La/Ba-based cobaltites as IT-SOFC cathodes: a discussion about the effect of crystal structure and microstructure on the O2-reduction reaction

This work discusses the effect of the crystal structure and microstructure on the electrocatalytic activity of Lanthanum-Barium cobaltite (LBC), evaluatedas IT-SOFC cathode. Two systems with similar microstructures (particle sizes dp≈1–5μm) are used to compare the effect of the crystal structure: the cubic La0.5Ba0.5CoO3-δ with La/Ba cations randomly distributed, and a tetragonal LaBaCo2O6-δ with La/Ba layered distribution. In addition, the effect of microstructure is studied by using a newsol-gel route which allows obtaining LaBaCo2O6-δ with smallest particle size (dp≈500nm). The electrode reaction isstudied by electrochemical impedance spectroscopy (EIS) as a function of temperature (T) and oxygen partial pressure (pO2). The electrode polarization resistance (RC,P) presents two contributions, a low frequency (RLF) and a high frequency Gerischer-resistance (RG). On one side, RLF, associated to the O2-gas diffusion across the gas layer boundary, presents low activation energy (Ea) and is proportional to pO2−1, presenting the same values regardless the microstructure.On the other side, RG is in agreement with anO2-reduction mechanismco-limited by O-surface exchange and O-bulk diffusion. RG is strongly influenced by structure (trough the diffusion coefficient DV) and microstructure (inversely proportional to dp). Besides, RG decreases by reducing dp and by modifying the La/Ba ordering from layered to randomly distributed.