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CoMoO4 nanocomposites with enhanced supercapacitor performance
Journal of Alloys and Compounds 616 (2014) 58–65
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microwave-assisted synthesis of graphene/CoMoO4 nanocomposites with enhanced supercapacitor performance Xiaowei Xu, Jianfeng Shen, Na Li, Mingxin Ye ⇑ Center of Special Materials and Technology, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 13 May 2014 Received in revised form 13 June 2014 Accepted 5 July 2014 Available online 17 July 2014 Keywords: Supercapacitor Cobalt molybdate Graphene Microwave Nanocomposite
a b s t r a c t A facile and efficient strategy for preparing reduced graphene oxide–cobalt molybdate (RGO/CoMoO4) nanocomposites assisted by microwave irradiation for the first time is demonstrated. The resulting nanocomposites are comprised of CoMoO4 nanoparticles that are well-anchored on graphene sheets by in situ reducing. The prepared RGO/CoMoO4 nanocomposites have been thoroughly characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy and X-ray photoelectron spectroscopy. Importantly, the prepared nanocomposites exhibit excellent electrochemical performance for supercapacitors. Results show that RGO/CoMoO4 nanocomposites exhibited much better electrochemical capability than pure-CoMoO4 and RGO/CoMoO4 for annealing. RGO/CoMoO4 nanocomposites with 37.4 wt% CoMoO4 content achieved a specific capacitance about 322.5 F g1 calculated from the CV plots at 5 mV s1, which was higher than that of pure-CoMoO4 (95.0 F g1) and RGO/CoMoO4 for annealing (102.5 F g1). The good electrochemical performance can be attributed to the synergistic effects of the individual components. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The fast-growing demand for energy and increasing concern about environmental problems urgently require the development of clean and high-efficient energy storage and conversion systems [1,2]. Supercapacitors, also known as electrochemical capacitors or ultracapacitors, have attracted considerable attention because of their high power density, fast charge–discharge process and long cycling stability [3]. Over the past decade, carbon materials, including graphene, carbon nanotube, active carbon, and mesoporous carbon, are widely used as electrode materials for supercapacitor applications [4,5]. However, the low energy density of supercapacitors limits their use in many important applications. Thus, growing interest in increasing their energy density has been extensively studied. Transition-metal oxides is considered as a kind of typical attractive supercapacitor material due to its multiple oxidation states that enable rich redox reactions for pseudocapacitance [6–9]. Among of these electrode materials, RuO2 is one of the most promising pseudocapacitive materials, which have shown the best performance [10,11]. Unfortunately, the high cost and rareness of Ru have greatly limited the commercial applications. Therefore, it
⇑ Corresponding author. E-mail address: [email protected] (M. Ye). http://dx.doi.org/10.1016/j.jallcom.2014.07.047 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
is still a great challenge to explore alternative and relatively low cost electrode materials with excellent properties. Recently, binary metal oxides such as NiCo2O4 [12,13], Zn2SnO4 [14], and CoFe2O4 [15], have shown improvements in electrochemical performance over single component oxides due to their achievable oxidation states and high electrical conductivities. Transition metal molybdates have been conceived gradually as promising, effective and scalable alternatives, since they offer many advantages such as low cost, abundant resources and environmental friendliness. In particular, cobalt molybdate (CoMoO4) has attracted great research interest because of its excellent catalytic and electrochemical characteristics [16–19]. As one of the most important compounds, CoMoO4 exhibits considerable specific capacitance and energy density. Mai et al. reported that the hierarchical MnMoO4/CoMoO4 heterostructures nanowires show a specific capacitance of 187.1 F g1 at a current density of 1 A g1 [17]. Because of the high surface/volume ratio, the hierarchical nanowires exhibit better electrochemical performance than CoMoO4 without MnMoO4 substrate. Xu et al. achieved the highest specific capacitance of 170 F g1 at 0.1 A g1 current density for the CoMoO4–CNTs composite with a high surface/volume ratio using carbon nanotube as the substrate [20]. Xia et al. used graphene as the substrate to get the high surface/volume ratio composites by hydrothermal method [21]. The prepared CoMoO4–graphene nanocomposites showed the highest specific capacitance of
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394.5 F g1 at 1 mV s1 scan rate. Most recently, Yu et al. developed a two-step process to prepare the nanohoneycomb-like strongly coupled CoMoO4–3D graphene hybrid, which exhibited excellent cyclic stability [22]. The excellent performances could be attributed to the 3D graphene network and the strong coupling between CoMoO4 and the 3D graphene. However, most processes for the synthesis of nanomaterials are even complicated and time-consuming. It is well-known that microwave synthesis has been developed as an important technique for preparing materials. Microwave heating often offers higher reaction rates and shorter reaction time compared to conventional heating processes. As such, microwave heating is becoming more and more popular. In this study, we present a one-pot, ‘‘green’’, effective microwave-assisted route for the synthesis of RGO/CoMoO4 nanocomposites on large scale in aqueous solution. For improving the electrochemical performance, finer structures with high surface/ volume ratios and more active sites are required. Graphene was used as the conductive substrate to obtain the high surface/volume ratio composites. Graphene oxide (GO) sheets, Na2MoO4 and Co(NO3)2 were used as the precursors. In the system, microwave irradiation not only provides sufficient heat needed for the reaction within very short time, but also reduces GO sheets to graphene sheets using ascorbic acid (Vc) as a reducer. And during the irradiation, the CoMoO4 nanoparticles were re-crystallized and simultaneously dispersed on the graphene sheets. To the best of our knowledge, this is the first report of the microwave-assisted synthesis of RGO/CoMoO4 nanocomposites. When evaluated as an electrode material, the resulting RGO/CoMoO4 nanocomposites material presented much improved electrochemical performance to the pure-CoMoO4, indicating that RGO/CoMoO4 could be a promising electrode material for supercapacitors.
2.3. Electrochemical measurement The working electrodes were prepared by thoroughly mixing the as-prepared composites, acetylene black, and poly(vinylidenefluoride) (PVDF) with the mass ratio 80:10:10, and were dispersed in N-methylpyrrolidone (NMP). The mixture was stirred adequately to form a homogeneous slurry, and then was coated and pressed onto nickel foam and dried under vacuum at 80 °C for 24 h. In a threeelectrode system, 6M KOH was used as the electrolyte, platinum foil and Ag/AgCl (KCl-saturated) electrodes were used as the counter and reference electrodes, respectively. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) measurements, and galvanostatic charge–discharge testing were performed by an electrochemical workstation (Autolab PGSTAT128 N). The scan rates of CV were in the range from 5 mV s1 to 100 mV s1 at the potential range of 0.6 to 0.4 V. EIS was recorded under the following conditions: AC voltage amplitude of 5 mV, frequency range of 1 105 to 0.1 Hz, and open circuit potential. The specific capacitance can be calculated from CV curves according to the following equation [25–28]:
R C¼
IdV
ð1Þ
mmV
where I is the current density, V is the potential, m is the potential scan rate, m is the mass of the electroactive materials in the electrodes.
