Molten Carbonate Fuel Cells
The essay below
outlines the technology and history of molten carbonate 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:
Molten Carbonate Fuel Cell Technology
In a molten carbonate fuel cell
(MCFC), carbonate salts are the electrolyte.
Heated to 650 degrees C (about 1,200 degrees F), the salts melt
and conduct carbonate ions (CO3) from the cathode to the anode.
At the anode, hydrogen reacts with the ions to produce
water, carbon dioxide, and electrons. The electrons travel through an external circuit, providing electrical power along the way, and return to the cathode.
There, oxygen from air and carbon dioxide recycled from the
anode react with the electrons to form CO3 ions that replenish the electrolyte and
transfer current through the fuel cell.
High-temperature MCFCs can extract hydrogen from a variety of fuels using either an internal or external reformer. They are also less prone
to carbon monoxide "poisoning" than lower temperature fuel cells, which makes
coal-based fuels more attractive for this type of fuel cell. MCFCs work well with catalysts made of nickel, which is much less expensive than platinum.
MCFCs exhibit up to 60 percent efficiency, and this can rise
to 80 percent if the waste heat is utilized for cogeneration.
Currently, demonstration units have produced up to 2 megawatts (MW),
but designs exist for units of 50 to 100 MW capacity.
A component module from a 1966 molten carbonate fuel cell made for the U.S. Army
Two major difficulties with molten carbonate technology puts it at a disadvantage compared to solid oxide cells. One is the complexity of working with a liquid electrolyte rather than a solid. The other stems from
the chemical reaction inside a molten carbonate cell. Carbonate ions from the electrolyte
are used up in the reactions at the anode, making it necessary to compensate by injecting carbon dioxide
at the cathode.
Molten Carbonate Fuel Cell History
A 100 watt molten carbonate fuel cell made for the Army around 1964
by Texas Instruments
Both molten carbonate and solid oxide
fuel cells are high-temperature devices. As such, the technical history of both cells
seems rooted in similar lines of research, with significant divergence appearing
in the late 1950s.
In the 1930s, Emil Baur and H. Preis in Switzerland experimented with high-temperature,
solid oxide electrolytes. They encountered problems with electrical
conductivity and unwanted chemical reactions between the
electrolytes and various gases (including carbon monoxide). The following decade, O. K.
Davtyan of Russia explored this area further, but with little success. By
the late 1950s, Dutch scientists G. H. J. Broers and J. A. A. Ketelaar began building on this
previous work and decided that limitations on solid oxides at that time made short-term
progress unlikely. They focused instead on electrolytes of fused (molten) carbonate salts.
By 1960, they reported making a cell that ran for six months using an electrolyte "mixture
of lithium-, sodium- and / or potassium carbonate, impregnated in a porous sintered disk
of magnesium oxide." However, they found that the molten electrolyte was slowly lost, partly
through reactions with gasket materials. About the same time, Francis T. Bacon was working
with a molten cell using two-layer electrodes on either side of a "free molten"
electrolyte. At least two groups were working with semisolid or "paste" electrolytes and most groups
were investigating "diffusion" electrodes rather than solid ones.
In the mid-1960s, the U.S. Army's Mobility Equipment Research and Development Center (MERDC) at Ft. Belvoir tested several molten carbonate cells made by Texas Instruments (see photo above). These ranged in size from 100 watts to 1,000 watts output and were designed to
run on "combat gasoline" using an external reformer to extract hydrogen. The Army especially
wanted to use fuels already available, rather than a special fuel that might be difficult to supply to field units.
Molten Carbonate Fuel Cell Applications
Molten carbonate fuel cells demand such
high operating temperatures that most applications for this kind of cell are limited
to large, stationary power plants. Yet consumers might benefit from this type of
cell, even if they never see it in their homes. The high operating temperature opens
the opportunity of using waste heat to make steam for space heating, industrial
processing, or in a steam turbine to generate more electricity. Many modern gas-fired
power plants exploit this type of system, called cogeneration.
In the early 1990s, Ishikawajima Heavy
Industries in Japan successfully operated a 1,000 watt molten carbonate fuel cell
power generator for 10,000 continuous hours. Now at least ten Japanese companies are
working on MCFCs. M-C Power Corporation of Illinois installed a 250 kw MCFC unit at
the Miramar Marine Corps Air Station in San Diego (see photo at left) in 1997. The
fuel cell ran briefly, producing about 160 mwh and generating steam for use on the
base. In spring 1999, the company installed a new 75 kw stack at Miramar and began a
test program intended to gradually scale up the installation--ultimately intending to
test a 300 kw prototype commercial plant.
MC-Power molten carbonate fuel cell power plant at Miramar, 1997
In 1996-97, Fuel Cell Energy Inc. (then Energy Research Corp.) operated a 2 mw
MCFC demonstration plant in Santa Clara, California. The 3,000-hour test program was
cosponsored by the U.S. Department of Energy and the electric industry research group
EPRI. The company hopes to construct units as large as 3 mw. More recently, Southern
Co., a large electric utility, announced a cooperative project with Mercedes Benz US
International to construct a 250 kw MCFC plant at Mercedes's new museum and visitor
center in Tuscaloosa, Alabama.
Large, stationary plants like these hold the promise of reducing the load on America's
stressed transmission grid. Placing power plants nearer consumers, a concept called
distributed generation, should improve transmission reliability and efficiencyespecially
if the plants are clean and quiet. As the electric industry is deregulated and utilities
become more reluctant to invest in new transmission lines, molten carbonate fuel cells may come to seem an increasingly attractive option.
©2001 Smithsonian Institution