MoS2 composite nanosheets with ultrahigh specific capacity for high-performance asymmetric supercapacitor

Journal of Alloys and Compounds 811 (2019) 151915

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Smart in situ construction of NiS/MoS2 composite nanosheets with ultrahigh specific capacity for high-performance asymmetric supercapacitor Jia Yan, Sichao Wang, Yuan Chen, Ming Yuan, Yunpeng Huang, Jiabiao Lian, Jingxia Qiu, Jian Bao, Meng Xie, Hui Xu**, Huaming Li*, Yan Zhao*** Institute for Energy Research, Key Laboratory of Zhenjiang, Jiangsu University, Zhenjiang, 212013, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 April 2019 Received in revised form 16 August 2019 Accepted 17 August 2019 Available online 17 August 2019

NiS/MoS2 composite nanosheets were designed and prepared via facile ion-exchange, freeze-drying, and Ostwald ripening processes. MoS2 nanosheets served as backbones to absorb Ni2þ ions and form NiS nanosheets. The electrode based on the NiS/MoS2 composite nanosheets exhibited an ultrahigh specific capacity of 343.5 mA h g1 at 9 A g1, which was superior to that of as-prepared MoS2 nanosheets (125.8 mA h g1) and Ni(OH)2 nanosheets (148.9 mA h g1). In addition, NiS/MoS2 composite nanosheets presented an enhanced rate performance and good life cycle due to their hierarchical structure, high surface area with numerous active sites, and synergistic performance. A NiS/MoS2//AC asymmetric supercapacitor device operated in a large potential window (0e1.5 V) delivered a high capacitance of 48.4 F g1 at 0.1 A g1, an excellent energy density of 15.1 Wh kg1 and a very high power density of 2.25 kW kg1. This smart in situ construction process opens a new route to synthesize binary metal sulfides for high-performance energy storage devices. © 2019 Elsevier B.V. All rights reserved.

Keywords: NiS/MoS2 Ion-exchange Ultrahigh specific capacity Freeze-drying Asymmetric supercapacitor

1. Introduction Recently, with the ever increasing and urgent desire for highperformance portable electronic devices, more highly efficient renewable clean energies have been explored [1e3]. As a renewable electrochemical energy storage technology, supercapacitors show higher power density than that of rechargeable batteries but low energy output [4,5]. Thus, the exploration of novel electrode materials with excellent performance is one of the core issues for advanced supercapacitors [6]. Materials with good electrical conductivity and high surface area are regarded as ideal electrodes for supercapacitors, such as carbon-based materials [7e9]. However, the low energy storage capacity of most carbon-based materials greatly prohibits their actual applications [10], and fabricating materials with high energy density to meet the ever-increasing energy requirements.

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (H. Xu), [email protected] (H. Li), yanzhao@ujs. edu.cn (Y. Zhao). https://doi.org/10.1016/j.jallcom.2019.151915 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Pseudocapacitive materials, including Ni(OH)2 [11], MnO2 [12], Co3O4 [13], Co9S8 [14], and related compounds [15] exhibiting faradaic charge storage at the electrode/electrolyte interface, have shown high theoretical capacity. However, they are enslaved to low electrical conductivity, slow rate capability, and an unsatisfactory cycling lifespan [16,17]. In recent years, many studies verified that building transition metal-based sulfides or multiple metal-based hybrid nanostructures could be effective methods to reinforce the electronic conductivity and enhance the performance of supercapacitor electrodes [18,19]. Among them, molybdenum disulfide (MoS2) has been used for supercapacitors given its graphite-like lamellar structure among layers and multiple redox states of Mo atom, which can store charge in interlayers and adjacent layers [20]. Frustratingly, MoS2 nanosheets prepared by chemical methods are likely to be stacked, which largely decreased the surface area and resulted in low electrical capacity [21,22]. To overcome these limitations, several strategies have been explored. Crystal phase transformation, structure design, and component control are three efficient methods. Acerce et al. reported that metallic 1T phase MoS2 nanosheets with high concentrations can electrochemically intercalate different ions and achieve excellent capacitance values [5]. Through the reaction of the vaporized sulfur

