Research progress of vanadium battery with mixed acid system: A review

The “double carbon” goal has accelerated the development of multiple energy integration. Due to the capricious nature of renewable energy resources, such as wind and solar, large-scale energy storage devices are increasingly required to make the best use of renewable power. Recently, vanadium redox flow battery (VRFB) has attracted extensive attention as a promising form of large-scale energy storage. However, its application is limited by issues such as low energy density. Mixed acid-supported electrolyte systems can greatly improve these issues. Here we summarized the preparation of VRFB electrolytes and the progress of comprehensive performance studies of vanadium electrolytes in mixed acid-supported electrolyte systems, such as H2SO4-HCl, H2SO4-CH3SO3H and H2SO4-H3PO4. The mixed acid system can expand the application temperature range of VRFB (−20–50 °C) and allow for a vanadium concentration as high as >2.5 M. The stability of the mixed acid system electrolyte is >10 days. At the same temperature and current density, the H2SO4-HCl system has the highest energy density (40 Wh/L) and the highest energy efficiency (85 %). The H2SO4-CH3SO3H system improves the redox reaction kinetics of vanadium. The energy density reached 39.87 Wh/L, but the system cost is increased. The electron transfer part of vanadium redox reaction in the H2SO4-H3PO4 system is greatly accelerated. The addition of H3PO4 prevented the formation of precipitation. Furthermore, we briefly describe the progress of research on electrode materials and exchange membranes in mixed acid systems. We also analyze and describe the problems and solutions to be addressed in the future study of mixed acid system.