Dense membranes made of mixed ionic and electronic conducting oxides have been widely applied for high-purity O2 production[1] and partial oxidation of CH4[2], as well as water splitting[3]. As one of the most promising membrane materials, doped alkaline earth titanates, e.g. SrTiO3 doped by Mg or Fe (≤ 25 mol%) at Ti site, showcase great advantages in chemical stability under reducing conditions over the barium/lanthanum strontium cobalt ferrite perovskite-type oxides[4, 5]. However, a conductivity minimum, caused by a depletion of charge carriers at intermediate oxygen partial pressures (~10-8-10-6 bar), may limit the performance of the doped alkaline earth titanates in application scenarios such as water splitting. One solution proposed here is to introduce additional dopants that sustain a mixed valence state for charge transfer over a broad range of oxygen partial pressure (Po2). In addition, the thickness reduction of membrane components also represents a state-of-the-art strategy for performance promotion due to the physically shortened transport passes[6]. The current work has explored and validated both of the two strategies based on characterizations of crystal structure, microstructure, and conductivity as well as oxygen permeation.

The doping strategy has been applied for CaTiO3 as an example. The Ti site is doped by Sc with a stable valence of 3+ together with Cr or Ru with variable valence states. The Sc dopant can introduce mobile charge carriers (oxygen vacancies) supporting ionic conduction[7], while the Cr or Ru dopant are expected to provide charge carriers that are mobile and sustainable over a wide Po2 range guaranteeing a high electronic conductivity[8, 9]. However, the synthesized materials with dual dopants show lower total conductivities as compared with the materials with a single dopant, suggesting a cancel-out effect likely caused by the charge balancing between two dopants. Further efforts to alleviate such an effect can rely on the concentration optimisation of the two dopants.

The strategy towards the membrane component has been conducted for the Sr0.98Ti0.75Fe0.25O3-δ materials using commercially available powders. As the free-standing membrane becomes highly fragile during production and handling when the thickness is continuously decreased to ~100μm, it is necessary to apply a thicker but porous layer as the mechanical support for the dense thin membrane layer, forming a so-called asymmetric membrane. By employing a well-developed tape casting technique, a two-layer tape has been cast in sequence and co-sintered, obtaining the asymmetric membrane. The thickness of the membrane layer has been successfully decreased to ~20 μm, which effectively enhances the oxygen permeation suggesting a promising prospect for application in water splitting. A further development of catalyst is required to overcome the increased limitation from the surface exchange.