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DOI: 10.1039/C5RA02177D (Paper) RSC Adv., 2015, 5, 38324-38329

Facile one-step synthesis of Co(OH)2 microsphere/graphene composites for an efficient supercapacitor electrode material†

Guixiang Du*, PanPan Liu, Xinhui Yang, Jingbo Zhang, Xiaoxu Wang and Xueshan Sun
Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry, Ministry of Education, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, College of Chemistry, Tianjin Normal University, Tianjin 300387, China. E-mail: dugx666@126.com; Fax: +86 22 23766516; Tel: +86 22 23766516

Received 4th February 2015 , Accepted 22nd April 2015

First published on 23rd April 2015

Abstract

A simple one-step hydrothermal method is developed for the synthesis of Co(OH)2/graphene (Co(OH)2/GN) composites, during which graphite oxide was reduced and Co(OH)2 particles were in situ formed on the GN sheets. The morphologies, structures and the electrochemical properties of the composites were investigated. The results show that flower-like Co(OH)2 microspheres, self-assembled by the acicular particles, are enveloped by the wrinkled GN sheets to form the Co(OH)2/GN composites. The composite electrode exhibits the highest specific capacitance (Cspec) of 433 F g−1 at a discharge current density of 0.1 A g−1 in 6 M KOH, which is much higher than that of pure Co(OH)2 (217 F g−1) and GN (187 F g−1), and shows excellent rate capability (the capacitance retains 90.3% at 10 A g−1) and a long cycle life along with 99% Cspec retained after continuous 1000 cycle tests at a current density of 1 A g−1. The enhanced electrochemical performance is attributed to the synergistic effects of the good redox activity of the Co(OH)2 particles combined with the high electronic conductivity of the GN sheets, which predicts the composite to be a highly promising candidate as an electrode material in energy conversion/storage systems.

1. Introduction

The growing concerns about environmental issues associated with fossil fuel consumption and increasing energy demand have stimulated intense research on energy storage and conversion from alternative energy sources.1 Energy storage devices like traditional capacitors and batteries are not capable of meeting the higher demand of future systems anymore owing to the relatively low power and energy density. Supercapacitors, also called electrochemical capacitors (EC), as new energy storage devices have attracted much attention due to their high power density, fast charge–discharge process, high cycle stability, environmental benignity and high reliability, which make them the most promising candidates for next-generation power devices.2–5 Based on different charge storage mechanisms, EC can be classified into two types: electric double-layer capacitor (EDLC), where capacitance arises from the pure electrostatic charges accumulated at the electrode-electrolyte interface, and pseudocapacitor, in which fast and reversible oxidation/reduction (redox) or faradaic charge reactions of the electroactive species take place on the surface of the electrode. Pseudocapacitor electrode materials such as transition metal oxides/hydroxides are promising and widely studied due to their variable oxidation states providing the possibility for charge transfer, which makes them higher energy density than that of the EDLC materials.6,7 Unfortunately, they deliver lower power density, bad cycling stability, and poor rate capability (dramatic drop of Cspec with an increase at a high scan rate) due to poor electrical conductivity.

An effective strategy to achieve high-performance electrode materials with high energy density without sacrificing the power density is to develop the composite materials that combined high conductivity of carbon materials with high Cspec of pseudocapacitor materials, which could avoid the shortcoming of single material.8,9 Among the pseudocapacitor materials, Co(OH)2 has been considered as one of the most promising electrode materials because of its low cost, environmental friendliness, ready availability, high theoretical Cspec and well-defined redox behavior.10–12 GN, as a new two-dimensional carbon material, offers the highest specific surface area along with high porosity and the highest conductivity among all the carbonaceous materials.13–15 Therefore, incorporating highly conductive GN sheets with Co(OH)2 to obtain the composite would be highly effective to achieve high performance electrode materials with high Cspec, excellent high-rate capability and good cycling stability by utilizing their positive synergistic effects. In the composite, GN sheets could not only provide a conductive support, but also efficiently buffer the volume expansion/contraction during the rapid charge–discharge process, and Co(OH)2 particles anchored on GN sheets may act as a spacer to effectively prevent GN sheets restacking.

Many cobalt-based GN composites and several Co(OH)2/GN composites have been reported as EC electrodes, and some achievements have been made.16–20 However, it still remains a challenge to develop simple, effective and low-cost approach to synthesis the high-performance electrode materials. In this paper, we reported a facile one-step surfactant-free hydrothermal method to fabricate Co(OH)2/GN composites, in which flower like Co(OH)2 microspheres are covered by the thin GN sheets. It is demonstrated that the composites exhibit excellent electrochemical properties with a large Cspec, excellent rate capability and good cyclic stability due to the positive synergistic effects of GN and Co(OH)2.

