Ionomer Degradation in Polymer Electrolyte Membrane Fuel Cells

Abstract

The structural degradation of the polymer electrolyte in both the bulk membrane and the cathode catalyst layer (CL) was investigated. An accelerated stress test (AST) was developed to degrade the ionomer in two membrane electrode assembly (MEA) designs, with cathode catalyst structures comprised of 23 and 33 wt % Nafion, respectively. During the AST the air cathode potential was held at (where RHE is reference hydrogen electrode) at and 100% relative humidity for up to 440 h. The MEA with 33 wt % Nafion had a greater platinum content in the membrane and a higher fluoride washout rate, suggesting the higher ionomer content in the cathode CL facilitated the mass transfer of contaminants (such as dissolved platinum) into the membrane. It is proposed that was produced at the anode, diffused into the membrane, and decomposed at the platinum and iron sites bound in the membrane structure. The decomposition products attacked the ionomer both in the bulk phase and CL, causing (i) membrane thinning, which exacerbated the crossover, (ii) lower membrane conductivity, and (iii) CL structure degradation manifested by enhanced reaction penetration depth into the CL and decreased effective oxygen diffusivity due to the changes in CL water content. These effects acting in synergy had profound negative repercussions on the fuel cell polarization for the MEA with 33 wt % Nafion in the cathode CL.