Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS2 and MoSe2 Joshua D. Wiensch,† Jimmy John,† Jesus M. Velazquez,†,∥ Daniel A. Torelli,† Adam P. Pieterick,# Matthew T. McDowell,†,§ Ke Sun,† Xinghao Zhao,# Bruce S. Brunschwig,‡ and Nathan S. Lewis*,†,‡,⊥ †
Division of Chemistry and Chemical Engineering, #Division of Engineering and Applied Science, ‡Beckman Institute, and ⊥Kavli Nanoscience Institute, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States S Supporting Information *
ABSTRACT: Single crystals of n-type MoS2 and n-MoSe2 showed higher electrocatalytic activity for the evolution of H2(g) in alkaline solutions than in acidic solutions. The overpotentials required to drive hydrogen evolution at −10 mA cm−2 of current density for MoS2 samples were −0.76 ± 0.13 and −1.03 ± 0.21 V when in contact with 1.0 M NaOH(aq) and 1.0 M H2SO4(aq), respectively. For MoSe2 samples, the overpotentials at −10 mA cm−2 were −0.652 ± 0.050 and −0.709 ± 0.073 V in contact with 1.0 M KOH(aq) and 1.0 M H2SO4(aq), respectively. Single crystals from two additional sources were also tested, and the absolute values of the measured overpotentials were consistently less (by 460 ± 250 mV) in alkaline solutions than in acidic solutions. When electrochemical etching was used to create edge sites on the single crystals, the kinetics improved in acid but changed little in alkaline media. The overpotentials measured for polycrystalline thin films (PTFs) and amorphous forms of MoS2 showed less sensitivity to pH and edge density than was observed for single crystals and showed enhanced kinetics in acid when compared to alkaline solutions. These results suggest that the active sites for hydrogen evolution on MoS2 and MoSe2 are different in alkaline and acidic media. Thus, while edges are known to serve as active sites in acidic media, in alkaline media it is more likely that terraces function in this role. the HER electrode at a current density of −10 mA cm−2 in contact with 0.5 M H2SO4(aq).16 Although MoS2 is well-established as an electrocatalyst of the HER in acidic solutions, electrocatalysis of the HER by MoS2 in alkaline electrolytes remains relatively unexplored. Active and stable electrocatalysis of the HER has been demonstrated recently for amorphous molybdenum sulfides and MoS2 nanoparticles in contact with 1 M KOH.17 Electrocatalysis of the HER by nanostructured sheets of MoS2 in contact with 0.1 M KOH and by MoS2−carbon composites in contact with 1 M NaOH has also been demonstrated.18,19 While edge sites have been established as the active electrocatalytic sites for MoS2 in contact with acidic aqueous solutions, a detailed picture of the catalytic mechanism for the HER at MoS2 surfaces in contact with aqueous alkaline solutions has not yet been established.
M
oS2 and MoSe2 are electrocatalysts of the hydrogenevolution reaction (HER).1,2 The electrocatalytic activity of MoS2 for the HER in acidic aqueous solutions has been examined in detail as part of an effort to identify a nonprecious metal alternative to Pt for use in watersplitting devices.3,4 MoS2 and MoSe2 possess a layered crystal structure, with the predominant catalytically active sites for the HER in acidic solution identified as the exposed step edges.5 Improvement of this class of electrocatalysts has focused on a number of strategies, including increasing the density of electrochemically accessible step edges,4,6−8 engineering defects,9,10 and utilizing phase transformations.7,11,12 Many improvements in MX2-based HER catalysis have occurred in a short period of time, and these improvements have been detailed in recent comprehensive reviews.13−15 In one system, when MoS2 nanoparticles are loaded onto an electrode and the number of active edge sites per geometric surface area is maximized, an overpotential, η, of −150 mV is required to drive © XXXX American Chemical Society
Received: August 4, 2017 Accepted: August 28, 2017
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Figure 1. Voltammograms of single crystals of n-MoS2 (left) and n-MoSe2 (right) in acid and base. Scan rate: 50 mV s−1.
