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PAF-1 as oxygen tank to in-situ synthesize edge-exposed O-MoS2 for highly efficient hydrogen evolution
Accepted Manuscript Title: PAF-1 as oxygen tank to in-situ synthesize edge-exposed O-MoS2 for highly efficient hydrogen evolution Authors: Jianing Guo, Feng Huo, Yuanhui Cheng, Zhonghua Xiang PII: DOI: Reference:
S0920-5861(18)30570-4 https://doi.org/10.1016/j.cattod.2018.05.003 CATTOD 11433
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
3-2-2018 23-4-2018 5-5-2018
Please cite this article as: Guo J, Huo F, Cheng Y, Xiang Z, PAF-1 as oxygen tank to in-situ synthesize edge-exposed O-MoS2 for highly efficient hydrogen evolution, Catalysis Today (2010), https://doi.org/10.1016/j.cattod.2018.05.003 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.
PAF-1 as oxygen tank to in-situ synthesize edge-exposed O-MoS2 for highly efficient hydrogen evolution
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Jianing Guoa, Feng Huob, Yuanhui Cheng* and Zhonghua Xianga*
State Key Laboratory of Organic-Inorganic Composites, College of Chemical
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Engineering, College of Energy, Beijing University of Chemical Technology, Beijing 100029, China.
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of
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Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of
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Sciences, Beijing 100190, China
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Email: [email protected] or [email protected]
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Graphical abstract
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We synthesized oxygen-incorporated MoS2 ultrathin nanosheets by oxygen
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Highlights
adsorbate.
The CPAF not only acts as a conductive template but also oxygen tank.
MoS2 nanosheets expose more catalytically active edges grown on the CPAF
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surface.
The O-MoS2-CPAF catalyst exhibited a low onset potential of 0.088 V for HER.
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Abstract
Developing highly-efficient and low-cost non-precious metal electrocatalysts towards
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the hydrogen evolution reaction (HER) is a promising strategy to solve the environmental pollution problems and energy demands. In this study, we synthesized oxygen-incorporated molybdenum disulfide (MoS2) ultrathin nanosheets with expanded interlayer spacing (∼9.3Å) induced by oxygen adsorbate, i.e., carbonized porous aromatic framework (CPAF), through a facile hydrothermal method. The
CPAF not only acts as a conductive template but also oxygen tank, which enhances the transport of electrons of the MoS2-based electrocatalysts and provides oxygen for O-MoS2 nanosheets. Owing to the high specific surface area, conductive template and oxygen source of CPAF, MoS2 nanosheets can grow densely and uniformly on the CPAF surface, exposing catalytically active edges effectively. The O-MoS2-CPAF catalyst exhibited significantly improved catalytic activity toward hydrogen evolution
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reaction (HER) with a low onset potential of 0.088 V, a large current density (10.0
mA cm-2 at 0.17 V), a small Tafel slope of 44 mV per decade as well as excellent
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stability and device performance.
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1. Introduction
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conductive template; active edges; HER catalysis
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Keywords: Oxygen-incorporated MoS2 nanosheets; expanded interlayer spacing;
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Hydrogen, an eco-friendly and high energy density carrier, may replace
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conventional fossil fuels in the future[1-6]. Electrochemical hydrogen evolution reaction (HER) is considered as one of the most promising techniques to combating
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global challenges of climate change and energy crisis[7-9]. Transition metal dichalcogenides (TMDs) are generally considered as a significant kind of material for
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application in environmental and energy technology, owing to their capability of accepting electrons and protons[10-13]. As one of the most interesting layered materials, molybdenum disulfide (MoS2) has been extensively demonstrated to be a promising non-precious electrocatalyst for hydrogen evolution[14]. It is known that the atomic hydrogen adsorption free energy
on the Mo-edge sites of MoS2 was found to be very close to zero, suggesting that HER active sites lay in the edges of MoS2 catalysts but not on their basal planes[15,16]. Based on this principle, research endeavors have been dedicated to designing nanostructured MoS2 with abundant and exposed edges to enhance the
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electrocatalytic activity of MoS2 based catalysts[17]. Defect-rich MoS2, edge exposed MoS2 nanoporous films and elemental incorporation in MoS2 nanostructures have
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been explored[18-20]. Among them, elemental incorporation has been widely used to
improve intrinsic conductivity and accelerate proton adsorption of MoS2
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electrocatalysts. Oxygen incorporation is considered as an effective method to
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improving the HER performance by changing the surface disorder of MoS2 with
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increasing additional active sites[21,22]. Meanwhile, oxygen incorporation can also
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reduce the bandgap and thus enhance the intrinsic conductivity of MoS2 catalyst[23].
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Although the increase of the edge sites can improve the HER activity of MoS2-based electrocatalysts, the electric conductivity of catalysts is another crucial
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factor to affect the electrocatalytic activity because a high conductivity ensures a fast
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electron transfer process[24-26]. In this regard, constructing hybrids of MoS2 with highly conductive templates is an efficient way to take full advantages of the excellent inner properties of HER active materials[27,28]. Ideal conductive materials such as
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carbon nanotubes (CNTs)[11,29], carbon nanofibers (CNFs)[24], graphene oxide (GO) [30-32]and other mesoporous carbon have been used to enhance the conductivity of the MoS2-based electrocatalysts, improving the electrical contact from the templates to the active sites.
On the basis of the above two key factors, we first demonstrate a one-pot hydrothermal method to prepare oxygen-incorporated MoS2 ultrathin nanosheets grown on carbonized porous aromatic framework(CPAF) substrates, realizing the balance between active site numbers and overall conductivity for HER catalysis. The
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CPAF substrate not only functions as a 3D network with large surface area, but also acts as the “oxygen tank” adsorption oxygen in the porous structure. In this work,
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oxygen incorporated in MoS2 comes from the adsorption oxygen in CPAF. The obtained O-MoS2-CPAF catalyst showed high electrocatalytic activity and strong
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durability toward the HER. The success of synergistically structural and electronic
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modulations in this work will pave a new pathway for pursuing efficient
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2.1. Materials
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2. Experimental section
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electrocatalysts for energy production and conversion.
All chemicals used in the synthesis of self-designed catalysts are analytical
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reagents (AR). 2,2'-dipyridyl, 1,5-cyclooctadiene and N, N-dimethylformamide (DMF) were purchased from J&K Chemical Technology. Tetrahydrofuran, chloroform,
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Sulfuric acid and hydrochloric acid were bought from Beijing Chemical Works. Ammonium tetrathiomolybdate and bis (1,5-cyclooctadiene) nickel(0) were supplied by Sigma-Aldrich Chemical Co., Ltd and Strem Chemical, respectively. Commercial Pt/C catalyst was bought from Alfa Aesar Chemical Co.Ltd. High-purity argon (99.99%) gas was obtained from Beijing AP BAIF Gases Industry Co., Ltd.
2.2. Materials Synthesis 2.2.1. Synthesis of CPAF PAF-1 was first prepared according to the Yamamoto-type Ullmann reaction.[33] The PAF-1 was placed into a crucible in a tube furnace, which was
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annealed to 350℃ for 2 h, and then at 900℃ for another 2 h under Ar protection at a heating rate of 6℃ min-1. This product is named CPAF.
