One-step strategy to a three-dimensional NiS-reduced graphene oxide hybrid nanostructure for high performance supercapacitors

Abstract

Metal sulfides are an emerging class of high-performance electrode materials for electrochemical energy storage devices. Here, a facile hydrothermal method is reported to assemble three-dimensional (3D) NiS-reduced graphene oxide (rGO) hybrid aerogels with strong coupling between the two compounds. It is intriguing to note that NiS nanoparticles are well anchored on the 3D porous and conductive scaffold constructed from wrinkled rGO nanosheets. When evaluated as binder-free electrode materials for supercapacitors, impressive electrochemical performances are presented. Specifically, the 3D NiS–rGO aerogel nanocomposite exhibits a high capacitance of 852 F g⁻¹, 526 F g⁻¹ based on the whole electrode mass (m_NiS : m_GO = 45 mg/50 mg) at a current density of 2 A g⁻¹ and 15 A g⁻¹, respectively. These satisfactory electrochemical behaviors, attributed to the introduction of reduced graphene oxide, suggest the great promise of fabricating graphene-supported hybrid electrode materials for high-performance energy applications.

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1. Introduction

The increasingly limited availability of fossil fuels and the deterioration of the environment have greatly affected the world economy and ecology. In these circumstances, the development of environmentally friendly high-power energy resources is becoming more and more significant.¹ Batteries, as the most common electrical energy-storage device, suffer from low power density, which are not applicative on special occasions where fast charge and discharge process coupled with high power is required.² Compared with conventional batteries, electrochemical capacitors, also known as supercapacitors, can provide higher specific power density, longer cycle life, and therefore are attracting a great deal of attention as an supplement to batteries in some specific energy storage field.^3,4 Among two types of common electrode materials for supercapacitors, faradaic capacitive materials (such as metal oxides, metal hydroxide and conductive polymers) stimulate extensive interests due to their rich redox reactions and high specific capacitance.^5,6

In recent years, a variety of metal sulfides with nanostructures such as NiCo₂S₄,⁷ Ni₃S₂,⁸ CuS,⁹ SnS₂,¹⁰ CoS₂,¹¹ CoS¹² have been widely investigated as a new type of faradaic capacitive electrode materials for energy storage due to their excellent redox reversibility, relatively high capacitance and unique physical and chemical properties (e.g., higher electrical conductivity, mechanical stability than those of corresponding metal oxides).^13,14 As one important member of the metal sulfides, nickel sulfide has aroused increasing attention because of its advantage such as relatively high electronic conduction, cost effectiveness and high theoretical capacity.^15,16 Therefore, different phases of nickel sulfide, such as NiS and Ni₃S₂ with various morphology, including hollow spheres,^17,18 nanochains,¹⁹ nanorods,²⁰ nanotube array,²¹ and nanoparticles,²² have been widely studied as electrode materials for supercapacitors and lithium ion batteries. However, their electrical conductivity is still unsatisfactory to support fast electron transport toward high rate capability, which will limit their practical applications to some extent. The mass integration of electrochemical active materials onto carbon nanomaterials is one of the feasible methods usually adopted to alleviate the problem.

Recently, mounting efforts have been focused on developing graphene-based macroscopic materials with three-dimensional (3D) porous networks.^23,24 Such 3D graphene framework, characterised by large surface area and the short diffusion paths for both electron and ions, shows their great potential applications in electrochemistry. Since prepared successfully, graphene oxide (GO) has emerged as a precursor offering the potential of cost-effective, large-scale production of graphene-based materials.²⁵ As one important derivative of the two-dimensional graphene family, reduced graphene oxide (rGO) is characterized by its excellent photoelectric properties, conductivity, mechanical strength and good stability.²⁶ Although series of reduced graphene oxide (rGO)/nickel sulfide hybrid nanocomposite have been reported,^27–30 3D conducting graphene scaffold were not constructed to support the pseudocapacitive materials. Furthermore, these materials are in powder morphology and thus need to be mixed with polymer binders, which may introduce undesirable interfaces and affect their electrochemical performance.

