Mixed conducting oxides

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

Fig. 3. Oxygen transport in solids. 02 is dissociated and ionized at the reduction interface to give O2 ions, which are transferred across the solid to the oxidation interface, at which they lose the electrons to return back to 02 molecules that are released to the stream, (a) In the solid electrolyte cell based on a classical solid electrolyte, the ionic oxygen transport requires electrodes and external circuitry to transfer the electrons from the oxidation interface to the reduction interface (b) in the mixed conducting oxide membrane, the ionic oxygen transport does not require electrodes and external circuitry to transfer the electrons to the reduction interface from the oxidation interface, because the mixed conductor oxide provides high conductivities for both oxygen ions and electrons.

Sr-doped La-manganates, cobaltates, nickelates, and ferrates are typical examples of such mixed conducting oxides. In fact,... 

Figure 1 Working principle of a mixed-conducting oxide membrane...

Figure 2 Ionic and electronic partial conductivities of selected mixed conducting oxides with various Crystal structures YSZ(10 m/o TiOi), (xdfZrQjTtQj)20y," SrFeCog Of at 1000°C, Ceafido,202.s > La 2 r 2 n02, r FeO, s > ...

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

Fig. 10.1. Different membrane concepts incorporating an oxygen ion conductor (a) mixed conducting oxide, (b) solid electrolyte cell (oxygen pump), and (c) dual-phase membrane. Also shown is the schematics of an asymmetric porous membrane (d), consisting of a support, an intermediate and a barrier layer having a graded porosity across the membrane.

P.J. Ceilings and H.J.M. Bouwmeester, Ion and mixed-conducting oxides as catalysts. 

Liou and W.L. Worrell, Electrical properties of novel mixed-conducting oxides. Appl. Phys. A., 49 (1989) 25-31. 

H.J.M. Bouwmeester, H. Kruidhof and A.J. Burggraaf, Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixed-conducting oxides. Solid State Ionics, 72 (1994) 185-194. 

M. Liu, Theoretical assessment of oxygen separation rates of mixed conductors, in T.A. Ramanarayanan and H.L. Tuller (Eds.), Ionic and Mixed Conducting Oxide Ceramics 91-12. Electrochemical Society Inc., NJ, 1991, pp. 95-109. 

S. Ling, M.P. Anderson and T.A. Ramanarayanan, Optimization of ionic transport through mixed conducting oxide ceramics. Solid State Ionics, 59 (1993) 33-45. 

W.L. Worrell, Electrical properties of mixed-conducting oxides having high oxygen-ion conductivity. Solid State Ionics, 52 (1992) 147-51. 

N. Miura, Y. Okamoto, J. Tamaki, K. Morinag and N. Yamazoe, Oxygen semipermeability of mixed-conductive oxide thick-film prepared by slip casting. Solid State Ionics, 79 (1995) 195-200. 

M.H.R. Lankhorst and H.J.M. Bouwmeester, Determination of oxygen nonstoichiometry and diffusivity in mixed conducting oxides by oxygen coulometric titration. Part 11 Oxygen nonstoichiometry and defect model for Lao.8Sro.2Co03-8. J. Electrochem. Soc. submitted... 

The use of a CTE is often insufficient for an adequate description of solids when substantial amounts of defects are formed at elevated temperatures. As an example, this is the case of mixed-conducting oxides where the lattice volume is a function of both temperature and oxygen vacancies concentration. Such strain variations can often be quantified in terms of both the standard volumetric CTE (a y) and volumetric chemical expansivity (Uc) induced by the vacancy formation [26]... 

K. E. Swider-Lyons, Mixed-Conducting Oxides in Electrochemical Power Sources, in High Temperature Materials In Honor of the 65 Birthday of Professor Wayne L. Worrell, ed. S. Singhal, The Electrochemical Proceedings Series, 2002, PV 2002-5, p. 124. 

K. E. Swider-Lyons, Mixed-Conducting Oxides in Electrochemical Power Sources, Symposium on High Temperature Materials In Honor of the... 

Preferred mixed conducting oxides for ITM oxygen compositions are generically represented by the formula LaxA COyFey-Cuy /Os-z where A is selected from Sr, Ba, Ca, or Mg, and z is a number that makes the composition charge neutral. Selection of A is based on achieving low expansion of the dense phase with temperature. Copper provides improved control over dimensional changes when used in the mix with cobalt and iron, and a coefficient of thermal expansion matches with the substrate. This composition also provides superior resistance when used in the presence... 

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