Redox flow battery systems having a supporting solution that contains Cl− ions can exhibit improved performance and characteristics. Furthermore, a supporting solution having mixed SO42− and Cl− ions can provide increased energy density and improved stability and solubility of one or more of the ionic species in the catholyte and/or anolyte. According to one example, a vanadium-based redox flow battery system is characterized by an anolyte having V2+ and V3+ in a supporting solution and a catholyte having V4+ and V5+ in a supporting solution. The supporting solution can contain Cl− ions or a mixture of SO42− and Cl− ions.