Research ArticleChemistry

Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: Development of highly efficient metal-free bifunctional electrocatalyst

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Science Advances  22 Apr 2016:
Vol. 2, no. 4, e1501122
DOI: 10.1126/sciadv.1501122
  • Fig. 1 Synthesis of N-GRW.

    (A) Synthesis steps: (1) polymerization at 600°C for 2 hours, and (2) pyrolysis and carbonization at 800° to 1000°C. (B) Digital photograph of the as-synthesized N-GRW. (C) Scanning electron microscopy (SEM) image of the as-synthesized N-GRW.

  • Fig. 2 Structure and composition of N-GRW.

    (A) High-resolution SEM image of the N-GRW. (B and C) Low-magnification (B) and high-magnification (C) TEM images of the N-GRW. (D) AFM image and height profile of the N-GRW on mica substrate (scale bar, 200 nm). (E) Barrett-Joyner-Halenda pore size distribution and pore volume of the N-GRW calculated from N2 desorption isotherm. (F) XPS spectrum of the N-GRW. Inset shows a high-resolution N1s spectrum with peaks deconvoluted into pyridinic (397.8 eV), pyrrolic (398.9 eV), quaternary (400.8 eV), and oxidized (402.0 eV) N species. a.u., arbitrary units.

  • Fig. 3 Electrochemical performances of N-doped graphene catalysts for ORR and OER.

    (A) LSV curves of N-doped graphene catalysts on RRDE in O2-saturated 1 M KOH at a rotation speed of 1600 rpm and a scan rate of 5 mV s1, with a constant potential of 1.5 V versus RHE applied on the ring. Disc current is displayed on the lower half of the graph, whereas the ring current (dotted line) is shown on the upper half. (B) Chronoamperometric (current-time) responses of Pt/C and the N-GRW for ORR at 0.7 V versus RHE, in 1 M KOH, at a rotation speed of 900 rpm. Inset shows the crossover effect of the N-GRW and Pt/C electrodes at 0.7 V versus RHE, followed by introduction of methanol (3 M) in O2-saturated 1 M KOH. (C) LSV curves of the N-GRW before and after ADT, performed in 1 M KOH at a scan rate of 50 mV s1 for the N-GRW and Pt/C. (D) LSV curves for OER on RDE for the N-GRW, N-HGS, and N-GS in O2-saturated 1 M KOH at a rotation speed of 1600 rpm and a scan rate of 5 mV s1. (E) Chronoamperometric (current-time) responses for OER at fixed overpotential of 320 mV (for Ir/C) and 360 mV (for the N-GRW). Inset shows chronopotentiometric (potential-time) response at a fixed current loading of 10 mA cm2. (F) LSV curves for OER before and after stability test for Ir/C and the N-GRW.

  • Fig. 4 Electronic characteristics and ORR/OER active sites of N-doped graphene catalysts.

    (A) UPS spectra collected using an He I (21.2 eV) radiation. Inset shows the enlarged view of the secondary electron tail threshold. (B and C) Carbon and nitrogen K-edge XANES spectra of N-GRW catalyst, acquired under ultrahigh vacuum, pristine (black line), after ORR (yellow line) and after OER (blue line). In carbon K-edge XANES spectra, A: defects, B: π*C=C, C: π*C–OH, D: π*C–O–C, C–N, E: π*C=O, COOH, F: σ*C–C. (D) Schematic diagram of ORR and OER occurring at different active sites on the n- and p-type domains of the N-GRW catalyst.

  • Fig. 5 Application of N-GRW bifunctional catalyst in rechargeable zinc-air batteries.

    (A) Schematic of a zinc-air battery at charging and discharging conditions. (B) Galvanodynamic charge/discharge profiles and power density curves of zinc-air batteries assembled from the N-GRW, Pt/C, Ir/C, and mixed Pt/C + Ir/C (1:1 by weight) air electrode (fig. S36), respectively. (C) Discharge curves of zinc-air batteries assembled from the N-GRW and Pt/C catalysts at 5 and 20 mA cm2 discharging rate. (D) Charging/discharging cycling at a current density of 2 mA cm2. Insets show the initial and after long time cycling testing charging/discharging curves of a zinc-air battery assembled from N-GRW as air catalyst. (E) Charging/discharging cycling curves of zinc-air batteries assembled from the N-GRW (yellow line) and mixed Pt/C + Ir/C air electrode (blue line) at a current density of 20 mA cm2 (catalyst loading amount: 0.5 mg cm2 for mixed Pt/C + Ir/C). (F) Photograph of an electrolysis cell powered by a zinc-air battery. Inset shows the bubble formation on both cathode and anode electrodes. All zinc-air batteries were tested in air at room temperature (catalyst loading amount: 0.5 mg cm2 for all zinc-air batteries).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/4/e1501122/DC1

    Experimental Section

    fig. S1. Molecular structures of precursors.