In this work, 3D self-assembled NiS–rGO aerogel was successfully prepared by a simple one-step hydrothermal method followed by supercritical drying. NiS nanoparticles were successfully incorporated into the 3D conductive graphene framework to take the advantages of both components. When evaluated as electrode materials for supercapacitors, no polymer binders and conductive agents were added. Impressively, the self-assembled NiS–rGO aerogel shows a high capacitance of 852 F g⁻¹ at a current density of 2 A g⁻¹, and a promising specific capacitance of 526 F g⁻¹ can still be delivered at 15 A g⁻¹, which exhibits the remarkable rate capability. The excellent performances make this hybrid nanostructure a promising candidate as the electrode materials for high performance supercapacitors.

2. Experimental section

Materials

All the reagents were of analytical grade and used without further purification. NiCl₂·6H₂O, thiourea, graphite (crystalline powder, 400 mesh), P₂O₅, K₂S₂O₇, 30% H₂O₂, HCl, H₂SO₄ and diethanol amine were obtained from Sino Pharm Chemical Reagent. The absolute ethanol and potassium hydroxide were purchased from Chinasun Specialty Products Co, Ltd.

Synthesis

Graphene oxide (GO) solution was initially prepared by a modified Hummers' method.³¹ In a typical procedure, a proper amount of GO was dispersed into distilled water by stirring, followed by the addition of 0.03 M NiCl₂·6H₂O and 0.125 M thiourea and diethanol amine (the volume ratio of H₂O and diethanol amine is 3 : 1). After stirring for 30 minutes, the hybrid was transferred to a 20 mL Teflon-lined autoclave and heated at 180 °C for 12 h. After the autoclave was allowed to cool to room temperature, the as-prepared hydrogels were taken out with tweezers, washed with distilled water for several times and supercritical dried into aerogels after sufficient substitution with ethanol. For comparison, pure NiS powder was prepared by the same procedure in the absence of graphene.

Materials characterization

The samples were characterized by field-emission scanning electron microscopy (FESEM, Quanta 400FEG, FEI), X-ray Energy Dispersive Spectrometer (EDS, Apollo40SDD) and Transmission electron microscope (TEM, Tecnai G2 F20 S-Twin, FEI). The structure of the products was examined by X-ray powder diffraction (XRD, D8Advance, Bruker AXS).

Electrochemical measurements

When used as working electrode, pure NiS was mixed with acetylene black and polytetrafluoro-ethylene (PTFE) binder (weight ratio of 80 : 10 : 10), pressed onto a nickel foam (NF) current collector and dried at 60 °C overnight. The prepared NiS–rGO aerogels were taken out to put onto the Nickel Foam, then pressed by Hydraulic pressure machine under the pressure of 10 Mpa as working electrode without any other additives. Electrochemical measurements (CHI 660D electrochemical workstation) were conducted in a three-electrode configuration at room temperature using 6 M KOH as electrolyte, a platinum wire as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge–discharge were performed in the potential range of −0.1 to 0.35 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an AC voltage with 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz at open circuit potential.

3. Results and discussion

The NiS/rGO 3D macroscopic hydrogel was formed in a hydrothermal process, and a subsequent supercritical drying process contributed to transforming it into corresponding aerogel (Fig. 1).

Fig. 1 Schematic illustration for the synthesis of three-dimensional NiS-reduced graphene oxide (rGO) hybrid aerogel.

Initially, the GO sheets are relatively well dispersed in water due to existence of hydrophilic oxygen-containing functional groups on their surface.²³ During the hydrothermally reduced process at 180 °C, the graphene oxide (GO) sheets become hydrophobic due to their diminished oxygenated functionalities, making GO sheets tend to shrink and flock together. During the formation of graphene hydrogel, plenty of positive Ni²⁺ are anchored on surface of GO sheet and then with the help of thiourea, NiS nanopraticles were gradually in situ grown on GO sheets. As presented above, GO is selected as the substrate for the in situ growth of NiS nanoparticles and thus the rGO network affords an efficient electron pathway to provide better electrical contact with generated NiS nanopraticles.

