Platinum Group Metal-Free Catalyst and Catalyst Layer Design for PEM Fuel Cells

Abstract

Facing the crises of energy shortage and climate change, it becomes increasingly urgent to find renewable and low carbon emission replacements for fossil fuels. Renewable energy such as wind power, hydropower, solar energy, geothermal energy, and bioenergy has been applied as power sources, mostly converted to electric power. In the area of transportation, gasoline and diesel are still the most widely used fuels, which are eco-unfriendly because of CO2, SOX and NOX emissions as the exhaust. In order to electrify the transportation process, electric power needs to be stored within appropriate media in the form of chemical energy, which must be able to easily transfer to electricity to power cars, buses, trains, cruises, airplanes, and other transportation modes. Lithium and hydrogen are the most promising candidates for transportation applications as the energy carrier, which puts the lithium-ion battery and hydrogen fuel cell into the dominance in the market. Restricted by the energy density of the lithium-ion battery, the driving range of a battery electric vehicle (BEV) is very limited. Comparing to BEV, Fuel cell electric vehicle (FCEV) has a much higher driving range benefit from the high energy density of compressed hydrogen.

There are challenges that FCEVs are confronting as well, among which the high cost of PEM fuel cell is one of the biggest challenges. The major reason that makes proton exchange membrane (PEM) fuel cell expensive is the platinum used to catalyze the anode and cathode reactions, mainly the cathode. The primary goal of the works for this thesis is to prepare catalysts that are: (1) highly active; (2) durable; (3) cost-effective; (4) scalable. The catalysts prepared will not only be tested in the “half-cell” simulated by the three-electrode system to verify their oxygen reduction reaction activity, but also be incorporated to the PEM fuel cell to see their performance in real applications. In the first work of this thesis, we prepared an ultra-high surface area hollow carbon sphere as the carbon support. By using the aminothiophenol as the N, S co-doping precursor, the hollow sphere structure was successfully retained in the final catalyst HCS-A, which also has a high surface area. HCS-A was also found to have high activity, especially in the alkaline medium. In the second work of this thesis, the heteroatom doping is restricted with nitrogen. However, we applied a secondary nitrogen doping precursor to enhance the nitrogen doping and boost the oxygen reduction reaction (ORR) activity. In order to meet the requirements for larger-scale applications, we have successfully scaled up the catalyst from milligram-scale to gram-scale, without any diminishment in the ORR activity. The scale-up catalyst with secondary nitrogen doping SU-PAU has demonstrated the state of art half-cell activity and PEM fuel cell performance. The last work of this thesis focuses on operating condition study and membrane electrode assembly (MEA) design optimization. Through a systematic study, we were able to obtain in-depth knowledge of how the operating parameters and design parameters may affect the performance. After careful optimization, the highest H2-air performance to date was achieved with a commercial size MEA.