Research On A CO₂-tolerant Mixed Ionic-electronic Conducting Oxygen Separation Membrane

The mixed ionic and electronic conducting （MIEC） oxides can transport oxygen ion andelectron simultaneously at high or intermediate temperatures. The dense membranes based onMIEC materials can separate oxygen from air selectively under the oxygen partial pressuregradient. The MIEC membranes have a great potential application in oxyfuel techniques forCO₂capture and storage technologies due to their good catalytic activity. Usually, theperovskite-type membranes contain alkaline earth metals on A site, which tend to reactreadily with CO₂and can’t sustain their oxygen transport property. In this paper, the chemical stability andoxygen permeability of a K₂NiF₄-type MIEC material based on(Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δmembrane under CO₂atmosphere are investigated indetail.The PLNCG powder was prepared through a sol-gel route based on citric acid and EDTAas complexing and gelation agents. X-ray diffraction analysis （XRD） indicated that thePLNCG powder samples still maintain the K₂NiF₄structure under pure CO₂in thetemperature range studied. The thermogravimetric curves of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δandPLNCG under CO₂atmospheres showed that PLNCG exhibit better stability under theCO₂-containing atmosphere than these perovkiste materials containing alkaline earth-metal.At975oC, when the sweep gas was changed from He to CO₂only a slight decrease of theoxygen permeation flux was observed. The oxygen permeation flux through the PLNCGmembrane swept by CO₂was0.32mL min^-1 cm^-2. Besides, a steady oxygen permeation fluxwas observed during more than230hours’ long term oxygen permeation test at975oC,indicating that PLNCG membrane exhibits good stability under CO₂atmosphere.The oxygen permeation fluxes through the PLNCG membranes with different thicknesseshad been studied. It was found that in low-temperature range of800-850oC, the oxygenpermeation of PLNCG membrane disk were controlled by the surface oxygen exchange.When the temperature increased to900oC or above, the bulk diffusion played a role in theoxygen permeation process. But surface oxygen exchange was still a main factor for thelimiting step of the PLNCG membrane. The oxygen permeability of PLNCG membrane can be improved by surface modification.Sulphuric acid and hydrochloric acid with various concentrations were employed to modifythe membrane surface. Experimental results revealed that a new membrane structure wasformed on the membrane surface after HCl or H₂SO₄etching. Comparatively, HCl was abetter acid for controlling the reaction to yield a porous surface with higher surface area. ThenLa_0.6Sr_0.4CoO_3-δslurry was brushed onto the air-side surface of the PLNCG membrane etchedby HCl. It’s found that the activation in the initial stage of oxygen permeation of themodified PLNCG membrane was much faster than that of the fresh one. At975oC, theoxygen permeation flux through the modified PLNCG membrane swept by CO₂was0.52mL min^-1 cm^-2with the enhancement factor of65%.