From cubic palladium to concave core-shell platinum palladium nanoparticles: Evolution of the structure and their electrochemical properties

Pt-Pd nanoparticles, from pure Pd nanocubes to Pd-Pt core-shell nanoparticles, are synthesized following different methods and intensively investigated, in view of a potential application in fuel cells, as catalysts for the oxygen reduction reaction (ORR). The galvanic replacement is an attractive method to prepare bimetallic particles with high catalytic activity and a high control of the size, shape and chemical composition of the particles, varying with the experimental conditions during the synthesis. The influence of the time with the transformation from pure Pd nanocubes to concave core-shell Pt-Pd nanoparticles synthesized by galvanic replacement (with a Pd core and a mix of Pt and Pd in the surface) is examined: after different times of preparation, the morphology of the particles was monitored by Transmission Electronic Microscopy (TEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS) for the chemical composition. Via X-Ray diffraction spectroscopy (XRD), the crystallographic structure and the variation of size, lattice parameters, d-spacing, and composition were determined. The Extended X-ray Fine Structure (EXAFS) measurements show the formation of a Pt-Pd alloy at the surface of the particles for all samples. Finally, the electrochemical determination of the catalytic activity and stability tests revealed two different particle types as candidates to replace pure Pt as catalyst in the Proton Exchange Membrane Fuel Cells (PEMFC) due to their enhanced stability, higher catalytic activity, and lower Pt content.