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MoS2 nanosheets array on carbon cloth as a 3D electrode for highly efficient electrochemical hydrogen evolution
Accepted Manuscript MoS2 Nanosheets Array on Carbon Cloth as a 3D Electrode for Highly Efficient Electrochemical Hydrogen Evolution ZhongCheng Xiang, Zhong Zhang, XiJin Xu, Qin Zhang, Chengwu Yuan PII:
S0008-6223(15)30381-X
DOI:
10.1016/j.carbon.2015.10.071
Reference:
CARBON 10435
To appear in:
Carbon
Received Date: 2 September 2015 Revised Date:
17 October 2015
Accepted Date: 21 October 2015
Please cite this article as: Z. Xiang, Z. Zhang, X. Xu, Q. Zhang, C. Yuan, MoS2 Nanosheets Array on Carbon Cloth as a 3D Electrode for Highly Efficient Electrochemical Hydrogen Evolution, Carbon (2015), doi: 10.1016/j.carbon.2015.10.071. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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MoS2 Nanosheets Array on Carbon Cloth as a 3D Electrode for Highly
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Efficient Electrochemical Hydrogen Evolution.
ZhongCheng Xianga, Zhong Zhanga,*, XiJin Xua, Qin Zhangb, Chengwu Yuanc
b
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School of Physics and Technology, University of Jinan, Jinan 250022, China
School of Science, Shandong Jiaotong University, Jinan 250357, China
c
BP America, 201 Westlake Park Blvd, Houston, Texas 77079, USA
Corresponding author: Tel: (0531)82765976, E-mail: [email protected]
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Abstract
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*
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a
We report a facile approach to the low-temperature hydrothermal fabrication of a
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2H-MoS2 nanosheets array on carbon cloth (2H-MoS2 NS/CC). Li-intercalation was used to prepare 1T-MoS2 sheets from pristine 2H-MoS2 NS/CC.
X-ray photoelectron
spectroscopy (XPS) and Raman spectra confirmed the local phase transitions from trigonal prismatic (2H-MoS2) into octahedrally coordinated Mo (1T-MoS2). Then, we investigated the HER activity of Li-MoS2 NS/CC as a 3D integrated electrode in 0.5 M H2SO4 at room temperature using a typical three-electrode setup.
The results exhibited
excellent catalytic activity in acidic media. The Li-MoS2 NS/CC 3D Electrode needs 1
ACCEPTED MANUSCRIPT overpotentials of 97 and 132 mV to approach 10 and 100 mA cm−2, respectively, which are smaller than the previously reported values. This performance of the Li-MoS2 NS/CC electrode suggests that it may hold great promise for practical applications.
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Keywords: Hydrothermal method; Hydrogen evolution reaction; MoS2; Carbon Cloth; Electrocatalysis; nanosheets
1. Introduction
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Fossil fuels not only create pollution and a carbon footprint but also are liable to
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depletion. Hydrogen is a promising alternative and renewable energy source with which to replace fossil fuels in the future. Electrochemical water splitting offers a simple way to produce highly pure hydrogen by converting such electrical energy into stable chemical bonds. Appropriate electrocatalysts, such as platinum (Pt) and its alloys, play a vital role
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in the H2 evolution reaction (HER) because they can catalyse the conversion from a pair of protons and electrons to H2 at high reaction rates and low overpotentials[1-3]. However, the prohibitive cost and scarcity of Pt pose severe limitations to its widespread
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use in industry [3]. Although earth-abundant nickel-based transition-metal alloys are
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capable of efficiently catalyzing the HER in alkaline media, they suffer from poor stability in acidic solutions because of their vulnerability to corrosion [4-6]. As such, it is highly desirable to develop and design highly active, acid-stable HER catalysts based on earth-abundant materials. Therefore, finding robust and efficient alternative catalysts that are geologically abundant is crucial to the future. Over the past few years, molybdenum disulfide (2H-MoS2) has been extensively studied as a catalyst for HER because of its special structures and electronic properties.
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ACCEPTED MANUSCRIPT It is composed of covalently bonded S–Mo–S sheets that are bound by weak van der Waals forces, similar to grapheme. In its bulk form, MoS2 is a semiconductor with an indirect bandgap of about 1 eV, and the monolayer material exhibits a direct gap of about
effects [7-8].
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1.8 eV due to interlayer interaction, quantum confinement, and long-range Coulomb The edges of MoS2 layers consist of zigzag edges and armchair edges
concurrently, due to the nonstoichiometric nature of the edges [9-10]. The Mo (1̅010)
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edge of MoS2 zigzag edges possesses a hydrogen-binding energy of approximately 0.08
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eV at 50% H coverage, very close to the optimum value of 0 eV, binding hydrogen neither too weakly nor too strongly for HER. Meanwhile, the basal plane sites of the MoS2 are catalytically inert [11-12].
In contrast, the metallic 1T polymorph is described
by a single S−Mo−S layer composed of edge-sharing MoS6 octahedra [13-14].
Voiry
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and co-workers [15] reported that the edges of the 1T-phase MoS2 nanosheets are not the main active sites and the basal plane could be catalytically active, so 1T-phase MoS2 for HER has been one of the most interesting to study.
Another important factor that affects
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the hydrogen evolution reaction (HER) is a variety of substrates. Tsai and co-workers [16]
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studied the support effects of MoS2 by DFT and suggested that the choice of support is crucial in determining the HER activity of supported MoS2 and that the catalytic activity at the Mo-edge could be optimally tuned by the choice of support. Lukowski and co-workers [17] reported that flowerlike MoS2 nanostructures with a high density of exposed edges directly on graphite, silicon/silicon oxide, glass, fluorine-doped tin oxide on glass, Mo foils, and carbon paper substrates respectively, via a simple chemical vapor deposition method, which showed enhanced HER catalysis.
