Optimisation of composite cathodes for intermediate temperature SOFC applications

The electrochemical properties of the interfaces between porous composites of La0.6Sr0.4Co0.2Fe0.8O3-delta/Ce0.9Gd0.1O2-delta cathodes and Ce0.9Gd0.1O 2-delta electrolytes have been investigated at intermediate temperatures (5 00-700 degrees C) using AC impedance spectroscopy. Results indicate that th e electrochemical properties of these composites are quite sensitive to the composition and the microstructure of the electrode. The optimum Ce0 9Gd0. 1O2-delta addition (36% by volume) to La0.6Sr0.4Co0.2Fe0.8O3-delta resulted in four times lower area specific resistivity, which classify this composi te as a promising material for solid oxide fuel cells based on Ce-0 9Gd0.1O 2-delta electrolytes. The observed high performance of the composite electr odes at this composition is consistent with the effective medium percolatio n theory which predicts the ambipolar transport behaviour of composite mixe d ionic-electronic conductors as a function of the volume fraction of each of the randomly-distributed constituent phases. Quantitatively, a slight di screpancy between measurements and theory was observed. This is believed to be due to the fact that the overall performance of a porous electrode is n ot only determined by the mixed conducting transport properties in the soli d phase of the electrode, but also by the inherent catalytic property of th e triple phase boundary, and by the gas transport to, or away, from the tri ple phase boundary. (C) 1999 Published by Elsevier Science B.V. All rights reserved.