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2.2.2. Synthesis of O-MoS2-CPAF
CPAF was first dried in a vacuum oven at 100℃ for 24 h. Then CPAF was putted
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into a closed container with the pressure of one atmosphere oxygen for 24 h. 40 mg
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CPAF adsorption of oxygen was dispersed in 40 ml of N, N dimethylformamide
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(DMF) and sonicated at room temperature for approximately 10 min. 80 mg
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(NH4)2MoS4 was put into the above solution and continuely sonicated for
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approximately 20 min until a homogeneous solution was achieved. After that, the reaction solution was transferred to a 100 mL Teflon-lined autoclave. It was heated in
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an oven at 200℃ for 10 h with no intentional control of ramping or cooling rate.
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Product was collected by centrifugation at 9250 rpm for 5 min, washed with DI water and recollected by centrifugation. The washing step was repeated for at least 5 times
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to ensure that ions and possible remnants were removed. Finally, product was dispersed in 10 ml of DI water, frozen by the refrigerator and freezen drying overnight. This product is named O-MoS2-CPAF. 2.2.3. Synthesis of MoS2-CPAF
CPAF was first dried in a vacuum oven at 100℃ for 24 h. Then 40 mg CPAF adsorption of oxygen was dispered in 40 ml of N, N dimethylformamide (DMF) and sonicated at room temperature for approximately 10 min. 80 mg (NH4)2MoS4 was put into the above solution and continuely sonicated for approximately 20 min until a
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homogeneous solution was achieved. After that, the reaction solution was transferred to a 100 mL Teflon-lined autoclave. It was heated in an oven at 200℃ for 10 h with no
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intentional control of ramping or cooling rate. Product was collected by centrifugation
at 9250 rpm for 5 min, washed with DI water and recollected by centrifugation. The
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washing step was repeated for at least 5 times to ensure that ions and possible
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remnants were removed. Finally, product was dispersed in 10 ml of DI water, frozen
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2.2.4. Synthesis of MoS2
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by the refrigerator and freezen drying overnight. This product named MoS2-CPAF.
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80 mg (NH4)2MoS4 was was dispersed in 40 ml of N, N dimethylformamide (DMF) and sonicated at room temperature for approximately 20 min until a
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homogeneous solution was achieved. After that, the reaction solution was transferred
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to a 100 mL Teflon-lined autoclave. It was heated in an oven at 200℃ for 10 h with no intentional control of ramping or cooling rate. Product was collected by centrifugation at 9250 rpm for 5 min, washed with DI water and recollected by centrifugation. The
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washing step was repeated for at least 5 times to ensure that ions and possible remnants were removed. Finally, product was dispersed in 10 ml of DI water, frozen by the refrigerator and freezen drying overnight. 2.3. Physical characterization
The morphologies and structures of catalysts were performed on a S4700 SEM instrument and a high-resolution transmission electron microscopy (HRTEM, 2100F). The crystalline structures of catalysts were recorded on a D/MAX 2000 X-ray diffractometer using Cu Kα line (λ=1.54178 Å) as the incident beam with the 2θ scan
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from 5° to 90° at a step of 0.02°. The X-ray photoelectron spectroscopy (XPS) was obtained on ESCALAB 250 operated at 150 W and 200 eV with monochromated Al
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Kα radiation. Raman spectra were collected on a Raman Spectroscopy (Renishaw)
using 514 nm laser. The thermogravimetric analysis (TGA) data were obtained on a
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DTG-60A (SHIMADZU) instrument at a heating rate of 10℃ min-1 under flowing air.
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performed on PS2-0722-analysis station A.
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BET was tested by the ASAP 2460. The adsorption isotherm of oxygen was
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2.4. Electrochemical measurements
All electrochemical
measurements
were conducted on a CHI 760E
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electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a three-electrode cell at room temperature. A glassy carbon electrode (GCE with the
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diameter of 5 mm, 0.19625 cm2) was used as the working electrode. Saturated calomel electrode and Pt wire as reference and counter electrodes, respectively. For
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comparison, A commercial Pt/C (20 wt% Pt ) catalyst was also tested in 0.5 M H2SO4 at room temperature. In order to make the working electrodes, 5 mg of catalyst powder was dispersed in 1 mL ethanol and 50 ul nafion solution (0.05 wt%) with 30 min of ultrasonication to generate homogeneous inks. Next, 10 µL of dispersion ink was transferred onto the GCE (~0.25 mg cm-2), and the as-modified RDE electrode
was then dried at room temperature. In HER tests, all electrodes were pretreated by taking continuous cyclic voltammograms(CVs) between -0.8 and 0.1 V versus a reversible hydrogen electrode (RHE) at a sweep rate of 100 mVs-1 under a flow of N2 to remove surface contamination before testing the HER activity. Then the potential
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was cycled from 0.1 V to 0.8 V (vs. RHE) at a scan rate 5 mV s-1 with the electrode rotation speed of 1600 rpm to record the polarization curves. All data were reported
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with IR compensation. 2.4.1. Double-layer capacitance
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The double-layer capacitance (Cdl) of catalyst can be calculated from the cyclic
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voltammograms in the region of 0.1-0.2 V vs. RHE. By plotting the Δj at 0.15 V vs.
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RHE against the scan rate, the slope is twice of Cdl. Electrochemical impedance
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spectroscopy was measured at the overpotential of 200 mV in the frequency range of
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106 Hz to 10-2 Hz with the amplitude potential of 5 mV. All of the potentials were calibrated to a reversible hydrogen electrode (RHE).
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2.4.2. Water splitting cell characterization
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Hydrogen evolution electrode was fabricated by dropping a well sonicated solution of O-MoS2-PAF with Nafion at a mass ratio of 9:1 onto carbon paper (1 cm×1 cm, thickness of 0.2 mm). Oxygen evolution electrode was fabricated by
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dropping a well-sonicated solution of IrO2 purchased from Alfa Aesar Chemical Co. Ltd., acetylene black and Nafion in a 3.6: 5.4: 1 wt ratio onto carbon paper (1 cm×1 cm, thickness of 0.2 mm). The loadings of O-MoS2-PAF and IrO2 are all 2 mg cm-2. Then, the two kinds of electrodes were immersed into a sealed electrolytic cell
containing 150 mL electrolyte (0.5 mol L-1 H2SO4). Inserted the electrode inside the 280 mL closed cell with 150 mL vitriol as electrolyte and put through power supply. After 10 mins, we took 0.5 mL of the sample into gas chromatography every five minutes to get the hydrogen concentration and repeated sampling 10 times in total.
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The water splitting cell was conducted at 10 mA cm-2 to produce hydrogen gas and oxygen gas.
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3. Results and discussion
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Fig.1 illustrated the schematic preparation process for O-MoS2-CPAF. CPAF first
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adsorbed oxygen in the pressure vessel (Fig. S1), then they were dissolved in
Fig. 1. Schematic illustration of the preparation of O-MoS2-CPAF
(NH2)4MoS4 solution and reacted under 200 ℃ gaining the final product. The morphology and hierarchical architecture of the hydrothermal fabricated flower
structured MoS2 ultrathin nanosheets are demonstrated in Fig.1. The structure of CPAF is uniformly dispersed sphere (Fig. S2a). The surface of the composed of dense curly 2D nanopetal that randomly crosswise was shown in Fig. 2a. In addition, MoS2 decorated on the CPAF structures and they combine each other very closer, forming
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the layered structure of staggered. Therefore, CPAF support would be beneficial for
Fig. 2. (a) SEM image of O-MoS2-CPAF. (b) and (c) HRTEM images of O-MoS2-CPAF. (d) The responding element mapping.
