Preparation of functionalized graphene sheets by a low-temperature thermal exfoliation approach and their electrochemical supercapacitive behaviors
Introduction
Supercapacitors have attracted considerable attention because they have higher power density and longer cycle life than common batteries [1]. On the basis of energy storage mechanisms, the supercapacitors are classified as faradaic pseudocapacitors and electrical double-layer capacitors (EDLCs) [1]. The faradaic pseudocapacitors store energy by a fast and reversible faradaic redox reaction at or near the electrode surface. Transition metal oxides, such as RuO2, NiO, Co3O4, and MnO2 [2], [3], [4], [5], [6], [7], have become promising alternative candidates as the electrodes of pseudocapacitors. In addition, EDLCs arise from the charge separation at the electrode/electrolyte interface. Porous carbonaceous materials are widely used for EDLCs because of their low cost and very high specific surface areas. Activated carbons, carbon blacks, carbon aerogels, carbon nanotubes, and mesoporous carbons have been investigated as electrodes in EDLCs [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].
Geim and Novoselov found an entirely new class of carbon which was named as graphene in 2004 [19]. Graphene, the basal plane of graphite, has attracted tremendous attention due to its unique physical, chemical, and mechanical properties [20], [21]. More recently, many meaningful research results about preparation of graphene as supercapacitor electrode materials have been obtained. Stoller et al. prepared graphene by means of suspending graphite oxide (GO) sheets in water and chemical reducing with hydrazine [22]. This kind of chemical modified graphene has many excellent performances, such as good conductivity, large BET surface area (705 m2 g−1), and good capacitance properties. The specific capacitance of the sample in 5.5 mol L−1 KOH aqueous electrolyte was about 135 F g−1 at a discharge current of 10 mA. Vivekchand et al. prepared the graphene by three different methods and investigated their electrochemical supercapacitor behaviors [23]. The highest specific capacitance value of the sample prepared by exfoliation of GO from CV method was about 117 F g−1 in 1 mol L−1 H2SO4 aqueous electrolyte. Wang et al. prepared graphene materials from GO and subsequently suffer a gas-based hydrazine reduction to restore the conducting carbon network [24]. The maximum specific capacitance value of the sample was 205 F g−1 at energy density of 28.5 Wh kg−1 in a 30 wt% KOH aqueous solution.
Recently, functionalized graphene sheets (FGS) have been prepared by thermal exfoliation of GO [25], [26], [27], [28]. It is well known that thermal exfoliation of GO, through oxidation-derived intercalation expansion and quick removal of functional groups, represents a very efficient approach for the preparation of graphene materials. McAllister et al. suggest a critical temperature of 550 °C for exfoliation of GO to occur [26]. However, the treatment temperature for exfoliation of GO is normally above 1000 °C in most of the reported work [25], [26], [28]. In this work, we prepared FGS using GO as the precursor via a low-temperature thermal exfoliation approach in air. The obtained samples possessed high BET surface areas and large specific capacitances. The sample was subsequently carbonized at higher temperatures in N2. Fourier transform infrared spectroscopy (FTIR) testing indicated that the surface of the FGS samples through low-temperature heat treatment in air had more functional groups than those of the samples with further high-temperature carbonization in N2. Although the BET surface areas of the carbonized samples increased evidently, the specific capacitances of the carbonized samples decreased obviously compared with the former. Whereas, the conductive performance of the carbonized samples improved significantly, which results in the high capacitance retention of the carbonized samples at large current density. The influence of functional groups on the surface of the FGS samples on the supercapacitive behaviors was discussed for the first time in this study.
Section snippets
Preparation of graphite oxide
GO was produced from natural graphite powders (universal grade, 99.985%) according to Hummers method [29], [30]. Firstly, natural graphite powders were treated by 5% HCl twice, then filtered, washed with distilled water thoroughly, and dried at 110 °C for 24 h. Secondly, graphite powders (10 g) were placed in cold (0 °C) concentrated H2SO4 (230 mL), KMnO4 (30 g) was added gradually with stirring and cooling, and the temperature of the solution was not allowed to go up to 20 °C. The mixture was stirred
Characterization of functionalized graphene sheets
GO is an oxidation product of graphite, and it has a lot of functional groups on the surface of carbon sheets, such as hydroxyl, carboxyl, and epoxyl groups [31], [32]. The XRD pattern of GO is shown in Fig. 1. The disappearance of the native graphite peak, between 2θ of 25° and 30°, reveals the complete oxidation of the starting graphite [25], [27]. The FGS samples were produced by thermal exfoliation of GO. For a successful exfoliation process, it increases the c-axis spacing by oxidation and
Conclusions
In summary, two kinds of FGS with high BET surface areas are produced by thermal exfoliation of GO. The first kind of FGS is obtained by thermal exfoliation of GO at low temperature in air. The second kind is prepared by the carbonization of the first kind of FGS at higher temperature in N2. The results of N2 adsorption–desorption analysis indicate that the second kind of FGS has higher BET surface areas than the first kind. The results of electrochemical tests indicate the following: the
Acknowledgments
This work was supported by Natural Science Foundation of Jiangsu Province (BK2006195), National Natural Science Foundation of China (50502020, 50701024), and Doctor Innovation Funds of Jiangsu Province (xm06-57). We thank Dr. Ping He of Energy Technology Research Institute of National Institute of Advanced Industrial Science and Technology of Japan for assistance.
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