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In this Letter, we report a facile approach to the low-temperature hydrothermal fabrication of a 2H-MoS2 nanosheets array on carbon cloth (2H-MoS2 NS/CC).
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Li-intercalation was used to prepare 1T-MoS2 sheets from pristine 2H-MoS2 NS/CC. X-ray photoelectron spectroscopy (XPS) and Raman spectra confirmed the local phase transitions from trigonal prismatic (2H-MoS2) into octahedrally coordinated Mo
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(1T-MoS2). We investigated the HER activity of MoS2 NS/CC as a 3D integrated
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electrode in 0.5 M H2SO4 at room temperature using a typical three-electrode setup. The results exhibited excellent catalytic activity in acidic media. The Li-MoS2 NS/CC 3D Electrode needs overpotentials of 97 and 132 mV to approach 10 and 100 mA cm−2, respectively, which are smaller than the previously reported values. This performance of
applications.
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the Li-MoS2 NS/CC electrode suggests that it may hold great promise for practical
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2. Experimental Details
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2.1 2H-MoS2 NS/CC Sample preparation We report a facile approach to the low-temperature hydrothermal fabrication of a
2H-MoS2 nanosheets array on carbon cloth (2H-MoS2 NS/CC). Before 2H-MoS2 nanosheets coating, CC was cleaned with ethanol and deionized water, respectively, and then treated in concentrated HNO3 at 100°C for 1h, followed by washing with deionized water. MoS2 NS/CC was prepared as follows: 0.5 g of sodium molybdate (Na2MoO4.2H2O) powder and 0.7 g of thiocarbamide (CH4N2S) powder were mixed and
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ACCEPTED MANUSCRIPT dissolved in 70 ml deionized water. After stirring the solution for 25 min, 0.47 g of citric acid (C6H8O7) was added to the foregoing mixed solution to adjust the pH value. After being magnetically stirred for 10 min, the homogeneous solution was obtained and
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transferred to a 100 ml Teflon-lined stainless steel autoclave with a piece of pre-treated carbon cloth. The autoclave was sealed and maintained at 180°C for 24 h. Then the autoclave was cooled down to room temperature; 2H-MoS2 NS/CC and black precipitates
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were collected and washed several times with deionized water and ethanol, respectively.
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After drying at 60°C for 6 h, the 2H-MoS2 NS/CC, and black powders were obtained. 2.2 n-Butyl lithium exfoliation treatment
Inside an argon-filled glove box, the carbon cloth substrates covered with an as-synthesized MoS2 nanosheets array were soaked in 3 mL n-butyl lithium (2.7 M in
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heptane) inside sealed vials for 48 h. The 2H-MoS2 NS/CC was retrieved by washing with hexane (100 ml) to remove excess lithium and organic residues. Then the substrates were reacted with deionized water and gently rinsed with water. Annealing was
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performed in argon gas at a desired temperature for 1 h. We denote these samples as
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Li-MoS2 NS/CC.
2.3 Characterizations
The morphologies and sizes of the represented MoS2 samples were analyzed by
SEM (Quanta FEG) at an accelerating voltage of 15 kV and Raman spectra were taken using a Thermo Scientific DXR Confocal Raman microscope using a 532 nm excitation laser. The chemical structure of the MoS2 samples was examined using X-ray
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ACCEPTED MANUSCRIPT photoelectron spectroscopy (XPS, K-alpha, Thermo UK) with 1486.6 eV Al Kα radiation.
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2.4 Electrochemical Measurements Electrochemical measurements were performed with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai) in a standard three-electrode system in 0.5 M
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H2SO4, with the use of a graphite rod and a saturated calomel electrode (SCE) as the
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counter and reference electrode, respectively. In all measurements, the SCE reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE). In 0.5 M H2SO4, E (RHE) = 0.242 + 0.059 pH. Linear sweep voltammetry measurements were conducted with a scan rate of 2 mV s-1 under strong stir in nitrogen-saturated solution.
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Electrodes were cycled at least 50 cycles prior to any measurements. Cyclic voltammetry experiments were carried out between 0 and 0.3 V vs. RHE.
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3. Results and Discussion
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Figure 1(a) shows the CC consists of conductive carbon fibers about 10µm in diameter and several millimeters in length. The carbon microfibers connect and fuse together to form a 3D conducting matrix, with about 13 times the surface area per geometric area [18]. Figure 1(b) shows the low-magnification scanning electron microscopy (SEM) image of 2H-MoS2 NS/CC, clearly indicating full coverage of CC by 2H-MoS2 NSs.
The high-magnification SEM image (Figure 1(c)) shows that the bended
MoS2 NSs array is perpendicular to the substrate and the range of the size in-plane
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ACCEPTED MANUSCRIPT direction is about 100–200 nm. Figure 1(d) shows the high-magnification SEM image of the Li-MoS2 NS/CC with n-Butyl lithium exfoliation treatment. The results indicated that the nanosheets array exist clustered and unkempt
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phenomena, these phenomena were due to the gliding atomic planes of sulphur and/or molybdenum in the lithiation and exfoliation processes, which lead to the 2H/1T phase transition [19-22]. However, this unkempt MoS2 NSs array is still perpendicular to the
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substrate and the space between the nanosheets becomes larger than before, which
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facilitates efficient diffusion of the electrolyte, and H2 gas evolved.
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ACCEPTED MANUSCRIPT Figure 1. (a) The low-magnification scanning electron microscopy (SEM) image of carbon cloth. (b-c) The low-magnification and high-magnification SEM image of MoS2 NS/CC. (d) The high-magnification SEM image of the Li-MoS2 NS/CC with
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n-Butyl lithium exfoliation treatment.
