Facile Synthesis of Co<sub>3</sub>O<sub>4</sub>/Mildly Oxidized Multiwalled Carbon Nanotubes/Reduced Mildly Oxidized Graphene Oxide Ternary Composite as the Material for Supercapacitors
Facile Synthesis of Co3O4/Mildly Oxidized Multiwalled Carbon Nanotubes/Reduced Mildly Oxidized Graphene Oxide Ternary Composite as the Material for Supercapacitors
Bulletin of the Korean Chemical Society. 2014. May, 35(5): 1349-1355
Copyright © 2014, Korea Chemical Society
  • Received : November 22, 2013
  • Accepted : January 10, 2014
  • Published : May 20, 2014
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About the Authors
Mei-yu, Lv
Kai-yu, Liu
Yan, Li
Lai, Wei
Jian-jian, Zhong
Geng, Su
College of Material Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China

A three-dimensional (3D) Co 3 O 4 /mildly oxidized multiwalled carbon nanotubes (moCNTs)/reduced mildly oxidized graphene oxide (rmGO) ternary composite was prepared via a simple and green hydrolysishydrothermal approach by mixing Co(Ac) 2 ·4H 2 O with moCNTs and mGO suspension in mixed ethanol/H 2 O. As characterized by scanning electron microscopy and transmission electron microscopy, Co 3 O 4 nanoparticles with size of 20-100 nm and moCNTs are effectively anchored in mGO. Cyclic voltammetry and galvanostatic charge-discharge measurements were adopted to investigate the electrochemical properties of Co 3 O 4 /moCNTs/ rmGO ternary composite in 6 M KOH solution. In a potential window of 0-0.6 V vs. Hg/HgO, the composite delivers an initial specific capacitance of 492 F g −1 at 0.5 A g −1 and the capacitance remains 592 F g −1 after 2000 cycles, while the pure Co 3 O 4 shows obviously capacitance fading, indicating that rmGO and moCNTs greatly enhance the electrochemical performance of Co 3 O 4 .
One of the great challenges for today’s information-rich, mobile society is providing high-efficient, low-cost, and environmentally friendly electrochemical energy conversion and storage devices for powering an increasingly diverse range of applications, ranging from portable electronics to electric vehicles (EVs) or hybrid EVS (HEVs). 1 2 As the performance of these devices depends intimately on the properties of their materials, considerable attention has been paid to the research and development of key materials. 3 - 10 Supercapacitors (also known as electrochemical capacitors or ultracapacitors) have drawn tremendous attention as an energy storage device for their high power density, good rate performance and long cycling life. They are playing an increasingly important role in various applications ranging from portable electronics to hybrid electric vehicles. 11 They are usually defined into electrochemical double layer capacitors (EDLCs) and pseudo-capacitors based on their different energy storage mechanisms.
Generally, the carbon and carbon nanotubes (CNTs) with high surface area and readily accessible mesopores are widely used for EDLCs, where the charge storage process is non-Faradic and energy storage is electrostatic. 12 Graphene, which is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, 13 has emerged as a promising material for electrochemical energy storage applications owing to its various superiorities in mechanical, electronic (with carrier mobility up to 200 000 cm 2 V −1 s −1 ), 14 15 thermal conductivity (~4840-5300 W m −1 K −1 ), 15 elasticity, 16 and specific surface area (~2600 m 2 g −1 ). 17 At the same time, the capacitance of the pseudo-capacitors is mainly from Faradic redox reaction. Their electrode materials generally involve various metal oxides 18 - 22 and conductive polymers. 23 - 30 In recent years, transition metal oxides have been drawn extensive research attentions for pseudo-capacitors since they could provide higher capacitance than carbon materials and longer cycling life than conductive polymers. 31 - 33 Among these materials, Co 3 O 4 shows an inviting prospect because of its ultra-high theoretical specific capacitance of 3560 F g −1 . Also, Co 3 O 4 is one of the most promising electrode materials for commercial supercapacitors due to their good efficiency, better stability, high abundance, low cost, environmental benignity, and relatively broad work potential window in aqueous solution. However, the poor electronic conductivity of Co 3 O 4 has hindered its further application for high-power electrochemical capacitor. Thus, it is of great importance to employ effective strategies to enhance the electronic conductivity of Co 3 O 4 and maintain high electrolyte penetration diffusion rates with the aim of improving its electrochemical capacitive performance. The common approach is either embedding Co 3 O 4 nanoparticles