Preparation And Electrochemical Performance Of The Nickel Cobalt Sulfide As Supercapacitor Electrode Material

Much attention has been paid to the study nickel-cobalt sulfide(CoNi2S4)owing to its richer redox reactions and high conductivity in the field of the supercapacitor electrode materials.In this paper,CoNi2S4 with different nickel-cobalt ratios and different morphologies are prepared by controlling the nickel-cobalt ratios,reaction solvents and sulfur source.The effect of nickel-cobalt ratio and morphology on the electrochemical performance of CoNi2S4 was investigated by comparing the electrochemical performance of prepared CoNi2S4.Then,high-performance anode material for supercapacitors was synthesized by CoNi2S4 incorporated with carbon nanofibers(CNFs-B)via the solvothermal method,in which the CNFs-B was directly carbonized from bacterial cellulose(BC).Finally,the CoNi2S4 were combined to BC and polypyrrole(PPy)to form flexible composite using solvothermal method and in situ polymerization oxidation method.The structure and morphology of the composite were characterized by scanning electron microscopy,transmission electron microscopy,X-ray diffraction,X-ray diffraction spectra,N2 physisorption measurements and Raman spectroscopy.Their electrochemical performance was tested by cyclic voltammetry,galvanostatic charge-discharge and alternating-current impedance methods.The results show that:(1)The reaction solvent and sulfur source have a great influence on the morphology of nickel-cobalt sulfide.When the solvent is ethylene glycol,the sulfur source is thiourea and the nickel-cobalt ratio is 2:1,the obtained nickel-cobalt sulfide exhibits nano-flower structure and has the best electrochemical performance(2048 F·g-1 at a current density of 1 A·g-1).(2)The carbon fiber(CNFs-B)with three-dimensional network structure obtained from carbonization of bacterial cellulose has a specific surface area of up to 197 m2·g-1.The CoNi2S4/CNFs-B composite obtained by combined CoNi2S4 with CNFs-B exhibits excellent physical and chemical properties.The CoNi2S4 nanoparticles were anchored on CNFs-B to form the integral structure and dispersed more uniformly,avoiding aggregation of CoNi2S4.Furthermore,CoNi2S4 nanoparticles anchored on CNFs-B leading the crystal lattices of CoNi2S4 overlapped the graphite lattices of CNFs-B to form the integral structure,which can shorten the distance of electron transport and reduce the charge-transfer resistance.The strong interaction between CNFs-B and CoNi2S4 nanoparticles is particularly favorable for the structural stability of composite and acceleration of the kinetic process of the ion diffusion in the electrode,benefiting for the long cycling stability and high capacitance.The CoNi2S4/CNFs-B composite exhibits a high specific capacitance(2272 F g"1 at current densities of 1 A·g-1),superior rate capability(71.2%at a current density of 20 A g-1)and outstanding cyclic stability(retains 99%of the initial capacitance after 5500 cycles).The asymmetric supercapacitor made of the CoNi2S4/CNFs-B composite and AC shows a high energy density of 65.25 Wh·kg-1 at the power density of 418.7 W·kg-1 and the excellent cycling stability with the retention of over 89.9%after 10,000 cycles.(3)High electrochemical performance flexibility electrode material for flexibility supercapacitors was synthesized by CoNi2S4 incorporated with bacterial cellulose and polypyrrole via the hydrothermal method and In situ polymerization oxidation method.Benefiting from the high redox activity of CoNi2S4 flexibility of bacterial cellulose and high conductivity of polypyrrole,the composite show a high specific capacitances(2412 F·g-1 at 1 A·g-1)and excellent electrochemical stabilities(retains 90.98%of the initial capacitance after 5200 cycles).