Capacitive behavior of MoS2 decorated with FeS2@carbon nanospheres

Chemical Engineering Journal 379 (2020) 122240

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Capacitive behavior of MoS2 decorated with FeS2@carbon nanospheres a

a

a

b

Xingliang Chen , Tao Shi , Kailiang Zhong , Guanglei Wu , Yun Lu

a,⁎

T

a Key Laboratory of High Performance Polymer Materials and Technology of MOE, State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Chemistry for Life Sciences, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210046, PR China b Institute of Materials for Energy and Environment, Qingdao University, Qingdao 266071, PR China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

A novel hybrid of MoS decorated • with FeS @C nanospheres were syn2

2

thesized.

capacitive behavior of this hybrid • The were investigated. optimized sample performs ex• The cellent capacitive performance. synthetic method is helpful to • The develop other multiple sulfides hybrid.

A R T I C LE I N FO

A B S T R A C T

Keywords: Ferrous disulfide Molybdenum disulfide Composite hybrid Hierarchical structure Capacitance performance

The composite hybrid with novel hierarchical structure composed of 1T/2H MoS2 nanoflower and watermelonlike FeS2@carbon nanospheres (FeS2@C@MoS2) were synthesized via a facile method. Morphologies and structures of the hybrid were investigated thoroughly by using SEM, TEM, XRD, Raman, XPS and BET. The optimized composite hybrid with an appropriate ratio of carbon, FeS2 and MoS2 performs a high specific capacitance (1321.4 F·g−1 at 2 A·g−1) and a capacitance retention rate of 81.2% at 6 A·g−1 after 1000 cycles charge-discharge. Such new material has the potential to be a strong competitor in high performance supercapacitor electrode materials, and this facile synthesized method can hopefully be applied or as a reference to develop other metal sulfide@MoS2 hybrid or multiple sulfides hybrid.

1. Introduction With the rapid development of the science and technology, there has been an ever-increasing and urgent requirement of environmental protection, efficient and sustainable energy storage devices [1–5]. As a new-type of energy storage devices, supercapacitors have shown great performance advantages such as high power density, fast charge-discharge rate capability and excellent cycle stability etc, on which the constitution of active electrode materials have a huge impact [6–11]. So far, metallic oxides, metal sulfides, carbon based materials and ⁎

conductive polymers have always been hot researched in this domain, and their special nanostructured design is attracting more and more attention [12–14]. As one of the most representative and widely studied materials of transition metal disulfides, MoS2 possesses a typical two-dimensional lamellar structure, and typically exists in two phases, 1T and 2H, corresponding to the octahedral metal phase and the triangular prismatic semiconductor phase, respectively [15–19]. Due to its higher intrinsic ionic conductivity than oxide [20] and theoretical specific capacitance than graphite [21] as well as larger specific surface area [5], MoS2 has

Corresponding author. E-mail address: [email protected] (Y. Lu).

https://doi.org/10.1016/j.cej.2019.122240 Received 9 April 2019; Received in revised form 16 June 2019; Accepted 13 July 2019 Available online 17 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 379 (2020) 122240

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water and ultrasound for 20 min. Then the mixed solution was transferred into a 100 mL teflon-lined stainless-steel autoclave and sealed at 200 °C for 20 h. After naturally cooling to room temperature, the FeS2@C@MoS2-100 was separated by 10,000 r.min−1 centrifugation, washed with deionized water and ethanol for several times and dried at 75 °C for about 8 h. Similarly, when the dosage of sodium molybdate were adjusted to 0 mg, 150 mg, 200 mg and 250 mg, and the corresponding additive amounts of thioacetamide were adjusted to 0.3 g, 0.6 g, 0.7 g and 0.8 g, FeS2@C, FeS2@C@MoS2-150, FeS2@C@MoS2200 and FeS2@C@MoS2-250 were obtained [33,34].

