Abstract
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.
Highlights
Electrochemical Capacitors (ECs) have been considered as promising electrochemical EnergyStorage Devices (ESDs) due to their inherent advantages, such as high power density, long cycle life, high safety factor and environmentally benign nature [1]
It was found that when the core-shell MnO2 @MnO2 composite was used as an electrode, well-dispersed amorphous MnO2 layers on α-MnO2 needles allowed a fast, reversible Faradic reaction and enabled ion diffusion
Scanning Electron Microscopy (SEM) images were obtained on Carl Zeiss Ultra Plus (Carl Zeiss Microscopy GmbH, Jena, Germany)
Summary
Electrochemical Capacitors (ECs) have been considered as promising electrochemical Energy. The authors found that the superior cycleability of the MnO2 @MnO2 material was due to the core-shell nanostructure in which the wire-like ß-MnO2 core provided a stable structural backbone [12]. Similar findings were recently reported by Ma and co-workers who showed that the α-MnO2 nanowires acted as the stable backbone for δ-MnO2 nano-sheets to form a hierarchical structured composite which exhibited excellent cycling stability values, e.g., 98.1% retention after 10,000 charge-discharge cycles [13]. It was found that when the core-shell MnO2 @MnO2 composite was used as an electrode, well-dispersed amorphous MnO2 layers on α-MnO2 needles allowed a fast, reversible Faradic reaction and enabled ion diffusion (due to the porous nature of the material). Our designed α-MnO2 @amorphous MnO2 hierarchical electrode exhibited a long-term stability during the cycling tests together with high specific capacitance and rate capability values
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