High Electrochemical Selectivity of Edge versus Terrace Sites in. Two-Dimensional Layered MoS 2 Materials

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1 Supporting Information for High Electrochemical Selectivity of Edge versus Terrace Sites in Two-Dimensional Layered MoS 2 Materials Haotian Wang 1, Qianfan Zhang 2 *, Hongbin Yao 3, Zheng Liang 3, Hyun-Wook Lee 3, Po-Chun Hsu 3, Guangyuan Zheng 4, and Yi Cui 3,5 * 1 Department of Applied Physics, Stanford University, Stanford, CA 94305, USA 2 School of Materials Science and Engineering, Beihang University, Beijing , P.R. China 3 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 4 Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA 5 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA *Correspondence should be addressed to: yicui@stanford.edu (Y. C.) and qianfan@buaa.edu.cn (Q. Z.) This PDF file includes: Materials and Methods Figures S1 to S11

2 Materials and Methods: Ab initio simulation. First-principles calculations were performed using the Vienna Ab Initio Simulation Package (VASP) in the framework of density functional theory (DFT). The projector augmented wave (PAW) pseudopotential was adopted and the GGA exchange-correlation function was described by erdew-burke-ernzerhof (PBE). To guarantee convergence, we chose 550 ev as the cut-off energy of the plane-wave basis. Polysulfide Li 2 S 8 solution synthesis. The Li 2 S 8 solution was prepared by dissolving a desired amount of stoichiometric S and Li 2 S in 1,3-dioxolane (DOL) / 1,2-dimethoxyethane (DME) solution (1:1 in volume) with the addition of LiNO 3 additive (5 wt.%). For the typical preparation of 5 M Li 2 S 8 solution, 0.56 g of sulfur and g of Li 2 S were added to 4 ml of DOL (2 ml) / DME (2 ml) mixed solution. The obtained suspension was stirred and heat at 80 o C overnight to yield red-brown Li 2 S 8 solution. The 0.5 M and 2 M Li 2 S 8 solution were prepared based on the 5 M solution. MoS 2 edge sites oxidation. The as-exfoliated MoS 2 nanosheets on GC substrate was put into O 2 saturated deionized water and kept under 80 on a hot plate for over 24 h. Rapid sulfurization. Mo or MoO 3 precursors was put into the hot center of a single-zone, 12-inch horizontal tube furnace (Lindberg/Blue M) equipped with a 1-inch-diameter quartz tube. The pressure and flow rate of the Ar gas were kept at 1, 000 mtorr and 100 sccm respectively. The heating center of the furnace was raised to reaction temperature of 600 in 10 minutes, and then held at reaction temperature for 20 minutes, followed by natural cool-down. CNF matrix by electrospinning. 0.5 g polyacrylonitrile (PAN, M w = 150,000) and 0.5 g polypyrrolidone (PVP, M w = 1,300,000) were dissolved in 10 ml of dimethylformamide (DMF) under 80 o C with constant stirring. The solution was electrospun using a conventional electrospinning set-up with the following parameters: 15 kv of static electric voltage, 18 cm of air gap distance, 3 ml PVP and PAN solution, and 0.5 ml/h flow rate. A carbon fiber paper (8 cm 8 cm) was used as the collection substrate. The electronspun polymer nanofibers on the carbon fiber paper was then heated up to 280 o C in 30 min in the box furnace, and kept under the temperature for 1.5 hours to oxidize the polymers. After the oxidization process, the nanofibers were self-detached from the carbon paper resulting in the freestanding film. Those nanofibers were carbonized under argon atmosphere at 900 o C for 2 hours to become low weight, high surface area, freestanding, and conducting substrate. V-MoS 2 and C-MoS 2 synthesis on the CNF. The CNF substrate was first treated by O 2 -plasma to make it hydrophilic and then dipped into the 20 wt% ammonium heptamolybdate ((NH 4 ) 6 Mo 7 O 24 4H 2 O) solution and dried in the vacuum oven under