X-ray photoelectron spectroscopy (XPS) was employed to characterize the
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samples at each synthesis stage, as shown in Figure 2(a). All of the spectra were
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calibrated by a C 1s peak located at 284.50 eV [18]. The spectrum (Figure 2(a)) shows 2H-MoS2 NS/CC peaks around 229.2 and 232.3 eV, corresponding to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components, respectively. For the Li-MoS2 NS/CC with n-Butyl lithium exfoliation treatment, Li intercalating into 2H-MoS2 changes and destabilizes the d-band
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structure of Mo. The local phase transitions from trigonal prismatic (2H-MoS2) to octahedrally coordinated Mo (1T-MoS2) occur to stabilize the structure [19, 23-25]. The chemically exfoliated local-phase-transitioned Li-MoS2 NS/CC exhibits coexisting
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2H-1T polymorphs. Figure 2(a) shows the Mo 3d XPS spectrum for the chemically
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exfoliated Li-MoS2 NS/CC after the deconvolution of the Mo 3d peaks. The spectrum shows 2H-MoS2 peaks (green line) around 229.2 and 232.3 eV, corresponding to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components, respectively. It also reveals 1T-MoS2 peaks (red line) around 228.3 and 231.5 eV, corresponding to structural distortion and binding energies shifting lower by about 0.9 eV because the local phase had transitioned from 2H-MoS2 into 1T-MoS2 [26]. The 1T concentration can be estimated through the areas of fitted Mo 3d5/2 peak, which were located at 229.3 and 228.4 eV for 2H and 1T,
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ACCEPTED MANUSCRIPT respectively. The ratio of MoS2 2H to the 1T phase is estimated to be 0.78. Additionally, the small peak in the Mo 3d spectrum around 235.1 eV, corresponding to Mo6+ 3d5/2, indicates that the Mo had slightly oxidized in the ambient air [26].
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The compositions and phases of the as-grown samples were further confirmed by Raman spectroscopy in Figure 2(b). The distinct peaks in the 2H-MoS2 Raman spectrum agree well with previous studies [17-18].
The Raman spectra in Figure 2(b) offer us
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more information about the morphology and phase transition of pristine and lithiated
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MoS2. Figure 2(b) illustrates the E1g, E12g, and A1g vibration modes located at 281, 377, and 403 cm−1, respectively, in 2H-MoS2 NS/CC, which is similar to pristine MoS2 [27]. The integrated intensity of A1g is nearly three times that of E12g, which suggests the texture of as-grown MoS2 with molecular layers vertically standing on the substrate that
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favors the vibration of A1g mode [28-29]. Li intercalation makes a large number of electrons donate from Li to MoS2 slabs, which results in the 2H to 1T phase transition to lower the Li-MoS2 NS/CC electronic energy [18]. Three additional peaks, located at 171,
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214, and 340 cm−1, respectively, indicate the 2H to 1T MoS2 phase transition [17-18].
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Figure 2.
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(a) XPS spectra of 2H-MoS2 NS/CC, and Li-MoS2 NS/CC. (b) Raman
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spectra of carbon cloth, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC. (c) Atomic vibration
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direction of E1g, E12g, and A1g Raman modes of MoS2.
We investigated the HER activity of MoS2 NS/CC (MoS2 loading: about 4 mg
cm−2) as a 3D integrated electrode in 0.5 M H2SO4 at room temperature using a typical three-electrode setup. The HER activities for bare CC, 2H-MoS2 (2H-MoS2 was immobilized on CC with the same loading using a Nafion solution), and 2H-MoS2 NS/CC were also examined for comparison purposes. To reflect the intrinsic electrocatalytic activity of the catalysts, an iR correction was applied to all initial data for 10
ACCEPTED MANUSCRIPT further analysis. Polarization curves (Figure 3(a)) of the current density (J) plotted against potential showed the HER activity of the bare CC, 2H-MoS2, and 2H-MoS2 NS/CC in comparison with Li-MoS2 NS/CC. Bare CC has poor HER activity.
In
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contrast, 2H-MoS2 exhibited the onset of HER activity near −150 mV vs. reversible hydrogen electrode (RHE), and significant H2 evolution (J = 10 mA cm-2) was not achieved until −190 mV vs. RHE. The 2H-MoS2 NS/CC prepared by the hydrothermal
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method showed dramatically improved HER activity, with J reaching 200 mA cm-2 at
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−180 mV vs. RHE (Figure 3(a)). Moreover, the onset of the catalytic activity shifted to a much lower overpotential, as significant H2 evolution (J = 10 mA cm-2) was observed at a voltage as low as −120 mV (Figure 3(a)).
However, Li-MoS2 NS/CC exfoliated after
lithium intercalation at 60°C for 48 h exhibits a more positive overpotential of 75 mV for
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the HER with much larger cathodic current densities (10 and 100 mA cm−2 at 97 and 132 mV, respectively). These overpotentials for MoS2 HER catalysts are smaller than the previous reported values about MoS2 with n-Butyl lithium exfoliation treatment [17-18,
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30], whose J reach 10 mA cm−2 at -187, -168 and -118mV, respectively.
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The Tafel slope is an inherent property of electrocatalysts, and a small Tafel slope leads to a strongly enhanced HER rate at a moderate increase of overpotential [31-32]. According to the HER kinetic models, Tafel slopes of about 30, 40, or 120 mV decade-1 will be achieved if the Tafel, Heyrovsky, or Volmer step is the rate-determining step, respectively [33]. The dramatic enhancement of catalytic activity was even more apparent upon comparison of the slopes of Tafel plots (Figure 3(b)) for 2H-MoS2, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC. So we investigated the Tafel plots for 2H-MoS2, 2H-MoS2
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ACCEPTED MANUSCRIPT NS/CC, and Li-MoS2 NS/CC. The resulting Tafel slope of Li-MoS2 NS/CC is 38 mV decade-1 after iR correction, which is much lower than that of 2H-MoS2 NS/CC (58 mV decade-1 after iR correction) and of 2H-MoS2 (81 mV decade-1 after iR correction).
