OXYGEN PERMEATION IN DENSE SRCO0.8FE0.2O3-DELTA MEMBRANES - SURFACE EXCHANGE KINETICS VERSUS BULK DIFFUSION

Oxygen permeation measurements were made on dense discs of SrCo0.8Fe0. 2O3-delta as a function of pressure gradient, 0.01 to 1 atm, temperatu re, 775 to 870 degrees C and disc thickness, 1 to 2.6 mm. Two differen t measuring configurations were used: with the pressure gradient appli ed parallel to the disc axis and with the pressure gradient applied be tween the edge and the two flat surfaces of the disc. This enabled us to take into account leakage effects due to the edge. A model is propo sed for the dependence of the surface exchange current on the chemical potential drop at the gas-solid interface. Analysis of the pressure g radient and membrane thickness dependence of the permeation shows that the permeation is controlled by both bulk diffusion of oxide ions as well as exchange kinetics between gas-phase oxygen molecules and oxide ions. Values for the surface exchange rate coefficient k(iO) and the ambipolar diffusion coefficient D-a are reported.