Fabricating dual redox electrolyte to achieve ultrahigh specific capacitance and reasonable Coulombic efficiency for biomass activated carbon

Adding redox mediators in an aqueous electrolyte is one of effective methods to enhance the energy storage capacity of biomass activated carbon, however, the Coulombic efficiency is dissatisfied to meet the applied demand. In this paper, we propose the challenge can be met by designing adjustable dual redox electrolyte. Biomass activated carbon (TAC) with a specific surface area of 2521.8 m2 center dot g- 1 is prepared by waste tea-leaves using pre-carbonization and activation with KOH. In a three-electrode system, the specific capacitance of TAC is 358.6 F center dot g- 1 in 1M H2SO4 at a current density of 1A center dot g- 1. After introducing 0.05M Na2MoO4, as a redox addictive, the specific capacitance is 856.67 F center dot g- 1, but the Coulombic efficiency is as high as 169.2% due to the irreversible reactions. To adjust the Coulombic efficiency, KBr is added to fabricate a dual redox electrolyte. The amazing results are that the specific capacitance of TAC is as high as 1901.36 F center dot g- 1, which is 5.3 times higher than that in 1M H2SO4, and the Coulombic efficiency is back to a rational value (100.7%). A symmetric supercapacitor (TAC-RE) with the optimized electrolyte is assembled, which exhibits an excellent energy density of 57.52 Wh center dot kg- 1 when a power density is 980 W center dot kg- 1. The application of powering three LED demonstrates TAC-RE has desirable capacitive performances, especially is a kind of low-cost energy storage devices.

Automated summary

This paper proposes the use of an adjustable dual redox electrolyte to enhance the energy storage capacity of biomass activated carbon. Experimental results show that adding KBr can significantly increase the specific capacitance of TAC and adjust the Coulombic efficiency to a rational value.