Transition metal oxides such as perovskites (ABO₃) are becoming an important class of materials in the context of oxygen electrocatalysis for alkaline fuel cells and metal-air batteries.^1,2 Although these materials have been extensively investigated in the field of solid oxide fuel cells, their properties as oxygen electrocatalysts at room temperature are far from being fully rationalised. Establishing structure-activity relationships based on the electronic structure of the B-site, widely recognised as the main active site, is one of the key challenge in this field. Such information is extremely important towards designing catalysts with optimal B-site orbital occupancy, coordination number and metal-oxygen bond length.

In a recent study, we have shown that LaMnO₃ exhibits over two orders of magnitude higher activity towards the ORR than other lanthanide perovskites (e.g. Co, Fe, Cr and Ni).³ Our analysis show that the origin of this high activity is linked to changes in the redox state at the Mn sites at potentials close to the formal ORR potential, leading to an increase in electron population at the key active site.

In this contribution, we shall focus on the structure-activity relationship of a family of La_xCa_1-xMnO₃ nanoparticles, assessing their activity as a function of the Mn-valency. We will provide a detailed analysis of the bulk and surface structure employing X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). Studies employing rotating ring-disc electrodes of the nanoparticles supported on mesoporous carbon films show that their activity towards the ORR are among the highest reported for transition metal oxides.⁴ We finally establish that the mean ORR activity of surface Mn sites increases as the Mn valency decreases.⁵ These studies highlight the need for independent assessment of key aspects such as the extent of A-site surface segregation and average Mn valency in order to establish consistent structure-activity relationships.

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

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