Facile synthesis of cobalt sulfide/carbon nanotube shell/core composites for high performance supercapacitors

Abstract

Novel CoS_x/functionalized multi-walled carbon nanotube (CoS_x/fMWCNTs) shell/core composites are successfully prepared via a simple hydrothermal route. The CoS_x nanoparticles with diameters of about 6 nm are decorated on the carbon nanotubes to form a shell. The electrochemical performance of the composites was investigated using a three-electrode system. The results show that CoS_x/fMWCNTs exhibit the highest specific capacitance of 1324 F g⁻¹ at a current density of 10 A g⁻¹, better than published results for supercapacitors based on CoS_x, CoS_x/CNTs, and CoS_x/graphene. Even at a higher current density of 50 A g⁻¹, the CoS_x/fMWCNTs composites still could deliver a relatively high specific capacitance of 796 F g⁻¹. The CoS_x/fMWCNTs composite electrodes show improved cyclic properties which show about 13% decay in available specific capacity after 2000 cycles. The facile synthesis of the CoS_x/fMWCNTs shell/core composites with superior electrochemical performance may provide a new candidate for energy storage devices with high efficiency.

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1 Introduction

Electrochemical capacitors, also called supercapacitors or ultracapacitors, have attracted much attention in the automotive and consumer electronics industry due to their high power density and long cycle life.^1–5 The growth in supercapacitors has arisen from displacing conventional battery and electrolytic capacitor products, and from new market applications where existing technologies cannot provide efficient solutions.^6,7 Pseudocapacitors, in which the charge is a process of energy storage via redox-based faradaic reactions, usually can deliver higher capacitance than electric double layer capacitors (EDLCs).^8,9 Metal oxide and conducting polymer can be used as the electrode of pseudocapacitors, such as MnO₂, Co₃O₄, Co(OH)₂, Fe₂O₃, polypyrrole, and polyaniline.^10–17 Hydrous RuO₂ has been recognized as the typical transition metal oxides electrode material for pseudocapacitors with high properties.^18–20 However, commercial application of RuO₂ is restricted due to its high cost, low porosity, and toxic nature.^21,22 Meanwhile, conducting polymer-based materials possess the disadvantages of low cycle-life and slow kinetics ion transport, because the redox sites in the polymer backbone are not sufficiently stable for many repeated redox processes.²³ Therefore, it is extremely important to develop alternative electrode materials with a combination of low cost and improved performance.

As electrode materials for supercapacitors, CoS_x showed high specific capacitance.^24–30 However, the poor conductance and mechanical instability of CoS_x blocked their practical application.^27,28 In order to enhance the properties of CoS_x, various CoS_x nanostructures have been investigated, such as nanoparticles, nanoplates, hollow and porous nanostructures.^25,27 Whereas, modifying the CoS_x with carbon materials to prepare composites is the most effective strategy to increase the conductivity as well as the electrochemical properties of CoS_x.^25,26,29 For example, graphene with high stability and good conductance were employed to prepare graphene/CoS_x composites with enhanced properties. The graphene based composites are easily to stack up and result in the poor diffusion of ions.³¹ One of the strategies to solve this issue is to fix the CoS_x on the conductive networks, such as the networks of carbon nanotubes.^25,29 The networks are facile for the diffusion of ions and the transfer of electrons to show the improved properties. A case in point was that Chia-Ying Chen et al. recently reported the preparation of carbon nanotubes (CNTs)/cobalt sulfide (CoS) nanoplates composites formed in the presence of poly(vinylpyrrolidone).²⁵ Cyclic voltammetry results revealed that the CNT/CoS composite electrodes yielded values of 1120 ± 80 F g⁻¹ for specific capacitance at a scan rate of 10 mV s⁻¹. To further improve the properties of CoS_x, some strategies should be developed to increase the load ration of CoS_x on the carbon nanotubes, including depositing the CoS_x coating on the carbon nanotubes.