of n-MoS2 samples showed no change as a result of electrolysis in acidic or alkaline electrolytes (Figure S2), indicating that the MoS2 was stable in both the acidic and alkaline electrolytes at the applied potentials. XPS of the MoS2 electrodes used in this work before and after electrochemical experiments in H2SO4 or NaOH showed no detectable contamination by other transition metals on the surface (Figure S3). The decrease in the absolute overpotentials required to drive the HER with n-MoS2 and nMoSe2 single crystals in alkaline electrolytes relative to acidic electrolytes is thus intrinsic to the n-MoS2 and n-MoSe2 samples. To quantify and compare the effects of pH on the kinetics of HER at MoS2 and MoSe2 surfaces, the electrocatalytic activities of at least four single-crystalline samples were compared from each of three different sources: natural; commercial synthetic; and n-type crystals grown in our lab. The overpotential, η, is defined as the value required to achieve a current density of −10 mA cm−2 based on the geometric surface area of the electrode, and we define ηacid and ηbase as the values of η measured in acid and base, respectively. The difference is thus Δη = ηacid − ηbase. Single crystals of MoS2 showed improved kinetics in base relative to that in acid (Table 1), Δη < 0, but the magnitude of the effect was highly sample dependent (Figure S4) and generally was greater for MoS2 than for MoSe2. The ∼−60 mV improvement in HER kinetics for n-MoSe2 in base relative to that in acid was much smaller than the ∼−260 mV improvement for n-MoS2 crystals, with Δη for n-MoSe2
Bulk single crystals are among the worst MX2-based hydrogen-evolution catalysts with respect to the overpotential required to drive the HER at a particular current density. For example, bulk single crystals of MoS2 possess negligible catalytic activity for the HER when operated in contact with acidic or neutral aqueous solutions, typically requiring the application of potentials more negative than −1.0 V vs the reversible hydrogen electrode (RHE) to drive the HER at a current density of −10 mA cm−2, whereas MoS2 nanoparticles require an overpotential of −150 mV to drive the HER at the same current density.16,20,21 However, the well-defined surfaces of single crystals provide unique opportunities for the rigorous study of electron−solution interfaces, active sites, and mechanisms of electrocatalysis.22 We compare herein the electrocatalytic activity of single crystals of MoS2 and MoSe2 for the HER in aqueous acidic and alkaline solutions and compare the dependence of their electrocatalytic activity on the density of step edges created by electrochemical etching. Furthermore, we compare the electrocatalytic activity of thin films of MoS2 and MoSe2 in acid and base. Single crystals of n-MoS2 and n-MoSe2 were grown in our laboratory using a chemical vapor transport (CVT) method known to produce the hexagonal (2H) crystal structure23 and were examined along with natural and commercial synthetic crystals of 2H-MoS 2 . Detailed descriptions of sample preparation, including synthesis of n-MoS2 and n-MoSe2 single crystals, instruments, and electrochemical experiments are provided in the Supporting Information. Figure 1 compares the HER activity of single-crystalline nMoS2 (left) and n-MoSe2 (right) samples operated in contact with either an aqueous acidic electrolyte (1.0 M H2SO4) or an aqueous alkaline electrolyte (1.0 M NaOH or 1.0 M KOH). When operated in contact with the acidic electrolyte, the nMoS2 sample required an absolute overpotential greater than 1.25 V to drive a current density of −10 mA cm−2 but required an absolute overpotential of approximately 0.80 V to drive the same current density when operated in contact with the alkaline electrolyte. The n-MoSe2 sample required overpotentials of −0.78 and −0.62 V to drive a current density of −10 mA cm−2 when in contact with acidic and alkaline electrolytes, respectively. Thus, the electrocatalytic activities of n-MoS2 and n-MoSe2 crystals in alkaline electrolytes were higher than their activities in an acidic electrolyte. Real-time mass spectroscopy confirmed the production of H2(g) with no detectable H2S by the n-MoS2 crystals, when operated in contact both with acidic and with alkaline electrolytes (Figure S1). X-ray photoelectron spectra (XPS)
Table 1. Comparison of the Electrocatalytic Activity for the HER of MoS2 and MoSe2 Single Crystals from Various Sources in Acidic and Alkaline Media (Δη = ηacid − ηbase)a crystal type n-MoS2 n-MoSe2 natural molybdenite commercial MoS2 MoS2 MoSe2 MoSx MoSex
ηacid/V
ηbase/V
−1.02 ± 0.21 −0.76 −0.71 ± 0.07 −0.65 −1.58 ± 0.65 −0.97 −1.48 ± 0.11 −0.95 Polycrystalline Thin Films −0.511 ± 0.024 −0.441 −0.492 ± 0.029 −0.557 Amorphous Materials −0.264 ± 0.010 −0.628 −0.238 ± 0.011 −0.393
± ± ± ±
Δη/V
0.14 0.05 0.49 0.24
−0.26 −0.06 −0.62 −0.53
± 0.054 ± 0.007
−0.07 +0.065
± 0.018 ± 0.006
+0.36 +0.16
a
A total of 4−7 single crystal samples, 4 polycrystalline samples, and 3 amorphous samples of each type were used to obtain average overpotential and error data. The overpotential is listed as the voltage versus RHE required to achieve a −10 mA cm−2 current density.
