Nghiên cứu tổng hợp và khảo sát tính chất của hệ vật liệu 2 chiều mos2rgo ứng dụng làm điện cực trong siêu tụ điện

Nowadays, many 2D composite materials are strongly attracted to research for the electrode materials of the supercapacitors because of their very unique structures, high electric conduct

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

-

Ngo Quang Minh

SYNTHESIS AND PROPERTIES OF 2D MATERIALS MOS2/GRAPHENE APPLIED FOR ELECTRODES IN

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

Độc lập – Tự do – Hạnh phúc

BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ

Họ và tên tác giả luận văn: Ngô Quang Minh

Đề tài luận văn: Nghiên cứu tổng hợp và khảo sát tính chất của vật

liệu 2 chiều MoS2/graphen ứng dụng làm điện cực trong siêu tụ điện

Chuyên ngành: Khoa học và kỹ thuật vật liệu điện tử

Mã số HV: CB160069

Tác giả, Người hướng dẫn khoa học và Hội đồng chấm luận văn xác nhận tác giả đã sửa chữa, bổ sung luận văn theo biên bản họp Hội đồng ngày 30/10/2018 với các nội dung sau:

 Tiêu đề chương để chữ in hoa, giữa dòng

 Sắp xếp lại phần chữ và hình để thu hẹp các khoảng giấy trống nhiều, dẫn tới số trang luận văn có sự thay đổi

 Tăng độ phân giải các hình ảnh bị mờ, 3.5; 3.6; 3.7; 3.8; 3.9; 3.10

 Đã chỉnh sửa lỗi về mô tả tiêu đề hình 3.11

Ngày 5 tháng 11 năm 2018

CHỦ TỊCH HỘI ĐỒNG

STATEMENT OF ORIGINAL AUTHORSHIP

I hereby declare that the results presented in the thesis are performed by the author The research contained in this thesis has not been previously submitted to meet requirements for an award at this or any higher education institutions

Hanoi, 30/9/2018 Signature

LIST OF PUBLICATIONS Ngo Quang Minh, Chu Manh Hung, Dang Thi Thanh Le, Nguyen Duc Hoa* and Nguyen Van Hieu (2018), “Synthesis and characterization of MoS2/rGO

nanocomposite for supercapacitor applications”, The 9th International Workshop on

Advanced Materials Science and Nanotechnology (IWAMSN 2018), Ninh Binh,

Vietnam (Submitted)

