Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes

Abstract Solid–gas interactions at electrode surfaces determine the efficiency of solid‐oxide fuel cells and electrolyzers. Here, the correlation between surface–gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La_0.8Sr_0.2Co_0.2Fe_0.8O₃. The gas‐exchange kinetics are characterized by synthesizing epitaxial half‐cell geometries where three single‐variant surfaces are produced [i.e., La_0.8Sr_0.2Co_0.2Fe_0.8O₃/La_0.9Sr_0.1Ga_0.95Mg_0.05O_3−δ/SrRuO₃/SrTiO₃ (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface‐orientation dependency of the gas‐exchange kinetics, wherein (111)‐oriented surfaces exhibit an activity >3‐times higher as compared to (001)‐oriented surfaces. Oxygen partial pressure ()‐dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas‐exchange kinetics, the reaction mechanisms and rate‐limiting steps are the same (i.e., charge‐transfer to the diatomic oxygen species). First‐principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron‐based, ambient‐pressure X‐ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin‐film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)‐surface exhibits a high density of active surface sites which leads to higher activity.