Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

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.