In this study, the novel CoS_x/fMWCNTs shell/core composites were synthesized by a simple hydrothermal method. The CoS_x nanoparticles with a diameter of about 6 nm were homogenously coated on the carbon nanotubes. The CoS_x/fMWCNTs composite electrodes displayed extremely high specific capacitance up to 1324 F g⁻¹ at a current density of 10 A g⁻¹ and showed improved cyclic properties. The rate capacitance of the composites was also enhanced compared with bare CoS_x. This composite may provide a direction toward solving the potential problems and promises for the next generation high-performance electrochemical electrodes.

2 Experiments

2.1 Carboxylation of CNTs

2.0 g pure CNTs was dispersed in an 80 mL solution, which comprised of 25% concentrated nitric acid (65%) and 75% concentrated sulfuric acid (98%). This procedure was similar to the literature.³² Then it was poured into a three mouth flask, and put in an oil bath. The mixture was refluxed gently with stirring at 120 °C for 12 hours. After that, the solution was diluted with deionized water and centrifuged several times, then leached for about 12 h until fMWCNTs was dried completely.

2.2 Synthesis of CoS_x/fMWCNTs

0.1 g fMWCNTs above was dispersed in 40 mL deionized water by ultrasonic treatment for 30 min. Then 0.5 mmol cobalt dichloride was dissolved in the solution. After stirring for 5 h, 2 mmol thiourea was dissolved in the solution, stirring for another 30 min. The obtained solution was transferred into a 50 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. After cooling naturally to the room temperature, the black product was collected by centrifugation, repeatedly washed with distilled water and ethanol to remove impurities. Finally, the product was dried in an oven at 60 °C overnight.

2.3 Fabrications of the electrodes for supercapacitors

The working electrodes were prepared by mixing 75 wt% of CoS_x/fMWCNTs powder with 20 wt% of acetylene black (>99.9%) and 5 wt% of poly(tetrafluoroethylene) dried powder (PTFE). The first two components were mixed together in an agate mortar until homogeneous black powder was achieved. PTFE was then added to the mixture with several drops of ethanol. The synthesized paste was pressed at 10 MPa into a nickel foam, and dried for 12 h under vacuum at 60 °C to remove the solvent totally. The mass of active material was about 2.0 mg in each working electrode.

2.4 Characterization

The morphologies of the obtained structures were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), with an accelerating voltage of 5 kV. The fine structures were characterized using transmission electron microscopy (FEI Tecnai F30, operated at 300 kV) and X-ray diffraction (XRD, Philips, X'pert pro, Cu Kα, 0.154056 nm). Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 F1 Iris. Fourier Transform Infrared (FT-IR) spectra in the range of 400–4000 cm⁻¹ were recorded in KBr pellets using a Thermo Nicolet FT-IR spectrophotometer.

A three-electrode system was employed to measure the cyclic voltammetry, constant current charge–discharge properties, and electrochemical impedance spectroscopy, where a platinum gauze and a saturated calomel electrode (SCE) were used as a counter electrode and reference electrode, respectively. All measurements were carried out in a 6 M KOH electrolyte. Electrochemical performance was characterized by cyclic voltammetry with voltage scan rates of 2, 5, 10, 50, and 100 mV s⁻¹. The impedance properties of the electrodes were examined by impedance spectroscopy at an applied potential of 5 mV by using a CHI660 electrochemical work station. Data were collected in the frequency range of 10⁵ to 10⁻¹ Hz at those applied potential. The galvanostatic charge–discharge of the electrode (effective area: 1.0 cm × 1.0 cm) was evaluated in the certain range of potential at current densities of 2, 5, 10, 20, 40, and 50 A g⁻¹. The specific capacitance (C), energy density (E) and power density (P) were calculated according to the following equations:²⁸

where Δu is the potential (V), i is the discharging current (A), t is discharge time (s), and m is the mass (g) of the CoS_x/fMWCNTs shell/core composites on the electrode. The current densities were defined based on the mass of CoS_x/fMWCNTs shell/core composite.

