Oxygen transport kinetics of the misfit layered oxide Ca₃Co₄O_9+δ

The oxygen transport kinetics of the misfit-layered cobaltite, Ca₃Co₄O_9+δ, known for its thermoelectric properties, was investigated by combined application of ¹⁸O/¹⁶O isotope exchange and electrical conductivity relaxation techniques. Although oxygen diffusion is found to be two orders of magnitude lower than in well-investigated lanthanum nickelates, e.g., La₂NiO_4+δ, the mixed ionic-electronic conductor Ca₃Co₄O_9+δ is found to exhibit fast surface exchange kinetics (k∗ = 1.6 × 10^-7 cm s^-1 at 700 °C to be compared to 1.3 × 10^-7 cm s^-1 for the nickelate), rendering it a promising electrode for application as an air electrode in solid oxide cells. In parallel, the chemical nature of the outermost surface of Ca₃Co₄O_9+δ was characterized by means of Low Energy Ion Scattering (LEIS) spectroscopy. The absence of cobalt at the sample's outermost surface suggests that the Ca₂CoO_3-δ rock salt layers in the structure may play a key role in the oxygen exchange mechanism.