One-pot synthesis of self-supported hierarchical urchin-like Ni3S2 with ultrahigh areal pseudocapacitance

Qian He a, Ying Wang b, Xiong Xiong Liu a, Daniel John Blackwood b and Jun Song Chen *ac
aSchool of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 610054, China. E-mail: jschen@uestc.edu.cn
bDepartment of Materials Science and Engineering, National University of Singapore, Singapore 117574
cCenter for Applied Chemistry, University of Electronic Science and Technology of China, Chengdu, 610054, China

Received 30th May 2018 , Accepted 10th July 2018

First published on 12th July 2018


Recently, nickel sulfides have been widely investigated as an electrode material for pseudocapacitors. Among them, self-supported nickel sulfides have been intensively studied; yet, their structures are rather limited to nanosheet or nanorod arrays. Herein, a facile one-step hydrothermal method was developed to synthesize self-supported Ni3S2 on a Ni foam substrate with an urchin-like hierarchical structure consisting of nanowire subunits with a diameter of about 20–30 nm, thus combining the advantages of both hierarchical and self-supported structures. Because of this unique structure, when the sample was applied as an electrode for supercapacitors, it demonstrated an ultrahigh areal capacitance of 21.54 F cm−2 at a current rate of 2 mA cm−2, and even at a very high current rate of 50 mA cm−2, a reversible capacitance of 5.40 F cm−2 can still be retained after 1000 charge–discharge cycles. When the sample was employed as the cathode to build an asymmetric supercapacitor (ASC) with activated carbon as the anode, the device exhibited an areal capacitance of 1.26 F cm−2 at a current rate of 2 mA cm−2 and a coulombic efficiency of 97.7%. Furthermore, this device showed great potential for practical application by lighting up 8 red light-emitting diodes.


1. Introduction

With the fast depletion of fossil fuels worldwide, the search for renewable energy has become an important task for sustainable development. At the same time, the ever growing demand for energy for daily activities requires significant advancement in energy storage technology. Together with other energy devices such as batteries and fuel cells, supercapacitors have played a critical role in tackling the energy crisis in recent decades.1–6

Owing to their long cycle life and high power density, supercapacitors have become an important focus in the field of materials and energy research.2,7–9 Based on the mechanisms of how the charges are stored, supercapacitors can be mainly classified into two categories: electric double layer capacitors (EDLCs) and pseudocapacitors.10,11 For EDLCs, the charges are adsorbed onto the surface of the electrode materials by electrostatic interaction.11–13 Because of this unique mechanism, the capacitance of EDLCs is, to a certain extent, related to the active surface area of the electrode materials. As a result, porous carbon materials, usually with a surface area of more than 1000 m2 g−1,14 have been intensively studied as electrode materials for EDLCs, because of their high electronic conductivity, good electrochemical stability and high porosity. For the second category of pseudocapacitors, the charges are stored through fast reversible faradic reactions at the surface or in the near-surface region of the electrodes.11,15–17 Materials such as metal oxides (Co3O4 and ZnCo2O4),18–22 metal hydroxides (Ni(OH)2 and Co(OH)2)23–26 and conducting polymers (polypyrrole, polyaniline, etc.)27–30 are commonly used in this type of supercapacitor.

Among these candidates, nickel-based materials with good thermal and chemical stabilities, high electrochemical activity, environmental friendliness, and low cost have great potential to be applied as the electrode material for high-performance pseudocapacitors.11,31 Moreover, nickel sulfides possess higher electronic conductivity compared to their oxide counterparts because of the lower electronegativity of sulfur than oxygen.32–36 Since pseudocapacitors rely on fast faradaic reactions to store charges, they require the electrode materials to have a high electrical conductivity to allow fast charge transfer during these redox reactions. As a result, self-supported materials have emerged to satisfy this particular requirement. By directly growing the electrochemically active material on the surface of the conductive substrate, this type of material offers an efficient charge transfer pathway with a reduced contact resistance between the two components, and they can be directly assembled into the device without the use of other additives.37 However, because of this special construction, self-supported materials need a high compatibility between the active material and current collector, and their morphologies are usually limited to nanosheet or nanorod arrays.38–40

