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 zi rconia (YSZ). Two kinds of cathode structure were studied by means of impedance spectroscopy and d.c. electrochemical methods. The measureme nts were performed in air at 700-1000 degrees C. Then, the electrode m icrostructures were examined ceramographically. The cathode performanc e 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 catho de was applied. The performance proved to be sensitive to structural c hanges at 700 and 850 degrees C, whereas the effect of the structure w as limited at 1000 degrees C. Values as low as 0.07 Omega cm(2) at 100 0 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 interpr etation of the results and for assessing the potential for further imp rovement of the performance by optimising the microstructure.