Supporting Information for ( ) ( ) ( )
Supporting Information for Highly Active and Stable Hybrid Catalyst of Cobalt-doped FeS2 Nanosheets-Carbon Nanotubes for Hydrogen Evolution Reaction Di-Yan Wang1,2,3#, Ming Gong1#, Hung-Lung Chou4, Chun-Jern Pan,5 Hsin-An Chen,6 Yingpeng Wu1, Meng-Chang Lin1, Mingyun Guan1, Jiang Yang1, Chun-Wei Chen6, Yuh-Lin Wang3, Bing-Joe Hwang5*, Chia-Chun Chen2,3* and Hongjie Dai1* 1
Department of Chemistry, Stanford University, Stanford, CA 94305, USA
2
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
3
Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan
4
Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 5
Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 6
Department of Materials Science and Engineering, National Taiwan University, Taipei, 10617, Taiwan
#
These authors contributed equally to this work
Corresponding Author * [email protected], [email protected], and [email protected]
Experiment Details Computational method: Computational method: All calculations in this work were performed using the CASTEP program [1,2], which employs the plane wave pseudopotential method to calculated the total energy within the framework of the Kohn-Shan DFT [3-5]. We used the PBE exchange-correlation functional [6] with TS dispersion correction [7] in describing the vdW interaction. The ion-electron interaction was modeled by the non-local real space, ultrasoft pseudopotential with a cutoff energy of 400 eV. DFT simulations were performed based on a Pt (111) slab model system. The surface is constructed as slab within the 3-D periodic boundary conditions, and models are separated from their images in the direction perpendicular to the surface by a 14 Å vacuum layer. To model the Pt (111) slab surface, we have adopted slabs with three layers, sixteen atoms per layer. The bottom layer was kept fixed to the bulk coordinates; full atomic relaxations were allowed for the top two layers. For these calculations, a 2 × 2 × 1 k-Point mesh was used in the 4 × 4 supercell. In the FeS2(110) (Fe:8, S:16) and FeCoS2(110) (Fe:7, Co:1, S:16) slab models, we have adopted slabs with six layers, four atoms per layer. The bottom two layers were kept fixed to the bulk coordinates; full atomic relaxations were allowed for the top four layers. For these calculations, a 3 × 3 × 1 k-Point mesh was used in the 2 × 2 supercell. For sake of comparison, the slab system was replaced Fe with a Co atom, the relaxation on internal coordinates at a variety of cutoff energies is repeated. All clusters are put on carbon nanotube-like substrates. The system would gradually become relaxed to achieve a balanced state with lowest energy. Total energy calculations were achieved when the residual total energy of 0.01 eV and the maximal force of 0.01 eV/ Å were reached.
The potential Energy diagram for Hydrogen Evolution Reaction: The energy per adatom can be calculated from the differential adsorption energy, Δ EH, and was derived as
∆E H =
1 n E (surf + nH ) − E (surf ) − E (H 2 ) , where n is the number of hydrogen atoms adsorbed n 2
on the surface and E(surf+nH) is the total energy of the surface with n atoms adsorbed. The negative sign S1
of EH corresponds to the energy gain of the system because of H atom adsorption. Our DFT results for the hydrogen evolution reaction gives a molecular level picture of the process, which is in good agreement with experimental observations. The finding that there is a direct interaction between the doping atom and the neighbouring S atoms in the plane of the layer leads to the conclusion that, when such doping atom is present acting to stabilize the pyrite structure, by replacing the weak dispersive forces between layers with chemically bonded interactions between the S and the doping Co atom. This is likely to have a significant impact upon the surface energies of the doped pyrite structure compared to the undoped. We can understand the onset of H adsorption and the sequent onset of hydrogen evolution as the potential (energy barrier) is decreased further. The overall energy profile is shown in Figure 4c. From the energy profile, one can see that cobalt in FeCoS2(110) system plays an important role in reducing adsorption energy. The removal of H2 is facilitated by cobalt, which act by weakening the S-H bond, and by promoting the adsorption