Self-buffered pH at carbon surfaces in aqueous supercapacitors

This paper presents a unique strategy aiming at extending the operational voltage range in supercapacitor working in the neutral aqueous electrolyte. In-depth analysis of the equilibria formed at the carbon/electrolyte interface gives the possibility to reach 1.8 V with excellent reversibility during cycling. To reach the goal, the donor - acceptor nature of carbon electrode surface has been taken into consideration. Controlled oxidation of electrode materials coupled with ammonia adsorption have been realized in order to adjust the optimal equilibrium conditions. Nitric acid oxidation of carbon leads to the formation of electrochemically active acidic sites promoting adsorption of ammonia. Modification with ammonia results in the introduction of self-controlled pH gradient within the capacitor system and formation of protective layer on electrodes, responsible for higher overpotentials of solvent decomposition. This, in turn, allows capacitor operation at higher voltages. Furthermore, this configuration eliminates the need for physical separation of ions (such as an ion-exchange membrane) to decelerate their mixing and improves the power density of the device.