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.

The attack by HO^• on the ionomer in the polymer electrolyte fuel cell (PEFC) can be mitigated by the incorporation of regenerative radical scavengers, such as cerium and manganese, into the membrane. The influence of the presence of the Ce and Mn ions on the reaction of HO^• with the perfluoroalkylsulfonic acid (PFSA) ionomer was studied in the framework of a kinetic model. Scavenging of HO^• by Ce³⁺ and Mn²⁺ leads to a decrease in the steady-state concentration of HO^• and, therefore, the rate of ionomer attack, where Ce is more effective than Mn. The HO^• scavenging yield was found to decrease with increasing reactive end-group concentration of the PFSA ionomer. The oxidized metal ions are rapidly reduced by both and HOO^•, which restores the scavenger and ensures that the majority (>99.99%) of metal ions is in the reduced, “HO^•-scavenging-active” oxidation state. This stabilization mechanism is not expected to be operational in hydrocarbon membranes, because HO^• reacts with aromatic units far too rapidly. Also, the effectiveness of the mitigation mechanism is limited under the conditions of an ex situ Fenton test due to the abundance of H₂O₂, which is confirmed by experimental data reported in the literature.