Preparation of manganese oxide /graphene composites by plasma-enhanced electrochemical exfoliation process and its electrochemical performance

Electrochemical supercapacitors, which can provide higher energy density than conventional capacitors and higher power density than batteries/fuel cells, have received significant attent

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN THANH HAI

PREPARATION OF MANGANESE

DIOXIDE/GRAPHENE COMPOSITES BY

PLASMA-ENHANCED ELECTROCHEMICAL EXFOLIATION PROCESS AND ITS ELECTROCHEMICAL

PERFORMANCE

MASTER’S THESIS

Hanoi, 2019

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN THANH HAI

PREPARATION OF MANGANESE

DIOXIDE/GRAPHENE COMPOSITES BY

PLASMA-ENHANCED ELECTROCHEMICAL EXFOLIATION PROCESS AND ITS ELECTROCHEMICAL

PERFORMANCE

Major: Nanotechnology Code: Pilot

Research supervisor:

Dr Phan Ngoc Hong MASTER’S THESIS Hanoi, 2019

TABLE OF CONTENTS

TITLE PAGE i

TABLE OF CONTENTS ii

LIST OF FIGURES iv

LIST OF TABLES vi

LIST OF ABBREVIATIONS vii

ACKNOWLEDGMENTS viii

DECLARATION ix

ABSTRACT x

INTRODUCTION 1

Chapter 1 OVERVIEW 3

1.1 Electrochemical energy storages 3

1.1.1 Supercapacitors 5

1.2 Electrode materials for supercapacitors 6

1.2.1 MnO 2 /graphene composites 7

1.2.1.1 Direct oxidation-reduction reaction 8

1.2.1.2 Solution-based mechanical mixing 10

1.2.1.3 The other methods 13

1.3 Current research in Vietnam 15

Chapter 2 MATERIALS AND METHODS 18

2.1 Chemicals and reagents 18

2.2 Preparation of MnO 2 /graphene composites 18

2.3 Preparation of graphene and GM1 electrodes 19

2.4 Preparation of symmetric supercapacitor (GM1//GM1) 20

2.5 Characterizations 20

2.6 Electrochemical analysis 21

Chapter 3 RESULTS AND DISCUSSION 23

3.1 Characterizations of MnO 2 /graphene composites 23

3.2 The proposed mechanism for PE 3 P method 29

3.3 Electrochemical performance 30

3.4 Symmetric supercapacitor 35

CONCLUSIONS 39

LIST OF PUBLICATIONS 40

REFERENCES 42

LIST OF FIGURES

Figure 1.1 A Ragone plot for various electrochemical energy storage devices [33] 4

Figure 1.2 The working principles of (a) electrochemical double layer capacitor (carbon as the electrode material) and (b) Pseudocapacitor (MnO2 as the electrode material) in Na2SO4 electrolyte [18] 5

Figure 1.3 (a) Schematic illustration for the synthesis of graphene–MnO2 composite (b) the comparison of specific capacitance with other materials [48] 8

Figure 1.4 Schematic representations of the experimental design of MnO2/rGO composite [53] 9

Figure 1.5 Schematic graphic of the synthesis process of the rGO/MnOx composite [41] 10

Figure 1.6 The formation mechanism for GO-MnO2 nanocomposites [2] 11

Figure 1.7 (a) Schematic representations for MnO2 anchoring on graphene through electrostatic attraction, (b,c) TEM image and (d) capacitance retention of MnO2/graphene [56] 12

Figure 1.8 Laser scribing of high-performance and flexible graphene/MnO2-based electrochemical capacitors [8] 13

Figure 1.9 (a) Schematic illustration for plasma-assisted electrochemical exfoliation method, (b) TEM image of graphene sheets and (c) XPS of C1s in graphene samples [37] 15

Figure 1.10 The detailed process of printing supercapacitor electrodes [7] 16

Figure 2.1 The schematic representation of the experimental design 19

Figure 3.1 SEM images of (a) graphene, (b) GM1 (1 mM KMnO4), (c) GM10 (10 mM KMnO4) and (d) MnO2 nanoparticles (1 mM KMnO4), respectively 23