2. Experimental

2.1 Synthesis of the Co(OH)2/GN composites

Graphite oxide (GO) was prepared by a modified Hummers method as described elsewhere.21 GO (20 mg) was dispersed in distilled water by sonication for 1 h. Subsequently, 4 mmol Co(NO3)2·6H2O aqueous solution was added into the dispersion, followed by sonication for another 0.5 h. Then, ethanolamine and ethylene glycol were added to above solution (molar ratio of Co2+, ethanolamine and ethylene glycol is 1[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1) under stirring for 30 min. The obtained suspension was sealed into the Teflon-lined autoclave and heated at 180 °C for 12 h. The obtained composite (Co(OH)2/GN-1) was filtered, washed with distilled water and dried in vacuum at 60 °C for 12 h. For comparison, 20 mg GO, 2 and 6 mmol Co(NO3)2·6H2O were also used to prepare other composites (molar ratio of Co2+, ethanolamine and ethylene glycol remains unchanged), which were denoted as Co(OH)2/GN-2 and Co(OH)2/GN-3, respectively. Pure Co(OH)2 and GN were also obtained by a similar method.

2.2 Materials characterization

The obtained samples were characterized by powder X-ray diffraction (XRD) (Bruker D8A) using a Cu Kα radiation (λ = 0.15418 nm) over a 2θ range of 5–80°. Raman spectra were obtained using a Renishaw invia Raman spectrometer under an excitation of the 532 nm laser. X-ray photoelectron spectra (XPS) were acquired with a PHI 5000 Versa Probe system. The morphologies and structures were observed with field-emission scanning electron microscopy (FESEM, FEI Nova Nano SEM 230) and transmission electron microscopy (TEM, FEI Tecnai G2 F20).

2.3. Preparation of electrodes and electrochemical tests

The working electrode was fabricated by mixing the obtained samples (85 wt%) with 10 wt% of acetylene black and 5 wt% of polytetrafluoroethylene binder. A small amount of ethanol was added to the mixture to produce a homogeneous paste. Then the resulting mixture was coated onto the nickel foam substrate (1 cm2), and followed by drying at 80 °C for 12 h in a vacuum oven.

The electrochemical measurements were performed in a three-electrode experimental setup. Platinum foil and a saturated calomel electrode were used as the counter and reference electrodes in a 6 M KOH aqueous electrolyte at room temperature, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on CHI660B electrochemical work station from shanghai CH Instrument Inc., and galvanostatic charge–discharge (GCD) tests were conducted on a Land cell tester. The CV measurement was conducted at a potential window of −0.05–0.45 V. EIS measurements were performed at an AC amplitude of 5 mV in a frequency range from 0.01 Hz to 100 kHz. GCD tests were performed in a potential range between −0.05–0.45 V.

3. Results and discussion

The XRD patterns of the GO, GN, Co(OH)2 and Co(OH)2/GN-1 composite are shown in Fig. 1. For GO, a strong peak at 12° (001) with an interlayer spacing of 0.74 nm is observed (Fig. 1a), suggesting the successful oxidation of graphite to GO due to the introduction of oxygen-containing groups on graphite sheets.22 For GN, a weak and broad diffraction peak centered at around 24° can be observed (Fig. 1b). Meanwhile the peak at 12° disappears, which suggests that most of the oxygen-containing groups are removed as the interlayer spacing varies from 0.74 to 0.37 nm during the hydrothermal process.23 The diffraction peaks of the synthesized pure Co(OH)2 (Fig. 1c) and Co(OH)2/GN-1 composite (Fig. 1d) are similar and can be mainly ascribed to β Co(OH)2 (JCPDS: 01-074-1057),24 and no obvious characteristic peaks assigned to GN or GO were found in the composite, indicating the effective exfoliation of the layered GN oxide in this simple one-step hydrothermal process.
image file: c5ra02177d-f1.tif
Fig. 1 XRD patterns of GO (a), GN (b), Co(OH)2 (c) and Co(OH)2/GN-1 composite (d).

