Master’s thesis Nanotechnology: Preparation of manganese dioxide/graphene composites by plasma enhanced electrochemical exfoliation process and its electrochemical performance

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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.

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Nội dung Text: Master’s thesis Nanotechnology: Preparation of manganese dioxide/graphene composites by plasma enhanced electrochemical exfoliation process and its electrochemical performance

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN THANH HAI PREPARATION OF MANGANESE DIOXIDE/GRAPHENE COMPOSITES BY PLASMA-ENHANCED ELECTROCHEMICAL EXFOLIATION PROCESS AND ITS ELECTROCHEMICAL PERFORMANCE MASTER’S THESIS Hanoi, 2019
VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN THANH HAI PREPARATION OF MANGANESE DIOXIDE/GRAPHENE COMPOSITES BY PLASMA-ENHANCED ELECTROCHEMICAL EXFOLIATION PROCESS AND ITS ELECTROCHEMICAL PERFORMANCE Major: Nanotechnology Code: Pilot Research supervisor: Dr. Phan Ngoc Hong MASTER’S THESIS Hanoi, 2019 i
TABLE OF CONTENTS TITLE PAGE .............................................................................................................i TABLE OF CONTENTS ........................................................................................ ii LIST OF FIGURES .................................................................................................iv LIST OF TABLES ...................................................................................................vi LIST OF ABBREVIATIONS ............................................................................... vii ACKNOWLEDGMENTS .................................................................................... viii DECLARATION .....................................................................................................ix ABSTRACT ............................................................................................................... x INTRODUCTION .................................................................................................... 1 Chapter 1 OVERVIEW............................................................................................ 3 1.1. Electrochemical energy storages .................................................................. 3 1.1.1. Supercapacitors ...................................................................................... 5 1.2. Electrode materials for supercapacitors ..................................................... 6 1.2.1. MnO2/graphene composites ................................................................... 7 1.2.1.1. Direct oxidation-reduction reaction ................................................... 8 1.2.1.2. Solution-based mechanical mixing .................................................. 10 1.2.1.3. The other methods ........................................................................... 13 1.3. Current research in Vietnam ..................................................................... 15 Chapter 2 MATERIALS AND METHODS ......................................................... 18 2.1. Chemicals and reagents .............................................................................. 18 2.2. Preparation of 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 3 RESULTS AND DISCUSSION ........................................................... 23 3.1. Characterizations of MnO2/graphene composites.................................... 23 3.2. The proposed mechanism for PE3P method ............................................. 29 3.3. Electrochemical performance .................................................................... 30 ii
3.4. Symmetric supercapacitor.......................................................................... 35 CONCLUSIONS ..................................................................................................... 39 LIST OF PUBLICATIONS ................................................................................... 40 REFERENCES........................................................................................................ 42 iii