3. Results and discussion Scheme 1 shows the formation process of the RGO/CoMoO4 nanocomposites. Microwave heating was favorable to promote the stoichiometric chemical reactions between the Co2+ and MoO42 ions, which are responsible for the formation of the crystalline CoMoO4 microcrystals, as shown in the following Eqs. (2)–(4): After dissolution of the salts at room temperature:
Na2 MoO4 2H2 OðsÞ CoðNO3 Þ2 6H2 OðsÞ
H2 Oð25 CÞ
2NaþðaqÞ þ MoO2 4ðaqÞ þ 2H2 O
ð2Þ
H2 Oð25 CÞ
Co2þ ðaqÞ þ 2NO3ðaqÞ þ 6H2 O
ð3Þ
!
!
After Microwave heating processing: 2. Experimental 2.1. Material synthesis All solvents and chemicals were of reagent quality and used as received without further purification. GO was obtained by the modified Hummers method as described elsewhere [23,24]. To prepare the hybrid RGO/CoMoO4 nanocomposites, the microwave-assisted one-pot synthesis was developed. In a typical procedure, 100 mg of GO was added to 100 mL deionized water. The mixture was sonicated for 1 h, followed by high-speed stirring for further 1 h. 100 mg of Vc was added to the GO solution to get part A. On the other hand, 1 mmol of Co(NO3)26H2O was dissolved in 20 mL deionized water, then 20 mL deionized water containing 1 mmol of Na2MoO42H2O was added dropwise and stirred for 2 h to form a pink suspension, which is denoted as part B. Subsequently, part A and part B were mixed. Then 1 mL of NH3H2O (25%) was added to the mixture. The mixture was placed in the center of a house-hold microwave oven (800 W) and irradiated for 5 min, and then the microwave was turned off and rested for 2 min, followed by turning on the microwave oven to irradiate the mixture for another 5 min. The product was collected by filtration, washed several times with deionized water and absolute ethanol respectively, and dried in a vacuum at 80 °C for 12 h. Free CoMoO4 was made through the same procedure except for adding GO.
2.2. Characterization Fourier transform infrared spectra (FTIR) were recorded on a Nicolet IS10 spectrometer. Solid samples were imbedded in KBr disks. The spectrum was generated, collected 16 times, and corrected for the background noise. Powder X-ray diffraction (XRD) analyses were performed on D/max-cB diffractometer using Cu Ka radiation. The Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514 nm laser excitation. Thermogravimetric analysis (TGA) was carried out in a Netzsch TG 209F1 under a nitrogen atmosphere at a heating rate of 10 °C/min. Before the tests, all the samples were carefully grinded to powders to ensure sufficient diffusion of heat. Morphological analyses of the samples were carried out on a scanning electron microscope (SEM, Philips XL30 FEG). X-ray photoelectron spectroscopy (XPS) was performed on XR 5 VG (UK) using a monochromatic Mg X-ray source.
2þ 2NaþðaqÞ þ MoO2 4ðaqÞ þ 2H2 O þ CoðaqÞ þ 2NO3ðaqÞ þ 6H2 O Microwave
! CoMoO4 þ 2NaþðaqÞ þ 2NO3ðaqÞ þ 8H2 O
ð4Þ
Completed the processing, the GO sheets transform into graphene through the reduction by Vc under microwave irradiation. Meanwhile, CoMoO4 particles are re-crystallized into nanoparticles, which are well-dispersed on graphene sheets. Fig. 1 shows FTIR spectra of GO, RGO, pure-CoMoO4 and RGO/CoMoO4. The characteristic IR features of GO indicate the presence of oxygen-containing functional groups on its surface (Fig. 1a). The characteristic bands at 1075 cm1, 1225 cm1, 1400 cm1, 1625 cm1 and 1725 cm1 correspond to the epoxy CAOAC stretching vibrations, the alkoxy CAO stretching peak, the OAH deformation of the CAO group and the carboxyl C@O and CAO stretching vibrations, respectively. The peak at 3200 cm1 is according to the AOH vibration stretching. As to RGO (Fig. 1b), after the GO was chemically reduced, the intensities of the FTIR peaks corresponding to the oxygen-containing functionalities decrease significantly, especially the peak at 3200 cm1 according to the AOH vibration. This result indicates that the majority of the oxygen-containing functionalities in GO have been successfully removed by the reduction process, which is consistent with the XRD result. Fig. 1c shows the typical CoMoO4 absorption features. As to RGO/CoMoO4 (Fig. 1d), some peaks of oxygen-containing functional groups such as epoxy still remains after reduction, which is beneficial to improve the dispersion of the composite and the stability of the CoMoO4 anchored onto the surface of RGO sheets. Besides, there still exhibit the typical absorption features of CoMoO4. This clearly confirms that CoMoO4 particles are strongly attached to the surface of RGO.
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Scheme 1. Illustration for the formation process of the RGO/CoMoO4 nanocomposites.
Fig. 1. FTIR spectra of GO (a), RGO (b), pure-CoMoO4 (c) and RGO/CoMoO4 (d).
Fig. 2 displays the typical XRD patterns of GO, pure-CoMoO4, RGO/CoMoO4 and the annealed sample of RGO/CoMoO4. The XRD pattern of GO reveals that the most intensive peak of GO appears at 2h = 10.5°, corresponding to an interlayer d-spacing of 0.8 nm.
Fig. 2. XRD patterns of GO (a), pure-CoMoO4 (b), RGO/CoMoO4 (c) and RGO/ CoMoO4-annealed (d).
Because of the introduction of oxygen-containing functional groups, the interlayer spacing is much wider than that of pristine graphite [29]. As for pure-CoMoO4 and RGO/CoMoO4, all the diffraction peaks are similar to the phase of CoMoO4, which are good in agreement with the previous study [21]. It is also well known that cobalt molybdate can occur in the forms of a-CoMoO4 and b-CoMoO4, and the transformation of a-CoMoO4 to b-CoMoO4 phase occurs between 330 °C and 410 °C [30,31]. The strongest peaks at 2h = 28° and 33.8° in the XRD patterns of pure-CoMoO4 and RGO/CoMoO4 are attributed to a-CoMoO4 (PDF No. 25-1434). Fig. 2d shows the XRD pattern of the annealed sample of RGO/ CoMoO4. The strongest diffraction peak at 2h = 26.5° corresponds to the reflection of (0 0 2) plane of the CoMoO4, which matches well to that for b-CoMoO4 (PDF reference No. 021-0868) [31]. Hence, after the high temperature anneal, the phase of CoMoO4 in RGO/ CoMoO4 transform from a-CoMoO4 and b-CoMoO4. However, the diffraction peaks of RGO did not appear. It is not surprising that the characteristic diffraction peaks of graphene were shielded by the strong diffraction peaks of CoMoO4. Raman spectroscopy, regarded as a powerful, nondestructive technology, has been extensively used for the characterization of carbonaceous materials. The Raman spectra of GO, RGO, pureCoMoO4 and RGO/CoMoO4 are showed in Fig. 3. In the spectrum
Fig. 3. Raman spectra of GO (a), RGO (b), pure-CoMoO4 (c) and RGO/CoMoO4 (d).
