Microwave-assisted Hydrothermal Synthesis Of MnO₂with Controlled Morphology And Its Electrochemical Properties

With the emission control and accelerated depletion of fossil fuel resourcescreate the urgent needs for seeking new alternative energies and the correspondingconversion devices. Electrochemical capacitors and batteries have been given everincreasing attention due to their high power density, rapid charging/discharging andlong cycle life. Nanostructured manganese dioxides are excellent electrode materialsfor capacitors/batteries and catalyst for batteries owing to their structural flexibilityand manganese valence variety.For recent years, much attention has been paid to rational design and control overthe structures and morphologies of manganese dioxides nanostructures, which willaffect the physical and chemical properties of manganese dioxides materials directly.Many routes for preparation of manganese dioxides have been reported, such assol-gel, hydrothermal, thermal decomposition, refluxing, electrodeposition methodsand microwave-assisted hydrothermal, etc. Among them, microwave-assistedhydrothermal method could provide a relatively simple, effective and green way togrow nanostructured manganese dioxides materials.The microwave-heating combining with hydrothermal can lead to enhancedkinetics of crystallization and promote the formation of new phase of product.Microwave heating is based on dipolar and electrical conductor mechanisms.Compared with the conventional heating procedure, it has been proved to be a fast and green method with high yields and reproducibility. Up to now,microwave-assisted hydrothermal is a popular route to synthesize inorganicnanostructured materials such as metallic nanostructures, transition metaloxide/chalcogenides and metal phosphates. The as-prepared materials have potentialapplications in optics, catalysis and luminescent. Here, this paper are committed tosynthesize the manganese dioxides with structure tuning and fine shape controlled bythe microwave-assisted hydrothermal method and study their application inelectrochemical fields.Birnessite-MnO₂（δ-MnO₂） is one kind of important electrode material forsupercapacitor due to their special layered structure. Birnessite-MnO₂microspherewas early synthesized via hydrothermal method. In this work, δ-MnO₂microspheresas well as δ-MnO₂microspheres/α-MnO₂nanorods mixture have been synthesizedusing microwave-assisted hydrothermal method by simply changing the reactiontemperature. The reaction time of the above hydrothermal methods is longer with100min while microwave-assisted hydrothermal only needs10min. Therefore, onevaluable advantage of microwave-assisted hydrothermal is shortening the reactiontime greatly. Powder X-ray diffraction, field-emission scanning electron microscopyand high-resolution transmission electron microscopy have been involved in provingthat the nanorod of mixture is α-MnO₂. Electrochemical performances of the sampleswere also examined. The results show that the δ-MnO₂microspheres possesses asmaller specific surface area while a higher specific capacitance than those of theδ-MnO₂micropheres/α-MnO₂nanorods mixture does. The presence of a few fractionα-MnO₂impurities increases the surface area but reduces the specific capacitance ofδ-MnO₂electrode. The lower capacitance of the mixture is very probably due to thelower electrochemical activity of the α-MnO₂, comparing to that of the layeredδ-MnO₂and the impurity leads to decrease of the conductivity of sample. In addition,the other electrochemical tests suggest that the δ-MnO₂microsphere is a promisingcandidate for electrochemical capacitor due to its good reversibility and high stability.Here, microwave-assisted hydrothermal is used to synthesize the MnO₂withdifferent structures and morphologies. It could be found that microwave irradiation played two roles here, that is, shorten the reaction time and get new shapes. In termsof reaction time, microwave heating can accelerate the reaction from hours to minutes.Meanwhile, in terms of new morphology, microwave heating not only obtain δ-MnO₂microspheres, α-MnO₂nanorods but also prepare γ-MnO₂nanosheets and β-MnO₂octahedrons or hollow microrods. More importantly, γ-MnO₂nanosheets and β-MnO₂octahedrons were prepared for the first time by a one-step microwave-assisted method.Therefore, exquisitely tuned and control over the structure and morphology ofmaterials can be easily obtained due to the concentration of HCl undermicrowave-assisted hydrothermal method. Electrocatalytic activities of thesynthesized MnO₂in KOH solution have been determined by cyclic voltammetrywhich show that all manganese dioxides can catalyze the oxygen reduction reaction（ORR） in alkaline medium with different catalytic activities. α-MnO₂nanorods appearto hold the highest catalytic activity due to their crystal phase and morphology withappropriate oxygen adsorption mode.Birnessite-type MnO₂has also been denoted as δ-MnO₂. It has a mixed-valencetwo-dimensional lamellar structure with an interlayer spacing of0.73nm and itscharacteristic of the birnessite is intercalated with Na+, K+or other cations and H2Omolecules. These cations in interlayer can maintain structural stability and balance ofchange. The interlayer spacing of birnessite-MnO₂can be tuned by species ofincorporation cations. The δ-MnO₂usually observed as the intermediate or precursorduring the format of tunnel structures. In our experimental conditions, branchedα-MnO₂nanorods were obtained by the reduction of potassium permanganate inhydrochloric acidic solution without using any catalysts and surfactants. To figure outthe formation mechanism of the branched α-MnO₂, time-dependent experiments wereperformed. The branched α-MnO₂was obtained by transformation of δ-MnO₂. Onbasis of the previous mechanism, the formation mechanism of the branch α-MnO₂nanorods can be concluded that “oriented attachment’’ and rolling-cum-phase process.Because of the δ-MnO₂nanosheets with the small size of crystalline domains aremetastable phase under high temperature and high acidity. In addition, theelectrochemical performances of the samples were examined by cyclic voltammetry tests. The results reveal that the specific surface area and the electrolyte will directlyimpact the specific capacitance.In short, combining the microwave irradiation with hydrothermal techniquerequired temperature for nanostructure growth can be rapidly achieved, which leads toenhanced kinetics of crystallization and promotes the formation of new phase ofproduct. The MnO₂with exquisitely tuned structures/morphologies have beensuccessfully synthesized by the microwave-assisted hydrothermal method. Theircrystal structures and morphologies were systematically studied. In addition, theelectrochemical properties of as-synthesized products were also studied in the area ofsupercapacitor and electrocatalysis.