Uniform MoS2 nanolayer with sulfur vacancy on carbon nanotube networks as binder-free electrodes for asymmetrical supercapacitor

Applied Surface Science 475 (2019) 793–802

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Uniform MoS2 nanolayer with sulfur vacancy on carbon nanotube networks as binder-free electrodes for asymmetrical supercapacitor ⁎

Peng Suna, Ruijing Wanga, Qiang Wangb, , Huanwen Wangc, Xuefeng Wanga,

T

⁎

a

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China c Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon nanotubes MoS2 S vacancy Asymmetric supercapacitor

Molybdenum sulfide (MoS2) is regarded as a promising material for supercapacitor applications but the intrinsically low electrical conductivity greatly limits its high specific capacitances. Herein, we introduce sulfur vacancy on MoS2 nanolayer (MoS2−x) by a pulsed laser deposition (PLD) process. By further using the highly conductive carbon nanotube (CNT) networks as the current collector, the as-fabricated defect-rich MoS2@CNTs/ Ni core/shell-structured electrode delivers an ultrahigh specific capacitance of 512F g−1 at 1 A g−1, excellent rate performance (342F g−1 at 30 A g−1) and long cycle life (no decay after 2000 cycles) in 1 M Na2SO4 electrolyte, which are among the best reported values for MoS2-based supercapacitors. Along with the experiment results, our DFT calculations further demonstrate that the S vacancy can create deep acceptor levels in the MoS2 monolayer, which can trap electrons and improve the electrons mobility. For practical application, we build an asymmetrical supercapacitor (ASC) with MoS2−x@CNTs/Ni as the positive electrode and CNT networks as the negative electrode, which exhibits a large energy density of 63 Wh kg−1 at 850 W kg−1 and an impressive power density of 25.5 kW kg−1 at 44.2 Wh kg−1. These results indicate that PLD is a very powerful technique to construct the binder-free film electrodes for energy storage applications.

1. Introduction As a new class of green energy storage devices, supercapacitors have gained considerable attention due to the increasing demand in portable devices, mobile phone, portable charger, digital camera and medical devices, etc. [1–3]. At present, carbon materials, such as graphene, carbon nanotube (CNT) and activated carbon (AC) are the most widely used electrode materials, but the low capacitance often fails to meet the demand. As compared to carbon materials, which store energy by electrostatic accumulation, transition metal oxides (TMOs, such as RuO2, VOx and MnO2) have been explored extensively as pseudocapacitive materials for supercapacitors owing to their large charge transferreaction capacitances [4,5]. In addition to TMOs, various transition metal sulfides and selenides shows the graphene-like architectures and excellent physical and electrochemical properties, which have been favored by many research groups [6–12]. Recently, MoS2-based nanomaterials have been intensively investigated as an supercapacitor material due to their unique 2D nanosheets feature [13]. However, the electronic conductivity of MoS2 is relatively low, which is largely limited in alone for energy storage applications; however, the sulfur

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vacancy in MoS2 materials can improve the electrical conductivity, surface reactivity and number of active sites [14–16]. Similarly oxygen deficiency can improve the conductivity of ZnO, In2O3, SnO2 and Y2O3doped CeO2 [16–18]. Another strategy to improve the electronic conductivity of MoS2 is combining MoS2 with highly conductive materials. Until now, lots of MoS2-based composites have been researched to increase the electrical conductivity of MoS2, including MoS2/graphene, [19,20] MoS2/RGO, [21] MoS2/CF, [22] MoS2/PANI and MoS2/CNTs [23–28]. CNT has been widely used as the skeleton for depositing different materials because of high electrical conductivity and one-dimensional (1D) tube nanostructure. A series of CNTs-based composites have been investigated in electrochemical energy storage, including anthraquinone/N-doped CNTs, [29] NiO/CNTs, [30] Ni(OH)2/CNTs, [31] Co (OH)2/CNTs, [32] MnO2/CNTs and NieCoeS/CNTs [33,34]. Until now, most of these hybrid materials are made from hydrothermal synthesis process or electro-deposition. Pulsed laser deposition (PLD) has emerged as a highly nonequilibrium hyperthermal particle deposition technique, which has a broad application in thin-film research, such as deposition of semiconductors and polymers, and synthesis of

Corresponding author. E-mail address: [email protected] (X. Wang).

https://doi.org/10.1016/j.apsusc.2019.01.007 Received 14 August 2018; Received in revised form 24 November 2018; Accepted 1 January 2019 Available online 02 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (A) The fabrication process of the MoS2−x@CNTs/Ni and (B) the schematic diagram of the PLD device.

2. Experimental

hybrid materials. The advantages of PLD include anhydrous deposition, reactive deposition from energetic plasma plume and inherent simplicity for constructing a multi-layer structure. Our group has demonstrated the feasibility of NiO, [35] MnO2 and Co3O4 [36,37] for supercapcitor applications by PLD. In this work we firstly utilize a PLD method to deposit sulfur-vacant MoS2 nanolayer (MoS2−x) on CNTs surface on Ni mesh to form MoS2−x nanolayer@CNTs (MoS2−x@CNTs) core-shell structure. CNTs were insitu grown on the surface of conductively Ni mesh, which further served as the substrate for MoS2−x nanolayer. The conductivity of MoS2−x is improved by both S vacancy and CNTs skeleton. Resulting from the synergistic effect of S vacancy and binder-free electrode configuration as well as the unique hierarchical one-dimensional nanotube structure, the as-obtained MoS2−x@CNTs/Ni electrode shows high supercapacitor performances (512F g−1 at 1 A g−1 in neutral electrolyte). The high capacitive performances are also supported by our theoretical calculations. Meanwhile, we also assemble an asymmetric supercapacitor (ASC) with the binder-free MoS2−x@CNTs/Ni as the positive electrode and CNTs@carbon cloth (CC@CNTs) as the negative electrode. The high energy density and power density of our ASC indicate the potential of MoS2−x@CNTs/Ni for energy storage devices.

