The limiting factor for oxygen exchange at the surface of fuel cell electrolytes

The outermost surface layer of the yttria stabilised zirconia (YSZ) electrolyte in the solid oxide fuel cell plays an important role in the performance of the cell. By combining 16O/18O exchange experiments with quantitative surface analysis by low energy ion scattering spectroscopy (LEIS), the relation between the composition of the outermost atomic layer and the oxygen kinetics at the surface of the electrolyte can be studied. The results suggest a linear relation between an increasing amount of impurity oxides present at the surface and a decreasing oxygen exchange. Ceramics invariably contain glassy impurities, which segregate to the grain boundaries during sintering. Our results, however, show that the accumulation is restricted to the outermost atomic layer and proceeds until complete coverage is obtained. This observation underlines the importance of the used surface analysis technique. The strong accumulation of the impurities at the surface is observed even in the purest YSZ materials available. A decrease of the total bulk impurity concentration by a factor of 10–100 is necessary to ensure that the YSZ surface cannot be covered completely by impurities. The amount of exchanged oxygen at the outermost surface layer reduces from 50% at a clean yttria stabilised zirconia surface to zero at a surface completely covered by impurity oxides. Although the oxygen exchange experiments pertain to a temperature of 500 °C it is believed that the impurity oxides also have a strong influence at higher temperatures.