Evaluation of La-Sr-Co-Fe-O perovskites for solid oxide fuel cells and gasseparation membranes

Perovskites based on alkaline-earth containing lanthanum cobaltites exhibit high electronic and ionic conductivities but their application as cathodes for solid oxide fuel cells (SOFC) or gas separation membranes may be limit ed by thermal expansion and low stability of certain compositions. To ident ify the optimum composition regime, the following three series in the syste m La1-xSrxCo1-yFeyO3-delta were studied: (a) La1-xSrxCoO3-delta, (b) La0.3S r1-yCo1-yFeyO3-delta and (c) La1-xSrxCo0.2Fe0.8O3-delta. All samples were i nvestigated by four probe de conductivity measurements and dilatometry. For samples in group (a), La1-xSrxCoO3-delta, the conductivity at 800 degreesC increases with Sr content to a peak value at x = 0.3 and then decreases co ntinuously. The thermal expansion coefficient (TEC) decreases first, then i ncreases again. A minimum of TEC with alpha (30-1000 degreesC) = 18.5 X 10( -6) K-1 was obtained at x = 0.2. For samples of series (b), both the electr ical conductivity and TEC decrease with iron content. Even though these mat erials have high electronic and ionic conductivities, it is difficult to ut ilize them as fuel cell cathode materials because their thermal expansion i s not compatible with other cell components. These materials are, however, promising for gas separation membranes. (C) 2000 Elsevier Science B.V. All rights reserved.