Yttria co-doping of scandia-zirconia electrolytes for SOFCs

Scandia stabilised zirconias offer much better performance than the conventional yttria stabilised materials and now it seems that availability and cost may not be hugely problematic. This offers a very important way forward to fuel cell manufacturing. It is shown here that a small addition of 2 mol% yttria to scandia stabilised zirconia results in formation of the cubic phase and so avoids major phase changes in scandia only substituted zirconia, which we believe is likely to be detrimental to long term electrolyte stability. This addition of yttria can be achieved without significant impairment of the electrical conductivity of the scandia stabilised zirconia. Samples which are cubic throughout the studied temperature range, i.e. either effectively quenched scandia only or yttria co-doped show two linear conductivity regions in Arrhenius conductivity plots. Importantly the low temperature activation energy decreases and the high temperature activation energy increases as yttrium content increases and scandium content decreases. This correlates with shortrange order effects observed by neutron diffraction. Thus we have identified a new range of composition for use in solid oxide fuel cell electrolytes, which exhibits a good compromise between phase stability and electrical properties.