Preparation And Electrochemical Properties Of Co₃O₄-based Supercapacitor Electrode Materials

Due to the limited reserves of fossil fuels and the fact that its combustion will pollute the environment.Human society has devoted to the development of renewable clean energy,and the problem of the storage of the clean energy has followed.Among devices for energy storage,supercapacitor stand out due to its advantages such as high power density,high specific capacity,and good cycle stability.The quality of the electrode material affects the output performance of the super capacitor directly.Co₃O₄ is favored by researchers because the easy availability,no pollution,and low cost advantages.However,Co₃O₄ is a semiconducting material and its conductivity is poor,which results in its poor cyclic stability.Therefore,it is significance to improve the electrochemical performance of Co₃O₄.This thesis mainly focuses on two aspects:firstly,the optimal preparation scheme of Co₃O₄ is explored by changing experimental conditions;secondly,Co₃O₄ is composited with graphene or NiO to obtain a composite material to improve the electrochemical performance.Co₃O₄ electrode materials which prepared by different methods have different morphological structures,which will lead to differences in the electrochemical properties.In this paper,Co₃O₄ and Co₃O₄composites which prepared by different methods were studied.Their morphology and crystal structure were characterized by SEM and XRD,and their electrochemical properties were tested in 6 mol?L^-11 KOH electrolyte.The main work is as follows:?1?The Co₃O₄ material was prepared by using a simple and easy-to-handle precipitation-calcination method.The effects of calcination temperature,calcination atmosphere,addition sequence,and feed rate on the electrochemical performance of Co₃O₄were discussed.And an optimal preparation condition was explored.The test results showed that the prepared Co₃O₄?Sample B3?which added the precursor of Co?NO₃?₂?6H₂O solution to the NH₃-NH₄⁺buffer solution dropwise slowly and then calcined in the air atmosphere at450?had the best electrochemical performance.When the current density is 1 A?g^-1,the specific capacitance is 425 F?g^-1.And when the current density is increased to 10 A?g^-1,the capacitance retains 51%of the capacitance at 1 A?g^-1.After 1000 cycles of charge and discharge at 1 A?g^-1,the specific capacitance retention rate was as high as 87%.The internal resistance of Co₃O₄ material is within 1?,indicating that the material has good conductivity.SEM and XRD show that Sample B3 has a flaky crystal structure with the best crystallinity.?2?Using a simple precipitation-calcination method,NH₃-NH₄⁺buffer solution as precipitant,combining Co₃O₄ with nitrogen-doped three-dimensional graphene?N-3DG?.The effect of pH of the buffer solution on the electrochemical performance of Co₃O₄/nitrogen doped 3D grahene?Co₃O₄/N-3DG?composites was investigated.Electrochemical test results show that when the buffer solution pH=9.75,the electrochemical performance of the composite material is best.When the current density increases from 1A?g^-11 to 2 A?g^-1,the specific capacitance decreases from 630.6 F?g^-11 to 545.8 F?g^-1,and the retention rate is 87%.After 1000 cycles of charge and discharge at 1 A?g^-1,the specific capacitance retention rate is as high as 87%,indicating that the sample has a good structural stability.The internal resistance of Co₃O₄/N-3DG composite is less than 1?,which shows the excellent electrochemical performance of the composite.XRD showed that the composite material was in an amorphous state.SEM proved that after Co₃O₄ and N-3DG were compounded,the particles became smaller and the material structure was more uniform.?3?Using a simple two-step method,Ni/Co binary oxide were prepared using NH₃-NH₄⁺buffer solutions at the Ni/Co ratios of 1:3,2:2,3:1 and 4:0.Through the electrochemical performance test,the influence of the ratio of Ni/Co on the electrochemical properties of Ni/Co binary oxide was investigated.The test results show that when the Ni/Co ratio is 3:1,the prepared sample?Sample C?has the best electrochemical performance.When the current density is increased to 2 A?g^-1,the specific capacitance is 545 F?g^-1.And the capacitance retention rate is 87%of the specific capacitance(627 F?g^-1)at 1 A?g^-1.The Ni/Co binary oxide was charged and discharged 1000 times at 1 A?g^-1,and the specific capacitance retention was measured as 65%.It had a small resistance within 1?fully exhibiting Ni/Co binary oxide good electrochemical performance.SEM shows that Sample C is granular,non-agglomerated,and has the most uniform structure.The XRD pattern shows that the composite Sample C has the most complete crystal.?4?Through a simple hydrothermal method,the Co₃O₄/N-3DG composites were synthesized in two steps using Co?NO₃?₂?6H₂O as the cobalt source.The precursors were prepared under different hydrothermal time and hydrothermal temperature.Then calcined in a tube furnace under an argon atmosphere at 600?for 4h to obtain composites.The SEM,XRD and IR results of the composites show that the method successfully synthesized the Co₃O₄/N-3DG composites and verified the feasibility of the method.Electrochemical results show that when the hydrothermal temperature is 80?and the hydrothermal time is 12h,the electrochemical performance of the Co₃O₄/N-3DG composite is best.When the current density is 1 A?g^-1,the specific capacitance of the composite is 189.97 F?g^-1.And when the current density is increased to 10 A?g^-1,the specific capacitance retention rate is 76%.After galvanostatic charge-discharge cycle 500 times at a current density of 1 A?g^-1,there is still a specific capacitance retention of 97.5%.The internal resistance of the material is within 1?,indicating that the composite material has a good rate performance and structural stability.