In this report, we developed a facile one-step hydrothermal method to synthesize self-supported urchin-like Ni3S2 on a nickel foam substrate (denoted as Ni3S2@Ni). Unlike the array structures reported in previous studies, these Ni3S2 particles contain a hierarchical structure which consists of nanowires as subunits, each with a diameter of about 20–30 nm. With such a unique structure, the as-prepared material combines the advantages of both hierarchical materials, such as the large number of active sites and good structural stability,41 and being a self-supported materialself-supported materials. Because of this good integration, when the sample was applied as the electrode material for supercapacitors, it exhibited a surprisingly high areal capacitance of 21.54 F cm−2 at a current rate of 2 mA cm−2. More significantly, a high capacitance of 5.40 F cm−2 can still be retained after 1000 charge–discharge cycles at a high current rate of 50 mA cm−2. Subsequently, the sample as the cathode was assembled into an asymmetric supercapacitor (ASC), with activated carbon as the anode, and the device manifested an areal capacitance of 1.26 F cm−2 at a current rate of 2 mA cm−2 and a coulombic efficiency of 97.7% during the first ten cycles. Then this ASC was capable of lighting up 8 red light-emitting diodes (LEDs), demonstrating promising potential for practical application.

2. Experimental section

2.1 Material synthesis

The urchin-like Ni3S2 was synthesized by a facile one-step hydrothermal method. The substrate Ni foam was first cleaned with deionized water and ethanol by ultrasonication for 10 min. Briefly, a 30 ml aqueous solution containing 1 mmol NiSO4·6H2O and 0.1 g thiourea was transferred into a Teflon-lined stainless steel autoclave, then a 1 × 2 cm2 piece of cleaned Ni foam was placed into the solution and then maintained at 150 °C for 12 h. After cooling down naturally to room temperature, the self-supported urchin-like Ni3S2 on Ni foam (Ni3S2@Ni) was carefully taken out and flushed with deionized water and ethanol several times, and then dried at 60 °C overnight. The increase in the mass of the sample after the hydrothermal reaction is 19.3 mg cm−2.

2.2 Material characterization

The morphology of the sample was analyzed using a field emission scanning electron microscope (FESEM; Zeiss) to which an energy dispersive spectroscope was attached and a high resolution transmission electron microscope (HRTEM; JEM2010F) with elemental mapping. Crystallographic information was collected by X-ray diffraction (XRD) using a Bruker D8 Advancer (Cu Kα, λ = 1.54 Å). The chemical state of the elements in the samples was determined using a Kratos Axis UltraDLD X-ray photoelectron spectrometer (XPS) equipped with an Al Kα X-ray source (1486.6 eV), and all peaks were referenced to the C 1s at 284.5 eV. The Raman spectroscopy measurements were performed on a LabRam HR Evolution Raman microscope using an Ar ion laser at 514 nm as the excitation source.

2.3 Electrochemical measurements

The electrochemical tests were conducted in a three-electrode electrochemical cell with a Pt wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, in 6 M NaOH aqueous solution with the aid of a CHI760e electrochemical workstation. The cyclic voltammetry was conducted with a voltage window of 0–0.6 V (vs. SCE) at different scan rates, while the galvanostatic charge–discharge tests were performed under various current rates. Electrochemical impedance spectroscopy (EIS) was carried out after charging at the open-circuit potential with a superimposed 5 mV sinusoidal (root-mean-square) perturbation over the frequency range from 100 kHz to 0.01 Hz. For the ASC, the anode was a 70[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]10 (wt%) mixture of activated carbon, carbon black Super-P-Li, and poly(vinylidene fluoride) (PVDF), and the mixture was subsequently pasted onto the Ni foam substrate.

