Computational and experimental study of the Volcano behavior of the oxygen reduction activity of PdM@PdPt/C (M = Pt, Ni, Co, Fe, and Cr) core–shell electrocatalysts

The activity of oxygen reduction electrocatalysts is governed by the Sabatier principle and follows a Volcano curve as a function of the oxygen-binding energy. Density functional theory calculations show that the oxygen-binding energy decreases in steps of about 10 kJ/mol in a series of core–shell Pd3M@Pd3Pt (M = Ni, Co, Fe, Mn, and Cr) electrocatalysts, leading to a gradual, Volcano-like variation in the oxygen reduction activity. A series of carbon-supported PdM@PdPt (M = Ni, Co, Fe, and Cr) nanoparticles with similar particle sizes were prepared by an exchange reaction between PdM nanoparticles and an aqueous solution of PtCl42-. The variation in the surface electronic structure of the core–shell structures was evaluated by Pt 4f7/2 X-ray photo-electron spectroscopy and by CO-stripping voltammetry and agrees with the first principle calculations. At 0.85 V, the PdM@PdPt/C core–shell electrocatalysts show a 6-fold variation in activity, following the Volcano trend predicted by the calculations. The Pt mass activity of the Volcano-optimal PdFe@PdPt/C catalyst is an order of magnitude higher than the activity of commercial 3.0-nm Pt/C catalysts. The core–shell catalysts also display a high methanol tolerance, which is important for use in direct methanol fuel cells. Calculated Pt–M segregation energies suggest that the Pd3M@Pd3Pt core–shell structures are stable, in particular in the presence of 1/4 ML CO. Adsorption of oxygen-containing species may induce surface segregation of the 3d transition metal, except for the Volcano-optimal ORR catalyst, Pd3Fe@Pd3Pt.

Variation of the 3d transition metal M in PdM@PdPt/C core–shell catalysts gradually changes the electronic structure of the surface Pt atoms, as evidenced by the Pt 4f7/2 binding energies. The gradual change in electronic structure causes a gradual change in the measured CO-stripping peak position and in the calculated oxygen affinity. The variation in oxygen affinity leads to a Volcano-like variation in the measured oxygen reduction activity, as expected from Sabatier’s principle.Figure optionsDownload high-quality image (227 K)Download as PowerPoint slideHighlights
► A series of PdM@PdPt/C core–shell catalysts with similar particle sizes were prepared.
► Pt XPS and CO-stripping peaks vary gradually over the series, in agreement with DFT.
► Oxygen-binding energies change in steps of 10 kJ/mol, leading to a Volcano curve for the predicted ORR activity.
► PdM@PdPt/C catalysts show a 6-fold variation in ORR activity, following the predicted trend.