Membrane reactors mixed ions-electrons conducting

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). 

Solid-state electrochemistry is an important and rapidly developing scientific field that integrates many aspects of classical electrochemical science and engineering, materials science, solid-state chemistry and physics, heterogeneous catalysis, and other areas of physical chemistry. This field comprises - but is not limited to - the electrochemistry of solid materials, the thermodynamics and kinetics of electrochemical reactions involving at least one solid phase, and also the transport of ions and electrons in solids and interactions between solid, liquid and/or gaseous phases, whenever these processes are essentially determined by the properties of solids and are relevant to the electrochemical reactions. The range of applications includes many types of batteries and fuel cells, a variety of sensors and analytical appliances, electrochemical pumps and compressors, ceramic membranes with ionic or mixed ionic-electronic conductivity, solid-state electrolyzers and electrocatalytic reactors, the synthesis of new materials with improved properties and corrosion protection, supercapacitors, and electrochromic and memory devices. 

Today, the majority of research investigations into CMRs are being conducted by many institutions, in addition to oil and chemical and utilities companies [5]. The use of mixed ionic-electronic membrane reactors for the partial oxidation of natural gas is undergoing active development by a number of consortia based around Air Products and Chemicals (USA), Praxair (USA), and/or Air Liquide (France). At present, the development of CMRs involving a pure ion-conducting electrolyte is restricted to a few reports of conceptual systems [12, 95]. 

Since the membrane conducts oxygen ions, in the great majority of the cases through a mechanism involving anion vacancy diffusion, an equivalent counterflux of electrons should take place for charge neutrality membrane materials should be mked conductors. Perovskite-type mixed-conducting materials are considered suitable candidates for use in dense membrane reactor chemical looping processes, since they fulfill most of the required characteristics. 

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). ... 

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... 

Some perovskite-type oxides having transition elements at B sites exhibit mixed conduction at elevated temperatures. A typical example is doped lanthanum cobaltite, in which oxide ions and holes are charge carriers. The electronic conductivity is a few orders of magnitudes higher than that of the oxide-ionic although the ionic conductivity itself is sufficiently high (>10 S cm at several 100°Q. This kind of mixed conductor is a promising candidate for the electrode materials of SOFCs and ceramic membrane reactors and is described in Chapters 7 and 8 in detail. 

Abstract Dense ceramic membrane reactors are made from composite oxides, usually having perovskite or fluorite structure with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. They combine the oxygen or hydrogen separation process with the catalytic reactions into a single step at elevated temperatures (>700°C), leading to significantly improved yields, simplified production processes and reduced capital costs. This chapter mainly describes the principles of various types of dense ceramic membrane reactors, and the fabrication of the membranes and membrane reactors. 

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