Effect of water on oxygen transport properties on electrolyte surface in SOFCs I. Surface Reaction Mechanism of Oxygen Isotope Exchange on Solid Oxide Electrolytes

The effect of water vapor on the oxygen isotope exchange property was investigated for yttria-stabilized zirconia (YSZ) oxide ion conducting ceramics, which is widely used as an electrolyte material for solid oxide fuel cells (SOFCs). A slight amount of water vapor addition made a great enhancement of surface exchange rate of oxygen isotope (O-18) on the surface of YSZ single crystal. The surface exchange rate constant increases with partial pressure of water vapor, whereas the oxygen isotope diffusivity has no dependence on water vapor. The temperature and water vapor pressure dependences of the enhancement of surface exchange rate are in good agreement with the dependences of water coverage on YSZ surface. Hence, the fast interaction between H2O molecule and oxygen vacancy in electrolytes was considered to be dominant in a humid atmosphere, and it resulted in the apparent high surface exchange rate constant of oxygen isotope. Relatively lower oxygen isotope diffusivity was obtained in humid atmospheres as compared to those measured in dry oxygen in the literature. The low diffusion coefficient was observed only for YSZ but not for other electrolytes such as gadolinium-substituted ceria. This is probably due to a temperature gradient between the sample and the thermocouple which is caused from low adsorption of YSZ single crystal for infrared. The effects of heating procedure, preannealing conditions, and materials used for sample holders were investigated. (C) 2003 The Electrochemical Society.