)

CY 4

CY 7

CY 10

CY 13

0

10

20

30

40

-1 0 1 2 3 4

|Z|

(Ω)

Log f (Hz)

4 Cy

7 Cy

10 Cy

13 Cy

0

20

40

60

80

-1 0 1 2 3 4

-ɸ (

deg

ree)

Log f (Hz)

4 Cy

7 Cy

10 Cy

13 Cy

(c)

CY4

CY7

CY10

CY13

(a)

(b)

CY4

CY7

CY10

CY13

89

(Figure 5.11, continued)

Figure 5.11 (a) Nyquist plot of all electrodes in frequency range from 10 mHz to 100 kHz at Na2SO4

electrolyte, (b) Bode plots of frequency dependence on the impedance magnitude (Z); (c) Bode plots

of frequency dependence on phase angle (), (d) Specific capacitance retention until the 1000th cycle

in 0.5 M Na2SO4 electrolyte with potential range of -1 to 1 V.

5.2.3.3 Electrochemical performance of MnO2-NiO (CY4) electrode in various

electrolytes

The electrochemical performance of CY4 electrode was further investigated

in different alkaline electrolytes. The CV of the CY4 electrode were recorded in 0.5

M Na2SO4, 0.5 M KOH, 0.04 M K3Fe(CN)6, and mix 0.5 M KOH/0.04 M

K3Fe(CN)6 electrolytes by sweeping the potential from -0.5 V to 0.5 V at a scan rate

of 5 mV s-1, as shown in Figure 5.12. In Na2SO4 electrolyte (Figure 5.12 (a)), CV

curve with a well-defined pair of anodic peaks (A0) and a cathodic peak (C0)

centered at around +0.20 and -0.05 V (vs. Ag/AgCl) is observed. These peaks are

referred as the redox reaction dominated by MnO2, according to Equations 5.2 and

5.3.

0

20

40

60

80

100

120

0 200 400 600 800 1000

Ret

enti

on

(%

)

Cycle number

(d)

90

The KOH electrolyte is known to be a better electrolyte for NiO material

than Na2SO4 aqueous solution, due to the higher OH- concentration in the electrolyte

solution. OH- ions play an important role in NiO reaction during

charging/discharging (Equation 5.1) (Wu et al., 2007). The combined contribution

of MnO2-NiO in the KOH electrolyte (Figure 5.12 (b)) is supported by Equation 5.1

to Equation 5.3. The increment of current response in the KOH electrolyte is also

attributed to the K+ ion’s smaller cation radius (3.31Å) compared to Na+ ions (3.35

Å), as well as the higher conductivity of K+ ions (73 cm2/Ω mol) than Na+ ions

(50 cm2/Ω mol). Easy passage of K+ ions into the electrode matrix during the

charging process is achieved because it has a smaller radius and faster ion

movements (Nithya et al., 2013).

When KOH was replaced with mix KOH/K3Fe(CN)6 electrolyte, as in Figure

5.12 (c), an additional pair of anodic peaks at around +0.25 V (A02) and a cathodic

peak at +0.17 V (C02) are detected, which can be attributed to the redox reaction of

K4Fe(CN)6 to K3Fe(CN)6, and it is consistent with the CV plot of bare K3Fe(CN)6

electrolyte as shown Figure 5.12 (d) (Zhao et al., 2013b). In this system, there are

two types of charge storage reaction that could contribute to the capacitance. The

first reaction originated from the redox couple of [Fe(CN)6]3- / [Fe(CN)6]

4- in the

electrolyte (Equation 5.4). The second type of charge storage can be derived from

the redox reaction in highly electroactive electrodes (Equation 5.5). The reaction

can be written as follows (Chen et al., 2014):

Redox electrolyte: [Fe(CN)6]3- + e- ↔ [Fe(CN)6]

4- (Equation 5.4)

Redox electrode: Mz+ ↔ M (z+n)+ + ne- (Equation 5.5)

Where M is the Ni2+ or Mn2+ cations, and 1≤n≤z.

91

Figure 5.12 CV curve of CY4 at scan rate of 5 mV s-1 within potential range of -0.5 V to 0.5 V at :

(a) 0.5 M Na2SO4, (b) 0.5 M KOH, (c) mix 0.5 M KOH/0.04 M K3Fe(CN)6 and (d) 0.04 M K3Fe(CN)6

electrolyte.

Other than the electrode redox reaction of MnO2-NiO in the KOH

electrolyte, the hexacyanoferrate ions also play a role as “electron shuttles” in the

charging/discharging process (Su et al., 2009). When the electrode is charged,

[Fe(CN)6]3- will accept the electron via the reduction of hexacyanoferrate (III) to

(II), the hexacyanoferrate ions of which act as “electron carriers” (Figure 5.13 (a)).

When the reaction is reversed, the hexacyanoferrate ions act as “electron donors”

and [Fe(CN)6]4- returns to [Fe(CN)6]

3-. This will provide electrons for the transition

process from Ni(III) to Ni (II) or Mn(III) to Mn(II) (Figure 5.13 (b)). This

-5

-3

-1

1

3

5

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t d

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

-13

-9

-5

-1

3

7

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t d

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

-56

-36

-16

4

24

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t d

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

-40

-25

-10

5

20

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t d

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

(Ao) (a) (b)

(c) (d)

(Co)

(Ao1)

(Co1)

(Co2)

(Ao2)

92

performance helps the active materials to lose and gain electrons smoothly and

improves the capacitive performance (Chen et al., 2014).

Comparing all the curves, the CY4 electrode in the mix KOH/K3Fe(CN)6

electrolyte has a bigger area under the curve, implying high specific capacitance.

The calculated specific capacitances from the CV at 5 mV s-1 are as follows: 474

F g-1, 780 F g-1, and 5130 F g-1 for Na2SO4, KOH and mix KOH/K3Fe(CN)6

electrolytes, respectively. The importance of K3Fe(CN)6 has been confirmed by the

enhancement of specific capacitance.

Figure 5.13 Schematic of the role of hexacyanoferrate (II) and (III) in the process of: (a) charge and

(b) discharge of CY4 electrode.

Figure 5.14 (a) displays the CDC profiles of CY4 in Na2SO4, KOH, and mix

KOH/ K3Fe(CN)6 electrolytes at a current density of 20 A g-1. The calculated

specific capacitance from discharge curve are 46 F g-1, 583 F g-1 and 3509 F g-1 in

Na2SO4, KOH and mix KOH/ K3Fe(CN)6 electrolytes respectively. High specific

capacitance is also obtained in mix KOH/ K3Fe(CN)6 electrolytes.

e

Ni(II)/ Mn(II)

Ni (III)/Mn(III)

Fe (CN)6 4-

Fe (CN)6 3-

Electrode Electrolyte

Ni(II)/ Mn(II)

Ni(III)/ Mn(III)

Fe (CN)6 4-

Fe (CN)63-

e

-

Electrode Electrolyte (a) (b)

93

The CDC profile of CY4 under different applied current densities is plotted

in Figure 5.14 (b). The presence of two potential plateaus at around -0.14 V and 0.18

V can be inferred as a result of the redox reaction of KOH and K3Fe(CN)6. The

energy and power density of CY4 in mix KOH/ K3Fe(CN)6 electrolytes are 1322

Wh kg-1 and 110.5 kW kg-1 at current density of 20 A g-1.

Figure 5.14 CDC profiles of CY4 electrode at: (a) current density of 20 A g-1 in three different

electrolytes and (b) different applied current densities in mix KOH/K3Fe(CN)6 electrolyte.

The cyclic stability tests for 1500 cycles at a scan rate of 10 mV s-1 are

performed in Figure 5.15.

Figure 5.15 Specific capacitance retention after 1500th cycle at scan rate of 10 mV s-1 in potential

range of -0.5 V to 0.5 V.

-0.5

-0.3

-0.1

0.1

0.3

0.5

0 100 200 300

Pote

nti

al

vs

Ag

/Ag

Cl

(V)

Time (s)

K3Fe(CN)6

KOH

Na2SO4-0.5

-0.3

-0.1

0.1

0.3

0.5

0 100 200 300 400

Pote

nti

al

vs

Ag

/Ag

Cl

Time (s)

20 Ag-1

30 Ag-1

40 Ag-1

50 Ag-1

0

20

40

60

80

100

120

0 500 1000 1500

Ret

enti

on

(%

)

Cycle

Na2SO4

KOH

Mix KOH/K3Fe(CN)6

-0.5

0

0.5

0 10 20

20 A g-1

30 A g-1

40 A g-1

50 A g-1

K3Fe(CN)6

Na2SO4

Na2SO4

Mix KOH/K3Fe(CN)6

(a) (b)

94

The retention of specific capacitance of CY4 in Na2SO4, KOH and mix

KOH/K3Fe(CN)6 electrolytes after 1500 cycles are 30 %, 36 %, and 44 %,

respectively. Low capacitance retention over 1500 cycles could be attributed to high

degradation of the electrode results from the high current passing through during the

cyclability test and volume loss of active materials (Engstrom & Doyle, 2013). The

retention of specific capacitance in KOH is remarkably enhanced when 0.4 M

K3Fe(CN)6 was added to the KOH electrolyte. The additional of K3Fe(CN)6

electrolyte enhance the specific capacitance and improves of electrode stability. This

suggests that the mixed electrolyte is the stable electrolyte for the CY4 electrode.

Although the specific capacitance and energy density in this system is extremely

high, the stability still needs to be improved.

5.3 Summary

The summary of this current work is listed below:

First, the binary MnO2-NiO electrode is successfully deposited on top of SS

using CP mode.

The NiO-25 electrode deposited from deposition electrolyte with

concentration ratio of 0.25 M Ni(CH3COO)2.4H2O mixed with 0.01 M

Mn(CH3COO)2.4H2O is found to have uniform interconnected flake porous

structure.

The electrochemical performance of NiO-25 confirmed that it has better

performance in ion diffusion with specific capacitance of 435 F g-1 at scan

rate of 1 mV s-1 at 0.5 M Na2SO4 electrolyte.

95

The optimum electrolyte ratio (0.25 M Ni(CH3COO)2.4H2O : 0.01 M

Mn(CH3COO)2.4H2O) is further used in different electrodeposition mode

studies include CA and CV modes.

The suitable electrodeposition modes for deposited MnO2-NiO electrodes is

CV mode.

The number of deposition cycles in CV mode influences the nucleation

process, morphology structure, deposit thickness and electrochemical

performance of deposited MnO2-NiO electrode.

The CV mode with 4 times deposition cycle (CY4 electrode) gave a better

electrochemical performance with specific capacitance of 769 F g-1 at scan

rate of 5 mV s-1 at 0.5 M Na2SO4 electrolyte.

The specific capacitance, energy density and power density of CY4 electrode

in mix KOH/ K3Fe(CN)6 electrolyte are 3509 F g-1, 1322 Wh kg-1 and 110.5

kW kg-1 at current density of 20 A g-1 with potential range from -0.5 to 0.5

V.

CY 4 electrode exhibits 44 % of specific capacitance retention after 1500

cycles in mix KOH/ K3Fe(CN)6 electrolyte with potential range from -0.5 to

0.5 V at scan rate of 10 mV s-1.

The additional K3Fe(CN)6 in KOH electrolyte has improved the specific

capacitance and electrode stability MnO2-NiO binary electrodes.

MnO2-NiO binary electrodes are cost efficient electrodes for supercapacitor

application.

96

CHAPTER 6: THE STUDIES OF Mn3O4-NiO-Co3O4 TERNARY

ELECTRODE SYSTEM

6.1 Introduction

Pseudocapacitor with high energy density, power density and high cycle

stability has become the necessary requirement in many applications (Pu et al.,

2013; Wang et al., 2014). Transition metal oxides are well known as one of the main

electrode materials that can produce high pseudocapacitance due to its reversibility

in oxidisation and reduction over a wide potential range (Rusi & Majid). However

the limitation of capacitive storage ability of single inexpensive metal oxide (MnO2,

NiO, Co3O4 and VO) as electrode is still a major problem. Thus, it has led to many

research works in this area (Wang et al., 2010b; Wei et al., 2011).

One of the simple approaches to improve the performance of single metal

oxide electrode is by incorporating two or more metal oxides in one electrode. This

method has become a popular method to produce high surface area electrode and it

can reduce the resistance and enhance the charge storage capability (Rusi & Majid,

2014b; Wang et al., 2013; Wang et al., 2010b; Wei et al., 2011). In our previous

studies, a binary electrode consisting of a binary metal oxides, MnO2-NiO electrode

has been successfully electrodeposited by the chronopotentiometry method (Rusi &

Majid, 2014b). The electrode exhibited an enhanced specific capacitance compared

with MnO2 deposits. The best specific capacitance has been found to be 435 F g-1 in

0.5 M Na2SO4 at scan rate of 1 mV s-1 (in a voltage range of -1 V to 1 V). The

capacitance of the MnO2-NiO electrode can be further improved by compositing

another metal oxide into this binary electrodes.

97

In this work, we synthesized a composite electrode containing three metal

oxides, Mn3O4, NiO and Co3O4, in thin film through the galvanostatic technique.

Mn3O4 is one of the most stable manganese oxides and has moderate specific

capacitance, long-term stability and good corrosion stability (Dubal et al., 2010).

Nickel oxide has high theoretical specific capacitance (~2573 F g-1) but relatively

low redox reversibility (Jena et al., 2013) while cobalt oxide has a relatively low

specific capacitance but has high redox activity and good reversibility (Hsu et al.,

2013). The combination of these features is beneficial for higher electrochemical

capacitance of the composite electrode (Wang et al., 2013).

The ternary of Mn3O4-NiO-Co3O4 nanostructures/SS electrode are also

evaluated in different electrolytes: 0.5 M Na2SO4, 0.5 M KOH and mixed 0.5 M

KOH/0.04 M K3Fe(CN)6 electrolytes. The use of the redox additive K3Fe(CN)6 in

alkaline electrolytes is designed to improve the storage capability performance (Su

et al., 2009; Zhao et al., 2013).

6.2 Results and discussion

6.2.1 Optimization of CoSO4.7H2O concentration for Mn3O4-NiO-Co3O4 electrode

system

6.2.1.1 Characterization of composition and morphology of Mn3O4-NiO-Co3O4

electrode

The formation of Mn3O4-NiO-Co3O4 ternary layer is generated from an

electrochemical reaction during the electrodeposition process and can be expressed

98

as follows (Kulkarni et al., 2013; Rusi & Majid, 2014b; Yousefi et al., 2012; Yuan

et al., 2010):

2H2O+ 2e → H2 + 2OH- (Equation 6.1)

Mn2+ + 2OH- → Mn(OH)2 Mn3O4 (Equation 6.2)

Ni2+ + 2OH- → Ni(OH)2 NiO (Equation 6.3)

Co2+ +2OH- → Co(OH)2 Co3O4 (Equation 6.4)

When the cathodic current is applied, the SS will lose its electron and the

electron will be gained by H2O to produce OH- on the cathodic surface. Then OH-

will react with Co2+, Mn2+ and Ni2+ to form metal hydroxide at the cathode. This

deposition process is in accordance with (Kulkarni et al., 2013). The annealing

process at 300 °C will transform these metal hydroxides to Mn3O4, Co3O4 and NiO.

