Durability of hydrocarbon ionomer-based cathode catalyst layers for PEM fuel cells under voltage cycling

Hydrocarbon (HC)-based proton-exchange membrane fuel cells have gained attention as a more sustainable alternative to conventional systems utilizing perfluorosulfonic acid (PFSA). While current optimizations have revealed performance parity of sulfo-phenylated poly(phenylene) ionomers in cathode catalyst layers (CLs), the long-term durability of CLs employing such HC ionomers remains insufficiently characterized. By applying a voltage cycling accelerated stress test (AST), this study finds a higher cathode CL degradation compared to a PFSA-based reference in a twofold sense: First, the CL's protonic resistance increases during the AST, which is not observed in the PFSA case. Second, the catalyst roughness factor decreases faster due to an accelerated Pt nanoparticle growth. While this decay of electrochemically active surface area scales expectedly with ionic conductivity (i.e., with ionomer volume fraction and relative humidity) within the same ionomer class, it is surprisingly found that even at a lower absolute number of sulfonic acid groups and lower ionic conductivity, the HC-based CL undergoes faster Pt agglomeration than the PFSA counterpart. As a speculative explanation, first evidence of a higher affinity of the utilized HC ionomer towards Pt ions is revealed, which may lead to an increased Pt content within the ionomer and hence to an enhanced ionic Pt transport.