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Fig. 4. TGA curves of GO (a), RGO (b), pure-CoMoO4 (c) and RGO/CoMoO4 (d).
of GO, the typical features of carbon materials as the G band at 1580 cm1 (which is related to the first-order scattering of the E2g phonons of the graphitic structure of carbon atoms) and the D band peak at 1350 cm1 (which is attributed to local defects and disorder especially at the edges of graphene and graphite platelets) can be clearly seen [32,33]. As for the spectrum of RGO, the ratio of D/G slightly increased from 1.43 for GO to 1.73 for RGO, indicating that GO was reduced during the synthesis procedure. It is also supposed that the reduction caused an increase in the number of defects by the removal of oxygencontaining functional groups, which resulted in the increase of D band [34]. The Raman spectrum of CoMoO4 exhibits an intense peak at 921 cm1 along with some medium intensity peaks appearing at 867 cm1, 809 cm1, 658 cm1 and 350 cm1, which can be attributed to the characteristic peaks of CoMoO4 [35]. The main peaks of CoMoO4 and GO are presented in the Raman spectrum of RGO/CoMoO4, indicating the successful synthesis of
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their composites. Interestingly, all the peaks of CoMoO4 appear slightly shifted (925 cm1, 871 cm1, 809 cm1, 657 cm1 and 356 cm1), indicating that graphene act not as a basal plane for the crystalline growth of CoMoO4, but instead there is some sort of chemical/electrostatic interaction between the CoMoO4 and graphene. This interaction is usually considered as the synergistic, which plays an important role in the electrochemical behavior. The chemical composition of the as-prepared RGO/CoMoO4 nanocomposites are determined using TGA. Fig. 4 shows the TGA curves of GO, RGO, pure-CoMoO4 and RGO/CoMoO4 nanocomposites. GO shows a relatively poor thermal stability with a rather low onset temperature for pyrolysis of the labile oxygen-containing functional groups over 180–300 °C (Fig. 4a). As to the curve of RGO (Fig. 4b), since the residual Vc is absorbed on RGO, its weight loss before 700 °C is about 24%. It is known that the weight loss of pure RGO is usually about 5% [24], thus the Vc content on prepared RGO is about 19%. It can be seen that after CoMoO4 nanoparticles were bonded onto RGO, the weight loss decreased. From the TGA curve of RGO/CoMoO4 (Fig. 4d), comparing with GO, RGO and pure-CoMoO4, the weight percentage of CoMoO4 nanoparticles of RGO/CoMoO4 is about 37.4 wt%. The surface morphologies and microstructure of pure CoMoO4 and RGO/CoMoO4 composites are imaged by SEM, as shown in Fig. 5. It can be seen from Fig. 5a that the pure-CoMoO4 phase is in the shape of polygonal nanoplatelets. These CoMoO4 nanoplatelets appear to be oriented relatively randomly. After graphene is incorporated in CoMoO4 matrix, RGO/CoMoO4 composites display an interesting and distinctive morphology. Fig. 5b and c shows that the RGO/CoMoO4 composites contain fine-separated and randomly distributed CoMoO4 nanoparticles on the graphene sheets. And it can be clearly seen that there are no bulk nanoplatelets on the graphene sheets, but instead small nanoparticles. This is an indication that crystal growth of CoMoO4 is restrained in the presence of RGO. Furthermore, compared with nanoplatelets CoMoO4, the fine-separated CoMoO4 nanoparticles on the graphene sheets in RGO/CoMoO4 composites could lead to better electrochemical
Fig. 5. SEM images of CoMoO4 (a), RGO/CoMoO4 at low magnification (b), RGO/CoMoO4 at high magnification (c), and EDX spectrum of RGO/CoMoO4 (d).
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Fig. 6. XPS survey spectrum (a), C 1s core level spectrum (b), Mo 3d core level spectrum (c), Co 2p core level spectrum (d) and O 1s core level spectrum (e) of RGO/CoMoO4.
performance with an increase in surface area and number of active sites. The elemental composition of the resulting hybrid was determined by an energy-dispersive spectrum (EDS) (Fig. 5d). Several peaks corresponding to C, O, Co, and Mo elements (Au peak originating from sputtered Au for the conductive coating) are observed. In order to achieve a better understanding of the elemental composition and the oxidation state of the as-prepared RGO/CoMoO4, XPS measurements were performed. Fig. 6a shows the wide range survey spectrum of RGO/CoMoO4 composites, displaying four peaks situated at 781.8 eV, 531.2 eV, 284.6 eV
and 232.4 eV, indicating Co 2p, O 1s, C 1s, and Mo 3d levels, respectively. According to the semi-quantitative analysis of XPS, the C/O atomic ratio of GO was about 2.3 [36]. In the as-prepared RGO/CoMoO4 composite, the C/O atomic ratio was found to be 2.2, including the content of oxygen from CoMoO4. This observation further indicates that most of the oxygen-containing functional groups from GO were reduced during the microwave process, whereas a few of the residual functional groups provide a stable dispersion of the nanoparticles in the RGO/CoMoO4 composites [37]. Additionally, in the RGO/CoMoO4 composites,
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Fig. 7. CV plots at 5 mV s1 of pure-CoMoO4 and RGO/CoMoO4 (a). CV plots at different scan rates (5, 10, 20, 50, 100 mV s1) of RGO/CoMoO4 (b) and pure-CoMoO4 (c). The relationship between the specific capacitance and the scan rate of RGO/CoMoO4 and pure-CoMoO4 (d).