Figure 3.2 EDX results of GM1 and their element mapping images 24

Figure 3.3 TEM images of (a) graphene and (b) GM1 25

Figure 3.4 Raman spectra of GM1 and graphene 25

Figure 3.5 XRD pattern of graphene and GM1 samples 27

Figure 3.6 XPS patterns of GM1, (a) survey, (b) C1s, (c) O1s and (d) Mn2p 28

Figure 3.7 Proposed mechanism for the formation of graphene/MnO2 composite 29

Figure 3.8 Cyclic voltammetry curves of (a) graphene and (b) GM1 electrodes in a 6 M KOH electrolyte at a different scan rate of 5, 10, 20, 50, 100 mV s-1 31

Figure 3.9 Charge-discharge curves of (a) graphene and (b) GM1 electrodes in a 6 M

KOH electrolyte at a different current density of 2, 5, 10, 20 A g-1 32

Figure 3.10 Cycling performances of (a) GM1 and (b) graphene electrodes at a current

density of 10 A g-1 34

Figure 3.11 (a) GCD curves of GM1//GM1 symmetric supercapacitor at a different

current density of 2.5, 5, 10 A g-1 and (b) the specific capacitance of GM1//GM1

symmetric supercapacitor 36

Figure 3.12 Ragone plot of GM1//GM1 symmetric supercapacitor 37 Figure 3.13 Cycle stability of GM1/GM1 symmetric supercapacitor at a current density of

5 A g-1 38

LIST OF TABLES

Table 3.1 Effect of concentration of KMnO4 on forming MnO2 nanoparticles 24

Table 3.2 The comparison of some vital parameters with other results 35

Cyclic voltammetry Galvanostatic charge/discharge Scanning electron microscopy Energy-dispersive X-ray Transmittance electron microscopy X-ray diffraction

X-ray photoelectron spectroscopy Saturated calomel electrode

PE3P

CNTs

Plasma-enhanced electrochemical exfoliation process Carbon nanotubes

ACKNOWLEDGMENTS

First of all, I would like to express my sincere gratitude to Dr Phan Ngoc Hong, Center for High Technology Development (HTD), Vietnam Academy of Science and Technology (VAST), for his extraordinary supervision, support and guidance throughout my research period My dissertation would not have been possibly conducted without his valuable advice and constructive comments I would like to express my appreciation to Assoc Prof Masashi Akabori, School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), for his excellent guidance, advice and support during the period of internship Especially Assoc Prof Masashi Akabori who has spent his precious time for training and helping me on characterizations and measurements

I would strongly give my sincere appreciation to Dr Dang Van Thanh, who always support and encourage me during all my research and future academic careers His energetic and enthusiastic attitudes towards research inspire me to overcome the research challenges Additionally, I also acknowledge Dr Nguyen Tuan Hong for allowing me to use the facilities and providing the best conditions when I did experiments I also appreciate Mr Pham Trong Lam and Mr Dang Nhat Minh for their kind help and fruitful discussion about data analysis I also thank Mr Le Hoang for TEM measurements

I would like to thank Nanotechnology Program staff, Ms Nguyen Thi Huong for being so nice and helping me with all the administrative and academic problems

This thesis is supported by National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.09-2017.360

Finally, special thanks go to my parents and my friends for being there, smiling

at me with love, good days or bad days

DECLARATION

I hereby declare that all the result in this document has been obtained and

presented in accordance with academic rules and ethical conduct I also declare that,

as required by these rules and conduct, I have fully cited and referenced all material

and results that are not original to this work

Author

Nguyen Thanh Hai

ABSTRACT

In this thesis, I have developed a low-cost, simple and one-step approach to synthesize MnO2/graphene composite with enhanced electrochemical performance MnO2/graphene composite was prepared via a plasma-assisted electrochemical exfoliation method with an electrolyte solution made of KMnO4 precursor MnO2/graphene composites were characterized by SEM, TEM, XRD, Raman, XPS and were employed for the examination of electrochemical behaviors MnO2/graphene composite displayed the specific capacitance of 217.0 F g−1 at current density of 2 A g-1, which is approximately three times higher than those of pristine graphene (47.0 F g−1) Interestingly, the capacitance retention was highly kept over 80% after 3000 cycles The enhanced electrochemical property might be due to the synergistic effect of MnO2 nanoparticles and graphene nanosheets With a view to practical applications, a symmetric supercapacitor has been fabricated and delivered the highest specific capacitance of 130.9 F g-1 at a current density of 2.5 A g-1 In general, this work provides a new approach to synthesize MnO2/graphene composite for supercapacitors applications by one-step, short-time and eco-friendly method

