Study On The Synthesis And Electrochemical Performance Of NiCo₂S₄ As Electrode Materials For Supercapacitors

With the advantages of high energy density,rapid charge/discharge,long cycle life and environmental frinendliness,supercapacitors have widely prospect in application as a new type of energy storage devices.However,the low energy density becomes the key problem that restricts its practical application.As the core of supercapacitors,the electrochemical performance of electrode materials has a decisive impact on the energy storage performance of supercapacitors.As a kind of transition metal based sulfide electrode material,nickel cobalt sulfide has become a potential supercapacitor electrode material with the advantages of high theoretical capacitance,rich source of raw materials and low cost.However,nickel cobalt sulfide still suffer from the unsatisfactory capacitance,low rate capability and poor electrochemical performance,which is attributed the limited electroactive sites and sluggish reaction kinetics.In order to solve the above problems,the high performance nickel cobalt sulfide electrode materials are prepared from two directions,which are the composite electrode materials with good core-shell structure and the doping of elements.Through simply and accurately controlling the concentration of sulfur source,various Core-shell nanostructures of NiCo₂S₄@Ni₃S₂ arrays grown in situ on Ni foam are successfully synthesized by a scalable and effective one-step strategy.The intriguing core-shell structures and integrated electrode configurations endow NiCo₂S₄@Ni₃S₂electrode a large electroactive sites,fast electron transport path and sufficient contacts with electrolyte.Serving as free-standing electrodes for supercapacitors,as-fabricated NiCo₂S₄@Ni₃S₂ composite electrode material exhibits the high specific capacitance(9.1F cm^-2at 2 mA cm^-2),good rate performance and good cycle stability.In order to improve the performance of NiCo₂S₄ material itself,the P-Ni Co₂S₄electrode material was prepared by phosphidation reaction in a tubular furnace.Then,the effects of phosphorus doping on the composition and electrochemical properties of NiCo₂S₄ electrode materials were studied.The experimental results show that some of the S atoms in NiCo₂S₄ are replaced by P atoms after the phosphating treatment.A proper amount of P doping will not change the morphology and structure of the Ni Co₂S₄electrode material.Electrochemical results demonstrate that P-doped NiCo₂S₄ nanotube arrays exhibit the better electrochemical performance than that of pristine Ni Co₂S₄,higher specific capacitance(2080 F g^-1at 0.5 A g^-1),better cycle stability and lower charge transfer resistance.As-assembled P-NiCo₂S₄//AC asymmetric supercapacitor achieved a high energy density up to 42.1 Wh kg^-1at the power densiy of 750 W kg^-1.Finally,free-standing nickel-cobalt sulfide/selenide nanotubes with controllable-component were directly synthesized on carbon cloth to improve the electrochemical performance of nickel cobalt sulfide by doping selenium.The whole synthesis process mainly involves growth of Ni Co-precursors on carbon cloth,following by the sulfuration/selenylation reaction under S/Se vapor to obtain nickel-cobalt sulfide/selenide nanotubes.Proper Se doping in nickel-cobalt sulfide can greatly increase the electrochemically active surface area and reduce the charge transfer resistance.Consequently,compared with pure nickel-cobalt sulfide,Se doped nickel-cobalt sulfide shows a better electrochemical performance in terms of high specific capacitance and good cycling stability.Electrochemical results demonstrate that NiCo₂S_2.2Se_1.8nanotube arrays exhibit the better electrochemical performance than that of pristine Ni Co₂S_3.6,higher specific capacitance(1739 F g^-11 at 2.5 A g^-1),better cycle stability and electrical conductivity.As-assembled NiCo₂S_2.2Se_1.8//AC asymmetric supercapacitor achieved a high energy density up to 39.6 Wh kg^-1at the power densiy of 1501 W kg^-1.