of 2Had H2. The synthesis of GO solution GO solution was prepared by Hummers method with some modifications.8 Raw graphite (2.5g) was mixed with of 1.5 g NaNO3 (purity 99%) and 67.5 ml of H2SO4 (purity 96%). The mixture was stirred while being cooled in an ice water bath. KMnO4 (9g, purity 99%) was gradually added within a hour. The mixture was kept in the ice-bath cooling for 2 hours and then was allowed to stand for five days at approximately 20 oC with gentle stirring. In order to wash out the excess reactant, 1L of 5 wt% H2SO4 aqueous solution was added to the resultant mixture and stirred for 2 hours. Afterward, 30 g of H2O2 (30 wt% aqueous solution) was added to reduce the excess KMnO4. Manganese ions from oxidant were removed by repeat wash with aqueous solution of 3 wt% H2SO4/0.5 wt% H2O2. DI-water was added to the final product and then vortex well to make a uniform suspension for storage. Preparation of Fe0.9Co0.1S2/rGO Hybrid catalysts: In a typical synthesis of Fe0.9Co0.1S2/rGO hybrid catalyst, ~2 mL oxidized GO solution were sonicated in 8 mL of anhydrous N,N-dimethylformamide (DMF) for 10 min, followed by addition of 0.8 mL of 0.2 M iron nitrite (Fe(NO)3), 2mL of 1M thioacetaminde (TAA) and 0.08 mL of 0.2 M cobalt acetate (Co(Ac)2) aqueous solution. The mixture was vigorously stirred at 90 °C in an oil bath for 24 hours. The suspension was centrifuged and washed with H2O twice to remove residue with no reacting and then the precipitant was obtained. The precipitant was re-dissolved into 8 mL of DMF. The suspension was heated to and maintained at 180 oC for 5 hours in an autoclave. After cooling down to room temperature, the sample was collected during centrifugation and washing with water for several times and finally lyophilized to get solid Fe0.9Co0.1S2/rGO hybrid catalyst. Preparation of Fe0.9Co0.1S2 catalysts: In a typical synthesis of Fe0.9Co0.1S2 catalyst, 0.8 mL of 0.2 M iron nitrite (Fe(NO)3), 2mL of 1M thioacetaminde (TAA) and 0.08 mL of 0.2 M cobalt acetate (Co(Ac)2) aqueous solution were mixed in 8 mL of anhydrous N,N-dimethylformamide (DMF) for 10 min. The mixture was vigorously stirred at 90 °C in an oil bath for 24 hours. The suspension was centrifuged and washed with H2O twice to remove residue with no reacting and then the precipitant was obtained. The precipitant was re-dissolved into 8 mL of DMF. The suspension was heated to and maintained at 180 oC for 5 hours in an autoclave. After cooling down to room temperature, the sample was collected during centrifugation and washing with water for several times and finally lyophilized to get solid Fe0.9Co0.1S2 catalyst. Estimation of turnover frequencies: To calculate the turnover frequency (TOF), the density of the sample was first calculated using the FeS2 unit cell. Pyrite crystallizes within the cubic space group (space group No. 205) with four formula units FeS2 per unit cell (a =5.418 Å). The unit cell indicated by the frame contains ideally four Fe atoms and eight S atoms. The measured BET surface areas were used as the “actual” surface area values in the calculations. The TOFs are reported as turnovers per second per
S2
surface atom. Since the active site of metal sulfide for HER is sulfur atom, here only sulfur atoms in unit cell were counted as hydrogen binding site. The procedure of our TOF calculation is provided below: Surface active atoms per square meter area (six planes and eight sulfur atoms in a unit cell):
Surface active atoms of Fe0.9Co0.1S2/CNT hybrid catalyst on glass carbon electrode (56 µg):
Turnover frequency at overpotential of 170 mV (an absolute current of 2 mA and a current density of 10 mA/cm2):
Fig. S1. (A) SEM image of Fe0.9Co0.1S2-CNT hybrid catalysts with sheet structure. (B) and (C) HRTEM images of Fe0.9Co0.1S2-CNT hybrid catalysts. The clear lattice image of Fe0.9Co0.1S2 and CNT materials was obtained.
S3
Fig. S2. (A) FT-EXAFS spectra at the Fe K-edge of FeS2/CNT and Fe0.9Co0.1S2/CNT hybrid catalysts and its fitted curve. (B) FT-EXAFS spectra at the Co K-edge of Fe0.9Co0.1S2/CNT hybrid catalysts and its fitted curve.
Fig. S3. (A) and (B) XANES spectra at Fe K-edge and Co K edge of a series of iron pyrite/CNT hybrid catalysts, respectively. (C) and (D) FT-EXAFS spectra at the Fe K-edge and Co K-edge of a series of iron pyrite/CNT hybrid catalysts, respectively.
