Electrodeposited Mo3S13 Films from (NH4)2Mo3S13Â·2H2O for

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Electrodeposited Mo3S13 Films from (NH4)2Mo3S13·2H2O for Electrocatalysis of Hydrogen Evolution Reaction Kuangzhou Du, Lirong Zheng, Tanyuan Wang, Junqiao Zhuo, Zhiwei Zhu, Yuanhua Shao, and Meixian Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

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ACS Applied Materials & Interfaces

Electrodeposited Mo3S13 Films from (NH4)2Mo3S13·2H2O for Electrocatalysis of Hydrogen Evolution Reaction Kuangzhou Du,† Lirong Zheng,‡ Tanyuan Wang,† Junqiao Zhuo,† Zhiwei Zhu, † Yuanhua Shao† and Meixian Li*†

† College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

‡ Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China *Corresponding author. Tel.: +86 10 62757953; fax: +86 10 62751708. E-mail address: [email protected]

ABSTRACT

Molybdenum sulfides are considered to be one kind of the promising candidates as cheap and efficient electrocatalysts for hydrogen evolution reaction (HER). But this is still a gap on electrocatalytic performance toward Pt. In order to further enhance electrocatalytic activity of molybdenum sulfides, in this work, Mo3S13 films with high ratio of sulfur to molybdenum were

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prepared by electrodeposition. The Mo3S13 films exhibit highly efficient electrocatalytic activity for HER and achieve a current density of 10 mA/cm2 at an overpotential of 200 mV with an onset potential of 130 mV vs. RHE and a Tafel slope of 37 mV/dec, which is superior to other reported MoS2 films. The highly electrocatalytic activity is attributed to high percentage of bridging S22- and apical S2- as well as good conductivity. This study provides an avenue for designing new molybdenum sulfides electrocatalysts.

KEYWORDS:

hydrogen

evolution

reaction,

molybdenum

sulfides, electrodeposition,

electrocatalysis, Mo3S13

1. INTRODUCTION Nowadays, energy crisis and environmental problems are becoming increasingly serious due to gradual depletion of non-renewable resources and carbon emission.1 Hydrogen has attracted great interest as an energy carrier or fuel since it has the highest mass energy density in all fuels and it burns cleanly. At present, the efficient production of hydrogen from water splitting by electrolysis and photolysis shows great application prospect. But involvement of catalysts is necessary in order to enhance efficiency. Pt is an ideal electrocatalyst for hydrogen evolution reaction (HER), however, its low abundance and high price limit its wide application in HER. Therefore, the investigation on earth-abundant, low-cost, and highly active catalysts for HER has aroused much more interest. Nanostructured molybdenum disulfide (MoS2) is considered as a promising substitute for Pt electrocatalyst since Nørskov and coworkers reported MoS2 nanoparticles as a catalyst for HER.1 Subsequent researches on MoS2 have revealed that the modifications of its structure could improve the electrocatalytic activity for HER, such as cubane-type [Mo3S4]4+,2 assembled


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[Mo3S4]4+ clusters on Au surfaces,3 MoS2 supported on graphene,4 core-shell MoO3-MoS2 nanowires,5 amorphous molybdenum sulfide films,6 and so on. Chorkendorff and coworkers proposed that the catalytic activity of MoS2 nanoparticles originated from its active edge sites,7 which revealed the size of nanostructured MoS2 influenced its electrocatalytic activity. Recent studies on electrocatalysts have concluded that various factors including active edge sites, conductivity, roughness and the attachment of catalysts on electrodes need to be wellcontrolled in order to obtain a more efficient electrocatalyst. 8, 9, 10, 11, 12 Among those, increase of active sites is crucial for improving electrocatalytic performance. According to our previous studies,13 MoSx with high ratio of S to Mo shows excellent electrocatalytic activity for HER. Recent report on electrocatalytic activity of the molecular clusters [Mo3S13]2- and [Mo2S12]2- for HER also verifies this.14, 15 Moreover, electrocatalytic activity of nanostructured MoSx is likely related to the percentage of bridging S22- and apical S2-,16 our previous work involved in polypyrrole and MoSx copolymer films also supports the conclusion.13 Based on the reported theoretical results that MoS4 (C2, 1A) is predicted to be the ground state and may be served as a neutral model for the sulfur-rich edge sites of the fresh MoS2 catalysts,17 herein, highly active Mo3S13 films for HER were fabricated by electrodeposition of (NH4)2Mo3S13·2H2O, and its structure was also characterized.

