Unraveling the Role of Lithium in Enhancing the Hydrogen Evolution Activity of MoS2: Intercalation versus Adsorption

Molybdenum disulfide (MoS2) is a highly promising catalyst for the hydrogen evolution reaction (HER) to realize large-scale artificial photosynthesis. The metallic 1T′-MoS2 phase, which is stabilized via the adsorption or intercalation of small molecules or cations such as Li, shows exceptionally high HER activity, comparable to that of noble metals, but the effect of cation adsorption on HER performance has not yet been resolved. Here we investigate in detail the effect of Li adsorption and intercalation on the proton reduction properties of MoS2. By combining spectroscopy methods (infrared of adsorbed NO, 7Li solid-state nuclear magnetic resonance, and X-ray photoemission and absorption) with catalytic activity measurements and theoretical modeling, we infer that the enhanced HER performance of LixMoS2 is predominantly due to the catalytic promotion of edge sites by Li.

M olybdenum disulfide (MoS 2 ) has demonstrated significant potential to replace noble-metal-based catalysts in electrochemical hydrogen production. Like other transition-metal dichalcogenides (TMDs), MoS 2 can exist in different polymorphs, that is, the 2H (trigonal prismatic D 3h ), 1T′ (octahedral O h ), and 3R (rhombohedral C 3v 5 ) phases. 1 By tuning the arrangement of the S atoms, MoS 2 can convert from the semiconducting 2H to the metallic 1T′ phase. Such a rearrangement of S atoms is typically caused by interlayer atomic plane gliding induced by electron donation or the intercalation of small molecules or cations. 2−6 Alkali metal cations, especially Li, are typically used to intercalate between the MoS 2 layers to induce the 2H to 1T′ phase conversion. 7 Despite many years of study of lithium-intercalated MoS 2 (1T′-Li x MoS 2 ), the 1T′ phase is metastable and can easily change to the 2H phase. 7−9 Furthermore, the quick hydration of Li in aqueous solution makes the stable operation of 1T′-MoS 2 under HER conditions challenging. 10−13 Upon Li intercalation, the crystal structure of MoS 2 is modified, shown by the emergence of broad diffraction peaks and a distinct red shift of Raman modes. 14 However, previous works have mostly only assumed a 2H to 1T′ phase conversion upon the adequate intercalation of Li ions without paying further attention to the behavior of adsorbed Li. 14,15 Even though there are several theoretical works in the literature investigating the structural transitions in MoS 2 monolayers induced by Li adsorption, 16−18 a systematic experimental study on the effect of Li adsorption is still lacking. Whereas, for instance, trace metal impurities are known to play an active role in determining the electrocatalytic properties of graphene, 19 the role of adsorbed Li in the MoS 2 -catalyzed HER remains ambiguous.
Since the discovery of 1T′-MoS 2 , it has emerged as a promising candidate for a broad range of applications, including photocatalysis, supercapacitors, and, in particular, as an electrocatalyst for the hydrogen evolution reaction (HER). 3,20−22 Bulk 2H-MoS 2 is a poor HER catalyst because the reaction is limited by the density of active sites, which are concentrated at the layer edges or edge-like defect sites on the (0001) basal planes. 23 Significant research efforts have been devoted to synthesis strategies that can expose more active (edge) sites to enhance the overall HER performance, for example, nanoparticulate MoS 2 , nanostructured MoS 2 , or MoS 2 basal planes with sulfur vacancies. 24−26 In contrast with its 2H counterpart, the significant catalytic improvement toward HER of 1T′-MoS 2 has been ascribed to the intrinsic activity of its basal planes. 8 Numerous studies have reported the structural change (extensive layer displacement or bond distortion) of 2H-MoS 2 to 1T′-MoS 2 after Li intercalation, and density functional theory (DFT) modeling suggests that the catalytic improvement of Liintercalated MoS 2 can be attributed to octahedral and distorted MoS 2 phases. 13,27−31 Nonetheless, a more direct role of the Li ions, which are inevitably present in 1T′-MoS 2 , in HER catalysis has never been shown. In most cases, excessive amounts of Li are used to induce the 2H-1T′ structural transformation. 11,15,32 However, considering that the local surface chemistry governs the catalytic performance, both excess Li and Li adsorbed on the catalysts may play a vital role during the catalytic reaction as well.