2.1. Preparation of the MoS2−x@CNTs/Ni CNTs/Ni (Ni mesh number: 200, 1 cm × 1 cm) were grown directly on Ni mesh via a chemical vapor deposition (CVD) method as reported previously [33]. Subsequently, MoS2 was deposited on CNTs/Ni (1 cm × 1 cm) by PLD method [35]. Briefly, the CNTs/Ni substrate and MoS2 target were fixed on a substrate holder and a target holder in the stainless steel chamber, respectively. The residual pressure of the chamber was pumped down to 10−4 Pa, and the Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 8 ns pulse width) was focused onto the MoS2 target for 1 h. An energetic plume generated from the surface of MoS2 target was deposited on CNTs/Ni substrate to form MoS2−x@CNTs/Ni composite. MoS2−x@CNTs/Ni was taken out and washed repeatedly with deionized water and dried in a vacuum at 60 °C for 12 h. The negative electrode CC@CNTs was also prepared by CVD method and the mass loading for CNTs was around 0.5 mg cm−2. 2.2. Materials characterization The morphology and microstructure of the products were characterized by field emission scanning electron microscopy (FESEM; 794

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Fig. 2. FESEM (A, B, D) and TEM (C, E) images of CNTs grown on Ni mesh (A–C), and MoS2−x@CNTs/Ni (D, E). The SAED pattern of MoS2−x@CNTs/Ni (F). EDX mapping of the samples: Mo (G) and S elements (H).

Fig. 3. XRD patterns (A) and Raman spectra (B) of the samples. XPS spectra of the MoS2−x@CNTs/Ni: Mo 3d (C), S 2p (D) and C 1s (E).

2.4. Calculation method

Hitachi S-4800) and transmission electron microscopy (TEM; JEOL, JEM-2100), X-ray diffraction (XRD; Bruker Focus D8 with Cu Kα radiation), Raman spectroscopy (Renishaw In via, 514 nm laser under ambient conditions) and X-ray photoelectron spectroscopy (XPS; AXIS Ultra DLD).

To understand the role of MoS2/C heterointerfaces on the supercapacitors, we carried out first-principles calculations on MoS2@graphene both with and without S vacancy. All DFT calculations are performed with Perdew-Burke-Ernzerhof that implemented in the Vienna Ab initio Simulation Package (VASP) [38–41]. The interactions between ion cores and valence electrons were calculated with the projector augmented wave (PAW) method [42]. For a better account of the weak interlayer attractions in the present layered superlattice, we have also performed PBE-D2 calculations [43]. A plane wave cutoff of 500 eV was used in the calculation, and the Brillouin zone of the surface calculations was sampled with 3 × 3 × 1 Monkhorst-Pack mesh [44]. During the structural relaxation, both the ion positions and lattice parameters were optimized with a convergence criterion set on the

2.3. Electrochemical measurement Electrochemical performance of MoS2−x@CNTs/Ni was tested in 1 M Na2SO4 aqueous on a CHI660D electrochemical working station. In the three-electrode system, the binder-free MoS2−x@CNTs/Ni (1 cm × 1 cm) electrode was directly used as working electrode, with platinum wire as the counter electrode and Hg/HgO as reference electrode. In the two-electrode configuration, the positive electrode is MoS2−x@CNTs/Ni and the negative electrode is CC@CNTs. 795

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Fig. 4. Electrochemical properties of the samples in three-electrode system. (A) CV curves of MoS2−x@CNTs/Ni, CNTs/Ni and Ni mesh at 20 mV s−1. (B) CV curves of MoS2−x@CNTs/Ni with different scan rates from 5 mV s−1 to 1 V s−1. (C) GCD curves of MoS2−x@CNTs/Ni and CNTs/Ni at 1 A g−1. (D) GCD curves of the MoS2−x@CNTs/Ni at different current densities. (E) Specific capacitance values versus current density of MoS2−x@CNTs/Ni. (F) Cycling performance of the MoS2−x@CNTs/Ni at 10 A g−1 for 2000 cycles.