ACKNOWLEDGEMENT 1

LIST OF TABLES 2

LIST OF ABBREVIATIONS 3

MOTIVATION FOR RESEARCHING 8

CHAPTER 1 INTRODUCTION 9

1.1 DOUBLE LAYERS MODEL 9

1.1.1 H ELMHOLTZ MODEL 9

1.1.2 G OUY -C HAPMAN MODEL 9

1.1.3 S TERN AND G RAHAME MODEL 10

1.1.4 C URRENT MODEL 11

1.2 BACKGROUND OF SUPERCAPACITOR 12

1.2.1 S EPARATOR 13

1.2.2 E LECTROLYTIC SOLUTIONS 14

1.2.3 E NERGY DENSITY AND POWER DENSITY 16

1.3 CLASSIFICATION OF SUPERCAPACITORS 19

1.3.1 E LECTRIC D OUBLE L AYER C APACITOR 20

1.3.2 P SEUDO - CAPACITOR 21

1.3.3 H YBRID SUPERCAPACITOR 23

1.4 ELECTRODE MATERIALS 24

1.4.1 P RINCIPLE , CLASSIFICATION AND RECENT DEVELOPMENT 24

1.4.2 M O S 2 / R GO- BASED ELECTRODE MATERIALS OF SUPERCAPACITOR 26

1.4.3 R ESEARCH QUESTION 30

1.5 OBJECTIVE RESEARCH AND OUTLINE OF THESIS 31

1.5.1 O BJECTIVE RESEARCH 31

1.5.2 O UTLINE OF THESIS 32

CHAPTER 2: EXPERIMENTAL SECTION 33

2.1 MATERIALS, EQUIPMENT AND STEPS OF PREPARING MATERIALS FOR ELECTRODE FABRICATION 33

2.1.1 M ATERIALS 33

2.1.2 P REPARATION OF MATERIALS POWDER FOR ELECTRODE FABRICATION 33

2.2 PREPARATION OF ELECTRODE AND ELECTROLYTIC SOLUTIONS 36

2.2.1 P REPARATION OF ELECTRODE 36

2.2.2 P REPARATION OF ELECTROLYTIC SOLUTIONS 36

2.3 METHODOLOGY OF STRUCTURAL CHARACTERIZATION AND CHEMICAL PROPERTIES ANALYSIS 36

CHAPTER 3: RESULTS AND DISCUSSION 38

3.1 MOS2 RESULTS 38

3.2 COMPOSITE MOS2/RGO RESULTS 42

3.2.1 C RYSTAL STRUCTURE , MORPHOLOGICAL PROPERTIES AND CV RESULTS 42

3.2.3 EIS ANALYSIS OF M O S 2 / R GO WITH MASS RATIO 1:3 55

REFERENCES 58

1

ACKNOWLEDGEMENT

First of all, I would like to express my greatest gratitude to my supervisor, Associate professor PhD Nguyen Duc Hoa for his friendliness, patience, and great support in the whole period of time doing the master thesis at ITIMS (International Training Institute for Materials Science), HUST (Hanoi university of Science and Technology) Without his dedicated guidance and encouragement, I might not be able to complete all of the work throughout this master thesis

Secondly, I would like to give my gratitude to other members in my group-iSensors, Associate professor PhD Nguyen Van Duy, PhD Chu Manh Hung, PhD Dang Thanh Le for giving me a lots support and encouragement throughout the period of doing my master thesis I also want to thank Assoc Prof., PhD Truong Thi Ngoc Lien (at Engineering Physics Department, HUST) for her great support in Electrochemical Imedance Spectroscopy measuring Moreover, I am also very grateful to my colleagues, PhD students: Nguyen Van Hoang, Nguyen Xuan Thai for supporting me a lot in experimental work I especially would like to show my gratitude to my colleagues, my classmates such as Miss Hong, Miss Phuoc, Mr Vuong and Mr Phu, Mr Son, who are always by my side for giving me a plenty of supports and advice in two years doing my master thesis at ITIMS

Last but not least, it is my family: my parents, my sibling elder sister, my grandparents, my uncles, my aunts, my cousins and my lover Thank you all a lot Without all of you, I could not go on such an easy way to complete another part of

my life

This research was partially funded by the Vietnam National Foundation for Science and Technology Development (Code: 103.02-2017.15)

2

LIST OF TABLES

3

LIST OF ABBREVIATIONS

Cdiff Capacitance of diffusive layer 17

4

5

LIST OF FIGURES

Figure 1 1 Models of the electrical double-layer at a positively charged surface: (a) the Helmholtz model, (b) the Gouy–Chapman model, and (c) the Stern model showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) ψ0 and

ψ are the potentials at the electrode surface and the electrode/electrolyte interface, respectively [13] 11 Figure 1 2 A double layer model including layers of solvent [15] 12 Figure 1 3 (a) Principal setup of an EDLC with porous carbon electrodes on current collectors separated by an ion conducting electrolyte (b), (c) and (d) Construction of a spirally would EDLC, the assembled device with 2600F in its housing and a flat 5F coin device [19] 13 Figure 1 4 Classification of electrolytes [30] 15 Figure 1 5 Diagram of effects of range of working temperature on electrolyte in some ways [12] 16 Figure 1 6 Ragone plot shows a comparison of some main types of energy storage devices in term of power density and energy density [19, 37] 18 Figure 1 7 Classification of supercapacitor based on electrode materials [39] 19 Figure 1 8 A schematic diagram of EDLCs and description of potential change through interface of electrode/electrolytic solutions when applied an external voltage [41] 20 Figure 1 9 Different types of pseudocapacitive behavior from B.E Conway: (a) under potential deposition, (b) redox pseudocapacitance, and (c) intercalation pseudocapacitance [13, 42] 22 Figure 1 10 An illustrative example of hybrid supercapacitor [30] 23

