Co-doping of scandia-zirconia electrolytes for SOFCs

Scandia stabilised zirconias offer much better electrical performance than conventional yttria stabilised materials; however, the limited availability and high cost of scandia have generally limited interest in its application in fuel cells. Political and economic changes over the last decade have significantly enhanced scandia's availability, rendering it worth considering for commercial application, even though there is still some uncertainty about its ultimate market price. A small addition of 2 mol% yttria to scandia stabilised zirconia results in stabilisation of the cubic phase and so avoids the major phase changes that occur on thermal cycling of scandia substituted zirconias, which might be expected to be detrimental to long term electrolyte stability. This addition of yttria does slightly impair the electrical conductivity of the scandia stabilised zirconia, although this can be reversed by further addition of ceria. Samples which are cubic throughout the studied temperature range basically show two linear conductivity regions in Arrhenius conductivity plots. A key observation is that the low temperature activation energy decreases and the high temperature activation energy increases as yttrium content increases and scandium content decreases. This correlates with the strength of short-range order as indicated by neutron and electron diffraction studies. Although scandia substitution increases conductivity and decreases high temperature activation energy, it also increases the tendency to short-range ordering at lower temperatures, resulting in a significant increase in activation energy for conduction. This is attributed to the ionic size of the Sc ion which favours a lower coordination number than that associated with ideal fluorite phases. It should also be realised that Zr, which has a similar size to Sc, also prefers a lower coordination number than is ideal for fluorite hence driving the tendency for short-range order in zirconia fluorites.