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The Tafel slope (38 mV decade-1) for Li-MoS2 NS/CC suggests that the HER proceeds via a Volmer−Tafel mechanism, for which the Tafel step is the rate-determining step [33].
The small Tafel slope and the early onset of significant H2 evolution confirm
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that Li-MoS2 NS/CC is among the most catalytic MoS2 materials.
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The advantage of the Li-MoS2 NS/CC became more apparent when we used electrochemical impedance spectroscopy (EIS) to investigate the electrode kinetics under HER operating conditions. Nyquist plots (Figure 4) revealed a dramatically decreased charge transfer resistance (RCT) for the Li-MoS2 NS/CC (about 4Ω) relative to the
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2H-MoS2 (about 85Ω). The structural characterization and EIS results show that Li-MoS2 NS/CC exhibits more facile electrode kinetics, a particularly useful feature for enhancing
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the catalytic activity.
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ACCEPTED MANUSCRIPT Figure 3. (a) Polarization curves for carbon cloth, 2H-MoS2, 2H-MoS2 NS/CC, Li-MoS2 NS/CC and Pt/C. (b) Tafel plots for carbon cloth, 2H-MoS2, 2H-MoS2 NS/CC,
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Li-MoS2 NS/CC and Pt/C.
Figure 4. (a) Nyquist plots of 2H-MoS2, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC
at overpotential of 150 mV. (b) Polarization curves for Li-MoS2 NS/CC in 0.5 M H2SO4 initially and after 1000 potential sweeps between −0.4 V and +0.1 vs RHE at a scan rate of 100 mV s−1, the insert is the time-dependent current density curves for Li-MoS2 NS/CC at fixed overpotential in 0.5 M H2SO4.
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ACCEPTED MANUSCRIPT To estimate the effective surface areas, we employed the CV method to measure the electrochemical double-layer capacitances (EDLCs), Cdl, as shown in Figure 5 [34]. The potential range where no faradic current was observed was selected for the catalysts.
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Slow voltage scan rates were chosen for accurate measurements of the large surface area electrodes. The halves of the positive and negative current density differences at the center of the scanning potential ranges are plotted versus the voltage scan rates in Figure
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5(d), in which the slopes are the EDLCs.
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The electrochemical effective surface area of the 2H-MoS2 NS/CC is increased by 4.5 times compared with the 2H-MoS2.
The EDLC is further improved by nearly 2.5
times when 2H-MoS2 NS/CC are lithiated and exfoliated to Li-MoS2 NS/CC. The lithiation and exfoliation processes open up the van der Waals gaps of MoS2 and expose
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more effective sites on the terrace surface. The Li intercalation process induces phase transition to 1T MoS2, in which the strained terrace sites have been shown to be active for HER [30,15]. The Li electrochemical tuning of MoS2 electronic structures not only helps
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to enhance the conductivity (Figure 4) of the catalyst but also creates HER active sites on
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the terrace surface other than the edge sites, which boosts the overall HER activity of the catalyst.
Furthermore, we demonstrated that the catalytic performance of the Li-MoS2
NS/CC is stable even though the 1T-phase is thermodynamically metastable. After 1000 cycling for potential sweeps between −0.4 V and +0.1 vs RHE, the similar polarization curves for Li-MoS2 NS/CC electrode are presented as show in figure 4(b), in which the insert is the time-dependent current density curves for Li-MoS2 NS/CC at an
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ACCEPTED MANUSCRIPT overpotential of 120mV in 0.5 M H2SO4. It can be observed that Li-MoS2 NS/CC retains its activity for at least 6 h.
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Factors that contribute to the excellent HER activity and durability of Li-MoS2 NS/CC are as follows. (1) The special texture of vertically aligned MoS2 nanosheets maximally exposes the active edge sites for HER. Furthermore, the lithiation and
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exfoliation processes open up the van der Waals gaps of MoS2 and expose more effective
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sites on the terrace surface and the Li intercalation process induces phase transition to 1T MoS2, in which the strained terrace sites have been shown to be active for HER. Meanwhile, the 3D configuration of this electrode facilitates efficient diffusion of the electrolyte, and H2 gas evolved. (2) The direct growth of MoS2 on CC offers intimate
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contact, good mechanical adhesion, and excellent electrical connection between them. (3) The absence of polymer binder for catalyst immobilization not only avoids the blocking of active sites and inhibition but also decreases the series resistance [35]. Electrochemical
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impedance spectroscopy measurements (Figure 4) confirm that Li-MoS2 NS/CC has
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much lower impedance and, hence, markedly faster HER kinetics than 2H-MoS2.
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Figure 5. Electrochemical double-layer capacitance measurements. (a-c) Electrochemical cyclic voltammogram of as-grown catalysts at different potential scanning rates. The selected potential range where no faradic current was observed is 0 to
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0.3 V vs. RHE. (d) Linear fitting of the capacitive currents of the catalysts vs scan rates.
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The calculated double-layer capacitances are 2, 9, and 21 mF for 2H-MoS2, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC, respectively.
5. Conclusion
In conclusion, a MoS2 NSs array has been developed on carbon cloth via a facile hydrothermal approach. Then Li-intercalation was used to prepare 1T-MoS2 sheets from pristine 2H-MoS2 NS/CC. This 3D architecture can be directly used as a nanoarray
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ACCEPTED MANUSCRIPT electrode for electrochemical hydrogen evolution in acidic solutions with high activity. The results exhibit excellent catalytic activity in acidic media. It needs overpotentials of 97 and 132 mV to approach 10 and 100 mA cm−2, respectively. This performance of the
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Li-MoS2 NS/CC electrode suggests that it may hold great promise for practical applications.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant No. 11304182 and 11304120).
