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Gold/Mining/Energy : SOFC vrs PEM Fuel Cells (Debate Forum) -- Ignore unavailable to you. Want to Upgrade?

To: Scoobah who wrote (23)	7/3/1999 11:09:00 AM
From: Scoobah	Respond to of 79

Basic fuel cell types: (clipped from "Fuel Cell Handbook") Polymer Electrolyte Fuel Cell (PEFC): The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymers) that is an excellent protonconductor. The only liquid in this fuel cell is water; thus, corrosion problems are minimal. Water management in the membrane is critical for efficient performance; the fuel cell must operate under conditions where the byproduct water does not evaporate faster than it is produced because the membrane must be hydrated. Because of the limitation on the operating temperature imposed by the polymer, usually less than 120°C, and because of problems with water balance, an H2-rich gas with minimal or no CO (a poison at low temperature) is used. Higher catalysts loading (Pt in most cases) than those used in PAFCs is required in both the anode and cathode. Alkaline Fuel Cell (AFC): The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature (~250°C), or less concentrated (35-50 wt%) KOH for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amountof CO2 in air must be considered with the alkaline cell. Phosphoric Acid Fuel Cell (PAFC): Concentrated to 100% phosphoric acid is used for the electrolyte in this fuel cell, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids; consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide (1), and the electrocatalyst in both the anode and cathode is Pt. Molten Carbonate Fuel Cell (MCFC): The electrolyte in this fuel cell is usually a combination of alkali carbonates or combination (Na and K), which is retained in a ceramic matrix of LiAlO2. The fuel cell operates at 600 to 700°C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to promote reaction. Noble metals are not required. Solid Oxide Fuel Cell (SOFC): The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3-stabilized ZrO2. The cell operates at 650 to 1000°C where ionic conduction by oxygen ions takes place. Typically, the anode is Co-ZrO2 or Ni-ZrO2 cermet, and the cathode is Sr-doped LaMnO3. In low-temperature fuel cells (PEFC, AFC, PAFC), protons or hydroxyl ions are the major charge carriers in the electrolyte, whereas in the high-temperature fuel cells, MCFC and SOFC, carbonate ions and oxygen ions are the charge carriers, respectively.