Thermodynamic and kinetic considerations for Bi₂O₃-based electrolytes

Development of bismuth oxide containing solid electrolytes for potential use in solid oxide fuel cells is the 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 intergration. The results indicate that the SrO-saturated rhombohedral phase in the SrO-Bi₂O₃ systems in 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 ≈ 600°C for ≥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, the 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 nade with such electrolytes should exhibit considerably higher power densities compared with all-zirconia electrolytes.