The increased interest in dual-phase membrane materials for oxygen separation leads to the continuous optimization of their composition. Rare-earth doped ceria is a promising candidate as the ion-conducting phase in the membrane. Spinel-structured FeCo2O4 was investigated as an electronic conducting phase forming an additional electronic conducting perovskite-structured phase during sintering when combined with Ce1−xLnxO2−δ. The influence of rare-earth lanthanide elements, i.e., Gd and Sm, as well as their concentration, i.e. x = 0.1 and 0.2, on the final phase composition and microstructure as well as its related functional properties in particular oxygen permeation is analyzed. 20 mol.% doping of either Gd or Sm reveals a multi-phase microstructure after sintering. Moreover, segregation of Gd/Sm, iron, and cobalt is found at the ceria-ceria grain boundaries in Ce0.8Sm0.2O2−δ- and Ce0.9Gd0.1O2−δ-based composites. In contrast, 10 mol.% Gd–doping leads to a dual-phase membrane material without the formation of any other phase. In all cases, the percolation threshold is reached at approx. 20 vol% of the electron-conducting phase in the system leading to similar maximum permeation rates determined by the ionic conductivity of the ceria phase.
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