TRANSPORT AND CATHODE DEGRADATION PHENOMENA IN HYDROXIDE-EXCHANGE-MEMBRANE FUEL CELLS

Abstract

Hydroxide-exchange-membrane fuel cells (HEMFCs) are an emerging alternative H2 fuel cell technology to the well-established proton-exchange-membrane fuel cells (PEMFCs). HEMFCs are potentially less expensive than PEMFCs due to active platinum-group-metal-free catalysts, simpler bipolar plates manufacturing, and other potential cost savings. The alkaline environment is less corrosive than the acidic environment which allows for less expensive metals and coating to be used for the bipolar plates. However, HEMFCs are a less mature technology with many key issues to understand and improve before commercialization. In this dissertation, we address some of the key issues: carbon dioxide purging, oxygen transport, water transport, and cathode degradation. It is well known that CO2 contamination in HEMFCs results in significant voltage losses. CO2 contaminated HEMFCs are self-purging, and the rate of CO2 purging increases at higher current densities, which can increase the rate of cathode and membrane degradation. Therefore, we developed a H2 starvation CO2 purge that regains approximately 94% of the voltage loss from low concentration CO2 contamination without going to high current densities. This H2 starvation CO2 purge shows less degradation than a higher current density CO2 purge due to maintaining a high cathode potential. With a HEMFC fully in hydroxide form, we measured the oxygen transport resistance using limiting-current analysis to improve overall cell performance under CO2-free air. By optimizing the gas-diffusion architecture without the presence of excess liquid water in the cathode, the microporous layer is removed and the triple serpentine flow field is replaced with nickel foam. We show that the GDL is critical for the electronic conductivity between the carbon support in the catalyst layer and the Ni foam. The decrease in the oxygen-transport resistance has a significant improvement on the polarization curves under CO2-free air. One of the most important challenges in HEMFCs is optimizing the water management. Maintaining high water content in the hydroxide-exchange-membrane and ionomer is crucial for high performance and long durability. Here, we show that the water diffusion from the anode, where water is generated, to the cathode, where water is consumed, is more than sufficient for the oxygen reduction reaction. As the cell polarization curve performance decreases, the overall water flux remains constant. Therefore, cell degradation does not affect the overall water flux, and the degradation is not affecting the water diffusivity in the membrane electrode assembly. The cathode has been identified as the bottleneck for HEMFC durability. Three durability tests were designed to probe different degradation mechanisms of the cathode: alkaline instability of the ionomer, carbon corrosion and ionomer oxidation by potential, and ionomer oxidation by reactive oxygen species. Ionomer oxidation by reactive oxygen species was found to be the primary mechanism. In an alkaline environment, the formation of reactive oxygen species is exacerbated by the catalytic activity of carbon for 2e- oxygen reduction reaction to peroxides. Identifying the dominant degradation mechanism and the possible exacerbation of it by the carbon support moves the field understanding and focus for improved durability for HEMFCs. This work gives sufficient evidence that research into more oxidative resistant ionomers and adding radical scavengers or sacrificial molecules that are stable in a HEMFC cathode while reducing system cost needs to be done to progress HEMFCs to long term durability and commercialization.