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, high energy efficiency, 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 (IEM) 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 (PEM) 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 (AEM) 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 proton selectivity. Vanadium/hydronium ion permeability, proton conductivity, area resistance, and ion exchange capacities of the membranes were measured. The single cell VRFB performance with the prepared membranes was evaluated and compared with the one using a commercial Nafion 212 membrane. The effect of the pendent group structure on the membrane ion selectivity and battery performance was also discussed.

It was found that the incorporation of aromatic structures on the pendent groups effectively improved the ion selectivity of membranes which can overcome the limitations of the trade-off relationship between the proton conductivity and selectivity. The membrane with aromatic pendent groups (BP-Ar-SH) shows 7 times higher vanadium/proton ion selectivity than that without aromatic groups (BP-SH) (5.18 vs. 0.75) and 3 times higher than Nafion 212 (5.18 vs. 1.78). As a result, the VRB single cell equipped with the BP-AR-SH membrane shows significantly better performance in comparison to the cell with Nafion membranes, as high as 90.05% EE (vs. 73% EE of Nafion) at 20 mA/cm². The cycle stability test shows that efficiencies of the membrane remain almost unchanged after running more than 50 cycles. Our results strongly suggest that the BP-Ar-SH membrane may provide a breakthrough toward the development of high-efficiency VRFB systems.