Effect of Phosphate Additive for Thermal Stability in a Vanadium Redox Flow Battery

Organic/inorganic materials are investigated as additives to improve the stability of a vanadium electrolyte for a vanadium redox flow battery (VRFB) at operating temperatures of 25 °C and 40 °C. Among these materials, the most effective additive is chosen based on the thermal stability and electrochemical performance with a long inhibition time. Through precipitation time and electrochemical measurements, the results show that the best inhibition effect is achieved by adding sodium pyrophosphate dibasic (SPD, H2Na2O7P2) as an additive at a considerably high H2SO4 concentration (3M) electrolyte, indicating an improved redox reversibility and electrochemical activity. Nonflow cell assembled with the SPD additive exhibits larger discharge capacity retentions of 40% than a blank solution with the retentions of 2% at 600 cycles at 40 °C. In the case of flow cell, the capacity retention on the SPD additive shows 55.4%, which is 5.3% higher than the blank solution at 40 °C and 180 cycles. The morphology of the precipitation is investigated by SEM, which exhibits more severe V2O5 precipitation amount on the carbon felt electrode used in the blank electrolyte at 40 °C, which causes larger capacity losses compared to cells assembled with the SPD additive electrolyte.