INTRODUCTION

Prosperous development of energy conversion and storage plays a crucial role

in our modern life Electrochemical supercapacitors, which can provide higher energy density than conventional capacitors and higher power density than batteries/fuel cells, have received significant attention in recent years as a promising alternative energy storage devices [15, 24, 43]

Owning to their low-cost, abundance, environmental friendliness and its ability to work under neutral pH, especially with high theoretical specific capacitance (1380 F g-1), MnO2 is generally considered to be an excellent candidate for supercapacitor applications [13, 39, 42, 55] However, MnO2 particles often tend to form big agglomerates, which noticeably reduce their electrochemical property and lower the efficiency as a result of the undone reaction of MnO2 nanostructures during the electrochemical reduction-oxidation reaction [36] To overcome this problem, researchers currently have developed a powerful route to strengthen the electrochemical property by combining MnO2 with transition metal oxides [16] or with graphene [2, 41, 48], which are benefited from the high electrical conductivity

of carbon materials as well as the high specific capacity of metal oxides Nevertheless, their works mostly utilize graphene oxide as a precursor to synthesize graphene and its derivatives via graphene oxide reduction process Graphene oxide has commonly been produced from graphite oxidation process i.e Hummers’ method, which requires hazardous chemicals such as strong acid (nitric acid, sulfuric acid) and strong oxidant agents (potassium permanganate, potassium chlorate) in order to partially oxidize graphite Notably, these as-mentioned chemicals are highly toxic and dangerously unstable, which can release toxic gases such as NO2, N2O4, and ClO2 Moreover, a large amount of wastewater containing acid waste and heavy metal ions has been considered to be risky for the environment Also, the present synthesis method is an extremely time-consuming process, which needs a few to hundreds of hours for oxidation and removing excessive acids and KMnO4 after the oxidation step Therefore, an alternative approach for preparing GO or graphene from

a commercial graphite source with the simple, rapid and inexpensive process is highly

necessary In the past few years, the electrochemical method has been proved to be a green and attractive approach to producing high-quality graphene due to its

environmentally friend and low-cost approach [37, 38] Dang et al [6] proposed a

novel and efficient method for the preparation of MoS2/graphene composite by modifying an electrolyte solution Thus, there is a great interest to change the electrolyte solution to obtain desirable metal oxide and graphene composites

In this thesis, a new, simple and environmentally friend approach for synthesizing MnO2/graphene composites will be demonstrated Plasma-assisted electrochemical exfoliation method that consists of graphite cathode and Pt anode under a high applied voltage will be conducted The purpose of this work is to find out new method for one-step synthesis of MnO2/graphene composites by plasma-assisted electrochemical exfoliation method in a short-time reaction, low-cost and time-saving process Besides, the MnO2/graphene composites will be tested electrochemical performance towards its supercapacitor applications This dissertation will be divided into the following Chapter

+ Introduction

+ Chapter 1: Overview of supercapacitors and current status on MnO2/graphene research

+ Chapter 2: Materials and Methods

+ Chapter 3: Results and Discussion

+ Conclusions

Chapter 1 OVERVIEW 1.1 Electrochemical energy storages

Energy is crucial to modern society for sustainable economies in both developed and developing countries Up to now, fossil fuels are still the major sources

of energy in spite of the increasing environmental pollution problems and ecological crisis caused by fossil fuels consumption Moreover, with the rapid expansion of the global economy, increasing environmental pollution worldwide, and the depletion of non-renewable fossil fuels, there has been an increasingly urgent demand for the development of not only efficient, clean, and sustainable sources of energy, but also high-performance, low-cost, and environmentally friendly energy conversion and storage devices Hence, electrochemical energy storages are indispensable and essential for us in order to contribute to new and constant energy supplements The electrochemical energy storage technology has been widely used for numerous applications such as portable electronic devices, electric vehicles, large-scale electric grids and stationary energy storage Figure 1.1 shows a Ragone plot for the energy storage systems, illustrating their relationship between power density and energy density [33]

