Statistical geometry of reaction space in porous cermet anodes based on ion-conducting electrolytes - Patterns of degradation

The statistical geometry of the electrochemically active triple-phase bound ary in solid oxide fuel cell (SOFC) anodes with oxygen-ion-conducting (zyrc onya type) electrolyte is analyzed by means of an 'effective-medium' theory and verified by Monte Carlo simulations. Variation of the triple-phase bou ndary with time due to spontaneous sintering of metal particles is describe d by kinetic effective-medium equations. Their solution reveals possible de gradation scenarios, as well as the factors that impede degradation, or eve n cause a rise of the active triple-phase boundary in the course of SOFC op eration. The cermet composition, i.e. the relative portion of electrolyte, metal and pores, is among these factors, It is shown that the 'best' compos ition before degradation may not be the one that provides the best performa nce after degradation. The latter depends on the probability of pore openin g in sintering of two metal grains. Rough estimates of this probability (an d determination of the 'optimum' composition) would be possible from a comp arison of the calculated porosity before and after degradation with experim ental data, which are not available so far. (C) 1999 Elsevier Science B.V. All rights reserved.