Hydrothermal synthesis of Ni₃S₂/graphene electrode and its application in a supercapacitor

Abstract

In this work, we demonstrate a simple hydrothermal synthesis of nickel sulfide/graphene nanosheets (Ni₃S₂/GNS) and use the nanocomposite as an electrode for a supercapacitor. X-ray diffraction, scanning electron microscopy and Raman spectroscopy were used to investigate the morphologies and microstructures of the resulting electrode materials. Detailed electrochemical characterization shows that the Ni₃S₂/GNS electrode exhibits high specific capacitance of about 1420 F g⁻¹ at a current density of 2 A g⁻¹. At a current density of 6 A g⁻¹, the specific capacitance of the Ni₃S₂/GNS electrode remains fairly constant at the initial value over 2000 cycles, obviously illustrating a relatively high cycling stability. The outstanding electrochemical properties of the Ni₃S₂/GNS nanocomposite suggest that it has great potential for practical applications in high-performance supercapacitors.

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1. Introduction

As a rising star, graphene has attracted the most intensive attention recently, due to its high specific surface area of 2600 m² g⁻¹, high electrical conductivity, chemical stability and excellent mechanical properties.^1,2 These outstanding properties guarantee its potentially significant utility in many fields, such as field-effect transistors, rechargeable lithium-ion batteries, fuel cells, solar cells, supercapacitors and electrochemical sensors.^3–17 Among them, supercapacitors, also known as electrochemical capacitors, are anticipant to have great potential applications in future energy storage devices, because of their high power density, long life span and fast discharge capability.^18–20 Recently, graphene-based carbonaceous materials used as electrodes for supercapacitors have been investigated extensively as alternatives to the conventional graphites.²¹ Although graphene-based electrodes for supercapacitors can store and release energy rapidly and reversibly, owing to the physical and reversible ion adsorption at the electrode–electrolyte interfaces, the low energy density of graphene limits its utility seriously. In order to address this problem, many pseudocapacitive transition-metal oxides such as MnO₂, RuO₂, and NiO have been widely studied, because they can display more excellent electrochemical performance as the result of rapid and multiple redox reactions.^22–26

In recent years, the fabrication of nickel sulfide used as a supercapacitor electrode has been investigated extensively. Among all kinds of nickel sulfide, Ni₃S₂ has become one of the ideal active materials due to its excellent special capacity.^27–29 To date, Xing et al. demonstrated the electrodeposition of Ni₃S₂ on a ZnO array,³⁰ and Chou et al. has synthesized a flaky Ni₃S₂ nanostructure by a facile potentiodynamic deposition approach and employed the material as an electrode for a supercapacitor exhibiting a specific capacitance of 717 F g⁻¹ at 2 A g⁻¹ as well as a remarkable rate capability and excellent cycling performance.³¹ Most recently, Zhou et al. reported a novel hydrothermal process for preparing Ni₃S₂ nanostructures³² and successfully deposited them on a three-dimensional (3D) graphene network, which exhibited excellent capacitive performance.³³ However, the 3D graphene template was synthesized by a chemical vapor deposition process requiring an experimental temperature as high as 1000 °C. Although the preparation of the graphene oxide (GO) precursor requires a complicated experimental procedure, solution-processed GO has been widely used to form graphene-based composites and 3D graphene networks via a simple hydrothermal route due to its mild synthetic conditions as well as simple manipulation.^34,35

In this paper, we demonstrate a simple and facile one-step hydrothermal route to fabricate nickel sulfide/graphene nanosheets (Ni₃S₂/GNS) nanocomposites, in which graphene provides anchors for nickel sulfide nanoparticles, presents the excellent advantages structure built by pseudocapacitively nanoparticles and conducting graphene nanosheets for electrochemical performance. The prepared electrode material exhibits very high specific capacitances of 1420 F g⁻¹ and 782 F g⁻¹ at current rates of 2 A g⁻¹ and 10 A g⁻¹, with superior cycling stability. The significant improvement in electrochemical performance is mainly attributed to the contribution of graphene that provides excellent electrical conductivity and mechanical stability.