    fig. S2. Temperature and time profile of pyrolysis and carbonization process.

    fig. S3. FESEM images of the sample after pyrolysis at different temperatures.

    fig. S4. Low-magnification FESEM image of the N-GRW.

    fig. S5. FESEM images of samples prepared with different melamine–to–l-cysteine ratios.

    fig. S6. XPS spectra of C3N4 and S-doped C3N4.

    fig. S7. TEM and FESEM images of C3N4 and S-doped C3N4.

    fig. S8. Nitrogen adsorption isotherms of C3N4 and S-doped C3N4.

    fig. S9. TGA and heat flow curves of S-doped C3N4.

    fig. S10. XPS spectra of S-doped C3N4 and S-doped C3N4 carbonized at 800°C.

    fig. S11. Nitrogen adsorption isotherms of S-doped C3N4 and S-doped C3N4 carbonized at 800°C.

    fig. S12. S2p core XPS spectra of S-doped C3N4 and S-doped C3N4 carbonized at 800° to 1000°C.

    fig. S13. FESEM and TEM images of N-HGS and N-GS.

    fig. S14. Nitrogen adsorption isotherms and pore size distribution of N-doped graphene samples.

    fig. S15. C1s, N1s, and O1s core level high-resolution XPS spectra of the N-GRW, N-HGS, and N-GS.

    fig. S16. XRD patterns and Raman spectra of the N-GRW, N-HGS, and N-GS.

    fig. S17. LSV of N-doped graphene samples at different rotation speeds in the ORR region.

    fig. S18. Linear sweeping voltammograms and Koutecky-Levich plots of different catalysts in the ORR region.

    fig. S19. Cyclic voltammograms of N-doped graphene catalysts and Pt/C (20%) in Ar- and O2-saturated 1 M KOH.

    fig. S20. Peroxide yield and electron transfer number of N-doped graphene catalysts.

    fig. S21. Tafel plots of N-doped graphene catalysts in ORR region.

    fig. S22. Cyclic voltammograms of Pt/C and the N-GRW electrode in O2-saturated 0.1 M KOH filled with methanol.

    fig. S23. Current-time response of Pt/C and N-GRW for ORR with/without introducing CO into the electrolyte.

    fig. S24. Cycling durability of Pt/C and N-GRW.

    fig. S25. Linear sweeping voltammograms and Tafel plots of N-doped graphene catalysts in the OER region.

    fig. S26. RRDE measurements for the detection of H2O2 and O2 generated during the OER process.

    fig. S27. OER Tafel plots of N-doped graphene catalysts.

    fig. S28. Cyclic voltammograms of the N-GRW in the OER region at different scan rates.

    fig. S29. Faraday efficiency of OER measurement.

    fig. S30. Mott-Schottky plots of N-doped graphene catalysts in two potential regions.

    fig. S31. XPS valence band spectra of the N-GRW.

    fig. S32. Nyquist plots of N-doped graphene samples in the ORR and OER regions.

    fig. S33. The assembly processes for preparation of hybrid air cathode.

    fig. S34. The assembly processes for the fabrication of a rechargeable zinc-air battery.

    fig. S35. ORR and OER performances of air cathode tested in half-cell configuration.

    fig. S36. Charge/discharge profiles and power density curves of zinc-air batteries assembled from mixed Pt/C + Ir/C air electrode.

    fig. S37. Open-circuit voltage profiles of zinc-air batteries.

    fig. S38. Charging/discharging cycling curves of the N-GRW–loaded electrode at charging/discharging current densities of 2 mA cm−2.

    fig. S39. Charging/discharging cycling curves of Pt/C and Ir/C electrodes at charging/discharging current densities of 20 mA cm−2.

    fig. S40. XRD patterns of materials detached from Ti foil after long time charging.

    note S1. XPS study of C3N4 and S-doped C3N4 samples.

    note S2. Deconvolution of C1s, N1s, and O1s XPS spectra of N-doped graphene samples.

    note S3. Average crystallite size of the sp2 domains of N-doped graphene samples from Raman spectra.

    note S4. Brief explanation of ac impedance of N-doped graphene samples.

    table S1. Structural and compositional parameters of nitrogen-doped graphene catalysts.

    table S2. Surface nitrogen and oxygen species concentrations of nitrogen-doped graphene catalysts.

    table S3. Comparison of ORR and OER performances of our N-doped graphene nanoribbon networks with the recently reported highly active bifunctional catalysts.

    video S1. The evolution of O2 bubbles at potentials from 1.6 to 1.8 V versus RHE.

    videos S2 and S3. The demonstration of water splitting driven by a single zinc-air battery.

