An Ideal Surface Reveals How Active It Is and Why It Degrades: SrTi _0.5 Fe _0.5 O _3−δ Model Thin Film with Atomically Flat Terraces─A Case Study

Surface cation segregation, specifically strontium (Sr), has been identified as a primary factor contributing to the performance degradation of perovskite-based oxide electrodes used in various energy conversion devices. However, due to the complex chemistry and structure of the perovskite oxide surfaces, the mechanisms behind surface segregation and its impact on electrode activity are only partially understood. Moreover, this phenomenon occurs during perovskite synthesis, further complicating the situation. To address this issue, this study implements a controlled approach using a model thin film system composed of SrTi_0.5Fe_0.5O_3-δ (STF50) with a stoichiometric surface and atomically flat terraces, enabling detailed examination. The evolution of surface structure, composition, and oxygen exchange kinetics are observed as a function of temperature and time. By integrating experiments and <i>ab initio</i> simulations, we tackle several fundamental questions, including the evaluation of reactivity for pristine perovskite oxide surface before surface segregation and the correlation between surface segregation at the surface with oxygen exchange kinetics. Our comprehensive analysis clearly reveals that the decline in performance of the perovskite oxide electrodes is primarily attributed to the detrimental effects of Sr-deficiency on the surface, thereby resolving longstanding debates in the field.