MIEC membranes transport

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. 

In the previous sections, it has been demonstrated that, in order to improve the properties of known MIEC membrane materials and to develop new materials, a fundamental understanding of their defect chemistry and the transport of defects is crucial. This is achieved through interpretation of experimental thermochemical and transport data by means of defect models. 

Figure 4.3 Schematic of the counter-current oxygen transport mechanism of a symmetric (dense) MIEC membrane exploited for oxygen separation applications (a) and the change in oxygen permeation flux with the membrane thickness (b). fSource Reproduced from Ref [ 10], with permission from Elsevier)...

Although oxides have a wide range of catalytic applications their transport properties are most obviously critical when they are used in the form of a membrane within a chemical or electrochemical reactor. As such their ionic conductivity must be high if they are going to support a reasonable ion flux. Such materials fall broadly into two classes those materials that exhibit a very low electronic conductivity and, if the electronic transport number is <0.01, are generally termed solid electrolytes (solid electrolytes are covered in a separate chapter) and those materials that exhibit an appreciable or high electronic conductivity as well as ionic conductivity and are hence termed mixed conductors. In the rest of this chapter we will focus on such mixed ionic and electronic conducting (MIEC) materials. First, we will address transport in MIEC membranes from a theoretical perspective... 

For MIEC membranes without external circuit, the overall charge balance is applied or Zz,/, = 0 and tbe local velocity of inert marker is negligible, o = 0. Accordingly, the transport flux of charged defects in the MIEC membrane at steady state can be derived (one-dimensional model) from Eqs (5.3)-(5.6) as [15]... 

The transport fluxes of charged defects at steady state in the MIEC membrane are given by Eq. (5.7), namely... 

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 process that is the subject of this article consists of the use of high temperature ceramic membranes that selectively transport oxygen. They are referred to with several acronyms of which ITM (Ion Transport Membranes), OTM (Oxygen Transport Membranes) and MIEC (Mixed Ionic Electronic Conducting) membranes prevail. We will use ITM throughout this article. 

Determination of nonstoichiometry in oxides is a key point in the search for new materials for electrochemical applications. In recent decades, owing to their current and potential applications (electrodes in fuel cells, insertion electrodes, membranes of oxygen separation, gas sensors, catalytic materials, etc.), various methods of precise characterization of MfECs have been proposed, either the measurement of the defect concentrations and the stoichiometric ratio as functions of the oxide composition, of the surroxmding oxygen pressure and of temperature, or the transport properties. There are different methods to determine the electrical properties of MIECs and, more specifically, the ionic and electronic contributions. The most appropriate method depends on different parameters, i.e., the total electrical conductivity of the studied oxides, the ionic and electronic transport numbers, the... 

The dense ceramic membranes applied in MRs can be either MIECs or pure ionic conductors (electrolytes). In the MIEC-MRs, the membrane itself serves as the internal circuit for electron transport, whereas an external circuit for electron transport has to be provided in the pure ionic conductor MRs (EMRs). The principles of the dense ceramic MRs are illustrated schematically in Eigure 5.5. 

Due to its adjustable transport properties, suitably doped ceria may be used as an electrolyte (which must be an electronic insulator and a good ion conductor), as discussed in depth in Section 12.5, or as a dense oxygen separation membrane (which needs to be a MIEC). A dense oxygen separation membrane must conduct both... 

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