3 Results and discussion

The morphology, nanosizes, and microstructure of the samples were characterized by SEM and TEM. Fig. 1A shows a typical SEM image of the fMWCNTs which keep their intact structure after concentrated sulfuric acid and nitric acid treatment.³² It can be found from the Fig. 1B that CoS_x has been successfully grown on a continuous fMWCNTs network because the diameter of the fiber-like nanomaterials in Fig. 1B is larger than the fMWCNTs. No aggregations of the CoS_x nanoparticles off the fMWCNTs scaffold are observed in the composites, indicating that the CoS_x were firmly fixed on the fMWCNTs. Additionally, transmission electron microscopy (TEM) images (Fig. 1C and D) reveal that the CoS_x/fMWCNTs shell/core nanostructures are synthesized. Detailed structural analysis of CoS_x nanoparticles reveals the crystalline grain with a lattice spacing of 0.24 nm which is very close to the (210) plane of CoS₂ (JCPDS# 41-1471). Similar composites about cobalt sulfide/CNT have been reported by Chen et al.²⁵ No surfactant was used in present procedure for the synthesis of the composites compared with the literature.²⁵ Besides, the cobalt sulfide was coated on the fMWCNTs to form the shell/core structure without any pieces of cobalt sulfide. The difference between the composites in present study and previous publication may be attributed the oxygen-containing group on the fMWCNTs. Obvious, the CoS_x coating is facile for the deposition of CoS₂ with a large load ratio compared with the decoration of nanoparticles. Moreover, the CoS_x nanoparticles are intertwined with highly conductive fMWCNTs, facilitating efficient electron transport and enabling effective electrolyte transport.

Fig. 1 (A) SEM image of fMWNTs, (B) SEM image of CoS_x/fMWNTs shell/core composites, (C) and (D) TEM images of CoS_x/fMWNTs shell/core composites.

As shown in Fig. 2A, the XRD pattern and relative intensities of CoS_x/fMWCNTs composites match well with those of CoS₂ (JCPDS# 41-1471), CoS (JCPDS# 75-605), and C (JCPDS# 75-444), demonstrating that the CoS_x/fMWCNTs composites are mainly composed of CoS₂, CoS, and C. It should be noticed that both CoS and CoS₂ could be used as active materials for pseudocapacitors. Thermogravimetry analysis (TGA) and derivative thermogravimetry (DTG) analysis were carried out in air to quantify the amount of components in the composites (Fig. 2B). The sample was heated from 40 °C to 900 °C at a rate of 10 °C min⁻¹. There are four stages of weight loss for CoS_x/fMWCNTs composites. The first weight loss appears at approximately 60–100 °C, which can probably be attributed to the evaporation of water remained in the sample.³³ The second one takes place at about 150–300 °C, where functional groups on the surface of fMWCNTs are thermally decomposed and evaporated.³⁴ The third one occurs between 500–660 °C, signifying the combustion of multi-walled CNTs.³⁵ The fourth one arises at approximately 700–820 °C, which are assigned to the oxidation of CoS_x to Co₃O₄.³⁶ By assuming that the remaining product after the TGA measurement is pure Co₃O₄, which has a weight percentage of approximately 29%, one can estimate that the CoS_x content in the initial CoS_x/fMWCNTs composites was approximately 47%. The introduced functional groups were detected by FT-IR. As shown in Fig. 2C, the FT-IR spectra verify the presence of some oxygen-containing groups in fMWCNTs, such as C–OH (3390 cm⁻¹), C–O–C (1230 cm⁻¹), and C=O (1730 cm⁻¹) in carboxylic acid moieties. All of the peaks in fMWCNTs are similar to those in published papers about fMWCNTs.³⁷

Fig. 2 (A) XRD pattern of the CoS_x/fMWCNTs composites, standard patterns of carbon (JCPDS# 75-444), CoS (JCPDS# 75-605) and CoS₂ (JCPDS# 41-1471), (B) TGA and DTG curves of the CoS_x/fMWCNTs composite and (C) FT-IR spectra of fMWCNTs.

