Reduced graphene oxide aerogel with high-rate supercapacitive performance in aqueous electrolytes
- Weijiang Si†1,
- Xiaozhong Wu†1,
- Jin Zhou1,
- Feifei Guo1,
- Shuping Zhuo1Email author,
- Hongyou Cui1 and
- Wei Xing2Email author
© Si et al.; licensee Springer. 2013
Received: 6 April 2013
Accepted: 7 May 2013
Published: 21 May 2013
Reduced graphene oxide aerogel (RGOA) is synthesized successfully through a simultaneous self-assembly and reduction process using hypophosphorous acid and I2 as reductant. Nitrogen sorption analysis shows that the Brunauer-Emmett-Teller surface area of RGOA could reach as high as 830 m2 g−1, which is the largest value ever reported for graphene-based aerogels obtained through the simultaneous self-assembly and reduction strategy. The as-prepared RGOA is characterized by a variety of means such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Electrochemical tests show that RGOA exhibits a high-rate supercapacitive performance in aqueous electrolytes. The specific capacitance of RGOA is calculated to be 211.8 and 278.6 F g−1 in KOH and H2SO4 electrolytes, respectively. The perfect supercapacitive performance of RGOA is ascribed to its three-dimensional structure and the existence of oxygen-containing groups.
As a novel energy storage device that bridges the gap between conventional capacitors and batteries, supercapacitor has attracted much attention for its high power density and long cyclic life . The studies about supercapacitor mainly focus on the electrode materials such as transition metal oxides, conducting polymers, and particularly carbon materials that are perfect electrode materials because of their good conductivity, cyclic stability, and large specific surface area [2–4]. Carbon materials with different structures such as carbon nanotubes, carbon nanofibers, hierarchical porous carbons, and ordered mesoporous carbons are widely studied in recent years [5–8]. Apart from these carbon materials, graphene and graphene-based materials have also been widely studied as electrode materials of supercapacitor [9–13]. Graphene is a two-dimensional sheet of sp2-hybridized carbon, which possesses many remarkable properties such as high surface area, excellent mechanical strength, and low electrical resistivity [14, 15]. However, the practical preparation (chemical reduction process) of graphene-based material is often accompanied by the sacrifice of graphene surface area because the graphene layers are easy to restack through a π-π interaction during the chemical reduction process.
In order to obtain graphene-based material with high specific surface area, many researchers have prepared graphene-based materials with three-dimensional architecture. As a typical three-dimensional graphene-based material that has attracted much attention of researchers, graphene aerogel is often synthesized mainly through two strategies currently: self-assembly during reduction process [16–20] and post-reduction process after self-assembly [21–24]. Employing the first method, Xu et al. prepared graphene aerogel via self-assembly of graphene oxide during a hydrothermal reduction process at 180°C . Chen synthesized graphene aerogel using various reductants such as NaHSO3, Na2S, vitamin C, and HI . The specific surface area of the as-prepared graphene aerogels could only reach up to 512 m2 g−1 because the reduction of graphene oxide was accompanied by the elimination of oxygen-containing groups in aqueous solution. This could lead to the hydrophobility increase of reduced graphene oxide, thus resulting in the restacking of graphene sheets. Adopting the second method, we prepared the graphene aerogel with a superhigh C/O molar ratio by hydrogen reduction . Worsley et al. synthesized a graphene aerogel through the self-assembly process in a basic solution followed by thermal reduction under nitrogen atmosphere. The Brunauer-Emmett-Teller (BET) surface area of the as-prepared graphene aerogel could reach as high as 1,300 m2 g−1, which is the largest value ever reported in the literatures . Although the graphene aerogels possess large BET surface area when employing the second strategy, the preparation procedure is complex due to the separated self-assembly and reduction processes. It usually takes 72 h to finish the separate self-assembly process . How to produce graphene aerogel with high surface area in a simple way is still a challenge currently.
Apart from the high surface area, the surface properties should also be taken into consideration while graphene-based material is used as electrode material in supercapacitor. The existence of surface functional groups is the characteristic surface properties of graphene-based materials made by Hummers' method. Graphene materials with functional surface often have a better dispersibility in aqueous electrolyte. Moreover, these functional groups may also generate pseudocapacitance in aqueous electrolytes. Xu's study indicates that graphene oxide is more suitable for supercapacitor application than graphene due to the existence of pseudocapacitance generated from the oxygen-containing groups . Our previous work also shows that graphene oxide aerogel possesses a higher specific capacitance than graphene aerogel at low current densities in KOH electrolyte . Thus, it would be promising to prepare high surface area graphene-based aerogels with functional surface for supercapacitor applications.
Herein, we synthesize a partially reduced graphene oxide aerogel (RGOA) through a simultaneous self-assembly and reduction process using hypophosphorous acid (HPA) and I2 as the reductants. Nitrogen sorption analysis shows that the specific surface area of the as-prepared RGOA could reach as high as 830 m2 g−1, which is the largest specific surface area ever reported for graphene aerogels obtained through the simultaneous self-assembly and reduction strategy. Electrochemical tests show that RGOA exhibits a high-rate supercapacitive performance in aqueous electrolytes. The specific capacitance of the RGOA can reach 211.8 and 278.6 F g−1 in KOH and H2SO4 electrolytes, respectively.
