MIEC oxides

This is not exactly the order as the electronic conductivity of the oxides, indicating that electrocatalytic activity of mixed ionic and electronic conducting (MIEC) oxides depends on other properties such as the oxygen exchange and ionic conductivities. 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

Mixed oxygen imfic and electronic conducting (MIEC) oxides find application in the electrodes of SOFCs, SOECs, SOERs, and as the membrane materials in ITMs. There are many materials... 

Reduced stabilized zireonia is black. As it oxidizes it become transparent. Following the propa tion of the front between the dark and transparent part in a single crystal b(0) has been determined. Instead of following the front, one can follow the integrated intensity changes due to diffusion of defects. This has been used to determine b(0) in SrTiOj doped with Fe impurities by following the absorption lines of the Fe ions. " The concentration of the optically active Fe ions increases at the expense of the Fe ions as the MIEC oxidizes. 

It has been proposed that D in the near to sttrface layers in MIEC oxides can be determined by following the change in the work function after a step change inR(02) has occttrred. ... 

As already mentioned, so far, three types of materials were used for SOFC anode, viz. (a) pure metal, (b) cermet and (c) MIEC-oxide. A host of metals such as Ni, Pt, Co and Ru have so far been considered as... 

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. 

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). 

Oxygen partial pressure between the two sides of the membrane is the driving force of oxygen permeation. Figure 3.1c shows a dual-phase MIEC membrane, which can be visualized as a dispersion of a continuous electronic conducting phase into an SE matrix. The electronic conducting phase is usually made from precious metal or metal oxides. 

Introduction of electronic conductivity to fluorite solid electrolyte was also attempted by dissolution of oxides having multivalent cations [34, 35], However, the electronic conductivity is often orders of magnitude less than those of perovskite based MIEC materials. 

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... 

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