A highly active molybdenum multisulfide electrocatalyst for the hydrogen evolution reaction

Abstract

We prepared a highly active molybdenum multisulfide (MoS_x) hybrid material by an arc-melting method, which exhibits superior activity in acid media with a low overpotential of 156 mV at j = 10 mA cm⁻², and a Tafel slope of 58 mV dec⁻¹, surpassing most existing molybdenum-sulfide-based catalysts.

---

The hydrogen evolution reaction (HER) from electrocatalytic water splitting offers a promising strategy to produce H₂.¹ However, the most efficient HER catalysts so far, platinum (Pt) and Pt-based material, are expensive and scarce, which impede their scale-up deployment.² To develop a scalable, environmentally friendly production of an inexpensive, highly active, stable HER catalyst remains a challenge.³ To replace the noble metal catalysts, cost-effective and active HER catalysts have been explored, including Ni and Ni-based alloys, transition metal sulfides, nitrides, phosphides, carbides, and selenides, as well as non-metal composites.^2d,4 Particularly, MoS₂ materials have been intensely studied for HER applications owing to its low cost, earth abundance, high catalytic activity and good stability.⁵ Many effort, such as maximally exposing edge sites using nanostructured MoS₂, introducing defects, and chemical doping, has been made to improve the performance of MoS₂ as HER catalysts.⁶ Dai et al. developed a solvothermal method to fabricate a MoS₂/graphene hybrid material, showing a remarkable HER.⁷ Zheng et al. reported that the monolayer 2H-MoS₂ by introducing sulphur (S) vacancies and straining yielded the optimal hydrogen adsorption free energy (ΔG_H) equivalent to 0 eV, leading to higher HER activity.⁸ New active phase of molybdenum multisulfide (MoS_x) has also been investigated as HER electrodes. However, they still exhibited poor performance compared with MoS₂ materials.⁹ Hu et al. reported that electrochemically-deposited MoS_x materials were efficient HER catalysts for the first time, showing significant geometric current densities (η = 242 mV at j = 10 mA cm⁻²).¹⁰ Markovic et al. combined the higher activity of CoS_x building blocks with the higher stability of MoS_x units into a compact and robust CoMoS_x chalcogel structure, to design a low-cost alternative for HER in both alkaline and acidic environments.¹¹ Moreover, previous reported procedures for synthesizing these catalysts usually involved low efficient fabrication process, like ultrahigh-vacuum processing, high-temperature treatment, sulfidization using H₂S gas, and electrodeposition, severely limiting the scale-up production of molybdenum-sulfide-based catalysts.¹²

Recently, we have developed a simple and efficient arc-melting technique for scale-up defective solid oxides production.¹³ The typical arc-melting process involves two steps: (1) the materials can be immediately heated beyond their melting point; (2) the molten can be subsequently cooled down to room temperature in a few seconds.¹⁴ We propose that such extremely high cooling process may also provide possibility to implant defects in molybdenum sulfides. Herein, we use the efficient and straightforward arc-melting method to fabricate a new molybdenum multisulfide (Mo₂S₃ and Mo₃S₄) hybrid material, introducing abundant active sites via the formation of defects within this material, leading to a significant improvement of the hydrogen evolution activity.

The A-MoS_x catalysts were prepared by the one-step arc-melting method. Firstly, the original MoS₂ powders were pressed into pellets and then were arc-melting treated in the closed arc furnace filled with Ar. During the treatment, the MoS₂ pellets were rapidly heated up to a high temperature over 3000 °C and melted, then cooled to room temperature within a few seconds on a copper substrate (maintained at 15 °C). The as-prepared black particles were ground to powders, followed by ball-milling.