The formations of these metal oxides are confirmed using XRD.

Figure 6.1 presents the XRD results of deposited electrode on stainless steel,

deposited powder scraped off from stainless steel and EDX data of deposited

powder. As can be seen from Figure 6.1 (a), no distinct diffraction peaks can be

assigned to deposits layer and only a stainless steel peak is observed, implying the

deposits formed is ultrathin. However, the when we scraped off the deposits formed

on stainless steel and examined it again using XRD, diffraction peaks of the deposits

is shown in Figure 6.1 (b). The diffraction peaks at 2Ө = 36.2°, 45° and 50.3° are

attributed to (211), (220) and (105) planes of Mn3O4, in good agreement with the

Joint Committee for Powder Diffraction Standards (JSPDS) card no.24-0734

(Moses Ezhil Raj et al., 2010). The broad peak at 2 = 43.7° is belonged to (200)

Annealing

Annealing

Annealing

99

diffraction planes of NiO (JCPDS card no. 04-0835) and the peak at 2Ө = 27° and

56° is assigned to (111) and (422) diffraction planes of Co3O4 (JCPDS card no.76-

1802), respectively (Jagadale et al., 2013; Wang et al., 2011; Zeng et al., 2012). The

low intensity and broad diffraction peaks implying poor crystallinity and the

dimensionality of the crystal size are in nanoscale (Dubal et al., 2012). Figure 6.1

(c) displays the EDX result for the deposited powder and confirms that the deposited

electrode consists of Ni, Co, Mn and O.

(a)

(b)

100

(Figure 6.1, continued)

Figure 6.1 XRD pattern for (a) all deposited electrode on the SS, (b) deposited powder of 0.15 M

scraped off from SS, (c) EDX of deposited powder.

Base on reaction above, it can be said that the formation of metal oxide is strongly

dependent on the cations in the deposition electrolyte solution. Babakhani and Ivey

(Babakhani & Ivey, 2011) have claimed that the concentration of cations in

deposition electrolyte strongly affects the nucleation process and growth mechanism

during the electrodeposition process which will influence the morphology structure

of oxide compound. This is in agreement with our present work where the different

concentrations of cobalt ions in deposition electrolyte can change the morphology

of deposits as shown in Figure 6.2. At low concentrations of cobalt ions (0.05 M),

the amount of cobalt oxide nuclei in the deposited metal hydroxide is less and a

network-like nanoflakes structure is easily formed (Figure 5.2(a)). The morphology

of 0.05 M electrode is almost similar with manganese-nickel oxide electrode (NiO-

25) as reported in our previous study (Rusi & Majid, 2014b), indicating that low

concentration of cobalt ions do not have much effect on the morphology of

manganese-nickel oxide electrode. When the concentration of cobalt ions increases

to 0.1 M (Figure 6.2 (b)), a denser network-like flakes structure and a noticeably

(c)

Elements Weight (%)

C 25.54

O 33.45

Mn 38.64

Co 1.50

Ni 0.88

101

porous structure is obtained. A layer of deposited electrode without an appreciable

nanoflakes structure and well-distributed porous structure is obtained with a 0.15 M

electrode (Figure 6.2 (c)), which may be due to the increased nucleation and growth

rate of cobalt oxide. A deposit thickness of the 0.15 M electrode is observed in the

range of 51 to 59 nm (Figure 6.2 (d)). The further increase of cobalt ion

concentrations to 0.2 M and 0.3 M results in the same morphology with a less porous

region which reduces the porosity of the electrode as shown in Figure 2(e-f). This

could be attributed to the complex reaction during electrodeposition. In order to

show a clear porous structure of the electrode, TEM images are captured. The TEM

images are shown in Figure 6.3.

10 nm

(a) (b)

(c) (d)

100 nm 100 nm

100 nm 100 nm

54.4 nm

58.8 nm 51.9 nm

58.8 nm 57.7 nm

102

(Figure 6.2, continued)

Figure 6.2 FESEM images of: (a) 0.05 M, (b) 0.1 M, (c) 0.15 M, (d) 0.15 M (cross-section), (e) 0.2

M and (f) 0.3 M.

Figure 6.3 displays the TEM images of a deposited electrode with 0.1 M and

0.15 M electrode concentrations and the corresponding interplanar lattice spacing.

The thin dark line represents the flake structure which can clearly be observed in the

deposited electrode of 0.1 M electrode concentration, as shown in Figure 6.3 (a).

The inset in Figure 6.3 (a) revealed the presence of a porous region in the deposited

Mn3O4-NiO-Co3O4 layer on the substrate (Zhou et al., 2009) and the dense flake

structure has a tendency to block the cation pathway into the electrode matrix. In

Figure 6.3 (b), no sharp flake shape can be observed in the deposited layer of 0.15

M electrode. This electrode has homogeneous distributed porous region compared

with the 0.1 M electrode. It is presumed that this region will lead to a better contact

between active electrode materials and cations transferred from the electrolyte and

enhances the pseudocapacitive behaviour of the electrode (Wu et al., 2008; Zhou et

al., 2009). The interplanar spacing corresponds to Mn3O4, NiO, and Co3O4 is shown

in Figure 6.3(c) which is in good agreement with the XRD result. The periodic lattice

fingers of 0.25 nm, 0.21 nm and 0.28 nm are attributed to the interplanar spacing of

(211) Mn3O4 plane, (200) plane of NiO and (220) plane of Co3O4, respectively

(Hotovy et al., 2006; Wang et al., 2010a; Xue et al., 2011).

(e) (f)

100 nm 100 nm

103

Figure 6.3 TEM images of: (a) 0.1 M, (b) 0.15 M and (c) the lattice fringes of 0.15 M deposited film.

6.2.1.2 Electrochemical performance of Mn3O4-NiO-Co3O4 electrode in Na2SO4

electrolyte

Figure 6.4 (a) presents the CV curves of all the prepared electrodes at a scan

rate of 5 mV s-1 in a 0.5 M Na2SO4 electrolyte in potential range of 0 V to 1 V. All

curves exhibit a quasi-rectangular shape. The storage capability of each electrode

can be determined by specific capacitance. The specific capacitance calculated

according to Equation 3.1 at a scan rate of 5 mV s-1 are 207 F g-1, 481 F g-1, 177

F g-1and 182 F g-1 with deposited mass loading of 60 µg.cm-2, 36 µg cm-2, 65

µg cm-2, 78 µg cm-2 for 0.1 M, 0.15 M, 0.2 M and 0.3 M electrodes respectively.

The deposited Mn3O4-NiO-Co3O4/SS nanostructure electrode of 0.15 M electrode

exhibits the highest specific capacitance. Figure 6.4 (b) shows the CDC results at a

Flake

Flake Flake

Flake

100 nm 100

nm

(a) (b)

(c)

(b)

(c)

100 nm 100 nm

5 nm

104

constant current density of 1 A g-1. All CDC curves are linear and symmetrical,

implying ideal capacitive behaviour. The CDC process operates in a constant range

potential reflecting the excellent storage capability of the electrodes (Hung et al.,

2013). The specific capacitances calculated from Equation 3.2 are 526 F g-1, 1428

F g-1, 417 F g-1and 370 F g-1 for 0.1 M, 0.15 M, 0.2 M and 0.3 M electrodes at 1

A g-1 respectively. The 0.15 M electrode again shows the highest specific

capacitance. Compared to the electrode without cobalt (Rusi & Majid, 2014b), the

specific capacitance of Mn3O4-NiO-Co3O4/SS has increased to 1071 F g-1 when 0.15

M cobalt ion is added into the deposition electrolyte solution, implying that the

cobalt oxide plays a role in the increment of specific capacitance. Although the

values of specific capacitances obtained from CV and CDC are different, the trends

are identical. The specific capacitance calculated from Equation 3.2 is higher than

the one obtained from Equation 3.1. This is because of the slower charging rate at

low current in the CDC test that allows more intercalation of electrolyte ions to occur

(Engstrom & Doyle, 2013). Thus more charges can be stored and contribute to the

higher specific capacitance.

The AC impedance measurement is performed in order to understand the ion

transportation and charge transfer mechanism. The Nyquist plots of the AC

impedance responses in the 0.5 M Na2SO4 electrolyte are shown in Figure 6.4 (c).

The measured impedances are analysed using Nova simulation software on the basis

of the equivalent circuit in Fig.6.4 (c, insert). Here, Rs is represented bulk resistance

which due to combination resistance of ionic resistance of electrolyte, contact

resistance and internal resistance of the materials. Rct is related to charge transfer

resistance in the electrode/electrolyte interface. The Warburg impedance (W) is

associated with the cation diffusion in the electrode, CPE1 represents the non-ideal

105

capacitive behaviour at the impermeable interface and CPE2 is the faradic

impedance which due to pseudocapacitance of electrode. (Chen et al., Wang et al.,

2013; Dirican et al., 2014; Hung et al., 2013; Zhang et al., 2014). According to the

equivalent circuit, the 0.15 M electrode has the smallest Rs which is 0.89 Ω, and Rs

obtained for 0.1 M, 0.2 M and 0.3 M electrode are 1.35 Ω, 1.32 Ω and 1.00 Ω

respectively. In addition, the obtained Rct are 1.70 Ω, 0.40 Ω, 1.06 Ω and 1.10 Ω for

0.1 M, 0.15 M, 0.2 M and 0.3 M electrode respectively. The 0.15 M electrode not

only exhibits the smallest Rs, but also smallest value of Rct indicates that it has the

highest electrical conductivity in this work. This result is consistent with CV and

CDC results. Hence, 0.15 M electrode has the best electrochemical performance and

highest conductivity electrode, which is believed to be owed to a well-dispersed

porous structure that promotes faster ion transportation/migration (Zhang et al.,

2013a).

-10

-6

-2

2

6

0 0.2 0.4 0.6 0.8 1

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

0.1M Co

0.15M Co

0.2M Co

0.3M Co0

100

200

300

400

500

0.1 0.15 0.2 0.25 0.3

Sp

ecif

ic c

ap

aci

tan

ce (

F g

-1)

[Co²+] (mol dm -³)

(a)

106

(Figure 6.4, continued)

Figure 6.4 (a) CV curves of all electrodes at scan rate of 5 mV s-1 (inset: graph of calculated specific

capacitance), (b) CDC profiles for electrodes at current density of 1 A g-1 (inset: graph of calculated

specific capacitance), (c) impedance spectra for all electrodes.

6.2.1.3 Electrochemical performance of Mn3O4-NiO-Co3O4 electrode in various

electrolytes

The electrochemical performance of the 0.15 M electrode has been further

investigated in different electrolytes. KOH aqueous solution is well known as a good

electrolyte for NiO and Co3O4 materials rather than Na2SO4 aqueous solution

because of its high OH- concentration in electrolyte solution. OH- ions contribute to

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500

Pote

nti

al

vs

Ag

/Ag

Cl

(V)

Time (s)

0.1M Co

0.15M Co

0.2M Co

0.3M Co

0

0.5

1

1.5

2

2.5

3

1 1.5 2 2.5 3 3.5 4

-Z

" (

Ω)

Z' (Ω)

0.15M Co

0.2M Co

0.3M Co

0.1M Co

0

400

800

1200

0.1 0.15 0.2 0.25 0.3

Sp

ecif

ic c

ap

aci

tan

ce (

F g

-1)

[Co²+] (mol dm -³)

(c)

(b)

107

storage energy of both NiO and Co3O4 reaction during charging/discharging, as

described by Equation 6.5 and Equation 6.6. However in the Na2SO4 electrolyte, the

high current response is mainly contributed by Mn3O4 (Equation 6.7 and Equation

6.8) and the electrical double layer capacitance at the electrode-electrolyte interface

which cause by less OH- ions (Wu et al., 2007) . In order to get high current response

, K3Fe(CN)6 is added to KOH electrolytes (Zhao et al., 2013a) and the CV responses

of the 0.15 M electrode in the 0.5 M Na2SO4 electrolyte, 0.5 M KOH electrolyte and

0.5 M KOH/ 0.04 M K3Fe(CN)6 electrolyte at a scan rate of 5 mV s-1 are displayed

in Figure 6.5. The oxidation and reduction peak in the Na2SO electrolyte (Figure 6.5

(a)) is owed to the Mn3O4 redox reaction. The Mn3O4 converts into MnO2 birnessite

during cycling in the Na2SO4 aqueous electrolyte (Equation 6.7), and then receives

electrons/protons (Equation 6.8) (Cao et al., 2012). In the KOH electrolyte, all the

Mn3O4-NiO-Co3O participate fully in the redox reaction according to Equation 6.5

to Equation 6.8 (Cao et al., 2012; Jagadale et al., 2013; Wu et al., 2007).

NiO + OH- ↔ NiOOH + e- (Equation 6.5)

Co3O4 + OH- + H2O ↔ 3CoOOH +e- (Equation 6.6)

Mn3O4(spinel) ↔ CδMnOx . n H2O (birnessite) (Equation 6.7)

CδMnOx + nH2O + y H+ + zC+ (y+x) e- ↔ Cδ+z MnOx . nH2O (Equation 6.8)

(Where C= Cation (K+ or Na+))

The redox peak of K3Fe(CN)6 can be observed at 0.1 V and 0.35 V, as shown

in Figure 6.5 (c) and the possible reaction is as follows:

K4Fe(CN)6 ↔ K3Fe(CN)6 + e- (Equation 6.9)

The CV response of the 0.15 M electrode in 0.5 M KOH/ 0.04 M K3Fe(CN)6

exhibits two pairs of redox peaks (Figure 6.5 (d)), indicating that redox reaction of

108

deposits in KOH aqueous electrolyte and K3Fe(CN)6 has simultaneously occurred.

The current peak owed to the K3Fe(CN)6 reaction is higher, because of the high

electrochemical activity of the electrode in K3Fe(CN)6 rather than the KOH

electrolyte. The current density increment on the y-axis in the KOH/K3Fe(CN)6

electrolyte reflects its high charge storage ability. The specific capacitances from

the CV discharge curve at a scan rate of 5 mV s-1 are 402 F g-1, 662 F g-1 and 3875

F g-1 for 0.5 M Na2SO4, 0.5 M KOH and 0.5 M KOH/0.04 M K3Fe(CN)6 electrolytes

respectively. The addition of K3Fe(CN)6 to KOH improve the specific capacitance.

