MIECs 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... 

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

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]. 

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. 

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). 

Figure 12.10 Various configurations of anodes. MIEC = mixed ionic electronic conductor Cermet — ceramic-metal mixture.

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... 

As follows from the previous chapters, a complex interface Metal/MIEC/Electrolyte (MIEC = mixed ion-electron conductor) appears in many processes related to the electrochemistry of polyvalent metals. The model of MIEC in terms of the concept of polyfunctional conductor (PFC) can be a useful approach to deal with the mechanisms of the processes in such systems. The qualitative classification of EPS has been given based on this approach. Further on, we are going to demonstrate that this concept is useful for quantitative (or at least, semi-quantitative) modelling of macrokinetics (dynamics) of the processes in highly non-equilibrium systems. Before doing this, it is worthwhile to outline some basic ideas related to the MIEC. These considerations will also show some restrictions and approximations that are commonly applied in electrochemical practice and which are no longer valid in such kind of systems. 

Key words membrane reactor, perovskite, proton conducting membrane, mixed ionic-electronic conductor (MIEC), partial oxidation of methane... 

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... 

The work carried out in our laboratory has led to the development of synthetic methods required for the preparation of MIEC block copolymers. It will now be necessary to obtain a clear understanding of the structure-microstructure-function interrelationship of the MIEC block copolymers. MIEC block copolymers of controlled structures may be ideal for testing the different hypotheses of coupling between the electronic and the ionic species in mixed conductors [56]. Methods are available to determine the diffusion constants of the ionic species in mixed conducting systems [65]. The diffusion constants of ionic species in mixed con-... 

Despite the apparent simplicity of this reaction, the process by which the oxygen reduction occurs followed by incorporation of the ionic species into the electrolyte is the subject of some debate and is dependent on the mode of operation of the cathode material. Two typical cathode types are currently utilized in SOFCs -electronic conductors and mixed ionic-electronic conductors (MIECs). The cathode reactions, while nominally the same in both types of materials, occur at different locations, and hence, the active region varies, leading to differences in the operating regime and ultimately performance. In the case of a single phase electronic conductor. 

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