Controllable Preparation Of Manganese Dioxide With Different Crystal Phases And Morphologies And Its Performance Of Supercapacitors

Manganese dioxide(MnO2)has the advantages of high theoretical specific capacitance(?1370 F·g-1),rich resources,low cost and the good capacitance performance in aqueous solution.Therefore,materials based MnO2 were considered as one of the most promising candidates for supercapacitors.However,due to the low conductivity and low material utilization,the actual specific capacitance of MnO2 is only about 1/5 to 1/6 of the theoretical values.The structure,morphology,specific surface area and particle size all have influence on the capacitance of MnO2·Hydrothermal method is widely applied for formation of MnO2 nanomaterials.So,it is challenging and important to obtain MnO2 with high capacitance and high cycling stability by controlling the preparation process.In this paper,the metal-organic framework[Mn(C8H4O4)(H2O)2n(manganese terephthalate dihydrate,Mn-MOF for short)was used as the precursor to synthesize MnO2 nanostructures with different crystalline phases(?-MnO2,?-MnO2 and ?-MnO2)and morphologies(rod,needle and sheet)by hydrothermal method.By optimizing the reaction temperature,pH and the molar ratio of Mn(?)and Mn(?)in the reactants,MnO2 with different structure and morphology can be formed.Moreover,the size of nanostructures can be adjusted by changing the proportion of Mn(?)/Mn(?).Finally,the MnO2 nanomaterials with small size,large specific surface area and stability were obtained.The materials were characterized by X-ray powder diffraction(XRD),field emission scanning electron microscopy(FESEM),X-ray photoelectron spectroscopy(XPS),N2 adsorption/desorption and other methods.The results show that higher molar ratio of Mn(?)/Mn(?)can decrease the particle size,increase the specific surface area and decrease the average oxidation state of manganese in MnO2.The characterization results provide useful information for the subsequent research on the capacitive properties of MnO2 materials.The electrochemical properties of the manganese dioxide nanomaterials(?-MnO2,?-MnO2 and ?-MnO2)were characterized by cyclic voltammetry(CV),galvanostatic charge and discharge(GCD)and electrochemical impedance spectroscopy(EIS).The results show that the larger the molar ratio of Mn(?)and Mn(?),the smaller the size of the products,the larger the specific surface area and the higher the specific capacitance.Among these samples,the layered ?-MnO2 nanostructures with n(Mn?)/n(Mn?)=2 show a specific surface area of 240 m2·g-1 and highest specific capacitance of 416 F·g-1 at 0.5 A·g-1,which was assembled with activated carbon(AC)to form an asymmetric supercapacitor.The energy density of the asymmetric supercapacitor is 23.2 W-h·kg-1 at a power density of 425 W·kg-1.These results indicate that the ?-MnO2//AC asymmetric supercapacitor is a promising candidate for high performance energy storage systems.Therefore,the preparation of manganese dioxide nanoscale electrode materials with different crystal phases and morphologies based on Mn-MOF is a new way for the investigation of supercapacitors.