The Phosphoric Acid Fuel Cell PAFC

Because of this extreme sensitivity, attention shifted to an acidic system, the phosphoric acid fuel cell (PAFC), for other applications. Although it is tolerant to CO, the need for liquid water to be present to facilitate proton migration adds complexity to the system. It is now a relatively mature technology, having been developed extensively for stationary power usage, and 200 kW units (designed for co-generation) are currently for sale and have demonstrated 40,000 hours of operation. An 11 MW model has also been tested. 

Similarly, the phosphoric acid fuel cell (PAFC), although still selling, is resisting efforts to get its capital costs down to the level at which it could be mass produced, and hence another chapter is eliminated. Reference is, however, made to the part played by the PAEC in evolving the catalysts for the polymer electrolyte fuel cell (PEFC). 

The phosphoric acid fuel cell (PAFC) uses a hydrocarbon as the original fuel, but a steam reformer is used first to convert the hydrocarbon to liydrogen and CO before the gas is pumped into the cell. The reaction involved, taking propane as an example, is... 

The electrolyte in the phosphoric acid fuel cells (PAFCs), which operate at 200 °C, is phosphoric acid and the electrodes are carbon black or graphite plates in which Pt particles are dispersed. The PAFCs are generally fed by natural gas, and CO has to be removed (only a 1 % concentration is accepted) to avoid Pt poisoning. PAFCs are mainly used for combined heat and power generation (up to 200 kW) in... 

In the phosphoric acid fuel cell (PAFC), the acid may react vigorously with the metals and consequently the main problem in the development of this type of cell is the choice of acid-resistant materials. The electrodes are made of platinum mounted on a carbon carrier. The working temperature is about 200°C. 

The phosphoric acid fuel cell (PAFC) has a quite similar construction and components as the PEMFC the electrolyte is liquid phosphoric acid in an inert matrix. The operation temperature of 200°C avoids formation of liquid water and improves CO tolerance of the electrocatalyst. For the catalyst properties, the same requirements are valid as for the PEMFC - nanoparticles with a high surface area and a good dispersion on the carbon carrier material are required. The application of PAFC typically is the combined heat and power supply in the 200-kW power range. 

Different catalysts are used for AFCs. For the hydrogen oxidation reaction, carbon-supported platinum and platinum-palladium catalysts (e.g., noble metal catalysts) are suitable. However, one of the advantages of the AFC compared with acid electrolyte fuel cells, including the phosphoric acid fuel cell (PAFC) and the... 

The stimulus for the alkaline fuel cells was provided by the NASA space program, while that for the phosphoric acid fuel cell (PAFC) came from the public service gas and electricity utilities. The gas utilities, who were... 

The phosphoric acid fuel cell (PAFC) was the first fuel cell to be commercialized and shares some technologies with the PEMFC, such as the porous electrodes and the platinum catalysts. The liquid phosphoric acid allows high operating temperatures, around 200 C. Fuels must be free of carbon monoxide, as with the PEMFCs. With rated power over 50 kW, PAFC systems are used for stationary applications. 

The polymer electrolyte fuel cell, PEFC, and the phosphoric acid fuel cell, PAFC, are acidic fuel cells the PEFC operates in the temperature range below and around 100 °C (PEFC), respectively, and the PAFC at around 200 °C. Hydrogen is the preferred fuel for both types. A PEFC can also be fed by liquid or gaseous methanol, called direct methanol fuel cell, DMFC. Other fuels based on alcohols, e.g., the direct ethanol fuel cell, DEFC, are subject to research. Fuel cell types utilizing an acidic aqueous electrolyte will not be considered here. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

There is a considerable drive to raise the operating temperatures of the PEMFC to above 100 °C. This would raise the system efficiency and dramatically improve the CO tolerance of Pt-based electrocatalysts. If the temperature can be raised to 160 °C, studies in the phosphoric acid fuel cell (PAFC) with 1 to 2% CO indicate... 

Because of the phosphoric acid-hlled catalyst layer, the HT-PEMFC encounters the same limitations as the phosphoric acid fuel cell (PAFC) in developing high performance MEAs. The sluggish oxygen reduction reaction (ORR) kinetics which are caused by the low O2 solubility in phosphoric acid and the strong adsorption... 

Having looked at the most important features that are common to all types of hot fuel cells, we will now look at the three major types of cells in this category, starting with the phosphoric acid fuel cell (PAFC). 

As it can be seen, low-temperature and high-temperature fuel cells can be distinguished. Low-temperature fuel cells are the Alkaline Fuel Cell (AFC), the Polymer Electrolyte Fuel Cell (PEMFC), and the Phosphoric Acid Fuel Cell (PAFC). The high-temperature fuel cells operate in the temperatures region from 500 to 1000 °C two different types have been developed the Molten Carbonate Fuel Cell (MCFC) and the Solid Oxide Fuel Cell (SOFC). They have the ability of using methane as fuel and thus present high inherent generation efficiency (45-60 % for common fuels such as natural gas, 90 % with heat recovery [3]). 

Fuel Cells Using Hydrocarbons - the Phosphoric Acid Fuel Cell (PAFC)... 

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