6

Figure 2 2 (A) Schemetic of the CV (three electrodes system) configuration, (B) real image of (A)-system 37

in 24h depends on temperature treatments 39

different reaction-time 24h, 36h and 48h 40 Figure 3 4 Reaction time (24h, 36h and 48h) dependence of specific capacitance

Figure 3 5 Raman spectra of: a) Graphite; b) Graphite oxide (GO) 42 Figure 3 6 a) X-ray diagram of rGO; b) SEM image of rGO; c) CV curve of rGO-based electrode 43 Figure 3 7 a) X-ray diagram of the nanocomposites MoS2/rGO 1:3; b) SEM image

of the nanocomposites MoS2/rGO 1:3; c) CV curve of the nanocomposites MoS2/rGO 1:3 -based electrode 44 Figure 3 8 a) Raman spectrum of the nanocomposites MoS2/rGO 1:1, b) SEM image of the nanocomposites MoS2/rGO 1:1, c) CV curve of the nanocomposites MoS2/rGO 1:1 -based electrode 45

the MoS2/rGO 3:1 composite c) CV curve of the MoS2/rGO 3:1 composite - based electrode 47

7

8

MOTIVATION FOR RESEARCHING

In the development of human civilization, energy is always one of the most burning topics because of its profound and comprehensive impacts on almost every aspects

of life such as science, medical, education and a lot of others It is completely true that energy crisis is a real big challenge in this century because human is facing the limitation of non-renewable fossil fuels as coal, gas, oil [1, 2] Besides, emissions from those sources are attributed to causing many negative problems to the environment, especially climate change, an urgently global issue, and greenhouse gas [41] That urges researchers and scientists attempt to develop renewable and clean energy sources for purpose of sustainable development parallel to friendliness with the environment Transportation is one of the pioneer sectors to apply the advances of renewable energy with hybrid vehicles manufactured [4–6] Solar energy [31] is more and more popular as a source of generating electricity In addition, enhancing storage and conversion energy capacity of materials and devices is also attracted huge research from many researchers Nevertheless, wind and solar energy is not kind of consistent sources creating energy in all cases Hybrid or electric vehicles would not be popular with all people without enhancing energy storage capacity systems, reducing the time of charging/discharging compared to batteries performance Fortunately, there is a very suitable solution namely “supercapacitor” [17], which is considered as the key factor to address issues mentioned in energy systems for spreading applications in real life

Nowadays, many 2D composite materials are strongly attracted to research for the electrode materials of the supercapacitors because of their very unique structures, high electric conductivity, high surface area, high stability and friendly with the

2D family, which have been focused on developing for the electrode materials in

composites is strongly dependent on their composition, geometry, surface area, and etc Herein, we dedicate on the utilizing this material for supercapacitor

9

CHAPTER 1 INTRODUCTION 1.1 Double layers model

1.1.1 Helmholtz model

In 1853, Helmholtz was the first scientist setting up the idea of an electric double layer There is an electrostatic charge separation at the interface between the solid electrode and electrolyte solutions when applying a voltage, figure 1.1 (a) Charges (electrons or ions) are not allowed to transfer reversibly from the solid electrode to electrolyte solutions [9, 10] This model is similar to a conventional capacitor and therefore, Helmholtz capacitance of the double layer can be calculated through the followed equation

C =

d

A

o .

It is true that the Helmholtz model obtains better results when applied for a high concentration of electrolyte solutions In practical, most supercapacitor is utilized with concentrated ion solutions That is the reason why this model is still being used

in some simple calculation

However, it is also true that the Helmholtz model about electric double layer does not consider to an effect of diffusion of other ions in the electrolyte solution to the first layer absorbed before Therefore, the obtained results in capacitance followed the model is not completely matched with the practical phenomenon

Therefore, scientists continued to develop other models for enhancing the accuracy compared to the real electrochemical process

1.1.2 Gouy-Chapman model

Georges Gouy discovered that capacitance of electric double layer depended on the applied voltage and random thermal motion Consequently, the concentration of reversible charge ions declined when going deeper (from electrode) into the

