A highly concentrated Vanadium Protic Ionic Liquid Electrolyte for the Vanadium Redox Flow Battery

A protic ionic liquid is designed and implemented for the first time as a solvent for a high energy density vanadium redox flow battery. Despite being less conductive than standard aqueous electrolytes, it is thermally stable on a 100° temperature window, chemically stable for at least four weeks, equally viscous and dense with typical aqueous solvents and most importantly able to solubilize to 6 mol L-1 vanadium sulfate, thus increasing the Vanadium Redox Flow Battery energy density by a factor of 2.5. Electrochemical measurements revealed quasi-reversible redox transitions for both catholyte and anolyte at 25°C while a proof-of-concept redox flow cell with the proposed electrolyte was tested for a total of 150 cycles, showing an open circuit potential of 1.39 V and energy and coulombic efficiencies of 65 and 93%, respectively. What’s more, the battery can be equally cycled at 45°C showing good thermal stability. This study underlines a new route to improve the energy-to-volume ratio of this promising energy storage system.