SEM image in Fig. 1a portrays that the morphology of A-MoS_x differs greatly from the original MoS₂, transforming into particles from layer structure (Fig. S1†). The majority of the particles are in the range of 50–500 nm, quite consistent with particle size analysis result (Fig. 1b). The average particle size of A-MoS_x is 224 nm, a little bigger than that of MoS₂ (Fig. S2†). Fig. 1c shows the X-ray diffraction (XRD) pattern of the as-synthesized A-MoS_x, indicating that the sample is composed of a mixture of several molybdenum multisulfide, in which Mo₂S₃ and Mo₃S₄ are dominate components. Notably, the intensity of the diffraction peaks from the MoS₂ obviously weakened after arc-melting treatment, indicating an almost complete conversion from MoS₂ to other molybdenum multisulfide. Transmission electron microscopy (TEM) was also applied to examine its morphology. The high-resolution TEM (HR-TEM) graph (Fig. 1d and f) exhibits a well-arrayed (101) plane of Mo₃S₄ with plane distance of 0.644 nm and (112) plane of Mo₂S₃ with a plane distance of 0.244 nm, quite consistent with XRD.

Fig. 1 (a) SEM image of the A-MoS_x. (b) Size analysis of the A-MoS_x. (c) XRD pattern of the A-MoS_x and MoS₂. (e) TEM image of the A-MoS_x. (d and f) HRTEM image of the A-MoS_x.

Raman and photoluminescence (PL) spectra further illustrate the structure changes from MoS₂ to A-MoS_x. As shown in Fig. 2a, the broadening of both A_1g and E¹_2g peaks in Raman spectra compared to MoS₂ are observed. Furthermore, after arc-melting, the peak position of A-MoS_x shifts from 407.91 cm⁻¹ to 405.02 cm⁻¹ for A_1g mode, and from 383.44 cm⁻¹ to 378.76 cm⁻¹ for E¹_2g mode. These results confirm the lattice distortion.¹⁵ Fig. 2b shows a PL image of A-MoS_x and MoS₂. The PL intensity of A-MoS_x obviously decreases, suggesting that defects and cracks were formed after arc-melting.^15,16 Fig. 2c and d shows the XPS profiles of Mo-3d and S-2p acquired from the MoS₂ and A-MoS_x, respectively. Compared to MoS₂, XPS spectrum of A-MoS_x shows variations in the observed more Mo valence states and core-level spectra with low binding energy feature peak (161.5 eV) being detected for S 2p core-levels. Fitting of the A-MoS_x Mo-3d data reveals four species: Mo²⁺ (229 eV), Mo³⁺ (230 eV), Mo⁶⁺ (232 eV), Mo⁴⁺ (236 eV).¹⁷ The peak of Mo²⁺, Mo³⁺ and Mo⁴⁺ are assigned to Mo₂S₃, Mo₃S₄, and residue MoS₂. And the peak of Mo⁶⁺ is attributed to Mo species that have been oxidized upon air exposure. Additionally, according to the analysis of S 2p XPS spectrum, the A-MoS_x with low binding energy feature peak (161.5 eV) would appear highly defective, consistent with the Raman and PL results.¹⁸

Fig. 2 (a) Raman spectra of A-MoS_x and MoS₂. (b) Photoluminescence spectra of the A-MoS_x and MoS₂. XPS spectra of (c) MoS₂; (d) A-MoS_x.

The catalytic performance for the HER of the catalysts was tested in 0.5 M sulfuric acid electrolyte using a three-electrode setup in a static state without rotation to mimic real industrial operation. Three candidates, including A-MoS_x, MoS₂, and commercial Pt/C, were examined. As shown in Fig. 3a, the prepared A-MoS_x displays a much lower overpotential, 156 mV at the current density of 10 mA cm⁻², which is more positive than original MoS₂ (330 mV at j = 10 mA cm⁻²) and even many previously reported molybdenum-sulfide-based catalysts (Table S1†), suggesting a prominent HER activity. On the other hand, the cathodic current density at η = 300 mV of the A-MoS_x catalysts is 42 mA cm⁻², which is about 5 times greater than that of the MoS₂ catalysts (7 mA cm⁻²). The Tafel plots (Fig. 3b) are used to determine Tafel slopes and exchange current densities by fitting to the Tafel equation η = b log(j) + log(j₀) (η is the overpotential, j is the current density, j₀ is the exchange current density, and b is the Tafel slope). The A-MoS_x catalysts resulted in a Tafel slope of 58 mV dec⁻¹ compared to Tafel slop of 39 and 90 mV dec⁻¹ for the Pt/C and the original MoS₂, respectively. The Tafel slope falls within the range of 40–120 mV dec⁻¹, indicating that the HER taking place on the A-MoS_x surface would proceed through a Volmer–Heyrovsky mechanism, and the desorption of hydrogen is the rate limiting step.²⁰ The HER exchange current density (j₀) for the A-MoS_x, calculated from the Tafel plots by extrapolation method, is 0.40 mA cm⁻², which outperforms that of the MoS₂ (0.19 mA cm⁻²), indicating a better catalytic activity. Fig. 3c portrays that I–V curves of the A-MoS_x are almost the same at scan rates from 5 to 300 mV s⁻¹, demonstrating its excellent stability. Electrochemical impedance spectroscopy (EIS) analyses are further performed to explore the HER kinetics at the electrode/electrolyte interface. The EIS data of the MoS₂ and A-MoS_x catalysts are illustrated by the Nyquist plots shown in Fig. 3d. The equivalent circuit we used in EIS analysis has been given in Fig. S6.† To decouple the ohmic resistance from the polarization resistance, we applied a model of ohmic resistance (R_s) in series with a module, where the polarization resistance (R_ct) is in parallel with a constant phase element (CPE).²¹ It is clearly seen that the radius of semicircle in the low frequency zone of the A-MoS_x catalysts is much smaller compared with that of the MoS₂, suggesting the better conductivity.

