A critical review on bismuth-based oxide ion electrolytes for low temperature solid oxide fuel cells: structure - chemical composition - ionic conductivity relationship

Cubic δ-Bi₂O₃, characterized by its fluorite structure, is a promising electrolyte material for solid oxide fuel cells (SOFCs) due to its exceptional ionic conductivity. However, its cubic phase is stable only within a narrow temperature range, and prolonged exposure at 550 °C induces oxygen ion lattice ordering, which reduces ionic conductivity. Additionally, δ-Bi₂O₃ is susceptible to reduction to metallic bismuth under reducing conditions, causing material degradation. Current research focuses on enhancing structural stability through doping and material modification. Doping δ-Bi₂O₃ with multiple elements, particularly in double-doped systems, can achieve high conductivity (up to 1.19 S cm⁻¹). Bilayer electrolytes, such as gadolinium-doped ceria (Ce_1-xGd_xO_2-δ, GDC)/doped Bi₂O₃ (DBO) and yttria-stabilized zirconia (Zr_1-xY_xO_2+δ, YSZ)/DBO, mitigate reduction issues. Single cells incorporating these bilayer electrolytes achieve peak power densities of 2–3 W cm⁻² between 600 and 700 °C. Composite cathodes, such as La_0.8Sr_0.2MnO₃ or La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, enhance performance by improving oxygen reduction kinetics and thermodynamic stability. Furthermore, composite electrolytes like Bi₂O₃-ScSZ lower sintering temperatures while maintaining high ionic conductivity. This review also explores other bismuth-based electrolyte materials, including Bi₂WO₆ and Bi₄V₂O₁₁, which exhibit favorable ionic conductivity at intermediate temperatures. These developments underscore the significant potential of bismuth-based materials for solid oxide fuel cells.