Development of CO2 Protective Layers by Spray Pyrolysis for Ceramic Oxygen Transport Membranes

Ceramic mixed ionic-electronic conducting (MIEC) membranes enable very selective oxygen separation from air at high temperatures. Two major potential applications of oxygen-transport membranes are: i) oxygen production for oxyfuel power plants, and, ii) integration within high-temperature catalytic membrane reactors for methane or alkane upgrading by selective oxidative conversions. However, these applications involve contact with carbon-bearing atmospheres and most state-of-the-art highly permeable MIEC membranes do not tolerate operation under CO2-rich environments due to carbonation processes. The present contribution shows our first attempts in the development of ceria-based protective thin layers on monolithic LSCF membranes. Gd-doped ceria (CGO) deposition is carried out by air blast spray pyrolysis on mirror-polished LSCF disc membranes. The layer thickness is maintained below 0.4 mu m in order to prevent the formation of cracks during thermal cycling and minimize limitations caused by the reduced oxygen permeability through the ceria layer. After optimization of the spraying process, smooth crack-free dense coatings are obtained with high crystallinity in the as-deposited state. The layers are characterized by XRD, SEM, AFM, DC-conductivity measurements, interferometry and optical microscopy. Oxygen separation is studied on coated LSCF using air as the feed and argon/CO2 mixtures as the sweep gas in the temperature range 650-1000 degrees C. The protected membrane exhibits a higher stability than the uncoated LSCF membrane, although the nominal oxygen flux is slightly reduced at temperatures below 850 C due to the limited ambipolar conductivity of doped ceria in the range of oxygen partial pressures investigated. Moreover, the protective layer (250 nm thickness) remains stable after the permeation testing.