The Investigation Of Co₉S₈ Nanotubes And CoS_x/Graphene Composite For Supercapacitors

Due to the unique physical and chemical properties,nanostructured metal sulfide materials have draw much attention to be used in supercapacitors.In this work,cobalt sulfide nanotubes were synthesized by a two-step hydrothermal method,while the size of them were controlled by adjusting the reaction pH.The results show that the size of nanotubes diminishes with the increase of pH.When the pH is 7.16,the nanotubes obtain the samllest size of 4 ?m in length and 212 nm in diameter.The electrochemical properties of samples were evaluated in a three-electrode cell.The results indicate that the specific capacitance of cobalt sulfide increases with the rise of pH.It can be known from the reports that the diffusion velocity of electrolyte ion become faster and the specific area of active materials become larger with the decrease of nanotube size.Co₉S₈-pH-7.16 have the largest specific capacitance of 369 F/g at the current density of 1 A/g.The electrochemical performance of Co₉S₈-pH-7.16 was further investigated.The results show that the specific capacitance at 5 A/g is 83% of that at 0.5 A/g and 84% retention of specific capacitance is achived after 500 cycles at 1 A/g,which suggests good rate capability and cycling stability.In order to improve the specific capacitance,rate capability and cycling stability of cobalt sulfide,a CoS_x/graphene composite was prepared by a two-step hydrothermal method.SEM measurements show that the cobalt sulfide nanotubes with the size of 8 ?m in length and 360 nm in diameter insert into graphene nanosheets.The size of nanotubes in CoS_x/graphene composite differs from that of nanotubes in Co₉S₈-pH-7.16,which may results from the interaction between Co²⁺ and oxygen-containing group on the graphene sheet.Cobalt sulfides are the mixture of Co₉S₈,Co3S4,and CoS1.097,in which the major phase corresponds to Co₉S₈.XRD and Raman measurements prove the partial reduction of graphene.Electrochemical analysis was carried out in a three-electrode cell.A specific capacitance of 790 F/g and 89% capacitance retention after 500 cylcles were observed for CoS_x/graphene at the current density of 1 A/g.The specific capacitance at 5 A/g is 88% of that at 0.5 A/g.Compared to Co₉S₈-pH-7.16,the specific capacitance,rate capability and cycling stability of CoS_x/graphene are all improved.The excellent performance of CoS_x/graphene maybe due to the better mechanical stability and electrical conductivity of graphene.Moreover,the insertion of CoS_x nanotubes into the graphene nanosheets can inhibit the agglomeration of graphene nanosheets,leading to the increase of specific area.The interaction bwteen CoS_x and graphene also plays an important role in promoting the transport of electrons during the charge-discharge process.