LIST OF FIGURES Figure 1.1. A Ragone plot for various electrochemical energy storage devices [33]. .......... 4 Figure 1.2. The working principles of (a) electrochemical double layer capacitor (carbon as the electrode material) and (b) Pseudocapacitor (MnO2 as the electrode material) in Na2SO4 electrolyte [18]. ........................................................................................................ 5 Figure 1.3. (a) Schematic illustration for the synthesis of graphene–MnO2 composite (b) the comparison of specific capacitance with other materials [48]. ........................................ 8 Figure 1.4. Schematic representations of the experimental design of MnO2/rGO composite [53]. ........................................................................................................................................ 9 Figure 1.5. Schematic graphic of the synthesis process of the rGO/MnOx composite [41]. ............................................................................................................................................. 10 Figure 1.6. The formation mechanism for GO-MnO2 nanocomposites [2]. ....................... 11 Figure 1.7. (a) Schematic representations for MnO2 anchoring on graphene through electrostatic attraction, (b,c) TEM image and (d) capacitance retention of MnO2/graphene [56]. ...................................................................................................................................... 12 Figure 1.8. Laser scribing of high-performance and flexible graphene/MnO2-based electrochemical capacitors [8]. ............................................................................................ 13 Figure 1.9. (a) Schematic illustration for plasma-assisted electrochemical exfoliation method, (b) TEM image of graphene sheets and (c) XPS of C1s in graphene samples [37] ............................................................................................................................................. 15 Figure 1.10. The detailed process of printing supercapacitor electrodes [7]. ..................... 16 Figure 2.1. The schematic representation of the experimental design. ............................... 19 Figure 3.1. SEM images of (a) graphene, (b) GM1 (1 mM KMnO4), (c) GM10 (10 mM KMnO4) and (d) MnO2 nanoparticles (1 mM KMnO4), respectively.................................. 23 Figure 3.2. EDX results of GM1 and their element mapping images................................. 24 Figure 3.3. TEM images of (a) graphene and (b) GM1. ..................................................... 25 Figure 3.4. Raman spectra of GM1 and graphene. ............................................................. 25 Figure 3.5. XRD pattern of graphene and GM1 samples. .................................................. 27 Figure 3.6. XPS patterns of GM1, (a) survey, (b) C1s, (c) O1s and (d) Mn2p. ................. 28 Figure 3.7. Proposed mechanism for the formation of graphene/MnO2 composite ........... 29 Figure 3.8. Cyclic voltammetry curves of (a) graphene and (b) GM1 electrodes in a 6 M KOH electrolyte at a different scan rate of 5, 10, 20, 50, 100 mV s-1. ................................ 31 iv
Figure 3.9. Charge-discharge curves of (a) graphene and (b) GM1 electrodes in a 6 M KOH electrolyte at a different current density of 2, 5, 10, 20 A g-1. ................................... 32 Figure 3.10. Cycling performances of (a) GM1 and (b) graphene electrodes at a current density of 10 A g-1. .............................................................................................................. 34 Figure 3.11. (a) GCD curves of GM1//GM1 symmetric supercapacitor at a different current density of 2.5, 5, 10 A g-1 and (b) the specific capacitance of GM1//GM1 symmetric supercapacitor. ................................................................................................... 36 Figure 3.12. Ragone plot of GM1//GM1 symmetric supercapacitor. ................................. 37 Figure 3.13. Cycle stability of GM1/GM1 symmetric supercapacitor at a current density of 5 A g-1. ................................................................................................................................. 38 v
LIST OF TABLES Table 3.1. Effect of concentration of KMnO4 on forming MnO2 nanoparticles ................. 24 Table 3.2. The comparison of some vital parameters with other results............................. 35 vi
LIST OF ABBREVIATIONS CVD Chemical vapor deposition EDLC Electrochemical double layer capacitor GO Graphene oxide rGO Reduced graphene oxide SCs Supercapacitors CV Cyclic voltammetry GCD Galvanostatic charge/discharge SEM Scanning electron microscopy EDX Energy-dispersive X-ray TEM Transmittance electron microscopy XRD X-ray diffraction XPS X-ray photoelectron spectroscopy SCE Saturated calomel electrode PE3P Plasma-enhanced electrochemical exfoliation process CNTs Carbon nanotubes vii