been extensively investigated as an electrode material. Although the real specific capacitance of pure MoS2 is not high [22], the combination of MoS2 and some other materials has been demonstrated to cause the resulted materials possess improved capacitance performance. For instance, hierarchical carbon@Ni3S2@MoS2 double core-shell nanorods deliver a specific capacitance of 1544 F·g−1 at a current density of 2 A·g−1 and excellent cycling stability (retaining 92.8% of the capacitance after 2000 cycles at a current density of 20 A·g−1) [23]. The Ni3S4@amorphous MoS2 nanospheres with a uniform core/shell architecture showed a high specific capacitance of 1440.9 F·g−1 at 2 A·g−1 [24]. Besides, many other materials such as CoS2@MoS2 [25], MoS2@graphene [26] and 3D Ni(OH)2 nanoplates@MoS2 [27] etc. have also been reported for application as supercapacitor electrode materials. On the other hand, iron disulfide (FeS2) have also attracted great interest as a potential energy storage material in recent years [28,29]. Compared with natural FeS2 crystals, the synthesized FeS2 by oxidationreduction have higher purity, smaller size and better designability, exhibiting predominant magnetic optical and electrochemical properties [30]. Nowadays FeS2 has been widely reported to be used as lithium ion battery and catalytic materials [31], but very few publications concern the application of FeS2 as supercapacitor electrode material. Thus it’s an interesting project to develop the potential of FeS2 and its composites on this respect. Herein, we report a novel composite hybrid of MoS2 nanoflower decorated with watermelon-like FeS2@carbon nanospheres (FeS2@C@ MoS2) for high-performance supercapacitors. A facile solvothermal reaction was adopted to prepare watermelon-like Fe3O4@C nanospheres, and then further prepare FeS2@C@MoS2 composite hybrid by achieving synchronously the synthesis of MoS2 and the sulfuration of Fe3O4 at the presence of sodium molybdate and thiacetamide. The structures, morphologies and capacitance performances of the resulting FeS2@C@MoS2 were then characterized. The ratio of carbon, FeS2 and MoS2 was adjusted and its impact on the performance of samples was discussed in detail. The optimized novel composite hybrid possesses high specific capacitance and good cyclical stability at a high current density.

2.4. Characterization The surface structures and morphology features of samples were investigated by using a JEOL JSM-7800F scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDS) and transmission electron microscope (TEM, JEOL JEM-2100). The chemical composition and crystal form of samples were checked using X-ray powder diffraction (XRD, PANalytical, Netherlands, Cu Kα, 10°–80°). Raman spectra of productions were obtained by using a Renishaw Ramascope (Renishaw, U.K.). X-ray photoelectron spectroscopy (XPS, Thermal Scientific Kα) measurement was carried out to investigate identify the elements and binding energy of samples. The surface area of samples was characterized by using liquid nitrogen adsorption-desorption based on Brunauer-Emmett-Teller (BET) method [35–39]. 2.5. Electrochemical measurements The working electrodes were fabricated by uniformly mixing the asprepared samples (FeS2@C, FeS2@C-100, FeS2@C@MoS2-150, FeS2@C@MoS2-200 and FeS2@C@MoS2-250), acetylene black and polyvinylidene fluoride binder in ethanol solution with a weight ratio of 8:1:1. The mixtures were then homogeneously coated onto a block of nickel foam with a mass loading of around 1.5 mg and dried at 100 °C for 10 h. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and cyclic stability of samples were measured by using a three-electrode system (CHI660E electrochemical workstation) in 6.0 M KOH solution at room temperature. The adopted reference electrode and counter electrode was mercuric oxide electrode (Hg/HgO) and Pt electrode, respectively. The specific capacitance values of samples were calculated by using the equation: C = (I*Δt)/(m*ΔV), where I stands for the discharge current, Δt represents the discharge time, m means the mass loading of active samples and ΔV is the potential drop during discharging, respectively [40,41].

2. Experimental 2.1. Materials All the reagents including sodium molybdate (Na2MoO4·2H2O), acetone, ferrocene, H2O2 (30%), thioacetamide (CH3CSNH2) and sodium dodecylsulphate were purchased from Aladdin Industrial Corporation and used without any further purification.

