Recently, enhancement of the energy density of a supercapacitor is restricted by the inferior capacitance of negative electrodes, which impedes the commercial development of high-performance symmetric and asymmetric supercapacitors. This article 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 a high-performance supercapacitor electrode in a symmetric device configuration. The high-surface wettability in the as-prepared state enables the formation of highly suspended active conducting material S-CNO ink, which eliminates the routine use of binders for the 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 pseudosites, 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 W h kg–1, and a power density of 1004 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 W h 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 applications. 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.

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In this work we report an economic, eco-friendly, high yielding and facile one-pot method for the large scale synthesis of few layer graphene (FLG) nanosheets directly from graphite in aqueous medium using a regenerative catalyst, sodium tungstate. This method is fast and makes use of environmental friendly chemicals and microwave radiation. The as-synthesized FLG nanosheets are characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) surface area analysis. Raman analysis indicates that the as-synthesized graphene is bilayered with a smaller domain size of 3.9 nm which is responsible for a higher specific surface area of FLG nanosheets (1103.62 m 2 g À1 ). Moreover, XPS analysis of FLG nanosheets shows a high C : O ratio (B9.6) which is the best among the graphene prepared from green chemicals. The electrochemical performance of as-synthesized FLG nanosheets is analysed by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in neat 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) electrolyte.The superior capacitive performance with large capacitance (219 F g À1 ), high energy density (83.56 W h kg À1 ) and excellent cyclability (3000 cycles) exhibited by these graphene nanosheets make them an excellent candidate for supercapacitor material.

A novel, highly efficient composite electrode containing earth-abundant elements (Co–Ni) and graphene has been developed for the electrocatalytic hydrogen evolution reaction in an alkaline medium.

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