Structure-Transport Relationships of Anion Exchange Membranes

Abstract

Anion exchange membrane (AEM) fuel cells are emerging energy conversion technologies. A significant challenge in these devices lies in the core component, the AEM, which must possess high ionic conductivity to minimize Ohmic losses and exhibit chemical stability in highly basic conditions. A significant amount of effort has been expended to develop new anion exchange polymers with enhanced transport functionality. However, understanding of how anion transport in these polymers is related to hydration and nano-morphology is far from adequate. In this work, we report a systematic study on PAP-TP-85, an AEM with promising cell performance.1 A commercial AEM (Fuma FAA3) is used for comparison in this study. Membrane water uptake and anion conductivity in liquid and vapor as well as the impact of various anion forms on these properties are investigated. Small-angle X-ray scattering (SAXS) is used to probe hydrated AEM nanostructure, which is correlated to the membrane uptake and transport properties. Our results show that these AEMs lack a phase-separated nanostructure, regardless of their counter-anion form and hydration level. This is associated with their weak electrolytes behavior, which also explains their lower ion conductivity values (compared to proton exchange membranes), despite having higher water content and greater ion exchange capacity (IEC). Water content plays a more significant role than the temperature in controlling the anion conductivity of these weak electrolytes in water vapor. Comparing the water content in liquid vs. saturated vapor, both AEMs show Schroeder’s paradox regardless of counter-anion form. This study also demonstrates the importance of hydration level and ion concentration for anion transport in amorphous AEMs and provides further understanding into their parameters governing their structure-transport relationship and sheds light into designing AEMs with improved performance. Wang, J.; Zhao, Y.; Setzler, B. P.; Rojas-Carbonell, S.; Ben Yehuda, C.; Amel, A.; Page, M.; Wang, L.; Hu, K.; Shi, L.; Gottesfeld, S.; Xu, B.; Yan, Y., Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells. Nature Energy 2019.