Ca, Sr or Mg-doped Ceria Electrolytes Prepared by Citrate-Nitrate Combustion Synthesis: Effect of Doping Concentration

In this work, it was aimed to synthesize and characterize rare earth metal-free cerium-based electrolytes that might be used in solid oxide fuel cells (SOFCs) by doping calcium, strontium, or magnesium to CeO₂. For this purpose, CeO₂, Ca_xCe_(1-x)O_(2−δ) (0.16 ≤ x ≤ 0.24), Sr_xCe_(1-x)O_(2−δ) (0.02 ≤ x ≤ 0.08) and Mg_xCe_(1-x)O_(2−δ) (0.07 ≤ x ≤ 0.13) were prepared by using citrate-nitrate combustion method. The solubility limits, microstructural and physical properties of the samples were characterized with XRD, SEM, TG-DTA and impedance analysis. It was found that all samples were in fluorite structure similar to the undoped ceria. The solubility limits of Ca²⁺, Sr²⁺ and Mg²⁺ were 21%, 6% and 12% (by mole) respectively based on XRD analysis results. The relative densities of sintered pellets at 1400 °C were more than 90%. Electrochemical impedance spectroscopy analysis, in which the ionic conductivities of the samples were measured, revealed that the Ca_0.2Ce_0.8O_2-δ (CCO20) sample sintered at 1400 °C showed the highest ionic conductivity value of 4.47 x10^-2 S.cm⁻¹ at 800 °C. It was determined that the O^2- ion conductivity decreased with the order of Ca²⁺ ≈ Sr²⁺ >> Mg²⁺. Conductivities increased with increasing dopant ratio, reached a maximum below the ratios of solubility limits, and then decreased. The obtained results showed that Ca or Sr doped electrolytes prepared by the citrate-nitrate method can show ionic conductivities close to the state-of-the-art Sm doped Ceria electrolytes. It has been determined that Mg doping is quite ineffective.