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 La0.8Sr0.2Co0.2Fe0.8O3. The gas‐exchange kinetics are characterized by synthesizing epitaxial half‐cell geometries where three single‐variant surfaces are produced [i.e., La0.8Sr0.2Co0.2Fe0.8O3/La0.9Sr0.1Ga0.95Mg0.05O3−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 (pO2)‐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.

Solid‐oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure–property relationships that would enable the rational design of better materials. Here, using epitaxial thin‐film growth, synchrotron radiation, impedance spectroscopy, and density‐functional theory, the impact of structural parameters (i.e., unit‐cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9Sr0.1Ga0.95Mg0.05O3–δ. As compared to the zero‐strain state, compressive strain reduces the unit‐cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit‐cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit‐cell volumes and octahedral rotations decrease migration barriers and create low‐energy migration pathways, respectively. The desired combination of large unit‐cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit‐cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion‐conducting perovskite electrolytes.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possibleAny correspondence concerning this service should be sent to the repository administrator: tech-oatao@listes-diff.inp-toulouse.fr This is an author's version published in: http://oatao.univ-toulouse.fr/24433