High-performance mixed-conducting membranes with nanoscaled functional layers

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Pure oxygen is a much-desired industrial resource commonly being produced through distillation of liquefied air. Using a mixed-conducting membrane, instead, that is able to separate oxygen from compressed air seems a far more energy-efficient alternative for oxygen production, thus opening up a wide range of applications, such as carbon-neutral power plants. Mixed-conducting perovskites are a very promising class of materials for such membranes. Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) exhibits unmatched oxygen permeation properties and is chemically stable even at low partial pressures, hence making large-scale oxygen production appear feasible. However, materials stability has to be ensured over several years under operating conditions (700...900 °C, oxygen partial pressures of 10E-3...1 bar) where the oxygen permeability of a polycrystalline BSCF membrane is influenced by the occurrence of non-cubic secondary phases, presumably owing to an oxidation of the B-site cation Co from a valence state of +2 to +2.7 which leads to a decrease of its ionic radius, a destabilization of the cubic phase and, hence, to a reduction of the oxygen transport properties. A possible means of stabilizing the cubic BSCF phase has been proven by very recent findings aiming at reducing the Co content through doping with elements such as Zr or Y that have a constant valence. Even small dopant concentrations are sufficient to increase the phase stability and preserve the excellent oxygen permeation of BSCF. Therefore, a thorough investigation of Y- or Zr-doped BSCF with respect to the stability limits of its cubic phase as a function of dopant level, temperature and oxygen partial pressure is considered very promising. Oxygen-transport membranes to be used for technical power-plant applications are usually composed of a porous support that is coated with mixed-conducting membrane layers less than 100 microns in thickness. This leads to the highest possible oxygen fluxes which exhibit surface-controlled oxygen exchange kinetics. A significant flux enhancement is now possible by a geometric increase of the gas/solid interface - or by a "catalytic" modification of the chemical surface composition ("hetero-interface"). The latter concept has successfully been developed by the applicants for mixed-conducting solid-oxide fuel cell cathodes made of (La,Sr)CoO3. By a purely geometrical surface enlargement, oxygen surface exchange could be increased by more than one decade, whereas the formation of a "hetero-interface" led to an additional enhancement by a factor of 50. The effect of such a (nanoscaled and/or catalytically active) functional layer on the performance of a mixed-conducting membrane shall be evaluated and verified within this project.