Solid electrolyte membrane materials

These applications will be briefly treated in this section. As will become evident, the solid electrolyte membrane materials are either stabilized oxides or mixed oxides. Further details of science and technology of electrocatalytic membrane reactors beyond the scope of this chapter can be found in a number of excellent reviews [Ceilings et al., 1988 Stoukides, 1988]. 

The successful application of the sol-gel route has been realized in the International technological practice of many branches of the industries—electronics, optics, construction of engines, nuclear energetics, chemical, and food industry equipment. Among these materials are ferro-, piezo-, and dielectrics, solid electrolytes, refractory materials, membranes, protective and decorative coatings, and also films with special optical and electrophysical properties, like hightemperature superconductors. 

Small-size starting particles with an appropriate size distribution that lead to a maximum packing density and final fired density are essential in the sintering step to form dense membranes. The required sintering temperatures are lower because of the active state of the Hne particulate materials used. For example, when particles of an average size of 5 nm made by the alkoxide approach are used to make stabilized zirconia, 1450X instead of the usual 2(X)0X is all that is necessary to produce fully dense material [Mazdiyasni et al., 1%7]. The amount of additives such as the stabilizers affects densification of the final solid electrolyte membranes as well. Generally there is an optimum amount of stabilizer for maximum densification. Excess addition actually can lead to lower densification. 

According to the third catalyst-membrane coupling possibility, represented in Fig. 5c, the surface of the membrane is deposited with some catalytic material. This setup is typical of solid-electrolyte membranes, where the catalyst is also playing the role of the electrode, necessary to drive the permeation of ions throughout the membrane at a desired rate. Problems may arise here concerning the fact that the catalyst per unit membrane surface is limited to some extent, and that several catalytic materials (e.g., metal oxides) are poor electricity conductors [26]. 

Concerning proton conductors, Govind and Zaho [100] stated that metal-based membranes could be outperformed by solid electrolyte membranes based on materials... 

All that we have mentioned above allows us to assume that the upper limit of the reversible work of the oxygen electrode with a solid electrolyte membrane made of YSZ is connected with the chemical stablility of the material of the membrane, which loses the zirconium dioxide both in strongly acidic and strongly basic media. 

The predominant importance of the cations in zeolites is that they form so-called active sites for selective interaction with guest molecules in sorption and catalytic processes. From the point of view of advanced material science [47] they play a significant role in the formation of quantum-sized clusters with novel optical or semiconducting properties. As they give rise to cationic conductivity, zeolites can be used as solid electrolytes, membranes in ion-selective electrodes and as host structures in solid-state batteries. Organometallic compounds and coordination complexes can be readily formed on these cations within the larger cages or channels and applied to gas separation, electron-transport relays and hybrid as well as shape-selective catalysis [48]. 

One of the current goals within the field of PEMFCs is to find a nonaqueous, proton-conducting solid electrolyte-membrane system for fuel cells. While these are not PlLs, they have similarities. Proton-exchange materials have been produced from imidazolium cations with inorganic anions, such as benzimidazole with TFSl, formed into a polymer with mai similarities to PlLs. 

Zeolites are essentially cationic conductors having temperature dependence of resistivity typically displayed by ionic conductors (higher mobility of ions at higher temperatures). By appropriate modifications of the zeolite composition, the conductivities of zeolites can be altered to allow their use as solid electrolytes, membranes in ion-selective electrodes, and as host stmctures for cathode materials in battery systems [84M1, 95A1]. 

Solids which have both ionic and electronic conductivity are called mixed conductors. Most of the known solids show such behavior ( Solid Electrolytes, Membrane Technology), depending on temperature and partial pressure of that gas which is in equilibrium with ions in the conductor. For the development of electrolytes, electrode materials, and ceramic membranes, it is important to know which charge carriers are responsible for the current transport and to which extent. The required range of the transference number depends on the desired application of the solids For solid electrolytes, a very low electronic conductivity is desirable (t, > 0.99) whereas for electrode materials, a high electronic conductivity combined with a low ionic conductivity is necessary (t, 0.10). For membranes, those solids are suited which exhibit nearly the same conductivity for electronic and ionic carriers (t,- 0.5). 

