He, Guanjie; (2018) Design of nanostructures and hybrids of transition metal derivatives for energy storage applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The research for high-performance energy storage devices, such as supercapacitors, Li-ion batteries, is continuing apace. These devices can store energy via electrochemical processes. However, further increases of capacity and stability of these devices to meet practical requirements remain a challenge. By using nanostructured electrode materials, several strategies were proposed in this thesis based on transition metals and their derivatives. The binder-free strategy was used across the entire project, which could increase the utilization and avoid “dead volume” of active materials. To improve the commonly reported electrochemical performance in aqueous of transition metal hybrids, such as nickel, cobalt or tungsten materials, bimetal nanostructured electrodes were proposed. Ni foam supported NiWO4 and CoWO4 electrodes were synthesized and increased the capacity and rate performance because of an increased electrical conductivity. Other methods for enhancing energy storage performance by means of increased electrical conductivities were found to be the sulfide or nitride counterparts of the corresponding oxide. To further improve the specific capacity and clarify the blurred energy storage mechanism of nickel or cobalt-based electrode materials, nickel cobalt sulfide nanostructures within sulfur and nitrogen-doped graphene frameworks were produced. Importantly, the redox reactions between materials and OH- groups were demonstrated as diffusion-controlled processes. In addition, the solid-state devices and properties of oxygen reduction were explored. Pseudocapacitor electrodes are attractive by bridging the power density and energy density between the electro-double layer capacitor and batteries. W2N@C core-shell structures on carbon cloth was synthesized by chemical vapour deposition via an ammoniation process and evaluated for supercapacitor performance. Combining with electrochemical analysis, in-situ electrical measurements and simulation, the merits of W2N materials were determined. The interesting nanostructured electrodes could be utilized in Li-ion batteries. Illuminated by above stratigies and considering the stable structures, capacity contribution and Li intercalation, a highly stable V2O5-based cathodes were developed.

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