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.

Automated summary

Imidazolium-based groups have a significant impact on the OH- transport and alkaline stability of anion exchange membrane (AEM) materials, with methyl or phenyl substituents at specific sites showing a sterically inhibitory effect on OH- transport but enhancing alkaline stability. Introduction of alkyl spacer chains promotes OH- transport efficiency, while the interaction of PPO with imidazolium weakens the alkaline stability of AEMs. A balance between OH- transport and alkaline stability is achieved with specific modifications, providing a design principle for fuel cell applications.