Temperature cells

While concentration and oxygen cells are responsible for perhaps 90 percent of the corrosion in soils and natural waters, other cells, such as the temperature cell shown in Fig. 7.28, may be still quite damaging when they get established. In such cells, the two electrodes are of the same metal, but one is maintained at a higher temperature than the other by some external means. In most cases, the electrode at the higher temperature becomes the anode to cause what has been called thermogalvanic corrosion. 

In the gas transmission line example shown in Fig. 7.28, the compressed gas coming out of the station is hot, and, as it travels down the line, it loses heat by transfer to the surrounding soil. The 

The hot pipe near the compressor is the anode The cooler pipe down the line is the cathode 

This is a particularly unfortunate type of cell because the high temperature just outside the compressor tends to damage the coating, so that the part of the line which is the anode is also likely to have the poorest coating. 

External corrosion of copper hot water tubing underslab has resulted in a great deal of construction defect litigations due to this type of corrosion cell [17]. Oil and gas well casings also experience similar cell attack. The pipe at some depth below the surface is at a higher temperature since temperature increases with depth and thus becomes the anode. The cooler pipe near the surface and the surface piping make up the cathode, the soil is the electrolyte, and the pipe is the electrical connection. 

---

Cells operating at low (2,80,81) and high (79,82) temperatures were developed first, but discontinued because of corrosion and other problems. The first medium temperature cell had an electrolyte composition corresponding to KF 3HF, and operated at 65—75°C using a copper cathode and nickel anodes. A later cell operated at 75°C and used KF 2.2HF or KF 2HF as electrolyte (83,84), and nickel and graphite as anode materials. 

The relation between reaction free energy, temperature, cell voltage, and reversible heat in a galvanic cell is reflected by the Gibbs-Helmholtz equation [Eq. (31)]. 

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. 

Figure 14. UV-vis-NIR absorption spectra of Cl2CH-C70+ generatedfrom the corresponding fuller enols in CF3SO3H at room temperature. Cell length 1 cm. (a) 1017 xlO 5 M. (b) 5.90 x 70° M. (Reproduced with permission from reference 15. Copyright 2005 Wiley- VCH.)...

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... 

PAFC systems achieve about 37 to 42% electrical efficiency (based on the LHV of natural gas). This is at the low end of the efficiency goal for fuel cell power plants. PAFCs use high cost precious metal catalysts such as platinum. The fuel has to be reformed external to the cell, and CO has to be shifted by a water gas reaction to below 3 to 5 vol% at the inlet to the fuel cell anode or it will affect the catalyst. These limitations have prompted development of the alternate, higher temperature cells, MCFC, and SOFC. 

Figure 2-1 shows the relation of E° to cell temperature. Because the figure shows the potential of higher temperature cells, the ideal potential corresponds to a reaction where the water product is in a gaseous state. Hence, E° is less than 1.229 at standard conditions when considering gaseous water product. 

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). 

Sulfur poisons catalytic sites in the fuel cell also. The effect is aggravated when there are nickel or iron-containing components including catalysts that are sensitive to sulfur and noble metal catalysts, such as found in low temperature cell electrodes. Sulfur tolerances are described in the specific fuel cell sections of this handbook." In summary, the sulfur tolerances of the cells of interest, by percent volume in the cleaned and altered fuel reformate gas to the fuel cells from published data, are ... 

There are high temperature and low temperature methods to remove sulfur from a fuel reformate stream. Low temperature cleanup, such as hydrodesulfurizing (limited to fuels with boiling end points below 205°C), is less difficult and lower in cost so should be used where possible, certainly with low temperature cells. Sulfur species in the fuel are converted to H2S, if necessary, then the H2S is trapped on zinc oxide. As previously mentioned, a minimum bed volume of the zinc oxide reactor is achieved at temperatures of 350 to 400°C. Simple... 

There are many types of coal with different compositions, including harmful species. The handling of these species is addressed earlier under Coal Processing. One common constituent, HCl, will cause formation of stable chlorides and corrosion in a MCFC, see the section on MCFC. There has not been much work in SOFC yet on this topic. It is doubtful whether low temperature cells will be fueled by coal. 

Although Clostridium perfringens strains did not grow at low temperatures, cells grown at 38 C were able to produce histamine at 4 C. 

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... 

Figure 3.4 Low temperature cell for HP IR spectroscopy in liquid noble gases (from Ref [10], reproduced by permission of The Royal Society of Chemistry).

---