PEM fuel cells have emerged as the most common type of fuel cell under development today. As stated above, they also are commonly referred to as proton exchange membrane fuel cells based on the key characteristic of the solid electrolyte membrane to transfer protons from the anode to the cathode. The solid electrolyte avoids problems caused by liquid electrolytes used in other systems, and the temperature range of <100°C enables rapid start-up under low temperature operation, with operation possible down to subfreezing temperatures. The lower temperature also allows a wider range of materials to be used and enables relatively easy stack design in terms of sealing issues and material selection. This type of fuel cell is the most feasible for use under transportation applications. 

Kumar et al. [36] recently published a report on solid-state lithium-air batteries. The lithium-ion, conductive solid electrolyte membrane is based on glass-ceramic (GC) and polymer-ceramic materials. This solid electrolyte is used as the ionic conductive membrane between the lithium electrode and the air electrode. It also is used in the soHd composite air cathode prepared from high-surface-area carbon. The cell exhibited excellent thermal stabihty in the 30-105 °C temperature range... 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

This presentation reports some studies on the materials and catalysis for solid oxide fuel cell (SOFC) in the author s laboratory and tries to offer some thoughts on related problems. The basic materials of SOFC are cathode, electrolyte, and anode materials, which are composed to form the membrane-electrode assembly, which then forms the unit cell for test. The cathode material is most important in the sense that most polarization is within the cathode layer. The electrolyte membrane should be as thin as possible and also posses as high an oxygen-ion conductivity as possible. The anode material should be able to deal with the carbon deposition problem especially when methane is used as the fuel. 

De Marco R, Mackey DJ, Zirino A (1997) Response of the jalpaite membrane copper(lI) ion-selective electrode in marine waters. Electroanalysis 9 330-334 Kozicki MN, Mitkova M (2006) Mass transport in chalcogenide electrolyte films - materials and applications. J Non-Cryst Solids 352 567-577... 

Materials similar to Nation containing immobilized —COO- or —NR3+ groups on a perfluorinated skeleton were also synthesized. These are available in the form of solid membranes or solutions in organic solvents the former can readily be used as solid electrolytes in the so called solid polymer electrolyte (SPE) cells, the latter are suitable for preparing ion-exchange polymeric films on electrodes simply by evaporating the polymer solution in a suitable solvent. 

Polymer electrolyte membrane and solid oxide fuel cells demonstration of systems and development of new materials. Activity leader National Research Council (CNR). Estimated activity cost 14 million. 

Suitable solid electrolytes can be employed as the electrolyte in an electrochemical cell. The electrolyte is used in the form of a membrane which is impermeable to gas phase transport. Electroactive materials, or electrodes, are deposited on both sides of the electrolyte to increase the rates of charge transfer across the electrolyte interface and it is important that the active molecules in the gas phase have easy access to the electrode/electrolyte interface where they can participate in the charge-transfer reactions. For this reason it is necessary, in most cases, to ensure that the electrode has a high porosity while, at the same time, remaining electrically continuous. 

This electrode, also called the glass electrode, is specific to H+ ions. Glass in this case does not refer to the material of the electrode body but to the membrane that ensures contact with the solution. The membrane is a thin wall of glass that has a very high sodium content (25%). In the presence of water, hydration occurs and the membrane s surface becomes comparable to a gel while its interior corresponds to a solid electrolyte. 

The cell anode and cathode spaces are separated by a solid-state electrolyte membrane. The anode is made of a porous, water-penetrable and current-conductive carrier material coated with an active layer. At the site where the active catalytic layer and the electrolyte membrane touch ozone is produced when a direct current of 3 to 6 V at currents of up to 50 A (corresponding with a current intensity of 0.2 to 3.0 A cm-2) are applied (Fischer, 1997). 

The central part of the electrochemical HDH reactor is the use of solid polymer electrolyte (SPE). A typical membrane material belongs to the fully fluorinated polyethene-based family polytetrafluoroethylene (PTFE) (known by the trade name Teflon), shown in Fig. 13.1. 

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