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Molten Carbonate Fuel Cell
www.MoltenCarbonateFuelCell.com
We
Provide Renewable Energy Project Development Services
including; Design, Engineering, Feasibility Studies
& Consulting Services
and Onsite Cogeneration & Trigeneration Power Plants that use
Molten Carbonate Fuel Cells
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direct inquiries to:
Email:
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Tel. (0813527)2775284-103072979
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Molten Carbonate Fuel
Cell
www.MoltenCarbonateFuelCell.com
We provide renewable
energy project development solutions; design, engineering, feasibility studies and consulting services that include
Onsite Cogeneration and Trigeneration power plants that are powered by Hydrogen
Fuel Cells such as Molten Carbonate Fuel Cells.
Hydrogen Fuel Cells
Hydrogen's potential use in fuel and energy applications includes powering vehicles, running turbines or fuel cells to produce electricity, and generating heat and electricity for buildings. The current focus is on hydrogen's use in fuel cells.
A fuel cell works like a battery but does not run down or need recharging. It will produce electricity and heat as long as fuel (hydrogen) is supplied. A fuel cell consists of two electrodes—a negative electrode (or anode) and a positive electrode (or cathode)—sandwiched around an electrolyte. Hydrogen is fed to the anode, and oxygen is fed to the cathode. Activated by a catalyst, hydrogen atoms separate into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they reunite with oxygen and the electrons to produce water and heat. Fuel cells can be used to power vehicles or to provide electricity and heat to buildings.
The primary fuel cell technologies under development are:
Phosphoric Acid Fuel Cells
A phosphoric acid fuel cell (PAFC) consists of an anode and a cathode made of a finely dispersed platinum catalyst on carbon paper, and a silicon carbide matrix that holds the phosphoric acid electrolyte. This is the most commercially developed type of fuel cell and is being used in hotels, hospitals, and office buildings. The phosphoric acid fuel cell can also be used in large vehicles, such as buses.
Proton-Exchange Membrane Fuel Cells
The proton-exchange membrane (PEM) fuel cell uses a fluorocarbon ion exchange with a polymeric membrane as the electrolyte. The PEM cell appears to be more adaptable to automobile use than the PAFC type of cell. These cells operate at relatively low temperatures and can vary their output to meet shifting power demands. These cells are the best candidates for light-duty vehicles, for buildings, and much smaller applications.
Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFC) currently under development use a thin layer of zirconium oxide as a solid ceramic electrolyte, and include a lanthanum manganate cathode and a
nickel-zirconia anode. This is a promising option for high-powered applications, such as industrial uses or central electricity generating stations.
Direct-Methanol Fuel Cells
A relatively new member of the fuel-cell family, the direct-methanol fuel cell
(DMFC) is similar to the PEM cell in that it uses a polymer membrane as an electrolyte. However, a catalyst on the DMFC anode draws hydrogen from liquid methanol, eliminating the need for a fuel reformer.
Molten Carbonate Fuel Cells
The molten carbonate fuel cell uses a molten carbonate salt as the electrolyte. It has the potential to be fueled with coal-derived fuel gases or natural gas.
Alkaline Fuel Cells
The alkaline fuel cell uses an alkaline electrolyte such as potassium hydroxide. Originally used by NASA on space missions, it is now finding applications in hydrogen-powered vehicles.
Regenerative Fuel Cells
This special class of fuel cells produces electricity from hydrogen and oxygen, but can be reversed and powered with electricity to produce hydrogen and oxygen. Hydrogen Fuel
Since the early 19th century, scientists have recognized hydrogen as a potential source of fuel. Current uses of hydrogen are in industrial processes, rocket fuel, and spacecraft propulsion. With further research and development, this fuel could also serve as an alternative source of energy for heating and lighting homes, generating electricity, and fueling motor vehicles. When produced from renewable resources and technologies, such as hydro, solar, and wind energy, hydrogen becomes a renewable fuel.
Composition of Hydrogen
Hydrogen is the simplest and most common element in the universe. It has the highest energy content per unit of weight—52,000 British Thermal Units (Btu) per pound (or 120.7 kilojoules per gram)—of any known fuel. Moreover, when cooled to a liquid state, this low-weight fuel takes up 1/700 as much space as it does in its gaseous state. This is one reason hydrogen is used as a fuel for rocket and spacecraft propulsion, which requires fuel that is low-weight, compact, and has a high energy content.
