Mass Transport and Durability of Proton-Exchange-Membrane Fuel Cell Electrodes
Fang, Zhengyuan Jung
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Large transport resistances at high current densities hinder the proton-exchange-membrane fuel cells from reaching performance-cost-durability targets set by the U.S. Department of Energy (DOE). In this dissertation, the effect of carbon corrosion on the electrode wettability and the effect of carbon surface functionalization on the fuel cell performance and durability are investigated. In the wettability study, commercial membrane electrode assemblies were employed and the surface roughness and porosity were fitted to surface texture models. It was found that cathode sustained its wettability after up to 35 wt% of carbon support loss, at which the cell performance dropped below the DOE’s durability-performance target. In the surface functionalization study, three schemes were investigated for either grafting positively charged nitrogen surface groups or negatively charged sulfonate groups for three types of carbon supports. In full-cell tests, improvements over high current densities were observed in samples reacted with para-phenylenediamine or ammonia, whereas the performance decreased after functionalization with sulfonate groups. The improvement at high current densities exceeded the mass-activity improvement and was attributed to reduced mass-transfer polarizations. Furthermore, a statistical approach was explored to examine the changes in ionomer surface coverage and ionomer coverage was found to increase after functionalization with nitrogen containing group. In addition, accelerated stress tests were performed to study the durability. Lastly, a modified agglomerate model was developed to study the effect of ionomer coverage on the electrode mass-transport resistance. The major contributions of this dissertation include understanding the role of electrode wettability in durability studies, providing high-performing carbon supports that can be incorporated to the state-of-the-art electrocatalysts, and exploring a novel approach to calculate nano-scale ionomer coverage on the electrocatalysts.