Novel Wet Chemical Syntheses of Graphene Oxide and Vanadium Oxide for Energy Storage Applications

Abstract

The ever-growing demand for high performance energy storage systems has become a driving force for seeking the ideal materials to deliver superior efficacy, and graphene oxide (GO) and vanadium oxide are such two promising nanostructured materials. However, neither of them has been widely adopted in the marketplace at the current stage, mainly limited by their costeffectiveness. While GO and vanadium oxide have been proved to outperform existing materials in the lab-scale studies, the more expensive and less scalable synthesis methods discourage industrial manufacturers from adopting the two materials. The research herein focuses on the novel low cost and scalable wet chemical synthesis methods, which may lead GO and vanadium oxide to greater commercial success. The PhD thesis generally is unfolded into two parts. In the first part, a simple hydrothermal method to synthesize tungsten-doped V6O13 is reported. The introduction of tungsten dopant can have a significant impact on the nanostructure evolution of vanadium oxide during hydrothermal reaction, which results in the formation of nanocrystalline structure. A realtime characterization of the hydrothermal reaction process was employed to reveal the complex phase changes of vanadium oxide in the course, which can be important guidance for controlling the product quality in larger-scale production. Moreover, when applied to lithium ion batteries (LIBs), the doped nanocrystalline V6O13-based electrode can provide better battery performance than the undoped V6O13. In the second part, graphite oxide route to synthesize graphene oxide is investigated in terms of the choices of graphite sources (expanded graphite, graphite intercalation compound and natural graphite), pre-treatment of expanded graphite (microwave-induced expansion of graphite in different atmospheres), reaction temperature, and post-processing of GO. It was found that the expanded graphite prepared in ambient air had higher dispersibility in organic solvent and finally led to higher GO yield, through the modified Hummers oxidation, than those prepared in pure carbon dioxide or argon. This is possibly due to the introduction of extra oxygen-containing functionalities accompanied by the rapid heating of graphite. We also found that graphite intercalation compound was a more suitable starting material for making large-sized GO at room temperature. One distinguishing feature of the GO produced at room temperature is that it has more thermal labile oxygen functional groups which allows the facile restoration of electrical conductivity via a mild thermal annealing. This characteristic will be very helpful to better combine GO with the electroactive particles in LIBs and thus benefit the overall battery performance. Finally, we further compared the cost-effectiveness between the room temperature synthesis method and the lower temperature method, using commercial expanded graphite powder as the graphite source. It revealed that the GO synthesized at room temperature could regain more conductive sp2 carbon and reached the same level of electrical conductivity through thermal or chemical reduction. Therefore, the room temperature method can produce conductive graphene for energy storage applications in a more cost-effective manner. On balance, this PhD thesis further develops the scalable wet chemical production of GO and vanadium oxide for energy storage by systematically investigating the key synthesis parameters and establishing the improved protocols. Ultimately, this work is anticipated to push forward the commercialization of GO and vanadium oxide in the field of energy storage in the near future.