Atomic insights into the ion-conducting channels of poly(arylene piperidinium) anion exchange membranes

Abstract

Anion exchange membranes (AEMs) are crucial in green energy devices such as water electrolyzers, fuel cells, and CO₂ electroreduction. Among the reported AEMs, the quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl) (QAPPT) AEM is notable for its high OH⁻ conductivity. However, the underlying mechanisms responsible for this high OH⁻ conductivity have remained elusive. Recent experiments have developed the poly(p-quaterphenyl dimethyl piperidinium) (QAQPP) AEM, which has a similar structure to the QAPPT AEM but exhibits much lower conductivity. This discrepancy further complicates our understanding of the superior performance of the QAPPT AEM. To clarify the origin of QAPPT AEM's enhanced conductivity, we perform molecular dynamics simulations to investigate the differences in the underlying causes of the OH⁻ conductivity between QAPPT and QAQPP AEMs. We observe higher OH⁻ diffusion with larger ion-conducting channels in the QAPPT AEM, which is in good agreement with experimental results. Further analysis of the conformations with varying water contents shows that the ion-conducting channels in QAPPT AEMs become larger due to polymer chain folding and dispersion, thereby enhancing OH⁻ transport. In contrast, the channels in QAQPP remain limited in size because the chains maintain their compactness regardless of the water content. Such pronounced conformational changes in QAPPT compared to QAQPP are essentially due to the greater sensitivity of its backbone phenyl rings to water. These findings highlight that conformational evolution in response to water is key to the enhanced conductivity of QAPPT AEMs, providing new insights for more efficient AEM design.