Synthesis of Birnessite Type MnO2 Nanotubes and Their Application in Aqueous Supercapacitors

Potassium birnessite nanotubes have been made through a templated hydrothermal route with specific surface areas and pore volumes as high as 110 m(2) g(-1) and 0.54 cm(3) g(-1) respectively. After casting into electrode films and assembling into hybrid supercapacitor cells, good cyclability has been demonstrated, with high coulombic efficiencies, with changes in the electrodes during cycling being monitored through electrochemical impedance spectroscopy. High specific capacitances have been observed, with values approaching 350 F g(-1) at current densities of 0.1 A g(-1). Energy densities approaching 90 W h g(-1) have been achieved at low current densities, with power densities over 75000 W kg(-1) at high current densities.