The Effect of Additives on the High-Temperature Stability of the Vanadium Redox Flow Battery Positive Electrolytes

Several inorganic additives are investigated as potential precipitation inhibitors for the positive half-cell electrolyte of the vanadium redox flow battery (VRB) at elevated temperatures. Electrolyte stability tests on the more concentrated 2m vanadium electrolyte show that of the additives tested, the best results are obtained with the addition of 1% phosphoric acid that give induction times of 40, 22 and 18 days for 80, 90 and 95% state of charge (SOC) solutions at 50 degrees C, compared with 5, 2 and 1 days respectively for the corresponding blank solution. Several additives are also evaluated at 45 degrees C in a vanadium redox flow cell operating with a 2m vanadium electrolyte. In the case of the cell employing the blank solution, a sudden drop in capacity, followed by pump failure is observed after around 70 cycles, while the cells with the additives continue to cycle without failure for 120 and 150 cycles for ammonium sulphate and ammonium phosphate additives respectively. A similar stabilising effect has previously been observed with these additives at low temperatures in the negative half-cell electrolyte of the VRB, confirming that these stabilising agents have the potential to extend the upper and lower operating temperature limits of a 2m vanadium electrolyte in the VRB to 45 and 5 degrees C respectively, while also significantly enhancing its energy density without the need for potentially hazardous chloride additions.