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the deposition and formation of MoS2 nanoparticles (Fig. S2b), and provide a good distribution for the exposed active surface of MoS2 nanoparticles. On the contrary, there is an obvious reunion of MoS2 for individual MoS2 catalyst (Fig. S2c). The HRTEM images in Fig. 2b-c give a close-up view of the MoS2 nanopetals on the CPAF structure. The HRTEM images of typical nanosheets suggest a building
structure of O-MoS2 thin as <10 layers with crystal dislocations and defects. The layered spacing of O-MoS2-CPAF can be identified to be around ∼0.93 nm which larger than that of 0.63 nm in the normally obtained 2H-MoS2, signifying an enlarged interlayer spacing. This result confirms the successful oxygen incorporation.
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Furthermore, the STEM image and corresponding EDS mapping analyses (Fig. 2d) reveal the homogeneous distribution of molybdenum, sulfur, and oxygen in the whole
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ultrathin nanosheets. The TG analysis of O-MoS2-CPAF (Fig. S6) shows three regions
of weight loss. The remaining part belongs to MoO3 from the decomposition of
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further oxidation of MoS2 to MoO3. Therefore, the weight fraction of MoS2 in
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investigate
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pore
structure
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In
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O-MoS2-CPAF can be calculated to be ~40%.
of
the
CPAF,
the
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adsorption-desorption isotherms of the CPAF was recorded (Fig. S3a). The
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corresponding BET surface area is found to be 571.5m2/g. Also, their corresponding pore size distributions show the coexistence of micropores, mesopores, and
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large-mesopores (Fig. S3b). This result indicates that the hierarchically porous
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structure has been successfully fabricated, which could shorten the pathway of mass transfer and be favorable for oxygen adsorption in CPAF. The average pore width is 3.29 nm. Meanwhile, we further tested the oxygen adsorption of CPAF, which shows
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that the CPAF had good adsorption performance on oxygen (Fig. 3a). The crystal structures of O-MoS2-CPAF were systematically investigated by X-ray diffraction (XRD) patterns. As shown in Fig. 3b, a new peak is observed at the low-angle region for O-MoS2-CPAF, which indicates the enlarged interlayer spacing
of oxygen incorporated MoS2 ultrathin nanosheets. The corresponding d spacing is 0.93 nm, indicating the formation of a new lamellar structure with larger interlayer spacing compared with that of 0.63 nm in pristine 2H-MoS2 (JCPDS Card No. 73-1508). Moreover, two strong diffraction peaks at high-angle region (32° and 56.3°)
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can be well indexed to (100) and (110) crystal planes of the pristine 2H-MoS2, respectively. The Raman spectrum in Fig. 3c reveals two dominant peaks located at
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1355 cm-1 and 1591 cm-1 which can be assigned to the typical D and G bands, respectively. The peaks located at 817 cm-1 and 992 cm-1 are attributed to Mo-O2, and
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Mo-O vibrations, respectively[34]. In addition, the characteristic peaks at 285cm-1 and
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334cm-1 corresponding to the B2g and B1g vibrational modes for Mo-O bonds of
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MoO3[21]. The existence of the Raman peaks for Mo-Ox bonds confirms the
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accordance with the XRD results.
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successful oxygen incorporation in the hybrid rather than the surface oxidation, in
Based on the above studies, a schematic structural model of oxygen-incorporated
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MoS2 with enlarged interlayer spacing is constructed in Fig. 3d, which reveals the
Fig. 3. (a) O2 uptake of CPAF at 301.5K. (b) XRD pattern of O-MoS2-CPAF. (c) Raman spectrum of O-MoS2-CPAF. (d) Structural models of the oxygen-incorporated MoS2 with enlarged interlayer spacing and the pristine 2H-MoS2. (e) XPS spectra for Mo3d of O-MoS2-CPAF. (f) XPS spectra for O1s of O-MoS2-CPAF.
new lamellar structure intuitively. In order to compare directly, a schematic model of
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pristine 2H-MoS2 with normal interlayer spacing of 0.63 nm is also presented. It is considered that, the O-MoS2 nanosheets possess the moderate degree of short-range
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disordering feature that can offer more active sites as well as enhance the intrinsic conductivity, thus leading to enhance HER activity.
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The X-ray photoelectron spectroscopy (XPS) was used to identify the
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compositions and chemical states in O-MoS2-CPAF. Fig. S4a clearly reveals the
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coexistence of C, O, Mo, and S elements in the hybrid. As seen from Fig. 3e, two
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major peaks at 228.8eV and 232.1eV are presented in high resolution Mo 3d spectrum,
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corresponding to the binding energies of Mo 3d5/2 and Mo 3d3/2, indicating the characteristic of Mo4+ in MoS2[23]. Moreover, a small peak at around 225.9eV can be
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observed, which is ascribed to the S 2s component in MoS2. For the high resolution
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S2p spectrum, the peaks located at 162.9eV and 161.7eV respectively refer to S 2p1/2 and S 2p3/2 orbits of divalent sulfide ions (S2+) (Fig. S4b). The peak located at 531.6 eV in the O1s spectrum (Fig. 3f) could be assigned to the successful oxygen
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incorporation into MoS2 rather than the surface oxidation, leading to the expansion of layer distance, which is in good accordance with the observation shown in XRD pattern and Raman spectrum[23]. Fig. 4a shows the polarization curves of the CPAF, MoS2, MoS2-CPAF,
O-MoS2-CPAF and Pt/C under IR drop compensation. Pt/C catalyst exhibits expected HER activity with a nearly zero over potential. In contrast, O-MoS2-CPAF catalyst exhibits small onset over potential of -88 mV and extremely low potential of -170mV at a current density of 10 mA cm-2, which are much smaller than that of CPAF, MoS2
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and MoS2-CPAF (the onset over potential of -150, -170 and -190 mV). To confirm the reaction mechanisms of our samples, we evaluated the Tafel slope from the low
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overpotential. It is believed that, the Tafel slope is an inherent property of
electrocatalytic materials that is determined by the rate-limiting step of the
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HER[35,36]. The HER mechanisms were studied in acidic media. Three possible
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steps have been suggested[37]. The first step is the Volmer step which is the primary
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discharge step. A proton associates an electron and attaches on the catalyst (*)
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H3O++ e-+*→H*+H2O (120 mV decade-1)
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After the Volmer step, there are two possible steps. The first one is the Heyrovsky reaction, which is the reaction of a hydrated proton with the adsorbed H
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from the electrolyte and receiving an electron from the catalyst surface to form H2.
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H*+H3O++e-→H2+H2O (40 mV decade-1) or a Tafel reaction, which is referred to the direct combination of the adsorbed H.
