Polybenzimidazole Membranes Doped with Sulfonic Acid-Containing Covalent Organic Frameworks and Polymers for Enhanced Performance in Vanadium Redox Flow Batteries

Polybenzimidazole (PBI)-based membranes in vanadium redox flow batteries (VRFBs) achieve high Coulombic efficiency due to their excellent ion selectivity; however, their energy efficiency remains relatively low due to restricted proton transport. Covalent organic framework (COF) and polymer (COP)-based proton-selective membranes have been developed to address the trade-off between proton permeability and selectivity, significantly improving the redox flow battery performance, including all-vanadium systems. These membranes achieve precise proton and vanadium ion separation with an optimized pore size, reducing vanadium ion crossover while maintaining low area resistance through their microporous structure and imine group functionalities. Single electrochemical cells with these membranes demonstrate high Coulombic efficiencies (94.18–99.19%) and energy efficiencies (75.37–88.50%) at current densities of 40–100 mA cm –2 . The integration of PBI with the sulfonic acid-bearing COFs and COPs enhances the membranes’ electrochemical performance as well as their chemical stability, ensuring long-term cycle durability. This work presents a promising approach for designing ultrahigh-proton-selective membranes, offering valuable insights for advanced redox flow batteries and other electrochemical systems.