Non-Monotonic Temperature Dependence of Hydroxide Ion Diffusion in Anion Exchange Membranes

Recent studies suggest that operating anion exchange membrane (AEM) fuel cells at high temperatures has enormous technological potential. However, obtaining a fundamental understanding of the effect of temperature on hydroxide conductivity and membrane stability remains a key hurdle to realizing the full potential of high-temperature AEM fuel cells. In this work, we present a combined theoretical and experimental study to explore the effect of temperature on hydroxide ion and water diffusivities in AEMs. Both fully atomistic ab initio molecular dynamics simulations and ¹H pulsed field gradient NMR measurements confirm that the OH^-diffusion changes non-monotonically with increasing temperature. Specifically, the D_OH^-versus T curve exhibits a region in which dD_OH^-/dT < 0, indicating the presence of a kink in the curve, which we refer to as a "diffusion kink". The simulations show that the underlying causes of this behavior vary with the hydration level. Furthermore, we were able to rationalize the conditions underlying this counterintuitive behavior and to suggest ways to identify the optimal operating temperature for each model AEM system. We expect that the discovery of this unusual temperature dependence of the diffusivity will play an important role in the design of new, stable, and highly conductive AEM-based devices such as electrolyzers, redox flow batteries, and fuel cells.