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or crack and operates at a temperature of about 80 degrees C (176 degrees F) which is low enough for the cell to be suitable for vehicles and homes. Nevertheless, the platinum catalyst used on both sides of the membrane is costly and the hydrogen for fuel must be purified. The direct methanol fuel cell is similar to the PEM cell because the electrolyte is a polymer membrane. However, the fuel is methanol instead of hydrogen. A catalyst on the anode draws hydrogen from liquid methanol, eliminating the need for a fuel reformer. This cell has an operating temperature of 50–100 degrees C (122–212 degrees F) (Society of Automo- tive Engineers, 2001) and has considerable potential value for use in vehicles. The unitized regenerative or reversible fuel cell functions alternately as a fuel cell, producing electricity and water, and as an electrolyzer, generating hydrogen and oxygen. This cell has a PEM electrolyte and “oxidation and reduction electrodes that reverse roles when switching from charge to discharge” (Walter, 1997). Water is moved from the anode to the cath- ode. A major advantage of the regenerative fuel cell, where combined with a lightweight hydrogen storage unit, is the comparatively low total weight, which is appropriate for use in a variety of vehicles. New Developments The widely recognized potential of fuel cells as sources of electricity for stationary power plants, for motors in vehicles, and for other purposes has encouraged investigations by government agencies and by large and small companies in several nations. Prototypes of micro fuel cells, which can be smaller than a deck of cards, are being prepared by several companies (Hill, 2002). Studies during the past decade have developed and described a variety of components and systems. One company produces an assortment of small fuel cells, one of which has an alkaline electrolyte and will function when any of several liquid or solid hydrogen-rich materials are dissolved in the electrolyte. They also offer the unusual aluminum/air cell that contains an aluminum anode, which is fuel for the cell, and an electrolyte consisting of salt water. In the fuel cell system of a recent concept-vehicle from a major manufacturer, the required hydrogen is extracted from sodium borohydride, a compound related to borax (Brown, 2002). For hydrogen storage, solid state materials include metal hydride alloys that reportedly absorb and desorb hydrogen rapidly at low pressures and at temperatures of 0–200 degrees C (0–392 degrees F) (Fuel cell investors portal, 2002). Carbon nanotubes, which are carbon molecules in the form of long thin tubes, are another potential hydrogen storage medium. They are also suitable for electrodes and can be used in sen- sors for hydrogen, especially for the smallest fuel cells (NEC Corporation press release, 2001). Another potential storage material is glass microspheres that will fill with hydrogen when their permeability is modified by changes in temperature (U.S. Department of Energy, 2002). Several major manufacturers have developed fuel cell systems that will produce enough hydrogen and electricity to power vehicles. The two systems commonly proposed include either an onboard fuel processor (fuel reformer) or the direct hydrogen system. The feedstocks currently preferred for these systems are gasoline or methanol for the onboard processor and natural gas for the direct hydrogen system (U.S. Depart- ment of Energy, 1999). A system using an onboard processor and gasoline is favored by many because a new and wide- spread infrastructure would not be required. The option of an onboard processor and methanol would need additional infrastructure but would be more readily implemented than the direct hydrogen system. However, present fuel reform- ers, which can extract hydrogen from several hydrogen-rich materials, increase the cost of the system, reduce the over- all efficiency, and produce carbon dioxide (Brown, 2002; Schrope, 2001). They also must be heated for several minutes before they can function. These characteristics contribute to a common opinion that fuel reformers would be part of an interim technology. Using the direct hydrogen system for passenger vehicles will require the development of a new national infrastructure for generating and supplying this gas. Hydrogen genera- tors, which can be steam reformers, partial oxidation units, or electrolyzers, will probably be widely distributed and can obtain hydrogen from natural gas, propane, alcohol, gasoline, or water (U.S. Department of Energy, 1999). Evaluations Several practical and fundamental issues concerning the applications of fuel cells have not been resolved. In cold weather, fuel cells can freeze and stop working (Baum, 2002). They are also fragile and could be damaged when powering vehicles on rough roads. Furthermore, the catalysts required in several fuel cell systems are expensive metals of the platinum group. For fuel cells that require pure hydrogen, the associated gas storage devices must be safe, efficient, and relatively inex- pensive. Storing and supplying hydrogen for cells in vehicles and in smaller mechanisms is a current technical problem. For a trip of 373 miles (600 kilometers) in the family car, using hydrogen and without refueling, a conventional pressurized storage tank would need to be much too large for the vehicle (Schrope, 2001). However, promising, newly developed, smaller storage tanks contain either metal hydride alloys, carbon nanotubes, or glass microspheres. Nevertheless, replac- ing most of the present combustion-based, energy-producing mechanisms throughout the world with less conventional, power-producing devices will probably require extensive and costly changes in sources of fuels, in the related national infra- structure for storage and distribution of fuels, and in associated manufacturing. Steele and Heinzel (2001) assumed “that fuel cells have to be designed for operation on hydrocarbon or alcohol fuels Evaluations 5PDF Image | An Introduction to Fuel Cells
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