A Versatile Capacity Balancer for Asymmetric Supercapacitors

Abstract Asymmetric supercapacitors (ASCs) with high theoretical energy density have attracted extensive attention during the past years. However, the huge capacity gap between the two electrodes greatly limits high energy density. Regulating electrode mass can make the capacity balanced, while sacrificing weight and volume. Herein, a soluble bipolar molecule, 4‐hydroxy‐2,2,6,6‐tetramethylpiperidinyloxyl (4‐OH‐TEMPO), is proposed as a versatile mediator in the electrolyte to balance the capacity gap in different types of ASCs. 4‐OH‐TEMPO is able to quickly obtain or lose electrons at different potentials regardless of the pH values, thus can contribute large redox capacity at the interface of capacitive electrode in ASCs in both positive or negative electrodes, acidic or alkaline systems. A case study of two typical ACSs is presented, Zn//activated carbon (AC) system with 4‐OH‐TEMPO for positive electrode enhancement in a mildly acidic electrolyte and AC//Ni(OH)₂ system with 4‐OH‐TEMPO for negative electrode enhancement in an alkaline electrolyte. Both demonstrate that the addition of 4‐OH‐TEMPO can effectively balance the capacity mismatching between two electrodes, and its capacity contribution can be adjusted by concentration. The energy density of the two ACSs with 4‐OH‐TEMPO can be greatly promoted without significant sacrifice of the device's volume or mass.