Electronic Supplementary Material (ESI) for Nanoscale. This journal is The Royal Society of Chemistry 2018 Supporting Information for Electrocatalytic Performance of Ultrasmall Mo 2 C Affected by Different Transition Metal Dopants in Hydrogen Evolution Reaction Feiyang Yu, a Ya Gao, a Zhongling Lang, a * Yuanyuan Ma, a Liying Yin, a Jing Du, a Huaqiao Tan, a * Yonghui Wang a and Yangguang Li a * Key Laboratory of Polyoxometalate Science of Ministry of Eduction, Faculty of Chemistry, Northeast Normal University, Changchun, 130024 (P. R. China) E-mail adress: langzl554@nenu.edu.cn; tanhq870@nenu.edu.cn; liyg658@nenu.edu.cn. S1
Table of content Section Additional characterization techniques Characterization of Anderson-type POMs Additional characterization of TM-Mo 2 C@C Additional electrochemical experiments of TM-Mo 2 C@C Theoretical calculation method Comparison of HER performance in acidic media for TM-Mo 2 C@C With other HER electrocatalysts Page S3 S4 S5 S15 S23 S28 S2
Structural characterization The transmission electron microscopy (TEM) was carried out a JEOL-2100F transmission electron microscope. The interrelated energy dispersive X-ray detector (EDX) spectra were performed on a SU8000 ESEM FEG microscope. The powder X-ray diffraction (PXRD) measurements were achieved by using a Rigaku D/max-IIB X-ray diffractometer with Cu-Kα radiation (λ=1.5418å). Raman spectra were obtained on a Raman spectrometer (JY, Labram HR 800). The X-ray photoelectron spectroscopy (XPS) measurement were recorded on an ESCALAB 250 spectrometer (Thermo Electron Corp) with A1 Kα radiation (hν=1486.6 ev) as the excitation source. The nitrogen sorption measurement was performed on an ASAP 2020 (Micromeritics, USA) Chemicals and Reagents All chemicals were purchased and used as received without further purification. Dicyandiamid (DCA) and (NH 4 ) 6 Mo 7 O 24 4H 2 O were purchased from Aladdin Industrial. Nafion solution (5 wt) and 20 Pt/C catalysts were purchased from Alfa Aesar China (Tianjin). The water used throughout all experiments was purified through a Millipore system. Anderson-type POMs ((NH 4 ) 4 [NiMo 6 O 24 H 6 ] 5H 2 O, (NH 4 ) 4 [CoMo 6 O 24 H 6 ] 5H 2 O, (NH 4 ) 3 [FeMo 6 O 24 H 6 ] 7H 2 O, and (NH 4 ) 3 [CrMo 6 O 24 H 6 ] 7H 2 O were synthesized according to a method previously described by Miwa s group [1] S3
Figure S1. (a-d) The IR spectrum of (NH 4 ) 4 [NiMo 6 O 24 H 6 ] 5H 2 O, (NH 4 ) 4 [CoMo 6 O 24 H 6 ] 5H 2 O, (NH 4 ) 3 [FeMo 6 O 24 H 6 ] 7H 2 O, (NH 4 ) 3 [CrMo 6 O 24 H 6 ] 7H 2 O, respectively. The region below 450 cm -1 belongs to the vibration peak of the central heteroatom TM-O (TM = Ni, Co, Fe, Cr). The characteristic peaks of Anderson-type POMs are located in the regions of 950-900 cm -1 and 650-500 cm -1. [1] S4
Figure S2. The SEM patterns of Ni-Mo 2 C@C with scale bar 5.00 µm (a), 2.00 µm (b), 1.00 µm (c) and 500 nm (d). S5
Figure S3. (a) TEM images of Co-Mo 2 C@C (inset: the particle size of distribution Co-Mo 2 C@C). (b and c) HRTEM images of Co-Mo 2 C@C. (d-h) Corresponding elemental mapping of Co, Mo, C, N. TEM images show that the ultrasmall Co-Mo 2 C@C are evenly distributed on the carbon and the average diameter of Co-Mo 2 C@C was 3.68 nm Co-Mo 2 C@C core is coated by N-doped carbon which prevent the catalyst particles from agglomerating and oxidizing, while the partial catalyst which is exposed can provide more active sites. The lattice fringes with an interplanar distance of 0.22 nm are consistent with the lattice plane (101) of Mo 2 C, meanwhile, a lattice spacing of ca. 0.34 nm indicated in figure corresponds to the typical layer spacing of graphitic carbon. [2] S6
