Interface confined hydrogen evolution reaction in zero valent metal nanoparticles-intercalated molybdenum disulfide

Interface confined reactions, which can modulate the bonding of reactants with catalytic centres and influence the rate of the mass transport from bulk solution, have emerged as a viable strategy for achieving highly stable and selective catalysis. Here we demonstrate that 1T′-enriched lithiated molybdenum disulfide is a highly powerful reducing agent, which can be exploited for the in-situ reduction of metal ions within the inner planes of lithiated molybdenum disulfide to form a zero valent metal-intercalated molybdenum disulfide. The confinement of platinum nanoparticles within the molybdenum disulfide layered structure leads to enhanced hydrogen evolution reaction activity and stability compared to catalysts dispersed on carbon support. In particular, the inner platinum surface is accessible to charged species like proton and metal ions, while blocking poisoning by larger sized pollutants or neutral molecules. This points a way forward for using bulk intercalated compounds for energy related applications.

, R-C s -H → R-C s -OH → R-C s =O → R-C s OOH → R-H + CO 2 (g); and IV. Catalyst deactivation of contaminant intermediates. As a result, the diameter of Pt nanoparticles after long-term stability test are much larger than that of as-received Pt/C. This is the major reason for the degraded performance of commercial Pt/C catalysts. (18,19) Supplementary Note 4 The strong dependence of R CT on the overpotential in both Tafel and semi-logarithmic plots suggests the Volmer-Tafel process of hydrogen evolution in our samples. (20)

Supplementary Note 5
Noble metals intercalated-MoS 2 are quite stable towards continuous operation while commercial Pt/C is rapidly degraded. Such improved stability can be attributed to 1) the inherently excellent mechanical resistance and stability of bulk, 2H-MoS 2 in an acidic and electroactive environment, thus it is very unlikely to be damaged by the H 2 bubbles produced from the intercalated nanoparticles; 2) the anchoring effect by adjacent MoS 2 layers that significantly reduces the dissolution or migration of Pt nanoparticles during operation, which is similar to the carbon coating methods in the literature (18,19). In such case, the leaching of Pt nanoparticles is not favorable as demonstrated by the stable HER performance and the TEM images in Supplementary Figure 16. The relatively poor stability of Pd-MoS 2 can be ascribed to the partial oxidation of Pd nanoparticles as evidenced by XPS spectra in Supplementary Figure 9B (21) since PdO has a much higher solubility in acidic condition.

