Activating and Optimizing Activity of CoS 2 for Hydrogen Evolution Reaction Through the Synergic Effect of N Dopants and S Vacancies

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1 Supporting information Activating and Optimizing Activity of CoS 2 for Hydrogen Evolution Reaction Through the Synergic Effect of N Dopants and S Vacancies Jingyan Zhang a, Wen Xiao b, Pinxian Xi c, Shibo Xi d, Yonghua Du d, Daqiang Gao a,b *, and Jun Ding b *. Experimental section Synthesis. N-doped CoS 2 catalysts were synthesized using the one-step sintering method by using the cobalt chloride and thiourea as the Co, S and N sources. In detail, 4 mmol cobalt chloride (CoCl 2 6H 2 O) and thiourea (CH 4 N 2 S; 1.2 mmol, 2.4 mmol, and 4.8 mmol denoted as sample NCS-1, NCS-2 and NCS 2-3, respectively) were firstly mixed in a beaker. Then excessive ethanol was slowly added into the beaker to dissolve the mixture under stirring condition for about 2 h. After drying the solution at 80 ºC, the precursor was finally sintered in a tube furnace at 550 ºC for 2 h under the argon gas of 100 sccm with the high-purity argon was first flowed for 20 min. After that, the furnace was cooled to room temperature naturally with a flow of 100 sccm argon. Pure CoS 2 is fabricated by the one-step hydrothermal method. 5 mmol cobalt chloride (CoCl 2 6H 2 O) and 5 mmol Sodium thiosulfate (Na 2 S 2 O 3 5H 2 O) were joined the beaker with 80 ml deionized water and stirred for half an hour by the magnetic stirrer. Then the red solution was moved it into 100 ml reaction tank for 12 h at 220 ºC. Finally, the products were washed with anhydrous alcohol. 1

2 To get the N doped CoS 2 catalysts with different defect concentrations, the sample NCS-2 was post-annealed at sulfur (under the argon gas of 100 sccm) or hydrogen (under the gas of 100 sccm with the hydorgen:argon=10:90) atmosphere, at 450 ºC for 2 h respectively. For the sulfuration progress, an alumina boat loaded with 50 mg of sample NCS-2 was placed at the downstream end of the tube furnace. Then, 1 g of sulfur powder in another alumina boat was positioned at the upstream end of the quartz tube. The distance between the two boats was about 30 cm. Characterization:The crystal structure of the N doped CoS 2 catalysts were tested by X-ray diffraction (XRD, X' Pert PRO PHILIPS with Cu Kα radiation) and Raman (Jobin-Yvon LabRam HR80 spectrometer Horiba Jobin Yvon, Inc. with a 532 nm line of Torus 50 mw diode-pumped solid-state laser under backscattering geometry). The samples' morphology and element mapping was obtained by the scanning electron microscope (SEM, Hitachi S-4800). The bonding characteristics and the elements contents of the catalysts were captured by the X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The Brunauer Emmett Teller (BET) nitrogen physisorption experiments were carried out on a Micromeritics ASAP 2010 system. A Varian E-112 spectrometer at microwave frequency of 9.11 GHz was used for electron spin resonance (ESR) measurements at ambient temperature. The temperature-dependent conductivity of the catalysts was measured through a four-terminal configuration on a Janis Research system and via Keithley 2600 semiconductor analysis. Transmission EXAFS spectra were collected at room temperature in the vicinity of the Co K edge at the XAFCA 2