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were still more active for the HER in 1.0 M NaOH(aq) than in 1.0 M H2SO4(aq). The effect of edge-site density on the electrocatalytic activity of MoS2 and MoSe2 in acidic and alkaline electrolytes was also investigated using samples that had substantially less crystallinity than the three types of single-crystal samples. Specifically, the HER behavior was determined for polycrystalline thin films (PTFs) of MoS2 or MoSe2 deposited on n+-Si,26 as well as for amorphous films of MoSx and MoSex. Data were collected from 3 to 7 samples of each type.27 Figure 3 summarizes the overpotentials measured for synthetic, PTFs, and amorphous MoS2 and MoSe2 in acidic and alkaline media, respectively. The overpotential difference, Δη, for PTF-MoS2 was much smaller than that for the n-MoS2 samples, with Δη = −0.07 and −0.27 V, respectively. Furthermore, amorphous MoSx samples showed a reversal of the pH effect, exhibiting a more negative (larger) overpotential in base than in acid (Δη = +0.36 V). For both PTF-MoSe2 and amorphous MoSex, Δη was >0. For the selenides, the PTF-MoSe2 overpotential difference (Δη = +0.07 V) was small but positive, although this difference was within the sample-to-sample error of the measurements, whereas for amorphous MoSex samples, Δη = +0.155 V. Figures S6 and S7 show Tafel plots for single crystals, PTF, and amorphous samples operated in contact with acid or base. As observed for iteratively etched single crystals, the direction and magnitude of the pH effect were largely determined by the improvement in the HER kinetics of MoS2 in acid as the crystallinity of the sample decreased (ηacid decreased from 1.03 V for n-MoS2 to 0.51 V for PTF-MoS2 and to 0.26 V for amorphous MoSx). Under alkaline conditions, for MoS2 materials, the variation in η base with crystallinity was substantially smaller than in acid and was nonmonotonic with a minimum value of ηbase (−0.44 V) observed for PTF-MoS2 samples. The smaller change in ηbase compared to ηacid as the crystallinity and number of step edges were varied is consistent with the behavior observed using iterative etching. Similarly, the variation in Δη correlated with changes in crystallinity for MoSe2, with the effect of crystallinity on ηbase being much weaker than the effect of decreased crystallinity on ηacid. These results are consistent with the hypothesis that, under alkaline conditions, strong hydroxide adsorption to metalterminated edge sites inhibits the formation of a Mo−H species through which hydrogen evolution presumably takes place in
falling within the sample-to-sample error of the measurement. The natural and commercial synthetic MoS2 crystals exhibited higher resistivities than the n-MoS2 and n-MoSe2 crystals and thus exhibited higher absolute overpotentials in both acid and base due to the higher uncompensated resistances relative to the doped n-type crystals. The effects of increased edge-site densities on Δη were investigated by electrochemically etching the single crystals of MoS2 at potentials between 0 and 1.5 V in 1.0 M H2SO4(aq). Anodic etching of MX2 surfaces is a well-known process that initiates at defect sites and proceeds anisotropically along the basal plane leaving triangular etch pits on the surface.24,25 Figure S5 shows optical images of the crystals before and after etching, showing an increase in microscopic step edges and wrinkles on the surface apparently from partial delamination of the layered crystals. The crystals were iteratively etched and tested for HER activity in acid or base. As a measure of the effect of increases in the step density, Figure 2 shows the
Figure 2. Effect of electrochemical etching on the HER catalytic activity of a natural single crystal of MoS2 in acid (0.10 M H2SO4(aq)) and base (1.0 M NaOH(aq)). η is the overpotential needed to achieve a current density of −10 mA cm−2.
