Sulfonated para‐Polybenzimidazole Membranes for Use in Vanadium Redox Flow Batteries

Abstract Ion conducting membranes play a crucial role in redox flow batteries, separating anolyte and catholyte while allowing proton transport to complete the circuit. However, most membranes are trapped in a trade‐off relation and show either low conductivity or high vanadium crossover. This study investigates the use of dense sulfonated para‐polybenzimidazole membranes for vanadium redox flow batteries (VRFBs), and analyzes the effects of membrane preparation process, membrane thickness and operating temperature on the VRFB performance. The results demonstrate superior performance of VRFBs utilizing fluorine‐free sulfonated para‐polybenzimidazole membranes compared to other types. Under optimal conditions, the VRFB exhibits high coulombic efficiency (>99%) and high energy efficiency (EE, 92.2% at a current density of 80 mA cm⁻²), and durability. The achieved EE represents one of the highest reported in the literature for VRFBs. In addition, it is shown that operation at 35 °C has benefits at high current densities (EE at 300 mA cm⁻² is over 80% at 35 °C but 72% at 25 °C), while the operation at 80 mA cm⁻² only shows a small temperature effect (91.8 and 92.2%, respectively).