  • Supplementary Materials

    This PDF file includes:

    • Experimental Section
    • fig. S1. Molecular structures of precursors.
    • fig. S2. Temperature and time profile of pyrolysis and carbonization process.
    • fig. S3. FESEM images of the sample after pyrolysis at different temperatures.
    • fig. S4. Low-magnification FESEM image of the N-GRW.
    • fig. S5. FESEM images of samples prepared with different melamine–to–L-cysteine ratios.
    • fig. S6. XPS spectra of C3N4 and S-doped C3N4.
    • fig. S7. TEM and FESEM images of C3N4 and S-doped C3N4.
    • fig. S8. Nitrogen adsorption isotherms of C3N4 and S-doped C3N4.
    • fig. S9. TGA and heat flow curves of S-doped C3N4.
    • fig. S10. XPS spectra of S-doped C3N4 and S-doped C3N4 carbonized at 800°C.
    • fig. S11. Nitrogen adsorption isotherms of S-doped C3N4 and S-doped C3N4 carbonized at 800°C.
    • fig. S12. S2p core XPS spectra of S-doped C3N4 and S-doped C3N4 carbonized at 800° to 1000°C.
    • fig. S13. FESEM and TEM images of N-HGS and N-GS.
    • fig. S14. Nitrogen adsorption isotherms and pore size distribution of N-doped graphene samples.
    • fig. S15. C1s, N1s, and O1s core level high-resolution XPS spectra of the N-GRW, N-HGS, and N-GS.
    • fig. S16. XRD patterns and Raman spectra of the N-GRW, N-HGS, and N-GS.
    • fig. S17. LSV of N-doped graphene samples at different rotation speeds in the ORR region.
    • fig. S18. Linear sweeping voltammograms and Koutecky-Levich plots of different catalysts in the ORR region.
    • fig. S19. Cyclic voltammograms of N-doped graphene catalysts and Pt/C (20%) in Ar- and O2-saturated 1 M KOH.
    • fig. S20. Peroxide yield and electron transfer number of N-doped graphene catalysts.
    • fig. S21. Tafel plots of N-doped graphene catalysts in ORR region.
    • fig. S22. Cyclic voltammograms of Pt/C and the N-GRW electrode in O2-saturated 0.1 M KOH filled with methanol.
    • fig. S23. Current-time response of Pt/C and N-GRW for ORR with/without introducing CO into the electrolyte.
    • fig. S24. Cycling durability of Pt/C and N-GRW.
    • fig. S25. Linear sweeping voltammograms and Tafel plots of N-doped graphene catalysts in the OER region.
    • fig. S26. RRDE measurements for the detection of H2O2 and O2 generated during the OER process.
    • fig. S27. OER Tafel plots of N-doped graphene catalysts.
    • fig. S28. Cyclic voltammograms of the N-GRW in the OER region at different scan rates.
    • fig. S29. Faraday efficiency of OER measurement.
    • fig. S30. Mott-Schottky plots of N-doped graphene catalysts in two potential regions.
    • fig. S31. XPS valence band spectra of the N-GRW.
    • fig. S32. Nyquist plots of N-doped graphene samples in the ORR and OER regions.
    • fig. S33. The assembly processes for preparation of hybrid air cathode.
    • fig. S34. The assembly processes for the fabrication of a rechargeable zinc-air battery.
    • fig. S35. ORR and OER performances of air cathode tested in half-cell configuration.
    • fig. S36. Charge/discharge profiles and power density curves of zinc-air batteries assembled from mixed Pt/C + Ir/C air electrode.
    • fig. S37. Open-circuit voltage profiles of zinc-air batteries.
    • fig. S38. Charging/discharging cycling curves of the N-GRW–loaded electrode at charging/discharging current densities of 2 mA cm−2.
    • fig. S39. Charging/discharging cycling curves of Pt/C and Ir/C electrodes at charging/discharging current densities of 20 mA cm−2.
    • fig. S40. XRD patterns of materials detached from Ti foil after long time charging.
    • note S1. XPS study of C3N4 and S-doped C3N4 samples.
    • note S2. Deconvolution of C1s, N1s, and O1s XPS spectra of N-doped graphene samples.
    • note S3. Average crystallite size of the sp2 domains of N-doped graphene samples from Raman spectra.
    • note S4. Brief explanation of ac impedance of N-doped graphene samples.
    • table S1. Structural and compositional parameters of nitrogen-doped graphene catalysts.
    • table S2. Surface nitrogen and oxygen species concentrations of nitrogen-doped graphene catalysts.
    • table S3. Comparison of ORR and OER performances of our N-doped graphene nanoribbon networks with the recently reported highly active bifunctional catalysts.
    • Legends for videos S1 to S3

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    Other Supplementary Material for this manuscript includes the following:

    • video S1 (.mp4 format). The evolution of O2 bubbles at potentials from 1.6 to 1.8 V versus RHE.
    • videos S2 and S3 (.mp4 format). The demonstration of water splitting driven by a single zinc-air battery.

    Download Videos S1 to S3

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