Phosphoric Acid Fuel Cells (PAFC)

Phosphoric Acid Fuel Cells
Phosphoric Acid Fuel Cells (PAFC) were the first fuel cells to be commercialized. Developed in the mid-1960s and field-tested since the 1970s, they have improved significantly in stability, performance, and cost. Such characteristics have made the PAFC a good candidate for early stationary applications.

Fuel cell operating temperature, ranges from 150˚C to 220˚C. The PAFC operates at greater than 40-45% efficiency in generating electricity. When operating in cogeneration applications, the overall efficiency is approximately 85%. Furthermore, at the operating temperature of PAFCs, the waste heat is capable of heating hot water or generating steam at atmospheric pressure.

PAFCs of up to 200 kw capacity are in commercial operation, and units of 11 MW capacity have been tested.

The charge carrier in this type of fuel cell is the hydrogen ion (H⁺, proton). This is similar to the PEFC where the hydrogen introduced at the anode is split into its protons and electrons. The protons migrate through the electrolyte and combine with the oxygen, usually from air, at the cathode to form water. The electrons are routed through an external circuit where they can perform useful work. This set of reactions in the fuel cell produces electricity and by-product heat.

Anode Reaction:
2 H₂ => 4 H⁺ + 4 e^-

Cathode Reaction:
O₂(g) + 4 H⁺ + 4 e^- => 2 H₂O

Overall Cell Reaction:
2 H₂ + O₂ => 2 H₂O

The formation of carbon monoxide (CO) around electrodes can "poison" a fuel cell. One advantage of PAFC cells is that at 200 degrees C they tolerate a CO concentration of about 1.5 percent. Another advantage is that concentrated phosphoric acid electrolyte can operate above the boiling point of water, a limitation on other acid electrolytes that require water for conductivity. The acid requires, however, that other components in the cell resist corrosion. Hydrogen for the fuel cell is extracted from a hydrocarbon fuel in an external reformer. If the hydrocarbon fuel is gasoline, sulfur must be removed or it will damage the electrode catalyst.

This system is already commercially available, with operational installations in buildings, hotels, hospitals and electric utilities. Also in New York, the Yonkers Waste Treatment Plant has been powered by a 200 kw ONSI unit since 1997.

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