Hellmann-Feynman forces of 0.02 eV Å−1. We imposed a commensurability condition between the graphene and MoS2 monolayer, where a 4 × 4 lateral periodicity of the graphene and 3 × 3 lateral periodicity of the MoS2 monolayer were employed, which leads to a small lattice mismatch.

conductive CNTs networks and the MoS2−x layer are combined by PLD method to construct a highly conductive hybrid structure. Comparing Fig. 2C with Fig. 2E, the thickness of MoS2−x nanolayer is about 35 nm. The results of the electron diffraction (SAED) pattern (Fig. 2F) did not find any diffraction rings of MoS2, indicating that the coated MoS2−x is amorphous. The results of EDX mapping were displayed in Fig. 2G, H, showing a homogeneous distribution of Mo and S elements throughout the whole sample. The physical structures of the samples were analyzed by X-ray diffraction (XRD) and Raman spectrum. As displayed in XRD patterns (Fig. 3A), a weak peak at 26° is assigned to the characteristic graphitic (0 0 2) of CNTs and 2θ = 41°, 44°, 52°, 77° are the typical diffraction peaks of metallic nickel substrate. Unfortunately, the XRD patterns of MoS2−x@CNTs/Ni did not show any characteristic peaks of MoS2, which further proves its amorphous nature and is consistent with the results of SAED pattern. The Raman spectra for pristine MoS2, CNTs/Ni and MoS2−x@CNTs/Ni were shown in Fig. 3B, respectively. Two strong peaks are observed in both of CNTs/Ni and MoS2−x@CNTs/Ni, which

3. Results and discussion 3.1. Positive electrode materials Fig. 1 shows the synthesis process of MoS2−x@CNT/Ni by CVD and PLD method using free-standing CNTs/Ni mesh as the substrate. Fig. 2A, B shows CNTs are interconnected with each other on the surface of the Ni substrate, which acts as the skeleton for the deposition of MoS2−x nanolayer. The diameter of CNTs can be seen from TEM image Fig. 2C, which is around 50 nm. The microscopic structure of the MoS2−x@CNTs/Ni was displayed in Fig. 2D. After PLD process, the MoS2−x nanolayer was uniformly coated on the surface of CNTs. The 796

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Fig. 5. (A–C) The interfacial compound model of MoS2−x, graphene and MoS2−x@graphene. (D–F) Band structure of MoS2−x, graphene and MoS2−x@graphene. (G–I) DOS of MoS2−x, graphene and MoS2−x@graphene.

are G band (1350 cm−1) and D band (1577 cm−1) of carbon in CNTs. G band is the vibrational mode of sp2 carbon and D band is caused by the structural defects and disorders [45]. The two peaks appeared at 285, 373.5 and 403.6 cm−1 in MoS2−x@CNTs/Ni are assigned to E1g, E12g (in-plane) and A1g (out-of-plane) for MoS2. The peak centered at 336 cm−1 appearing with MoS2−x@CNTs/Ni bands is caused by S vacancy in MoS2 [46]. Surprisingly, the intensity of the peak at 285 cm−1 has significantly enhanced compared with the pristine MoS2. This phenomenon indicates that S is truly lost in the PLD process, which is consistent with the previous reports [46,47]. The composition and valence state of MoS2−x@CNTs/Ni were analyzed by X-ray photoelectron spectroscopy (XPS). Fig. 3C–E are Mo 3d, S 2p and C 1s spectra of MoS2−x@CNTs/Ni, respectively. There are four peaks in Mo 3d spectra, in which two strong peaks at 228.6 and 232.0 eV are assigned to Mo 3d5/2 and Mo 3d3/2 of Mo+4 and another two weak peaks at 226.0 and 235.6 eV are attributed to S 2s in MoS2 and Mo 3d5/2 of Mo-O, respectively [48]. The only two peaks at 162.1 and 163.0 eV observed in S 2p spectra are assigned to S 2p3/2 and S 2p1/ 2− in MoS2. As shown in C 1s spectra, the characteristic peak of 2 of S C]C was observed at 284.6 eV and another peak at 285.5 eV was caused by CeOH [21,23]. Compared with the Mo 3d and S 2p peaks for pristine MoS2 (Fig. S1), the XPS spectra of MoS2−x@CNTs/Ni has lower binding energies (BE shifts = 0.37 eV for Mo 3d5/2 and 0.62 eV for Mo 3d3/2), which can be attributed to the combination of CNT and the

MoS2−x [15,16]. The MoS2−x@CNTs/Ni composite was firstly tested in the threeelectrode system in 1 M Na2SO4 electrolyte. Fig. 4A, B are the CV curves of the samples. We performed CV tests with MoS2−x@CNTs/Ni (1 cm × 1 cm), CNTs/Ni (1 cm × 1 cm) and Ni mesh (1 cm × 1 cm) at 20 mV s−1 to prove that the main contribution is from MoS2−x. It is obvious that the CV area of MoS2−x@CNTs/Ni is much larger than that of CNTs/Ni and Ni mesh, which means the much higher specific capacitance of MoS2−x@CNTs/Ni. Fig. 4B exhibits CV curves for MoS2−x@CNTs/Ni with different scan rates and the well-retained CV shape at 1 V s−1 shows the excellent rate capability of MoS2−x@CNTs/ Ni. Fig. 4C and D shows the GCD curves of the samples. According to the equation S1, the calculated specific capacitance of MoS2−x@CNTs/ Ni and CNTs/Ni is 512F g−1 and 73F g−1 at 1 A g−1, respectively. Similarly, the specific capacitances of MoS2−x@CNTs/Ni at different current densities are calculated from Fig. 4D. As shown in Fig. 4E, the maximum capacitance of 1 A g−1 is 512F g−1, which is maintained 342F g−1 even at 30 A g−1. The results obtained in our work are superior to similar materials reported previously. For example, Huang et al. reported a MoS2@CNTs composite prepared by a hydrothermal method, and the result was 412F g−1 at 1 A g−1 [28]. In addition, the result of a MoS2@CC composite was 368F g−1 at 1 A g−1 [49]. As shown in Table S2 in supporting information, we compared the results 797

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Fig. 6. (A–C) FESEM images of the CNTs grown on CC. (D) TEM image of the CNTs on CC and the inset for Raman spectra of CC@CNTs.

