Performance/structure correlation for composite SOFC cathodes

Many solid oxide fuel cell (SOFC) developers use composite electrodes. The cathode is often a mixture of La(Sr)MnO3 and yttria stabilised zirconia (YSZ). Two kinds of cathode structure were studied by means of impedance spectroscopy and d.c. electrochemical methods. The measurements were performed in air at 700-1000 degrees C. Then, the electrode microstructures were examined ceramographically. The cathode performance was improved by increasing the thickness of the composite electrode, and the polarisation resistance was decreased by extending the active triple phase boundary line through the application of a coarse layer of YSZ particles on the electrolyte surface before the composite cathode was applied. The performance proved to be sensitive to structural changes at 700 and 850 degrees C, whereas the effect of the structure was limited at 1000 degrees C. Values as low as 0.07 Omega cm(2) at 1000 degrees C and 0.50 Omega cm(2) at 850 degrees C were obtained in air at an overvoltage of -50 mV. Two simple models relating performance, percolation and triple phase boundary length were used for the interpretation of the results and for assessing the potential for further improvement of the performance by optimising the microstructure.