Electron-conducting oxide

When platinum is made the anode in an aqueous solution, a protective electron-conducting oxide film is formed by the following reaction ... 

Figure 2. Common strategies for SOFC cathodes (a) porous single-phase electronically conductive oxide such as (La,Sr)Mn03 (LSM) (b) porous single-phase mixed conductor (c) porous two-phase composite. The SEM micrograph of LSM on YSZ in a is adapted from ref 84. (Adapted with permission from ref 84. Copyright 1997 Swiss Federal Institute of Technology.)...

In addition to being able to catalyze the dissociation of O2. the material used for the cathode must be electronically conductive in the presence of air at high temperature, a property found primarily in noble metals and electronically conductive oxides. Ionic conductivity is also desirable for extending the reaction zone well into the electrode since the ions must ultimately be transferred to the electrolyte. Since precious metals are prohibitively expensive when used in quantities sufficient for providing electronic conductivity, essentially all SOFC prototypes use perovskite-based cathodes, with the most common material being a Sr-doped LaMnOs (LSM). In most cases, the cathode is a composite of the electronically conductive ceramic and an ionically conductive oxide, often the same material used in the electrolyte. 

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

In these representations, the electron-transporting phase is usually a metal however, in certain cases it can be an electron-conducting oxide, other compound, or other material, such as graphite. Furthermore, two categories of electron-transporting phases may be encountered ... 

Figures 25-28. IWo comparisons of plots of the signals from a Bosch lean lambda sensor (upper plots) and from a lambda sensor with specially prepared electronically conducting oxidic powders. Both sensors were placed together in automobile exhaust gas during periodic variations of the oxygen concentration at about the air/fuel equivalence point (83].

Cathodic debonding requires electron-conducting oxides and an interface that allows electrochemical reactions to occur. The latter usually requires the presence of ions and water molecules at the interface and therefore will dominate at high relative humidity. 

Defect-structure-wise there are two main routes to a mixed proton-electron conducting oxide. The simplest would be to dissolve protons compensated by electrons originating directly from hydrogen gas ... 

As the mixed proton and electron conductive oxide membrane becomes sufficiently thin, surface kinetics will become important, and difiusion of protons across the membrane will no longer be rate deterrriining for the overall hydrogen flux. Bouwmeester et al. [10] defined a characteristic thickness, L, for membranes where surface kinetics and bulk kinetics are equally important to the flux. Decreasing a membrane s thickness below gives essentially no increase in the flux. 

To the best of our knowledge, there is yet no literature example where surface kinetics has been proven to limit the hydrogen permeation across a mixed pro-ton-electron conducting oxide, and seemingly membrane thicknesses below Lc have thus never been reached. 

With the requirement of electronic conductivity, oxides containing cations with mixed valence and, in particular, reducible cations are preferable. Oxides containing transition metals are therefore appropriate alternatives. There are indications based on conductivity measurements that Ti02 could be a possible candidate [83], but no direct measurements of hydrogen permeability have been reported. Tita-nates, in general, however, are interesting because there are a number of materials classes that accommodate oxygen vacancies and may dissolve protons. 

These results are of great importance considering the oxidation or hot corrosion of alloys where the scale consists of electronic conducting oxides and a melt. 

FIGURE 30.8 Temperature dependence of the electrical conductivity of several electronically conducting oxides. 

V Transparent electronic conductors are required in certain applications. A thin layer of an electronically conducting oxide deposited on a glass substrate is then used. 

Kim, S., Yang, Y.L., Jacobson, A.J. and Abeles, B. (1998) Diffusion and surface exchange coefficients in mixed ionic electronic conducting oxides from the pressure dependence of oxygen permeation. Solid State Ionics, 106 (3-4), 189-195. 

This mechanism may also explain the attainability of reversible potential of reaction (I) on highly oxidized Pt electrodes in very pure solutions saturated with oxygen. According to Hoare, highly oxidized Pt is covered with a uniform, nonporous, electronically conductive oxide layer which blocks further oxidation of the metal, and therefore only reaction (I) is active in establishing the rest potential. The Pt has been essentially rendered passive. According to Hoare " " such electronically conductive oxide layers which are nonper-meable to metal ions are formed only on Pt and Rh, and only for these metals has the reversible potential of the oxygen electrode been obtained. 

Oxygen separation by using a membrane is expected to be a real possibility, thanks to developments in mixed ionic and electronic conductors (MIECs). With mixed ionic and electronic conduction, oxide-ion conductors selectively permeate oxygen as a form of oxide ion. The mixed oxide-ion and electronic conductors used for this purpose are referred to as oxygen-permeable membranes. An oxygen-permeable membrane subjected to an oxygen potential gradient at elevated temperatures of around 700—1000 °C leads to the ambipolar conduction of oxide ions and electrons, as shown... 

It has proved much more difficult to select satisfactory electrode materials for the oxidising environments prevailing at the cathode in fuel cells and at the anode in electrolysers. The noble metals are too expensive and it is generally agreed that the only practical class of materials to consider for the relevant electrodes are electronically conducting oxides. 

In the above results, stable coke-free SOFC operation was enabled by balancing the hydrocarbon fuel with H2O, CO2, or O2 in order to achieve a composition where sohd carbon is not stable (Fig. 1). It has also been demonstrated that SOFCs can be operated with reasonable stability in a range of pure hydrocarbons. With Ni-based anodes, it is well known that carbon fibers form that can damage the anode [24, 62, 63]. Furthermore, the presence of steam may not prevent this process, even in amotmts beyond those normally expected to prevent coking [24]. Thus, direct operation with higher hydrocarbons requires replacement of Ni with other electronically conducting oxides or a metal such as Cu. Indeed, when operated with higher hydrocarbons such as -butane, polyaromatic compounds (tars) formed... 

The most commonly utilized SOFC electrolyte is 8 mol% yttria-stabilized zirconia (YSZ). YSZ is utilized as it is an almost pure oxygen ion conductor with acceptable conductivity, and is stable in both anode and cathode gas environments. The cathode [7] is a mixture of an electron or mixed oxygen ion and electron conducting oxide and the YSZ electrolyte. Typical cathode materials include Lao.2Sro.8Mn03 5 (LSM) and the Lai xSrxCoi yFey03 s (LSCF) family. The traditional anode is a combination of Ni metal and YSZ, where Ni provides electronic conductivity and electrocatalytic activity [3]. 

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