The oxygen surface exchange coefficient (k) dictates the efficiency and response time of many mixed conductors, so its accurate, continuous measurement in realistic conditions, enabling rational tailoring, is necessary. However, recent results showed that k values determined by a thin-film optical transmission relaxation (OTR) method were orders of magnitude lower than those extracted from the cross-cell AC-impedance spectroscopy (AC-IS) approach, and similar discrepancies among methods exist in the literature. OTR has also detected dramatic increases in k in situ during crystallization. Therefore, in this work, we sought to establish whether k values from OTR are reliable, and to gain further insight into crystallization-induced changes, via comparison to the electrical conductivity relaxation (ECR) method. We performed simultaneous OTR and ECR measurements on the same region of an as-grown amorphous SrTi 0.65 Fe 0.35 O 2.825+δ (STF) film, prepared by pulsed laser deposition and characterized by Rutherford backscattering spectrometry, during thermal treatment to induce crystallization and a large increase in k. We also compared cross-cell AC-IS vs OTR on an as-grown amorphous film during crystallization and OTR vs ECR on a crystalline-grown film. Simultaneous measurements eliminate variability in k between samples or due to different thermal/gas history. OTR and ECR methods yielded the same k values, and the same crystallization temperature, within error. Both isothermal optical absorption and electrical conductivity changes are proportional to the hole and oxygen concentration changes under the conditions of this study. However, while OTR was able to measure optical absorption changes under all of the conditions tested, ECR was not viable in the high-resistance regime. Cross-cell AC-IS k values were elevated vs OTR values, were less stable over time, and were only accessible in limited conditions. We discuss the potential impacts of current collectors and oxygen exchange driving force on k values determined by cross-cell AC-IS vs ECR vs OTR.

Solid-state heterointerfaces are of interest for emergent local behavior that is distinct from either bulk parent compound. One technologically relevant example is the case of mixed ionic/electronic conductor (MIEC)–metal interfaces, which play an important role in electrochemistry. Metal–MIEC composite electrodes can demonstrate improved catalytic activity vs single-phase MIECs, improving fuel cell efficiency. Similarly, MIEC surface reaction kinetics are often evaluated using techniques that place metal current collectors in contact with the surface under evaluation, potentially altering the response vs the native surface. Techniques enabling direct and local in situ observation of the behavior at and around such heterointerfaces are needed. Here, we develop a spatially resolved optical transmission relaxation (2D-OTR) method providing continuous evaluation of local, high-temperature, controlled atmosphere defect kinetics across a ∼1 cm2 sample area simultaneously in a contact-free manner. We apply it to observe the spatial variance of oxygen incorporation and evolution rates at ∼525–620 °C, in response to step changes in oxygen partial pressure, on MIEC SrTi0.65Fe0.35O3–x films as a function of distance from porous Pt and Au layers. Using this model geometry, we find significant enhancements in kinetics adjacent to the metals that decay over a few millimeter distance. To extract kinetic parameters, we fit the short-term optical data (initial portion of relaxations) with an exponential decay function appropriate for surface-exchange-limited kinetics, yielding apparent surface exchange coefficients (k chem) with spatial resolution, decreasing with distance from the metal. To understand the kinetic processes governing the complete (long-term) optical relaxations, we performed COMSOL simulations, which demonstrated that a combination of laterally varying k chem and in-plane diffusion controls the observed kinetics over the full time range. Further support for spatially varying k chem comes from demonstrations of changing surface and bulk chemistry vs distance from the metal–MIEC interface, by X-ray photoelectron and optical absorption spectroscopies, respectively. Although microporous Pt and Au are not excellent electrodes in isolation, both metals exert a synergistic effect on the oxygen surface exchange rate in the presence of the mixed conducting film.

Crystallization of a perovskite mixed conductor is coupled to oxidation, which drives defect chemistry, ion coordination, polyhedra alignment, hierarchical microstructure, and property evolution – including a boost to electrical conductivity.