Modulation of the electronic structure of metal cationic centers in non-stoichiometric mixed metal oxides for enhanced oxygen electrocatalysis

The design of inexpensive but highly active electrocatalysts as alternatives for the state-of-the-art Pt-based materials remains a high-priority research direction in oxygen reduction reaction (ORR) for fuel cells. A promising group of electrocatalysts for ORR in alkaline environment are non-stoichiometric mixed metal oxides. The most studied among these oxides are 3d transition metal based perovskites, for which the ORR is found to depend on both the oxide composition and crystal structure, with rhombohedral LaMnO₃ exhibiting the highest activity. However, the electrocatalytic activity of the best performing rhombohedral LaMnO₃ perovkite is still inferior to Pt/C.

To enhance the electrochemical ORR activity of perovskites, we have incorporated highly dispersed 4d/5d transition metal cations (i.e., Pd, Pt, Rh, Ru, Ire, and Os) into 3d transition metal based-perovskite hosts as an approach to tune the electronic structure of these cations through variations in oxide composition. We have used a combination of quantum chemical calculations, well-controlled synthesis, and electrochemical studies to show the effect of incorporating these cations into 3d based perovskites towards enhancing the ORR performance. A significant decrease in ORR overpotential is observed, specifically in the case of Rh incorporated into LaNiO₃ (LaNi_1-xRh_xO₃, x<=0.01). This was ascribed to the modulation of the binding of ORR intermediates on Rh cations via their tailored electronic structure through changes in the perovskite composition. This study demonstrates an approach for tailoring the catalytic reactivity of cationic centers in non-stoichiometric mixed metal oxides for targeted electrochemical reactions.