10

solutions until getting the same value [3], figure 1.1 (b) Chapman, in 1913, used both Poisson's equation and Boltzmann distribution relationship to formulate mathematically the diffusion layer and eventually came up with followed equation [3]:

2 ( 2

T o T

o

u

z ch u

qn

(2)

concentration at thermodynamics equilibrium, Boltzmann constant and the absolute temperature, respectively

This model obtains better results compared to the Helmholtz model Nevertheless, considering ions to be point charges causes another problem that they could infinitely close to the interface of the solid electrode It is obviously unpractical in the experiment that why a more precise model needs to be developed

1.1.3 Stern and Grahame model

By collaborating both Helmholtz and Gouy-Chapman models, Stern has developed and shown a new counterpart in 1923 The main point was that he assumed ions had

a finite size, which tackled them forward infinitely close to the interface of the electrode, figure 1.1 (c) In addition, this model separated the double layers into two parts: a uniform part of ions layer close to the electrode (Stern layer) and another part formed by diffusion of ions into electrolytic solutions forward to the electrode [58]

Stern’s theory is mathematically described in the following equation, which shows that capacitance of the solid electrode depends on two factors [14]:

11

d

C C

11

1  

(4)

In 1947, Grahame implemented a specific adsorption of ions into the Stern’s model for more matching with results in experiments After that, Stern-Chapmann consisted of three regions: the Inner Helmholtz (IHP), the Outer Helmholtz (OHP) and the diffuse layer [60]

Figure 1 1 Models of the electrical double-layer at a positively charged surface:

(a) the Helmholtz model, (b) the Gouy–Chapman model, and (c) the Stern model showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) ψ0 and

ψ are the potentials at the electrode surface and the electrode/electrolyte interface,

respectively [14]

1.1.4 Current model

Bockris, Devanathan and Muller suggested the BDM model, which is the most accurate model applied for electric double layer (EDL) nowadays, which is an

12

upgrade version of Stern-Chapmann model with an implementation of solvents molecules [43] Considering water as solvents molecules, this model proposed that some of them were absorbed within the inner Helmholtz plane of the electrode surface Formation of the dipole of water molecules depends on their distance to the electrode An alignment of the dipole is fixed due to interaction with the electronics charges of the electrode Other layer water molecules would follow the first one without being fixed their dipole alignment The model is presented in the Figure 1.2

Figure 1 2 A double layer model including layers of solvent [43]

1.2 Background of supercapacitor

Supercapacitors, which also called ultracapacitors, is fundamentally electrochemical

13

capacitors, have attracted plenty of research interest in several decades up to now because of their excellent cycle life stability, higher power density (rapidly charge/discharge) than batteries and higher energy density compared to conventional capacitors [15–18] A simplest electrochemical capacitor constructed

by a pair of solid electrodes connect directly to current collectors, electrolytic solutions, and a separator which prevents short circuit happens The more detailed information of each part is discussed in below sections The Figure 1.3 depicts a fundamental structure of supercapacitors in term of theoretical modeling (with ELD phenomena presented) and practical appearance

Figure 1 3 (a) Principal setup of an EDLC with porous carbon electrodes on

current collectors separated by an ion conducting electrolyte (b), (c) and (d) Construction of a spirally would EDLC, the assembled device with 2600F in its

housing and a flat 5F coin device [53]

1.2.1 Separator

It is very necessary to have a separator in an electrochemical capacitor in order to prevent a short circuit between the positive and negative electrode Therefore,

(a )

14

materials of separators need to get high electrical resistance On the other hand, it is required that ions in the electrolytic solutions can be still permeable through it easily in charge/discharge processes That means the separator not only have a conductivity tailored with ionic transference but also is porous enough to permit ions to go through it

Many studies reported were concentrated on this issue to figure out suitable materials to prepare separator in the supercapacitors [20–27] Actually, separator strongly depends on what kind of electrolytic solutions was utilized For organics electrolyte, polymers and papers separators show excellent performance, while it is better to use ceramic or glass fiber as material separators with aqueous electrolytic solutions