Figure S4. (a) TEM images of Fe-Mo 2 C@C (inset: the particle size of distribution Fe-Mo 2 C@C). (b and c) HRTEM images of Fe-Mo 2 C@C. (d-h) Corresponding elemental mapping of Fe, Mo, C, N. TEM images show that the ultrasmall Fe-Mo 2 C@C are evenly distributed on the carbon and the average diameter of Fe-Mo 2 C@C was 3.77 nm Fe-Mo 2 C@C core is coated by N-doped carbon which prevent the catalyst particles from agglomerating and oxidizing, while the partial catalyst which is exposed can provide more active sites. The lattice fringes with an interplanar distance of 0.22 nm are consistent with the lattice plane (101) of Mo 2 C, meanwhile, a lattice spacing of ca. 0.34 nm indicated in figure corresponds to the typical layer spacing of graphitic carbon. [2] S7
Figure S5. (a) TEM images of Cr-Mo 2 C@C (inset: the particle size of distribution Cr-Mo 2 C@C). (b and c) HRTEM images of Cr-Mo 2 C@C. (d-h) Corresponding elemental mapping of Cr, Mo, C, N. TEM images show that the ultrasmall Cr-Mo 2 C@C are evenly distributed on the carbon and the average diameter of Cr-Mo 2 C@C was 3.79 nm Cr-Mo 2 C@C core is coated by N-doped carbon which prevent the catalyst particles from agglomerating and oxidizing, while the partial catalyst which is exposed can provide more active sites. The lattice fringes with an interplanar distance of 0.22 nm are consistent with the lattice plane (101) of Mo 2 C, meanwhile, a lattice spacing of ca. 0.34 nm indicated in figure corresponds to the typical layer spacing of graphitic carbon. [2] S8
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Figure S6. The energy dispersive X-ray (EDX) spectra of (a) Ni-Mo 2 C@C, (b) Co-Mo 2 C@C, (c) Fe- Mo 2 C@C, (d) Cr-Mo 2 C@C. The observed N element suggest that TM-Mo 2 C@C contain N dopants. Figure S7. The Raman spectra of Ni-Mo 2 C@C. S10
Figure S8. (a) The Raman spectrum of Ni-Mo 2 C@C, the value of I G /I D was 1.05. (b) The Raman spectrum of Co-Mo 2 C@C, the value of I G /I D was 1.04. (c) The Raman spectrum of Fe-Mo 2 C@C, the value of I G /I D was 0.97 The Raman spectrum of Cr-Mo 2 C@C, the value of I G /I D was 0.93 Figure S9. (a) Full-scan XPS spectrum of Ni-Mo 2 C@C. Full-scan XPS spectrum indicates that the S11
catalyst consists of carbon, nitrogen, oxygen, nickle and molybdenum elements. (b) Full-scan XPS spectrum of Co-Mo 2 C@C. Full-scan XPS spectrum indicates that the catalyst consists of carbon, nitrogen, oxygen, cobalt and molybdenum elements. (c) Full-scan XPS spectrum of Fe-Mo 2 C@C. Full-scan XPS spectrum indicates that the catalyst consists of carbon, nitrogen, oxygen, iron and molybdenum elements. (d) Full-scan XPS spectrum of Cr-Mo 2 C@C. Full-scan XPS spectrum indicates that the catalyst consists of carbon, nitrogen, oxygen, chromium and molybdenum elements. Figure S10. The XPS spectrum of (a) Co 2p, (b) Mo 3d, (c) C 1s, (d) N 1s of Co-Mo 2 C@C. The peaks of Co 2p at 780.0 ev and 795.5 ev, corresponding to Co 2p 3/2 and 2p 1/2, respectively. S12
