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 image. 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.