Understanding the defect chemistry of thin films, including deviations from bulk defect chemistry, is of great importance for mechanistic investigations of the oxygen reduction and evolution reactions on perovskites. Not only are defects vital for mass and charge transport, they are also the reaction partners in oxygen exchange reactions, such as the cathodic oxygen reduction on a solid oxide fuel cell. One method particularly suited for investigating the defect chemistry of thin films is measuring the chemical capacitance. In this study, we use voltage and oxygen partial pressure dependent chemical capacitance measurements to obtain defect chemical data for thin (La,Sr)FeO3 (LSF) films.
Thin LSF films were deposited onto yttria stabilized zirconia by pulsed laser deposition and characterized by electrochemical impedance spectroscopy (EIS). A variation of film thickness revealed a dominating volume specific chemical capacitance, and an additional area specific capacitance from the LSF/YSZ interface. The voltage and partial pressure dependence of the chemical capacitance showed that the chemical capacitance and thus the defect concentrations depend solely on the oxygen chemical potential inside the film.
From temperature dependent chemical capacitance measurements reaction enthalpies and entropies for the oxygen incorporation and electron/hole pair formation were extracted. Comparison with LSF defect chemical bulk data from literature showed good agreement for vacancy formation energies but suggest larger electronic defect concentrations in the films. Furthermore, deviations from simple chemical capacitance models at high pressures suggest defect interactions.