Figure S11. The XPS spectrum of (a) Fe 2p, (b) Mo 3d, (c) C 1s, (d) N 1s of Fe-Mo 2 C@C. Two peaks of Fe 2p indicated two oxidation states for Fe, which are Fe 0 (706.75 ev and 719.95 ev) and Fe 3+ (710.7 ev and 724.3 ev).. Figure S12. The XPS spectrum of (a) Cr 2p, (b) Mo 3d, (c) C 1s, (d) N 1s of Cr-Mo 2 C@C. The peak at S13
576.6 ev and 596.3 ev can be attributed to Cr 2p 3/2 and 2p 1/2 in Cr-Mo 2 C@C, respectively. Figure S13. (a) N 2 sorption isotherms of Ni-Mo 2 C@C,which presents a typical IV hysteresis loop and the BET surface area is 113.133 m 2 g -1, (b) N 2 sorption isotherms of Co-Mo 2 C@C,which presents a typical IV hysteresis loop and the BET surface area is 125.778 m 2 g -1, (b) N 2 sorption isotherms of Fe- Mo 2 C@C,which presents a typical IV hysteresis loop and the BET surface area is 99.779 m 2 g - 1, (b) N 2 sorption isotherms of Cr-Mo 2 C@C,which presents a typical IV hysteresis loop and the BET surface area is 106.298 m 2 g -1. S14
Figure S14. Cyclic voltammograms (CVs) (a) Ni-Mo 2 C@C (b) Co-Mo 2 C@C (c) Fe-Mo 2 C@C (d) Cr- Mo 2 C@C without redox current density peaks in 0.5 M H 2 SO 4 S15
Figure S15. Nyquist plots of electrochemical impedance spectra (EIS) of (a) Ni-Mo 2 C@C, (b) Co- Mo 2 C@C, (c) Fe-Mo 2 C@C and (d) Cr-Mo 2 C@C recorded in 0.5 M H 2 SO 4 aqueous solution. (e): Twotime-constant model equivalent circuit used for data fitting of EIS spectra (Rs represents the overall series resistance, CPE1 and CPE2 represent the constant phase element and resistance related to surface porosity Rp, and Rct represents the charge transfer resistance related to HER process). S16
Figure S16. The PXRD patterns of TM-Mo 2 C@C and after 1000 cycles. The PXRD patterns of TM- Mo 2 C@C catalyst show negligible changes, which means that the structure of TM-Mo 2 C@C can remain after 1000 cycles HER test. Figure S17. (a) and (b) TEM images of Ni-Mo 2 C@C before and after 1000 cycles test. The images show that the morphology of Ni-Mo 2 C@C catalyst has negligible changes. S17
Figure S18. FEs of Ni-Mo 2 C@C towards the HER in 0.5 M H 2 SO 4 at the overpotential of 200 mv. S18
Figure S19. (a) and (b) The HER polarization curves of Ni-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm NiSO 4. (c) and (d) The HER polarization curves of Ni-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm CoSO 4. (e) and (f) The HER polarization curves of Ni-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm FeSO 4 S19
Figure S20. (a) and (b) The HER polarization curves of Co-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm NiSO 4. (c) and (d) The HER polarization curves of Co-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm CoSO 4. (e) and (f) The HER polarization curves of Co-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm FeSO 4 S20
Figure S21. (a) and (b) The HER polarization curves of Fe-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm NiSO 4. (c) and (d) The HER polarization curves of Fe-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm CoSO 4. (e) and (f) The HER polarization curves of Fe-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm FeSO 4 S21
