Controllable Preparation Of Nickel Based/Graphene Composite Materials And Their Electrochemical Properties

With the progress of industry,the shortage of energy and environmental pollution are becoming more and more serious.The supercapacitor has the advantages of fast charging,high charging and discharging efficiency,friendly environment,small internal resistance and maintenance free.Nickel based materials are widely recognized by many researchers as electrode material,which is rich in raw materials,low in cost,high in theory capacitance,and pollution-free to the environment.However,poor conductivity and cycle stability limit their further development.In order to improve the shortcomings of the nickel base material,researchers often combine it with other high-performance materials to make better electrode materials.In this study,various morphologies Ni?OH?₂/rGO composite and Ni?OH?₂/Ni?HCO₃?₂/rGO composite materials were successfully prepared by solvothermal method and hydrothermal method.Various samples were characterized by XRD,Raman,SEM,TEM and so on.And samples were used as electrode materials for electrochemical tests.The main results are as follows:?1?Using a mixed solution of distilled water and glycerin as a reaction solvent and nickel acetate as a nickel source to prepare a series of rose-like Ni?OH?₂.The solvothermal time of the final compound reaction was determined to be 3.5 h by comparing the TEM photos,cyclic voltammetric curves,and galvanostatic discharge curves of samples obtained at different solvothermal times.The graphene was dispersed in the reaction solvent,and the rose-like Ni?OH?₂/rGO composite was successfully prepared in one step with the solvothermal reaction time of 3.5 h.The prepared Ni?OH?₂/rGO composites exhibit a rose-like morphology with a diameter of about 3?m.The Ni?OH?₂/rGO composites are assembled from a sheet of Ni?OH?₂ and graphene with uniform and layered overlap.In this state,graphene does not act as a substrate that is different with most of the literature,but is uniformly distributed in the composite material with Ni?OH?₂.That achieves a large-area contact between the graphene sheet and Ni?OH?₂ sheet,which has a greater improvement in the stability of the material.Specifically,after 1000 times of charge and discharge,the electrode material has the capacitance retention rate of 94.5%.?2?A series of hollow tubular Ni?OH?₂ was successfully prepared by solvothermal method by heating the glycol water bath,adding nickel chloride,sodium acetate and polyethylene glycol,distilled water in the state of water bath.The solvothermal time of the final compound reaction was determined to be 6 h by comparing the TEM photos,cyclic voltammetric curves and galvanostatic discharge curves of samples obtained at different solvothermal times.Graphene was dispersed in the ethylene glycol required for the reaction.The Ni?OH?₂/rGO composite was prepared in one step with a solvothermal reaction of 6 h.The composite material presents a hollow tubular structure,and the hollow tube has a network-like structure inside and outside,and this network structure also forms many kinds of pore-like structures.The specific capacitance reachs 439.5 F/g at the current density of 4 A/g.After galvanostatic charge and discharge 1000 times at the current density of 8 A/g,the capacitance retention rate is 75.6%,which is22.5%higher than that of pure Ni?OH?₂,indicating that Ni?OH?₂/rGO composites have higher cycle stability.?3?Chips assembled cuboid-like Ni?OH?₂/Ni?HCO₃?₂/rGO composites have been successfully synthesized by a simple hydrothermal method with nickel nitrate as the nickel source,urea as the precipitant,and L-cysteine as the template.According to the results of various characterizations,the obtained Ni?OH?₂/Ni?HCO₃?₂/rGO composites have good crystalline,chips assembled cuboid-like structure?size is about 1?m?and big specific surface area?140.6m²/g?.And the Ni?OH?₂/Ni?HCO₃?₂/rGO composites exhibit high specific capacitance of 1161.6 F/g at 4 A/g in the 3 M KOH,which is superior to the Ni?OH?₂/Ni?HCO₃?₂ composites.Also,this electrode shows good rate capability with 62.1%capacitance retention from 2 A/g to 10 A/g.And 72.1%specific capacitance have being retained after 1000 times of cycling at the high current density of 10 A/g.It indicates that the Ni?OH?₂/Ni?HCO₃?₂/rGO composites have high cycle stability and long cycle life.?4?Nickel nitrate was used as the nickel source,urea was the alkali source,and polyethylene glycol-4000 was used as the template.The flower-like Ni?OH?₂/Ni?HCO₃?₂ composites were prepared by hydrothermal method.By comparing the SEM photos,cyclic voltammetric curves,and galvanostatic discharge curves of samples obtained at different hydrothermal times,the hydrothermal time of the final composite reaction was determined to be 22 h.The graphene was dispersed in the deionized water solution required for the reaction and flower-like Ni?OH?₂/Ni?HCO₃?₂/rGO composites were successfully prepared after 22 h hydrothermal method.When it was used as an electrode material,the specific capacitance of the material is higher than that of the Ni?OH?₂/Ni?HCO₃?₂ composite material.At the same time,the electrical conductivity and the cycle stability of the material have also been improved.The specific capacitance reached 1222.0 F/g at the current density of 4 A/g.The capacitance retention rate at the current density of 8 A/g reaches 69.1%after galvanostatic charge and discharge 1000 times,which is 18.4%higher than that of Ni?OH?₂/Ni?HCO₃?₂ composite material,indicating Ni?OH?₂/Ni?HCO₃?₂/rGO composite has higher cycle stability.