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Co3O4 composite for supercapacitors
Electrochimica Acta 55 (2010) 6973–6978
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Rapid microwave-assisted synthesis of graphene nanosheet/Co3 O4 composite for supercapacitors Jun Yan a,c , Tong Wei a , Wenming Qiao b , Bo Shao a , Qiankun Zhao a , Lijun Zhang c , Zhuangjun Fan a,∗ a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China c College of Automation, Harbin Engineering University, Harbin 150001, China
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Article history: Received 7 March 2010 Received in revised form 25 June 2010 Accepted 29 June 2010 Available online 6 July 2010 Keywords: Microwave-assisted Graphene nanosheets Cobalt oxide Supercapacitor
a b s t r a c t Graphene nanosheet (GNS)/Co3 O4 composite has been rapidly synthesized by microwave-assisted method. Field emission scanning electron microscopy and transmission electron microscopy observation reveals the homogeneous distribution of Co3 O4 nanoparticles (3–5 nm in size) on graphene sheets. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. A maximum specific capacitance of 243.2 F g−1 has been obtained at a scan rate of 10 mV s−1 in 6 M KOH aqueous solution for GNS/Co3 O4 composite. Furthermore, the composite exhibits excellent long cycle life along with ∼95.6% specific capacitance retained after 2000 cycle tests. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Climate change and the decreasing availability of fossil fuels call for not only urgent development of sustainable, renewable resources and emission control of global warming gases, but also more advanced energy storage and management devices worldwide [1,2]. Supercapacitors (also called electrochemical capacitors or ultracapacitors), attracted much attention because of their pulse power supply, long cycle life (>100,000 cycles), simple principle, and high dynamic of charge propagation, are probably the most important next generation energy storage device [3,4]. With a high power capability and relatively large energy density compared to conventional capacitors, supercapacitors offer a promising approach to meet the increasing power demands of energy storage systems in the 21st century. Currently, supercapacitors have played an important role in complementing or replacing batteries in the energy storage field, such as for uninterruptible power supplies, consumer electronics and industrial power and energy management [1,3,4]. Graphene, a single layer of carbon atoms tightly packed into a two-dimensional honeycomb sp2 carbon lattice, has attracted tremendous attention from both the experimental and theoretical scientific communities in recent years [5–7]. Its intriguing
∗ Corresponding author. Tel.: +86 451 82569890; fax: +86 451 82569890. E-mail address: [email protected] (Z. Fan). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.081
properties such as large thermal conductivity, superior mechanical properties, superior chemical stability, large surface-to-volume ratio and unusual electronic properties [5,6], holds great promise for potential applications in many technological fields such as nanoelectronics [8], sensors [9,10], nanocomposites [11,12], batteries [13], supercapacitors [14] and hydrogen storage [15]. More recently, graphene-based materials as electrodes for supercapacitors have been reported with the specific capacitance ranging from 117 to 205 F g−1 in aqueous solution [14,16,17]. It is wellknown that metal oxide such as RuO2 , IrO2 , MnO2 and NiOx can improve the electrochemical performance of carbon-based supercapacitors, as they can contribute pseudocapacitance to the total capacitance apart from the double-layer capacitance from carbon materials. Recently, graphene nanosheet (GNS)/ZnO and GNS/SnO2 composites have been synthesized for supercapacitors in aqueous electrolyte [18,19], however, their specific capacitance is very low (11–43 F g−1 ) and still need to be enhanced. Cobalt oxide is reported to be a promising electrode material for supercapacitors because of its relatively low cost, high redox activity, high theoretical specific capacitance (∼3560 F g−1 ) and its great reversibility [20]. Nevertheless, to the best of our knowledge, there are few reports on the GNS/Co3 O4 composite as electrode materials for supercapacitors. In this paper, GNS/Co3 O4 composite was synthesized by microwave-assisted method. GNS/Co3 O4 composite exhibited high specific capacitance (243.2 F g−1 at 10 mV s−1 ) and excellent long cycle life (∼95.6% capacitance retained after 2000 cycles). Furthermore, the effect of microstructure on electrochemical performances of the composite was also investigated.
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2. Experimental 2.1. Synthesis of GNS/Co3 O4 composite All the chemicals were of analytical grade and were used without further purification. Graphite oxide (GO) was synthesized from natural graphite (Qingdao Graphite Company) by a modified Hummers method as described elsewhere [21]. As-synthesized GO was suspended in water to give a brown dispersion, which was subjected to dialysis to completely remove residual salts and acids. The synthesis of GNS/Co3 O4 composite was performed by microwave-assisted method as follows. First, 1.28 g of cobalt nitrate hexahydrate and 1.32 g of urea were added into 200 mL of GO suspension (0.5 mg mL−1 ) and magnetically stirred for 10 min. Then the above mixture was heated using a household microwave oven (Haier, 2450 MHz, 700 W) for 10 min and then cooled to room temperature naturally. Subsequently, the brownish black precipitation was filtered, washed with distilled water and absolute alcohol for several times, and dried at 100 ◦ C for 12 h in a vacuum oven. Finally, the samples were calcined in a muffle furnace at a temperature of 320 ◦ C for 1 h in air, and then cooled to room temperature. The resulting black powder was collected for the following characterization. The mass percentage of Co3 O4 was about 24.4%, which was
calculated from the data of XPS analysis. For control experiments, graphene was prepared by reduction of GO with hydrazine hydrate as described elsewhere [21].
