| Manganese oxides with different crystal structures and morphologies are considered as one of the most potential materials for supercapacitors due to their abundant resources, relatively low cost, environmentally friendly nature and high theoretical specific capacitance. In this paper, a series of manganese oxide nanomaterials with different crystal structures and morphologies have been prepared by the hydrothermal treatment method, and the hydrothermal treatment conditions are discussed systematically. On the basis of the preparation of manganese oxide nanomaterials with different crystal structures and morphologies, the effects of crystal structure, specific surface area, morphology and crystallinity on the capacitive properties of manganese oxide nanomaterials have also been investigated. The research contents are as follows:(1) KMnO4as manganese source and organic amine/ammonium and inorganic ammonium as reducing agent, a simple preparation method of manganese oxide nanomaterials with large surface area and layered structure is developed.Layered manganese oxide with plate-like morphology has been hydrothermally prepared at90℃for24h in a reaction system of KMnO4and urea. The BET specific surface area of the layered manganese oxide with plate-like morphology is230m2/g. Research results indicate that urea plays a crucial role for the formation of the manganese oxide nanomaterial with large surface area and layered structure. And hydrothermal temperature has great effects on the crystal phase and morphology of the prepared materials. When (NH4)2SO4is used as reductant, birnessite-type manganese oxide with flower-like microsphere morphology and large specific surface area has been prepared by hydrothermal treating a mixture solution of KMnO4and (NH4)2SO4at90℃for24h. Results indicate that the birnessite-type manganese oxide shows novel flower-like microsphere morphology and a specific surface area of280m2/g, and the flower-like microsphere consists of the thin nano-platelets. On the basis of the optimizing experiments, the optimization preparation conditions of the manganese oxide nanomaterial with large surface area and layered structure is a molar ratio of KMnO4to (NH4)2SO4=1:0.5. This preparation approach is a green strategy for synthesizing layered manganese oxide nanomaterials because no organic solvent or surfactant is added in the reaction system. When cetyltrimethylammonium hydroxide (CTAOH) is used as reductant, hierarchical manganese oxide nanomaterial has been simply prepared via a hydrothermal treatment technology in a mixed solution of KMnO4and cetyltrimethylammonium hydroxide (CTAOH) at90℃for12h. The obtained material has a specific surface area of105m2/g and the average pore size is7.8nm. CTAOH serves as both a reductant and a surfactant, and it plays a very important role in the formation of the hierarchical structure. Hydrothermal temperature has an obvious influence for the crystalline and morphology of the obtained materials. In company with the hydrothermal treatment temperature to180℃, the layered birnessite structure has completely transformed into purity y-MnOOH with rod-like morphology.(2) Birnessite manganese oxide material with belt-like morphology and high crystallinity has been prepared through two-step hydrothermal reaction.Firstly, MnOOH is prepared by reacting KMnO4and ethanol at140℃for24h. Then, the obtained MnOOH is hydrothermally treated at140℃for72h after it is well dispersed in10mol/L KOH aqueous solution. The specific surface area of the belt-like manganese oxide nanomaterial is53m2/g. Hydrothermal treatment temperature, reaction time and the amout of MnOOH all can affect the crystalline phases of the obtained materials. A transformation process has been observed. MnOOH is dissolved and followed by recrystallizing, and finally transformed into the birnessite manganese oxide material with belt-like morphology in a strong basic solution.(3) A new preparation method of both α-MnO2nanowires and β-MnO2nanorods has been developed.Manganese powder is used as manganese source and K2S2O8is used as oxidant. a-MnO2nanowires and β-MnO2nanorods are controllably prepared with a hydrothermal method at150℃and180℃for24h, separately. The method is simple for no catalysis or template reagent is added, and the products have high purity. Research results show that hydrothermal temperature, reaction time and the concentration of K+ions in the reaction system affect the crystal phase and moiphology of the products. K+ions serve as both template and structure stabilizer for a-MnO2. The crystal phase of a-MnO2maintains unchanged in aqueous solution with high K+ions concentration when treated under higher hydrothermal temperature. Under certain conditions, α-MnO2nanowires can transform into β-MnO7nanorods. Higher hydrothermal temperature and H+ions concentration favor the the formation of β-MnO2. (4) The electrochemical properties of the prepared manganese oxide nanomaterials with different crystallinities and morphologies have been systematically investigated.Research results indicate that the capacitive properties of the prepared manganese oxide nanomaterials with different crystallinities and morphologies connects with their crystal phase, crystallinity, specific surface area and morphology. Generally, the specific capacitance of layered manganese oxides and tunnel manganese oxides with larger cavity sizes is higher than that of tunnel manganese oxides with smaller cavity sizes. The capacitive behavior of manganese oxide nanomaterials with low crystallinity is superior to than those with high crystallinity due to their loose lattice. Manganese oxide nanomaterials possessing high specific surface area show high specific capacitance because they can provide more redox electroactive sites. Morphology of manganese oxide materials is related to their specific surface area and also affects the transport/diffusion path lengths for ions and electrons. Hierarchical structure manganese oxide materials have found to be one of the best systems for energy storage, because it offers both advantages of nanosized building blocks and submicrometer-sized structures.(5) Manganese oxide nanomaterial with hierarchical structure has good capacitance.The perfect electrochemical property of hierarchical manganese oxide nanomaterial is ascribed to the unique hierarchical structure, poor crystalline and relatively high specific surface area. Manganese oxide nanomaterial with hierarchical structure exhibits not only high specific capacitance of347F/g, but also excellent cycle stability (97.5%capacitance retention after10000cycles at a scan rate of20mV/s). An asymmetric supercapacitor based on the obtained manganese oxide as a positive electrode and graphene as a negative electrode is assembled. The assembled asymmetrical supercapacitor gives a high energy density of20.9Wh/kg at a power density of400W/kg. The hierarchical manganese oxide nanomaterial is a very promising electrode material for assembling supercapators with good capacitive performance. |
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