Oxide Ion Conduction

These studies were not intended to seek a good ionic conductor but to confirm the conduction species to clarify the phenomena characteristic to the ferroelectric or pyroelectric materials. It should be noted that the conductivity of ferroelectric materials mentioned above is very low and most of the researchers in those days took no notice of the value of conductivity itself. Studies on highly conductive ionic conductors of perovskite-type compounds were started in the second half of the 1960s to search for a good oxide ion-conducting electrolyte for fuel cells and oxygen sensors. These are described in the following sections. 

There are several methods to confirm oxide ion conduction in the oxide specimens. One of the most convenient methods is to examine the discharge performance of the oxygen concentration cell. If the conduction in the sample is oxygen ionic, a steady and stable current with a reasonable value can be taken from the oxygen concentration cell, which is composed of the specimen oxide as a solid electrolyte. Therefore, for example, one can regard the conduction as the oxide ion conduction, if the oxygen concentration cell shown in Cell [1] gives rise to stable emf and a steady and reasonable current can be taken from the cell. 

CaTiOs is a typical 2 4-type perovskite composed of large-sized divalent cation Ca and small-sized tetravalent cation Ti, whereas the aforementioned LaAlOs is a typical 3 3 type composed of large trivalent cation La and small trivalent cation Al. The excellent high oxide ion conductor that was discovered by Ishihara and reported in 1994 [18] is also based on 3 3-type perovskite LaGaOs, as described in detail in Chapter 4. 

Oxide ion conductors with the perovskite structure mentioned above belong to so-called single perovskites, which can be expressed as the simple form, ABO3. Besides these, there are different types of perovskite-related oxides and some of them are known to show oxide ion conduction. One of them is Brownmillerite, Ba2ln205. This composition can be written as Baln02.5, and 

A characteristic feature of perovskite-type oxide ion conductors is that they are often accompanied with p-type electronic conduction under an oxidizing atmosphere such as air at elevated temperatures. As described in Section 3.2, the contribution of electronic conduction depends on P02 in the atmosphere and temperature. As a typical example, Fig. 3.4 shows the P02 dependence of conductivities of CaTiOs- and SrTiOs-based solid solutions at 800°C [16]. P-type electronic conduction appears in the region of high P02 and n-type one under low oxygen partial pressure, i.e., a reducing atmosphere. The shape of the curve Incr lnPo2 is essentially the same as that shown schematically in Fig. 3.1. In many fluorite-type oxide ion conductors such as stabilized zirconias and 

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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],... 

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