High temperature electrocatalytic properties of the SOFC air electrode La0.8Sr0.2MnO3/YSZ

The electrocatalytic properties of the air electrode La0.8Sr0.2MnO3 in solid oxide fuel cells were examined by using complex impedance spectroscopy under the conditions of oxygen partial pressure, P(O-2), between 1 and 10(-4) atm and temperatures, T,between 800 and 1000 degrees C. The electrodes, O-2/La0.8Sr0.2MnO3/YSZ, were prepared by co-firing at 1350, 1400, 1435, and 1500 degrees C in air. In the measurements at 800-1000 degrees C, sigma(E), was nearly proportional to P(O-2)(1/2) at high P(O-2)s and was proportional to P(O-2) at low P(O-2)s. In the region where sigma(E) was proportional to P(O-2), sigma(E) showed nearly the same value irrespective of the different electrode microstructures, suggesting that the electrode reaction was controlled by O-2 gas diffusion. In the region where sigma(E) proportional to P(O-2)(1/2), the relationship between sigma(E) and P(O-2) was interpreted by the model that the electrode reaction was controlled by the surface diffusion of adsorbed oxygen atoms, O-ad, from the electrode surface to the electrode/electrolyte boundary. The results obtained in the present work meant that La2Zr2,O-7, formed on the La0.8Sr0.2MnO3 surface, disturbed the surface diffusion of O-ad from the La0.8Sr0.2MnO3 surface to the three phase boundary of O-2, La0.8Sr0.2MnO3, and Y2O3 stabilized ZrO2 (YSZ). Increasing the co-firing temperature, the surface composition of the electrode material changed, and hence the oxygen adsorption process might be changed.