Figure 6.5 CV curve of 0.15 M electrode in voltage range of -0.5 V to 0.5 V at scan rate of 5 mV s-1

in: (a) 0.5 M Na2SO4 electrolyte, (b) 0.5 M KOH electrolyte, (c) 0.04 M K3Fe(CN)6 electrolyte, (e)

0.5 M KOH/0.04 M K3Fe(CN)6 electrolyte.

Figure 6.6 (a-b) displays the CDC profiles of Mn3O4-NiO-Co3O4 in KOH

and 0.5 M KOH/0.04 M K3Fe(CN)6 electrolytes at different current densities in the

-5

-3

-1

1

3

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

-10

-6

-2

2

6

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

-25

-15

-5

5

15

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

-45

-30

-15

0

15

30

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

A g

-1)

Potential vs Ag/AgCl (V)

(a) (b)

(c) (d)

109

voltage range from -0.5 V to 0.5 V. The CDC profile shows a slightly non-linear

curve attributable to faradic redox reaction (Inamdar et al., 2011). The redox

reaction of KOH and K4Fe(CN)6 can be inferred from the presence of two potential

plateaus at -0.2 V and 0.2 V respectively (Figure 6.6 (b)) as a result of electron

exchange during the reaction and reproduces what is observed in CV curves. The

specific capacitance, energy and power density calculated from the discharging

curve at current density of 20 A g-1 are 489 F g-1, 68 Wh kg-1 and 84 kW kg-1 for the

KOH electrolyte and 7404 F g-1, 1028 Wh kg-1 and 99 kW kg-1 for the 0.5 M

KOH/0.04 M K3Fe(CN)6 electrolyte respectively. The cycling stability of Mn3O4-

NiO-Co3O4 in 0.5 M Na2SO4, 0.5 M KOH and 0.5 M KOH/0.04 M K3Fe(CN)6

electrolytes are also performed at a scan rate of 5 mV s-1 over 1800 cycles as shown

in Figure 6.6 (c). Capacitance retention of 16 % and 18 % after 1800 cycles is

achieved in Na2SO4 and KOH electrolytes. Notably, the addition of 0.04 M

K3Fe(CN)6 to KOH has improved the stability, indicates the importance of

K3Fe(CN)6 as a redox electrolyte. However the capacitance retention reaches 50 %

after 1800 cycles. The high degradation in the capacitance after long cycling may

be owed to the high current passing through the electrode during the cyclability test

and volume loss of active materials (Engstrom & Doyle, 2013) as can be observed

from Figure 6.6 (d). Further improvement can be done by deposit the Mn3O4-NiO-

Co3O4 on carbon cloth or graphene electrode due to its unique properties such as

good electrical conductivity, high surface area and high chemical stability which can

help to improve the stability and electrochemical performance of electrode (Wei et

al., 2011).

110

Figure 6.6 CDC profiles of 0.15 M electrode at different current densities in: (a) 0.5 M KOH

electrolyte, (b) 0.5 M KOH/0.04 M K3Fe(CN)6 electrolyte, (c) electrochemical stability of electrode

in three different electrolytes, (d) 0.15 M electrode after 1800 cycles in 0.5 M KOH/0.04 M

K3Fe(CN)6 electrolyte.

6.3 Summary

The summary of this chapter is listed below:

A ternary Mn3O4-NiO-Co3O4 electrode has been successfully prepared by

a simple and versatile electrodeposition method.

The concentration of cobalt ions in the deposition electrolyte is strongly

influenced by the morphology of the deposits.

The Mn3O4-NiO-Co3O4/SS electrode of 0.15 M exhibited a homogeneous

network-like nanoflakes structure with a thickness of 51 to 59 nm.

0

20

40

60

80

100

0 500 1000 1500 2000

Ret

enti

on

(%

)

Number of Cycles

Mix KOH with K3FeCN6

KOH

Na2SO4

Mix KOH/ K3Fe(CN)6

KOH

Na2SO4

(c) (d)

100 nm

111

0.15 M electrode exhibited highest specific capacitance of 7404 F g-1 in a

mixed KOH/K3Fe(CN)6 electrolyte with high energy and power density of

1028 Wh kg-1 and 99 kW kg-1 at current density of 20 A g-1 within potential

range of -0.5 V-0.5 V respectively

0.15 M electrode showed 50 % capacitance retention after 1800 cycles for

a mixed KOH/K3Fe(CN)6 electrolyte at scan rate of 10 mV s-1 within

potential range of -0.5 V-0.5 V.

The incorporation of K3Fe(CN)6 in the KOH electrolyte improves the

energy density, power density and cyclability of the electrode.

112

CHAPTER 7: THE STUDIES OF RGO-MnO2 BASED

NANOCOMPOSITE ELECTRODE SYSTEM

7.1 Introduction

Carbon based electrodes have excellent rate of capability, good reversibility,

and superior cyclability but suffer from low capacitance value (Wang et al., 2014).

Besides, the transition metal oxides and polymer-based electrodes have produced

high capacitance through a fast faradic reaction but have a poor rate of capability

and stability (Yang et al., 2014a; Yang et al., 2012). Therefore, hybrid electrode

materials such as carbon-metal oxide-based electrodes have become necessary for

producing high capacitive performance and good cyclability. GO holds great

potential to be coupled with MnO2 because it has high conductivity, good chemical

stability, and a large surface area which can improve the drawbacks of MnO2

electrode. The surfaces of graphene are capable of reversible pseudo-reaction and

electrochemical double layer formation, which is beneficial to the electrochemical

performance of MnO2/GO composite materials (Liu et al., 2014; Sawangphruk et

al., 2013; Yang et al., 2014a; Yang et al., 2012).

Herein, we report a facile and green method to prepare RGO-MnO2 with a

glucose carbon composite (i.e., an in-situ electrodeposition technique). The

deposited electrode was heated to exceed the decomposition temperature of glucose

with the hope that the presence of carbon from glucose in the electrode would

increase the effectiveness of cation pathways from electrolyte to electrode. This

study also found that a suitable concentration of D (+) glucose in the deposition

113

electrolyte which could slow down the nucleation process of MnO2 particles and

lead to uniform and ultrathin nanoflakes structure.

7.2 Results and discussions

7.2.1 Optimization of RGO-MnO2 electrode by various the ratio of

GO:Mn(CH3COO)2.4H2O in deposition electrolyte solution

7.2.1.1 Schematic illustration of electrodeposited RGO-MnO2 electrode

The schematic diagram of in-situ electrodeposition of RGO-MnO2 electrode

is displayed in Figure 7.1. The dispersed GO in water is negatively charged due to

some ionization of carboxyl and hydroxyl functional groups on the GO surface (Li

et al., 2008) . Those functional groups act as anchor sites, which allow the positive-

charged ions to be absorbed on the surface and edge of the negatively-charged GO

sheets (Moradi Golsheikh et al., 2013). When the Mn(CH3COO)2·4H2O is added to

the GO suspension solution, the Mn2+ ion is bonded with the oxygen atom of the

negatively-charged residual oxygen-containing functional groups on the GO via the

electrostatic force. Then, upon electrodeposition the GO sheets with absorbed Mn2+

ions are deposited together to form manganese hydroxide (Mn(OH)2) and reduced

to graphene oxide (RGO). Mn(OH)2 will be converted to MnO2 after being heated

at 300°C for 6 hours.

114

Figure 7.1 A schematic illustration of RGO-MnO2 mechanism via electrodeposition.

7.2.1.2 The composition and morphology studies of RGO-MnO2 electrode

It is essential to confirm that the RGO-MnO2 has been deposited on top of

SS. The XRD, Raman and EDX studies have been used to investigate the

compositions and elemental analysis. The XRD patterns of as prepared RGO-MnO2

deposits on top of SS and pure SS as references are shown in Figure 7.2. Compared

to the XRD pattern of stainless steel, there is an additional peak at 2θ=28.3° in the

diffractograms of deposited M30, M60, and M90 electrodes, Figure 7.2 (a). This

peak is attributed to the (310) plane of MnO2 and the intensity of the peak increased

as the content of manganese ion in the deposition electrolyte increased. For clearer

evidence of deposited RGO-MnO2, the XRD diffractogram for scraped-off deposits

powder of M30, M60, and M90 are carried out and is shown in Figure 7.2 (b). The

characteristic peaks of MnO2 observe at 2= 28.7°, 36.9°, 42.9°, and 50.3° are

115

attributed to the (310), (211), (301) and (411) planes, which can be indexed to a

tetragonal phase of α-MnO2 with lattice constant a=9.7847 Å, c=2.8630 Å (JCPDS

44-0141) (Kim et al., 2014; Zhang et al., 2012b) . In all XRD patterns of RGO-

MnO2 scraped-off powders, an appreciable peak is observed at 2=42.9°, which is

associated with the (301) plane of MnO2. The peaks of GO in the deposit powders

of the M30, M60, and M90 electrodes are hardly noticeable, suggesting that

reduction of GO has taken place (Kim et al., 2013a; Li et al., 2014c).

To investigate the vibrational properties of deposited RGO-MnO2 electrode,

Raman spectroscopy analysis is performed and is shown in Figure 7.2 (c). The

characteristics peaks of GO centered at 1358 cm−1, 1579 cm−1, and 2675 cm−1 are

attributed to D-band, G-band, and 2D-band respectively. The D-band is related to

the vacancies, edge defects, grain boundaries and disordered carbon species in

graphite layers. The G-band is due to the vibration of sp2 hybridized C-C bonds in

two dimensional hexagonal lattices (Cançado et al., 2004; Niu et al., 2014). The

intensity ratio of the D and G-bands (ID/IG) can be used to evaluate the sp2 domain

size of a carbon structure and partially ordered crystal structure of graphene (Li et

al., 2014b). The ID/IG ratio of GO powder is 0.78 and the M30 and M60 electrodes

results in increments of the ID/IG ratio to 2.37 and 1.78 which can be attributed to an

increase in defects on the surface of the reduced GO that were induced during the

synthesis process. Furthermore, the G band and D band in the deposited electrode

have shifted to lower wave numbers of around 1242 cm-1 and 1568 cm-1 for the M30

and M60 electrode, revealing that RGO are deposited (Zhang et al., 2013c). In

addition, the presence of sharp distinct peak located at 650 cm-1 is belong to MnO2

which is attributed to stretching vibrations of MnO6 octahedral (Kim et al., 2013a).

116

Figure 7.2 XRD pattern of: (a) M30, M60, and M90 on top of SS, (b) scraped-off deposits powder

of M30, M60, and M90; (c) Raman spectroscopy of selected electrodes.

The elements of deposits on top of SS have been studied using EDX. The

EDX of empty SS, pristine MnO2 and M30 are shown in Figure 7.3. The presence

of Cr, Fe, Ni, Si, and Mo are belonged to SS (Figure 7.3 (a)) and the presence of

new elements such as Mn and O is assigned to the formation of MnO2 in electrode.

The increment of carbon weight percentage in the M30 can be seen in Figure 7.3

(c). The increment of carbon content in the M30 electrode is due to the RGO carbon.

5 10 15 20 25 30 35 40 45 50 55 60

Inte

nsi

ty (

a.u

.)

2θ(°)

SS

M30

M60

M90

MnO2

SS

(a)

-10

40

90

140

190

-5

0

5

10

15

20

25

30

35

5 10 15 20 25 30 35 40 45 50 55 60

Inte

nsi

ty (

a.u

.)

2θ (°)

M90M60M30MnO2GO

MnO2 (b)

100 600 1100 1600 2100 2600

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

MnO2

GO

M30

(c)

Elements Weight

(%)

C 3.63

Mn 21.69

O 20.08

(a) (b)

Elements Weight

(%)

C 2.20

Mn -

O -

Energy (KeV) Energy (KeV)

G D

MnO2

MnO2

M60

117

(Figure 7.3, continued)

Figure 7.3 EDX spectrum of: (a) empty SS, (b) MnO2 without GO, (c) M30.

FESEM analysis are carried out to characterize the morphology structures of

as-heated RGO-MnO2 deposited from different deposition electrolyte contents as

shown in Figure 7.4. The FESEM image of GO-free deposited MnO2 (Figure 7.4

(a1, a2)) exhibit round shape particles covered with nanoflake-like structures on the

top. The thickness of the deposited particles is in the range of 550 nm to 801 nm,

Figure 7.4 (a2). When GO is added into the deposition electrolyte, M30 electrode

has grew with less flake-like structures, which may due to the slow nucleation

process of MnO2 flakes and is caused by the presence of RGO sheets as shown in

Figure 7.4 (b1). The cross-section of the M30 electrode in Figure 7.4 (b2) indicates

less agglomerated MnO2 and more uniform thickness within a range from 560 nm

to 630 nm. When Mn ion in deposition electrolyte is further increased, the structure

of M60 and M90 electrodes almost look like pristine MnO2 structure as shown in

Figure 7.4 (c1, d1). The size of deposited electrode is thicker as observed in the

cross-section images in Figure 7.4 (d2). The thick, dense structure generally can limit

the diffusion of electrolyte cations toward the entangled oxide, resulting in low

utilization of MnO2 (Li et al., 2013).

Elements Weight

(%)

C 4.47

Mn 9.19

O 16.98

(c) (c)

Energy (KeV)

118

Figure 7.4 FESEM morphology images and cross section thickness of: (a1, a2) pristine MnO2, (b1,

b2) M30, (c1, c2) M60, and (d1, d2) M90.

(a1) (a2)

(b1) (b2)

(c1)

(d2) (d1)

(c2)

100 nm 100 nm

100 nm

100 nm 100 nm

100 nm

100 nm

100 nm

677nm

688nm 681nm

801nm

557nm

561nm

570nm 596nm 623nm 581nm

518nm

611nm 435nm

407nm

420nm

641nm 641nm 593nm

362nm

686nm

119

The morphological studies of M30 and M60 deposits are further investigated

by using high magnification TEM as shown in Figure 7.5. The MnO2 has dispersed

uniformly on the RGO sheets. The TEM image of the M30 electrode displays an

RGO sheet with a thin flakes structure, whereas the M60 electrode shows thicker

MnO2 flakes which is in agreement with the FESEM results.

Figure 7.5 TEM images of: (e) M30 and (f) M60.

7.2.1.3 Electrochemical performance of RGO-MnO2 electrode in Na2SO4 electrolyte

The electrochemical properties of RGO-MnO2 electrode were characterized

by CV and EIS studies. CV has been considered to be a suitable technique to

investigate the occurrence of faradic or non-faradic reactions in the electrode (Dong

et al., 2014). The CV curve of pristine MnO2, M30, M60, and M90 electrodes in the

potential range from -1 V to +1 V at scan rate of 5 mV s-1 in the 0.5 M Na2SO4

electrolyte are shown in Figure 7.6 (a). A pair of distinct anodic and cathodic peaks

can be clearly observed around 0.2 V and -0.1 V, while other less-intense anodic

and cathodic peaks are around 0.9 V/-0.7 V. These peaks are believed to be derived

mainly from the redox pairs of Mn2+/Mn3+. The current response for M30, M60, and

M90 is much higher than that of pure MnO2, inferring that RGO-MnO2 electrodes

RGO sheet

RGO sheet RGO sheet

(a) (b)

50 nm 100 nm

120

have better charge transfer kinetics, due to higher utilization of active Mn species.

