Supplementary Materials for

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1 advances.sciencemag.org/cgi/content/full/4/3/eaap7970/dc1 Supplementary Materials for In situ formation of molecular Ni-Fe active sites on heteroatom-doped graphene as a heterogeneous electrocatalyst toward oxygen evolution Jiong Wang, Liyong Gan, Wenyu Zhang, Yuecheng Peng, Hong Yu, Qingyu Yan, Xinghua Xia, Xin Wang This PDF file includes: Published 9 March 2018, Sci. Adv. 4, eaap7970 (2018) DOI: /sciadv.aap7970 fig. S1. High-resolution TEM images of HG-Ni. fig. S2. SEM image of HG-Ni with EDX analysis. fig. S3. TEM image of HG-Ni with EDX analysis. fig. S4. FTIR spectra of Ni(acac)2, HG, and HG-Ni. fig. S5. UV-visible spectra of Ni(acac)2, HG, and HG-Ni. fig. S6. Schematic illustrations of electrochemical tests. fig. S7. CVs of HG-NiFe in 1 M KOH before and after removal of FeCl3. fig. S8. GC spectrum of O2 detection. fig. S9. OER durability on HG-NiFe. fig. S10. TEM image of HG-NiFe with EDX analysis. fig. S11. TEM images of HG-NiFe. fig. S12. XRD patterns of HG, HG-Ni, and HG-NiFe. fig. S13. EDTA treatment of HG-NiFe. fig. S14. CVs of HG in 1 M KOH containing FeCl3. fig. S15. Comparisons of CVs of HG-NiFe and HG-Fe. fig. S16. DFT model of Ni-Fe sites. fig. S17. Analysis of HG-NiFe in Laviron equation. fig. S18. Analysis of HG-NiFex in Laviron equation. fig. S19. Analysis of HG-Ni in Laviron equation. fig. S20. Pourbaix diagram of HG-Ni. fig. S21. CO poisoning of HG-NiFe. fig. S22. CV and TOF analysis of a control sample. fig. S23. Analysis of a control sample in Laviron equation.

2 fig. S1. High-resolution TEM images of HG-Ni. High resolution TEM images of HG-Ni, enlarged at the regions shown in the Fig. 1A, left. fig. S2. SEM image of HG-Ni with EDX analysis. A SEM image of HG-Ni, and the corresponding elemental mapping results analyzed by EDX spectrum.

3 fig. S3. TEM image of HG-Ni with EDX analysis. A TEM image of HG-Ni, and the corresponding elemental mapping results by EDX analysis. fig. S4. FTIR spectra of Ni(acac)2, HG, and HG-Ni.

4 fig. S5. UV-visible spectra of Ni(acac)2, HG, and HG-Ni. UV-visible spectra of HG-Ni, HG and Ni(acac)2 collected in ethanol. fig. S6. Schematic illustrations of electrochemical tests. Scheme illustrates the electrochemical tests of HG-Ni performed in (A) 1 M KOH without any treatment, corresponding to the CVs in Fig. 2A; (B) in purified 1 M KOH, a plastic vessel was applied as the electrochemical cell, corresponding to the CVs in Fig. 2B; (C) in 1 M KOH with addition of FeCl3, corresponding to the CVs in Fig. 2C and E.

5 fig. S7. CVs of HG-NiFe in 1 M KOH before and after removal of FeCl3. CVs of HG-NiFe in 1 M KOH with the addition of 12 μm FeCl3 (black solid) and in the freshly prepared 1 M KOH (red dashed), respectively. fig. S8. GC spectrum of O2 detection. A representative GC spectrum for the detection of O2 product of OER on HG-NiFe in 1 M KOH, potential: 1.58 V. Inset is the corresponding digital photo during the OER measurement focusing on the anode.

6 fig. S9. OER durability on HG-NiFe. Current retention of HG-NiFe loaded on a carbon fiber cloth in 1 M KOH at fixed potential, the initial current density was 10 ma cm -2. fig. S10. TEM image of HG-NiFe with EDX analysis. A typical TEM image of HG-NiFe with corresponding elemental mapping results by EDX analysis.

7 fig. S11. TEM images of HG-NiFe. TEM images of HG-NiFe at different magnifications. fig. S12. XRD patterns of HG, HG-Ni, and HG-NiFe.