overpotential at −10 mA cm−2 to assess the electrocatalytic activity of a natural MoS2 crystal in contact with acidic or alkaline electrolytes. Prior to etching this particular crystal, the value of ηacid was −1.20 V and ηbase was −0.49 V. After etching once, ηacid = −1.03 V and ηbase = −0.51 V, whereas after two rounds of etching, ηacid = −0.89 V and ηbase = −0.53 V. Etching improved the HER kinetics for MoS2 crystals in contact with 1.0 M H2SO4(aq) by ∼0.3 V, consistent with step edges being the catalytically active sites.5 However, the etching barely changed the overpotential in base, implying that etching did not substantially change the number of sites that are catalytically active for the HER in base. Even after etching, the electrodes
Figure 3. Effect of crystallinity on the kinetics for the HER on MoS2 and MoSe2 materials in 1.0 M H2SO4(aq)) or 1.0 M base (NaOH(aq) for n-MoS2 and PTF MoS2; 1.0 M KOH(aq) for amorphous MoSe2). η is the overpotential at −10 mA cm−2 of current density, and Δη= ηacid ηbase (where ηacid and ηbase are values of η in acid and base respectively). The mean values of the quantities, η and Δη are plotted, and error bars are ±1 standard deviation. 0.1 M H2SO4(aq) and 1.0 M NaOH(aq) were used to test amorphous MoSx samples. 2236
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acid. Instead, in alkaline solution, hydrogen evolution may proceed predominantly through X−H (i.e., sulfhydryl and selenohydryl groups) moieties, which are the dominant functionalities present on terrace sites. These hydrogenbound-to-chalcogen motifs have been observed by neutronscattering experiments after adsorption of hydrogen on MoS2 surfaces.28 Additionally, the activity of Mo−S-containing formate dehydrogenase is characterized by a mechanism involving a MoS−H motif and multiple proton and electron transfers, and the activity is substantially inhibited by an increase in pH.29 At the same Mo site in formate dehydrogenase, the rate of solvent exchange decreases substantially at higher pH.29 This mechanism invokes the dominant functionality on the surface of single-crystalline MX2s as active sites for the HER, while the active sites for HER in acid are step edges that account for only a very small fraction of the threedimensional surface area. Therefore, this proposed mechanism implies that the density of active sites for the HER on singlecrystalline samples is orders of magnitude higher in base than in acid and that the turnover frequency per active site in acid is much higher than in base. The active site under alkaline conditions could also present itself as single chalcogen vacancies on terraces of MX2s. Chalcogen vacancies at terrace sites of single-layer and polycrystalline MoS2 have been proposed to have catalytic activity for the HER.9,30−32 The dominant hydrogen-evolution site shifts from the edges in acid to terraces in alkaline media. The proposed active sites do not depend on the density of open transition-metal sites at edges as neither the number of chalcogen vacancies nor exposed sulfur or selenium sites are strongly correlated with crystallinity. In summary, single crystals of MoS2 or MoSe2 exhibited enhanced kinetics for the HER in contact with alkaline solutions relative to acidic solutions. Although in acidic solutions the hydrogen-evolution activity was sensitive to the crystallinity of MoS2 materials, with decreasing crystallinity improving the HER activity, in contact with alkaline solutions the electrocatalytic activity of MoS2 materials was relatively insensitive to the crystallinity of the materials. The electrocatalytic activity of MoSe2 was also less sensitive to the crystallinity of the materials in alkaline solutions than in acidic solutions. These results suggest that in acid and alkaline solutions the HER reactions on MoS2 and MoSe2 proceed through different active sites and likely through different mechanisms. Although in acidic media the reaction processes are associated with edge sites, in alkaline solution the terraces are likely more active.
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Matthew T. McDowell: 0000-0001-5552-3456 Xinghao Zhao: 0000-0001-9229-7670 Nathan S. Lewis: 0000-0001-5245-0538 Present Addresses ∥
J.M.V.: Department of Chemistry, UC Davis, CA 95616. M.T.M.: Woodruff School of Mechanical Engineering and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332. §
Author Contributions
J.D.W. and J.J. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.J. thanks the Camille and Henry Dreyfus Foundation for support through its postdoctoral fellowship program in environmental chemistry. This material is based upon work supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-FG02-03ER15483. J.M.V. acknowledges support through an NRC Ford Foundation Postdoctoral Fellowship. D.A.T. acknowledges support through the NSF Graduate Research Fellowships Program. Single-crystal MoS2 growth and sputtering of Mo was performed at the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993. X-ray photoelectron spectroscopy was performed at the Molecular Materials Research Center of the Beckman Institute at Caltech.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00700. Details of the synthetic methods followed for preparing various MoS2 and MoSe2 materials studied; procedures for making electrodes; experimental protocols for the electrochemical measurements; instrumentation; and cyclic voltammograms obtained on and optical images of electrochemically etched MoS2 single crystals (PDF) 2237
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