Grotthuss versus Vehicular Transport of Hydroxide in Anion-Exchange Membranes: Insight from Combined Reactive and Nonreactive Molecular Simulations

Combined reactive and nonreactive polarizable molecular dynamics simulations were used to probe the transport mechanisms of hydroxide in hydrated anion-exchange membranes (AEMs) composed of poly(p-phenylene oxide) functionalized with the quaternary ammonium cationic groups. The direct mapping of membrane morphologies between two models allowed us to investigate the contributions of vehicular and Grotthuss mechanisms in hydroxide motion and correlate these mechanisms with the details of local structure. In AEMs with nonblocky polymer structure, where anion transport occurs through narrow (subnanometer size) percolating water channels, simulations indicate the importance of the Grotthuss mechanism. In nonreactive simulations, in order to diffuse through bottlenecks in the water channels, the hydroxide anion has to lose part of its hydration structure, therefore creating a large kinetic barrier for such events. However, when the Grotthuss mechanism is involved, the hydroxide transport through these bottlenecks can easily occur without loss of anion hydration structure and with a much lower barrier.