3 60 for 8 h. The substrate was then heated up to 600 in 30 min under Ar atmosphere in a tube furnace and kept there for 1.5 h, under which the ammonium heptamolybdate decomposed to produce MoO 3 followed by the rapid sulfurizaiton process to become V-MoS 2. The C-MoS 2 can be obtained by changing the sulfurization temperature and reaction time. Typically, we increase the temperature to 800 in 1 h and keep it there for 5 h. Li-S battery assembly type coin cells (MTI) were assembled using lithium metals as the counter/reference electrode. The V-MoS 2 -CNF, C-MoS 2 -CNF, and bare CNF substrates were punched into discs with ~ 1 cm 2 area and directly used as the electrode free of additional current collector. The bare CNF disc weighs ~ 1.5 mg and the MoS 2 -CNF disc weighs ~ 2.2 mg. The 5 M Li 2 S 8 solution was used as the active material. For a sulfur mass loading of 2 mg, 12.5 µl of 5 M Li 2 S 8 solution was added into a cell, sandwiched by two identical as-prepared electrodes for thoroughly absorption. The electrolyte was a freshly prepared solution of lithium bis(trifluoromethanesulfonyl)imide (1 M) in 1 : 1 v/v 1,2-dimethoxyethane and 1,3-DOL containing LiNO 3 (1 wt%). 20 µl of electrolyte was added in one cell type coin cells were assembled in an argon-filled glove box and galvanostatic cycling of cells was carried out using a 96-channel battery tester (Arbin Instruments). HER sample preparation and testing. The obtained V-MoS 2 -CNF, C-MoS 2 -CNF, and bare CNF were ball-milled into small parts and mixed with nafion and super P by the weight ratio of 80 : 5 : 15. The mixture was then dispersed into ethanol solution with a concentration of 5 mg/ml. 1 mg of the mixture was then drop casted onto 1 cm 2 carbon fiber paper substrate. The Pt/C catalyst was mixed with nafion by the weight ratio of 95 : 5 and dispersed into ethanol solution with the same concentration discussed above. 0.1 mg of the mixture was then drop casted onto 1 cm 2 glassy carbon substrate. The catalysts were tested in 0.5 M H 2 SO 4 solution (de-aerated by N 2 ) using a typical three-electrode electrochemical cell setup, with a saturated calomel electrode (E(RHE) = E(SCE) V after calibration) as the reference electrode and a graphite rod (99.999%, from Sigma Aldrich) as the counter electrode. Electrochemically inert kapton tape was used to define the 1 cm 2 electrode area. Linear sweep voltammetry at 5 mv/s is recorded by a Biologic VSP potentiostat. Li electrochemical tuning of the as-prepared catalysts was performed in a pouch battery cell with a discharging current of 0.1 ma/cm 2 and a cut-off voltage of 0.8 V vs Li + /Li. Characterization. An FEI XL30 Sirion scanning electron microscope (SEM) with an FEG source and energy dispersive spectroscopy detector was used for SEM characterization. An FEI Tecnai G2 F20 X-TWIN transmission electron microscope (TEM) was used for TEM characterization. Raman spectroscopy (531 nm excitation laser, cut-off around 175 cm -1, WITEC Raman spectrometer).

4 Figure S1. The Configurations of the bonding between Li 2 S and the different MoS 2 atomic sites. The binding energies between Li 2 S and different MoS 2 atomic sites are: 4.48 ev for Mo-edge, 2.70 ev for S-edge, 2.70 ev for 50% S covered Mo-edge, 1.40 ev for 100 % S covered Mo-edge, and 0.87 ev for terrace site.

5 Figure S2. SEM images of as-exfoliated H-MoS 2 on GC substrate with clear edges.

6 Figure S3. AFM measurement of H-MoS 2 nanosheet on GC substrate. The height profile shown in (c) corresponds to the dashed line in (a), with a height of ~ 15 nm. The arrow represents a higher magnification.

7 Figure S4. (a) Electrochemical deposition of S on MoS 2 nanosheets. The S particles tend to be decorated along the edges which suggests the same selectivity of edge sites on S. (b) Li 2 S deposition after terminating the dangling bonds of the edge sites. No obvious edge effects are shown, indicating that the edge sites lose their electrochemical selectivity after the mild oxidation treatment.

8 Figure S5. Li 2 S electrochemical deposition on patterned electrode. (a) Schematic of GC substrate with V-MoS 2 nanofilm patterning, followed by Li 2 S electrodeposition to show the edge site selectivity. (b) SEM images of the patterning before and after Li 2 S deposition. The µm V-MoS 2 thin film squares separated by 40 µm wide carbon lines was obtained by sputtering 10 nm thick Mo film through a nickel grid mask onto the GC substrate followed by the rapid sulfurization process. More concentrated polysulfide solution (2 M) and higher discharge current density (20 μa/cm 2 ) are selected to have more deposited Li 2 S for observable patterning effects. On the left figure the bright area represents V-MoS 2 and the dark area is GC. On the right one, the densely packed, white dots are Li 2 S NPs deposited onto the V-MoS 2 surface. Few Li 2 S NPs are observed on the carbon surface. The insets are SEM images with lower magnification. (c), (d) Zoom in SEM images of the edge and carbon surfaces after Li 2 S deposition. (e), (f) Zoom in SEM images of the edge and carbon surfaces before Li 2 S deposition.

9 Figure S6. SEM images of the CNF by electrospinning of PVP and PAN followed by the carbonization process.

10 Figure S7. SEM images with low and high magnification of V-MoS2, C-MoS2, and bare CNF with Li2S deposition.

11 Figure S8. Electrochemical double layer capacity measurements of V-MoS 2 -CNF and C-MoS 2 -CNF. The capacity of V-MoS 2 -CNF is 16.0 mf, significantly higher than that of C-MoS 2 -CNF with 1.3 mf.

12 Figure S9. Potentiostatic electrolysis of V-MoS 2 -CNF catalyst for over 2 h. The potential we applied is around mv vs RHE after ir-correction. The cathodic current remains stable during the operation, indicating that the catalyst is stable under HER conditions.

13 Figure S10. TEM image of C-MoS 2 after 10 ma/cm 2 HER constant current operation for 2 hours. The MoS 2 cage is partially opened during this hydrogen evolution process.

14 Figure S11. TEM images of V-MoS 2 -CNF and C-MoS 2 -CNF catalysts after Li electrochemical tuning. The red arrows in (b) indicate the broken parts of the MoS 2 bucky-ball.