Figure 1.1 A Ragone plot for various electrochemical energy storage devices [33]

As shown in Figure 1.1, the fuel cells show the highest energy density, but their power density is the lowest among these promising devices Similar to the fuel cells, batteries also get high energy density, but the practical applications of batteries are still limited due to its low power density and cycle stability Thus, supercapacitors will be a prospective nominee for electrochemical energy storage that could bring higher energy density and higher power density than conventional capacitors and batteries, respectively However, the current limitation of the supercapacitor is low energy density compared to batteries in actual applications For instance, carbon-based supercapacitors commonly possess energy density less than 10 Wh kg-1, which

is much lower than that of lead-acid batteries (33-42 Wh kg-1) and lithium-ion batteries (100–265 Wh kg-1) [33] Because of its low energy density, supercapacitors are not able to meet the demand for energy storage devices for the next power generation The enhanced energy and power density of supercapacitors (electrochemical capacitors) are very crucial to fulfill the higher and higher requirements of energy storage devices

1.1.1 Supercapacitors

Recently, supercapacitors have drawn significant concern of scientists particularly thank to their high-power density, long cycle life and fast charge-discharge processes [31, 43] Supercapacitors preserve an essential position in the Ragone plot since they can fill the gap of energy-power density between conventional capacitors and batteries With a reasonably high energy density and power density, supercapacitors have been extensively applied in practical applications ranging from portable consumer electronic devices, back-up memory systems, automotive, to industrial power and energy management, and many more Dependence on the charge storage mechanisms, the electrochemical supercapacitors might be classified into two kinds of supercapacitors: electrical double-layer capacitor (EDLC) and pseudocapacitor [13]

Figure 1.2 The working principles of (a) electrochemical double layer capacitor

(carbon as the electrode material) and (b) Pseudocapacitor (MnO2 as the electrode

material) in Na2SO4 electrolyte [18]

A conventional electrostatic capacitor is a passive device with two electrodes, which are separated by a dielectric layer Static charge is stored by polarizing the electrodes within an electric field, providing a mechanism for delivering very high-power density, but low energy density (few microFarads per gram) Electrochemical

double layer capacitors (EDLCs) store charge by physical electrostatic where reversible adsorption of ions from the electrolyte onto the active material The active materials will adsorb ions on its surface to form a double layer at the electrode-electrolyte interface (Figure 1.2a) [24] The EDLCs mechanism is commonly presented for carbon-based materials due to its high BET surface area The absence

of a redox reaction (non-Faradaic process) allows fast charge/discharge cycles, which produce high power density and long cycle life since there is no mechanical stress caused by changes in the volume of the electrode However, as the energy storage strongly depends on the surface area of the active material, they exhibit limited energy density

Pseudocapacitive electrode materials store charge based on a fast and reversible surface oxidation-reduction reaction (Faradaic process) by electron transfer in addition to the formation of the double layer (Figure 1.2b) [15] Common pseudocapacitive materials are conducting polymers (e.g polypyrene, polyaniline, polythiophenes) and metal oxides (e.g., MnO2, RuO2) The capacitance of these electrodes is between 10-100 times higher than EDCLs; however, the power density and cycle life are lower because Faradic processes are slower than electrostatic processes and change in the volume of the electrode upon cycling (swelling and shrinking) tend to cause mechanical stress, degrading the materials When electrodes

of different nature are used as the cathode (e.g., pseudocapacitive material) and the anode (e.g., capacitive material) the supercapacitor is called hybrid capacitor

1.2 Electrode materials for supercapacitors

A variety of materials has been carefully studied as active materials for electrochemical capacitors Among them, carbon nanomaterials (e.g graphene, carbon nanotubes, amorphous…) [19, 29, 45] with different microstructures have been comprehensively explored as electrode materials for high-performance supercapacitors owning to their high specific surface area, interconnected pore structure, controlled pore size, high electrical conductivity, excellent chemical stability, and good environmental compatibility Due to a purely physical ion adsorption-desorption process, there is a considerable hurdle for carbon-based

materials in meeting the requirements of high-performance supercapacitors even though with very high surface area [33, 59]