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intercalation in MoS2 and related compounds. Can. J. Phys. 1983, 61, 76–84. [25]
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Wang, H.; Lu, Z.; Kong, D.; Sun, J.; Hymel, T. M.; Cui, Y. Electrochemical
Tuning of MoS2 Nanoparticles on Three-Dimensional Substrate for Efficient
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S0008-6223(15)30381-X
DOI:
10.1016/j.carbon.2015.10.071
Reference:
CARBON 10435
To appear in:
Carbon
Received Date: 2 September 2015 Revised Date:
17 October 2015
Accepted Date: 21 October 2015
Please cite this article as: Z. Xiang, Z. Zhang, X. Xu, Q. Zhang, C. Yuan, MoS2 Nanosheets Array on Carbon Cloth as a 3D Electrode for Highly Efficient Electrochemical Hydrogen Evolution, Carbon (2015), doi: 10.1016/j.carbon.2015.10.071. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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MoS2 Nanosheets Array on Carbon Cloth as a 3D Electrode for Highly
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Efficient Electrochemical Hydrogen Evolution.
ZhongCheng Xianga, Zhong Zhanga,*, XiJin Xua, Qin Zhangb, Chengwu Yuanc
b
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School of Physics and Technology, University of Jinan, Jinan 250022, China
School of Science, Shandong Jiaotong University, Jinan 250357, China
c
BP America, 201 Westlake Park Blvd, Houston, Texas 77079, USA
Corresponding author: Tel: (0531)82765976, E-mail: [email protected]
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Abstract
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*
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a
We report a facile approach to the low-temperature hydrothermal fabrication of a
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2H-MoS2 nanosheets array on carbon cloth (2H-MoS2 NS/CC). Li-intercalation was used to prepare 1T-MoS2 sheets from pristine 2H-MoS2 NS/CC.
X-ray photoelectron
spectroscopy (XPS) and Raman spectra confirmed the local phase transitions from trigonal prismatic (2H-MoS2) into octahedrally coordinated Mo (1T-MoS2). Then, we investigated the HER activity of Li-MoS2 NS/CC as a 3D integrated electrode in 0.5 M H2SO4 at room temperature using a typical three-electrode setup.
The results exhibited
excellent catalytic activity in acidic media. The Li-MoS2 NS/CC 3D Electrode needs 1
ACCEPTED MANUSCRIPT overpotentials of 97 and 132 mV to approach 10 and 100 mA cm−2, respectively, which are smaller than the previously reported values. This performance of the Li-MoS2 NS/CC electrode suggests that it may hold great promise for practical applications.
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Keywords: Hydrothermal method; Hydrogen evolution reaction; MoS2; Carbon Cloth; Electrocatalysis; nanosheets
1. Introduction
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Fossil fuels not only create pollution and a carbon footprint but also are liable to
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depletion. Hydrogen is a promising alternative and renewable energy source with which to replace fossil fuels in the future. Electrochemical water splitting offers a simple way to produce highly pure hydrogen by converting such electrical energy into stable chemical bonds. Appropriate electrocatalysts, such as platinum (Pt) and its alloys, play a vital role
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in the H2 evolution reaction (HER) because they can catalyse the conversion from a pair of protons and electrons to H2 at high reaction rates and low overpotentials[1-3]. However, the prohibitive cost and scarcity of Pt pose severe limitations to its widespread
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use in industry [3]. Although earth-abundant nickel-based transition-metal alloys are
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capable of efficiently catalyzing the HER in alkaline media, they suffer from poor stability in acidic solutions because of their vulnerability to corrosion [4-6]. As such, it is highly desirable to develop and design highly active, acid-stable HER catalysts based on earth-abundant materials. Therefore, finding robust and efficient alternative catalysts that are geologically abundant is crucial to the future. Over the past few years, molybdenum disulfide (2H-MoS2) has been extensively studied as a catalyst for HER because of its special structures and electronic properties.
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ACCEPTED MANUSCRIPT It is composed of covalently bonded S–Mo–S sheets that are bound by weak van der Waals forces, similar to grapheme. In its bulk form, MoS2 is a semiconductor with an indirect bandgap of about 1 eV, and the monolayer material exhibits a direct gap of about
effects [7-8].
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1.8 eV due to interlayer interaction, quantum confinement, and long-range Coulomb The edges of MoS2 layers consist of zigzag edges and armchair edges
concurrently, due to the nonstoichiometric nature of the edges [9-10]. The Mo (1̅010)
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edge of MoS2 zigzag edges possesses a hydrogen-binding energy of approximately 0.08
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eV at 50% H coverage, very close to the optimum value of 0 eV, binding hydrogen neither too weakly nor too strongly for HER. Meanwhile, the basal plane sites of the MoS2 are catalytically inert [11-12].
In contrast, the metallic 1T polymorph is described
by a single S−Mo−S layer composed of edge-sharing MoS6 octahedra [13-14].
Voiry
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and co-workers [15] reported that the edges of the 1T-phase MoS2 nanosheets are not the main active sites and the basal plane could be catalytically active, so 1T-phase MoS2 for HER has been one of the most interesting to study.
Another important factor that affects
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the hydrogen evolution reaction (HER) is a variety of substrates. Tsai and co-workers [16]
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studied the support effects of MoS2 by DFT and suggested that the choice of support is crucial in determining the HER activity of supported MoS2 and that the catalytic activity at the Mo-edge could be optimally tuned by the choice of support. Lukowski and co-workers [17] reported that flowerlike MoS2 nanostructures with a high density of exposed edges directly on graphite, silicon/silicon oxide, glass, fluorine-doped tin oxide on glass, Mo foils, and carbon paper substrates respectively, via a simple chemical vapor deposition method, which showed enhanced HER catalysis.
