Fig. 1
Schema of oxygen transport through a perovskite membrane with mixed conductivity (copyright Caro, Uni Hannover): Molecular oxygen is split at the membrane side with high oxygen partial pressure (normally air) into atomic oxygen species O* which are adsorbed at the surface. After ionization, the oxygen ions O2− enter the mixed conducting membrane material. By a diffusional hopping process among the vacancies in the oxygen lattice, the oxygen ions can enter the membrane side facing a low oxygen partial pressure. Here the reverse process takes place: The oxygen ions decompose into atomic surface oxygen O* and electrons, and the latter diffuse to the air side where they are consumed in the oxygen ionization. The surface oxygen species O* dimerize and form molecular oxygen which is desorbed into the gas face. To keep the oxygen partial pressure low on the permeate side (driving force for permeation), the molecular oxygen has to be removed by pumping or a sweep gas; it can be also consumed in a chemical oxidation reaction (Shao et al. 2001)