In a free state and under normal conditions, hydrogen is a colorless, odorless, and tasteless gas. The basic hydrogen (H) molecule exists as two atoms bound together by shared electrons. Each atom is composed of one proton and one orbiting electron. Since hydrogen is about 1/14 as dense as air, some scientists believe it to be the source of all other elements through the process of nuclear fusion. It usually exists in combination with other elements, such as oxygen in water, carbon in methane, and in trace elements as organic compounds. Because it is so chemically active, it rarely stands alone as an element.
When burned (or combined) with pure oxygen, the only by products are heat and water. When burned (or combined) with air, which is about 68% nitrogen, some oxides of nitrogen (Nitrogen Oxides or
NOx) are formed. Even then, burning hydrogen produces less air pollutants relative to fossil fuels.
Producing HydrogenHydrogen bound in organic matter and in water makes up 70% of the earth's surface. Breaking up these bonds in water allows us produce hydrogen and then to use it as a fuel. There are numerous processes that can be used to break these bonds. Described below are a few methods for producing hydrogen that are currently used, or are under research and development.
Most of the hydrogen now produced in the United States is on an industrial scale by the process of steam reforming, or as a byproduct of petroleum refining and chemicals production. Steam reforming uses thermal energy to separate hydrogen from the carbon components in methane and methanol, and involves the reaction of these fuels with steam on catalytic surfaces. The first step of the reaction decomposes the fuel into hydrogen and carbon monoxide. Then a "shift reaction" changes the carbon monoxide and water to carbon dioxide and hydrogen. These reactions occur at temperatures of 392° F (200 ° C) or greater.
Another way to produce hydrogen is by electrolysis. Electrolysis separates the elements of water—H and oxygen (O)—by charging water with an electrical current. Adding an electrolyte such as salt improves the conductivity of the water and increases the efficiency of the process. The charge breaks the chemical bond between the hydrogen and oxygen and splits apart the atomic components, creating charged particles called ions. The ions form at two poles: the anode, which is positively charged, and the cathode, which is negatively charged. Hydrogen gathers at the cathode and the anode attracts oxygen. A voltage of 1.24 Volts is necessary to separate hydrogen from oxygen in pure water at 77° Fahrenheit (F) and 14.7 pounds per square inch pressure [25° Celsius (C) and 1.03 kilograms (kg) per centimeter squared.] This voltage requirement increases or decreases with changes in temperature and pressure.
The smallest amount of electricity necessary to electrolyze one mole of water is 65.3 Watt-hours (at 77° F; 25 degrees C). Producing one cubit foot of hydrogen requires 0.14 kilowatt-hours (kWh) of electricity (or 4.8 kWh per cubic meter).
Renewable energy sources can produce electricity for electrolysis. For example, Humboldt State University's Schatz Energy Research Center designed and built a stand-alone solar hydrogen system. The system uses a 9.2 kilowatt (KW) photovoltaic (PV) array to provide power to compressors that aerate fish tanks. The power not used to run the compressors runs a 7.2 kilowatt bipolar alkaline
electrolyzer. The electrolyzer can produce 53 standard cubic feet of hydrogen per hour (25 liters per minute). The unit has been operating without supervision since 1993. When there is not enough power from the PV array, the hydrogen provides fuel for a 1.5 kilowatt proton exchange membrane fuel cell to provide power for the compressors.
Steam electrolysis is a variation of the conventional electrolysis process. Some of the energy needed to split the water is added as heat instead of electricity, making the process more efficient than conventional electrolysis. At 2,500 degrees Celsius water decomposes into hydrogen and oxygen. This heat could be provided by a concentrating solar energy device. The problem here is to prevent the hydrogen and oxygen from recombining at the high temperatures used in the process.
Thermochemical water splitting uses chemicals such as bromine or iodine, assisted by heat. This causes the water molecule to split. It takes several steps—usually three—to accomplish this entire process.
Photoelectrochemical processes use two types of electrochemical systems to produce hydrogen. One uses soluble metal complexes as a catalyst, while the other uses semiconductor surfaces. When the soluble metal complex dissolves, the complex absorbs solar energy and produces an electrical charge that drives the water splitting reaction. This process mimics photosynthesis.
The other method uses semiconducting electrodes in a photochemical cell to convert optical energy into chemical energy. The semiconductor surface serves two functions, to absorb solar energy and to act as an electrode. Light-induced corrosion limits the useful life of the semiconductor.
Researchers at the University of Tennessee and U.S. Department of Energy's (DOE) Oak Ridge National Laboratory are researching ways to use photosynthesis to produce hydrogen from sunlight. The researchers extracted two photosynthetic complexes from spinach plants; called Photosystem I and Photosystem II. The two work together to produce carbohydrates for the plant. By attaching platinum atoms to the Photosystem I complexes, the researchers were able to produce hydrogen from visible light. Unfortunately, the process required the use of an added chemical that makes the overall process impractical, but the achievement shows potential. The researchers are working to combine the
platinum-Photosystem I complexes with the Photosystem II complexes, forming a molecular system that can convert light and water directly into hydrogen, without help from an added chemical.