According to above results, the strategy of synthesize CoS_x/fMWCNTs shell/core composites is schematically shown in Fig. 3. Firstly, there are some oxygen-containing functional groups on fMWCNTs. The Co²⁺ in the solution are preferentially adsorbed on these sites due to electrostatic force between Co²⁺ ions and polar oxygen functional groups introduced by acid treatment.³² Secondly, the adsorbed Co²⁺ acted as anchoring sites or nucleation sites for the growth of CoS_x. Thirdly, the CoS_x layer coated the fMWCNTs with the growth of CoS_x. In such a composite, CoS_x offers the attractive high specific capacitance and CNTs framework provides the improved electrical conductivity and mechanical stability.³⁸ The synthesis processes are complicated, since the interfacial chemistry condition may affect the nature of the deposited CoS_x. Fortunately, hydrothermal processing has proven to be a useful method for not only dispersing and functionalizing CNTs, but also preparing metal sulfide nanoparticles.

Fig. 3 Schematic illustration of the formation of CoS_x/fMWCNTs composites.

Cyclic voltammetry (CV) measurements are conducted to evaluate the electrochemical characteristics of CoS_x/fMWCNTs shell/core composites. Fig. 4A shows the CV curves of electrodes fabricated from CoS_x/fMWCNTs in 6 M KOH solution electrolyte with various scan rates. The shape of CV curves reveals that the capacitance characteristic of the CoS_x/fMWCNTs is distant from that of the electric double-layer capacitance, which would produce a CV curve with close to an ideal rectangular shape.³⁹ Two quasi-reversible electron-transfer processes are visible in curves, indicating that the capacity mainly results from the pseudocapacitive capacitance. At a low scan rate of 2 mV s⁻¹, the anodic peak at 0.29 V is due to the oxidation process, and the cathodic peak at about 0.21 V is related to its reverse process. The electrochemical redox reaction can be assumed as the eqn (4) and (5). The redox peak potentials and the profiles of the CV results are consistent with those results in the previous literature.^27,40 Furthermore, it is obvious that the redox peaks positions shifted with the increase of the scan rate from 2 to 100 mV s⁻¹. It may be that the charge transfer kinetics is the limiting step of the reaction.^41,42 The CoS_x nanoparticles used for the comparison were synthesized with a similar method but without the addition of fMWCNTs. As can be seen from Fig. 4B, the severely distorted shape of CV curve from pure CoS_x nanoparticles indicates its intrinsically poor electric conductivity.⁴³ It implies that the electrochemical properties of CoS_x nanoparticles have been greatly improved by introducing fMWCNTs as current collectors.

CoS_x + OH⁻ ⇌ CoS_xOH + H₂O + e⁻ (4)

CoS_xOH + OH⁻ ⇌ CoS_xO + H₂O + e⁻ (5)

Fig. 4 (A) The CV curves of the electrodes fabricated from CoS_x/fMWCNTs in 6 M KOH solution electrolyte with various scan rates: 2, 5, 10, 50, and 100 mV s⁻¹, (B) the CV curves of the electrodes fabricated from CoS_x/fMWCNTs and CoS_x at a scan rate of 10 mV s⁻¹, (C) the CV curves of the electrodes fabricated from fMWCNTs in 6 M KOH solution electrolyte with various scan rates: 2, 5, 10, 50, and 100 mV s⁻¹ (from inner to outer) and (D) Nyquist plot of the EIS of the CoS_x/fMWCNTs, CoS_x, and fMWCNTs composites.

