Investigating the Influence of the Spacer Length on Anion-Exchange Membrane Properties Using a Reactive Molecular Model
Résumé
This study investigates the influence of 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) have been 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 simulated materials, where water channel emerges to enable OH -transport through Grotthuss and classical mechanisms. We find that the number of hydrogen bonds formed between OH -and H 2 O molecules is a key-parameter governing diffusion properties of OH -. At the 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 the higher hydration level, the three membranes exhibit identical diffusion properties, irrespective of their spacer length, as OH -molecule approaches its bulk-like behaviour.
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