The phase stability and conductivity of Ho₂O₃–Gd₂O₃ co-doped electrolyte for solid oxide fuel cell

In this study, Bi₂O₃ ternary system sample materials were synthesized by using solid state reaction method. Two oxides, Ho₂O₃ and Gd₂O₃, were used to stabilize the δ (cubic) Bi₂O₃ phase used as an electrolyte of solid oxide fuel cell applications. The characteristics and electrical properties of these electrolyte samples for solid oxide fuel cells were evaluated by X-ray diffraction (XRD), scanning electron microscope and the four-point probe technique. According to XRD measurements, δ-phase (x = 0.05–0.1, y = 0.05, 0.1, 0.15, 0.2) samples were obtained with a stable structure. The results of the electrical measurements of the (Bi₂O₃)_1−x−y(Ho₂O₃)_x(Gd₂O₃)_y ternary system show that the electrical conductivity increase with decreasing amount of Gd₂O₃ molar ratio at a fixed molar ratio of Ho₂O₃. The value of highest conductivity was found as 2.53 × 10⁻¹ S cm⁻¹ for the (Bi₂O₃)_1−x−y(Ho₂O₃)_x(Gd₂O₃)_y ternary system (x = 0.05 mol% and y = 0.05 mol%) at 973 K. It was found that activation energy at low temperature varied from 1.10 to 1.29 eV and the activation energy at high temperature varied from 0.61 to 0.70 eV.