Anion Conduction and Water Percolation Effects in Polynorbornene-based Thin Film Membranes

Anion exchange membranes (AEMs) are at the heart of many electrochemical driven processes such as fuel cells, water electrolysers, reverse electrodialysis, and redox flow batteries. It requires a fundamental understanding of ion transport in AEMs at different hydration states to efficiently operate these systems with long term durability. To allow for better understanding, combining experimental characterizations with targeted simulations reveals new insights on the interplay of water and ion transport in hydrated AEMs. Here, we have synthesized and fabricated polynorbornene-based anion exchange thin films as our model polymers due to their high alkaline stabilities and ionic conductivities via vapor infiltration reactions (VIRs). We customize an in situ ellipsometer to understand thin film expansion at different hydration levels and water uptakes of AEMs has been measured by dynamic water sorption. We investigate bromide ion (Br^-) transport by measuring thin film electrochemical impedance as a function of relative humidity and temperature. Br^- conductivities show Arrhenius behaviors and activation energy has been extracted as a function of relative humidity. By combining experimental characterizations, percolation theory, and atomistic molecular dynamics simulations, we quantitatively identify two transport regimes (site hopping mechanism and vehicular mechanism) from low to high relative humidity.