277868 Steady State Temperature Dependent Carbon Oxidation Kinetics for Proton Exchange Membrane Fuel Cells
Vehicle applications for proton exchange membrane (PEM) fuel cells require low cost, durable materials to compete with incumbent propulsion technology. As a result, recent research has focused on reducing the quantity of precious metal needed to catalyze the oxygen reduction reaction (ORR) in the cathode electrode of the fuel cell. Some of the most promising low-loaded catalysts have been developed on high surface area carbon (HSC) supports, which enable fine dispersion of the precious metal catalyst, leading to higher mass activities. Unfortunately, HSC supports (such as Ketjen) are also more corrosion susceptible than previously used catalyst supports, such as Vulcan XC-72. Corrosion of the HSC support can promote catalyst sintering and increase electrode transport resistance. Consequently, vehicle systems are being developed to mitigate the most damaging events to the HSC support. One such event is cathode air storage during vehicle off times, where data has indicated that the cathode potential can reach about 0.96-1.0 V, well above the equilibrium corrosion potential of 0.207 V. As a result, it is critical to understand how much carbon loss can result from an air storage event. This work is focused on developing an empirical kinetic model for carbon loss under steady state high potential (> 1.0 V) conditions at various operating temperatures.
Low-loaded MEAs (0.2 mgPt/cm2 30% Pt-alloy catalyst on HSC support) were exposed to steady-state high potential (1.0-1.4 V) operation at various temperatures (30°C, 55°C, 80°C) to induce HSC support oxidation to carbon dioxide (CO2). The cathode outlet gas was analyzed using an in-line nondispersive infrared (NDIR) carbon dioxide detector at 1 Hz to establish a corrosion current vs. time profile. A form of the empirical equation for air storage corrosion current as a function of potential, temperature, and time was proposed, and an attempt was made to fit three coefficients to the collected data using multivariate linear regression. These kinetics were then used to perform a sensitivity analysis of number of air storage events versus duration of air storage events, comparing total predicted carbon loss from air storage over the life of each vehicle usage profile.
Finally, the characteristic decay of CO2 concentration over time during the steady state potential hold was used to hypothesize the possible impact of surface coverage (both catalyst and carbon) on carbon oxidation kinetics. Based on this hypothesis, future potential cycling testing to measure carbon oxidation is proposed.