On the other hand, pseudocapacitive materials such as metal oxides (e.g MnO2 and RuO2), which are capable of fast and reversible redox reactions at the electrode surface, resulted in much higher capacitances compared to carbon-based materials alone However, the rapid degradation of capacitance in pseudocapacitive materials is mostly due to their low conductivity, low structural and chemical stability [13, 33] By introducing pseudomaterials and carbon materials, it is believed that these nanocomposites by taking electrical double layer capacitance and pseudocapacitance can effectively improve the capacitance and energy density of supercapcitors without compromising the power density and cycling stability of the resulting supercapcitors

Among electrode materials for pseudocapacitors, MnO2 has been selected as

an outstanding candidate due to their low-cost, abundance, environmental friendliness and its ability to work under neutral pH, especially with high theoretical specific capacitance (1380 F g-1) [13, 42] In this thesis, the experimental conditions and state-of-the-art of MnO2/graphene materials for SCs electrode materials are only considered

1.2.1 MnO 2 /graphene composites

Graphene, a one-atom-thick 2D single layer of sp2-bonded carbon, has become

a new star in material science since they are firstly isolated from bulk graphite by using “scotch tape” method Owing to its abundant raw material resources, excellent electrochemical stability, large theoretical specific area (up to 2630 m2 g-1) and high electrical conductivity (104 S cm-1), graphene has been pointed as an attractive material for the development of high-performance of supercapacitors [15, 45, 59] Thus, these exciting properties of graphene and MnO2 can produce a synergistic effect, which could overcome their current obstacles, thus enhancing the capacitance and property of supercapacitors [12, 32, 47, 53, 54] Until now, there are numerous

of literature have been carried out to (i) develop a simple, scalable, and reliable method, (ii) prepare MnO2 nanostructure with desired morphology and mass

percentage comparing with graphene, (iii) improve electrical and mechanical property between two components There are two favorable ways for preparing MnO2/graphene nanocomposites: direct oxidation-reduction reaction and solution-based mechanical mixing of two components

1.2.1.1 Direct oxidation-reduction reaction

The first approach is that the direct redox reaction between KMnO4 and

graphene / graphene oxide For instance, Yan et al [48] proposed a fast and effective

method to prepare MnO2/graphene nanocomposites through the self-limiting deposition of nanoscale MnO2 on the surface of graphene under microwave irradiation In their experiment, they mixed the graphene solution with particular KMnO4 precursor together (Figure 1.3) Then, by taking advantage of microwave irradiation, the following reaction will occur: 𝑀𝑛𝑂$%+ 3𝐶 + 𝐻*𝑂 Û 4𝑀𝑛𝑂*+

𝐶𝑂-*%+ 2𝐻𝐶𝑂-% The author stated that MnO2/graphene nanocomposite (78 wt.% MnO2) exhibited the maximum specific capacitance of 310 F g-1 at a scan rate of 2

mV s-1 and kept a reasonable capacitance of 228 F g-1 at a scan rate of 500 mV s-1 Moreover, the cycle stability of the nanocomposite was also performed by repeatedly carrying out cycle voltammetry tests and showed a very perfect capacitance retention

of 95.4% after over 15 000 cycles

Figure 1.3 (a) Schematic illustration for the synthesis of graphene–MnO2

composite (b) the comparison of specific capacitance with other materials [48]

