Understanding of hydroxide transport in poly(arylene indole piperidinium) anion exchange membranes: Effect of side-chain position

Abstract

The introduction of long side chains to improve hydroxide transport in anion exchange membranes (AEMs) is a common strategy. However, the mechanism of the effect of side-chain position on hydroxide transport in the currently emerging poly(arylene indole piperidinium) AEMs remains unclear. Molecular dynamics simulation was performed to investigate the effect of the side-chain position on hydroxide transport. Three typical long side-chain poly(arylene indole piperidinium) AEMs were constructed, which were tadpole-chain type (t-PITPC10), pendant-chain type (p-PITPC10), and side chains attached onto the backbone separated from cationic groups type (a-PITPC10). The simulation results suggest that the self-diffusion coefficients of hydroxide follow the order: t-PITPC10 < p-PITPC10 < a-PITPC10 at the same hydration number. The strong interaction between the hydrophobic indole groups and hydroxides in t-PITPC10 and p-PITPC10 hinders the efficient transport of hydroxide. In contrast, this interaction is weaker in a-PITPC10 due to side chains being connected at indole groups. The overlapping region of the hydration shell around adjacent cationic groups is larger in a-PITPC10. Meanwhile, the connected aqueous phase and ion transport channels in a-PITPC10 promote hydroxide transport. Hence, the AEM with side chains attached onto the backbone separated from cationic groups (a-PITPC10) is superior to the AEMs with side chains linked to cationic groups (t-PITPC10 and p-PITPC10) in the improvement of hydroxide transport.