Preparation Of Nickel-based Pseudocapacitive Composite Electrode Materials And Electrochemical Performance

Nowadays,more and more people are paying attention to various power devices for clean and efficient energy storage,such as lithium-ion batteries,supercapacitors,fuel cells.Among them,supercapacitors are favored by many researchers for their high-power density,fast charge/discharge capability,and long life.However,the relatively low energy density of traditional electric double layer capacitors have limited their use as power sources.Therefore,recent research has focused on pseudocapacitive materials,such as RuO2,NiO,Ni(OH)2,Co3O4,MnO2,and NiCo2O4.They show high specific capacitance and energy density,which is mainly due to the faraday redox reaction.However,their poor conductivity results in slower electron transfer rates.On the other hand,the availability of active materials is low,further impairing their electrochemical performance.In order to solve the above problems,there are mainly two methods widely used at present.One is to directly incorporate a material with high pesduocapacitance into conductive carbon materials,such as activated carbon,mesoporous carbon,carbon nanotubes or graphene,but usually introduce some functional groups on the carbon materials to better interact with the pseudocapacitive materials,which results in a decrease in the electrical conductivity of the carbon materials.Another approach is to construct hybrid nanostructure of pseudocapacitive materials/conductive matrix.The matrix with a porous conductive network increases the electron transfer rate,thereby improving the performance of the supercapacitors.The traditional preparation methods have certain defects in designing special structure.However,due to the diversity of building units and compositions,metal-organic framework materials(MOFs)can introduce different metal centers and functional groups through reasonable design.On the other hand,MOFs materials pyrolysis under certain conditions can directly obtain porous carbon,metal/carbon,metal oxide/carbon composite materials.At the same time,MOFs can also be used as templates to prepare composite materials with complex structure,such as hollow structure and yolk-shelled structure.This thesis designs and synthesizes new nickel-based pseudocapacitive composites with high specific capacitance,excellent cycling life and rate performance.The main research contents of this paper are as follows:(1)In order to improve the capacitance of Ni(OH)2,a nano-flower spherical NiAl LDHs electrode material is prepared by a simple hydrothermal method.When the content of Al(NO3)3 is 0.3705 g,NiAl LDHs-3 displays a capacitance of 1213 F g-1 after 10,000 cycles at a current density of 10 A g-1 in the three-electrode system.NiAl LDHs@PDA/rGO composite is synthesized to further improve the cycling stability and rate performance of NiAl LDHs.The composite material is grown with tremella-like NiAl LDHs nanosheets on a three-dimensional polydopamine modified reduced graphene oxide sheet(3D PDA/rGO).The NiAl LDHs@PDA/rGO displays specific capacitance of 1924.4 F g-1(10 A g-1),and it can still give a specific capacitance of 1582 F g-1 at a large current density of 40 A g-1,also the specific capacitance maintained 98.7%of the initial capacitance after 15,000 cycles(20 A g-1),which is mainly due to the synergy of NiAl LDHs and PDA/rGO.The NiAl LDHs@PDA/rGO//AC hybrid supercapacitor displays high specific capacitance(165.8 F g-1 at a current density of 1 A g-1)and excellent cycling performance(it can still maintain 93.5%of the initial capacitance after 10,000 cycles at 10 A g-1).(2)In order to alleviate the volume effect on the capacitance of layered double hydroxide materials,hollow NiCo LDHs electrode material is synthesized by a combinating one-step hydrothermal method with ion exchange method.In the ion exchange process,when the content of Ni(NO3)2 reaches 150 mg,the precursor ZIF-67 can be completely etched to form a hollow polyhedron nanocage structure,which displays a capacitance of 1120 F g-1 after cycling 10,000 times at a current density of 20 A g-1 in the three-electrode system.The hollow NiCo LDHs/CNTs composite is synthesized by in-situ growth to further improve the cycling stability of hollow NiCo LDHs.The NiCo LDHs/CNTs displays a specific capacitance of 1731.3 F g-1,and maintain 94.3%of the initial capacitance after 20,000 cycles at 20 A g-1,showing that the composite material has good cycling stability.The NiCo LDHs/CNTs//AC hybrid supercapacitor exhibits high specific capacitance of 156.3 F g-1 at a current density of 1 A g-1,and it can still maintain 89.6%of the initial capacitance after 12,000 cycles at 10 A g-1.(3)To improve the problem of poor self-conductivity caused by the properties of NiO semiconductor,a yolk-shelled structure Ni/NiO composite is synthesized by simple hydrothermal method and in-situ carbonization method,and the reaction mechanism in the preparation process is studied.At the same time,by changing the content of oxalic acid in the precursor,the effect of Ni content in the composite on its electrochemical performance is studied.Electrochemical tests show that the composite has the optimal electrochemical performance,when the Ni content is 0.4 g.As an indication,the YS Ni/NiO displays superior charge storage performance in an alkaline electrolyte with a high specific capacitance of 1357.2 F g-1 at 15 A g-1,and maintain 94.3%of the initial capacitance after 10,000 cycles at 10 A g-1,which is sgnificantly superior to previously reported NiO electrode materials.The YS Ni/NiO//AC hybrid supercapacitor displays high specific capacitance(122.9 F g-1 at a current density of 1 A g-1),and excellent cycling performance(it can still maintain 93.5%of the initial capacitance after 10,000 cycles at 5 A g-1).(4)Ni MOFs prepared by hydrothermal method is used as precursor,then in-situ carbonization was performed at a certain temperature to prepare Ni/N-doped PC composite,and explore the reaction mechanism.Next,the effect of carbonization temperature on the electrochemical performance of the composite is studied.Electrochemical tests show that the composite has the optimal electrochemical performance,when the carbonization temperature is 500?.As an indication,The Ni/N-doped PC displays superior charge storage performance in an alkaline electrolyte with a high specific capacitance of 1114.8 F g-1,and maintain 91.5%of the initial capacitance after 10,000 cycles at 10 A g-1.The high capacitance is mainly because Ni nanoparticles can be converted into active substances(NiO or Ni(OH)2)in a strong alkaline solution,which improves the utilization rate of active substances.The Ni/N-doped PC//AC hybrid supercapacitor displays high specific capacitance of 115.7 F g-1 at a current density of 1 A g-1,and it can still maintain 94.1%of the initial capacitance after 10,000 cycles at 5 A g-1.