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4.1 Cell Components 4.1.1 State-of-the-ArtComponents The concentration of KOH in an immobilized electrolyte typically used in the space program varies from 35 to 50 wt percent KOH for low temperature (<120 °C) operation to 85 wt percent KOH in cells designed for operation at high temperature (~260 °C). The electrolyte is retained in a matrix (usually asbestos), and a wide range of electro-catalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals) to promote reaction. The cylindrical AFC modules used in the U.S. Apollo Space Program had a 57 cm diameter, a 112 cm height, weighed about 110 kg, produced a peak power of 1.42 kW at 27 to 31 V, and operated at an average power of 0.6 kW. These cells operated on pure H2 and O2 and concentrated electrolyte (85 percent KOH) at a moderate pressure (4 atmospheres reactant gas pressure) without electrolyte boiling. With this concentrated electrolyte, cell performance was not as high as in the less-concentrated electrolyte; consequently, the operating temperature was increased to 260 oC. The typical performance of this AFC cell was 0.85 V at 150 mA/cm2, comparing favorably to the performance of the Bacon cell operating at about 10 times higher pressure. The state-of-the-art alkaline fuel cell stacks in the Space Shuttle Orbiter are rectangular with a width of 38 cm, a length of 114 cm, and a height of 35 cm. They weigh 118 kg, produce a peak power of 12 kW at a minimum of 27.5 V (end of life), and operate at an average power of 7 kW. They operate in the same pressure range as the Apollo cells (4 atmospheres), but at a lower temperature (85 to 95 °C) and higher current density (0.88 V at 470 mA/cm2; UTC Fuel Cells has demonstrated 3.4 W/cm2 at 0.8 V and 4,300 mA/cm2, Reference (8)). The electrodes contain high loadings of noble metals: 80 percent Pt – 20 percent Pd anodes are loaded at 10 mg/cm2 on Ag-plated Ni screen; 90 percent Au – 10 percent Pt cathodes are loaded at 20 mg/cm2 on Ag- plated Ni screen. Both are bonded with PTFE to achieve high performance at the lower temperature of 85 ot 95 oC. A wide variety of materials (e.g., potassium titanate, ceria, asbestos, zirconium phosphate gel) have been used in the micro-porous separators for AFCs. The electrolyte is 35 percent KOH and is replenished via a reservoir on the anode side. Gold-plated magnesium is used for the bipolar plates. Sheibley and Martin (10) provide a brief survey of the advanced technology components in AFCs for space applications. An advanced cell configuration for underwater application was developed using high surface area Raney nickel anodes loaded at 120 mg/cm2 (1 to 2 percent Ti) and Raney silver cathodes loaded at 60 mg/cm2 containing small amounts of Ni, Bi, and Ti (11). The efforts of Union Carbide Corporation have formed the basis for most of today’s terrestrial applications of AFCs with circulating liquid electrolytes. Companies like Da Capo Fuel Cell Ltd. (which bought ZeTek Power (formerly Zevco and Elenco)), Astris Energy, and Apollo Energy System Inc. are developing circulating electrolyte cells for motive and backup power primarily based on that technology. A typical configuration (Apollo, Figure 4-2) uses carbon- based plastic-bonded gas diffusion electrodes with a current collector (nickel) inside. Due to the ease of preparation, the electrodes in present stacks use noble metals loaded to less than 0.5mg/cm2. The 0.3 cm thick cells are stacked in a monopolar order and are commonly 4-5PDF Image | Fuel Cell Handbook (Seventh Edition)
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