The crystal phase and chemical structure of the product were characterized by XRD measurements. Fig. 2 shows the broad XRD patterns of NiS/GR hybrid aerogel. It is clear to see that both position and intensity of the main diffraction peaks were matched to those of NiS (JCPDS no. 086-2281). Thus, we can easily conclude that the NiS prepared were pure and single phase. To further validate the chemical composition, the energy dispersive X-ray spectroscopy (EDS) measurement was carried out. The corresponding elemental analysis shows the atomic ratio of Ni to S is nearly close to 1 : 1 (Fig. S1†), which further confirms that the as-prepared aerogels are merely composed of graphene and NiS without any other unexpected by-products.

Fig. 2 XRD pattern of self-assembled NiS–rGO aerogel.

Fig. 3 shows the typical SEM images and TEM images of the self-assembled NiS–rGO aerogel. As shown in Fig. 3a and b, the graphene sheets construct interconnected 3D porous and loose network architecture with numerous NiS nanoparticles dispersed on them as had been supposed. On one hand, such unique architecture is electrically conductive and could protect the NiS nanoparticles from pulverization during the charging and discharging process. On the other hand, the NiS nanoparticles existed on rGO nanosheets may contribute to effectively preventing the agglomeration of rGO nanosheets in the process of hydrothermal reaction, and consequently could contribute to taking fully advantages of pseudocapacitance.³² TEM studies were carried out to further characterize the structure of the graphene/NiS hybrid aerogels (Fig. 3c and d). Clearly, nanosized NiS particles with an average size of 25 nm are uniformly anchored on rGO scaffold, which is shown in Fig. 3c. The high magnification TEM image of the NiS particle visibly reveals its lattice fringes with a lattice spacing of 0.63 nm, which is corresponding to the (110) plane of NiS.

Fig. 3 FESEM images of self-assembled NiS–rGO (50 mg GO was added) aerogel (a and b); Typical TEM images of self-assembled NiS–rGO aerogel (50 mg GO was added).

In order to investigate the influence of GO and diethanol amine on the architecture of the finally prepared NiS, a series of experiments were carried out. Fig. 4 presents FESEM images of pure NiS and self-assembled NiS–rGO aerogel which was prepared without the addition of diethanol amine. Firstly, pure NiS powder was prepared under the same condition as the one in the typical process except the addition of GO. It can be noted that the NiS nanoparticles tend to get together and become larger without the involvement of GO (Fig. 4a and b), which may decrease the specific surface area of NiS and thus could be bad for the fully utilization of NiS as the electrochemical active material during electrochemical process. While effectively prevents the aggregation of NiS particles, pure GO without the involvement of diethanol amine fails to have a good control of the size of the NiS particles, which is shown in Fig. 4c and d. After comparison with the product prepared in a typical process, we may come to the conclusion that H₂DEA and its ionized analogs could adopt a chelating mode to bind metal ions and as a result NiCl₂ could react with H₂DEA to form a H₂DEA–NiCl₂ complex,³³ avoiding the growth of NiS nanoparticles into large particles. Therefore the existence of graphene oxide and DEA together helps to control the morphology of nickel sulfide and ensure a well dispersion state and intimate interaction between the NiS and the rGO nanosheets matrix, which may contribute to the improvement of its electrochemical performance.

Fig. 4 FESEM images of NiS (a and b); FESEM images of self-assembled NiS–rGO aerogel without the addition of EDA.

