Vanadium redox flow batteries (VRFBs) have received extensive attention due to its attractive features for the large-scale energy storage because of their design flexibility, absence of intercalation/deintercalation and stress build-up in electrodes, and safe operation. One of the most critical components in VRFB is an ion exchange membrane which can prevent cross mixing of the multivalence vanadium ions, while it still allows the transport of protons (or anions) to complete the circuit. However, the trade-off relationship of proton exchange membrane limits the energy efficiency (EE) of the VRFB due to low coulombic efficiency (CE) caused by vanadium ion cross over through the membrane or high membrane resistances. Anion exchange membrane suffers from poor chemical stability and low voltage efficiency due to low ion conductivity.

In this study, we synthesized a series of novel sulfonated aromatic polymer membranes having high proton conductivity as well as selectivity which surpass the proton conductivity-selectivity trade-off relationship. The effect of the pendent group structure on the membrane ion selectivity and battery performance was investigated. It was found that the incorporation of aromatic structures on the pendent groups effectively improved the proton conductivity and selectivity. The membrane with aromatic pendent groups (BPAr-F4) shows 3 times higher vanadium/proton ion selectivity than Nafion 212. As a result, the VRFB single cell equipped with the 80um-thick BPAr-F4 membrane shows significantly better performance in comparison to the cell with Nafion membranes, as high as 97.18% CE, 95.91% VE and 93.20% EE (vs. 75.14% CE, 96.12% VE and 72.22% EE of Nafion 212; 91.30% CE, 95.63% VE and 87.62% EE of Nafion 117) at 20 mA/cm². Small-angle X-ray scattering (SAXS) studies indicates that their unique morphologies with narrow aqueous ionic domains which are well organized with acidic -SO3H groups results in the high proton selectivity and VRFB performance.