S4
Fig. S4. FT-EXAFS spectra at the Fe K-edge of bulk iron pyrite materials.
Fig. S5. (A) Fe 2p, (B) Co 2p and (C) S 2p spectra of XPS measurement on the as-prepared Fe0.9Co0.1S2/CNT hybrid catalysts.
S5
Fig. S6. SEM image of pure FeS2 catalysts prepared by hydrothermal method.
Fig S7. The optimized geometries of the H2 molecule on the (a) Fe0.9Co0.1S2 (b) FeS2 nanocluster with a conductive carbon nanotube-like substrate, respectively
S6
Fig. S8. The energy barrier profiles of hydrogen evolution reaction on Pt (111) catalyst.
S7
Table S1. EXAFS Fitting Parameters at the Fe K-Edge and Co K-Edge of FeS2/CNT and Fe0.9Co0.1S2/CNT hybrid catalysts.a
Samples
shell
N
Rj (A)
σ2(×10-3) (A2)
∆E0 (eV)
Fe K-edge Fe-S
6.0(±0.4)
2.2678(±0.0116)
5.8(±0.6)
5.7
Fe-Fe
12.0(±2.0)
3.8571(±0.0300)
8.8(±1.1)
8.3
Fe-S
4.83(±0.11)
2.2411(±0.0066)
9.0
-0.14
Fe-Fe/Co
6.10(±0.45)
3.8041(±0.0260)
13.1
-1.29
Fe-S
4.43(±0.28)
2.2411(±0.0139)
9.0
4.65
Fe-Fe/Co
6.49(±1.16)
3.8041(±0.0459)
13.1
-3.55
8.7
-6.0
Bulk FeS2
Fe0.9Co0.1S2/CNT
Pure FeS2/CNT
Co K-edge Co-S
5.57(±0.56)
Co-Fe
5.12(±1.47)
2.2373(±0.0065)
Fe0.9Co0.1S2/CNT 3.8517(±0.0722) 17.9 5.4 aN, coordination number; R , bonding distance; s2, Debye−Waller factor; ∆E , inner potential shift. j 0
.
Table S2. H and H2 binding on Pt(111) slab and FeS2(110) and FeCoS2(110) slab surfaces; adsorption energies and optimized geometries.
System
H binds on site of FeS2 (110)
H2 binds on S site of FeS2 (110)
H binds on S sites of FeCoS2 (110)
H2 binds on S site of FeCoS2(110)
H binds on site of Pt (111)
H2 binds on S site of Pt (111)
Adsorption energy (eV)
-0.35
-0.32
-0.41
-0.23
-0.19
-0.21
H-S: 1.36
H-S: 1.36
H-S: 1.36
H-S: 1.36
H-Pt: 1.57
H-Pt: 1.58
Fe-S: 2.16
Fe-S: 2.219
Fe-S: 2.17 Co-S: 2.22
Fe-S: 2.16 Co-S: 2.27
Pt-Pt: 2.77
Pt-Pt: 2.77
Bond length (Å)
S8
Fig. S9. XPS spectra of CNTs with oxidation degrees of (a) 1x and (b) 4.5x. The oxygen content of 1xCNT and 4.5xCNT were estimated around 20% and 30%, respectively.
Fig. S10.Electrochemical impedance spectroscopy data for CNT with oxidation degree of 1x, 2x and 4.5x in 0.5M H2SO4. The data were collected for the electrodes under HER overpotential~ 100mV.
S9
Fig. S11. Raman spectrum of Fe0.9Co0.1S2/CNT hybrid catalyst.
Fig. S12. Polarization curves and amount of hydrogen gas were recorded on Ti foil with loading of 7 mg/cm2. The hydrogen gas generated on the hybrid catalyst on Ti foil was collected in the seal glass container under different applying voltages. The gas sample (1mL) was collected by a micro syringes and measured under exsitu gas chromatography with TCD detector. Also, the calculated Faraday yield of hydrogen production was over 99 % during 10 min of electrolysis.
