Preparation And Electrochemical Performances Of Ni-based Metal Phosphides(Selenides)

With the continuously consumption of fossil energy and the urgent demand for new and renewable energy,a variety of energy storage devices have been produced.Supercapacitors have recently attracted considerable attention due to their high power density,long cycle life,high safety and rapid charging and discharging performance.Although supercapacitors have many advantages,their lower energy density limits their application.In order to obtain supercapacitors with high energy density,in this dissertation,nickel-based metal phosphides/selenides were synthesized and combined with graphene to prepare composites,which were used as positive electrodes to assemble asymmetric supercapacitors,and then the electrochemical performances were tested in detail.Nano-flowered NiCo layered double hydroxides?NiCo-LDH?were prepared by one-step solvothermal method.Then RGO/NiCo-LDH composite materials was prepared with RGO nanosheets as the deposition substrate.The results showed that the NiCo-LDH grows on the surface of RGO to form a flaky sheet-like structure with large specific surface area,which improves the electrochemical properties of the material.The specific capacitance of RGO/NiCo-LDH composite is 2362 F g^-1 at the current density of 1 A g^-1 and 67.2%capacitance is retained at 120 A g^-1.With RGO/NiCo-LDH composites as cathode and N-RGO as anoth,the asymmetric supercapacitor was assembled.The maximum energy density of the asymmetric supercapacitor is 111.0 Wh kg^-1 when the power density is 1066W·kg^-1.The capacitance retention ratio is 94.0%after 100,000 continuous cycles under the current density of 20 A g^-1,indicating excellent cycling stability.RGO/NiCo-LDH was phosphatized into RGO/bimetallic Ni-Co phosphide composite after loading on graphene by the same phosphating process.Such composite exhibits high specific capacitance of 1668 F g^-1 when the current density is 1 A g^-1,and the capacity retention rate is 66.4%when the current density is 120 A·g^-1.The asymmetric supercapacitor is assembled with RGO/bimetallic Ni-Co phosphide and N-RGO as cathode and anode,respectively.The measured energy density of the asymmetric supercapacitor is 48.8 Wh kg^-1when the power density is 1064 W kg^-1,and 90.2%of the initial specific capacitance is retained after 10,000 cycles at the current density of 20 A g^-1,indicating good cyclic stability.With RGO as the deposition substrate,RGO/Ni-Co-Se-S-0.33 composite was prepared by solvothermal approach.The specific capacitance of the RGO/Ni-Co-Se-S-0.33 composite is 1913 F g^-1 at a current density of 1 A g^-1,and the capacitance retention ratio is 60.0%at 120A g^-1.With RGO/Ni-Co-Se-S-0.33 composite as the positive electrode and RGO/Bi₂O₃prepared by solvothermal method as the negative electrode,asymmetric supercapacitor was assembled.The energy density of the RGO/Ni-Co-Se-S-0.33//RGO/Bi2O3 asymmetric supercapacitor is 31.9 Wh kg^-1 when the power density is 1065 W kg^-1,and the capacitance retention ratio is 80.0%after 10000 cycles.