Mixed Electronic-Ionic Conductors

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

D Mixed ionic electronic conductor (MIEC) o Triple-phase boundaries (TPB s)... 

Wan J, Goodenough JB, and Zhu JH. Nd2 xLaxNi04+5, a mixed ionic/electronic conductor with interstitial oxygen, as a cathode material. Solid State Ionics 2007 178 281-286. 

Heterophase assemblages of mixed ionic/electronic conductors of the type A/AX/AY/A under an electric load are the simplest inhomogeneous electrochemical systems that can serve to exemplify our problem. Let us assume that the transport of cations and electrons across the various boundaries occurs without interface polarization and that the transference of anions is negligible. For the other transference numbers we then have... 

Conductor — is a qualitative term reflecting the capability of a substance to conduct an electrical -> current. Depending on the type of sole or prevailing - charge carriers, - solid materials can be classified into ionic, electronic, and mixed ionic-electronic conductors. 

The class of ionic conductors is not unambiguously defined in literature. Depending on context, this term maybe used either for solid electrolytes where the ion transference number is higher than 0.99, or for any solid material where ions are mobile, including mixed ionic-electronic conductors where the partial ionic and electronic diffusivities are comparable. The latter term is used for materials where the ion transference numbers are lower than 0.95-0.99, and also in conditions when a minor contribution to the total conductivity (ionic or... 

Similar approaches are used for most steady-state measurement techniques developed for mixed ionic-electronic conductors (see -> conductors and -> conducting solids). These include the measurements of concentration-cell - electromotive force, experiments with ion- or electron-blocking electrodes, determination of - electrolytic permeability, and various combined techniques [ii-vii]. In all cases, the results may be affected by electrode polarization this influence should be avoided optimizing experimental procedures and/or taken into account via appropriate modeling. See also -> Wagner equation, -> Hebb-Wagner method, and -> ambipolar conductivity. 

Ionic and mixed ionic-electronic conductors — Ionic conductors are solid systems that conduct electric current by movement of the ions. Mixed ionic-electronic conductors are those also conducting by the passage of electrons or holes (like metals or semiconductors). Usually only one type of ion (cation or anion) is predominantly mobile and determines conductivity. 

Refs. [i] Tubandt C (1932) Leitfdhigkeit und Uberfuhrungszahlen infes-ten Elektrolyten. In Wien W, Harms F, Fajans K (eds) Elektrochemie. Handbuch der Experimentalphysik, vol. 12, part 1. Akadem Verlags-ges, Leipzig, pp 381 [ii] Riess I (1997) Electrochemistry of mixed ionic-electronic conductors. In Gellings PJ, Bouwmeester HJM (eds) Solid state electrochemistry. CRC Press, Boca Raton, pp 223... 

Wagner equation — denotes usually one of two equations derived by -> Wagner for the flux of charged species Bz under an -> electrochemical potential gradient, and for the - electromotive force of a -> galvanic cell with a mixed ionic-electronic -> conductor [i-v] ... 

Majkic, G. et al.. High-temperature deformation of La 2Sro.8Feo.8Cro.203 5 mixed ionic-electronic conductor. Solid State Ionics, 146, 393-404 (2002). 

Figure 15.1. Illusuation of the difference in location of the electrode reaction on two different SOFC electrode types. Upper In an electrode where the electrode material is exclusively an electronic conductor, the reaction zone is restrained to the vicinity of the triple phase boundary (TPB). Lower In a mixed ionic-electronic conductor (MIEC) the electrode reaction can take place on the entire electrode surface...

It is obvious that a highly permeable membrane material must exhibit large con-ductivies for both ionic and electronic charge carriers. Partial conductivities of various, so-called mixed ionic electronic conductors (MIEC), as calculated or directly obtained from Refs. 9-21, are presented in Figure 2. 

The use of mixed ionic-electronic conductors (MIECs) as ORR electrocatalysts is quite common in solid-state electrochemistry [125], because the reaction zone is extended over the entire electrode/gas interface, contrary to the case of metal electrodes where the reaction is, to a large extent, restricted to the tpb zone [23]. 

Y. Shen, M. Liu, D. Taylor, S. Balagopal and A. Joshi, Mixed ionic electronic conductors based on Bi-Y-O-Ag metal-ceramic system, in T.A. Ramanarayan et al. (Eds.), Proceedings of the 2nd International Symposium on Ionic and Mixed Conductors. Proceedings Vol. 94.12, The Electrochemical Society Inc., pp. 574r-583. 

In this section, a brief overview is given of major membrane concepts and materials. Besides membranes made from a mixed ionic-electronic conductor (MIEC), other membranes incorporating an oxygen ion conductor are briefly discussed. Data from oxygen permeability measurements on selected membrane materials are presented. 

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