Radical (HO•, H• and HOO•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Abstract

Formation of radicals, such as HO•, H• and HOO•, in the membrane of the polymer electrolyte fuel cell and their attack on perfluoroalkylsulfonic acid (PFSA) and poly(styrenesulfonic acid) (PSSA) ionomers was simulated based on a kinetic framework with H2O2 as “parent” molecule and with contaminating Fe as parameter. Analysis under quasi-steady state conditions yielded radical concentrations of around 10−19 M for H•, 10−16 M for HO• and 10−10 M for HOO•. H• is formed via the reaction of HO• with H2 dissolved in the membrane. The attack of the PFSA ionomer was assumed to proceed via weak carboxylic end-groups. The corresponding calculated fluoride emission rate (FER) showed good agreement with experimental data under ex situ Fenton test conditions. The predicted FER under fuel cell operating conditions was underestimated by 2–3 orders of magnitude. It is likely that degradation via side-chain attack is prevalent during open circuit voltage hold tests. The oxidative degradation of PSSA ionomer follows an entirely different pathway, because, in addition to α-hydrogen abstraction by HO•, the aromatic ring effectively scavenges HO• to form an OH-adduct. Follow-up reactions lead to chain scission and formation of a stable hydroxylated degradation product.