Supercapacitor electrode with a homogeneously Co3O4-coated multiwalled carbon nanotube for a high capacitance

Cobalt oxide (Co3O4) was homogeneously coated on multiwalled carbon nanotube through a simple chemical deposition method and employed in supercapacitor electrodes. SEM image indicated the uniform distribution of Co3O4 nanoparticles on the surface of the multiwalled carbon nanotube. A maximum specific capacitance of 273 Fg−1 was obtained at the charge–discharge current density of 0.5 Ag−1. After 500 cycles of continuous charge–discharge process, about 88% of the initial capacity could be retained. Electronic supplementary material The online version of this article (doi:10.1186/s11671-015-0915-2) contains supplementary material, which is available to authorized users.


Background
Electrochemical capacitors (ECs) are causing great concern due to their long cycle life and safety tolerance to high-rate charge and discharge [1]. The electrochemical capacitors have higher power density than secondary batteries and higher energy density than conventional capacitors. With the development of nanoscience and technology, nanoscaled cobalt oxide (Co 3 O 4 ) has received great attention for its use in diverse applications such as catalysis, energy storage devices, and electrochemical sensors due to its peculiar properties and controllable morphology compared with the bulk phase [2][3][4]. In particular, extensive efforts have been devoted to utilize Co 3 O 4 for supercapacitors because of its high reversibility and theoretical specific capacitance (3560 Fg −1 ) [5,6]. Recently, cobalt oxide has been proven to be a potential alternate to expensive RuO 2 which is broadly used as the electrochemically active material in electrochemical capacitors [7][8][9][10][11][12].
It is well known that Co 3 O 4 is an important p-type semiconductor. Co 3 O 4 has been used in lithium-ion batteries, heterogeneous catalysis, electrochemical capacitor devices, and other applications. For this purpose, Co 3 O 4 has been synthesized using a variety of methods such as sol-gel, reflux, microwave, and hydrothermal methods [13][14][15][16]. Furthermore, much work has been done on the controlled synthesis of nanostructure Co 3 O 4 and Co 3 O 4 cubes, rods, wires, tubes, and sheets [17][18][19][20][21]. Although electrochemical capacitors based on Co 3 O 4 have shown excellent electrochemical capacity, its practical application in supercapacitors is still limited in part due to its poor electrical conductivity. In order to improve the electrical conductivity, one of the most common ways is to mix the Co 3 O 4 with conductive additives. Introduction of carbon-based composites may be a promising way to improve the electrical conductivity of Co 3 O 4 . Carbonaceous materials, such as activated carbon, carbon nanotubes (CNTs), and grapheme nanosheets (GNs), can provide matrices for structural stability and fine electron transfer property due to their excellent mechanical flexibility and high electrical conductivity [22][23][24]. Fu et al. [25] synthesized spherical cobalt oxide nanoparticles along CNTs in supercritical fluid (containing ethanol and CO 2 ) and studied their electrical transport properties as a Schottky-junction diode. Huang et al. and Tang et al. obtained hybrid MnO 2 /carbon nanotube through facile redox and hydrothermal methods, respectively, which both showed high-rate capacitility and fine stability [26,27]. Wang synthesized Co 3 O 4 @MWCNT composites through a hydrothermal procedure. This hybrid showed superior electrochemical performance as a cathode material in aqueous supercapacitors, which gave 590 Fg −1 at 15 Ag −1 in 0.5 M KOH aqueous solution [28]. Su et al. [29] electrodeposited Co 3 O 4 and NiO on the carbon nanotube and obtained a high capacitance of 52.6 mF cm −2 .
In this work, we describe a general method to synthesize Co 3 O 4 /MWCNTs through a simple chemical deposition method. Co 3 O 4 nanoparticles can be evenly and tightly attached on the surface of multiwalled carbon nanotubes (MWCNTs), through a long time of constant temperature heating. The obtained samples showed high specific capacitance (273 Fg −1 at a current density of 0.5 Ag −1 ) though just few Co 3 O 4 was deposited on the MWCNTs. This method could significantly decrease the consumption of rare cobalt element.

