Preparation of manganese dioxide graphene composites by plasma enhanced electrochemical exfoliation process and its electrochemical performance

a Schematic illustration for plasma-assisted electrochemical exfoliation method, b TEM image of graphene sheets and c XPS of C1s in graphene samples [37] .... In this thesis, I have dev

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

LIST OF ABBREVIATIONS

PE3P Plasma-enhanced electrochemical exfoliation process

First of all, I would like to express my sincere gratitude to Dr Phan NgocHong, Center for High Technology Development (HTD), Vietnam Academy ofScience and Technology (VAST), for his extraordinary supervision, support andguidance throughout my research period My dissertation would not have beenpossibly conducted without his valuable advice and constructive comments I wouldlike to express my appreciation to Assoc Prof Masashi Akabori, School ofMaterials 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 fortraining and helping me on characterizations and measurements

I would strongly give my sincere appreciation to Dr Dang Van Thanh, whoalways support and encourage me during all my research and future academiccareers His energetic and enthusiastic attitudes towards research inspire me toovercome the research challenges Additionally, I also acknowledge Dr NguyenTuan Hong for allowing me to use the facilities and providing the best conditionswhen I did experiments I also appreciate Mr Pham Trong Lam and Mr Dang NhatMinh 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 Huongfor being so nice and helping me with all the administrative and academic problems

This thesis is supported by National Foundation for Science and TechnologyDevelopment (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

In this thesis, I have developed a low-cost, simple and one-step approach tosynthesize MnO2/graphene composite with enhanced electrochemical performance.MnO2/graphene composite was prepared via a plasma-assisted electrochemicalexfoliation method with an electrolyte solution made of KMnO4 precursor.MnO2/graphene composites were characterized by SEM, TEM, XRD, Raman, XPS andwere employed for the examination of electrochemical behaviors MnO2/graphenecomposite 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 3000cycles The enhanced electrochemical property might be due to the synergistic effect ofMnO2 nanoparticles and graphene nanosheets With a view to practical applications, asymmetric supercapacitor has been fabricated and delivered the highest specificcapacitance of 130.9 F g-1 at a current density of 2.5 A g-1 In general, this workprovides a new approach to synthesize MnO2/graphene composite for supercapacitorsapplications by one-step, short-time and eco-friendly method

Prosperous development of energy conversion and storage plays a crucialrole in our modern life Electrochemical supercapacitors, which can provide higherenergy density than conventional capacitors and higher power density than batteries/fuel cells, have received significant attention in recent years as a promisingalternative 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 supercapacitorapplications [13, 39, 42, 55] However, MnO2 particles often tend to form bigagglomerates, which noticeably reduce their electrochemical property and lower theefficiency as a result of the undone reaction of MnO2 nanostructures during theelectrochemical reduction-oxidation reaction [36] To overcome this problem,researchers currently have developed a powerful route to strengthen the electrochemicalproperty 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 aswell as the high specific capacity of metal oxides Nevertheless, their works mostlyutilize graphene oxide as a precursor to synthesize graphene and its derivatives viagraphene oxide reduction process Graphene oxide has commonly been produced fromgraphite oxidation process i.e Hummers’ method, which requires hazardous chemicalssuch as strong acid (nitric acid, sulfuric acid) and strong oxidant agents (potassiumpermanganate, potassium chlorate) in order to partially oxidize graphite Notably, theseas-mentioned chemicals are highly toxic and dangerously unstable, which can releasetoxic gases such as NO2, N2O4, and ClO2 Moreover, a large amount of wastewatercontaining acid waste and heavy metal ions has been considered to be risky for theenvironment Also, the present synthesis method is an extremely time-consumingprocess, which needs a few to hundreds of hours for oxidation and removing excessiveacids and KMnO4 after the oxidation step Therefore, an alternative approach forpreparing GO or graphene from a commercial graphite source with the simple, rapidand 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 bymodifying an electrolyte solution Thus, there is a great interest to change theelectrolyte solution to obtain desirable metal oxide and graphene composites

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

MnO2/graphene research

Chapter 1 OVERVIEW1.1 Electrochemical energy storages

Energy is crucial to modern society for sustainable economies in bothdeveloped and developing countries Up to now, fossil fuels are still the majorsources of energy in spite of the increasing environmental pollution problems and

