Optimized La1-xSrxGa1-yMgyO3-δ (0.1 ≤x, y ≤ 0.3) Electrolytes for Solid Oxide Fuel Cells Applications: Synthesis, Structural Characterization and Properties

This work explores the synthesis, structural characterization and properties measurements of La₁₋ₓSrₓGa₁₋ᵧMgᵧO₃₋δ (0.1 ≤x, y ≤ 0.3) compounds obtained with the aid of the wet chemical synthesis route (Pechini Method) and which were calcined for 10 hours at 1300 °C. La0.9Sr0.1Ga0.8Mg0.2O3-δ was successfully prepared as a single phase compound while La₀.₈Sr₀.₂Ga₀.₈Mg₀.₂O₃₋δ, La₀.₉Sr₀.₁Ga₀.₉Mg₀.₁O₃₋δ, and La₀.₉Sr₀.₁Ga₀.₇Mg₀.₃O₃₋δ exhibited high perovskite phase purity as secondary phases (SrLaGa₃O₇, SrLaGaO₄) were found to be less than 10%. However, La₀.₇Sr₀.₃Ga₀.₈Mg₀.₂O₃₋δ was obtained with significantly low perovskite content (57%) implying that the applied doping content lies outside the single-phase region for the La₂O₃ - Ga₂O₃ - SrO - MgO phase diagram. Rietveld refinement in combination with X-ray diffraction (XRD) analysis established the predominance of cubic structure (Pm3m) of La₁₋ₓSrₓGa₁₋ᵧMgᵧO₃₋δ compounds when doping with Sr2+ and Mg2+. An exception was the La0.9Sr0.1Ga0.8Mg0.2O3-δ that crystallizes in a combination of monoclinic (I12/A1, 80%) and cubic (Pm3m, 20%) perovskite phases. All compounds obtained crystallites at the nano scale (<100 nm) and open lattice pathways for ionic conduction. SEM analysis identified polygonal and spherical grains in Sr and Mg-substituted compounds, respectively while Mg-doped samples exhibited improved sintering and homogeneity. A 23 % increase in thermal expansion coefficient was observed when Mg doping increased from 0.1 to 0.2. However, Sr substitution decreased thermal expansion coefficients by 13 %. Increase Sr and Mg doping levels enhances the conductivity of the compounds (e.g. 0.12 S/cm at 800oC for La₀.₈Sr₀.₂Ga₀.₈Mg₀.₂O₃₋δ), yet the presence of secondary phases and crystal lattice distortion affects conductivity. Nevertheless, the presence of impurities and crystal lattice distortion can negatively affect ion conduction.