2.2. Characterization methods The crystallographic structures of the materials were determined by a powder X-ray diffraction system (XRD, TTR-III) equipped with Cu K␣ radiation ( = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with a monochromated Al K␣ radiation (h = 1486.6 eV). All XPS spectra were corrected using the C 1s line at 284.6 eV. Curve fitting and background subtraction were accomplished using Casa XPS version 2.3.13 software. Raman spectra were recorded with a Raman Station 400F (Perkin Elmer) using a near infrared laser operating at 785 nm with a CCD detector. The microstructure of the samples was investigated by a scanning electron microscopy (SEM, Camscan Mx2600FE) and transition electron microscopy (TEM, JEOL JEM2010). Specific surface area of the samples was characterized by physical adsorption of N2 at 77 K (Micromeritics ASAP 2010) and obtained by the Brunauer–Emmett–Teller (BET) method.
Fig. 1. (a) AFM image of exfoliated GO sheets on mica surfaces with height profiles. (b) TEM image of GO. SEM images of (c) GNS and (d) GNS/Co3 O4 composite. (e) TEM image of GNS/Co3 O4 composite, the inset is the high-resolution TEM image.
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2.3. Preparation of electrodes and electrochemical measurement The fabrication of working electrodes was carried out as follows. Briefly, the electroactive materials, carbon black and poly(tetrafluoroethylene) were mixed in a mass ratio of 75:20:5 and dispersed in ethanol. Then the resulting mixture was coated onto the nickel foam substrate (1 cm × 1 cm) with a spatula, which was followed by drying at 100 ◦ C for 12 h in a vacuum oven. All electrochemical measurements were done in a threeelectrode setup: Ni foam coated with GNS/Co3 O4 composite as the working electrode, platinum foil and saturated calomel electrode (SCE) as the counter and reference electrodes. The measurements were carried out in a 6 M KOH aqueous electrolyte at room temperature. Cyclic voltammograms (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were measured by a CHI 660C electrochemical workstation. CV tests were done between 0 and 0.4 V (vs. SCE) at different scan rates of 10, 20, 50 and 100 mV s−1 . Galvanostatic charge/discharge curves were measured in the potential range of 0–0.4 V (vs. SCE) at different current densities of 5, 10, 20 and 50 mA cm−2 , and EIS measurements were also carried out in the frequency range from 100 kHz to 0.1 Hz at open circuit potential with an ac perturbation of 5 mV.
3. Results and discussion 3.1. Microstructure characterizations The morphology and microstructure of the as-prepared products are shown in Fig. 1. It can be observed that GO has wrinkles and folds of edges with a lateral dimensions of about 0.5–1 m and an apparent thickness of ∼1 nm (Fig. 1a and b), which corresponds to a single layer GO. GNS (267 m2 g−1 ) agglomerate with each other to form irreversible layered dense aggregates through van der Waals interactions (Fig. 1c) due to the remove of oxygen-containing functionalities (epoxide, hydroxyl, carbonyl and carboxyl groups) [7,22]. In addition, a layered structure can be observed at the edge of the agglomerates. By contrast, for GNS/Co3 O4 composite (253 m2 g−1 ), after thermal treatment, GO is reduced into crumpled GNS [23]. GNS have loose morphology duo to the well-dispersed Co3 O4 particles on the graphene surfaces as nanoscale spacers inhibiting the agglomeration of graphene and increasing the spacing between adjacent graphene sheets (Fig. 1d). The surfaces of graphene sheets are rough on the nanoscopic scale with some wrinkles, which may be attributed to the residual functional groups and the resultant defects during the preparation of GO [24]. In addition, Co3 O4 nanoparticles can hardly be observed from the SEM image of GNS/Co3 O4 (Fig. 1c), indicating that the synthesized Co3 O4 particles are in nanoscale. But their presence is confirmed by TEM as shown in Fig. 1e, Co3 O4 particles with the size of 3–5 nm disperse fairly homogeneously on the surface of GNS. This may be favorable for the improvement of the electrochemical performance as an electrode for supercapacitors. Fig. 2 shows the XRD patterns of pristine natural graphite, GO, GNS and GNS/Co3 O4 composite. The most intense peak at around 2 = 10.0◦ corresponds to the (0 0 1) reflection of GO, and the interlayer spacing (0.88 nm) is much larger than that of natural graphite (about 0.34 nm) due to the introduction of oxygen-containing functional groups on the graphite sheets. GNS show a broad diffraction peak for C (0 0 2) at 2 = 24.8◦ representing an interlayer spacing of 0.36 nm, which is slightly higher than that of well-ordered natural graphite. This can be interpreted in terms of short-range order of the graphene sheets along the stacking direction and the residual functional groups which may be present between the graphene layers. Whereas, for GNS/Co3 O4 composite, the diffraction peak for C (0 0 2) is relatively low indicating that significant face-to-face stack-
Fig. 2. Typical XRD patterns of natural graphite, GO, GNS and GNS/Co3 O4 composite.
