graphene composites by plasma-enhanced electrochemical exfoliation process and its electrochemical performance

Among them, carbon nanomaterials (e.g. graphene, carbon nanotubes, amorphous…) [19, 29, 45] with different microstructures have been comprehensively explored as electr[r]

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

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

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

Chapter OVERVIEW

1.1 Electrochemical energy storages

1.1.1 Supercapacitors

1.2 Electrode materials for supercapacitors

1.2.1 MnO2/graphene composites

1.2.1.1 Direct oxidation-reduction reaction

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 MATERIALS AND METHODS 18

2.1 Chemicals and reagents 18

2.2 Preparation of MnO2/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 RESULTS AND DISCUSSION 23

3.1 Characterizations of MnO2/graphene composites 23

3.2 The proposed mechanism for PE3P method 29

3.4 Symmetric supercapacitor 35

CONCLUSIONS 39

LIST OF PUBLICATIONS 40

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]

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

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

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

LIST OF TABLES

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

LIST OF ABBREVIATIONS CVD

EDLC

Chemical vapor deposition

Electrochemical double layer capacitor GO rGO SCs CV GCD SEM EDX TEM XRD XPS SCE Graphene oxide

Reduced graphene oxide Supercapacitors

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

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

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

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 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

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), MnO

2 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

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 OVERVIEW 1.1 Electrochemical energy storages

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

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]

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

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 MnO2/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

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

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 MnO

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 g-1

Figure 1.4 Schematic representations of the experimental design of MnO2/rGO

composite [53]

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 MnO x

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 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

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 graphene-wrapped MnO2 prepared by co-assembling and controlling by the electrostatic

interactions of negatively charged graphene nanosheets and positively charged MnO2

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

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-MnO

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

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 MnO

2

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 MnO

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

400 F g-1 In other words, when increasing MnO

2 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

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

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

high-performance of supercapacitor based on buckypaper/polyaniline composite electrode by in situ method [27]

Chapter

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 MnO2/graphene composites

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

Figure 2.1 The schematic representation of the experimental design

After 60 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, 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

pasted onto a graphite substrate (1x1 cm) and further dried at 40°C in an oven for 48h The mass density of each electrode was approximately mg cm-2 For

comparison, 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 be used for separation of two electrodes Before the assembling process, the two electrodes and the cellulose based-separator were immersed in M KOH solution in ambient condition for 120 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 field emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, Hitachi 9500) For SEM measurement, the samples were dropped on Cu tape For TEM measurement, the samples were dispersed in ethanol and then a few drops of each solution were placed on Cu grid This measurement was performed at Institute of Materials Science, Vietnam Academy of Science and Technology

The structures of the graphene and GM1 samples were characterized by using 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

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-300 multichannel electrochemical workstation (Bio-Logic)

For the three-electrode system, a piece of cm2 of graphene and GM1

electrodes were used immediately as the working electrode The counter and reference electrodes were a platinum wire and saturated calomel electrode (SCE), respectively, in a M KOH electrolyte solution The CV measurements were carried out in a potential range of -0.8 to 0.2 V at various scan rates of 5, 10, 20, 50 and 100 mV s-1 The GCD measurements were conducted between -0.8 and 0.2 V at various

current densities of 2, 5, 10 and 20 A g-1

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

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

𝐶 = ∫ 𝐼(𝐸)𝑑𝐸

67 68

2𝑚𝑣(𝐸;− 𝐸=)

Where: C is the specific capacitance (F/g), ∫ 𝐼(𝐸)𝑑𝐸67

68 is the integrated area

in the voltammograms, m is the mass of the active sample (including binder) (g), v is the scan rate (V/s), (E2-E1) is the potential window (V),

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

𝐶 = 𝐼D𝑡 ∆𝑉𝑚

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 active sample (including binder) (g) and DV is the potential window (V)

(2.1)

The energy density (E) and the power density (P) were determined using the following equations [23]:

𝐸 =𝐶 × (∆𝑉)*

2 ×

1000 3600 𝑃 = 𝐸 × 3600

∆𝑡

Where: E is energy density (Wh kg-1), P is power density (W kg–1), and Dt is

the discharge time in the potential window (s) and DV is the potential window (V) (2.3)