2. EXPERIMENTAL SECTION 2.1 Synthesis of (NH4)2Mo3S13·2H2O Thiomolybdate (NH4)2Mo3S13·2H2O was synthesized by (NH4)6Mo7O24·4H2O reacting with an ammonium polysulfide solution according to the method reported by Müller et al. 18 40 g of sulfur was dissolved in 130 mL of (NH4)2S aqueous solution with stirring to prepare saturated ammonium polysulfide solution, and then 4g of


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(NH4)6Mo7O24·4H2O was added into it. The resulting solution was heated to 90 °C and maintained for 15 h, and then cooled down to room temperature naturally. The filtered precipitation was washed with CS2 and acetone to remove sulfur formed in the synthetic process. X-ray diffraction (XRD) characterization is shown in Figure S1, and it is consistent with the reported results14, which indicates that the obtained product is (NH4)2Mo3S13·2H2O. 2.2 Preparation of Mo3S13 modified electrodes The Mo3S13 films were electrodeposited respectively on a highly oriented pyrolytic graphite (HOPG) electrode, a glassy carbon electrode (GCE) or a fluorine doped tin oxide (FTO) electrode in 0.1 M NaClO4 aqueous solution containing 1.0 mM (NH4)2Mo3S13·2H2O by controlled-potential electrolysis at 0.15 V (vs. SCE) for 100 s. 2.3 Characterization Polarization curves were acquired by linear sweep voltammetry with a scan rate of 2 mV/s in 0.5 M H2SO4 on a CHI 660D electrochemical workstation (Shanghai Chenhua Instrument Co., China) at room temperature (around 25 °C) under inert environment. The modified electrode and a graphite rod electrode were used as the working and counter electrodes, respectively. A saturated calomel electrode (SCE) was used as the reference electrode. Prior to measurement, a resistance was test, and then the iR compensation was done using the CHI software. All the potentials were calibrated to the reversible hydrogen electrode (RHE) after measurement, similar to our previous work.19 For stability test, 3 µL of 0.1% Nafion was coated on the electrodeposited Mo3S13 films modified electrode. X-ray diffraction (XRD) patterns were performed by a XRD-6000 diffractometer using Cu Kα radiation (Shimadzu, Japan). Scanning electron microscopy (SEM) images and Energydispersive X-ray spectroscopy (EDS) images were photographed on an S-4800 electron microscope (Hitach, Japan). The voltages for SEM images and EDS element mappings were 1


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kV, 5 kV and 15 kV, respectively. Electrochemical impedance spectra (EIS) were recorded on an Autolab PGSTAT 302 (ECO Chemie, Netherlands) in 0.5 M H2SO4. The frequency ranged from 100000 Hz to 0.1 Hz and the applied voltage was 5 mV. X-ray photoelectron spectra (XPS) were obtained on an Axis Ultra spectrometer (Kratos Analytical Ltd., Japan) with C 1s peak calibrated at 284.8 eV. Inductively coupled plasma - atomic emission spectrometry (ICP-AES) was performed on a Leeman PROFILE SPEC. X-ray absorption spectroscopy (XAS) characterization was performed on Beijing Synchrotron Radiation Facility (BSRF). The measurements were carried out in fluorescence mode at the Mo K-edge at ambient temperature. Data were analyzed using Athena, and curve fitting was performed with Artemis and IFEFFIT software.20

3. RESULTS AND DISCUSSION

Figure 1. (a) Cyclic voltammogram of (NH4)2Mo3S13 in 0.1 M NaClO4 solution at a scan rate of 100 mV/s. (b) Digital image of electrodeposited films modified on a FTO electrode for easy observation. Hu’s group has reported that amorphous molybdenum sulfide films prepared with (NH4)2MoS4 as the reactant by electrodeposition are a class of highly active catalysts for