ing is absent [25] due to the introduction of Co3 O4 nanoparticles on both sides of graphene sheets. All the other diffraction peaks can be unambiguously indexed to the cubic spinel Co3 O4 with a lattice parameter of a = 8.01 Å, which are in good agreement with the standard values in the standard card of Co3 O4 (JCPDS card No. 42-1467) [26]. But the relative intensity of these peaks is very low, which demonstrates that the size of Co3 O4 particles is very small. Additionally, no peaks from other phases have been detected indicating that the product is of high purity. According to the Bragg formula, the calculated interplanar spacing d from the (2 2 0) reflection is about 0.290 nm, which is in good agreement with the high resolution TEM image (inset of Fig. 1e). Important information about the chemical composition of the products can be further provided by XPS measurements, as shown in Fig. 3. The binding energies obtained in the XPS analyses were corrected for specimen charging by referencing the C 1s peak to 284.60 eV. Fig. 3a shows the XPS spectra of GO and as-prepared GNS/Co3 O4 composite. Compared to that of GO, the XPS spectra of GNS/Co3 O4 composite exhibit a relatively low O 1s peak and two additional N 1s and Co 2p peaks (Fig. 3a), the N comes from the precursor of cobalt nitrate hexahydrate. The C 1s XPS spectrum of GO (Fig. 3b) clearly indicates a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups: the nonoxygenated ring C, the C in C O bonds, the carbonyl C and the carboxylate carbon (O C O). Although the C 1s XPS spectrum of the GNS/Co3 O4 composite also exhibits these same oxygen-containing functionalities, their peak intensities are much smaller than those in GO, confirming that most of the epoxide, hydroxyl, and carboxyl functional groups are successfully removed [21]. The Co 2p XPS spectra shows two major peaks with binding energies at 779.85 and 794.82 eV, corresponding to Co 2p3/2 and Co 2p1/2 , respectively, with a spin-energy separation of 14.97 eV (Fig. 3c), which is the characteristic of a Co3 O4 phase and in good agreement with the reported data [27,28]. This result is consistent with the XRD analysis as mentioned above. Raman spectroscopy is a non-destructive approach to characterize graphitic materials, in particular to determine ordered and disordered crystal structures of graphene [29]. Fig. 4 shows the Raman spectrum of GO and GNS/Co3 O4 composite. A broad D band (1350 cm−1 ) and a broad G band (1590 cm−1 ) are observed in the both samples. In the Raman spectrum, the G band represents the in-plane bond-stretching motion of the pairs of C sp2 atoms (the E2g phonons); while the D band corresponds to breathing modes of rings or K-point phonons of A1g symmetry [30,31]. Compared with GO, the decreased D/G intensity ratio for GNS/Co3 O4 com-
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Fig. 4. Raman spectra of GNS and GNS/Co3 O4 composites (excitation at 780 nm).
Fig. 5 shows the variation in the specific capacitance of asprepared samples as a function of scan rates. It can be found that the specific capacitance decreases with the increase of scan rates from 10 to 100 mV s−1 . This maybe because at high scan rates, diffusion limits the movement of electrolyte ions due to the time constraint, and only the outer active surface is utilized for charge storage. At lower scan rates, however, all the active surface area can be utilized for charge storage and the electrochemical utilization of Co3 O4 . The specific capacitance of GNS/Co3 O4 sample is much higher than that of pure graphene at the same scan rates. The maximum specific capacitance of 243.2 F g−1 is obtained at 10 mV s−1 in 6 M KOH aqueous solution for GNS/Co3 O4 composite, compared to 169.3 F g−1 for pure GNS. The improvement is probably attributed to the unique structure of composite. Firstly, the well-dispersed nanoscale Co3 O4 particles on graphene could not only effectively inhibit the stacking/agglomerating of GNS, resulting in high doublelayer capacitance, but also improve the electrochemical utilization of Co3 O4 . Secondly, GNS also provide a highly conductive network for electron transport during the charge and discharge processes. Finally, the excellent interfacial contact and increased contact area between Co3 O4 and GNS can significantly improve the accessibility of this composite to the electrolyte ions and shorten the ion diffusion and migration pathways. Therefore, the specific capacitance of the GNS/Co3 O4 composite is still 174.9 F g−1 at a scan rate of 100 mV s−1 . Fig. 6 shows the CV curves of GNS/Co3 O4 composite at different scan rates of 10, 20, 50 and 100 mV s−1 in 6 M KOH solution. It could
Fig. 3. (a) XPS survey spectra and (b) C 1s XPS spectra of GO and GNS/Co3 O4 composite. (c) Co 2p XPS spectra of GNS/Co3 O4 composite.
posite indicating the removal of oxygen-containing functionalities, which is in accordance with the XPS analysis. 3.2. Electrochemical behavior The specific capacitance of the electrode can be calculated according to the following equation:
C=
I dV
mV
(1)
where C is the specific capacitance based on the mass of electroactive materials (F g−1 ), I is the response current density (A cm−2 ), V is the potential (V), is the potential scan rate (mV s−1 ), and m is the mass of the electroactive materials in the electrodes (g).
Fig. 5. Specific capacitance of as-prepared GNS and GNS/Co3 O4 samples at different scan rates from 10 to 100 mV s−1 .
