Oxygen permeation properties of dense Bi1.5Er0.5O3-Ag cermet membranes

Oxygen permeation experiments were performed on dense mixed-conducting ceramic-metal composite membranes (thickness 0.2 to 2 mm) Bi1.5Er0.5O3-Ag with 10.0, 27.8, and 40.0 volume percent (v/o) silver, respectively, in the temperature range 873 to 993 K and oxygen partial pressure range 10(-3.5) to 1 bar O-2. The oxygen fluxes increased with increasing silver content. In the cermets with a nonpercolative silver phase (10.0 and 27.8 v/o), the increased oxygen flux relative to that of pure Bi1.5Er0.5O3 was attributed to faster kinetics of surface oxygen exchange in the presence of silver. Percolativity of the silver phase in the 40 v/o Ag composition enhances the ambipolar diffusion of oxygen ions and electrons. High oxygen fluxes (similar to 0.25 mmol m(-2) s(-1) at 873 K) were observed with the latter composition, which were shown to be fully limited by the surface exchange kinetics. The activation energy for oxygen permeation in the temperature range 848 to 1003 K is about 85 to 95 kJ/mol for the compositions without percolativity of silver and 115 kJ/mol for the composite with 40 v/o Ag, which reflects a change of the rate-limiting step upon passing the percolation threshold. Results from both permeation and isotopic exchange measurements on the composition with Ag percolativity indicated the kinetic order of the surface process in oxygen to be 1/4, indicating a process fundamentally different from that on pure Bi1.5Er0.5O3.