Research ArticleELECTROCHEMISTRY

In situ formation of molecular Ni-Fe active sites on heteroatom-doped graphene as a heterogeneous electrocatalyst toward oxygen evolution

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Science Advances  09 Mar 2018:
Vol. 4, no. 3, eaap7970
DOI: 10.1126/sciadv.aap7970
  • Fig. 1 Characterizations of HG-Ni.

    (A) TEM images of HG-Ni at different magnifications. (B) Normalized Ni K-edge XAS data of HG-Ni and Ni(acac)2. The inset presents the pre-edge region. a.u., arbitrary units. (C) Fourier-transformed EXAFS curves at Ni K-edge. (D) A DFT calculation–derived model to indicate the conjugation of Ni(acac)2 onto HG. The dopant of HG was a sulfoxide group as one representative interacting site. The gray, white, red, and yellow spheres represent C, H, O, and S atoms, respectively. The Ni atom is represented by a dark blue sphere.

  • Fig. 2 Electrochemical tests of HG-Ni.

    CV polarization curves of HG-Ni conducted in (A) a glass cell, 1 M KOH without any treatment; (B) a plastic vessel, 1 M KOH with removal of Fe impurity. (C) Steady CV curves of HG-Ni performed in a glass cell in 1 M KOH solution containing different contents of FeCl3 from 2.4 to 12 μM and (D) the corresponding TOF analysis after the potentials were compensated for iR drop. (E) Comparison of the initial and steady CV curves of HG-Ni conducted in a glass cell, 1 M KOH + 12 μM FeCl3. Scan rate, 50 mV s−1; rotation rate, 2000 rpm.

  • Fig. 3 OER assessments of HG-NiFes.

    (A) iR-compensated LSVs of HG-NiFe and HG-NiFex in 1 M KOH (5 mV s−1; 2000 rpm), corresponding Tafel slopes (B) and TOF analysis (C).

  • Fig. 4 Formation of molecular Ni-Fe sites.

    (A) Normalized Ni K-edge XAS data of HG-Ni and HG-NiFe. The inset presents the pre-edge region. (B) Fourier-transformed EXAFS curves at Ni K-edge. (C) Normalized Fe K-edge XAS data of HG-NiFe, FeOOH, and FeCl3. (D) Fourier-transformed EXAFS curves at Fe K-edge.

  • Fig. 5 Adsorption of HO ions at Ni-Fe sites.

    (A) ks of metal redox in HG-Ni, HG-NiFex, and HG-NiFe. (B) Pourbaix diagram, formal potentials of redox versus pH values of KOH solutions. The inset shows the proposed configuration for the adsorption of HO ions onto Ni-Fe sites before OER.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaap7970/DC1

    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.

  • Supplementary Materials

    This PDF file includes:

    • 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.

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