ACKNOWLEDGMENTS First of all, I would like to express my sincere gratitude to Dr. Phan Ngoc Hong, Center for High Technology Development (HTD), Vietnam Academy of Science and Technology (VAST), for his extraordinary supervision, support and guidance throughout my research period. My dissertation would not have been possibly conducted without his valuable advice and constructive comments. I would like to express my appreciation to Assoc. Prof. Masashi Akabori, School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), for his excellent guidance, advice and support during the period of internship. Especially Assoc. Prof. Masashi Akabori who has spent his precious time for training and helping me on characterizations and measurements. I would strongly give my sincere appreciation to Dr. Dang Van Thanh, who always support and encourage me during all my research and future academic careers. His energetic and enthusiastic attitudes towards research inspire me to overcome the research challenges. Additionally, I also acknowledge Dr. Nguyen Tuan Hong for allowing me to use the facilities and providing the best conditions when I did experiments. I also appreciate Mr. Pham Trong Lam and Mr. Dang Nhat Minh for their kind help and fruitful discussion about data analysis. I also thank Mr. Le Hoang for TEM measurements. I would like to thank Nanotechnology Program staff, Ms. Nguyen Thi Huong for being so nice and helping me with all the administrative and academic problems. This thesis is supported by National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.09-2017.360. Finally, special thanks go to my parents and my friends for being there, smiling at me with love, good days or bad days. viii
DECLARATION I hereby declare that all the result in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work. Author Nguyen Thanh Hai ix
ABSTRACT In this thesis, I have developed a low-cost, simple and one-step approach to synthesize MnO2/graphene composite with enhanced electrochemical performance. MnO2/graphene composite was prepared via a plasma-assisted electrochemical exfoliation method with an electrolyte solution made of KMnO4 precursor. MnO2/graphene composites were characterized by SEM, TEM, XRD, Raman, XPS and were employed for the examination of electrochemical behaviors. MnO2/graphene composite displayed the specific capacitance of 217.0 F g−1 at current density of 2 A g-1, which is approximately three times higher than those of pristine graphene (47.0 F g−1). Interestingly, the capacitance retention was highly kept over 80% after 3000 cycles. The enhanced electrochemical property might be due to the synergistic effect of MnO2 nanoparticles and graphene nanosheets. With a view to practical applications, a symmetric supercapacitor has been fabricated and delivered the highest specific capacitance of 130.9 F g-1 at a current density of 2.5 A g-1. In general, this work provides a new approach to synthesize MnO2/graphene composite for supercapacitors applications by one-step, short-time and eco-friendly method. x
INTRODUCTION Prosperous development of energy conversion and storage plays a crucial role in our modern life. Electrochemical supercapacitors, which can provide higher energy density than conventional capacitors and higher power density than batteries/fuel cells, have received significant attention in recent years as a promising alternative energy storage devices [15, 24, 43]. Owning to their low-cost, abundance, environmental friendliness and its ability to work under neutral pH, especially with high theoretical specific capacitance (1380 F g-1), MnO2 is generally considered to be an excellent candidate for supercapacitor applications [13, 39, 42, 55]. However, MnO2 particles often tend to form big agglomerates, which noticeably reduce their electrochemical property and lower the efficiency as a result of the undone reaction of MnO2 nanostructures during the electrochemical reduction-oxidation reaction [36]. To overcome this problem, researchers currently have developed a powerful route to strengthen the electrochemical property by combining MnO2 with transition metal oxides [16] or with graphene [2, 41, 48], which are benefited from the high electrical conductivity of carbon materials as well as the high specific capacity of metal oxides. Nevertheless, their works mostly utilize graphene oxide as a precursor to synthesize graphene and its derivatives via graphene oxide reduction process. Graphene oxide has commonly been produced from graphite oxidation process i.e. Hummers’ method, which requires hazardous chemicals such as strong acid (nitric acid, sulfuric acid) and strong oxidant agents (potassium permanganate, potassium chlorate) in order to partially oxidize graphite. Notably, these as-mentioned chemicals are highly toxic and dangerously unstable, which can release toxic gases such as NO2, N2O4, and ClO2. Moreover, a large amount of wastewater containing acid waste and heavy metal ions has been considered to be risky for the environment. Also, the present synthesis method is an extremely time-consuming process, which needs a few to hundreds of hours for oxidation and removing excessive acids and KMnO4 after the oxidation step. Therefore, an alternative approach for preparing GO or graphene from a commercial graphite source with the simple, rapid and inexpensive process is highly 1