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H*+H*→H2 (30 mV decade-1) From Fig. 4b, O-MoS2-CPAF exhibits a Tafel slope of 44 mV decade-1, implying
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Fig. 4. (a) The HER polarization curves and (b) the corresponding Tafel plots of various catalysts (CPAF, MoS2, O-MoS2-CPAF, MoS2-CPAF and Pt/C) with a scan rate of 5 mV s-1 in 0.5 mol L-1
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H2SO4. (c) Electrochemical impedance spectroscopy of MoS2, O-MoS2-CPAF and MoS2-CPAF at
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η = 200 mV in the frequency range of 105 to 10-2 Hz with the amplitude of 5 mV. (d) LSV curves
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of O-MoS2-CPAF before and after 1000 cycles test. The insert is the corresponding Tafel plots of
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before and after 1000 cycles for O-MoS2-CPAF, respectively. (e) Long-term stability test of O-MoS2-CPAF carried out under a constant pressure of -0.3V vs RHE. (f) The ratio of current
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density with various scan rates of CPAF, MoS2, MoS2-CPAF and O-MoS2-CPAF.
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that the HER reaction on O-MoS2-CPAF electrode is probably via the
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Volmer-Heyrovsky mechanism, indicating the electrochemical desorption is the rate-determining step[38]. Compared with CPAF (283 mV decade-1), MoS2 (91 mV decade-1), and MoS2-CPAF (62 mV decade-1), O-MoS2-CPAF shows the lowest Tafel
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slopes of 44 mV decade-1, indicating that the O-MoS2-CPAF electrode possesses the fastest proton discharge kinetics. As displayed in Fig. 4c, electrochemical impedance spectroscopy (EIS) was conducted to further investigate the electrocatalytic activity of the catalysts for the HER[29,39]. The semicircle in the high-frequency range
contributes to the charge-transfer resistance (Rct). Generally, the value of Rct varies inversely to the electrocatalytic reaction rate[24,40]. Apparently, O-MoS2-CPAF exhibits the lowest Rct among all four catalysts, demonstrating the rapid electron-transfer rate during the HER[41], which can be attributed to the excellent
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catalytic activity. This result may be attributed to oxygen-incorporated MoS2 ultrathin nanosheets, which improves the intrinsic conductivity, promoting fast mobility of the
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electron along the MoS2 nanosheets.
In addition to the onset potential and Tafel slope, another important criterion for
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a good electrocatalyst is high durability[42]. A stability test of the O-MoS2-CPAF
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catalyst was performed by cycling the potential between -0.1 and 0.4 V (vs. RHE) in
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0.5 M H2SO4 at a scan rate of 100 mV/s. To assess this, we cycled our O-MoS2-CPAF
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hybrid catalyst continuously for 1000 cycles. At the end of cycling, the catalyst
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afforded similar i-v curves as before, with negligible loss of the cathodic current, indicating that O-MoS2-CPAF is a stable catalyst for HER (Fig. 4d). Moreover, we
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also characterized the stability of the O-MoS2-CPAF catalyst by i−t curve at an
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overpotential of ∼300 mV in 0.5 M H2SO4 (Fig. 4e). In this test, current density of the O-MoS2-CPAF electrode displays only a slight degradation after a 10 h testing period,
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suggesting extraordinary durability of the O-MoS2-CPAF composite for HER. To gain insights into the intrinsic activity of the catalysts, we measured the
double-layer capacitances (Cdl) to estimate the electrocatzalytic active surface areas (ECSAs)[43,44]. The Cdl was calculated via cyclic voltammetry plots in the range of 0.1-0.2 V, in which no faradic reactions occurred with various scan rates from 20 to
120 mV s-1 (Fig. S5). The value of the Cdl was obtained by plotting Δj/2 at 150mV against the scan rates. As shown in Fig. 4f, the O-MoS2-CPAF has the highest Cdl value (16.7mF cm-2), indicating the highest catalytically relevant surface area. This result may be mainly attributed to the oxygen incorporation and more exposed active
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sites. Inspired by the excellent HER performance of O-MoS2-CPAF catalyst, we
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further used the IrO2/C and O-MoS2-CPAF as anode and cathode for simulating the
real water splitting cell in 0.5 M H2SO4 in a two-electrode electrolyzer (Fig. 5a).
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Remarkably, the polarization curve for overall water splitting achieved a cell voltage
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of 1.69 V at a current density of 10 mA cm-2, only 460 mV higher than the theoretical
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reversible potential of water splitting reaction (1.23 V) (Fig. 5b). The inset in Fig. 5a
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is the photograph picture of the electrolyzer system, where the left electrode is the
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anode for O2 production and the right one is the cathode for H2 production, as shown in Movie S1. Furthermore, the O-MoS2-CPAF shows excellent stability upon
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long-term testing with only a negligible deactivation at 10 mA cm-2 after ~110 h (Fig.
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5c). We further use gas chromatography to determine the hydrogen production, it is
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Fig. 5. (a) Schematic illustration of two-electrode cell for water splitting using O-MoS2-CPAF and
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IrO2/C as cathode and anode, respectively. (b) LSV polarization curve of O-MoS2-CPAF
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electrocatalyst for overall water splitting in 0.5 M H2SO4 (without IR-compensation). (c)
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Long-term stability test of overall water splitting at constant current densities of 10 mA cm-2. The inset is the digital photograph of the two-electrode configuration. (d) Experimental (Red dot) and
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fitting (black line) H2 production versus time for O-MoS2-CPAF in 0.5 M H2SO4.
can be seen from Fig. 5d that the hydrogen production rate of O-MoS2-CPAF catalyst
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is stable, which again proves the good stability of this catalyst.
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Therefore, the significantly enhanced electrochemical HER activity of O-MoS2-CPAF catalyst can be ascribed to the following reasons: (i) the incorporated oxygen from CPAF increases the defect and vertical edge ratio, giving more exposed
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Mo and S edges to participate catalytic process; (ii) the oxygen-incorporated MoS2 ultrathin nanosheets improve the intrinsic conductivity, promoting fast mobility of the electron along the MoS2 nanosheets; (iii) CPAF with high specific surface area can prevent MoS2 reunion, so O-MoS2 would expose more the edge active sites; (iv) in
situ synthesis of O-MoS2-CPAF through hydrothermal method results in the pronounced synergetic effect between MoS2 nanosheets and CPAF, ensuring the stability during long-term electrocatalysis.
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4. Conclusions In summary, O-MoS2-CPAF has been successfully synthesized via a facile
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solvothermal approach. As is known to all, the number of active sites and conductivity are two key factors that affect the HER performance. Oxygen incorporation in MoS2
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ultrathin nanosheets can provide more edge active sites and enhance the intrinsic
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conductivity. In this work, CPAF ont only served as the conductive template, but also
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acted as oxygen-element donor to enlarge the layer-to-layer distance of MoS2
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nanosheets. Thus, rich active sites and high conductivity were synergistically
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regulated by modulating the optimized oxygen incorporated MoS2 catalyst, exhibiting superior HER performance. The O-MoS2-CPAF catalyst with expanded layer distance
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of crystal planes has been shown to be a highly HER active with a small onset
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overpotential of ∼-88mV, a large current density of 10 mA/cm2 at -170 mV, a small Tafel slope as low as 44 mV/decade, and high durability. This research proposes a
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new perspective to open a new strategy to synthesize other transition metal sulfides with excellent HER performance.
