High-performance mixed-conducting membranes with nanoscaled functional layers

纯氧是一种非常需要的工业资源，通常通过液化空气的蒸馏来生产。相反，使用能够从压缩空气中分离氧气的混合导电膜似乎是一种更节能的氧气生产替代方案，从而开辟了广泛的应用，例如碳中和发电厂。混合导电钙钛矿是用于这种膜的非常有前途的一类材料。Ba0.5Sr0.5Co0.8Fe0.2O3（BSCF）表现出无与伦比的氧渗透性能，并且即使在低分压下也是化学稳定的，因此使得大规模氧气生产似乎是可行的。然而，材料的稳定性必须在几年内确保在操作条件下（700…900 ° C，氧分压10E-3...其中多晶BSCF膜的氧渗透性受到非立方次级相的出现的影响，推测是由于B位阳离子Co从+2价态氧化到+2.7价态，这导致其离子半径减小，立方相不稳定，因此导致氧传输性能降低。稳定立方BSCF相的一种可能的方法已经被最近的发现所证明，该发现旨在通过掺杂具有恒定化合价的元素如Zr或Y来降低Co含量。即使是小的掺杂剂浓度也足以增加相稳定性并保持BSCF的优异氧渗透性。因此，Y-或Zr-掺杂的BSCF相对于其立方相的稳定性极限作为掺杂剂水平，温度和氧分压的函数的彻底调查被认为是非常有前途的。 用于技术动力装置应用的氧输送膜通常由多孔支撑体组成，该多孔支撑体涂覆有厚度小于100微米的混合导电膜层。这导致最高可能的氧通量，其表现出表面控制的氧交换动力学。一个显着的通量增强是现在可能的几何增加的气体/固体界面-或通过"催化"改性的化学表面组成（"异质界面"）。申请人已经成功地开发了后一种概念，用于由（La，Sr）CoO3制成的混合导电固体氧化物燃料电池阴极。通过一个纯粹的几何表面扩大，氧表面交换可以增加超过十年，而形成的“异质界面”导致额外的增强了50倍。这种（纳米级和/或催化活性）功能层对混合导电膜性能的影响应在本项目中进行评估和验证。

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.