Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide

Jan 6, 2016 - The apical S2– atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (ΔGH) in excess of +1 eV, and wer...
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Research Article pubs.acs.org/acscatalysis

Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction Louisa Rui Lin Ting,†,‡ Yilin Deng,†,‡ Liang Ma,†,‡ Yin-Jia Zhang,§ Andrew A. Peterson,∇ and Boon Siang Yeo*,†,‡ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 § Department of Chemistry, Brown University, 324 Brook Street, Providence, Rhode Island 02912, United States ∇ School of Engineering, Brown University, 184 Hope Street, Providence, Rhode Island 02912, United States ‡

S Supporting Information *

ABSTRACT: The catalytic activities of sulfur sites in amorphous MoSx for the electrochemical hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H2SO4 electrolyte. Using X-ray photoelectron spectroscopy and linear sweep voltammetry, we found the turnover frequency for H2 production to increase linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S2− and/or bridging S22−. To distinguish the catalytic performances of these two types of atoms, we turn to quantum chemical simulations using density functional theory. The apical S2− atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (ΔGH) in excess of +1 eV, and were thus ruled out as reaction sites for HER. In situ Raman spectroscopy of the model [Mo3S13]2− cluster further demonstrate the higher catalytic reactivity of the bridging S22− over terminal S22− (which have lower electron binding energy) for proton reduction. KEYWORDS: amorphous molybdenum sulfide, hydrogen evolution reaction, electrocatalysis, linear sweep voltammetry, X-ray photoelectron spectroscopy, density functional theory At η ≈ 200 mV, amorphous MoSx was shown to catalyze HER at TOFs of 0.3 s−1 and give a H2 current of ∼10 mA/cm2.12 Further improvements to their HER activities could be made by depositing them onto highly conductive substrates or preparing films with large electrochemically active surface areas.12−15 The inclusion of Fe, Co, and Ni dopants could also improve the HER activities of the MoSx films. Based on these considerations, amorphous MoSx catalysts are arguably more costeffective and suitable for large-scale water splitting operations, compared to single-layer MoS2 polygons supported on Au substrates. The structure of amorphous MoSx has been proposed to be polymeric (Figure 1). The structure of one model contains chains of MoVS6 octahedra units (Figure 1a).16 Another proposition consists of aggregates of Mo3IV−S clusters (Figure 1b).17 The S atoms in these polymers are known to be in various bonding environments, namely, bridging S22−, apical S2−, unsaturated S2−, and terminal S22− (differentiated by their colors in Figure 1). However, the degree of catalytic activity

1. INTRODUCTION The electrolysis of water using solar electricity is a sustainable and green method for producing hydrogen gas fuel.1,2 Protons are reduced to H2 in acidic electrolytes via 2H+ + 2e− → H2 (the hydrogen evolution reaction, HER). This process is most effectively catalyzed by platinum metal.3 However, platinum is one of the rarest and most expensive elements on Earth. Therefore, extensive efforts have been devoted to the development of HER catalysts based on Earth-abundant materials. A highly promising alternative to platinum is crystalline molybdenum disulfide (MoS2), which has stacked S-Mo-S layers similar to graphite.4−6 Single-layer MoS2 polygons deposited on Au(111) electrodes catalyzed H2 production with turnover frequencies (TOFs) of 10 s−1 per active edge site at an overpotential (η) of 160 mV.4,7 However, the synthesis of such active MoS2 catalysts typically requires ultrahigh vacuum apparatus, sulfidation using corrosive H2S and annealing at high temperatures.4,8 This makes them less scalable. Amorphous molybdenum sulfides (MoSx), on the other hand, can be easily prepared by electrodeposition (using a [MoS4]2− precursor) onto virtually any kind of working electrode under ambient temperatures and pressures.9−11 They can also be synthesized via wet chemistry methods.12 © XXXX American Chemical Society

Received: October 21, 2015 Revised: December 17, 2015

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nanoparticles (see Figures 2a and 2b). Their morphologies were consistent with past imaging data of electrodeposited

Figure 1. Proposed structures for amorphous MoSx (a) Mo2S9 chain unit and (b) Mo3 cluster units in a polymeric chain. The different types of sulfur ligands are indicated in different colors: bridging S22− (green), apical S2− (blue), unsaturated S2− (red), and terminal S22− (gold).

