Enhanced Cycleability of Amorphous MnO 2 by Covering on α-MnO 2 Needles in an Electrochemical Capacitor

摘要

An allomorph MnO 2 @MnO 2 core-shell nanostructure was developed via a two-step aqueous reaction method. The data analysis of Scanning Electron Microscopy, Transmission Electron Microscopy, X-Ray Diffraction and N 2 adsorption-desorption isotherms experiments indicated that this unique architecture consisted of a porous layer of amorphous-MnO 2 nano-sheets which were well grown onto the surface of α-MnO 2 nano-needles. Cyclic voltammetry experiments revealed that the double-layer charging and Faradaic pseudo -capacity of the MnO 2 @MnO 2 capacitor electrode contributed to a specific capacitance of 150.3 F·g ?1 at a current density of 0.1 A·g ?1 . Long cycle life experiments on the as-prepared MnO 2 @MnO 2 sample showed nearly a 99.3% retention after 5000 cycles at a current density of 2 A·g ?1 . This retention value was found to be significantly higher than those reported for amorphous MnO 2 -based capacitor electrodes. It was also found that the remarkable cycleability of the MnO 2 @MnO 2 was due to the supporting role of α-MnO 2 nano-needle core and the outer amorphous MnO 2 layer.