Complementary side chain promotes microphase separation in the membranes for alkali fuel cells

Abstract

Anion exchange membranes (AEMs) play a crucial role in alkali electrolyte membrane fuel cells. Side chain grafted AEMs can undergo microphase separation and form good pathways to promote hydroxide ion transport; such AEMs typically have uniform side chain length at all repeat units of backbone, which may suffer from steric hindrance during cation aggregation and thus lead to insufficient microphase separation. In this work, we propose a complementary side chain structure, where a small amount of short side chain (C1) quaternized poly (dimethyl phenylene oxide) (qPPO-C1) is blended with long side chain AEM (qPPO-C6). Our results suggest that a blend AEM containing 5% qPPO-C1, whose IEC (ion exchange capacity) is as low as 1.45 mmol/g, exhibits an impressive conductivity of 37.5 mS/cm (30 °C) while its counterpart AEM with purely long side chains (qPPO-C6) displays an inferior conductivity (31.4 mS/cm) at a similar IEC. Its hydrogen/oxygen fuel cell yields a peak power density of 192 mW/cm2 when operating at 60 °C. Our work discloses, for the first time to our best knowledge, the advantage of complementary side chain in promoting microphase separation, in particular, at low IECs; it may offer new opportunities for further improvement of AEM performance.