Role of Water Molecules on Vehicular Transport Mechanism in Polynorbornene-based Anion Exchange Membranes

Ion conducting polymer electrolytes are at the heart of fuel cells and water electrolyzers. The rise in anion exchange membranes (AEMs) research is attributed to the viability of using platinum-group-metal free electrocatalysts. However, the complex role of water molecules in facilitating ion transport is still in debate. Here, we investigated polynorbornene-based AEMs as our model polymers due to their high alkaline stabilities and outstanding fuel cell performance. We employ atomistic molecular dynamics (MD) simulations to decouple site-hopping and the vehicular mechanism by monitoring bromide ion transport. The conductivity-temperature relationship at different relative humidity (RH) conditions follows an Arrhenius behavior. We find that with increasing RH, the activation energy drops dramatically at around 55% RH and then remains constant. In addition to the common site-hopping mechanism, we use a quantitative model to show that the formation of water percolation can significantly change ion solvation and facilitate vehicular transport. To enable fast vehicular transport, both percolated water networks and dynamic water molecules are needed. We expect these new findings will provide useful guidance for the design of novel polymer membranes.