Acknowledgements This work was supported by the NSF of China (51502012; 21676020; 21620102007);
Beijing Natural Science Foundation (2162032); The Start-up fund for talent introduction of Beijing University of Chemical Technology (buctrc201420; buctrc201714); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; The Fundamental Research Funds for the Central Universities (ZD1502);
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Distinguished scientist program at BUCT (buctylkxj02) and the ‘‘111” project of China (B14004).
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S0920-5861(18)30570-4 https://doi.org/10.1016/j.cattod.2018.05.003 CATTOD 11433
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
3-2-2018 23-4-2018 5-5-2018
Please cite this article as: Guo J, Huo F, Cheng Y, Xiang Z, PAF-1 as oxygen tank to in-situ synthesize edge-exposed O-MoS2 for highly efficient hydrogen evolution, Catalysis Today (2010), https://doi.org/10.1016/j.cattod.2018.05.003 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.
PAF-1 as oxygen tank to in-situ synthesize edge-exposed O-MoS2 for highly efficient hydrogen evolution
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Jianing Guoa, Feng Huob, Yuanhui Cheng* and Zhonghua Xianga*
State Key Laboratory of Organic-Inorganic Composites, College of Chemical
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Engineering, College of Energy, Beijing University of Chemical Technology, Beijing 100029, China.
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of
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Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of
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Sciences, Beijing 100190, China
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Email: [email protected] or [email protected]
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Graphical abstract
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We synthesized oxygen-incorporated MoS2 ultrathin nanosheets by oxygen
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Highlights
adsorbate.
The CPAF not only acts as a conductive template but also oxygen tank.
MoS2 nanosheets expose more catalytically active edges grown on the CPAF
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surface.
The O-MoS2-CPAF catalyst exhibited a low onset potential of 0.088 V for HER.
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Abstract
Developing highly-efficient and low-cost non-precious metal electrocatalysts towards
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the hydrogen evolution reaction (HER) is a promising strategy to solve the environmental pollution problems and energy demands. In this study, we synthesized oxygen-incorporated molybdenum disulfide (MoS2) ultrathin nanosheets with expanded interlayer spacing (∼9.3Å) induced by oxygen adsorbate, i.e., carbonized porous aromatic framework (CPAF), through a facile hydrothermal method. The
CPAF not only acts as a conductive template but also oxygen tank, which enhances the transport of electrons of the MoS2-based electrocatalysts and provides oxygen for O-MoS2 nanosheets. Owing to the high specific surface area, conductive template and oxygen source of CPAF, MoS2 nanosheets can grow densely and uniformly on the CPAF surface, exposing catalytically active edges effectively. The O-MoS2-CPAF catalyst exhibited significantly improved catalytic activity toward hydrogen evolution
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reaction (HER) with a low onset potential of 0.088 V, a large current density (10.0
mA cm-2 at 0.17 V), a small Tafel slope of 44 mV per decade as well as excellent
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stability and device performance.
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1. Introduction
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conductive template; active edges; HER catalysis
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Keywords: Oxygen-incorporated MoS2 nanosheets; expanded interlayer spacing;
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Hydrogen, an eco-friendly and high energy density carrier, may replace
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conventional fossil fuels in the future[1-6]. Electrochemical hydrogen evolution reaction (HER) is considered as one of the most promising techniques to combating
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global challenges of climate change and energy crisis[7-9]. Transition metal dichalcogenides (TMDs) are generally considered as a significant kind of material for
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application in environmental and energy technology, owing to their capability of accepting electrons and protons[10-13]. As one of the most interesting layered materials, molybdenum disulfide (MoS2) has been extensively demonstrated to be a promising non-precious electrocatalyst for hydrogen evolution[14]. It is known that the atomic hydrogen adsorption free energy
on the Mo-edge sites of MoS2 was found to be very close to zero, suggesting that HER active sites lay in the edges of MoS2 catalysts but not on their basal planes[15,16]. Based on this principle, research endeavors have been dedicated to designing nanostructured MoS2 with abundant and exposed edges to enhance the
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electrocatalytic activity of MoS2 based catalysts[17]. Defect-rich MoS2, edge exposed MoS2 nanoporous films and elemental incorporation in MoS2 nanostructures have
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been explored[18-20]. Among them, elemental incorporation has been widely used to
improve intrinsic conductivity and accelerate proton adsorption of MoS2
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electrocatalysts. Oxygen incorporation is considered as an effective method to
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improving the HER performance by changing the surface disorder of MoS2 with
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increasing additional active sites[21,22]. Meanwhile, oxygen incorporation can also
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reduce the bandgap and thus enhance the intrinsic conductivity of MoS2 catalyst[23].
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Although the increase of the edge sites can improve the HER activity of MoS2-based electrocatalysts, the electric conductivity of catalysts is another crucial
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factor to affect the electrocatalytic activity because a high conductivity ensures a fast
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electron transfer process[24-26]. In this regard, constructing hybrids of MoS2 with highly conductive templates is an efficient way to take full advantages of the excellent inner properties of HER active materials[27,28]. Ideal conductive materials such as
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carbon nanotubes (CNTs)[11,29], carbon nanofibers (CNFs)[24], graphene oxide (GO) [30-32]and other mesoporous carbon have been used to enhance the conductivity of the MoS2-based electrocatalysts, improving the electrical contact from the templates to the active sites.
On the basis of the above two key factors, we first demonstrate a one-pot hydrothermal method to prepare oxygen-incorporated MoS2 ultrathin nanosheets grown on carbonized porous aromatic framework(CPAF) substrates, realizing the balance between active site numbers and overall conductivity for HER catalysis. The
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CPAF substrate not only functions as a 3D network with large surface area, but also acts as the “oxygen tank” adsorption oxygen in the porous structure. In this work,
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oxygen incorporated in MoS2 comes from the adsorption oxygen in CPAF. The obtained O-MoS2-CPAF catalyst showed high electrocatalytic activity and strong
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durability toward the HER. The success of synergistically structural and electronic
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modulations in this work will pave a new pathway for pursuing efficient
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2.1. Materials
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2. Experimental section
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electrocatalysts for energy production and conversion.
All chemicals used in the synthesis of self-designed catalysts are analytical
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reagents (AR). 2,2'-dipyridyl, 1,5-cyclooctadiene and N, N-dimethylformamide (DMF) were purchased from J&K Chemical Technology. Tetrahydrofuran, chloroform,
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Sulfuric acid and hydrochloric acid were bought from Beijing Chemical Works. Ammonium tetrathiomolybdate and bis (1,5-cyclooctadiene) nickel(0) were supplied by Sigma-Aldrich Chemical Co., Ltd and Strem Chemical, respectively. Commercial Pt/C catalyst was bought from Alfa Aesar Chemical Co.Ltd. High-purity argon (99.99%) gas was obtained from Beijing AP BAIF Gases Industry Co., Ltd.
2.2. Materials Synthesis 2.2.1. Synthesis of CPAF PAF-1 was first prepared according to the Yamamoto-type Ullmann reaction.[33] The PAF-1 was placed into a crucible in a tube furnace, which was
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annealed to 350℃ for 2 h, and then at 900℃ for another 2 h under Ar protection at a heating rate of 6℃ min-1. This product is named CPAF.
