Review of mechanistic origins of proton exchange membrane fuel cell degradation: Radical chemistry, coupled causal chains, and microstructural evolution

Proton exchange membrane fuel cells serve as an important enabling technology for the conversion of clean energy, yet their commercial success is restricted by durability degradation. Although many studies have found individual failure mechanisms can be identified in isolation for aging, such studies overlook non-linear interactions and feedback loops governing real-world aging effects. The present review undertakes a systematic, mechanisms-based review of PEMFC degradation down to the nano-scale and micro-scale. The review begins by exploring the chemical mechanisms for perfluorosulfonic acid membrane attack by hydroxyl radicals, from a chemical perspective including the four fundamental chain-scission mechanisms and humidity dependence. We then describe three closely paired causal chains that are causative of performance decay; they are (i) platinum dissolution leading to radical induced membrane degradation, (ii) carbon corrosion resulting in oxygen transport failure, and (iii) water management mismanagement amplifying peroxide formation. The three chains are evaluated with respect to kinetics, positive feedback loops, and cross-coupling with each other. Aside from the above core chains, the review also covers platinum resource constraints, loading trade-offs, and contaminant deactivation. The spatial distribution of radical generation is mapped over three zones: the catalyst layer interface, the bulk membrane, and the local thinning zone, where the self-amplifying feedback causes the failure to become more rapid. Catalyst layer instability is explored in conjunction with carbon support corrosion, including pore collapse, tortuosity increase, and platinum detachment. Finally, thermo-mechanical hotspot formation is examined by humidity cycling, gas crossover, and stress concentration, which synergistically accelerates local degradation. By synthesizing evidence from more than 200 recent studies, it is demonstrated that these intertwined mechanisms form a network of positive feedback that can accelerate end-of-life far beyond predictions from single-mechanism models. This review establishes a comprehensive mechanistic foundation for understanding PEMFC degradation – a necessary step toward designing more durable fuel cells.