Preparation Of Ni/Co-based Composite Sulfides And Their Capacitive Behavior

With the development of society,the demand for energy storage and power supply system is increasing,resulting in great challenges to environment and energy.Therefore,it is urgent to design pollution-free and sustainable energy storage equipment.Supercapacitors have great application potential in large-scale energy storage due to its good cycling performance,super charge and discharge speed,high energy density,and economical and environmental friendliness.As well known,the performance of supercapacitors is mainly determined by the electrode material,so the research on high-performance materials has become a key point.To be specific,transition metal sulfide has been widely studied because of its high theoretical capacitance and good conductivity.In this paper,based on the nickel-cobalt sulfide and the synergistic effect among different elements,the micro-nano composite materials with unique structure were prepared.The main research contents are as follows:(1)Construction of NiS2@MoS2 composite and its capacitive behavior.The ball-flower like composite(NiS2@MoS2)was synthesized by a easy-to-operate two-step hydrothermal method using Ni(NO3)2-6H2O and(NH4)6Mo7O24·4H2O as precursors.MoS2 nano-sheets,uniformly generated on NiS2 surface,increase the surface roughness and porosity and accelerate the electrons and ions transfer.The synergistic effect of Mo and Ni improves the conductivity of the materials.Furthermore,by controlling the molar ratio of Mo to Ni in precursors,the composite MN-1(the molar ratio of MoS2 to NiS2 is 1:1)was obtained with the best capacitance performance.The sample MN-1,as the active electrode material of supercapacitor,achieved a high specific capacitance in 6 M KOH(1413.6 F g-1 at current density of 1 A g-1)and good cycling stability(83.1%specific capacitance retention after 10000 GCD cycles)in 6 M KOH.Furthermore,an asymmetric supercapacitor MN-1//AC was assembled.When the power density is 0.8 kW kg-1,its energy density reaches up to 37.2 Wh kg-1.(2)Preparation and capacitance behavior of hollow dodecahedron Co3S4@NiO.Hollow polyhedral composite Co3S4@NiO was prepared by self-template,and the rhombohedral dodecahedral zeolitic imidazolate frameworks-67(ZIF-67)was synthesized from Co(NO3)2·6H2O as Co source.The growth of NiO makes the composite expose more active sites.In addition,its heterogeneous hollow structure promotes electrolyte free diffusion and shortens electron transfer path in electrochemical reaction process,making the performance of Co3S4@NiO higher than that of Co3S4.In the 6 M KOH electrolyte,the specific capacitance of Co3S4@NiO is 1877.93 F g-1 at the specific current 1 A g-1.In particular,the composite exhibits good cyclic stability with 92.6%capacity retention after 10000 cycles.Meanwhile,the asymmetric supercapacitors Co3S4@NiO//AC delivered high energy density 54.99 Wh kg-1 as for the power density 0.78 kW kg-1.(3)Fabrication of double-layer hollow dodecahedron Zn-Co-S and its capacitive behavior.Double-layer heterogeneous metal sulfide(2L-ZnCoS)was successfully prepared at the expense of cobalt-nickel organic framework.The double-shell structure confines the electrolyte between the shells,providing a higher driving force for the redox reaction.On the other hand,the gap between layers can alleviate material volume effect during charge-discharge process,thus achieving a longer cycle life than that of the monolayer sulfide.The test results showed its specific capacitance high up to 1618.35 F g-1 at the current density of 1 A g-1,and 90.3%specific capacitance retention after 10000 cycles.An asymmetric supercapacitor 2L-ZnCoS//AC was assembled.As for the power density 0.80 kW kg-1,the energy density is as high as 51.76 Wh kg-1.In addition,2L-ZnCoS//AC showed good cycling stability.