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hydrogen evolution.6,21 Recently, thiomolybdate [Mo3S13]2- nanoclusters was reported to show highly efficient electrocatalytic activity for HER on graphite paper, but its adsorption on HOPG electrode is poor. In order to increase adsorption, electrochemical behavior of (NH4)2Mo3S13 was investigated in an aqueous solution containing 0.1 M NaClO4, and a few redox peaks were observed, as shown in Figure 1a. In order to understand redox of (NH4)2Mo3S13, different potential ranges were applied, as shown in Figure S2. Two reduction peaks and one reoxidation peak were obtained in the potential range of 0 ∼ -1.2 V; in the meantime, two irreversible oxidation peaks appeared from 0 to 1.2 V. But it is relatively complicated to understand electrochemical reaction mechanism, which needs further study. Moreover, we observed thin films on the electrode surface by naked eyes after electrochemical cycling scan, similar to MoSx (x ≈ 2 or 3) films prepared from (NH4)2MoS4.6 As shown in Figure 1b, the films were electrodeposited on a FTO electrode for easy observation, and it displayed thin and uniform structure. In addition, the films electrodeposited on a HOPG electrode showed electrocatalytic activity for HER. In order to obtain the electrodeposited films with the best electrocatalytic activity for HER, different deposition conditions including electrodeposition potential and time were tested. Effect of the deposition potential was complicated since different deposition potentials resulted in different compositions and structures of the films. The results proved that the optimal deposition potential was 0.15 V vs. SCE (Figure 2a). We also optimized the electrodeposition time as shown in Figure 2b, and increase in electrodeposition time from 0.1 s to 100 s benefited enhancement of the catalytic activity of the films due to more loadings and more active sites. But when the electrodeposition time extended to 1500 s, no improvement could be found. The reason for this might be the thicker films with larger resistance at longer electrodeposition time So the


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electrodeposited films (denoted by MoSx) on the HOPG electrode were prepared in 1.0 mM (NH4)2Mo3S13 containing 0.1 M NaClO4 by controlled-potential electrolysis at 0.15 V vs. SCE for 100 s.

Figure 2. (a) Polarization curves of the different modified electrodes deposited at 0.15 V, 0.1 V, 0.2 V, 0.4 V, 0.8 V, and 1.2 V (vs. SCE). (b) Polarization curves of the modified electrodes deposited at 0.15 V (vs. SCE) for different time. Polarization curves (c) and Tafel plots (solid lines) with fitting curves (dash lines) (d) of electrodeposited MoSx films modified electrode and (NH4)2Mo3S13 films modified electrode. Figures 2c, d display the polarization curves and Tafel plots of the electrodeposited MoSx films and drop-casting (NH4)2Mo3S13 films for HER. Compared to electrocatalytic activity of the


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(NH4)2Mo3S13 films with an onset potential of 170 mV vs. RHE and a Tafel slope of 57 mV/dec, which was similar to the reported results

14

, the electrodeposited MoSx films showed highly

efficient electrocatalytic activity for HER with a lower onset potential of 130 mV vs. RHE, a Tafel slope of 37 mV/dec and a current density of 10 mA/cm2 at an overpotential of 200 mV. Based on the kinetic principle of HER, a Tafel slope of 37 mV/dec means that HER mechanism at the electrodeposited MoSx films might be Volmer–Heyrovsky mechanism with Heyrovsky reaction as the rate determining step. In order to further understand the micro morphology of the electrodeposited MoSx films, SEM images were photographed (Figure 3). The electrodeposited MoSx films were made up of flakes covered on the surface of the HOPG electrode, which was obviously different from that of the blank HOPG electrode (Figure S3).

Figure 3. SEM images of electrodeposited MoSx films on the HOPG electrode. The scale bars are 4 µm (a) and 2 µm (b), respectively. Various characterizations were performed in order to illustrate the origin of highly electrocatalytic activity of the electrodeposited MoSx films,. XPS survey spectrum shows that elemental compositions of the electrodeposited MoSx films are Mo and S, and the element N


