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it is made of porous graphite. During operation, the pores are filled with liquid water that communicates directly with the coolant stream. Product water flows from the cathode through the pores into the coolant stream (a small pressure gradient between reactant and the coolant stream is needed). The water in the coolant stream is then routed to a reservoir. Removal of water by the porous membrane results in the reactant flow stream being free of any obstructions (liquid water). The flooded pores serve a second purpose of supplying water to the incoming reactant gases and humidifying those gases. This prevents drying of the membrane, a common failure mode, particularly at the anode. Control of the amount of area used to humidify the inlet gases has eliminated the need to pre-humidify the reactant gases. Reasons for removing the water through the porous plate are: (1) there is less water in the spent reactant streams; (2) this approach reduces parasitic power needs of the oxidant exhaust condenser; (3) the cell can operate at high utilizations that further reduce water in the reactant streams; (4) higher temperatures can be used with higher utilizations so that the radiator can be smaller,9 and (5) the control system is simplified. In fact, in-stack water conservation is even more important in arid climates, where there may exist a significant challenge to achieve water balance at the system level without supplying water or refrigerating the exhaust stream. Hand-in-hand with water management goes the thermal management of the stack. Temperatures within the stack must be kept within a narrow range in order to avoid local dehydration and hot- spots as well as local dead zones. This is particularly challenging when one recognizes the narrow temperature zone and the relatively small temperature difference between the cell operating temperature and the ambient temperature. Electrode-Catalyst Layer In intimate contact with the membrane and the backing layer is the catalyst layer. This catalyst layer, integral with its binder, forms the electrode. The catalyst and binder electrode structure is applied either to the membrane or to the backing layer. In either case, the degree of intimacy of the catalyst particles and the membrane is critical for optimal proton mobility. The binder performs multiple functions. In one case, it “fixes” the catalyst particles within a layered structure, while a second function is to contribute to the overall architecture of the electrode. This architecture has a direct bearing on performance. There are two schools of thought on the electrode composition, in particular, the binder. In the original hydrophobic, porous, gaseous electrodes developed by Union Carbide and later advanced by General Electric, the Dow Chemical Company, and others, the binder was polytetrafluoroethylene: a non-wetting component within the electrode itself. The second school of electrode science developed a hydrophyllic electrode in which the binder was perfluorosulfonic acid. The driver for this development was to enhance the membrane/catalyst contact to minimize the platinum loading requirements (11). In most state-of-the-art PEFC membrane electrode assemblies (MEAs), the catalyst is largely embedded in a solution of electrolyte monomer, which provides high solubility for protons as well as oxygen, and thus effective use of the platinum catalyst surface. 9. Higher average temperature operaton is possible because of the reduction of hot spots within the cell. Water will evaporate through the porous plate in the vicinity of a hot spot. Conversely, a local cool spot can produce a concentration of water. This water is quickly removed through the porous plate. 3-5PDF Image | Fuel Cell Handbook (Seventh Edition)
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