Anion Exchange Membranes with Low Hydration Conditions from an Ab Initio Molecular Dynamics Perspective

Abstract

Regardless of the initial hydration level, a typical Anion Exchange Membranes (AEMs) fuel cell device would operate ultimately under extremely low hydration values. It is therefore vital to fully explore the hydroxide diffusion in AEMs under such conditions. In this work, we present fully atomistic ab initio molecular dynamics (AIMD) simulations to obtain a molecular-level understanding of the hydroxide solvation complexes and diffusion mechanism in low hydration AEMs. By changing the polymer electrolyte cation spacing and the hydration values we create three different AEMs models. We find that under extreme low hydration conditions, the water molecules attain a non-uniform distribution. We further see that the unique water distribution results in distinct hydroxide diffusion mechanisms varies from a stationary behavior, a vehicular diffusion, and a mixture of structural and vehicular diffusion, depending on the existence of a water oxygen at the hydroxide second solvation shell. Therefore, we find the water density to be a better descriptor than hydration values when exploring AEM under low hydration condition. Furthermore, as the hydroxide transport is essentially different compared to bulk solution, we provide an idealized hydroxide diffusion mechanism for the three diffusion categories. We believe the results presented in this study enable us to define the terms required for achieving high hydroxide conductivity in high-performance AEM fuel cell devices under extremely low hydration conditions.