Conductivity Improvement Of Nanostructured Metal(Hydro-) Oxides For Enhanced Supercapacitor Performances

With the rapid increase in human energy consumption and the severe environmental issues it brought up in the last decades,facilitating clean energy sources such as solar power,wind power,tide power,etc.becomes more and more urgent.However,a common problem for these sustainable energies sources is that they don't exist all the time but obey certain patterns.Hence,energy storage devices that collects these energy from time to time attracts scientists' attention.As one of the most popular energy storage devices,supercapacitors,possessing both high power density and energy density,fill the gap between the traditional capacitors with high power density and the lithium secondary batteries with large energy denisty,together with lithium-ion batteries.Compared to lithium-ion batteries,supercapacitor features even higher power density,fast charging and long cycle life properties,making it widely used in many fields.Nanomaterials,with their unique structures and chemical/physical characteristics,has been the research focus for material scientists in the past decades.In supercapacitor researches,the high specific area and shorter transfer/diffusion path for ions and electrons provided can both facilitate faster reaction rates and larger capacitances.Moreover,electrode materials in nano dimension with a better tolerance in the structure deformation enabling a longer cycle life,have been proved to be strong candidates for high performance supercapacitor active materials.Furthermore,with the progress in tunable sizes,morphology and composition in nanomaterials,especially inorganic nanomaterials in the past years makes the optima supercapacitance from the precisely control of nano electrode materials possible.Metal oxides and hydroxides are one of the most significant electrode materials for supercapacitors:the reversible redox reactions coming from the metal's multiple valence states make them the "hotspo" in supercapacitor research with the more favorable pseudocapacitances other than double layer capacitances.However,despite the high theoretical specific capacitance for the metal(hydro-)oxides,they can't actually obtain such high values,mostly due to the poor electron conductivities.In practical applications,the loading density of electrode materials has to reach certain level causing the metal(hydro-)oxides stacking together and lowering their specific capaictances because of the high internal resistance.Therefore,in order to enhance the capacitive performance of metal(hydro-)oxides,one feasible way is to improve their conductivities.In the first chapter of this thesis,the classification,principle and evaluation methods of supercapacitors are briefly introduced.In the following three chapters,we propose three different solutions for the conductivity improvement of metal(hydro-)oxides:In chapter two,for metal oxide nanoparticles,we synthesize dumbbell like metal-metal oxide nanoparticles by introducing Au seeds,successfully altered the disadvantage in traditional metal/metal oxide or metal oxide/metal core/shell structrues,exposing both metal and metal oxide parts to the electrolyte and achieving a bettercapacitive performance in the composite nanoparticle.In chapter three,also aiming at metal oxide nanoparticles,we facilitate metal salts as reducing agent for the redox reaction of graphene oxide,transferring the poorly conductive graphene oxide into highly conductive reduced graphene oxide(rGO),and obtaining metal oxide nanoparticles on rGO at the same time with the increased specific capacitances.In the last chapter,emploiting reverse thinking,we synthesize ultrathin cobalt hydroxides nanosheets in the first place and then load the metal nanoparticles onto the nanosheets improving the nanosheets' conductivity.Moreover,as the metal nanoparticles employed as spacers between the nanosheets,the lost specific area from the nanosheets restacking issue can be avoided,providing a better wetting degree for the electrolyte.Accompanied by these advantages,the material's capacitive perfomance and rate property are significantly improved.