Chapter

RESULTS AND DISSCUSSION 3.1 Characterizations of MnO2/graphene composites

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

The morphology of graphene, GM1, GM10 and MnO2 was characterized by

using FE-SEM As evidence in Figure 3.1a, the graphene sheets possessed a layer-by-layer structure Meanwhile, the MnO2 sample showed a self-agglomeration as a

cluster (Figure 3.1d) In term of graphene/MnO2 composites, it is obviously seen that

the GM1 sample also exhibited graphene layers and consisted of MnO2 nanoparticles

decorated on graphene surface uniformly Besides, the low-magnification image of GM10 indicated the graphene sheet was almost covered by MnO2 nanoparticles due

to the higher concentration of KMnO4 precursor Furthermore, a variety of

Table 3.1 To obtain the uniform structure, the optimal concentration for KMnO4 was

chosen at mM for further investigations

Table 3.1 Effect of concentration of KMnO4 on forming MnO2 nanoparticles

Electrolyte

solution Concentration Morphology

KMnO4

0.1 mM Hardly notice MnO2 on surface

1 mM The uniform structure

5 mM MnO2 tended to form moderate cluster

10 mM MnO2 tended to form big cluster

To further confirm the homogeneity of GM1 composite, energy dispersive X-ray (EDX) mapping was taken in SEM The results could be verified that the C, Mn and O elements were homogenously distributed, proving that MnO2 nanoparticles

would be dispersed throughout the entire surface of graphene sheets In contrast, the existence K element might come from the crystal structure of MnO2, in which K

element might be filled in the tunnel of MnO2 nanoparticles [10] In order to

investigate the morphology at higher resolution, TEM analysis was taken out Regarding to Figure 3.3, GM1 sample revealed the MnO2 nanoparticles were

decorated uniformly on graphene sheets, while the TEM image of graphene showed corrugated morphology with few-layer graphene

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

Figure 3.4 Raman spectra of GM1 and graphene

cm-1, which are indexing to D, G and 2D (secondary order of D-band) bands,

respectively [30, 37] Specifically, the G band is well-known for stretching mode of sp2 bonded carbon (C=C), while the D band is recognized as a first-order zone

boundary phonon mode and presented the defects/disorders in graphitic material Thus, low intensity of D band would be indicated a small of defects or disorders of the graphitized structure Additionally, the intensity ratio of D band to G band (ID/IG)

unveils the level of the defects/disorders in the graphene surface Compared to the Raman spectrum of GM1 sample, the new peaks located at 490, 579 and 635 cm-1,

which might be assigned to the Mn-O symmetric stretching vibration and Mn-O stretching vibration in the MnO6 group and basal plane of MnO6, respectively[20,

21] Interestingly, a similar intensity ratio of ID/IG for the obtained MnO2/graphene

composite could be explained that there is no chemical bonding between MnO2 and

graphene We suggest that the interaction of MnO2 and graphene might simply be

Figure 3.5 XRD pattern of graphene and GM1 samples

XRD measurement was a powerful tool for determining the crystal phase Figure 3.5 showed the XRD pattern of GM1 and graphene samples The sharp and prominent peak located at 2q value of 26.6° indicated the reflection of graphene sheets [6, 38]; the other three main peaks centered at 2q values of 12.3°, 37.5° and 65.5° could be attributed to the (001), (110) and (020) crystal phases of the layered structure of monoclinic birnessite-type MnO2 (containing K) [22] However, in the

GM1 pattern, the peak located at 2q value of around 26° was broadened due to the overlapping signal of MnO2 nanoparticles and graphene sheets Moreover, the lattice

constants of a, b, c and b were 5.150 Å, 2.844 Å, 7.159 Å and 100.64 Å, respectively The XRD analysis was similar to the standard pattern of monoclinic birnessite-type MnO2 (JCPDS 43-1317) The d001 value was calculated and found to be 0.73 nm,

Figure 3.6 XPS patterns of GM1, (a) survey, (b) C1s, (c) O1s and (d) Mn2p Figure 3.6a displayed the XPS survey spectra of GM1 sample According to the wide scan XPS spectra of GM1, it is clearly seen the presence of C, O, Mn and K elements in the sample, demonstrating that our sample did not contain any impurities other The deconvolution of the C1s spectrum can be divided into three main peaks at 284.5, 285.3 and 286.3 eV, which are associated with the non-oxygenated and oxygenated carbon components For instance, the peak at 284.5 and 285.3 eV corresponded to C=C (sp2-hybridized carbon atoms) and C–C bonds (sp3-hybridized

C-O/C=O bond at 532.2 eV for oxygen-containing groups in graphene surface [40] Furthermore, the high resolution of Mn2p spectrum showed the binding energies of the Mn2p3/2 and Mn2p1/2 peaks centered at 642.4 eV and 653.8 eV, respectively