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could not be detected (Figure 4a-c). The ratio of S to Mo for the MoSx films was calculated to be 4.3:1, which is in accordance with the result obtained by ICP-AES (the ratio of S to Mo is 4.22). Therefore, we presume that the electrodeposited films are Mo3S13, which is further verified by binding energies of the elements Mo and S. Binding energies of Mo 3d3/2 and Mo 3d5/2 in Mo3S13 located at 229.4 eV and 232.5 eV are the same as those in (NH4)2Mo3S13·2H2O, which suggests existence of Mo(IV) (Figure 4d-e). We further analyzed S 2p region by peak fitting, and two doublets (2p3/2, 2p1/2) were observed, which indicates multiple chemical states of sulfur (Figure 4f). One doublet at 2p3/2=163.5 eV and 2p1/2=164.7 eV is attributed to the bridging S22- and apical S2- (I S); the other doublet at 2p3/2=162.1 eV and 2p1/2=163.3 eV results from the terminal S22- (II S). These are similar to those in (NH4)2Mo3S13·2H2O, but residual sulfur (III S) from (NH4)2Sn existing in (NH4)2Mo3S13·2H2O (Figure 4g) could not be seen in Mo3S13. Moreover, the intensity of the S 2p3/2 peak at 163.5 eV that represents bridging S22- and apical S2- is higher than that of the S 2p3/2 peak at 162.1 eV that represents terminal S22- in Mo3S13 compared with those in XPS spectra of (NH4)2Mo3S13·2H2O, which benefits electrocatalysis because more bridging S22- and apical S2- are considered as active sites for HER. 14, 15, 16, 22


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Figure 4. XPS survey spectra for Mo3S13 (a) and (NH4)2Mo3S13·2H2O (b). XPS spectra for Mo3S13 and (NH4)2Mo3S13·2H2O in the region of N 1s (c). XPS spectra for Mo3S13 (d) and (NH4)2Mo3S13·2H2O (e) in Mo 3d region. XPS spectra for Mo3S13 (f) and (NH4)2Mo3S13·2H2O (g) in S 2p region.


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Figure 5. Mo K-edge XANES spectra of Mo3S13 and (NH4)2Mo3S13·2H2O (a). Mo K-edge Fourier transform EXAFS (k3-weighted) of Mo3S13 and (NH4)2Mo3S13·2H2O (b). In order to further verify structure of Mo3S13, X-ray absorption spectroscopy (XAS) was done, as shown in Figure 5 and Table 1. The Mo K-edge X-ray absorption near edge spectra (XANES) were recorded in situ on the electrodeposited Mo3S13 films. For the Mo K-edge spectra, an E0 value of 20004.6 eV was used. For comparison, the XANES spectra of (NH4)2Mo3S13·2H2O is shown in Figure 5a. Negligible difference in Mo K-edge XANES spectra is reflected, which shows that the valence state and the chemical environment of the electrodeposited Mo3S13 and (NH4)2Mo3S13·2H2O are similar. The Mo K-edge EXAFS spectra, which recorded in situ on the electrodeposited Mo3S13 films and (NH4)2Mo3S13·2H2O for comparison (Figure 5b), show two peaks corresponding to Mo−S bond and Mo−Mo distance. The EXAFS curve fitting results of the electrodeposited Mo3S13 films and (NH4)2Mo3S13·2H2O as prepared are summarized in Table 1, and the k-space curves are shown in Figure S4. For the electrodeposited Mo3S13 films, a Mo−S bond with a distance of R = 2.44 Å and an N value of 7.2 was included in modelling the peak at short R’ value (R is the actual distance and R’ is the apparent distance). The best fit was obtained with a Mo−Mo


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distance of 2.73 Å and an N value of 2.7 (R’ = 2.4 Å). For (NH4)2Mo3S13·2H2O, the peak at R’ = 2.0 Å was fit with a Mo−S bond (R = 2.43 Å, N = 7.0), while the low intensity one at R’ = 2.4 Å was fit with a Mo−Mo distance (R = 2.75 Å, N = 2.0). The parameters of the Mo−S bond and Mo−Mo distance of Mo3S13 are in good accordance with those of (NH4)2Mo3S13·2H2O, which was used as the fitting reference.23 Table 1. EXAFS fitting parameters for Mo3S13 and (NH4)2Mo3S13·2H2O. (N: coordination numbers; R: bond distance; σ2: Debye-Waller factors; ∆E0: the inner potential correction. R factor: goodness of fit.)

