The key motivation of this study is to investigate the potential of graphene/metal oxides nanocomposites as electrodes for electrochemical capacitor applications. It is envisioned that the positive synergistic effect between graphene and metal oxides (where novel graphene material acts as a highly conductive platform for ease of ion transfer kinetics and metal oxide acts as spacers to avoid the restacking of graphene sheets to make available more active surface areas) results in excellent electrode material for high performance electrochemical capacitor. In this thesis, a series of hybrid composites comprising of graphene and low cost transition metal oxides were synthesised and characterised for their potential as electrode for electrochemical capacitor applications. In order to achieve this, the graphene used in the preparation of the hybrid composites was successfully synthesised from highly pyrolytic graphene in a proper ratio of ethanol and water before the integration of the metal oxides via a solvothermal route. A parametric study was carried out in a step by step approach to validate the success of the composite synthesis before the electrochemical stage. X-ray Diffraction, Field emission and Transmission scanning electron microscopy, energy-dispersive X-ray and Raman spectroscopy, cyclic voltammetry and galvanostatic charge/discharge tests were used to verify the integrity of the as-produced graphene/metal oxide composites and their applicability to electrochemical capacitors. Upon the completion of the experimental work, the electrochemical tests demonstrated that the introduction of graphene to the metal oxide improved the electrochemical performance in-terms of capacitance, energy density, power density, equivalent series resistance and cycling stability. The results also indicated that the ratio of graphene to metal-oxide plays a significant role in the electrochemical performance of the composite. In comparison with the different graphene/Zinc oxide (ZnO) nanocomposites studied, the electrode material with a weight ratio of 1:8 (graphene: ZnO) displayed a specific capacitance of 236 F/g at a scan rate of 10 mV/s with energy and power densities of 11.80 Wh/kg and 42.48 kW/kg respectively. The specific capacitance of the graphene-Manganese oxide (MnO2) composite electrode material with a weight ratio of 1:16 (graphene: MnO2) demonstrated the best performance of 380 F/g at a scan rate of 5 mV/s among the four ratios studied. The G1Co4 composite electrode with a weight ratio of 1:8 (graphene: Co3O4) demonstrated a superior specific capacitance of 384 F/g at a current density of 0.3 A/g coupled with retention of 80% of its capacitance after 1000 cycles among the graphene-cobalt composites. The Graphene-Nickel cobaltite composite electrode with weight ratio of 1:8 (graphene: NiCo2O4) labelled G-8NC2 displayed a superior specific capacitance (698 F/g at a current density of 0.5 A/g) and good cycling stability (74% capacity retention after 5000 cycles at current density of 1 A/g). The 1:8 ratio exhibited well attached Nickel molybdate nanorods on the surface and edges of the graphene sheets with the highest specific capacitance of 670 F/g at 0.3 A/g, as compared to other tested composites. The significance of these findings details a synthesis route that provides an effective, simple and practical method of preparing graphene-metal oxide composite materials for electrochemical capacitor applications.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:757567 |
| Date | January 2018 |
| Creators | Ezeigwe, Ejikeme Raphael |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/52517/ |
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