On the Impact of the Degree of Fluorination on the ORR Limiting Processes within Iron Based Catalysts: A Model Study on Symmetrical Films of Barium Ferrate

In this study, symmetrical films of BaFeO$_{2.67}$, BaFeO$_{2.33}$F$_{0.33}$ and BaFeO$_{2}$F were synthesized and the oxygen uptake and conduction was investigated by high temperature impedance spectroscopy under an oxygen atmosphere. The data were analyzed on the basis of an impedance model designed for highly porous mixed ionic electronic conducting (MIEC) electrodes. Variable temperature X-ray diffraction experiments were utilized to estimate the stability window of the oxyfluoride compounds, which yielded a degradation temperature for BaFeO$_{2.33}$F$_{0.33}$ of 590 °C and a decomposition temperature for BaFeO$_{2}$F of 710 °C. The impedance study revealed a significant change of the catalytic behavior in dependency of the fluorine content. BaFeO$_{2.67}$ revealed a bulk-diffusion limited process, while BaFeO$_{2.33}$F$_{0.33}$ appeared to exhibit a fast bulk diffusion and a utilization region δ larger than the electrode thickness L (8 μm). In contrast, BaFeO$_{2}$F showed very area specific resistances due to the lack of oxygen vacancies. The activation energy for the uptake and conduction process of oxygen was found to be 0.07/0.29 eV (temperature range-dependent), 0.33 eV and 0.67 eV for BaFeO$_{2.67}$, BaFeO$_{2.33}$F$_{0.33}$ and BaFeO$_{2}$F, respectively.