into or depositing Co 3 O 4 nanoparticles on a highly conductive porous matrix to form composites. To date, many Co 3 O 4 / carbon composites systems have been investigated for supercapacitor application. But all of these results are far from meeting the needs of commercial application due to their complexity in synthesis, relatively low capacitance, or insufficient stability. Thus, the facile synthesis of Co 3 O 4 composite with better electrochemical performance is still a hot issue to address. Currently, graphene/metal oxide composites are drawing increasing attention as electrode materials since the ultrathin flexible graphene sheets can provide a support to anchor metal oxide nanoparticles and serve as a highly conductive matrix. 34 - 35 There are many studies of graphene based composites such as graphene/MnO 2 (315 F g −1 in 1 M Na 2 SO 4 ), 36 graphene/Co 3 O 4 (548 F g −1 in 6 M KOH). 37 But the complicated synthesis processes greatly limited its commercial application. Graphene composites via one-step solvothermal process exhibited a specific capacitance of 147 F g −1 . 38 Myoungho Pyo et al . also reported that the SnO 2 - anchored graphene was subjected to thermal reduction in order to enhance the crystallinity of SnO 2 and the electrochemical conductivity of graphene. 39 40 Noting that the utilization of toxic and oxidative dimethyl sulfoxide (DMSO) as solvent made it environmentally unfriendly and probably could result in incomplete reduction of graphite oxide to graphene.
Consequently, it is still a big challenge to develop a facile simple and rapid approach to synthesize better performance Co 3 O 4 /graphene composites. It is known that the electrochemical properties of the composites greatly depend on the structural features of graphene and the degree of homogenous dispersion. Herein, Co 3 O 4 nanoparticles are homogeneously embedded into the graphene nanosheets via a simple solvothermal process which does not require the reduction of graphite oxide to graphene at first and no any toxic organic solvent is used.
Chemicals . Multiwalled carbon nanotubes with 40−60 nm in diameter and 5−15 μm in length were purchased from Shenzhen Nanotech Port Co. Ltd. Cobalt acetate [Co(CH 3 COO) 2 ᐧ4H 2 O], KMnO 4 , NaNO 3 , H 2 O 2 were purchased from Sinopharm Chemical Reagent (Co., Ltd). All chemicals were used as received from vendors without further purification. Deionized water was prepared in our laboratory.
Material Synthesis and Characterization . The mildly oxidized CNTs (moCNT) were made by a modified Hummer’s method. 41 Mildly oxidized graphene oxide (mGO) was made from flake graphite (200 mesh) by a modified Hummer’s method 42 using a lower concentration of oxidizing agent. Typically, as-prepared mGO sample was dissolved in an aqueous solution (0.8 mg/mL). For the first step synthesis of hybrid, 62.5 mL of mGO aqueous solution was ultrasonicated for 2 h. Then 12.5 mL of 0.24 M Co(Ac) 2 aqueous solution was added to 200 mL of moCNT (25 mg) ethyl alcohol suspension by ultrasonic agitation for 2 h. The mixture solution was then dropped into the mGO aqueous solution. The reaction was kept at 80 ℃ under magnetic stirring for 8 h. Another 475 mL of anhydrous ethanol was added to the mixed solution four times in every two hours. In the first step, Co 3 O 4 nanoparticles and moCNTs were grown on mildly oxidized GO sheets (mGO) freely suspended in solution by hydrolysis and oxidation of cobalt acetate. After that, the reaction mixture from the first step was transferred to a 100 mL autoclave for hydrothermal reaction at 150 ℃ for 8 h. This subsequent hydrothermal step also led to form crystallization of Co 3 O 4 and reduction of mGO to form the Co 3 O 4 /moCNTs/rmGO hybrid. The resulted product was collected by vacuum filtration and washed by deionized water 3 times and anhydrous ethanol 3 times. The resulting hybrid was ~150 mg after lyophilization.
The crystal structures of samples were determined by Xray diffractometer (XRD, Rigaku D/max 2550VB + ) from 10° to 80° with Cu Kα radiation (λ = 1.54056 Å). The Fourier transform infrared (FT-IR) spectra were recorded on Nicolet 560 spectroscopy with KBr pellet technique. Thermal-gravimetric analysis was carried out on NETZSCH STA449C thermalanalyzer under air atmosphere at temperature ranging from 30 ℃ to 900 ℃ with a heating rate of 10 ℃ min −1 . Raman spectra were tested with Dior LABRAM-1B instrument. The surface morphology was characterized by scanning electron microscope (SEM; JEOL JSM-6360 LV), transmission electron microscope (TEM; JEOL JEM-2100 F) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010) at an acceleration voltage of 200 kV.
