THERMODYNAMIC AND KINETIC CONSIDERATIONS FOR BI2O3-BASED ELECTROLYTES

Development of bismuth oxide containing solid electrolytes for potential use in solid oxide fuel cells is thc principal objective of this work. Bismuth-oxide-based solid electrolytes containing alkaline-earth or rare-earth oxides are known to exhibit high ionic conductivity. In the CaO-Bi2O3, SrO-Bi2O3, and BaO-Bi2O3 systems, there exist several compounds. A rhombohedral phase in these systems exhibits high ionic conductivity. Experiments were conducted to assess the thermodynamic stability of these materials in reducing environments. Galvanic cells with alkaline-earth fluorides as electrolytes were constructed for determining activity-composition diagrams in these three systems. Specifically, the activities of the alkaline-earth oxides were measured and those of Bi2O3 were determined by Gibbs-Dunhem integration. The results indicate that the SrO-saturated rhombohedral phase in the SrO-Bi2O3 system is more stable than the rhombohedral phase in the other two systems. The addition of Y2O3 or rare-earth oxides to Bi2O3 is known to stabilize the high-temperature CaF2-type phase to lower temperatures. However, annealing at approximately 600-degrees-C for greater-than-or-equal-to 200 h destabilizes the cubic phase. This destabilization can be suppressed by the addition of aliovalent dopants to suppress the cation interdiffusion. It has been observed that in Y2O3-Bi2O3 and Er2O3-Bi2O3 as well as other rare-earth oxide-Bi2O3 systems. the destabilization can be suppressed by adding ZrO2 as a dopant. This result is rationalized on the premise that cation interstitials are more mobile compared with cation vacancies in cubic Bi2O3. The potential use of Bi2O3-based electrolytes in solid oxide fuel cells is examined. It is shown that by a careful manipulation of conduction properties, thc interface partial pressure of oxygen in two-layer electrolytes consisting of a thin layer of zirconia on Bi2O3-based materials can be maintained high enough to prevent electrolyte reduction. Fuel cells made with such electrolytes should exhibit considerable higher power densities compared with all-zirconia electrolytes.