Effect of co-doping with Sm3+, Bi3+, La3+, and Nd3+ on the electrochemical properties of hydrothermally prepared gadolinium-doped ceria ceramics

The structure, the thermal expansion coefficient, ionic and electronic conductivities of Ce0.8Gd0.2−xMxO2−δ (for M: Sm, x = 0–0.1, and for M: Bi, La, and Nd, x = 0.05) solid solutions, prepared for the first time hydrothermally, are investigated. The uniformly small particle size (23–65 nm) of the materials allows sintering of the samples into highly dense ceramic pellets at 1300–1400 °C, significantly lower temperature, compared to that at 1600–1650 °C required for ceria solid electrolytes prepared by solid state techniques. The maximum conductivity, σ700 °C ∼ 6.50 × 10−2 S cm−1, Ea = 0.59 eV, is found at x = 0.1 for Sm-codoping. Among various metal co-dopings for x = 0.05, the maximum conductivity is found for Sm-co-doping (σ700 °C ∼ 5.13 × 10−2 S cm−1, with Ea = 0.65 eV). The electrolytic domain boundary (EDB) of Ce0.8Gd0.1Bi0.1O2−δ is found to be 1.2 × 10−19 atm, which is relatively lower than that of singly doped samples. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.7 ± 0.6 × 10−6 K−1 for the CeO2, 12.2 ± 0.6 × 10−6 K−1 for Ce0.8Gd0.2O2−δ, and increase with co-doping to 13.07 ± 0.6 × 10−6 K−1 for Ce0.80Gd0.15Sm0.05O2−δ. These results suggest that co-doping can further improve the electrical performance of ceria-based electrolytes.