Sulfur Doping: Unique Strategy to Improve Supercapacitive Performance of Carbon Nano-Onions.

Recently, enhancement of the energy density of supercapacitor is restricted by the inferior capacitance of the negative electrodes, which impedes the commercial development of high- performance symmetric and asymmetric supercapacitors. This paper introduces the in-situ bulk-quantity synthesis of hydrophilic, porous, graphitic sulfur-doped carbon nano-onions (S-CNO) using a facile flame-pyrolysis technique and evaluated its potential applications as high-performance supercapacitor electrode in a symmetric device configuration. The high surface wettability in the as-prepared state enables the formation of the highly suspended active conducting material S-CNO ink, which eliminates the routine use of binders for electrode preparation. The as-prepared S-CNO displayed encouraging features for electrochemical energy storage applications with a high specific surface area (950 m2 g-1), ordered mesoporous structure (~3.9 nm), high S-content (~3.6 at.%), and substantial electronic conductivity, as indicated by the ~80% sp2 graphitic carbon content. The in-situ sulfur incorporation into the carbon framework of the CNO resulted in a high-polarized surface with well-distributed reversible pseudo-sites, increasing the electrode-electrolyte interaction and improving the overall conductivity. The S-CNOs showed a specific capacitance of 305 F g-1, an energy density of 10.6 Wh kg-1, and a power density of 1,004 W kg-1 at an applied current density of 2 A g-1 in a symmetrical two-electrode cell configuration, which is approximately three times higher than that of the pristine CNO-based device in a similar electrochemical testing environment. Even at 11 A g-1, the S-CNO||S-CNO device rendered an energy density (6.1 Wh kg-1) at a deliverable power density of 5.5 kW kg-1, indicating a very good rate capability and power management during peak power delivery application. Furthermore, it showed a high degree of electrochemical reversibility with excellent cycling stability, retaining ~95% of its initial capacitance after more than 10,000 repetitive charge-discharge cycles at an applied current density of 5 A g-1.