Fig. 8. Galvanostatic charge/discharge curves of RGO/CoMoO4 (a) and pure-CoMoO4 (b) at the current densities of 0.5, 1, 1.5, and 2 A g1.
the Co to Mo atomic ratio was 1.2, which is closely to the stoichiometry of CoMoO4, indicating that the formation of CoMoO4 and the reduction of GO in the synthesis route. Fig. 6b shows the C 1s core level spectrum of the as-prepared RGO/CoMoO4 composites, which can be curve-fitted into three peaks with binding energies of 284.8 eV, 287.2 eV and 289.2 eV, corresponding to the sp2 hybridized carbon in graphene, CAOH and O@CAOH functional groups, respectively [38]. The Mo 3d core level spectrum (Fig. 6c) shows two peaks with binding energies of 232.4 eV and 235.5 eV,
corresponding to Mo 3d5/2 and Mo 3d3/2, respectively. The binding energy peaks of Mo 3d are separated by 3.1 eV, which is in good agreement with previous study for Mo6+ [21]. The Co 2p3/2 corelevel spectrum (Fig. 6d) shows the binding energy of 781.8 eV, which is a signature of Co2+ oxidation state [21]. The O 1s core level spectrum (Fig. 6e) contains two peaks at binding energies of 530.8 eV and 532.5 eV, representing the O 1s level in CoMoO4. The peak at 532.5 eV is likely attributed to the surface OAH group [39] and the water molecule attached to the CoMoO4.
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Fig. 9. EIS measurement of pure-CoMoO4 and RGO/CoMoO4.
The CV plots of RGO/CoMoO4 and pure-CoMoO4 at the scan rate of 5 mV s1 are shown in Fig. 7a. The CV plots of both the RGO/CoMoO4 and pure-CoMoO4 are similar in nature, with two couples of redox peaks during the positive and negative sweep indicating the pseudocapacitive nature. In alkaline electrolyte, the redox peaks are associated with the reversible redox reaction between the Co(II)/Co(III) and Co(III)/Co(IV) states. Due to the high conductivity of graphene, the RGO/CoMoO4 nanocomposite exhibit the large current density responses at the same scan rate, indicating that the RGO/CoMoO4 electrode have a much higher specific capacitance. Fig. 7b and c show the CV plots of RGO/CoMoO4 and pure-CoMoO4 at different scan rates of 5, 10, 20, 50, 100 mV s1. It is noted that the total peak current density of RGO/CoMoO4 increases obviously with increasing potential scan rate, demonstrating the good rate property and excellent capacitance behavior. As shown in Fig. 7d, the pure-CoMoO4 exhibited specific capacitances of 95.0 F g1, 86.3 F g1, 56.6 F g1, 28.3 F g1, 15.1 F g1 at scan rates of 5 mV s1, 10 mV s1, 20 mV s1, 50 mV s1 and 100 mV s1, respectively. It is supposed that the gradual decreasing of the specific capacitance with increasing scan rate results from the slow redox reaction at high scan rate. However, the RGO/CoMoO4 nanocomposites exhibited increased specific capacitance of 322.5 F g1, 215.0 F g1, 186.9 F g1, 100.3 F g1, 58.8 F g1 at respective scan rates of 5 mV s1, 10 mV s1, 20 mV s1, 50 mV s1 and 100 mV s1. The highest electrochemical performance of the RGO/CoMoO4 may result from the fine-designed
nanostructure and the synergistic effects among graphene and CoMoO4. Firstly, the addition of graphene can greatly improve the electrical conductivity and control the morphology and nanostructure of CoMoO4 nanoparticles, which can lead to high electrode/electrolyte contact areas and high rates of electrode reaction, resulting in enhanced electrochemical performance [40]. Besides, graphene can also increase the double-layer capacitance contribution to the overall capacitance. Secondly, CoMoO4 nanoparticles may prevent the restacking of the graphene sheets and provide Faradaic processes to increase the total capacitance. Fig. 8a–b illustrates the galvanostatic charge/discharge plots of RGO/CoMoO4 and pure-CoMoO4. As can be seen in the galvanostatic charge/discharge plots, the discharge time of RGO/CoMoO4 is significantly longer compared with that of pure-CoMoO4 at both high and low current densities, indicating that RGO/CoMoO4 composites offer a much higher capacitance. This is coincidence with the CV results. With increasing current density, the specific capacitance gradually decreases, this is a consequence of the fact that at high currents, the electrolyte ions suffer from low diffusion and can get access to only some of the total available reaction sites, which results in an incomplete insertion reaction and low specific capacitance [38]. From the galvanostatic charge/discharge plots, the coulombic efficiency was calculated using the following equation:
Coulombic efficiency ðgÞ ¼ ðtD =t C Þ 100%
ð5Þ
where tD and tC correspond to the discharging time and charging time, respectively. The highest coulombic efficiencies of 93.9% and 94.4% were calculated at a discharge current density of 1.5 mA g1 for the pure-CoMoO4 and RGO/CoMoO4, respectively. For an ideal capacitor, the galvanostatic charge/discharge plots should be triangular in nature. However, the galvanostatic charge/discharge plots of both the pure-CoMoO4 and RGO/CoMoO4 deviate from linearity, indicating a pseudocapacitive nature of the electrode material, which also supports the CV test results. The electrochemical performances of the pure-CoMoO4 and RGO/CoMoO4 were further investigated by electron impedance spectroscopy (EIS). Fig. 9 shows the EIS plots of the pure-CoMoO4 and RGO/CoMoO4. The higher resistance of pure-CoMoO4 is due to the poor electrical conductivity of CoMoO4 in KOH electrolyte. Due to the introduction of graphene, RGO/CoMoO4 nanocomposites achieved the low resistance, indicating the better electrical conductivity of RGO/CoMoO4 nanocomposites than that of pureCoMoO4. Besides, the existence of graphene also provides easy access and more space for electrolyte diffusion. In order to investigate the effects of crystalline phase on the electrochemical performance of RGO/CoMoO4, the prepared RGO/CoMoO4 composites were annealed at 500 °C for 5 h in N2
Fig. 10. CV plots at different scan rates (5, 10, 20, 50, 100 mV s1) of RGO/CoMoO4 for annealing (a) and galvanostatic charge/discharge curves of RGO/CoMoO4 for annealing at the current densities of 0.5, 1, 1.5, and 2 A g1 (b).
