This study investigates the large voltage range of symmetric carbon/carbon capacitors in environmentally friendly aqueous lithium sulfate electrolyte. A high over-potential related to the hydrogen sorption mechanism at the negative electrode contributes usefully to enhance the operating voltage up to 1.9 V with an excellent stability during 10000 charge/discharge cycles. Such a voltage value is two times higher than the values generally demonstrated with symmetric carbon/carbon capacitors in conventional aqueous media, while avoiding the disadvantages of the corrosive properties of acidic and basic electrolytes. Temperature programmed desorption analysis of the electrodes after long-term cycling gives the evidence that the maximum voltage is essentially limited by an irreversible electro-oxidation process at the positive electrode. If the potential of the positive electrode goes beyond a given value during cell operation, a massive electro-oxidation of carbon leads to a further deleterious increase of the maximum potential of the electrode and an increase of electrode resistance resulting in a decrease of capacitance. Inconvenience can be sidestepped by performing a controlled chemical oxidation of the carbon surface using hydrogen peroxide. As a consequence, the maximum potential of the electrode remains stable during operation of the cell at 1.9 V, and the system can be charged/discharged during 10000 cycles with very moderate loss of capacitance or increase of resistance.
