Study On The Nanoconstruction And Electrochemical Performance Of Nickel-based Sulfides

Supercapacitor is a new energy storage device,which can be divided into double-layer capacitors,pseudocapacitors and hybrid capacitors.The inorganic electrode materials of pseudocapacitors mainly include transition metal hydroxides/oxides,transition metal sulfides?TMS?.TMS will play an important role in energy storage systems due to their high electrochemical activity,low cost and environmental friendliness.Among these transition metal sulfides,nickel sulfides have become the target electrode materials in our works because of their excellent physical and chemical properties,low cost and facile synthesis.It is known that the specific capacitance of nickel-based sulfides as electrode materials in supercapacitors is still lower than that of corresponding hydroxides and its cycle stability and rate performance need to be further improved.Therefore,we focus on the two dimension nanosheets design and the construction of hierarchical nanostructures of nickel sulfides in order to improve the electrochemical performance of nickel-based sulfides.The main conclusion is listed as follows:?1?Preparation and electrochemical properties of Ni₃S₂ 2D nanosheets:Nickel sulfidenanosheets with high electrochemical performance were successfully synthesized by an electrochemical deposition method.It is interesting to notice that the size and surface area were simply tailored by adjusting the initial potential.The highest electrochemical performance and excellent high rate performance were achieved on the nickel sulfide sample prepared at initial potential of-0.9 V.?2?Preparation and electrochemical properties of NiS/Ni₃S₄ 2D nanosheets:The NiS/Ni₃S₄ 2D nanosheets were prepared by hydrothermal sulfuration on the?-Ni?OH?₂ monolayer nanosheets that were produced by grinding the mixture of nickel salt and morpholine at room temperature.The results of TEM indicated that the NiS/Ni₃S₄ prepared at 140? had sheet-like smooth morphology,presented high-specificcapacitance(2070.0 F g^-1 at current density of 2.5 A g^-1)and excellent electrochemical stability?86%retention of initial capacitance after 10,000 cycles?.The high electrochemical performance of the NiS/Ni₃S₄ 2D nanosheets could be attributed to its sheet-like morphology and the introduction of Ni₃S₄ phase.?3?Preparation and electrochemical properties of hierarchical nanostructure NiCo₂S₄:Thenanostructured NiCo₂S₄ with nickel foam as substrate was synthesized by microwave hydrothermal/solvothermal.The hierarchical nanostructure of NiCo₂S₄ was tuned through the supersaturation determined by the polarity of solvent and the solubility of product in solvent.The results of electrochemical measurement indicate that the as-prepared NiCo₂S₄ nanospheres with ordered nanosheets grown on have high electrochemical performance and cycling stability.The material presents high specific capacitance of 2070.5 F g^-1 at current density of 2.2 A g^-1.It also exhibits super-high cycling stability with 81.2%capacitance retention after 100,000 electrochemical charge-discharge cycles.The asymmetric supercapacitors with nanostructured NiCo₂S₄ and active carbon used respectively as positive and negative electrode materials also present high energy density 53.6 Wh kg^-1 at power density of 601.5 W kg^-1.Theseresults indicate that the nanostructure-tuned NiCo₂S₄ could be a promising electrodematerial for high performance supercapacitors.?4?Preparation and electrochemical properties of hierarchical nano-porous Ni_xS_y structure:The porous structure of three-dimensional nickel foam skeleton was treated by electrodeposition method.The results show that the porous structure is composed of interlinked metal nickel nanosheets on the three-dimensional nickel foam skeletonwhich effectively increases the specific surface area of the nickel foam.Then,hierarchical nano porous structure of Ni_xS_y with different specific surface area and poresize were controlled by various electrodeposition scan rate.The results ofelectrochemical tests show that the electrode materials prepared at the scanning rate of15 mV s^-1 exhibit the best electrochemical performance(the specific capacitance is upto 2418.87 F g^-1 at current density 3.97 A g^-1).This is mainly attributed to its highspecific surface area and appropriate porous structure,which could effectively improvethe specific capacitance of the material.The energy density of asymmetric supercapacitors is up to 40.69 Wh kg^-1 at low power density.