Fig. 3 (a) The HER polarization curves of various catalysts (Pt/C, MoS₂ and A-MoS_x) with a scan rate of 50 mV s⁻¹ in 0.5 M H₂SO₄. (b) The corresponding Tafel plots of various catalysts (Pt/C, MoS₂ and A-MoS_x). Each green dashed line is the fitting slope of the corresponding Tafel plot. (c) The polarization curves of the A-MoS_x with different scan rate from 5 to 300 mV s⁻¹ in 0.5 M H₂SO₄. (d) Electrochemical impedance spectra of the A-MoS_x and MoS₂ in 0.5 M H₂SO₄. (e) The HER current density at 10 mA cm⁻² versus overpotential for various catalysts in acid media.^{6c,7,8,12a,15a,19}

Over the last decade, molybdenum-sulfide-based materials have gained growing attention for use as HER electrocatalysts.²² We list a number of latest literature reports molybdenum-sulfide-based materials and compare their HER performance. As summarized in Fig. 3e, the A-MoS_x catalysts performs better than most of molybdenum-sulfide-based catalysts reported previously for the overpotential at j = 10 mA cm⁻² in acid media.

We further investigated the HER performance of the A-MoS_x in 1.0 M KOH. It only requires 356 mV to achieve 10 mA cm⁻² (Fig. 4a) and shows a Tafel slope of 101 mV dec⁻¹ (Fig. 4b) in basic media. These values compare favorably to the original MoS₂, which displays an overpotential of 551 mV at j = 10 mA cm⁻² and a Tafel slope of 165 mV dec⁻¹. These results suggest an enhanced HER activity. According to the Tafel plots, the HER exchange current density for the A-MoS_x is 6.30 × 10⁻³ mA cm⁻², a slightly higher than the original MoS₂ (5.01 × 10⁻³ mA cm⁻²). The I–V curves of the A-MoS_x and MoS₂ at scan rates from 5 to 300 mV s⁻¹ is also tested (Fig. 5). The A-MoS_x almost remains unchanged at different scan rates, demonstrating its excellent stability. It should be noted that molybdenum-sulfide-based electrocatalysts rarely reported to apply to HER in basic media because of the relatively lower activity in basic media due to the limited amount of hydrogen ions for proton reduction reaction in basic solution. Thus, our work represents a new break for advanced A-MoS_x electrocatalysts in a basic media for HER.

Fig. 4 (a) The HER polarization curves of various catalysts (Pt/C, MoS₂ and A-MoS_x) with a scan rate of 50 mV s⁻¹ in 1.0 M KOH. (b) The corresponding Tafel plots of various catalysts (Pt/C, MoS₂ and A-MoS_x). Each green dashed line is the fitting slope of the corresponding Tafel plot.

Fig. 5 The polarization curves of (a) the A-MoS_x; (b) MoS₂ with scan rates from 5 to 300 mV s⁻¹ in 1.0 M KOH.

