Oxygen surface exchange kinetics and stability of (La,Sr)2CoO4±δ/La1−xSrxMO3−δ (M = Co and Fe) hetero-interfaces at intermediate temperatures

Abstract

Heterostructured oxide interfaces created by decorating Ruddlesden–Popper (RP) phases on ABO₃ perovskites have shown not only pronounced cation segregation at the interface and in the RP phase but also enhanced kinetics for oxygen electrocatalysis at elevated temperatures. In this study, combining experimental and theoretical approaches, we report and compare the time-dependent surface exchange kinetics and stability of (La_0.5Sr_0.5)₂CoO_4±δ (LSC₂₁₄)-decorated La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF₁₁₃) and La_0.8Sr_0.2CoO_3−δ (LSC₁₁₃) thin films. While LSC₂₁₄ decoration on LSC₁₁₃ greatly reduced the degradation in the surface exchange kinetics as a function of time relative to undecorated LSC₁₁₃, LSCF₁₁₃ with LSC₂₁₄ coverage showed comparable surface exchange kinetics and stability relative to undecorated LSCF₁₁₃. This difference is attributed to stabilization of the LSC₁₁₃ surface by LSC₂₁₄ decoration and greater stability of LSCF₁₁₃ against decomposition into secondary phases than LSC₁₁₃. This hypothesis is supported by density functional theory (DFT) computation, revealing greater surface Sr segregation for LSCF₁₁₃, which is predicted to have an SrO termination, than LSC₁₁₃, which is predicted to have a less Sr enriched (La_0.25Sr_0.75)O termination. Furthermore, DFT also showed a lower energy gain to move Sr from LSCF₁₁₃ into LSC₂₁₄ relative to the LSC₂₁₄-LSC₁₁₃ surface, and predicted the stability of LSCF₁₁₃, LSC₁₁₃, and LSC₂₁₄ with 100% Sr substitution in their top (001) surface. The stability differences of Sr substitution (with La) in LSCF₁₁₃, LSC₁₁₃, and LSC₂₁₄, along with the assessed DFT decomposition free energies of fully Sr substituted LSCF₁₁₃, LSC₁₁₃, and LSC₂₁₄, correlate with the experimental observation of surface stability trends in surface particle formation of LSCF₁₁₃ and LSC₁₁₃ without and with LSC₂₁₄ decoration.