Preparation And Capacitance Properties Of Nanostructured Manganese Dioxide

Manganese dioxides have been widely used as traditional electrode active materials. Nanostrctured manganese dioxides exhibit excellent electrochemical properties owning to their unique physical and chemical characteristics. Supercapacitors are new energy-storage devises with extensive applications, and their electrode active materials have drawn much attention. Preparation and capacitance properties of nanostructured manganese dioxides were mainly researched in this paper.Different nanostructured manganese dioxides were prepared by sol-gel, hydrothermal and coprecipitation methods, respectively. Several techniques such as XRD, TEM, BET, cyclic voltammetry(CV) and chronopotentiomtry were used to study the physical and chemical properties and electrochemical performances of the synthesized materials.During the sol-gel process, when the molar ratio of manganese acetate to citric acid was 0.5:1, in the absence of KNO3, and the pH value of aqueous solution was adjusted to 6, followed by annealing the dry-gel at 300℃. And the as-prepared MnOx was acidified by H2SO₄, finally MnO₂ nano-fibers with slightly aggregate were obtained. In 0.5 mol·L^-1 Na2SO₄ aqueous electrolyte, the product could provide a specific capacitance of 133.58 F·g^-1 within the range of -0.15⁰.85 V (vs·SCE) at a discharge current density of 66.5 mA·g^-1.Under hydrothermal conditions, tunnel structuredα-MnO₂ was synthesized at lower molar ratios of KMnO₄ to MnSO₄ while layeredδ-MnO₂ at higher molar ratios, and the latter was more beneficial for the intercalation/deintercalation of H+ or Na+ because of its layer structure. When KMnO₄ and MnSO₄ with the molar ratio of 1:1 were treated hydrothermally at 120℃for 6h, uniformα-MnO₂ nanorods with diameters 20³0 nm and length up to 1.0μm were obtained. However, KMnO₄ and MnSO₄ with the molar ratio of 3:1 reacted at 120℃for 2h, the obtained sample wasδ-MnO₂ with poor crystallinity, and the average particle size was 8.3nm. The results showed that it possessed a specific capacitance of 259.19 F·g^-1 between -0.2 ¹.0 V (vs·SCE) in 0.5 mol·L^-1 Na2SO₄ aqueous electrolyte at a discharge current density of 70 mA·g^-1.Amorphousδ-MnO₂ was synthesized by simple coprecipitation of KMnO₄ to MnSO₄ with the molar ratio of 4:1 at room temperature. This material had a specific capacitance up to 290.03 F·g^-1 at a discharge current density of 94mA·g^-1 in the 0. 5 mol·L^-1 Na2SO₄ solution within -0.2 ¹.0 V (vs·SCE); and the specific capacitance could reach 195.49F·g^-1while the discharge current density was1880mA·g^-1. Its cycling behavior was studied by technique of chronopotentiomtry at the current density of 375mA·g^-1, the fist cycle showed the highest discharge specific capacitance of 227.84 F·g^-1, after 100 cycles it still possessed 200.12 F·g^-1, about 88% of its initial capacitance.