Homologous flexible multi-cationic cross-linkers modified poly(aryl ether sulfone) anion exchange membranes for fuel cell applications

Abstract

Issues of unsatisfactory ionic conductivity and chemical stability should be solved for anion exchange membranes (AEMs) in practical fuel cell applications. A series of flexible spacers, homologous multi-cationic cross-linkers with different lengths, were designed and synthesized from 1,4-diazabicyclo(2.2.2)octane (DABCO) and 1,6-dibromohexane, and subsequently used to fabricate a series of flexible multi-cationic cross-linked poly(aryl ether sulfone) (CQPAES) AEMs. The CQPAES membranes fabricated by simultaneous cross-linking and membrane formation are tough and pliable. The length and number of cations of the cross-linkers show noticeable effects on the comprehensive membrane performance. The CQPAES membranes display more distinct nano-phase separation morphology and well-developed ion transfer channels due to the higher mobility and hydrophilicity of the flexible long-chain multi-cationic segments. As a result, the CQPAES membranes exhibit gradually increased water uptake and ionic conductivity with the increase of cross-linker length. Furthermore, DABCO and hexyl segments in the cross-linker greatly enhance the steric hindrance and electron cloud density of the quaternary ammonium groups, which inhibit the Hoffmann elimination reaction. Consequently, the CQPAES membranes display a significant improvement in alkali stability than conventional benzyl-substituted quaternary ammonium group type AEMs.