Preparation And Electrochemical Properties Of Carbon Dots-Based Supercapacitor Electrode Materials

Supercapacitors have received more and more attention due to their long cycle life,wide working temperature range and the ability to provide higher power density and fast charging rate.It is well accepted that the capacitive performance greatly relies on the electrode materials of supercapacitors.Therefore,at present,the preparation of low-cost,environmentally friendly and high-performance electrode materials is the focus of research on supercapacitors.Owing to their unique electronic conductivity,high chemical stability,and strong capability to combine with other materials,carbon dots have attracted increasing attention.In this paper,a series of carbon dot-based nanocomposites were prepared to obtain electrode materials with high specific capacitances,long cycle life and high energy density.The as-prepared materials are characterized by XRD（X-ray Diffraction）,SEM（Scanning Electron Microscopy）,TEM（Transmission Electron Microscopy）,Raman spectroscopy,XPS（X-ray Photoelectron Spectroscopy）and BET（Brunauer-Emmett-Teller）.And then,the electrochemical capacitance performances of the composites are systematically investigated by cyclic voltammetry,constant current charge and discharge,etc.The main points are as follows:1.Nitrogen-doped carbon dots modified reduced graphene oxide（RGO/NCDs）nanosheets were successfully prepared by a simple one-pot hydrothermal method.It is found that NCDs play an important role as a structure-directing agent in tuning the morphology of the Ni（OH）₂ and improving the performance of supercapacitors.The Ni（OH）₂/NCDs nanosheets have a large specific surface area and a rich porosity.As electrode materials for supercapacitors,the Ni（OH）₂/NCDs exhibit a higher specific capacitance of 1711.2 F g^-1 at a current density of 1 A g^-1,which is twice than that of bare Ni（OH）₂.In addition,the hybrid supercapacitor device was fabricated by using Ni（OH）₂/NCDs nanosheets and three-dimensional graphene（3DGE）as the positive and negative electrode,respectively.The device has a specific capacitance of 127 F g^-1 at a current density of 1 A g^-1 and energy densities of 23.8-34.6 Wh kg^-1 at power densities of 700-7000 W kg^-1.Therefore,the prepared Ni（OH）₂/NCDs nanosheets have potential application in the field of energy storage.2.Reduced graphene oxide/nitrogen-doped carbon dots（RGO/NCDs）nanosheets were successfully prepared by a simple one-pot hydrothermal method.In the RGO/NCDs composites,ultra-small sized NCDs are uniformly dispersed on the surface of RGO sheets,providing rich active sites for charge storage.When used as electrode material for supercapacitor,the specific capacitance of RGO/NCDs is as high as 350 F g^-1 at a current density of 1 A g^-1,which is much higher than that of bare RGO(249 F g^-1).In addition,the RGO/NCDs nanosheets also show good rate performance and cycle stability.The RGO/NCDs electrode was also assembled into a symmetrical supercapacitor,and the specific capacitance of the device is 85.6 F g^-1 at a current density of 1 A g^-1.The specific capacitance almost keeps constant over35000 cycles at a current density of 5 A g^-1.The excellent electrochemical properties of RGO/NCDs nanocomposites make it a good candidate material for supercapacitor electrode.3.A facile hydrothermal method was demonstrated to synthesize three-dimensional flower-like CoS/NCDs composites using ethylene glycol as solvent.The structure,chemical states and surface morphology of the prepared composites were characterized by XRD,XPS,SEM and TEM.When used as electrode material for supercapacitor,the CoS/NCDs delivers enhanced specific capacitance of about 697 F g^-11 at a current density of 1 A g^-1,which is much higher than that of pure CoS nanoflower.The enhanced capacitive performance is attributed to the synergistic effect between CoS and NCDs.The hybrid supercapacitor device was assembled using CoS/NCDs nanoflowers as positive electrode and RGO/NCDs as negative electrode.The device exhibites a specific capacitance of 103 F g^-1 at a current density of 1 A g^-1 and an excellent energy density of about 36.6 Wh kg^-1 at a power density of800 W kg^-1.These results indicate that CoS/NCDs nanocomposites are promising cathode materials for supercapacitors.4.The NiO/Co₃O₄/NCDs ternary nanocomposite was successfully prepared by simple hydrothermal and calcination method.The prepared ternary nanocomposites consist of a large number of ultra-thin nanosheets.When used as electrode materials for supercapacitor,the NiO/Co₃O₄/NCDs exhibits a specific capacitance of 1775 F g^-1at a current density of 1 A g^-1.The NiO/Co₃O₄/NCDs nanocomposites also show excellent cycling stability,about 92.4%of the specific capacitance was retained even after 10000 continuous cycles at 10 A g^-1.The hybrid supercapacitor device was assembled using NiO/Co₃O₄/NCDs nanosheets as positive electrode and RGO/NCDs as negative electrode,which exhibits higher energy density of 41.6 Wh kg^-1 at a power density of 800 W kg^-1.