Face-centered tetragonal (FCT) Fe and Co alloys of Pt as catalysts for the oxygen reduction reaction (ORR): A DFT study.

Abstract

Proton exchange membrane fuel cells (PEMFCs) are promising candidates for alternate energy conversion devices owing to their various advantages including high efficiency, reliability, and environmental friendliness. The performance of PEMFCs is fundamentally limited by the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. Various studies have addressed myriads of Pt-based alloys as potential catalysts for ORR. However, most of these studies only focus on the cubic-structured Pt-based alloys which require further improvements especially in terms of stability and required loading. In this work, we perform first-principle density functional theory calculations to explore Fe and Co alloys of Pt in a different face centered tetragonal (L10) geometry as potential catalysts for ORR. The work focuses on understanding the reaction mechanism of ORR by both dissociative and associative mechanisms on L10-FePt/Pt(111) and L10-CoPt/Pt(111) surfaces. The binding pattern of each reaction intermediate is studied along with the complete reaction free energy landscape as a function of Pt overlayers. The L10-FePt/Pt(111) and L10-CoPt/Pt(111) surfaces show higher calculated surface activity for ORR as compared to the native fcc Pt(111) surface. The decrease in the required overpotential (η) for the alloys with respect to the unstrained Pt(111) surface is found to be in the range (0.04 V-0.25 V) assuming the dissociative mechanism and (0.02 V-0.10 V) assuming the associative mechanism, where the variation depends on the thickness of Pt overlayers. We further correlate the binding behavior of the reaction intermediates to the applied biaxial strain on the Pt(111) surface with the help of a mechanical eigenforce model. The eigenforce model gives a (semi-) quantitative prediction of the binding energies of the ORR intermediates under applied biaxial strain. The numerical values of the limiting potential (UL) obtained from the eigenforce model are found to be very close to ones obtained from electronic structure calculations (less than 0.1 V difference). The eigenforce model is further used to predict the ideal equi-biaxial strain range required on Pt(111) surfaces for optimum ORR activity.