In order to highlight the advantages of the NiS/rGO nanocomposite as an electrode for supercapacitors, further comparison between NiS/rGO hybrid aerogel and pure NiS powder was conducted as shown in Fig. 5. The CV profiles of NiS/rGO composite aerogel and pure NiS powder share the same characteristics (see Fig. 5a), but the area covered by the former is larger due to the contribution of both redox reaction of the NiS and the double-layer charging/discharging process of the rGO, indicating that the introduction of rGO have significant influence on electrochemical performance of NiS in the hybrid aerogel.³² Fig. 5b shows the galvanostatic charge/discharge curves of the electrodes prepared with pure NiS and NiS/rGO nanocomposite at a current density of 5 A g⁻¹. Obviously, for pure NiS, the charge/discharge duration is relatively short, which could be ascribed to the limited utilization of NiS active material. The comparison of rate performance between the NiS powder and NiS/rGO electrodes is presented in Fig. 5c. It's obvious to note that pure NiS powder exhibits an inferior rate capability when compared with the NiS/rGO hybrid aerogel. The specific capacitance of NiS powder is 462 F g⁻¹ at a current density of 2 A g⁻¹, whereas the specific capacitance is only 80 F g⁻¹ at a high current density of 15 A g⁻¹. While the corresponding values of NiS/rGO hybrid aerogel electrode were 852 F g⁻¹ and 526 F g⁻¹ based on the whole electrode mass (m_NiS : m_GO = 45 mg/50 mg), respectively. Besides, we conducted the electrochemical measurement of pure rGO aerogel under the same condition (Fig. S2†). CV and Galvanostatic charge–discharge test demonstrates that: the capacitance of pure rGO (33 F g⁻¹ at 5 A g⁻¹) is far lower than the value of NiS–rGO hybrid (800 F g⁻¹ at 5 A g⁻¹). Thus, we may safely draw the conclusion that the contribution of the rGO to the whole capacitance is negligible in the potential range of −0.1 to 0.35 V. In order to investigate the charge and ionic transfer characteristics of the NiS powder and NiS/rGO hybrid aerogel electrode, EIS measurements were carried out, which is shown in Fig. 5d. It is observed that the hybrid aerogel electrode shows a much smaller radius of semicircle in the Nyquist plots as compared to that of the NiS powder electrode, suggesting that the aerogel exhibits a much lower charge transfer resistance (R_ct) because of the presence of rGO. The lower R_ct of the NiS/rGO hybrid aerogel could contribute to the enhancement of rapid charge transfer of the electrodes.^1,34 Furthermore, in the low frequency, the straight line in the EIS spectra inclines at an angle of nearly 90°, indicating that the NiS/rGO hybrid aerogel electrode has good electrochemical double-layer formation on its surface, which is beneficial for increasing the area of electrochemical reaction.³⁵

Fig. 5 CV curves at 5 mV s⁻¹ (a) and charge and discharge profiles at an current density of 5 A g⁻¹ (b) of pure NiS powder and NiS–rGO aerogel separately; the rate performance (c) and Niquist plots (d) of NiS powder and NiS–rGO, respectively.