3 beamline of the Singapore Synchrotron Light Source (SSLS), Singapore 1. Energy was calibrated using a neodymium foil. EXAFS data was analyzed using the WINXAS 3.1 code. Data was normalized and transformed from energy space to momentum (k) space, background absorption was removed and the (k) function was extracted in the range of 1.5~10.6 Å -1. The Fourier transform of (k) weighted by k 3, (i.e., k 3 x(k) in R space) was performed using the Bessel function. 2 Electrocatalytic Hydrogen Evolution. To fabricate the electrochemical testing electrode, firstly, 3 mg of catalysts and carbon black (Vulcan XC72) were dispersed in 1.47 ml N, N-Dimethyl Formamide with 30 µl Nafion-117 solution. Then, 12 µl catalyst ink was dropped onto a glassy carbon electrode (0.071 cm 2 geometrical area, Pine Research Instrument) and dried at room temperature where the loading amount is estimated to be 0.32 mg/cm 2 for the samples. Electrochemical measurements were carried out with the electrochemical workstation (CHI 660E, USA) in a standard three-electrode cell. In our test, the working electrode (WE) was the glassy carbon (GC) coated with various catalysts. Ag/AgCl was the reference electrode and carbon rod was the counter electrode. After purging by high purity N 2 gas, the linear sweep voltammetry (LSV) was performed in 0.5 M H 2 SO 4 with the scan rate of 2 mv/s at the room temperature. The cyclic voltammetry (CV) measurements were carried out at the scan rate of 100 mv/s. During the electrochemical testing, all the electrochemical measurements were corrected by the equation of E(RHE)=E(Ag/AgCl)+197 mv. The polarization curves were ir-corrected by the equation of E ir -corrected=e original +j R solution The Nyquist plots were tested at 3

4 overpotential of 200 mv with frequencies changing from 10 khz to 0.1 Hz. Calculation details. Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) and the Perdew Burke Ernzerhof exchange correlation functional correction. 3 The atomic structures for the models were fully relaxed with self-consistency accuracy of 10-4 ev and the residual forces were within 0.02 ev/å for geometry optimizations. The plane-wave basis set was 450-eV kinetic energy cutoff, and the Monhorst Pack k-point sampling was used. 4 All the surface models contained the 2 2 supercell of the CoS 2 (001) surface with the calculated lattice parameters of a = b =c= Å, where the vacuum slab of 15 Å was used for surface isolation to prevent interaction between two neighboring surfaces. The top three surface atomic layers were relaxed with all other layers fixed to simulate bulk structure. The Gibbs free energy change ( G H* ) shows the following express: = +,where, and are the adsorption energy of atomic hydrogen on the given surface, zero point energy correction and entropy change of H* adsorption, respectively. The zero point energy correction is estimated by the equation = H 1/2 H, where H and H are calculated by vibration frequency calculation. At 1 bar and 300 K, T S is approximately 0.2 ev. The value of is calculated as =E tot E sub 1/2E H2, where E tot and E sub are the energies of H absorbed systems and the clean given surface, respectively, and E H2 is the energy of molecular H 2 in the gas phase. 5 Based on the optimized structure, the charge distribution can be obtained by Bader charge analysis.6 4

5 (1) Du, Y.; Zhu, Y.; Xi, S.; Yang, P.; Moser, H.O.; Breese, M.B.H.; Borgna, A. XAFCA: a new XAFS beamline for catalysis research, Journal of Synchrotron Radiation, 2015, 22, (2) Flot, D.; Mairs, T.; Giraud, T.; Guijarro, M.; Lesourd, M.; Rey, V.; van Brussel, D.; Mitchell, E. The ID23-2 structural biology microfocus beamline at the ESRF, J Synchrotron Radiat. 2010, 17, (3) Kresse, G.; Furthmüller, J.; Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, (4) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, (5) Jiao, Y.; Zheng, Y.; Davey, K.; Qiao, S. Z. Activity origin and catalyst design principles for electrocatalytic hydrogen evolution on heteroatom-doped graphene. Nat Energy 2016, 1, (6) Wang, J. H.; Cheng, Z.; Brédas, J. L. Liu, M.; Electronic and vibrational properties of nickel sulfides from first principles, J. Chem. Phys. 2007, 127,

6 Figure S1. Nitrogen adsorption desorption isotherms for the sample NCS-1, NCS-2 and NCS-3. Figure S2. (a) XRD, (b) Raman, (c) ESR, (d) SEM image and (e) R-T curve of pure CoS 2 fabricated by hydrothermal method. 6

