Ion Conducting Membranes

B.C.H. Steele. Dense Ceramic ion conducting membranes in Oxygen ion and mixed conductors and their technological applications, (1997) Erice, Italy Kluwer. 

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

This type of electrolytic cell consists of anodes and cathodes that are separated by a water impermeable ion-conducting membrane. Brine is fed through the anode where chlorine gas is generated and sodium hydroxide solution collects at the cathode. Chloride ions are prevented from migrating from the anode compartment to the cathode compartment by the membrane and this, consequently, leads to the production of sodium hydroxide, free of contaminants like salts. The condition of the membrane during operation requires more care. They must remain stable while being exposed to chlorine and strong caustic solution on either side they must allow, also, the transport of sodium ions and not chloride ions. 

Lackner, K.S., West, A.C., and Wade, J.L., Ion Conducting Membranes for Separation of Molecules, U.S. Patent Publication Number WO2006113674, 2006. 

One of the most important parts of the fuel cell is the electrolyte. For polymer-electrolyte fuel cells this electrolyte is a single-ion-conducting membrane. Specifically, it is a proton-conducting membrane. Although various membranes have been examined experimentally, most models focus on Nafion. Furthermore. it is usually necessary only to modify property values and not governing equations if one desires to model other membranes. The models presented and the discussion below focus on Nafion. 

C.K. Shin, Block copolymer ionomers for ion conducting membranes, Verl. Dr. Hut, 2002, Originally published as PhD Thesis. 

The following example illustrates the potential of membrane-separation processes for precombustion carbon capture in an IGCC. This approach avoids using an expensive air-separation unit (asu) or a difficult-to-implement high-temperature mixed-ion conducting membrane process however, it still enables capture of C02 at purities suitable for commercial use or sequestration. 

Electrolysis of water — This is a process of electrochemical decomposition of water into -> hydrogen and -> oxygen. Apart from alkaline electrolyzers using 25% KOH solution [i], devices with polymer, or ceramic ion-conducting -> membranes have been developed for industrial applications [ii]. 

PEM technology was originally developed as part of the Gemini space program.16 In a PEM electrolyzer, the electrolyte is contained in a thin, solid ion-conducting membrane rather than the aqueous solution in the alkaline electrolyzers. This allows the H+ ion (proton) or hydrated water molecule (HsO+) to transfer from the anode side of the membrane to the cathode side, and separates the hydrogen and oxygen... 

Imperial College s Centre for Ion Conducting Membranes is working on perm-selective membranes, and perhaps could develop the concentration cells mentioned in Appendix A (Williams, 2002), pp. 878-891. 

Ehrenberg, S.G. et al.. Fuel Cell Incorporating Novel Ion-Conducting Membrane, U.S. Patent 5,679,482, October 21, 1997. 

FIGURE 6.10 Different membrane concepts for oxygen-ion conducting membranes, (a) Dense mixed conducting membrane top-layer supported on an asymmetric macroporous support (b) dense self-supported mixed conducting membrane with graded porous interfaces and (c) solid electrolyte cell (oxygen pump). 

Dense ceramic ion-conducting membranes (CICMs) are emerging as an important class of inorganic membranes based on fluorite- or perovskite-derived crystalline structures [18]. Most of the ion-conducting ceramics discovered to date exhibit a selective ionic oxygen transport at high temperatures >700°C. Ionic transport in these membranes is based on the following successive mechanisms [25] ... 

Ismail AF and David LIB. A review on the latest development of carbon membranes for gas separation. J. Membr. Sci. 2001 193 1-18. Kilner JA, Benson S, Lane J, and Waller D. Ceramic ion conductive membranes for oxygen separation. Chem. Ind. 1997 17 November 907-911. 

Steele B. Ceramic ion conducting membranes and their technological application. C.R. Acad. Set (Paris) 1998 Parti, Series IIc 533M-3. 

Tricoli V and Nannetti F. Zeolite-Nafion composites as ion conducting membrane materials. Electrochim Acta 2003 48 2625-2633. Libby B, Smyrl WH, and Cussler EL. Polymer-zeolite composite membranes for direct methanol fuel ceUs. AIChE J 2003 49 991-1001. 

In the membrane process the cathode and anode chambers are separated by a water-impermeable ion-conducting membrane (see Fig, 1.7-11). 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

Porous membranes, especially ceramic and carbon compositions, are the focus of intense development efforts. Perhaps, the least studied of the group, at least for hydrogen separations, are the ion-conducting membranes (despite the fact that many fuel cells incorporate a proton-conducting membrane as the electrolyte), and this class of membranes will not be discussed further in this chapter. 

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