J. Yan et al. / Electrochimica Acta 55 (2010) 6973–6978
Fig. 6. CV curves of GNS/Co3 O4 composite at different scan rates of 10, 20, 50 and 100 mV s−1 in 6 M KOH solution.
be found that the area surrounded by CV curves for GNS/Co3 O4 electrode is apparently larger than that of the pure graphene electrode at the same san rate (see Fig. 6), implying the higher specific capacitance of GNS/Co3 O4 electrode. The shapes of the curves for GNS/Co3 O4 composite indicate that the capacitive characteristic is different from that of electric double-layer capacitance, for which the CV curve is close to a rectangular shape. For each curve, a pair of redox peaks with an anodic peak at around 0.14 V and a cathodic peak at about 0.28 V can be found, indicating that reversible and continuous faradic redox reactions of Co3 O4 are involved during charge and discharge processes. Within the potential scan range, the electrochemical reactions can be expressed as follows: Co3 O4 + OH− + H2 O 3CoOOH + e− −
−
CoOOH + OH CoO2 + H2 O + e
(2) (3)
It should also be noted that with an increase of scan rates, a positive shift of oxidation peaks and a negative shift of reduction peaks are observed, which is mainly due to the resistance of the electrode. Fig. 7 shows the galvanostatic charge/discharge curve of the asprepared GNS/Co3 O4 sample. There are two clear voltage stages are included in the charge/discharge curve: 0.4–0.2 and 0.2–0 V, respectively. The former exhibits short charge/discharge duration, which is purely ascribed to the electric double-layer capacitance
Fig. 7. Galvanostatic charge/discharge tests of GNS/Co3 O4 composite within the potential window of 0–0.4 V (vs. SCE) at a current density of 10 mA cm−2 .
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Fig. 8. Variation of the specific capacitance of GNS/Co3 O4 composite as a function of cycle number measured at 200 mV s−1 in 6 M KOH aqueous solution.
from the charge separation on the electrode/electrolyte interface. The latter is just different. Since Faradaic redox reaction is usually accompanied by the double-layer charge/discharge process, the combination of electric double-layer capacitance and Faradaic pseudocapacitance is responsible for the longer charge/discharge duration. Long cycle life of supercapacitors is a crucial parameter for their practical applications [1]. The long-term cycle stability of the GNS/Co3 O4 composite was also evaluated in this study by repeating the CV test between 0 and 0.4 V (vs. SCE) at a scan rate of 200 mV s−1 for 2000 cycles (Fig. 8). The electrode is found to exhibit excellent long cycle life over the entire cycle numbers. Interestingly, the specific capacitance of the sample increases by 13.9% after 250 cycle tests, instead of decreasing in most cycle life tests, which indicates that the electrode is fully activated and reach the optimum condition. After 2000 cycles, the capacitance decreases only 4.4% of initial capacitance demonstrating that GNS/Co3 O4 electrode exhibits excellent cycle stability and a very high degree of reversibility in the repetitive charge/discharge cycling. The reason may be that flexible GNS in composites can not only efficiently buffer the volume change of cobalt oxide during charge and discharge processes but also provide the high electrical conductivity of the overall electrode due to the high conductivity of graphene. The EIS analysis has been recognized as one of the principal methods examining the fundamental behavior of electrode materials for supercapacitors [32,33]. For further understanding, impedance of the GNS/Co3 O4 composite after the 1st and 2000th cycles were measured in the frequency range of 100 kHz to 0.1 Hz at open circuit potential with an ac perturbation of 5 mV (Fig. 9). It could be obviously seen that the impedance spectra are almost similar in shape, composed of one semicircle at high-frequency end followed by a linear part at the low-frequency end. The measured impedance spectra were analyzed using the CNLS fitting method [33] on the basis of the equivalent circuit, which is given in the inset of Fig. 9. At very high frequencies, the intercept at real part (Z ) is a combinational resistance of ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface (Re ) [34]. This value is almost the same for both electrodes. A major difference is the semicircle in the high-frequency range, which corresponds to the charge-transfer resistance (Rct ) caused by the Faradaic reactions and the doublelayer capacitance (Cdl ) on the grain surface. The slope of the 45◦ portion of the curve is called the Warburg resistance (ZW ) and is a result of the frequency dependence of ion diffusion/transport in the
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Acknowledgements This work was supported by the Key Project of Chinese Ministry of Education (No. 106011), the Fundamental Research Funds for the Central Universities (HEUCF101006) and the Key Project of National High-Tech Research and Development Plan of China (2008AA03A186). References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 9. Nyquist plots of experimental impedence data (scattering dot) and fitting results (solid line) for GNS/Co3 O4 electrode in the frequency range of 100 kHz to 0.1 Hz measured during the cycle life testing. Inset is the electrical equivalent circuit used for fitting impedance spectra.
[9] [10] [11]
Table 1 The calculated values of Re , Cdl , Rct , ZW and CL through CNLS fitting of the experimental impedance spectra based upon the proposed equivalent circuit in Fig. 9.
After 1st cycle After 2000th cycle
Re ()
Cdl (F)
Rct ()
ZW
CL (F)
0.3404 0.3388
0.4775 0.3291
1.668 5.603
0.08929 0.04698
0.002437 0.001335
electrolyte to the electrode surface [14]. CL is the limit capacitance [35,36]. The values of Re , Cdl , Rct , ZW and CL were calculated from CNLS fitting of the experimental impedance spectra and presented in Table 1. After 2000 cycles, the calculated charge-transfer resistance for the electrode is increased from 1.668 to 5.063 , which is probably due to the loss of adhesion of some active material with the current collector during the long-term charge/discharge cycling.
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
4. Conclusions In summary, GNS/Co3 O4 nanocomposite has been rapidly synthesized by microwave-assisted method. The well-dispersed nanoscale Co3 O4 particles (3–5 nm) on GNS greatly improves the electrochemical utilization of Co3 O4 and double-layer capacitance from the interconnected open channels between graphene layers. Moreover, GNS not only efficiently buffer the volume change of cobalt oxide during charge and discharge processes, but also preserve the high electrical conductivity of the overall electrode. Therefore, GNS/Co3 O4 composite exhibits high specific capacitance (243.2 F g−1 at 10 mV s−1 ) and excellent long cycle life (∼95.6% specific capacitance retained after 2000 cycles), suggesting very suitable and promising electrode materials for supercapacitors.
