Importance of Ligand Effects Breaking the Scaling Relation for Core-Shell Oxygen Reduction Catalysts

Abstract

Tremendous recent efforts have been made toward developing highly active oxygen reduction reaction (ORR) catalysts with a minimized usage of noble metal Pt by using Pt alloys and core-Pt shell structures. A main computational framework for such a goal has been the search for a new material with the *OH binding slightly weaker than Pt based on the conventional volcano relation of ORR activity versus *OH binding energy. In this work, by using carbides and nitrides as core materials, we demonstrate that the conventional scaling relation between *OH and *O can be completely broken owing to a significant ligand-Pt orbital interactions in the core-Pt shell structure, and in such cases, the usual catalyst design strategy of tuning the *OH binding energy of Pt to a weaker leg of the volcano can mislead the prediction. In these cases, one needs to consider all reaction intermediates to appropriately predict the activity of ORR catalysts. We additionally show that, although the transition metal nitrides and carbides studied here as core materials all induce an undesired tensile strain to the Pt overlayers with a stronger *OH binding, a proper tuning of the ligand (core) effects in the Pt-1 and Pt-2 overlayers core-shell configurations can lead to an activity comparable to or slightly better than Pt.