L16-fuecelll2-1 - Fuel Cells 1 8/19/2005 LECTURE LECTURE...

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Unformatted text preview: Fuel Cells 1 8/19/2005 LECTURE LECTURE LECTURE LECTURE # # # # 16 16 16 16 TOPIC TOPIC TOPIC TOPIC: F u e l C e l l s Fuel Cells 2 8/19/2005 Types of Fuel Cell s Types of fuel cells differ primarily by the type of electrolyte which in turn, determines the operating temperature s High-temperature fuel cells operate at greater than 1100 F (600 C) s These high temperatures permit the spontaneous internal reforming of light hydrocarbon fuels s This reaction occurs at the anode over a nickel catalyst provided that adequate heat is always available s Internal reforming eliminates the need for a separate fuel processor, and can use fuels other than pure hydrogen s These significant advantages lead to an increase in overall efficiency by as much as 15% Fuel Cells 3 8/19/2005 s High-temperature fuel cells also generate high-grade waste heat, which can be used in downstream processes for cogeneration purposes s High-temperature fuel cells react easily and efficiently without an expensive noble metal catalyst, such as platinum s On the other hand, the amount of energy released by the electrochemical reaction degrades as the reaction temperature increases s High-temperature fuel cells suffer from severe materials problems s Only few materials can work for extended periods without degradation within a chemical environment at high temperature Types of Fuel Cell Fuel Cells 4 8/19/2005 s High-temperature operation does not lend itself easily to large-scale operations and is not suitable where quick startup is required s Current high-temperature fuel cells applications have focused on stationary power plants where the efficiencies of internal reforming and co-generative capabilities outweigh the disadvantages of material breakdown and slow startup s High-temperature fuel cells operate at greater than 1100 F (600 C) s The most prominent high-temperature fuel cells are S molten carbonate S solid oxide Fuel Cell Technology Fuel Cells 5 8/19/2005 s Low-temperature fuel cells typically operate below 480 F (250 C) s These low temperatures do not permit internal reforming, and therefore require an external source of hydrogen s They exhibit quick startup, suffer fewer materials problems and are easier to handle in vehicle applications s The most prominent low-temperature fuel cells are S alkaline S phosphoric acid S proton exchange membrane (or solid polymer) Fuel Cell Technology Fuel Cells 6 8/19/2005 s Molten carbonate fuel cells use an electrolyte that conducts carbonate (CO 3 2 ) ions from the cathode to the anode s This is the opposite of many other types of fuel cells, which conduct hydrogen ions from the anode to the cathode s The electrolyte is composed of a molten mixture of lithium and potassium carbonates s This mixture is retained by capillary forces within a ceramic support matrix of lithium aluminate s At the fuel cell operating temperature, the electrolyte structure is a thick paste which provides gas seals at the cell edges s Molten carbonate fuel cells operate at about 1200 F...
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L16-fuecelll2-1 - Fuel Cells 1 8/19/2005 LECTURE LECTURE...

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