Electrode Preparation and Electrochemical Analysis . The working electrode was prepared by mixing 85 wt % active materials, 10 wt % acetylene black as conductive agent, and 5 wt % poly(tetrafluoroethylene) (PTFE) as binder to form a slurry, which was pressed onto a nickel foam. The electrodes were dried under vacuum at 60 ℃ for 12 h. Cyclic voltammetry (CV) test and galvanostatic charge-discharge (CD) test were carried out on a CHI660C electrochemistry workstation, and the electrochemical impedance spectroscopy (EIS) was carried on PARSTAT2273 workstation. The cycle performance of the active material was tested on Land 5.9 version workstation. All electrochemical measurements were finished in a three-compartment cell: a working electrode, a platinum plate as counter-electrode, and a Hg/HgO electrode as reference electrode. The electrolyte was 6 M KOH aqueous solutions.
Results and Discussion
Structural and Morphological Characterization . Figure 1 shows the XRD patterns of the prepared samples Co 3 O 4 , Co 3 O 4 /rmGO, Co 3 O 4 /moCNTs /rmGO. All diffraction peaks in Figure 1(a) can be indexed to face-centered cubic (fcc) Co 3 O 4 phase (JCPDS card no. 43-1003). 43 No impurity phases were observed, demonstrating that the Co precursor was completely transformed into Co 3 O 4 after hydrothermal reaction. Figure 1(b) displays the XRD pattern of Co 3 O 4 / rmGO. A broad peak appears at 24°, confirming the conversion of GO to graphene. The as-prepared Co 3 O 4 /moCNTs/ rmGO composite was manifested in Figure 1(c) . The peak due to graphene can also be identified. Besides, there were some peaks corresponding with the Co(OH) 2 phase (JCPDS card no. 30-0443), implying that the oxidation was incomplete to some extent.
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XRD pattern of Co3O4 (a), the product of Co3O4/rmGO (b), Co3O4/moCNTs /rmGO (c).
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FT-IR spectra of moCNTs, Co3O4, Co3O4/rmGO, Co3O4/moCNTs/rmGO composite.
The FT-IR spectra of the products are presented in Figure 2. The strong and sharp peaks at 668 and 583 cm −1 in Figure 2(b) , 2(c) , and 2(d) are attributed to the vibrations of the Co- O. 42 In Figure 2(b) , peaks around 3,405 and 1,643 cm −1 are assigned to the stretching and bending vibration of water molecules, which is probably due to the absorbed moisture from the air during storage. No other impurities are detected, which indicates the formation of highly pure Co 3 O 4 . From Figure 2(a) , a broad absorption band at 3,437 cm −1 is attributed to the hydroxyl group, which is due to water molecules and/or OH functional groups remaining even after reduction of CNT and GO. 45 46 The peak at 1,639 cm −1 is due to C=C stretching of the moCNTs and the peak at 1,384 cm −1 signifies the O-H bending deformation in -COOH. A small peak at 1,086 cm −1 is assigned to C-O bond stretching. Thus, the reminiscence of -OH and -COOH groups on moCNTs or rmGO due to functionalization is observed.