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In this Letter, we report a facile approach to the low-temperature hydrothermal fabrication of a 2H-MoS2 nanosheets array on carbon cloth (2H-MoS2 NS/CC).
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Li-intercalation was used to prepare 1T-MoS2 sheets from pristine 2H-MoS2 NS/CC. X-ray photoelectron spectroscopy (XPS) and Raman spectra confirmed the local phase transitions from trigonal prismatic (2H-MoS2) into octahedrally coordinated Mo
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(1T-MoS2). We investigated the HER activity of MoS2 NS/CC as a 3D integrated
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electrode in 0.5 M H2SO4 at room temperature using a typical three-electrode setup. The results exhibited excellent catalytic activity in acidic media. The Li-MoS2 NS/CC 3D Electrode needs overpotentials of 97 and 132 mV to approach 10 and 100 mA cm−2, respectively, which are smaller than the previously reported values. This performance of
applications.
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the Li-MoS2 NS/CC electrode suggests that it may hold great promise for practical
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2. Experimental Details
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2.1 2H-MoS2 NS/CC Sample preparation We report a facile approach to the low-temperature hydrothermal fabrication of a
2H-MoS2 nanosheets array on carbon cloth (2H-MoS2 NS/CC). Before 2H-MoS2 nanosheets coating, CC was cleaned with ethanol and deionized water, respectively, and then treated in concentrated HNO3 at 100°C for 1h, followed by washing with deionized water. MoS2 NS/CC was prepared as follows: 0.5 g of sodium molybdate (Na2MoO4.2H2O) powder and 0.7 g of thiocarbamide (CH4N2S) powder were mixed and
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ACCEPTED MANUSCRIPT dissolved in 70 ml deionized water. After stirring the solution for 25 min, 0.47 g of citric acid (C6H8O7) was added to the foregoing mixed solution to adjust the pH value. After being magnetically stirred for 10 min, the homogeneous solution was obtained and
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transferred to a 100 ml Teflon-lined stainless steel autoclave with a piece of pre-treated carbon cloth. The autoclave was sealed and maintained at 180°C for 24 h. Then the autoclave was cooled down to room temperature; 2H-MoS2 NS/CC and black precipitates
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were collected and washed several times with deionized water and ethanol, respectively.
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After drying at 60°C for 6 h, the 2H-MoS2 NS/CC, and black powders were obtained. 2.2 n-Butyl lithium exfoliation treatment
Inside an argon-filled glove box, the carbon cloth substrates covered with an as-synthesized MoS2 nanosheets array were soaked in 3 mL n-butyl lithium (2.7 M in
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heptane) inside sealed vials for 48 h. The 2H-MoS2 NS/CC was retrieved by washing with hexane (100 ml) to remove excess lithium and organic residues. Then the substrates were reacted with deionized water and gently rinsed with water. Annealing was
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performed in argon gas at a desired temperature for 1 h. We denote these samples as
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Li-MoS2 NS/CC.
2.3 Characterizations
The morphologies and sizes of the represented MoS2 samples were analyzed by
SEM (Quanta FEG) at an accelerating voltage of 15 kV and Raman spectra were taken using a Thermo Scientific DXR Confocal Raman microscope using a 532 nm excitation laser. The chemical structure of the MoS2 samples was examined using X-ray
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ACCEPTED MANUSCRIPT photoelectron spectroscopy (XPS, K-alpha, Thermo UK) with 1486.6 eV Al Kα radiation.
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2.4 Electrochemical Measurements Electrochemical measurements were performed with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai) in a standard three-electrode system in 0.5 M
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H2SO4, with the use of a graphite rod and a saturated calomel electrode (SCE) as the
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counter and reference electrode, respectively. In all measurements, the SCE reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE). In 0.5 M H2SO4, E (RHE) = 0.242 + 0.059 pH. Linear sweep voltammetry measurements were conducted with a scan rate of 2 mV s-1 under strong stir in nitrogen-saturated solution.
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Electrodes were cycled at least 50 cycles prior to any measurements. Cyclic voltammetry experiments were carried out between 0 and 0.3 V vs. RHE.
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3. Results and Discussion
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Figure 1(a) shows the CC consists of conductive carbon fibers about 10µm in diameter and several millimeters in length. The carbon microfibers connect and fuse together to form a 3D conducting matrix, with about 13 times the surface area per geometric area [18]. Figure 1(b) shows the low-magnification scanning electron microscopy (SEM) image of 2H-MoS2 NS/CC, clearly indicating full coverage of CC by 2H-MoS2 NSs.
The high-magnification SEM image (Figure 1(c)) shows that the bended
MoS2 NSs array is perpendicular to the substrate and the range of the size in-plane
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ACCEPTED MANUSCRIPT direction is about 100–200 nm. Figure 1(d) shows the high-magnification SEM image of the Li-MoS2 NS/CC with n-Butyl lithium exfoliation treatment. The results indicated that the nanosheets array exist clustered and unkempt
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phenomena, these phenomena were due to the gliding atomic planes of sulphur and/or molybdenum in the lithiation and exfoliation processes, which lead to the 2H/1T phase transition [19-22]. However, this unkempt MoS2 NSs array is still perpendicular to the
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substrate and the space between the nanosheets becomes larger than before, which
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facilitates efficient diffusion of the electrolyte, and H2 gas evolved.
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ACCEPTED MANUSCRIPT Figure 1. (a) The low-magnification scanning electron microscopy (SEM) image of carbon cloth. (b-c) The low-magnification and high-magnification SEM image of MoS2 NS/CC. (d) The high-magnification SEM image of the Li-MoS2 NS/CC with
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n-Butyl lithium exfoliation treatment.