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Rapid microwave-assisted synthesis of graphene nanosheet/Co3 O4 composite for supercapacitors Jun Yan a,c , Tong Wei a , Wenming Qiao b , Bo Shao a , Qiankun Zhao a , Lijun Zhang c , Zhuangjun Fan a,∗ a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China c College of Automation, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 7 March 2010 Received in revised form 25 June 2010 Accepted 29 June 2010 Available online 6 July 2010 Keywords: Microwave-assisted Graphene nanosheets Cobalt oxide Supercapacitor
a b s t r a c t Graphene nanosheet (GNS)/Co3 O4 composite has been rapidly synthesized by microwave-assisted method. Field emission scanning electron microscopy and transmission electron microscopy observation reveals the homogeneous distribution of Co3 O4 nanoparticles (3–5 nm in size) on graphene sheets. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. A maximum specific capacitance of 243.2 F g−1 has been obtained at a scan rate of 10 mV s−1 in 6 M KOH aqueous solution for GNS/Co3 O4 composite. Furthermore, the composite exhibits excellent long cycle life along with ∼95.6% specific capacitance retained after 2000 cycle tests. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Climate change and the decreasing availability of fossil fuels call for not only urgent development of sustainable, renewable resources and emission control of global warming gases, but also more advanced energy storage and management devices worldwide [1,2]. Supercapacitors (also called electrochemical capacitors or ultracapacitors), attracted much attention because of their pulse power supply, long cycle life (>100,000 cycles), simple principle, and high dynamic of charge propagation, are probably the most important next generation energy storage device [3,4]. With a high power capability and relatively large energy density compared to conventional capacitors, supercapacitors offer a promising approach to meet the increasing power demands of energy storage systems in the 21st century. Currently, supercapacitors have played an important role in complementing or replacing batteries in the energy storage field, such as for uninterruptible power supplies, consumer electronics and industrial power and energy management [1,3,4]. Graphene, a single layer of carbon atoms tightly packed into a two-dimensional honeycomb sp2 carbon lattice, has attracted tremendous attention from both the experimental and theoretical scientific communities in recent years [5–7]. Its intriguing
∗ Corresponding author. Tel.: +86 451 82569890; fax: +86 451 82569890. E-mail address: [email protected] (Z. Fan). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.081
properties such as large thermal conductivity, superior mechanical properties, superior chemical stability, large surface-to-volume ratio and unusual electronic properties [5,6], holds great promise for potential applications in many technological fields such as nanoelectronics [8], sensors [9,10], nanocomposites [11,12], batteries [13], supercapacitors [14] and hydrogen storage [15]. More recently, graphene-based materials as electrodes for supercapacitors have been reported with the specific capacitance ranging from 117 to 205 F g−1 in aqueous solution [14,16,17]. It is wellknown that metal oxide such as RuO2 , IrO2 , MnO2 and NiOx can improve the electrochemical performance of carbon-based supercapacitors, as they can contribute pseudocapacitance to the total capacitance apart from the double-layer capacitance from carbon materials. Recently, graphene nanosheet (GNS)/ZnO and GNS/SnO2 composites have been synthesized for supercapacitors in aqueous electrolyte [18,19], however, their specific capacitance is very low (11–43 F g−1 ) and still need to be enhanced. Cobalt oxide is reported to be a promising electrode material for supercapacitors because of its relatively low cost, high redox activity, high theoretical specific capacitance (∼3560 F g−1 ) and its great reversibility [20]. Nevertheless, to the best of our knowledge, there are few reports on the GNS/Co3 O4 composite as electrode materials for supercapacitors. In this paper, GNS/Co3 O4 composite was synthesized by microwave-assisted method. GNS/Co3 O4 composite exhibited high specific capacitance (243.2 F g−1 at 10 mV s−1 ) and excellent long cycle life (∼95.6% capacitance retained after 2000 cycles). Furthermore, the effect of microstructure on electrochemical performances of the composite was also investigated.
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2. Experimental 2.1. Synthesis of GNS/Co3 O4 composite All the chemicals were of analytical grade and were used without further purification. Graphite oxide (GO) was synthesized from natural graphite (Qingdao Graphite Company) by a modified Hummers method as described elsewhere [21]. As-synthesized GO was suspended in water to give a brown dispersion, which was subjected to dialysis to completely remove residual salts and acids. The synthesis of GNS/Co3 O4 composite was performed by microwave-assisted method as follows. First, 1.28 g of cobalt nitrate hexahydrate and 1.32 g of urea were added into 200 mL of GO suspension (0.5 mg mL−1 ) and magnetically stirred for 10 min. Then the above mixture was heated using a household microwave oven (Haier, 2450 MHz, 700 W) for 10 min and then cooled to room temperature naturally. Subsequently, the brownish black precipitation was filtered, washed with distilled water and absolute alcohol for several times, and dried at 100 ◦ C for 12 h in a vacuum oven. Finally, the samples were calcined in a muffle furnace at a temperature of 320 ◦ C for 1 h in air, and then cooled to room temperature. The resulting black powder was collected for the following characterization. The mass percentage of Co3 O4 was about 24.4%, which was
calculated from the data of XPS analysis. For control experiments, graphene was prepared by reduction of GO with hydrazine hydrate as described elsewhere [21].