Besides, the separation energy of 11.4 eV is strong evidence for the formation of MnO2, which is in good agreement with previous scholarly publication [22] The

above results could suggest that the 2D layered of birnessite-type MnO2 was

successfully formed and uniformly decorated on graphene layers 3.2 The proposed mechanism for PE3P method

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

The proposed mechanism of plasma-assisted electrochemical exfoliation method was illustrated in Figure 3.7, described as follows (i) By applying the bias voltage of 60-70V and current of 0.5-1A, the plasma phenomenon can be immediately observed due to the high electric field being generated at a small tip [37, 38] (ii) In the primary reaction zone at the cathode, H2O will be reduced and ionized

for rapidly producing H2 gas (eq 3.1) and the free radical (OH•, O•…) [37] (iii) These

oxidized edge sites and grain boundaries of graphene sheets to introduce oxygen-containing functional groups, which is in good concurrence with Raman and XPS analysis (v) At the same time, MnO$% ions are reduced to generate MnO

2 material on

graphite cathode (eq 3.2) [35] Then, MnO2 will be anchored on the surface of

graphite cathode to form MnO2 anchored-graphite rod As above described, by taking

advantage of H2 gaseous bubble at the cathode, MnO2 anchored-graphite will be

exfoliated to generate graphene/MnO2 composite and eventually disperse in the

electrolyte solution [6]

Cathode Anode

H2O + 4e- ® H2 + OH- (3.1)

MnO$% + 4H+ + 3e- ® MnO2 + H2O (3.2)

2H2O ® O2 + 4H+ + 4e-

(3.3) 3.3 Electrochemical performance

Based on the above results and thanks to its good electrical contact, low diffusion resistance, easy electrolyte penetration and high surface area GM1 would be expected as an excellent candidate alternating to current material for developing the high-performance supercapacitor So far, the proposed mechanism for charge-discharge of MnO2-based electrodes in electrolyte could be attributed to reversible

intercalation/deintercalation of alkali metal ions such as K+ in the electrode, in which

involved the redox reaction between the III and IV oxidation states of Mn [28, 36] The following equation is presented for this process:

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

Figure 3.8a and b show the CV curves for graphene and GM1 at different scan rates of 2, 5, 10, 20, 50, 100 mV s-1 The CV curve of graphene was fairly rectangular

shape without any redox peaks, suggesting that graphene had ideal capacitive behavior (electrochemical double layer capacitors) Interestingly, the CV profile of GM1 also showed a non-rectangular shape with two peaks, which is attributed to anodic (oxidation) and cathodic (reduction) peaks during the analyses These peaks indicated that the charge storage mechanism is pseudocapacitive Generally, the CV profile of graphene/MnO2 composites often showed a severe distortion at higher scan

Additionally, Figure 3.8c showed the correspondence between the specific capacitance and different scan rates in 6M KOH electrolyte As might be seen in GM1 sample, the highest specific capacitance of 119 F g-1 was achieved at mV s-1 (even

49 F g-1 at 100 mV s-1), which is nearly two times higher than individual one (48 F g -1) at the same scan rate of mV s-1 The specific capacitance decreased with the

increasing scan rate could be explained that there is inadequate time for the electrolyte to insert and remove on the electrode surface at a higher scan rate [26]

Figure 3.9 Charge-discharge curves of (a) graphene and (b) GM1 electrodes in a M KOH electrolyte at a different current density of 2, 5, 10, 20 A g-1

GM1 electrode showed pseudo-capacitor behavior, which also confirmed with CV results of GM1 electrode At the current density of A g-1, the discharge time of the

GM1 electrode was close to 5.5 times longer than that of graphene electrode due to the presence of MnO2 nanoparticles Moreover, the specific capacitances of graphene

and GM1 were calculated based on the discharge times The calculated specific capacitance of GM1 displayed an enhanced specific capacitance of 217 F g-1 while

only 47 F g-1 was observed for the pure graphene By increasing current density, the

specific capacitance will be lessened due to the short time for interaction of diffusion ions and electrodes in the electrolyte [16]

There are several possibilities that could lead to the excellent electrochemical property of graphene/MnO2 composites: (1) the distribution of uniformly

nanoparticles on the graphene layers, which make graphene/MnO2 composites easily

to access to the electrolyte; (2) the appropriate size of MnO2 particles at nanoscale,

which could not only reduce the diffusion path of electrolyte ions but also prevent effective agglomeration of graphene layers; (3) the excellent connection between nanoparticles and graphene layers, resulted in the excellent interfacial interaction and electrical contacts (4) the synergistic effect of MnO2 nanoparticles and graphene

nanosheets in their composites

Figure 3.10 Cycling performances of (a) GM1 and (b) graphene electrodes at a current density of 10 A g-1