3. Results and discussion 2.2. Synthesis of Fe3O4@C nanospheres Fig. 1 schematically presents the formation process of FeS2@C@ MoS2 composite hybrids. First, the typical decomposition of ferrocence was carried out via a simple solvothermal reaction. Through this process, the Fe3O4@C nanospheres with core-shell structure or pitaya-like structure have been prepared by adjusting the reaction time and temperature [31,42]. In our case, the watermelon-like Fe3O4@C nanospheres was achieved by the synchronous reactions of both oxidation of the iron atoms in ferrocence into magnetite and the growing of Fe3O4 nanoparticles in carbon phase. During the second one-pot hydrothermal reaction, the formation/crystallization of molybdenum disulfide nanoflower and reduction of Fe3O4 nanoparticle were completed simultaneously, thus obtaining the FeS2@C@MoS2 composite hybrid. Carbon layer on the surface of Fe3O4 played a role of effective protective layer to ensure that the structure of composite nanospheres not be destroyed during the reaction process. The main reactions occurred in the whole process were summarized as Eqs. (1)–(4). Such facile synthesized method can hopefully be applied or as a reference to develop other metal sulfide@MoS2 hybrid or multiple sulfides hybrid.

The synthetic procedure of Fe3O4@C nanospheres was based on a facile solvothermal reaction [32]. 750 mg ferrocene was uniformly dispersed in acetone by 5 min ultrasonic processing, into which 3.75 mL H2O2 (30%) was added and stirred for 15 min. Then the miscible liquid was transferred into a 100 mL teflon-lined stainless-steel autoclave and sealed at 240 °C for 24 h. After naturally cooling to room temperature, the synthesized Fe3O4@C nanospheres were separated by 10,000 r.min−1 centrifugation, washed with ethanol for several times and dried at 75 °C for about 8 h. The obtained nanospheres were further calcined under nitrogen atmosphere for 4 h at 500 °C in order to increase carbonization degree. 2.3. Synthesis of FeS2@C@MoS2 composite hybrid 150 mg of the as-synthesized Fe3O4@C nanospheres, 100 mg sodium molybdate, 0.5 g thioacetamide and 100 mg of sodium dodecylsulphate (SDS) were uniformly dispersed in 80 mL of deionized 2

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Fig. 1. The formation process of FeS2@C@MoS2 composite hybrid.

Fig. 2. XRD patterns and Raman spectra of samples. XRD patterns of Fe3O4@C and FeS2@C (a) and FeS2@C@MoS2 (b and c), Raman spectra of FeS2@C (d) and FeS2@C@MoS2 (e and f). 3

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Fig. 3. Electrochemical performance of samples. (a) CV curves at a scan rate of 10 mV·s−1, (b) GCD curves at a current density of 6 A·g−1, (c) specific capacitances at different current density, (d) GCD curves of FeS2@C@MoS2-200 at different current density.

Fig. 4. Electrochemical performance of FeS2@C and FeS2@C@MoS2-200. (a) CV curves at a scan rate of 10 mV·s−1, (b) GCD curves at a current density of 2 A·g−1, (c) specific capacitances at different current density, (d) EIS spectra of samples, (e) the corresponding cyclical stability, (f) Ragone plots of samples (energy density vs. power density).

4

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Fig. 5. SEM images of Fe3O4@C (a) and FeS2@C@MoS2-200 (b and c), EDS pattern (d), EDX elemental mapping (e and f) of FeS2@C@MoS2-200.

CH3 CSNH2 + 2H2 O → CH3 COOH + H2 S + NH3

CH3 COOH ⇋ CH3

COO−

+

H+

(1)

change illustrates the transition from Fe3O4 to FeS2. And in Fig. 2b, compare with FeS2@C, another new peak attributing to (0 0 2) crystal plane of MoS2 emerges, verifying the ingredients of the newly formed composite hybrids at the presence of sodium molybdate. With the increase of the contents of MoS2, some crystal plane diffraction peaks of samples show a slight shift in varying degrees (Fig. 2c), suggesting the occurrence of possible lattice distortion during the sulfuration reaction [23]. The molecular structure and components of the samples were further characterized by Raman spectroscopy (Fig. 2d to f). Two distinguishable peaks at 1340 and 1590 cm−1 can be observed for the asprepared FeS2@C (Fig. 2d), assigning to D-mode and G-mode of typical carbon based materials [45]. Meanwhile, another three peaks located at 320, 344 and 379 cm−1 arise. The latter two stand for the typical Eg and Ag vibration modes of FeS2 crystal, respectively, and the former represents marcasite phase of FeS2, implying that the obtained FeS2 may exist in pyrite-marcasite hybrid phase [46,47]. The Raman spectra of