Sample

Shell

R(Å)

N

σ2(103 Å2)

∆E0 (eV)

Mo-S

2.44

7.2

4.4

2.7

R factor (%)

0.58

Mo3S13 Mo-Mo

2.73

2.7

3.4

5.2

Mo-S

2.43

7.0

4.6

3.0 0.04

(NH4)2Mo3S13·2H2O Mo-Mo

2.75

2.0

2.9

7.4

XRD technique was also employed to characterize the structure of Mo3S13 films, but we could not obtain any information since the crystallinity of Mo3S13 is poor, which is similar to the reported poorly crystalline Mo3S12 and the amorphous Mo3S13.5 in the literature. 24 Electrochemical impedance spectroscopy (EIS) was used to study charge-transfer resistance, and Nyquist plots are presented in Figure 6. Based on the electrical equivalent circuit shown in Figure S5, a quite small charge-transfer resistance of 8 Ω ·cm2 at an overpotential of 196 mV was obtained for the electrodeposited Mo3S13 films, which is much smaller than that obtained from the drop-casted (NH4)2Mo3S13. The superior electrical conductivity of Mo3S13 modified films results in its high electrocatalytic activity.


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Figure 6. Nyquist plots of the electrodeposited Mo3S13 films modified electrode (a) and (NH4)2Mo3S13 films modified electrode (b). Additionally, compared to the previous work on electrocatalytic activity of the molecular clusters [Mo3S13]2- 14 and [Mo2S12]2- 15 as well as MoS2 5, 6, 9, 25-30 for HER, as shown in Table S1, the electrodeposited Mo3S13 films show a smaller Tafel slope. A small Tafel slope is desirable for practical application since it will be helpful for a faster increase in the rate of HER with increase of the overpotential. Moreover, electrocatalytic performance of the Mo3S13 films is comparable to that of the reported [Mo3S13]2- on graphite paper.14 As discussed above, the high HER efficiency of the electrodeposited Mo3S13 films can be attributed to the following reasons: (1) The high percent of bridging S22− and apical S2− benefits the electrocatalytic process of the electrodeposited Mo3S13 films for HER. (2) The in situ electrodeposited Mo3S13 films have good electrical conductivity, which ensures the effective electrons transport during the electrochemical process.


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Figure 7. (a) Stability test of the electrodeposited Mo3S13 films by cyclic voltammetry. (b) SEM image of the electrodeposited films after hydrogen evolution reaction. The stability of the electrodeposited Mo3S13 films was assessed by scanning potential for 3000 cycles from 0.2 V to -0.2 V vs. RHE at a scan rate of 100 mV/s. The HER activity showed slight decrease in current density after CV scans (Figure 7a). Moreover, no film exfoliation was observed after hydrogen evolution reaction (Figure 7b) compared to SEM image of the films before HER, which imply the good durability of the electrodeposited Mo3S13 films.

4. CONCLUSION

This work reports the preparation of a novel electrocatalyst Mo3S13 using a facile electrodeposition. The electrodes modified with Mo3S13 films exhibit highly activity for HER with an onset potential of 130 mV vs. RHE and a Tafel slope of 37 mV/dec, achieving a current density of 10 mA/cm2 at an overpotential of only 200 mV. The Mo3S13 films are attested to have high ratio of sulfur to molybdenum and high percent of bridging S22- and apical S2- as well as good conductivity, resulting in highly efficient HER performance which is superior to the reported MoS2 films.29-30 Our work demonstrates the importance of active sites in catalysis and


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provides a practical strategy to design highly-efficient catalysts based on molybdenum sulfides. In addition, this study lays a foundation for investigation on HER catalysts based on other metal polysulfides.

ASSOCIATED CONTENT Supporting Information. Some supplemental figures and a table, including XRD pattern of (NH4)2Mo3S13·2H2O; cyclic voltammograms of (NH4)2Mo3S13; SEM images of electrodeposited Mo3S13 films and EDS element mappings; the k-space curves for the electrodeposited Mo3S13 films and (NH4)2Mo3S13·2H2O; electrical equivalent circuit for the simulation of all modified electrodes; comparison of HER performance of the related works. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected].

ACKNOWLEDGMENTS We appreciate the National Natural Science Foundation of China (No. 21475003, 21675003, 11375229 and 21335001) for financial support. X-ray absorption spectroscopy characterization was performed on Beijing Synchrotron Radiation Facility (BSRF). The authors gratefully acknowledge the assistance of scientists of 1W1B beamline during the experiments. We thank Weizhen Li and Siyu Yao for assistance with X-ray absorption fine structure.


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X.;

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GRAPHICAL TABLE OF CONTENT


20

Electrodeposited Mo3S13 Films from (NH4)2Mo3S13Â·2H2O for

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