Biological and photobiological processes can use algae and bacteria to produce hydrogen. Under specific conditions, the pigments in certain types of algae absorb solar energy. The enzyme in the cell acts as a catalyst to split the water molecules. Some bacteria are also capable of producing hydrogen, but unlike algae they require a substrate to grow on. The organisms not only produce hydrogen, but can clean up pollution as well.
Research funded by DOE has led to the discovery of a mechanism to produce significant quantities of hydrogen from algae. Scientists have known for decades that algae produce trace amounts of hydrogen, but had not found a feasible method to increase the production of hydrogen. Scientists from the University of California
(UC), Berkeley, and the U.S. DOE's National Renewable Energy Laboratory found the key. After allowing the algae culture to grow under normal conditions, the research team deprived it of both sulfur and oxygen, causing it to switch to an alternate metabolism that generates hydrogen. After several days of generating hydrogen, the algae culture was returned to normal conditions for a few days, allowing it to store up more energy. The process could be repeated many times. Producing hydrogen from algae could eventually provide a cost-effective and practical means to convert sunlight into hydrogen.
Another source of hydrogen produced through natural processes is methane and ethanol. Methane (CH4) is a component of "biogas" that is produced by anaerobic bacteria. Anaerobic bacteria occur widely throughout the environment. They break down or "digest" organic material in the absence of oxygen and produce biogas as a waste product. Sources of biogas include landfills, and livestock waste and municipal sewage treatment facilities. Methane is also the principal component of "natural gas" (a major heating and power plant fuel) produced by anaerobic bacteria eons ago. Ethanol is produced by the fermentation of biomass. Most fuel ethanol produced in the United States is made from corn.
Chemical engineers at the University of Wisconsin-Madison have developed a process to produce hydrogen from glucose, a sugar produced by many plants. The process shows particular promise because it occurs at relatively low temperatures, and can produce fuel-cell-grade hydrogen in a single step. Glucose is manufactured in vast quantities from corn starch, but can also be derived from sugar beets or low-cost waste streams like paper mill sludge, cheese whey, corn stover or wood waste.
The United States, Japan, Canada, and France have investigated thermal water splitting, a radically different approach to creating hydrogen. This process uses heat of up to 5,430°F (3,000°C) to split water molecules.
Potential Uses for Hydrogen
When properly stored, hydrogen as a fuel burns in either a gaseous or liquid state. Motor vehicles and furnaces can be converted to use hydrogen as a fuel. Hydrogen has actually been used in the transportation, industrial, and residential sectors in the United States for many years. Many people in the late 19th century burned a fuel called "town gas," which is a mixture of hydrogen and carbon monoxide. Several countries, including Brazil and Germany, still distribute this fuel. Hydrogen was used in early "hot-air" balloons, and later in airships (dirigibles) during the early 1900's. Gaseous hydrogen was used in 1820 as fuel for one of the earliest internal combustion engines. The U.S. Air Force had a secret, multi-million dollar program during the 1950's, code-named "Suntan," to develop hydrogen as a fuel for airplanes. Currently, industries use large quantities of hydrogen for refining petroleum, and for producing ammonia and methanol. The Space Shuttle uses hydrogen as fuel for its rockets. Automobile manufacturers have developed hydrogen-powered cars.
Burning hydrogen creates less air pollution than gasoline or diesel. Hydrogen also has a higher flame speed, wider flammability limits, higher detonation temperature, burns hotter, and takes less energy to ignite than gasoline. This means that hydrogen burns faster, but carries the danger of pre-ignition and flashback. While hydrogen has its advantages as a vehicle fuel it still has a long way to go before it can be used as a substitute for gasoline. This is mainly due to the investment required to develop a hydrogen production and distribution infrastructure.
However, things are getting started in this regard. Vehicle manufacturers Honda and BMW have set up hydrogen fueling stations as part of their efforts to develop fuel cell powered cars. At Honda's research and development center in Torrance, California, a PV array electrolyses hydrogen from water. The array generates enough hydrogen to power one fuel-cell vehicle. Additional power from the power grid is used to increase the hydrogen production capacity. The new station is supporting Honda's fuel cell vehicle development program for hydrogen production, storage, and fueling. Honda and a fuel cell developer are also working together on a "home" hydrogen refueling system for fuel cell vehicles. BMW opened a hydrogen fueling station at the company's engineering and emissions control test center in Oxnard, California. BMW is taking a different approach than most car companies, burning hydrogen directly in advanced internal-combustion engines, and is testing these vehicles at the Oxnard facility.