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atmosphere. The CV plots and galvanostatic charge/discharge plots of the annealed sample are shown in Fig. 10. It is obvious that the specific capacitance decreased from 322.5 F g1 to 102.5 F g1 at 5 mV s1, and the discharge time is dramatically short for annealed RGO/CoMoO4 nanocomposites. According to the above results of electrochemical tests, we explain that the crystalline phase of CoMoO4 is important for the electrochemical performance of RGO/CoMoO4. The results also reveal that a-CoMoO4 is favorable to improve the electrochemical performance in comparison with b-CoMoO4. 4. Conclusion We have successfully developed a facile and efficient microwave-assisted reaction system to prepare RGO/CoMoO4 nanocomposites. The RGO sheets provide a favorable surface for depositing the CoMoO4 nanoparticles and preventing their agglomeration. In addition, graphene further enhances the conductivity of the composites and therefor provides fast charge transport. The asprepared nanocomposites exhibit higher specific capacitance, lower resistance and better rate capability than those of pureCoMoO4 and annealed RGO/CoMoO4, which makes it a promising candidate for next generation supercapacitors. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microwave-assisted synthesis of graphene/CoMoO4 nanocomposites with enhanced supercapacitor performance Xiaowei Xu, Jianfeng Shen, Na Li, Mingxin Ye ⇑ Center of Special Materials and Technology, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 13 May 2014 Received in revised form 13 June 2014 Accepted 5 July 2014 Available online 17 July 2014 Keywords: Supercapacitor Cobalt molybdate Graphene Microwave Nanocomposite
a b s t r a c t A facile and efficient strategy for preparing reduced graphene oxide–cobalt molybdate (RGO/CoMoO4) nanocomposites assisted by microwave irradiation for the first time is demonstrated. The resulting nanocomposites are comprised of CoMoO4 nanoparticles that are well-anchored on graphene sheets by in situ reducing. The prepared RGO/CoMoO4 nanocomposites have been thoroughly characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy and X-ray photoelectron spectroscopy. Importantly, the prepared nanocomposites exhibit excellent electrochemical performance for supercapacitors. Results show that RGO/CoMoO4 nanocomposites exhibited much better electrochemical capability than pure-CoMoO4 and RGO/CoMoO4 for annealing. RGO/CoMoO4 nanocomposites with 37.4 wt% CoMoO4 content achieved a specific capacitance about 322.5 F g1 calculated from the CV plots at 5 mV s1, which was higher than that of pure-CoMoO4 (95.0 F g1) and RGO/CoMoO4 for annealing (102.5 F g1). The good electrochemical performance can be attributed to the synergistic effects of the individual components. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The fast-growing demand for energy and increasing concern about environmental problems urgently require the development of clean and high-efficient energy storage and conversion systems [1,2]. Supercapacitors, also known as electrochemical capacitors or ultracapacitors, have attracted considerable attention because of their high power density, fast charge–discharge process and long cycling stability [3]. Over the past decade, carbon materials, including graphene, carbon nanotube, active carbon, and mesoporous carbon, are widely used as electrode materials for supercapacitor applications [4,5]. However, the low energy density of supercapacitors limits their use in many important applications. Thus, growing interest in increasing their energy density has been extensively studied. Transition-metal oxides is considered as a kind of typical attractive supercapacitor material due to its multiple oxidation states that enable rich redox reactions for pseudocapacitance [6–9]. Among of these electrode materials, RuO2 is one of the most promising pseudocapacitive materials, which have shown the best performance [10,11]. Unfortunately, the high cost and rareness of Ru have greatly limited the commercial applications. Therefore, it
⇑ Corresponding author. E-mail address: [email protected] (M. Ye). http://dx.doi.org/10.1016/j.jallcom.2014.07.047 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
is still a great challenge to explore alternative and relatively low cost electrode materials with excellent properties. Recently, binary metal oxides such as NiCo2O4 [12,13], Zn2SnO4 [14], and CoFe2O4 [15], have shown improvements in electrochemical performance over single component oxides due to their achievable oxidation states and high electrical conductivities. Transition metal molybdates have been conceived gradually as promising, effective and scalable alternatives, since they offer many advantages such as low cost, abundant resources and environmental friendliness. In particular, cobalt molybdate (CoMoO4) has attracted great research interest because of its excellent catalytic and electrochemical characteristics [16–19]. As one of the most important compounds, CoMoO4 exhibits considerable specific capacitance and energy density. Mai et al. reported that the hierarchical MnMoO4/CoMoO4 heterostructures nanowires show a specific capacitance of 187.1 F g1 at a current density of 1 A g1 [17]. Because of the high surface/volume ratio, the hierarchical nanowires exhibit better electrochemical performance than CoMoO4 without MnMoO4 substrate. Xu et al. achieved the highest specific capacitance of 170 F g1 at 0.1 A g1 current density for the CoMoO4–CNTs composite with a high surface/volume ratio using carbon nanotube as the substrate [20]. Xia et al. used graphene as the substrate to get the high surface/volume ratio composites by hydrothermal method [21]. The prepared CoMoO4–graphene nanocomposites showed the highest specific capacitance of
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394.5 F g1 at 1 mV s1 scan rate. Most recently, Yu et al. developed a two-step process to prepare the nanohoneycomb-like strongly coupled CoMoO4–3D graphene hybrid, which exhibited excellent cyclic stability [22]. The excellent performances could be attributed to the 3D graphene network and the strong coupling between CoMoO4 and the 3D graphene. However, most processes for the synthesis of nanomaterials are even complicated and time-consuming. It is well-known that microwave synthesis has been developed as an important technique for preparing materials. Microwave heating often offers higher reaction rates and shorter reaction time compared to conventional heating processes. As such, microwave heating is becoming more and more popular. In this study, we present a one-pot, ‘‘green’’, effective microwave-assisted route for the synthesis of RGO/CoMoO4 nanocomposites on large scale in aqueous solution. For improving the electrochemical performance, finer structures with high surface/ volume ratios and more active sites are required. Graphene was used as the conductive substrate to obtain the high surface/volume ratio composites. Graphene oxide (GO) sheets, Na2MoO4 and Co(NO3)2 were used as the precursors. In the system, microwave irradiation not only provides sufficient heat needed for the reaction within very short time, but also reduces GO sheets to graphene sheets using ascorbic acid (Vc) as a reducer. And during the irradiation, the CoMoO4 nanoparticles were re-crystallized and simultaneously dispersed on the graphene sheets. To the best of our knowledge, this is the first report of the microwave-assisted synthesis of RGO/CoMoO4 nanocomposites. When evaluated as an electrode material, the resulting RGO/CoMoO4 nanocomposites material presented much improved electrochemical performance to the pure-CoMoO4, indicating that RGO/CoMoO4 could be a promising electrode material for supercapacitors.
2.3. Electrochemical measurement The working electrodes were prepared by thoroughly mixing the as-prepared composites, acetylene black, and poly(vinylidenefluoride) (PVDF) with the mass ratio 80:10:10, and were dispersed in N-methylpyrrolidone (NMP). The mixture was stirred adequately to form a homogeneous slurry, and then was coated and pressed onto nickel foam and dried under vacuum at 80 °C for 24 h. In a threeelectrode system, 6M KOH was used as the electrolyte, platinum foil and Ag/AgCl (KCl-saturated) electrodes were used as the counter and reference electrodes, respectively. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) measurements, and galvanostatic charge–discharge testing were performed by an electrochemical workstation (Autolab PGSTAT128 N). The scan rates of CV were in the range from 5 mV s1 to 100 mV s1 at the potential range of 0.6 to 0.4 V. EIS was recorded under the following conditions: AC voltage amplitude of 5 mV, frequency range of 1 105 to 0.1 Hz, and open circuit potential. The specific capacitance can be calculated from CV curves according to the following equation [25–28]:
R C¼
IdV
ð1Þ
mmV
where I is the current density, V is the potential, m is the potential scan rate, m is the mass of the electroactive materials in the electrodes.
