Possible Repair Mechanism for Hydrocarbon-Based Ionomers Following Damage by Radical Attack
Tym de Wild, Tamás Németh, Tom M. Nolte, Thomas J. Schmidt, Thomas Nauser, Lorenz Gubler
Abstract
Polymer electrolyte fuel cell (PEFC) membranes are subject to radical-induced degradation. Antioxidant strategies for hydrocarbon-based ionomers containing aromatic units can focus on intermediates that are formed upon attack by hydroxyl radicals (HO · ). Among the different intermediates, the cation radical P ·+ is the most promising target for repair, for example by cerium(III). For the “repair” reaction of Ce(III) with radicals of a poly( α -methylstyrene sulfonate) oligomer we determined an activation energy of (9 ± 2) kJ mol −1 and a rate constant of 1.6 · 10 8 M −1 s −1 at 80 °C by pulse-radiolysis. For the reduction of Ce(IV) by hydrogen peroxide the activation energy was determined by stopped-flow as (30 ± 1) kJ mol −1 with a rate constant of 4.8 · 10 6 M −1 s −1 at 80 °C. These parameters are fed into a kinetics model to estimate the efficacy of the cerium (III)/(IV) redox couple as a catalytic repair agent in hydrocarbon-based fuel cell membranes. While cerium can mitigate polymer degradation, repair efficacy depends on the polymer degradation pathway and the nature and lifetime of the intermediates.