Structure–property relationships in Sm–Eu–Yb co-doped δ-Bi₂O₃ ceramics synthesized for advanced solid electrolyte applications

In this study, we investigated the structural, thermal, and electrical properties of Sm³⁺–Eu³⁺–Yb³⁺ co-doped bismuth oxide (Bi₂O₃) ceramics for use as solid electrolytes in intermediate-temperature solid oxide fuel cells (IT-SOFCs). Solid solutions with the formula (Bi₂O₃)₁₋ₓ₋ᵧ₋𝓏(Sm₂O₃)ₓ(Eu₂O₃)ᵧ(Yb₂O₃)𝓏 were synthesized via solid-state reaction and sintered at 750 °C for 100 hours. X-ray diffraction (XRD) analysis confirmed the formation and retention of the high-conductivity δ-phase structure after high-temperature electrical testing. Dense and uniform microstructures were observed by SEM, with only minor grain coarsening when relatively high amounts of Yb³⁺ were incorporated into the ceramic materials. TG/DTA data confirm the thermal stability of these materials as they exhibit very little mass loss and no phase transitions to 850 °C. The electrical conductivity data demonstrate thermal activation of conduction, with the most conductive samples containing 5 mol% Yb₂O₃; further addition of Yb₂O₃ caused clustering of defects, which resulted in reduced conductivities. The results of this investigation further demonstrate that triple-doping with rare-earth oxides can effectively provide thermal and structural stability to the δ-Bi₂O₃ phase and, therefore, allow for the development of thermally and structurally stable electrolytes as potential candidates for IT-SOFCs. This work provides the basis for the use of these sustainable functional ceramics in future energy applications.