Decoding the Influence of Microstructure on the Stability and Dynamical Properties of the HMT-PMBI Anion Exchange Membrane Using Molecular Dynamics Simulation

This work utilizes all-atom simulations to investigate the thermodynamic and transport properties of the hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) membrane with chloride anions, focusing on its stability and performance across different temperatures and hydration levels (λ). By employing generalized AMBER force field (GAFF) and density functional theory (DFT)-derived partial charges, we examine key microstructural features such as the radial distribution function (RDF) and pore size distribution (PSD) and their influence on membrane properties such as density, diffusivity, and conductivity. Our findings show that the cation–anion RDF peak values are reduced by nearly half, and the PSD distribution range expands 2-fold with a 4-fold increase in λ. This leads to faster decorrelation between cations and anions, as indicated by a significant decrease in cation–anion residence times from 40 ns to less than 0.1 ns, resulting in nearly a 20 times increase in diffusivity and conductivity at higher λ. Additionally, at low λ, anions exhibit subdiffusive behavior due to pore sizes smaller than their molecular diameter and the formation of segregated ion conduction channels in multiple clusters rather than a continuous percolated structure. Despite this, the chloride ion conductivity reaches 13 mS/cm at T = 300.15 K and λ = 20.0, the highest hydration level examined. A comparison with our previous study on Sustainion anion-exchange membrane (AEM) reveals that although HMT-PMBI demonstrates superior chemical stability, its conductivity is lower, mainly due to the cationic group incorporated into the polymer backbone. These insights into the structure–property relationships of anion-exchange membranes (AEMs) are essential for the development of next-generation membranes that balance both durability and performance efficiency.