Reference: [1] Cambridge Serial Total Energy Package, distributed by Accelrys Inc., San Diego. [2] Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H. Int. J. Quantum Chem. 2000, 77, 895. S10
[3] Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 136, 864. [4] Kohn, W.; Shan, L. Phys. Rev. 1965, 140, A1133. [5] Payne, M. C.; Teter, M.; Allan, D.; Arias, T.; Joannopoulos, J. Rev. Mod. Phys. 1992, 64, 1045. [6] Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868. [7] Tkatchenko; A.; Scheffler, M. Phys. Rev. Lett. 2009, 102, 073005. [8] Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.
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Department of Chemistry, Stanford University, Stanford, CA 94305, USA
2
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
3
Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan
4
Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 5
Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 6
Department of Materials Science and Engineering, National Taiwan University, Taipei, 10617, Taiwan
#
These authors contributed equally to this work
Corresponding Author * [email protected], [email protected], and [email protected]
Experiment Details Computational method: Computational method: All calculations in this work were performed using the CASTEP program [1,2], which employs the plane wave pseudopotential method to calculated the total energy within the framework of the Kohn-Shan DFT [3-5]. We used the PBE exchange-correlation functional [6] with TS dispersion correction [7] in describing the vdW interaction. The ion-electron interaction was modeled by the non-local real space, ultrasoft pseudopotential with a cutoff energy of 400 eV. DFT simulations were performed based on a Pt (111) slab model system. The surface is constructed as slab within the 3-D periodic boundary conditions, and models are separated from their images in the direction perpendicular to the surface by a 14 Å vacuum layer. To model the Pt (111) slab surface, we have adopted slabs with three layers, sixteen atoms per layer. The bottom layer was kept fixed to the bulk coordinates; full atomic relaxations were allowed for the top two layers. For these calculations, a 2 × 2 × 1 k-Point mesh was used in the 4 × 4 supercell. In the FeS2(110) (Fe:8, S:16) and FeCoS2(110) (Fe:7, Co:1, S:16) slab models, we have adopted slabs with six layers, four atoms per layer. The bottom two layers were kept fixed to the bulk coordinates; full atomic relaxations were allowed for the top four layers. For these calculations, a 3 × 3 × 1 k-Point mesh was used in the 2 × 2 supercell. For sake of comparison, the slab system was replaced Fe with a Co atom, the relaxation on internal coordinates at a variety of cutoff energies is repeated. All clusters are put on carbon nanotube-like substrates. The system would gradually become relaxed to achieve a balanced state with lowest energy. Total energy calculations were achieved when the residual total energy of 0.01 eV and the maximal force of 0.01 eV/ Å were reached.
The potential Energy diagram for Hydrogen Evolution Reaction: The energy per adatom can be calculated from the differential adsorption energy, Δ EH, and was derived as
∆E H =
1 n E (surf + nH ) − E (surf ) − E (H 2 ) , where n is the number of hydrogen atoms adsorbed n 2
on the surface and E(surf+nH) is the total energy of the surface with n atoms adsorbed. The negative sign S1
of EH corresponds to the energy gain of the system because of H atom adsorption. Our DFT results for the hydrogen evolution reaction gives a molecular level picture of the process, which is in good agreement with experimental observations. The finding that there is a direct interaction between the doping atom and the neighbouring S atoms in the plane of the layer leads to the conclusion that, when such doping atom is present acting to stabilize the pyrite structure, by replacing the weak dispersive forces between layers with chemically bonded interactions between the S and the doping Co atom. This is likely to have a significant impact upon the surface energies of the doped pyrite structure compared to the undoped. We can understand the onset of H adsorption and the sequent onset of hydrogen evolution as the potential (energy barrier) is decreased further. The overall energy profile is shown in Figure 4c. From the energy profile, one can see that cobalt in FeCoS2(110) system plays an important role in reducing adsorption energy. The removal of H2 is facilitated by cobalt, which act by weakening the S-H bond, and by