In situ synchrotron measurements of surface compensation mechanisms in La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$ thin films

With its desirable combination of thermal stability, catalytic activity, and electronic and ionic conductivity, La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$ (LSCF) is rapidly becoming the standard cathode material for solid oxide fuel cells (SOFCs). Prior electrical measurements have isolated oxygen reduction at the cathode as a primary rate-limiting step in the performance of SOFCs. To better understand the nature of oxygen reduction at the high temperature, atmospheric oxygen partial pressure (pO2) conditions of a working SOFC, we study epitaxial LSCF thin films grown on (001)p-oriented NdGaO$_3$ and SrTiO$_3$ using in situ x-ray scattering and spectroscopy methods. We find that at sufficiently high temperatures, LSCF forms surface reconstructions at atmospheric pO2 levels. Using grazing-incidence spectroscopy methods, we also find that strontium segregates to the surface and that the minority B-site cation, cobalt, responds to changes in pO2 and temperature. We discuss the interplay between these changes in surface composition and structure and its implication on oxygen reduction in SOFCs.