As shown in Figure 3.10, the specific capacitance of GM1 electrode retained 80.1% of its original value specific capacitance in 6M KOH electrolyte after 3000 cycles, which is much higher than that of the graphene electrode (56.9% of its initial value after 3000 cycles) in the same aqueous electrolyte The above results would demonstrate that the combination of graphene nanosheets and MnO2 nanoparticles

has notably improved cycling stability when compared with the graphene electrode The comparison of our works with other reports for MnO2-based materials was

Table 3.2 The comparison of some vital parameters with other results

Material Electrolyte

Maximum specific capacitance

(F g-1)

Cycle stability Ref

MnO2/CNT

1M Na2SO4

199 F g-1 at

0.1 A g-1

97% after 20 000

cycles [51]

Sponge@RGO@MnO2

1M Na2SO4

205 F g-1 at

0.1 A g-1

> 90% after 3000

cycles [11]

Birnessite-type MnO2

1M Na2SO4

191 F g-1 at

mV s-1 - [55]

MnO2/CNT

1M Na2SO4

167.5 F g-1 at

77 mA g-1

>88% after 3000

cycles [4]

graphene@NiO-MnO2 6M KOH

242.15 F g-1

at 0.2 mV s-1 - [16]

MnO2 nanowires -

180.0 F g-1 at

1 A g-1

> 99% after 1000

cycles [17]

3D network b-MnO2

1M Na2SO4

180.0 F g-1 at

2 mV s-1

> 99% after 10 000

cycles [57]

MnO2/CNT

1M Na2SO4

325.5 F g-1 at

0.3 A g-1 - [14]

3D graphene/MnO2

network

1.5M Li2SO4

326.33 F g-1

at 200 mV s-1

92% after 1200

cycles [47]

Graphene/MnO2 6M KOH

217.0 F g-1 at

2 A g-1

80% after 3000 cycles

This work - : not available

3.4 Symmetric supercapacitor

In order to investigate the excellent performances of graphene/MnO2

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

GCD tests were also examined with a potential range of 0–1 V at different current densities The charge and discharge curves were fairly symmetrical to each other, suggesting excellent reversibility and good charge propagation between them Furthermore, the specific capacitance of SC was determined by e.q (2.2) based on the discharge curves and was shown in Figure 3.11b The calculated specific capacitance could deliver up to 130.9 F g-1 at a current density of 2.5 A g-1 and 51.66

Figure 3.12 Ragone plot of GM1//GM1 symmetric supercapacitor

The energy density and power density are two critical parameters in determining the property of supercapacitor Their relationship was presented in a Ragone plot Figure 3.12 represented the Ragone plot of GM1//GM1 symmetric supercapacitor and compared its E and P with those for other reported data of some typical MnO2 carbon-based supercapacitors The highest energy density of 18.18 Wh

kg−1 at a power density of 2500 W kg−1 was achieved at a current density of 2.5 A g -1 Furthermore, the energy density remained an acceptable value of 7.175 Wh kg−1

when increasing the power density up to 10 000 W kg−1, which are comparable with

Figure 3.13 Cycle stability of GM1/GM1 symmetric supercapacitor at a current density of A g-1

Similarly, the stability of symmetric supercapacitor is a crucial parameter in order to evaluate the lifetime and chemical stability of SC Thus, GCD tests were conducted to validate these characteristics The cycle stability of GM1/GM1 symmetric supercapacitor was displayed in Figure 3.13 In particular, the capacitance retention rates remained nearly 77% after over 3000 times of charging/discharging cycles During charge/discharge cycles, the rapid intercalation of K+ ions in electrode

caused the mechanical expansion of MnO2, or the dissolution of some parts

(graphene, MnO2) could lead to decrease the specific capacitance of the device

CONCLUSIONS

In this study, we presented a simple, low-cost and one-step method to synthesize graphene/MnO2 composites via plasma-enhanced electrochemical

exfoliation process and its electrochemical performance The morphologies, structures and chemical compositions of the obtained composites were characterized by SEM, EDX, TEM, XRD, Raman and XPS MnO2 nanoparticles distributed

uniformly on graphene sheets in our composite with KMnO4 concentration and

reaction time of mM and hour, respectively The cyclic voltammetry and galvanostatic charge/discharge measurements showed EDLC behavior for graphene and pseudocapacitive behavior for graphene/MnO2 composites According to CV

results, the specific capacitance of graphene and GM1 was calculated and found to be about 119 and 48 F g-1 at mV s-1, respectively After 3000 cycles, the capacitance

retention of graphene/MnO2 composite was remained at 80% of its initial value,

indicating excellent electrochemical stability By symmetric configuration, the packing cell of graphene/MnO2 supercapacitor has been performed in a range of 0-1