3. Results and discussion Scheme 1 shows the formation process of the RGO/CoMoO4 nanocomposites. Microwave heating was favorable to promote the stoichiometric chemical reactions between the Co2+ and MoO42 ions, which are responsible for the formation of the crystalline CoMoO4 microcrystals, as shown in the following Eqs. (2)–(4): After dissolution of the salts at room temperature:
Na2 MoO4 2H2 OðsÞ CoðNO3 Þ2 6H2 OðsÞ
H2 Oð25 CÞ
2NaþðaqÞ þ MoO2 4ðaqÞ þ 2H2 O
ð2Þ
H2 Oð25 CÞ
Co2þ ðaqÞ þ 2NO3ðaqÞ þ 6H2 O
ð3Þ
!
!
After Microwave heating processing: 2. Experimental 2.1. Material synthesis All solvents and chemicals were of reagent quality and used as received without further purification. GO was obtained by the modified Hummers method as described elsewhere [23,24]. To prepare the hybrid RGO/CoMoO4 nanocomposites, the microwave-assisted one-pot synthesis was developed. In a typical procedure, 100 mg of GO was added to 100 mL deionized water. The mixture was sonicated for 1 h, followed by high-speed stirring for further 1 h. 100 mg of Vc was added to the GO solution to get part A. On the other hand, 1 mmol of Co(NO3)26H2O was dissolved in 20 mL deionized water, then 20 mL deionized water containing 1 mmol of Na2MoO42H2O was added dropwise and stirred for 2 h to form a pink suspension, which is denoted as part B. Subsequently, part A and part B were mixed. Then 1 mL of NH3H2O (25%) was added to the mixture. The mixture was placed in the center of a house-hold microwave oven (800 W) and irradiated for 5 min, and then the microwave was turned off and rested for 2 min, followed by turning on the microwave oven to irradiate the mixture for another 5 min. The product was collected by filtration, washed several times with deionized water and absolute ethanol respectively, and dried in a vacuum at 80 °C for 12 h. Free CoMoO4 was made through the same procedure except for adding GO.
2.2. Characterization Fourier transform infrared spectra (FTIR) were recorded on a Nicolet IS10 spectrometer. Solid samples were imbedded in KBr disks. The spectrum was generated, collected 16 times, and corrected for the background noise. Powder X-ray diffraction (XRD) analyses were performed on D/max-cB diffractometer using Cu Ka radiation. The Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514 nm laser excitation. Thermogravimetric analysis (TGA) was carried out in a Netzsch TG 209F1 under a nitrogen atmosphere at a heating rate of 10 °C/min. Before the tests, all the samples were carefully grinded to powders to ensure sufficient diffusion of heat. Morphological analyses of the samples were carried out on a scanning electron microscope (SEM, Philips XL30 FEG). X-ray photoelectron spectroscopy (XPS) was performed on XR 5 VG (UK) using a monochromatic Mg X-ray source.
2þ 2NaþðaqÞ þ MoO2 4ðaqÞ þ 2H2 O þ CoðaqÞ þ 2NO3ðaqÞ þ 6H2 O Microwave
! CoMoO4 þ 2NaþðaqÞ þ 2NO3ðaqÞ þ 8H2 O
ð4Þ
Completed the processing, the GO sheets transform into graphene through the reduction by Vc under microwave irradiation. Meanwhile, CoMoO4 particles are re-crystallized into nanoparticles, which are well-dispersed on graphene sheets. Fig. 1 shows FTIR spectra of GO, RGO, pure-CoMoO4 and RGO/CoMoO4. The characteristic IR features of GO indicate the presence of oxygen-containing functional groups on its surface (Fig. 1a). The characteristic bands at 1075 cm1, 1225 cm1, 1400 cm1, 1625 cm1 and 1725 cm1 correspond to the epoxy CAOAC stretching vibrations, the alkoxy CAO stretching peak, the OAH deformation of the CAO group and the carboxyl C@O and CAO stretching vibrations, respectively. The peak at 3200 cm1 is according to the AOH vibration stretching. As to RGO (Fig. 1b), after the GO was chemically reduced, the intensities of the FTIR peaks corresponding to the oxygen-containing functionalities decrease significantly, especially the peak at 3200 cm1 according to the AOH vibration. This result indicates that the majority of the oxygen-containing functionalities in GO have been successfully removed by the reduction process, which is consistent with the XRD result. Fig. 1c shows the typical CoMoO4 absorption features. As to RGO/CoMoO4 (Fig. 1d), some peaks of oxygen-containing functional groups such as epoxy still remains after reduction, which is beneficial to improve the dispersion of the composite and the stability of the CoMoO4 anchored onto the surface of RGO sheets. Besides, there still exhibit the typical absorption features of CoMoO4. This clearly confirms that CoMoO4 particles are strongly attached to the surface of RGO.
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Scheme 1. Illustration for the formation process of the RGO/CoMoO4 nanocomposites.
Fig. 1. FTIR spectra of GO (a), RGO (b), pure-CoMoO4 (c) and RGO/CoMoO4 (d).
Fig. 2 displays the typical XRD patterns of GO, pure-CoMoO4, RGO/CoMoO4 and the annealed sample of RGO/CoMoO4. The XRD pattern of GO reveals that the most intensive peak of GO appears at 2h = 10.5°, corresponding to an interlayer d-spacing of 0.8 nm.
Fig. 2. XRD patterns of GO (a), pure-CoMoO4 (b), RGO/CoMoO4 (c) and RGO/ CoMoO4-annealed (d).
Because of the introduction of oxygen-containing functional groups, the interlayer spacing is much wider than that of pristine graphite [29]. As for pure-CoMoO4 and RGO/CoMoO4, all the diffraction peaks are similar to the phase of CoMoO4, which are good in agreement with the previous study [21]. It is also well known that cobalt molybdate can occur in the forms of a-CoMoO4 and b-CoMoO4, and the transformation of a-CoMoO4 to b-CoMoO4 phase occurs between 330 °C and 410 °C [30,31]. The strongest peaks at 2h = 28° and 33.8° in the XRD patterns of pure-CoMoO4 and RGO/CoMoO4 are attributed to a-CoMoO4 (PDF No. 25-1434). Fig. 2d shows the XRD pattern of the annealed sample of RGO/ CoMoO4. The strongest diffraction peak at 2h = 26.5° corresponds to the reflection of (0 0 2) plane of the CoMoO4, which matches well to that for b-CoMoO4 (PDF reference No. 021-0868) [31]. Hence, after the high temperature anneal, the phase of CoMoO4 in RGO/ CoMoO4 transform from a-CoMoO4 and b-CoMoO4. However, the diffraction peaks of RGO did not appear. It is not surprising that the characteristic diffraction peaks of graphene were shielded by the strong diffraction peaks of CoMoO4. Raman spectroscopy, regarded as a powerful, nondestructive technology, has been extensively used for the characterization of carbonaceous materials. The Raman spectra of GO, RGO, pureCoMoO4 and RGO/CoMoO4 are showed in Fig. 3. In the spectrum
Fig. 3. Raman spectra of GO (a), RGO (b), pure-CoMoO4 (c) and RGO/CoMoO4 (d).