Here we report a study of the role of Li in the MoS 2 -catalyzed HER. The influence of Li adsorption on the MoS 2 2H-to-1T′ phase transformation was systematically investigated by X-ray photoelectron (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopies. With the assistance of insitu IR spectroscopy with NO as a probe molecule as well as 7 Li MAS nuclear magnetic resonance (NMR) spectra, we were able to identify the interaction between Li ions and MoS 2 . Interestingly, Li-adsorbed 2H-Li x MoS 2 (0 < x < 0.5) presents much higher activity than 1T′-Li x MoS 2 (x ≈ 1 or 2), which sheds new light on understanding the intrinsic activity of lithiated TMDs. This systematic investigation on the adsorption and promotion effects of Li on MoS 2 in the electrocatalytic HER will provide a new platform for designing effective TMD-based catalysts.
We adopt a typical impregnation method to prepare a series of carbon-supported Li x MoS 2 catalysts with a precisely controlled Li content ( Figure S1). As indicated in Figure 1a, Li is expected to preferentially adsorb on the surface of MoS 2 at low concentrations, whereas at high Li concentrations, the structure undergoes a transformation from 2H-to 1T′-MoS 2 . 17 HR-TEM images (Figure 1b,c and Figure S2) show that the molybdenum sulfide phase is well distributed across the carbon support, and the influence of particle dispersion upon Li addition on HER activity could be ruled out. Because the catalytic activity of MoS 2 is known to be significantly enhanced by edge-terminated surfaces, 24,33,34 we predict here through first-principles (DFT) calculations the surface formation energy of a (0001) monolayer of 1T′-MoS 2 with a pristine Mo edge and how it is stabilized through adsorbed Li atoms in increasing concentration (Li x MoS 2 ). As shown in Figures S3 and S4, Li adsorption on the Mo-edge surface is found to have a stabilizing effect on the monolayer, as reflected in the monotonic decrease in the surface formation energies with increasing adsorbed Li concentration. The stabilization of the Mo-edge monolayers can be rationalized by considering the fact that the adsorption acts to coordinate the Li to the under-coordinated Mo ions, thus providing a closer match to the bulk coordination of the edge species. Moreover, we characterized the electronic structure of Li x MoS 2 by means of X-ray photoelectron spectroscopy (XPS) (Figure 1d,e, Figures S5 and S6, and Tables S4 and S5). The Mo 3d core-level spectra present a shift to lower binding energy for Li 1.00 MoS 2 and Li 2.06 MoS 2 as compared with samples with lower Li loading, indicating the formation of 1T′-MoS 2 . 3,35,36 Because the S 2p binding energy of 1T′-phase sulfur overlaps with that of Li 2 S, 3,32,33 we cannot quantify the amount of 1T′ phase present based on the S 2p spectra. Consistently, our DFT-calculated core-level binding-energy shifts (Table S5)  Solid-state 7 Li MAS NMR has been used to study the local coordination environments of Li in the Li x MoS 2 samples. As indicated in Figure 2a, the chemical shift at around −7 ppm reveals the interaction between Li and MoS 2 , which is distinctly different from Li adsorbed on a carbon support (Li/C−He) or mobile Li 2 S (Li/C−H 2 S) species; the latter are presumed not to interact with MoS 2 . Furthermore, on the basis of a deconvolution of the quantitative NMR spectra ( Figures S7  and S8), we have analyzed the composition of the Li species. Table 1 (Table S6) indicate that the adsorption process is characterized by a charge transfer from the Li atoms to the S and Mo atoms. Consistently, from the differential charge density isosurface plots in Figure 3g, where the pink and cyan blue contours indicate an electron density increase and decrease by 0.02 e − Å −3 , respectively, it is obvious that the electron densities of the Li atoms (cyan contours) were transferred to the S-2p and Mo-3d orbitals (pink contours) in the process of Li adsorption. The electron transfer from the Li atoms to the S and Mo atoms is responsible for the observed distortions in the Mo−S and Mo−Mo bonds of the Li x MoS 2 monolayers, as obtained from EXAFS fitting ( Table 2) and confirmed by DFT results (Table S7).