The calculated specific capacitances from the CV curve for pristine MnO2, M30,

M60, and M90 electrodes are 167 F g-1, 264 F g-1, 220 F g-1, and 175 F g-1

respectively. The highest specific capacitance is found in the M30 electrode, which

can be attributed to the combined contribution of pseudocapacitance of MnO2 and

the electrical double layer capacitance of the RGO. The improved performance of

the M30 electrode may also be due to a high content of graphene carbon, which is

beneficial to shorten the cation path into the electrode matrix and reducing the

transfer resistance.

The characteristic of ion transport resistance for all electrodes was

investigated by electrochemical impedance spectra (EIS). The Nyquist plot of RGO-

MnO2 deposited electrode in 0.5 M Na2SO4 electrolyte is shown in Figure 7.6 (b).

The equivalent circuit in accordance with Nyquist plots is fitted using Nova software

and parameters are listed in Table 7.1.

Table 7.1 Equivalent circuit parameters deducted by fitting Nyquist plots of M30, M60 and M90.

Base on the equivalent circuit, there are some important parameters have

been used to fit the system i.e. Rs, Rct, CPE1 and CPE2. The intercept of the arc on

the x-axis at high frequency region is called Rs which represents combination

resistance of ionic resistance of electrolyte, contact resistance and internal resistance

of the materials. This value is almost same for all electrodes within the logical

Electrode L

(x10-7 H)

Rs

(Ω)

Rct

(Ω)

CPE1

(x10-3 Ω)-1

CPE2

(x10-3 Ω)-1 n1 n2

M30 5.23 1.01 0.55 0.41 50.50 0.78 0.81

M60 8.98 1.10 0.71 2.98 47.40 0.62 0.78

M90 8.33 1.48 0.92 1.89 56.00 0.67 0.76

121

magnitude error. The semicircle region in high frequency region corresponds to

charge transfer resistance, Rct (Chen et al., 2013). The two constant phase elements

of CPE1 and CPE2 in this system are used to replace the double layer capacity and

Warburg impedance resistance for semi-infinite linear diffusion respectively (Yang

et al., 2014b). M30 electrode has lowest series resistance and transfer resistance

among all RGO-MnO2 electrodes, indicating M30 has a better electrochemical

performance and lead to high specific capacitance.

Figure 7.6 (a) CV curve in 0.5 M Na2SO4 electrolyte at a scan rate of 5 mV s-1 of : pristine MnO2,

M30, M60, and M90, and (b) Nyquist plot of : (c) M30, M60, and M90.

-3.5

-1.5

0.5

2.5

4.5

-1 -0.5 0 0.5 1

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential Vs Ag/AgCl (V)

MnO2

M30

M60

M90

0

1

2

3

0 1 2 3

-Z"

(Ω

)

Z' (Ω)

M30

M60

M90

MnO2 (a)

(b)

122

7.2.2 Optimization of D(+)glucose content in deposition electrolyte solution for

RGO-MnO2-glucose carbon electrode system

The best performance RGO-MnO2 electrode is determined using deposition

electrolyte containing GO: Mn(CH3COO)2.4H2O (10 ml : 30 ml) as described in

section 7.2.1. In order to further improve this electrode, the introduction of

D(+)glucose into deposition electrolyte is investigated. The research has been

carried out with different concentrations of glucose into the deposition electrolyte.

The possible mechanism with additional of glucose in GO : Mn(CH3COO)2.4H2O

is shown in Figure 7.7.

Figure 7.7 A schematic illustration of RGO-MnO2 mechanism via electrodeposition with glucose.

In the early case, Mn2+ is formed and bind with negatively charged GO and

both are deposited together. It is believed that adding glucose causes some

molecules of Mn2+ will bind together with the hydroxyl group of glucose via

electrostatic interaction and will also deposit together in the SS. The glucose will

convert into carbon when the electrode heat at 300°C.

123

7.2.2.1 The composition and morphology studies of RGO-MnO2-glucose carbon

electrode

The XRD pattern of deposited RGO-MnO2 on top of SS with the presence

of different concentrations of glucose molecules in the deposition electrolyte is

shown Figure 7.8 (a). The XRD reveals that the addition of glucose molecules did

not change the compositions of the electrode, since only one peak is observable that

centred at 28.3° which is attributed to the (310) plane of MnO2. Raman

spectroscopes in Figure 7.8 (b) reveal that the addition of glucose content in the

deposition electrolyte has resulted in an increase of D band intensity, indicating an

increase of disorder carbon in the graphite layers. The increment of D peak intensity

might be attributed to the bands combination of D1, D2, D3 and D4 in the region

from 1000 to 1800 cm-1. The deconvoluted of Raman spectra in this region for

selected electrodes M30 and G03 is displayed in Figure 7.8 (c,d). The deconvolution

results for both electrodes clearly show that the peak at around 1560-1598 cm-1 is

related to the G peak. D1 and D2 peaks can be observed at around 1301-1317 cm-1

and 1599-1624 cm-1, respectively. Another two peaks at around 1489-1545 cm-1 and

1127-1200 cm-1 corresponded to D3 and D4 peaks respectively (Sadezky et al.,

2005). The ID/IG ratio (area) and vibration mode of M30 and G03 are listed in Table

7.2.

124

Table 7.2 Raman bands, IDx/IG ratio and vibration modes of M30 and G03 electrode.

Bands

Raman shift

(cm-1) Ratio (IDx/IG)

Vibration mode

M30 G03 M30 G03

G 1573.76 1547.81 - - Ideal graphitic lattice (E2g-

symmetry)

D1 1294.93 1315.29 3.09 11.49

Disordered graphitic lattice

(graphene layer edges, A1g-

symmetry)

D2 1617.56 1600.88 1.26 0.48

Disordered graphitic lattice

(surface graphene layers, E2g-

symmetry)

D3 1529.42 1517.35 1.57 0.36 Amorphous carbon (Gaussian or

Lorentzian line shape)

D4 1202.22 1196.59 7.07 8.16

Disordered graphitic lattice (A1g-

symmetry), polyenes, ionic

impurities

The increment of the D band intensity mainly arises from overlapping of D1

and D4 peaks in the band region of 1100-1400 cm-1, suggesting that the disordered

carbon in graphitic lattice has increased. Again, there is the presence of a sharp peak

at 650 cm-1 which is corresponds to MnO2 in all deposited electrodes (Kim et al.,

2013a).

The elemental study of G03 is shown in Figure 7.8 (e). This EDX result

reveals that there is an increment of carbon weight percentage in G03 electrode

compared to M30 electrode (Figure 7.3 (d)). The increment of carbon content in

G03 electrode is believed from glucose decomposes carbon. Glucose is considered

to be the one of the most organic compounds that will decompose to a carbon solid

element (known as carbon sources) (Yamaoka et al., 2002). Figure 7.8 (f) displays

the TGA analysis of D (+) glucose, in which the decomposition temperature of

glucose is found to be around 250 °C. The heating temperature of our deposited

electrodes is 300 °C, which exceeds the glucose decomposition temperature.

125

Therefore, the increase of carbon content of the G03 electrode in the Raman and

EDX analyses is believed originated from the decomposition of glucose.

Figure 7.8 (a) XRD pattern of G01, G03 and G06 electrode, (b) The Raman spectroscopy of G01,

G03 and G06 electrode, (c) deconvolution of M30 G03 in the range of 1000 to 1800 cm-1, (d)

deconvolution of G03 in the range of 1000 to 1800 cm-1, (e) EDX result of G03 electrode, (f) TGA

of D-(+)-glucose.

5 15 25 35 45 55

Inte

nsi

ty (

a.u

.)

2θ (°)

SS

0.01M

0.03M

0.06M

(a)

200 600 1000 1400 1800

Inte

nsi

ty (

a.u

.)

Raman Shift ( cm-1)

0.01M

0.03M

0.06M

G01

G03

G06

(b)

1000 1200 1400 1600 1800

inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

1000 1200 1400 1600 1800

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

-1

0

1

2

3

4

5

0

20

40

60

80

100

0 200 400 600 800

Wei

gh

t (%

)

Temperature (°C)

Weight %

Deriv. Weight %/°CElements Weight

(%)

C 6.32

Mn 3.43

O 7.03

(e) (f)

G01

G03

G06

Energy (KeV)

(c) (d)

D4 D1

G

D3

D2

D4

D1

D3

G

D2

126

The morphological change of M30 electrode is can be clearly observed when

glucose molecules are added to the electrolyte deposition as shown in Figure 7.9.

When 0.01 M glucose is added into the deposition electrolyte, the MnO2 bunches no

longer exists and shows that the porous structure is formed and regularly arranged

(Figure 7.9 (a1)). The pore diameter is in the range of 60-100 nm (Figure 7.9 (a2)).

The pore diameter decreases dramatically (~30 nm) when the glucose concentration

is increased to 0.03 M as shown in Figure 7.9 (b1), indicating that the pore structure

of RGO-MnO2 electrodes can be tuned by adjusting the glucose concentration in the

electrodeposition electrolyte. The average thickness of the G03 electrode is also

reduced to 265 nm as seen in Figure 7.9 (b2). A high number of pores that built from

interconnected nanoflakes structures in the G03 electrode are believed to improve

the porosity and provide a unique conductive network. This observation might be

due to a slower rate of MnO2 electrocrystallization, which allows the atoms to

arrange themselves at the lowest energy site. According to Babakhani & Ivey,

(2011), the influential factors, such as the concentration of the deposition electrolyte,

would affect the electrocrystallization rate of MnO2 (Babakhani & Ivey, 2011).

When glucose concentration 0.06 M (G06) is further increased, the deposition rate

becomes slower and this leads to non-uniform growth of flakes as shown in Figure

7.9 (c1).

(a1) (a2)

100 nm 100 nm

585nm

527nm 548nm 555nm

127

(Figure 7.9, continued)

Figure 7.9 FESEM morphology images and cross section of: (a1, a2) G01, (b1, b2) G03, (c1, c2)

G06.

The TEM image of G03 electrode is shown in Figure 7.10. The deposits have

a uniform and well-spread RGO sheet, which is covered by MnO2 nanoflakes. The

interconnected structure, which creates the porous structure, can also be clearly

observed at a high magnification of TEM (Figure 7.10 (b)). This unique structure

has several advantages: (i) the porous structure greatly facilitates ion diffusion from

the electrolyte into the electrode matrix, which promotes the specific capacitance

due to high utilization of MnO2; (ii) thin deposited materials that are able to shorten

the diffusion path of electrons and ions; (iii) the interconnected flakes structure

without agglomeration could exhibit the excellent electrochemical performance as

an electrode for a supercapacitor (Liu et al., 2014).

(b1) (b2)

(c1) (c2)

100 nm

100 nm

100 nm 100 nm

265nm 231nm

218nm 226nm 199nm

341nm 304nm 326nm 298nm

253nm

128

Figure 7.10 TEM images of G03 M in: (a) low-magnification, (b) high-magnification.

7.2.2.2 Electrochemical performance of RGO-MnO2-glucose carbon electrode in

Na2SO4 electrolyte

The CV curve of the deposited G03 electrode at a scan rate of 5 mV s-1 in

Na2SO4 electrolyte is shown in Figure 7.11 (a). Compared to the CV curve of M30,

CV curve of G03 electrode has a similar shape and potential position of anodic and

cathodic peaks. However, the current response of G03 is higher, indicating that the

effective utilization of the MnO2 (Li et al., 2013). The calculated specific

capacitances are 377 F g-1, 430 F g-1, and 361 F g-1 for G01, G03 and G06

respectively at a scan rate of 5 mV s-1. Electrode G03 exhibits 63 % specific

capacitance improvement compared to M30, which is attributed to the uniform

morphology structure, less thickness and low transfer resistance. A further increase

of glucose concentration the capacitance has decreased to around 37 % (G06),

which might be due to less MnO2 available for the reaction.

The Nyquist plot of G01, G03 and G06 electrodes in 0.5 M Na2SO4

electrolyte is shown in Figure 7.11 (b). Table 7.3 shows the fitted parameters of

equivalent circuit in accordance with Nyquist plots using Nova software.

(a) (b)

200 nm 50 nm

129

Table 7.3 Equivalent circuit parameters deducted by fitting Nyquist plots of G01, G03 and G06.

Overall, G03 electrode has lowest transfer resistance among all electrodes,

indicating G03 has a better electrochemical performance. The incorporation of 0.03

M glucose in M30 electrode leads to improvise the access for

intercalation/deintercalation of cation to electrode matrix. In this system, CPE2

represents the faradic impedance which is due to redox transition within the

electrode (Hu & Chu, 2001). As discuss in previous reports (Girija &

Sangaranarayanan, 2006; Yang et al., 2014b), two constant phase elements can be

describe as ZCPE1= [Q(jω)n1]-1 and ZCPE2= [Q(jω)n2]-1 with -1 ≤ n ≤ 1. The component

n is correction factor represents the roughness of electrode and it has value ranging

from 0 to 1. Pure capacitance yields n=1, pure resistance yields n=0, while n=0.5

represents Warburg impedance. The value of n1 ~ 0.8 in G03 indicates that G03 has

a nature porous of electrode in agreement with the TEM results.

-5

-3

-1

1

3

5

-1 -0.5 0 0.5 1

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential Vs Ag/AgCl (V)

0.01M

0.03M

0.06M

Electrode L

(x10-7 H)

Rs

(Ω)

Rct

(Ω)

CPE1

(x10-3 Ω)-1

CPE2

(x10-3 Ω)-1 n1 n2

G01 9.98 1.10 0.40 1.08 35.60 0.64 0.78

G03 3.14 1.10 0.32 0.39 32.5 0.81 0.83

G06 3.79 1.30 0.36 0.56 28.3 0.76 0.85

G01

G03

G06

(a)

130

(Figure 7.11, continued)

Figure 7.11 (a) CV curve in 0.5 M Na2SO4 electrolyte at a scan rate of 5 mV s-1 of G01, G03, and

G06, and (b) Nyquist plot of G01, G03, and G06.

7.2.2.3 Electrochemical performance of RGO-MnO2-glucose carbon electrode in

various electrolytes

In order to get more informations on the capacitive performance of the best

prepared G03 electrode, G03 electrode is selected to study the performance in three

different electrolytes (i.e., 0.5 M Na2SO4, 0.5 M KOH, and a 0.5 M KOH/0.04 M

K3Fe(CN)6). The capacitive performance is believed to be influenced by the size of

the cation, cation mobility, and rate of adsorption/desorption at the electrode-

electrolyte surface (Nithya et al., 2013). All CV and CDC performances were

studied in a potential range of -0.5 V to 0.5 V. The CV curve of the G03 electrode

in three different electrolytes at a scan rate of 5 mV s-1 is shown in Figure 7.12.