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J. Yan et al. / Journal of Alloys and Compounds 811 (2019) 151915

powder and Mo oxide films, edge-oriented MoS2 nanoporous films were constructed for an all-solid-state supercapacitor with superior energy-storage properties [23]. Moreover, Ni3S4@MoS2 hierarchical material with a tunable core diameter and shell thickness was prepared by Zhang et al., revealing a high capacity of 1440.9 F g1 at 2 A g1 [24]. Therefore, electrode materials with appropriate nanostructures prepared by combining the advantages of MoS2 and other transition metal sulfides may overcome the inherent limitations of MoS2 and enhance the activity of highcapacity supercapacitors. However, related MoS2-based composite materials have seldom been fabricated hitherto. Ion exchange reactions represent a low-cost and efficient strategy to liberally introduce the desired ions into the host material to modify their composition and improve their performance [25,26]. Liu et al. synthesized hierarchical core-shell (MoO3@MoS2) nanowires by a direct anion-exchange reaction of the MoO3 precursor [27]. The unique nanostructures provide numerous diffusion channels for Liþ and greatly improved Liþ storage properties. Ni(OH)2 is highly considered as a promising candidate material to coordinate with MoS2 for HER and supercapacitor applications [28,29]. For instance, Hao and coworkers reported a hybrid of MoS2@Ni(OH)2 using a single mode microwave-assisted hydrothermal technique [28]. Ni(OH)2 nanosheets serve as a supporting backbone, and flower-like MoS2 nanosheets are directly grown on the surface of Ni(OH)2. This hierarchical nanostructure not only effectively weakens the restacking of MoS2 layers but also ensures the high-rate and long-life charge-discharge process. Nickel sulfide (NixSy) typically exhibits higher electroconductivity and better theoretical capacitive contribution than Ni(OH)2 [26,30]. Therefore, the introduction of Ni ions into MoS2 can produce a NiSx/MoS2 composite that reveals the advantages of MoS2 and NixSy and improves their performance. With the above considerations, in this paper, a facile ion exchange approach was used to enhance the energy storage properties of MoS2 via the decoration of Ni. In this hybrid, MoS2 nanosheets served as backbones to absorb Ni2þ ions and form NiS nanosheets. The high specific surface area and low ion resistance of NiS/MoS2 composite nanosheets with more active sites yields a high capacity of 343.5 mA h g1 at 9 A g1. This specific capacity of NiS/MoS2 composite is significantly increased compared with MoS2 (125.8 mA h g1) and Ni(OH)2 (148.9 mA h g1) nanosheets at the same current density. The electrode fabricated by this method also presents a good rate performance and Coulombic efficiency. Moreover, a NiS/MoS2//AC asymmetric supercapacitor device with a large potential window (0e1.5 V) delivered high energy output and showed a long cycle lifespan. 2. Experimental 2.1. Materials Molybdenum trioxide (MoO3, 99.9%), potassium thiocyanate (KSCN, 99.0%), nickel (II) chloride hexahydrate (NiCl26H2O, 99.9%), potassium hydroxide (KOH, 99.9%) and sodium hydroxide (NaOH, 98%) were commercially available from Aladdin Chemical Co., Shanghai, China. Activated carbon was purchased from XF Nano. Ltd, Shanghai, China. All materials were used without further purification, while the nickel foam was cleaned beforehand with acetone, 3 M HCl and deionized water to remove the oxide layer and impurities. 2.2. Synthesis of MoS2 and NiS/MoS2 composite nanosheets To prepare the composite of NiS/MoS2 nanosheets, MoS2 should be first synthesized as follows: MoO3 (4 mmol) was added into a