(2)

2H2 S + 4H+ + Fe3 O4 → FeS2 + 2Fe 2 + + 4H2 O

(3)

MoO42 − + 3H2 S → MoS2 + 3H2 + SO42 −

(4)

The XRD patterns of Fe3O4@C, FeS2@C and four FeS2@C@MoS2 samples containing different proportions of MoS2 are exhibited in Fig. 2 (a, b and c). As shown in Fig. 2a, the as-synthesized Fe3O4@C nanosphere retains the all crystal plane diffraction peaks of Fe3O4 typically for (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 0 0) at 2θ = 30.2°, 35.6°, 43.3°, 53.6°, 57.3° and 62.9°, respectively (JCPDS Card no. 190629) [43]. After vulcanization without sodium molybdate, the diffraction peaks of Fe3O4 were replaced by another 9 peaks, which belong to (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 1 1), (2 2 2), (2 3 0) and (3 2 1) crystal plane of FeS2 (JCPDS Card no. 42-1340) [44]. This 5

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Fig. 6. TEM image and SAED pattern of Fe3O4@C (a), TEM images of FeS2@C@MoS2-200 and FeS2@C (b), SAED pattern of FeS2@C@MoS2 (c), MoS2 lattice spacing of FeS2@C@MoS2-200 (d).

FeS2 + OH− ⇋ FeS2 OH + e−

FeS2@C@MoS2-100, −150, −200 and −250 samples are exhibited in Fig. 2e and f. Compare with FeS2@C, the FeS2@C@MoS2 samples show a new peak located at around 405 cm−1, which could be ascribed to the typical A1g vibration mode of the hexagonal MoS2 crystal [48]. With the increase of MoS2 content in the samples, this peak is enhanced stepwise accompanying by the decrease of the Eg vibration peak intensity of FeS2, verifying the chemical composition of the FeS2@C@MoS2 composite hybrid again. The electrochemical behaviors of FeS2@C@MoS2 composite hybrids were evaluated based on the cyclic voltammetry (CV) and galvanostatic charging-discharging (GCD) test results. Fig. 3 shows the CV patterns and GCD patterns of FeS2@C@MoS2 samples with varied MoS2 content, from which their specific capacitances were calculated. It can been observed from CV curves (Fig. 3a), different from three other samples performing two reduction peaks of FeS2 and MoS2 respectively (Eqs. (5) and (6)), the sample FeS2@C@MoS2-200 shows only one reduction peak, suggesting its best matching effect for the reduction of FeS2 and MoS2. Fig. 3b illustrates the GCD curves of samples measured at a current density of 6 A·g−1, which indicates that all these samples perform pseudocapacitor behaviors. According to their GCD curves, the corresponding specific capacitances of these samples were calculated via C = (I*Δt)/(m*ΔV) and the results are displayed in Fig. 3c. When the imposed current density is 2 A·g−1, the specific capacitance values of FeS2@C@MoS2-100, −150, −200 and −250 are 505.6 F·g−1, 993.2 F·g−1, 1321.4 F·g−1 and 743.5 F·g−1, respectively, indicating that with the increase of MoS2, the specific capacitance value of FeS2@C@ MoS2 hybrid increases first and then decreases. FeS2@C@MoS2-200 sample exhibits the optimal energy storage performance, which can be attributed to the synergistic effect of appropriate constitute proportions of FeS2, carbon and MoS2. As the optimum performance specimen, the GCD curves of FeS2@C@MoS2-200 at different current density are shown in Fig. 3d, state clearly that the specific capacitance of the sample can maintain above 900 F·g−1 when the imposed current density increased to 8 A·g−1, highlighting its excellent charge acceptance [49–51].

