Temperature range, fuel cells

Sayers R, Liu J, Rustumji B, Skinner SJ (2008) Novel K2NiF4-type materials for solid oxide fuel cells compatibility with electrolytes in the intermediate temperature range. Fuel Cells 8(5) 338-343... 

Figure 6.49. In situ AC impedance spectroscopy at a frequency range of 3500 to 0.1 Hz at 0.91 A/cm2, 100% RH, and 30 psig pressure at 80°C, 100°C, and 120°C [44]. (Reproduced by permission of ECS—The Electrochemical Society, and of the authors, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, MacKinnon S, Peckham T, Li J, McDermid S, Kozak P, Temperature dependent performance and in situ AC impedance of high temperature PEM fuel cells using the Nafionl 12 membrane.)...

Different material combinations for practical realization of fuel cells have been developed in the past few decades [3] comprehensive overviews about the technology have been published [4], Possible operation temperatures of fuel cells range from ambient temperature to 1,000°C. The operation principle of the different fuel cells is depicted in Fig. 1. The main components are the same for all types of fuel cells and comprise an electrolyte, catalytically active electrodes, and a cell frame for gas distribution and current collection. Regular nanostrucmres are not typically used until now, but nanomaterials for preparation of layers are frequently the best base materials. Some examples will be given in the description of the five types of fuel cells in this introductory chapter. 

Rosa et al. [251] set up a complete 5-kW diesel fuel processor based on autothermal reforming and catalytic carbon monoxide clean-up, which was dedicated to a low temperature PEM fuel cell. The breadboard system was composed of the autothermal reformer operated between 800 and 850 °C with a ruthenium/perovskite catalyst (see Section 4.2.8), a single water-gas shift reactor containing platinum/titania/ceria catalyst operated between 270 and 300 °C (see Section 4.5.1), and a preferential oxidation reactor containing platinum/alumina catalyst operated between 165 and 180 °C. Figure 9.54 shows the gas composition and reactor temperatures achieved. The hydrogen content of the reformate was in the range from 40 to 44 vol.% on a dry basis. The carbon monoxide content of the reformate was 7.4 vol.% and could be reduced to values of between 0.3 and 1 vol.% after the water-gas shift reactor and to below 100 ppm after the preferential oxidation reactor. 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Solid oxide fuel cells consist of solid electrolytes held between metallic or oxide elecU odes. The most successful fuel cell utilizing an oxide electrolyte to date employs Zr02 containing a few mole per cent of yttrium oxide, which operates in tire temperature range 1100-1300 K. Other electrolytes based... 

There are a whole variety of types of fuel cell, named after the electrolyte used, each operating at a preferred temperature range with its own feedstock purity criteria (Table 6.3). 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

Finally, for relevant fuel cell modeling, the kinetic model studies should be performed under similar temperature and pressure conditions, i.e., at temperatures in the range of 80-120 °C and pressures up to 3 bar, and at comparable space velocities. The first flow-cell and DBMS measurements under such temperature and pressure conditions are currently underway in our laboratory. 

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