Fig. 7. Electrochemical properties of the samples in three-electrode system. (A) CV curves of CC@CNTs with different scan rates from 5 mV s−1 to 300 mV s−1. (B) GCD curves of the CC@CNTs at different current densities. (C) Specific capacitance values versus current densities for CC@CNTs.

result was obtained by the GCD curves in Fig. S2. Such good electrochemical performance of MoS2−x@CNTs/Ni composite is mainly due to its unique MoS2−x@CNTs core–shell structure. The synergy between the MoS2−x and CNTs makes MoS2−x@CNTs/Ni have incredible electrochemical performance. In addition, the high specific capacitance is also enhanced by S vacancy in MoS2 nanolayer, which is confirmed by our theoretical calculations.

with other works, which shows the electrochemical performance of the MoS2−x@CNTs/Ni is better than other well-balanced MoS2 based electrodes. The long-term stability of the MoS2−x@CNTs/Ni composite was performed by GCD cycling at 10 A g−1 for 2000 cycles in Fig. 4F. After 2000 cycles, the capacitance of MoS2−x@CNTs/Ni did not have any decay but a slight increase, indicating the advantages of PLD to construct bind-free electrode with excellent cycling stability. The same 798

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Fig. 8. Electrochemical performance of the MoS2−x@CNTs/Ni//CC@CNTs ASC in 1 M Na2SO4 aqueous solution. (A) CV curves of MoS2−x@CNTs/Ni and CC@CNTs at 10 mV−1. (B) CV curves of MoS2−x@CNTs/Ni//CC@CNTs ASC at different voltage windows. (C) CV curves of MoS2−x@CNTs/Ni//CC@CNTs ASC at 1.7 V with different scan rates from 10 to 500 mV−1. (D) GCD curves of MoS2−x@CNTs/Ni//CC@CNTs ASC from 1 A g−1 to 30 A g−1. (E) Specific capacitances of MoS2−x@CNTs/Ni//CC@CNTs ASC at different current densities. (F) Cycling performance of MoS2−x@CNTs/Ni//CC@CNTs ASC at 15 A g−1 for 3000 cycles.

However, the calculated band gap of S vacancy MoS2 is 0.85 eV, which is much narrower than that of pristine MoS2 monolayer owing to the defective state as show in Fig. 5D. The band gap of pure graphene is at high symmetry K-points in the first Brillouin zone [52,53]. From the inset in Fig. 5F, the π and π* bands repulse each other at the K point, forming a small energy gap. This suggests that electronic property of graphene has changed by the MoS2−x@graphene. As shown in Fig. 5F, the as-calculated band gap of MoS2−x@graphene is 28 meV (MoS2@ graphene: 48 meV), which indicates that the conductivity of the MoS2−x@graphene is far beyond that of MoS2@graphene and isolated MoS2 monolayer. The electronic density of states of MoS2−x, graphene and MoS2−x@graphene are shown in Fig. 5G–I. It can be obtained from the results that the S vacancy can create deep acceptor levels for the MoS2 monolayer due to the Mo dangling bonds. These deep acceptor states can trap electrons and improve the electrons mobility. The

To further elucidate the improved electrochemical properties of MoS2−x@CNT composite, we have performed DFT calculations. As shown in Figs. 5(A–C) and S3 the MoS2/C bilayer with or without S vacancy was used as calculation models. During MoS2 deposition the sulfur atom is lost more than Mo atom because of mass difference between Mo and S, so it is reasonable concluded that S vacancy in MoS2 nanolayer is formed by our PLD method. In our calculation the MoS2 monolayer is modeled with a single S atom removed from 3 × 3 supercell. The band structure and total density of states (DOS) of MoS2, graphene and MoS2@graphene are calculated to compare the electrochemical reactivity of these structures. For the comparison, the band structures and DOS results of isolated MoS2 monolayer without S vacancy and pristine graphene were shown in Fig. S3(D–I). From our calculations the direct K → K gap of isolated MoS2 monolayer is 1.62 eV, which is in agreement with the previous reports [50,51]. 799

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Fig. 9. (A) The optical photograph of one MoS2−x@CNTs/Ni//CC@CNTs ASC device powering one red LED. (B) Ragone plot for the MoS2−x@CNTs/Ni//CC@CNTs ASC with other ASCs and SCs reported previously. (C) Schematic structure of MoS2−x@CNTs/Ni//CC@CNTs ASC and three devices can light up 12 LEDs in series. (D) One ASC device can power a digital clock with a working voltage of 1.5 V for more than 20 mins.