promoting the adsorption of 2Had H2. The synthesis of GO solution GO solution was prepared by Hummers method with some modifications.8 Raw graphite (2.5g) was mixed with of 1.5 g NaNO3 (purity 99%) and 67.5 ml of H2SO4 (purity 96%). The mixture was stirred while being cooled in an ice water bath. KMnO4 (9g, purity 99%) was gradually added within a hour. The mixture was kept in the ice-bath cooling for 2 hours and then was allowed to stand for five days at approximately 20 oC with gentle stirring. In order to wash out the excess reactant, 1L of 5 wt% H2SO4 aqueous solution was added to the resultant mixture and stirred for 2 hours. Afterward, 30 g of H2O2 (30 wt% aqueous solution) was added to reduce the excess KMnO4. Manganese ions from oxidant were removed by repeat wash with aqueous solution of 3 wt% H2SO4/0.5 wt% H2O2. DI-water was added to the final product and then vortex well to make a uniform suspension for storage. Preparation of Fe0.9Co0.1S2/rGO Hybrid catalysts: In a typical synthesis of Fe0.9Co0.1S2/rGO hybrid catalyst, ~2 mL oxidized GO solution were sonicated in 8 mL of anhydrous N,N-dimethylformamide (DMF) for 10 min, followed by addition of 0.8 mL of 0.2 M iron nitrite (Fe(NO)3), 2mL of 1M thioacetaminde (TAA) and 0.08 mL of 0.2 M cobalt acetate (Co(Ac)2) aqueous solution. The mixture was vigorously stirred at 90 °C in an oil bath for 24 hours. The suspension was centrifuged and washed with H2O twice to remove residue with no reacting and then the precipitant was obtained. The precipitant was re-dissolved into 8 mL of DMF. The suspension was heated to and maintained at 180 oC for 5 hours in an autoclave. After cooling down to room temperature, the sample was collected during centrifugation and washing with water for several times and finally lyophilized to get solid Fe0.9Co0.1S2/rGO hybrid catalyst. Preparation of Fe0.9Co0.1S2 catalysts: In a typical synthesis of Fe0.9Co0.1S2 catalyst, 0.8 mL of 0.2 M iron nitrite (Fe(NO)3), 2mL of 1M thioacetaminde (TAA) and 0.08 mL of 0.2 M cobalt acetate (Co(Ac)2) aqueous solution were mixed in 8 mL of anhydrous N,N-dimethylformamide (DMF) for 10 min. The mixture was vigorously stirred at 90 °C in an oil bath for 24 hours. The suspension was centrifuged and washed with H2O twice to remove residue with no reacting and then the precipitant was obtained. The precipitant was re-dissolved into 8 mL of DMF. The suspension was heated to and maintained at 180 oC for 5 hours in an autoclave. After cooling down to room temperature, the sample was collected during centrifugation and washing with water for several times and finally lyophilized to get solid Fe0.9Co0.1S2 catalyst. Estimation of turnover frequencies: To calculate the turnover frequency (TOF), the density of the sample was first calculated using the FeS2 unit cell. Pyrite crystallizes within the cubic space group (space group No. 205) with four formula units FeS2 per unit cell (a =5.418 Å). The unit cell indicated by the frame contains ideally four Fe atoms and eight S atoms. The measured BET surface areas were used as the “actual” surface area values in the calculations. The TOFs are reported as turnovers per second per
S2
surface atom. Since the active site of metal sulfide for HER is sulfur atom, here only sulfur atoms in unit cell were counted as hydrogen binding site. The procedure of our TOF calculation is provided below: Surface active atoms per square meter area (six planes and eight sulfur atoms in a unit cell):
Surface active atoms of Fe0.9Co0.1S2/CNT hybrid catalyst on glass carbon electrode (56 µg):
Turnover frequency at overpotential of 170 mV (an absolute current of 2 mA and a current density of 10 mA/cm2):
Fig. S1. (A) SEM image of Fe0.9Co0.1S2-CNT hybrid catalysts with sheet structure. (B) and (C) HRTEM images of Fe0.9Co0.1S2-CNT hybrid catalysts. The clear lattice image of Fe0.9Co0.1S2 and CNT materials was obtained.
S3
Fig. S2. (A) FT-EXAFS spectra at the Fe K-edge of FeS2/CNT and Fe0.9Co0.1S2/CNT hybrid catalysts and its fitted curve. (B) FT-EXAFS spectra at the Co K-edge of Fe0.9Co0.1S2/CNT hybrid catalysts and its fitted curve.
Fig. S3. (A) and (B) XANES spectra at Fe K-edge and Co K edge of a series of iron pyrite/CNT hybrid catalysts, respectively. (C) and (D) FT-EXAFS spectra at the Fe K-edge and Co K-edge of a series of iron pyrite/CNT hybrid catalysts, respectively.
S4
Fig. S4. FT-EXAFS spectra at the Fe K-edge of bulk iron pyrite materials.
Fig. S5. (A) Fe 2p, (B) Co 2p and (C) S 2p spectra of XPS measurement on the as-prepared Fe0.9Co0.1S2/CNT hybrid catalysts.