2. Experimental section

2.1 Materials preparation

Graphene oxide was prepared from graphite power by a simplified Hummer's method.³⁶ The Ni₃S₂/GNS electrode was obtained via the simple hydrothermal technique as follows: typically, nickel foam was ultrasonically washed with acetone, 2 M HCl solution, deionized water, and absolute ethanol, sequentially, for 15 minutes, to ensure a clean nickel foam surface. The freshly washed nickel foam was then totally immersed into a 50 ml Teflon autoclave with a homogeneous solution containing 20 ml water, 10 mg GO, and 30 mg thioacetamide (TAA, C₂H₅NS). Then the mixture was heated at 180 °C for 6 h. Afterward, the autoclave was naturally cooled down to room temperature, and the obtained materials were taken out, rinsed with deionized water and ethanol for several times, and dried at −55 °C for 12 h under vacuum. For comparison, pure Ni₃S₂ and pure GNS deposited on nickel foam were also prepared under the same procedure.

2.2 Structural characterizations

Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/MAX-2500 diffractometer. The surface morphologies and microstructures of the resulting products were characterized by scanning electron microscopy (SEM, JEOL, JSM-6360). Raman measurements were carried out by employing Renishaw inVia micro Raman spectrometer, excited at room temperature with laser light at 532 nm.

2.3 Electrochemical tests

The electrochemical properties of the as-obtained electrode material were investigated in a three-electrode configuration in which the electrode covered with Ni₃S₂/GNS nanocomposite was used as the working electrode, and platinum foil and Ag/AgCl served as the counter and reference electrodes, respectively. All the electrochemical measurements including cyclic voltammograms and galvanostatic charge–discharge tests were carried out in 1 M KOH aqueous solution with a CHI660D (ChenHua, China) electrochemistry workstation. Electrochemical impedance spectroscopy (EIS) measurement was also carried out in the frequency range of 100 kHz to 0.01 Hz with a perturbation amplitude of 5 mV.

3. Results and discussion

Fig. 1(a) shows the XRD pattern of the sample prepared in this study. Several distinct diffraction peaks at 21.7°, 31.1°, 37.7°, 50.1° and 55.3° are observed, which belong to (101), (110), (003), (211), and (300) of the nickel sulfide (Ni₃S₂), respectively (JCPDS no. 44-1418). Furthermore, the other peaks can be indexed to the single nickel foam (JCPDS no.04-0850). Therefore, it is reasonable to think that the Ni₃S₂ nanoparticles have been fabricated via the hydrothermal approach. In order to further examine the crystal structures of the as-prepared samples, the Raman spectra of the Ni₃S₂/GNS nanocomposite, sole graphene, and pure Ni₃S₂ were obtained, as shown in Fig. 1(b). Obviously, the Raman spectrum of pure Ni₃S₂ shows various peaks at ∼200 cm⁻¹, ∼222 cm⁻¹, ∼305 cm⁻¹, ∼324 cm⁻¹, and 350 cm⁻¹, which can be attributed to vibration of Ni₃S₂.³⁷ Additionally, two significant Raman peaks can be observed at 1355 cm⁻¹ (D band) and 1573 cm⁻¹ (G band) for GNS, corresponding to the breathing mode of A_1g symmetry involving photons near the K zone boundary and assigned to the E_2g mode of sp²-bonded carbon atoms, respectively,³⁸ strongly indicating the presence of graphene. Compared to the pure Ni₃S₂ and GNS, the above characteristic bands are observed for all of the as-prepared composite samples, confirming that the Ni₃S₂/GNS hybrid nanostructures had been successfully fabricated.

Fig. 1 (a) X-ray diffraction pattern of Ni₃S₂/GNS. (b) Raman spectra of pure Ni₃S₂, GNS and Ni₃S₂/GNS nanocomposite.