necessary. In the past few years, the electrochemical method has been proved to be a green and attractive approach to producing high-quality graphene due to its environmentally friend and low-cost approach [37, 38]. Dang et al. [6] proposed a novel and efficient method for the preparation of MoS2/graphene composite by modifying an electrolyte solution. Thus, there is a great interest to change the electrolyte solution to obtain desirable metal oxide and graphene composites. In this thesis, a new, simple and environmentally friend approach for synthesizing MnO2/graphene composites will be demonstrated. Plasma-assisted electrochemical exfoliation method that consists of graphite cathode and Pt anode under a high applied voltage will be conducted. The purpose of this work is to find out new method for one-step synthesis of MnO2/graphene composites by plasma- assisted electrochemical exfoliation method in a short-time reaction, low-cost and time-saving process. Besides, the MnO2/graphene composites will be tested electrochemical performance towards its supercapacitor applications. This dissertation will be divided into the following Chapter. + Introduction. + Chapter 1: Overview of supercapacitors and current status on MnO2/graphene research. + Chapter 2: Materials and Methods + Chapter 3: Results and Discussion. + Conclusions. 2
Chapter 1 OVERVIEW 1.1. Electrochemical energy storages Energy is crucial to modern society for sustainable economies in both developed and developing countries. Up to now, fossil fuels are still the major sources of energy in spite of the increasing environmental pollution problems and ecological crisis caused by fossil fuels consumption. Moreover, with the rapid expansion of the global economy, increasing environmental pollution worldwide, and the depletion of non-renewable fossil fuels, there has been an increasingly urgent demand for the development of not only efficient, clean, and sustainable sources of energy, but also high-performance, low-cost, and environmentally friendly energy conversion and storage devices. Hence, electrochemical energy storages are indispensable and essential for us in order to contribute to new and constant energy supplements. The electrochemical energy storage technology has been widely used for numerous applications such as portable electronic devices, electric vehicles, large-scale electric grids and stationary energy storage. Figure 1.1. shows a Ragone plot for the energy storage systems, illustrating their relationship between power density and energy density [33]. 3
Figure 1.1. A Ragone plot for various electrochemical energy storage devices [33]. As shown in Figure 1.1, the fuel cells show the highest energy density, but their power density is the lowest among these promising devices. Similar to the fuel cells, batteries also get high energy density, but the practical applications of batteries are still limited due to its low power density and cycle stability. Thus, supercapacitors will be a prospective nominee for electrochemical energy storage that could bring higher energy density and higher power density than conventional capacitors and batteries, respectively. However, the current limitation of the supercapacitor is low energy density compared to batteries in actual applications. For instance, carbon- based supercapacitors commonly possess energy density less than 10 Wh kg-1, which is much lower than that of lead-acid batteries (33-42 Wh kg-1) and lithium-ion batteries (100–265 Wh kg-1) [33]. Because of its low energy density, supercapacitors are not able to meet the demand for energy storage devices for the next power generation. The enhanced energy and power density of supercapacitors (electrochemical capacitors) are very crucial to fulfill the higher and higher requirements of energy storage devices. 4
1.1.1. Supercapacitors Recently, supercapacitors have drawn significant concern of scientists particularly thank to their high-power density, long cycle life and fast charge- discharge processes [31, 43]. Supercapacitors preserve an essential position in the Ragone plot since they can fill the gap of energy-power density between conventional capacitors and batteries. With a reasonably high energy density and power density, supercapacitors have been extensively applied in practical applications ranging from portable consumer electronic devices, back-up memory systems, automotive, to industrial power and energy management, and many more. Dependence on the charge storage mechanisms, the electrochemical supercapacitors might be classified into two kinds of supercapacitors: electrical double-layer capacitor (EDLC) and pseudocapacitor [13]. Figure 1.2. The working principles of (a) electrochemical double layer capacitor (carbon as the electrode material) and (b) Pseudocapacitor (MnO2 as the electrode material) in Na2SO4 electrolyte [18]. A conventional electrostatic capacitor is a passive device with two electrodes, which are separated by a dielectric layer. Static charge is stored by polarizing the electrodes within an electric field, providing a mechanism for delivering very high- power density, but low energy density (few microFarads per gram). Electrochemical 5