As we have discussed, CNTs serve both as the electroactive material and the scaffold for the deposition of the CoS_x nanoparticles, thus both of their surface area and electrical conductivity are critical for obtaining high performance. The ideal CNTs should have high surface area as well as high conductivity. Most of the times, these two properties do not exist together. Normally, to obtain high surface area, CNTs are functionalized with functional groups to improve suspension stability and to reduce the bundle size before the fabrication of composites.⁴⁴ However, electrical conductivity of CNTs decreases if too much functional groups are created on the sidewalls of nanotubes. Therefore, to achieve both high capacitance and good rate performance, a proper balance between the specific surface area and the electrical conductivity must be achieved through well controlled functionalization steps. To investigate the chemical functionalization effect of carbon nanotubes on the electrochemical performance of electrodes by our functionalization method, electrodes of fMWCNTs have been fabricated. Fig. 4C shows the cyclic voltammograms for fMWCNTs networks over a range of scan rates from 2 to 100 mV s⁻¹. The rectangular shapes of the CV curves reveal that fMWCNTs are highly reversible as ideal capacitors. The area surrounded by CV curves of fMWCNTs is larger than that of unfunctionlized multi-walled CNTs, indicating that fMWCNTs obtain higher specific capacitance.⁴⁵ The enhanced electrochemical performance of the CoS_x/fMWCNTs shell/core composites electrode was further confirmed by the electrochemical impedance spectroscopy (EIS) measurements. Fig. 4D shows the Nyquist plots of the EIS spectra of CoS_x/fMWCNTs, CoS_x, and fMWCNTs, respectively. The equivalent series resistance, which is a combination of the inherent resistance of the electroactive material, bulk resistance of electrolyte, and contact resistance at the interface between electrolyte and electrode, is 0.35 Ω for CoS_x/fMWCNTs, showing good conductivity in aqueous electrolytes. From the slope of the EIS curve in the low frequency range, Warburg resistance of the CoS_x/fMWCNTs composite electrodes is smaller than that of the CoS_x electrodes. The CoS_x/fMWCNTs composite electrodes show a much smaller radius of semicircle in the Nyquist plots as compared to that of the CoS_x electrodes. This result indicates that the fMWCNTs not only improve the conductivity of the electrode, but also largely enhance the electrochemical activity of the composite electrodes during the cycling processes. Additionally, the resistance of fMWCNTs is much higher than that of unfunctionalized multi-walled CNTs, indicating that the functionalization increases the resistance of multi-walled CNTs, similar to previous results.⁴⁶

Rate capability is one of the most important factors to evaluate the applications of supercapacitors. An excellent electrochemical energy storage device is required to provide a high specific capacitance at a high charge–discharge rate. The galvanostatic charge–discharge curves of the as-prepared CoS_x/fMWCNTs composites at different current densities are shown in Fig. 5A. The nonlinearity in the charge–discharge curves, unlike battery and double-layer capacitors,^47,48 shows representative pseudocapaciance behavior of CoS_x/fMWCNTs arising from the electrochemical adsorption/desorption or redox reaction at the electrode/electrolyte interface. The specific capacitances derived from the charge–discharge curves at different current densities are shown in Fig. 5B. CoS_x/fMWCNTs composites not only exhibit high specific capacitance but also maintain those values at high current density compared to other electrodes. As shown in Fig. 5B, the CoS_x/fMWCNTs composites deliver 52.4% of their specific capacitance (from 1717 to 900 F g⁻¹) as the current density increasing from 2 to 40 A g⁻¹. However, the specific capacitance of CoS_x (788 F g⁻¹) are not only much lower than the composites but also decrease significantly with the increase of current densities (e.g., from 788 to 309 F g⁻¹ at current density of 2 to 40 A g⁻¹). The superior rate capability in the composite electrodes can be attributed to the reduced diffusion path of ions, high surface area and increased electrical conductivity. Because of the synergetic contribution from fMWCNTs and CoS_x, the high-surface area and porous network structure allow a higher rate of solution infiltration and facilitate the ions insertion/extraction and electrons transport in the electrode. On the contrary, the severer aggregation, lower conductivity, and poor mechanical stability in CoS_x block the ion diffusion and electron transport, compromising their electrochemical performance. Table 1 summarizes some recently papers about supercapacitors. On comparison of these values, our CoS_x/fMWCNTs shell/core composites electrodes possess a higher value of specific capacitance. Those results also were better than that of Co(OH)₂ nanosheets for supercapacitors.¹⁷

