In view of long term reliability of SOFCs, the prediction of voltage degradation is essential. In yttria-stabilized zirconia (YSZ) generally adopted as electrolyte, the degradation of electrical conductivity is known to be caused by the phase transformation of the cubic to the tetragonal phase over the long term of several thousand hours. Using model YSZ electrolyte under SOFC operating condition, we have so far examined the parameters dominating the behavior of phase transformation. We clarified that the acceleration of phase transformation was caused by the dissolution of NiO into YSZ electrolyte and the following formation of low oxygen potential in YSZ electrolyte, and that the acceleration was significantly suppressed with decreasing temperature. Based on such effects on the phase transformation, we tried to predict the conductivity degradation due to phase transformation of electrolyte during long term operation.
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