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Fig. 4. TGA curves of GO (a), RGO (b), pure-CoMoO4 (c) and RGO/CoMoO4 (d).
of GO, the typical features of carbon materials as the G band at 1580 cm1 (which is related to the first-order scattering of the E2g phonons of the graphitic structure of carbon atoms) and the D band peak at 1350 cm1 (which is attributed to local defects and disorder especially at the edges of graphene and graphite platelets) can be clearly seen [32,33]. As for the spectrum of RGO, the ratio of D/G slightly increased from 1.43 for GO to 1.73 for RGO, indicating that GO was reduced during the synthesis procedure. It is also supposed that the reduction caused an increase in the number of defects by the removal of oxygencontaining functional groups, which resulted in the increase of D band [34]. The Raman spectrum of CoMoO4 exhibits an intense peak at 921 cm1 along with some medium intensity peaks appearing at 867 cm1, 809 cm1, 658 cm1 and 350 cm1, which can be attributed to the characteristic peaks of CoMoO4 [35]. The main peaks of CoMoO4 and GO are presented in the Raman spectrum of RGO/CoMoO4, indicating the successful synthesis of
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their composites. Interestingly, all the peaks of CoMoO4 appear slightly shifted (925 cm1, 871 cm1, 809 cm1, 657 cm1 and 356 cm1), indicating that graphene act not as a basal plane for the crystalline growth of CoMoO4, but instead there is some sort of chemical/electrostatic interaction between the CoMoO4 and graphene. This interaction is usually considered as the synergistic, which plays an important role in the electrochemical behavior. The chemical composition of the as-prepared RGO/CoMoO4 nanocomposites are determined using TGA. Fig. 4 shows the TGA curves of GO, RGO, pure-CoMoO4 and RGO/CoMoO4 nanocomposites. GO shows a relatively poor thermal stability with a rather low onset temperature for pyrolysis of the labile oxygen-containing functional groups over 180–300 °C (Fig. 4a). As to the curve of RGO (Fig. 4b), since the residual Vc is absorbed on RGO, its weight loss before 700 °C is about 24%. It is known that the weight loss of pure RGO is usually about 5% [24], thus the Vc content on prepared RGO is about 19%. It can be seen that after CoMoO4 nanoparticles were bonded onto RGO, the weight loss decreased. From the TGA curve of RGO/CoMoO4 (Fig. 4d), comparing with GO, RGO and pure-CoMoO4, the weight percentage of CoMoO4 nanoparticles of RGO/CoMoO4 is about 37.4 wt%. The surface morphologies and microstructure of pure CoMoO4 and RGO/CoMoO4 composites are imaged by SEM, as shown in Fig. 5. It can be seen from Fig. 5a that the pure-CoMoO4 phase is in the shape of polygonal nanoplatelets. These CoMoO4 nanoplatelets appear to be oriented relatively randomly. After graphene is incorporated in CoMoO4 matrix, RGO/CoMoO4 composites display an interesting and distinctive morphology. Fig. 5b and c shows that the RGO/CoMoO4 composites contain fine-separated and randomly distributed CoMoO4 nanoparticles on the graphene sheets. And it can be clearly seen that there are no bulk nanoplatelets on the graphene sheets, but instead small nanoparticles. This is an indication that crystal growth of CoMoO4 is restrained in the presence of RGO. Furthermore, compared with nanoplatelets CoMoO4, the fine-separated CoMoO4 nanoparticles on the graphene sheets in RGO/CoMoO4 composites could lead to better electrochemical
Fig. 5. SEM images of CoMoO4 (a), RGO/CoMoO4 at low magnification (b), RGO/CoMoO4 at high magnification (c), and EDX spectrum of RGO/CoMoO4 (d).
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Fig. 6. XPS survey spectrum (a), C 1s core level spectrum (b), Mo 3d core level spectrum (c), Co 2p core level spectrum (d) and O 1s core level spectrum (e) of RGO/CoMoO4.
performance with an increase in surface area and number of active sites. The elemental composition of the resulting hybrid was determined by an energy-dispersive spectrum (EDS) (Fig. 5d). Several peaks corresponding to C, O, Co, and Mo elements (Au peak originating from sputtered Au for the conductive coating) are observed. In order to achieve a better understanding of the elemental composition and the oxidation state of the as-prepared RGO/CoMoO4, XPS measurements were performed. Fig. 6a shows the wide range survey spectrum of RGO/CoMoO4 composites, displaying four peaks situated at 781.8 eV, 531.2 eV, 284.6 eV
and 232.4 eV, indicating Co 2p, O 1s, C 1s, and Mo 3d levels, respectively. According to the semi-quantitative analysis of XPS, the C/O atomic ratio of GO was about 2.3 [36]. In the as-prepared RGO/CoMoO4 composite, the C/O atomic ratio was found to be 2.2, including the content of oxygen from CoMoO4. This observation further indicates that most of the oxygen-containing functional groups from GO were reduced during the microwave process, whereas a few of the residual functional groups provide a stable dispersion of the nanoparticles in the RGO/CoMoO4 composites [37]. Additionally, in the RGO/CoMoO4 composites,
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Fig. 7. CV plots at 5 mV s1 of pure-CoMoO4 and RGO/CoMoO4 (a). CV plots at different scan rates (5, 10, 20, 50, 100 mV s1) of RGO/CoMoO4 (b) and pure-CoMoO4 (c). The relationship between the specific capacitance and the scan rate of RGO/CoMoO4 and pure-CoMoO4 (d).
Fig. 8. Galvanostatic charge/discharge curves of RGO/CoMoO4 (a) and pure-CoMoO4 (b) at the current densities of 0.5, 1, 1.5, and 2 A g1.
the Co to Mo atomic ratio was 1.2, which is closely to the stoichiometry of CoMoO4, indicating that the formation of CoMoO4 and the reduction of GO in the synthesis route. Fig. 6b shows the C 1s core level spectrum of the as-prepared RGO/CoMoO4 composites, which can be curve-fitted into three peaks with binding energies of 284.8 eV, 287.2 eV and 289.2 eV, corresponding to the sp2 hybridized carbon in graphene, CAOH and O@CAOH functional groups, respectively [38]. The Mo 3d core level spectrum (Fig. 6c) shows two peaks with binding energies of 232.4 eV and 235.5 eV,
corresponding to Mo 3d5/2 and Mo 3d3/2, respectively. The binding energy peaks of Mo 3d are separated by 3.1 eV, which is in good agreement with previous study for Mo6+ [21]. The Co 2p3/2 corelevel spectrum (Fig. 6d) shows the binding energy of 781.8 eV, which is a signature of Co2+ oxidation state [21]. The O 1s core level spectrum (Fig. 6e) contains two peaks at binding energies of 530.8 eV and 532.5 eV, representing the O 1s level in CoMoO4. The peak at 532.5 eV is likely attributed to the surface OAH group [39] and the water molecule attached to the CoMoO4.
