Investigating the Influence of the Spacer Length on Anion-Exchange Membrane Properties Using a Reactive Molecular Model

Abstract

This study investigates the influence of the spacer length within anion-exchange membranes (AEMs) on their diffusion properties. Using reactive molecular dynamics with ReaxFF, three polyphenylene oxide (PPO)-based AEMs functionalized with trimethylamine (TMA) cationic groups at two hydration levels (λ = 10 and 20) were simulated at the molecular scale through three alkyl spacer chains lengths: methyl (PPO1-TMA), pentyl (PPO5-TMA), and decyl (PPO10-TMA). Our simulations capture the nanophase separation due to the amphiphilic nature of the simulated materials, where a water channel emerges to enable OH^– transport through Grotthuss and classical mechanisms. We find that the number of hydrogen bonds formed between OH^– and H₂O molecules is a key parameter governing the diffusion properties of OH^–. At a lower hydration level, PPO1-TMA has the lowest diffusion properties, whereas PPO5 and PPO10-TMA membranes show the same diffusion because pentyl and decyl spacers bend and remain stuck on the backbone because of their hydrophobicity. At a higher hydration level, the three membranes exhibit identical diffusion properties, irrespective of their spacer length, as the OH^– molecule approaches its bulk-like behavior.