Cation deficiency enables reversal of dopant segregation at perovskite oxide surfaces under anodic potential

Abstract

Surface instability of perovskite oxides caused by aliovalent dopant segregation and resultant formation of insulating surface layers and precipitates, such as SrO_x, limits the performance and durability of these materials in energy conversion, including in solid oxide fuel and electrolysis cells. Previous studies showed that reversal (re-incorporation) of such dopant segregation layers can occur under both cathodic and anodic potential in Sr-doped lanthanum manganite (LSM) as a model perovskite system. Sr segregation under anodic and reversal of segregation under cathodic potential could be explained by a defect equilibrium reaction involving A-site cation vacancies and Mn oxidation state. Sr segregation under cathodic and reversal of segregation under anodic potential is also known to take place, but it has been poorly understood and its governing mechanism is not known. In this study, we show that defect reactions involving cation vacancies and oxygen holes at high oxygen pressures can account for re-incorporation of the segregated dopant from the surface into the bulk of lanthanum manganite under anodic conditions. We use Ca-doped lanthanum manganite (LCM) as a model perovskite oxide. We have demonstrated with X-ray photoelectron and absorption spectroscopy that the precipitation and re-incorporation of the surface CaO_x under cathodic and anodic potential, respectively, are coupled with the reduction and oxidation of the perovskite surface. Under oxidizing, anodic conditions, cation-deficient perovskite unit cells form on/near the surface, allowing for the dissolution of the excess Ca at the surface back into the perovskite lattice. Notably, applying anodic potential could remove half of the surface Ca precipitates within a few minutes, thus improving the oxygen exchange kinetics of the LCM surface significantly. These results establish a highly efficient oxidative route for activating perovskite oxide surfaces and advance the quantitative understanding of dopant segregation and re-incorporation reactions.