Nanoscopic Surface Decomposition of Pr_0.5Ba_0.5CoO_3-δ Perovskites Turns Performance Descriptors Ambiguous

The surface electronic structure of a material is frequently used to identify simple descriptors for its catalytic efficacy and other properties. To harness the predictive ability of such descriptors, structural and chemical evolutions of the material when exposed to operating conditions, such as oxidizing environments and high temperatures, need to be considered. These evolutions occur at length scales not easily observable, leading to averaging over short-range variations and thus misinterpretation of the property in question. Here, we investigate perovskite Pr_0.5Ba_0.5CoO_3-δ as a prototypical mixed ionic-electronic conductor that exhibits promising catalytic properties toward the oxygen evolution reaction in electrochemical cells, which have been characterized by such descriptors. We employ spatially resolved X-ray absorption spectroscopy and find a Cahn-Hilliard-type decomposition process at sub-micrometer length scales after mere hours at operating or processing conditions. The observation is in contrast to the thermodynamic stability of the Pr_0.5Ba_0.5CoO_3-δ bulk, suggesting the decomposition to be confined to the surface. Our results showcase a considerable lateral inhomogeneity of the surface electronic structure, emphasizing that descriptors derived through spatially averaging techniques have to be heavily scrutinized.