Tunable OH– Transport and Alkaline Stability by Imidazolium-Based Groups of Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes: A Molecular Dynamics Simulation

Imidazolium-based groups are promising organic cations in anion exchange membrane (AEM) materials. To investigate the effect of the imidazolium structure on OH– transport and alkaline stability of AEMs, we performed molecular dynamics simulation studies on hydrated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) AEMs with imidazoliums modified by various substituents and/or alkyl pendent/spacer chains. Imidazoliums with the methyl or phenyl substituent at the C2, C4, and C5 sites or alkyl pendant chains at the N3 site show a steric effect on the distribution of water and OH– around imidazoliums, which inhibits the OH– transport but increases the alkaline stability of AEMs. By introducing alkyl spacer chains, the enhanced hydration structure of imidazolium promotes OH– transport, but the weakened steric effect of PPO on imidazolium decreases alkaline stability. We elucidate that the PPO AEMs modified by 1,2,4,5-tetramethylimidazolium and alkyl spacer chains with six or eight aliphatic carbons show good balance between OH– transport and alkaline stability of AEMs. Moreover, the complete hydration shells of both imidazolium and OH– enhance the OH– transport efficiency and decrease the possibility of imidazolium degradation with the hydration level more than six. Our work provides a design principle of imidazolium-based AEMs in fuel cell applications.