The CV curves of the G03 electrode in 0.5 M Na2SO4 and 0.5 M KOH

electrolyte solutions are shown in Figure 7.12 (a,b). The electrode reaction occurred

according to Equation 7.1 and Equation 7.2 (Lu et al., 2014). The current response

in 0.5 M KOH is found to be higher than in the 0.5 M Na2SO4 electrolyte. This

0

1

2

3

0 0.5 1 1.5 2 2.5 3

-Z"

(Ω

)

Z' (Ω)

G01

G03

G06

(b)

131

behaviour might be due to the smaller K+ size which can enhance the

chemisorptions reaction rate, thus optimizing the pseudocapacitance (Nithya et al.,

2013). The specific capacitance of G03 in the Na2SO4 and KOH electrolyte solutions

calculated from the CV curve are 370 F g-1 and 804 F g-1.

MnO2 + H+ + e- ↔ MnOOH (Equation 7.1)

MnO2 + C+ + e- ↔ MnOOC (Equation 7.2)

Where C is Na2+ or K+

The best performance of the G03 electrode in KOH is obtained. The redox

mediator electrolyte, 0.04 M of K3Fe(CN)6, is then added into a 0.5 M KOH

electrolyte solution with a volume ratio of 1:1. The CV curve in Figure 7.12 (d)

reveals that the additional of redox mediator has increased the current response

drastically. The highest anodic/cathodic peaks at 0.27 V/0.15 V is assigned to the

charging and discharging process of K4Fe(CN)6 to K3Fe(CN)6 (Figure 7.12 (c))

which undergoes the reaction shown in Equation 7.3 (Zhao et al., 2013a). The other

less-intensely observed redox peaks is originated from electrode reactions with

KOH electrolytes. The calculated specific capacitance is 5135 F g-1. The specific

capacitance is found to increase to 538 % after an addition of 0.04 M K3Fe(CN)6

electrolyte solution into 0.5 M KOH. In this system, the high capacitance could

attribute from the couple of [Fe(CN)6]3- / [Fe(CN)6]

4- in the electrolyte and highly

electroactive electrode as describe by Equation 7.4. These mechanisms have been

explained clearly in previous Chapter 5 and Chapter 6.

K4Fe(CN)6 ↔ K3Fe(CN)6 + e- (Equation 7.3)

Redox electrode, Mz+ ↔ M (z+n)+ + ne- (Equation 7.4)

Where M is Mn2+ cations, and 1≤n≤z.

132

Figure 7.12 CV curve of G03 at a scan rate of 5 mV s-1 within potential scan of -0.5 V to 0.5 V in :

(a) 0.5 M Na2SO4, (b) 0.5 M KOH, (c) 0.04 M K3Fe(CN)6, and (d) 0.5 M KOH/0.04 M K3Fe(CN)6

electrolyte solution.

Figure 7.13 (a) displays the CDC profile of the G03 electrode in three

different electrolytes at current density of 20 A g-1. The CDC profile shows a slightly

non-linear curve, which indicates the occurrence of a redox reaction within this

voltage range (Dubal et al., 2013). The CDC curve of G03 in the Na2SO4 and KOH

electrolyte solutions has a small plateau potential around -0.1 V in the discharging

curve, which corresponds to the reduction of MnO2. In the KOH/K3FeCN6

electrolyte solution, two potential plateaus are found. The first potential at -0.1 V is

due to the reduction of MnO2, and the second potential at 0.2 V corresponds to the

redox reaction of K4Fe(CN)6. Because both reactions occurred, the G03 electrode in

the KOH/K3Fe(CN)6 electrolyte solution at a current density of 20 A g-1 has a higher

-5

-3

-1

1

3

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential Vs Ag/AgCl (V)

Na2SO4

-15

-10

-5

0

5

10

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential Vs Ag/AgCl) (V)

KOH

-70

-50

-30

-10

10

30

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential Vs Ag/AgCl (V)

K3Fe(CN)6

-50

-30

-10

10

30

-0.5 -0.3 -0.1 0.1 0.3 0.5

Cu

rren

t D

ensi

ty (

Ag

-1)

Potential Vs Ag/AgCl (V)

Mix KOH/

K3Fe(CN)6

Na2SO4

K3Fe(CN)6

(c) (d)

(a) (b)

KOH/K3Fe(CN)6

133

specific capacitance of 13,333 F g-1, with a power density of 68.35 kW kg-1 and an

energy density of 1851 Wh kg-1.

The stability test of the 0.03 M electrode in Na2SO4, KOH, and KOH/

K3Fe(CN)6 electrolyte solutions was continuously performed in a voltage range

from -0.5 V to 0.5 V at a scan rate of 10 mV s-1 until 2000 cycles as shown in Figure

7.13 (b). The specific capacitance of G03 in the KOH/K3Fe(CN)6 electrolyte

solution has retained up to 46 % of its initial specific capacitance after 2000th cycle.

It has good stability compared to other electrolytes. In comparison with past studies,

RGO-MnO2 electrode prepared using hydrothermal and sol gel methods (Li et al.,

2014c; Yan et al., 2010) show good stability but low specific capacitance which is

due to high carbon content in the electrode. In our work, the high capacitance, energy

and power density can be assigned to high redox activity of the electrode. The high

degradation of the capacitance retention may be due to a high current pass through

the electrode during cyclability test and loss of its active materials (Engstrom &

Doyle, 2013; Kim et al., 2013a).

Figure 7.13 (a) CDC curve of G03, and (b) cyclability test of G03 electrode in three different

electrolytes.

-0.5

-0.3

-0.1

0.1

0.3

0.5

0 200 400

Volt

ag

e (V

)

Time (s)

in Mix KOH/K3FeCN6

KOH

Na2SO4

0

20

40

60

80

100

120

0 500 1000 1500 2000

Ren

ten

tion

(%

)

cycle

Mix

Na2SO4

KOH

(a) (b)

Na2SO4

KOH

Mix KOH/K3Fe(CN)6 Mix KOH/K3Fe(CN)6

Na2SO4

KOH

134

7.3 Summary

An ultrathin electrode with uniform nanoflakes structure was deposited from

GO:Mn(CH3COO)2·4H2O:0.03 M of glucose (10 ml : 30 ml: 30 ml)

deposition electrolyte solution.

From EDX and Raman spectrocopy studies, the deposition of glucose onto

SS electrodes is also found to increase the carbon content of electrode. This

helps to reduce the transfer resistance of cation diffusion paths to the electrode

matrix.

The 0.03 M of RGO-MnO2-glucose carbon electrode exhibited optimum

specific capacitance of 13,333 F g-1 with an energy density of 1851 Wh kg-1

and power density of 68.35 kW kg-1 at a current density of 20 A g-1 in mixed

KOH/K3Fe(CN)6 electrolyte solution at potential range of -0.5 V - 0.5 V.

The 0.03 M exhibits a retention percentage of 46 % after 2000 cycles at a scan

rate of 10 mV s-1 within potential range of -0.5 V - 0.5 V.

The preparation method of G03 electrode is a simple, low-cost, and

environmentally-friendly that holds great potential for producing cost-

effective and high-energy-density supercapacitors.

135

CHAPTER 8: CONCLUSIONS AND SUGGESTIONS FOR

FUTURE WORK

8.1 Conclusions

In summary, the main objectives of the present study were achieved as

mentioned in Chapter 1. This thesis focuses on the development of composite

manganese oxide based electrode with secondary/ternary transition metal oxides and

conductive carbon for supercapacitor’s electrodes. There are four electrode systems

that have been produced and tested successfully and reported in this thesis, as listed

below:

i. MnO2 electrode,

ii. MnO2-NiO binary electrode,

iii. Mn3O4-NiO-Co3O4 ternary electrode,

iv. RGO-MnO2-glucose carbon nanocomposite electrode.

During the first part of our work, different MnO2 structures have been

observed by varying the concentration of manganese acetate tetrahydrate

(Mn(CH3COO)2·4H2O) in deposition electrolyte (Chapter 4). The concentration of

cations source in deposition electrolyte is one of the important parameters that

influence the electrodeposition rate and nucleation process, thus resulted in various

morphologies. At small concentration of Mn(CH3COO)2·4H2O (<0.01 M), the

deposited MnO2 particles were less formed which is due to slow electrodeposition

rate. The deposition electrolyte of 0.01 M Mn(CH3COO)2·4H2O was identified as a

suitable concentration to produce nano flower-like structure electrode through

chronopotentiometry mode. The diameter size of spherical flower is in the range of

136

200 to 400 nm with thickness around 300 to 400 nm. It exhibited the highest specific

capacitance among others deposited electrodes and has good cycling stability. The

specific capacitance of deposited MnO2 with varied Mn(CH3COO)2·4H2O

concentration at scan rate of 5 mV s-1 is displayed in Table 8.1.

Table 8.1 The specific capacitance of all deposited MnO2 electrode in 0.5 M KOH solution in at scan

rate of 5 mV s-1

Single MnO2 electrode Specific capacitance (F g-1)

0.0025 M 116

0.005 M 128

0.01 M 143

In the next part of this research, the effect of secondary metal oxide on the

capacitive behaviour of single metal oxide electrode was investigated. The

optimization of nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) concentration in

0.01 M Mn(CH3COO)2·4H2O as deposition electrolyte for binary MnO2-NiO

electrode was carried out using chronopotentiometry mode (Chapter 5). Addition of

Ni(CH3COO)2·4H2O into 0.01 M Mn(CH3COO)2·4H2O solution has resulted in the

formation of MnO2-NiO electrode with unique interconnected nanoflakes structure.

The deposition solution which contains 0.25 M of Ni(CH3COO)2·4H2O mixed with

0.01 M Mn(CH3COO)2·4H2O (sample was designated as NiO-25) was found to be

the best deposition electrolyte for MnO2-NiO binary electrode system. The specific

capacitance of all deposited MnO2-NiO electrode with different concentrations of

Ni(CH3COO)2·4H2O/ Mn(CH3COO)2·4H2O is summarized in Table 8.2.

137

Table 8.2 The specific capacitance of all deposited MnO2-NiO electrode in 0.5 M Na2SO4 electrolyte

in potential range of 0 to 1 V at scan rate of 5 mV s-1.

MnO2-NiO binary

electrode

Specific capacitance (F g-1)

NiO-15 139

NiO-20 152

NiO-25 173

NiO-30 166

NiO-40 154

From Table 8.2, NiO-25 electrode exhibited highest specific capacitance of

240 F g-1 at scan rate of 1 mV s-1 in 0.5 M Na2SO4 electrolyte. The high specific

capacitance of NiO-25 electrode is due to the good arrangement of thin layer of

flakes structure which generate more porous network structure with pore size in the

range of 10 nm to 15 nm. The uniform nano-sized porous structure led to easy access

of electrolyte cation transport to electrode matrix, hence produce a better ion

diffusion performance.

In order to study the effect of electrodeposition modes on MnO2-NiO

electrode, different electrodeposition modes such as CP, CA and CV methods are

investigated. The CV mode with four cycle’s deposition number (CY4 electrode)

exhibited less agglomeration, highly distributed pores and less flakes in the deposits

with thickness in the range of 80 nm to 110 nm. CY4 electrode exhibited optimum

specific capacitance of 3509 F g-1 at current density of 20 A g-1 with potential range

from -0.5 V to 0.5 V in mix KOH/K3Fe(CN)6 electrolytes. The specific capacitance

retained 44 % after 1500 cycles at scan rate 10 mV s-1.

138

The effect of different concentrations of cobalt sulphate (CoSO4·7H2O) on

the electrochemical performance of 0.25 M Ni(CH3COO)2·4H2O/0.01 M

Mn(CH3COO)2·4H2O electrode was evaluated. In this new system, ternary metal

oxides electrode was prepared using chronopotentiometry mode (Chapter 6). The

interconnected flakes structure of MnO2-NiO electrode has changed into

interconnected network-like structure without noticeable flakes when 0.15 M cobalt

sulphate was added into the deposition solutions. A layer of deposited electrode

without an appreciable nanoflakes structure (no sharp flake shape) and well-

distributed porous structure was obtained with deposit thickness in the range of 51

nm to 59 nm. The thickness of the best MnO2 and MnO2-NiO electrode from

previous studies was larger than the obtained Mn3O4-NiO-Co3O4 electrode (0.15 M

electrode). The thinner film with well distributed porous of 0.15 M electrode lead to

a better contact between active electrode materials and cations transferred from the

electrolyte and enhances the pseudocapacitive behaviour. The specific capacitances

of Mn3O4-NiO-Co3O4 electrode system are tabulated in Table 8.3.

Table 8.3 The specific capacitance of Mn3O4-NiO-Co3O4 electrode in 0.5 M Na2SO4 electrolyte in

potential range of 0 to 1 V at scan rate of 5 mV s-1.

Mn3O4-NiO-Co3O4 ternary

electrode

Specific capacitance (F g-1)

0.1 M 207

0.15 M 481

0.2 M 177

0.3 M 182

139

The best electrode from the ternary metal oxides system, NiO-15 was further

investigated in three different electrolytes, 0.5 M Na2SO4, 0.5 KOH and mix 0.5 M

KOH/0.04 M K3Fe(CN)6. The highest specific capacitance of 7404 F g-1 of NiO-15

was achieved in a KOH/K3Fe(CN)6 electrolyte with high energy and power density

of 1028 Wh kg-1 and 99 kW kg-1 at 20 A g-1, respectively. The specific capacitance

retained 50 % after 1800 cycles at scan rate 10 mV s-1.

Finally, the incorporation of conductive carbon into MnO2 was carried out to

produce high performance of manganese oxide based electrode as discussed in

Chapter 7. An ultrathin-deposited RGO-MnO2 electrode with uniform nanoflakes

structure was obtained in M30 electrode sample that was prepared from electrolyte

that contains GO:Mn(CH3COO)2·4H2O (10 ml : 30 ml). M30 exhibited optimum

performance than others deposited electrode. The specific capacitance for all

deposited RGO-MnO2 electrodes are listed in Table 8.4.

Table 8.4 The specific capacitance of RGO-MnO2 electrode in 0.5 M Na2SO4 electrolyte in potential

range of -1 to 1 V at scan rate of 5 mV s-1.

RGO-MnO2

composite electrode

Specific capacitance (F g-1)

Pristine MnO2 167

M30 264

M60 220

M90 175

The capacitance value of GO: Mn(CH3COO)2·4H2O (10 ml : 30 ml) sample

was then further improved when D (+) glucose anhydrous (C6H12O6) was added in

140

the deposition electrolyte solution. In this approach, we found that the addition of

glucose was able to slow down the nucleation process of MnO2 and enhanced the

carbon content of the electrode which helps to reduce the transfer resistance of cation

diffusion paths to the electrode matrix. The deposited electrode by using deposition

solution of 0.03 M glucose mixed 10 ml GO : 30 ml Mn(CH3COO)2·4H2O (G03)

exhibited uniform and well distributed porous structure within 60-100 nm of pore

diameter size. The average thickness of G03 electrode was around 265 nm. The

specific capacitances of all deposited electrode with additional of glucose, G01, G03

and G06 electrode in Na2SO4 electrolyte were 377 F g-1, 430 F g-1 and 361 F g-1,

respectively at scan rate of 5 mV s-1 in potential range of -1 to 1 V. The unique

structure of G03 gives an optimum electrochemical performance of specific

capacitance.