exhibited by these S atoms for proton reduction is less certain. Hu and co-workers reported that anodic electrodeposition produced amorphous MoS 3 which they believe is a precatalyst.18 This material was reduced to the active form of MoS2+x during HER. On the basis of in situ X-ray absorption spectroscopy data, the terminal S22− units in these films have been recently shown to be involved in proton reduction.19 The rate-limiting step of the HER was thought to be the reduction and protonation of these terminal S22− units. However, in another study of amorphous MoSx grown on graphene-coated nickel foams, bridging S22− and/or apical S2− have been implicated as active species.20 So far, theoretical simulations have shown that the proton reduction activities of various MoSx samples are intimately linked to the type of S atoms present.21,22 However, there is still an absence of experimental data to characterize the HER activities of these sites. This knowledge is necessary to tailor-design MoSx materials with a higher population of active S sites, which will ultimately result in a more efficacious HER catalyst. Here, we investigate the catalytic activities of S atoms in amorphous MoSx films used in the electrochemical hydrogen evolution reaction. Films with different compositions were facilely prepared by electrodeposition and modification with cyclic voltammetry (CV) oxidative stripping. Their electrochemical HER activities were assessed by linear sweep voltammetry (LSV) in aqueous 0.5 M H2SO4 electrolyte. Xray photoelectron spectroscopy (XPS) was used to characterize the types of S atoms present in these films, which was correlated with their activity. We show that the TOF for H2 production increases linearly with the percentage of S atoms with higher electron binding energies (by XPS); these S atoms could be apical S2− and/or bridging S22−. Through electronic structure calculations, we ruled out the apical S atoms as reaction sites for HER as they adsorb H weakly, compared with a target of ΔGH = 0. These results were corroborated by in situ Raman spectroscopy of the model [Mo3S13]2− compound.

Figure 2. SEM images of (a) MoSx-AE and (b) MoSx-CE films deposited on glassy carbon electrodes (GCEs); (c) X-ray diffractograms of the MoSx-AE and MoSx-CE films.

MoSx films.9,10 Elemental analysis by energy-dispersive X-ray spectroscopy revealed that the average S:Mo atomic ratios for MoSx-AE and MoSx-CE are 2.7 (±0.1) and 1.7 (±0.3), respectively. These stoichiometries agree with the expected products produced by these respective anodic and cathodic electrodeposition processes:11,18 1 [MoS4 ]2 − → MoS3 + S8 + 2e− (anodic) 8 [MoS4 ]2 − + 2H 2O + 2e− → MoS2 + 2HS− + 2OH− (cathodic)

The amorphous nature of the films was confirmed by the absence of the relevant reflections in their X-ray diffractograms (Figure 2c). The compositional difference of the MoSx-AE and MoS x-CE films can also be ascertained during cyclic voltammetry where they oxidize to MoOx at 0.95 and 0.70 V, respectively (see Figure 3a).23 2.2. Hydrogen Evolution Activities of MoSx Films. The HER activities of freshly prepared MoSx films were assessed by linear sweep voltammetry in 0.5 M H2SO4 electrolyte (Figure 3b, 2 mV/s scan rate, all potentials in this work are cited with respect to the reversible hydrogen electrode (RHE)). Online gas chromatography was also used to probe the gas compounds formed and only H2 was detected (see Supporting Information S1). The Faradic efficiency for HER was determined to be ∼100%. A reduction peak at −0.09 V appeared in the first LSV scan of the MoSx-AE film (Figure 3b). This peak was not seen in subsequent LSV scans, which indicate that the film (or part of it) had undergone irreversible reduction.11 This reduction consisted of the removal of S atoms which changed the atomic composition of the films from MoS3 to MoS2.18 No reduction peak could be discerned in the LSV of MoSx-CE. These observations agree with previous studies of MoSx prepared by electrodeposition or wet synthesis..11,18,21 The onset potential for H2 evolution occurred at −0.12 V to −0.16 V (Figure 3b). At a representative η = 200 mV, the HER

2. RESULTS AND DISCUSSION 2.1. Materials Characterization of MoS x Films. Amorphous MoSx films were anodically or cathodically deposited onto glassy carbon electrodes (GCEs) from an aqueous electrolyte consisting of 2 mM (NH4)2[MoS4] in 0.1 M KCl. Scanning electron microscopy of the anodically deposited (MoSx-AE) and cathodically deposited (MoSx-CE) films show that both catalysts consisted of 20−30 nm sized 862

