Complex transition metal oxides such as perovskites have been extensively investigated in the context of oxygen electrocatalysis.^1,2 These materials represent a vast family of compounds with electronic properties sharply dependent on their composition, crystal phase and oxygen content. Consequently, the establishment of structure-activity relationships is of paramount importance in the search of highly active and chemically stable electrocatalysts for alkaline fuel cells. Indeed, the strongly correlated nature of electrons in these materials, as opposed to metallic phases, introduces a high degree of complexity for material screening from first-principle calculations. We have recently examined a range of lanthanide perovskites (LaBO₃, with B: Co, Fe, Cr, Mn and Ni) synthesised by a versatile synthetic method capable of generating nanoparticles with a high degree of phase purity.³ We have shown that Mn based oxides show over two orders of magnitude higher activity than any other perovskite towards the 4-electron oxygen reduction reaction (ORR);^3,4,5 a difference significantly larger than recognised in previous studies.^2,6

In this contribution, we will investigate the origin of the exceptional activity of Mn based perovskites. Our key hypothesis centres around the fact that Mn sites undergo changes in their redox state at potentials close to the formal ORR potential. These redox transitions are key in the oxygen bond breaking step and the overall reactivity of the material. Our studies are underpinned by detailed analysis of the bulk and surface structure employing X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and in-operando X-ray absorption spectroscopy (XAS). We will also show that the intrinsic activity of these materials can be strongly affected by the nature of the A-site⁷ and the temperature of oxide phase formation.

References:

1- A. Grimaud et al. Nat. Mater. 2016, 15, 121

2- J. Suntivich et al. Nat. Chem. 2011, 3, 647

3- V. Celorrio et al. ChemElectroChem 2016, 3, 283

4- V. Celorrio et al. Catal. Sci. & Technol. 2016, 6, 7231

5- V. Celorrio et al. J. Phys. Chem. C. 2016, 120, 22291

6- K.A. Stoerzinger, ACS Catalysis 2015, 5, 6021

7- V. Celorrio, MRS Commun. 2017, DOI: 10.1557/mrc.2017.22