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

T Flottat and B Latour and F Goujon and P Hauret and P Malfreyt, JOURNAL OF PHYSICAL CHEMISTRY C, 129, 343-352 (2024).

DOI: 10.1021/acs.jpcc.4c07011

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 (lambda = 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.

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