Oxygen permeation through composites consisting of four well-known oxide-ion conductors and a noble metal, Pd or Ag, is reported. The oxides were Zr0.9Y0.1O1.95 (YSZ), (Bi1.75Y0.25O3)0.95(CeO2)0.05 (BYC5), Ce0.8Sm0.2O1.9 (SSC), and La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM). The results show that (BYC5 + Ag) yields the highest oxygen permeation flux, but the composite deteriorates with time. The composites (SSC + Pd), (LSGM + Pd), and (YSZ + Pd) give stable, but relatively lower oxygen permeation flux in the order of (SSC + Pd) \u3e (LSGM + Pd) \u3e (YSZ + Pd). The composite microstructures indicate that (BYC5 + Ag) has the best percolating network for both oxide-ion and electronic pathways while (SSC + Pd) has the longest triple-phase boundary lengths with the smallest grains, which is beneficial to the surface oxygen exchange. It is shown that the microstructure of the composites, which strongly influences the competition between surface reaction and bulk diffusion, is technically as important as the oxide-ion conductivity. The activation energy appears to be related more to the morphology of the metallic phase than to that of the oxide phase. These results suggest that (SSC + Pd) is a promising composite mixed conductor for applications requiring oxygen separation
