Novel ether-free sulfonated poly(biphenyl) tethered with tertiary amine groups as highly stable amphoteric ionic exchange membranes for vanadium redox flow battery

The development of cost-effective, ion-selective and highly stable non-fluorinated hydrocarbon membranes is crucial to the vanadium redox flow battery (VRFB). Herein, we report a facile approach to designing a novel ether-free amphoteric ionic exchange membrane (AiEM) grafted with long flexible alkyl side chains with zwitterionic groups, prepared by superacid-catalyzed polycondensation with post-modification. Both FTIR and H1NMR spectra confirm the polymer's chemical structure. An acid-base interaction that can form between sulfonic group and tertiary amine group contributes to the high stability, low swelling ratio and vanadium ion permeability of the AiEM. Owing to the unique amphoteric structure and connected ion channels, optimized TA15-SPBP membrane achieves a high proton conductivity of 0.116 S cm-1 @ 25celcius together with superb ion selectivity of 14.7 x 105 S min cm-3 compared to that of Nafion212 membrane. Consequently, the VRFB assembled with the TA15-SPBP membrane exhibits superior coulombic/energy efficiencies of 99.1%/87% at 80 mA cm-2 with respect to Nafion212 (92.9%/78.2%). Importantly, the highly stable ether-free backbone and acid-base interaction greatly improve the structural stability of the membrane. During an 1188-cycling test, the TA15-SPBP membrane keeps almost unchanged efficiencies at 80 mA cm-2 with excellent capacity retention (80.5%) in comparison with Nafion212 (11.3%) during the first 200 cycles, thus demonstrating the promising prospects for VRFB applications.

Automated summary

The development of a novel ether-free amphoteric ionic exchange membrane with long alkyl chains and zwitterionic groups shows high stability, low swelling ratio, and excellent proton conductivity. Compared to Nafion212 membrane, the optimized TA15-SPBP membrane exhibits higher ion selectivity and better electrical efficiencies.