Structurally ordered intermetallic platinum–cobalt core–shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts

Journal name:
Nature Materials
Volume:
12,
Pages:
81–87
Year published:
DOI:
doi:10.1038/nmat3458
Received
Accepted
Published online

Abstract

To enhance and optimize nanocatalyst performance and durability for the oxygen reduction reaction in fuel-cell applications, we look beyond Pt–metal disordered alloys and describe a new class of Pt–Co nanocatalysts composed of ordered Pt3Co intermetallic cores with a 2–3 atomic-layer-thick platinum shell. These nanocatalysts exhibited over 200% increase in mass activity and over 300% increase in specific activity when compared with the disordered Pt3Co alloy nanoparticles as well as Pt/C. So far, this mass activity for the oxygen reduction reaction is the highest among the Pt–Co systems reported in the literature under similar testing conditions. Stability tests showed a minimal loss of activity after 5,000 potential cycles and the ordered core–shell structure was maintained virtually intact, as established by atomic-scale elemental mapping. The high activity and stability are attributed to the Pt-rich shell and the stable intermetallic Pt3Co core arrangement. These ordered nanoparticles provide a new direction for catalyst performance optimization for next-generation fuel cells.

At a glance

Figures

  1. XRD and HAADF-STEM images.
    Figure 1: XRD and HAADF-STEM images.

    a, XRD patterns of Pt/C, Pt3Co/C-400 and Pt3Co/C-700. The inset shows the enlarged region of the Pt (220) diffraction peaks, with the black dotted line corresponding to the peak position of pure Pt. The red vertical lines indicate the peak positions of the intermetallic Pt3Co reflections (PDF card # 04-004-5243). b, Atomic-resolution ADF-STEM image of Pt3Co/C-700 after Richardson-Lucy deconvolution (for details see Supplementary Fig. S5), with yellow arrows indicating the Pt-rich shell. A smaller particle (lower left) overlaps the larger particle in projection. The inset shows the projected unit cell along the [001] axis. c, Diffractogram of the centre particle in b. d, A crop of the super lattice feature from b. e, The simulated ADF-STEM image of L12 ordered Pt3Co along [001] by a simple incoherent linear imaging model. f, Multislice simulated ADF-STEM (100 kV, probe forming angle  =  27.8 mrad, ADF collection angles  =  98–295 mrad) image of the idealized nanoparticle as shown in bg, The idealized atomic structure of the Pt3Co core–shell nanoparticle. The white and blue spheres in e,g present Pt and Co atoms, respectively.

  2. ADF-STEM image of one nanoparticle and elemental mapping.
    Figure 2: ADF-STEM image of one nanoparticle and elemental mapping.

    a, ADF-STEM image of a Pt3Co/C-700 nanoparticle, with two parallel lines along with arrow marks indicating {100} lattice spacing. bd, 2D EELS maps of Pt (b), Co (c) and the composite Pt versus Co map (d). e, Line profiles extracted from the boxed area in b,c across the facet showing that the Pt shell is ~0.5 nm thick.

  3. Electrochemical characterization.
    Figure 3: Electrochemical characterization.

    a, ORR polarization curves for Pt/C, Pt3Co/C-400 and Pt3Co/C-700 in O2-saturated 0.1 M HClO4 at room temperature, with rotation rate, 1,600 r.p.m. and sweep rate, 5 mV s−1. b, The Koutecky–Levich plots from ORR data for Pt3Co/C-700 at different potentials. The inset in b shows the rotation-rate-dependent current–potential curves. c, Comparison of mass activities for Pt/C, Pt3Co/C-400 and Pt3Co/C-700 at 0.85 and 0.9 V. d, Comparison of specific activities (Ik).

  4. Characterization of the surface area changes and stability for ORR.
    Figure 4: Characterization of the surface area changes and stability for ORR.

    a,b, Cyclic voltammetry curves of Pt3Co/C-400 (a) and Pt3Co/C-700 (b) nanoparticles in N2-purged 0.1 M HClO4 solution at room temperature for various numbers of potential cycles, as indicated, at a scan rate of 5 mV s−1. c, ECSA as a function of the number of cyclic voltammetry cycles for Pt3Co/C-400 and Pt3Co/C-700 catalysts. d, Comparative ORR activities of Pt3Co/C-400 and Pt3Co/C-700 catalysts before and after 5,000 potential cycles.

  5. Structural stability.
    Figure 5: Structural stability.

    a, ADF-STEM image of a Pt3Co/C-700 nanoparticles after 5,000 electrochemical cycles. b, EELS maps of Pt, Co and the composite Pt versus Co map from the selected region in a. c,d, Two more particles and their Pt, Co elemental mapping to show the Pt-rich shell and PtCo intermetallic core after electrochemical cycling. e, Further particles demonstrating that the ordered structure is maintained after electrochemical cycling. Scale bars, 1 nm.

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Author information

  1. These authors contributed equally to this work

    • Deli Wang &
    • Huolin L. Xin

Affiliations

  1. Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, USA

    • Deli Wang,
    • Hongsen Wang,
    • Yingchao Yu,
    • Francis J. DiSalvo &
    • Héctor D. Abruña
  2. Department of Physics, Cornell University, Ithaca, New York 14853, USA

    • Huolin L. Xin
  3. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA

    • Robert Hovden &
    • David A. Muller
  4. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA

    • David A. Muller
  5. Present address: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Huolin L. Xin

Contributions

D.W. and H.L.X. conceived and designed the experiments. D.W. performed synthesis and electrochemical characterizations. H.L.X. performed STEM and EELS mapping experiments. D.W. and H.L.X. wrote the manuscript with assistance from R.H.D.W. and H.L.X. contributed equally to this work. R.H. participated in analysis of the data. All authors discussed the results and commented on the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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