3. Results and discussion

Fig. 1 shows the scanning electron microscope (SEM) images of the as-prepared Ni3S2 sample. It is apparent that numerous sphere-like particles were formed on the surface of the nickel foam substrate with a relatively uniform distribution (Fig. 1A). With a closer look (Fig. 1B), it can be observed that these particles are loosely arranged with ample space in between one another, forming a highly porous layer. Under an even higher magnification (Fig. 1C), the particles are shown to have an urchin-like morphology with a hierarchical structure composed of radially-oriented nanowires as subunits. By focusing on the tip of a single nanowire constituent (Fig. 1D), it can be measured that the diameter of the nanowire is about 30 nm. Because of the loose packing of the urchin-like particles as well as the hierarchical assembly of the nanowires, this sample allows the easy penetration of the electrolyte and provides a large number of active sites, thus facilitating the charge transfer process at the electrode–electrolyte interface.
image file: c8ta05050c-f1.tif
Fig. 1 Characterization results of the urchin-like Ni3S2 grown on the Ni foam substrate (Ni3S2@Ni): Scanning electron microscopy (SEM) images (A–D) at different magnifications.

A single particle was also viewed under a transmission electron microscope (TEM; Fig. 2A). It shows a looser organization in the peripheral regions and a denser core at the center, thus confirming the hierarchical urchin-like structure. The high-resolution TEM (HRTEM) image (Fig. 2B) depicts a portion of the nanowire with a diameter of around 24 nm. With a zoomed-in view (Fig. 2B, inset), some visible lattice fringes at the side of the nanowire can be clearly observed with an interplanar distance of 0.23 nm, corresponding to the (023) plane of Ni3S2. The chemical composition of the sample was investigated by elemental mapping coupled with TEM. From Fig. 2C, it can be seen that a single Ni3S2 nanowire exhibits even distributions of Ni and S, confirming the uniform chemical composition of the sample.


image file: c8ta05050c-f2.tif
Fig. 2 Transmission electron microscopy (TEM) image (A) of a single urchin-like structure. High-resolution TEM (HRTEM) image (B) and elemental mapping (C) of a single Ni3S2 nanowire from the urchin-like structure.

The crystalline structures of the Ni3S2@Ni sample were investigated by X-ray diffraction (XRD), with the results shown in Fig. 3A. The diffraction peaks marked with asterisks at 44.5° and 51.9° can be indexed to the (111) and (200) planes of the Ni foam substrate (JCPDS no. 87-0712), respectively. The other peaks detected at 21.8°, 31.2°, 37.8°, 38.4°, 49.8° and 55.3° can be assigned to the (101), (110), (003), (021), (113) and (300) planes of rhombohedral Ni3S2 (JCPDS no. 44–1418; space group, R32; a0 = b0 = 5.7454 Å; c0 = 7.1350 Å), respectively.42 No impurity peaks can be observed, suggesting the absence of other crystalline phases. The Raman spectrum is shown in Fig. 3B, and six distinct peaks located at 184, 196, 217, 297, 317 and 345 cm−1 can be unambiguously assigned to the Ni3S2 phase,43 while the smaller peak at 243 cm−1 could be attributed to the NiS phase,44 suggesting the possible presence of amorphous NiS in the sample.


image file: c8ta05050c-f3.tif
Fig. 3 Characterization results of Ni3S2@Ni: X-ray diffraction (XRD) pattern (A); Raman spectra (B), and X-ray photoelectron (XPS) of Ni 2p (C) and S 2p (D). The asterisks in A mark the peaks corresponding to the Ni foam substrate.

The surface element and the valence state of Ni and S in the Ni3S2@Ni sample were verified by X-ray photoelectron microscopy (XPS). As shown in Fig. 3C, the peaks at a binding energy of 855.1 eV belongs to Ni 2p3/2, and the peak at 872.7 eV can be assigned to Ni 2p1/2, followed by two shake-up satellite peaks at 860.6 and 879.0 eV (labeled “sate.” in the figure). The two doublets are consistent with the characteristic peak of Ni3S2.45 The binding energy at 851.8 eV in Ni 2p3/2 agrees with the characteristic of Ni2+.46 The two peaks at 160.2 and 161.2 eV in Fig. 3D belong to S 2p3/2, and the peak at 167.4 eV can be indexed to atomic S in the Ni3S2@Ni sample.43,47,48