The G03 electrode was also studied in mixed 0.5 M KOH/ 0.04 M

K3Fe(CN)6 electrolyte. It exhibited specific capacitance of 13,333 F g-1 with a power

density of 68.35 kW kg-1, and an energy density of 1851 Wh kg-1 at a current density

of 20 A g-1. The extremely high specific capacitance in mixed KOH/ K3Fe(CN)6

electrolyte solution is attributed from the [Fe(CN)6]3- / [Fe(CN)6]

4- in the electrolyte

and highly redox reaction from electroactive electrode. Although the specific

capacitance and energy density if G03 in mix 0.5 M KOH/ 0.04 M K3Fe(CN)6

electrolyte was higher than earlier study, the specific capacitance retention is still

need to be improved. A retention percentage of 46 % was observed after 2000 cycles

at a scan rate of 10 mV s-1. High degradation is due to loss of active material and

high current. The obtained G03 electrode with optimum ratio of RGO-MnO2-

glucose carbon has been achieved in this study. All optimized electrode in each

141

system of MnO2 based electrode has been studied in mix 0.5 M KOH/ 0.04 M

K3Fe(CN)6 electrolyte.

8.2 Suggestions for future work

In this work, the concentration and deposition parameters have been

optimized for manganese oxide based electrode systems. Overall, the high specific

capacitance and energy density was obtained in our research which is suitable for

supercpacitor application. However, the cycling performance of the electrode is still

needs to be improved. There are some future works which can be used to improve

the electrochemical performance of electrodes as listed below:

The cycling performance and power density of electrode is believed can be

improved by deposited the nanostructure manganese oxide based electrode

with carbon based substrate like graphite cloth, graphite sheet, carbon cloth,

or CNT based electrode.

The adhesion between the electrode surface and active material can be

improved by adding the sodium dodecyl sulfate (SDS) into deposition

solution.

The optimized electrode in our studies can be used as cathode electrode for

hybrid supercapacitor and high conductivity of carbon materials as anode

electrode. This combination able to give good electrochemical performance

for flexible supercapacitor devices.

The optimized electrode can be further used to investigate their

electrochemical performance in different electrolytes viz. gel polymer

electrolyte or solid polymer electrolyte.

142

References

Aghazadeh, M., Golikand, A. N., & Ghaemi, M. (2011). Synthesis, characterization,

and electrochemical properties of ultrafine β-Ni(OH)2 nanoparticles.

International Journal of Hydrogen Energy, 36(14), 8674-8679

Alvi, F., Ram, M. K., Basnayaka, P. A., Stefanakos, E., Goswami, Y., & Kumar, A.

(2011). Graphene–polyethylenedioxythiophene conducting polymer

nanocomposite based supercapacitor. Electrochimica Acta, 56(25), 9406-

9412

Augustyn, V., Simon, P., & Dunn, B. (2014). Pseudocapacitive oxide materials for

high-rate electrochemical energy storage. Energy & Environmental Science,

7(5), 1597-1614

Babakhani, B., & Ivey, D. G. (2010). Anodic deposition of manganese oxide

electrodes with rod-like structures for application as electrochemical

capacitors. Journal of Power Sources, 195(7), 2110-2117

Babakhani, B., & Ivey, D. G. (2011). Effect of electrodeposition conditions on the

electrochemical capacitive behavior of synthesized manganese oxide

electrodes. Journal of Power Sources, 196(24), 10762-10774

Beaudrouet, E., Le Gal La Salle, A., & Guyomard, D. (2009). Nanostructured

manganese dioxides: Synthesis and properties as supercapacitor electrode

materials. Electrochimica Acta, 54(4), 1240-1248

Benhaddad, L., Makhloufi, L., Messaoudi, B., Rahmouni, K., & Takenouti, H.

(2011). Reactivity of Nanostructured MnO2 in Alkaline Medium Studied

with a Microcavity Electrode: Effect of Oxidizing Agent. Journal of

Materials Science & Technology, 27(7), 585-593

Broughton, J. N., & Brett, M. J. (2005). Variations in MnO2 electrodeposition for

electrochemical capacitors. Electrochimica Acta, 50(24), 4814-4819

Brousse, T., Toupin, M., Dugas, R., Athouël, L., Crosnier, O., & Bélanger, D.

(2006). Crystalline MnO2 as Possible Alternatives to Amorphous

Compounds in Electrochemical Supercapacitors. Journal of the

Electrochemical Society, 153(12), A2171-A2180

Buchholz, D. B., Liu, J., Marks, T. J., Zhang, M., & Chang, R. P. H. (2009). Control

and Characterization of the Structural, Electrical, and Optical Properties of

Amorphous Zinc−Indium−Tin Oxide Thin Films. ACS Applied Materials &

Interfaces, 1(10), 2147-2153

Cai, Y., Wang, Y., Deng, S., Chen, G., Li, Q., Han, B., Han, R., Wang, Y. (2014).

Graphene nanosheets-tungsten oxides composite for supercapacitor

electrode. Ceramics International, 40(3), 4109-4116

143

Cançado, L. G., Pimenta, M. A., Neves, B. R. A., Dantas, M. S. S., & Jorio, A.

(2004). Influence of the Atomic Structure on the Raman Spectra of Graphite

Edges. Physical Review Letters, 93(24), 247401

Cao, J., Wang, Y., Zhou, Y., Jia, D., Ouyang, J.-H., & Guo, L. (2012). Performances

of high voltage electrochemical capacitor using ball-milled graphite/Mn3O4

composite electrodes. Journal of Electroanalytical Chemistry, 682, 23-28

Chan, P. Y., Rusi, & Majid, S. R. (2014). RGO-wrapped MnO2 composite electrode

for supercapacitor application. Solid State Ionics, 262, 226-229

Chen, K., Song, S., & Xue, D. (2014). An ionic aqueous pseudocapacitor system:

electroactive ions in both a salt electrode and redox electrolyte. RSC

Advances, 4(44), 23338-23343

Chen, Y.-S., & Hu, C.-C. (2003). Capacitive Characteristics of Binary Manganese-

Nickel Oxides Prepared by Anodic Deposition. Electrochemical and Solid-

State Letters, 6(10), A210-A213

Chen, Y., Wang, J.-W., Shi, X.-C., & Chen, B.-Z. (2013). Pseudocapacitive

characteristics of manganese oxide anodized from manganese coating

electrodeposited from aqueous solution. Electrochimica Acta, 109, 678-683

Cheng, Q., Tang, J., Ma, J., Zhang, H., Shinya, N., & Qin, L.-C. (2011). Graphene

and nanostructured MnO2 composite electrodes for supercapacitors. Carbon,

49(9), 2917-2925

Conway, B. E. (1999). Electrochemical Supercapacitors: Scientific Fundamentals

and Technological Applications. New York: Kluwer-Plenum.

Conway, B. E., Birss, V., & Wojtowicz, J. (1997). The role and utilization of

pseudocapacitance for energy storage by supercapacitors. Journal of Power

Sources, 66(1–2), 1-14

Cordente, N., Respaud, M., Senocq, F., Casanove, M.-J., Amiens, C., & Chaudret,

B. (2001). Synthesis and Magnetic Properties of Nickel Nanorods. Nano

Letters, 1(10), 565-568

Cross, A., Morel, A., Cormie, A., Hollenkamp, T., & Donne, S. (2011). Enhanced

manganese dioxide supercapacitor electrodes produced by electrodeposition.

Journal of Power Sources, 196(18), 7847-7853

Devaraj, S., & Munichandraiah, N. (2008). Effect of Crystallographic Structure of

MnO2 on Its Electrochemical Capacitance Properties. The Journal of

Physical Chemistry C, 112(11), 4406-4417

Derek, P. (2009). A first course in electrode process 2nd edition. RSC

publishing:Cambridge UK.

Dharmaraj, N., Prabu, P., Nagarajan, S., Kim, C. H., Park, J. H., & Kim, H. Y.

(2006). Synthesis of nickel oxide nanoparticles using nickel acetate and

144

poly(vinyl acetate) precursor. Materials Science and Engineering: B, 128(1–

3), 111-114

Ding, R., Qi, L., Jia, M., & Wang, H. (2013). Facile and large-scale chemical

synthesis of highly porous secondary submicron/micron-sized NiCo2O4

materials for high-performance aqueous hybrid AC-NiCo2O4

electrochemical capacitors. Electrochimica Acta, 107, 494-502

Dirican, M., Yanilmaz, M., & Zhang, X. (2014). Free-standing polyaniline-porous

carbon nanofiber electrodes for symmetric and asymmetric supercapacitors.

RSC Advances, 4(103), 59427-59435

Dong, X., Wang, K., Zhao, C., Qian, X., Chen, S., Li, Z., Liu, H., Dou, S. (2014).

Direct synthesis of RGO/Cu2O composite films on Cu foil for

supercapacitors. Journal of Alloys and Compounds, 586, 745-753

Dongale, T. D., Jadhav, P. R., Navathe, G. J., Kim, J. H., Karanjkar, M. M., & Patil,

P. S. (2015). Development of nano fiber MnO2 thin film electrode and cyclic

voltammetry behavior modeling using artificial neural network for

supercapacitor application. Materials Science in Semiconductor Processing,

36, 43-48

Du Pasquier, A., Laforgue, A., Simon, P., Amatucci, G. G., & Fauvarque, J.-F.

(2002). A Nonaqueous Asymmetric Hybrid Li4Ti O12 / 

Poly(fluorophenylthiophene) Energy Storage Device. Journal of the

Electrochemical Society, 149(3), A302-A306

Dubal, D. P., Dhawale, D. S., Gujar, T. P., & Lokhande, C. D. (2011). Effect of

different modes of electrodeposition on supercapacitive properties of MnO2

thin films. Applied Surface Science, 257(8), 3378-3382

Dubal, D. P., Dhawale, D. S., Salunkhe, R. R., Pawar, S. M., & Lokhande, C. D.

(2010). A novel chemical synthesis and characterization of Mn3O4 thin films

for supercapacitor application. Applied Surface Science, 256(14), 4411-4416

Dubal, D. P., Gund, G. S., Holze, R., Jadhav, H. S., Lokhande, C. D., & Park, C.-J.

(2013). Solution-based binder-free synthetic approach of RuO2 thin films for

all solid state supercapacitors. Electrochimica Acta, 103, 103-109

Dubal, D. P., Jagadale, A. D., Patil, S. V., & Lokhande, C. D. (2012). Simple route

for the synthesis of supercapacitive Co–Ni mixed hydroxide thin films.

Materials Research Bulletin, 47(5), 1239-1245

El-Kady, M. F., & Kaner, R. B. (2013). Scalable fabrication of high-power graphene

micro-supercapacitors for flexible and on-chip energy storage. Nat Commun,

4, 1475

Engstrom, A. M., & Doyle, F. M. (2013). Exploring the cycle behavior of

electrodeposited vanadium oxide electrochemical capacitor electrodes in

various aqueous environments. Journal of Power Sources, 228, 120-131

145

Feng, L., Zhu, Y., Ding, H., & Ni, C. (2014). Recent progress in nickel based

materials for high performance pseudocapacitor electrodes. Journal of

Power Sources, 267, 430-444

Frackowiak, E., Jurewicz, K., Delpeux, S., & Béguin, F. (2001). Nanotubular

materials for supercapacitors. Journal of Power Sources, 97–98, 822-825

Frackowiak, E., Metenier, K., Bertagna, V., & Beguin, F. (2000). Supercapacitor

electrodes from multiwalled carbon nanotubes. Applied Physics Letters,

77(15), 2421-2423

Geim, A. K., & Novoselov, K. S. (2007). The rise of graphene. Nat Mater, 6(3),

183-191

Ghodbane, O., Pascal, J.-L., & Favier, F. (2009). Microstructural Effects on Charge-

Storage Properties in MnO2-Based Electrochemical Supercapacitors. ACS

Applied Materials & Interfaces, 1(5), 1130-1139

Girija, T. C., & Sangaranarayanan, M. V. (2006). Analysis of polyaniline-based

nickel electrodes for electrochemical supercapacitors. Journal of Power

Sources, 156(2), 705-711

Han, Z. J., Seo, D. H., Yick, S., Chen, J. H., & Ostrikov, K. (2014). MnOx/carbon

nanotube/reduced graphene oxide nanohybrids as high-performance

supercapacitor electrodes. NPG Asia Mater, 6, e140

Hasa, I., Buchholz, D., Passerini, S., & Hassoun, J. (2015). A Comparative Study of

Layered Transition Metal Oxide Cathodes for Application in Sodium-Ion

Battery. ACS Applied Materials & Interfaces, 7(9), 5206-5212

Ho, C.-L., & Wu, M.-S. (2011). Manganese Oxide Nanowires Grown on Ordered

Macroporous Conductive Nickel Scaffold for High-Performance

Supercapacitors. The Journal of Physical Chemistry C, 115(44), 22068-

22074

Hotovy, I., Huran, J., Spiess, L., Romanus, H., Buc, D., & Kosiba, R. (2006). NiO-

based nanostructured thin films with Pt surface modification for gas

detection. Thin Solid Films, 515(2), 658-661

Hsu, H.-Y., Chang, K.-H., Salunkhe, R. R., Hsu, C.-T., & Hu, C.-C. (2013).

Synthesis and characterization of mesoporous Ni–Co oxy-hydroxides for

pseudocapacitor application. Electrochimica Acta, 94, 104-112

Hu, C.-C., Hung, C.-Y., Chang, K.-H., & Yang, Y.-L. (2011a). A hierarchical

nanostructure consisting of amorphous MnO2, Mn3O4 nanocrystallites, and

single-crystalline MnOOH nanowires for supercapacitors. Journal of Power

Sources, 196(2), 847-850

Hu, C. C., & Chu, C. (2001). Electrochemical impedance characterization of

polyaniline-coated graphite electrodes for electrochemical capacitors -

146

effects of film coverage/thickness and anions. Journal of Electroanalytical

Chemistry, 503(1), 105-116

Hu, L., Chen, W., Xie, X., Liu, N., Yang, Y., Wu, H., Yao, Y., Pasta, M., Alshareef

H. N., Cui, Y. (2011b). Symmetrical MnO2–Carbon Nanotube–Textile

Nanostructures for Wearable Pseudocapacitors with High Mass Loading.

ACS Nano, 5(11), 8904-8913

Hughes, M., Chen, G. Z., Shaffer, M. S. P., Fray, D. J., & Windle, A. H. (2002).

