Solid mixed ionic-electronic conductors

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. 

Some MIECs exhibit metallic properties. These materials can have different concentration of the mobife ioiflc species, compared with that of electrons and holes. Silver chalcogenides, Ag2+sX (X = S, Se, or Te) is an example of a metallic MIEC that conduct electrons and silver ions. These materials are good electronic conductors (close to metallic) and show interesting electronic behavior as a function of temperature as 

MIECs may be made nonuniform to the extent that they become n-type on one side and p-type on the other side, thus forming pn or pin (/ = intrinsic) junctions. Zr02 + 10 mol % Y2O3 subject to an oxygen partial pressure, Pq, gradient at elevated temperatures becomes p-type near the high (P —l atm) side and n-type near the low P, 

4 ELECTROCHEMICAL REACTIONS AT INTERFACES WITH SOLID ELECTROLYTES 

In this section we treat some electrochemical reactions at interfaces with solid electrolytes that have been chosen for both their technological relevance and their scientific relevance. The understanding of the pecularities of these reactions is needed for the technological development of fuel cells and other devices. Investigation of hydrogen or oxygen evolution reactions in some systems is very important to understand deeply complex electrocatalytic reactions, on the one hand, and to develop promising electrocatalysts, on the other. 

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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. 

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... 

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). 

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]. 

L. Heyne, Electrochemistry of mixed-ionic electronic conductors, in S. Geller (Ed.), Solid Electrolytes, Topics in Applied Physics. Springer, Berlin, Heidelberg, New York, 1977, pp. 167-221. 

Y.S. Shen, A. Joshi, M. Liu and K. Krist, Structure, microstructure and transport properties of mixed ionic-electronic conductors based on bismuth oxide. Part 1. Bi-Y-Cu-O system. Solid State Ionics, 72 (1994) 209-217. 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

The first volume of this Handbook contains brief reviews dealing with the general methodology of solid-state electrochemistry, with the major groups of solid electrolytes and mixed ionic-electronic conductors, and with selected applications for electrochemical cells. Attention is drawn in particular to the nanostructured solids, superionics, polymer and hybrid materials, insertion electrodes, electroanalysis and sensors. Further applications, and the variety of interfacial processes in solid-state electrochemical cells, will be examined in the second volume. 

Liu, M. and Z. Wu, Significance of interfaces in solid-state cells with porous electrodes of mixed ionic-electronic conductors. Sohd State Ionics, 1998. 107 pp. 105-110... 

I. Riess Electrochemistry of mixed ionic electronic conductors, in CRC Handbook of Solid State Electrochemistry (Eds. P. J. Gellings, H. J. M. Boumeester), CRC Press, Boca Raton, 1997, pp. 223-268. 

Solids are mixed conductors that means electronic and ionic charge carriers show mobility in the lattice. One speaks of preferential ionic condnctivily if the electronic transference nnmber is t <0.01. The electronic condnctivity increases exponentially with the temperature and, for oxides, depends on the partial pressnre of oxygen. Materials with preferential ionic condnctivity can be found only in a certain temperature and pressure region. Materials with comparable ionic as well as electronic conductivity are called MIECs (mixed ionic electronic conductors). These materials have become especially interesting for applications. As an example, the ratio of electronic conductivity to ionic conductivity... 

Z. Wu and M. Liu [1997] Modelling of Ambipolar Transport Properties of Composite Mixed Ionic-Electronic Conductors Solid State Ionics 93, 65-84. 

Riess, I., Godickemeier, M., and Gauckler, L.J. (1996). Characterization of solid oxides fuel cells based on solid electrolytes or mixed ionic electronic conductors. Solid State Ionics 90 91-104. 

Reiss, I., The possible use of mixed ionic electronic conductors instead of electrolytes in fuel cells. Solid State Ionics, 52, 127-134 (1992). 

Solid state ionic cathodes are mixed ionic/electronic conductors, generally with layer or tunnel structures in which ionic diffusion is rapid. The phenomenon of mixed con g ion in solids has been discussed in theoretical terms by Heyne and a review of suitable compounds with ynnel and layer structures is included in a paper by Whittingham ... 

Heyne L (1977) Electrochemistry of mixed ionic-electronic conductors. In Geller S (ed) Solid electrolytes. Springer, Berlin/Heidelberg/New York, pp 202-217... 

V.V. Khaiton, F.M.B. Marques, Mixed ionic-electronic conductors Effects of Ceramic Microstructure on Transport Properties, Curr. Opin., Solid State Mater. Sci., 6,261-269 (2002)... 

Kawasaki T, Tokuhiro M., Kimizuka N., Kunitake T. Hierarchical self-assembly of chiral complementary hydrogen-bond networks in water. J. Am. Chem. Soc. 2001 123 6792-6800 Kharton V.V., Marques F.M.B. Mixed ionic-electronic conductors effects of ceramic microstructure on transport properties. Curr. Opin. Solid State Mater. Sci. 2002 6(3) 261-269 Kikkinides E.S., Stoitsas K.A., Zaspalis V.T. Correlation of structural and permeation properties in sol-gel-made nanoporous membranes. J. Colloid Interface Sci. 2003 259 322-330 Kilner J., Benson S., Lane J., Waller D. Ceramic ion conducting membranes for oxygen separation. Chem. Ind. November 1997 907-911... 

Chebotin s scientific interests were characterized by a variety of topics and covered nearly all aspects of solid electrolytes electrochemistry. He made a significant contribution to the theory of electron conductivity of ionic crystals in equilibrium with a gas phase and solved a number of important problems related to the statistical-thermodynamic description of defect formation in solid electrolytes and mixed ionic-electronic conductors. Vital results were obtained in the theory of ion transport in solid electrolytes (chemical diffusion and interdiffusion, correlation effects, thermo-EMF of ionic crystals, and others). Chebotin paid great attention to the solution of actual electrochemical problem—first of all to the theory of the double layer and issues related to the nature of the polarization at the interface of the solid electrol34e and gas electrode. 

The electrode processes in solid-electrolyte systems consist always of a number of serial and/or parallel steps. The characteristic steps of the gas electrode reactions include transport in the gas phase to (or from) the gas/electrode or gas/electrolyte interface, adsorption (or desorption) at these surfaces, diffusion to (or from) the reaction zone, and transfer reactions [14-24]. As a rule, the electrochemical reaction is believed to occur in the vicinity (within a few microns) of triple-phase boundary (TPB), the junction of the gas, electronic or mixed ionic-electronic conductor (electrode), and ionic conductor (electrolyte) the TPB length is mostly determined by the electrode microstructure formed during the cell fabrication. Actual location of the electrochemicaUy active sites depends generally on the bulk and surface transport properties of the electrode and solid-electrolyte materials. When the current I is passed or drawn through the cell, the working electrode potential vve deviates from the equilibrium value E. This deviation is characterized by the quantity of overpotential r] = we (see Chap. 1). The electrode polarization resistance defined as... 

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