Abstract
Oxygen permeation through composites consisting of four well‐known oxide‐ion conductors and a noble metal, 
or 
, is reported. The oxides were 
(YSZ) 
(BYC5), 
(SSC), and 
(LSGM). The results show that 
yields the highest oxygen permeation flux, but the composite deteriorates with time. The composites 
, 
, and 
give stable, but relatively lower oxygen permeation flux in the order of 
. The composite microstructures indicate that 
has the best percolating network for both oxide‐ion and electronic pathways while 
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 
is a promising composite mixed conductor for applications requiring oxygen separation. ©1999 The Electrochemical Society