Phase Stability and Permeation Behavior of a Dead-End Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δTube Membrane in High-Purity Oxygen Production

Phase stability and oxygen permeation behavior of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF) dead-end tube membranes were investigated in long-term oxygen production at 950 and 750 °C. At 950 °C, the BSCF tube membranes exhibit good long-term phase stability and a stable oxygen permeation flux. However, at the intermediate temperature of 750 °C, both the oxygen permeation flux and the oxygen purity decrease continuously. This behavior is related to the formation of two secondary phases that are a hexagonal perovskite, Ba 0.5± x Sr 0.5± x CoO 3−δ, and a trigonal mixed oxide, Ba 1– x Sr x Co 2– y Fe y O 5, that evolved in the ceramic membrane made of cubic BSCF perovskite during the dynamic flow of oxygen through it. Tensile stress as a result of phase formation causes the development of cracks in the membrane, which spoil the purity of the permeated oxygen. The partial degradation of cubic BSCF perovskite in the intermediate temperature range (750 °C) was more pronounced under the strongly oxidizing conditions on the oxygen supply (feed) side than on the oxygen release (permeate) side of the membrane. The structural instability of BSCF is attributed to an unsuitable redox state of cobalt, that exhibits an ionic radius that is too small to be tolerated by the cubic perovskite structure, which then becomes unstable. The phase stability of cubic BSCF (i.e., the proper redox states of cobalt) can be maintained by operating the membrane in the high temperature regime (950 °C).