The Raman spectrum is an essential tool to characterize graphite and GN materials. Fig. 2a show the Raman spectrum of GO and Co(OH)2/GN-1 composite. The samples exhibit two distinct peaks located at around 1360 cm−1 and 1590 cm−1, which correspond to the D band and G band vibrations, respectively. The D band origin from the disorder induced features of carbon and structural defects, while the G band corresponds to the sp2 carbon-bonded graphitic structure, which is beneficial for enhancing the electrical conductivity of carbon materials.25,26 The appearance of G band indicates that the obtained Co(OH)2/GN-1 composite material would be in favor of improving electrical conductivity when used as electrode materials of the energy storage devices. The ID/IG intensity ratio is usually used to semi-quantitatively determine the disorder degree and average size of the sp2 domains of the graphite materials. Compared to that of GO, the increased ID/IG ratio (0.89 for Co(OH)2/GN-1 and 0.80 for GO) of Co(OH)2/GN-1 suggests a decrease in the average size of the sp2 domains and a typical GN structure obtained by the one-step hydrothermal reduction, which is associated with the XRD results.27 The important information about the chemical composition, chemical state and binding energy of the Co(OH)2/GN-1 composite were analyzed by XPS measurement. From the XPS spectrum (Fig. 2b), it can be observed that the Co(OH)2/GN-1 composite mainly contains carbon, oxygen and cobalt species. The C 1s spectrum of the Co(OH)2/GN-1 composite (Fig. 2c) contains three components at 284.4 eV, 285.4 eV and 287.0 eV, which are assigned to the C–C or C[double bond, length as m-dash]C bonds, C–O bond and C[double bond, length as m-dash]O bond, respectively.28 It suggests that there are still some residual oxygen-containing functional groups on the GN sheets due to the incomplete reduction. The XPS spectrum of Co 2p (Fig. 2d) contains two main peaks at 797.2 eV and 781.7 eV, which are attributed to Co 2p1/2 and Co 2p3/2, respectively, with an energy separation of 15.5 eV, in good agreement with the reported data of Co 2p1/2 and Co 2p3/2 in Co(OH)2.29 The results further confirm that the Co(OH)2 have been successfully anchored on to the GN sheets in the composite, and are consistent with the XRD analysis discussed above.


image file: c5ra02177d-f2.tif
Fig. 2 (a) Raman spectra of GO and Co(OH)2/GN-1 composite, (b) XPS survey spectrum of Co(OH)2/GN-1 composite, (c) C 1s and (d) Co 2p spectrum of Co(OH)2/GN-1 composite.

The detailed morphologies and structures of the obtained GN/Co(OH)2-1 were examined by SEM and TEM (Fig. 3). From the SEM images of GN/Co(OH)2-1 composite (Fig. 3a), we can clearly see that some of the Co(OH)2 microspheres are exposed on the surface of GN sheets, and some of them are wrapped or covered by the highly transparent and thin GN sheets, resulting in the wrinkled and rough textures to form sandwich-like structure to avoid the restack of the thick GN sheets. Most of the Co(OH)2 microspheres in the composite are about 2–3 μm in diameter and exhibit the pompon-like or flower-like structures. The magnified images (Fig. 3b and the inset) suggest that the Co(OH)2 hierarchical architecture is self-assembled by several tens of small attached sheets. Fig. 3c illustrates the TEM images of the GN/Co(OH)2-1 composite, in which the veil-like GN sheets are quite thin, and the flower-like Co(OH)2 microspheres self-assembled by the many sheets can be obviously observed. The selected area diffraction (SAED) pattern of the Co(OH)2/GN-1 composite (the inset in Fig. 3c) demonstrates the polycrystalline nature.16 The obvious lattice fringes in Fig. 3d are separated by a distance of about 0.46 nm representing the (001) crystalline plane, which is associated with the obtained strongest peak (001) in the XRD pattern. The unique structure possibly results in an increase of the surface area and provides better access for the electrolyte into the GN sheets, which would facilitate electrolyte diffusion and intercalation and ensure that a sufficient faradaic reaction takes place especially at high charge–discharge rates. The open pore structure implies that the Co(OH)2/GN-1 composite maybe have potential application in energy storage. Fig. 4 presents the morphologies of the composites with the changing of the concentration of Co2+. When the concentration decreases, the composite Co(OH)2/GN-2 are shown in Fig. 4a and b, in which the size of the flower-like Co(OH)2 slightly decreases, and many small unassembled particles are presented. In contrast, the increase of the concentration results in the size of the flower-like Co(OH)2 increases, and some of them agglomerate (Fig. 4c and d). The size change and the dispersion of the Co(OH)2 on the GN sheets maybe have an effect on the corresponding electrochemical energy storage.


image file: c5ra02177d-f3.tif
Fig. 3 Morphology of Co(OH)2/GN-1 composite. (a and b) SEM images, (c) TEM image with the SAED pattern. (d) High-resolution image.

image file: c5ra02177d-f4.tif
Fig. 4 SEM images of Co(OH)2/GN-2 (a and b) and Co(OH)2/GN-3 (c and d) composites.

