A high-performance dual-redox electrochemical capacitor using stabilized Zn2+/Zn anolyte and Br3-/Br- catholyte

Redox electrochemical capacitors, using soluble redox couples to utilize the "dead weight" of the electrolyte to add faradaic charge, are considered as an effective strategy to increase the energy storage capacity of the electrochemical double-layer capacitors. In this work, we report a new aqueous dual-redox electrochemical capacitor, using Zn2+/Zn as the anolyte and Br-3(-)/Br- as the catholyte. The electrolyte contains 1 M ZnBr2, 0.1 M TPABr and 2 M Zn(OTf)(2). The theoretical voltage gap of the two redox couples is larger than 1.7 V, which is expected to be an advantage for higher specific energy. Tetrapropyl ammonium cation (TPA(+)) has been used as a complex agent to solve the Br-3(-) cross diffusion issue. To avoid the generation of [TPA](2)[ZnBr4] precipitation, an appropriate amount of Zn(OTf)(2) has been added to the electrolyte to adjust the Zn/Br ratio. After adding these two additives, the coulombic efficiency of the dual-redox EC has been increased from 90% to 99%, and the open-circuit voltage loss (24 h) has been dropped from 31% to 5%. The newly configured Zn/OTf/TPA/Br redox EC exhibits a specific energy of 110 W h/kg at 1 A/g, a high power density (7 kW/kg) and stable cycling over 6000 cycles.