Rationalizing the Effect of the Redox-Inactive Metals on the Activity of Oxides in Oxygen Electrocatalysis

Abstract

The large-scale implementation of renewable energy conversion and storage technologies is a key challenge to transition to a sustainable society. One of the most critical elements towards this goal is the discovery of active, stable and inexpensive catalysts for electrochemical energy conversion processes, such as the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are central for the efficiency of water-splitting electrolyzers, fuel cells and metal–air batteries.1 Manganese-based spinels, MxMn3-xO4, and ruthenium pyrochlores, M2Ru2O7 − δ, are known as active catalysts for the ORR and OER, respectively, and were studied here as model systems to rationalize the effect of the incorporation (doping) of redox-inactive elements (M) into the structure on the catalytic performance of complex oxides in oxygen electrocatalysis.We found a direct correlation between the ORR/OER activity of Mn3O4:M spinels (M = Mn, Cu, Zn, Mg, Ca, Sr),2 and Y1.8M0.2Ru2O7 − δ pyrochlores (M = Y, Fe, Co, Ni, Cu),3 the lattice oxygen binding strength/lability and the formation enthalpy of the binary MO x oxides. We found experimentally that the activity of Mn3O4:M and Y1.8M0.2Ru2O7 − δ in, respectively, ORR and OER, varies significantly depending on the nature of M. We observed that metals M with higher enthalpies of formation for the respective oxide weaken the oxygen binding on the surface of the complex oxide that correlates in turn with changes in ORR/OER activity. DFT studies captured changes in the electronic structure upon M substitution and allowed to correlate trends in the electronic band structure and the oxygen vacancies formation energies with the ORR/OER performance of the Mn3O4:M/Y1.8M0.2Ru2O7 − δ catalysts. These activity trends are subsequently discussed in the framework of theoretically established correlations between the electrocatalytic activity and adsorption energies of catalytic intermediates. We demonstrated that the introduction of redox-inactive metal substituents/dopants into the host structure of a metal oxide is a highly versatile approach to tune the binding strength of reaction intermediates (determining the position of the catalyst on the ORR/OER activity volcano), and in turn improve the ORR/OER activity and also selectivity. This approach is a general concept that is transferrable to a broad selection of materials and processes associated with adsorption and redox reactions involving oxygen species.References(1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, eaad4998.(2) Wu, Y.-H.; Mehta, H.; Willinger, E.; Yuwono, J. A.; Kumar, P. V.; Abdala, P. M.; Wach, A.; Kierzkowska, A.; Donat, F.; Kuznetsov, D. A.; Müller, C. R. Angew. Chem. Int. Ed. 2023, 62, e202217186.(3) Kuznetsov, D. A.; Naeem, M. A.; Kumar, P. V.; Abdala, P. M.; Fedorov, A.; Müller, C. R. J. Am. Chem. Soc. 2020, 142, 7883-7888.