magnification in Fig. 6B, C, the interlaced CNTs forms a highly conductive 3D network, which is beneficial for charge accumulation on its surface and diffusion of electrolyte ions. Fig. 6D shows the same result with Fig. 2C, in which the diameter of the CNTs is approximately 50 nm. It can be seen from the inset in Fig. 6D that G band and D band were also observed in Raman spectra of CC@CNTs. Fig. 7A, B are the CV and GCD curves of the CC@CNTs with a potential window of −1 ∼ 0 V, respectively. The good rate capability of CC@CNTs was proved by the quasi-rectangular CV curves at 200 mV s−1 in Fig. 7A. According to the GCD tests (Fig. 7B) for CC@CNTs from 1 to 20 A g−1, the specific capacitance can be calculated up to 154F g−1 at 1 A g−1 and retains 99.4F g−1 (63.3%) at 20 A g−1 in Fig. 7C. Carbon cloth did not have a large capacitance contribution so the ideal electrochemical performance of CC@CNTs can be attributed (1) the main capacitance that derived from the highly conductive CNTs networks; (2) large contact area between the electrode and electrolyte; (3) fast diffusion of electrolyte ions at the interface of electrode. Hence, the flexible CC@CNTs negative electrode synthesized by in-suit growth of CNTs has a great potential for wearable energy devices.

Table 1 Performance of ASCs and SCs in various device configurations. Electrode

Device performance

Positive

Negative

Energy density (Wh kg−1)

Power density (W kg−1)

MoS2@CC MoS2@TiO2/Ti MoS2@PPY NiO@CNTs Ni(OH)2@CNTs Co(OH)2@CNTs MnO2@CNTs MOF@CNTs/Ni MoS2−x@CNTs/Ni

MoS2@CC MoS2@TiO2/Ti MoS2@PPY PCPs 3DGNs CNTs/Ni AC rGO-C3N4 CC@CNTs

5.42 2.7 2.6 25.4 44 19 13.3 33.6 63

128 530.9 212 400 800 113 600 480 850

Number

Ref.

1 2 3 4 5 6 7 8 Our work

[50] [54] [55] [30] [31] [32] [56] [57]

computations revealed that the C 2p states of graphene and the atomic orbitals of MoS2 are overlapped, which have obviously hybridization across the whole region, leading to enhance the electrochemical reactivity of MoS2−x@graphene by filling its valence band. These results are in good agreement with our experimental results.

3.3. Asymmetric supercapacitors 3.2. Negative electrode materials The MoS2−x@CNTs/Ni as a positive electrode and CC@CNTs as a negative electrode were used to assemble an asymmetric supercapacitors (ASC). The CV (Fig. 8A–C) and GCD (Fig. 8D) tests were performed for MoS2−x@CNTs/Ni//CC@CNTs ASC. The specific capacitance values of MoS2−x@CNTs/Ni//CC@CNTs ASC are calculated by the active materials of both two electrodes, and the results are displayed in Fig. 8E. The specific capacitance of MoS2−x@CNTs/Ni// CC@CNTs ASC is 153.1F g−1 at 1 A g−1 and retains 109.9F g−1 at 30 A g−1, respectively. The cycling test for MoS2−x@CNTs/Ni//

Carbon materials, with high electrical conductivity, large contact area with electrolyte ions and superior cycling stability, are often used as the negative electrode for asymmetric supercapacitors. CC@CNTs was prepared by CVD method and selected as the negative electrode for assembling the asymmetric supercapacitors. As a current collector for most electrode materials, carbon cloth has advantages of high strength, low density and corrosion resistance, etc. The uniformly distribution of CNTs on the carbon cloth was shown in Fig. 6A. With a high 800

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CC@CNTs ASC was performed by GCD (Fig. 8F) at 15 A g−1 with a retention of 91% after 3000 cycles. The outstanding electrochemical performance of individual MoS2−x@CNTs/Ni and CC@CNTs enables the excellent energy storage capability of the ASC. The Ragone plot was shown in Fig. 9B, in which the power density and energy density of MoS2−x@CNTs/Ni//CC@CNTs ASC were calculated by equation S3 and S4. As shown in Table 1, the energy density of as-assembled ASC is 63 Wh kg−1 at the power density of 850 W kg−1, which is much higher than reported works, such as MoS2@CC//MoS2@CC, [50] MoS2@TiO2/ Ti//MoS2@TiO2/Ti, [54] MoS2@PPY//MoS2@PPY, [55] NiO@CNTs// PCPs, [17] Ni(OH)2@CNTs//3DGNs, [18] Co(OH)2@CNTs//CNTs/Ni, [16] MnO2@CNTs//AC, [56] and MOF@CNTs/Ni//rGO-C3N4 [57]. To further demonstrate the practical application for our ASC, We show the optical photographs of one MoS2−x@CNTs/Ni//CC@CNTs ASC device powering a red LED in Fig. 9A and three of our ASC devices light up 12 LEDs in series at the same time in Fig. 9C. Otherwise, we use one ASC device powering a digital clock with a working voltage of 1.5 V for more than 20 mins (Fig. 9D).

[6]

[7]

[8]

[9]

[10]

[11]

[12]

4. Conclusion

[13] [14]

In summary, CNTs/Ni was prepared by CVD method and MoS2 nanolayer with S vacancy was deposited on CNT networks by PLD process. In the three-electrode system, the specific capacitance of MoS2−x@CNTs/Ni is 512F g−1 at 1 A g−1 and without any decay after 2000 cycles. We developed an ASC based on a binder-free MoS2−x@CNTs/Ni positive electrode and CNT networks negative electrode, exhibiting a high power density of 25.5 kW kg−1 and energy density of 63 Wh kg−1. Furthermore, DFT calculations were performed with a MoS2−x/graphene bilayer as calculation model to demonstrate the band gap of MoS2−x@graphene has changed by S vacancy, which means the enhanced conductivity of MoS2. The synergy between S vacancy and MoS2−x@CNTs core-shell structure gives MoS2−x@CNTs/Ni excellent electrochemical performance. The binder-free MoS2−x@CNTs/Ni will be a promising candidate for energy storage devices.