water solution of KSCN (10 mmol/40 mL H2O) and subjected to ultrasonic treatment for 0.5 h. After vigorous stirring for an additional 0.5 h, the mixture was poured into a Teflon-lined autoclave (volume: 50 mL) and heated at 180  C for 24 h. Subsequently, it was cooled down, washed, centrifuged, and dried in a freezer dryer at 65  C overnight. The obtained MoS2 sample (16 mg) was added into a light green solution of NiCl2 (0.02 mmol/10 mL water) under a strong stirring process. NaOH was slowly added dropwise to adjust the pH to 14. The final solution was stirred for 1 h, poured into a 25-mL autoclave and heated at 160  C for 8 h. Then, the as-prepared sample was centrifuged and washed to remove impurities. The precursor sample was dried in a freezer dryer at 65  C overnight. The precursor sample was finally calcined under N2 gas protection at 400  C for 2 h in a muffle furnace at a rate of 5  C min1. Then, the molar ratio of 20% NiS in MoS2 composites was obtained, which we termed 20% NiS/MoS2 (shorted name as NiS/MoS2). In addition, binary composites with NiS molar ratios of 10% and 30% in MoS2 were obtained by employing the same methods, and we termed them as 10% NiS/MoS2 and 30% NiS/MoS2, respectively. 2.3. Synthesis of Ni(OH)2 nanosheets This method was similar to the preparation process of NiS/MoS2 composite nanosheets with some changes. The MoS2 sample was not added at the beginning, and the precipitate was collected after the freeze-drying treatment. 2.4. Characterization Crystal phase structures were determined by powder X-ray diffraction with Cu-Ka radiation (l ¼ 1.5418 Å). Sample morphology and microstructure were performed using a scanning electron microscope (SEM Hitachi, Japan). The chemical composition was investigated through X-ray photo electron (XPS) spectra (Kratos AXIS Ultra DLD). N2 adsorption isotherms were measured on a Quantachrome Autosorb 6B system at 77 K. 2.5. Electrochemical measurements Typically, electrochemical measurements were performed using a working station (CHI 660E, Chenghua) with KOH (3 M) as the electrolyte. A Pt plate and SCE were used as the counter electrode and the reference electrode, respectively. The working electrodes were prepared by pasting a mixture of active materials (MoS2, Ni(OH)2, NiS/MoS2 nanosheets, or activated carbon (AC) 72 wt%), poly(tetrafluoroethylene) (5 wt%) and carbon black (23 wt%) onto a Ni foam substrate (1*4 cm2). Then, it was dried at 120  C for 3 h and pressed at 10 MPa. Electrochemical impedance spectroscopy (EIS) measurements were recorded. The asymmetric supercapacitor (ASC) was assembled using our as-synthesized NiS/MoS2 and AC as electrodes and 3 M KOH as an electrolyte. To maintain the charge balance of positive (mþ) and negative (m) electrodes, the loading mass ratio (mþ/m) was estimated using the following equations [31].

Qþ ¼ Q

(1)

Q ¼ C  DU  m

(2)

mþ C  DU ¼ m Cþ  DUþ

(3)

