Self-Assembled Water Channels in Fluorine-Free Polymers for Fast Proton Conductivity*

While perfluorosulfonic acid polymers are widely used as proton-exchange membranes (PEMs), concerns arising from fluorine have stimulated interest in hydrocarbon-based PEMs. In previous work, we studied a linear polyethylene with a phenylsulfonic acid pendant group precisely on every fifth carbon and found the proton conductivity to exceed 0.1 S/cm above 65% relative humidity at 40 C. This study explores related polymers with lower sulfonations levels to improve the processability and mechanical properties. By combining novel synthesis, X-ray scattering, FT-IR spectroscopy, pulse-field gradient NMR, electrochemical impedance spectroscopy, and all-atom molecular dynamics simulations, we are establishing design rules to produce nanoscale water channels with high proton conductivity. From the simulations we extract quantitative information about the self-assembled water channels in the hydrated polymers via cluster analysis, channel width distributions, surface area per sulfonate group, and fractal dimensions. From the spectroscopy methods, we probe the local structure and relaxations of water in the hydrated membranes. Notably, a moderate reduction in sulfonation level enhances the mechanical toughness while maintaining high proton conductivity in these new fluorine-free PEMs.