Preparation Of Co₃O₄ Nanostructures And Study On Their Supercapacitor Properties

With the rapid development of modern technology,energy issues have become increasingly prominent.While studying and exploiting various energy storage devices,supercapacitors have been extensively studied by researchers because of their unique electrochemical properties.As one of the performance complementary devices to battery,they have the advantages of environmental friendly,good cycle stability,high power density and fast charge and discharge.They are currently used in aerospace,urban transportation and energy power systems.However,compared with other storage devices,the applications of supercapacitors have severely been limited due to the shortcoming of the low energy density,which is far from the needs of industrial development.In addition,the study on capacitor electrode materials is also facing bottlenecks.The main reason is that the study on cathode materials is relatively lagging and the performance of the cathode materials cannot meet the demand of high-performance supercapacitor.As a common transition metal oxide,Co₃O₄ has been applied to the negative electrode of supercapacitors for its own electrochemical properties.It is well known that Co₃O₄ nanomaterials with different morphologies and structures exhibit different electrochemical properties.In this thesis,we pay attention to the preparation of supercapacitor cathode materials and the specific capacitance properties.Co₃O₄ is selected as the research object.Its different nanostructure electrode materials are prepared under different experimental conditions and their capacitance properties are tested.The studying contents are as follows:?1?The spherical Co₃O₄ nanostructures assembled by hexagonal flakes and the urchin-like Co₃O₄ nanostructures are obtained by hydrothermal method combined with thermal decomposition method.The experimental results show that the additive amounts of PVP and urea directly affect the morphology of the products.The obtained products have different morphology at different additive contents.The obtained precursors of hexagonal flakes and the assembled structures are annealed in an air atmosphere to obtain the final products,which all are porous.The optimum annealing temperature of the urchin-like nanostructure precursors is 400°C.The final products of the hexagonal sheet assembled spherical structure and the urchin-like nanostructure powders are made into a single electrode by doctor-blade method.The results of electrochemical tests show that the as-prepared Co₃O₄ nanostructures all have unique specific capacitance performance.?2?In order to eliminate the process of blade-coating and avoid the introduction of conductive active agent which without any contributions to the capacitance,nickel foam is added to the hydrothermal system to form a Ni-based self-support structure to improve the performance of the entire electrode systems.For the self-support structure of Ni-based urchin-like/nanowire Co₃O₄,the CV,GCD and EIS tests show that the self-support structure electrode exhibits superior specific capacitance and rate performance compared to the doctor-coating electrode.Due to the influence of the preparation process,Ni-based self-support structure of the final product has fault phenomena,so the EIS spectrum shows a bulk resistance of about 1.28?.In order to eliminate this higher bulk resistance,ethylene glycol is used as a complex molecule instead of glycerol to participate in the hydrothermal reaction to form Ni-based emission-like mesoporous Co₃O₄ nanowires self-support structure precursors.The SEM images show that the fault phenomena are significantly reduced.The electrochemical tests show that the specific capacitance is about 2477 Fg^-1 at the scan rate of 2 mVs^-1,and the EIS spectrum show the inner resistance increases from 0.92?to 0.96?at the first and 5,000^th cycle galvanostatic charge and discharge at 10 Ag^-1respectively,this slight change further indicates the stability of the as-prepared Co₃O₄nanostructures.?3?Using Co?NO₃?₂?6H₂O and Ce?NO₃?₃?6H₂O as cobalt and cerium source,Ni-based Co₃O₄/CeO₂ composite self-support nanostructure electrodes are prepared under appropriate hydrothermal reaction conditions.Due to the synergistic effect of two metal oxides and the unique electrode structure,the excellent electrochemical performance is obtained.i.e.,the obtained nanowire structure electrode has a specific capacitance of about 1179 Fg^-1 at a current density of 1 Ag^-1.In order to study the performance and energy density of the Ni-based composite electrode in an asymmetric device,it is assembled into an asymmetric device with activated carbon.The CV and GCD results reveal that when the energy density is 20 W h kg^-1,the power density is 176 W kg^-1;when the energy density is attenuated to 12 W h kg^-1,the power density increases to 1413 W kg^-1.Due to its superior performance,it can meet the needs of future high performance supercapacitor cathode materials.