7 Figure S3. (a) The lattice parameter dependence on S vacancy concentration in bulk CoS 2, as well as the optimized atomic structures. (b) The changes of bader charge for Co and S as the increasing of the S vacancies. The calculated model is supercell. Figure S4. Variation of N concentration and S vacancy concentration in the three samples. 7

8 Figure S5. The calculated crystal structure and lattice parameters for pure CoS 2 and N doped CoS 2 with a supercell The formation energy of N replaced S atom was performed in neutral states: =, where,, and are the total energy of the supercell with N substitution (N S ), total energy of pristine supercell, chemical potentials of isolated S and N atoms. Figure S6. The formation energy of Vs-CoS 2 and Ns-Vs-CoS 2 versus S chemical potential. The formation energy of S vacancy was performed in neutral states: =, where,, are the total energy of the supercell with one S vacancy, total energy of pristine supercell, chemical potentials of isolated S atom. The shows the value between the 1/2E E and E. Further, we also calculated the formation energy of S vacancy in N doped CoS 2 (Ns-Vs-CoS 2 ), where the results are shown as below. As can be seen that the formation energy of S vacancy in N doped CoS 2 is smaller than that in pure CoS 2, indicating that material structure accommodate S vacancy formation by incorporating N atoms. 8

9 Figure S7. Density of states (DOS) plots of CoS 2, Ns-CoS 2 and Ns-Vs-CoS 2. Figure S8. LSV polarization curves of N-doped CoS 2 and platinum in 0.5 M H 2 SO 4 with the scan rate of 2 mv/s. Figure S9. Cyclic voltammetry curves of sample NCS-1, NCS-2 and NCS-3 in the region of V vs. SCE. 9

10 Figure S10. (a) CVs of bare GBE and N-doped CoS 2 catalysts in ph=7 phosphate buffer between -0.2 V and 0.4V vs. RHE at a scan rate of 50 mv/s. (c) Turnover frequency of the N-doped CoS 2 catalysts in 0.5 M H 2 SO 4. Figure S11. (a) LSV polarization curves for N-doped CoS 2 catalysts before and after CV cycles between V and V performed at a scan rate of 100 mv/s. (b) The time-dependent current density as a function of time at a static overpotential of 200 mv without ir correction. Figure S12. Polarization curves recorded for sample NCS-2 before and after 5000 CV cycles in (a) 1.0 M PBS and (c) 1.0 M KOH. Inset show the time-dependent current density as a function of time at a static overpotential of 200 mv without ir correction. 10

11 Catalyst G H* (ev) Co site N site S site CoS N s -CoS V s -CoS N s -V s -CoS V Co -CoS N s -V Co -CoS N s represents that N substituted S in CoS 2, V s, V Co represents the S or Co defects in CoS 2, respectively. Table S1. Free energy change ( G H* ) of hydrogen adsorption on various structural sites for pure, N-doped CoS 2, V s -CoS 2, N s -V s -CoS 2, V Co -CoS 2 and N s -V Co -CoS 2. Figure S13. The calculation models for CoS 2, N s -CoS 2 and N s -V s -CoS 2 with (001) surface. 11

12 Figure S14. Calculated adsorption free energy of H* ( G H* ) on Co and N atoms of different sites (in units of ev) for the N s -V s -CoS 2.. Figure S15. ESR results of catalyst NCS-2, H+NCS-2, and S+NCS-2. 12

13 Figure S16. (a) LSV polarization curves of catalysts NCS-2, post-annealing NCS-2 under the S and H 2 atmosphere, and platinum in 0.5 M H 2 SO 4 (scan rate of 2 mv/s). (b) Tafel plots of catalysts NCS-2, post-annealing NCS-2 under the S and H 2 atmosphere and platinum catalysts in 0.5 M H 2 SO 4. (c) Nyquist plots and (d) The differences in current density ( j= ja-jc) at 0.15 V vs. RHE plotted against scan rate fitted to a linear regression allow for the estimation of C dl for the NCS-2, post-annealing NCS-2 under the S and H 2 atmosphere. Inset in figure S15 c is the fitting circuit. 13