Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Dr. Andy (Xueliang) Sun

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are non-polluting and efficient energy conversion devices that are expected to play a dominant role in energy solutions of the future. However, due to high cost and known degradation issue of Pt electrocatalyst, more durable, efficient, and inexpensive electrocatalysts are required before fuel cells can become commercially viable. This research is revolving around the development of electrocatalysts such as non-noble metal oxygen reduction reaction (ORR) catalyst, new alternative supports, and novel Pt nanostructures to address the above-mentioned challenges in PEMFCs.

Firstly, we report the synthesis of nitrogen doped carbon nanotubes (CNx) and nitrogen doped graphene (N-graphene) with the various nitrogen contents. The relationship between structures and ORR activity is investigated in detail. We identified the real active site by the study. Most importantly, CNx and N-graphene have the comparable ORR activity even the improved durability compared with a platinum-based catalyst, showing the potential to replace costly Pt/C catalyst in alkaline fuel cells.

Secondly, due to the advantages of N-graphene as not only a support of Pt but the non-noble metal ORR catalyst, we developed three different methods to prepare it: (i) post-treatment of graphene with ammonia (ii) from CNx to N-graphene directly (iii) one-step solvothermal process. Especially, by the solvothermal method, for the first time, nanoflower-like N-graphene was obtained with pure sp2 hybridized carbon and the controllable nitrogen types. Importantly, the synthesized materials exhibit much higher durability as Pt support for fuel cells than commercial carbon powder.

Thirdly, previous results have shown that star-like Pt nanowires have both good catalytic activity and durability for ORR. However, there is a limitation in scale up and the controlling length and shape of Pt nanowire for previous method. Here we report a universal method to address the challenge. It is a very simple, green and efficient wet chemical route without any surfactant and template to produce urchin-shaped Pt nanostructures in high yield.

In summary, the discoveries in this thesis contribute to development of fuel cell cathode electrocatalysts and make the improvement in electrocatalyst cost and stability.

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