(87n) Understanding Water Absorption, Percolation, and Ion Transport in an Anion Exchange Membrane

Understanding ion transport mechanisms in an anion exchange membrane (AEM) is crucial for designing efficient electrochemical-driven processes such as fuel cells, water electrolyzers, reverse electrodialysis, and redox flow batteries. A key challenge in understanding the ion transport properties of AEMs is the lack of a methodology that captures the ion solvation environment, water network, and ion transport at different time and length scales. 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. Bromide ion conductivities in polynorbornene-based thin films are measured as a function of temperature and relative humidity using electrochemical impedance spectroscopy. Bromide ion transport shows Arrhenius behaviors and activation energy (E_a) is used for the first time as an indicator for detecting the transition of transport mechanisms. Percolation theory is examined and enriched by combing experimental results and atomistic molecular dynamics simulations. We quantitatively demonstrate that the transition of a slower site hopping mechanism and a faster transport mechanism is aided by better solvation environments of anions and more percolated water pathways at a certain RH.