Low, Wei Hau
(2021)
Hybridisation of mixed transition metal oxide with graphene as advanced electrode material for symmetric supercapacitor application.
PhD thesis, University of Nottingham.
Abstract
Nowadays, imminent deficiency of fossil fuels and rising environmental concerns have triggered the immense research enthusiasm for the development of green and renewable energy sources. Lately, supercapacitors are emerging as one of the competent energy storage devices owing to their excellent capacitive properties, long term cycling stability as well as remarkable power density. In this sense, the promising family of mixed transition metal oxides (MTMO) possesses felicitous as active electrode materials for supercapacitor due to their multiple oxidation states and ions, leading to superior specific capacitance. Moreover, many researchers are dedicated in hybridising the MTMO with graphene nanosheets as advanced electrode materials due to the peculiar properties of graphene, which ameliorate the electrical conductivity and enlarge the specific surface area of the nanocomposite for Faradaic redox reaction. The current thesis emphasis on the latest evolution of MTMO and the integration of MTMO with graphene nanosheets as active electrode materials, with a comprehensive study of their synthetic approaches. In addition, the energy storage mechanism of distinct types of supercapacitor and the critical factors affecting the electrochemical activities of transition metal oxide-based materials are also delineated.
Based on the literature review, solvothermal technique was acknowledged as one of the effective synthetic routes for MTMO/graphene nanocomposites, which impress by its versatility in controlling the morphology and growth mechanism (nucleation and diffusion process) of the reactants. Additionally, solvothermal method is also a simple and cost effective process with high scalability potential. In chapter 4, graphene/nickel vanadate (Ni3V2O8) nanocomposites were successfully synthesised through the solvothermal method. The nanocomposites integrate separately the advantages of graphene sheet and pseudocapacitive nature of Ni3V2O8, leading to outstanding electrochemical performance than the pristine Ni3V2O8. Optimisation on the weight ratios between the graphene and Ni3V2O8 were conducted to determine the perfect synergistic effect between the highly conductive graphene sheet and the pseudocapacitive Ni3V2O8. Among the graphene/Ni3V2O8 nanocomposites, G-4NVO as promising electrode material performed an eminent specific capacitance of 368 Fg-1 at a current density of 0.5 Ag-1 and energy density of 51 Wh/kg at a power density of 920 W/kg. An excellent cycling stability with 92 % capacitance retention and coulombic efficiency of nearly 100 % were achieved after 3000 charge-discharge cycles at 1Ag-1.
A series of graphene/cobalt vanadate (Co3V2O8) nanomaterials with multiple weight ratios were prepared and their respective physical and electrochemical performances were evaluated in chapter 5. The nanocomposite with weight ratio of 1:4 (G-4CVO) exhibited the highest specific capacitance of 275.2 Fg-1 at 0.8 Ag-1. A specific capacitance of 216 Fg-1 was obtained at high current density of 1.2 Ag-1. The electrode material maintained 81 % of its charge storage capability after 3000 cycles, signifying its outstanding cycling stability. Additionally, G-4CVO delivered remarkable energy and power densities, signifying its potential as advanced electrode material for supercapacitor application.
In chapter 6, the energy storage capability of the electrode material can be improved by combining graphene with zinc vanadate (Zn3V2O8) to form the “sheet on sheet” architecture. For comparison, different weight ratios were applied to prepare the nanocomposites by using the solvothermal method. The synergistic effect between graphene and Zn3V2O8 resulted in enhanced capacitive performance. Among the nanocomposites, G-3ZVO delineated a specific capacitance of 313.6 Fg-1 at 0.8 Ag-1. Besides, 80 % of the initial capacitance was retained as the current density increased to 1.2 Ag-1, suggesting its good rate capability. G-3ZVO achieved a specific capacitance of 246.8 Fg-1 (86 % retention) after experiencing 3000 repeated galvanostatic charge-discharge cycles, confirming its exceptional cycling stability. Furthermore, this symmetrical supercapacitor exhibited a maximum energy density of 43.55 Wh/kg.
A distinctive, nanobelts like manganese vanadate (MnV2O6) decorated on graphene nanaosheets was developed via the solvothermal method as electrode material for symmetric supercapacitor (Chapter 7). The effect of weight ratio (graphene:MnV2O6) on the structural and electrochemical performances was also determined to study the detail synergistic effect between graphene and MnV2O6. G-8MVO revealed the maximum specific capacitance of 348 Fg-1 at the current density of 0.5 Ag-1. A capacitance retention of 88 % was achieved after 3000 cycles, suggesting its excellent cycling stability. In addition, G-8MVO obtained a maximum energy density of 48.33 Wh/kg at a power density of 880.6 W/kg, highlighting the potential of this material to bridge the gap between the battery and supercapacitor.
In chapter 8, graphene/aluminium vanadate (AlV3O9) nanocomposites were developed through the deposition of 3D lion’s mane like AlV3O9 microspheres on graphene sheets. Different physical and electrochemical properties were obtained by varying the weight ratios between graphene and AlV3O9. Notably, nanocomposite with the weight ratio 1:5 (G-5AlV) showed a specific capacitance of 268.8 Fg-1 at 0.8 Ag-1. A specific capacitance of 192 Fg-1 was obtained as the current density increased to 1.2 Ag-1, suggesting its remarkable rate capability. Besides, a capacitance retention of 82 % was achieved together with a coulombic efficiency of nearly 100 % after 3000 repeated charge-discharge cycles, confirming its excellent cycling stability and good reversibility. The remarkable energy and power densities of G-5AlV suggest that it has a great potential in hybrid supercapacitor application.
In short, five desired graphene/MTMO nanocomposites were successfully synthesised in this work and their physical, chemical and electrochemical properties were analysed comprehensively. Besides, the effect of weight ratio between graphene and MTMO was well determined to study the best synergistic effect between both nanomaterials for advanced supercapacitor performance. Thus, it proves that all the research objectives in this PhD study have been achieved accordingly.
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