Synthesis And Electrochemical Capacitive Performance Of Cobalt Oxide-based Electrode Materials

The supercapacitor is a novel energy storage device that bridge the gap between electrolytic capacitors and rechargeable batteries.Electrode material is one of the key factors that affect the performance of supercapacitors.Therefore,the development of new materials with various component,structure,morphology and porosity can potentially improve their performance in supercapacitor applications.Cobalt-based metal oxides have attracted much attention due to their ultrahigh theoretical specific capacity and excellent rate capability.In this thesis,two kinds of cobalt-based metal oxides,layered stacked manganese-cobalt composite oxide（MnCo₂O₄）and cobalt oxide nanoneedles（Co₃O₄）,were synthesized and characterized.Their pseudocapacitance performances were studied in detail.1)Synthesis and electrochemical supercapacitor performance of layered stacked MnCo2O4.Layered stacked MnCo₂O₄ was synthesized by solvothermal method.Cobalt acetate tetrahydrate and manganese acetate tetrahydrate served as precursors,urea served as complexant and a mixture of ethylene glycol and deionized water as solvent in the synthesis.It was found that the structure of lamellar stacking of MnCo2O4 was obtained under the condition of a molar ratio of 0.25 mmol : 0.5mmol（manganese acetate/cobalt acetate）,reaction temperature of 150 °C and reaction time of 5.5 h.It has been suggested that the morphologies of MnCo2O4 are greatly influenced by reaction conditions including the amount of reactants and reaction temperature.MnCo₂O₄ with layered structure can exhibit a higher capacitive performance than the other forms of MnCo2O4.A specific capacitance of480.5 F g^-1 is obtained at a current density of 1 A g^-1.An ultrahigh capacity retention of 75.5% even at a current density of 25 A g^-1 is reached.Furthermore,the layered stacked MnCo₂O₄ delivered long cycling life with a capacity retention of 96.6%after 3000 cycles.The formation mechanism of the multi-layered stacked MnCo2O4 was further discussed and found that the layered structure of MnCo2O4 rooting in the combined effect of the Mn element and Co element.The products exhibited different morphologies in a continuous reaction times because the reaction rates of Mn2+ and Co2+ are constantly changing during the reaction.On the overall appearance of MnCo2O4,however,its cube structure mainly came from the role of Co elements and its layered structure derived from the role of Mn elements.2)Synthesis of Co₃O₄ nanoneedles on Ni foam for high-performance electrochemical capacitors.Co₃O₄ nanoneedles,as a electrodes for supercapacitors,are grown on a cellularnickel foam（Co₃O₄/NF）using a cost-effective hydrothermal procedure and cobalt nitrate hexahydrate served as precursors.It was found that the optimum conditions of Co₃O₄/NF are as follows,the concentration of cobalt nitrate for 0.1 M,the reaction temperature of 120 °C and the reaction time is 4 h.XRD results indicated that the prepared products were the Co₃O₄ supported on Ni foam.SEM images showed that Co₃O₄ had a structure of nanoneedles uniformly.The as-fabricated Co₃O₄/NF presented an excellent specific capacitance of 751.1 F g^-1 at a current density of 1 m A cm^-2 and good rate capability(specific capacitance can maintain81.3% when the current density increased to 20 m A cm^-2).Furthermore,it also delivered 88.6% retention at a current density of 5 m A cm^-2 after 6000 cycles.The excellent electrochemical performance made the Co₃O₄/NF a promising electrode material for electrochemical capacitors.