Research ArticleENERGY RESOURCES

N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells

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Science Advances  27 Feb 2015:
Vol. 1, no. 1, e1400129
DOI: 10.1126/sciadv.1400129
  • Fig. 1 Fabrication of MEA of VA-NCNT arrays and its performance in a PEM fuel cell.

    (A) Schematic drawings for the fabrication of MEA from VA-NCNT arrays (0.16 mg cm−2) and the electrochemical oxidation to remove residue Fe. C.E., counter electrode; R.E., reference electrode; W.E., working electrode. (B) Typical SEM image of the VA-NCNT array. (C) Digital photo image of the used MEA after durability test with the cross-section SEM images shown in the inserts. (D) Polarization curves as the function of the areal current density after accelerated degradation by repeatedly scanning the cell from OCV to 0.1 V at the rate of 10 mA s−1. (E) Polarization and power density as the function of the gravimetric current density. Cathode catalyst loading 0.16 mg cm−2, Nafion/VA-NCNT = 1/1. H2/O2: 80°C, 100% relative humidity, 2-bar back pressure.

  • Fig. 2 Morphological features of the N-G-CNT electrodes with and without the addition of Ketjenblack.

    (A to D) Cross-section SEM images of (A and B) the densely packed catalyst layer of N-G-CNT/Nafion (0.5/0.5 mg cm−2) and (C and D) the porous catalyst layer of N-G-CNT/KB/Nafion (0.5/2/2.5 mg cm−2). Purple arrows in (D) indicate the parallelly separated N-G-CNT sheets with interdispersed porous KB agglomerates. (E and F) BET surface areas (E) and pore volume distributions (F) of a piece of 5-cm2 GDL, GDL with KB (2 mg cm−2), GDL with N-G-CNT (0.5 mg cm−2), and GDL with N-G-CNT/KB (0.5/2 mg cm−2) as indicated in the figures. (G and H) Schematic drawings of the MEA catalyst layer cross section, showing that O2 efficiently diffused through the carbon black separated N-G-CNT sheets (G) but not the densely packed N-G-CNT sheets (H).

  • Fig. 3 Electrocatalytic activities of the carbon-based metal-free catalysts in half-cell tests.

    (A) CVs of the N-G-CNT in O2- or N2-saturated 0.1 M KOH. (B) Linear sweep voltammetry (LSV) curves of the N-G-CNT compared with Pt/C (20%) electrocatalyst by RRDE in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm. (C and D) LSV curves of the N-G and N-CNT compared with the N-G-CNT in O2-saturated 0.1 M KOH (C) and 0.1 M HClO4 (D).

  • Fig. 4 Power and durability performance of N-G-CNT with the addition of KB in PEM fuel cells.

    (A) Polarization curves of N-G-CNT with loadings: 2, 0.5, or 0.15 mg cm−2 plus KB (2 mg cm−2) for each cathode. The weight ratio of (N-G-CNT/KB)/Nafion = 1/1. (B) Cell polarization and power density as the function of gravimetric current for the N-G-CNT/KB (0.5/2 mg cm−2) with the weight ratio of (N-G-CNT/KB)/Nafion = 1/1. (C) Durability of the metal-free N-G-CNT in a PEM fuel cell measured at 0.5 V compared with a Fe/N/C catalyst (see the Supplementary Materials for preparation details). Catalyst loading of N-G-CNT/KB (0.5 mg cm−2) and Fe/N/C (0.5 and 2 mg cm−2). Test condition: H2/O2: 80°C, 100% relative humidity, 2-bar back pressure.

  • Table 1 The gravimetric activities of various transition metal–derived NPMCs compared with the metal-free VA-NCNT and N-G-CNT + KB in PEM fuel cells.

    All the data in the table have also been scaled by the electrode surface area.

    MaterialsCurrent at 0.8 V
    (A g−1)
    Current at 0.2 V
    (A g−1)
    Peak power density
    (W g−1)
    Catalyst loading
    (mg cm−2)
    O2-H2 back
    pressure (bars)
    Reference
    FeCo/N/C1570020021.0(14)
    Fe/N/C8/100800/2500233/4003.9/0.90.5(11)
    Fe/N/C153258041.3(45)
    VA-NCNT3515503200.161.5This work
    N-G-CNT + KB3015003000.51.5This work

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/1/e1400129/DC1

    Fig. S1. Characterization of VA-NCNTs.

