Guidelines for selecting coating materials for a high oxygen permeation flux in a fluorite-rich dual-phase membrane

It is necessary to understand the role of surface modification in the oxygen permeation mechanism because the active coating layer on the ceramic membrane is important for dramatically enhancing the oxygen permeation flux. The effect of coating layers on oxygen permeation in a fluorite-rich dual-phase membrane [80 vol% Ce0.9Gd0.1O2−δ (GDC): 20 vol% La0.6Sr0.4Co0.2Fe0.8O3−δ(LSCF)] has been investigated as a function of membrane thickness and temperature to elucidate the extent of oxygen flux improvement due to surface modification. Various perovskite materials such as La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), SrTi0.5Fe0.5O3-δ (STF), La0.7Sr0.3MnO3-δ (LSM), as well as the LSM/Ce0.9Gd0.1O2−δ (GDC)composite, have been adopted as the coating layers to enhance the oxygen permeation flux. The role of surface modification in the dual-phase membrane on the enhancement of oxygen flux has been systematically studied from the point of view of oxygen exchange kinetics and bulk diffusion. The oxygen permeation fluxes increased in the order of LSC > STF ≈ LSCF > LSM/GDC > LSM coating materials. The surface exchange kinetics coefficient (k) of the membrane with various coating materials can be calculated based on the characteristic thickness and bulk diffusion coefficient. The k values of the coated dual-phase membrane follow the k values of the coating materials. This result implies that the remarkable enhancement of oxygen flux in the dual-phase membrane with surface modification is closely related to the surface exchange kinetics of the coating materials.