In this work, a novel redox additive of calcon carboxylic acid sodium salt (abbr. CS) has been implemented for the first time in KOH electrolyte for largely improving the electrochemical performance of a supercapacitor. By directly carbonizing potassium citrate monohydrate at 800 degrees C, giving rise to the Carbon-800 sample, and the resulting specific capacitance can reach up to 128.4 F g(-1) at 3 A g(-1) when measured in a three-electrode system with the conventional 6 mol L-1 KOH electrolyte. To further elevate these capacitances, certain amount of CS as a redox additive is introduced into KOH electrolyte. Remarkably, the Carbon-800-15 sample by adding CS 15 mmol L-1 into 6 mol L-1 KOH electrolyte displays a highly improved specific capacitance of 237.0 F g(-1) at 3 A g(-1) in a three-electrode system, which is almost the 2.1 times than that of the pristine one. In addition, the Carbon-800-15 sample also possesses an elevated energy density of 11.4 Wh kg(-1) at the power density of 1.5 kW kg(-1) when measured in a two-electrode system, much larger than the 73 Wh kg(-1) of the Carbon-800 sample. The improved performance incurred by CS additive is attributed to the fast redox reactions at the electrolyte/electrode interface. Apparently, the present efficient and low-cost redox additive of CS is expected to be implemented for largely improving the performances of a supercapacitor. (C) 2016 Elsevier B.V. All rights reserved.