Investigation On Inonic Channel Morphology Of PPO-Im Membrane And OH-Conduction Mechanism

In anion exchange membrane,OH~-conduction has close relationship with the affinity of functional group and hydrogen bond network,which changes with varying the water uptake of the membrane.In order to explore the effect of water uptake on the affinity of functional group to OH~-and hydrogen bond network,a series of molecular dynamics simulations based on all-atom force field were performed on the imidazolium-grafted PPO(PPO-Im)membranes with different water uptakes.The local distributions of OH~-and water around imidazolium group indicate that increasing water uptake enhances the hydration structure of the imidazolium group and weakens the affinity of the imidazolium group to OH~-.The critical water saturation of imidazolium group could produce proper affinity to the surrounding OH~-.When imidazolium group is water saturated,further increasing water uptake is not conductive to retain the affinity to OH~-.As a result,the critical water saturation of imidazolium group makes a balance between the affinity to OH~-and the transfer of OH~-in the hydrated PPO-Im membrane.Furthermore,it could also produce percolated hydrophilic channel and keeps relatively high mechanical strength of the PPO-Im membrane.Therefore,the imidazolium groups should be maintained under the critical water saturation,where two and eight water molecules should be respectively kept in the first and second hydration shells.OH~-conduction involved in the hydrophilic channel of anion exchange membrane strongly depends on the water uptake.By means of local structural properties and pair-potential energy,reasonable hydrogen bond criteria were determined to describe the hydrogen bond network confined in the membrane.Increasing water uptake enhances the hydration structures of water and OH~-,and facilitates the reorganization of the hydrogen bond network.Water and OH~-are nearly saturated with water when the water uptake reaches?=10,where well-connected hydrogen bond network is produced.Further increasing water uptake has much less contribution to improving the hydrogen bond network,but inevitably swells the membrane channel.This work provides a molecular-level understanding of the effect of the hydration structure of imidazolium group on OH~-conduction and hydrogen bonding structures of the PPO-Im membrane.It also provides potential guidance for maintaining high performance in anion exchange membrane fuel cell.