Steric-hindrance benzimidazole constructed highly conductive and robust membrane for vanadium flow battery

New benzimidazole functionalized adamantane-containing poly(aryl ether ketone) (BAPEK) membranes were prepared for use in vanadium flow batteries. The BAPEK membranes were pre-swelled in H3PO4, and the pre-swelling temperature significantly affected the conductivity of the BAPEK membrane. The area resistance of BAPEK-130 membranes (0.09 Ω cm2, pre-swelled at 130 °C) was nearly one-eleventh less than the virgin BAPEK-40 membrane (1.04 Ω cm2, pre-swelled at 40 °C). The synergistic effect of the steric hindrance of the benzimidazole groups and the highly rigid, hydrophobic adamantane-containing backbone of the polymer produced low swelling in the BAPEK membranes. Moreover, this low swelling and the Donnan exclusion effect (between positively charged benzimidazole groups and vanadium ions), produced high selectivity in the BAPEK membranes. Consequently, VRFB utilizing a BAPEK-130 membrane exhibited remarkably high energy efficiency (91.1% at 80 mA cm−2; 81.1% at 280 mA cm−2). The stability of the BAPEK membrane was confirmed by its stable performance during in-situ (2000 cycles, at 160 mAcm−2 and 200 mAcm−2) and ex-situ stability tests. These results suggest that the BAPEK membranes are promising candidates for high-power-density vanadium flow batteries.