| Nanotechnology , one of important technologies in modern material science , has extend -ed its applications in the field of power sources. MnO2 has been used as electrode materials in chemical power sources over 100 years. Nanocrystalline MnO2 maybe improve its electrochemical activity and may extend its applications in chemical power sources.The structure, composition and the performance of two commercial electrolytic manga -nese dioxide(EMD) were compared. Nanocrystalline a -MnO2(KMn8O16) was prepared by a low-heating solid state reaction (at room temperature) between KMnO4 and MnCl2 4H2O. Nanocrystalline single crystal β -MnO2 was obtained by a hydrothermal process through redox reaction between MnSO4 solution and (NH4)2S2O8 solution. Based on the preparation of β -MnO2, nanocrystalline a -MnO2[(NH3)2Mn8O16] and γ-MnO2 were obtained throughadjusting the concentration of SO42- ion by simply adding (NH4)2SO4 to the reaction system. The physical and chemical performances in KOH solution were investigated by several methods such as cyclic voltammetry, AC impedance, chronopotentiometry, X-ray diffraction(XRD), transmission electron microscopy(TEM), chemical analysis et al. Powder microelectrode and powder composite electrode technique were used in the electrochemical experiment.There are differences in the structures, composition and the performances of two comer -cial EMDs including the crystallization degree of crystal facet, content of combined water, content of Mn02, oxidation state of manganese, and polarization at different current densities. The HMD with higher content of combined water was found to have less polarization than that of the other EMD. This performance can be ascribed to the more OH groups in its crystal lattice, which act as "proton bridge" and can decrease the energy needed for proton to diffuse in crystal lattice.Both the KMn8O16 prepared by low-heating solid state reaction and the (NH3)2Mn8O16prepared by hydrothermal method belong to tetragonal system, but the cell volume of KMn8O16 is smaller than that of (NH3)2Mn8O16. Both have similar cyclic voltammogram in the range 0.4- -0.8V (vs.Hg/HgO) of the two a -MnO2 . The discharging capacity of KMn8O16 is higher than that of (NH3)2Mn8O16 because the particle size of the former is smaller than that of the later. It results from the mechanism of dissolution-reduction -deposition in the heterogeneous discharge process of MnO2 . Smaller particle materials have a higher specific surface. More Mn(III) ions dissolve at the surface .The dissolved Mn(III)ions are easier to diffuse to the surfaces of conductors and then be reduceed there. In Imol/L KOH, both KMn8O16 and (NH3)Mn8O16 have excellent cyclic performances in the range -0.25V~ 0.45V(vs.Hg/HgO), however, voltammograms of them are different. This may result from the differences in their reduction mechanisms of Mn4+ having different energy in different crystal structures. The specific capacity of (NH3)2Mn8O16 is hingher than that of KMn8O16, which may the result from difference of the cell parameter.In the synthesis process of nanocrystalline 3 -MnO2, while fixing the concentration of MnSO4 and (NH4)2S2O8, the final product tends to be 1 X 1 tunnel structure( 3 -MnO2) with the increase of reaction time and system pressure. Moreover, 1X2 tunnel structure can be formed when the reaction time is shorter or the system pressure is lower. Nano-phase -MnO2 has good cyclic performance in the range -0.2~0.5V(vs.Hg/HgO), but the specific capacity of 3 -MnO2 was lower than that of a -MnO2.In the reaction system of MnSO4 and (NH4)2S2O8, the concentration of SO42- has an influence on the growing rate and the joint mode of [MnO6] octahedron, which consequently affects crystallinity of crystal facet, content of combined water, content of MnO2, oxidation degree of -MnO2 The nm- MnO2 sample with higher content of combined water has better specific capacity in 1 mol/LKOH solution in the range -0.25~0.45V (vs.Hg/HgO) .
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