2.2. Characterization methods The crystallographic structures of the materials were determined by a powder X-ray diffraction system (XRD, TTR-III) equipped with Cu K␣ radiation ( = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with a monochromated Al K␣ radiation (h = 1486.6 eV). All XPS spectra were corrected using the C 1s line at 284.6 eV. Curve fitting and background subtraction were accomplished using Casa XPS version 2.3.13 software. Raman spectra were recorded with a Raman Station 400F (Perkin Elmer) using a near infrared laser operating at 785 nm with a CCD detector. The microstructure of the samples was investigated by a scanning electron microscopy (SEM, Camscan Mx2600FE) and transition electron microscopy (TEM, JEOL JEM2010). Specific surface area of the samples was characterized by physical adsorption of N2 at 77 K (Micromeritics ASAP 2010) and obtained by the Brunauer–Emmett–Teller (BET) method.
Fig. 1. (a) AFM image of exfoliated GO sheets on mica surfaces with height profiles. (b) TEM image of GO. SEM images of (c) GNS and (d) GNS/Co3 O4 composite. (e) TEM image of GNS/Co3 O4 composite, the inset is the high-resolution TEM image.
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2.3. Preparation of electrodes and electrochemical measurement The fabrication of working electrodes was carried out as follows. Briefly, the electroactive materials, carbon black and poly(tetrafluoroethylene) were mixed in a mass ratio of 75:20:5 and dispersed in ethanol. Then the resulting mixture was coated onto the nickel foam substrate (1 cm × 1 cm) with a spatula, which was followed by drying at 100 ◦ C for 12 h in a vacuum oven. All electrochemical measurements were done in a threeelectrode setup: Ni foam coated with GNS/Co3 O4 composite as the working electrode, platinum foil and saturated calomel electrode (SCE) as the counter and reference electrodes. The measurements were carried out in a 6 M KOH aqueous electrolyte at room temperature. Cyclic voltammograms (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were measured by a CHI 660C electrochemical workstation. CV tests were done between 0 and 0.4 V (vs. SCE) at different scan rates of 10, 20, 50 and 100 mV s−1 . Galvanostatic charge/discharge curves were measured in the potential range of 0–0.4 V (vs. SCE) at different current densities of 5, 10, 20 and 50 mA cm−2 , and EIS measurements were also carried out in the frequency range from 100 kHz to 0.1 Hz at open circuit potential with an ac perturbation of 5 mV.
3. Results and discussion 3.1. Microstructure characterizations The morphology and microstructure of the as-prepared products are shown in Fig. 1. It can be observed that GO has wrinkles and folds of edges with a lateral dimensions of about 0.5–1 m and an apparent thickness of ∼1 nm (Fig. 1a and b), which corresponds to a single layer GO. GNS (267 m2 g−1 ) agglomerate with each other to form irreversible layered dense aggregates through van der Waals interactions (Fig. 1c) due to the remove of oxygen-containing functionalities (epoxide, hydroxyl, carbonyl and carboxyl groups) [7,22]. In addition, a layered structure can be observed at the edge of the agglomerates. By contrast, for GNS/Co3 O4 composite (253 m2 g−1 ), after thermal treatment, GO is reduced into crumpled GNS [23]. GNS have loose morphology duo to the well-dispersed Co3 O4 particles on the graphene surfaces as nanoscale spacers inhibiting the agglomeration of graphene and increasing the spacing between adjacent graphene sheets (Fig. 1d). The surfaces of graphene sheets are rough on the nanoscopic scale with some wrinkles, which may be attributed to the residual functional groups and the resultant defects during the preparation of GO [24]. In addition, Co3 O4 nanoparticles can hardly be observed from the SEM image of GNS/Co3 O4 (Fig. 1c), indicating that the synthesized Co3 O4 particles are in nanoscale. But their presence is confirmed by TEM as shown in Fig. 1e, Co3 O4 particles with the size of 3–5 nm disperse fairly homogeneously on the surface of GNS. This may be favorable for the improvement of the electrochemical performance as an electrode for supercapacitors. Fig. 2 shows the XRD patterns of pristine natural graphite, GO, GNS and GNS/Co3 O4 composite. The most intense peak at around 2 = 10.0◦ corresponds to the (0 0 1) reflection of GO, and the interlayer spacing (0.88 nm) is much larger than that of natural graphite (about 0.34 nm) due to the introduction of oxygen-containing functional groups on the graphite sheets. GNS show a broad diffraction peak for C (0 0 2) at 2 = 24.8◦ representing an interlayer spacing of 0.36 nm, which is slightly higher than that of well-ordered natural graphite. This can be interpreted in terms of short-range order of the graphene sheets along the stacking direction and the residual functional groups which may be present between the graphene layers. Whereas, for GNS/Co3 O4 composite, the diffraction peak for C (0 0 2) is relatively low indicating that significant face-to-face stack-
Fig. 2. Typical XRD patterns of natural graphite, GO, GNS and GNS/Co3 O4 composite.
