Chloride ions as an electrolyte additive for high performance vanadium redox flow batteries

Employing chloride ions as additive in electrolytes is proposed for all vanadium redox flow batteries. The influences of different concentrations of chloride ions in the electrolyte on the battery performance are investigated. It is found that the chloride ions improve the reaction activity of VO²⁺/VO₂⁺ redox couples. At a current density of 200 mA cm⁻², the battery with an optimum chloride ion concentration of 0.04 M attains an energy efficiency and vanadium utilization ratio of 82.5% and 86.3%, which are 2% and 4.1% higher than those without the additives. Further, at 400 mA cm⁻², the battery with 0.04 M chloride ions shows a high rate capability by delivering the specific discharge capacity of 5.95 Ah L⁻¹. Moreover, the battery with 0.04 M chloride ions exhibits the capacity retention rate of 83.1% after one hundred cycle test, which is nearly 12% higher than that of pristine battery. The results suggest that chloride ions, as low-cost additives in electrolytes, offer great promise for high-performance vanadium redox flow battery application.