Preparation Of Transition Metal Electrode Materials And Study On Supercapacitor Performance

Supercapacitors have received a lot of attention because of their properties such as high-power density and fast charge/discharge capability.It is well known that the type of electrode material has a very important position in supercapacitors,which includes different kinds of carbon materials,conducting polymers and transition metal compounds.When transition metal compounds are used as electrode materials,the energy density can be significantly increased by effectively utilizing the reversible reactions between the oxidation states of various metals.In order to further improve the performance of transition metal compound electrode materials,the energy storage performance of supercapacitors can be improved by modulating the parameters such as electrical conductivity,morphological structure,and specific surface area of the materials.In this paper,we have designed electrode materials with excellent performance based on nickel-cobalt based multi compound and utilized the direct synergistic effect of transition metal elements as follows:1.The synergistic advantage of different metal ions in two or more metal compounds improves the electrochemical properties compared to monometallic compounds.The improved energy storage capacity of polymetallic compounds is due to the redox reactions of different active centers,and the multivalency of transition metals can improve the electrical conductivity.In this thesis,Cu Co₂S₄ with yolk-shell structure was grown on NiO nanosheets by hydrothermal and solvothermal methods,which combined the synergistic effect between Ni,Co and Cu transition metal elements and modulated the morphology of the composites by the amount of thiourea and significantly enhanced the electrochemical properties.When the molar ratio of Cu²⁺:Co²⁺:thiourea in the precursor solution was 1:2:6,the obtained NiO@Cu Co₂S₄ showed the best electrochemical performance with a specific capacitance of 1658 F g^-1 at 1 A g^-1.The flexible asymmetric supercapacitor（FASC）composed of NiO@Cu Co₂S₄ cathodes achieves an energy density of 73 Wh kg^-1 at a power density of 802 W kg^-1 and excellent capacitance stability（91%retention after 5000 cycles）.2.The capacitive performance of electrode materials can be improved by combining different morphologies and microstructures of the same metal oxides,and adjusting the morphology of transition metal compounds can effectively improve their performance.In this work,Ni Co₂O₄@Ni Co layered double hydroxide composites with nano-sheet,nano-grass and nano-flower morphologies were synthesized on carbon cloth by controlling the hydrothermal reaction time.The performance of these three composites was investigated and compared.The nanograss morphology has better electrochemical performance than nanosheets and nanoflowers because of network structure and reduced volume expansion,and the nanograss morphology achieved 81 Wh kg^-1 at a power density of 0.85 k W kg^-1 when used as a positive electrode material for asymmetric supercapacitors（ASC）,and high repetitive charge/discharge stability（88%after 5000 cycles）.3.Zeolite imidazole skeletons（ZIFs）based materials have uniform pores,large specific surface area and simple structures,especially,the conductivity of Co-based ZIF-67 is more than 1000 times higher than that of Zn-based ZIF-8,making cobalt-based ZIF-67 widely studied as an energy storage material.In this thesis,Co-ZIF-67@NiMo-LDH composites with hollow core-shell were synthesized,and electrochemical studies showed that the specific capacitance of Co-ZIF-67@NiMo-LDH composites was 1734 F g^-1(1 A g^-1),and the Co-ZIF-67@NiMo-LDH//AC flexible asymmetric supercapacitor also exhibited excellent capacitive performance.