V, and the energy density delivered up to 18.18 Wh kg−1, suggesting its potential for

practical application The exceptional electrochemical performance could be assigned to the synergy effect of MnO2 nanoparticles and graphene nanosheets Thus,

graphene/MnO2 composites possibly show a noticeable active material for practical

LIST OF PUBLICATIONS

1 Xuan Linh Ha, Thi Thu Ngo, Quoc Toan Tran, Tra Huong Do, Syamone Somxayasine, Huynh Ky Phuong Ha, Hong-Tham T Nguyen, Thi Kim Chung Nguyen, Tri Khoa Nguyen, Thanh Hai Nguyen, “Fast and effective route for removing of methylene blue from aqueous solution by using red mud-activated graphite composites”, submitted to Journal of Chemistry (accepted)

2 Ha Xuan Son, Pham Van Hao, Hac Van Vinh, Nguyen Thanh Hai, Nguyen Thi Kim Ngan, Dang Nhat Minh, Phan Ngoc Minh, Phan Ngoc Hong, Dang Van Thanh, “Removal of arsenic from water using crumpled graphite oxide”, Green Processing and Synthesis, 2018, 7, 404-408

3 Chien Nguyen Van, Nguyen Thanh Hai, Jiri Olejnicek, Petra Ksirova, Michal Kohout, Michaela Dvorakova, Pham Van Hao, Phan Ngoc Hong, Manh Cuong Tran, Do Hoang Tung, Dang Van Thanh, “Preparation and photoelectrochemical performance of porous TiO2/graphene nanocomposite films”, Materials Letters, 2018, 213, 109–118

4 Lưu Việt Hùng, Nguyễn Thanh Hải, Nguyễn Thành Trung, Đỗ Trà Hương, Nguyễn Phương Chi, Nghiêm Thị Hương, Đặng Văn Thành “Hấp phụ Mn(II) môi trường nước sử dụng nano bentonit chế tạo phương pháp hoạt hóa có hỗ trợ siêu âm”, Tạp chí Hóa học, tập 56, số 3e, 2018, tr 27-32

4 Nguyen Thanh Hai, Ha Xuan Linh, Nguyen Thi Thuy, Nguyen Nhat Huy, Le Phuoc-Anh, Phung Thi Oanh and Dang Van Thanh, “Electrochemical activation of graphite using red mud slurry for enhancing electrochemical performances”, Vietnam- Japan Science and Technology Symposium 2019 (VJST2019), Ha Noi, Vietnam, May 4th 2019

6 Nguyen Thanh Hai, Dang Nhat Minh, Tran Viet Thu, Le Trong Lu, Nguyen Tuan Hong, Phan Ngoc Minh and Phan Ngoc Hong, “Preparation of MnO2/graphene composites by plasma-enhanced electrochemical exfoliation method and its electrochemical properties”, The 9th International Workshop on Advanced Materials Science and Nanotechnology, IWAMSN 2018 Ninh Binh, Vietnam, November 7-11 2018

REFERENCES

[1] An, G., Yu, P., Xiao, M., Liu, Z., Miao, Z., Ding, K., & Mao, L (2008) Low-temperature synthesis of Mn(3)O(4) nanoparticles loaded on multi-walled carbon nanotubes and their application in electrochemical capacitors

Nanotechnology, 19(27), 275709 doi:10.1088/0957-4484/19/27/275709

[2] Chen, S., Zhu, J., Wu, X., Han, Q., & Wang, X (2010) Graphene oxide MnO2 nanocomposites for supercapacitors ACS nano, 4(5), 2822-2830 doi:10.1021/nn901311t

[3] Cheng, Y., Lu, S., Zhang, H., Varanasi, C V., & Liu, J (2012) Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for high-performance supercapacitors Nano letters, 12(8), 4206-4211 doi:10.1021/nl301804c

[4] Chou, S.-L., Wang, J.-Z., Chew, S.-Y., Liu, H.-K., & Dou, S.-X (2008) Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors Electrochemistry

communications, 10(11), 1724-1727 doi:10.1016/j.elecom.2008.08.051

[5] Dai, K., Lu, L., Liang, C., Dai, J., Liu, Q., Zhang, Y., Zhu, G., & Liu, Z (2014) In situ assembly of MnO2 nanowires/graphene oxide nanosheets composite with high specific capacitance Electrochimica Acta, 116, 111-117 doi:10.1016/j.electacta.2013.11.036

