Synthesis And Electrochemical Properties Of Nanostructured Co-based Transition Metal Oxides Composite Films

As the fossil fuel depletion and environmental pollution problems have become more and more serious, the ever-increasing energy needs have urgently called for the development of sustainable and high efficient electrochemical energy storage devices and systems. Among the promising electrochemical energy storage devices, electrochemical capacitors ?supercapacitors? and lithium ion batteries have attracted wide attention. Transition metal oxides have been investigated as promising electrode materials for supercapacitors and lithium ion batteries owing to their ease to large-scale preparation, rich redox reactions and superior electrochemical performance. Therefore, the development of novel cobalt-based transition metal oxide with excellent electrochemical performance has important significance. Based on the above points, nanostructured Co-based transition metal oxides composite film materials are synthesized by various methods, and their electrochemical properties were investigated in this thesis.The Co₃O₄ nanosheets array film is synthesized by the facile hydrothermal method. These 2D Co₃O₄ nanosheets with the thickness of 100 nm perpendicular to the substrates, and were interconnected with each other. The Co₃O₄ nanosheets array can provide easy access of the surfaces to liquid electrolyte, offering more active sites for the pseudocapacitive reactions and shortening ion diffusion paths. The free-standing film structure not only allows a good electronic connection between each nanosheets and the nickel foam, but also eliminates the need for binders and conductive additives required in a normal pasted electrode, thus the electrochemical performances are enhanced. The specific capacitances of the as-fabricated Co₃O₄ nanosheets array electrode at the current densities of 1 and 20 A g^-1 are as high as 1705.8 and 1186.9 F g^-1, respectively.The thin Co₃O₄ nanosheets array on 3D porous graphene/nickel foam is synthesized by the electrophoretic deposition of graphenes on nickel foam and the subsequent hydrothermal growth of Co₃O₄ nanosheets array on the graphene/nickel foam substrate. These 2D Co₃O₄ nanosheets with the thickness of 40?50 nm lie aslant or perpendicular to the substrates, and are interconnected with each other. It is also noted that the Co₃O₄ nanosheets array on the graphene/nickel foam has a much thinner nanosheet and larger pore size than that on the bare nickel foam. The Co₃O₄ nanosheets array on the graphene/nickel foam electrode shows superior pseudocapacitive performance. The specific capacitances of the electrode at the current densities of 1 and 20 A g^-1 are as high as 3533 and 2222 F g^-1, respectively. Remarkably, the electrode deliveres a specific capacitance of 2459 F g^-1 after 2000 cycles at the current density of 8 A g^-1. The existence of the graphene layer may effectively inhibit the volumetric change of the Ni?OH?2 surface film on nickel foam during the charge/discharge, which could play an important role in ensuring the excellent electrochemical cycling stability of the film electrode.Various nanostructured Co₃O₄ films on nickel foam substrate are synthesized by the facile solvothermal method. The morphologies and dimensions of the Co₃O₄ films can be effectively tuned by tailoring the solvent compositions in the solvothermal reaction solutions. The effect mechanism of solvent composition on the morphologies of nanostructured Co₃O₄ films is analyzed. We also investigate the pseudocapacitance performance of various nanostructured Co₃O₄ films. It is demonstrated that the transformation of Go3O4 from 1D nanowire to 2D nanosheet can be effectively tuned by increasing the ratio of ethylene glycol/water in the solvothermal reaction solutions. The 3D hierarchically porous Co₃O₄ network film synthesized in the solvothermal reaction solution with a medium ethylene glycol/water volume ratio ?1:29? shows prominent pseudocapacitive performance, such as large specific capacitance, excellent rate capability and good cyclic stability. The specific capacitances of the Co₃O₄ network film electrode at the current densities of 0.870 and 17.391 A g^-1 are 2817 and 1948F g^-1, respectively. Furthermore, the electrode exhibits a specific capacitance of 1628 F g^-1 after 3500 cycles at a current of 4.348A g^-1.Based on the electrochemical corrosion, the Co₃O₄ nanowires array is synthesized by the facile hydrothermal method. Different from the previous hydrothermal synthesis, we adopt the cobalt metal film as precursor materials. The Co₃O₄ nanowires array is obtained through the electrochemical corrosion of cobalt metal in the urea-containing solution. The nanowire formation mechanism is also investigated. NH4F not only plays an important role in the formation of well-aligned vertically nanowires array, but also provides a tight adhesion between nanowires array and the substrate. The reversible areal capacities of the Co₃O₄ nanowires array electrode at the current densities of 0.05 and 0.8 mA cm-2 are 0.52 and 0.26 mAh cm-2, respectively. Moreover, the electrode still displays a reversible areal capacity of 0.52 mAh cm-2 after 200 cycles at a current of 0.05 mA cm-2.The 3D hierarchical porous flower-like and dandelion-like ZnCo2O4 micro-nanostructures films are synthesized by the facile solvothermal method. They are obtained in different reaction solvents, which means that the solvent plays an important role in the morphologies of ZnCo2O4. The as-synthesized 3D ZnCo2O4 micro-nanostructures films electrodes exhibit superior lithium storage properties. The reversible capacities of the 3D dandelion-like ZnCo2O4 micro-nanostructures film electrode at the current densities of 100 and 800 mA g^-1 are 2819 and 2781 mAh g^-1, respectively. Moreover, the electrode still displays a reversible capacity of 2088 mAh g^-1 after 100 cycles at a current of 400 mA g^-1. The excellent lithium storage properties of the electrodes can be ascribed to the 3D hierarchically porous micro-nanostructures, Zn doping and the synergetic effect of Co and Zn elements.