    Fig. S2. Electrocatalytic activities of the VA-NCNT catalyst in alkaline electrolyte (O2-saturated 0.1 M KOH) by half-cell tests.

    Fig. S3. Electrocatalytic activities of the VA-NCNT catalyst in acidic electrolyte (O2-saturated 0.1 M HClO4) by half-cell tests.

    Fig. S4. Typical cross-section SEM images of the GDL with the MEA of VA-NCNTs as the cathode catalyst layer, Nafion membrane (N211) as the separator, and Pt/C as the anode.

    Fig. S5. SEM (A) and TEM (B) images of N-CNT bundles.

    Fig. S6. Typical cross-section SEM images of the GDLs with the MEAs of (A to C) N-G-CNT (2 mg cm−2) and (D to F) N-G-CNT + KB (0.5 + 2 mg cm−2) as the cathode catalyst layers, respectively.

    Fig. S7. Tafel plot (A) and electron transfer number (B) for the N-G-CNT and Pt/C (20%) as the function of electrode potential by RRDE in oxygen-saturated 0.1 M KOH solution at a scan speed of 5 mV s−1 and a rotation speed of 1600 rpm.

    Fig. S8. Long-time stability and tolerance to methanol/carbon monoxide of metal-free catalyst N-G-CNT.

    Fig. S9. SEM images of catalyst layer cross sections used in RDE measurements.

    Fig. S10. Electrocatalytic activities of the carbon-based metal-free N-G-CNT catalysts in acidic electrolyte (O2-saturated 0.1 M HClO4) by half-cell tests.

    Fig. S11. Optimization of cathode catalyst layer composition.

    Fig. S12. Single-cell performance comparison between N-G-CNT and Fe/N/C catalysts at the same catalyst layer composition: catalyst (0.5 mg cm−2)/KB (2 mg cm−2)/Nafion (2.5 mg cm−2).

    Fig. S13. Polarization curves of the N-G-CNT and individual components of N-G or N-CNT.

    Fig. S14. Durability of the catalyst layer composed of metal-free N-G-CNT (2 mg cm−2) + KB (2 mg cm−2) in a PEM fuel cell measured at 0.5 V.

    Fig. S15. The metal-free character of N-G-CNT catalyst.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Characterization of VA-NCNTs.
    • Fig. S2. Electrocatalytic activities of the VA-NCNT catalyst in alkaline electrolyte (O2-saturated 0.1 M KOH) by half-cell tests.
    • Fig. S3. Electrocatalytic activities of the VA-NCNT catalyst in acidic electrolyte (O2-saturated 0.1 M HClO4) by half-cell tests.
    • Fig. S4. Typical cross-section SEM images of the GDL with the MEA of VANCNTs as the cathode catalyst layer, Nafion membrane (N211) as the separator, and Pt/C as the anode.
    • Fig. S5. SEM (A) and TEM (B) images of N-CNT bundles.
    • Fig. S6. Typical cross-section SEMimages of the GDLswith the MEAs of (A to C)N-G-CNT (2mg cm−2) and (D to F) N-G-CNT + KB (0.5 + 2 mg cm−2) as the cathode catalyst layers, respectively.
    • Fig. S7. Tafel plot (A) and electron transfer number (B) for the N-G-CNT and Pt/C (20%) as the function of electrode potential by RRDE in oxygen-saturated 0.1 M KOH solution at a scan speed of 5 mV s−1 and a rotation speed of 1600 rpm.
    • Fig. S8. Long-time stability and tolerance to methanol/carbon monoxide of metalfree catalyst N-G-CNT.
    • Fig. S9. SEM images of catalyst layer cross sections used in RDE measurements.
    • Fig. S10. Electrocatalytic activities of the carbon-based metal-free N-G-CNT catalysts in acidic electrolyte (O2-saturated 0.1 M HClO4) by half-cell tests.
    • Fig. S11. Optimization of cathode catalyst layer composition.
    • Fig. S12. Single-cell performance comparison between N-G-CNT and Fe/N/C catalysts at the same catalyst layer composition: catalyst (0.5 mg cm−2)/KB (2 mg cm−2)/Nafion (2.5 mg cm−2).
    • Fig. S13. Polarization curves of the N-G-CNT and individual components of N-G or N-CNT.
    • Fig. S14. Durability of the catalyst layer composed of metal-free N-G-CNT (2 mg cm−2) + KB (2 mg cm−2) in a PEM fuel cell measured at 0.5 V.
    • Fig. S15. The metal-free character of N-G-CNT catalyst.

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