Effects of Hydration Level and Hydrogen Bonds on Hydroxide Transport Mechanisms in Anion Exchange Membranes.

Abstract

The transport of hydroxide in anion exchange membranes (AEMs) is generally determined by multiple factors, including hydration levels, pore morphologies, and the hydration shells of cationic groups and hydroxides. Thus, clarifying the working mechanisms would benefit the proposal of strategies for enhancing the hydroxide transport, thereby enabling a rational design of high-performance AEMs. Herein, by using ReaxFF MD simulations and RDAnalyzer, we explored the straightforward but effective correlations for steric hindrance versus hydration shell, hydration level versus Free/Associated diffusion, and strong (short) hydrogen bond (SHB) versus Vehicular/Grotthuss diffusion. Our theoretical investigations indicate that higher steric hindrance of cationic groups results in less water in the first hydration shell of cationic groups in AEMs. Meanwhile, a higher hydration level facilitates wider hydrophilic pores of AEMs and increases the ratio of the Free diffusion mechanism of hydroxides. Interestingly, we found a strong correlation between the number of SHBs and the Grotthuss diffusion, thereby enhancing the understanding of the high conductivity of COF-based AEMs that contain obvious SHBs. This work provides a theoretical view for fine-tuning the Free/Associated and Vehicular/Grotthuss transport of hydroxide in AEMs.