Oxygen Conductors

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], 

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], 

The Physical Chemistry of Materials Energy and Environmental Applications 

FIGURE 8.11 Schematic representation of the eight fluorite unit cells required to represent the pyrochlore structure. 

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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],... 

Proton-conducting materials [38-47], analogous to oxygen conductors but with stationary oxygen anions, can show mixed protonic-electronic conductivity, without considerable oxygen transport in hydrogen or water atmospheres [40,41], These materials have not been widely studied in comparison... 

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

There has been a strong effort to rationalise and elucidate a structural principle which will account for all the anion-deficienl, fluorite-related, mixed-valent binary oxides of cerium, praseodymium and terbium. This is a key step not only for the solid-state chemistry of these materials but also for a large class of fluorite-related materials involved in applications such as fast oxygen conductors and as catalysts. The two main theoretical approaches to the problem were developed by Martin and by Kang and Eyring, and will be illustrated in the following sections. 

Sensors may also use multiple electrolytes (referredto as electrolyte chains), and two examples of these are shown in Figure 13.Id and e. In both examples, a Na2SO4 auxiliary electrode is used with a sodium ion conductor so that the sensing electrode reaction is given by Equation (13.5). The difference between the two cases is that, in Figure 13.Id, the sodium ion conductor is used with another cation (strontium) conductor, whereas in Figure 13.le the sodium ion conductor is used with an anion (oxygen) conductor. In both cases, an equilibrium reaction is required to relate the... 

The most important aspects of the study of oxygen conductors are the abilities to enhance their ionic conductivity and reaction kinetics. Both features are essential for the development of electrochemical devices including fuel cells, gas sensors and ionic membranes. These devices have the potential to deliver high economic and ecological benefits however to achieve satisfactory performance, it is necessary to optimize the ionic conductivity of the solid electrolytes. 

The ability to reach ionic conductivities of the level of lOOS/cm in nanoscaled YSZ thin films is quite unique and is not possible to obtain in conventional materials as their diffusivity is limited by the lattice [7, 31]. The increase of the conductivity of epitaxial YSZ thin films offers new opportunity for oxygen conductors whose properties can be effectively controlled by the thickness and epitaxy level of these materials. 

The nature of the mobile ionic species was questionable for a long period of time. For passive Al, Verwey [47] assumed in 1935 that exclusive transport of Al-cations occurs in a fixed oxygen matrix. The idea of mobile cations dominated the oxide formation theories for the next 30 years. It seemed to be reasonable, as the volumes of cations are much smaller than -anions (e.g. by a factor of 20 for AP+), even if the experimental results indicated a combined transport. Marker experiments in the sixties proved cation-transference numbers in the range from 0.3 to 0.7 for many systems (Al, Be, Nb, Ta, Ti, V, W) coming closer to 0.5 with increasing current density, that is, cations and anions move in fact simultaneously (Table 1). This indicates that effects of charge distribution become more important than individual ion properties like size or polarizability [25]. Exceptions are the crystalline oxides on Hf and Zr, which are pure oxygen conductors. 

Le6n-Reina L, Porras-Vdzquez J M, Losilla E R, Sheptyakov D V, Llobet A and Aranda MAG (2007), Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen , Dalton Trans, 20,2058-2064. 

Weppner, W. (1992). Tetragonal zirconia polycrystals—a high performance solid oxygen conductor. Solid State Ionics 32 13-21. 

All of the above-mentioned solid electrolytes are oxygen conductors. An automatic consequence of this is that, as in molten carbonate fuel cells, the products of electrochemical reactions all end up on the anode side. While is beneficial for internal reforming and water gas shift reaction (which utilizes the water produced as a reactant), it dilutes the fuel, and at high utilization it can significantly reduce the Nernst potential. 

Figure 14.3 Conductivity of various fluorine and oxygen conductors as a function of the ionic transfer activation energy. (Reprinted with permission from [2]. (1 -fluorite-likephases, 2-tysonite-Uke phases). Copyright (2007) Pleiades Publishing Inc.)...

Let us consider two compartments where we have different chemical potentials of oxygen, separated by a solid electrolyte membrane, an oxygen conductor. The chemical potentials may be fixed with the help of either a gas mixture (Ar + Oj, CO + CO2, H2 + H2O for example) ... 

I. Kosacki, Nanoscale oxygen conductors for energy conversion Presentation at Imperial College Eondon. 2006, Eondon... 

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