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Fig. 9. EIS measurement of pure-CoMoO4 and RGO/CoMoO4.
The CV plots of RGO/CoMoO4 and pure-CoMoO4 at the scan rate of 5 mV s1 are shown in Fig. 7a. The CV plots of both the RGO/CoMoO4 and pure-CoMoO4 are similar in nature, with two couples of redox peaks during the positive and negative sweep indicating the pseudocapacitive nature. In alkaline electrolyte, the redox peaks are associated with the reversible redox reaction between the Co(II)/Co(III) and Co(III)/Co(IV) states. Due to the high conductivity of graphene, the RGO/CoMoO4 nanocomposite exhibit the large current density responses at the same scan rate, indicating that the RGO/CoMoO4 electrode have a much higher specific capacitance. Fig. 7b and c show the CV plots of RGO/CoMoO4 and pure-CoMoO4 at different scan rates of 5, 10, 20, 50, 100 mV s1. It is noted that the total peak current density of RGO/CoMoO4 increases obviously with increasing potential scan rate, demonstrating the good rate property and excellent capacitance behavior. As shown in Fig. 7d, the pure-CoMoO4 exhibited specific capacitances of 95.0 F g1, 86.3 F g1, 56.6 F g1, 28.3 F g1, 15.1 F g1 at scan rates of 5 mV s1, 10 mV s1, 20 mV s1, 50 mV s1 and 100 mV s1, respectively. It is supposed that the gradual decreasing of the specific capacitance with increasing scan rate results from the slow redox reaction at high scan rate. However, the RGO/CoMoO4 nanocomposites exhibited increased specific capacitance of 322.5 F g1, 215.0 F g1, 186.9 F g1, 100.3 F g1, 58.8 F g1 at respective scan rates of 5 mV s1, 10 mV s1, 20 mV s1, 50 mV s1 and 100 mV s1. The highest electrochemical performance of the RGO/CoMoO4 may result from the fine-designed
nanostructure and the synergistic effects among graphene and CoMoO4. Firstly, the addition of graphene can greatly improve the electrical conductivity and control the morphology and nanostructure of CoMoO4 nanoparticles, which can lead to high electrode/electrolyte contact areas and high rates of electrode reaction, resulting in enhanced electrochemical performance [40]. Besides, graphene can also increase the double-layer capacitance contribution to the overall capacitance. Secondly, CoMoO4 nanoparticles may prevent the restacking of the graphene sheets and provide Faradaic processes to increase the total capacitance. Fig. 8a–b illustrates the galvanostatic charge/discharge plots of RGO/CoMoO4 and pure-CoMoO4. As can be seen in the galvanostatic charge/discharge plots, the discharge time of RGO/CoMoO4 is significantly longer compared with that of pure-CoMoO4 at both high and low current densities, indicating that RGO/CoMoO4 composites offer a much higher capacitance. This is coincidence with the CV results. With increasing current density, the specific capacitance gradually decreases, this is a consequence of the fact that at high currents, the electrolyte ions suffer from low diffusion and can get access to only some of the total available reaction sites, which results in an incomplete insertion reaction and low specific capacitance [38]. From the galvanostatic charge/discharge plots, the coulombic efficiency was calculated using the following equation:
Coulombic efficiency ðgÞ ¼ ðtD =t C Þ 100%
ð5Þ
where tD and tC correspond to the discharging time and charging time, respectively. The highest coulombic efficiencies of 93.9% and 94.4% were calculated at a discharge current density of 1.5 mA g1 for the pure-CoMoO4 and RGO/CoMoO4, respectively. For an ideal capacitor, the galvanostatic charge/discharge plots should be triangular in nature. However, the galvanostatic charge/discharge plots of both the pure-CoMoO4 and RGO/CoMoO4 deviate from linearity, indicating a pseudocapacitive nature of the electrode material, which also supports the CV test results. The electrochemical performances of the pure-CoMoO4 and RGO/CoMoO4 were further investigated by electron impedance spectroscopy (EIS). Fig. 9 shows the EIS plots of the pure-CoMoO4 and RGO/CoMoO4. The higher resistance of pure-CoMoO4 is due to the poor electrical conductivity of CoMoO4 in KOH electrolyte. Due to the introduction of graphene, RGO/CoMoO4 nanocomposites achieved the low resistance, indicating the better electrical conductivity of RGO/CoMoO4 nanocomposites than that of pureCoMoO4. Besides, the existence of graphene also provides easy access and more space for electrolyte diffusion. In order to investigate the effects of crystalline phase on the electrochemical performance of RGO/CoMoO4, the prepared RGO/CoMoO4 composites were annealed at 500 °C for 5 h in N2
Fig. 10. CV plots at different scan rates (5, 10, 20, 50, 100 mV s1) of RGO/CoMoO4 for annealing (a) and galvanostatic charge/discharge curves of RGO/CoMoO4 for annealing at the current densities of 0.5, 1, 1.5, and 2 A g1 (b).
X. Xu et al. / Journal of Alloys and Compounds 616 (2014) 58–65
atmosphere. The CV plots and galvanostatic charge/discharge plots of the annealed sample are shown in Fig. 10. It is obvious that the specific capacitance decreased from 322.5 F g1 to 102.5 F g1 at 5 mV s1, and the discharge time is dramatically short for annealed RGO/CoMoO4 nanocomposites. According to the above results of electrochemical tests, we explain that the crystalline phase of CoMoO4 is important for the electrochemical performance of RGO/CoMoO4. The results also reveal that a-CoMoO4 is favorable to improve the electrochemical performance in comparison with b-CoMoO4. 4. Conclusion We have successfully developed a facile and efficient microwave-assisted reaction system to prepare RGO/CoMoO4 nanocomposites. The RGO sheets provide a favorable surface for depositing the CoMoO4 nanoparticles and preventing their agglomeration. In addition, graphene further enhances the conductivity of the composites and therefor provides fast charge transport. The asprepared nanocomposites exhibit higher specific capacitance, lower resistance and better rate capability than those of pureCoMoO4 and annealed RGO/CoMoO4, which makes it a promising candidate for next generation supercapacitors. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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