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2.2.2. Synthesis of O-MoS2-CPAF
CPAF was first dried in a vacuum oven at 100℃ for 24 h. Then CPAF was putted
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into a closed container with the pressure of one atmosphere oxygen for 24 h. 40 mg
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CPAF adsorption of oxygen was dispersed in 40 ml of N, N dimethylformamide
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(DMF) and sonicated at room temperature for approximately 10 min. 80 mg
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(NH4)2MoS4 was put into the above solution and continuely sonicated for
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approximately 20 min until a homogeneous solution was achieved. After that, the reaction solution was transferred to a 100 mL Teflon-lined autoclave. It was heated in
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an oven at 200℃ for 10 h with no intentional control of ramping or cooling rate.
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Product was collected by centrifugation at 9250 rpm for 5 min, washed with DI water and recollected by centrifugation. The washing step was repeated for at least 5 times
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to ensure that ions and possible remnants were removed. Finally, product was dispersed in 10 ml of DI water, frozen by the refrigerator and freezen drying overnight. This product is named O-MoS2-CPAF. 2.2.3. Synthesis of MoS2-CPAF
CPAF was first dried in a vacuum oven at 100℃ for 24 h. Then 40 mg CPAF adsorption of oxygen was dispered in 40 ml of N, N dimethylformamide (DMF) and sonicated at room temperature for approximately 10 min. 80 mg (NH4)2MoS4 was put into the above solution and continuely sonicated for approximately 20 min until a
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homogeneous solution was achieved. After that, the reaction solution was transferred to a 100 mL Teflon-lined autoclave. It was heated in an oven at 200℃ for 10 h with no
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intentional control of ramping or cooling rate. Product was collected by centrifugation
at 9250 rpm for 5 min, washed with DI water and recollected by centrifugation. The
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washing step was repeated for at least 5 times to ensure that ions and possible
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remnants were removed. Finally, product was dispersed in 10 ml of DI water, frozen
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2.2.4. Synthesis of MoS2
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by the refrigerator and freezen drying overnight. This product named MoS2-CPAF.
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80 mg (NH4)2MoS4 was was dispersed in 40 ml of N, N dimethylformamide (DMF) and sonicated at room temperature for approximately 20 min until a
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homogeneous solution was achieved. After that, the reaction solution was transferred
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to a 100 mL Teflon-lined autoclave. It was heated in an oven at 200℃ for 10 h with no intentional control of ramping or cooling rate. Product was collected by centrifugation at 9250 rpm for 5 min, washed with DI water and recollected by centrifugation. The
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washing step was repeated for at least 5 times to ensure that ions and possible remnants were removed. Finally, product was dispersed in 10 ml of DI water, frozen by the refrigerator and freezen drying overnight. 2.3. Physical characterization
The morphologies and structures of catalysts were performed on a S4700 SEM instrument and a high-resolution transmission electron microscopy (HRTEM, 2100F). The crystalline structures of catalysts were recorded on a D/MAX 2000 X-ray diffractometer using Cu Kα line (λ=1.54178 Å) as the incident beam with the 2θ scan
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from 5° to 90° at a step of 0.02°. The X-ray photoelectron spectroscopy (XPS) was obtained on ESCALAB 250 operated at 150 W and 200 eV with monochromated Al
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Kα radiation. Raman spectra were collected on a Raman Spectroscopy (Renishaw)
using 514 nm laser. The thermogravimetric analysis (TGA) data were obtained on a
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DTG-60A (SHIMADZU) instrument at a heating rate of 10℃ min-1 under flowing air.
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performed on PS2-0722-analysis station A.
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BET was tested by the ASAP 2460. The adsorption isotherm of oxygen was
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2.4. Electrochemical measurements
All electrochemical
measurements
were conducted on a CHI 760E
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electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a three-electrode cell at room temperature. A glassy carbon electrode (GCE with the
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diameter of 5 mm, 0.19625 cm2) was used as the working electrode. Saturated calomel electrode and Pt wire as reference and counter electrodes, respectively. For
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comparison, A commercial Pt/C (20 wt% Pt ) catalyst was also tested in 0.5 M H2SO4 at room temperature. In order to make the working electrodes, 5 mg of catalyst powder was dispersed in 1 mL ethanol and 50 ul nafion solution (0.05 wt%) with 30 min of ultrasonication to generate homogeneous inks. Next, 10 µL of dispersion ink was transferred onto the GCE (~0.25 mg cm-2), and the as-modified RDE electrode
was then dried at room temperature. In HER tests, all electrodes were pretreated by taking continuous cyclic voltammograms(CVs) between -0.8 and 0.1 V versus a reversible hydrogen electrode (RHE) at a sweep rate of 100 mVs-1 under a flow of N2 to remove surface contamination before testing the HER activity. Then the potential
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was cycled from 0.1 V to 0.8 V (vs. RHE) at a scan rate 5 mV s-1 with the electrode rotation speed of 1600 rpm to record the polarization curves. All data were reported
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with IR compensation. 2.4.1. Double-layer capacitance
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The double-layer capacitance (Cdl) of catalyst can be calculated from the cyclic
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voltammograms in the region of 0.1-0.2 V vs. RHE. By plotting the Δj at 0.15 V vs.
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RHE against the scan rate, the slope is twice of Cdl. Electrochemical impedance
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spectroscopy was measured at the overpotential of 200 mV in the frequency range of
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106 Hz to 10-2 Hz with the amplitude potential of 5 mV. All of the potentials were calibrated to a reversible hydrogen electrode (RHE).
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2.4.2. Water splitting cell characterization
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Hydrogen evolution electrode was fabricated by dropping a well sonicated solution of O-MoS2-PAF with Nafion at a mass ratio of 9:1 onto carbon paper (1 cm×1 cm, thickness of 0.2 mm). Oxygen evolution electrode was fabricated by
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dropping a well-sonicated solution of IrO2 purchased from Alfa Aesar Chemical Co. Ltd., acetylene black and Nafion in a 3.6: 5.4: 1 wt ratio onto carbon paper (1 cm×1 cm, thickness of 0.2 mm). The loadings of O-MoS2-PAF and IrO2 are all 2 mg cm-2. Then, the two kinds of electrodes were immersed into a sealed electrolytic cell
containing 150 mL electrolyte (0.5 mol L-1 H2SO4). Inserted the electrode inside the 280 mL closed cell with 150 mL vitriol as electrolyte and put through power supply. After 10 mins, we took 0.5 mL of the sample into gas chromatography every five minutes to get the hydrogen concentration and repeated sampling 10 times in total.
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The water splitting cell was conducted at 10 mA cm-2 to produce hydrogen gas and oxygen gas.
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3. Results and discussion
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Fig.1 illustrated the schematic preparation process for O-MoS2-CPAF. CPAF first
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adsorbed oxygen in the pressure vessel (Fig. S1), then they were dissolved in
Fig. 1. Schematic illustration of the preparation of O-MoS2-CPAF
(NH2)4MoS4 solution and reacted under 200 ℃ gaining the final product. The morphology and hierarchical architecture of the hydrothermal fabricated flower
structured MoS2 ultrathin nanosheets are demonstrated in Fig.1. The structure of CPAF is uniformly dispersed sphere (Fig. S2a). The surface of the composed of dense curly 2D nanopetal that randomly crosswise was shown in Fig. 2a. In addition, MoS2 decorated on the CPAF structures and they combine each other very closer, forming
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the layered structure of staggered. Therefore, CPAF support would be beneficial for
Fig. 2. (a) SEM image of O-MoS2-CPAF. (b) and (c) HRTEM images of O-MoS2-CPAF. (d) The responding element mapping.
