(547e) Residence Time Distribution of Anode Impurity Pulses in a 47-Cell Commercial Pem Stack

Pulses of various impurities were injected into the anode feed stream of a 47-cell Ballard® Nexa? PEM stack to measure the residence time distribution associated with each species. Inert gases with different diffusivities (helium, nitrogen, and argon) were tested in addition to reversible poisons (ammonia and carbon monoxide) at pulse levels of up to 20 cc. Feeds composed of 99.99% hydrogen are typically used in PEM stacks; however, impurity tolerances were increased for this study by leaving the exhaust valve partially open, instead of allowing the system to periodically purge the exhaust line as it was designed. Separate voltage taps were applied to each MEA in the stack to measure the time-dependent potential and concentration of impurities. A residence time distribution model was constructed to fit the experimental data and predict the outcome of future experiments. The aforementioned technique is useful for performing cell diagnostics, designing a manifold flow scheme, and understanding gas distribution within the stack feed manifold and on the catalyst surface of each MEA.