1.2.2 Electrolytic solutions

The electrolyte located between two electrodes is one of the central parts constructing a supercapacitor It is a source of ions and allows ions to transfer reversibly to the electrodes (in EDLCs) or to fast reversible redox (in pseudocapacitor) [49] Many efforts have been focused on developed to figure out the electrolytes tailored with active materials as the electrodes [12] The electrolytes could be classified into following core-group (with representatives corresponding to every group) Classification of electrolytes used in supercapacitor is summarized in Figure 1.4

15

Figure 1 4 Classification of electrolytes [84]

It is believed that electrolyte is the component affecting very much to qualifications

of the supercapacitors Energy density and power density are two very important parameters generally used to evaluate qualification of a supercapacitor Firstly, expressions of the energy density and the power density (being expressed in following sub-section 1.2.3) show that they definitely depend on operating potential, larger voltage window corresponding to a larger value of energy and power density of the device Nevertheless, the maximum value of the working potential is strongly sticky with the stability of the electrolyte utilized [35] Secondly, originally from the expression of the power density (below sub-section), its value is inversely proportional to the value of equivalent series resistance Such value of the resistance is strongly dependent on kinds of electrolyte [84]

Moreover, ranges of temperature, in which the supercapacitors could still operate, is reported that it is also affected by what kind of electrolyte used [1] Some ways giving its effect is summarized into the afterward Figure 1.5

1.2.3 Energy density and power density

Energy density and power density are two main factors not only supercapacitor but also all kind of energy storage systems/devices to evaluate their qualifications The

17

energy density is represented by a capability of energy storage, whereas power density shows performance in term of how fast they charge or discharge

Moreover, it is very crucial to pay attention to another parameter when considering

a supercapacitor, capacitance The reason is that it is used to calculate energy and power density values Consequently, most of the reports about electrochemical capacitor, the authors focus on show their results in term of capacitance values Based on the model presented, the total capacitance of a simple supercapacitor can

be determined in the expression:

n p

C

111





(5)

For simple calculation of capacitance of supercapacitors, the capacitance of each electrode can be treat as one electrostatic capacitor, rely on the Helmholtz model And therefore,

d

A C

o

i

With i = p, n

For better results, it can be applied for calculating the capacitance of each electrode

by using BDM modern discussed above, which treats Helmholtz as IHP and OHP For that reason,

diff OHP

IHP

C

11

11

1 2

1

U C QU

18

Q is total charges stored at the electrodes of ECs

determined by:

2 2

1

U C

The power density of the supercapacitors is given in a common way by the express:

V I

P

ESR



Figure 1 6 Ragone plot shows a comparison of some main types of energy storage

devices in term of power density and energy density [19, 37]

19

The given Ragone plot (Figure 1.6) is a very unique way to understand the energy storage capability of various devices The dashed line illustrates for the period of time required to charge uptake or delivery process of devices The conventional capacitors possess very high power density but much lower energy density compared to batteries or fuel cells [33]

Figure 1 7 Classification of supercapacitor based on electrode materials [22]

20

1.3.1 Electric Double Layer Capacitor

Figure 1 8 A schematic diagram of EDLCs and description of potential change

through interface of electrode/electrolytic solutions when applied an external

voltage [52]

EDLCs are defined as electrochemical devices, which store the electric energy directly in the double-layer at the electrode/electrolyte interface [14] The electric double layer was formed by an accumulation of charges (ions) at the interface between the solid electrode and electrolytic solutions In term of principle, there are not any faradaic reactions occurring at the surface of the electrodes Consequently, this type of supercapacitor assesses higher energy uptake than the conventional capacitors and better power performance compared to all kinds of batteries Besides, cycle life could reach up to millions of times of charge/discharge, while it is only thousands of cycles with some of the best kinds of batteries In addition, one of the very important thing noted is isolation between electrolyte used and the mechanism

21

of charge storage, whereas it is ascribed to existence in Li-ion batteries at electrolyte interphase [53] This means that a variety of kinds of solvents could be utilized in electrochemical capacitors in cases of different environments operated

solid-It is a truth in term of the experiment that there is a vacuum with a length of several Angstroms separating the electrolytic ions layer and surface of electrode [73]

