Highly active N, S Co-Doped Ultramicroporous Carbon for High-Performance Supercapacitor Electrodes

N, S-doped ultramicroporous carbons (NSUC-x) with a high nitrogen/sulfur content and a narrow pore-size distribution of around 0.55 nm were firstly prepared using L-cysteine as a nitrogen and sulfur source. The phase, graphitization degree, morphology, specific surface area, pore structure and surface condition of NSUC-x are investigated to analyze the key role in electrochemical performance. Such an ultramicroporous structure and N, S doping not merely provide a high-specific surface area and a suitable pore size, but also induce a good wettability for the fast transport and adsorption of electrolyte ions. Due to the above strategies, the typical NSUC-0.4 exhibits a high gravimetric capacitance of 339 F g^-1 at 0.5 A g^-1 as well as a capacity retention of 91.6% after 10,000 cycles in a three-electrode system using a 6 M KOH electrolyte. More attractively, a NSUC-0.4-assembled symmetrical supercapacitor delivers an energy output of 7.4 Wh kg^-1 at 100 W kg^-1 in 6 M KOH as well as a capacity retention of 92.4% after 10,000 cycles, indicating its practical application prospect. Our findings open up new prospects for the design and electrochemical application of N, S-doped ultramicroporous carbons.