In recent decades, tuning the anionic sublattice in oxides with hydrogen, nitrogen, fluorine, chlorine etc., is attracting growing interests. Among them, fluorine with the similar ionic radius as oxygen ions and able to occupy the oxygen sites in the lattice bringing anionic disorder, is of particular interest for applications in solid oxide cells (SOCs) or oxygen transport membranes (OTMs). Moreover, its lower electronic charge is expected to result in the concomitant reduction of the oxidation state of transition metal cations to maintain the electronic neutrality bringing changes in electronic behaviour. The strong electronegativity of F is predicted to destabilise the M-O bond, potentially beneficial for increasing oxygen diffusion kinetics. Previous investigations [1,2] on different oxide materials have demonstrated that a small amount of F doping is capable of improving the electrochemical and oxygen transport performance for materials applied in high temperature devices. Despite of the promising results, thermal stability of the F-doped materials is scarcely investigated considering the material preparation and device operation are both at elevated temperatures. Furthermore, post mortem chemical analysis on the materials to verify whether F stays in the oxide lattice after the high temperature annealing and studies on the mechanism of the improved electrochemical performance, are even rarely seen.

In this work, a state-of-art mixed electronic and ionic conducting (MIEC) material La_0.6Sr_0.4C_o0.2Fe_0.8O_3-δ (LSCF6428) as the oxygen electrode in the SOCs was chosen as the parent oxide material. The F-doped La_0.6Sr_0.4C_o0.2Fe_0.8O_3-δF_x powders (LSCFFx, x = 0.05, 0.10 and 0.20) was obtained via low temperature fluorination with PVDF binder. The incorporation of F in the oxide lattice has been carefully studied via X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS) measurements. The incorporated F concentration was further quantified with the fluoride selective electrode. Symmetric cells were prepared with LSCF and LSCFFx materials, and the cell performance was characterised with electrochemical impedance spectroscopy. Furthermore, the thermal stability of the LSCFFx oxyfluoride has been carefully studied with TGA-MS measurement and a comprehensive post mortem chemical analysis has also been carried out with various techniques. The absence of F is confirmed for the symmetric cells while an improvement in electrochemical performance is indeed observed. The oxygen surface exchange behaviour of the LSCFF materials with verified fluorine presence was further studied via pulsed isotopic exchange (PIE). The results raise the doubt if the fluorine incorporation is truly beneficial for the electrochemical performance of the materials with applications in solid oxide cells. Structural and chemical investigation (XRD, Mossbauer spectroscopy, and XPS analysis) were carried out on the materials after fluorine loss to understand the effect of preliminary PVDF treatment on the electrochemical properties. The main results of this work will be detailed during this presentation.