Electrochemical performance of the Co(OH)2/GN-1 composite and Co(OH)2 are investigated by the means of CV tests in a three-electrode system with a 6 M KOH aqueous solution. Fig. 5a represents the CV plots of the Co(OH)2 and Co(OH)2/GN-1 at scan rates of 50 mV s−1. CV patterns of Co(OH)2 sample show two pairs of intense redox peaks, which are typical of pseudocapacitance behavior due to the occurrence of the following faradaic redox reactions:30

Co(OH)2 + OH− = CoOOH + H2O + e−

CoOOH + OH− = CoO2 + H2O + e−


image file: c5ra02177d-f5.tif
Fig. 5 CV curves of Co(OH)2 and Co(OH)2/GN-1 composite at the scan rate of 50 mV s−1 (a). CV curves of Co(OH)2/GN-1 composite at various scan rates.

In the Co(OH)2/GN-1 composite, only a pair of obvious redox peak can be clearly observed, possibly due to the two anodic and cathodic peaks combing together form a single intense anodic and cathodic peak, which is maybe attributed to the synergistic effect between the GN sheets and Co(OH)2.27 The current density response for the Co(OH)2/GN-1 composite electrode is much larger than that of Co(OH)2 at the same scan rate, indicating that Co(OH)2/GN-1 has a higher Cspec. Fig. 5b displays the CV plots of Co(OH)2/GN-1 at scan rates of between 5 mV s−1 and 50 mV s−1. The obvious increase of the current with the scan rate reveals the desirable rate capability of the Co(OH)2/GN-1 composite electrode. In addition, no obvious distortion in the shape of the CV curves with the increase of the scan rate should result from the improved mass transportation and electron conduction.

The performance of the composite electrode materials are further analyzed by GCD method. The Cspec was calculated from the following equation

Cspec = i × t/m × Δv
where Cspec is the specific capacitance (F g−1), i/m is constant charging and discharging current density (A g−1), Δv is the potential difference (V). Fig. 6a displays the discharge curves of GN, Co(OH)2 and the Co(OH)2/GN composites at 0.1 A g−1, and Fig. 6b show the discharge curves of Co(OH)2/GN-1 at different current density. The nonlinear nature of the discharge curves of Co(OH)2 and the Co(OH)2/GN composites demonstrates the effective contribution of pseudocapacitance to the total capacitance,31 which is also in agreement with the CV test results. The Cspec of the Co(OH)2/GN composites are much higher than that of pure Co(OH)2 (217 F g−1) and GN (187 F g−1), which indicates that the synergistic effect of GN sheets and Co(OH)2 in the composites cannot only improve the electron conductivity but also prevent agglomeration or irreversible restacking of GN sheets, thus ensuring that a sufficient faradaic reaction take place easily and providing better access for the electrolyte into the entire structure. Furthermore, the Cspec values of Co(OH)2/GN-1 (433 F g−1) is slightly higher than that of Co(OH)2/GN-2 (308 F g−1) and Co(OH)2/GN-3 (414 F g−1). It is possibly due to more uniform particle size distribution of the Co(OH)2 on the GN sheets for Co(OH)2/GN-1 and most optimized amount of GN in Co(OH)2/GN composites, which is in agreement with the SEM images, resulting in intimate interaction between GN sheets and well-dispersed Co(OH)2 particles and minimizing the interfacial resistance of the charge transfer process. In addition, similar experiment (another two composites have been synthesized and characterized, in which the amount of GN is increased or decreased based on that in Co(OH)2/GN-1 composite with the best capacitive behavior) is designed and further confirm the role of GN played in the composite, which is described in ESI.† With the increase of current density, the Cspec value of Co(OH)2 and the composites exhibit an overall decrease (Fig. 6c), which results from the partial active material having minimum time to respond or insufficient time available for ion diffusion and adsorption at high current density.31 The Cspec values of the Co(OH)2/GN-1 were 433, 434, 413, 413.6, 394 and 391 F g−1 at a discharge current density of 0.1, 0.5, 1, 2, 5 and 10 A g−1, respectively. For Co(OH)2/GN-1, Co(OH)2/GN-2 and Co(OH)2/GN-3, 90.3%, 90% and 79% retention at 10 A g−1 are obtained, respectively, indicating that the composites have good rate capability, which is crucial for the electrode materials of supercapacitors to achieve both high power and energy densities. However, only 65% Cspec retention at 10 A g−1 for Co(OH)2 was obtained.