where DU (V) is the voltage of our ASC device, and Q (coulomb) and

J. Yan et al. / Journal of Alloys and Compounds 811 (2019) 151915

C (F g1) are the charge and capacity of the ASC device, respectively. The mass ratio of the electrodes was calculated as mþ/m- ¼ 2:19. 3. Results and discussions The synthetic route is schematically shown in Fig. 1a. First, we prepared MoS2 nanosheets using a hydrothermal strategy and freeze-drying process. We take advantage of ion exchange and coordination interactions to ensure the conversion of MoO3 particles and formation of thin-layer MoS2 nanosheets after the additional freeze-drying process. Then, NiMo-precursors were achieved in an alkaline solution after another hydrothermal method, and the stability increased by using spatially confined Ostwald ripening. After calcination in an N2 atmosphere, a composite of NiS/MoS2 was finally obtained. To better understand the morphologies changes of NiS/MoS2 composite, a comparison of SEM images of NiS/MoS2, MoS2, and as-synthesized Ni(OH)2 (prepared by the hydrothermal method without addition of MoS2) was performed. The SEM images of Ni(OH)2 at different magnifications were detected, as shown in Fig. 1b and c. Many overlapping nanosheets with a size distribution of hundreds of nanometers were observed. The aggregated nanosheets are stacked together to form micron-sized clusters. Similarly, the MoS2 sample also presents a stacking of nanosheets, while the average size distribution of MoS2 sample (less than one hundred nanometers) was smaller than Ni(OH)2 nanosheets (Fig. 1d and e). Compared with the morphologies of MoS2 and Ni(OH)2 nanosheets, the novel nanosheets with a uniform shape and an average size of 200 nm were observed for the NiS/MoS2 composite (Fig. 1f and g). This special NiS/MoS2 composite material can effectively reduce the stacking phenomenon of raw materials and improve the structural

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stability of the MoS2 nanosheets. Fig. 2a exhibits the XRD patterns of pure MoS2 and NiS/MoS2 samples. The peaks at 14.6 , 33.0 , 39.5 , and 57.9 are identified as (002), (100), (103), and (110) planes of MoS2, respectively (JCPDS: 65-7025). The composite material has four main peaks at 30.0 , 34.9 , 46.1, and 53.8 , which are related to the (100), (101), (102), and (110) planes of NiS, respectively (JCPDS: 65-0830). The other peaks overlapped with peaks of MoS2. The XRD results reveal the successful formation of NiS/MoS2 composite. The surface areas, the pore size contribution, and the pore volumes of NiS/MoS2 composite and MoS2 were analyzed by N2 adsorption-desorption curves, as shown in Fig. 2b. The Brunauer-Emmett-Teller (BET) results show that a specific surface area of 41 m2 g1 was detected for the NiS/MoS2 composite. Furthermore, the average pore size contribution of NiS/MoS2 composite is 3.1 nm (inset of Fig. 2b). In comparison, a smaller surface area of 19.0 m2 g1 with the pore size of 3.4 nm can be detected for the MoS2 sample. In addition, the pore volume of the composite is 0.246 cm3 g1, which is almost twice of that for the MoS2 sample (0.124 cm3 g1, Table 1). Therefore, it could be further confirmed that the composite material may present better electrochemical properties than that of its single monomer. XPS characterization was performed to further explore the chemical composition of NiS/MoS2 (Fig. 2cef). In Fig. 2c, the primary chemical compositions of Ni, Mo, and S can be verified from the survey spectra of NiS/MoS2 [32]. In Fig. 2d, the peaks of Mo 3p1/2 and Mo 3p3/2 in the NiS/MoS2 composite shift to the low-energy region from 412.9 to 395.3 eV to 412.0 and 394.8 eV, respectively, compared with pure MoS2 nanosheets. In addition, the Mo 3d3/2 and Mo 3d5/2 signals also shift to the low-energy region from 235.9,

Fig. 1. (a) Scheme of the process to prepare the NiS/MoS2 composite. Low and high magnification SEM images of (b, c) Ni(OH)2, (d, e) MoS2, and (f, g) NiS/MoS2 samples.

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J. Yan et al. / Journal of Alloys and Compounds 811 (2019) 151915

Fig. 2. Chemical composition analysis of MoS2 and NiS/MoS2 samples: (a) XRD, (b) BET, (c) XPS full spectra and high resolution (d) Mo 3p, (e) Mo 3d, (f) S 2p XPS spectra.