[6] Dang, M N., Ung, T D T., Phan, H N., Truong, Q D., Bui, T H., Phan, M N., Nguyen, L Q., & Tran, P D (2017) A novel method for preparation of molybdenum disulfide/graphene composite Materials Letters, 194, 145-148 doi:10.1016/j.matlet.2017.02.018

[7] Dung, H T., Dung, N T., Dung, T Q., Tung, D T., Yen, N T., Tam, L T T., Thu, T V., Hong, P N., & Lu, L T (2018) Fabrication and characterization of supercapacitor electrode by 3D printing Vietnam Journal

[8] El-Kady, M F., Strong, V., Dubin, S., & Kaner, R B (2012) Laser scribing of high-performance and flexible graphene-based electrochemical capacitors

Science, 335(6074), 1326-1330 doi:10.1126/science.1216744

[9] Fan, Z., Chen, J., Wang, M., Cui, K., Zhou, H., & Kuang, Y (2006) Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials Diamond and related materials, 15(9), 1478-1483 doi:10.1016/j.diamond.2005.11.009

[10] Gao, T., Glerup, M., Krumeich, F., Nesper, R., Fjellvåg, H., & Norby, P (2008) Microstructures and spectroscopic properties of cryptomelane-type manganese dioxide nanofibers The Journal of Physical Chemistry C, 112(34), 13134-13140 doi:10.1021/jp804924f

[11] Ge, J., Yao, H.-B., Hu, W., Yu, X.-F., Yan, Y.-X., Mao, L.-B., Li, H.-H., Li, S.-S., & Yu, S.-H (2013) Facile dip coating processed graphene/MnO2 nanostructured sponges as high performance supercapacitor electrodes Nano

Energy, 2(4), 505-513 doi:10.1016/j.nanoen.2012.12.002

[12] He, Y., Chen, W., Li, X., Zhang, Z., Fu, J., Zhao, C., & Xie, E (2012) Freestanding three-dimensional graphene/MnO2 composite networks as ultralight and flexible supercapacitor electrodes ACS nano, 7(1), 174-182 doi:10.1021/nn304833s

[13] Huang, M., Li, F., Dong, F., Zhang, Y X., & Zhang, L L (2015) MnO 2-based nanostructures for high-performance supercapacitors Journal of

Materials Chemistry A, 3(43), 21380-21423 doi:10.1039/C5TA05523G

[14] Huang, M., Mi, R., Liu, H., Li, F., Zhao, X L., Zhang, W., He, S X., & Zhang, Y X (2014) Layered manganese oxides-decorated and nickel foam-supported carbon nanotubes as advanced binder-free supercapacitor electrodes Journal of Power Sources, 269, 760-767 doi:10.1016/j.jpowsour.2014.07.031

[16] Hwang, S.-G., Hong, J.-E., Kim, G.-O., Jeong, H M., & Ryu, K.-S (2013) Graphene anchored with NiO-MnO2 nanocomposites for use as an electrode material in supercapacitors ECS Solid State Letters, 2(1), M8-M11 doi:10.1149/2.010301ssl

[17] Jiang, H., Zhao, T., Ma, J., Yan, C., & Li, C (2011) Ultrafine manganese dioxide nanowire network for high-performance supercapacitors Chemical

Communications, 47(4), 1264-1266 doi:10.1039/C0CC04134C

[18] Jost, K., Dion, G., & Gogotsi, Y (2014) Textile energy storage in perspective

Journal of Materials Chemistry A, 2(28), 10776-10787 doi:10.1039/C4TA00203B

[19] Jost, K., Stenger, D., Perez, C R., McDonough, J K., Lian, K., Gogotsi, Y., & Dion, G (2013) Knitted and screen printed carbon-fiber supercapacitors for applications in wearable electronics Energy & Environmental Science,

6(9), 2698-2705 doi:10.1039/C3EE40515J

[20] Julien, C., Massot, M., Baddour-Hadjean, R., Franger, S., Bach, S., & Pereira-Ramos, J (2003) Raman spectra of birnessite manganese dioxides Solid State

Ionics, 159(3-4), 345-356 doi:10.1016/S0167-2738(03)00035-3

[21] Julien, C., Massot, M., & Poinsignon, C (2004) Lattice vibrations of manganese oxides: Part I Periodic structures Spectrochimica Acta Part A:

Molecular and Biomolecular Spectroscopy, 60(3), 689-700 doi:10.1016/S1386-1425(03)00279-8

