Helox Study of Transport Limitations in Non-PGM Cathodes for PEM Fuel Cells

A study of mass transfer limitations in a non-PGM cathode MEA were undertaken by studying the limiting current as a function of flow rate with varying oxygen carrier gases. Previous reports have indicated that the mass transfer coefficient for oxygen in the gas phase and electrolyte phase can be elucidated by studying the limiting current behavior in the presence of carrier gases of varying molecular weight. [1] We apply this approach to a non-PGM cathode using helium, nitrogen, and argon as carrier gases. Under conditions of low fractional O2 conversion, mass transfer limitations were found to exist mainly in the catalyst layer and flow field, not in the gas diffusion layer. Combined with mercury porosimetry studies to elucidate the macropore size distribution, this data is modeled using a previously-develop code [2] to provide an estimate of the gas-phase mass transfer coefficient, and by extension the fraction of hydrophobic, gas-phase pores in the catalyst layer.