Molecular modeling of OH− transport in poly(arylene ether sulfone ketone)s containing quaternized ammonio-substituted fluorenyl groups as anion exchange membranes

We present classical molecular dynamics (MD) and first principles molecular dynamics (FPMD) studies to investigate the transport mechanisms of hydroxide ions through poly(arylene ether sulfone ketone)s containing quaternized ammonio-substituted fluorenyl groups (QPE) as anion exchange polymers used in applications such as alkaline fuel cells. The effect of the number of repeating units in QPE on the ion conductivity of hydroxide ions was investigated by classical MD. Calculated ion conductivities are almost the same regardless of the number of repeating units in QPE and are consistent with the experimental values. The microscopic structures of water distribution in hydrated QPE were also investigated. Analysis of radial distribution functions (RDFs) of atoms calculated by classical MD implies the possibility of two different interacting conformations of hydroxide ions and ammonium groups in hydrated QPE. Distinct peaks in the RDF of O(hydroxide ion)-N(QPE) means that diffusion of hydroxide ions in hydrated QPE is mainly governed by surface diffusion, i.e., hopping between the ammonium groups of QPE, when the vehicle transport occurs. Transport of hydroxide ions based on the Grotthuss mechanism was also investigated by FPMD. The results show that hydroxide ion transport occurs with the formation of H3O2− through the hydrogen network of the water molecules. The formed H3O2− cleaves into H2O and OH−, resulting in proton exchange between a water molecule and a hydroxide ion, which is similar to the proton transfer mechanism observed in proton exchange membranes.