Voltage jump during polarization of a PEM fuel cell operated at low relative humidities

A PEM fuel cell with a Nafion 211 membrane-based membrane electrode assembly (MEA) was tested with an H2/air stoichiometry of 2/4 at 25%, 50%, 75%, and 100% relative humidities. A voltage jump on the polarization curve was observed when the cell was operated at a lower humidity. This phenomenon may be explained by the water back-diffusion from the cathode into the membrane resulting in both a non-uniform water distribution in the membrane and a liquid-equilibrated interface between the membrane and the anode catalyst layer. Experimental results obtained by AC impedance spectroscopy measuring the MEA resistance (membrane+catalyst ionomer layers) at different current densities as well as collected polarization data at high feed-gas flow rates (or at low backpressures) and high temperatures all confirmed the validity of the proposed water back-diffusion hypothesis.