All-vanadium sulfate redox flow battery systems have a catholyte and an anolyte comprising an aqueous supporting solution including chloride ions and phosphate ions. The aqueous supporting solution stabilizes and increases the solubility of vanadium species in the electrolyte, allowing an increased vanadium concentration over a desired operating temperature range. According to one example, the chloride ions are provided by MgCl.sub.2, and the phosphate ions are provided by (NH.sub.4).sub.2HPO.sub.4.
ACKNOWLEDGMENT OF GOVERNMENT SUPPORT
 This invention was made with government support under DE-AC0576RL01830 awarded by the U.S. Department of Energy. The government has certain rights in the invention.