X-ray photoelectron spectroscopy (XPS) was employed to characterize the
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samples at each synthesis stage, as shown in Figure 2(a). All of the spectra were
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calibrated by a C 1s peak located at 284.50 eV [18]. The spectrum (Figure 2(a)) shows 2H-MoS2 NS/CC peaks around 229.2 and 232.3 eV, corresponding to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components, respectively. For the Li-MoS2 NS/CC with n-Butyl lithium exfoliation treatment, Li intercalating into 2H-MoS2 changes and destabilizes the d-band
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structure of Mo. The local phase transitions from trigonal prismatic (2H-MoS2) to octahedrally coordinated Mo (1T-MoS2) occur to stabilize the structure [19, 23-25]. The chemically exfoliated local-phase-transitioned Li-MoS2 NS/CC exhibits coexisting
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2H-1T polymorphs. Figure 2(a) shows the Mo 3d XPS spectrum for the chemically
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exfoliated Li-MoS2 NS/CC after the deconvolution of the Mo 3d peaks. The spectrum shows 2H-MoS2 peaks (green line) around 229.2 and 232.3 eV, corresponding to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components, respectively. It also reveals 1T-MoS2 peaks (red line) around 228.3 and 231.5 eV, corresponding to structural distortion and binding energies shifting lower by about 0.9 eV because the local phase had transitioned from 2H-MoS2 into 1T-MoS2 [26]. The 1T concentration can be estimated through the areas of fitted Mo 3d5/2 peak, which were located at 229.3 and 228.4 eV for 2H and 1T,
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ACCEPTED MANUSCRIPT respectively. The ratio of MoS2 2H to the 1T phase is estimated to be 0.78. Additionally, the small peak in the Mo 3d spectrum around 235.1 eV, corresponding to Mo6+ 3d5/2, indicates that the Mo had slightly oxidized in the ambient air [26].
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The compositions and phases of the as-grown samples were further confirmed by Raman spectroscopy in Figure 2(b). The distinct peaks in the 2H-MoS2 Raman spectrum agree well with previous studies [17-18].
The Raman spectra in Figure 2(b) offer us
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more information about the morphology and phase transition of pristine and lithiated
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MoS2. Figure 2(b) illustrates the E1g, E12g, and A1g vibration modes located at 281, 377, and 403 cm−1, respectively, in 2H-MoS2 NS/CC, which is similar to pristine MoS2 [27]. The integrated intensity of A1g is nearly three times that of E12g, which suggests the texture of as-grown MoS2 with molecular layers vertically standing on the substrate that
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favors the vibration of A1g mode [28-29]. Li intercalation makes a large number of electrons donate from Li to MoS2 slabs, which results in the 2H to 1T phase transition to lower the Li-MoS2 NS/CC electronic energy [18]. Three additional peaks, located at 171,
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214, and 340 cm−1, respectively, indicate the 2H to 1T MoS2 phase transition [17-18].
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Figure 2.
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(a) XPS spectra of 2H-MoS2 NS/CC, and Li-MoS2 NS/CC. (b) Raman
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spectra of carbon cloth, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC. (c) Atomic vibration
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direction of E1g, E12g, and A1g Raman modes of MoS2.
We investigated the HER activity of MoS2 NS/CC (MoS2 loading: about 4 mg
cm−2) as a 3D integrated electrode in 0.5 M H2SO4 at room temperature using a typical three-electrode setup. The HER activities for bare CC, 2H-MoS2 (2H-MoS2 was immobilized on CC with the same loading using a Nafion solution), and 2H-MoS2 NS/CC were also examined for comparison purposes. To reflect the intrinsic electrocatalytic activity of the catalysts, an iR correction was applied to all initial data for 10
ACCEPTED MANUSCRIPT further analysis. Polarization curves (Figure 3(a)) of the current density (J) plotted against potential showed the HER activity of the bare CC, 2H-MoS2, and 2H-MoS2 NS/CC in comparison with Li-MoS2 NS/CC. Bare CC has poor HER activity.
In
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contrast, 2H-MoS2 exhibited the onset of HER activity near −150 mV vs. reversible hydrogen electrode (RHE), and significant H2 evolution (J = 10 mA cm-2) was not achieved until −190 mV vs. RHE. The 2H-MoS2 NS/CC prepared by the hydrothermal
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method showed dramatically improved HER activity, with J reaching 200 mA cm-2 at
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−180 mV vs. RHE (Figure 3(a)). Moreover, the onset of the catalytic activity shifted to a much lower overpotential, as significant H2 evolution (J = 10 mA cm-2) was observed at a voltage as low as −120 mV (Figure 3(a)).
However, Li-MoS2 NS/CC exfoliated after
lithium intercalation at 60°C for 48 h exhibits a more positive overpotential of 75 mV for
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the HER with much larger cathodic current densities (10 and 100 mA cm−2 at 97 and 132 mV, respectively). These overpotentials for MoS2 HER catalysts are smaller than the previous reported values about MoS2 with n-Butyl lithium exfoliation treatment [17-18,
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30], whose J reach 10 mA cm−2 at -187, -168 and -118mV, respectively.
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The Tafel slope is an inherent property of electrocatalysts, and a small Tafel slope leads to a strongly enhanced HER rate at a moderate increase of overpotential [31-32]. According to the HER kinetic models, Tafel slopes of about 30, 40, or 120 mV decade-1 will be achieved if the Tafel, Heyrovsky, or Volmer step is the rate-determining step, respectively [33]. The dramatic enhancement of catalytic activity was even more apparent upon comparison of the slopes of Tafel plots (Figure 3(b)) for 2H-MoS2, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC. So we investigated the Tafel plots for 2H-MoS2, 2H-MoS2
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ACCEPTED MANUSCRIPT NS/CC, and Li-MoS2 NS/CC. The resulting Tafel slope of Li-MoS2 NS/CC is 38 mV decade-1 after iR correction, which is much lower than that of 2H-MoS2 NS/CC (58 mV decade-1 after iR correction) and of 2H-MoS2 (81 mV decade-1 after iR correction).