The morphologies of plain nickel foam (Supporting Information, Fig. S1†), pristine graphene electrode (Fig. S2†) and pure Ni₃S₂ (Fig. S3†) were characterized by Scanning Electron Microscope (SEM) observation. Fig. 2 shows the SEM observations of the Ni₃S₂/GNS hybrid structure. At a low magnification (inset in Fig. 2(b)), the porous nickel foam network is found to be densely and homogeneously covered by Ni₃S₂/GNS composite. Furthermore, acting as a support and spacer to prevent the agglomeration of Ni₃S₂, the graphene nanosheets (indicated by red arrows) could be strongly and uniformly anchored by Ni₃S₂ nanoparticles, forming the special conjugated network nanostructure. The resulting electrode with a unique characteristic structure may provide not only channels for electrolyte diffusion and ionic conduction toward nickel sulfide nanoparticles, but also improvement in the active sites for redox reaction, due to the intrinsically high electrical conductivity and large network nanostructure of graphene nanosheets. All these excellent properties are proposed to benefit the reversible capacity and rate performance, leading to potentially extensive utilization of the obtained material as an electrode for supercapacitors. The EDS spectrum of the Ni₃S₂/GNS nanocomposite in Fig. S4† clearly illustrates the presence of carbon (C), nickel (Ni) and sulfide (S) elements, which is in good agreement with the XRD and Raman results. According to the above results, we present the following synthesis mechanism. Via a mildly hydrothermal process, graphene oxide could be reduced to graphene nanosheets and deposited on the surface of Ni foam, forming an individual 3D graphene network simultaneously, for use as a template for the anchor of an active material. Meanwhile, after the hydrothermal treatment, a Ni source is applied, and the Ni foam exposed to a solution for absorption of the active species (S ions) released from thioacetamide (TAA) to form small Ni₃S₂ nanoparticles anchored on the graphene nanosheets. Compared with that in a previous report,³³ this one-step hydrothermal formation of Ni₃S₂/GNS hybrid is more facile with simple manipulation and mild synthetic conditions, and because the active material is in situ grown on a nickel foam current collector, the electrode preparation does not require an additional polymer binder and coating procedure.³⁹

Fig. 2 Scanning electron microscope (SEM) images of Ni₃S₂/GNS nanocomposite (inset is the overview with low-magnification).

According to the cyclic voltammetry (CV) curves of Ni₃S₂/GNS at various scan rates as shown in Fig. 3(a), a couple of obvious redox peaks can be observed, corresponding to the reaction between Ni²⁺/Ni³⁺ and anions OH⁻,^30,39 indicating the typically reversible pseudocapacitive reactions, which largely contribute to the overall specific capacitance. That is very different from the fairly rectangular CV curves of graphene-based electrodes. For instance, the CV plots of pure GNS prepared under the same procedure are shown in Fig. S5(a),† and their shape is caused by the charge separation occurring at the electrode and electrolyte interface, known as double-layer capacitance behavior.^1,40,41 Moreover, with an increase in scan rate, the height of the redox reaction peaks increases linearly and a progressive shift of the peaks to a higher voltage is presented, demonstrating that the fast redox reactions occur at the interface between the active material and electrolyte. In addition, the linear relationship between the square root of the scan rate and the oxidation peak current at different scan rates is shown in Fig. 3(b), implying that the process is diffusion-controlled and reversible. Even when the scan rate is increased to 20 mV s⁻¹, the shape of the CV curve is retained well, illustrating that the as-obtained electrode material has desirable pseudocapacitive properties, which is generally required for use in power devices. For comparison, the CV curve for pure Ni₃S₂ was also obtained, and clearly similar redox peaks were observed [Fig. S5(b)†].

Fig. 3 (a) Cyclic voltammetric curves of Ni₃S₂/GNS composite at various scan rates from 2 to 20 mV s⁻¹. (b) The relationship of the square root of the scan rate and the oxidation peak current at different scan rates.

Fig. 4(a) and Fig. S6(b)† present the typical discharge curves of the Ni₃S₂/GNS nanocomposite and pure Ni₃S₂ at galvanostatic current densities of 2, 3, 4, 6, 8, and 10 A g⁻¹ in the potential range of 0–0.5 V, respectively. Apparently, a well-defined plateau region during the discharge processes can be observed, which is consistent with the results of the above CV measurements, suggesting that the as-prepared sample has excellent pseudocapacitive behavior. Different from the above results, the charge–discharge curves for the pure GNS electrode (Fig. S6(a)†) were fairly symmetric and linear, again demonstrating the double-layer capacitance behavior. In contrast, Fig. 4(b) shows the specific capacitance of Ni₃S₂/GNS and pure Ni₃S₂ electrode materials in accordance with the charge–discharge curves at different current densities. The Ni₃S₂/GNS electrode delivers specific capacitances of 1420 F g⁻¹ and 782 F g⁻¹ at the current densities of 2 A g⁻¹ and 10 A g⁻¹ in 1 M KOH aqueous solution, which are much higher than those of the sole Ni₃S₂ electrode at the same current densities, implying a superior high-rate capability of the hybrid nanostructure.

