Effect of Sm co-doping on structural, mechanical and electrical properties of Gd doped ceria solid electrolytes for intermediate temperature solid oxide fuel cells

Abstract

Two series of samarium co-doped in 10 mol% Gd doped ceria Ce0.90Gd0.1O1.95(GDC10) formulated as Ce0.9-xSmxGd0.1O2-δ(CS) and Ce0.9Gd0.1-xSmxO2-δ(GS) (x = 0.0, 0.03, 0.05 0.10) were synthesized by glycine nitrate auto combustion method. The structural effects of Sm substitution in the GDC10 solid solution have been studied by X-ray diffraction (XRD) and Raman spectroscopy techniques. An increased lattice constant (5.407 Å to 5.414 Å) with decreasing crystallite size (28.6 nm 22.5 nm) was observed in both the Ce substituted by Sm (CS) and Gd substituted by Sm (GS) in GDC10 crystal system. The highest oxygen vacancy concentration (VO⋅⋅ = 2.65 × 1021 cm−3) was found for 10 mol % Sm and 10 mol % Gd doped ceria (Ce0.8 Sm0.1Gd0.1O1.90) composition (CS10) evaluated by spatial correlation model from Raman spectroscopy. The microhardness of the sintered pellets was investigated by Vickers hardness measurement. The highest fracture toughness was found to be 1.85 ± 0.27 MPa m1/2 for Ce substituted by 3 mol% Sm composition (CS3) among the overall compositions. The surface morphology and elemental composition of the CS and GS compositions were analyzed by FESEM. The morphology and composition of optimized electrolyte (CS10) was further analyzed by HRTEM and XPS respectively. AC impedance spectroscopy revealed that CS10 composition has an improved ionic conductivity (σ800 = 0.147 × 10−3 S‧cm−1) with significantly reduced activation energy (0.85 eV) in the temperature range of 673–1073 K under air atmosphere. A mechanism for conductivity enhancement by oxygen vacancy for CS compositions has been proposed. The effect of Sm as a secondary co-dopant in the GDC10 electrolyte was studied in detail to establish a candidate electrolyte material for operating solid oxide fuel cells in the intermediate temperature range (673–1073 K).