Segregated Chemistry and Structure on (001) and (100) Surfaces of (La_1-xSr_x)₂CoO₄ Override the Crystal Anisotropy in Oxygen Exchange Kinetics

Attaining fast oxygen exchange kinetics on perovskite and related mixed ionic and electronic conducting oxides is critical for enabling their applications in electrochemical energy conversion systems. This study focuses on understanding the relationship between surface chemistry and the surface oxygen exchange kinetics on epitaxial films made of (La_1-xSr_x)₂CoO₄, a prototypical Ruddlesden-Popper structure that is considered as a promising cathode material for fuel cells. The effects of crystal orientation on the surface composition, morphology, oxygen diffusion, and surface exchange kinetics were assessed by combining complementary surface-sensitive analytical techniques, specifically low energy ion scattering, X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and secondary ion mass spectroscopy. The films were grown in two different crystallographic orientations, (001) and (100), and with two different Sr compositions, at x = 0.25 (LSC25) and 0.50 (LSC50), by using pulsed laser deposition. In the as-prepared state, a Sr enriched layer at the top surface and a Co enriched subsurface layer were found on films with both orientations. After annealing at elevated temperatures in oxygen, the Sr enrichment increased, followed by clustering into Sr-rich secondary phase particles. Both the LSC25 and LSC50 films showed anisotropic oxygen diffusion kinetics, with up to 20 times higher oxygen diffusion coefficient along the ab-plane compared that along the c-axis at 400-500 °C. However, no dependence of surface oxygen exchange coefficient was found on the crystal orientation. This result indicates that the strong Sr segregation at the surface overrides the effect of the structural anisotropy that was also expected for the surface exchange kinetics. The larger presence of Co cations exposed at the LSC25 surface compared to that at the LSC50 surface is likely the reason for the faster oxygen surface exchange kinetics on LSC25 compared to LSC50. This work demonstrated the critical role of surface chemistry on the oxygen exchange kinetics on perovskite related oxides, which are thus far underexplored at elevated temperatures, and provides a generalizable approach to probe the surface chemistry on other catalytic complex oxides. (Graph Presented).