The California Fuel Cell Partnership (CaFCP) is also building a hydrogen infrastructure. The CaFCP commissioned its first "satellite" hydrogen fueling system in late October 2002, in Richmond, California, about 70 miles from the CaFCP headquarters and a primary refueling facility in West Sacramento. This extends the range over which the CaFCP's prototype fuel cell vehicles can be driven. The fueling system uses electrolysis to generate hydrogen from water and includes a storage unit capable of holding 104 pounds (47 kilograms) of hydrogen. It is capable of fueling a small fleet of vehicles and requires only one or two minutes per refueling.
In November 2002, the world's first hydrogen energy station that can provide fuel for vehicles and also produce electricity opened in Las Vegas Nevada. The station is located in the city's vehicle maintenance and operation service center. It combines an on-site hydrogen generator, compressor, liquid and gaseous hydrogen storage tanks, dispensing systems, and a stationary fuel cell. It is capable of dispensing hydrogen, hydrogen-enriched natural gas, and compressed natural gas. DOE is also working with the city to convert municipal vehicles to operate on hydrogen.
Fuel cells are a type of technology that use hydrogen to produce useful energy. In fuel cells, electrolysis is reversed by combining hydrogen and oxygen through an electrochemical process, which produces electricity, heat, and water. The U.S. space program has used fuel cells to power spacecraft for decades. Fuel cells capable of powering automobiles and buses have been and are being developed. Several companies are developing fuel cells for stationary power generation. Most major automobile manufacturers are developing fuel cell powered automobiles.
Hydrogen could be considered a way to store energy produced from renewable resources such as solar, wind, biomass, hydro, and geothermal. For example, when the sun is shining, solar photovoltaic systems can provide the electricity needed to separate the hydrogen (as described above regarding Humboldt State University's Research Center). The hydrogen could then be stored and burned as fuel, or to operate a fuel cell to generate electricity at night or during cloudy periods.
Storing Hydrogen
In order to use hydrogen on a large scale, safe, practical storage systems must be developed, especially for automobiles. Although hydrogen can be stored as a liquid, it is a difficult process because the hydrogen must be cooled to -423° Fahrenheit (-253° Celsius). Refrigerating hydrogen to this temperature uses the equivalent of 25% to 30% of its energy content, and requires special materials and handling. To cool one pound (0.45 kg) of hydrogen requires 5 kWh of electrical energy.
Hydrogen may also be stored as a gas, which uses less energy than making liquid hydrogen. As a gas, it must be pressurized to store any appreciable amount. For large-scale use, pressurized Hydrogen gas could be stored in caverns, gas fields, and mines. The hydrogen gas could then be piped into individual homes in the same way as natural gas. Though this means of storage is feasible for heating, it is not practical for transportation because the pressurized metal tanks used for storing hydrogen gas for transportation are very expensive.
A potentially more efficient method of storing hydrogen is in hydrides. Hydrides are chemical compounds of hydrogen and other materials. Research is currently being conducted on magnesium hydrides. Certain metal alloys such as magnesium nickel, magnesium copper, and iron titanium compounds, absorb hydrogen and release it when heated. Hydrides, however, store little energy per unit weight. Current research aims to produce a compound that will carry a significant amount of hydrogen with a high energy density, release the hydrogen as a fuel, react quickly, and be cost-effective.
A company in Utah, Power Ball Technologies, has developed a process in which sodium metal is pelletized and encapsulated with polyethylene plastic. The pellets can then be containerized, transported, and then opened in a patented hydrogen generator to produce hydrogen gas. According to the company, each gallon of these pellets is capable of producing 1,307 gallons of hydrogen gas, which is an equivalent hydrogen storage density more than 7 times greater by volume than a compressed hydrogen tank storing hydrogen at 3,000
psi.
The Cost of Hydrogen
Currently the most cost-effective way to produce hydrogen is steam reforming. According to the U.S. Department of Energy, in 1995 the cost was $7.39 per million Btu ($7.00 per
gigajoule) in large plant production. This assumes a cost for natural gas of $2.43 per million Btu ($2.30 per
gigajoule). This is the equivalent of $0.93 per gallon ($0.24 per liter) of gasoline. The production of hydrogen by electrolysis using hydroelectricity at off peak rates costs between $10.55 to $21.10 per million Btu ($10.00 to $20.00 per
gigajoule).