ecological crisis caused by fossil fuels consumption Moreover, with the rapidexpansion of the global economy, increasing environmental pollution worldwide,and the depletion of non-renewable fossil fuels, there has been an increasinglyurgent demand for the development of not only efficient, clean, and sustainablesources of energy, but also high-performance, low-cost, and environmentallyfriendly energy conversion and storage devices Hence, electrochemical energystorages are indispensable and essential for us in order to contribute to new andconstant energy supplements The electrochemical energy storage technology hasbeen 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 relationshipbetween 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 theirpower density is the lowest among these promising devices Similar to the fuelcells, batteries also get high energy density, but the practical applications ofbatteries are still limited due to its low power density and cycle stability Thus,supercapacitors will be a prospective nominee for electrochemical energy storagethat could bring higher energy density and higher power density than conventionalcapacitors and batteries, respectively However, the current limitation of thesupercapacitor is low energy density compared to batteries in actual applications.For instance, carbon-based supercapacitors commonly possess energy density lessthan 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 energydensity, supercapacitors are not able to meet the demand for energy storage devicesfor the next power generation The enhanced energy and power density ofsupercapacitors (electrochemical capacitors) are very crucial to fulfill the higher andhigher requirements of energy storage devices

1.1.1 Supercapacitors

Recently, supercapacitors have drawn significant concern of scientistsparticularly thank to their high-power density, long cycle life and fast charge-discharge processes [31, 43] Supercapacitors preserve an essential position in theRagone plot since they can fill the gap of energy-power density betweenconventional capacitors and batteries With a reasonably high energy density andpower density, supercapacitors have been extensively applied in practicalapplications ranging from portable consumer electronic devices, back-up memorysystems, automotive, to industrial power and energy management, and many more.Dependence on the charge storage mechanisms, the electrochemical supercapacitorsmight be classified into two kinds of supercapacitors: electrical double-layercapacitor (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 theelectrodes 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 wherereversible adsorption of ions from the electrolyte onto the active material Theactive materials will adsorb ions on its surface to form a double layer at theelectrode-electrolyte interface (Figure 1.2a) [24] The EDLCs mechanism iscommonly presented for carbon-based materials due to its high BET surface area.The absence of a redox reaction (non-Faradaic process) allows fast charge/dischargecycles, which produce high power density and long cycle life since there is nomechanical stress caused by changes in the volume of the electrode However, asthe energy storage strongly depends on the surface area of the active material, theyexhibit limited energy density.

Pseudocapacitive electrode materials store charge based on a fast and reversiblesurface oxidation-reduction reaction (Faradaic process) by electron transfer in addition

to the formation of the double layer (Figure 1.2b) [15] Common pseudocapacitivematerials are conducting polymers (e.g polypyrene, polyaniline, polythiophenes) andmetal 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 lowerbecause Faradic processes are slower than electrostatic processes and change in thevolume of the electrode upon cycling (swelling and shrinking) tend to cause mechanicalstress, degrading the materials When electrodes of different nature are used as thecathode (e.g., pseudocapacitive material) and the anode (e.g., capacitive material) thesupercapacitor is called hybrid capacitor

1.2 Electrode materials for supercapacitors

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

materials in meeting the requirements of high-performance supercapacitors eventhough 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 theelectrode surface, resulted in much higher capacitances compared to carbon-basedmaterials alone However, the rapid degradation of capacitance in pseudocapacitivematerials is mostly due to their low conductivity, low structural and chemicalstability [13, 33] By introducing pseudomaterials and carbon materials, it isbelieved that these nanocomposites by taking electrical double layer capacitance andpseudocapacitance can effectively improve the capacitance and energy density ofsupercapcitors without compromising the power density and cycling stability of theresulting supercapcitors

Among electrode materials for pseudocapacitors, MnO2 has been selected as

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

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, excellentelectrochemical stability, large theoretical specific area (up to 2630 m2 g-1) and highelectrical conductivity (104 S cm-1), graphene has been pointed as an attractive materialfor the development of high-performance of supercapacitors [15, 45, 59] Thus, theseexciting properties of graphene and MnO2 can produce a synergistic effect, whichcould overcome their current obstacles, thus enhancing the capacitance and property ofsupercapacitors [12, 32, 47, 53, 54] Until now, there are numerous of literature havebeen carried out to (i) develop a simple, scalable, and reliable method, (ii) prepareMnO2 nanostructure with desired morphology and mass

percentage comparing with graphene, (iii) improve electrical and mechanicalproperty between two components There are two favorable ways for preparingMnO2/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 limiting deposition of nanoscale MnO2 on the surface of graphene under microwaveirradiation In their experiment, they mixed the graphene solution with particularKMnO4 precursor together (Figure 1.3) Then, by taking advantage of microwaveirradiation, the following reaction will occur: $% + 3 + * Û 4 * + -*% + 2

self % 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-1and 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 byrepeatedly carrying out cycle voltammetry tests and showed a very perfectcapacitance 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

8

property indicated that the N-doped composites exhibited a remarkable improvementthan non-doped one Especially, N-doped composites reached specific capacitance of257.1 F g-1 while undoped composite delivered 217 F g-1 at the same current density of0.2 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 anysurfactants, 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