MoS2 +

OH−

⇋ MoS2 OH +

e−

(5) (6)

The sample showing optimal performance, FeS2@C@MoS2-200, was taken as the research object to conduct a comprehensive electrochemical test and the comparison of FeS2@C@MoS2-200 with FeS2@C in capacitance properties is illustrated in Fig. 4. It is interesting to note that FeS2@C@MoS2-200 composite hybrid possesses a specific capacitance value of 1321.4 F.g−1 at 2 A·g−1, far higher than FeS2@C (419 F·g−1 at 2 A·g−1) at any current density (Fig. 4a–c), which can be mainly attributed to the introduction of appropriate ratio of MoS2. Fig. 4d exhibits the electrochemical impedance spectra (EIS) of these two samples in the frequency region from 1000 Hz to 0.01 Hz. Obviously, the X axis intercepts of these two samples which presents the equivalent series resistance exhibit a certain of difference, implying the important role of MoS2 in improving the conductivity. After 1000 cycles charging-discharging with a current density of 6 A·g−1, the samples FeS2@C@MoS2-200 and FeS2@C perform a capacitance retention of 81.2% and 88.9% respectively (Fig. 4e), indicating good cyclical stability most probably due to the excellent chemical/electrochemical stability and the high mechanical strength of carbon and MoS2 framework. Also, the comparison of the EIS curves of FeS2@C and FeS2@C@ MoS2-200 measured before and after cycles were provided, they all showed less significant change in solution resistance (Rs), and partly increase in diffusion resistances. The equivalent series resistance of FeS2@C@MoS2-200 electrode measured before and after cycles are all less than that of FeS2@C electrode, which further confirm the superior electrochemical properties of the FeS2@C@MoS2-200 hybrid (Fig. S3). In addition, the Ragone plot relating the powder and energy density of FeS2@C@MoS2-200 composite hybrid and FeS2@C nanosphere was employed to evaluate their application values through the Eqs. (7) and (8), where E (Wh·kg−1) is the energy density, P (W·kg−1) is the power density, C (F·g−1) is the specific capacitance of the supercapacitor, V (V) is the operating voltage of the device and t (s) is the time of discharge process. When the power density is 0.5 Kw·kg−1, the energy density of the sample FeS2@C@MoS2-200 can reach up to 6

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Fig. 7. XPS and BET results of FeS2@C@MoS2-200 composite hybrid. Survey scan (a), Narrow scan of C1s (b), Fe2p (c), Mo3d (d), S2p (e) and liquid nitrogen adsorption-desorption isotherms and the related pore size distribution (f).

29.4 Wh·kg−1 (Fig. 6f), suggesting the promising potentials of the composite hybrid in application of energy storage [52–56].

E = 0.5*C *ΔV 2

(7)

P = E /t

(8)

of around 150 nm, which is composed of countless superfine size Fe3O4 nanoparticles with an extremely thin carbon coating (Figs. 5a and 6a). The characteristic (2 2 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0) diffraction peaks for Fe3O4 can be observed in corresponding electron diffraction pattern (Fig. 6a). After the sulfurization reaction at presence of sodium molybdate and thiacetamide, a unique three dimensional MoS2 nanoflower decorated with watermelon-like FeS2@C nanospheres was constructed (Figs. 5b and 6b). The composite hybrid is uniform with a petal thickness of as-synthesized MoS2 nanoflower of less than 10 nm (Fig. 5c). The MoS2 nanoflower may be interconnected with FeS2@C nanospheres by Fe-S-Mo chemical bonds. EDS pattern and elemental

The morphologies, microstructures and element composition of the as-prepared Fe3O4@C and FeS2@C@MoS2-200 were investigated via SEM and TEM, and the images, corresponding EDS pattern, EDX elemental mapping, electron diffraction and lattice spacing of samples are shown in Figs. 5 and 6. SEM and TEM results indicate that the Fe3O4@C possesses a homogeneous watermelon-like morphology with a diameter 7

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Fig. 8. Schematic illustration of the charge process for FeS2@C@MoS2 composite hybrid. Table 1 Comparison of morphology, the maximum specific capacitance (Cs), electrolytes and cycle stability of some reported similar materials and the present work. Materials

Morphology

Cs (Current density/scan rate)

MoS2 nano flower

196.9 F g

MoS2@C

−1

(0.5 A g

−1

)

