Advancing Polybenzimidazole Membrane by Knitting Soft Poly(vinyl imidazolium) for Vanadium Redox Flow Batteries

Polybenzimidazole (PBI)‐based membranes are well known for their potential in electrochemical applications. However, these membranes often have poor conductivity. The modification of N ‐alkylation of benzimidazole unit improves the conductivity of membranes, which are mostly found in alkaline water electrolysis and fuel cells. Only a few membranes are incorporated in vanadium redox flow battery (VRFB). Herein, a new class of polyimidazolium‐knitted PBI‐based membranes are synthesized via de novo approach using 3,3′‐diaminobenzidine and 4,4′‐biphenyldicarboxylic acid with infused grafting of halato‐telechelic poly(vinyl imidazolium) (PVIm) for VRFB. As evidenced, the PVIm‐grafted membranes show increase in the electrochemical properties. OPBI‐VIm‐ x demonstrates improved water uptake attributes and bulk ion conductivities compared to Nafion‐212 (N‐212). Further, OPBI‐VIm‐ x membranes are assessed based on several key parameters like vanadium crossover, ion selectivity, and vanadium single‐cell setup. Among them, OPBI‐VIm‐1.25 achieves lowest vanadium permeability and highest ion selectivity with maximum single‐cell performance (Coulombic efficiency ≈98%, voltage efficiency ≈71%, and energy efficiency ≈70%) with stable cycling up to 300 cycles. As evidence, a promising strategy to address limitations of traditional OPBI involves the functional tethering of stable poly‐cationic moieties, which enhance hydrophilicity while preserving the inherent thermal and chemical stability of pristine PBI membranes.