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

hydrothermal process The resulting composites demonstrated fantasticcharacteristics such as high electrical conductivity, high specific surface area andquick 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

low-that GO sheets were synthesized by Hummer’s method and further served as a scaffoldfor 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 alsofractured into Mn4+, Mn3+ or Mn2+ species to form multivalent MnOx nanoparticlesconcurrently [41] The partial reduction of MnO2 nanosheets might lose somevacancies in the MnOx nanoparticles due to the missing oxygen atoms Fortunately, thebonding between rGO and MnOx nanoparticles are believed to occur through oxygen-bridge and thus enhance the conductivity of the rGO/MnOx hybrid The enhancedelectrical conductivity of the hybrid is advantageous; this could lead to delivering aspecific capacitance of 202 F g−1 and exhibited exceptional cycling stability of almost100% 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 andgraphene nanosheets in solution, in which the formation of MnO2 nanostructuresand graphene nanosheets do not depend on the presence of each other The MnO2

nanostructures could be uniformly and firmly anchored with graphene throughphysical electrostatic attraction, where graphene sheets will present as negativelycharged surface and MnO2 nanostructures will present as positively charge surface[39] For example, Chen et al [2] synthesized GO-MnO2 nanocomposites via asimple chemical route in a water-isopropyl alcohol system The formation

mechanism of the nanocomposite was proposed in Figure 1.6 The GO contains alot of oxygen-contain functional groups that act as anchoring sites for creatingbonding between GO and MnO2, and eventually forming GO-MnO2

nanocomposites The author stated that the optimized composite expressed anamazing electrochemical property with the specific capacitance of 216 F g-1 at acurrent density of 0.15 A g-1 After over 1000 cycles, the capacitance retentionpreserved 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 electrostaticinteractions 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 highconductivity 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 ofMnO2/graphene [56]

Zhang et al [52] prepared rGO-MnO2 nanocomposites via an electrostaticcoprecipitation method At first, rGO was synthesized by reducing GO solution withpoly(diallyldimethylammonium chloride) (PDDA), which aimed to transfer thesurface 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 precursorsolution 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 Theoutstanding cycling stability was obtained and found to be 93.6% capacitanceretention after over 10 000 cycles Preparation of graphene nanosheets/MnO2

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

1.2.1.3 The other methods

The 3D graphene network with highly electrical conductivity was grown bychemical vapor deposition (CVD) using as a sacrificial template such as a commercialproduct 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 andconductive 3D graphene networks The author concluded that the 3D graphene networkwould 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 totalelectrode) 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 Byexploiting benefits of high surface area and electrical conductivity of graphene andpseudocapacitive 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 willlose contact with graphene and subsequently lessen electrical conductivity

MnO2/graphene-based supercapacitors have been held a great potential tobecome a rising star for future energy storage systems due to its excellent powerand energy density However, developing an effortless, reasonable, eco-friend andextensible way to produce graphene–MnO2 composites still remain a challenge.Among these fantastic methods to produce high-quality and large-quantity, theelectrochemical 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 exfoliationmethod The method was based on the employment of two graphite rods as cathodeand anode electrodes in KOH 10%, then applying DC power at high voltage andhigh current into cathode and anode, which resulted in the formation of graphenesheets (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 electrolytesolution into metal salt solutions By changing the electrolyte solution, we havesuccessfully prepared and characterized MoS2/graphene composites [6] Variousmetal oxides such as MnO2, MoS2, MoOx, Co3O4… have been thoroughly studied

by our group at Center for High Technology Development, Vietnam Academy ofScience and Technology

1.3 Current research in Vietnam

To the best of my knowledge, the publications of MnO2/graphene orMnO2/GO composite are still limited in Vietnam There is only one report by Tuong

and MnO2 nanoparticles prepared by precipitation method The obtained composite

was further used as an adsorbent to remove heavy metal ions from an aqueoussolution 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 amaximum adsorption capacity of 333.3 mg g-1, 208.3 mg g-1 and 99.0 mg g-1 forPb(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 MnFe2O4hybrids 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 Theink suspension was prepared by mixing CNTs and CoFe2O4 nanoparticles in phenolsolvent 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 industriallevel 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 METHODS2.1 Chemicals and reagents

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

All chemicals and reagents were used directly as received and no furtherpurification 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 electrolytesolution was ultra-sonicated for 10 minutes to gain a homogeneous solution HighlyOrdered Pyrolytic Graphite (HOPG) rod and Pt electrode were used as cathode andanode electrode, respectively Importantly, the cathode tip head was sharpened to 1

mm in diameter and immersed in an electrolyte solution The distance between thecathode tip and electrolyte solution were approximately 1 mm, while the anode wassubmerged deeply Two electrodes were connected with DC bias (110V-3A, KikusuiElectronics Corporation, Japan) with an applied maximum voltage of 70 V, which resulted inthe generation of “plasma” region and amount of gaseous bubbles on Pt electrode Thecurrent density was maintained in a range of 0.4 – 0.6 A Due to the fact that the cathode tipwill 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, theHOPG rod gradually dipped into a beaker at a rate of 1mm/min controlled by a scissor liftingsystem To ensure the homogeneity of the reaction and improve exfoliation process, thebeaker was partially placed in ultrasonication bath during the period of reaction Theultrasonic 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

18

Figure 2.1 The schematic representation of the experimental design.