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voltammogram of the post-LSV MoSx-AE film. This affirms that the MoSx-AE film had indeed been partially reduced during the polarization scan (Supporting Information S2).11,12,18 Interestingly, the integrated charges below its CV peaks decreased by 30% after only one LSV sweep. While partial peeling of the MoSx films from the GCE could occur, the decrease in charges could be more satisfactorily, related to the loss of S atoms. To investigate how different components in a reduced MoSxAE film contribute to the HER activity, post-LSV MoSx-AE films (after one LSV scan to H2 evolving potentials) were oxidatively stripped to different anodic potentials (Supporting Information S3). LSV was then used to assess the HER activities of the remaining films (Figure 3c). Partial stripping to 0.75 V or 0.95 V was found to change their HER activities. A complete anodic stripping to 1.2 V oxidizes all the MoSx to HER-inactive MoOx, which gave a negligible HER current.24 Previous work on nanocrystalline MoS2 has also showed that performing CVs to oxidize parts of the catalyst can result in its deactivation to various degrees.23 The MoSx films exhibited similar Tafel slopes of 38−40 mV/ dec (Supporting Information S4). This suggests that H2 evolution could have occurred through a fast proton discharge H+ + e− → Hads (Volmer) step, followed by a rate-determining Hads + H+ + e− → H2 (Heyrovsky) step.11 While our measured Tafel slopes match those found for other amorphous MoSx films, it is lower than the 110−120 mV/dec value of crystalline MoS2.5,23 This variation could be attributed to structural differences between our amorphous MoSx films and that of crystalline MoS2, which consists of hexagonally packed layered structures.22 2.3. Correlating the Types of Sulfur Atoms in MoSx to Their HER Activities. Here, we present a correlation between the chemical environment of the S atoms in the MoSx catalysts with their TOF for HER (see Table 1, as well as Supporting Information S5). The TOF of H2 production is a suitable measure to compare the intrinsic catalytic activities of the MoSx films. This is especially so since the often-used current density (expressed as mA/cm2) does not consider differences in catalyst loading. The amount of H2 produced is deduced from the LSV. The active sites present in each sample was quantified from the catalyst’s cyclic voltammogram, which was recorded immediately after the LSV scan. Inaccuracies resulting from subsequent desorption and degradation of the catalyst can thus be minimized. The catalytic sites were characterized by XPS (see Figure 4). Electrodeposited MoSx films contain S atoms with electron binding energy (BE) signals at 163.8/165.0 and 162.5/163.7 eV, respectively (two sets of doublets). The higher BE signals are assigned to bridging S22− and apical S2−, while the lower BE signals are assigned to unsaturated S2− and terminal S22−.7,25 A finer distinction between the two types of S present at a particular BE is difficult.7,20 MoSx-AE has the same percentage of each type of S atoms (Table 1). However, a post-LSV MoSxAE stripped to 0.75 V has a lower percentage of S atoms with lower BE (33%−37%). This could occur because the Mo−S bonds of these S atoms are weaker, and are thus readily oxidized to MoOx during CV (electrons with lower binding energy can be more easily removed). An analogous finding has also been previously made for metal−fluorine compounds where the strength of the metal−fluorine bonds is proportional to the BE of fluorine.26 MoSx-CE has the smallest percentage of S atoms with high BE (41%). The presence of MoIV signals in

Figure 3. (a) Cyclic voltammograms of the MoSx-CE and MoSx-AE films before and after LSV polarization scans. (b) First LSV scan of MoSx-CE, first two LSV scans of MoSx-AE film, and the 300th LSV scan of MoSx-AE. (c) LSV scans of stripped MoSx-AE films.

currents of MoSx-AE films at −1.2 mA/cm2 is ∼2 times larger than that of MoSx-CE at −0.63 mA/cm2. The HER currents of MoSx-AE films decrease with subsequent LSV scans. For reference, we include the 300th LSV scan of MoSx-AE. Its HER activity is similar to that of the MoSx-CE. A cyclic voltammogram was recorded immediately after the LSV scan (Figure 3a). While the post-LSV MoSx-CE film had retained its CV profile, an additional oxidation peak emerged at 0.60 V in the 863

