Construction of bSPI/GO@ZIF67 composite membranes with excellent proton conductivity and selectivity for vanadium redox flow battery application

Series of novel branched sulfonated polyimide/graphite oxide@zeolite imidazolate framework-67 (bSPI/GO@ZIF67) composite membranes were prepared by a solution-casting method for vanadium redox flow battery (VFB) application. ATR-FTIR, EDS and XPS analyses confirm the successful fabrication of the bSPI/GO@ZIF67 composite membranes. The optimized bSPI/GO@ZIF67–1.5 % composite membrane shows outstanding performance, including a favorable area resistance (0.15 Ω cm²), reduced vanadium ion permeability (1.64 × 10⁻⁷ cm² min⁻¹), and exceptional proton selectivity (1.97 × 10⁵ min cm⁻³) compared with Nafion 212. Meanwhile, the bSPI/GO@ZIF67–1.5 % composite membrane shows superior coulomb and energy efficiencies (97.5 %–99.3 % and 88.1 %–73.7 %, respectively) at 100 mA cm⁻²–300 mA cm⁻², along with an extended self-discharge duration of 50.4 h, outperforming Nafion 212. Furthermore, the membrane maintains stability over 800 charge/discharge cycles at 160 mA cm⁻². Theoretical calculations reveal strong hydrogen bonding interactions between the nitrogen in the imidazole group of ZIF67 and the hydrogen in the sulfonic group of bSPI, with a bond distance of 1.06 Å and an interaction energy of −12.42 kcal mol⁻¹ individually. This interaction enhances the chemical stability of bSPI while effectively balancing proton conduction and vanadium ion blocking due to the incorporation of GO@ZIF67 fillers. Overall, the bSPI/GO@ZIF67–1.5 % composite membrane demonstrates significant potential for use in VFBs, offering enhanced efficiency, durability, and stability.