8 fig. S13. EDTA treatment of HG-NiFe. (A) CV curves of HG-NiFe before and after being treated with 2 mm EDTA solution for 12 h, 1 M KOH, scan rate: 50 mv s -1, rotation rate: 2000 rpm. (B) UV-visible spectra of EDTA solution reacted with HG-NiFe surface, Ni(acac)2, FeCl3, bulk Ni(OH)2 and Ni(OH)2 nanostructures. (C) The electron microscopic images of Ni(OH)2 nanostructures, it showed the assembly of nanosheets with edge sizes of about 5 nm. fig. S14. CVs of HG in 1 M KOH containing FeCl3. (A) Scheme illustrates the electrochemical tests of HG in 1 M KOH with the addition of FeCl3. (B) CV curves of HG before and after addition of 12 μm FeCl3, scan rate: 50 mv s -1, rotation rate: 2000 rpm.

9 fig. S15. Comparisons of CVs of HG-NiFe and HG-Fe. (A) Comparisons of CVs of HG-NiFe (up) and HG-Fe (bottom) in 1 M KOH, 50 mv s -1, 2000 rpm. (B) Corresponding OER assessments of HG-Fe. In this control experiment, we tried to have a rough estimation on the redox potential of Fe 2+/3+ couple at the HG based environment. The HG-Fe was synthesized using the same strategy as the synthesis of HG-Ni. Namely, mixing the HG with 0.1 M Fe(acac)3 in DMF at 80 o C for 12 h. By such efforts, it was still difficult to conjugate Fe 3+ ions to HG as suggested by the low redox currents. But it was found that the redox potential of Fe 2+/3+ couples could be close to that of Ni 2+/3+ couple. For OER, the catalytic currents on HG-Fe were very low with high overpotentials, which could be assigned to the low coverage of electroactive Fe sites (i.e., ~ mol cm -2 ; in contrast, the electroactive Ni-Fe sites of HG-NiFe were ~ mol cm -2 ). The HG-Fe exhibited instability that serious degradation of anodic current was observed. We roughly estimated the TOFs of Fe site on HG-Fe at high overpotentials based on the first LSV curves. It was found that the TOF of Fe sites could reach 0.57 s -1 at an overpotential of 0.38 V (1.12 s -1 for HG-NiFe, and 0.18 s -1 for HG-NiFex). Such a result might imply that the Fe sites were at least more OER active than the Ni sites, so that it could further supply an implication that in the configurations of Ni-Fe dual sites, the Fe sites could be more responsible for proceeding the OER catalysis.

10 fig. S16. DFT model of Ni-Fe sites. A DFT calculation derived model for indicating the formation of dual Ni-Fe clusters. The gray, white and green spheres represent C, H and Cl atoms, respectively. The dark-blue and blue spheres represent Ni and Fe atoms respectively. fig. S17. Analysis of HG-NiFe in Laviron equation. (A) CVs of HG-NiFe with scan rates from 5, 10, 20, 50, 100, 200, 300, 400, 500, 600 to 700 mv s -1, 1 M KOH. (B) The plot of the redox peak currents densities versus the square root of scan rates. (C) The plot of the redox peak potentials versus the logarithm of scan rates.

11 fig. S18. Analysis of HG-NiFex in Laviron equation. (A) CVs of HG-NiFex with scan rates from 5, 10, 20, 50, 100, 200, 300, 400, 500, 600 to 700 mv s -1, 1 M KOH. (B) The plot of the redox peak currents densities versus the square root of scan rates. (C) The plot of the redox peak potentials versus the logarithm of scan rates. fig. S19. Analysis of HG-Ni in Laviron equation. (A) CVs of HG-Ni with scan rates from 5, 10, 20, 50, 100, 200, 300, 400, 500, 600 to 700 mv s -1, purified 1 M KOH. (B) The plot of the redox peak currents densities versus the square root of scan rates. (C) The plot of the redox peak potentials versus the logarithm of scan rates.

12 fig. S20. Pourbaix diagram of HG-Ni. Pourbaix diagram of HG-Ni collected in the plastic cell, formal potentials of redox (E1/2) versus ph values of purified KOH solutions. fig. S21. CO poisoning of HG-NiFe. CV polarization curves of HG-NiFe before and after being treated with CO for 4 h, 1 M KOH, scan rate: 50 mv s -1, rotation rate: 2000 rpm.

13 fig. S22. CV and TOF analysis of a control sample. (A) CVs of HG-Ni in absence of OER polarization, 1 M KOH with the addition of 12 μm FeCl3, 50 mv s -1 scan rate, 2000 rpm. (B) LSV and TOFs, collected at the steady state, fresh 1 M KOH, 5 mv s -1 scan rate, 2000 rpm. fig. S23. Analysis of a control sample in Laviron equation. (A) CVs of the above steady electrode (fig. S22) with scan rates from 5, 10, 20, 50, 100, 200, 300, 400, 500, 600 to 700 mv s -1, fresh 1 M KOH. (B) The plot of the redox peak currents densities versus the square root of scan rates. (C) The plot of the redox peak potentials versus the logarithm of scan rates.