Influence of yttria concentration upon electrical properties and susceptibility to ageing of yttria-stabilised zirconias

The most widely used electrolyte in solid oxide fuel cell (SOFC) devices is 8 mol% yttria-stablished zirconia (YSZ). Recent investigations have questioned the long term stability of this material at both the SOFC operating temperature of 1000 degrees C and during cycling between room temperature and 1000 degrees C. One possible method of improving the stability is by increasing the yttria concentration. Previous studies, utilising a number of different commercial YSZ powders have suggested that this may be possible, however, there were other differences between the samples in addition to yttria-content, notably impurity concentration. Thus the importance of yttria concentration has not been unambiguously demonstrated and the optimal content of yttria has not been determined In this study, the mechanical strength and ionic conductivity of a range of ultra-pure compositions from the (ZrO2)(1-x)(Y2O3)(x) system close to 8 mol% yttria-stabilised zirconia, x = 0.08, have been investigated. 8.5 mol% YSZ was found to have comparable mechanical strength and ionic conductivity to conventional 8 mol% YSZ, but showed improved stability. On ageing at typical fuel cell operating temperatures, i.e. 1000 degrees C, the conductivity of samples with yttria contents of 8.5 mol%, or higher, changed little; however, conductivity values for compositions with lower yttria contents decreased by 10-15%. These results strongly suggest that optimal properties are obtained for the minimum yttrium composition required to fully stabilise the cubic polymorph at the temperature of operation. (C) 1998 Elsevier Science Limited. All rights reserved.