Rationally designed hierarchical MnO 2 -shell/ZnO-nanowire/carbon-fabric for high-performance supercapacitor electrodes

High-performance supercapacitor electrodes with a novel hierarchical structure of MnO2/ZnO/carbon-fabric were rationally designed, and prepared by a simple three-step-solution method. The design comprises ZnO nanowires radially grown on each micron-size fiber of a carbon-fabric electrode, with a thin MnO2 shell on each ZnO nano-core. This multi-scale hierarchical structure yields: (a) high specific area of pseudo-capacitive MnO2 to maximize specific capacitance; (b) effective MnO2-electrolyte interface to facilitate fast charging/discharging; and (c) conductive MnO2 ZnO-electrode path to reduce energy loss. In addition, the overall capacitor performance is optimized by choosing proper thickness of MnO2 shell and aspect ratio of ZnO nano-core. The design was realized and validated with the development of a simple three-step-solution method: (a) radial deposition of nano-ZnO on carbon fabric; (b) coating ZnO by a thin layer of carbon; and (c) reduction of MnO4- and replacement of this carbon overlayer by MnO2. With this design and method, high specific capacitance of 886 F g(-1) was found from electrodes with 5 nm MnO2 on ZnO having an average diameter of 50 nm and aspect ratio of 30. These samples showed specific energy of 16 Wh kg(-1) and specific power of 27 kW kg(-1) at current density of 20 mA cm(-2), and good long-term cycling stability. (C) 2014 Elsevier B.V. All rights reserved.