Anchoring CuO nanoparticles on nitrogen-doped reduced graphene oxide nanosheets as electrode material for supercapacitors
Introduction
As the result of increasing concern about environmental issues and the depletion of fossil fuels, alternative energy storage/conversion devices have attracted much attention over the past few years. Supercapacitors are considered as the most important energy storage device due to their higher power density and longer cycle life than those of secondary batteries, and greater energy density than that of conventional dielectric capacitors [1], [2], [3], [4], [5]. In order to design high performance electrode materials in terms of high power capability and relatively large energy density, it is of great importance to understand the charge storage mechanism of surpercapacitors. Generally, supercapacitors can be classified as: (1) electrical double layer capacitors (EDLCs) that are based on electrostatic charge diffusion and accumulation at the electrode/electrolyte interface. Owing to the fast and stable process between electrodes and electrolytes, EDLCs usually own ultrahigh power density and excellent cycle life [6], [7]. However, the energy density and specific capacitance are restricted by the poor electrical charge separation at electrode/electrolyte interface and limited surface area of active materials; (2) pseudocapacitors that are dominated by Faradaic reactions on electrode materials. Compared with EDLCs, the pseudocapacitors could provide excellent specific capacitances and high energy density [8], [9], [10]. But the poor electroconductivity of the pseudocapacitors materials inevitably results in the unsatisfactory rate capability and reversibility. Therefore, combining these two charge storage mechanisms together to fabricate hybrid capacitor electrode materials is promising to achieve remarkable capacitive performance in the future. Up to now, carbon materials [11], [12], [13] and metal oxides/hydroxides [14], [15], [16] are the two typical active materials for EDLCs and pseudocapacitors, respectively. The researches based on the composites of novel carbon materials such as carbon nanotube and graphene with transition metal oxides have shown enhanced electrochemical performance [17], [18], [19].
As one type of carbon materials, reduced graphene oxide (RGO) has attracted significant attention because of its large specific surface area and fascinating chemical, electronic and mechanical properties. In order to further increasing its conductivity and surface active sites, nitrogen doping has become a key-enabling technology to functionalize reduced graphene oxide [20], [21], [22]. It has been assumed that nitrogen functionalities can change the electron donor/acceptor characteristics of graphene or reduced graphene oxide, contributing to the enhancement of capacitance property. Nowadays, chemical vapor deposition (CVD) [23], thermal treatment with NH3 [24], [25], nitrogen plasma treatment [26], and hydrazine hydrate treatment [27] are commonly used to fabricate nitrogen-doped graphene or reduced graphene oxide nanosheets. However, some problems and challenges still remain. For example, the nitrogen precursors of NH3 and pyridine used in the CVD process are toxic, and careful treatments are necessary. In the preparation of nitrogen-doped graphene, special equipments and rigorous conditions are certainly required since the preparation process is much more complicated. In addition, it is difficult to prepare nitrogen-doped graphene with uniform and high-concentration nitrogen doping when using NH3 as the N source. Therefore, it is of great interest to develop a simple, green and scalable method to synthesis N-RGO.
Herein, we report, for the first time, an easily-controllable, facile and scalable method to fabricate metal oxide/N-RGO composites for supercapacitor. In this paper, we chose CuO as a metal oxide material due to its high theoretical specific capacity, excellent chemical stability, low cost and non-toxicity [28], [29]. CuO/N-RGO composite is prepared by refluxing in ammonia solution, followed by a low temperature annealing. The effect of the loadings of CuO on the electrochemical performance has also been investigated. The as-obtained CuO/N-RGO composite with 15.1Ā wt% CuO loading electrode exhibits highest specific capacitance of 340Ā FĀ gā1 at 0.5Ā AĀ gā1 and excellent long cycle life.
Section snippets
Synthesis of CuO/N-RGO composite
Fig. 1 summarily describes the entire procedure for preparing CuO/N-RGO composite, which starts from the graphite oxide (GO) that was prepared by using a modified Hummers method [30]. Briefly, 1Ā g GO was dispersed into 60Ā mL isopropanol solution. The mixed solution was dealt with ultrasonic treatment for 3Ā h until the GO was evenly dispersed. Then, 5Ā mmol Cu(NO3)2 and 5Ā mmol NH4NO3 were added into 60Ā mL deionized water, with vigorous stirring for 30Ā min. Next, concentrated aqueous ammonia was added
Structure and morphology of the CuO/N-RGO composite
Fig. 2A shows the XRD patterns of GO and N-RGO. The most intensive peak (2ĪøĀ =Ā 12.1Ā°) of GO corresponds to the (0Ā 0Ā 1) diffraction. Based on the Bragg equation, owing to the introduction of oxygen-containing functional groups on the graphite sheets, the interlayer spacing (0.76Ā nm) of GO is much larger than that of pristine graphite (0.34Ā nm) [31]. After the GO was treated with ammonia and low temperature heating, a large part of oxygenic functional groups were removed from the nanosheets. The
Conclusions
In this study, a facile, green and cost-effective method has been developed to fabricate the CuO/N-RGO composites. Results show that the 10Ā mmol CuO/N-RGO composite demonstrates better specific capacitance (340Ā FĀ gā1 at 0.5Ā AĀ gā1) within a wide potential window of 1.4Ā V. In addition, the 10Ā mmol CuO/N-RGO composite also exhibits enhanced rate capability, improved electrochemical stability (ā¼80% after 500 cycles). The elevated capacitive performance can be attributed to the contributions of terrific
Conflict of interest
We declare that we have no conflict of interest.
Acknowledgements
We gratefully acknowledge the financial support of this research by the Heilongjiang Postdoctoral Fund (LBH-Z13059) and Fundamental Research Funds for the Central Universities (HEUCF20130910013).
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