232.3, 229.0, and 226.1 eV to 234.8, 231.5, 228.1, and 225.7 eV, as shown in Fig. 2e. This finding can be explained by the fact that the Ni replaces a part of the Mo in MoS2, resulting in a decrease in the binding energy [33,34]. In Fig. 2f, two peaks of MoS2 at 163.1 and 162.0 eV are detected, corresponding to the S 2p1/2 and S 2p3/2, respectively. Regarding the NiS/MoS2 composite, the peaks shifted to 162.1, and 161.2 eV in the S 2p XPS spectrum [35]. The changes in the binding energy values for the NiS/MoS2 composite strongly suggest that electrons are transferred from the surface of NiS to the MoS2 matrix, suggesting the establishment of coupling interfaces between NiS and MoS2 [36]. Fig. 3a shows the CV curves of the optimum NiS/MoS2 composite electrode at different scan rates from 2 mV s1 to 10 mV s1 in the potential range of 0e0.6 V (vs. SCE) in 3 M KOH, respectively. Interestingly, two pairs of peaks are visible in each voltammogram, indicating the successful formation of the NiS/MoS2 hybrid and further confirming the battery-type characteristics of these sulfide electrodes [37]. In addition, the shape of the curves was maintained, and the oxidation and reduction peaks of the NiS/MoS2 composite electrode gradually shifted to a wider potential window with increasing scan rate because the capacitance characteristics are mainly controlled by Faradaic reactions [38]. The redox process may be described as follow:

NiS þ OH 4NiSOH þ e

(4)

MoS2 þ OH 4MoS2 OH þ e

(5)

To further verify the advantages of the NiS/MoS2 composite material for advanced supercapacitor electrode, we compared the

CV curves of Ni(OH)2, MoS2, and NiS/MoS2 materials at a scan rate of 5 mV s1, as shown in Fig. 3b. As expected, the NiS/MoS2 electrode exhibits much higher current densities and larger areal capacity compared with Ni(OH)2 and MoS2 electrodes, indicating a substantial improvement of electrochemical capacitance by ion exchange and proving the advantage of the synergistic effect [39]. To further discuss the influence of electrochemical properties of the composites, we measured the CV curves of the NiS/MoS2 nanocomposites for different ratios between Ni and Mo at 10%, 20%, and 30%, as shown in Figs. S1aec (Supporting information). The 20% NiS/MoS2 composites exhibited the largest integral area compared with the other two composites, suggesting a superior specific capacity. These conclusions were consistent with the galvanostatic charge-discharge (GCD) results. Fig. 3c shows the GCD curves of NiS/MoS2 electrode within a potential range of 0e0.45 V. The nonlinear shape of the GCD curves revealed the reversible pseudocapacitive features [40]. With increasing current density, a rapid potential drop appeared, and the discharge time decreases significantly due to the slow potential attenuation caused by internal resistance and Faradaic redox reaction [41]. Moreover, the NiS/ MoS2 composite electrode can still work at 28 A g1, suggesting good rate stability [42]. Compared with the GCD curves of MoS2 and Ni(OH)2 electrodes, the NiS/MoS2 composite material shows the longest discharge time of 138 s at 9 A g1 in Fig. 3d, indicating superior capacity [43]. In Fig. 4a, the specific capacitances of NiS/MoS2 electrode were 343.5, 287.3, 253, 211.5, 178, 154.6, and 133.8 mA h g1 at 9, 11, 13, 16, 20, 24, and 28 A g1, respectively. The maximum specific capacitances at 9 A g1 of three electrodes were calculated as 343.5 mA h g1 (for NiS/MoS2), 148.9 mA h g1 (for Ni(OH)2

Table 1 BET surface area, pore volume, and BJH average pore size of the as-synthesized MoS2 and NiS/MoS2 samples. Materials

Surface area (m2 g1)

BJH average pore size (nm)

Pore volume (cm3 g1)

MoS2 NiS/MoS2

19.0 41.0

3.4 3.1

0.124 0.246

J. Yan et al. / Journal of Alloys and Compounds 811 (2019) 151915

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Fig. 3. (a) CV curves of NiS/MoS2 composite electrode at different scan rates. (b) Comparison of CV curves of Ni(OH)2, MoS2 and NiS/MoS2 at a scan rate of 5 mV s1. (c) GCD curves of NiS/MoS2 composite electrode at various current densities and (d) comparison of Ni(OH)2, MoS2 and NiS/MoS2 electrodes at a current density of 9 A g1.

