Oxide-ion conductors

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. 

CeOj and ThOj have the cubic fluorite structure. Fig. 2.15, and can be doped with large amounts of, for example, Ca, La or Gd to give extensive ranges of cubic solid solutions. ZrOj is cubic only above 2400°C, however, and requires 8% of dopant to stabilise the cubic form to room temperature (as in YSZ, yttria-stabilised zirconia). 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

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. 

Some pyrochlore, A2B2O7, phases are moderately good oxide ion conductors. The pyrochlore structure may be regarded as a fluorite derivative in which g of the oxygens are missing but since the oxygen sublattice, ideally, is fully ordered, it is necessary to introduce defects to achieve high conductivity. 

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Solid Oxide Fuel Cell In SOF(7s the electrolyte is a ceramic oxide ion conductor, such as vttriurn-doped zirconium oxide. The conduetKity of this material is 0.1 S/ern at 1273 K (1832°F) it decreases to 0.01 S/ern at 1073 K (1472°F), and by another order of magnitude at 773 K (932°F). Because the resistive losses need to be kept below about 50 rn, the operating temperature of the... 

In oxide ion conductors, current flow occurs by the movement of oxide ions through the crystal lattice. This movement is a result of thermally activated hopping of the... 

FIGURE 25.5 Arrhenius plots of some oxide ion conductors. (Courtesy of Prof. Tatsumi Ishihara, Kyushu University, Japan.)... 

Oxide ion conductors have found widespread apphcations in our modem society. The devices based on oxide ion conductors include oxygen sensors, solid oxide fuel cells (SOFCs), and oxygen pump. 

Lawless [19] has patented a multi-cell device with various solid-oxide ion-conductors as electrolyte. Some are described to be viable at temperatures as low as 400 °C, but the conductivity is so low (er T = 0.9 K/Cl cm) as to make the voltage demand unrealistically high at economic current densities (e.g. i > 100 mA/cm2). 

The compounds BajInjOj, Agl and PbFj illustrate ionic conductivity in stoichiometric compounds. The first is a fast oxide-ion conductor above a first-order order-disorder transition at 930 °C that leaves the Bain array unchanged the second is a fast Ag -ion conductor above a first-order transition at which the I -ion array changes from close-packed to body-centred cubic and the third exhibits a smooth transition to a fast F ion conductor without changing the face-centred-cubic array of Pb " ions. 

Figure 25. Proton conductivity of various oxides, as calculated from data on proton concentrations and mobilities, according to Norby and Larring (the type of dopant is not indicated see ref 187 for source data). The conductivity of oxides with a perovskite-type structure are shown by bold lines, and the conductivity of the oxide ion conductor YSZ (yttria-stabilized zirconia) is shown for comparison, (reproduced with the kind permission of Annual Reviews, http //www.AnnualReviews.org).

One of the exceptions was the discovery of high ionic conductivity in appropriately doped FaGa03.128 129 As in the other oxide ion conductors, its ionic conductivity depends on both the dopant level as well as on the nature of the dopant. A major difference to ceria and zirconia is the presence of two cations that can be substituted the detailed defect chemistry of such solid solutions is far from being fully understood. Co-doping of Sr on A sites and Mg on B-sites leads to an ionic conductivity of ca. 0.12—0.17 S cm 1 at 800°C,130-133 which is similar to doped ceria but considerably exceeds the value of YSZ (ca. 0.03 S cm 1 at 800°C80 81). The activation energy also varies with composition and can be as low as ca. 0.6 eV.130 131 At about 600-700°C, the... 

K. Huang, S. Tichy, J. Goodenough, Superior perovsite oxide-ion conductor strontium and magnesium-doped I a(. aOc I, phase relationships and electrical properties. J. Am. Ceram. Soc., 1998,... 

Oxide ion conductors have been extensively investigated for their applications in fuel cells, oxygen sensors, oxygen pumps, and oxygen permeable membranes [81-108], The ion conduction effect was discovered more than a century ago by Nernst in zirconia products [83,84], To use zirconia, it... 

For oxide ion conductors, vacancy hopping is the major transport mechanism consequently, the materials should contain oxygen vacancies to conduct. To obtain oxide conduction properties, a part of the Zi4 must be substituted by another cation with a lower valence state, that is, Ca2+, Sc3+, Y3+, or a rare-earth cation [84,86],... 

Ample studies on pyrochlore oxide electrolytes have been carried out, particularly on Gd2Ti207- and Gd2Zr207-based conductors, where the Gd2Ti. Zrx07 solid solution is of great interest because the x=0 member is an ionic insulator whereas the x = 1 end member is a good oxide ion conductor [96,97],... 

Finally, two remarks regarding terminology. If an electrochemical reaction displays a negligible resistance, the corresponding electrode is called a reversible electrode . Reversible electrodes are known for cation conductors, but have not been reported for oxide ion conductors. The term electrode resistance denotes the electrical resistance due to the electrochemical reaction, or to the transfer through the space charge, rather than the resistance of the electrode material itself. 

Crystalline solid electrolytes such as a-Agl, ji-alumina, NASICON, and LISICON, LLN, oxide ion conductors such as yttria-stabilized zirconia, etc ... 

Boivin J. C. and Mairesse G., Recent Material Developments in Fast Oxide Ion Conductors, Chem. Mater. 10(10) (1998) pp. 2870-2888. 

Kendall K.R., Navas C., Thoams J.K. and zur Loye H.-C., Recent Developments in Perovskite-based Oxide ion Conductors, Solid State Ionics. 82 (1995) pp. 215-223. 

Lacorre Ph., Foutenoire F., Bohnke O., Retoux R. and Laligant Y., Designing fast oxide-ion conductors based on La2Mo209, Nature. 404 (2000) pp. 856-858. 

Oxides containing bismuth oxygen atom double layers include the bismuth calcium copper oxide series of superconductors exemplified by Bi2Sr2CaCu20g (Bi-2212) and the beta phase oxide ion conductors based on the composition Sr Big j 0(2 j)/2. While the details of these two stmcture types are quite different, they have the common feature of containing BiMT double layers that are only weakly bonded. In Bi-2212, the bismuth atoms are four + one coordinated by oxygen atoms whereas, in Srj Bi9 j 0(2 j)/2 they are three + one coordinated and form square and hexagonal layers of Bi-O bonds. Both classes of compounds and be intercalated by iodine atoms that are inserted between the Bi-O double layers. 

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