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the deposition and formation of MoS2 nanoparticles (Fig. S2b), and provide a good distribution for the exposed active surface of MoS2 nanoparticles. On the contrary, there is an obvious reunion of MoS2 for individual MoS2 catalyst (Fig. S2c). The HRTEM images in Fig. 2b-c give a close-up view of the MoS2 nanopetals on the CPAF structure. The HRTEM images of typical nanosheets suggest a building
structure of O-MoS2 thin as <10 layers with crystal dislocations and defects. The layered spacing of O-MoS2-CPAF can be identified to be around ∼0.93 nm which larger than that of 0.63 nm in the normally obtained 2H-MoS2, signifying an enlarged interlayer spacing. This result confirms the successful oxygen incorporation.
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Furthermore, the STEM image and corresponding EDS mapping analyses (Fig. 2d) reveal the homogeneous distribution of molybdenum, sulfur, and oxygen in the whole
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ultrathin nanosheets. The TG analysis of O-MoS2-CPAF (Fig. S6) shows three regions
of weight loss. The remaining part belongs to MoO3 from the decomposition of
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further oxidation of MoS2 to MoO3. Therefore, the weight fraction of MoS2 in
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the
pore
structure
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In
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O-MoS2-CPAF can be calculated to be ~40%.
of
the
CPAF,
the
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adsorption-desorption isotherms of the CPAF was recorded (Fig. S3a). The
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corresponding BET surface area is found to be 571.5m2/g. Also, their corresponding pore size distributions show the coexistence of micropores, mesopores, and
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large-mesopores (Fig. S3b). This result indicates that the hierarchically porous
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structure has been successfully fabricated, which could shorten the pathway of mass transfer and be favorable for oxygen adsorption in CPAF. The average pore width is 3.29 nm. Meanwhile, we further tested the oxygen adsorption of CPAF, which shows
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that the CPAF had good adsorption performance on oxygen (Fig. 3a). The crystal structures of O-MoS2-CPAF were systematically investigated by X-ray diffraction (XRD) patterns. As shown in Fig. 3b, a new peak is observed at the low-angle region for O-MoS2-CPAF, which indicates the enlarged interlayer spacing
of oxygen incorporated MoS2 ultrathin nanosheets. The corresponding d spacing is 0.93 nm, indicating the formation of a new lamellar structure with larger interlayer spacing compared with that of 0.63 nm in pristine 2H-MoS2 (JCPDS Card No. 73-1508). Moreover, two strong diffraction peaks at high-angle region (32° and 56.3°)
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can be well indexed to (100) and (110) crystal planes of the pristine 2H-MoS2, respectively. The Raman spectrum in Fig. 3c reveals two dominant peaks located at
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1355 cm-1 and 1591 cm-1 which can be assigned to the typical D and G bands, respectively. The peaks located at 817 cm-1 and 992 cm-1 are attributed to Mo-O2, and
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Mo-O vibrations, respectively[34]. In addition, the characteristic peaks at 285cm-1 and
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334cm-1 corresponding to the B2g and B1g vibrational modes for Mo-O bonds of
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MoO3[21]. The existence of the Raman peaks for Mo-Ox bonds confirms the
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accordance with the XRD results.
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successful oxygen incorporation in the hybrid rather than the surface oxidation, in
Based on the above studies, a schematic structural model of oxygen-incorporated
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MoS2 with enlarged interlayer spacing is constructed in Fig. 3d, which reveals the
Fig. 3. (a) O2 uptake of CPAF at 301.5K. (b) XRD pattern of O-MoS2-CPAF. (c) Raman spectrum of O-MoS2-CPAF. (d) Structural models of the oxygen-incorporated MoS2 with enlarged interlayer spacing and the pristine 2H-MoS2. (e) XPS spectra for Mo3d of O-MoS2-CPAF. (f) XPS spectra for O1s of O-MoS2-CPAF.
new lamellar structure intuitively. In order to compare directly, a schematic model of
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pristine 2H-MoS2 with normal interlayer spacing of 0.63 nm is also presented. It is considered that, the O-MoS2 nanosheets possess the moderate degree of short-range
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disordering feature that can offer more active sites as well as enhance the intrinsic conductivity, thus leading to enhance HER activity.
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The X-ray photoelectron spectroscopy (XPS) was used to identify the
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compositions and chemical states in O-MoS2-CPAF. Fig. S4a clearly reveals the
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coexistence of C, O, Mo, and S elements in the hybrid. As seen from Fig. 3e, two
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major peaks at 228.8eV and 232.1eV are presented in high resolution Mo 3d spectrum,
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corresponding to the binding energies of Mo 3d5/2 and Mo 3d3/2, indicating the characteristic of Mo4+ in MoS2[23]. Moreover, a small peak at around 225.9eV can be
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observed, which is ascribed to the S 2s component in MoS2. For the high resolution
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S2p spectrum, the peaks located at 162.9eV and 161.7eV respectively refer to S 2p1/2 and S 2p3/2 orbits of divalent sulfide ions (S2+) (Fig. S4b). The peak located at 531.6 eV in the O1s spectrum (Fig. 3f) could be assigned to the successful oxygen
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incorporation into MoS2 rather than the surface oxidation, leading to the expansion of layer distance, which is in good accordance with the observation shown in XRD pattern and Raman spectrum[23]. Fig. 4a shows the polarization curves of the CPAF, MoS2, MoS2-CPAF,
O-MoS2-CPAF and Pt/C under IR drop compensation. Pt/C catalyst exhibits expected HER activity with a nearly zero over potential. In contrast, O-MoS2-CPAF catalyst exhibits small onset over potential of -88 mV and extremely low potential of -170mV at a current density of 10 mA cm-2, which are much smaller than that of CPAF, MoS2
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and MoS2-CPAF (the onset over potential of -150, -170 and -190 mV). To confirm the reaction mechanisms of our samples, we evaluated the Tafel slope from the low
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overpotential. It is believed that, the Tafel slope is an inherent property of
electrocatalytic materials that is determined by the rate-limiting step of the
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HER[35,36]. The HER mechanisms were studied in acidic media. Three possible
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steps have been suggested[37]. The first step is the Volmer step which is the primary
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discharge step. A proton associates an electron and attaches on the catalyst (*)
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H3O++ e-+*→H*+H2O (120 mV decade-1)
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After the Volmer step, there are two possible steps. The first one is the Heyrovsky reaction, which is the reaction of a hydrated proton with the adsorbed H
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from the electrolyte and receiving an electron from the catalyst surface to form H2.
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H*+H3O++e-→H2+H2O (40 mV decade-1) or a Tafel reaction, which is referred to the direct combination of the adsorbed H.