Electrochemical Capacitance of a Nanoporous Composite of Carbon

Nanotubes and Polypyrrole. Chemistry of Materials, 14(4), 1610-1613

Hung, C. J., Lin, P., & Tseng, T. Y. (2013). Electrophoretic fabrication and

pseudocapacitive properties of graphene/manganese oxide/carbon nanotube

nanocomposites. Journal of Power Sources, 243, 594-602

Hwang, B. J., Santhanam, R., & Lin, Y. L. (2000). Nucleation and Growth

Mechanism of Electropolymerization of Polypyrrole on Gold/Highly

Oriented Pyrolytic Graphite Electrode. Journal of the Electrochemical

Society, 147(6), 2252-2257

Inamdar, A. I., Kim, Y., Pawar, S. M., Kim, J. H., Im, H., & Kim, H. (2011).

Chemically grown, porous, nickel oxide thin-film for electrochemical

supercapacitors. Journal of Power Sources, 196(4), 2393-2397

Jagadale, A. D., Kumbhar, V. S., & Lokhande, C. D. (2013). Supercapacitive

activities of potentiodynamically deposited nanoflakes of cobalt oxide

(Co3O4) thin film electrode. Journal of Colloid and Interface Science, 406,

225-230

Jena, A., Munichandraiah, N., & Shivashankar, S. A. (2013). Carbonaceous nickel

oxide nano-composites: As electrode materials in electrochemical capacitor

applications. Journal of Power Sources, 237, 156-166

Jiang, R., Huang, T., Liu, J., Zhuang, J., & Yu, A. (2009). A novel method to prepare

nanostructured manganese dioxide and its electrochemical properties as a

supercapacitor electrode. Electrochimica Acta, 54(11), 3047-3052

Kim, H., & Popov, B. N. (2003). Synthesis and Characterization of MnO2-Based

Mixed Oxides as Supercapacitors. Journal of the Electrochemical Society,

150(3), D56-D62

Kim, M., Hwang, Y., & Kim, J. (2013a). Graphene/MnO2-based composites

reduced via different chemical agents for supercapacitors. Journal of Power

Sources, 239, 225-233

Kim, M., Yoo, M., Yoo, Y., & Kim, J. (2014). Capacitance behavior of composites

for supercapacitor applications prepared with different durations of

graphene/nanoneedle MnO2 reduction. Microelectronics Reliability, 54(3),

587-594

147

Kim, S.-I., Lee, J.-S., Ahn, H.-J., Song, H.-K., & Jang, J.-H. (2013b). Facile Route

to an Efficient NiO Supercapacitor with a Three-Dimensional Nanonetwork

Morphology. ACS Applied Materials & Interfaces, 5(5), 1596-1603

Kim, Y.-T., Tadai, K., & Mitani, T. (2005). Highly dispersed ruthenium oxide

nanoparticles on carboxylated carbon nanotubes for supercapacitor electrode

materials. Journal of Materials Chemistry, 15(46), 4914-4921

Kötz, R., & Carlen, M. (2000). Principles and applications of electrochemical

capacitors. Electrochimica Acta, 45(15–16), 2483-2498

Kulkarni, S. B., Jagadale, A. D., Kumbhar, V. S., Bulakhe, R. N., Joshi, S. S., &

Lokhande, C. D. (2013). Potentiodynamic deposition of composition

influenced Co1−xNix LDHs thin film electrode for redox supercapacitors.

International Journal of Hydrogen Energy, 38(10), 4046-4053

Leng, Y. (2008). Materials characterization: Introduction to microscopic and

spectroscopic methods. Willey: Singapore

Li, D., Muller, M. B., Gilje, S., Kaner, R. B., & Wallace, G. G. (2008). Processable

aqueous dispersions o f graphene nanosheets. Nat Nano, 3(2), 101-105

Li, H., Wang, J., Chu, Q., Wang, Z., Zhang, F., & Wang, S. (2009). Theoretical and

experimental specific capacitance of polyaniline in sulfuric acid. Journal of

Power Sources, 190(2), 578-586

Li, H., Zhu, S., Xi, H. a., & Wang, R. (2006). Nickel oxide nanocrystallites within

the wall of ordered mesoporous carbon CMK-3: Synthesis and

characterization. Microporous and Mesoporous Materials, 89(1–3), 196-

203

Li, J., Yang, Q. M., & Zhitomirsky, I. (2008). Nickel foam-based manganese

dioxide–carbon nanotube composite electrodes for electrochemical

supercapacitors. Journal of Power Sources, 185(2), 1569-1574

Li, L., Zhang, Y., Shi, F., Zhang, Y., Zhang, J., Gu, C., Wang, X., Tu, J. (2014a).

Spinel Manganese–Nickel–Cobalt Ternary Oxide Nanowire Array for High-

Performance Electrochemical Capacitor Applications. ACS Applied

Materials & Interfaces, 6(20), 18040-18047

Li, M., Bo, X., Mu, Z., Zhang, Y., & Guo, L. (2014b). Electrodeposition of nickel

oxide and platinum nanoparticles on electrochemically reduced graphene

oxide film as a nonenzymatic glucose sensor. Sensors and Actuators B:

Chemical, 192, 261-268

Li, Q.-Y., Li, Z.-S., Lin, L., Wang, X. Y., Wang, Y.-F., Zhang, C.-H., & Wang, H.-

Q. (2010). Facile synthesis of activated carbon/carbon nanotubes compound

for supercapacitor application. Chemical Engineering Journal, 156(2), 500-

504

148

Li, S.-H., Liu, Q.-H., Qi, L., Lu, L.-H., & Wang, H.-Y. (2012a). Progress in

Research on Manganese Dioxide Electrode Materials for Electrochemical

Capacitors. Chinese Journal of Analytical Chemistry, 40(3), 339-346

Li, S.-M., Wang, Y.-S., Yang, S.-Y., Liu, C.-H., Chang, K.-H., Tien, H.-W., Wen,

N.-T., Ma, C.-C., Hu, C.-C. (2013). Electrochemical deposition of

nanostructured manganese oxide on hierarchically porous graphene–carbon

nanotube structure for ultrahigh-performance electrochemical capacitors.

Journal of Power Sources, 225, 347-355

Li, X., Xu, X., Xia, F., Bu, L., Qiu, H., Chen, M., Zhang, L., Gao, J. (2014c).

Electrochemically active MnO2/RGO nanocomposites using Mn powder as

the reducing agent of GO and the MnO2 precursor. Electrochimica Acta, 130,

305-313

Li, Y., Diao, P., Jin, T., Sun, J., & Xu, D. (2012b). Shape-controlled

electrodeposition of standing Rh nanoplates on indium tin oxide substrates

and their electrocatalytic activity toward formic acid oxidation.

Electrochimica Acta, 83, 146-154

Li, Y., Xie, H., Wang, J., & Chen, L. (2011). Preparation and electrochemical

performances of α-MnO2 nanorod for supercapacitor. Materials Letters,

65(2), 403-405

Li, Z., Liu, Z., Li, B., Li, D., Li, Q., & Wang, H. (2014d). MnO2 nanosilks self-

assembled micropowders: Facile one-step hydrothermal synthesis and their

application as supercapacitor electrodes. Journal of the Taiwan Institute of

Chemical Engineers, 45(6), 2995-2999

Lin, H.-K., Chiu, H.-C., Tsai, H.-C., Chien, S.-H., & Wang, C.-B. (2003). Synthesis,

Characterization and Catalytic Oxidation of Carbon Monoxide over Cobalt

Oxide. Catalysis Letters, 88(3-4), 169-174

Liu, E.-H., Li, W., Li, J., Meng, X.-Y., Ding, R., & Tan, S.-T. (2009). Preparation

and characterization of nanostructured NiO/MnO2 composite electrode for

electrochemical supercapacitors. Materials Research Bulletin, 44(5), 1122-

1126

Liu, N., Shen, J., & Liu, D. (2013). Activated high specific surface area carbon

aerogels for EDLCs. Microporous and Mesoporous Materials, 167, 176-181

Liu, Y., Yan, D., Li, Y., Wu, Z., Zhuo, R., Li, S., Feng, J., Wang, J., Yan, P., Geng,

Z. (2014). Manganese dioxide nanosheet arrays grown on graphene oxide as

an advanced electrode material for supercapacitors. Electrochimica Acta,

117, 528-533

Liu, Y., Zhang, M., Zhang, J., & Qian, Y. (2006). A simple method of fabricating

large-area α-MnO2 nanowires and nanorods. Journal of Solid State

Chemistry, 179(6), 1757-1761

149

Lu, X., Yu, M., Wang, G., Tong, Y., & Li, Y. (2014). Flexible solid-state

supercapacitors: design, fabrication and applications. Energy &

Environmental Science, 7(7), 2160-2181

Mai, L.-Q., Yang, F., Zhao, Y.-L., Xu, X., Xu, L., & Luo, Y.-Z. (2011). Hierarchical

MnMoO4/CoMoO4 heterostructured nanowires with enhanced

supercapacitor performance. Nat Commun, 2, 381

Moradi Golsheikh, A., Huang, N. M., Lim, H. N., Zakaria, R., & Yin, C.-Y. (2013).

One-step electrodeposition synthesis of silver-nanoparticle-decorated

graphene on indium-tin-oxide for enzymeless hydrogen peroxide detection.

Carbon, 62, 405-412

Moses Ezhil Raj, A., Victoria, S. G., Jothy, V. B., Ravidhas, C., Wollschläger, J.,

Suendorf, M., Neumann, M., Jayachandran, M., Sanjeeviraja, C. (2010).

XRD and XPS characterization of mixed valence Mn3O4 hausmannite thin

films prepared by chemical spray pyrolysis technique. Applied Surface

Science, 256(9), 2920-2926

Nithya, V. D., Kalai Selvan, R., Kalpana, D., Vasylechko, L., & Sanjeeviraja, C.

(2013). Synthesis of Bi2WO6 nanoparticles and its electrochemical

properties in different electrolytes for pseudocapacitor electrodes.

Electrochimica Acta, 109, 720-731

Niu, L., Wang, J., Hong, W., Sun, J., Fan, Z., Ye, X., Wang, H., Yang, S. (2014).

Solvothermal Synthesis of Ni/Reduced Graphene Oxide Composites as

Electrode Material for Supercapacitors. Electrochimica Acta, 123, 560-568

Nouzu, N., Ashida, A., Yoshimura, T., & Fujimura, N. (2010). Control of cathodic

potential for deposition of ZnO by constant-current electrochemical method.

Thin Solid Films, 518(11), 2957-2960

Obreja, V. V. N. (2008). On the performance of supercapacitors with electrodes

based on carbon nanotubes and carbon activated material—A review.

Physica E: Low-dimensional Systems and Nanostructures, 40(7), 2596-2605

Pang, S. C., Anderson, M. A., & Chapman, T. W. (2000). Novel electrode materials

for thin-film ultracapacitors: comparison of electrochemical properties of

sol-gel-derived and electrodeposited manganese dioxide. Journal of the

Electrochemical Society, 147(2), 444-450

Park, B.-H., & Choi, J.-H. (2010). Improvement in the capacitance of a carbon

electrode prepared using water-soluble polymer binder for a capacitive

deionization application. Electrochimica Acta, 55(8), 2888-2893

Pauporté, T., Goux, A., Kahn-Harari, A., de Tacconi, N., Chenthamarakshan, C. R.,

Rajeshwar, K., & Lincot, D. (2003). Cathodic electrodeposition of mixed

oxide thin films. Journal of Physics and Chemistry of Solids, 64(9–10),

1737-1742

150

Prasad, K. P. S., Dhawale, D. S., Joseph, S., Anand, C., Wahab, M. A., Mano, A.,

Sathish, C. I., Balasubramaniam, V. V., Sivakumar, T., Vinu, A. (2013).

Post-synthetic functionalization of mesoporous carbon electrodes with

copper oxide nanoparticles for supercapacitor application. Microporous and

Mesoporous Materials, 172, 77-86

Pu, J., Wang, J., Jin, X., Cui, F., Sheng, E., & Wang, Z. (2013). Porous hexagonal

NiCo2O4 nanoplates as electrode materials for supercapacitors.

Electrochimica Acta, 106, 226-234

Rajendra Prasad, K., & Miura, N. (2004). Electrochemically synthesized MnO2-

based mixed oxides for high performance redox supercapacitors.

Electrochemistry Communications, 6(10), 1004-1008

Ramesh, T. N., & Kamath, P. V. (2008). Nickel oxyhydroxide/manganese dioxide

composite as a candidate electrode material for alkaline secondary cells.

Journal of Power Sources, 175(1), 625-629

Ramya, R., Sivasubramanian, R., & Sangaranarayanan, M. V. (2013). Conducting

polymers-based electrochemical supercapacitors—Progress and prospects.

Electrochimica Acta, 101, 109-129

Reddy, R. N., & Reddy, R. G. (2003). Sol–gel MnO2 as an electrode material for

electrochemical capacitors. Journal of Power Sources, 124(1), 330-337

Rudge, A., Raistrick, I., Gottesfeld, S., & Ferraris, J. P. (1994). A study of the

electrochemical properties of conducting polymers for application in

electrochemical capacitors. Electrochimica Acta, 39(2), 273-287

Rusi, & Majid, S. R. Electrodeposited Mn3O4-NiO-Co3O4 as a composite electrode

material for electrochemical capacitor. Electrochimica Acta(0)

Rusi, & Majid, S. R. (2013). Synthesis of MnO2 particles under slow cooling process

and their capacitive performances. Materials Letters, 108, 69-71

Rusi, & Majid, S. R. (2014a). Controllable synthesis of flowerlike α-MnO2 as

electrode for pseudocapacitor application. Solid State Ionics, 262, 220-225

Rusi, & Majid, S. R. (2014b). High performance super-capacitive behaviour of

deposited manganese oxide/nickel oxide binary electrode system.

Electrochimica Acta, 138, 1-8

Sadezky, A., Muckenhuber, H., Grothe, H., Niessner, R., & Pöschl, U. (2005).

Raman microspectroscopy of soot and related carbonaceous materials:

Spectral analysis and structural information. Carbon, 43(8), 1731-1742

Sawangphruk, M., Srimuk, P., Chiochan, P., Krittayavathananon, A., Luanwuthi, S.,

& Limtrakul, J. (2013). High-performance supercapacitor of manganese

oxide/reduced graphene oxide nanocomposite coated on flexible carbon

fiber paper. Carbon, 60, 109-116

151

Shen, L., Yu, L., Wu, H. B., Yu, X.-Y., Zhang, X., & Lou, X. W. (2015). Formation

of nickel cobalt sulfide ball-in-ball hollow spheres with enhanced

electrochemical pseudocapacitive properties. Nat Commun, 6

Shirale, D. J., Gade, V. K., Gaikwad, P. D., Kharat, H. J., Kakde, K. P., Savale, P.

