Glucose-Derived Carbon Layer Coated on Mixed-Phase\nNickel Sulfide Nanoparticles for Hybrid Supercapacitors

Glucose\ncan form carbon material under hydrothermal conditions.\nThus, carbon-layer-coated nickel sulfide nanoparticles were prepared\nusing a one-step hydrothermal method with nickel sulfide as the primary\nphase. The effect of the glucose mass on the structure and electrochemical\nproperties of nickel sulfide was studied in detail. The size of nanoparticles\ndecreased after the nickel sulfide surface was coated by using the\ncarbon layer, forming uniform spherical particles of 200–300\nnm. The carbon layer thickness on the surface was ∼3 nm. The\nnitrogen adsorption–desorption test revealed that the specific\nsurface area of the pristine nickel sulfide was 2.7 m<sup>2</sup> g<sup>–1</sup>. Under optimal conditions, the prepared carbon-layer-coated\nnickel sulfide (NixSy-0.5) depicted a surface area of 15.2 m<sup>2</sup> g<sup>–1</sup>. The high specific surface area provided additional\nelectrochemically active sites. The glucose-derived carbon layer could\nbe utilized as a conductive agent to enhance electron transport in\nthe electrode. Electrochemical test results depicted a high capacitance\nof NixSy-0.5 of 612.5 C g<sup>–1</sup> at 1 A g<sup>–1</sup>, higher than that of pristine nickel sulfide (381.7 C g<sup>–1</sup>). The hybrid supercapacitor (HSC) assembled with NixSy-0.5 as the\npositive electrode and commercial activated carbon (AC) as the negative\nelectrode exhibited a high energy density of 47 Wh kg<sup>–1</sup>. The capacitance retention rate is high (99%) after 20 000\ncycles. This study demonstrated the simultaneous preparation of nickel\nsulfide–carbon layer composites and provided a feasible strategy\nfor preparing economical and efficient electrode materials.