Towards high alkaline stability and fuel cell performance in anion exchange membranes via backbone−cation alkylene spacer tuning for quaternized poly(biphenylene alkylene)s

Abstract

A major challenge in anion exchange membrane (AEM) fuel cells is the development of high-performed AEMs that can simultaneously meet the criteria of alkaline stable, swelling resistant, and ion conductive. Pentyltrimethylammonium-tethered poly(arylene alkylene)s are emerging as one of the most promising candidates in AEM community, however, they also suffer from conductivity−water uptake trade-offs and improvement-needed alkaline stability. Herein, a series of poly(biphenylene alkylene)-based membranes, PBPA-n-QAs (n = 4, 5, 6), with different alkylene spacer length are synthesized by hydroxyalkylation polymerization and the subsequent amination. The membranes are found to have an improved phase separation as the number of carbon atoms of the alkylene spacers increased from 4 to 6. The ordered morphology and continuous hydrophilic channels make the PBPA-6-QA membrane with outstanding properties including suppressed water uptake, high ion conductivity, and improved alkaline stability. Specifically, the PBPA-6-QA membrane yields the highest hydroxide conductivity (180 mS cm−1 at 80 °C), which is considerably higher than that of PBPA-5-QA and PBPA-4-QA. Furthermore, a single cell using PBPA-6-QA shows a peak power density of 0.97 W cm−2 and can operates stably at a constant current density of 0.2 A cm−2 for 90 h without membrane degradation.