Electrochemical Properties of Nanocrystalline La_0.5Sr_0.5CoO_3−x Thin Films

As part of 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 La 0.5 Sr 0.5 CoO 3− 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 °C) observed is because the synthesis method gives a very high surface area (80 μm −1 ).