Study On The Hydrothermal Synthesis And Electrochemical Properties Of MnO₂

Supercapacitor as a new type of energy storage components has receivedmuch attention in development of clean energies because of its high powerdensity, long service life and some other characteristics. EnvironmentallyFriendly transition metal oxides are generally assumed to be a candidate of asupercapacitor electrode materials. In this thesis, manganese dioxide, whichbears high theory specific capacitance, was hydrothermally synthesized toinvestigate their electrochemical properties by cyclic voltammetry (CV),electrochemical impedance spectroscopy (EIS), and galvanostaticcharge-discharge (CP). The morphology and structure of the products were alsocharacterized by X-ray diffraction (XRD) spectrometry and scanning electronmicroscopy (SEM). The main experimental conclusions can be drawn asfollows:(1) We conducted the synthesis of manganese dioxide under a series ofreaction temperatures at120℃,140℃,160℃, and180℃, respectively. Theexperimental results revealed that at120℃,140℃and160℃, the resultantmanganese dioxide exhibited the poor crystalline. However, the increasing of thereaction temperature to180℃led to the high crystalline of manganese oxide.Analysis of galvanostatic charge-discharge curves of the four samples at different reaction temperature showed that at120℃,140℃,160℃and180℃, the specific capacitance of manganese oxide is323F·g-1,354F·g-1,331F·g-1,282F·g-1, respectively. This indicates that reaction temperature hassignificant effect on the specific capacitance of MnO2and the maximum specificcapacitance was achieved to be354F·g-1at140℃.(2) With optimized condition of reaction temperature in hand, we next toexplore the reaction time influence on hydrothermal synthesis of MnO2at140℃. Changing of the reaction time from1h to24h leads to the strongerintensity of XRD diffraction peak of MnO2, indicating that the crystallization ofthe sample turns to be much higher. Galvanostatic charge-discharge curves ofthe seven samples showed that at1h,2h,4h,6h,8h,12h and24h, thespecific capacitance of manganese oxide is237F·g-1,375F·g-1,354F·g-1321F·g-1,287F·g-1,261F·g-1and238F·g-1, respectively. Among the reaction timesexamined, reaction at2h gave the best result with maximum specificcapacitance of375F·g-1.(3) We than screen the influence of the proportion of layered graphenein MnO2on their electrochemical performance. In this work, MnO2compositeelectrodes with the concentration of layered graphene ranging from20%,40%,60%to80%were prepared. The experimental results revealed that compositeelectrode with40%layered graphene as being optimal that gave the maximumspecific capacitance of451F·g-1, suggesting a significant effect of layeredgraphene involved in MnO2. As compared with the naked MnO2electrode, the specific capacitance of our optimized composite electrode was increased by17%.