Redox Flow Batteries with an Amphoteric Sulfonated Poly(styrene-co-2-vinyl benzimidazole) Membrane

The membrane plays a crucial role in redox flow batteries by selectively allowing charge-balancing ions to pass through while preventing the crossover of redox-active molecules. However, in real-world applications, ions with similar charges, in addition to charge-balancing ions, may pass through the membrane, compromising cycling stability over time. Much of the research in flow batteries focuses on optimizing the transport of charge-balancing ions and redox-active molecules. Monopolar membranes, such as perfluorosulfonic acid–based membranes like Nafion-117, as well as sulfonated or quaternized thermoplastic polymer-based membranes, allow counterions to pass through while blocking co-ions. However, optimal control is achieved with mixed-charge membranes, which can selectively allow the passage of charge-balancing protons and anions while preventing the passage of cations. This has led to growing attention in recent years toward amphoteric membranes, which offer better control in this regard. In this study, we demonstrate the design and synthesis of an amphoteric membrane made from a copolymer of styrene and 2-vinyl benzimidazole. The polymer was synthesized through postmodification of polystyrene-co-acrylic acid. Detailed electrochemical and physicochemical characterization revealed its excellent suitability for flow battery applications, as demonstrated in vanadium redox flow batteries (VRFB) and aqueous organic redox flow batteries (AORFB). In VRFB, the membrane achieved a Coulombic efficiency (CE) of 98.13% and an energy efficiency (EE) of 78.50% at 150 mA cm^–2, with 99.75% capacity retention over 200 charge/discharge cycles. In the AORFB, the membrane delivered 220 stable cycles at 100 mA cm^–2, with an average CE of 97%, EE of 67%, and a capacity retention of 99.86%, highlighting its excellent potential for flow battery applications.