Deposition of MnO2 nanoneedles on carbon nanotubes and graphene nanosheets as electrode materials for electrochemical capacitors

Abstract

Hierarchical carbon nanotubes (CNTs) and graphene nanosheets (GNs) supported MnO2 nanoneedles have been synthesized through a facile chemical-wet route without any further thermal treatment. The electrochemical capacitive performances of MnO2-based composite electrodes are analyzed using cyclic voltammetry, galvanostatic charge–discharge cycling, and impedance spectroscopy. It is found that the electrochemical utilization of MnO2 nanoneedles is effectively enhanced by the synergistic effect that combines the MnO2 crystals and the carbon supports, raising more active sites available for formation of charge transfer and electric double-layer in the hybrid architecture. The maximal specific capacitance of capacitors attains as high as 440 F g−1. The specific energy of MnO2/GN capacitor can reach as high as ∼40 Wh kg−1 at a specific power of 20,000 W kg−1, analyzed by the Ragone plot. The improved performance can described to the fact that the robust design of hybrid structure is capable of (i) maximizing the utilization of MnO2 nanoneedles and (ii) leading to fast chemical reaction kinetics including ionic electro-sorption and charge transfer. This method provides a straightforward approach to deposit MnO2 onto GNs as electrode materials for various energy-storage devices.