Improved energy density and temperature range of vanadium redox flow battery by controlling the state of charge of positive electrolyte

The properties of electrolytes with different concentration compositions and state of charge (SOC) are investigated to improve energy density and temperature range of vanadium redox flow battery. The stability tests indicate that the electrolyte with 2.0 mol/L (M) vanadium concentration, 5.5 M sulfate concentration and SOC range of 0–90% can keep stable at the temperature range of −10 to 40 °C and the decrease of SOC can greatly improve the thermal stability of positive electrolyte but the improvement is not just caused by the decrease of V(V) ions concentration. The conductivity increases and viscosity decreases with the increase of SOC but the changing tendency are nonlinear. The electrochemical activity and reversibility of positive electrolyte achieve minimum at 50% SOC. The UV–Vis, Raman spectra and excess spectra analysis show that a mixed valence compound [V2O3(H2O)7]3+ forms in the mixture of V(IV) and V(V) electrolyte which leads to the improvement of stability and decrease of electrochemical performance. In the SOC range of 0–90%, the battery with electrolyte of 2.0 M vanadium concentration, 5.5 M sulfate concentration can operate stable at temperature range of −10 to 40 °C and the energy efficiency (EE) can stay between 75% and 80%.