A High-Performance Redox Electrochemical Capacitor: Confinement of Sodium Anthraquinone-2-Sulfonate in Porous Carbons with Electric-Field Assistance

Redox electrolytes bridge the energy density gap between carbon-based supercapacitors and batteries. However, across-diffusion of soluble redox couples results in subpar rate performance, therefore, how to deal with this impasse becomes the focus of the field's attention. In this study, we put forth the idea that the problem can be solved by using activated carbon nanopores as reservoirs to retain redox species and prevent their diffusion. The electric field is used to help confine sodium anthraquinone-2-sulfonate (AQS, size is around 1.634 × 0.634 × 0.864 nm3) to the nanopores of the tea-leaf biomass carbon electrode (TAC, 59.65% of micropores). We show that AQS molecules occupy small micropores (less than 1 nm), resolving the problem of cross-diffusion that arises naturally in redox-enhanced electrochemical capacitors. Thus, in the 1 M H2SO4 electrolyte, a high specific capacitance of 733.3 F g–1 is produced in a three-electrode system at a current density of 1 A g–1, and only 7% of the specific capacitance decays at a current density of 10 A g–1. Besides, the TAC/AQS-5 electrode-based symmetric device delivers a maximum energy density of 41.48 Wh kg–1 and excellent capacitance retention of 86.2% after 5000 charge and discharge cycles at 2 A g–1. The maximum energy density obtained by our symmetrical supercapacitor is higher than that of currently reported aqueous electrolyte supercapacitors.