Boosting the performance of hybrid supercapacitors through redox electrolyte-mediated capacity balancing

Abstract

Hybrid supercapacitors (HSCs) are promising new energy storage devices offering both high power density and good energy density. However, because of the energy storage mechanism difference in two electrodes, the capacity imbalance issue is challenging which often results in limited specific capability in the HSCs. The conventional approach by balancing the mass of electrode materials could extend the operating voltage window but sacrificing the specific capacity. To fundamentally address this problem, here we propose a new concept of asymmetric electrolyte design in the HSCs where NiCo layered double hydroxide (LDH) battery-type electrode operates in the KOH electrolyte while electrolyte-soluble redox couples are deliberately introduced to the carbon capacitive electrode. The redox couples contribute extra faradic capacity to the capacitive carbon electrode, resolving the capacity imbalance problem in the two electrodes with equal mass loading. The optimized HSC delivers extraordinary high specific energy of 79.6 Wh/kg, which is about 4 times the value in the pristine unbalanced device (20.3 Wh/kg). This new conceptual design could be extended to other energy storage systems to further improve performance.