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The Tafel slope (38 mV decade-1) for Li-MoS2 NS/CC suggests that the HER proceeds via a Volmer−Tafel mechanism, for which the Tafel step is the rate-determining step [33].
The small Tafel slope and the early onset of significant H2 evolution confirm
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that Li-MoS2 NS/CC is among the most catalytic MoS2 materials.
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The advantage of the Li-MoS2 NS/CC became more apparent when we used electrochemical impedance spectroscopy (EIS) to investigate the electrode kinetics under HER operating conditions. Nyquist plots (Figure 4) revealed a dramatically decreased charge transfer resistance (RCT) for the Li-MoS2 NS/CC (about 4Ω) relative to the
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2H-MoS2 (about 85Ω). The structural characterization and EIS results show that Li-MoS2 NS/CC exhibits more facile electrode kinetics, a particularly useful feature for enhancing
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the catalytic activity.
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ACCEPTED MANUSCRIPT Figure 3. (a) Polarization curves for carbon cloth, 2H-MoS2, 2H-MoS2 NS/CC, Li-MoS2 NS/CC and Pt/C. (b) Tafel plots for carbon cloth, 2H-MoS2, 2H-MoS2 NS/CC,
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Li-MoS2 NS/CC and Pt/C.
Figure 4. (a) Nyquist plots of 2H-MoS2, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC
at overpotential of 150 mV. (b) Polarization curves for Li-MoS2 NS/CC in 0.5 M H2SO4 initially and after 1000 potential sweeps between −0.4 V and +0.1 vs RHE at a scan rate of 100 mV s−1, the insert is the time-dependent current density curves for Li-MoS2 NS/CC at fixed overpotential in 0.5 M H2SO4.
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ACCEPTED MANUSCRIPT To estimate the effective surface areas, we employed the CV method to measure the electrochemical double-layer capacitances (EDLCs), Cdl, as shown in Figure 5 [34]. The potential range where no faradic current was observed was selected for the catalysts.
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Slow voltage scan rates were chosen for accurate measurements of the large surface area electrodes. The halves of the positive and negative current density differences at the center of the scanning potential ranges are plotted versus the voltage scan rates in Figure
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5(d), in which the slopes are the EDLCs.
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The electrochemical effective surface area of the 2H-MoS2 NS/CC is increased by 4.5 times compared with the 2H-MoS2.
The EDLC is further improved by nearly 2.5
times when 2H-MoS2 NS/CC are lithiated and exfoliated to Li-MoS2 NS/CC. The lithiation and exfoliation processes open up the van der Waals gaps of MoS2 and expose
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more effective sites on the terrace surface. The Li intercalation process induces phase transition to 1T MoS2, in which the strained terrace sites have been shown to be active for HER [30,15]. The Li electrochemical tuning of MoS2 electronic structures not only helps
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to enhance the conductivity (Figure 4) of the catalyst but also creates HER active sites on
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the terrace surface other than the edge sites, which boosts the overall HER activity of the catalyst.
Furthermore, we demonstrated that the catalytic performance of the Li-MoS2
NS/CC is stable even though the 1T-phase is thermodynamically metastable. After 1000 cycling for potential sweeps between −0.4 V and +0.1 vs RHE, the similar polarization curves for Li-MoS2 NS/CC electrode are presented as show in figure 4(b), in which the insert is the time-dependent current density curves for Li-MoS2 NS/CC at an
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ACCEPTED MANUSCRIPT overpotential of 120mV in 0.5 M H2SO4. It can be observed that Li-MoS2 NS/CC retains its activity for at least 6 h.
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Factors that contribute to the excellent HER activity and durability of Li-MoS2 NS/CC are as follows. (1) The special texture of vertically aligned MoS2 nanosheets maximally exposes the active edge sites for HER. Furthermore, the lithiation and
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exfoliation processes open up the van der Waals gaps of MoS2 and expose more effective
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sites on the terrace surface and the Li intercalation process induces phase transition to 1T MoS2, in which the strained terrace sites have been shown to be active for HER. Meanwhile, the 3D configuration of this electrode facilitates efficient diffusion of the electrolyte, and H2 gas evolved. (2) The direct growth of MoS2 on CC offers intimate
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contact, good mechanical adhesion, and excellent electrical connection between them. (3) The absence of polymer binder for catalyst immobilization not only avoids the blocking of active sites and inhibition but also decreases the series resistance [35]. Electrochemical
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impedance spectroscopy measurements (Figure 4) confirm that Li-MoS2 NS/CC has
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much lower impedance and, hence, markedly faster HER kinetics than 2H-MoS2.
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Figure 5. Electrochemical double-layer capacitance measurements. (a-c) Electrochemical cyclic voltammogram of as-grown catalysts at different potential scanning rates. The selected potential range where no faradic current was observed is 0 to
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0.3 V vs. RHE. (d) Linear fitting of the capacitive currents of the catalysts vs scan rates.
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The calculated double-layer capacitances are 2, 9, and 21 mF for 2H-MoS2, 2H-MoS2 NS/CC, and Li-MoS2 NS/CC, respectively.
5. Conclusion
In conclusion, a MoS2 NSs array has been developed on carbon cloth via a facile hydrothermal approach. Then Li-intercalation was used to prepare 1T-MoS2 sheets from pristine 2H-MoS2 NS/CC. This 3D architecture can be directly used as a nanoarray
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ACCEPTED MANUSCRIPT electrode for electrochemical hydrogen evolution in acidic solutions with high activity. The results exhibit excellent catalytic activity in acidic media. It needs overpotentials of 97 and 132 mV to approach 10 and 100 mA cm−2, respectively. This performance of the
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Li-MoS2 NS/CC electrode suggests that it may hold great promise for practical applications.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant No. 11304182 and 11304120).
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