Kinetic Models of Radical Attack on Sulfonated Hydrocarbon Proton Exchange Membranes

The chemical stability of proton exchange membranes (PEMs) remains a critical limitation for the long-term durability of fuel cells. Although perfluorosulfonic acid membranes such as Nafion offer excellent durability and proton conductivity, their cost and possible environmental concerns motivate the development of sulfonated hydrocarbon-based alternatives. In this work, density functional theory calculations were employed to study radical-induced degradation mechanisms, with emphasis on C–S bond cleavage of the sulfonic acid functional group. Our results indicate that direct aryl sulfonation is inherently more prone to desulfonation under hydroxyl radical attack than fluorinated C–S linkages. Introducing a methylene spacer between the aromatic ring and the sulfonate group substantially reduces this susceptibility. However, spacer insertion shifts the degradation pathway, as benzylic C–C bond cleavage becomes energetically favorable due to radical stabilization at the aromatic–alkyl junction. Replacement of the aromatic ring with a saturated cyclohexane unit suppresses this vulnerability and significantly increases resistance to C–C scission. These results reveal a mechanistic trade-off in hydrocarbon PEM design and provide molecular-level insight into how electronic structure governs radical-induced degradation pathways.