To explore the underlying effect of GO content on NiS–rGO nanocomposite's electrochemical performance, cyclic voltammograms (CV), galvanostatic charge/discharge techniques and electrochemical impedance spectroscopy were conducted in a three-electrode system. Fig. 6a shows the CV analysis of NiS–rGO nanocomposite with different GO content involved in hydrothermal reaction at a scan rate of 5 mV s⁻¹. The CV curves clearly exhibit representative pseudocapacitive characteristics, which are substantially different from the nearly rectangular CV shapes arisen from electric double-layer capacitive materials.³⁶ The peaks appeared at around 0.1 V and 0.25 V at a scan rate of 5 mV s⁻¹ can be ascribed to Faradaic redox reaction of NiS, indicating that the electrochemical capacitance of the electrode mainly results from the pseudocapacitance. NiS is supposed to own the similar redox mechanism to metal oxide or metal hydroxide in alkaline electrolyte, which could be expressed as follows:³⁷ NiS + OH⁻ ↔ NiSOH + e⁻. From Fig. 6a, it is easy to come to the conclusion that the area covered by the CV curve of NiS-GR (50 mg GO) is the largest, meaning that the corresponding sample possesses the best electrochemical reactivity and that the highest capacitance can be reached.³⁸ Fig. 6b shows the representative CV curves of NiS–rGO (50 mg GO) hybrid aerogel electrode at different scan rates ranging from 1 mV s⁻¹ to 20 mV s⁻¹. It can be observed that all of the redox peaks are nearly symmetrical under different scan rates, indicating the excellent reversibility of redox reaction on the nanocomposite electrode surface.³⁹ Such reversibility may help us to confirm that the existence of rGO in the composite assists in maintaining high electrical conductivity of the electrode as an integral. Galvanostatic charge/discharge profiles of NiS–rGO (50 mg GO) at various current densities ranging from 2 A g⁻¹ to 15 A g⁻¹ within a potential window of −0.1 to 0.35 V vs. SCE are presented in Fig. 6c. The shape of the charge/discharge curves shows the capacitance mainly results from pseudo-capacitance, which are in accordance with the CV results. The anodic and cathodic peak at about 0.25 and 0.1 V from the CV curves is manifested as a plateau in the corresponding charge and discharge curves severally. When higher current densities are induced, the charge plateau would shift to a higher voltage due to the problem of polarization.⁴⁰ The specific capacitance of the NiS/rGO electrode at different current densities can be calculated according to the following equation: C = IΔt/mΔV, where I is the discharge current, Δt is the discharge time, ΔV is the potential window during the charge–discharge process, m is the total mass of the electrode material. Impressively, the NiS/rGO electrode shows a specific capacitance of 852 F g⁻¹ at a current density of 2 A g⁻¹. Even at a much higher current density of 15 A g⁻¹, 526 F g⁻¹ is still delivered. So far, a variety of relative metal sulfides with different nanostructures have been investigated as a new type of faradaic capacitive electrode materials for supercapacitors, such as Ni₃S₂ nanoflakes (664 F g⁻¹ at 4 A g⁻¹),⁴¹ NiS hollow spheres (927 F g⁻¹ at 4 A g⁻¹),⁴² NiS nanoparticles/rGO nanocomposite (800 F g⁻¹ at 1 A g⁻¹),²⁷ NiS nanorods/rGO (628 F g⁻¹ at 2 A g⁻¹).⁴³ Fortunately, our NiS–rGO hybrid nanostructure prepared in this study is comparable to or even higher than these literature results. Moreover, all of the charge–discharge profiles display the almost symmetrical features at various current densities, indicating its excellent pseudocapacitive behavior and good reversible redox reaction characteristics. At the same time, the effect of the GO content in NiS/rGO hybrid aerogel on rate performance of the NiS/rGO nanocomposite electrodes is shown in Fig. 6d. Firstly, the surface area and conductivity of NiS active material improves with the increase of rGO doping. Thus, more active sites of redox reactions could function, which is responsible for the improvement of specific capacitance of the NiS/rGO nanocomposite (when the GO increases from 30 mg to 50 mg). However, as the amount of GO increases further (60 mg), the content of NiS in the NiS/rGO nanocomposite reduces further. As the contribution of the rGO to the whole capacitance is negligible, the reduced NiS active sites would make specific capacitance of NiS/rGO decrease. Furthermore, the restack of rGO may occur, blocking the transport of electrolyte ion and electron into inner part of electrode materials, which could also result in the decrease of capacitance. In a word, the capacitance of NiS/rGO nanocomposite may depend on the match between rGO and NiS. Only when the content of GO is appropriate (50 mg), can NiS active materials and rGO best play their synergistic effect.

Fig. 6 CV curves of the NiS–rGO nanocomposite obtained with different GO content involved in hydrothermal reaction (a); CV curves of NiS–rGO (50 mg GO was added) at different scan rates; charge and discharge profiles of NiS–rGO (50 mg GO was added) at various current densities (c); the rate performance of the NiS–rGO nanocomposite gained with different GO content (d).

What's more, the cyclic performance of NiS–rGO nanocomposite at a current density of 2 A g⁻¹ was studied (Fig. S3†). It can be observed that the initial capacitance is relatively low due to the failure to take fully advantage of the active materials. After proper charge/discharge cycles, the electrochemically active NiS/GO sites pressed inside the nickel foam would be fully exposed to the electrolyte, which may be responsible for the increasement of its capacitance. However, the capacitance decreased after about 150 cycles, which may result from the dissolution of electrode material into the electrolyte. It is worthy to mention that the NiS–rGO nanocomposite exhibits excellent cycling performance during the following hundreds of cycles. Fortunately, nearly 82% of the initial capacitance is retained for the NiS–rGO sample after 1000 cycles. At the same time, the aerogel microstructure could be well kept during the electrochemical tests.

