Oxygen Non-Stoichiometry of Bulk and Thin Film of Co-Based Mixed Ionic and Electronic Conductors

Oxygen nonstoichiometry is a critical property of mixed ionic and electronic conductor oxides. However, for certain oxides, the oxygen vacancy concentration (δ) in thin film differs from that in the bulk material. To investigate the cause of this discrepancy, dense films of La0.6Sr0.4CoO3-δ (LSC64), La0.6Ca0.4CoO3-δ (LCC64), and La0.8Ca0.2CoO3-δ (LCC82) were prepared on Ce0.9Gd0.1O1.95 electrolytes, and their chemical capacitance values (Cchem) were measured as a function of oxygen partial pressure (p(O2)) and temperature using electrochemical impedance spectroscopy. The results revealed a significant deviation in Cchem at high p(O2) for LSC64, while the LCC64 exhibited a behavior consistent with that of bulk. A detailed analysis of the film composition revealed Sr-O segregation in the LSC64. Cross-sectional analysis of the thin film electrodes indicated inhomogeneous distribution of Co in the LSC film, whereas Ca-Co-O was observed on the LCC64. On the other hand, evaluation of a relatively stable composition of LCC82 also revealed a discrepancy between Cchem film and bulk electrode, despite no cation segregation was observed at near film surface. These results underscore the differences in mechanism of the compositional change which causes the deviation of δ in the film from the bulk.