Mass-selected nanoparticles of PtxY as model catalysts for ​oxygen electroreduction

Journal name:
Nature Chemistry
Volume:
6,
Pages:
732–738
Year published:
DOI:
doi:10.1038/nchem.2001
Received
Accepted
Published online

Abstract

Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of ​platinum, currently the best-known catalyst for reducing ​oxygen in terms of both activity and stability. One solution is to decrease the amount of ​platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy PtxY is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt−1 at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental ​platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.

At a glance

Figures

  1. Surface characterization of the PtxY nanoparticles before and after ORR activity measurements.
    Figure 1: Surface characterization of the PtxY nanoparticles before and after ORR activity measurements.

    a, Schematic representation of the cluster source. bd, Representative surface characterization data for the as-prepared PtxY 9 nm particles. eg, Equivalent data for the PtxY 9 nm particles following exposure to the ORR conditions. b,e, ISS spectra. c,f, Detailed XPS survey of Pt 4f core-level region spectra. d,g, Detailed XPS survey of Y 3d core-level region spectra. Based on the XPS data, the Pt:Y ratio in the near-surface region of the as-prepared particles is four, whereas it is 14.3 for the particles after the ORR. a.u., arbitrary units; SEM, scanning electron microscope; UHV, ultrahigh vacuum.

  2. Particle-size distributions and representative TEM micrographs for the PtxY catalysts.
    Figure 2: Particle-size distributions and representative TEM micrographs for the PtxY catalysts.

    a, 4 nm. b, 5 nm. c, 7 nm. d, 9 nm.

  3. Catalyst activity of PtxY in nanoparticles in comparison to Pt.
    Figure 3: Catalyst activity of PtxY in nanoparticles in comparison to ​Pt.

    a,b, Surface specific activity (a) and mass activity (b) of PtxY (red). All data are taken at 0.9 V, recorded at 50 mV s−1, 1,600 revolutions per minute and 23 ± 1 °C in ​O2-saturated 0.1 M ​HClO4, taken from the anodic cycle and corrected for mass-transport limitations. For comparison, the previously published mass and specific activities of pure ​Pt nanoparticles, prepared in the same way, and an extended surface of polycrystalline ​Pt (Pt pc) are also plotted (black)40. The Supplementary Information gives details of the quantification of mass and surface area40. The Pt3Y pc data represent a sputter-cleaned extended surface of polycrystalline Pt3Y tested under the same conditions16. Each data point corresponds to the mean value from at least three independent activity tests; the error bars show the standard deviation in the electrochemical measurements and the PSD. The lines serve as a guide for the eye.

  4. High-angle annular dark-field (HAADF)-STEM images and EDS elemental maps for PtxY 9 nm.
    Figure 4: High-angle annular dark-field (HAADF)-STEM images and EDS elemental maps for PtxY 9 nm.

    Images for the as-prepared sample (ae) and for the catalyst after ORR conditions (fj). a,f, HAADF-STEM images. b,g, EDS elemental maps of ​Y Kα. c,h, EDS elemental maps of ​Pt Lα. d,i, Combined elemental maps of ​Pt + ​Y. e,j, EDS intensity line profiles extracted from the spectrum image data cube along the purple lines drawn on a and f. We estimated the ​Pt-shell thickness of the nanoparticles by extracting ten line profiles across each particle of f and obtained a value of 1.0 ± 0.3 nm.

  5. XAS analysis.
    Figure 5: XAS analysis.

    a, Average nearest-neighbour Pt–Pt distance measured by EXAFS as a function of the particle size for both as-prepared (black) and after ORR (red) recorded on PtxY. For the purpose of comparison, EXAFS measurements were also performed on as-prepared (blue) and electrochemically tested (red) ​Pt nanoparticles. Measurements were performed on a ​Pt foil (continuous grey horizontal line; the dashed grey horizontal lines show the error from the fitting software) as a reference. The left y axis shows the nearest neighbour Pt–Pt distance; the right axis represents the % strain in the Pt–Pt distance, relative to that of the bulk ​Pt-foil. The y-axis represents the % strain in the Pt–Pt distance relative to that in the ​Pt foil. b, Surface specific activity, extracted from Fig. 3, as a function of the average compressive strain in the PtxY particles relative to bulk ​Pt, based on the data shown in a. The error bars are produced by the fitting software (full details can be found in Supplementary Table 2). The continuous curved lines serve as a guide for the eye.

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Author information

Affiliations

  1. Center for Individual Nanoparticle Functionality, Department of Physics, Technical University of Denmark, Kgs Lyngby DK-2800, Denmark

    • Patricia Hernandez-Fernandez,
    • Federico Masini,
    • David N. McCarthy,
    • Christian E. Strebel,
    • Paolo Malacrida,
    • Anders Nierhoff,
    • Anders Bodin,
    • Jane H. Nielsen,
    • Ifan E. L. Stephens &
    • Ib Chorkendorff
  2. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, MS31, Menlo Park CA 94025, USA

    • Daniel Friebel,
    • Anna M. Wise &
    • Anders Nilsson
  3. Center for Electron Nanoscopy, Technical University of Denmark, Kgs Lyngby DK-2800, Denmark

    • Davide Deiana &
    • Thomas W. Hansen

Contributions

I.C. and I.E.L.S. conceived the experiments. P.H-F. performed the electrochemical experiments. F.M., D.N.M., C.E.S., P.M. and A.N. performed the UHV experiments. D.F., A.B. and A.M.W performed the XAS measurements. D.D. performed the microscopy. P.H-F. designed the figures. P.H-F. and I.E.L.S. wrote the first draft of the paper. All authors discussed the results and commented on the manuscript.

Competing financial interests

I.C. and I.E.L.S. have a patent on the catalyst material PtxY, PCT/DK2010/050193. The other authors declare no competing financial interests.

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