Hydrothermal Synthesis Of Manganese Oxides And Their Electrochemical Properties

The excessive use and exploit of fossil fuels has caused a series of environmental issues,arousing worldwide concerns from experts and scholars.Therefore,it is imperative for human beings to find a new type of innovative energy storage device to meet the demand of people's daily activities and industrial productions as well.Recently,supercapacotors,which are also know as electrochemical capacitors,have been considered as one of the most promising renewable energy storage device among researchers because of their high power density,environmental protection,excellent reversible charge-discharge characteristic and long cycling life.Furthermore,the quality of electrode materials can significantly change the whole performance as well as commercial application of supercapacitors to a large extent.Traditional electrode materials can be broadly divided into three categories: carbon materials,conductive polymeric materials and transition metal oxide materials in term of their reaction mechanisms.In comparison to carbonaceous materials and conducting polymers,transition metal oxides have often been employed as promising candidates for the design of supercapacitors owing to their high energy density,variable oxidation states,natural abundance and good stability during the faradaic reaction.In this thesis,various nanosized manganese oxides were successfully synthesized via a facile hydrothermal method.The relevant morphologies and crystal structures had been investigated by Field emission scanning electron microscopy?FESEM?,high resolution transmission electron microscopy?HRTEM?and X-ray diffraction?XRD?,respectively.Meanwhile,electrochemical characteristics of as-prepared sample for the supercapacotors application had also been checked in detail.The main contents and conclusions of this thesis are listed as follows:Firstly,three dimensional urchin-like and flower-like manganese dioxides with different morphologies were successfully synthesized by a simple hydrothermal strategy,and experimental results revealed that:?1?Due to the increase of reaction temperature,we can observe the obvious morphology evolution of urchin-like manganese dioxides.During the electrochemical test,the sample obtained at 110 oC shows good electrochemical performances such as a high capacitance of 151.5 F g-1 at the current density of 1 A g-1 and the good capacitance retention characteristic?93.4% after 1000 cycles?.This mainly because the regular and ordered rod-like structure on the surface of materials.?2?With the increase of the concentration of reactants,the thickness of as-prepared manganese dioxide nanoflowers changed gradually.Obviously,the sample derived from high KMnO4 concentration exhibits superior electrochemical property.It is mainly because this multi-branch hierarchical structure which provides abundantly active sites and larger contact areas allowing the electrolyte to be accessible.Electrochemical tests demonstrate that the electrode obtained at the high KMnO4 concentration shows a capacitance of 197.3 F g-1 at the current density of 1 A g-1,which is much higher than the lower one,and the former also has the smaller faradaic impedance as well as the better cycling stability than the latter.All of the results indicate that 3D manganese dioxide nanomaterials have excellent electrochemical performances and wide application prospects.Secondly,the nickel doped and chromium doped manganese dioxide nanostructures had been prepared via the hydrothermal method.Owing to the effect of metal ions,faradaic impedances of materials decrease significantly and stabilities improve as well,thereby showing outstanding electrochemical properties.The Ni-doped manganese dioxide presents ideal electrochemical behaviors,such as a high specific capacitance?153.6 F g-1 at the current density of 1 A g-1?and the outstanding cycling stability?the capacitance even shows a retention of 103 % after 1000 cycles?.Meanwhile,the Cr-doped sample has a high specific capacitance of 202.5 F g-1 and the superior cycling stability?6.8 % capacitance decay after 1000 cycling test?.As a consequence,these metal ions doped manganese dioxides have a huge application in energy storage devices.Finally,by using CuCl₂·2H₂O and KMnO4 as reactant sources,the Cu-doped manganese dioxide microsphere is rationally synthesized via a simple and facile hydrothermal process.BET test shows that the specific surface area of the sample is calculated to be 30.18 m2 g-1 and presents the typical mesoporous structure.In order to investigate its electrochemical performances and practical applications thoroughly,the manganese dioxide microsphere,flexible carbon clothes and the LiCl-PVA solid electrolyte were assembled into a solid supercapacitor.This device is expected to deliver a high specific capacitance?300 F g-1 at the scan rate of 5 mV s-1?,a maximum energy density of 47.4 Wh kg-1 at a power density of 259 W kg-1 and the long-term cycling stability?89.7% capacitance retention after 5000 times repetition at the current density of 5 A g-1?.Moreover,three charged supercapacitors connected in series with each other can light a blue light emitting diode?LED?in tens of seconds.In summary,this sort of solid state device is inexpensive,excellent and has a large significance in industry.