From chain length to ion diffusion: Molecular insights into phosphonium-based polymerized ionic liquid membranes for energy applications

Abstract

The development of high-performance anion exchange membranes (AEMs) is essential for advanced electrochemical technologies such as alkaline fuel cells and water electrolysis. Here, we investigate the influence of phosphonium side chain length on structural hydration and chloride ion transport in polymerized ionic liquid (MPIL) membranes. Atomistic molecular dynamics simulations are conducted on hydrated MPIL systems with ethyl, butyl, and octyl n-alkyl substituents, and simulation results are validated against experimental data for water uptake and ionic conductivity. Shorter side chains (ethyl) significantly enhance water uptake (approximate to 81 wt%) and promote the formation of interconnected hydrophilic channels, resulting in markedly higher Cl- ionic conductivity. In contrast, longer chains (octyl) restrict water accessibility and confine ion diffusion within localized hydrophilic domains, favoring ion retention tendency at the expense of transport efficiency. Intermediate chain length (butyl) yields a balanced morphology, combining moderate hydration with controllable ion mobility. Quantitative analyses, including pore connectivity descriptors, ion-ion association free energies from RDF integration, and backbone-water interaction profiles, consistently confirm that steric hindrance modulates hy-dration shell formation, ion pairing, and channel percolation. This molecular-level insight suggests that alkyl chain engineering provides a tunable parameter for optimizing trade-offs between ion conductivity and relative mobility control in MPIL-based AEMs. The combined computational and experimental results provide practical guidelines for designing next-generation membranes for desalination, electrochemical conversion, and energy storage.