Bias‐Triggered Conductivity Relaxation (BCR): A Unique Tool to Simultaneously Investigate Thermodynamics, Kinetics, and Electrostatic Effects of Oxygen Reactions in MIEC Thin Films

ABSTRACT Mixed ionic‐electronic transfer (MIET) reactions, such as the oxygen reduction reaction (ORR) at oxide surfaces, are of paramount importance to manifold technologically highly relevant processes, and fundamental understanding must be developed to improve performance and tailor highly efficient electrodes and catalysts. Understanding such complex multi‐step reactions requires the study of kinetic processes, underlying thermodynamic properties, i.e., ionic and electronic defect concentrations, and electrostatic surface effects. However, conventional techniques struggle to uncover the complete picture within the same sample/measurement. Here, we overcome this limitation by introducing bias‐triggered conductivity relaxation (BCR) as a novel tool to investigate MIET reactions on oxides. It is based on alternating out‐of‐plane coulometric titration/polarization and in‐plane electrical conductivity relaxation measurements, providing simultaneous electronic, ionic, and extraordinarily rich surface kinetics information. This innovative combination of electrical and chemical driving forces synergizes information depth, with enhanced time resolution, versatility, and speed, yet it lifts the weaknesses of the individual approaches, while remaining cost‐effective and surprisingly simple. Furthermore, BCR allows to disentangle overpotential induced electrostatic modifications of the surface kinetics in a unique manner. We showcase the advantages of BCR in this work by studying the ORR in model (La, Sr)FeO 3‐δ thin film electrodes and reporting on their thermodynamic and kinetic properties.