Furthermore, the asymmetric supercapacitor was fabricated, using the prepared NiS–rGO and activated carbon as positive and negative electrodes respectively, in our home-made two-electrode test system. Fig. 7a shows a CV curve of the asymmetric capacitors at a scan rate of 10 mV s⁻¹, whose shape is relatively full. To further evaluate the performance of the cell, galvanostatic charge–discharge testing was measured at various current densities (Fig. 7b). The asymmetric capacitor exhibits a specific capacitance of 79.7 F g⁻¹ at a current density of 0.2 A g⁻¹, and 49.5 F g⁻¹ could still be retained at 4 A g⁻¹, which is shown in Fig. 7c. As shown in the Fig. 7d, the fabricated asymmetric supercapacitor shows a relatively high energy density of 18.7 W h kg⁻¹ at a power density of 124 W kg⁻¹ and 11.6 W h kg⁻¹ can still be kept even at a power density of 2900 W kg⁻¹, which shows potential application in the area of future energy storage.

Fig. 7 CV profile of the fabricated asymmetric supercapacitor at 10 mV s⁻¹ (a); charge and discharge profiles of the fabricated asymmetric supercapacitor at various current densities (b); the rate performance (c) and Ragone plot (d) of the asymmetric supercapacitor.

The excellent supercapacitive property of the NiS/rGO hybrid aerogel could be attributed to the following three reasons. Firstly, NiS nanoparticles are directly anchored on rGO sheets, resulting in a good contact between the nanoparticles and rGO sheets. Such fascinating nanostructure affords a facile electron transport in the composite, which is the key to both high specific capacitance and the rate capability of the self-assembled NiS/rGO aerogel electrodes.¹⁷ Secondly, three-dimensional porous network nanostructure contributed by rGO is favorable to the rapid diffusion of ions by providing low-resistance pathways for the ions through the electrode materials. Last but not the least, the binder-free nature of the electrode avoids the polymer binding coating on its electrochemical active surface, which is beneficial for full utilization of NiS in electrochemical reactions.

4. Conclusions

In this work, we proposed a facile strategy to conduct the synthesis of self-assembly NiS/rGO hybrid aerogel. The NiS nanoparticles are uniformly distributed on the three-dimensional porous conductive rGO nanoscaffold. This binder-free NiS/rGO hybrid aerogel possesses excellent electrochemical capacitive performance with a specific capacitance value of 852 F g⁻¹ at a current density of 2 A g⁻¹ and 526 F g⁻¹ at a much higher current density of 15 A g⁻¹, exhibiting excellent electrochemical capacity under large current densities. The high conductivity of rGO, the three-dimensional porous nanostructure and the good contact between NiS nanoparticles and rGO sheets are supposed to be responsible for their improved electrochemical performances. Thus the adopted method in this study could be a favourable approach to the synthesis of other metal sulfides (or oxide)/rGO hybrid nanocomposite. Furthermore, the 3D self-assembled NiS/rGO hybrid aerogel is a promising electrode material for the development of high-performance energy storage devices.

Acknowledgements

The project was supported by the National Science Foundation of China (no. 21203238), the National Basic Research Program (no. 2010CB934700), and Production and Research Collaborative Innovation Project of Jiangsu Province, China (no. BY2011178).

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Footnote

† Electronic supplementary information (ESI) available: EDS measurement, CV curves at 5 mV s⁻¹ (a) and charge and discharge profiles at an current density of 5 A g⁻¹ (b) of pure rGO and NiS–rGO aerogel separately, the cycling performance of NiS–rGO nanocomposite at a current density of 2 A g⁻¹. See DOI: 10.1039/c5ra02058a

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