Materials preparation
MWCNTs (purity, >95%; diameter, 40 to 60 nm; specific surface area, 200 m 2 g −1 ) were purchased from Chengdu Organic Chemicals Co. Ltd., Chengdu, China. All of the other chemicals were of analytical grade and were used as purchased without further purification. Firstly, MWCNTs were acid-treated with concentrated nitric at 140°C for 10 h. The treated MWCNTs were rinsed with distilled water until the PH was 7 and dried at 60°C for 24 h. It is well known that the surface of MWCNTs possesses a great deal of functional carboxyl groups and becomes negatively charged after functioned with nitric acid [30]. This extraordinary change of the tubular structure for MWCNTs was familiar to be coated with inorganic nanomaterials. Secondly, 80 mg of acidtreated MWCNTs was dispersed into 50 ml ethanol by stirring and ultrasonic treatment, then 2.5 ml of 0.5 M Co(OAC) 2 aqueous solution was added to the above solution in a state of agitation, followed by the addition of 1 ml of NH 4 OH (30% solution) and 1.4 ml of distilled water an hour later. Thirdly, the reaction was kept at 80°C with stirring for 10 h. After that, the reaction mixture was transferred and sealed in a 100-ml Teflon-lined stainless steel autoclave for a hydrothermal reaction at 150°C for 3 h. After cooling to room temperature, the product was collected by centrifugation and rinsed with deionized water and absolute ethyl alcohol in sequence several times until pH was equal to 7, then dried at 80°C for 12 h. The content of Co 3 O 4 on the surface of MWCNTs was controlled through the regulation of the Co(OAC) 2 content. The Co(OAC) 2 contents were controlled to be 0.125, 0.25, 0.5, and 1 mmol. The prepared samples were denoted as Co 3 O 4 -0.125/MWCNTs, Co 3 O 4 -0.25/MWCNTs, Co 3 O 4 -0.5/MWCNTs, and Co 3 O 4 -1/ MWCNTs, accordingly. Pure Co 3 O 4 sample was also prepared through the same preparation process as the Co 3 O 4 /MWCNTs samples.

Structural characterization and electrochemical measurements
The morphology and structure of the samples were characterized by JSM-7001 F field emission scanning electron microscope (FESEM) and DX-2700 X-ray diffractometer (XRD) with a monochromatized Cu K irradiation (k = 0.154145 nm), respectively. The composition was characterized by the thermogravimetric (TG) analysis method through Netzsch-STA 449C, from 25°C to 900°C at a heating rate of 10°C min −1 in air.
The electrochemical measurements were carried out using a three-electrode system with a 6 M KOH electrolyte in which platinum foils and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The working electrodes were fabricated by mixing the as-prepared composite, acetylene black, and polytetrafluoroethylene (1% wt.) with a mass ratio of 85:10:5. N-methyl pyrrolidinone (NMP) was also added to form slurry for the spreading on nickel sheets (1.0 cm × 1.0 cm). The working electrodes were pressed at 10 MPa and dried under vacuum at 60°C for 24 h [31]. Cyclic voltammetry (CV) measurement was performed with a CHI660B (Chen Hua Co., Shanghai, China) workstation. The scan rates of CV were in the range from 5 to 100 mVs −1 . Electrochemical impedance spectroscopy (EIS) measurement was performed with the electrochemistry workstation IM6 (Zahner Co., Kronach, Germany). accordingly. When the added Co(OAC) 2 content was 0.25 mmol, new diffraction peaks appeared. These diffraction peaks appeared at 18.9, 31.3, 36.8, 44.9, 59.4, and 65.4°. These peaks belong to the characteristic peaks of spinel Co 3 O 4 , which could be indexed with a JCPDS card (No.43-1003). These diffraction peaks were corresponded to the reflection planes (111), (220), (311), (400), (511), and (440), respectively. These peaks were all in accordance with the pure Co 3 O 4 nanomaterial, except the peak at 2θ = 26°, which is corresponding to the (002) reflection of the MWCNTs. This indicated that the coated Co 3 O 4 on the surface of MWCNTs has the same crystal phase with the pure Co 3 O 4 nanoparticles.

