Controllable Synthesis And Properties Research Of Manganese Oxides Functional Materials

This paper carried out based on the controllable synthesis and propertiles research of manganese oxides functional materials. Manganese oxide micro/nanostructures with various morphology and structures have been synthesized by hydrothermal reaction of potassium permanganate and glucose. The growth process, structures and compositions of the products were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The applications in capacitor, electrocatalysis, and catalytic activities of CO oxidation area were investigated, and the influences on their properties of introduction of doping ions, hollow-mesoporous structures, and composite structures were discussed. The main contents were summarized as follows:In the second chapter, pristine and transition-metal-doped Mn3O4 spinel-structure nanocrystals shaped in octahedrons have been successfully synthesized by hydrothermal reduction of potassium permanganate. XRD, SEM/TEM, XPS, and electrochemical experiments were employed to study the doping effect on crystal, morphology and electrochemical properties. The results reveal that a multistep reduction process is taking place, and the effectiveness of Cr ions doping can promote the formation of single-phase Mn3O4 by preventing the presence of the MnOOH phase and effectively improve the capacitive performance and the phase stability during cycling. Cr-doped Mn3O4 nanocrystals exhibited a maximum specific capacitance of 272 F·g-1 at a current density of 0.5 A·g-1, and suggesting an almost 100% capacitance retention at 1 A·g-1 for 1000 cycles. These results have verified the ability of doping to improve capacitive performances of spinel-structured transition-metal oxides.In the third chapter, in situ activation process of Mn3O4 electrode (Mn3O4/NF) with nickel foam (NF) as current collector has been investigated. Depended on the instability of the two components, electrodes exhibited excellent electrochemical performances by activated in neutral electrolyte containing alkaline earth metal ion. The morphology and electrochemical properties of electrodes during activated process have also been observed. It was found that electrochemical activation didn’t change the morphology, but improved the electrochemical activity of the material, and the soaking process was the key period for the morphology changes of NF electrode and Mn3O4 particles. The effects of the electrolyte, voltage window, activation method, and activation time on the morphology and electrochemical properties of activated electrode were discussed, and adopted cyclic voltammetry method at 5 mV·s-1 for 50 cycles from 0.0-1.0 V in MgSO4 aqueous solution as the optimum activation condition. The obtained electrode exhibited high capacitance, excellent rate, and good cycle stability of 1511 mF·cm-2 at a current density of 0.5 mA·cm-2. Commercial Mm3O4 electrode with nickel foam (NF) as current collector has also been attempted; the results verified the practicability of the in situ activation method.In the fourth chapter, hollow-mesoporous octahedral nanocrystals with various morphology and internal structures have been synthesized in acidic KMnO4 system by controlled the amount of KMnO4 and reflux time, with spinel-structure Mn3O4 octahedrons nanocrystals as the crystal nucleus. The formation mechanisms and influence factors have also been investigated based on the structures and compositions of the products collected at different reaction times. The results reveal that the products composited of Mn3O4/MnO2, its formation can be regarded as oxidation-lattice matched growth-etching process. During this process, competition exists between the redox reaction of KMnO4 and the disproportionate reaction of Mn3O4 in acid solution, and the morphology of the hollow-mesoporous nanocrystals depends on the difference speed of the two reactions. The catalytic activities of the products for oxygen evolution reaction (OER) have been examined. The results show that, for hollow-mesoporous nanocrystals, the catalytic properties depend on the specific surface area and mesoporous size. Larger size of mesoporous is beneficial to the diffusion of gas molecules, thus improve their catalytic performance.In the fifth chapter, a series of C@MnC2O4/MnCO3 core/shell microstructures were fabricated via a self-templated method, in which carbon spheres obtained by a carbonization process in the same solution served as cores and templates. The shape of C@MnC2O4/MnCO3 evolved from rhombi, to ellipsoids, to dumbbells, and to twinsphere particles, depending on the setting time and temperature of the pre-mixing period. The carbon core was completely removed after calcination at 550℃, and a phase transformation to Mn2O3 hollow microstructures was achieved, retaining their original shapes. The catalytic activities of the as-obtained Mn2O3 hollow microstructures for CO oxidation were examined. The catalytic activities decreased in the order of Mn2O3 ellipsoids≈dumbbells>twin spheres>rhombi.