Factors controlling the energetics of the oxygen reduction reaction on the Pd-Co electro-catalysts: Insight from first principles

TL;DR

This study uses density functional theory to analyze how electronic structure and alloy composition of Pd-Co catalysts influence the oxygen reduction reaction, revealing key factors that enhance catalytic efficiency.

Contribution

It provides new insights into the electronic factors controlling ORR energetics on Pd-Co alloys, especially the role of d-band hybridization and surface segregation effects.

Findings

dPd - dCo hybridization shifts d-band, weakening intermediates' bonds

Surface segregation affects reactivity, with non-segregated surfaces being too reactive

Co-adsorption of intermediates and water significantly improves ORR energetics

Abstract

We report here results of our density functional theory based computational studies of the electronic structure of the Pd-Co alloy electrocatalysts and energetics of the oxygen reduction reaction (ORR) on their surfaces. The calculations have been performed for the (111) surfaces of pure Pd, Pd0.75Co0.25 and Pd0.5Co0.5 alloys, as well as of the surface segregated Pd/Pd0.75Co0.25 alloy. We find the hybridization of dPd and dCo electronic states to be the main factor controlling the electrocatalytic properties of Pd/Pd0.75Co0.25. Namely the dPd - dCo hybridization causes low energy shift of the surface Pd d-band with respect to that for Pd(111). This shift weakens chemical bonds between the ORR intermediates and the Pd/Pd0.75Co0.25 surface, which is favorable for the reaction. Non-segregated Pd0.75Co0.25 and Pd0.5Co0.5 surfaces are found to be too reactive for ORR due to bonding of the intermediates to the surface Co atoms. Analysis of the ORR free energy diagrams, built for the Pd and Pd/Pd0.75Co0.25, shows that the co-adsorption of the ORR intermediates and water changes the ORR energetics significantly and makes ORR more favorable. We find the onset ORR potential estimated for the configurations with the O - OH and OH - OH co-adsorption to be in very good agreement with experiment. The relevance of this finding to the real reaction environment is discussed.