[15]

[16]

[17] [18]

[19]

[20]

[21]

Acknowledgements [22]

This work was supported by the National Natural Science Foundation of China (No. 21873070) and the Ministry of Science and Technology of China (No. 2012YQ220113-7).

[23]

Appendix A. Supplementary material

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.01.007.

[25]

References

[26]

[1] Q. Zhang, J. Sun, Z. Pan, J. Zhang, J. Zhao, X. Wang, C. Zhang, Y. Yao, W. Lu, Q. Li, Y. Zhang, Z. Zhang, Stretchable fiber-shaped asymmetric supercapacitors with ultrahigh energy density, Nano Energy 39 (2017) 219–228. [2] Q. Zhang, X. Wang, Z. Pan, J. Sun, J. Zhao, J. Zhang, C. Zhang, L. Tang, J. Luo, B. Song, Z. Zhang, W. Lu, Q. Li, Y. Zhang, Y. Yao, Wrapping aligned carbon nanotube composite sheets around vanadium nitride nanowire arrays for asymmetric coaxial fiber-shaped supercapacitors with ultrahigh energy density, Nano Lett. 17 (2017) 2719–2726. [3] Q. Zhang, W. Xu, J. Sun, Z. Pan, J. Zhao, X. Wang, J. Zhang, P. Man, J. Guo, Z. Zhou, B. He, Z. Zhang, Q. Li, Y. Zhang, L. Xu, Y. Yao, Constructing ultrahigh-capacity zinc–nickel–cobalt oxide@Ni(OH)2 core-shell nanowire arrays for high-performance coaxial fiber-shaped asymmetric supercapacitors, Nano Lett. 17 (2017) 7552–7560. [4] X. Wang, Q. Zhang, J. Sun, Z. Zhou, Q. Li, B. He, J. Zhao, W. Lu, C. Wong, Y. Yao, Facile synthesis of hierarchical porous manganese nickel cobalt sulfide nanotube arrays with enhanced electrochemical performance for ultrahigh energy density fiber-shaped asymmetric supercapacitors, J. Mater. Chem. A 6 (2018) 8030–8038. [5] J. Guo, Q. Zhang, J. Sun, C. Li, J. Zhao, Z. Zhou, B. He, X. Wang, P. Man, Q. Li, J. Zhangb, L. Xie, M. Mingxing Lia, Y. Yao, Direct growth of vanadium nitride nanosheets on carbon nanotube fibers as novel negative electrodes for high-energy-

[27]

[28]

[29]

[30]

[31]

[32]

[33]

