Chemical resilience of hydrocarbon-based PEMs in fuel cells: effect of aromatic electron density on molecular degradation and in-situ aging

Radicals formed during operation of polymer electrolyte fuel cells, particularly during accelerated stress tests (AST), chemically degrade hydrocarbon proton exchange membranes (HC PEMs). In HC PEMs, the path from initial chemical attack over the evolution of membrane damage to failure is not yet well understood. This is evident when assessing different classes of HC PEMs in OCV hold ASTs: while chemical degradation occurs in all systems, expression of the evolution of membrane damage can differ vastly. This work combines an extensive OCV hold study with a complimentary gamma-radiolysis investigation on model small molecules for the first time. Our experiments reveal that the in-situ AST lifetime trends of HC PEMs correlate with the susceptibility of phenyl sulfonates – featuring to some extent comparable electronic configurations – to undergo degradative chain-reactions when exposed to the highly oxidizing radicals generated during the radiolysis of aqueous solutions. Our conclusions aim to incite the discussion on underlying radical stability of different HC PEMs, and we propose that electron-rich, poly(phenylene) type materials more likely undergo chemical degradation pathways that delay OCV failure, i.e. “suppress” membrane damage, when compared to electron-poor poly(phenylene sulfone) and poly(ether ether ketone) type materials. • First comparative OCV hold assessment of HC PEMs focusing on evolution of inevitable membrane damage. • For hydrocarbon constituents, Ce(III) acts as a repair agent and not as radical scavenger. • Radical-induced degradation is suppressed for compounds with low ionization potential. • PEM chemistry may limit high frequency resistance measurements as metric for membrane's state-of-health. • Complexity of cell environment constrains translation of findings from small molecules to membrane degradation.