Optimized mixed ionic–electronic conductivity in two-phase ceria–zirconia composite with cobalt oxide and Na2CO3 as suitable additives

Abstract

Composite membranes consisting of tantalum doped ceria (TDC) and yttria doped zirconia (YSZ) were investigated, aiming at an optimized mixed ionic–electronic conduction. A particular interest was to find suitable additives that minimize the usually high interface resistance for both electron and oxygen ion transport at ceria–zirconia two phase interfaces. Two types of additives were investigated: (a) CoO, to improve the electronic conduction and (b) Na₂CO₃, SrCO₃ and BaCO₃, to achieve a favorable influence on the ionic transport at and through grain boundaries. As compared to cobalt oxide free composites, ∼1.6 wt% CoO (1 mol%) increased the electronic conductivity by one decade. Regarding the ion transport, SrCO₃ and BaCO₃ exhibited no favorable influence, whereas the addition of 20 wt% Na₂CO₃ caused a clear increase in ionic conductivity by three orders of magnitude. The combined effect of cobalt oxide and Na₂CO₃ addition was optimized finally in a sample with ∼1.6 wt% CoO doping and ∼14.3 wt% Na₂CO₃ addition which showed a conductivity value nearly 1000 times higher than the additive free TDC–YSZ composite. Ionic and electronic conductivities for this sample were comparatively high and enough to achieve a good MIEC composite membrane. A consistent model is given to explain the transport mechanism which supports a continuous oxygen ion transport through a Na₂CO₃ interlayers between two YSZ grains. It is based on sodium ion mobility, exchange of O²⁻ ions and reversible formation of Na₂O and CO₂ in the same way as known from corresponding Na₂CO₃ based CO₂ sensors.