Investigating contributions from vehicular and Grotthus transport on hydroxide ion conductivity in anion exchange membranes from molecular dynamics simulations

Anion exchange membranes (AEMs) used in alkaline fuel cells rely on the selective transport of hydroxide ions through the membrane to produce electricity. The low conductivity of hydroxide ions in AEMs has prevented their widespread use and is of significant fundamental and industrial relevance. An understanding of the hydroxide transport mechanisms through the polymer can help in the design of membranes with improved conductivity. In prior work, we leveraged atomistic molecular dynamics (MD) simulations to capture correlations in hydroxide transport across four polyethylene AEMs with different functional groups. While the trends in conductivity were consistent with experiments, there was a significant difference in the magnitude. We ascribed this to the inability of our model to capture the reactive Grotthuss transport (proton hopping) mechanism which is dominant in these systems. In this work, to account for Grotthuss transport, we use the heuristic reaction protocol REACTER that allows us to model reactions within the framework of classical simulations. We find that the simulated hydroxide conductivity is in better agreement with experimental values after the inclusion of Grotthuss transport. We extend our analysis across different AEM chemistries, including polyethylene, polysulfone, and polynorbornene, and find that the balance between vehicular and Grotthuss transport strongly depends on the polymer's microstructure and water uptake.