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H*+H*→H2 (30 mV decade-1) From Fig. 4b, O-MoS2-CPAF exhibits a Tafel slope of 44 mV decade-1, implying
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Fig. 4. (a) The HER polarization curves and (b) the corresponding Tafel plots of various catalysts (CPAF, MoS2, O-MoS2-CPAF, MoS2-CPAF and Pt/C) with a scan rate of 5 mV s-1 in 0.5 mol L-1
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H2SO4. (c) Electrochemical impedance spectroscopy of MoS2, O-MoS2-CPAF and MoS2-CPAF at
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η = 200 mV in the frequency range of 105 to 10-2 Hz with the amplitude of 5 mV. (d) LSV curves
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of O-MoS2-CPAF before and after 1000 cycles test. The insert is the corresponding Tafel plots of
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before and after 1000 cycles for O-MoS2-CPAF, respectively. (e) Long-term stability test of O-MoS2-CPAF carried out under a constant pressure of -0.3V vs RHE. (f) The ratio of current
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density with various scan rates of CPAF, MoS2, MoS2-CPAF and O-MoS2-CPAF.
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that the HER reaction on O-MoS2-CPAF electrode is probably via the
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Volmer-Heyrovsky mechanism, indicating the electrochemical desorption is the rate-determining step[38]. Compared with CPAF (283 mV decade-1), MoS2 (91 mV decade-1), and MoS2-CPAF (62 mV decade-1), O-MoS2-CPAF shows the lowest Tafel
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slopes of 44 mV decade-1, indicating that the O-MoS2-CPAF electrode possesses the fastest proton discharge kinetics. As displayed in Fig. 4c, electrochemical impedance spectroscopy (EIS) was conducted to further investigate the electrocatalytic activity of the catalysts for the HER[29,39]. The semicircle in the high-frequency range
contributes to the charge-transfer resistance (Rct). Generally, the value of Rct varies inversely to the electrocatalytic reaction rate[24,40]. Apparently, O-MoS2-CPAF exhibits the lowest Rct among all four catalysts, demonstrating the rapid electron-transfer rate during the HER[41], which can be attributed to the excellent
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catalytic activity. This result may be attributed to oxygen-incorporated MoS2 ultrathin nanosheets, which improves the intrinsic conductivity, promoting fast mobility of the
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electron along the MoS2 nanosheets.
In addition to the onset potential and Tafel slope, another important criterion for
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a good electrocatalyst is high durability[42]. A stability test of the O-MoS2-CPAF
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catalyst was performed by cycling the potential between -0.1 and 0.4 V (vs. RHE) in
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0.5 M H2SO4 at a scan rate of 100 mV/s. To assess this, we cycled our O-MoS2-CPAF
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hybrid catalyst continuously for 1000 cycles. At the end of cycling, the catalyst
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afforded similar i-v curves as before, with negligible loss of the cathodic current, indicating that O-MoS2-CPAF is a stable catalyst for HER (Fig. 4d). Moreover, we
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also characterized the stability of the O-MoS2-CPAF catalyst by i−t curve at an
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overpotential of ∼300 mV in 0.5 M H2SO4 (Fig. 4e). In this test, current density of the O-MoS2-CPAF electrode displays only a slight degradation after a 10 h testing period,
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suggesting extraordinary durability of the O-MoS2-CPAF composite for HER. To gain insights into the intrinsic activity of the catalysts, we measured the
double-layer capacitances (Cdl) to estimate the electrocatzalytic active surface areas (ECSAs)[43,44]. The Cdl was calculated via cyclic voltammetry plots in the range of 0.1-0.2 V, in which no faradic reactions occurred with various scan rates from 20 to
120 mV s-1 (Fig. S5). The value of the Cdl was obtained by plotting Δj/2 at 150mV against the scan rates. As shown in Fig. 4f, the O-MoS2-CPAF has the highest Cdl value (16.7mF cm-2), indicating the highest catalytically relevant surface area. This result may be mainly attributed to the oxygen incorporation and more exposed active
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sites. Inspired by the excellent HER performance of O-MoS2-CPAF catalyst, we
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further used the IrO2/C and O-MoS2-CPAF as anode and cathode for simulating the
real water splitting cell in 0.5 M H2SO4 in a two-electrode electrolyzer (Fig. 5a).
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Remarkably, the polarization curve for overall water splitting achieved a cell voltage
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of 1.69 V at a current density of 10 mA cm-2, only 460 mV higher than the theoretical
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reversible potential of water splitting reaction (1.23 V) (Fig. 5b). The inset in Fig. 5a
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is the photograph picture of the electrolyzer system, where the left electrode is the
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anode for O2 production and the right one is the cathode for H2 production, as shown in Movie S1. Furthermore, the O-MoS2-CPAF shows excellent stability upon
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long-term testing with only a negligible deactivation at 10 mA cm-2 after ~110 h (Fig.
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5c). We further use gas chromatography to determine the hydrogen production, it is
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Fig. 5. (a) Schematic illustration of two-electrode cell for water splitting using O-MoS2-CPAF and
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IrO2/C as cathode and anode, respectively. (b) LSV polarization curve of O-MoS2-CPAF
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electrocatalyst for overall water splitting in 0.5 M H2SO4 (without IR-compensation). (c)
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Long-term stability test of overall water splitting at constant current densities of 10 mA cm-2. The inset is the digital photograph of the two-electrode configuration. (d) Experimental (Red dot) and
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fitting (black line) H2 production versus time for O-MoS2-CPAF in 0.5 M H2SO4.
can be seen from Fig. 5d that the hydrogen production rate of O-MoS2-CPAF catalyst
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is stable, which again proves the good stability of this catalyst.
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Therefore, the significantly enhanced electrochemical HER activity of O-MoS2-CPAF catalyst can be ascribed to the following reasons: (i) the incorporated oxygen from CPAF increases the defect and vertical edge ratio, giving more exposed
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Mo and S edges to participate catalytic process; (ii) the oxygen-incorporated MoS2 ultrathin nanosheets improve the intrinsic conductivity, promoting fast mobility of the electron along the MoS2 nanosheets; (iii) CPAF with high specific surface area can prevent MoS2 reunion, so O-MoS2 would expose more the edge active sites; (iv) in
situ synthesis of O-MoS2-CPAF through hydrothermal method results in the pronounced synergetic effect between MoS2 nanosheets and CPAF, ensuring the stability during long-term electrocatalysis.
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4. Conclusions In summary, O-MoS2-CPAF has been successfully synthesized via a facile
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solvothermal approach. As is known to all, the number of active sites and conductivity are two key factors that affect the HER performance. Oxygen incorporation in MoS2
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ultrathin nanosheets can provide more edge active sites and enhance the intrinsic
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conductivity. In this work, CPAF ont only served as the conductive template, but also
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acted as oxygen-element donor to enlarge the layer-to-layer distance of MoS2
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nanosheets. Thus, rich active sites and high conductivity were synergistically
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regulated by modulating the optimized oxygen incorporated MoS2 catalyst, exhibiting superior HER performance. The O-MoS2-CPAF catalyst with expanded layer distance
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of crystal planes has been shown to be a highly HER active with a small onset
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overpotential of ∼-88mV, a large current density of 10 mA/cm2 at -170 mV, a small Tafel slope as low as 44 mV/decade, and high durability. This research proposes a
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new perspective to open a new strategy to synthesize other transition metal sulfides with excellent HER performance.
Acknowledgements This work was supported by the NSF of China (51502012; 21676020; 21620102007);
Beijing Natural Science Foundation (2162032); The Start-up fund for talent introduction of Beijing University of Chemical Technology (buctrc201420; buctrc201714); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; The Fundamental Research Funds for the Central Universities (ZD1502);
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Distinguished scientist program at BUCT (buctylkxj02) and the ‘‘111” project of China (B14004).
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