Determination of oxygen vacancy concentration in a thin film of La_0.6Sr_0.4CoO_3-δ by an electrochemical method

Equivalent circuit analysis was undertaken on a mixed conductor electrode/solid oxide electrolyte system. In a limited case where surface reaction is the predominant rate-controlling process, the equivalent circuit was simplified to parallel connection of a resistor (surface reaction resistance) and a capacitor (chemical capacitance due to oxygen nonstoichiometry). Equilibrium oxygen vacancy concentration was correlated with the chemical capacitance. The model was applied to a dense film of La_0.6Sr_0.4CoO_3-δ deposited on a sintered plate of Ce_0.9Gd_0.1O_1.95 by a laser ablation method. Frequency response of the electrochemical impedance was measured under dc bias at 873-1073 K in O₂-Ar gas mixtures. The observed capacitance was extremely large, e.g., around 0.1 to 1 F cm^-2 for the 1.5 μm thick film. The oxygen vacancy concentration in the film was calculated from the capacitance and compared with the literature data measured by thermogravimetry. The film was found to show smaller oxygen nonstoichiometry. The enthalpy for oxygen vacancy formation in the film was about 40 kJ mol^-1 larger than the bulk.