Unsteady-state permeation and surface exchange of dual-phase membranes

► Unsteady-state permeation in the initial stage of dual-phase membranes were investigated. ► The unsteady period can be shortened or eliminated by coating the active porous layers on membrane surfaces. ► The active porous layer can improve the oxygen flux and decrease the permeation activation energy. ► The changes of surface microstructures are related to the unsteady-state permeation.