Yang et al [49] produced N-doped graphene/MnO2 composites by employing

a one-step hydrothermal method at a moderate temperature around 120oC The MnO2

nanosheets were tightly anchored on graphene sheets Their results in electrochemical

property indicated that the N-doped composites exhibited a remarkable improvement than non-doped one Especially, N-doped composites reached specific capacitance of 257.1 F g-1 while undoped composite delivered 217 F g-1 at the same current density

of 0.2 A g-1 Dai et al [5] demonstrated a gram-scale approach to prepare a uniform

graphene oxide/MnO2 nanowires through a hydrothermal process without using any surfactants, catalysts or templates The morphological analysis demonstrated that a-MnO2 nanowires were obtained with diameters and lengths of 20–40 nm and 0.5–2

mm and were fairly distributed throughout the sample In other words, a-MnO2

nanowires were well-dispersed on the surfaces of GO sheets Besides, the specific capacitance of the composite was calculated and determined to be 360 F g-1 Zhang

et al [53] presented a one-step way to prepared MnO2/rGO composite by hydrothermal process The resulting composites demonstrated fantastic characteristics such as high electrical conductivity, high specific surface area and quick diffusion of electrolyte ions These properties could lead to an excellent capacitance of 255 F g-1 at a current density of 0.5 A g-1 and approximately 84.5% of original capacitance was maintained after 10 000 cycles at a current density of 10 A

low-that GO sheets were synthesized by Hummer’s method and further served as a scaffold for growing of MnO2 nanosheets on the surface of GO Then, the GO will

be reduced to rGO by using hydrazine vapor treatment, while MnO2 nanosheets were also fractured into Mn4+, Mn3+ or Mn2+ species to form multivalent MnOx

nanoparticles concurrently [41] The partial reduction of MnO2 nanosheets might lose some vacancies in the MnOx nanoparticles due to the missing oxygen atoms Fortunately, the bonding between rGO and MnOx nanoparticles are believed to occur through oxygen-bridge and thus enhance the conductivity of the rGO/MnOx hybrid The enhanced electrical conductivity of the hybrid is advantageous; this could lead

to delivering a specific capacitance of 202 F g−1 and exhibited exceptional cycling stability of almost 100% of its initial capacitance after 115 000 cycles

Figure 1.5 Schematic graphic of the synthesis process of the rGO/MnOx composite

[41]

1.2.1.2 Solution-based mechanical mixing

Another approach is the mechanical mixture of MnO2 nanostructures and graphene nanosheets in solution, in which the formation of MnO2 nanostructures and graphene nanosheets do not depend on the presence of each other The MnO2

nanostructures could be uniformly and firmly anchored with graphene through physical electrostatic attraction, where graphene sheets will present as negatively charged surface and MnO2 nanostructures will present as positively charge surface

[39] For example, Chen et al [2] synthesized GO-MnO2 nanocomposites via a simple chemical route in a water-isopropyl alcohol system The formation

mechanism of the nanocomposite was proposed in Figure 1.6 The GO contains a lot

of oxygen-contain functional groups that act as anchoring sites for creating bonding between GO and MnO2, and eventually forming GO-MnO2 nanocomposites The author stated that the optimized composite expressed an amazing electrochemical property with the specific capacitance of 216 F g-1 at a current density of 0.15 A g-1 After over 1000 cycles, the capacitance retention preserved 84.1% of its original value of capacitance

Figure 1.6 The formation mechanism for GO-MnO2 nanocomposites [2]

Zhu and co-workers [56] studied a nanocomposite consisting of wrapped MnO2 prepared by co-assembling and controlling by the electrostatic interactions of negatively charged graphene nanosheets and positively charged MnO2

graphene-nanospheres The obtained composites exhibited an improved specific capacitance (210 F g-1 at 0.5 A g-1), which could be explained due to the synergistic effect of the high conductivity of graphene and a pseudomaterial of MnO2 nanospheres

Figure 1.7 (a) Schematic representations for MnO2 anchoring on graphene through electrostatic attraction, (b,c) TEM image and (d) capacitance retention of

MnO2/graphene [56]

Zhang et al [52] prepared rGO-MnO2 nanocomposites via an electrostatic coprecipitation method At first, rGO was synthesized by reducing GO solution with poly(diallyldimethylammonium chloride) (PDDA), which aimed to transfer the surface charge of rGO from negative charge to positive charge Then, rGO-MnO2

nanocomposites would be achieved by dispersing with negatively charged MnO2

nanosheets The results illustrated improved capacitances than pure rGO and MnO2, and over 89% of initial capacitance was held after 1000 cycles Li and co-workers [58] described a simple procedure to construct a 3D hierarchical rGO/b-MnO2

nanobelt hybrid hydrogel by a hydrothermal reaction Subsequently, the GO precursor solution was prepared for dispersing of as-prepared b-MnO2 nanobelts under 180oC for 12 h The obtained hybrid hydrogel with 54.2% ultrathin b-MnO2

nanobelts achieved a significant specific capacitance of 362 F g-1 at 1.0 A g-1 The outstanding cycling stability was obtained and found to be 93.6% capacitance retention after over 10 000 cycles Preparation of graphene nanosheets/MnO2

nanowires was reported by Cheng and co-workers through the solution-phase assembly of MnO2 nanowires and graphene sheets [44]

