General considerations on degradation of Solid Oxide Fuel Cell anodes and cathodes due to impurities in gases

The degradation mechanism of cathodes and anodes due to gaseous impurities has been considered to provide generalized understanding of materials behavior involved in degradation and hopefully to possible recovery processes. Considerations can be made in terms of (1) the chemical reactivity of gaseous impurities with a main component of electrode materials, (2) diffusivity of main components associated with chemical reactions, and (3) electrochemical reduction/oxidation processes of gaseous impurities in competing with recovery processes due to those electrochemical reactions or chemical reactions which lead to the volatilization of deposited substance and removal of poisons from the electrochemically active sites. The chemical reactivity of the main component with gaseous impurities can be well characterized in terms of the thermodynamic properties such as interaction parameters for intermetallic compounds or the stabilization energy/the valence stability of the transition-metal oxides for oxide electrodes. Diffusivity of main components should be discussed with the enhancement due to surface coverage of impurities. The electrochemical reduction/oxidation of the gaseous impurities provides the mechanism of impurities being deposited and accumulated at the electrochemical reaction sites. Possible recovery process could be examined by whether corresponding electrochemical or chemical process for the deposited substances to be volatilized is available.