Hydrothermal preparation and the capacitance of hierarchical MnO2 nanoflower

Hierarchical MnO2 nanoflower has been simply prepared via a hydrothermal treatment technology in a mixed solution of KMnO4 and cetyltrimethylammonium hydroxide at 90 °C for 12 h. The morphology and structure of the obtained material are examined by XRD, SEM, TEM, HRTEM, SAED, and N2 adsorption–desorption. The electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy in 1.0 M Na2SO4 aqueous solution. The obtained material has layered birnessite-type structure and shows flower-like hierarchical morphology. The hierarchical nanostructure is assembled by manganese oxide nanosheets with a diameter of about 30 nm and a thickness of 3–5 nm. The hierarchical MnO2 nanoflower exhibits not only high specific capacitance of 347 F g−1, but also excellent cycle stability (97.5% capacitance retention after 10,000 cycles at a scan rate of 20 mV s−1). An asymmetric electrochemical capacitor based on the obtained hierarchical MnO2 nanostructures as a positive electrode and graphene as a negative electrode is assembled. The assembled asymmetrical electrochemical capacitor can cycle reversibly in a voltage of 0–1.6 V and gives a high energy density of 20.9 Wh kg−1 at a power density of 400 W kg−1.