Figure S22. (a) and (b) The HER polarization curves of Cr-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm NiSO 4. (c) and (d) The HER polarization curves of Cr-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm CoSO 4. (e) and (f) The HER polarization curves of Cr-Mo 2 C@C and 20 Pt/C in ph 1.05 electrolyte with 10 mm FeSO 4. Figure S14-S17 indicated that compared to Pt/C, the activity of TM-Mo 2 C@C catalysts almost remains unchanged in the presence of Ni 2+, Co 2+ and Fe 2+. S22
Theoretical calculation methods Computational Methods and Models The calculation was performed using Vienna ab initio simulation package (VASP). All DFT calculations were treated within the generalized gradient approximation (GGA) with the PBE functional for the exchange and correlation effects of the electrons. A cutoff energy of 400-eV for the grid integration was utilized and the convergence threshold for force and energy was set 0.02Å and 10-5 ev, respectively. Ion cores are described by projector augmented wave PAW potentials. Monkhorst-Pack grids of 3 3 1 k points were used for all calculations. For geometry optimization, the top two layers of Ni-Mo 2 C@C (101) and graphene were allowed to relax, while the rest of Ni- Mo 2 C@C (101) (the bottom two layers) remained fixed. In addition, the systems that involve TM atom were calculated with spin-polarization. In this work, Mo 2 C surface is modeled. The correlative theoretical models were constructed to probe the HER activity of Ni-Mo 2 C@C and TM-Mo 2 C@C. S23
Figure S23. The theoretical models of H adsorbed on: (a) Ni-Mo 2 C@C, (b) Co-Mo 2 C@C, (c) Fe- Mo 2 C@C, (d) Cr-Mo 2 C@C, (e) Mo 2 C, and (f) Ni-Mo 2 C@C. The gray, blue, white, darkcyan, green, Magenta, purple, and dark green balls represent C, N, H, Mo, Ni, Co, Fe, and Cr atoms, respectively. In order to compare the catalytic activity of different catalysts, the free energies of the intermediate is calculated by the formula ΔG (H*) = ΔE (H*) + ΔZPE - TΔS, where H* represents the H atom adsorbed on the surface, ΔE (H*), ΔZPE and ΔS represent the binding energy, zero energy and entropy change between adsorption H and gas phase, respectively. Therefore, ΔZPE can be calculated by the formula EZPE = ZPE (H*) - 1 / 2ZPE (H 2 ). The gas entropy of H is taken from the literature. Combining the analysis of the Bader charge on the surface atoms and the charge density difference (CDD) plot. We selected several adsorption sites on each surface to study the ability of H to adsorb on different surfaces, including an N atom (N site), several C sites adjacent to the N atom, and one C site away from the N atoms. The calculated binding energies, zero point energies and the S24
free energies for H adsorption on different surfaces are listed in Table S1. If ΔG (H*) 0, the catalyst has good HER performance. The results show that the C atom adjacent to N in Ni-Mo 2 C@C has a good HER catalytic activity, which is consistent with the blue area in the CDD plot of Ni-Mo 2 C @C. Figure S24. Models of the structures of Ni-Mo 2 C@C and its charge density difference ρ with the red and blue areas denoting low and high charge density, respectively. The gray, blue, white, darkcyan, green, balls represent C, N, H, Mo, Ni atoms, respectively. The results demonstrate that the carbon atoms adjacent to N atom in Ni-Mo 2 C@C could possess a good activity for