ing is absent [25] due to the introduction of Co3 O4 nanoparticles on both sides of graphene sheets. All the other diffraction peaks can be unambiguously indexed to the cubic spinel Co3 O4 with a lattice parameter of a = 8.01 Å, which are in good agreement with the standard values in the standard card of Co3 O4 (JCPDS card No. 42-1467) [26]. But the relative intensity of these peaks is very low, which demonstrates that the size of Co3 O4 particles is very small. Additionally, no peaks from other phases have been detected indicating that the product is of high purity. According to the Bragg formula, the calculated interplanar spacing d from the (2 2 0) reflection is about 0.290 nm, which is in good agreement with the high resolution TEM image (inset of Fig. 1e). Important information about the chemical composition of the products can be further provided by XPS measurements, as shown in Fig. 3. The binding energies obtained in the XPS analyses were corrected for specimen charging by referencing the C 1s peak to 284.60 eV. Fig. 3a shows the XPS spectra of GO and as-prepared GNS/Co3 O4 composite. Compared to that of GO, the XPS spectra of GNS/Co3 O4 composite exhibit a relatively low O 1s peak and two additional N 1s and Co 2p peaks (Fig. 3a), the N comes from the precursor of cobalt nitrate hexahydrate. The C 1s XPS spectrum of GO (Fig. 3b) clearly indicates a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups: the nonoxygenated ring C, the C in C O bonds, the carbonyl C and the carboxylate carbon (O C O). Although the C 1s XPS spectrum of the GNS/Co3 O4 composite also exhibits these same oxygen-containing functionalities, their peak intensities are much smaller than those in GO, confirming that most of the epoxide, hydroxyl, and carboxyl functional groups are successfully removed [21]. The Co 2p XPS spectra shows two major peaks with binding energies at 779.85 and 794.82 eV, corresponding to Co 2p3/2 and Co 2p1/2 , respectively, with a spin-energy separation of 14.97 eV (Fig. 3c), which is the characteristic of a Co3 O4 phase and in good agreement with the reported data [27,28]. This result is consistent with the XRD analysis as mentioned above. Raman spectroscopy is a non-destructive approach to characterize graphitic materials, in particular to determine ordered and disordered crystal structures of graphene [29]. Fig. 4 shows the Raman spectrum of GO and GNS/Co3 O4 composite. A broad D band (1350 cm−1 ) and a broad G band (1590 cm−1 ) are observed in the both samples. In the Raman spectrum, the G band represents the in-plane bond-stretching motion of the pairs of C sp2 atoms (the E2g phonons); while the D band corresponds to breathing modes of rings or K-point phonons of A1g symmetry [30,31]. Compared with GO, the decreased D/G intensity ratio for GNS/Co3 O4 com-
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Fig. 4. Raman spectra of GNS and GNS/Co3 O4 composites (excitation at 780 nm).
Fig. 5 shows the variation in the specific capacitance of asprepared samples as a function of scan rates. It can be found that the specific capacitance decreases with the increase of scan rates from 10 to 100 mV s−1 . This maybe because at high scan rates, diffusion limits the movement of electrolyte ions due to the time constraint, and only the outer active surface is utilized for charge storage. At lower scan rates, however, all the active surface area can be utilized for charge storage and the electrochemical utilization of Co3 O4 . The specific capacitance of GNS/Co3 O4 sample is much higher than that of pure graphene at the same scan rates. The maximum specific capacitance of 243.2 F g−1 is obtained at 10 mV s−1 in 6 M KOH aqueous solution for GNS/Co3 O4 composite, compared to 169.3 F g−1 for pure GNS. The improvement is probably attributed to the unique structure of composite. Firstly, the well-dispersed nanoscale Co3 O4 particles on graphene could not only effectively inhibit the stacking/agglomerating of GNS, resulting in high doublelayer capacitance, but also improve the electrochemical utilization of Co3 O4 . Secondly, GNS also provide a highly conductive network for electron transport during the charge and discharge processes. Finally, the excellent interfacial contact and increased contact area between Co3 O4 and GNS can significantly improve the accessibility of this composite to the electrolyte ions and shorten the ion diffusion and migration pathways. Therefore, the specific capacitance of the GNS/Co3 O4 composite is still 174.9 F g−1 at a scan rate of 100 mV s−1 . Fig. 6 shows the CV curves of GNS/Co3 O4 composite at different scan rates of 10, 20, 50 and 100 mV s−1 in 6 M KOH solution. It could
Fig. 3. (a) XPS survey spectra and (b) C 1s XPS spectra of GO and GNS/Co3 O4 composite. (c) Co 2p XPS spectra of GNS/Co3 O4 composite.
posite indicating the removal of oxygen-containing functionalities, which is in accordance with the XPS analysis. 3.2. Electrochemical behavior The specific capacitance of the electrode can be calculated according to the following equation:
C=
I dV
mV
(1)
where C is the specific capacitance based on the mass of electroactive materials (F g−1 ), I is the response current density (A cm−2 ), V is the potential (V), is the potential scan rate (mV s−1 ), and m is the mass of the electroactive materials in the electrodes (g).
Fig. 5. Specific capacitance of as-prepared GNS and GNS/Co3 O4 samples at different scan rates from 10 to 100 mV s−1 .
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Fig. 6. CV curves of GNS/Co3 O4 composite at different scan rates of 10, 20, 50 and 100 mV s−1 in 6 M KOH solution.
be found that the area surrounded by CV curves for GNS/Co3 O4 electrode is apparently larger than that of the pure graphene electrode at the same san rate (see Fig. 6), implying the higher specific capacitance of GNS/Co3 O4 electrode. The shapes of the curves for GNS/Co3 O4 composite indicate that the capacitive characteristic is different from that of electric double-layer capacitance, for which the CV curve is close to a rectangular shape. For each curve, a pair of redox peaks with an anodic peak at around 0.14 V and a cathodic peak at about 0.28 V can be found, indicating that reversible and continuous faradic redox reactions of Co3 O4 are involved during charge and discharge processes. Within the potential scan range, the electrochemical reactions can be expressed as follows: Co3 O4 + OH− + H2 O 3CoOOH + e− −
−
CoOOH + OH CoO2 + H2 O + e
(2) (3)
It should also be noted that with an increase of scan rates, a positive shift of oxidation peaks and a negative shift of reduction peaks are observed, which is mainly due to the resistance of the electrode. Fig. 7 shows the galvanostatic charge/discharge curve of the asprepared GNS/Co3 O4 sample. There are two clear voltage stages are included in the charge/discharge curve: 0.4–0.2 and 0.2–0 V, respectively. The former exhibits short charge/discharge duration, which is purely ascribed to the electric double-layer capacitance
Fig. 7. Galvanostatic charge/discharge tests of GNS/Co3 O4 composite within the potential window of 0–0.4 V (vs. SCE) at a current density of 10 mA cm−2 .
