A Model for Water Transport in the Membrane and an Impedance Spectroscopy Study of the Effect of Relative Humidity on PEM Fuel Cell Parameters

Effective water management is essential for the optimal performance of PEM fuel cells. We have developed an impedance model for liquid water transport through the membrane and coupled it with the two-phase model for cathode side impedance. The complete model was fitted to experimental spectra measured at anode/cathode relative humidities (RH) of 32/32%, 50/50% and 100/100% within a current density range of 100 to 1000 mA cm −2 and an air flow stoichiometry of 2. Cathode catalyst layer (CCL) saturation decreases with current density due to a growing liquid pressure gradient. For all RH values, the CCL oxygen diffusivity increases dramatically with cell current due to progressive involvement of larger pores into the proton current conversion. The Tafel slope of the oxygen reduction reaction increases with the cell current. At RH 32/32% the Tafel slope is significantly larger than an the other two RH combinations. Higher RH leads to higher double layer capacitance, which indicates that liquid water increases the electrochemical surface area.