1.3.2 Pseudo-capacitor

Unlike EDLCs, this is a kind of supercapacitor based on a faradaic reaction (or electrochemical reversible redox reactions) to store energy When an external voltage is applied to current collectors, very quickly and highly reversible redox occurs on closely surface or on the surface of the electrodes This process, which is similar to mechanism happened in all kinds of batteries, relates to a transference of charges between the electrodes and the electrolytes Phenomena taken place at both positive and negative electrodes can be illustrated as [59]:

With their meaning:

electrodes)

respectively

22

In the 1970s, B.E Conway showed that three electrochemical phenomena can be consequences of pseudo-capacitive process: (1) under potential deposition, (2) redox pseudocapacitance and (3) intercalation pseudocapacitance, Figure 1.9 [2] The first phenomenon is the formation of an adsorbed monolayer on the surface of another metal above their redox potential One of the most featured of under potential deposition is the Langmuir-type electrosorption of Hydrogen ions in electrolytic solutions on the noble metal active materials such as Pt, Rh and Ru The

electrochemically on (or nearly) surface of active materials and faradaic charge transference happen simultaneously For instance, some classical representatives of

electrolytes intercalate into tunnels or layers of the redox materials, parallel with charges transfer but still maintain the crystallographic phase

Figure 1 9 Different types of pseudocapacitive behavior from B.E Conway: (a) under potential deposition, (b) redox pseudocapacitance, and (c) intercalation

23

Faradic processes occurring together with EDL charge storage increase the specific capacitance of an electrode The capacitance of a pseudocapacitor can be 10–100 times higher than that of an EDLC Nevertheless, the power performance of a pseudocapacitor is usually lower than that of EDLCs, due to the slower Faradic

processes involved [13]

1.3.3 Hybrid supercapacitor

Figure 1 10 An illustrative example of hybrid supercapacitor [84]

In order to limit the disadvantages of electrochemical capacitors and amplify their advantages, scientists have constructed another type of supercapacitor, which is hybrid supercapacitor, Figure 1.10 The hybrid supercapacitors are a combination of

and another negative electrode relied on electric-double-layer materials (carbon)

24

in both electric double layer capacitance stored by the porous carbon electrode and pseudo-capacitance stored by metal oxide or conducting polymer [24] Nowadays, many research are concentrated on hybrid supercapacitors due to their interesting features combined with both EDLCs and pseudo-capacitors[30, 61] Hybrid supercapacitors could be divided into three types based on electrode configuration: asymmetric hybrid, composite hybrid and a buttery-type hybrid supercapacitor

1.4 Electrode materials

1.4.1 Principle, classification and recent development

First of all, it is necessary to make sense the general principle of developments of the electrode materials with the purpose of enhancing energy density capacity

On the one hand, the energy density value of the electrode is proportional to capacitance value as mentioned previously in equation 8 On the other hand, it could

be derived from the equation 6, that the value of capacitance is definitely proportional to surface area and absolute permittivity (proportional to conductivity), whereas its value is reverse proportional to the distance between the ions and surface electrode according to Helmholtz model

Consequently, the larger surface area and higher conductivity that a type of electrode materials possess whether it is EDLCs or pseudo-capacitor types, the more capacitance value it could generate Relied on this principle, tremendous research efforts have been devoted to figuring out new kinds of the electrode materials to improve energy capacity of the supercapacitor However, many research [59] also pointed out that capacitance value was not rising linearly with a total surface area of the electrode materials due to correlating dimension with ions

in the solutions used After that, a term “electrochemically accessible surface area” was proposed to determine the surface area relating directly to the capacitance

Table 1.1 Carbon-based the electrode materials for supercapacitors

Table 1.2 Metal oxide-based the electrode materials for supercapacitors

Table 1.3 Conductive polymers-based the electrode materials for supercapacitors

26

research interest for close to a decade up to now due to its exceptional characteristics