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Fig. 8. Variation of the specific capacitance of GNS/Co3 O4 composite as a function of cycle number measured at 200 mV s−1 in 6 M KOH aqueous solution.
from the charge separation on the electrode/electrolyte interface. The latter is just different. Since Faradaic redox reaction is usually accompanied by the double-layer charge/discharge process, the combination of electric double-layer capacitance and Faradaic pseudocapacitance is responsible for the longer charge/discharge duration. Long cycle life of supercapacitors is a crucial parameter for their practical applications [1]. The long-term cycle stability of the GNS/Co3 O4 composite was also evaluated in this study by repeating the CV test between 0 and 0.4 V (vs. SCE) at a scan rate of 200 mV s−1 for 2000 cycles (Fig. 8). The electrode is found to exhibit excellent long cycle life over the entire cycle numbers. Interestingly, the specific capacitance of the sample increases by 13.9% after 250 cycle tests, instead of decreasing in most cycle life tests, which indicates that the electrode is fully activated and reach the optimum condition. After 2000 cycles, the capacitance decreases only 4.4% of initial capacitance demonstrating that GNS/Co3 O4 electrode exhibits excellent cycle stability and a very high degree of reversibility in the repetitive charge/discharge cycling. The reason may be that flexible GNS in composites can not only efficiently buffer the volume change of cobalt oxide during charge and discharge processes but also provide the high electrical conductivity of the overall electrode due to the high conductivity of graphene. The EIS analysis has been recognized as one of the principal methods examining the fundamental behavior of electrode materials for supercapacitors [32,33]. For further understanding, impedance of the GNS/Co3 O4 composite after the 1st and 2000th cycles were measured in the frequency range of 100 kHz to 0.1 Hz at open circuit potential with an ac perturbation of 5 mV (Fig. 9). It could be obviously seen that the impedance spectra are almost similar in shape, composed of one semicircle at high-frequency end followed by a linear part at the low-frequency end. The measured impedance spectra were analyzed using the CNLS fitting method [33] on the basis of the equivalent circuit, which is given in the inset of Fig. 9. At very high frequencies, the intercept at real part (Z ) is a combinational resistance of ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface (Re ) [34]. This value is almost the same for both electrodes. A major difference is the semicircle in the high-frequency range, which corresponds to the charge-transfer resistance (Rct ) caused by the Faradaic reactions and the doublelayer capacitance (Cdl ) on the grain surface. The slope of the 45◦ portion of the curve is called the Warburg resistance (ZW ) and is a result of the frequency dependence of ion diffusion/transport in the
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Acknowledgements This work was supported by the Key Project of Chinese Ministry of Education (No. 106011), the Fundamental Research Funds for the Central Universities (HEUCF101006) and the Key Project of National High-Tech Research and Development Plan of China (2008AA03A186). References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 9. Nyquist plots of experimental impedence data (scattering dot) and fitting results (solid line) for GNS/Co3 O4 electrode in the frequency range of 100 kHz to 0.1 Hz measured during the cycle life testing. Inset is the electrical equivalent circuit used for fitting impedance spectra.
[9] [10] [11]
Table 1 The calculated values of Re , Cdl , Rct , ZW and CL through CNLS fitting of the experimental impedance spectra based upon the proposed equivalent circuit in Fig. 9.
After 1st cycle After 2000th cycle
Re ()
Cdl (F)
Rct ()
ZW
CL (F)
0.3404 0.3388
0.4775 0.3291
1.668 5.603
0.08929 0.04698
0.002437 0.001335
electrolyte to the electrode surface [14]. CL is the limit capacitance [35,36]. The values of Re , Cdl , Rct , ZW and CL were calculated from CNLS fitting of the experimental impedance spectra and presented in Table 1. After 2000 cycles, the calculated charge-transfer resistance for the electrode is increased from 1.668 to 5.063 , which is probably due to the loss of adhesion of some active material with the current collector during the long-term charge/discharge cycling.
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
4. Conclusions In summary, GNS/Co3 O4 nanocomposite has been rapidly synthesized by microwave-assisted method. The well-dispersed nanoscale Co3 O4 particles (3–5 nm) on GNS greatly improves the electrochemical utilization of Co3 O4 and double-layer capacitance from the interconnected open channels between graphene layers. Moreover, GNS not only efficiently buffer the volume change of cobalt oxide during charge and discharge processes, but also preserve the high electrical conductivity of the overall electrode. Therefore, GNS/Co3 O4 composite exhibits high specific capacitance (243.2 F g−1 at 10 mV s−1 ) and excellent long cycle life (∼95.6% specific capacitance retained after 2000 cycles), suggesting very suitable and promising electrode materials for supercapacitors.
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