HER, which are corresponding to the blue area in the CDD picture of Ni-Mo 2 C@C Figure S25. Nyquist plots of electrochemical impedance spectra (EIS) of Ni-Mo 2 C@C, and Ni-Mo 2 C; Nyquist plots of EIS of Ni-Mo 2 C@C and Ni-Mo 2 C tested with the overpotential of 200 mv. the charge transfer impedance (Rct) for Ni-Mo 2 C@C and Ni-Mo 2 C are 14.7 Ω and 204.8 Ω with overpotential of 200 mv in 0.5 M H 2 SO 4, which indicates the N-doped carbon can reduce the grain boundary resistance between the Ni-Mo 2 C nanoparticles and promote the charge transfer in the composite. S25
Figure S26. (a) HER polarization curves for Ni-Mo 2 C@C samples prepared at the mass ratio of 1:3 (173 mv), 1:4 (124 mv), 1:5 (72 mv), 1:6 (104 mv), 1:7 (105 mv), 1:8 (121 mv), which were measured in the 0.5 M H 2 SO 4 solution; (b) LSV curves of a series of catalysts annealed at different temperatures (750 o C (187 mv), 800 o C (72 mv) and 850 o C (149 mv)). Table S1 The C, N, O, Mo, and Ni components of Ni-Mo 2 C@C recorded from the EDX quantitative analyses (three parallel measurements). 1 2 3 Average C 38.22 50.66 56.43 48.44 N 40.57 31.65 22.31 31.51 O 20.37 10.38 14.96 15.24 Mo 0.7 6.29 5.50 4.16 Ni 0.14 1.02 0.80 0.65 Table S2 The C, N, O, Mo, and Co components of Co-Mo 2 C@C recorded from the EDX quantitative analyses (three parallel measurements). 1 2 3 Average C 70.48 72.41 64.15 69.01 N 9.16 9.51 22.04 13.57 O 12.87 10.46 12.53 11.96 Mo 6.55 6.43 1.10 4.69 Co 0.94 1.19 0.18 0.77 Table S3 The C, N, O, Mo, and Fe components of Fe-Mo 2 C@C recorded from the EDX quantitative S26
analyses (three parallel measurements). 1 2 3 Average C 75.14 71.62 66.94 71.23 N 9.02 10.03 11.31 10.13 O 8.85 15.97 8.54 11.12 Mo 5.93 2.04 11.32 6.43 Fe 1.06 0.34 1.89 1.09 Table S4 The C, N, O, Mo, and Cr components of Cr-Mo 2 C@C recorded from the EDX quantitative analyses (three parallel measurements). 1 2 3 Average C 70.69 68.94 70.41 70.02 N 9.53 9.33 10.31 9.72 O 12.46 12.9 13.45 12.94 Mo 6.39 7.38 5.02 6.26 Cr 0.93 1.45 0.81 1.06 Table S5 The ΔE(H*), ZPE(H*) and ΔG(H*) values of the H* adsorbed on different surfaces of different catalysts Models ΔE(H*)/eV ΔZPE(H*)/eV ΔG(H*)/eV Mo 2 C -0.96 0.037066-0.72 Ni-Mo 2 C -0.50-0.00922-0.30 Fe-Mo 2 C -0.66 0.030371-0.43 Co-Mo 2 C -0.63 0.027687-0.40 Cr-Mo 2 C -0.77 0.036711-0.53 Ni-Mo 2 C@C -0.20 0.177051 0.19 S27
Table S6 Comparison of HER performance in acidic media for TM-doped Mo 2 C with other HER electrocatalysts Catalyst Current Overpotential Tafel Slope Ref density (ma (mv) (mv dec -1 ) cm -2 ) N-Mo 2 C@C 10 72 65.6 This work Co-Mo 2 C@C 10 122 80.9 This work Fe-Mo 2 C@C 10 129 102.4 This work Cr-Mo 2 C@C 10 147 114.2 This work Ni-Mo 2 C/C MF 10 99 73 Chem. Eur. J 2017,23,4644-4650 Ni/Mo 2 C@C 10 169 100 J. Mater. Chem. A. 2017, 5. 5000-5006 Ni-decorated Hollow 10 123 83 Chem. Mater. 2016, 28, 6313-6320 MoC x @C Co-doped Mo 2 C NW Fe 3 C/Mo 2 C@ NPGC Fe 3 C- 10 140 39 Adv. Funct. Mater. 2016, 26, 5590-5598 10 98 45.2 J. Mater. Chem. A. 2016, 4, 1202-1207 10 116 43 DOI: 10.1002/cssc.201700207 Mo 2 C/NC Mo 2 C@PC 10 177 96 Angew. Chem. Int. Ed. 2016, 55, 12854-12858 ß -Mo 2 C nanosheet 10 172 62 Angew. Chem. Int. Ed. 2015, 54, 15395-15399 Mo 2 C x 10 142 53 Nat. Commun. 6, 6512 S28
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