Acid Site Design for Low Hydration Proton Exchange Membranes

ABSTRACT Hydrocarbon proton exchange membranes (PEMs) are promising PFSA‐free candidates for proton exchange membrane‐based electrochemical devices, but high conductivity is typically accompanied by excessive hydration and swelling. Here we introduce, for the first time in a polymer electrolyte membrane, a sulfonic acid group bearing a difluoromethylene (─CF 2 −) substituent directly adjacent to the acid site. Density functional theory calculations indicate that α‐CF 2 substitution effectively lowers the pK a of alkylsulfonic acid, suggesting enhanced proton dissociation under reduced hydration. Guided by this concept, we synthesized the hydrocarbon membrane TPCF2SA and compared it with the alkylsulfonic acid analogue TPSA. TPCF2SA exhibits much lower water uptake (20 vs. 51% at 80°C), lower hydration number (λ = 6.2 vs. 15), and lower swelling, while maintaining high proton conductivity (87 vs. 79 mS cm −1 ). Morphology analysis reveals a more distinct hydrated nanostructure for TPCF2SA at 80°C, consistent with efficient proton‐transport pathways at reduced water content. In water electrolyzers, TPCF2SA delivers 3.0 A cm −2 at 1.79 V, strongly outperforming TPSA and approaching NR212. These results highlight acid‐group design as a powerful strategy for low‐hydration, high‐performance hydrocarbon PEMs.