Electrochemical Properties of Nanocrystalline La0.5Sr0.5CoO3-x Thin Films

Abstract

As part or an investigation of new cathode materials for intermediate temperature solid oxide fuel cells, we have investigated several perovskite oxides with cobalt ions on the B sites in both bulk and thin film forms. Of particular interest is the composition La0.5Sr0.5CoO3-x (LSCO) which has exceptional properties for oxygen reduction at intermediate temperatures in ceria based fuel cells. Thin films of LSCO were deposited on both sides of a dense polycrystalline gadolinia doped ceria substrate by pulsed laser deposition under conditions that lead to the formation of nanocrystalline films. The electrochemical properties for oxygen reduction were determined in a symmetric electrochemical cell by alternating current (AC) impedance spectroscopy. The results were analyzed using the Adler-Lane-Steele (ALS) model to obtain the diffusion and surface exchange coefficients and the thermodynamic factor. We show that the thermodynamic factor, a measure of how easy it is to create oxygen vacancies, is much higher than observed in conventional cathodes. As a result, the electrode composition changes little with temperature and oxygen partial pressure, the large chemical contribution to the thermal expansion is reduced, and the electrode has good stability. The use of a nanostructured electrode does not significantly affect the fundamental material parameters (surface exchange and diffusion coefficients), and the very low area specific resistance (0.09 ohm cm(2) at 600 degrees C) observed is because the synthesis method gives a very high surface area (80 mu m(-1)).