nanosheets), and 125.8 mA h g1 (for MoS2 nanosheets). To the best of our knowledge, this performance was also comparable or superior to some reported NixSy- or MoS2-based electrodes listed in Table 2 [44e48], such as Ni3S2 (161.6 mA h g1 at 1 A g1) [46], Ni1.5Co1.5S4 (347.5 mA h g1 at 0.5 A g1) [47], and NiSe@conductive fabric (119.6 mA h g1 at 2 A g1) [48]. Fig. 4b displays the Nyquist plots of the NiS/MoS2, MoS2, and Ni(OH)2 electrodes. The tiny quasi semicircle loop in medium-high frequency area (2153 Hze146 Hz) indicates the charge-transfer resistance (Rct), whereas the X-intercept represents the series resistance (Rs) derived from the electrode and the electrolyte [49]. Rs values of MoS2, Ni(OH)2, and NiS/MoS2 electrodes are 0.44, 0.57, and 0.49 U, respectively. Clearly, the NiS/ MoS2 electrode retains lower diffusion resistance than that of the Ni(OH)2 electrode, suggesting the improved electronic conductivity of the electrode material [50]. The slopes of the curves (Zw) of the three electrodes present different trends. From the inset of Fig. 4b, we can conclude that the Zw of the Ni(OH)2 electrode is much lower than the other two electrodes, which indicates sufficient ions diffusive routes, high porosity and short diffusive distance of the hydroxide ions into the electrode surface [51,52]. Although the Rs and Zw of the NiS/MoS2 electrode do not have the highest values compared to MoS2 and Ni(OH)2 electrodes, the NiS/MoS2 electrode combined the advantages of MoS2 and Ni(OH)2 electrodes and exhibited a synergetic effect. Fig. S1d displays the Nyquist plots of 10%, 20%, and 30% NiS/MoS2 electrodes. The 20% NiS/MoS2 electrode displays the lowest Zw compared with the other two composites, suggesting the improved electronic conductivity of the electrode material. The cyclability test of the NiS/MoS2 composite measured at 10 A g1 is presented in Fig. 4c. The capacity retention is greater than 90% after 3000 cycles and is maintained at 83.13% after 5000 cycles, indicating good cycling stability. It should be mentioned that the active material was prone to collapse when the electrode was subject to rapid charging and discharging at high current [53]. Furthermore, the NiS/MoS2 composite electrode presents approximately 93% Coulombic efficiency over 5000 cycles. Fig. 4d shows

typical EIS spectra obtained after zero and 5000 cycles in the Nyquist format. The Rs of the NiS/MoS2 electrode after cycling exhibited almost no change. However, the Zw of this electrode changed considerably due to the destruction of the NiS/MoS2 composite [37]. Accordingly, the remarkable electrochemical properties of the NiS/MoS2 electrode are attributed to the following reasons (Fig. 4e): (1) the largely increased surface area and pore volume can efficiently enhance the infiltration of the electrolyte to the electrode [54]; (2) the sufficient contact between the electrolyte and the electrode can facilitate the transport of electrons and ions [55]; (3) the hierarchical structure with homogeneous chemical distribution provides numerous active sites and serves as an effective charge transport pathway [56]. We also evaluated the capacitive performance of our NiS/MoS2 electrode in an ASC device to further evaluate potential practical application using NiS/MoS2 as the positive electrode and AC as the negative electrode. As shown in Fig. 5a, the AC and NiS/MoS2 electrodes are tested at the potential range of 1.0-0 V and 0e0.50 V at 30 mV s1 accordingly, demonstrating that the optimal potential window of the ASC device is 1.5 V. The shapes of CV curves in Fig. 5b were not deformed with increasing sweep speeds, demonstrating good rate performance. Moreover, no obvious internal resistance reduction was detected in Fig. 5c, implying the fast I-V response [57]. The calculated specific capacitances of our ASC device are 48.4, 46.0, 40.5, 32.1, 25.6, and 20.5 F g1 at 0.1, 0.2, 0.5, 1, 2, and 3 A g1, respectively (Fig. 5d). The Ragone plot is displayed in Fig. 5e. The energy density is calculated as 15.1, 14.4, 12.7, 10.0, 8.0, and 6.4 Wh kg1 at a power density of 75, 150, 375, 750, 1500, and 2250 W kg1, respectively. As demonstrated in Fig. 5f, the ASC device can maintain greater than 81.5% of its initial value after 5000 charge/discharge times, indicating high reversibility and stability. These results further indicated the good electrochemical performance and possibility for future practical applications of our NiS/ MoS2//AC ASC device.

