Non-Monotonic Change of Hydroxide Ion Diffusion with Increasing Temperature in Anion Exchange Membranes

Abstract

The development of reliable, cost-effective polymer architectures for use as anion exchange membranes (AEMs) is an important challenge for emerging electrochemical device technologies. In this work, we present a fully atomistic ab initio molecular dynamics study of carefully designed idealized AEM models using nano-confined structures, in order to obtain a molecular-level understanding of the effect of temperature on hydroxide ion diffusion mechanisms in AEMs. We find that the hydroxide ion diffusion coefficient changes non-monotonically with increasing temperature. While this is consistent for all systems, the specific reasons underlying this behavior depends on the level of hydration. Furthermore, while the diffusion of water molecules and hydroxide ions do not necessarily share the same trends, we find that the water structure, distribution, and mobility have a major impact on the hydroxide ion diffusion at all temperatures. The results presented in this study enable us to define the optimal temperature requirements to achieve high hydroxide conductivity, for each setup of AEM model.