Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors

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Abstract

MnO2 nanowires were electrodeposited onto carbon nanotube (CNT) paper by a cyclic voltammetric technique. The as-prepared MnO2 nanowire/CNT composite paper (MNCCP) can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that the MNCCP electrode displayed specific capacitances as high as 167.5 F g−1 at a current density of 77 mA g−1. After 3000 cycles, the composite paper can retain more than 88% of initial capacitance, showing good cyclability. The CNT paper in the composite acted as a good conductive and active substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO2 could facilitate the contact of the electrolyte with the active materials, and thus increase the capacitance.

Introduction

Supercapacitors as one of the most important energy devices need to be designed with not only high energy density, but also high levels of important mechanical properties, especially flexibility to meet the various design and power needs of modern gadgets [1], [2]. From the materials point of view, three families of materials have been used in supercapacitors so far, that is, carbon, conducting polymer, and metal oxide [3], [4]. Each kind of material has its own advantages and disadvantages. Carbon materials have long cycle life and good mechanical properties (especially carbon nanotubes (CNTs)), but low specific capacitance [5]. Conducting polymers are famous for their high flexibility, but have poor cyclability and relatively low capacitance [6]. As for the metal oxides, MnO2 is considered as the most promising material for the next generation of supercapacitors because of its low cost, environmentally friendly nature, and ideal capacitor performance [7], [8], [9], [10]. The problem is that the real specific capacitance value of MnO2 is far from the theoretical specific capacitance value (1110 F g−1, calculated by transferring 1 mol electrons) [9b]. Moreover, it has been found that the thin film form has the highest specific capacitance, and the specific capacitance dramatically dropped when the MnO2 thickness increased [11]. It is evident that the capacitance strongly depends on the morphology and surface area. Therefore, considerable research has been done on the synthesis of nanostructured [12], [13], [14], [15] and porous [16], [17], [18], [19] MnO2 materials to improve the electrochemical performance. On the other hand, coating MnO2 onto conducting agents (e.g. carbon nanotubes) also results in significant improvement of electrochemical performance [20], [21], [22], [23], [24], [25]. However, these MnO2 or its composite materials that have been previously reported were mostly powder-based or non-flexible, which hindered the potential application in flexible supercapacitors.

Herein we used CNT paper which is prepared by a simple filtration method as the matrix and electrodeposited MnO2 nanowires into/onto the CNT paper to obtain a free-standing, totally flexible with mechanically tough, high-capacitance, and long-life electrode for supercapacitors.

Section snippets

Experimental section

Carbon nanotube (CNT) paper was prepared via filtration using double wall CNTs (DW0923, Carbon Nanotechnologies Incorporated, USA). The detailed procedure for preparing CNT paper is described elsewhere [26]. The MnO2 nanowires were electrodeposited by a cyclic voltammetric technique in the potential range between +0.60 and +0.30 V with reference to a standard calomel electrode (SCE) at the scan rate of 500 mV s−1 onto the carbon nanotube paper [15]. The pure MnO2 nanowires were also prepared by

Results and discussion

Fig. 1 shows SEM images and EDS patterns of the as-prepared MnO2 nanowire/CNT composite paper (MNCCP). Fig. 1a shows a cross-sectional view of the as-prepared sample. It can be seen that there are three different layers, labeled as layer 1 to layer 3. Layer 1, about 1 μm thick, which is the top layer of the electrode, is composed of MnO2 nanowires. The top view of layer 1 shown in Fig. 1b illustrates the cross-linked MnO2 nanowires, which are about 30 nm in diameter, in good agreement with

Conclusions

Free-standing, totally flexible MnO2 nanowire/CNT composite paper (MNCCP) was successfully prepared by electrochemical deposition of MnO2 nanowires into/onto CNT paper through a CV technique. The MNCCP electrode displayed high specific capacitance with good cyclability. Based on our studies, the improved electrochemical performance of the composite electrode could be (1) the CNT paper act as a highly conductive, flexible, and active substrate for supercapacitor electrode, and (2) the nanowire

Acknowledgments

Financial support provided by the Australian Research Council (ARC) through ARC Centre of Excellence funding (CE0561616) is gratefully acknowledged. The authors thank Dr. T. Silver at the University of Wollongong for critical reading of the manuscript.

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