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NFCRC Tutorial: Phosphoric Acid Fuel Cell (PAFC)

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The electrolyte consists of concentrated phosphoric acid and a silicon carbide matrix is used to retain the acid while both the electrodes which also function as catalysts are made from Pt or its alloys. The operating temperature is maintained between 300 to 430 deg F or 150 to 220 deg C, at lower temperatures, phosphoric acid tends to be a poor ionic conductor and CO poisoning of the Pt electrocatalyst in the anode becomes severe.

The phosphoric acid fuel cell has demonstrated its use in a number of commercial building. Worldwide, there are over 40 MW of demonstrators in various phases of testing or under fabrication, mostly in the 50 to 200 kW capacity range. However, larger plants of 1 MW and 5 MW output are also being constructed. The major industrial participant in the U.S is International Fuel Cells Corporation, and in Japan are Fuji Electric Corporation, Toshiba Corporation, and Mitsubishi Electric Corporation.

The electrochemical reactions occurring in a PAFC are:
at the anode:
H2 = 2H+ + 2e-
at the cathode:
1/2O2 + 2H+ + 2e- = H2O
with the overall cell reaction: l/2O2 + H2 = H20

Note that the fuel cell operates on H2 while CO is a poison when present in a concentration greater than 0.5%. If a hydrocarbon such as natural gas is used as a fuel, reforming of the fuel by the reaction:
CH4 + H2O = 3H2 + CO,
and shift of the reformate by the reaction:
CO + H2O = H2 + CO2
are required to generate the required fuel by the cell. Any sulfur compounds present in the fuel have to be removed prior to use in the cell (upstream of the reformer)to a concentration of <0.1 ppmV. The fuel cell itself, however, can tolerate a maximum of 50 ppmV of sulfur compounds. There is a limit on the CO concentration in the fuel gas also which is 0.5%.

Temperatures of about 390 deg F or 200 deg C and acid concentrations of 100% H3PO4 are commonly used now while the operating pressure has exceeded 8 atm in a 11 MW electric utility demonstration plant.

The porous electrodes used in PAFCs contain a mixture of the electrocatalyst supported on carbon black and a polymeric binder to bind the carbon black particles together forming an integral structure A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The composite structure consisting of a carbon black/binder layer on carbon paper substrate forms a three phase interface, with the electrolyte on one side and the reactant gases on the other side of the carbon paper.

Several designs for the bipolar plate and stack components are being used. For example, in the multicomponent bipolar plates, a thin impervious plate serves to separate the reactant gases in adjacent cells within the stack, and separate porous plates with ribbed channels direct the gas flow. The porous structure allows rapid gas permeability and also provides storage for additional acid to replace the acid lost by evaporation during the operating life of the cell.

Heat generated during cell operation is removed by either liquid or gas coolants which are routed through cooling channels in the cell stack. Complex manifolds and connections are required for liquid cooling but better heat transfer is achieved than with air cooling. Gas cooling on the other hand has simplicity, reliability, and relatively low cost.

The conversion efficiency of fuel bound energy to electricity of a PAFC is typically 40 to 47% on a fuel (natural gas) LHV basis.

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