Ceria-based composites for application in oxygen transport membranes

Among the gas membrane separation technologies, inorganic dense membranes, like oxygen transport membranes (OTMs), are regarded as one of the most potential and promising methods and gain considerable attention over the past decades. OTMs represent an energy-efficient and economical alternative to conventional cryogenic air separation for producing pure oxygen for oxyfuel combustion technologies as they provide high oxygen selectivity in a single-step process. Mixed ionic-electronic conductors (MIEC) are applied mostly as materials for OTMs. Compared to diverse single-phase materials, dual-phase composite materials, i.e., pure ionic-conductor Ce0.8Gd0.2O2 δ (CGO20) and electronic-conductor FeCo2O4 (FCO), may potentially fulfil these requirements completely. At the low FCO fraction (<25 wt.%) in the composite, a sufficient percolation with the targeted 30 vol.% of the electronic (e)-conducting phase is achieved, thanks to phase interaction and formation to the perovskite phase. This emerged donor-doped GdFeO3 phase contributes to the electronic conductivity providing additional paths for the charge carriers. Furthermore, this Gd0.85Ce0.15Fe0.75Co0.25O3 phase was synthesized separately and exploited as an electron-conducting phase and added to the commercially available CGO20. This approach reveals a dual-phase structure after sintering, which is expected to be more chemically stable compared to FCO in particular at low pO2.

The transport properties of the MIEC system are closely related to the composition and conducting properties of the individual phases in the composite, as well as microstructure features, such as density, porosity, grain size, and the presence of defects. Therefore, the objective of this work is to determine the relationship between the composition as well as phase morphology of the permeation properties in the fluorite-spinel ceramic composite by varying the ion- and electron-conducting materials in the composite. Finally, the reduction in the thickness to µm range is an important milestone for the development of CGO-FCO composite, thus the asymmetric configuration is done by using the tape casting technique.