Oxide ion conductivity

In the case of O3 perovskites, the largest suitable trivalent cation is La,  

based on reported experimental data, it can be argued that lattice distortion (lattice stress and deviation from cubic symmetry) due to ion radii mismatch determines the ionic conductivity to a very large extent, and that lattice distortion is of much greater importance than many other proposed parameters. At the same time, the fully symmetric and stress-free lattice, in which the oxide vacancy has the highest mobility and the lowest activation energy for the vacancy hopping, is a more proper candidate for high-ion conductors. 

---

R. DiCosimo, J.D. Burrington, and R.K. Grasselli, Oxidative dehydrodimerization of propylene over a Bi203-La203 oxide ion-conductive catalyst, J. Catal. 102, 234-239 (1986). 

X-ray photoelectron spectroscopic (XPS) studies of Ag63,64 and Pt6,56-62 films deposited on YSZ under positive current application conditions have confirmed the proposition2-4 that NEMCA with oxide ion conducting solid electrolytes is due to an electrochemically induced and controlled backspillover of oxide ions on the catalyst surface. 

Bismuth sesquioxide, BijOj, exhibits a high oxide ion conductivity at high temperature without doping of aliovalent cations. The oxide transforms from the monoclinic... 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

Figure 6.2 Ionic transport number for oxide ion conductivity in the pyrochlore phases Lu2Ti207, Lu2.096Tii.904O6.952> and Lu2286Tii.7i406.857- [Data adapted from A. V. Shlyakhtina, J. C. C. Abrantes, A. V. Levchenko, A. V. Knot ko, O. K. Karyagina, and L. G. Shcherbakova, Solid State Ionics, 177, 1149-1155 (2006).]...

The stabilized zirconia family of oxides, especially calcia-stabilized zirconia, are solids in which oxide ion conductivity has been increased to the extent that they are widely used solid electrolytes (Section 1.11.6, Section 4.4.5, and Section 6.8). 

Suzuki M, Sasaki H, and Kajimura A. Oxide ion conductivity of doped lanthanum chromite thin film interconnects. Solid State Ionics 1997 96 83-88. 

Conductivity of Nation in comparison to some intermediate-temperature proton conductors and the oxide ion conductivity of YSZ (yttria-stabilized zirconia). (From Kreuer, K. D. et al 2004. Chemical Reviews 104 4637-4678.)... 

Anion conduction, particularly oxide and fluoride ion conduction, is found in materials with the fluorite structure. Examples are Cap2 and Zr02 which, when doped with aliovalent impurities. Fig. 2.2, schemes 2 and 4, are F and 0 ion conductors, respectively, at high temperature. The 3 polymorph of 61303 has a fluorite-related structure with a large number of oxide vacancies. It has the highest oxide ion conductivity found to date at high temperatures, > 660 °C. 

The most well-studied and useful materials to date are those with fluorite-related structures, especially ones based on ZrOj, ThOj, CeOj and Bi203 (Steele, 1989). To achieve high oxide ion conductivity in ZrOj, CeOj and ThOj, aliovalent dopants are required that lead to creation of oxide vacancies. Fig. 2.2, scheme 4. The dopants are usually alkaline earth or trivalent rare earth oxides. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

Intrinsic Frenkel disorder, in which some of the oxygens are displaced into normally unoccupied sites, is responsible for the oxide ion conduction in, for example, Zr2Gd207, Fig. 2.11. The interstitial oxygen concentration is rather low, however, and is responsible for the low value of the preexponential factor and for the rather low (by -Bi203 standards ) conductivity. 

High temperature solid oxide fuel cells (SOFCs) have become of great interest as a potentially economical, clean and efficient means of producing electricity in a variety of commercial and industrial applications (Singhal, 1991). A SOFC is based upon the ability of oxide ions to be conducted through a solid at elevated temperatures. Oxide ion conductivity was observed in Zr02 9 mol% YjOj by Nernst as early as 1899. In 1937, Bauer... 

Platinum and Palladium Based Catalysts. Researchers at ANL have also developed an ATR catalyst formulation comprised of a transition metal element supported on an oxide ion-conducting substrate, such as ceria. 

Oxide-ion conducting supports such as ceria, doped ceria (with Sm or Gd), or perovskites are found to be effective for reducing carbon formation on the catalyst during reforming of liquid hydrocarbons. 

Another group of materials that has displayed high oxide ion conductivity is based upon a layered bismuth perovskite-based structure, first reported by Aurivillius in 1949 [90-92], The so-called Aurivillius phases are chemically expressed normally as Bi2A B 03 +3 [82], where A is a large 12-coordinated cation and B a small 6-coordinated cation. The structure is formed by n perovskite-like layers, (A 1B 03n+1)2, sandwiched between bismuth-oxygen fluorite-type sheets, (Bi/) 2 [93,94],... 

An SOFC cathode normally consists of a porous matrix cast onto an oxide ion-conducting electrolyte substrate (see Figure 8.24), where the cathode porosities are typically 25-40 vol% [66,123,137], Besides, the cathode must be an electron conductor and catalytically active for the oxygen reduction reaction. However, because it is not an oxygen conductor, it must be porous with an optimized three-phase interface at which the reduction reaction takes place [33],... 

---