Altering oxygen binding by redox-inactive metal substitution to control catalytic activity: oxygen reduction on manganese oxide nanoparticles as a model system

Abstract

Electrocatalytic transformations of oxygen, i.e., the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), are key processes in renewable energy conversion, defining to a large extent, the efficiency of numerous energy conversion technologies, such as fuel cells, metal-air batteries or water electrolyzers. However, the development of highly effective, stable and inexpensive materials for such conversion processes is a bottleneck. Hence, establishing generic catalyst design principles by identifying structural features of catalysts that influence their performance would constitute a major step towards the rational engineering of advanced electrocatalysts. In this study, by investigating a series of metal-substituted manganese oxide (spinel), Mn3O4:M (M = Sr, Ca, Mg, Zn, Cu), nanoparticles as a model system, we demonstrate experimentally and rationalized the dependence of the activity of Mn3O4:M for ORR and the oxygen binding strength in Mn3O4:M oxides on the properties of the M substituent, viz. the enthalpy of formation of the binary MO oxide and the Lewis acidity of the M2+ substituent. Incorporation of elements M that have a low enthalpy of formation of MO (i.e., highly exothermic oxides featuring relatively strong M‒O bonding) enhances the oxygen binding strength in Mn3O4:M, which increases its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species on the catalyst surface. Our work provides a new perspective on the design of new compositions for oxygen electrocatalysis relying on the substitution by redox-inactive elements affecting the binding energy of oxygen to the surface of the complex metal oxide catalysts (ORR/OER activity descriptor). We speculate that this concept is general and transferrable to a broad selection of materials and processes involving oxygen adsorption and redox, beyond electrocatalysis.