- Title
- Synthesis, structure and application of coal-derived few-layer graphene composite materials
- Creator
- Islam, Faridul
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Higher Doctorate - Doctor of Philosophy (PhD)
- Description
- Graphene and its composite materials have attracted wide attention due to their unique structures and outstanding properties, such as their excellent thermal and conductive properties, along with their mechanical properties and high surface areas. In recent years, high-performance energy resources, such as rechargeable lithium-ion batteries (LIBs), have been in demand in the world economy due to their increasing usage. The performances of the LIBs have been developed to improve their capacity, stability, cycling ability, safety, and fabrication costs, along with enhancing their battery applications in electric vehicles (EVs) and other electronic devices. Graphene is utilized as an anode material in LIBs. In general, graphite is used as the raw material to produce graphene. The chemical vapor deposition (CVD) technique is frequently used to fabricate high-quality graphene films over large areas on different metal substrates. However, this technique involves the use of toxic chemicals and is time-consuming. Moreover, solid carbon sources like many polymers and food waste have also been used as precursors for graphene synthesis films. These raw materials and synthesis methods are not feasible for the large-scale production of graphene. The growth of graphene structures using catalytic graphitization is crucially associated with the quality of the few-layer graphene (FLG) produced. Due to the limited global resources of graphite, coal has become a cheap raw material for making graphene due to its high carbon content and being plentifully available. The synthesis process for FLG is influenced by the coal particle, catalyst, and microwave heating parameters during the pyrolysis process. Therefore, understanding the changes in the physical and chemical structures during the growth of FLG films is a core part of what governs the number of graphene layers and its ultimate effectiveness as an anode material in LIBs. Therefore, this study is mainly focused on: (1) developing a simple synthesis technique using coal loading catalysts and microwave heat treatment; (2) the transformation of amorphous carbon to ordered carbon during the graphitization period; (3) the conversion of the chemical structures of the coal composites during the formation of the graphitic layers; (4) clarifying the mechanisms of the catalytic graphitization in the presence of a transitional metal catalyst; (5) structural investigation of FLG during pyrolysis; and (6) investigating the electrochemical properties and performances of the FLG composites as potential anode materials for LIBs. To achieve these objectives, a microwave oven and a high-temperature type B thermocouple was used to modify the composite samples and measure the sample temperatures. The FLG samples were scrutinized using many analytical techniques, XRD, SEM, TEM, Raman spectroscopy, BET, XPS, and AFM, to determine the FLG sample properties. Furthermore, the electrochemical behaviours of the FLG composite materials were measured using galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), rate capability, cycling stability, coulombic efficiency, and electrochemical impedance spectroscopy (EIS) analyses. In the first section, the source material was bituminous coal, which was first activated through a steam gasifier and then loaded with the catalyst (iron) by weight percentages. The pyrolysis runs were carried out at a temperature range of 800 - 1300 °C with several catalyst percentages and numerous holding times during the graphitization period. The highest degree of graphitization and a well-developed pore structure was achieved by S10% Fe catalyst at a heating temperature of 1300 °C for 20 minutes. The high-resolution transmission electron microscopy (HRTEM) analysis suggested that the fabricated S10% sample consisted of 3 - 6 layers of graphene nanosheets. Following these findings, a mechanism for the growth of FLG on a coal substrate during the microwave graphitization period was proposed. The findings demonstrated that the supersaturation point of the carbon matrix and the Fe particles was reached quickly during the high heating temperatures in the microwave oven. Moreover, the oven was fast, and the automated on/off mode can heat the absorption desorption process and facilitate the layer-by-layer growth of the graphene. At the Fe C eutectic point, the amorphous carbon was dissolved into the particles of the Fe catalyst at a constant microwave heating rate for 20 minutes, which initiated the layer growth. The HRTEM images confirmed that the interlayer spaces of the graphitic layers were around 0.34 nm, which corresponded to the (002) plane of FLG. The second section investigates the few-layer graphene structure by Raman study and AFM. Coal was activated using potassium hydroxide (KOH) and then loaded with the iron catalyst by weight percentages. The findings showed that three bands in the Raman spectra of graphene were considered, the D, G, and 2D bands, and the defect levels present in the samples. At a higher annealing temperature (1300 °C), the P10% Fe-loaded sample, when heated for 20 min, produced a unique morphology with 2 - 7 graphitic layers with lower defect levels, which demonstrated that more regular and continuous thin sheets of graphene were formed. The Raman mapping measurements showed that at 1300 °C, the P10% Fe-loaded sample was homogeneously distributed. Moreover, the AFM technique measured that the thickness of the FLG was around 4.5 nm. In the third section, the dual catalyst (iron and nickel) is used to synthesize the few-layer graphene materials through steam activated and also compared the single and dual catalyst for LIBs application. It was found that the S5% dual catalyst, when heated at a temperature of 1300 °C for 20 min, synthesized an FLG sample that contained 2 - 7 graphitic layers, which was confirmed by the analysis of the HRTEM images. The Raman spectroscopy, fast Fourier transform (FFT), selected area electron diffraction (SAED), TEM, and XPS analyses confirmed the formation of the crystalline FLG films. Finally, the coal-derived few-layer composites were evaluated as potential anode materials for LIB applications. The single and double composite materials were compared for their potential applications in anode electrodes. The analysis showed that the dual S5% Fe–Ni catalyst samples delivered a high reversible capacity of 287.91 mAhg-1 at 0.1 C with a higher coulombic efficiency of 99.9%. In contrast, the single catalyst of S10% Fe contained a reversible capacity of 260.13 mAhg-1 at 0.1 C with a coulombic efficiency of 97.96%. After 100 cycling performances, the dual catalyst capacity was 187 mAhg-1, and the single catalyst capacity was 67.5 mAhg-1. Moreover, the dual catalyst loaded sample retention capacity was 95% after 100 cycles. These unique characteristics of the FLG significantly supported the findings that stable SEI formed, the conductivity improved,and made the short ion diffusion path during the cycling period. In addition, the EIS and cyclic voltammetry results for the prepared dual catalyst (S 5% Fe-Ni) loaded FLG materials revealed excellent stable electrochemical properties. The findings presented in this study provide a fundamental understanding of the growth of coal-derived FLG through catalytic graphitization mechanism under microwave heat treatment and compare the single and dual catalyst with the potential applications of FLG composite material in LIBs.
- Subject
- few-layer graphene (FLG); coal-derived; composite materials; lithium-ion batteries (LIBs)
- Identifier
- http://hdl.handle.net/1959.13/1509168
- Identifier
- uon:56219
- Rights
- This thesis is currently under embargo and will be available from 31.12.2024, Copyright 2022 Faridul Islam
- Language
- eng
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