Redox flow batteries store electrical energy in reduced and oxidized species dissolved in two separate electrolyte solutions. The anolyte and the catholyte circulate through a cell electrode separated by a porous membrane. Redox flow batteries are advantageous for energy storage because they can tolerate fluctuating power supplies, repetitive charge/discharge cycles at maximum rates, overcharging, over discharging, and because cycling can be initiated at any state of charge. However, several disadvantages exist for many redox couples upon which redox flow batteries are based. For example, many systems utilize redox species that are unstable, are highly oxidative, are difficult to reduce or oxidize, precipitate out of solution, and/or generate volatile gases. The existing approaches to addressing these disadvantages have been ad hoc and can include imposing restrictive operating conditions, the use of expensive membranes, the inclusion of catalysts on the electrodes, and/or the addition of external heat management devices. These approaches can significantly increase the complexity and the cost of the total system. Therefore, a need exists for improved redox flow battery systems.

To meet this need, scientists at PNNL have developed a redox flow battery system with a supporting solution that comprises Cl- anions. 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.