Several forms of activated carbons have already been implemented and widely investigated as electrodes for electrochemical capacitors. Aqueous solutions based on inorganic salts demonstrated effective operating voltages around 1.6 V, and several interesting strategies have been furthermore implemented to improve these systems, including the redox activity of the electrolyte.

This work is focused on the electrochemical performance of the activated carbon electrodes operating in various electrolytes containing redox couple based on the ascorbic acid and sodium ascorbate. Since sulphate (SO₄^2-) and nitrate (NO₃^-) -based aqueous solutions have already been widely investigated, this study will discuss these electrolytes modified by the redox-active additive and their interaction with a carbon electrode. The results obtained clearly indicated that redox mediators play an important role in the charge accumulation process and has an excellent redox chemistry. Dependently on the electrolyte pH, the ionic specimen might be easily reduced or oxidized and perfectly preserves the activated carbon surface against oxidation.

A variety of electrochemical methods used in the study confirmed that a specific formulation of the electrolyte with ascorbate-based redox shuttle might remarkably enhance the electrode capacitance (up to 260 F g^-1) and the operating voltage (up to 1.8 V). Moreover, operando Raman spectroscopy proved that several bonds are reversibly created while the charge transfer occurs. Furthermore, high specific energy (ca. 14 Wh kg^-1) maintained at excellent power rates (1 kW kg^-1) has been preserved during 8 000 charge/discharge cycles with reversibility of 94%.

Acknowledgements

European Research Council is acknowledged for financial support within the Starting Grant project (GA 759603) under European Unions’ Horizon 2020 research and innovation programme.