Guo, Jian; (2020) Design of carbon-based heterostructures for oxygen electrocatalysis. Doctoral thesis (Ph.D), UCL (University College London).

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Text PhD Thesis_Jian Guo-16042750-UCL Chemistry-July 2020-Submitted version.pdf - Accepted Version Download (11MB) | Preview

Abstract

The research for developing sustainable and clean energy conversion and storage technologies (such as rechargeable metal−air batteries, fuel cells, etc.) has attracted tremendous attention over past decades. Among which, the development of cost-effective and high-performance electrocatalysts for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial but still challenging for practical applications. Therefore, the primary objective of this thesis was to rationally design carbon-based ORR and OER electrocatalysts for applications in zinc-air batteries by a cost-effective strategy. The research background and theoretical knowledge are summarized in Chapter 1 and Chapter 2. Several novel strategies for the preparation of oxygen electrocatalysts and air-cathodes in zinc-air batteries are proposed and presented in the following Chapters. Firstly, considering intrinsic microporosity, high specific surface area and nitrogen rich ligands in ZIF-67 (zeolitic imidazolate framework-67, is a sub-family of metal-organic frameworks with zeolite-like structure, consist of cobalt metal centres and imidazolate linkers), and the hierarchical pore structure with lower oxygen content in thermal-shock exfoliated graphene oxide (EGO), a series of ZIF-67@EGO hybrid nanostructures were successfully synthesised via a highly controllable and facile method. The influence of ZIF-67 loading ratio in the hybrids and the carbonization temperature for the preparation of the final oxygen electrocatalysts was systematically investigated, an optimal hybrid nanostructure was achieved to produce highly efficient bifunctional oxygen electrocatalyst. The zinc-air battery based on this catalyst exhibits a high peak power density of 175 mW cm-2 and specific capacity of 767 mAh g-1, as well as superior long-term cycling stability (Chapter 3). Subsequently, inspired by the first study, a further low-temperature thermolysis strategy was developed to produce ultra-small Co3O4/Co nanoparticles in nitrogen-doped hyperporous graphenic networks (Co3O4/Co@N-G). By utilising the residual oxygen functionalities of the EGO and low-gasification point (≈350 °C) of the ZIF-67, the catalyst, Co3O4/Co@N-G, was developed at moderate conditions, ≈450 °C in nitrogen only atmosphere. The as-synthesised catalyst without any further acid washing or oxidation process, exhibits excellent ORR performance. In addition, this low-temperature route yields a high amount of the catalyst (>65 wt%), which is far higher than the commonly reported (<30 wt%) high-temperature derivatives. This study shows not only a new method of producing high-performance ORR catalyst with high yield and low energy consumption, but also an effective way of controlling metal aggregation and in-situ oxidation of MOF (ZIF) structures (Chapter 4). Moreover, a self-activation phenomenon of carbon paper cathode substrates in zinc-air batteries was discovered and systematically explored. It was found that the air-cathodes generated from electrochemical activation (during the galvanostatic discharge/charge process) of normal carbon paper substrates without any additional electrocatalysts can be directly used in zinc-air batteries. Therefore, this new method for making air-cathodes is scalable, extremely facile and low-cost. The self-activated carbon paper substrate exhibits an impressive cycling stability (more than 165 hours for 1,000 cycles) and a small discharge-charge voltage gap. After the activation, the maximum power density and electrochemical surface area were increased by over 40 and 1920 times, respectively. The mechanism behind this enhancement was revealed by multiscale simulations and comprehensive characterizations (Chapter 5). Overall, step-by-step research was performed to develop new strategies and new nanomaterials with reduced cost for application in oxygen electrocatalysis. The achievements in this thesis pave a novel and cost-effective pathway for rational design of carbon-based heterostructures as high-performance oxygen electrocatalysts and can be further applied in diverse energy conversion and storage technologies.

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