Phosphoric acid fuel -cells

Phosphoric acid fuel cell Phosphoric acid H+ 220°C Pure H2 (tolerates C02, approx. 1% CO) 40% Stationary (200kW)... 

Acid Fuel Cells Phosphoric Acid Electrolyte - Fickett, A.P., Fuel Cell Electrolysis Where Have We Failed Proc. of the Symp. on Electrode Materials and Processes for Energy Conversion and Storage, J.D.E. McIntyre, S. Srinivasan and F. G. Will (Eds.), Vol. 77-6, 546-558. Electrochemical Society, Princeton, Mew Jersey, 1977. 

Several types of fuel cells have been developed and are classified according to the electrolytes used alkaline fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells (PAFCs), PEMFCs, and solid oxide fuel cells (SOFCs). As shown in Figure 1.3, the optimum operation temperatures of these fuel cells are different, and each type has different advantages and disadvantages. 

PAFCs. See fuel, cells, phosphoric acid paraffins, 61, 477, 498-500, 511 PC. See coal, pulverized power plants, pulverized coal... 

Oka Y (2009) Fuel Cells—Phosphoric acid fuel cells cells and stacks, encyclopedia of electrochemical power sources 568-578. doi 10.1016/ B978-044452745-5.00277-X... 

There are various types of fuel cells that are under development. The most noticeable ones are polymer electrolyte membrane (PEM) fuel cells, phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), and solid oxide fuel cells (SOFC). PEM fuel cells are mainly being targeted toward transportation needs due to their ability to provide high power densities at reasonable operating temperatures ( 100°C). PAFCs and MCFCs are being developed primarily for stationary applications since their power densities are lower than PEM. SOFCs are currently being developed for both stationary applications and transportation applications but high-temperature material development is needed before they become commercially viable. 

Alkaline fuel cell Phosphoric acid fuel cell... 

Phosphoric acid (H3PO4) is the only common inorganic acid that has enough thermal stability, chemical, and electrochemical stabihty, and low enough volatihty (above about 150°C) to be considered as an electrolyte for fuel cells. Most importantly, phosphoric acid is tolerant to CO2 in the fuel and oxidant, unlike the alkaline fuel cell. Phosphoric acid was therefore chosen by the American company. United Technologies (later the spin-off... 

Fhosphoric acid does not have all the properties of an ideal fuel cell electrolyte. Because it is chemically stable, relatively nonvolatile at temperatures above 200 C, and rejects carbon dioxide, it is useful in electric utility fuel cell power plants that use fuel cell waste heat to raise steam for reforming natural gas and liquid fuels. Although phosphoric acid is the only common acid combining the above properties, it does exhibit a deleterious effect on air electrode kinetics when compared with other electrolytes ( ) including such materials as sulfuric and perchloric acids, whose chemical instability at T > 120 C render them unsuitable for utility fuel cell use. In the second part of this paper, we will review progress towards the development of new acid electrolytes for fuel cells. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Hydrogen use as a fuel in fuel cell appHcations is expected to increase. Fuel cells (qv) are devices which convert the chemical energy of a fuel and oxidant directiy into d-c electrical energy on a continuous basis, potentially approaching 100% efficiency. Large-scale (11 MW) phosphoric acid fuel cells have been commercially available since 1985 (276). Molten carbonate fuel cells (MCFCs) ate expected to be commercially available in the mid-1990s (277). 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

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. 

In a simple version of a fuel cell, a fuel such as hydrogen gas is passed over a platinum electrode, oxygen is passed over the other, similar electrode, and the electrolyte is aqueous potassium hydroxide. A porous membrane separates the two electrode compartments. Many varieties of fuel cells are possible, and in some the electrolyte is a solid polymer membrane or a ceramic (see Section 14.22). Three of the most promising fuel cells are the alkali fuel cell, the phosphoric acid fuel cell, and the methanol fuel cell. 

If an acid electrolyte is used, water is produced only at the cathode. An example is the phosphoric acid fuel cell ... 

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