Active material crossover suppression with bi-ionic transportability by an amphoteric membrane for Zinc–Bromine redox flow battery

Zinc-bromine redox flow batteries (Zn/Br₂ RFBs) are gaining attention as a next-generation energy storage system with the advantages of a cost-effective redox couple material price, high output, and high energy density. However, bromine (Br₂) crossover through a commercial porous membrane causes self-discharge to lower the capacity retention. Nafion, a commercial ion exchange membrane, can lower the crossover but has low voltage efficiency due to high membrane resistance. To address this trade-off, the amphoteric functionalized silica (Am-SiO₂) is introduced into the Nafion membrane (Nafion/Am-SiO₂). It suppresses the crossover of active materials such as Br₂ and polybromide (Br_n⁻) and possesses high ionic conductivity due to the quaternary ammonium and sulfonic groups on the Am-SiO₂ surface. In addition, increasing the water content in the membrane prevents the expansion of the water cluster size, which could help balance bi-ionic transport. As a result, the composite membrane showed 83.3 and 19.0 times higher ion selectivity than the commercial porous membrane (SF600) and ion-exchange membrane (NRE-212). Compared to SF600 and NRE-212, the energy efficiency of Nafion/Am-SiO₂ was improved by 4.2 and 6.4%, respectively. Balancing anion and cation transport can be successfully applied in Zn/Br₂ RFBs by introducing an amphoteric group into an ion-exchange membrane.