Enhanced proton conductivity of main/side chain bi-sulfonated polybenzimidazoles via embedment of fluorinated modified MOF-801 for vanadium redox flow batteries

The development of low-cost membranes with high ion-selectivity is crucial for facilitating the practical application of vanadium redox flow batteries (VRFBs). In this study, sulfonated polybenzimidazole grafted with benzenesulfonic acid side chains was used as the polymer matrix, and fluorine-containing modified MOF-801 (FM) was incorporated to prepare a series of membranes for VRFB applications. The enhancement of proton conductivity and reduction of area resistance can be attributed to several factors: acid-base interactions, the microphase-separated structure of the grafted side chains, the intrinsic conductivity of MOF-801, and the extensive hydrogen-bonding network established through fluoride modification. Notably, the incorporation of fluorine or fluorine-containing groups into the hydrocarbon polymer matrix significantly improves its oxidative stability. The sPBI A5 -FM X membrane, with a FM content of 3 wt%, demonstrated the most remarkable overall performance, achieving a proton conductivity of 33.5 mS cm −1 at 30 °C and 100 % relative humidity (RH). At the same time, it has outstanding oxidation stability and excellent ion selectivity (5.23 × 10 6  S min cm −3 ). In addition, the sPBI A5 -FM 3 membrane exhibited excellent cell performance, with energy efficiency (EE) from 84.80 % to 86.50 % at current densities of 40–120 mA cm −2 . This study confirms the significant potential of fluoride-modified MOF-801 to enhance ion selectivity and provides an effective strategy for optimizing proton transport channels.