The enhanced HER activity of A-MoS_x can be attributed to the following reasons. The A-MoS_x shows the better conductivity. That enables simple and effective electrical transfer to minimize parasitic ohmic loss. Also, low resistance correspond to a more favorable HER kinetic over the A-MoS_x catalysts. Most importantly, after arc-melting, large number of defects were created in the A-MoS_x. Thus it is effective to introduce more active sites for H⁺ adsorption via the formation of defects, leading to an improvement of the hydrogen evolution activity.

In conclusion, we have prepared a defective A-MoS_x catalyst, showing excellent HER active both in acid and basic media. The catalyst mainly consists of Mo₂S₃ and Mo₃S₄ and exhibits a superior activity in acid condition, with a low overpotential of 156 mV at j = 10 mA cm⁻², Tafel slope of 58 mV dec⁻¹, and a large exchange current density of 0.40 mA cm⁻². The results surpass most of MoS₂ catalysts previously reported. Our work demonstrates that the new molybdenum multisulfide hybrid material can be used as an active, low-cost, non-noble metal electrocatalyst for the HER and the arc-melting method is simple and straightforward technique to implant defects, which could be easily extended to synthesize other catalysts for the HER.

Acknowledgements

This work was supported by National Basic Research of China (Grant No. 2015CB932500 and 2013CB632702) and NSF of China (Grant No. 51302141 and 61274015).

