Optimizing rhombohedral Bi2O3 conductivity for low temperature SOFC electrolytes

Abstract

Rhombohedral Bi2O3 has not been extensively evaluated as an electrolyte material for solid oxide fuel cells (SOFCs) due to the greater potential of the more highly conductive cubic phase. Due to the higher structural stability of rhombohedral Bi2O3, improving the conductivity of the material was undertaken to improve the performance of the electrolyte. Since the rhombohedral phase is generally only assessed in Bi2O3 with high total dopant concentrations, the phase stability window was mapped out in the lower dopant regime. Reported herein is a phase diagram of the rhombohedral Bi2O3 at a total dopant concentration below 10%. Within the phase stability window, the rhombohedral phase was also optimized in terms of the dopant cation and the total cation substitution rate. It was found that increasing the size of the cation dopant and decreasing the total dopant concentration required to stabilize the rhombohedral phase, ultimately maximized the conductivity. Overall, the La and Y double-doped Bi2O3 exhibited a conductivity of 3 × 10−2 S/cm at 500 °C, which is over three times higher than the conductivity of the more traditional SOFC electrolyte GDC.