Synthesis And Electrochemical Properties Based On Cobalt-nickel Compounds

Supercapacitor,which is one of the new types of energy storage devices,has varied properties that depend highly on the component and structure of electrode materials.Up to now,much attention has been focus on the development of composite electrodes that are composed of transition metal oxides and carbon substrate.Spinel nickel cobaltice(NiCo2O4)is a type of ideal electrode material that possesses several advantages such as high catalytic activity,low cost and low energy consumption.Graphite felt,as a kind of new carbon material,is prominent to be served as substrate material for the growth of transition metal oxides because of its characters in terms of high specific surface area,good mechanical strength and low cost.Therefore,we focus on the preparation of nano-structure NiCo2O4 and subsequent decoration of graphite felt with NiCo2O4,in order to synthesize a composite electrode with comprehensive electrochemical performance such as high specific capacitance and good cycle stability.In this thesis,NiCo2O4 with different morphologies have been synthesized at different hydrothermal temperatures.The NiCo2O4 that was synthesized at 110 ?exhibits the morphology of porous nanowires with high specific surface area.When used as the electrode material,the pure NiCo2O4 electrode can exhibit a specific capacitance of 1554 F·g-1 at 1 A·g-1,as well as a remarkable cycling stability with capacitance retentions of 90.4%after 2000 cycles.The nanowires and nanosheets of NiCo2O4 were loaded on the graphite felt by hydrothermal method to obtain the composites with different properties.Compared with NiCo2O4 nanosheets@graphite felt electrode,NiCo2O4 nanowires@graphite felt electrode possesses neatly needle array structure which provides a large contact area of electrolyte and the facilitated diffusion path of electrolyte charge.When employed as electrodes,the NiCo2O4 nanowires@graphite felt exhibited excellent capacitive properties,such as high specific capacitance(1782 F·g-1 at 1 A·g-1)and outstanding cycling stability(88.5%after 2000 cycles),which demonstrate the potential applications for electrode of supercapacitors.To further improve the capacitance of the composite electrode,theNiCo2S4@graphite felt electrode was subsequently synthesized via a two-step hydrothermal method.It's known that the conductivity will be enhanced when the oxygen atoms of NiCo3O4 are replaced by sulfur atoms.Therefore,the NiCo2S4@graphite felt shows excellent capacitance characteristics with a specific capacitance of 1974 F·g-1 at 1 A-g-1 and a remarkable cycling stability with capacitance retentions of 91.2%after 2000 cycles.