Investigation of the stability of ceria-gadolinia electrolytes in solid oxide fuel cell environments

Doped ceria-based materials are potential electrolytes for use in lower ope rating temperature (500-700 degrees C) solid oxide fuel cells because of th eir high ionic conductivity. In this study, impedance behaviour and microst ructure of the (Ce0.8Gd0.2)O-1.9 exposed to mild fuel environments (H-2-N-2 mixtures) have been investigated. The exposure of specimens to H-2-N-2 mix tures at 1000 degrees C resulted in a substantial expansion of the lattice as a consequence of the reduction of Ce4+ to Ce3+, which in turn led to the development of microcracks and loss of continuity at the grain boundary re gion and increase in both the grain boundary (major effect) and the lattice (minor effect) resistivity. The behaviour for the grain boundary resistivi ty after the 800 degrees C exposure was somewhat similar although expansion of the lattice at 800 degrees C (or lower temperatures) was considerably l ess conspicuous. After exposure to H-2-N-2 atmosphere at lower temperatures (650 and 500 degrees C), although no significant increase in the grain bou ndary resistivity for exposures up to 1000 h was observed, the shape of the grain boundary are was clearly affected. The large increase in the grain b oundary resistivity in reduced specimens has been attributed to the observe d microcracking, loss of continuity between grains and possibly the formati on of new phase regions with extremely poor oxygen-ion conductivity along g rain boundaries during the reduction. The disruption to the microstructure is not recovered on subsequent oxidation in air. (C) 1999 Elsevier Science B.V. All rights reserved.