Electrolyte

Stability

Refs. −1

3 M KOH

91.9% (4 A g

, 900 cycles)

[22]

411 F g−1 (1 A g−1)

1 M Na2SO4

93.2% (1 A g−1, 1000 cycles)

[54]

Ni3S2@MoS2

848 F g−1 (5 A g−1)

2 M KOH

91% (8 A g−1, 2000 cycles)

[63]

NiS/MoS2/carbon nanotube

757 F g−1 (0.5 A g−1)

3 M KOH

96% (0.5 A g−1, 2000 cycles)

[56]

CoS2/MoS2/C

406 F g−1 (5 mV s−1)

0.5 M Na2SO4

95.2% (100 mV s−1, 10,000 cycles)

[65]

rGO-FeS2

112.41 mF cm−2 (5 mV s−1)

1 M Na2SO4

90% (5 mV s−1, 10,000 cycles)

[53]

FeS2@C@MoS2

1321.4 F g−1 (2 A g−1)

6 M KOH

81.2% (6 A g−1, 1000 cycles)

This work

structure factors that could offer excellent capacitive properties [57–60]. SEM and TEM images, EDX elemental mapping and EDS pattern of the FeS2@C@MoS2-200 electrode after cycling show the uniform element composition and morphology (Fig. S4), which reflects the superior cycling stability of the electrode material from another aspect. XPS measurement was also adopted to evaluate the surface elemental composition and chemical states of FeS2@C@MoS2-200 composite hybrid. It can be seen from the XPS survey scan spectrum that C,

mapping images (Fig. 5d and e) indicate the surface elemental composition of the sample FeS2@C@MoS2-200, suggesting that the proportion of elements of C, S, Fe and Mo is 28.66, 34.09, 20.98 and 16.27 in wt%, respectively. Besides, the typical (2 0 0), (2 1 0), (2 1 1), (2 2 0) and (3 1 1) crystal plane diffraction peaks for FeS2 (Fig. 6c) and the (0 0 2) plane lattice spacing with a spacing distance of 0.62 nm for MoS2 (Fig. 6d) confirm the component of the composite hybrid once more. The above results forcefully proved that the FeS2@C@MoS2 composite hybrid with special hierarchical structure and morphology is of internal 8

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1000 cycles), revealing its performance advantages and application potential as an ideal capacitor electrode material.