The as-prepared Ni3S2@Ni sample was then applied as the electrode material for supercapacitors in a three-electrode system. Cyclic voltammograms (CVs) at different scan rates were first conducted with the results shown in Fig. 4A, where a pair of redox peaks being obviously identified at 0.32 V (positive sweep) and 0.07 V (negative sweep) at a scan rate of 2 mV s−1 between 0.0 and 0.6 V. This pair of redox peaks can be regarded as the signature of pseudocapacitive capacitance, and the corresponding faradaic reactions can be described as follows:32,49

 
Ni3S2 + 3OH ↔ Ni3S2(OH)3 + 3e(1)


image file: c8ta05050c-f4.tif
Fig. 4 Supercapacitor performance of Ni3S2@Ni in a three-electrode system: cyclic voltammetry (CV) at different scan rates (A) and the corresponding capacitance calculated from A (B). Galvanostatic charge–discharge curves at different current rates (C) and the corresponding discharge capacitance calculated from C (D). The long term charge–discharge performance at a current rate of 50 mA cm−2 (E).

The resulting areal capacitance C (F cm−2) can be calculated from the area of the CVs (Fig. 4B) using the equation below:35

 
image file: c8ta05050c-t1.tif(2)
where I is the current density, v is the scan rate and ΔV is the voltage window of the scan. As a result, the areal capacitances of 15.69, 9.21, 6.17, 3.10 and 1.73 F cm−2 can be calculated at the scan rates of 2, 5, 10, 25 and 50 mV s−1, respectively.

Fig. 4C shows the galvanostatic charge–discharge curves at different current densities, from which the corresponding discharge capacitance can be obtained using the following equation:50,51

 
image file: c8ta05050c-t2.tif(3)
where C is the areal capacitance in F cm−2, I is the current density in A cm−2, Δt is the discharge time in s, and ΔV is the discharge voltage window in V. The areal capacitance can be calculated to be 21.54, 20.50, 17.72 and 12.22 F cm−2 at the current rates of 2, 5, 10 and 25 mA cm−2, respectively (Fig. 4D). Particularly, the Ni3S2@Ni still shows a fairly high capacitance of 9.11 F cm−2 even at the highest current rate of 50 mA cm−2, which confirms the superior electrochemical performance of the self-supported urchin-like Ni3S2. Fig. 4E demonstrates the long-term cycling stability test of the sample carried out at a constant current rate of 50 mA cm−2. It can be observed that the capacitance increased slightly after 100 cycles. This phenomenon would probably be attributed to an activation process, which is commonly observed in batteries and supercapacitors.52–54 During the charge–discharge process, the structure of the electrode material will experience some changes, which may lead to the exposure of more active sites, or the opening up of new charge transfer channels.42 These factors would most likely give rise to a higher electrochemical activity, leading to a brief increase in the capacitance. Overall it is a gradual decay in the reversible capacitance upon the 1000 charge–discharge cycles, and it still delivers a high areal capacitance of 5.40 F cm−2 at the end of the test. Some typical capacitance values of reported nickel sulfide-based materials are compared in Table 1.53,55–58 It is quite clear that our Ni3S2@Ni electrode yields significantly higher areal capacitance at both high and low current densities in comparison to other reported data. These results further confirm that the self-supported urchin-like Ni3S2 holds great promise in the field of supercapacitors.

Table 1 Comparison of performance of the nickel sulfide-based material
Electrode materials Current density/mA cm−2 Areal capacitance/F cm−2 Ref.
Ni 3 S 2 @Ni 2 21.54 This work
50 9.11
Ni3S2 nanosheets 1 4.57 53
20 2.86
Ni3S2@CdS 2 3.15 55
10 2.88
Ni3S2@CoS 4 4.89 56
32 2.68
Ni3S2@MoS2 2.5 5.29 57
10 4
Ni3S2–Co9S8 5 5.37 58
50 3.07