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ACS Catalysis Table 1. Turnover Frequencies for H2 Evolution Exhibited by Mo−S Catalysts sample

substrate

η (mV)

turnover frequency, TOF (s−1)

% higher BE sulfur

% lower BE sulfur

MoSx-CE MoSx-AE, after 300 LSV cycles MoSx-AE MoSx-AE MoSx-AE, partially stripped to 0.95 V MoSx-AE, partially stripped to 0.75 V MoSx-AE, partially stripped to 0.75 V [Mo3S4]4+ cluster [Mo3S13]2−cluster bulk MoS2

glassy carbon glassy carbon glassy carbon carbon paper glassy carbon carbon paper glassy carbon glassy carbon glassy carbon glassy carbon

200 200 200 200 200 200 200 200 200 300

0.17 0.20 0.29 0.30 0.36 0.44 0.51 0.02 0.37 5.0 × 10−4

41 44 50 51 53 63 67 25 54 0

59 56 50 49 47 37 33 75 46 100

MoV peaks (3d5/2, BE ≈ 231 eV27,28) also rules out the chainlike MoVS6 model (Figure 1a).9,29 A linear relationship was found between the TOFs for H2 production of our prepared MoSx samples (at η = 200 mV, TOF normalized to per active site) and their percentage of S atoms with higher BE (see Figure 5, as well as Table 1). For

Figure 5. Plot of the turnover frequencies of the amorphous MoSx films versus the percentage of S atoms with high electron binding energy. The orange and green points are the data for the [Mo3S13]2− and [Mo3S4]4+ clusters, respectively. The blue dotted line is drawn as a guide to the eye.

instance, when the percentage of S atoms with high BE increased from 41% (in the case of the MoSx-CE film) to 67% (in the case of the MoSx-AE film partially stripped to 0.75 V), the TOF tripled from 0.17 s−1 to 0.51 s−1. The TOFs and XPS data of reference compounds, namely, [Mo3S13]2−, [Mo3S4]4+ and bulk MoS2 (Supporting Information S6) supports this relationship. Specifically, [Mo3S4]4+, which alongside a small TOF of 0.02 s−1, has the smallest number of S atoms with higher BE (25%). The TOF of 0.37 s−1 by [Mo3S13]2−, which has 54% S atoms with high BE, is also in excellent agreement with the observed trendline. Bulk MoS2, which exhibits only S atoms with low BE, has the smallest TOF at 5.0 × 10−4 s−1, even at a higher η of 300 mV. We note here that control experiments made on MoSx deposited on carbon paper showed that the substrate does not significantly influence the HER activities. No correlation between the TOF and the intensity of the MoIV signal could be drawn (Supporting Information S7). Further evidence for the superior activity of bridging S22−, one of the two types of S atoms with high BE, was obtained

Figure 4. X-ray photoelectron spectra of representative amorphous MoSx catalysts.

the XPS data shows that the structures of MoSx-AE, MoSx-CE, and their derivatives correspond to the Mo3 cluster model depicted in Figure 1b.17 This assignment agrees with previous structural studies of electrodeposited MoSx. The absence of 864

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Figure 6. Configurations of periodic MoS2 slab, 1-D MoSx polymers, and MoSx clusters.

from the in situ Raman spectroscopy of the model [Mo3S13]2− cluster during HER (Supporting Information S8). The intensity of the clusters’ υ(S−S)bridging band at 552 cm−1 was found to decrease more significantly than the υ(S−S)terminal band at 522 cm−1 during H2 evolution.30,31 This shows that the bridging S22− is of higher reactivity than the terminal S22− for HER. Because of the insufficient limits of detection afforded by our spectrometer, we could not clearly discern the signal and, hence, the reactivity of the apical S atom (υ(Mo−S)apical = 461 cm−1). The question remains on whether there is a difference in catalytic performance of the bridging and apical S atoms toward HER. To address this, we turn to density functional theory (DFT) simulations carried out with a grid-based projectoraugmented wave (GPAW) calculator (Supporting Information S9).32 Because of the variety of motifs likely to be present in an amorphous preparation, we conducted a range of simulations using MoSx clusters and MoSx one-dimensional (1-D) polymers to approach the real structure of amorphous MoSx (see Figure 6). As a reference, a MoS2 slab (Mo edge with 0.5 ML S monomers) was also investigated. According to the local structural environment at the hydrogen binding sites, we categorized all results into five types: 1-terminal (1t), 2-terminal (2t), 1-bridging (1b), 2-bridging (2b), and apical (Figure 7). For the 1t and 1b sites, only one S atom is bonded to the corresponding Mo atom(s); for the 2t and 2b sites, another S is also bonded to the same Mo atom(s). The HER descriptor of the Gibbs binding energy of H, ΔGH, was calculated on more than 50 different binding sites and plotted by site type (see Figure 7). The same type sites show some similarities in local structure environment, although they may not have exactly the

