Oxygen exchange at the surface of mixed ionic and electronic conductors (MIECs) plays an integral role in the efficiency and lifetime of many electrochemical devices and is often the efficiency-limiting process at intermediate temperatures (300-600 °C). Despite development of many MIEC perovskite compositions for this purpose, there is still a need to deeply understand processing-structure-property relationships in relation to optimizing the surface exchange coefficient. In recent work, we observed by a new optical transmission relaxation (OTR) approach that perovskite SrTi_0.65Fe_0.35O_3-d (STF35) films deposited by pulsed laser deposition at room temperature in the amorphous state, and subsequently annealed at the crystallization temperature, show immeasurably fast oxygen surface exchange kinetics in the intermediate temperature range [1,2]. In order to better understand this effect, our ongoing work has three aims: 1) to characterize the structural changes occurring in STF35 thin films upon crystallization using X-ray absorption spectroscopy 2) to relate the structural evolution to emergent functional properties, and 3) to evaluate the accuracy of the OTR technique for the measurement of oxygen surface exchange kinetics during crystallization.

Towards this last goal, surface exchange coefficients (k_chem) were analyzed using simultaneous electrical conductivity relaxation (ECR) and optical transmission relaxation (OTR) techniques on amorphous and crystalline films at various temperature points from 300-600 °C and as a function of oxygen activity in the films by varying the oxygen partial pressure (pO₂). Our prior work has shown that the change in optical absorption of the films is directly proportional to the change in their oxygen concentration and change in hole concentration (via Fe valence changes). Therefore, we hypothesized that as the films oxidize or reduce, the amount of light absorbed increases or decreases in direct correspondence to the increase or decrease in p-type electronic conductivity in the film, assuming minimal changes in the electronic mobility with pO₂ [3]. Across the different measurement conditions, our results showed excellent agreement in relaxation profiles and fitted k_chem values between these two techniques (OTR and ECR) for crystalline films (within a factor of 2), demonstrating the applicability of the optical technique to the STF35 system. Further, both ECR and OTR showed the emergence of rapid oxygen exchange upon in situ crystallization, validating the earlier optically-based observation of this effect. This novel OTR technique provides a continuous, contact-free, and in situ measurement of the native surface kinetics in realistic operating conditions that has the potential to be applied to many other systems.

[1] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach” Journal of Materials Chemistry A, 5, 23006-23019 (2017).

[2] T. Chen, G.F. Harrington, J. Masood, K. Sasaki, and N.H. Perry, “Emergence of Rapid Oxygen Surface Exchange Kinetics During In Situ Crystallization of Mixed Conducting Thin Film Oxides” ACS Applied Materials and Interfaces (2019).

[3] N.H. Perry, N. Kim, E. Ertekin, H.L. Tuller, “Origins and control of optical absorption in a non-dilute oxide solid solution: Sr(Ti,Fe)O3-x perovskite case study” Chemistry of Materials (2019).