Graphite powder was purchased from Qingdao Ruisheng Graphite Co., Ltd. (Shandong, China). All other chemicals were purchased from Shanghai Chemical Reagents Company (Shanghai, China) and used directly without further purification. Graphite oxide was prepared according to Hummers' method . Graphene oxide solution (5 mg mL−1) was acquired by dispersing graphite oxide in deionized water under ultrasonication. The reduced graphene oxide hydrogel was prepared according to Phams' method . In a typical experiment, 5 g I2 was dissolved in 100 g HPA solution (50 wt.%), and then a 100-mL graphene oxide solution was added and sonicated for 5 min before transferred into an oven and aged at 90°C for 12 h. The obtained product was washed twice with acetone in a Soxhlet extractor (ISOPAD, Heidelberg, Germany) for 12 h to get reduced graphene oxide gels. The wet gels were dried with supercritical CO2 to obtain reduced graphene oxide aerogel, which was labeled as RGOA.
The microstructure of the samples was characterized by X-ray diffraction (XRD, D8 Advance, Bruker Optik Gmbh, Ettlingen, Germany) and Raman spectroscopy (RM2000, Renishaw, Gloucestershire, UK). The thickness of graphite oxide sheet was examined using an atomic force microscope (AFM, Multimode NS3A, Veeco Instruments Inc., Plainview, NY, USA). The microscopic morphology of the samples was observed using a scanning electron microscope (SEM, FEI, Eindhoven, The Netherlands) and a transmission electron microscope (TEM, JEOL2010, Akishima, Tokyo, Japan). The surface properties of the samples were characterized by X-ray photoelectron spectroscopy (XPS, Escalab 250, Thermo VG Scientific, Waltham, MA, USA) and Fourier transform infrared spectroscopy (FT-IR, Nicolet 5700, Thermo Electron Corporation, Waltham, MA, USA). Nitrogen sorption measurement was performed with an ASAP 2020M analyzer (Micromeritics, Norcross, GA, USA) to obtain the specific surface area and pore structure parameters of the sample.
Working electrodes were made by pressing RGOA onto the nickel foam and titanium mesh for 6 M KOH and 1 M H2SO4 electrolytes, respectively. The mass of active materials in each electrode was about 2 mg. In order to ensure that the electrode materials were thoroughly wetted with the electrolyte, the working electrodes were vacuum-impregnated with the electrolytes before electrochemical tests. The electrochemical capacitive performances of the sample were studied on a CHI660D electrochemical workstation. Electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS) were performed in a three-electrode system using a platinum film as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. Potential windows of −1 ~ 0 V and 0 ~ 1 V vs. SCE reference electrode were applied to the electrochemical measurements in KOH and H2SO4 electrolytes, respectively. In addition, the electrochemical performance of RGOA was also evaluated using a two-electrode system in H2SO4 electrolyte with a potential window of 0 ~ 1.2 V.
Results and discussion
Evolution of surface properties
Electrochemical capacitive performances
In addition, the current density at each scan rate in H2SO4 electrolyte is higher than that in KOH electrolyte, which indicates that oxygen-containing groups exhibit more pseudocapacitance in acid electrolyte. Therefore, as shown in Figure 4b, the specific capacitance calculated from CV curves displays that RGOA possesses larger capacitance in H2SO4 electrolyte when the scan rates are lower than 100 mV s−1. However, RGOA maintains a higher capacitance in KOH electrolyte when the scan rates exceed 100 mV s−1, which is probably due to the higher ionic concentration of KOH electrolyte than that of H2SO4 electrolyte. The galvanostatic charge–discharge curves of RGOA in different electrolytes are composed of two parts: the first part is within the potential window of 0.0 ~ −0.3 V in KOH electrolyte and 0.6 ~ 1.0 V in H2SO4 electrolyte, which is attributed to the electric double-layer capacitance. The other part exhibits a longer duration time, indicating the existence of pseudocapacitance besides the electric double-layer capacitance. As shown in Figure 4d, capacitance retention ratios of RGOA remain 74% and 63% in KOH and H2SO4 electrolytes when current density increases from 0.2 to 20 A g−1, exhibiting a high-rate capacitive performance. This high-rate performance is mainly attributed to the three-dimensional structure, which is beneficial for the ionic diffusion of electrolyte to the inner pores of bulk material. As shown in Figure 4d, the specific capacitances are calculated to be 211.8 and 278.6 F g−1 in KOH and H2SO4 electrolytes at the current density of 0.2 A g−1. The specific capacitances per surface area are calculated to be 25.5 and 33.6 μF cm−2 in KOH and H2SO4 electrolytes, respectively, indicating more pseudocapacitance in H2SO4 electrolyte. These results coincide well with the cyclic voltammetry measurements.
A simultaneous self-assembly and reduction method is adopted to successfully synthesize the reduced graphene oxide aerogel with the specific surface area of 830 m2 g−1, which is the largest value ever reported for graphene-based aerogels obtained through the simultaneous self-assembly and reduction strategy. Systematic characterizations suggest that the as-prepared RGOA is a three-dimensional mesoporous material with functionalized surface. Electrochemical tests show that RGOA exhibits high-rate supercapacitive performance. Its specific capacitances reach as high as 211.8 and 278.6 F g−1 in KOH and H2SO4 electrolytes, respectively. The perfect supercapacitive performance of RGOA is ascribed to its three-dimensional structure and the existence of oxygen-containing groups.
This work was financially supported by the Natural Science Foundation of China (51107076), Distinguished Young Scientist Foundation of Shandong Province (JQ201215), China University of Petroleum (13CX02004A), Outstanding Young Scientist Foundation of Shandong Province (BS2009NJ014), and Key Sci-Tech Development Project of Shandong Province (2009GG10007006).
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