Ionic liquid-mediated aqueous redox flow batteries for high voltage applications

• An aqueous electrolyte with an electrochemical stability window of 3 V has been achieved.
• 2 V couples of metal acetylacetonates have been tested in 10 m 1-butyl-3-methylimidazolium chloride aqueous electrolytes.
• Reversible Zn2 +/Zn and Ce4 +/Ce3 + redox reactions have been demonstrated in a flow battery with a 2 V cell voltage.

Aqueous redox flow batteries with high cell voltages represent a promising approach for low-cost, high safety and high energy density applications. However, water breakdown is a major concern because it limits cell voltage. For the first time, we report the use of a highly concentrated aqueous ionic liquid electrolyte, 1-butyl-3-methylimidazolium chloride (BMImCl)-H2O, in an aqueous flow battery operating with a broad electrochemical stability window of 3 V. The proof-of-concept was demonstrated using 2 V redox couples of metal acetylacetonates and a hybrid Zn/Ce flow battery.