Results and discussion
To quantify the amount of Co 3 O 4 in the composites, TG analysis was carried out in air. The sample was heated from 25°C to 900°C at a rate of 10°C min −1 . Figure 2 gives the TG curves for the Co 3 Figure 3b, we can see that the as-prepared Co 3 O 4 particle is small and homogeneous. But the small particles agglomerate with each other to form large powders, which is inconvenient for contact with the electrolyte during the charging and discharging process. It can be seen from Figure 3a that the pure MWCNTs are smooth and flexible, forming strong intertwined entanglements with a three-dimensional (3-D) network structure. Compared with the pure MWCNTs, the surface of Co 3 O 4 -particle-coated MWCNTs (Co 3 O 4 -0.5/MWCNTs) became unsmooth with well-distributed small particles. The particle size is in the range of 5 to 10 nm.  Figure S1). In order to evaluate the supercapacitor performance of the electrodes, electrochemical studies were performed using cyclic voltammetry in 6 M KOH aqueous electrolyte. Figure 4a illustrates the CV curves of pure MWCNT electrode at different scan rates in the voltage range of −1 to 0 V. The pure MWCNT electrode has deviated from idealized double layer because of the redox reactions of the functional groups on the surface. The paragraph shows high symmetry between the negative curves and the positive ones, so the MWCNT electrode behaves as a pseudocapacitor. With the increase of the sweep rate, the CV curves have no obvious distortion, indicating a highly reversible system. Figure 4b shows the cyclic voltammograms of CoOOH Compared with the pure MWCNT electrode, the  Figure 5. The charge-discharge current density is 0.5 Ag −1 . Figure 5a shows the charge-discharge curve of pure MWCNT electrode within a potential window of −1 to 0 V. The shape of the curve is closely linear and shows a typical triangle symmetrical distribution indicating a good double layer capacitive property. Figure 5b shows the charge-discharge curve of the pure Co 3 O 4 electrode within a potential window of −0.4 to 0.35 V. It can be seen that the curve has significant bend which indicates a pseudocapacitive capacity for the electrodes. Figure 5c shows the charge-discharge of the Co 3 O 4 /MWCNT composites with different cobalt content in the potential range of −0.4 to 0.35 V. The shape of the charge-discharge curves is similar with that of the pure Co 3 O 4 electrode. The average specific capacitances for the electrodes can be calculated on the basis of Equation 3: where C is the specific capacitance (Fg −1 ), i (A) is the discharge current, △V (V) is the potential window during the discharge process, △t (s) is the discharge time, and m (g) is the mass of electroactive material [33].  Figure 5d displays the charge-discharge curves of the Co 3 O 4 -0.5/MWCNT composite at various current densities in the range of 0.5 to 1.5 Ag −1 . The specific capacitances were calculated to be about 273, 160, 134, 88, and 69 Fg −1 , respectively. It can be seen that the specific capacitances gradually decreases with the increasing of the discharging current density. This phenomenon might be due to the diffusion limits of the OH − ion movement.
The electrochemical performance of the electrode was further investigated by the EIS measurements. Figure 6 shows the Nyquist plots of the EIS spectra of pure Co 3 O 4 electrode and Co 3 O 4 -0.5/MWCNT electrode, in the frequency range of 1 Hz to 100 KHz. The obtained EIS spectra are composed of a half semicircle at high frequency and a line at low frequency. The small arc observed at the high frequency is related to the process at the electrode material electrolyte interface (R ct ), and the line at the low frequency indicates a capacitive behavior related to the charging mechanism. High-frequency intercepts of the real axis gives the serial resistance (R s ) for the working electrode. It can be seen that the Co 3 O 4 -0.5/MWCNT has smaller R ct and R s , indicating a lower electrochemical reaction resistance and electron transfer resistance. Furthermore, the straight line of the Co 3 O 4 -0.5/MWCNT spectra is more close to 90°compared with the pure Co 3 O 4 spectra which shows that the Co 3 O 4 -0.5/MWCNTs possess a more ideal capacitive behavior. Figure 7 shows the cycle life of the pure Co 3 O 4 and Co 3 O 4 -0.5/MWCNT electrodes at 0.5 Ag −1 . It is clearly seen that the specific capacitance of the Co 3 O 4 -0.5/ MWCNT electrode is much higher than that of pure  the first 500 cycle times. The process of charging and discharging are both relatively stable. After 500 times charge-discharge, the specific capacity of Co 3 O 4 -0.5/ MWCNT electrodes remains to be about 219 Fg −1 which is about 88% of the first discharge capacity.

Conclusions
Co 3 O 4 was homogeneously coated on a multiwalled carbon nanotube through a simple chemical deposition method. The contents of Co 3 O 4 on the surface of MWCNTs were handily controlled through the regulation of the Co(OAC) 2 content. Furthermore, the Co 3 O 4 nanoparticles were homogeneously distributed on the surface of multiwalled carbon nanotubes. The coating of Co 3 O 4 could significantly increase the specific capacity of MWCNTs. The optimized samples were obtained when the Co(OAC) 2 content was 0.5 mmol. The maximum specific capacitance of 273 Fg −1 was obtained at the charge-discharge current density of 0.5 Ag −1 . After 500 cycles of charge-discharge process, about 88% of the initial capacity could be retained.