Sulfur modification of carbon materials as well as the redox additive of Na2S for largely improving capacitive performance of supercapacitors

Heteroatom doping is an alternative method for obtaining a supercapacitor with high specific capacitance and high energy density. Here we have designed an effective strategy. Firstly, the surface of the activated carbon is modified by sulfur powder at 700 °C to produce sulfur-containing groups on the surface, and it exhibits good electrochemical performance (207 F g−1 at 1 A g−1, and 7.2 Wh kg−1). Next, the redox additive Na2S introduced and the sulfur-containing group on the surface of the carbon material undergo a redox reaction (at the positive electrode) and form a chemical bond. As a result, the specific capacitance can still reach 643 F g−1 when the current density is as high as 15 A g−1 and has a high energy density of 12.6 Wh kg−1. This improvement is attributed to the synergistic action of the sulfur-containing groups on the surface of the carbon material with S2− in the redox additive and forms a covalent bond. Therefore, our strategy (surface modification and synergy of redox additives) provides an effective means of improving the performance of carbon-based supercapacitors.