Anion exchange membranes based on long side-chain quaternary ammonium-functionalized poly(arylene piperidinium)s for vanadium redox flow batteries

A new series of poly(arylene piperidinium)-based anion exchange membranes (AEMs) are proposed for vanadium redox flow batteries (VRFBs). The AEMs are fabricated via the Menshutkin reaction between poly(arylene piperidine) without ether bonds in the backbone and various quaternizing agents, including iodomethane, 1-bromopentane, and (5-bromopentyl)-trimethylammonium bromide. The properties of the AEMs are investigated in terms of sulfuric acid doping content, swelling, vanadium permeability, ion selectivity, area-specific resistance, mechanical properties, VRFB performance, and cyclic testing. Particularly, a method of measuring the H+ permeability of the AEM is developed. It demonstrates that the poly(p-terphenyl-N-methylpiperidine)-quaternary ammonium (PTP-QA) membrane with a QA cation-tethered alkyl chain exhibits high H+ permeability, resulting in low area resistance. Combined with its low vanadium permeance, the PTP-QA membrane achieves nearly 370 times higher ion selectivity than Nafion 115. The VRFB based on PTP-QA-based AEM displays high Coulombic efficiencies above 99% at current densities of 80–160 mA cm−2. The higher energy efficiency of 89.8% is achieved at 100 mA cm−2 (vs. 73.6% for Nafion 115). Meanwhile, the PTP-QA-based AEM shows good cycling stability and capacity retention, proving great potential as the ion exchange membrane for VRFB applications.