| Nowadays,most of researches have been focusing on the development of transition metal oxides due to their redox reaction which exhibits high specific capacitance and good electrochemical stability for pseudocapacitors.Among various available metal oxides,MnO2 materials have many outstanding characteristics,such as environmental friendliness,low cost and non-toxicity,and their charge storage mechanism is widely recognized according to the surface adsorption/desorption of electrolyte cations and the intercalation/deintercalation of alkali cations in the bulk.But their poor electrical conductivity and low structural stability restrict their practical application.To address these problems,many researchers have focused on fabricating novel MnO2 materials via unique approaches.This thesis attaches importance to the examination about MnO2.The electrode materials with different morphologies have been synthesized by different methods.The structures/morphologies and capacitive properties were investigated by XRD,SEM,cyclic voltammetry galvanostatical charge/discharge and electrochemical impedance spectroscopy.The main research contents are as follows:1.MnO2 with different morphologies were synthesized by liquid-phase precipitation method,water bath liquid-phase method and hydrothermal method.We found that MnO2 prepared by hydrothermal method was ?-MnO2,which was nanorod-like in the micromorphology,and was about 30-40 nm in diameter and 1 ?m in length.Used liquid-phase precipitation method,?-MnO2 can be obtained.Nanospheres with folds on the diameter of 500 nm.Used the liquid-phase heating method,?-MnO2 can be obtained.The surface was covered with fold nanospheres on the surface,with a diameter of 200 nm.At the same time,three kinds of MnO2 were prepared as electrodes,and different electrochemical tests were used a threeelectrode system with 1 mol/L Na2SO4 eletrolyte,including cyclic voltammetry,galvanostatic charge and discharge,and AC impedance.As a result,?-MnO2 exhibited the highest specific capacitance of 256 F/g at the current density of 0.5 A/g.And ?-MnO2 also showed excellent structural stability which cycled 2000 times at the current density of 8 A/g,the capacitance retention rate was 86%.2.Firstly,?-MnO2 nanorods were synthesized by hydrothermal method,then NiCo2O4 nanosheets were coated by chemical reaction to form MnO2/NiCo2O4 composites with coreshell structures.The samples were characterized by XRD and SEM,the results showed that NiCo2O4 nanosheets uniformly grew on the surface of MnO2 nanorods to form a core-shell composite structure with larger specific surface areas.Electrochemical performance tests represented that MnO2/NiCo2O4 had relatively higher specific capacitance and excellent structural stability.The specific capacitance of the MnO2/NiCo2O4 composites can reach 434 F/g.After cycling 2000 times at current density of 8 A/g,the capacity retention rate was 91.8%.3.MnO2 nanorods were synthesized by ordinary hydrothermal method,and Ce doped during the reaction to prepare modified MnO2.The antimony-doped manganese oxide had excellent electrochemical performance as a tantalum capacitor electrode material,which was directly relevant to its good electrical conductivity and large specific surface area.It was found that ytterbium ions doped into the MnO2 crystals and altered the morphology of MnO2,the morphology of MnO2 changed from nanorods to nanosheets.Electrochemical performance tests demonstrated that the specific capacitance of the 10% antimony-doped MnO2 electrode was as high as 748 F/g.And the cycle stability was also more than good.After 2000 cycles,the specific capacitance retention was about 100%.The results showed that yttrium can significantly improve the specific capacitance and electrochemical stability of MnO2,and can promote the conversion of MnO2 nanorods into nanosheets. |
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