The shape of the Co 3 O 4 nanoparticle on rmGO sheets was confirmed by SEM images in Figure 3(a), (b) and the corresponding transmission electron microscope (TEM) image was also shown in Figure 3(c), (d) . The folded graphene sheets with crumples wrapping the relatively uniform Co 3 O 4 nanoparticles. In order to investigate the structural information of Co 3 O 4 /moCNTs/rmGO nanoparticle, TEM, highresolution transmission electron microscopy (HRTEM) was investigated and presented in Figures 4 and Figure 5 . The Co 3 O 4 /moCNTs/rmGO exhibits interconnected 3-D network in SEM image of different magnifications ( Fig. 4(a), (b), (c), (d) ). From Figure 5(a) , it can be obviously observed that Co 3 O 4 and moCNTs are scattered on the graphene sheets. The moCNTs are not found in Figure 5(b) , and a few cobalt oxide nanoparticles were loosely decorated on the graphene nanosheet edge, resulting from the pretreatment of a strong ultrasonic. HRTEM image ( Fig. 5(c) ) shows a clear cubic lattice. The HRTEM image was recorded through the (111) direction. The (111) lattice spacing was 0.4777 nm. By using the formula for a cubic lattice: a 0 = d(h 2 + k 2 + l 2 ) 1/2 , a 0 is calculated to be 0.808 nm, corresponding to the given value in the JCDS 43-1003 card. 47 Figure 5(d) shows the amorphous structure of moCNTs. On the basis of the nanostructural observations made from the above SEM and TEM images, the overall fabrication procedures of the Co 3 O 4 /moCNTs/ rmGO composite are schematically 48 illustrated in Figure 6. The moCNTs that synthesized simply had introduced many carboxyl functional groups to imporve the dispersion stability and the functional groups on the outer walls to nucleate and anchor nanocrystals, while retaining intact inner walls for highly conducting network. Besides the moCNTs had negative charge in the surface, which was in favour of combination with Co ion. To some extent, the dispersed particles can effective alleviate the aggregation of rmGO. As a result, the composite had greatly improved in cyclability and rate capability.
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(a), (b) SEM image of Co3O4/rmGO and (c), (d) corresponding TEM images of Co3O4/rmGO after agitation for 1 h.
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SEM images of Co3O4/moCNTs /rmGO.
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(a, b) TEM images of Co3O4/moCNTs/rmGO composites after agitation for 1 h; (c, d) HRTEM images of the Co3O4 nanoparticle surface and the moCNTs viewed from the broad plane. The inset in c shows the corresponding fast Fourier transform pattern.
The content of moCNTs and rmGO in Co 3 O 4 /moCNTs/ rmGO was about 17.18% based on the weight loss before 700 ℃ in TGA ( Fig. 7 ) for the decomposition of hydroxy group and the release of CO 2 from moCNTs and rmGO. The weight loss before 240 ℃ was attributed to the removal of surface water.
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Schematic illustration for the synthesis of the Co3O4/ moCNTs/rmGO composite.
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TGA curve of Co3O4/moCNTs/rmGO in air at 10 ℃ min−1.
Electrochemical Properties . The CV curves of Co 3 O 4 / moCNTs/rmGO composite at various scan rates of 5, 10 and 30 mV s −1 are displayed in Figure 8 . There are a couple of redox peaks at about 0.351 and 0.485 V ( vs Hg/HgO), indicating that pesudocapacitance originating from the electrochemically active Co 3 O 4 component is dominant in the whole capacitance. 49 During the electron transfer procedure, only one oxidation peak can be clearly observed, possibly due to the production of CoOOH as an intermediate form which just existed for quite short time and then converted into CoO 2 rapidly. As the decreases of the scan rate, two oxidation peaks could appear, proving that CoOOH has relatively enough time to be detected in a given time scale, which can be changed into CoO 2 . The peaks are related to the reactions between different oxidation states of Co according to the following equations: 50 51
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If the scan rate is increased, the anodic peaks shift toward positive potential and the cathodic peaks move to negative potential. The potential difference between anodic and cathodic peaks is around 0.134 V at a scan rate of 10 mV s −1 , which indicates that the quasi-reversible feature of redox couples. The corresponding CV curves for Co 3 O 4 /rmGO and Co 3 O 4 /moCNTs/rmGO electrodes at 30 mV s −1 are displayed in Figure 9. The area of CV curve for Co 3 O 4 /moCNTs/ rmGO is apparently larger than that of Co 3 O 4 /rmGO.
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Cyclic voltammograms of Co3O4/moCNTs/rmGO electrode in 6 M KOH solution at various scan rates.
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Cyclic voltammograms of Co3O4/rmGO, Co3O4/ moCNTs/rmGO electrodes in 6 M KOH solution at a scan rate of 30 mV s−1.
In addition, the rate performance of electrode materials is also crucial for supercapacitors. Galvanostatic charge-discharge curves of pure Co 3 O 4 and Co 3 O 4 /moCNTs/rmGO composite are shown in Figure 10(a) and (b) . It can be seen that the discharge curves consist of a sudden potential drop from 0.5 to 0.4 V and a slow potential decay from 0.4 to 0.3 V. It is in good agreement with CV results. The specific capacitance (SC), based on galvanostatic charge/discharge measurement, can be calculated from the following equation:
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Galvanostatic discharge curves of pure Co3O4 (a) and Co3O4/moCNTs/rmGO composite (b) electrode in 6 M KOH solution at different current densities.