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J. Yan et al. / Journal of Alloys and Compounds 811 (2019) 151915

Fig. 4. (a) Specific capacitance of Ni(OH)2, MoS2 and NiS/MoS2 electrodes at different current densities. (b) Comparison of enlarged Nyquist plots of Ni(OH)2, MoS2 and NiS/MoS2 electrodes at an initial potential of 0 V and the EIS at the low-frequency region (inset). (c) Cycling performance of NiS/MoS2 composite electrode at a current density of 10 A g1. (d) Impedance Nyquist plots of the NiS/MoS2 composite electrode before and after 5000th cycle. (e) Schematic illustration of the advantages of the NiS/MoS2 electrode for highperformance energy storage properties.

Table 2 Comparison of specific capacitance of NiS/MoS2 with other reported NixSy- or MoS2based electrodes. Materials

Potential (V)

Cs (mAh g1)

Ref.

Ni3S2/MWCNT Ni@rGO-Ni3S2 Ni3S2 Ni1.5Co1.5S4 NiSe@CF NiS/MoS2

0e0.45 0e0.55 0e0.45 0e0.45 0e0.45 0e0.45

128 (1024 F g¡1) 151 (987.8 F g¡1) 161.6 (1293 F g¡1) 347.5 119.6 343.5

44 45 46 47 48 This work

CF: conductive fabric.

4. Conclusions In summary, we developed the hierarchical NiS/MoS2 composite nanosheets via a facile strategy. The NiS/MoS2 composite presents an increased specific surface area and reduced contact resistance compared with pure MoS2 nanosheets. Moreover, the NiS/MoS2 composite exhibited the highest specific capacitance of 343.5 mA h g1, which is better than its monomers and superior to some of the reported NixSy- and MoS2-based values. In addition, the hybrid electrode also maintained good stability at 83.13% after 5000 charge-discharge cycles. The above results are mainly attributed to

Fig. 5. (a) CV curves of the negative and positive electrodes; electrochemical properties of NiS/MoS2//rGO/FeOOH ASC: (b) CV curves, (c) GCD curves, (d) specific capacitances, (e) Ragone plot, and (f) cycle lifespan.

J. Yan et al. / Journal of Alloys and Compounds 811 (2019) 151915

the hierarchical structure, the largely increased surface area and pore volume with enormous active sites, and the synergistic effect of the two compounds. An asymmetric (NiS/MoS2//AC) supercapacitor displays satisfactory energy density (15.1 Wh kg1 at 75 W kg1) and brilliant cycle stability even after 5000 cycles. Beyond the reported NiS/MoS2 composite, the design concept, including the in situ construction process, will positively impact the development of electrode materials for energy-related applications. Acknowledgments This work is financially supported by the Natural Science Foundation of Jiangsu Province (BK20180887), the China Postdoctoral Science Foundation (2017M621654). Young Talents Program of Jiangsu University, High-tech Research Key laboratory of Zhenjiang (SS2018002), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

[18]

[19]

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[21]

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[23]

[24]

Appendix A. Supplementary data

[25]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.151915.

[26]

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