[22] Ko, W.-Y., Chen, Y.-F., Lu, K.-M., & Lin, K.-J (2016) Porous honeycomb structures formed from interconnected MnO sheets on CNT-coated substrates for flexible all-solid-state supercapacitors Scientific Reports, 6, 18887 doi:10.1038/srep18887

[23] Le, P.-A., Nguyen, V.-T., Yen, P.-J., Tseng, T.-Y., & Wei, K.-H (2019) A new redox phloroglucinol additive incorporated gel polymer electrolyte for flexible symmetrical solid-state supercapacitors Sustainable Energy & Fuels,

[24] Long, J W., Bélanger, D., Brousse, T., Sugimoto, W., Sassin, M B., & Crosnier, O (2011) Asymmetric electrochemical capacitors—Stretching the limits of aqueous electrolytes Mrs Bulletin, 36(7), 513-522 doi:10.1557/mrs.2011.137

[25] Mạnh Tường, N., Thị Chi, H., Đình Trinh, T., & Văn Nội, N Nghiên cứu khả hấp phụ số kim loại nặng nước vật liệu nano compsite GO/MnO2 Tạp chí Phân tích Hóa, Lý Sinh học, 22(3),

[26] Mujawar, S H., Ambade, S B., Battumur, T., Ambade, R B., & Lee, S.-H (2011) Electropolymerization of polyaniline on titanium oxide nanotubes for supercapacitor application Electrochimica Acta, 56(12), 4462-4466 doi:10.1016/j.electacta.2011.02.043

[27] Ngo, L T., Tran, T P., Ngo, V K T., & Do, Q H (2019) High performance supercapacitor based on thick buckypaper/polyaniline composite electrodes

Advances in Natural Sciences: Nanoscience and Nanotechnology, 10(2),

025004 doi:10.1088/2043-6254/ab1b77

[28] 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 doi:10.1149/1.1393216

[29] Park, S., An, J., Suk, J W., & Ruoff, R S (2010) Graphene‐based actuators

Small, 6(2), 210-212 doi:10.1002/smll.200901877

[30] Parvez, K., Wu, Z.-S., Li, R., Liu, X., Graf, R., Feng, X., & Müllen, K (2014) Exfoliation of graphite into graphene in aqueous solutions of inorganic salts

Journal of the American Chemical Society, 136(16), 6083-6091

doi:10.1021/ja5017156

[32] Peng, L., Peng, X., Liu, B., Wu, C., Xie, Y., & Yu, G (2013) Ultrathin two-dimensional MnO2/graphene hybrid nanostructures for high-performance, flexible planar supercapacitors Nano letters, 13(5), 2151-2157 doi:10.1021/nl400600x

[33] Simon, P., & Gogotsi, Y (2010) Materials for electrochemical capacitors

Nanoscience And Technology: A Collection of Reviews from Nature Journals,

320-329 doi:10.1038/nmat2297

[34] Thu, T V., Van Nguyen, T., Le, X D., Le, T S., Van Thuy, V., Huy, T Q., & Truong, Q D (2019) Graphene-MnFe2O4-polypyrrole ternary hybrids with synergistic effect for supercapacitor electrode Electrochimica Acta, 314, 151-160 doi:10.1016/j.electacta.2019.05.042

[35] Tigor, A R., Sakiynah, N m., & Setyawan, H (2014) A versatile

electrochemical method to produce nanoparticles of manganese oxides by KMnO4 electrolysis Paper presented at the AIP Conference Proceedings

doi:10.1063/1.4866744

[36] Toupin, M., Brousse, T., & Bélanger, D (2004) Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor Chemistry of

Materials, 16(16), 3184-3190 doi:10.1021/cm049649j

[37] Van Thanh, D., Li, L.-J., Chu, C.-W., Yen, P.-J., & Wei, K.-H (2014) Plasma-assisted electrochemical exfoliation of graphite for rapid production of graphene sheets RSC Advances, 4(14), 6946-6949 doi:10.1039/C3RA46807K

[38] Van Thanh, D., Oanh, P P., & Le, P H (2017) Ultrasonic-assisted cathodic electrochemical discharge for graphene synthesis Ultrasonics sonochemistry,

34, 978-983 doi:10.1016/j.ultsonch.2016.07.025

[39] Wang, J.-G., Kang, F., & Wei, B (2015) Engineering of MnO2-based nanocomposites for high-performance supercapacitors Progress in Materials

Science, 74, 51-124 doi:10.1016/j.pmatsci.2015.04.003

high-performance supercapacitors Journal of Materials Chemistry, 22(33), 16943-16949 doi:10.1039/C2JM33364C

