Navigation Bar link to Fuel Cell site hompage link to Project Overview Page link to Glossary of Terms
link to Historical Information page link to Alkali Fuel Cells link to Molten Carbonate page Phosphoric Acid page link to Proton Exchange Membrane page link to Solid Oxide page

Phosphoric Acid Fuel Cells

Technology History Applications

The essay below outlines the technology and history of phosphoric acid fuel cells. If you have artifacts, photos, documents, or other materials that would help to improve our understanding of these devices be sure to respond to the questionnaire:
Collecting Fuel Cell History


Phosphoric Acid Fuel Cell Technology

Phosphoric acid fuel cells (PAFC) operate at temperatures around 150 to 200 C (about 300 to 400 degrees F). As the name suggests, PAFCs use phosphoric acid as the electrolyte. Positively charged hydrogen ions migrate through the electrolyte from the anode to the cathode. Electrons generated at the anode travel through an external circuit, providing electric power along the way, and return to the cathode. There the electrons, hydrogen ions and oxygen form water, which is expelled from the cell. A platinum catalyst at the electrodes speeds the reactions.

photo of phosphoric acid fuel cell and development team, about 1965
"Project team for 5 kw fuel cell system, Allis-Chalmers, 1965."
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. Efficiencies of PAFCs average 40 to 50 percent, but this can rise to about 80 percent if the waste heat is reused in a cogeneration system. PAFCs of up to 200 kw capacity are in commercial operation, and units of 11 MW capacity have been tested.



Phosphoric Acid Fuel Cell History
photo of demonstration phosphoric acid fuel cell plant, 1979
The 40 kw phosphoric acid fuel cell demonstration plant in South Windsor, Connecticut, 1979

Experimenters have used acids as electrolytes since the time of William Grove's first gas battery in 1842 – he used sulfuric acid. But phosphoric acid, a poor conductor of electricity, was not as attractive, and PAFCs were slower to develop than other types of fuel cells. In 1961, G. V. Elmore and H. A. Tanner revealed new promise in phosphoric acid electrolytes in their paper "Intermediate Temperature Fuel Cells." They described their experiments using an electrolyte that was 35 percent phosphoric acid and 65 percent silica powder pasted into a Teflon gasket. "Unlike sulfuric," they noted, "phosphoric acid is not reduced electrochemically under cell operating conditions." Also, their PAFC ran on air, rather than pure oxygen. "An acid cell was operated for six months at a current density of 90 [milliamps per square centimeter] and 0.25 v. with no apparent deterioration."

Experiments with sulfuric acid electrolytes were underway in 1963 at both the California Research Corporation and the Surface Processes Research and Development Corporation. PAFCs are almost absent, however, from the papers in George J. Young's two volume compilation of a fuel cell symposia held in 1959 and 1961. In the late 1960s and 1970s, major advances in electrode materials and lingering problems with other types of fuel cells spurred new interest in PAFCs.

In the mid 1960s, the U.S. Army explored the potential for PAFCs that ran on "logistic fuels," meaning fuels commonly available to units in the field. For the Army's tests, a cell was produced by Allis-Chalmers and used an Engelhard Industries steam reformer and an electrical inverter from Varo, Inc. (See photo at top of page.) Engelhard's O. J. Adlhart developed a "plastic-bonded electrolyte" during this program.

Work at Union Carbide by Karl Kordesch and R. F. Scarr yielded a thin electrode made of "carbon paper as substrate and a Teflon-bonded carbon layer as catalyst carrier." An industry partnership known as TARGET, or Team to Advance Research for Gas Energy Transformation, Inc., also supported some significant research. Sponsored primarily by Pratt & Whitney and the American Gas Association, TARGET research led to fuel cell power plants from about 15 kw in 1969 to nearly 5 mw in 1983.



Phosphoric Acid Fuel Cell Applications
photo of bus powered by a phosphoric acid fuel cell, 1996
This bus runs on a phosphoric acid fuel cell.

The energy crises of the 1970s inspired researchers at Los Alamos National Laboratory to begin studying fuel cells. With an eye toward developing electric vehicles, they designed a golf cart powered by a phosphoric acid fuel cell.

H-Power, Georgetown University, and the U.S. Department of Energy adapted a 50 kw Fuji Electric PAFC for transit buses (photo at left), and began running these buses in 1994. Four years later, Georgetown, Nova BUS, and the U.S. Department of Transportation began tests on a bus powered by a 100 kw PAFC from International Fuel Cells Corporation (a joint venture of Toshiba and United Technologies). PAFCs currently require an extended warm-up period, however, so their usefulness in private cars remains limited.

PAFCs have supplied stationary power for nearly 10 years. A model PC25 power plant from ONSI Corp. recently began supplying supplemental power in the new Conde Nast Building at 4 Times Square in New York City. During the next blackout in New York City, when this building remains lighted, it should provide some powerful publicity for fuel cells. Also in New York, the Yonkers Waste Treatment Plant has been powered by a 200 kw ONSI unit since 1997. This plant reforms sewage methane as a fuel, and the stacks have an estimated life of 5 to 6 years (they cost about $100,000 to replace).

The military's interest in PAFCs led in 1993 to a program of purchasing these units for various bases where air quality is an issue. Between 1993 and 1997, 15 PAFCs from International Fuel Cells Corp. were placed in service through this program.

[Editor's note: the above comment about a blackout in New York was written in early 2001. During the blackout that ocurred in August 2003 the fuel cells in the Conde Nast Building apparently functioned as intended. A fuel cell powered police station in Central Park also remained in service. Any role that this publicity may have played in pursuading potential users to adopt fuel cell technology is not yet clear.]


©2004 Smithsonian Institution
(Copyright Statement)


Site Map Sources