Doped ceria electronic conductivity

Replacement of most or all of the Ni is also investigated using an anode catalyst, such as doped ceria, that is stable in contact with YSZ in SOFC fuel conditions [10]. Ceria offers improved stability, though its electronic conductivity is much less than that of Ni. As the electronic current passing from the metal support to the active electrode area is carried by the ceria catalyst coating, cell performance may be limited by ceria electronic conductivity. This situation can be improved by addition of a conductive component to the anode backbone, such as copper or stainless steel particles. Ris0 has demonstrated the latter choice and promising durability of the cell is reported. 

Pure and doped ceria are mixed ionic and electronic conductors under reducing atmospheres, due to considerable p-type electronic conductivity under reducing conditions. The electronic conduction is due to the reduction of Cc4 to Ce3+ under... 

To introduce a barrier layer when utilizing doped ceria electrolytes (SDC, GDC, or lanthanum-doped ceria, LDC) to prevent the reduction of Ce4+ to Ce3+. Reduction of cerium cations results in unwanted electronic conductivity that lowers fuel efficiency [34], and mechanical degradation that results from the volume expansion of cerium ions upon reduction [35],... 

The intermediate- and low-temperature materials exhibit both significant oxygen ion and electron conductivity. This means that the oxygen reduction can take place everywhere in the cathode and is not restricted to the TPBs as in the LSM-type cathodes. An option for compensating the relatively low oxygen ion conductivity of LSC and LSCF at operating temperatures around 600 °C is by mixing the material with (doped) ceria [79, 80],... 

Finally, another possibility often discussed in the literature is that cation dopants from the electrode may enhance the electronic conductivity of the gas-exposed surface of the electrolyte in the vicinity of the TPB, thereby extending the reduction zone along the electrolyte surface via mixed conduction. The surface exchange rate of oxygen on both YSZ- and rare-earth-doped ceria (as measured by isotope methods) is only about 1 order of magnitude lower than on LSM at 700 Thus, if there were sufficient... 

Ceria, particularly when doped with Gd203 or SmzOs," has received some attention for direct hydrocarbon conversion in SOFC. Dating back to Steele and co-workers,interesting properties have been demonstrated for ceria-based anodes in direct utilization of methane. Later work suggested that the performance of ceria-based anodes in hydrocarbons could be improved by the addition of precious-metal catalysts, at dopant levels,but the performance of these cells was still too low for practical considerations. The problem with doped ceria is likely that its electronic conductivity is not sufficient. In general, the electrode material should have a conductivity greater than 1 S/cm in order to be practical since a conductivity of 1 S/cm would lead to a cell resistance of 0.1 Q cm for an electrode thickness of 1 mm, even... 

As with doped ceria, low electronic conductivity is an issue with perovskite anodes as well. Some perovskites show very high electronic conductivities under oxidizing environments, but most of these materials are much less conductive under the reducing environments to which they will be exposed at the anode. For example, the conductivity of Lao.75-Sro.25Cro.5Mno.5O3, a promising material developed by Tao and Irvine, decreases by almost 2 orders of... 

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]. 

The conductivities of some ceria-containing compounds obtained from the literature have been compiled and shown as solid lines in Fig. 2.12. The main dopants for ceria belong to the alkaline earth or rare-earth metal series and the majority of doped samples exhibit conductivity values which fall into a rather limited band (gray band in Fig. 2.12), which points to a similar behaviour for all doped ceria samples. Exceptions are pure ceria and ceria doped with redox elements like Pr and Tb which give rise to electronic contribution to conductivity. An important requirement is that a homogeneous solid solution forms between the two oxides, which maintains the fluorite structure since the presence of a second phase or phase inhomogeneity due to insufficient solubility can affect ionic conductivity. The very low values of conductivity found for BaO and MgO-doped ceria were in fact attributed to the low solubility of these oxides into the lattice of When the... 

Whereas the ionic conductivity is always much lower than the electronic conductivity in pure reduced ceria, the situation is quite different in ceria doped with oxides of two- or three-valent metals due to the introduction of oxide ion vacancies, cf eqs. 15,2 and 15.3. A high vacancy concentration will shift eq. 15.1 to the left. This means that the ionic domain extended down to 10 atm or even lower in the temperature range of 600 - 1000 0. The electronic conductivity in air may be very low, and the doped cerias are under these conditions excellent electrolytes. The conductivity mechanism is the hopping of oxide ions to vacant sites, and the ionic conductivity, a may be expressed as... 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

At 1000°C, significant levels of electronic conduction in titania-doped YSZ, as in ceria-doped specimens, are foimd only under strongly reducing atmospheres. Data of oxygen permeability have been presented for the Zr02-Y203-Ti02 system by Arashi and Naito [127] (see also Table 10.1). By virtue of its... 

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