[41] Wang, Y., Lai, W., Wang, N., Jiang, Z., Wang, X., Zou, P., Lin, Z., Fan, H J., Kang, F., & Wong, C.-P (2017) A reduced graphene oxide/mixed-valence manganese oxide composite electrode for tailorable and surface mountable supercapacitors with high capacitance and super-long life Energy &

Environmental Science, 10(4), 941-949 doi:10.1039/C6EE03773A

[42] 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 doi:10.1039/C0CS00127A

[43] Winter, M., & Brodd, R J (2004) What are batteries, fuel cells, and supercapacitors? , 104(10), 4245-4270 doi:10.1021/cr020730k

[44] Wu, Z.-S., Ren, W., Wang, D.-W., Li, F., Liu, B., & Cheng, H.-M (2010) High-energy MnO2 nanowire/graphene and graphene asymmetric electrochemical capacitors ACS nano, 4(10), 5835-5842 doi:10.1021/nn101754k

[45] Wu, Z S., Parvez, K., Feng, X., & Müllen, K (2013) Graphene-based in-plane micro-supercapacitors with high power and energy densities Nature

communications, 4, 2487 doi:10.1038/ncomms3487

[46] Xie, X., & Gao, L (2007) Characterization of a manganese dioxide/carbon nanotube composite fabricated using an in situ coating method Carbon,

45(12), 2365-2373 doi:10.1016/j.carbon.2007.07.014

[47] Xiong, C., Li, T., Khan, M., Li, H., & Zhao, T (2015) A three-dimensional MnO 2/graphene hybrid as a binder-free supercapacitor electrode RSC

Advances, 5(104), 85613-85619 doi:10.1039/C5RA14411F

[49] Yang, S., Song, X., Zhang, P., & Gao, L (2013) Facile synthesis of nitrogen-doped graphene–ultrathin MnO2 sheet composites and their electrochemical performances ACS applied materials & interfaces, 5(8), 3317-3322 doi:10.1021/am400385g

[50] Ye, K.-H., Liu, Z.-Q., Xu, C.-W., Li, N., Chen, Y.-B., & Su, Y.-Z (2013) MnO2/reduced graphene oxide composite as high-performance electrode for flexible supercapacitors Inorganic Chemistry Communications, 30, 1-4 doi:10.1016/j.inoche.2012.12.035

[51] Zhang, H., Cao, G., Wang, Z., Yang, Y., Shi, Z., & Gu, Z (2008) Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube arrays for high-rate electrochemical capacitive energy storage Nano letters, 8(9), 2664-2668 doi:10.1021/nl800925j

[52] 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 doi:10.1021/jp200724h

[53] Zhang, Q., Wu, X., Zhang, Q., Yang, F., Dong, H., Sui, J., & Dong, L (2019) One-step hydrothermal synthesis of MnO2/graphene composite for electrochemical energy storage Journal of Electroanalytical Chemistry, 837, 108-115 doi:10.1016/j.jelechem.2019.02.031

[54] Zheng, Y., Pann, W., Zhengn, D., & Sun, C (2016) Fabrication of functionalized graphene-based MnO2 nanoflower through electrodeposition for high-performance supercapacitor electrodes Journal of the

Electrochemical Society, 163(6), D230-D238 doi:10.1149/2.0341606jes

[55] Zhou, J., Yu, L., Sun, M., Yang, S., Ye, F., He, J., & Hao, Z (2013) Novel synthesis of birnessite-type MnO2 nanostructure for water treatment and electrochemical capacitor Industrial & Engineering Chemistry Research,

[56] Zhu, J., & He, J (2012) Facile synthesis of graphene-wrapped honeycomb MnO2 nanospheres and their application in supercapacitors ACS applied

materials & interfaces, 4(3), 1770-1776 doi:10.1021/am3000165

[57] Zhu, J.-J., Yu, L.-L., & Zhao, J.-T (2014) 3D network mesoporous beta-manganese dioxide: Template-free synthesis and supercapacitive performance Journal of Power Sources, 270, 411-417 doi:10.1016/j.jpowsour.2014.07.093

[58] Zhu, S., Zhang, H., Chen, P., Nie, L.-H., Li, C.-H., & Li, S.-K (2015) Self-assembled three-dimensional hierarchical graphene hybrid hydrogels with ultrathin β-MnO2 nanobelts for high performance supercapacitors Journal of

Materials Chemistry A, 3(4), 1540-1548 doi:10.1039/C4TA04921G