1.2.1.3 The other methods

The 3D graphene network with highly electrical conductivity was grown by chemical vapor deposition (CVD) using as a sacrificial template such as a commercial product Ni foams or Cu foils Then, the MnO2 nanoparticles were uniformly

deposited on highly conductive 3D graphene by electrodeposition method He et al

[12] developed a flexible supercapacitor consisting of MnO2-coated flexible and conductive 3D graphene networks The author concluded that the 3D graphene network would show an idea supporter for deposition of MnO2 nanoparticles and can load a large amount of MnO2 up to 9.8 mg cm-2 (nearly got 92.9% the weight of total electrode) To further evaluate its application, the flexible symmetric supercapacitor was fabricated and displayed a high specific capacitance of 130 F g-1 Furthermore, the flexible symmetric supercapacitor revealed an energy and power density of 6.8

W h kg-1 and 62 W kg-1 under the working potential 0-1 V

Figure 1.8 Laser scribing of high-performance and flexible graphene/MnO2-based

electrochemical capacitors [8]

Kaner and co-workers [8] applied a laser scribing method to convert rGO films

to highly electrically conductive graphene, including a highly specific surface area (1520 m2 g−1) After that, the hybrid electrode was obtained by deposition MnO2

microflowers through electrodeposition method into the laser-scribed graphene By exploiting benefits of high surface area and electrical conductivity of graphene and pseudocapacitive mechanism of MnO2 microflowers, a very high capacitance (1145

F g-1) was achieved at a very low mass content of MnO2 (~0.3 mg cm-2) Interestingly,

the mass loading of MnO2 was reached 2 mg cm-2, the capacitance was reduced to

400 F g-1 In other words, when increasing MnO2 contents, the excessive MnO2 will lose contact with graphene and subsequently lessen electrical conductivity

MnO2/graphene-based supercapacitors have been held a great potential to become a rising star for future energy storage systems due to its excellent power and energy density However, developing an effortless, reasonable, eco-friend and extensible way to produce graphene–MnO2 composites still remain a challenge Among these fantastic methods to produce high-quality and large-quantity, the electrochemical method has attracted much attention due to its ease of operation,

single-step, and environmentally friendly A few years ago, Thanh et al [37]

introduced firstly a novel method called plasma-assisted electrochemical exfoliation method The method was based on the employment of two graphite rods as cathode and anode electrodes in KOH 10%, then applying DC power at high voltage and high current into cathode and anode, which resulted in the formation of graphene sheets (the detailed mechanism for this method might be discussed in Chapter 3)

Figure 1.9 (a) Schematic illustration for plasma-assisted electrochemical

exfoliation method, (b) TEM image of graphene sheets and (c) XPS of C1s in

graphene samples [37]

Our group recently has developed this technique and modified the electrolyte solution into metal salt solutions By changing the electrolyte solution, we have successfully prepared and characterized MoS2/graphene composites [6] Various metal oxides such as MnO2, MoS2, MoOx, Co3O4… have been thoroughly studied by our group at Center for High Technology Development, Vietnam Academy of Science and Technology

1.3 Current research in Vietnam

To the best of my knowledge, the publications of MnO2/graphene or MnO2/GO composite are still limited in Vietnam There is only one report by Tuong

et al [25] He synthesized GO/MnO2 composite based on graphene oxide precursor and MnO2 nanoparticles prepared by precipitation method The obtained composite

was further used as an adsorbent to remove heavy metal ions from an aqueous solution such as Pb(II), Cu(II), Ni(II) Their results demonstrated that the GO/MnO2

composite exhibited an excellent potential for adsorption of heavy metal ions with a maximum adsorption capacity of 333.3 mg g-1, 208.3 mg g-1 and 99.0 mg g-1 for Pb(II), Ni(II) and Cu(II), respectively

