A low vanadium permeability sulfonated polybenzimidazole membrane with a metal-organic framework for vanadium redox flow batteries

A proton selective membrane based on a metal-organic framework (MOF) and polybenzimidazole (PBI) with a controllable sulfonation degree is designed for vanadium redox flow battery (VRFB) applications. The size of UiO-66 is between hydrated multivalent vanadium ions and stokes radius of protons, and its suitable pore size can obstruct the penetration of vanadium ions, resulting in a high vanadium resistance rate. Simultaneously, the imidazole groups and sulfonic acid groups in the sulfonated polybenzimidazole compose the acid-base pairs, which block the penetration of vanadium ions. Therefore, sulfonated PBI together with UiO-66 forms a denser network structure for as a vanadium ion barrier. Compare with the Nafion117 film, the vanadium transmittance of prepared composite film is lower (5.15 × 10−7 cm2 min−1 vs. 2.95 × 10−9 cm2 min−1). Compare with the Nafion117 membrane, VRFB with MOF polymer exhibits higher voltage efficiency (94.54%–74.93%) and energy efficiency (84.82%–73.98%) at 40-200 mA•cm−2.