A., Hussaini, S. S., Dhumane, N. R., Shirsat, M. D. (2006). The influence of

electrochemical process parameters on the conductivity of poly(N-

methylpyrrole) films by galvanostatic method. Materials Letters, 60(11),

1407-1411

Shown, I., Ganguly, A., Chen, L.-C., & Chen, K.-H. (2015). Conducting polymer-

based flexible supercapacitor. Energy Science & Engineering, 3(1), 2-26

Simon, P., & Gogotsi, Y. (2008). Materials for electrochemical capacitors. Nat

Mater, 7(11), 845-854

Snook, G. A., Kao, P., & Best, A. S. (2011). Conducting-polymer-based

supercapacitor devices and electrodes. Journal of Power Sources, 196(1), 1-

12

Su, L.-H., Zhang, X.-G., Mi, C.-H., Gao, B., & Liu, Y. (2009). Improvement of the

capacitive performances for Co-Al layered double hydroxide by adding

hexacyanoferrate into the electrolyte. Physical Chemistry Chemical Physics,

11(13), 2195-2202

Su, X., Yu, L., Cheng, G., Zhang, H., Sun, M., & Zhang, X. (2015). High-

performance α-MnO2 nanowire electrode for supercapacitors. Applied

Energy, 153, 94-100

Sugimoto, W., Iwata, H., Yokoshima, K., Murakami, Y., & Takasu, Y. (2005).

Proton and Electron Conductivity in Hydrous Ruthenium Oxides Evaluated

by Electrochemical Impedance Spectroscopy:  The Origin of Large

Capacitance. The Journal of Physical Chemistry B, 109(15), 7330-7338

Sun, Z., Liao, T., Dou, Y., Hwang, S. M., Park, M.-S., Jiang, L., Kim, J. H., Dou, S.

X. (2014). Generalized self-assembly of scalable two-dimensional transition

metal oxide nanosheets. Nat Commun, 5

Takahashi, K., Wang, Y., & Cao, G. (2005). Ni−V2O5·nH2O Core−Shell Nanocable

Arrays for Enhanced Electrochemical Intercalation. The Journal of Physical

Chemistry B, 109(1), 48-51

Tsai, W.-Y., Lin, R., Murali, S., Li Zhang, L., McDonough, J. K., Ruoff, R. S.,

Taberna, P. L., Gogotsi, Y., Simon, P. (2013). Outstanding performance of

activated graphene based supercapacitors in ionic liquid electrolyte from −50

to 80 °C. Nano Energy, 2(3), 403-411

Vijayakumar, S., Nagamuthu, S., & Muralidharan, G. (2013). Porous NiO/C

Nanocomposites as Electrode Material for Electrochemical Supercapacitors.

ACS Sustainable Chemistry & Engineering, 1(9), 1110-1118

152

Wang, B., Park, J., Wang, C., Ahn, H., & Wang, G. (2010a). Mn3O4 nanoparticles

embedded into graphene nanosheets: Preparation, characterization, and

electrochemical properties for supercapacitors. Electrochimica Acta, 55(22),

6812-6817

Wang, G., Liu, L., Zhang, L., & Zhang, J. (2013). Nickel, cobalt, and manganese

oxide composite as an electrode material for electrochemical

supercapacitors. Ionics, 19(4), 689-695

Wang, G., Zhang, L., & Zhang, J. (2012a). A review of electrode materials for

electrochemical supercapacitors. Chemical Society Reviews, 41(2), 797-828

Wang, H., Gao, Q., & Hu, J. (2010b). Asymmetric capacitor based on superior

porous Ni–Zn–Co oxide/hydroxide and carbon electrodes. Journal of Power

Sources, 195(9), 3017-3024

Wang, J.-G., Kang, F., & Wei, B. (2015). Engineering of MnO2-based

nanocomposites for high-performance supercapacitors. Progress in

Materials Science, 74, 51-124

Wang, L., Zheng, Y., Chen, S., Ye, Y., Xu, F., Tan, H., Li, Z., Hou, H., Song, Y.

(2014). Three-Dimensional Kenaf Stem-Derived Porous Carbon/MnO2 for

High-Performance Supercapacitors. Electrochimica Acta, 135, 380-387

Wang, M., Tan, W., Feng, X., Koopal, L. K., Liu, M., & Liu, F. (2012b). One-step

synthesis of sea urchin-like α-MnO2 using KIO4 as the oxidant and its

oxidation of arsenite. Materials Letters, 77, 60-62

Wang, X., Wang, X., Huang, W., Sebastian, P. J., & Gamboa, S. (2005). Sol–gel

template synthesis of highly ordered MnO2 nanowire arrays. Journal of

Power Sources, 140(1), 211-215

Wang, X., Yu, L., Wu, X.-L., Yuan, F., Guo, Y.-G., Ma, Y., & Yao, J. (2009).

Synthesis of Single-Crystalline Co3O4 Octahedral Cages with Tunable

Surface Aperture and Their Lithium Storage Properties. The Journal of

Physical Chemistry C, 113(35), 15553-15558

Wang, Y., Shi, J.-C., Cao, J.-L., Sun, G., & Zhang, Z.-Y. (2011). Synthesis of Co3O4

nanoparticles via the CTAB-assisted method. Materials Letters, 65(2), 222-

224

Wei, D., Scherer, M. R. J., Bower, C., Andrew, P., Ryhänen, T., & Steiner, U.

(2012). A Nanostructured Electrochromic Supercapacitor. Nano Letters,

12(4), 1857-1862

Wei, W., Cui, X., Chen, W., & Ivey, D. G. (2011). Manganese oxide-based materials

as electrochemical supercapacitor electrodes. Chemical Society Reviews,

40(3), 1697-1721

Wintterlin, J., & Bocquet, M. L. (2009). Graphene on metal surfaces. Surface

Science, 603(10–12), 1841-1852

153

Wu, C.-H., Ma, J.-S., & Lu, C.-H. (2012). Synthesis and characterization of nickel–

manganese oxide via the hydrothermal route for electrochemical capacitors.

Current Applied Physics, 12(4), 1190-1194

Wu, M.-S., Huang, Y.-A., Jow, J.-J., Yang, W.-D., Hsieh, C.-Y., & Tsai, H.-M.

(2008a). Anodically potentiostatic deposition of flaky nickel oxide

nanostructures and their electrochemical performances. International

Journal of Hydrogen Energy, 33(12), 2921-2926

Wu, M.-S., Huang, Y.-A., Yang, C.-H., & Jow, J.-J. (2007). Electrodeposition of

nanoporous nickel oxide film for electrochemical capacitors. International

Journal of Hydrogen Energy, 32(17), 4153-4159

Wu, M.-S., Yang, C.-H., & Wang, M.-J. (2008b). Morphological and structural

studies of nanoporous nickel oxide films fabricated by anodic

electrochemical deposition techniques. Electrochimica Acta, 54(2), 155-161

Xiong, G., Hembram, K. P. S. S., Reifenberger, R. G., & Fisher, T. S. (2013). MnO2-

coated graphitic petals for supercapacitor electrodes. Journal of Power

Sources, 227, 254-259

Xue, W. J., Wang, Y. F., Li, P., Liu, Z.-T., Hao, Z. P., & Ma, C. Y. (2011).

Morphology effects of Co3O4 on the catalytic activity of Au/Co3O4 catalysts

for complete oxidation of trace ethylene. Catalysis Communications, 12(13),

1265-1268

Yamaoka, S., Shaji Kumar, M. D., Kanda, H., & Akaishi, M. (2002). Thermal

decomposition of glucose and diamond formation under diamond-stable

high pressure–high temperature conditions. Diamond and Related Materials,

11(1), 118-124

Yan, D., & Cui, W. (1999). Preparation and properties of no-binder electrode Ni/MH

battery. Journal of Alloys and Compounds, 293–295, 780-783

Yan, J., Fan, Z., Wei, T., Qian, W., Zhang, M., & Wei, F. (2010). Fast and reversible

surface redox reaction of graphene–MnO2 composites as supercapacitor

electrodes. Carbon, 48(13), 3825-3833

Yang, Q., Li, Q., Yan, Z., Hu, X., Kang, L., Lei, Z., & Liu, Z.-H. (2014a). High

performance graphene/manganese oxide hybrid electrode with flexible holey

structure. Electrochimica Acta, 129, 237-244

Yang, W., Gao, Z., Song, N., Zhang, Y., Yang, Y., & Wang, J. (2014b). Synthesis

of hollow polyaniline nano-capsules and their supercapacitor application.

Journal of Power Sources, 272, 915-921

Yang, W., Gao, Z., Wang, J., Wang, B., Liu, Q., Li, Z., Mann, T., Yang, P., Zhang,

M., Liu, L. (2012). Synthesis of reduced graphene nanosheet/urchin-like

manganese dioxide composite and high performance as supercapacitor

electrode. Electrochimica Acta, 69, 112-119

154

Yousefi, T., Golikand, A. N., Mashhadizadeh, M. H., & Aghazadeh, M. (2012).

Template-free synthesis of MnO2 nanowires with secondary flower like

structure: Characterization and supercapacitor behavior studies. Current

Applied Physics, 12(1), 193-198

Yu, G., Xie, X., Pan, L., Bao, Z., & Cui, Y. (2013). Hybrid nanostructured materials

for high-performance electrochemical capacitors. Nano Energy, 2(2), 213-

234

Yuan, L., Lu, X.-H., Xiao, X., Zhai, T., Dai, J., Zhang, F., Hu, B., Wang, X., Gong,

L., Chen, J., Hu, C., Tong, Y., Zhou, J., Wang, Z. L. (2012). Flexible Solid-

State Supercapacitors Based on Carbon Nanoparticles/MnO2 Nanorods

Hybrid Structure. ACS Nano, 6(1), 656-661

Yuan, Y. F., Xia, X. H., Wu, J. B., Gui, J. S., Chen, Y. B., & Guo, S. Y. (2010).

Electrochromism in mesoporous nanowall cobalt oxide thin films prepared

via lyotropic liquid crystal media with electrodeposition. Journal of

Membrane Science, 364(1–2), 298-303

Yuan, Y. F., Xia, X. H., Wu, J. B., Yang, J. L., Chen, Y. B., & Guo, S. Y. (2011).

Nickel foam-supported porous Ni(OH)2/NiOOH composite film as advanced

pseudocapacitor material. Electrochimica Acta, 56(6), 2627-2632

Zeng, W., Miao, B., Lin, L.-y., & Xie, J.-y. (2012). Facile synthesis of NiO

nanowires and their gas sensing performance. Transactions of Nonferrous

Metals Society of China, 22, Supplement 1, s100-s104

Zhang, J., Jiang, J., & Zhao, X. S. (2011). Synthesis and Capacitive Properties of

Manganese Oxide Nanosheets Dispersed on Functionalized Graphene

Sheets. The Journal of Physical Chemistry C, 115(14), 6448-6454

Zhang, J., Wang, X., Ma, J., Liu, S., & Yi, X. (2013a). Preparation of cobalt

hydroxide nanosheets on carbon nanotubes/carbon paper conductive

substrate for supercapacitor application. Electrochimica Acta, 104, 110-116

Zhang, L. L., & Zhao, X. S. (2009). Carbon-based materials as supercapacitor

electrodes. Chemical Society Reviews, 38(9), 2520-2531

Zhang, M., Guo, S., Zheng, L., Zhang, G., Hao, Z., Kang, L., & Liu, Z.-H. (2013b).

Preparation of NiMn2O4 with large specific surface area from an epoxide-

driven sol−gel process and its capacitance. Electrochimica Acta, 87, 546-

553

Zhang, M., Xie, J., Sun, Q., Yan, Z., Chen, M., Jing, J., & Hossain, A. M. S. (2013c).

In situ synthesis of palladium nanoparticle on functionalized graphene sheets

at improved performance for ethanol oxidation in alkaline media.

Electrochimica Acta, 111, 855-861

Zhang, X., Ji, L., Zhang, S., & Yang, W. (2007). Synthesis of a novel polyaniline-

intercalated layered manganese oxide nanocomposite as electrode material

for electrochemical capacitor. Journal of Power Sources, 173(2), 1017-1023

155

Zhang, Y.-Z., Zhao, J., Xia, J., Wang, L., Lai, W.-Y., Pang, H., & Huang, W. (2015).

Room temperature synthesis of cobalt-manganese-nickel oxalates

micropolyhedrons for high-performance flexible electrochemical energy

storage device. Scientific Reports, 5, 8536

Zhang, Y., Feng, H., Wu, X., Wang, L., Zhang, A., Xia, T., Dong, H., Li, X., Zhang,

L. (2009). Progress of electrochemical capacitor electrode materials: A

review. International Journal of Hydrogen Energy, 34(11), 4889-4899

Zhang, Y., Li, J., Kang, F., Gao, F., & Wang, X. (2012a). Fabrication and

electrochemical characterization of two-dimensional ordered nanoporous

manganese oxide for supercapacitor applications. International Journal of

Hydrogen Energy, 37(1), 860-866

Zhang, Y., Yang, Y., Zhang, Y., Zhang, T., & Ye, M. (2012b). Heterogeneous

oxidation of naproxen in the presence of α-MnO2 nanostructures with

different morphologies. Applied Catalysis B: Environmental, 127, 182-189

Zhang, Y. X., Huang, M., Li, F., Wang, X. L., & Wen, Z. Q. (2014). One-pot

synthesis of hierarchical MnO2-modified diatomites for electrochemical

capacitor electrodes. Journal of Power Sources, 246, 449-456

Zhao, C., Zheng, W., Wang, X., Zhang, H., Cui, X., & Wang, H. (2013). Ultrahigh

capacitive performance from both Co(OH)2/graphene electrode and

K3Fe(CN)6 electrolyte. Scientific Reports, 3, 2986

Zhao, S., Liu, T., Hou, D., Zeng, W., Miao, B., Hussain, S., Pheng, X., Javed, M. S.

(2015a). Controlled synthesis of hierarchical birnessite-type MnO2

nanoflowers for supercapacitor applications. Applied Surface Science, 356,

259-265

Zhao, S., Liu, T., Shi, D., Zhang, Y., Zeng, W., Li, T., & Miao, B. (2015b).

Hydrothermal synthesis of urchin-like MnO2 nanostructures and its

electrochemical character for supercapacitor. Applied Surface Science, 351,

862-868

Zhou, W.-J., Xu, M.-W., Zhao, D.-D., Xu, C.-L., & Li, H.-L. (2009).

Electrodeposition and characterization of ordered mesoporous cobalt

hydroxide films on different substrates for supercapacitors. Microporous

and Mesoporous Materials, 117(1–2), 55-60

Zhu, Y., Li, L., Zhang, C., Casillas, G., Sun, Z., Yan, Z., Ruan, G., Pheng, Z., Raji,

R. J. O., Kittrel, C., Hauge, R. H., Tour, J. M. (2012). A seamless three-

dimensional carbon nanotube graphene hybrid material. Nat Commun, 3,

1225