Figure 1.10 The detailed process of printing supercapacitor electrodes [7]

Several groups in Vietnam have done the preparation of electrode materials

for supercapacitor purposes For instance, Thu et al [34] grew a conductive polymer

chemically with various polypyrrole (PPy) contents on graphene-supported MnFe2O4

hybrids materials Hence, the combination of PPy and graphene-supported MnFe2O4

hybrids could result in enhanced capacitive performance and cycle stability Lu et al

[7] used a novel 3D printing technique to fabricate electrode for supercapacitor The ink suspension was prepared by mixing CNTs and CoFe2O4 nanoparticles in phenol solvent The detailed process can be seen more in Figure 1.10 This technique is up-and-coming to replace a typical method by lower cost and ability to scale up for industrial level A research group in Saigon Hi-Tech Park also developed high-

performance of supercapacitor based on buckypaper/polyaniline composite electrode

by in situ method [27]

Chapter 2 MATERIALS AND METHODS 2.1 Chemicals and reagents

Highly Ordered Pyrolytic Graphite 99.999% (HOPG), Potassium permanganate (KMnO4) and Potassium hydroxide (KOH) were purchased from Sigma-Aldrich DI water was used as a solvent for all experiments

All chemicals and reagents were used directly as received and no further purification was needed

2.2 Preparation of MnO 2 /graphene composites

The solution, containing 150 mL KOH (3%) and 0.0237g KMnO4 (1mM) at a

pH of approximately 12, was used as an electrolyte solution Then, the electrolyte solution was ultra-sonicated for 10 minutes to gain a homogeneous solution Highly Ordered Pyrolytic Graphite (HOPG) rod and Pt electrode were used as cathode and anode electrode, respectively Importantly, the cathode tip head was sharpened to 1

mm in diameter and immersed in an electrolyte solution The distance between the cathode tip and electrolyte solution were approximately 1 mm, while the anode was submerged deeply Two electrodes were connected with DC bias (110V-3A, Kikusui Electronics Corporation, Japan) with an applied maximum voltage of 70 V, which resulted in the generation of “plasma” region and amount of gaseous bubbles on Pt electrode The current density was maintained in a range of 0.4 – 0.6 A Due to the fact that the cathode tip will be worn out by reaction time, which could be a cause of reduced plasma performance Thus, in order to maintain the dipping distance between the HOPG rod and electrolyte, the HOPG rod gradually dipped into a beaker at a rate

of 1mm/min controlled by a scissor lifting system To ensure the homogeneity of the reaction and improve exfoliation process, the beaker was partially placed in ultrasonication bath during the period of reaction The ultrasonic cleaner in this experiment was maintained steadily at 50/60 kHz under the power of 280 W Figure 2.1 was presented the schematic representation of the experimental design

Figure 2.1 The schematic representation of the experimental design

After 60 min reaction, the product (denoted as GM1) was obtained by vacuum filtration through a polyvinylidene fluoride (PVDF) membrane with a pore size of 0.2

µm, then washed with DI water until reaching to neutral pH Finally, the obtained sample was dried at 80oC for 24 hours and stored in a drying box at 25°C For comparison, the electrolyte solution, containing only 150mL KOH (3%) and KMnO4

(0.1, 5 and 10 mM), was used to synthesize graphene/MnO2 composites by the same procedure as described above Also, the graphene sheets were also prepared in the same approach, but the electrolyte without KMnO4 was used Generally, the as-prepared material would achieve 15 mg after 60 minutes

2.3 Preparation of graphene and GM1 electrodes

The working electrodes were fabricated by the following process Firstly, the obtained composites (GM1), carbon nanotubes (CNTs) and polyvinylidene difluoride (PVDF) were mixed in a weight ratio of 80:10:10 and dispersed in N-methyl pyrrolidinone (NMP) solvent To obtain a homogeneous slurry, the mixture solution was sonicated continuously for 6 hours Eventually, the resulting slurry was then