Phosphoric Acid Fuel Cells
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
Acid Fuel Cell Technology
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.
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
team for 5 kw fuel cell system, Allis-Chalmers, 1965."
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.
Acid Fuel Cell History
kw phosphoric acid fuel cell demonstration plant in
South Windsor, Connecticut, 1979
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
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.
Acid Fuel Cell Applications
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
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.]