In 2013, Ke-Jing Huang et al [28] showed some very first results relating to the

To prepare GO as an intermediate substance to synthesize the composite, the

reactants (a kind of very flammable substance) and up to 96 hours for stirring time Besides, the composites were synthesized by using the hydrothermal method with

hours Consequently, the EIS results indicated that the composites had very low Rct (demonstrating their excellent electron conductivity) while BET results convinced that specific surface area was much larger than that of pure rGO That was the reason why the electrochemical properties were enhanced a lot [28]

composites prepared by hydrothermal method with l-cysteine as a source of sulfur (they called this as a modified l-cysteine-assisted solution-phase method) However,

6.5 by NaOH 0.1M solution After transferring the two mixtures into every

27

after 1000 cycles The authors claimed that 3D structure after loading sheets of

transfer through the electrode [29]

In 2014, Firmiano et al [51] prepared the electrode of supercapacitor by directly

kinds of samples were synthesized with 5.6% (low), 17.6% (intermediate) and

counterparts The specific capacitances measured at 10mA/g were 128, 265 and 148

respectively, while the value for rGO was 40F/g This feature for the electrode with

with medium one was 70% after 1000 cycles The authors provided a relationship between the storage charge and nanostructure of the hybrid materials First, the

significant contribution of fast faradaic reactions [51]

In the same year 2014, there was a very interesting publication about

synthesis time less than 1 minute by using a hybrid microwave annealing method Indeed, they still had to prepare GO by the modified Hummer method as a medium

28

substance in the process After that, GO, thiourea (source of sulfur), ethanol and

strongly stirring in 1 hour Eventually, they utilized a drying oven to remove residual ethanol and then used a 1000W household microwave oven to irradiate materials in 45 seconds The authors stated that their synthetic method has some advantages: (1) Reduction of GO to GR occurs simultaneously with the

(3) Thermal treatment time is extremely short, thus this process is simple, ultrafast and energy-economic, (4) Scale-up is very easy In term of electrochemical

lead to enhance charge transfer capability [74]

In 2015, a very interesting paper was published by Peng Chen et al [56] about

in hybrid fibers form The researchers prepared multi-wall carbon nanotube

method [77] GO was still synthesized by the modified Hummer method like other previous publications, whereas reduction process from GO to rGO underwent some different points compared to other methods before [45] Samples (rGO-MWCNT,

nanosheets on MWCNT nanosheets The electrochemical properties showed that the voltage window was ranged from -0.6V to 0.8V The volumetric capacitance of

29

Still in 2015, Mark A Bisset et al reported the article namely “characterization of

(1:1, 1:3 and 3:1) The electrochemical performance showed that every composite got better specific capacitance compared to pure one, while the 1:3 composite

that specific capacitance strongly increased by 250% after 10000 cycles and figure

cycles (still maintained after 7000 cycles) EIS results indicated that equivalent

figures for the 1:3 composites, graphene were 0.7 and 0.96 Ohm, respectively

In 2016, Nguyen Van Hoa and his collaborators published a research in which

high performance in storage capacity of the electrode of supercapacitor [7]

hybrid layer-structure GO was still the modified Hummer method However, they

The specific capacitance of the composite was 575 F/g compared to 309 F/g of bare

were explained owing to ERS (in EIS results) increasing compared to the uncycled electrodes

composite as a hollow sphere by hydrothermal to apply for the electrode of supercapacitor [68] Before that, GO sheets were bought as a commercial one with

30

(source of sulfur) and HF 40% were used as precursors to create the mixture

microspheres The CV results showed that the specific capacitance of the

1A/g in 2M KOH solution This number was maintained at 92% after 1000 cycles compared to that of the uncycled electrode

In 2017, Arnab Ghosh et al [32] came up with very systematical results in the

after 5000 cycles Demonstrations proposed in this article based on EIS were very remarkable as a highly reliable reference

1.4.3 Research question

While working out with the overview information, I have realized that recent researches for the electrode materials of the supercapacitor have been focusing on the enhancement of its surface area and electric conductivity to improve the supercapacitors performance Moreover, new environmental friendly electrode materials are desired to develop by scientists As among of promises candidates for