Simultaneously improved H+ conductivity and H+/V4+ selectivity for all vanadium flow battery by ZIF-8 induced physical cross-linking

One of the major challenges in all vanadium redox flow battery (VRFB) is the trade-off between proton conductivity and vanadium ion cross-mixing. Here, we simultaneously enhanced proton conductivity and sharply reduced the vanadium crossover by introducing ZIF-8 into a sulfonated polyimide (6FTMA-100) to prepared a high performance VRFB membrane. Compared with the pristine membrane, the proton conductivity increased from 34.6 to 47.4 mS cm−1, and the vanadium ion permeation rates decreased from 3.56 × 10−7 to 0.82–1.88 × 10−7 cm2 min−1 with different ZIF-8 doping concentrations. The ZIF-8 doped membranes also showed increased viscosity, reduced membrane swelling ratio and increased Young's modulus. We propose this is due to the strong hydrogen bonding (−106.68 Kcal/mol) between the N in imidazole group and H in -SO3H. Battery performance confirmed that the optimized MMM-2 (2 % ZIF-8 loading) displayed the energy efficiency of 80 % at 160 mA cm−2 that much higher than its pristine 6FTMA-100, N117, and most of the reported sulfonated polyimides. This idea of creating physical crosslinked network provided an efficient way for achieving highly efficient VRFB membranes.