Effects of an Electric Field on Hydroxide Diffusion in Aqueous Anion Exchange Membranes

Abstract

Anion exchange membrane (AEM)-based water electrolysis represents a promising pathway for sustainable hydrogen production, leveraging nonprecious metal catalysts. A comprehensive understanding of hydroxide ion (OH–) transport mechanisms within AEMs is crucial to achieving high-performance electrolysis. However, the influence of an electric field on OH– diffusion in aqueous AEMs remains inadequately explored. This study employs reactive force field (ReaxFF) molecular dynamics (MD) simulations to investigate OH– diffusion through four distinct mechanisms: Free-Vehicular, Free-Grotthuss, Associate-Vehicular, and Associate-Grotthuss. Three poly(biphenyl alkylene) (PBPA) AEMs with varying hydration levels and hydrophilicity were analyzed under conditions with and without an applied electric field. Results indicate that the Associate-Grotthuss mechanism predominantly governs OH– diffusion across all three AEMs. Higher hydration levels enhance OH– diffusion rates, while the application of an electric field significantly increases the total diffusion coefficient of OH–. Microstructural analysis further reveals that AEMs with higher hydration levels and greater hydrophilicity promote OH– diffusion by preserving water channel continuity and pore width. Additionally, the electric field improves the orientation order of water molecules, further facilitating OH– transport. These findings provide valuable insights into the enhancement of hydroxide ion diffusion via applied electric fields and offer theoretical guidance for designing high-conductivity AEMs to advance water electrolysis technology.