Mixed ionic and electronic conductivity MIEC membranes

A single material membrane based on a material exhibiting mixed ionic and electronic conductivity (MIEC membrane). 

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... 

In the last 20 years, the mixed oxygen-ionic and electronic conducting (MIEC) ceramic membranes such as Lai Sr Coi Fe Os perovskite that exhibit appreciable oxygen ionic and electronic conductivity at elevated... 

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... 

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. 

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. 

Sunarso J, Baumann S, Serra JM, Meulenberg WA, Liu S, Lin YS, and Diniz da Costa JC. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation. J. Membr. Sci. 2008 320 13-41. 

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. 

Continuous air separation by an oxygen-conducting membrane which constitutes the wall of a CPO reactor is another approach which has received much interest, also from industry. Two types of membrane materials have been studied zirkonia-based membranes, which are efficient oxygen ion conductors but require electrodes to transfer electrons to the reduction interface, and perovskites (of general formula ABO3, with dopants in the A and/or B site), which are mixed ionic/electronic conductors (MIEC). ... 

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