Doped ceria membrane

The performance of acceptor-doped ceria membranes, composite ceria-based membranes, and membranes with redox active dopants was reviewed in Sections 12.6.4.2, 12.6.4.3, 12.6.4.4, and 12.6.4.5, respectively. Variations in sample geometry, membrane thickness, as well as test conditions make comparison of performance somewhat difficult. Also for some studies, catalysts have been added to the surface of the samples to enhance the flux, whereas in other studies the as-prepared surface is used in the experiments. However, in particular, studies of thin-film membranes, both the... 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

SEM observations of the surface and the cross-section of the Gd doped ceria coated with YSZ (YSZ GDC) show that the thin YSZ film is porous and the YSZ particles are fairly uniform in size. The thickness of YSZ film was about 20 pm. The conductivity of this composite membrane was 4.39X10 S cm at 500 C under air atmosphere, which was almost same order as that of Sm doped ceria coated with YSZ (YSZISDC) (1.51X10 4 Sxm ). However, either value was lower than that of GDC alone (4.03 X lO Sxm ) or SDC alone (4.66 X lO Sxm ). This may be due to the low conductivity of YSZ film. 

Propene oxidation was carried out by using an electrochemical reactor constructed from a Sm doped ceria electrolyte coated with YSZ (YSZ 1 SDC) as a membrane. In a blank test where nitrogen gas alone was passed over the Au anode instead of the reaction gas at 450 C, it was confirmed that the oxygen pumping was well controlled by the applied current, i.e., the amount of oxygen gas evolved at the anode coincided well the value calculated from the electric current by using Faraday s law. 

In order to clarify the reaction site of the partial oxidation of propene using the ceria-based solid electrolyte coated with YSZ as a membrane, we have studied the dependence of the selectivities to oxygenates on the thickness of YSZ. When the Sm doped ceria coated with YSZ (YSZ SDC), each selectivity of the oxidation products did not dependent on the thickness of YSZ, as shown in Figure 4. 

Although the mixed conduction is detrimental to fuel cell application, the high electronic conductivity of doped ceria in low p02 conditions can be exploited in hydrogen separation membrane technology. The chemical, thermal expansion and processing compatibilities of the two oxides in the composite make pervoskite-ceria combination suitable for the mixed conducting membrane application. 

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

Hwang, H. J., Moon, J. W., Awano, M. (2004). Electrochemical removal of NOx by scandium doped zitconia membrane reactor with ceria buffer layer. Journal of Electroceramics, 13, 727-732. 

Best known examples of oxides with pure ionic conductivity are yttria-stabilized zirconia (YSZ), gadolinia-doped ceria (CGO) with fluorite structure, and strontia-doped lanthanum gal-lates (LSG) with perovskite structure. These oxides exhibit high thermodynamic stability because of the absence of transition-metal ions. These ionic conductors differ in their high oxide-ionic conductivity as follows LSG > CGO > YSZ. They show low electronic conductivity and low catalytic activity. For their utilization as oxygen membrane materials, an electronic part of conductivity is required. 

Norton, T.T., Lu, B., and Lin, Y.S. (2014) Carbon dioxide permeation properties and stability of samarium-doped-ceria carbonate dual-phase membranes. /. Membr. Sci., 467,... 

Due to its adjustable transport properties, suitably doped ceria may be used as an electrolyte (which must be an electronic insulator and a good ion conductor), as discussed in depth in Section 12.5, or as a dense oxygen separation membrane (which needs to be a MIEC). A dense oxygen separation membrane must conduct both... 

Due to its high ionic conductivity and good chemical stability, acceptor-doped ceria can potentially be used in dense ceramic membranes for the separation of oxygen from air. 

Membranes based on acceptor-doped ceria may represent a promising alternative to Ln,Tm-based perovskites. Ceria has a much wider pOg stability range and a higher ionic conductivity than most of the Tm-based perovskites at 700 °C the ionic conductivity of Ceo.gGdo.iOi 95 is 0.04 while the ionic conductivity of... 

As pointed out above, the performance-limiting factor of a Gcj. Gdy02—y/2 membrane is electronic conductivity. Different ways of enhancing electronic conductivity in acceptor-doped ceria have... 

Under certain conditions acceptor-doped ceria itself exhibits significant electronic conductivity and addition of an electronic conducting second phase is not necessary. This is for instance the case with the classical acceptor-doped materials Cei yGdy02 y/2 and Cei y Smy02 y/2 under wide p02 membrane applications (with strong reduction on the low p02 side). Also, other redox active elements may be substituted in ceria to enhance further either the p-type or n-type conductivity. The former can be achieved with e.g. the addition of Pr or Tb, and numerous studies exist for Pr substitution - - - - or Pr and Gd substitution (see also Section 12.4.2.2). Fewer studies are available where enhanced n-type conduction has been introduced by substitution with redox active elements, but recently... 

As expected from the discussion in the previous section, the electronic conductivity in acceptor-doped ceria (e.g. Sm-doped or Gd-doped ceria) and ceria doped with redox active elements such as Pr or Tb, as well as for materials with multiple substituents (e.g. Gd and Pr substitution) may be sufficient for use as an oxygen membrane. Several studies give measurements of the oxygen fluxes through dense samples of doped ceria. A subset of measured oxygen fluxes as well as the experimental conditions under which they were measured is collected in Table 12.6. The performance of the purely acceptor-doped materials (materials 17,18, and 19) was discussed in Section 12.6.4.3. The performance of membrane materials vdth redox active dopants (materials 15 and 16) and with redox active dopants -l- Co addition (materials 20 and 21) is discussed in this section. 

Lobera, M. R, Balaguer, M., Garcia-Fayos, J., and Serra, J. M. Rare earth-doped ceria catalysts for ODHE reaction in a catalytic modified MIEC membrane reactor. ChemCatChem 4,2102-2111 (2012). 

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

---