Enhanced Cycleability of Amorphous MnO2 by Covering on α-MnO2 Needles in an Electrochemical Capacitor

An allomorph MnO₂@MnO₂ 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 N₂ adsorption-desorption isotherms experiments indicated that this unique architecture consisted of a porous layer of amorphous-MnO₂ nano-sheets which were well grown onto the surface of α-MnO₂ nano-needles. Cyclic voltammetry experiments revealed that the double-layer charging and Faradaic pseudo-capacity of the MnO₂@MnO₂ 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 MnO₂@MnO₂ 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 MnO₂-based capacitor electrodes. It was also found that the remarkable cycleability of the MnO₂@MnO₂ was due to the supporting role of α-MnO₂ nano-needle core and the outer amorphous MnO₂ layer.