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This paper discusses methodologies to evaluate durability of catalyst and carbon-support materials used in Polymer Electrolyte Membrane (PEM) fuel cells under relevant automotive accelerated test conditions. Durability of carbon-supported Pt and Pt-alloy catalyst is evaluated under an accelerated voltage- cycling protocol, developed by analyzing idle-to-peak power load-transients of various automotive drive-cycles. Results indicate that Pt catalysts on conventional carbon supports (Pt/C) are unlikely to meet automotive durability target; however, given no loss in specific-activity over time, Pt- alloys are likely to be successful. Shutdown/startup of fuel cell stack and local fuel starvation are recognized as an accelerating mechanism for carbon-support corrosion. Conventional and corrosion-resistant supports are evaluated under an accelerated protocol (1.2V vs. RHE). Corrosion of these currently used supports induces unacceptable mass- transport related performance loss at high current densities. Implementation of corrosion-resistant supports in high- performance electrodes, combined with controlled system strategies, will most likely lead to automotive carbon support durability targets being met.

Conventional carbon MEAs and graphitized carbon MEAs were evaluated for the resistance to carbon corrosion and startup/shutdown durability in this paper. Graphitized carbon MEAs show higher resistance to carbon corrosion than conventional carbon MEAs by a factor of 35 at a point where 5% weight loss had occurred. A graphitized carbon MEA yielded lower degradation rate than that of a conventional carbon MEA by a factor of 5 after 1,000 startup/shutdown cycles. The kinetics of carbon corrosion over both conventional carbon MEAs and graphitized carbon MEAs were measured, and carbon corrosion during startup/shutdown was explained and modeled. The model results correlate to what we have measured from our startup/shutdown durability test. Overall, MEAs with corrosion resistant carbon supports are one of major materials approaches to mitigate cell voltage degradation due to fuel cell startup/shutdown. We believe that a combination of corrosion resistant materials and system operating mitigation strategies is the path to attain the strict automotive durability targets.

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This paper discusses fundamental models developed to predict cathode carbon support corrosion induced by start-stop and local H 2 starvation in a PEM (proton exchange membrane) fuel cell. The model incorporating the electrode pseudo-capacitance agrees well with controlled start/stop experiments. When the pseudo-capacitive effect is included, the model not only captures the difference in CO 2 evolution between start and stop, but also matches the measured spatially resolved mass activity and limiting current distribution of the damaged cathode electrode. For long H 2 /airfront residence times during start/stop, commonly used in accelerated materials tests, the electrode damage predicted by the capacitive model is similar to that predicted by previously published models that neglect pseudo-capacitive effects. However, for application relevant shorter residence times, significantly lower damages are predicted when capacitive effects are included, consistent with experiments. Because local H 2 starvation occurs on a longer time scale, pseudo-capacitive effects are less significant.

Carbon blacks such as Vulcan XC-72 are widely used to support platinum ͑Pt͒ or Pt alloy catalysts in proton exchange membrane fuel cells. Despite their widespread use, carbon blacks are susceptible to corrosion during fuel cell operations. In this work, the corrosion behaviors of platinized Vulcan XC-72 nanoparticles under thermal and electrochemical conditions were monitored by transmission electron microscopy ͑TEM͒. The thermal corrosion experiment was carried out in a gas-cell TEM, which allows for a direct observation of the thermal oxidation behavior of the nanoparticles. The electrochemical corrosion experiment was performed outside of the TEM by loading the nanoparticles on a TEM grid and then electrochemically corroding them step by step followed by taking TEM images from exactly the same nanoparticles after each step. This work revealed four types of structural changes: ͑i͒ total removal of structurally weak aggregates, ͑ii͒ breakdown of aggregates via neck-breaking, ͑iii͒ center-hollowed primary particles caused by an inside-out corrosion starting from the center to outer region, and ͑iv͒ gradual decrease in the size of primary particles caused by a uniform removal of material from the surface. These structural changes took place in sequence or simultaneously depending on the competition of carbon corrosion dynamical processes. The results obtained from this work provide insight on carbon corrosion and its effects on fuel cells' long-term performance and durability.

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