Characterization of structural degradation in a polymer electrolyte membrane fuel cell cathode catalyst layer

Abstract

This study investigated the structural degradation of a polymer electrolyte membrane fuel cell (PEMFC) due to carbon corrosion and ionomer degradation. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and polarization analyses were completed to characterize and correlate the structural degradation to the performance. Accelerated stress tests (AST) were used to produce the different known degradation mechanisms. Both failure mechanisms had unique fingerprints on the performance degradation. The carbon corrosion results showed a clear thinning of the cathode catalyst layer (CCL) and gas diffusion carbon sub-layer, and a reduction in the effective platinum surface area caused by the carbon support oxidation. The degradation of the CCL and carbon sub-layer altered the water management, as evidenced by an increase of the voltage losses associated with oxygen mass transport and CL ohmic resistance. The ionomer degradation AST showed that greater ionomer in the CCL resulted in greater platinum content in the membrane and a higher fluoride washout rate, suggesting the higher ionomer content facilitated the mass transfer of contaminants (such as dissolved platinum and iron) into the membrane. It is proposed that H2O2 was produced at the anode, diffused into the membrane, and decomposed at the platinum and/or iron sites bound in the membrane structure. The decomposition products attacked the ionomer both in the bulk phase and CCL causing: i) membrane thinning which exacerbated H2 crossover, ii) lower membrane conductivity, and iii) CL structure degradation, resulting in increased reaction penetration into the CL and decreased effective oxygen diffusivity due to changes in CL water content. A method using an electrochemical quartz microbalance (EQCM) was investigated to further evaluate ionomer degradation. Mimicking the ionomer films in the CCL on the EQCM would enable a quantitative method to further evaluate the degradation reactions and overall mechanism. While this technique was not fully developed, background on the EQCM and the work to date is presented as a starting point for future development.