Er–Tm co-doped δ-Bi₂O₃ electrolytes: structural stability and high oxide-ion conductivity for IT-SOFC applications

Abstract

In this study, (Bi₂O₃)₁₋ₓ₋ᵧ(Er₂O₃)ₓ(Tm₂O₃)ᵧ ternary solid solutions were synthesized via a solid-state reaction method to achieve high oxygen-ion conductivity and phase stability for intermediate-temperature solid oxide fuel cells (IT-SOFCs). High-purity Bi₂O₃, Er₂O₃, and Tm₂O₃ powders were mixed in stoichiometric ratios and subjected to calcination, pressing, and sintering steps. Their structural, thermal, and electrical properties were then examined in detail. X-ray diffraction (XRD) analysis confirmed that all samples retained the δ-Bi₂O₃ phase at room temperature, indicating the formation of a single-phase crystalline structure. Crystallite size calculations revealed that increasing the dopant concentration reduced the grain size to the 35–45 nm range and intensified lattice defects. Electrical conductivity measurements exhibited Arrhenius-type behavior with distinct activation energies in the low- and high-temperature regimes. Notably, the E5T composition (20 mol% Er₂O₃ – 5 mol% Tm₂O₃) achieved the highest conductivity of approximately 1.14 × 10⁻¹ Ω⁻¹·cm⁻¹ at 750 °C and the lowest activation energy of 1.33 eV. Thermogravimetric (TG) and differential thermal analysis (DTA) results showed no significant mass loss between 100 and 600 °C and revealed no clear endothermic or exothermic peaks associated with phase transitions, confirming excellent thermal stability. These findings demonstrate that co-doping with Er and Tm enhances both the oxygen-ion conductivity and the long-term thermal durability of δ-Bi₂O₃-based systems. Such characteristics position these materials as a strong alternative to conventional YSZ electrolytes for high-performance IT-SOFC applications operating at lower temperatures.