Determining the rate-limiting step during oxygen semi-permeation of CaTi0.9Fe0.1O3-δ oxygen transport membranes

Oxygen transport membranes are dense ceramic oxides that have the ability to exchange oxygen with the surrounding atmosphere and to transport it along a chemical potential gradient. To improve performance, it is important to determine whether the oxygen transport is limited by the surface exchange or by bulk diffusion. Because this analysis involves very specific testing conditions, it is difficult to generalize and compare literature results. Recent studies have shown that CaTi0.9Fe0.1O3−δ (CTF) offers a reasonable compromise between performance and long-term stability, making it a good candidate for the rigorous conditions of industrial applications. Although the baseline flux performance of CTF has been established, the rate-limiting step for oxygen transport remains a matter of debate. In the present investigation, the oxygen activity on both surfaces of the membrane is measured simultaneously with the permeation flux to determine the limiting step in the oxygen semipermeability of CTF. The limiting step varies with temperature: below 750 °C oxygen transport is limited both by bulk diffusion and by surface exchanges, with bulk diffusion limitations dominating at higher temperatures, although surface polarization remains influential. Moreover, the flux is limited by ionic conductivity at low temperatures and by electron hole conductivity at high temperatures.