Study On The Preparation Of Ni-based Composite And Its Capacitance Performance

Supercapacitors have been successfully used in trams and buses due to their advantages such as fast charging and discharging rate,long cycle life,high power density and environmental friendliness.However,the electrode materials of commercial supercapacitors are mainly activated carbon and graphene,which still have the disadvantages of low energy density and high cost.Therefore,it is particularly important to explore high-performance electrode materials with low price.Nickel-based compounds have become a research hotspot due to their high theoretical specific capacitance and low cost,but at the same time,they still suffer from poor electrical conductivity and particle agglomeration,which result in low initial specific capacitance,poor rate capability and unsatisfactory cycling stability.In order to improve the comprehensive capacitance performance of nickel-based compounds,this paper firstly selects different carbon materials as carriers to investigate the effects of the specific surface area and different dimensions of carbon materials on the capacitance performance of Ni(OH)2/C composites.The experimental results show that the capacitance of the composite is much better than that of Ni(OH)2,but its rate capability and cyclic stability are still not ideal.Therefore,sulfur element is used to replace oxygen element to prepare nickel sulfide composites with better conductivity.On the basis of the nickel sulfide composite with better capacitance performance,the nickel-cobalt sulfur composites and nickel-cobalt sulfur/C composites were further prepared by adding cobalt.The specific capacitance,rate capability and cycle life of the composite can meet the requirements of electrode materials for supercapacitors.The main research contents and results are as follows:1.Effects of different dimensional carbon materials on the performances of Ni(OH)2/C composites.Four different conductive carbon materials are selected as carriers,including zero-dimensional XC-72,one-dimensional carbon nanotubes(CNT),two-dimensional graphene(G)and three-dimensional expanded graphite(EG).Nickel hydroxide nanoparticles were deposited on the corresponding conductive carbon substrate by a simple solvethennal method(pH=11.5,T=120?,t=18 h,VEG:VH20=1:1).The results showed that compared with XC-72,carbon nanotubes and expannded graphite,graphene,due to its high conductivity,large specific surface area and special two-dimensional layered structure,was more conducive to the transport of electrons,infiltration of ions and the exposure of active sites,so that the Ni(OH)2/G composite exhibited better comprehensive capacitance performance.It can achieve 2089.4 F g-1 at 1 A g-1,and still possess 72.6%of initial capacitance at 20 A g-1,and after 1000 cycles at 10 A g-1,the capacitance retention can still reach 81.7%.2.Study on the capacitance of nickel sulfide composites.Considering the difference of electronegativity between sulfur and oxygen,nickel sulfide has higher conductivity than nickel oxide/hydroxide.In this paper,nickel sulfide is also synthesized by microwave heating to improve the poor conductivity of nickel hydroxide.The results show that the specific capacitance of the synthesized nickel sulfide is 1574.4 F g-1 at 5 A g-1,when the current density increases to 30 A g-1,the specific capacitance is still 1222.5 F g-1,the rate capability remains at 77.6%,and the capacitance retention rate still reaches 79.2%after 1000 cycles at a high current density of 30 A g-1.Although compared with nickel hydroxide and its compounds,nickel sulfide has better rate capability and cycling stability under high current density,but its comprehensive capacitance performance still has room for improvement.3.Study on the capacitance of nickel-cobalt-sulfur/C composites.Considering that the capacitance performance of single metal sulfide is still not very ideal,while doping cobalt can not only further improve the conductivity of the comosite,but also bimetal sulfide has multiple oxidation states during charging/discharging processes,and smaller band gaps for richer oxidation reactions.We also prerared Ni2CoS4 and Ni2CoS4/G,the results show that Ni2CoS4/G has excellent electrochemical performance,the initial capacitance can achieve 2056.8 F g-1 at 5 A g-1,even under the high current density of 30 A g-1,the rate capability is still as high as 93.5%,and after 2000 cycles at 30 A g-1,the capacitance retention rate is still 94.4%,compared with that of Ni(OH)2 and Ni3S4,the electrochemical performances have a qualitative leap.Ni2CoS4/G,AC,non-woven fabric,6 M KOH were employed as positive electrode material,negative electrode material,separator and electrolyte respectively to assemble Ni2CoS4/G//AC asymmetric supercapacitor,the device exhibits a high specific capacitance of 120.3 F g-1@0.5 A g-1,an superior cycle life(91%@5 A g-1 for 5000 cycles),and an extremely high energy density of 52 Wh kg-1 @477 W kg-1.In summary,all the three methods adopted in this paper can improve the capacitance performance of nickel-based complex,and the Ni2CoS4/G//AC asymmetric supercapacitor exhibits an excellent capacitance performance,which is closer to commercial application.