Understanding the Role of Water Molecules and Ion Transport Mechanisms in Anion Exchange Membranes

Understanding the ion transport mechanisms of anion exchange membranes (AEMs) is crucial for the design of efficient polymer electrolytes for fuel cells and other electrochemical technologies. A key challenge in understanding the ion transport properties of AEMs is the lack of a methodology that combines macroscopic experimental characterizations with simulation results to probe ion transport mechanisms at different hydration states from a molecular level. Here we develop a methodology to investigate site hopping and vehicular transport mechanisms using anion exchange thin films, interdigitated electrodes, and atomistic molecular dynamics (MD) simulations. 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 site hopping and vehicular transport mechanisms. Percolation theory is examined and enriched by combining experimental results and MD simulations. We quantitatively demonstrate that the transition of site hopping and vehicular mechanisms is aided by better solvation environments of anions and more percolated water pathways at 55% RH. The change in conductivity observed by both experiments and simulations could be attributed to the microstructural change in the robustness of percolation. These results have important implications for the design of AEMs as efficient ion-conducting membranes.