double layer capacitors (EDLCs) store charge by physical electrostatic where reversible adsorption of ions from the electrolyte onto the active material. The active materials will adsorb ions on its surface to form a double layer at the electrode- electrolyte interface (Figure 1.2a) [24]. The EDLCs mechanism is commonly presented for carbon-based materials due to its high BET surface area. The absence of a redox reaction (non-Faradaic process) allows fast charge/discharge cycles, which produce high power density and long cycle life since there is no mechanical stress caused by changes in the volume of the electrode. However, as the energy storage strongly depends on the surface area of the active material, they exhibit limited energy density. Pseudocapacitive electrode materials store charge based on a fast and reversible surface oxidation-reduction reaction (Faradaic process) by electron transfer in addition to the formation of the double layer (Figure 1.2b) [15]. Common pseudocapacitive materials are conducting polymers (e.g. polypyrene, polyaniline, polythiophenes) and metal oxides (e.g., MnO2, RuO2). The capacitance of these electrodes is between 10-100 times higher than EDCLs; however, the power density and cycle life are lower because Faradic processes are slower than electrostatic processes and change in the volume of the electrode upon cycling (swelling and shrinking) tend to cause mechanical stress, degrading the materials. When electrodes of different nature are used as the cathode (e.g., pseudocapacitive material) and the anode (e.g., capacitive material) the supercapacitor is called hybrid capacitor. 1.2. Electrode materials for supercapacitors A variety of materials has been carefully studied as active materials for electrochemical capacitors. Among them, carbon nanomaterials (e.g. graphene, carbon nanotubes, amorphous…) [19, 29, 45] with different microstructures have been comprehensively explored as electrode materials for high-performance supercapacitors owning to their high specific surface area, interconnected pore structure, controlled pore size, high electrical conductivity, excellent chemical stability, and good environmental compatibility. Due to a purely physical ion adsorption-desorption process, there is a considerable hurdle for carbon-based 6
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 effect, which could overcome their current obstacles, thus enhancing the capacitance and property of supercapacitors [12, 32, 47, 53, 54]. Until now, there are numerous of literature have been carried out to (i) develop a simple, scalable, and reliable method, (ii) prepare MnO2 nanostructure with desired morphology and mass 7
percentage comparing with graphene, (iii) improve electrical and mechanical property between two components. There are two favorable ways for preparing MnO2/graphene nanocomposites: direct oxidation-reduction reaction and solution- based mechanical mixing of two components. 1.2.1.1. Direct oxidation-reduction reaction The first approach is that the direct redox reaction between KMnO4 and graphene / graphene oxide. For instance, Yan et al. [48] proposed a fast and effective method to prepare MnO2/graphene nanocomposites through the self-limiting deposition of nanoscale MnO2 on the surface of graphene under microwave irradiation. In their experiment, they mixed the graphene solution with particular KMnO4 precursor together (Figure 1.3). Then, by taking advantage of microwave irradiation, the following reaction will occur: 𝑀𝑛𝑂$% + 3𝐶 + 𝐻* 𝑂 Û 4𝑀𝑛𝑂* + 𝐶𝑂-*% + 2𝐻𝐶𝑂-% . The author stated that MnO2/graphene nanocomposite (78 wt.% MnO2) exhibited the maximum specific capacitance of 310 F g-1 at a scan rate of 2 mV s-1 and kept a reasonable capacitance of 228 F g-1 at a scan rate of 500 mV s-1. Moreover, the cycle stability of the nanocomposite was also performed by repeatedly carrying out cycle voltammetry tests and showed a very perfect capacitance retention of 95.4% after over 15 000 cycles. Figure 1.3. (a) Schematic illustration for the synthesis of graphene–MnO2 composite (b) the comparison of specific capacitance with other materials [48]. Yang et al. [49] produced N-doped graphene/MnO2 composites by employing a one-step hydrothermal method at a moderate temperature around 120oC. The MnO2 nanosheets were tightly anchored on graphene sheets. Their results in electrochemical 8
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]. Notably, both of the high-temperature thermal reduction and the low- temperature chemical reduction by acid or alkali (e.g., reduction by HI acid or hydrazine) can affect both GO and MnOx. For instance, Yang and co-workers stated 9