S5
Fig. S6. SEM image of pure FeS2 catalysts prepared by hydrothermal method.
Fig S7. The optimized geometries of the H2 molecule on the (a) Fe0.9Co0.1S2 (b) FeS2 nanocluster with a conductive carbon nanotube-like substrate, respectively
S6
Fig. S8. The energy barrier profiles of hydrogen evolution reaction on Pt (111) catalyst.
S7
Table S1. EXAFS Fitting Parameters at the Fe K-Edge and Co K-Edge of FeS2/CNT and Fe0.9Co0.1S2/CNT hybrid catalysts.a
Samples
shell
N
Rj (A)
σ2(×10-3) (A2)
∆E0 (eV)
Fe K-edge Fe-S
6.0(±0.4)
2.2678(±0.0116)
5.8(±0.6)
5.7
Fe-Fe
12.0(±2.0)
3.8571(±0.0300)
8.8(±1.1)
8.3
Fe-S
4.83(±0.11)
2.2411(±0.0066)
9.0
-0.14
Fe-Fe/Co
6.10(±0.45)
3.8041(±0.0260)
13.1
-1.29
Fe-S
4.43(±0.28)
2.2411(±0.0139)
9.0
4.65
Fe-Fe/Co
6.49(±1.16)
3.8041(±0.0459)
13.1
-3.55
8.7
-6.0
Bulk FeS2
Fe0.9Co0.1S2/CNT
Pure FeS2/CNT
Co K-edge Co-S
5.57(±0.56)
Co-Fe
5.12(±1.47)
2.2373(±0.0065)
Fe0.9Co0.1S2/CNT 3.8517(±0.0722) 17.9 5.4 aN, coordination number; R , bonding distance; s2, Debye−Waller factor; ∆E , inner potential shift. j 0
.
Table S2. H and H2 binding on Pt(111) slab and FeS2(110) and FeCoS2(110) slab surfaces; adsorption energies and optimized geometries.
System
H binds on site of FeS2 (110)
H2 binds on S site of FeS2 (110)
H binds on S sites of FeCoS2 (110)
H2 binds on S site of FeCoS2(110)
H binds on site of Pt (111)
H2 binds on S site of Pt (111)
Adsorption energy (eV)
-0.35
-0.32
-0.41
-0.23
-0.19
-0.21
H-S: 1.36
H-S: 1.36
H-S: 1.36
H-S: 1.36
H-Pt: 1.57
H-Pt: 1.58
Fe-S: 2.16
Fe-S: 2.219
Fe-S: 2.17 Co-S: 2.22
Fe-S: 2.16 Co-S: 2.27
Pt-Pt: 2.77
Pt-Pt: 2.77
Bond length (Å)
S8
Fig. S9. XPS spectra of CNTs with oxidation degrees of (a) 1x and (b) 4.5x. The oxygen content of 1xCNT and 4.5xCNT were estimated around 20% and 30%, respectively.
Fig. S10.Electrochemical impedance spectroscopy data for CNT with oxidation degree of 1x, 2x and 4.5x in 0.5M H2SO4. The data were collected for the electrodes under HER overpotential~ 100mV.
S9
Fig. S11. Raman spectrum of Fe0.9Co0.1S2/CNT hybrid catalyst.
Fig. S12. Polarization curves and amount of hydrogen gas were recorded on Ti foil with loading of 7 mg/cm2. The hydrogen gas generated on the hybrid catalyst on Ti foil was collected in the seal glass container under different applying voltages. The gas sample (1mL) was collected by a micro syringes and measured under exsitu gas chromatography with TCD detector. Also, the calculated Faraday yield of hydrogen production was over 99 % during 10 min of electrolysis.
Reference: [1] Cambridge Serial Total Energy Package, distributed by Accelrys Inc., San Diego. [2] Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H. Int. J. Quantum Chem. 2000, 77, 895. S10
[3] Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 136, 864. [4] Kohn, W.; Shan, L. Phys. Rev. 1965, 140, A1133. [5] Payne, M. C.; Teter, M.; Allan, D.; Arias, T.; Joannopoulos, J. Rev. Mod. Phys. 1992, 64, 1045. [6] Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868. [7] Tkatchenko; A.; Scheffler, M. Phys. Rev. Lett. 2009, 102, 073005. [8] Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.
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