The HER performance of different Li x MoS 2 catalysts on glassy carbon was evaluated using a standard three-electrode   Figures S14 and S15). The polarization curves (Figure 4a) show that a small amount of Li adsorption (Li 0.14 MoS 2 and Li 0.29 MoS 2 ) greatly decreases the onset overpotential and improves the current density for HER as compared with pure MoS 2 . Interestingly, the cathodic current was lower in the case of Li 0.48 MoS 2 and decreased sharply for Li 1.00 MoS 2 and Li 2.06 MoS 2 . Tafel slopes in Figure 4b reveal the same trend, that is, that an optimum amount of Li loading dramatically improves the HER activity (lower Tafel slope and higher cathodic current density), whereas an excess of Li hinders the electrocatalytic reaction. To quantify the catalytic activity, we measured the actual number of active sites using the IR NO titration method (for further details see the experimental section of the SI). On the basis of this method, we have determined the number of active sites to be ∼3.0 × 10 15 sites cm −2 (based on geometric electrode area; Table S8 and Figures S16 and S17). The turnover frequency (TOF) (s −1 ) of the hydrogen evolution was calculated, as shown in Figure 4c.      suggests that their hydrogen− surface bonds are neither too strong nor too weak (i.e., |ΔG H* | ≈ 0) to limit the recombination of the adsorbed H atoms to evolve molecular hydrogen via a Volmer−Tafel or Volmer−Heyrovsky mechanism, 48 therefore resulting in the observed increase in HER activity.

ACS Energy Letters
As Li can easily hydrolyze in H 2 O, 49 we have employed 7 Li MAS NMR spectroscopy to probe the local coordination environments of Li in the presence of H 2 O. As shown in Figure  4e, a small portion of Li migrates to the carbon support (chemical shift: around −1 ppm) for Li 0.29 MoS 2 once in contact with H 2 O, whereas most Li remains adsorbed on MoS 2 (chemical shift: ∼−7 ppm). Additionally, and in contrast with MoS 2 and Li 2.06 MoS 2 , Li 0.29 MoS 2 exhibits outstanding long-term electrochemical stability at −23 mA/cm 2 with an increase in overpotential of only 10 mV after 24 h (Figure 4f and Figures  S18 and S19). The spent Li 0.29 MoS 2 catalyst after 24 h of stability testing was further subjected to an NMR analysis. As shown in Figure 4g, the presence of Li species for Li 0.29 MoS 2 after long-term HER measurements indicates the strong interaction between Li and MoS 2 , further illustrating the promotion effects of Li adsorption for MoS 2 -catalyzed HER.
In conclusion, we have systematically employed a suite of complementary experimental and computational techniques to investigate the effect of Li adsorption on the phase conversion and HER activity of MoS 2 catalysts. The promoting effect of Li adsorption on 2H-MoS 2 in enhancing the electrocatalytic hydrogen evolution was shown for the first time. With the assistance of IR spectroscopy using NO as a probe molecule, we experimentally determined the number of active sites for Li x MoS 2 catalysts, which allowed us to determine the TOF of the catalysts. Both experimental and theoretical results indicate that, next to Li intercalation, Li adsorption plays a key role in describing the high HER activity of Li x MoS 2 electrocatalysts. Whereas Li intercalation causes a phase transition from 2H-to 1T′-MoS 2 and, with that, impacts on electronic properties such as conductivity, Li adsorption leads a promotion of the HER active edge sites by changing ΔG H* in a favorable direction. Thus the overall influence of Li in the MoS 2 -catalyzed HER appears to be more complex than initially reported. Following these results, we believe that an appropriate amount of adsorbed Li or other alkali cations on TMDs would change their corresponding electron density, resulting in a beneficial tuning of the activity in electrocatalytic reactions involving proton adsorption and reduction.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00945.
Experimental and theoretical details as well as supplementary figures and discussions (PDF) ing and Physical Sciences Research Council is acknowledged for funding (grant number EP/K009567).