Mean Intrinsic Activity of Single Mn Sites at LaMnO3 Nanoparticles Towards the Oxygen Reduction Reaction

LaMnO3 has been identified as one of the most active systems towards the 4-electron oxygen reduction reaction (ORR) under alkaline conditions, although the rationale for its high activity in comparison to other perovskites remains to be fully understood. LaMnO3 oxide nanoparticles are synthesised by an ionic-liquid based method over a temperature range of 600 to 950 degrees C. This work describes a systematic study of the LaMnO3 properties, from bulk to the outermost surface layers, as a function of the synthesis temperature to relate them to the ORR activity. The bulk and surface composition of the particles are characterised by transmission electron microscopy, X-ray diffraction, X-ray absorption and X-ray photoemission spectroscopy (XPS), as well as low-energy ion scattering spectroscopy (LEIS). The particle size and surface composition are strongly affected by temperature, although the effect is non-monotonic. The number density of redox active Mn sites is obtained from electrochemical measurements, and correlates well with the trends observed by XPS and LEIS. ORR studies of carbon-supported LaMnO3 employing rotating ring-disk electrodes show a step increase in the mean activity of individual surface Mn sites for particles synthesised above 700 degrees C. Our analysis emphasises the need to establish protocols for quantifying turn-over frequency of single active sites in these complex materials to elucidate appropriate structure-activity relationships.