Polymer Electrolyte Membrane Fuel Cell Active Area Utilization Dependence on Relative Humidity Measured via AC Impedance High Frequency Resistance

Abstract

In this paper, we report experimental data and analysis which suggests that the observed performance reduction of PFSA PEMFCs as reactant relative humidity is reduced below saturation is due to a reduction in the utilized active area of the membrane electrode assembly (MEA), rather than the typically reported mode involving increased bulk ionic resistances of the PFSA PEM and cathode electrode. This hypothesis is supported by the observation that the large measured performance decrease with reduced RH reactants can be very largely explained by assuming that the increase in measured AC impedance high frequency resistance (HFR) is due to decreased PEM conductive area rather than increased PEM bulk resistivity, using the HFR to estimate the actually-utilized active area and renormalizing the measured current density to the HFR-estimated active area. A simple qualitative model, consistent with the experimental data and analysis, suggests that performance loss with decreased RH may be primarily due to decreased areal density of ionically-conductive connections between the PFSA PEM and cathode electrode, leading to decreased electrocatalyst utilization in the direction parallel to the PEM-electrode interface.