Notes and references

1. (a) H. M. Chen, C. K. Chen, R.-S. Liu, L. Zhang, J. Zhang and D. P. Wilkinson, Chem. Soc. Rev., 2012, 41, 5654; (b) H. A. Gasteiger and N. M. Marković, Science, 2009, 324, 48; (c) H. I. Karunadasa, C. J. Chang and J. R. Long, Nature, 2010, 464, 1329; (d) M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446; (e) M. S. Dresselhaus and I. L. Thomas, Nature, 2001, 414, 332; (f) M. Zeng and Y. Li, J. Mater. Chem. A, 2015, 3, 14942; (g) X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148; (h) Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev., 2015, 44, 2060.
2. (a) V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A. Lucas, G. Wang, P. N. Ross and N. M. Markovic, Nat. Mater., 2007, 6, 241; (b) H. B. Gray, Nat. Chem., 2009, 1, 7; (c) M. Mavrikakis, Nat. Mater., 2006, 5, 847; (d) W.-F. Chen, K. Sasaki, C. Ma, A. I. Frenkel, N. Marinkovic, J. T. Muckerman, Y. Zhu and R. R. Adzic, Angew. Chem., Int. Ed., 2012, 51, 6131.
3. (a) C. G. Morales-Guio, L.-A. Stern and X. Hu, Chem. Soc. Rev., 2014, 43, 6555; (b) M. S. Faber and S. Jin, Energy Environ. Sci., 2014, 7, 3519; (c) H. Vrubel and X. Hu, Angew. Chem., Int. Ed., 2012, 51, 12703.
4. (a) E. J. Popczun, C. G. Read, C. W. Roske, N. S. Lewis and R. E. Schaak, Angew. Chem., Int. Ed., 2014, 53, 5427; (b) X. Wang, Y. V. Kolen'ko, X. Q. Bao, K. Kovnir and L. Liu, Angew. Chem., Int. Ed., 2015, 54, 8188; (c) Q. Lu, G. S. Hutchings, W. Yu, Y. Zhou, R. V. Forest, R. Tao, J. Rosen, B. T. Yonemoto, Z. Cao, H. Zheng, J. Q. Xiao, F. Jiao and J. G. Chen, Nat. Commun., 2015, 6, 6567; (d) J. Tian, Q. Liu, A. M. Asiri and X. Sun, J. Am. Chem. Soc., 2014, 136, 7587; (e) C. He and J. Tao, Chem. Commun., 2015, 51, 8323.
5. (a) A. B. Laursen, S. Kegnaes, S. Dahl and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 5577; (b) M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh and H. Zhang, Nat. Chem., 2013, 5, 263.
6. (a) D. Merki and X. Hu, Energy Environ. Sci., 2011, 4, 3878; (b) B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Nørskov, J. Am. Chem. Soc., 2005, 127, 5308; (c) D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao and Y. Cui, Nano Lett., 2013, 13, 1341; (d) J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807; (e) H. Wang, C. Tsai, D. Kong, K. Chan, F. Abild-Pedersen, J. K. Nørskov and Y. Cui, Nano Res., 2015, 8, 566.
7. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296.
8. H. Li, C. Tsai, A. L. Koh, L. Cai, A. W. Contryman, A. H. Fragapane, J. Zhao, H. S. Han, H. C. Manoharan, F. Abild-Pedersen, J. K. Norskov and X. Zheng, Nat. Mater., 2016, 15, 48.
9. (a) Y. H. Chang, C. T. Lin, T. Y. Chen, C. L. Hsu, Y. H. Lee, W. Zhang, K. H. Wei and L. J. Li, Adv. Mater., 2013, 25, 756; (b) A. B. Laursen, P. C. Vesborg and I. Chorkendorff, Chem. Commun., 2013, 49, 4965.
10. D. Merki, S. Fierro, H. Vrubel and X. Hu, Chem. Sci., 2011, 2, 1262.
11. J. Staszak-Jirkovský, C. D. Malliakas, P. P. Lopes, N. Danilovic, S. S. Kota, K.-C. Chang, B. Genorio, D. Strmcnik, V. R. Stamenkovic, M. G. Kanatzidis and N. M. Markovic, Nat. Mater., 2016, 15, 197.
12. (a) Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara and T. F. Jaramillo, Nano Lett., 2011, 11, 4168; (b) T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 2007, 317, 100.
13. G. Ou, D. Li, W. Pan, Q. Zhang, B. Xu, L. Gu, C. Nan and H. Wu, Adv. Mater., 2015, 27, 2589.
14. L. A. Jacobson and J. McKittrick, Mater. Sci. Eng., R, 1994, 11, 355.
15. (a) G. Ye, Y. Gong, J. Lin, B. Li, Y. He, S. T. Pantelides, W. Zhou, R. Vajtai and P. M. Ajayan, Nano Lett., 2016, 16, 1079; (b) N. Kang, H. P. Paudel, M. N. Leuenberger, L. Tetard and S. I. Khondaker, J. Phys. Chem. C, 2014, 118, 21258.
16. M. R. Islam, N. Kang, U. Bhanu, H. P. Paudel, M. Erementchouk, L. Tetard, M. N. Leuenberger and S. I. Khondaker, Nanoscale, 2014, 6, 10033.
17. B. Cao, G. M. Veith, J. C. Neuefeind, R. R. Adzic and P. G. Khalifah, J. Am. Chem. Soc., 2013, 135, 19186.
18. S. McDonnell, R. Addou, C. Buie, R. M. Wallace and C. L. Hinkle, ACS Nano, 2014, 8, 2880.
19. (a) D. J. Li, U. N. Maiti, J. Lim, D. S. Choi, W. J. Lee, Y. Oh, G. Y. Lee and S. O. Kim, Nano Lett., 2014, 14, 1228; (b) J. D. Benck, Z. Chen, L. Y. Kuritzky, A. J. Forman and T. F. Jaramillo, ACS Catal., 2012, 2, 1916; (c) J. Kibsgaard, Z. Chen, B. N. Reinecke and T. F. Jaramillo, Nat. Mater., 2012, 11, 963.
20. Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher and X. Wang, J. Mater. Chem. A, 2015, 3, 131.
21. (a) X. Xie, L. Lin, R.-Y. Liu, Y.-F. Jiang, Q. Zhu and A.-W. Xu, J. Mater. Chem. A, 2015, 3, 8055; (b) X. Xie, R. Yu, N. Xue, A. B. Yousaf, H. Du, K. Liang, N. Jiang and A.-W. Xu, J. Mater. Chem. A, 2016, 4, 1647; (c) C.-Z. Yuan, Y.-F. Jiang, Z. Wang, X. Xie, Z.-K. Yang, A. B. Yousaf and A.-W. Xu, J. Mater. Chem. A, 2016, 4, 8155.
22. Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher and X. Wang, J. Mater. Chem. A, 2015, 3, 131.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental section and additional experimental data. See DOI: 10.1039/c6ra18106f

‡ L. F. Zhang and G. Ou contributed equally to this work.

---