Relating Microstructure to Surface Exchange Kinetics Using in Situ Optical Absorption Relaxation

Abstract

Reversible solid oxide cells (R-SOCs) are promising devices, which could directly convert chemical fuels to electrical power (Solid Oxide Fuel Cells, SOFCs) and store energy as chemical fuels (Solid Oxide Electrolysis Cells, SOECs). Nonetheless, the polarization resistance of the oxygen electrode, often originating in sluggish surface oxygen incorporation/ excorporation kinetics, causes large efficiency losses. Seeking to understand and ultimately improve surface exchange kinetics, many previous studies have focused on controlling the bulk defect chemistry (oxygen vacancy and/or electron content) or electronic structure through tailoring chemistry or strain. Fewer studies have focused on understanding the role of microstructural features in controlling both the surface exchange coefficient, k, and its deterioration over time (aging), especially for mixed ionic-electronic conductor (MIEC) electrode materials. SrTi0.65Fe0.35O3-δ (STF35) is a mixed conductor with fast k, which serves as a model system in our studies, to study the impact of microstructure (particularly crystallinity) on k and the aging behavior. In this work, a series of STF35 thin films were grown by Pulsed Laser Deposition (PLD) using different growth parameters (temperature, time) to vary structural aspects such as crystallinity in a well-defined and controlled manner. Structural characterization of the films’ thickness, orientation, crystal quality, and grain size was performed using X-ray reflectivity and diffraction (coupled scans, rocking curves) plus atomic force microscopy. The surface exchange kinetics (k) of the series of STF35 thin films were measured in situ and continuously using a novel optical transmission relaxation approach, to investigate the impact of the aforementioned structural features on the performance. The approach takes advantage of the different optical absorption signatures of Fe in different valence states. Both XRD and AFM results showed that the STF35 thin films grown on yttria-stabilized zirconia at temperatures up to 400 ºC are amorphous, while the films become crystalline and oriented around 500-550 °C, with larger grains at higher temperatures. There is no obvious optical response to changing gas content for the as-grown amorphous films below 500 °C; however, upon heating these films at 500-550 °C, where they begin to crystallize, rapid oxygen exchange could be observed optically in the same films. Subsequently, the films optically demonstrated an ability to exchange oxygen at temperatures as low as 350 °C. The results indicate that crystalline thin films showed faster oxygen exchange kinetics compared to the amorphous thin films. The aging of STF thin films’ surface exchange kinetics was also studied optically as a function of microstructure, which in every case demonstrated a rapid decline in k during the initial 50 hours of measurement. Implications for electrode design will be discussed.