Supercapacitors Based On Hierarchical Transition Metal Oxide And Sulfide Nanostructures For Practical Applications

With the popularization of various kinds of portable electronics and electric vehicles,the requirement for high energy density,high power density and long cycle life energy storage equipment is becoming unprecedented urgent.Currently,the portable energy storage equipment is mainly lithium ion battery.However,slow charging speed,short cycle life and insufficient power density are the shortcomings of lithium ion batteries to meet the demand for more advanced,multifunctional electronics.Supercapacitors,a new type of energy storage device which can store charge based on faster energy storage mechanism,endow it the advantages of high power density and long cycle life,and are expected to play a vital role in the future energy storage field.In recent years,renewable green energy such as solar energy,wind energy and tidal energy have been gradually developed and used for practical applications.Unlike traditional hydropower and thermal power,which are stable with the change of time,solar,wind and tidal energies are unstable due to the change of sunlight illumination,wind speed and tidal cycle.The unstable power output is unsolicited for the state grid,so the generated power should be transported in other ways.One possible solution is to store the energy electrochemically and then redistribute it.With high power density and long cycle lifetime,supercapacitors could be applied as the energy storage equipment for such unstable power sources and provide a solution for power transmission.Hierarchical nanomaterials have unique advantages in the field of electrochemical energy storage devices.Hierarchical structure further expands the surface area of nanomaterial and forms effective charge transfer channel,which is the key to improve the electrochemical performance of electrode materials.The main content of this thesis is to synthesize hierarchical nanomaterials on the current collector directly to overcome the agglomeration problem of powdered electrode materials.The main research contents are as follows:1.Two step hydrothermal method was employed to synthesize the hierarchical NiCo2O4@MnMoO4 core shell nanostructure.NiCo2O4@MnMoO4 was built with high conductive NiCo2O4 nanorod core and high capacitance MnMoO4 nanoflake shell.NiCo2O4 nanorod core was grown on current collector directly.In such a structure,MnMoO4 nanoflake is directly contact with the electrolyte and realize energy storage through electrochemical reaction.NiCo2O4 nanorod serves as high transport path for electron transport.Moreover,the exposed part of NiCo2O4 nanorod contact with electrolyte can also take part in energy storage.The hierarchical structure of NiCo2O4@MnMoO4 can not only achieve high surface area,but also realize synergistic effect through the cooperation of core and shell.Our further assembled supercapacitor prototype devices also show that NiCo2O4@MnMoO4 exhibit good energy storage performance.2.Hierarchical NiCo2O4 nanoarrays have been synthesized by two step hydrothermal method.Hierarchical NiCo2O4 is composed of NiCo2O4 nanorod as core and NiCo2O4 nanoflake as shell,namely,homogeneous core shell structure.Compared with NiCo2O4 nanorod arrays,hierarchical nanoarrays possess superior surface area and introduce more active materials,which improve the area mass load of electrode.Moreover,the introduction of active material is realized by constructing hierarchical structure,therefore,dead volume by stacking can be avoid.Compared with sole NiCo2O4 nanoflake,the NiCo2O4 nanoflake in the hierarchical structure can anchor on the NiCo2O4 nanorod and distributed orderly in the space,which is beneficial for electrochemical reaction.During electrochemical reaction,both NiCo2O4 nanorod and NiCo2O4 nanoflake can react with electrolyte and contribute for charge storage.With NiCo2O4 nanorod directly grown on current collector,NiCo2O4 nanorod also acts as high transport path for electron,which is beneficial for high performance electrode.3.Hierarchical,three dimensional CoSx nanonets have been synthesized by two step hydrothermal method.Hierarchical CoSx nanonets are formed by one dimensional CoSx interwoven in the space,while one dimensional CoSx are composed of CoSx nanowire covered with CoSx nanosheets.CoSx nanowires are grown on the current collector directly without the introduction of binder.The three dimensional hierarchical structure enable efficient contact with electrolyte and the CoSx nanowire can serve as effective path for electron transport.In such a way,the hierarchical CoSx nanonets can achieve rapid electrochemical reactions and charge transfer rate,thus realize excellent electrochemical performance.Supercapacitors were fabricated using hierarchical CoSx nanonets and the prototype exhibit excellent performance.4.To further explore the application of supercapacitors in the field of frequency devices,we analyzed the alternating current filtering performance of NiCo2O4//AC asymmetric supercapacitors.The asymmetric supercapacitor was constructed with NiCo2O4 nanorod array as cathode and active carbon as anode,a frequency of 50 Hz alternating current was applied as the input signal.The main advantage of asymmetric supercapacitor as filter element is its high specific capacitance compared with commercial aluminum electrolytic capacitor,but the disadvantage is that the response speed is not fast enough.In order to reduce this shortcoming as much as possible,we choose high conductive NiCo2O4 nanorod arrays as electrode materials,hoping that it can achieve faster frequency response as far as possible.The results infer that although the output quality of signals was not up to the mark,but it proves the possibility for applicability of asymmetric supercapacitors in frequency devices.We believe by further modifying the structure and composition of electrode materials,higher quality direct signal can be obtained.