High temperature cells

A large number of low and high temperature cells have been described in the literature (28). 

Diffuse reflectance FTIR (DRIFT) spectra were recorded on a Bio-Rad FTIR spectrometer (EXCALIBUR FTS3000). A high-temperature cell was attached to a flow system that allows in-situ sample treatment, adsorption and desorption of probe molecules at different temperatures. 

G) In reality, CO with H20 shifts H2 and C02, and CH4 with H20 reforms to H2 and CO faster than reaction as a fuel at the electrode. CO is a poison for lower temperature fuel cells, but is used as a fuel in the high-temperature cells (e.g., SOFC, MCFC). CO may not actually react electrochemically within these cells. It is commonly understood that CO is consumed in the gas phase through the water-gas shift reaction as CO + H20 = C02 + H2. The H2 formed in this reaction is subsequently consumed electrochemically. 

Carbon Monoxide. Carbon monoxide, a fuel in high-temperature cells (MCFC and SOFC), is preferentially absorbed on noble metal catalysts that are used in low-temperature cells (PAFC and PEFC) in proportion to the hydrogen-to-CO partial pressure ratio. A particular level of carbon monoxide yields a stable performance loss. The coverage percentage is a function of temperature, and that is the sole difference between PEFC and PAFC. PEFC cell limits are < 50 ppm into the anode major U.S. PAFC manufacturers set tolerant limits as < 1.0% into the anode MCFC cell limits for CO and H20 shift to H2 and C02 in the cell as the H2 is consumed by the cell reaction due to a favorable temperature level and catalyst. 

Another high-temperature cell (Fig. 8 g, up to 2400 °C, can be produced from tungsten. Tungsten Knudsen cells are used primarily for high-temperature vapor pressure measurements, e.g. for metal oxides. They are suitable also for metals when graphite linings are applied to the inner surface. The vapor pressure can be determined... 

In this analysis of concentration polarization, the activation polarization is assumed to be negligible. The charge transfer reaction has such a high exchange current density that the activation polarization is negligible in comparison with the concentration polarization (most appropriate for the high temperature cells). 

Two earlier reviews were published on high temperature cells and batteries based on molten salt and solid electrolytes. The first one (69) describes the Li/Cl2 cells, particularly the LiA.l/LiCl-KCl/Cl2 cell with gaseous CI2. Li cells with chalcogenides as cathode materials are mentioned, as well as some details of construction. This review, and the 26 references attached to it, reflects the state of the Li molten salt batteries to the end of 1970 (69). The second review (70), prepared two years later is more comprehensive. It discusses in detail some theoretical problems, the thermodynamics and rate processes in electrochemical cells, and presents tables and... 

Cathode materials of both high-temperature cells are composed of chemically and morphologically relatively stable oxide ceramics. For molten carbonate cells, lithiated NiO is used and the cathode of oxide ceramic cells usually is made of porous LaMnOj. 

The physical properties of lithium metal were given in Table 4.4. Despite its obvious attractions as an electrode material, there are severe practical problems associated with its use in liquid form at high temperatures. These are mainly related to the corrosion of supporting materials and containers, pressure build-up and the consequent safety implications. Such difficulties were experienced in the early development of lithium high temperature cells and led to the replacement of pure lithium by lithium alloys, which despite their lower thermodynamic potential remained solid at the temperature of operation and were thus much easier to use. 

Of the various solid intermetallic lithium compounds which might be used in high temperature cells, the Li-Al system has been most studied. The Li-Al phase diagram is shown in Fig. 8.1. An a phase which consists... 

Sodium is also a very reactive metal, and with a melting point even lower than that of lithium, presents in principle problems similar to those of lithium. However, the fortunate discovery of ceramic materials which show high stability to molten sodium together with good sodium ionic conductivity at high temperature has permitted the reliable fabrication of sodium-based cells. In some sodium high temperature cells, the liquid metal is housed in closed, shaped ceramic containers. In the others, the... 

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