801

density wearable fiber-shaped asymmetric supercapacitors, J. Power Sources 382 (2018) 122–127. K. Gopalakrishnan, S. Sultana, A. Govindaraj, C.N.R. Rao, Supercapacitors based on composites of PANI with nanosheets of nitrogen-doped RGO, BC1.5N, MoS2 and WS2, Nano Energy 12 (2015) 52–58. N. Choudhary, C. Li, H.S. Chung, J. Moore, J. Thomas, Y. Jung, High-performance one-body core/shell nanowire supercapacitor enabled by conformal growth of capacitive 2D WS2 layers, ACS nano 10 (2016) 10726–10735. C.C. Mayorga-Martinez, J. Moo, B. Khezri, P. Song, A.C. Fisher, Z. Sofer, M. Pumera, Self-propelled supercapacitors for on-demand circuit configuration based on WS2 nanoparticles micromachines, Adv. Funct. Mater. 26 (2016) 6662–6667. W. Li, S. Wang, L. Xin, M. Wu, X. Lou, Single-crystal β-NiS nanorod arrays with a hollow-structured Ni3S2 framework for supercapacitor applications, J. Mater. Chem. A 4 (2016) 7700–7709. N. Yu, M.Q. Zhu, D. Chen, Flexible all-solid-state asymmetric supercapacitors with three-dimensional CoSe2/carbon cloth electrodes, J. Mater. Chem. A 3 (2015) 7910–7918. T. Chen, S. Li, J. Wen, P. Gui, Y. Guo, C. Guan, J. Liu, G. Fang, Rational construction of hollow core-branch CoSe2 nanoarrays for high-performance asymmetric supercapacitor and efficient oxygen evolution, Small 14 (2018) 1700979. L. Hou, Y. Shi, C. Wu, Y. Zhang, Y. Ma, X. Sun, J. Sun, X. Zhang, C. Yuan, Monodisperse metallic NiCoSe2 hollow sub-microspheres: formation process, intrinsic charge-storage mechanism, and appealing pseudocapacitance as highly conductive electrode for electrochemical supercapacitors, Adv. Funct. Mater. 28 (2018) 1705921. D. Kong, H. Wang, J.J. Cha, M. Pasta, K.J. Koski, J. Yao, Y. Cui, Synthesis of MoS2 and MoSe2 films with vertically aligned layers, Nano Lett. 13 (2013) 1341–1347. S. Yang, Y. Liu, Y. Hao, X. Yang, W.A. Goddard III, X.L. Zhang, B. Cao, Oxygenvacancy abundant ultrafine Co3O4/graphene composites for high-rate supercapacitor electrodes, Adv. Sci. 5 (2018) 1700659. Y. Xu, L. Wang, X. Liu, S. Zhang, C. Liu, D. Yan, S. Luo, Monolayer MoS2 with S vacancies from interlayer spacing expanded counterparts for highly efficient electrochemical hydrogen production, J. Mater. Chem. A 4 (2016) 16524–16530. H. Li, C. Tsai, A.L. Koh, L. Cai, A.W. Contryman, A.H. Fragapane, J.K. Nørskov, Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies, Nat. Mater. 15 (2016) 48. S. Lany, A. Zunger, Dopability, intrinsic conductivity, and nonstoichiometry of transparent conducting oxides, Phys. Rev. Lett. 98 (2007) 045501. M. Burbano, S.T. Norberg, S. Hull, S.G. Eriksson, D. Marrocchelli, P.A. Madden, G.W. Watson, Oxygen vacancy ordering and the conductivity maximum in Y2O3doped CeO2, Chem Mater. 24 (2011) 222–229. M.A. Bisset, I.A. Kinloch, R.A.W. Dryfe, Characterization of MoS2-graphene composites for high-performance coin cell supercapacitors, ACS Appl. Mater. Interf. 7 (2015) 17388–17398. Y. Wang, S. Chou, D. Wexler, H. Liu, S. Dou, High-performance sodium-ion natteries and sodium-ion pseudocapacitors based on MoS2/Graphene composites, Chem-Eur J. 20 (2014) 9607–9612. G. Sun, X. Zhang, R. Lin, J. Yang, H. Zhang, P. Chen, Hybrid fibers made of molybdenum disulfide, reduced graphene oxide, and multi-walled carbon nanotubes for solid-state, flexible, asymmetric supercapacitors, Angew. Chem. 54 (2015) 4651–4656. C. Zhao, J. Kong, X. Yao, X. Tang, Y. Dong, S.L. Phua, X. Lu, Thin MoS2 nanoflakes encapsulated in carbon nanofibers as high-performance anodes for lithium-ion batteries, ACS Appl. Mater. Interf. 6 (2014) 6392–6398. K.J. Huang, L. Wang, Y. Liu, H. Wang, Y. Liu, L. Wang, Synthesis of polyaniline/2dimensional graphene analog MoS2 composites for high-performance supercapacitor, Electrochim. Acta 109 (2013) 587–594. L. Ren, G. Zhang, Z. Yan, L. Kang, H. Xu, F. Shi, Z. Lei, Z.H. Liu, Three-dimensional tubular MoS2/PANI hybrid electrode for high rate performance supercapacitor, ACS Appl. Mater. Interf. 7 (2015) 28294–28302. J. Wang, Z. Wu, K. Hu, X. Chen, H. Yin, High conductivity graphene-like MoS2/ polyaniline nanocomposites and its application in supercapacitor, J. Alloys Compd. 619 (2015) 38–43. J. Zhu, W. Sun, D. Yang, Y. Zhang, H.H. Hoon, H. Zhang, Q. Yan, Multifunctional architectures constructing of PANI nanoneedle arrays on MoS2 thin nanosheets for high-energy supercapacitors, Small 11 (2015) 4123–4129. T. Sun, X. Liu, Z. Li, L. Ma, J. Wang, S. Yang, Graphene-wrapped CNT@MoS2 hierarchical structure: synthesis, characterization and electrochemical application in supercapacitors, New J. Chem. 41 (2017) 7142–7150. K. Huang, L. Wang, J.Z. Zhang, L. Wang, Y.P. Mo, One-step preparation of layered molybdenum disulfide/multi-walled carbon nanotube composites for enhanced performance supercapacitor, Energy 67 (2014) 234–240. X. Chen, H. Wang, H. Yi, X.F. Wang, Anthraquinone on porous carbon nanotubes with improved supercapacitor performance, J. Phys. Chem. C 118 (2014) 8262–8270. H. Yi, H. Wang, Y. Jing, T. Peng, X.F. Wang, Asymmetric supercapacitors based on carbon nanotubes@NiO ultrathin nanosheets core-shell composites and MOF-derived porous carbon polyhedrons with super-long cycle life, J. Power Sources 285 (2015) 281–290. H. Yi, H. Wang, Y. Jing, T. Peng, Y. Wang, J. Guo, Q. He, Z. Guo, X.F. Wang, Advanced asymmetric supercapacitors based on CNT@Ni(OH)2 core-shell composites and 3D graphene networks, J. Mater. Chem. A 3 (2015) 19545–19555. T. Peng, H. Wang, H. Yi, Y. Jing, P. Sun, X.F. Wang, Co(OH)2 nanosheets coupled with CNT arrays grown on Ni mesh for high-rate asymmetric supercapacitors with excellent capacitive behavior, Electrochim. Acta 176 (2015) 77–85. P. Sun, H. Yi, T. Peng, Y. Jing, R. Wang, H. Wang, X.F. Wang, Ultrathin MnO2

Applied Surface Science 475 (2019) 793–802

P. Sun et al.

[34]

[35]

