A long-side-chain sulfonated ether-free copolybenzimidazole membrane containing alicyclic structure for vanadium redox flow batteries

The stability and selectivity (balance between ionic conductivity and vanadium permeability) of the ion exchange membrane in vanadium redox flow batteries (VRFB) are critical factors that directly impact the battery's performance and lifetime. Herein, we synthesized an ether-free polybenzimidazole copolymer (mcPBI) with rigid benzene ring and flexible alicyclic structures in the polymer's backbone via solution condensation from 3,3′-diaminobenzidine, isophthalic acid and 1,4-cyclohexanedicarboxylic acid monomers. A series of sulfonated polybenzimidazoles (mcPBI-S-x) with long side chains and different grafting degrees were synthesized through grafting reactions, and membranes were prepared by the solution casting method. Microphase separation structure created by grafting accelerates ion transport. Protonated imidazole in an acidic environment enhances proton transport while impeding vanadium penetration due to the Donnan effect. Additionally, the ionic cross-linking between the sulfonic acid group and the imidazole group is in favor of dimensional stability maintenance. The ether-free polymer backbone is conducive to maintaining stability. The results show that all mcPBI-S-x membranes exhibit excellent ion selectivity. Specifically, the mcPBI-S-32% membrane demonstrates optimal ion selectivity (9.06 × 10⁷ S s cm⁻³), low area resistance of 0.45 Ω cm², vanadium permeability (0.76 × 10⁻¹⁰ cm² s⁻¹) and swelling ratio in sulfuric acid (4.3%). The battery with the mcPBI-S-32% membrane demonstrates a coulomb efficiency of 90.50%, a voltage efficiency of 85.69%, and an energy efficiency of 77.55% at a current density of 60 mA cm⁻². What's more, the membrane shows excellent chemical stability, and the chemical structure of mcPBI-S-32% characterized by ¹H NMR does not change after 200 cycles at 120 mA cm⁻².