Three-dimensional MnO₂ nanowire/ZnO nanorod arrays hybrid nanostructure for high-performance and flexible supercapacitor electrode

Highlights

• Binder-free flexible ZnO nanorod array/MnO₂ nanowire composite capacitor electrodes are grown on carbon cloth.

• The electrochemical performance of MnO₂ nanowire electrode is greatly enhanced with the supporting of ZnO nanorod arrays.

• The hybrid nanostructure electrode delivers a high specific capacitance of 746.7 F g⁻¹ at 2 mV s⁻¹.

• The specific capacitance of ZnO nanorod/MnO₂ nanowire hybrid electrode shows only losses 6.5% after 1000 cycles.

• The hybrid electrode demonstrates excellent mechanical stability under different bending angles.

Abstract

Pure MnO₂ nanowires and MnO₂ nanowire/ZnO nanorod array hybrid nanostructure grown on carbon cloth are synthesized through a low temperature solution method for flexible and high performance supercapacitor applications. The MnO₂ nanowire/ZnO nanorod hybrid nanostructured electrodes exhibit more than two times higher specific capacitance, and better capacitance retention than those of pure MnO₂ nanowire electrodes. For the three-dimensional MnO₂ nanowire/ZnO nanorod array hybrid electrode, a high specific capacitance of 746.7 F g⁻¹ (areal capacitance ∼41.5 mF cm⁻²) is obtained at a scan rate of 2 mV s⁻¹, while the specific capacitance of pure MnO₂ nanowire electrode is 319.6 F g⁻¹. The electrochemical impedance spectroscopy measurements also confirm MnO₂ nanowire/ZnO nanorod array hybrid electrode has better electrochemical character. The MnO₂ nanowire/ZnO nanorod array hybrid electrode shows great cycle stability, it only losses 6.5% of the initial capacitance after 1000 cycles. The energy density and power density of the hybrid electrode at 2 A g⁻¹ are 63.1 Wh kg⁻¹ and 950 W kg⁻¹, respectively. It is illustrated that the electrochemical performance of MnO₂ nanowire electrode has been greatly enhanced with the supporting of ZnO nanorod arrays.

Introduction

With the increasing power and energy demand in applications ranging from portable electronics to hybrid electric vehicles, it is essential for the utilization of clean and renewable energy. Of the various power source devices, supercapacitors have attracted considerable attention because of their unique characteristics, such as high power density, fast charge/discharge rates, and long cyclic life [1], [2], [3]. Many noble or transition metal oxides can show pseudocapacitive behavior [4], [5], [6], [7]. Among these transition metal oxides, manganese oxides (MnO₂) are regarded as one of the most candidated electrode materials for supercapacitors due to its high theoretical specific capacitance (∼1370 F g⁻¹), natural abundance, environmental friendliness and low cost [8], [9]. However, the poor conductivity (∼10⁻⁵ to 10⁻⁶ S cm⁻¹) of MnO₂ is a crucial factor to limit its electrochemical performance [10]. Additionally, the textural characteristics and crystal forms of MnO₂ also affect the electrochemical performances [11], [12]. To produce the best possible performances of MnO₂, great efforts have been devoted in designing and developing the new framework of electrode material. One effective and straightforward approach is to exploit binary or ternary composites based on MnO₂/electrical conductive materials [13], [14], [15], [16].

Novel three-dimensional hybrid nanostructured electrodes based on one-dimensional nanowire/nanorod arrays have attracted considerable attentions due to the synergic effect of three-dimensional nanostructures. One-dimensional nanorod arrays could not only serve as a conducting scaffold for supporting electrochemically active materials, but also serve as effective channels for electrons transport [17], [18]. Some hybrid nanostructures of TiO₂/MnO₂ [19], CoO/NiHON [20], CNT/RuO₂ [21], MnO₂/RGO/CNT [22] have been investigated for supercapacitor applications and improved performances have been obtained.

ZnO nanowire/nanorod is one of the most attractive functional semiconductor materials and has a small capacity, so it can function as efficient mechanical support and electron conducting pathway because of its high chemical stability, conductivity, and mechanical flexibility. Mao's group has reported ZnO@MnO₂ core@shell nanostructure electrode on titanium substrate, and obtains specific areal capacitances of be 31.30 mF cm⁻² [17]. Yang et al. has fabricated ZnO@MnO₂ core–shell nanocables, and it authentically improves the electrochemical performance of supercapacitor electrode by high temperature annealing and hydrogenating treatment [23].

In this paper, we demonstrate a novel binder-free and flexible supercapacitor electrode with only low temperature hydrothermal in situ grown processes. Three-dimensional MnO₂ nanowire/ZnO nanorod hybrid nanostructured arrays are in situ grown by hydrothermal processes on carbon cloth. The carbon cloth provides a good electrical conductive path and a light, flexible, and stable substrate for composites growth. As a binder-free flexible electrode for supercapacitor, the MnO₂ nanowire/ZnO nanorod array hybrid electrode exhibits a very high specific capacitance of 746.7 F g⁻¹ (areal capacitance ∼41.5 mF cm⁻²) and good cycling stability.