After 60 min reaction, the product (denoted as GM1) was obtained byvacuum filtration through a polyvinylidene fluoride (PVDF) membrane with a poresize of 0.2 µm, then washed with DI water until reaching to neutral pH Finally, theobtained 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%) andKMnO4 (0.1, 5 and 10 mM), was used to synthesize graphene/MnO2 composites bythe same procedure as described above Also, the graphene sheets were alsoprepared 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, theobtained composites (GM1), carbon nanotubes (CNTs) and polyvinylidene difluoride(PVDF) were mixed in a weight ratio of 80:10:10 and dispersed in N-methylpyrrolidinone (NMP) solvent To obtain a homogeneous slurry, the mixture solutionwas sonicated continuously for 6 hours Eventually, the resulting slurry was then

pasted onto a graphite substrate (1x1 cm) and further dried at 40°C in an oven for48h The mass density of each electrode was approximately 1 mg cm-2 Forcomparison, the graphene electrode was also prepared by using the same procedure

as described above

2.4 Preparation of symmetric supercapacitor (GM1//GM1)

Two identical GM1 electrodes, which is described in Section 2.3, were used

to construct symmetric supercapacitor (SC) A cellulose based-separator would beused for separation of two electrodes Before the assembling process, the twoelectrodes and the cellulose based-separator were immersed in 6 M KOH solution inambient condition for 120 min to ensure the complete wetting in both electrodes.Then, the two electrodes were fabricated in a sandwich-type cell

2.5 Characterizations

The morphology of graphene and GM1 samples were characterized by fieldemission scanning electron microscopy (FESEM, Hitachi S-4800) and transmissionelectron microscopy (TEM, Hitachi 9500) For SEM measurement, the sampleswere dropped on Cu tape For TEM measurement, the samples were dispersed inethanol and then a few drops of each solution were placed on Cu grid Thismeasurement was performed at Institute of Materials Science, Vietnam Academy ofScience and Technology

The structures of the graphene and GM1 samples were characterized byusing a D2 X-ray diffractometer equipped with a Cu Ka tube and a Ni filter (l =0.1542 nm) This measurement was conducted at Institute of Materials Science,Vietnam Academy of Science and Technology

Surface chemical compositions of graphene and GM1 samples wereevaluated by using Raman spectra (LabRam HR Evolution, Laser 532nm) and X-ray photoelectron spectroscopy (XPS, S-Probe TM2803) XPS spectra were fitted

by using Originlab Pro 9.0 software, where a Shirley background was presumed.The Raman and XPS measurements were conducted at Hanoi National University

of Education, Vietnam and Japan Advanced Institute of Science and Technology,Japan, respectively

2.6 Electrochemical analysis

The electrochemical tests of graphene and GM1 electrodes were examined

by using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD)measurements All electrochemical performances were evaluated on a VSP-300multichannel electrochemical workstation (Bio-Logic)

For the three-electrode system, a piece of 1 cm2 of graphene and GM1electrodes were used immediately as the working electrode The counter andreference electrodes were a platinum wire and saturated calomel electrode (SCE),respectively, in a 6 M KOH electrolyte solution The CV measurements werecarried out in a potential range of -0.8 to 0.2 V at various scan rates of 5, 10, 20, 50and 100 mV s-1 The GCD measurements were conducted between -0.8 and 0.2 V atvarious current densities of 2, 5, 10 and 20 A g-1

For the symmetric supercapacitor (GM1//GM1), the GCD measurementswere only conducted in a range of 0 to 1 V at various current densities In addition,the electrochemical parameters were calculated as follows

From the CV curve, the specific capacitance was calculated using thefollowing equation [23]:

∫ 6 8 ( )

=

2 ( ; − =)

is the mass of the active sample (including binder) (g), v is the scan rate (V/s), (E 2 -E 1 ) is the potential window (V),

From the GCD curve, the specific capacitance was calculated using thefollowing equation [23]:

=

D

∆

Where: C is the specific capacitance (F/g), I is the discharge current (A), and

Dt is the discharge time in the potential window (s), m is the mass of the activesample (including binder) (g) and DV is the potential window (V)

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