Supplementary Note 6
Cu UPD is unique for its full, monolayer coverage on noble metal surface while Cu OPD (over-potential deposition, also known for bulk deposition) can happen on any surface including bulk MoS 2 with multilayer deposition. Therefore, one should carefully examine the contribution from Cu OPD when using Cu UPD to calculate the ECSA of Pt-based catalysts.
Based on the Nernst equation, equilibrium potential for Cu bulk deposition in 0.5 M H 2 SO 4 with 1 mM Cu 2+ is ca. 0.25 V vs. RHE. From a thermodynamic point of view, UPD and OPD of Cu will take place on the potential region more positive and negative than 0.25 V. As shown in Supplementary Figure 19, if we hold at a OPD potential (e.g., 0.18 V vs. RHE), we can observe the Cu OPD peak at around 0.34 V vs. RHE as well as the Cu UPD peak at 0.42 V. However, this OPD peak is not observed when we hold at a UPD potential (e.g., 0.275 mV vs. RHE), suggesting only Cu UPD happens in such condition. (19) Because Cu UPD is unique to noble metals, the peak at 0.42 V should not be observed in bulk MoS 2 as demonstrated in Supplementary Figure 17B. In contrast, we can observe the OPD peak at 0.34 V for bulk MoS 2 (not shown here).
To further confirm this Cu UPD peak, we performed Cu UPD with various holding time in Figure 5C and Supplementary Figure 20. The Cu UPD will finally reach a saturation because of its full, monolayer coverage on active Pt sites. In contrast to Cu UPD, we didn't observe any saturation of OPD peak even after 300 s. The calculated ECSA from Cu OPD (3.94 cm 2 ) is also much higher than that from Cu UPD (0.85 cm 2 ) or from the literature. (22,23) Finally, this Cu UPD peak is supported by its scan rate dependence in Supplementary Figure  22 and Supplementary Table 3. The ECSAs at various scan rates after monolayer saturation should be similar (e.g., 0.851 vs. 0.852 cm 2 ), while Cu OPD never get similar results due to the nature of multilayer deposition.
Based on all these observations, we conclude the peak at ~ 0.42 V vs. RHE in Pt-MoS 2 should be Cu UPD peak and the mismatched ECSAs from CO stripping and Cu UPD may attribute to the different accessibility of inner and outer Pt nanoparticles to CO gas and cupric ions.
For reference, Li x MoS 2 powders were taken out from glove box and added to cooled ultrapure water under hydrogen evolution. Homogeneous suspension (~1 mg mL -1 ) was sonicated for 30 min and further purified using exhaustive dialysis for 7 days to obtain exfoliated MoS 2 nanosheets. Hydrated compounds were necessary to facilitate the sluggish metal intercalation process. Feeding ratio was carefully controlled to avoid the exfoliation of MoS 2 flakes. Successful Pt loading was also observed using anhydrous K 2 PtCl 4 (> 99.9%) with extended reaction time (1 week), but many large Pt aggregates were found on the surface of MoS 2 .
All electrochemical measurements were performed at room temperature in a three-electrode cell using an Ag/AgCl electrode and a Pt wire as the reference and the counter electrode, or in a three-electrode H-cell using a saturated Hg/HgSO 4 electrode, a carbon rod and a Nafion ® -117 membrane as the reference, the counter electrode and separated membrane. A glassy carbon (GC) electrode for working electrode (3 mm diameter) was polished using 3 μm, 1 μm diamond and 0.05 μm alumina slurries, followed by rinsing with ultrapure water, ethanol, acetone and ultrapure water. Finally, the GC electrode was dried under a continuous nitrogen stream. The catalyst ink was prepared by dispersing 2.0 mg catalyst in 2 mL 4:1 ethanol/water mixture with 5% Nafion ® solution (20 μL) and sonicated for at least 2 hours. A quantity of 5 μL of the mixture was pipetted onto the GC electrode surface (70 μg cm -2 loading). Working electrode was then dried at room temperature in air for a few hours.
For HER measurements, linear sweep voltammetry (LSV) with a scan rate of 2 mV s -1 was recorded in 0.5 M H 2 SO 4 electrolyte on a CHI 660E electrochemical workstation at room temperature. An average of at least 5 LSV curves was employed to calculate the Tafel slope.
Chronoamperometry was tested at 50 mA cm -2 for designed period (60,000 or 120,000 s). LSV curves were recorded after operation. Electrochemical impedance spectroscopy (EIS) were taken from 4 MHz to 0. Charges obtained from CO stripping or Cu UPD were corrected for double layer capacity by subtracting the charge obtained for the same electrode under the same condition in N 2 without cupric. Electrochemically active surface area (ECSA) was calculated with an empirical value of 0.7 ML for saturated CO coverage, 152 μC cm -2 for CO monolayer oxidation as well as 420 μC cm -2 for Cu monolayer deposition. LSV curve was recorded prior to any measurement to ensure the HER activity.
ToF-SIMS was performed on ION-TOF SIMS 5 with Bi + primary beam (25 keV with 80x80 um 2 spot size) and Cs + secondary gun (2 keV, 70 nA with 230 x230 um 2 analyse area). A prolonged n-BuLi pretreatment (1 week) and Pt precursor reaction (3 days) were adopted for single crystal MoS 2 flake due to relatively slow intercalation. Pt-MoS 2 flake was then peeled off using Scotch tape method and transferred onto 300 nm SiO 2 /Si substrates for mapping.
30 mg sample was first ground into fine powder using mortar and pestle in glove box, mixed with 90 mg boron nitride and pressed into a 13 mm pellet. Sample pellet was then coated with one or two drops of paraffin wax and stored under vacuum to protect from the exposure of air. XAFS measurements were performed at the 1W1B-XAFS beamline of the Beijing Synchrotron Radiation Facility (BASF). Data analysis and simulation were performed on Athena, Artemis and Hephaestus (Version 0.9.23). (26) Pt-MoS 2 flake prepared from single-crystal MoS 2 was also used for GIXRD and SAXS. GIXRD was performed on a Bruker GADDS diffractometer with an area detector under Cu K α (1.5418 Å) radiation (40 kV, 40 mA) at room temperature. The incident angle of primary beam to sample surface was moved from 0.5 to 4°, with a detection angle of 1.3 to 30°.
SAXS measurement was conducted on SAXSess mc2 (Anton Paar) from 0.08 to 5° with Cu K α (1.5418 Å) at 40 kV and 50 mA. Pt-MoS 2 and single-crystal MoS 2 flakes were directly placed on test holder for both GIXRD and SAXS measurements. (27)