Efforts to lower the operating temperature of solid oxide fuel cells include producing electrolytes that are sufficiently conductive and stable below 600 °C. Doped ceria is one such electrolyte being considered. During this study, codoped ceria powders (Ce0.8Sm0.2-xMxO2-δ, M = Bi3+, Zn2+ and x = 0, 0.05, 0.1, 0.15, 0.2) were prepared via coprecipitation by the addition of sodium carbonate and annealed at 800 and 1200 °C, respectively. Poor solubility of the codopants in the ceria was observed for samples annealed at 800 °C, resulting in a mixed-phase product including stable phases of the oxides of these codopants. A second-stage partial incorporation of these codopants into the ceria lattice was observed when the annealing temperature was increased to 1200 °C, with both codopants forming cubic-type phases of their respective oxides. Materials were characterized using X-ray diffraction (XRD), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR), as well as scanning electron microscopy (SEM) for structural and morphological investigations. The oxide ion conductivity was evaluated using electrochemical impedance spectroscopy between 550 and 750 °C. Fuel cell performance tests of selected samples (annealed at 1200 °C) showed remarkable improvement in peak power densities when the test temperature was increased from 500 to 600 °C (∼720 mW/cm2 for Ce0.8Sm0.15Bi0.05O2-δ and ∼1230 mW/cm2 for Ce0.8Sm0.15Zn0.05O2-δ), indicating possible contribution from the distinct cubic-type oxide phases of the codopants in performance enhancement.
Keywords: codoping; conductivity; electrolyte; fuel cell; phase change; power density.