Influence of KNO3 on Capacitive Properties of Active Carbon in Potassium Ferricyanide Solutions

Electrolyte plays an integral role in electrochemical capacitors with active carbon electrodes. A critical challenge in electrolyte formula is to achieve a decent balance between voltage window and capacitance of the capacitor. In this study, redox-active aqueous electrolytes, which consist of neutral KNO3 aqueous solution and K-4[Fe(CN)(6)], were used to extend the voltage window of active carbon (AC) capacitors and improve their performance. Cyclic voltammetry test, galvanostatic charge-discharge test and electrochemical impedance spectroscopy test were conducted to investigate the pseudocapacitive effects of KNO3-K-4[Fe(CN)(6)] on AC electrodes, which were respectively arranged in three-electrode and two electrode capacitor configuration. Faradaic redox reactions of [Fe(CN)(6)](3-)/[Fe(CN)(6)](4-)occurred on the positive electrodes. The working voltage window of the electrodes in the neutral electrolyte was larger than that in the control electrolyte, resulting in considerable performance enhancement. The specific capacitance, power density and energy density respectively reached 250 F g(-1), 332 W kg(-1) and 14.4 Wh kg(-1) at a current density of 1.0 A g(-1) in the redox-active aqueous electrolyte for a symmetric AC capacitor. The most favorable balance among energy density, power density and cycle life was realized when the K-4[Fe(CN)(6)] concentration was 0.033 M.