Research On Transition Metal Oxides And Their Composites For Supercapacitors

Supercapacitors are a new type of energy storage device between static capacitor and secondarybatteries. Because of their high power density, excellent pulse charge-discharge characteristics andlong cycle life, in recent years, supercapacitors have attracted great interest in electrochemical energystorage applications. Pseudocapacitive transition metal oxides are promising for next-generationsupercapacitors due to their high theoretical specific capacitance, good electrochemical reversibilityand environmentally benign nature. However, since many transition metal oxides have low electronicconductivity, the peudocapacitors with such electrodes invariably suffer from low capacitance andpoor rate capability at high current densities. Based on the above analysis, in this thesis, in order toimprove their electrochemical performance, the transition metal oxides are doped with other species,incorporated into highly conductive carbons （e.g., carbon nanofibers） or directly deposited on metalsubstrates.（1） We have prepared three-dimension （3D） flower-like Ni-Co oxides by thermaldecomposition of layered double hydroxides （Ni-Co LDHs） precursors in air, which were synthesizedby a5-sulfosalicylic acid （SSA）-assisted hydrothermal process. Co₃O₄doping improves the electricalproperties of the electrode materials. Furthermore, it is found that Ni-Co oxides have large specificsurface area and abundant mesoporous structure. In the hybrid supercapacitors, Ni-Co oxides workedas the positive electrode and actived carbon （AC） or ordered mesoporous carbon （OMC） as thenegative electrode in a KOH aqueous electrolyte. The performance of asymmetric capacitors is foundto be affected by the properties of carbon materials used in the negative electrode. When OMC is usedas the negative electrode, the hybrid system shows better rate performance, while the Ni-Cooxide//AC hybrid capacitor exhibits higher specific capacitance. The cell delivered an impressivespecific capacitance of162F·g^-1and energy density of50.6Wh·kg^-1based on the total weight of theactive materials. In addition, it keeps73%of initial capacity over1000cycles.（2） Mesoporous Co₃O₄nanowire （NW） arrays freely standing on Ni foam substrate areprepared via two-step strategy: precipitating hydroxides followed by calcinating process. The usage ofNi substrate significantly improves electrical conductivity and ion transfer efficiency. The mesoporousCo₃O₄NWs and three-dimensional （3D） network architecture of Ni substrate composed a unique3Dhierarchical structure. Cyclic voltammetry, chronopotentiometry, and electrochemical impedancemeasurements are applied to investigate the performance of the Co₃O₄NW arrays. Electrochemicaltests show that Co₃O₄NW arrays deliver a specific capacitance （SC） of1160F g^-1at2A g^-1, and even 820F g^-1at20A g^-1. Also, the SC degradation is only9.6%after5000continuous charge-dischargecycles at8A g^-1, indicating their excellent electrochemical stability. We have constructed a hybridsupercapacitor by using the Co₃O₄NW arrays as the positive electrode and activated carbon （AC） asthe negative electrode. This hybrid system exhibits a superior performance with energy density of37.8Wh·kg^-1and power density of19.5kW·kg^-1.（3） Carbon-Co₃O₄composite nanofibers were prepared via electrospunning and subsequentcarbonization. The mass loading of Co₃O₄in the composite nanofibers can be easily controlled bytuning the ratio of polyacrylonitrile （PAN） and cobalt compound （Co（acac）2） used in polymersolutions. For all C-Co₃O₄composite nanofibers, large amount of Co₃O₄hollow spheres are found tobe well dispersed and embedded in the carbon nanofibers. The Kirkendall effect is responsible for theformation of the Co₃O₄hollow spheres. Furthermore, the nanofibers can cross-link formingfreestanding membranes. Due to good electrical conductivity and unique nanostructure of each fiber,the membrane electrodes exhibit large specific capacitance, excellent rate capability and good cyclingstability. The maximum specific capacitance of1730F g^-1for Co₃O₄species is obtained fromC-Co₃O₄nanofiber membrane with15.6wt.%Co₃O₄loading. And increasing the mass loading to35.9wt.%, the specific capacitance of the C-Co₃O₄nanofiber membrane is556F·g^-1.