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Cycling performances of Co3O4/moCNTs/rmGO composite and pure Co3O4 under a current density of 1 A g−1.
where I is the discharge current (A), Δ t is the discharge time (s), m is the mass of the electrode materials (g), and Δ V is the discharge potential range. In this paper, the voltage of galvanostatic charge-discharge tests are all ranging from 0 V to 0.5 V. Therefore, the value of specific capacitance for the Co 3 O 4 /moCNTs/rmGO composite is calculated to be 502, 492, 464 F g −1 at the current densities of 0.5, 1, 2 A g −1 , respectively. Compared to 286.3, 271.6, 245.2 F g −1 for pure Co 3 O 4 , the capacitance of the composite is remarkably enhanced and the utilization has doubled increasing from 8.04% to 16.85%, which is also higher than that surfactantassisted synthesized Co 3 O 4 /reduced graphene oxide (163.8 F g −1 ). 52 The higher specific capacitance of the Co 3 O 4 /moCNTs/ rmGO composite than pure Co 3 O 4 can be ascribed to the highly conductive network for electron transport, the better kinetic of electrode in the composite, and the ion transport channel between graphene and Co 3 O 4 nanoparticles.
The long-term cycle stability of Co 3 O 4 /moCNTs/rmGO and pure Co 3 O 4 electrodes were evaluated by repeating charge-discharge testing in 6 M KOH electrolyte at a current density of 1 A g −1 for 2,000 cycles. As shown in Figure 11 , the specific capacitance of Co 3 O 4 /moCNTs/rmGO is increased by 17% after 500 cycles, then remains almost at the same value during the following 1,500 cycles ( Fig. 11(a) ). In contrast, the pure Co 3 O 4 presents a significant decrease in capacitance and just 88.6% of the initial capacitance can be maintained after 2000 cycles ( Fig. 11(b) ). The phenomenon could be attributed to the reasons as follows: at the initial cycle, the nanostructures wrapped by graphene have not been fully activated. After repeated charge-discharge cycling, the electrochemically active sites will be fully accessible by the electrolyte.
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Nyquist plots for Co3O4/moCNTs/rmGO composite and pure Co3O4 eletrodes.
The EIS data of the pure Co 3 O 4 and Co 3 O 4 /moCNTs/ rmGO composites at open circuit potential are shown in Figure 12 . It is found that Co 3 O 4 /moCNTs/rmGO shows a smaller radius of semicircle in the Nyquist plots as compared to Co 3 O 4 , suggesting that the Co 3 O 4 /moCNTs/rmGO possesses higher conductivity because of the presence of rmGO and moCNTs. These results indicate that the Co 3 O 4 / moCNTs/rmGO composites are suitable for fast charging and discharging. At very high frequency, the crossover point of the semicircle on the real part is a combinational resistance of the electrolyte resistance, intrinsic resistance of substrate, and contact resistance between the active materials and current collector. Meanwhile, it can be also found that charge-transfer resistance of pure Co 3 O 4 is much larger than that of sample Co 3 O 4 /moCNTs/rmGO. Evidently, it indicates that the conductivity of sample Co 3 O 4 /moCNTs/rmGO is greatly improved in comparison with pure Co 3 O 4 , in good agreement with the all above discussion.
The rmGO plays as the base for the growth of moCNTs and Co 3 O 4 . The moCNTs and Co ion, to some extent, as the dispersed particles can effective alleviate the aggregation of rmGO. As a result, the composite exhibited greatly improved cyclability and rate capability. The composite had an outstanding conducting network structure and exhibited a high capacitance of 502 F g −1 in 6 M KOH electrolyte at 1 A g −1 , much higher than that of pure Co 3 O 4 . The rmGO and moCNTs also effectively restrain the volume expansion of Co 3 O 4 during cycling and improve the conductivity of electrode, so the composite exhibits the excellent cycling performance without capacitance fading after 2000 cycles. The ternary composite is a promising candidate as the electrode for supercapacitors due to its high capacitance and excellent capacity retention. Taking the advantages of the new structure into consideration, we believe that the strategy could be readily applicable to other M x O y /moCNTs/rmGO (M=Fe, Co, Ni, Mn) composites.
We thank the National Natural Science Foundation of China (No. 21071153) for their financial support.
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