image file: c5ra02177d-f6.tif
Fig. 6 (a) Discharge curves of GN, Co(OH)2 and Co(OH)2/GN composites at the current density of 0.1 A g−1, (b) discharge curves of Co(OH)2/GN-1 composite at various current densities. (c) The Cspec values of Co(OH)2 and Co(OH)2/GN composites as a function of current density, (d) the cycling performance of Co(OH)2 and Co(OH)2/GN composites at a constant current density of 1 A g−1.

Good cyclic stability of the electrode is crucial for a supercapacitor. Fig. 6d shows the retention of Cspec over 1000 cycles of the composites and Co(OH)2 at a discharge current density of 1 A g−1. Co(OH)2/GN-1,2,3 display the higher cycle stability (99%, 118% and 98%, respectively) than that of Co(OH)2 (87%). It indicates that in the composites GN sheets without restacking and aggregation play a significant role in ensuring high conductivity and cycling stability and inhibiting the volume change and detachment of the Co(OH)2. Interestingly, for Co(OH)2/GN-2, the capacitance keeps increasing during cycling, which is similar to the previous reports.32–34 We think that it may be ascribed to electrochemical activation of the composite electrode, because in general electrolytes require a certain period of time to penetrate the entire inner space of active electrode material. We estimate that the interior surface of the composite is unable to be fully utilized during a few initial cycles. As the electrolyte gradually penetrates into the interior of the composite, more and more material become activated, which finally results in the increase of the capacitance. EIS analysis is a tool to examine the electrical conductivity and ion transfer of the electrode materials. Fig. 7 shows Nyquist plots of the composites and Co(OH)2 with a frequency range from 0.01 to 100 kHz in 6 M KOH solution. All the impedance spectra of the samples are almost similar, which contain a high-frequency semicircle and a low-frequency sloping straight line. The electrode series resistance (Rs) is derived from the high frequency intersection in the real axis.35 It can be calculated that the Rs values for Co(OH)2/GN-1,2,3 are 0.48, 0.49 and 0.60 Ω, respectively, which suggests that the superior electronic conductivity of GN sheets is preserved after loading of Co(OH)2 particles. The diameter of semicircle represents the charge transfer resistance (Rct), and the post-semicircle straight line in the low frequency region reflects the diffusion of the electroactive species, in which the line with higher slope signifies lower diffusion resistance.28 The Rct value of Co(OH)2 is 1.0 Ω, which is much higher than all the Co(OH)2/GN composites. The Co(OH)2/GN-1 has the smallest value (0.49 Ω) and the low-frequency line is closer to the vertical, which suggest that low charge transfer resistance and facile ion diffusion process through the Co(OH)2/GN-1 composite electrode material, resulting from GN sheets minimizing the interfacial resistance of the charge transfer process. It is consistent with the GCD results, in which the unique structure of the Co(OH)2/GN-1 composite leads to efficient utilization of such pseudo- and double-layer capacitors to result in the excellent supercapacitor performance.


image file: c5ra02177d-f7.tif
Fig. 7 EIS of Co(OH)2 and Co(OH)2/GN composites.

4. Conclusion

In summary, the Co(OH)2/GN composites have been successfully synthesized in a facile one-step hydrothermal process. Due to the unique structure, in which the flower-like Co(OH)2 microspheres, self-assembled by the acicular particles, are evenly dispersed and in contact with the wrinkled GN sheets, the composite Co(OH)2/GN-1 exhibited excellent capacitive behavior with a Cspec of 433 F g−1 at 0.1 A g−1, good rate performance (90.3% retention at 10 A g−1) and long cyclic stability (99% retention after continuous 1000 cycles at a current density of 1 A g−1). The facile synthesis method and good electrochemical properties suggest that the composite could be a good electrode candidate in energy storage.

Acknowledgements

This work was supported by National Natural Science Foundation of China (nos 51102180 and 21273160), and supported by the Program for innovative Research Team in University of Tianjin (no. TD12-5038).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02177d
This journal is © The Royal Society of Chemistry 2015
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