Unusual strain effect of a Pt-based L1 face-centered tetragonal core in core/shell nanoparticles for the oxygen reduction reaction.

Nanoparticles with a low-Pt content core and a few-layer thick Pt skin are attractive catalysts toward the oxygen reduction reaction (ORR) not only for their low cost, but also because their activity can be enhanced by judiciously choosing the core alloy. Achieving the optimal ORR performance would require fine tuning of the core composition and structure. Previous work studying the enhancement effects has primarily focused on core alloys with a cubic structure, (i.e. disordered alloy or L1 ordered structure) which limits the tuning to composition alone. In this work, using ab initio calculations, we have systemically investigated a new class of PtM (M = V, Cr, Fe, Co, Ni and Cu) core alloy that has a face-centered tetragonal L1 intermetallic structure. We have calculated the adsorption energies of O, OH and OOH on various Pt skins and the underlying tetragonally structured alloys, which allows us to not only predict the optimal number of pure Pt skin layers but also tune the activity of the catalysts toward the peak of the ORR volcano plot. More importantly, using adsorption energies on intermediate structures, we are able to decompose the enhancement factor into the ligand, normal and shear strain effects, and reveal the significant contribution of the shear strain that is only possible with a tetragonal core but not a cubic one. Our results point to a new direction in designing tetragonally structured intermetallic core-shell nanoparticles for ORR applications.