Preparation And Electrochemical Performance Of Graphene/Nickel-based Composites

Supercapacitors,as a novel type of energy storage device,have called the focus of thefield of research because of its high power density,short charging time and other merits.Nonetheless,the poor energy density is an important factor that hinders its practical application.It has been found that Ni-based pseudocapacitive electrode materials such as Ni?OH?₂ and Ni₃?NO₃?₂?OH?₄ have relatively high theoretical specific capacitance and energy density,showing the broad space for development in supercapacitors.The short cycle life,poor conductivity and the aggregation for the nickel-based composites are major bottlenecks to impede the development of electrochemical performance.The unique structure of graphene makes it possess many excellent properties,such as high mechanical strength,fast carrier migration and large specific surface area,which is widely used in the field of supercapacitors.By combining the carbon material and the pseudocapacitive material,not only the specific surface area but also the conductivity and stability of the material can be improved.Moreover,the embedding of transition metal elements into the composite is also a good way to improve the conductivity of the material.In this paper,we successfully prepare Ni-based functional materials directly grown on RGO as a substrate and systematically study the electrochemical performance of the materials.The main contents of this paper are as follows:The Co-doped ?-Ni?OH?₂ nanosheets grown directly on RGO substrate were prepared successfully by one-step solvothermal method.Herein,methanol acted as solvent and it also could be oxidized to release OH-without any additional alkaline source.Then the morphologies and structure of the materials were characterized and the influence of Co doping amount on the capacitance performance was studied in detail.The experimental results showed that Co doping significantly improved the charge transfer efficiency and the discharge performance of the material.In addition,using RGO as the growth site for Co-doped?-Ni?OH?₂ nanosheets greatly reduced the auto-deposition of the nanosheets and evaluated the utilization levels of electrode materials.At the same time,the strong adsorption between Co-doped ?-Ni?OH?₂ and RGO effectively relieved the volume expansion of the metal hydroxide in the alkaline electrolyte,which improved the stability of the material.When thedoping ratio of Co to Ni was 0.5,the Co-doped ?-Ni?OH?₂/RGO composite displayed the best performance,with a specific capacitance of 2322 F g-1.The discharge capacity could still maintain 87.9% of the initial value after 1000 cycles.An asymmetric supercapacitor was manufactured with the Co-doped ?-Ni?OH?₂/RGO as a positive electrode and activated carbon as a negative electrode,which delivered a maximum energy density of 38.9 Wh kg-1.In order to further explore the crystallization and growth mechanism of materials in the solvothermal process,we controlled the type of solvents to study the effect of different structure-directing agents in the reaction system.The RGO/Ni-based composites were synthesized using cyclohexanol,ethylene glycol and isopropanol as solvents.The results showed that solvents had a great influence on the structure and morphologies of the products.The nanomaterials with controllable morphology by this method were prepared without any surfactant or organic polymer.Compare with other samples,the electrochemical performance of Co-doped Ni₃?NO₃?₂?OH?₄ nanoparticles obtained in isopropanol was best.The capacitance could reach 2158 F g-1at a current density of 1 A g-1and the power performance exceeded70%.Moreover,the energy density of CNG-iso//AC device was as high as 54.3 Wh kg-1.