Novel synthesis of stabilized Bi1–x–yGdxDyyO1.5 solid electrolytes with enhanced conductivity for intermediate temperature solid oxide fuel cells (SOFCs)

Abstract

Herein, we report the synthesis of a Dy–Gd co-doped cubic phase-stabilized Bi2O3 solid electrolyte system via solid-state processing under atmospheric conditions. Doping with Dy3+ and Gd3+ has been observed to significantly enhance the densification process during sintering for stabilization purposes, thereby improving the electrical properties of δ-Bi2O3-type polymorphs. The synthesized ceramics were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy-energy dispersive X-ray spectroscopy (FESEM-EDX), thermal gravimetry/differential thermal analysis (TG/DTA), and the four-point probe technique (4PPT). XRD analysis reveals that the samples Bi1–x–yGdxDyyO1.5 (y = 0.05/x = 0.05, 0.10, 0.15, and 0.20, and x = 0.05/y = 0.10, 0.15, and 0.20) exhibit a stable face-centered cubic δ-phase and a mixed-phase crystallographic structure. The XRD analysis of the stabilized δ-phase suggests that the prepared oxides show a face-centered cubic (FCC) structure with a space group of Fm-3m. FESEM micrographs reveal that the composition Bi0.90Gd0.05Dy0.05O1.5 has no significant holes. Nevertheless, an evident increase in the pore formation is observed as the amount of Gd2O3 increases until it reaches 20%. This finding suggests that dense pellets are formed during the sintering process at 900–1000 °C. The DTA analyses were performed to verify the phase stability, which agrees with the XRD results. The electrochemical performance of the synthesized Dy–Gd co-doped Bi2O3 solid electrolyte system was evaluated and analyzed in detail by using the electrochemical impedance spectroscopy (EIS) technique. Based on EIS and conductivity measurements, Bi0.75Gd0.20Dy0.05O1.5 exhibits the lowest activation energy of 0.519 eV and the highest conductivity value of 0.398 S/cm at 627 °C compared to the other samples; this composition can be used as a solid electrolyte for intermediate-temperature solid oxide fuel cells (SOFCs).