Adjusting the electrode surface functionality to improve the cell voltage of aqueous electrolyte carbon/carbon supercapacitors

Improving the performance of supercapacitors in terms of energy density is a major technical challenge, especially in aqueous media where the operating voltage is limited by the electrochemical stability window of water and by undesirable reactions at the electrode/electrolyte interface. Carbon/carbon supercapacitors using 1.0 mol L −1 Li 2 SO 4 as electrolyte can achieve cell voltages of 1.8 V. Beyond this value, long-term supercapacitor operation is limited by positive electrode degradation due to irreversible oxidation reactions at the carbon/electrolyte interface. Such degradation processes can be minimized by selectively modifying the surface functionality of the porous carbon active electrode material. An in-depth study of the effect of different surface functionalities on the ageing of the supercapacitor, combining quantitative analysis of the gas generated during operation with electrochemical techniques and physical characterization of the carbon electrode, showed that among the different processes, the chemical grafting of low amounts of phenyl functionalities is the most effective in preventing the oxidation of the carbon. The origin of the resistance to oxidation was a combined effect of the blocking of active sites and the reduction of the local pH at the interface. The cell voltage can be increased to 2.2 V and the energy density more than doubled by using an asymmetric system with the modified carbon as the positive electrode and an unmodified carbon as the negative electrode.