In order to investigate the charge transfer processes during the electrochemical reaction, electrochemical impedance spectroscopy (EIS) was conducted before and after the long-term test. Fig. 5A shows the Nyquist impedance plots, and it is apparent that both have very similar impedance profiles which are composed of a depressed semicircle in the high-frequency region and an up-sloping near straight line at the low-frequency section.59,60 Furthermore, the equivalent circuit shown in Fig. 5B was employed to model the resulting EIS spectra: Rs is the series resistance consisting of the intrinsic resistance of the Ni foam, contact resistances and the uncompensated solution resistance; Rct represents the faradaic interfacial charge-transfer resistance related to the redox reaction; CPE is a constant phase element representing the double layer capacitance; W is a finite length Warburg impedance arising from the diffusion of ions through the film; and CF corresponds to the faradaic pseudocapacitance. From the fitting data, the value of Rs was largely unchanged before (1.15 Ω) and after (1.01 Ω) the test. Moreover, the Rct value was increased significantly from 1.02 Ω to 1.74 Ω after 1000 cycles, which implies more retarded redox reactions, giving rise to a lower capacitance at the end of the test. Such an observation is in good agreement with the data shown in Fig. 4E.


image file: c8ta05050c-f5.tif
Fig. 5 Electrochemical Impedance Spectroscopy (EIS) studies: Nyquist plots of the sample before and after 1000 charge–discharge cycles (A), and the equivalent circuit used to model the experimental data (B). The symbols in A denote experimental data, and the continuous lines represent the modeled data.

We subsequently assembled an asymmetric supercapacitor (ASC) by using the as-prepared Ni3S2@Ni as the cathode and activated carbon (AC) as the anode (Ni3S2‖AC). Fig. 6A shows the CV scans with a pair of current peaks at around 1.0 V and 1.3 V (at 50 mV s−1), and the corresponding areal capacitances are calculated to be 740, 650, 500, 360 and 260 mF cm−2 at the scan rates of 2, 5, 10, 25 and 50 mV s−1, respectively (Fig. 6B). Fig. 6C shows the galvanostatic charge–discharge curves at different current rates, and the corresponding discharge capacitances of 1260, 860, 600, 360 and 260 mF cm−2 can be obtained at the current rates of 2, 5, 10, 25 and 50 mA cm−2, respectively (Fig. 6D). The galvanostatic charge–discharge voltage profiles of the first ten cycles are shown in Fig. 6E, which exhibits a very high coulombic efficiency of 97.7%. Then to further investigate the practical use of the ASC device, it was used to light up 8 red LEDs (Fig. 6F), which is significantly desirable for energy storage applications.


image file: c8ta05050c-f6.tif
Fig. 6 Performance of the Ni3S2@Ni‖AC ASC: CVs at different scan rates (A) and the corresponding capacitance calculated from A (B). Galvanostatic charge–discharge curves at different current rates (C) and the corresponding discharge capacitance calculated from C (D). The first ten charge–discharge cycles of the ASC at a current density of 10 mA cm−2 (E) and a photograph of the ASC powering up 8 red LEDs arranged in two rows (F).

4. Conclusions

In this work, a facile one-step hydrothermal method was developed to synthesize self-supported Ni3S2. The sample contains urchin-like particles with a hierarchical assembly of nanowires with a diameter of 20–30 nm, and it thus offers a high electrical conductivity because of the self-supported structure, and a large number of active sites due to the hierarchical construction. As a result, the as-prepared Ni3S2 exhibited a high capacitance of 21.54 F cm−2 at a current rate of 2 mA cm−2 when applied as the electrode material for pseudocapacitors, and delivered a superior capacitance of 5.40 F cm−2 after 1000 charge–discharge cycles at a high current rate of 50 mA cm−2. These values compare very favorably to other nickel sulfide-based electrodes reported in the literature. The above prominent electrochemical properties can be attributed to urchin-like Ni3S2 composed of numerous nanowires grown radially from the center, which facilitates the penetration of electrolyte and allows the fast reversible redox reaction at the electrolyte–electrode interface. At the same time, the self-supported structure grants a highly efficient charge transfer pathway, further enhancing the electrical conductivity of the material. An asymmetric supercapacitor (ASC) based on the as-prepared material was subsequently assembled with activated carbon, and this device was able to light up 8 red LEDs, suggesting that the current Ni3S2 sample has great potential for practical applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to the University of Electronic Science and Technology of China for financial support by providing the start-up fund.

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