Figure 7. ΔG of hydrogen evolution via an adsorbed H on various hydrogen binding sites.

same structure. These calculations revealed that the general order of S−H binding strengths decreases from terminal > bridging > apical sulfur sites, crossing ΔGH = 0 between the terminal and bridge sites. Apical S atoms have the weakest hydrogen binding energies >1 eV, and can thus be ruled out as HER catalytic sites. We found that the binding of H on terminal S atoms was generally too strong, which indicates that their poor HER catalytic activity could be due to the S sites being passivated by H. The bridging S22− sites are of a similar motif to the MoS2 slab edge sites with a moderately positive hydrogen binding energy, suggesting that they are probably the most active sites in the system. Our results are in good agreement with a recent theoretical study of the [Mo2S12]2− cluster, which 865

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ACS Catalysis also indicates that its bridging S22− are more active for HER, compared to its terminal S22− atoms.33 Interestingly, our simulations revealed that most S−S bonds of the terminal S22− atoms remained intact after one H atom is adsorbed, while the S−S bonds of the bridging S22− atoms broke after H adsorption. These findings are consistent with our in situ Raman spectroscopy study of the [Mo3S13]2− cluster, which showed that the intensity of its υ(S−S)bridging band decreased more significantly than its υ(S−S)terminal band during HER (Supporting Information S8). Understanding the reactivity of S catalytic sites in MoSx is an important requisite for improving its functionality for H+ reduction. We found that bridging S22− motifs (high BE), although unstable, are albeit more catalytic-active for electrochemical HER than their terminal S22− (lower BE) counterparts (see Table 1 and Figure 3, as well as Supporting Information S10). We thus suggest that further work should focus on stabilizing the bridging S22− atoms. The use of metallic dopants could lead to both improvements in the intrinsic activity and durability of the MoSx catalysts.

4.2.2. MoSx-CE. Identical to MoSx-AE, except that cathodic electrodeposition (CE) at −1.37 V for 100 s was used.34 4.2.3. MoSx-AE-stripped to 0.75 V. Partially stripped MoSx-AE films were prepared by first subjecting a freshly prepared MoSx-AE film to one LSV sweep (+0.15 V → −0.30 V at 2 mV/s) in 0.5 M H2SO4, followed by a oxidative CV scan from 0 to 0.75 V at a scan rate of 10 mV/s. The stripping process oxidized part of the films to MoOx, which will dissolve in the acidic electrolyte.23 4.2.4. MoSx-AE-stripped to 0.95 V. Identical to MoSx-AE-stripped to 0.75 V, except that the final oxidative CV scan was conducted from 0 to 0.95 V. 4.2.5. (NH4)2[Mo3S13]·nH2O. This compound was synthesized according to a published procedure.35 One milligram (1 mg) of the prepared compound was sonicated in a solution containing 800 μL of deionized water, 200 μL of ethanol, and 100 μL of carbon ink (2 mg of carbon black (Black Pearl 2000, Cabot Corp) sonicated in a solution of 800 μL of deionized water, 150 μL of ethanol, and 50 μL of Nafion 117 solution) to obtain a homogeneous ink. Five microliters (5 μL) of the ink was drop-casted onto polished GCEs and dried in air. 4.2.6. [Mo3S4(H2O)9]Cl4. This compound was synthesized according to a published procedure, and then loaded onto a Vulcan-72 carbon support.36,37 Four milligrams (4 mg) of the carbon-supported catalyst were then added to 800 μL of deionized water, 150 μL of ethanol, and 50 μL of Nafion 117 solution (Sigma−Aldrich). The mixture was sonicated to produce a homogeneous ink. Four microliters (4 μL) of this ink was drop-casted onto polished GCEs and dried in air. 4.2.7. Bulk MoS2. Two milligrams (2 mg) of bulk MoS2 (powder,