Study On The Preparation And Performance Of Nickel-cobalt Composites With Electrochemical Deposition Method As Supercapacitors

Supercapacitors have attracted great attention in recent years due to their distinct merits of ultrahigh power density,fast charging rate,long cycling life.However,the relatively low energy density hinders extensive application of supercapacitors.Currently,the enhancement of energy density for supercapacitors can be achieved by developing advanced materials with high specific capacitances or assembling asymmetric systems to extend the operating voltage window.Nickel and cobalt binary composites have emerged as a promising class of active materials due to their excellent redox reversibility and relatively high capacity.However,these materials normally have either low conductivity or poor electrochemical stability.Graphene(G)is a carbon material with a two-dimensional structure,large surface,and integrated conductive network that is suitable for loading other materials.With the help of RGO,the capacitance of electrode materials is enhanced.Moreover,the introduction of graphene can also reduce the volume and particle aggregation in the process of charging and discharging.In addition,the three-dimensional porous structure obtained by the combination of the two methods is helpful to the increase of the ion transfer rate and the electron transfer rate,which further enhances the overall electrochemical performance.Herein,a facile and efficient hydrothermal-electrodeposition method is developed to fabricate binder-free NiCo binary compounds/three-dimensional(3D)graphene composite electrode material,and preparedasymmetric supercapacitor.The morphologies and chemical structures of the obtained samples were characterized by FESEM,XPS,XRD and the electrochemical performances of the samples were studied in a three-electrode system using CV,GCD and EIS.1.Based on the three-dimensional continuous porous foam nickel as current collector and a conductive substrate,where 3D graphene was prepared by hydrothermal reduction of graphene oxide solution with a piece of Ni foam in the autoclave,and then NiCo binary compounds were electrodeposited on the free-standing RGO/NF,fabricatedthe NCG composite electrode materials.(1)NCG30,NCG50,NCG70 were obtained by changing the deposition time(30min,50 min,70min)with RGO/NF as the substrate after hydrothermal deposition of RGO.Among them,NCG50 has ordered porous petaloid structure and the best electrochemical performance,at the scan rate of 1mA/cm2,the specific capacitance as high as 2856 mF/cm2.After the charge-discharge rate was increased to 15 mA/cm2,the specific capacitance of the 2285 mF/cm2,with a retention of 80.1%,indicating its good rate capability.(2)The NCG composites with different Ni/Co molar ratios were prepared by changing the molar ratio of nickel to cobalt under the condition of(1).It can be seen from the SEM that the different molar ratio of Ni and Co has a great influence on the structure of NCG composites.NCG2 showed a three-dimensional ordered porous structure and the electrochemical performance was the best.When the current density was 1mA/cm2,the area specific capacitance was as high as 3172 mF/cm2.After 5000 cycles of continuous charging and discharging,the capacitance retention rate is still the initial value of 92%.(3)The asymmetric supercapacitor with NCG2 as the positive electrode and RGO as the negative electrodecan deliver a maximum power density of 5.6 kW/kg and energy density of 50 Wh/kg.Meanwhile,the potential window of the homemade asymmetric capacitor device expanded to 1.4V,and can continue to light up LED nearly 5min after fast charge processing.2.The NCS/G composites were prepared by a two-step electrodeposition method under nickel foam as current collector and a conductive substrate.The results are as follows:NCS/G composites exhibited flower-like structureandthe specific capacitance up to 5540 mF/cm2 even at a charge-discharge rate of 50 mA/cm2,with a retention of 81%,much higher than NS/G(70%)and CS/G(68%).Moreover,it exhibits good capacity retention only9% loss after 3000 cycles at 5 mA/cm2,the specific capacitance of the NCS/G was 5347mF/cm2.