[36] [37]

[38] [39] [40] [41] [42] [43] [44] [45]

[46]

[47] P. Agaskar, F.A. Cotton, I.F. Fraser, R.D. Peacock, Configuration chirality of metalmetal multiple bonds: preparation and circular dichroism spectrum of tetrachlorobis [(S, S)-2, 3-(diphenylphosphino) butane] dimolybdenum [Mo2Cl4(S, Sdppb)2], J. Am. Chem. Soc. 106 (1984) 1851–1853. [48] Y. Yang, H. Fei, G. Ruan, C. Xiang, J.M. Tour, Edge-oriented MoS2 nanoporous films as flexible electrodes for hydrogen evolution reactions and supercapacitor devices, Adv. Mater. 26 (2014) 8163–8168. [49] M.S. Javed, S. Dai, M. Wang, D. Guo, L. Chen, X. Wang, C. Hua, Y. Xi, High performance solid state flexible supercapacitor based on molybdenum sulfide hierarchical nanospheres, J. Power Sources 285 (2015) 63–69. [50] Y. Li, Z. Zhou, S. Zhang, Z. Chen, MoS2 nanoribbons: high stability and unusual electronic and magnetic properties, J. Am. Chem. Soc. 130 (2008) 16739–16744. [51] J. Zhang, J.M. Soon, K.P. Loh, J. Yin, J. Ding, M.B. Sullivian, P. Wu, Magnetic molybdenum disulfide nanosheet films, Nano Lett 7 (2007) 2370–2376. [52] C. Xu, P.A. Brown, J. Lu, K.L. Shuford, Electronic properties of halogen-adsorbed graphene, J. Phys. Chem. C 119 (2015) 17271–17277. [53] X.D. Li, S. Yu, S.Q. Wu, Y.H. Wen, S. Zhou, Z.Z. Zhu, Structural and electronic properties of superlattice composed of graphene and monolayer MoS2, J. Phys. Chem. C 117 (2013) 15347–15353. [54] X. Li, X.D. Li, J. Cheng, D. Yuan, W. Ni, Q. Guan, L. Gao, B. Wang, Fiber-shaped solid-state supercapacitors based on molybdenum disulfide nanosheets for a selfpowered photodetecting system, Nano Energy 21 (2016) 228–237. [55] A.K. Thakur, R.B. Choudhary, M. Majumder, G. Gupta, M.V.J. Shelke, Enhanced electrochemical performance of polypyrrole coated MoS2 nanocomposites as electrode material for supercapacitor application, Electroanal. Chem. 782 (2016) 278–287. [56] L. Li, Z.A. Hu, N. An, Y.Y. Yang, Z.M. Li, H.Y. Wu, Facile synthesis of MnO2/CNTs composite for supercapacitor electrodes with long cycle stability, J. Phys. Chem. C 118 (2014) 22865–22872. [57] P. Wen, P. Gong, J. Sun, J. Wang, S. Yang, Design and synthesis of Ni-MOF/CNT composites and rGO/carbon nitride composites for an asymmetric supercapacitor with high energy and power density, J. Mater. Chem. A 3 (2015) 13874–13883.

nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performance, J. Power Sources 341 (2017) 27–35. T. Peng, H. Yi, P. Sun, Y. Jing, R. Wang, H. Wang, X.F. Wang, In situ growth of binder-free CNTs@Ni-Co-S nanosheets core/shell hybrids on Ni mesh for high energy density asymmetric supercapacitors, J. Mater. Chem. A 4 (2016) 8888–8897. H. Wang, H. Yi, X. Chen, X. Wang, Asymmetric supercapacitors based on nanoarchitectured nickel oxide/graphene foam and hierarchical porous nitrogen-doped carbon nanotubes with ultrahigh-rate performance, J. Mater. Chem. A 2 (2014) 3223–3230. H. Wang, Y. Wang, X. Wang, Pulsed laser deposition of large-area manganese oxide nanosheet arrays for high-rate supercapacitors, New J. Chem 37 (2013) 869–872. Y. Wang, H. Wang, X. Wang, The cobalt oxide/hydroxide nanowall array film prepared by pulsed laser deposition for supercapacitors with superb-rate capability, Electrochim. Acta 92 (2013) 298–303. J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865. G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. G. Kresse, J. Hafner, Ab initio molecular dynamics for open-shell transition metals, Phys. Rev. B 48 (1993) 13115. P.E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 17953. H.J. Monkhorst, J.D. Pack, Special points for brillouin-zone integrations, Phys. Rev. B 16 (1976) 5188. S. Grimme, Semiempirical GGA-type density functional constructed with a longrange dispersion correction, J. Comput Chem. 27 (2006) 1787–1799. L. Wang, T. Maxisch, G. Ceder, Oxidation energies of transition metal oxides within the GGA+ U framework, Phys. Rev. B 73 (2006) 195107. H. Wang, H. Yi, C. Zhu, X.F. Wang, H.J. Fan, Functionalized highly porous graphitic carbon fibers for high-rate supercapacitive electrodes, Nano Energy 13 (2015) 658–669. Y. Liu, Y. Xie, L. Liu, J. Jiao, Sulfur vacancy induced high performance for photocatalytic H2 production over 1T@2H phase MoS2 nanolayers, Catal. Sci. Technol. 7 (2017) 5635–5643.

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