Energetics of dysprosia-stabilized bismuth oxide electrolytes

With ionic conductivities superior to conventional doped zirconia and ceria at intermediate temperatures (IT, 700-800 °C), bismuth oxide (BiO _1.5) materials based on the defect fluorite structure are promising electrolyte candidates for solid oxide fuel cells (SOFCs) operating at reduced temperatures. In order to investigate the energetics of stabilized BiO _1.5 in the fluorite structure, DyO _1.5-stabilized BiO _1.5 (DSB) over a range of compositions was synthesized by solid state reaction and quenched. Using high temperature oxide melt solution calorimetry in molten 3Na ₂O·4MoO ₃ at 702 °C, enthalpies of formation at 25 °C were determined. Relative to the phases of the oxide end-members stable at room temperature (monoclinic BiO _1.5 and C-type DyO _1.5), the formation of Bi _1-xDy _xO _1.5 is endothermic at x < 0.30 and becomes slightly exothermic toward the upper phase boundary (x = 0.50). These data suggest that this system is slightly stabilized, and there is only a moderate (1-2 orders of magnitude) decrease in the volatility and susceptibility to reduction of bismuth oxide at high temperatures. However, high conductivity still makes the system potentially useful at 700 °C and below. Similar to findings for rare earth-doped zirconia, hafnia, and ceria, a negative interaction parameter for mixing in the solid solution suggests a tendency for short-range ordering, and the increasingly exothermic ΔH _mix with increasing x parallels the conductivity decrease with increasing dopant content.