Pt-Decorated Composition-Tunable Pd-Fe@Pd/C Core-Shell Nanoparticles with Enhanced Electrocatalytic Activity toward the Oxygen Reduction Reaction.

Design of electrocatalysts with both a high-Pt-utilization efficiency and enhanced electrochemical activity is still the key challenge in the development of proton exchange membrane fuel cells. In the present work, Pd-Fe/C bimetallic nanoparticles (NPs) with an optimal Fe composition and decorated with Pt are introduced as promising catalysts toward the oxygen reduction reaction. These bimetallic nanoparticles have a Pd-Fe@Pd core-shell structure with a surface Pt decoration as established through the use of electron energy loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) spectroscopy. These catalysts exhibit excellent electrocatalytic activity ( E1/2 = 0.866 V vs RHE), increasing the mass activity by more than 70% over that of Pt/C in terms of the total mass of Pt and Pd and by 14 times if only Pt is considered. Simple geometrical calculations, based on spherical core-shell models, indicate that Pd-Fe@Pt has a surface Pt decoration rather than a complete Pt monolayer. Such calculations applied to other examples in the literature point out the need for careful and rigorous arguments about claimed "Pt monolayer/multilayers". Such calculations must be based on not only elemental mapping data but also on the Pt/Pd and other metal atomic ratios in the precursors. Our analysis predicts a minimal Pt/Pd atomic ratio in order to achieve a complete Pt monolayer on the surface of the core materials.