Scandia–zirconia electrolytes for intermediate temperature solid oxide fuel cell operation

Conductivity of several compositions in the scandia–zirconia system (Sc2O3 content between 7.0 and 11.0 mol%) has been investigated as a function of temperature and time at the nominal solid oxide fuel cell operating temperature of 850°C. For comparison, the stability of electrolyte compositions was also studied at 1000°C by monitoring conductivity as a function of time. All compositions showed conductivity degradation as a function of time both at 850 and 1000°C, however, it was much lower in compositions containing Sc2O3 between 9.0 and 11.0 mol% especially at 1000°C. The maximum conductivity was observed for materials with compositions close to 9 mol% Sc2O3–ZrO2. The 10.0 and 11.0 mol% Sc2O3–ZrO2 compositions showed the presence of a rhombohedral-phase (Sc2Zr7O17, the beta-phase) at room temperature. Hysteresis loops were observed in the conductivity–temperature plots around 500–600°C due to the transformation of the low conducting beta-phase to high conductivity cubic-phase on heating and the reverse on cooling. The contribution of grain boundary impedance in as-sintered and annealed specimens was relatively small for all specimens studied.