S, Fe, Mo and O elements exist on the surface of the sample (Fig. 7a), which is consistent with the results of EDS pattern and EDX elemental mapping. And the details of chemical states can be obtained from the narrow spectrum of each element. For examples, the three peaks in Fig. 7b could be attributed to CeC, CeO and C]C binding energy respectively. The two peaks centered at 710.5 eV and 725 eV in Fig. 7c could be assigned to Fe 2p3/2 and Fe 2p3/2 binding energy, respectively, and also the two peaks located at 707.3 eV and 719.9 eV in Fig. 7c might belong to (Fe-S)2p3/2 and (Fe-S)2p1/2 binding energy, respectively. In addition, Fig. 7d displays the peaks related to Mo3d binding energy including two pairs peaks located at 228.9 eV, 232.1 eV and 229.4 eV, 233.1 eV corresponding to 1T and 2H MoS2 polymorphs with +4 oxidation states respectively, and one pair of peaks at 232.7 eV and 236.1 eV assigning to Mo6+ 3d5/2 and Mo6+ 3d3/2. Fig. 7e reveals the peaks concerned S2p binding energy such as Fe-S2p3/2 (162.1 eV) and Fe-S2p1/2 (162.8 eV), as well as Mo-S2p3/2 (161.6 eV) and Mo-S2p1/2 (164 eV) for 1T MoS2, Mo-S2p3/2 (163.2 eV) and Mo-S2p1/2 (164.6 eV) for 2H MoS2. Besides, the peak at around 169 eV for S2p regions can be attributed to unsaturated S atoms. Combine the above discussion, XPS spectra further manifest the constitute and structure of FeS2@C@MoS2200 composite hybrid, and suggest that the optimal matching of components helps to improve capacitance properties [56,61–63]. Except for XPS measurement, the specific surface area of FeS2@C@ MoS2-200 composite hybrid was examined by using BET. The results obtained from its adsorption curve, desorption curve and pore distribution curve (Fig. 7f) indicate that the specific surface area value of the hybrid is 94.7 m2·g−1. Also, the hybrid possesses uniform mesoporous structure with a pore size distribution of 2 nm to 50 nm, which is very beneficial to the infiltration of electrode materials and the improvement of effective specific surface area [64]. Through the above discussion, the as-synthesized FeS2@C@MoS2200 composite hybrid exhibits a high specific capacitance and good cycling stability for high-performance supercapacitor electrode materials. Such superior electrochemical performance can mainly be attributed to the following factors. Firstly, the introduction of 1T/2H hybrid MoS2 framework can effectively improve the electrical conductivity of the material, which is more favorable for ions transfer. Secondly, a most suitable proportion of carbon, FeS2 and MoS2 can lead to the best matching effect of synergistic reaction of FeS2 and MoS2 on accelerating ion exchange. Thirdly, the space derived from neighboring 2D MoS2 nanosheets could function as ‘‘ion reservoir” to facilitate the ion transportation near the interface of active materials and electrolyte, and buffer the volume expansion during charging and discharging. Fourth, the mesoporous structure of the composite hybrid can offer large effective specific surface area and more channels for promoting adequate entry of electrolyte and fast transfer of ions. It is these factors that codetermine the excellent capacitive properties of this hybrid [48,56,66]. The schematic illustration of the charge process for FeS2@C@MoS2 composite hybrid is detailed in Fig. 8. Comparing with the other reported similar electrode materials, including MoS2 nanoflower [22], MoS2@mesoporous carbon [54], Ni3S2@MoS2 [63], NiS/ MoS2/carbon nanotube [56], CoS2/MoS2/carbon fiber [65] and rGo/ FeS2 [53], the present as-synthesized composite hybrid exhibits an advantageous specific capacitance value and cyclical stability at a high current density (Table 1).

Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No. 21374046), Program for Changjiang Scholars and Innovative Research Team in University (IRT1252), the Fundamental Research Funds for the Central Universities and the Testing Foundation of Nanjing University. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122240. References [1] K. Wang, H. Wu, Y. Meng, Z. Wei, Conducting polymer nanowire arrays for high performance supercapacitors, Small 10 (2014) 14–31. [2] S. Gong, Z. Jiang, P. Shi, J. Fan, Q. Xu, Y. Min, Noble-metal-free heterostructure for efficient hydrogen evolution in visible region: Molybdenum nitride/ultrathin graphitic carbon nitride, Appl. Catal. B: Environ. 238 (2018) 318–327. [3] A. Kafy, A. Akther, L. Zhai, H.C. Kim, J. Kim, Porous cellulose/graphene oxide nanocomposite as flexible and renewable electrode material for supercapacitor, Synth. Met. 223 (2017) 94–100. [4] X. Li, T. Qian, J. Zai, K. He, Z. Feng, X. Qian, Co stabilized metallic 1Td MoS2 monolayers: bottom-up synthesis and enhanced capacitance with ultra-long cycling stability, Mater. Today Energy 7 (2018) 10–17. [5] G.F. Ma, H. Peng, J.J. Mu, H.H. Huang, X.Z. Zhou, Z.Q. Lei, In situ intercalative polymerization of pyrrole in graphene analogue of MoS2 as advanced electrode material in supercapacitor, J. Power Sources 229 (2013) 72. [6] T. Wu, J. Fan, Q. Li, P. Shi, Q. Xu, Y. Min, Palladium nanoparticles anchored on anatase titanium dioxide-black phosphorus hybrids with heterointerfaces: highly electroactive and durable catalysts for ethanol electrooxidation, Adv. Energy Mater. 8 (2018) 1701799. [7] L. Jiang, J.W. Yan, L.X. Hao, R. Xue, G.Q. Sun, B.L. Yi, High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors, Carbon 56 (2013) 146–154. [8] P. Liu, J. Yan, X. Gao, Y. Huang, Y. Zhang, Construction of layer-by-layer sandwiched graphene/polyaniline nanorods/carbon nanotubes heterostructures for high performance supercapacitors, Electrochim. Acta 272 (2018) 77–78. [9] X. Li, J. Zai, S. Xiang, Y. Liu, X. He, Z. Xu, K. Wang, Z. Ma, X. Qian, Regeneration of metal sulfi des in the delithiation process: the key to cyclic stability, Adv. Energy Mater. 6 (2016) 1601056. [10] Y. Min, G. He, Q. Xu, Y. Chen, Dual-functional MoS2 sheet-modified CdS branch-like heterostructures with enhanced photostability and photocatalytic activity, J. Mater. Chem. A 2 (2014) 2578. [11] C. Yang, Z. Chen, I. Shakir, Y. Xu, H. Lu, Rational synthesis of carbon shell coated polyaniline/MoS2 monolayer composites for high-performance supercapacitors, Nano Res. 9 (2016) 951–962. [12] X. Bai, Q. Liu, J. Liu, H. Zhang, Z. Li, X. Jing, P. Liu, J. Wang, R. Li, Hierarchical Co3O4@Ni(OH)2 core-shell nanosheet arrays for isolated all-solid state supercapacitor electrodes with superior electrochemical performance, Chem. Eng. J. 315 (2017) 35–45. [13] M.W. Shi, Y.Y. Zhang, M.D. Bai, B.M. Li, Facile fabrication of polyaniline with corallike nanostructure as electrode material for supercapacitors, Synth. Met. 223 (2017) 74–78. [14] Y.R. Zhu, X.B. Ji, X.B. Wu, Y. Liu, NiCo2S4 hollow microsphere decorated by acetylene black for high-performance asymmetric supercapacitor, Electrochim. Acta 186 (2015) 562–571. [15] M. Zhang, J. Zai, J. Liu, M. Chen, Z. Wang, G. Li, X. Qian, L. Qian, X. Yu, A hierarchical CoFeS2/reduced graphene oxide composite for highly efficient counter electrodes in dye-sensitized solar cells, Dalton Trans. 46 (2017) 9511. [16] D. Wang, Y. Xiao, X. Luo, Z. Wu, Y.J. Wang, B. Fang, Swollen ammoniated MoS2 with 1T/2H hybrid phases for high-rate electrochemical energy storage, ACS Sustainable Chem. Eng. 5 (2017) 2509–2515. [17] Y. Li, C. Xu, J. Qin, W. Feng, J. Wang, S. Zhang, L. Ma, J. Cao, P. Hu, W. Ren, L. Zhen, Tuning the excitonic states in MoS2/graphene van der waals heterostructures via electrochemical gating, Adv. Funct. Mater. 26 (2016) 293–302. [18] N. Zheng, X. Bu, P. Feng, Synthetic design of crystalline inorganic chalcogenides exhibiting fast-ion conductivity, Nature 426 (2003) 428–432. [19] J. Xiao, D. Choi, L. Cosimbescu, P. Koech, J. Liu, Exfoliated MoS2 nanocomposite as an anode material for lithium ion batteries, Chem. Mater. 22 (2010) 4522–4524. [20] F. Zhang, C. Zhang, H. Peng, H. Cong, H. Qian, Near-infrared photocatalytic upconversion nanoparticles/TiO2 nanofibers assembled in large scale by electrospinning, Part. Part. Syst. Charact. 33 (2016) 248–253. [21] X. Li, Z. Feng, J. Zai, Z. Ma, X. Qian, Incorporation of Co into MoS2/graphene nanocomposites: one effective way to enhance the cycling stability of Li/Na storage, J. Power Sources 373 (2018) 103–109.

4. Conclusion A new composite hybrid with novel hierarchical structure constructed by three dimensional 1T/2H MoS2 nanoflower decorated with watermelon-like FeS2@carbon nanospheres was facilly synthesized. Due to the synergistic effect of appropriate ratio of MoS2 framework, carbon layer and FeS2 and its mesoporous structure, the FeS2@C@ MoS2-200 composite hybrid performs high specific capacitance (1321.4 F·g−1 at 2 A·g−1) and good cyclical stability at a high current density (specific capacitance retention rate of 81.2% at 6 A·g−1 after 9

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