Fig. 4 (a) Discharge curves of Ni₃S₂/GNS nanocomposites at different current densities (2, 3, 4, 6, 8, or 10 A g⁻¹). (b) Specific capacitance for the Ni₃S₂/GNS electrode and sole Ni₃S₂ at various current densities (2, 3, 4, 6, 8, 10, or 12 A g⁻¹).

Cycling capability or cycling life is a significant parameter to examine the electrochemical performance of an electrode for supercapacitor application. In order to examine the cyclability of the as-prepared electrode, consecutive galvanostatic charge–discharge experiments for 2000 cycles at a high current density of 6 A g⁻¹ were conducted in 1 M KOH with a potential window of 0–0.50 V. As shown in Fig. 5(a), although a slight decrease in the initial stage is observed because the active materials could not be fully used, the overall specific capacitance remains fairly static and is still able to achieve 994.4 F g⁻¹ after 2000 cycles. The last 20 charge–discharge curves of Ni₃S₂/GNS are displayed (inset in Fig. 5(a)), which indicates that the as-obtained Ni₃S₂/GNS electrode offers excellent long-term electrochemical cyclability even under fast charge–discharge conditions. All together, the above experimental data demonstrate that the as-prepared Ni₃S₂/GNS composite is a promising candidate for designing high-performance supercapacitors.

Fig. 5 (a) Cycling measurements at a current density of 6 A g⁻¹ (inset shows the galvanostatic charge–discharge curves for the last 20 cycles). (b) Nyquist plots for the Ni₃S₂/GNS electrode and sole Ni₃S₂ (insets are the corresponding equivalent circuit and the partial, enlarged Nyquist plots).

The electrochemical performance was further investigated by electrochemical impedance spectroscopy (EIS). A Nyquist plot was obtained in 1 M KOH at frequency from 100 kHz to 0.01 Hz with an amplitude of 5 mV and is illustrated in Fig. 5(b), with a fitted equivalent circuit (inset) and an enlarged view (inset). Similar to that in a previous report,⁴² the Nyquist plot is composed of three parts: the intersection point with the real x axis indicative of the internal resistance of the electrode (R_s), the semicircle at a relative high frequency section implying the charge-transfer resistance (R_ct) at the electrode–electrolyte interface and a linear region at low frequency part corresponding to typical capacitor behavior. Notably, the Ni₃S₂/GNS nanocomposites present a smaller internal resistance (R_s) (1.14 Ω) than that of pure Ni₃S₂ (1.72 Ω), a semicircle with a smaller diameter that is representative of lower charge transfer resistance, and a steeper straight line with a phase angle of 74.48° larger than that (56.88°) of pure Ni₃S₂, showing the pronounced capacitive behavior with less diffusion resistance, due to the addition of GNS. Therefore, the low charge-transfer resistance and superior capacitive performance of the Ni₃S₂/GNS nanocomposite enable fast redox reaction and easier electron transport, implying the broad potential of the nanocomposite for application in supercapacitors.

The improvement of electrochemical performance and high cycling stability of the Ni₃S₂/GNS nanocomposite is proposed to be attributed to the following aspects. First, graphene nanosheets in the composite can provide not only enough anchors for nickel sulfide nanostructures, but also effective pathways allowing for the fast and facile ion charge transfer, due to the large network nanostructure and superior conductivity of graphene. Furthermore, the Ni₃S₂/GNS hybrid structure has been directly deposited on a nickel foam substrate, which used as a binder-free electrode can avoid the increase in contact resistance between the active material and collector.

4. Conclusion

In summary, we have fabricated an Ni₃S₂/GNS nanocomposite and employed it as an electrode material for supercapacitors. With excellent cycle stability, the obtained electrode delivers high specific capacitances of 1420 F g⁻¹ and 782 F g⁻¹ at constant current rates of 2 A g⁻¹ and 10 A g⁻¹, respectively. Owing to the accompanying graphene nanosheets, the Ni₃S₂/GNS electrode exhibits excellent improvements in electrochemical properties, due to its large network nanostructure and high conductivity. Therefore, we conclude that the as-prepared nanocomposites have exciting potential in electrochemical energy storage applications.

Acknowledgements

This work was supported by grants from the National Natural Science Foundation of China (nos 51002129 and 51172191), Open Fund based on the innovation platform of Hunan colleges and universities (no. 13K045), the Provincial Natural Science Foundation of Hunan (no. 14JJ3079), a China Postdoctoral Science Foundation funded project (no. 20100480068) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13093).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05078a

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