Study On The Synthesis And Performance Of New Manganese Compound Based Electrode Materials

There are seven oxidation states (Mn⁷⁺,Mn⁶⁺,Mn⁵⁺,Mn⁴⁺,Mn³⁺,Mn²⁺,Mn⁺) in manganese oxides, and their corresponding and mixed valence oxides show extraordinary physical and chemical properties in electricity, magnetic and catalysts. At present, it is also a kind of important power sources materials which can be used widely. In particular, the spring-up of nanomaterials further fuels-up the activity of the research and development of manganese oxides.In this thesis, we have reviewed the recent developments of manganese oxides in synthesis and application. Through the means of improving synthetic method, optimizing preparation condition, we have obtained a series of manganese oxides with different crystal structure and novel morphologies. Besides, we have investigated the structure properties, electrochemical performance and the correlations between them using different testing technologies such as TG-DTG, XRD, FESEM, TEM, HRTEM, BET as well as some electrochemical measurements. The main content is as follows:(â… ) Various hollow sphere and urchin structuredα-MnO₂ have been synthesized by a simple and facile hydrothermal method for the first time. The effect of reaction time and reaction temperature on the structure and morphology of samples has also been studied systemically. The experimental results indicated that the products grew from solid sphere to hollow sphere, further to nanorods gradually with the increasing of reaction time, which is the result of Ostwald ripening process. With the increasing of hydrothermal reaction temperature,α-MnO₂ transformed intoβ-MnO₂, its morphology also changed correspondingly, from nanorods into polyhedral particles. The results indicated different shapedα-MnO₂ materials were kinetically controlled products, which were formed at lower temperature, whereasβ-MnO₂ was a thermodynamically controlled product, which was formed at higher temperature. The novel hollow sphere or urchin structuredα-MnO₂ materials possessed high loosely mesoporous cluster structure consisting of thin plates or nanowires and exhibited enhanced rate capacity and cycleability. The good cycleability and high rate capability coupled with the low cost and environmentally benign nature of manganese may make this material attractive for large applications. Furthermore, the hydrothermal method has excellent reliability, selectivity, and efficiency for synthesizing inorganic materials with uniform and distinct morphologies.(â…¡) Usingγ-MnO₂ sphere as raw material, by controlling reaction temperature and reaction atmosphere, different nanostructured products were prepared via hydrothermal treatment. The results of FESEM investigation indicated that the sample can maintain its spherical morphology of starting materials in ammonia solution, while it unlayed into nanowires in- water atmosphere, which revealed that the reaction atmosphere has significantly influence on the microstructure and morphology of the resulting products.(â…¢)β-MnO₂ nanotube was first proposed as catalyst supporting material in our experiment. Pd/β-MnO₂ nanotubes composites were prepared via a simple and facile reductive process. Electrochemical study indicated that Pd/β-MnO₂ composites had a better electrocatalytic activity than that of Pd/C for methanol oxidation in NaOH solution. It implies that Pd/β-MnO₂ nanotubes may be a good candidate for noble metal catalyst supports and have potential application in preparing catalysts for direct methanol fuel cells. It open a new way for searching good catalyst supporting material.(â…£) Mesoporous amorphous MnO₂ was synthesized by an improved reduction reaction and using supramolecular as template. Its amorphous structure was determined by TEM and XRD analysis. The amorphous MnO₂ annealed at 200℃exhibited the maximum specific capacitance of 298.7 F/g in a 2 mol/L KOH electrolyte at 5 mA. The prepared mesoporous amorphous MnO₂ showed ideal capacitive nature, high specific capacitance and long cycle life, so it is suitable to be used as electrode material for electrochemical capacitors. It is believed that the main part of the capacitance comes from the pseudocapacitive surface redox process. When the specific surface area of MnO₂ electrode material increase, not only the double layer capacitance will increase, the redox active sites will also increase subsequently, so the pseudocapacitance will increase significantly.(â…¤) Spinel-type LiCr_0.15Mn_1.85O₄ has been efficiently fabricated via improved microwave assisted sintering method. The prepared LiCr_0.15Mn_1.85O₄ powders have ideal spinel structure, regular shape and narrow particle size distribution. The results of the experiments indicated that microwave not only reduced the sintering time, but also saved the cost significantly. In the microwave irradiation field, since the microwave heated not from the outside but from the inside of the precursor and thus it provided a uniform heating environment which could shorten the synthesizing time, effectively avoided the growing of crystal grain exceptionally and overcome the agglomeration of particles.(â…¥) Mesoporous carbon with large specific surface area (1020 m²/g) and uniform pore size distribution (3-6 nm) has been successfully synthesized by using SBA-15 as template, sucrose as carbon sources. The result of electrochemical measurement indicated that it is a good electrode material for double-layer capacitor.(â…¦) The electrochemical performance of LiCr_0.15Mn_1.85O₄ and mesoporous carbon in aqueous electrolyte was studied respectively. A hybrid aqueous supercapacitor using LiCr_0.15Mn_1.85O₄ as anode and mesoporous carbon as cathode materials was setup in our experiment. The results of electrochemical measurement indicated that LiCr_0.15Mn_1.85O₄ has a potential as good electrode materials in hybrid aqueous supercapacitor.