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

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.