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 per-fluoroalkylsulfonic 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 rad-ical concentrations of around 1019 M for H, 1016 M for HO and 1010 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 cor-responding calculated fluoride emission rate (FER) showed good agreement with experimental data under ex situ Fenton test con-ditions. 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 a-hydrogen abstraction by HO, the aromatic ring effectively scav-enges HO to form an OH-adduct. Follow-up reactions lead to chain scission and formation of a stable hydroxylated degradation product. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3581040] All rights reserved. Manuscript submitted February 8, 2011; revised manuscript received March 28, 2011. Published April 27, 2011. The proton conducting membrane in the polymer electrolyte fuel cell (PEFC) is exposed to considerable oxidative stress due to th