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Nickel cobalt sulfide nanoparticles embedded in holey defect graphene hydrogel (HGH) that exhibit highly porous structures and uniform nickel cobalt sulfide nanoparticle sizes are successfully prepared by a facile solvothermal-hydrothermal method. As an electrode material for supercapacitors, the as-prepared NiCo S @HGH shows ultra-high specific capacitances of 1000 F g and 800 F g at 0.5 and 6 A g , respectively, owing to the outstanding electrical conductivity of HGH and high specific capacitance of NiCo S . After 2100 charge/discharge cycles at a current density of 6 A g , 96.6 % of the specific capacitance was retained, signifying the superb durability of NiCo S @HGH. Moreover, remarkable specific capacitance (312.6 F g ) and capacity retention (87 % after 5000 cycles) at 6 A g were displayed by the symmetric solid-state supercapacitor fabricated by using NiCo S @HGH electrodes. These auspicious supercapacitor performances demonstrate that the as-developed solvothermal-hydrothermal approach can be widely used to prepare graphene-coupled binary metal sulfides for high-performance supercapacitor applications.

We report successfully synthesizing two-dimensional (2D) and nanocrystalline (NC) Fe2Ni2N/rGO nanohybrid sheets (NHSs) via ammonolysis of as-prepared 2D Ni2.25Fe0.75[Fe­(CN)6]2/rGO precursors. We compared the electrochemical properties of the 2D-NC Fe2Ni2N/rGO NHSs as non-precious-metal nitride and graphene nanohybrid electrocatalysts for an oxygen evolution reaction (OER) with those of NiFe-based composition. The overpotential and Tafel plot of the 2D-NC Fe2Ni2N/rGO NHSs had their lowest values of 290 and 49.1 mV dec–1, respectively, at a current density of 10 mA cm–2 (0.1 M, KOH). The 2D-NC Fe2Ni2N/rGO NHS catalyst was stable under OER conditions, and X-ray photoelectron spectroscopy and scanning transmission electron microscopy confirmed the stability of the catalysts after electrochemical testing (24 h). The synergistic interactions between the transition-metal nitride and graphene represent unique 2D nanostructured, metallic properties, and graphene sheets with nanocrystalline Fe2Ni2N on them are significantly more efficient and active electrocatalysts. The presented strategy of transition-metal nitride/graphene hybrid nanostructures provides potential for more efficient and outstanding OER electrocatalysts.

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