Fig. 5 (A) Galvanostatic charge–discharge curves of CoS_x/fMWCNTs at various current densities, (B) specific capacitance of CoS_x/fMWCNTs composite and CoS_x at different charge–discharge current densities, (C) charge–discharge cycle test of CoS_x/fMWCNTs, CoS_x and fMWCNTs at current densities of 10 A g⁻¹ and (D) Ragone plot of the estimated energy density and power density at various charge–discharge rates.

Table 1 Recently supercapacitors based on CoS_x with high capacitances

Material	C (F g^−1)/scan rates (mV s^−1)	C (F g^−1)/current density (A g^−1)	Ref.
CoS/CNT	1120/10	—	25
Annealed CoS/CNT	2140/10		25
CoSx	—	650/4	30
β-CoS/G	—	1184/10	28
CoS2/G	—	314/0.5	26
CoSx/fMWCNTs	—	334/0.4	29
CoSx/fMWCNTs	—	1324/10	This work

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Furthermore, the long-term cyclic stability of the CoS_x/fMWCNTs shell/core composites is investigated using galvanostatic charge–discharge measurement at a current density of 10 A g⁻¹ in a potential window of −0.1 to 0.405 V. Fig. 5C illustrates that the CoS_x/fMWCNTs electrodes show about 13% decay (from 1324 to 1148 F g⁻¹) in available specific capacity after 2000 cycles, while CoS_x electrodes show about 23% decay (from 534 to 412 F g⁻¹), indicating that the CoS_x/fMWCNTs electrodes display excellent stability. The power density (P) and energy density (E) are generally used as important parameters to characterize the electrochemical performance of the supercapacitors. The energy densities and power densities can be further calculated from the galvanostatic discharge curves using the previous eqn (2) and (3). In Fig. 5D, the Ragone plot of the estimated specific energy and specific power at the various current densities. The energy densities are 61, 57, 53, 47, 40, 32, 28, 21, and 20 W h kg⁻¹, while the specific powers are 0.14, 0.21, 0.35, 0.7, 1.43, 2.91, 3.5, 5.53, and 6.94 kW kg⁻¹ at current densities of 2, 3, 5, 10, 20, 40, 50, 80, and 100 A g⁻¹, respectively. High energy (ca. 61 W h kg⁻¹) and high power (6.94 kW kg⁻¹) densities were achieved with slow and fast charge–discharge rates, respectively.

4 Conclusions

A simple and cost-effective approach is developed to fabricate CoS_x/fMWCNTs shell/core nanocomposites. In such a composite, each component provides much critical function for efficient use of metal sulfide for energy storage. fMWNTs not only provide high surface for the deposition of CoS_x nanospheres but also improve the electrical conductivity and the mechanical stability of the composites, while the CoS_x nanoparticles provide high specific capacitances. As electrodes for supercapacitors, the CoS_x/fMWCNTs shell/core nanocomposites could deliver a capacitance of 1148 F g⁻¹ after 2000 cycles, much higher than those of CoS_x and fMWCNTs. Besides, the composites showed an energy density of 20 W h kg⁻¹ with a power density of 6.94 kW kg⁻¹, which was very higher than that of CoS_x. We believe this design concept can be generalized toward other electrochemical materials containing metal sulfide, opening a new avenue for a large spectrum of device applications.

Acknowledgements

This work was partly supported from the National Natural Science Foundation of China (Grant no. 61376073, 21003041), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20120161110016), the Hunan Provincial Natural Science Foundation of China (Grant no. 11JJ7004), and the Hunan Provincial Major Project of Science and Technology Department (Grant no. 2012TT1004).

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