Hydrogen Research in the United States
Recognizing the potential for hydrogen fuel, the U.S. Department of Energy (DOE) and private organizations have funded research and development (R&D) programs for several years. DOE has a major effort to develop hydrogen as a major fuel within the next few decades.
Types
of Fuel Cells
Fuel
cells are classified primarily by the kind of electrolyte they employ. This
determines the kind of chemical reactions that take place in the cell, the
kind of catalysts required, the temperature range in which the cell operates,
the fuel required, and other factors. These characteristics, in turn, affect
the applications for which these cells are most suitable. There are several
types of fuel cells currently under development, each with its own advantages,
limitations, and potential applications.
What
are Molten Carbonate Fuel Cells?
Molten
Carbonate Fuel Cells (MCFC) evolved from work in the 1960's aimed at
producing a fuel cell which would operate directly on coal. While direct
operation on coal seems less likely today, operation on coal-derived fuel
gases or natural gas is viable.
Molten
Carbonate Fuel Cell Design and Operation
Molten
Carbonate Fuel Cells use a molten carbonate salt mixture as its
electrolyte. The composition of the electrolyte varies, but usually consists
of lithium carbonate and potassium carbonate. At the operating temperature of
about 1200°F (650°C), the salt mixture is liquid and a good ionic conductor.
The electrolyte is suspended in a porous, insulating and chemically inert
ceramic (LiA102) matrix.
The
Molten Carbonate Fuel Cell
reactions that occur are:
The
anode process involves a reaction between hydrogen and carbonate ions (CO3=)
from the electrolyte which produces water and carbon dioxide (CO2) while
releasing electrons to the anode. The cathode process combines oxygen and CO2
from the oxidant stream with electrons from the cathode to produce carbonate
ions which enter the electrolyte. The need for CO2 in the oxidant stream
requires a system for collecting CO2 from the anode exhaust and mixing it with
the cathode feed stream.
As
the operating temperature increases, the theoretical operating voltage for a
fuel cell decreases and with it the maximum theoretical fuel efficiency. On
the other hand, increasing the operating temperature increases the rate of the
electrochemical reaction and thus the current which can be obtained at a given
voltage. The net effect for the Molten
Carbonate Fuel Cell is that the real operating voltage is higher than the
operating voltage for the Phosphoric
Acid Fuel Cell at the same current density.
The
higher operating voltage of the Molten
Carbonate Fuel Cell means that more power is available at a higher fuel
efficiency from a Molten Carbonate
Fuel Cell than from a Phosphoric
Acid Fuel Cell of the same electrode area. As size and cost scale roughly
with electrode area, this suggests that a Molten Carbonate Fuel Cell should be
smaller and less expensive than a "comparable" Phosphoric
Acid Fuel Cell.
The
Molten Carbonate Fuel Cell also
produces excess heat at a temperature which is high enough to yield high
pressure steam which may be fed to a turbine to generate additional
electricity. In combined cycle operation, electrical efficiencies in excess of
60% (HHV) have been suggested for mature Molten
Carbonate Fuel Cell systems.
The
Molten Carbonate Fuel Cell
operates at between 1110°F (600°C) and 1200°F (650°C) which is necessary
to achieve sufficient conductivity of the electrolyte. To maintain this
operating temperature, a higher volume of air is passed through the cathode
for cooling purposes.
As
mentioned above, the high operating temperature of the Molten
Carbonate Fuel Cell offers the possibility that it could operate directly
on gaseous hydrocarbon fuels such as natural gas. The natural gas would be
reformed to produce hydrogen within the fuel cell itself.
The
need for CO2 in the oxidant stream requires that CO2 from the spent anode gas
be collected and mixed with the incoming air stream. Before this can be done,
any residual hydrogen in the spent fuel stream must be burned. Future systems
may incorporate membrane separators to remove the hydrogen for recirculation
back to the fuel stream.
At
cell operating temperatures of 1200°F (650°C) noble metal catalysts are not
required. The anode is a highly porous sintered nickel powder, alloyed with
chromium to prevent agglomeration and creep at operating temperatures. The
cathode is a porous nickel oxide material doped with lithium. Significant
technology has been developed to provide electrode structures which position
the electrolyte with respect to the electrodes and maintain that position
while allowing for some electrolyte boil-off during operation. The electrolyte
boil-off has an insignificant impact on cell stack life. A more significant
factor of life expectancy has to do with corrosion of the cathode.
The
Molten
Carbonate Fuel Cell
operating temperature is about 1200°F (650°C). At this temperature the salt
mixture is liquid and is a good conductor. The cell performance is sensitive
to operating temperature. A change in cell temperature from 1200°F (650°C)
to 1110°F (600°C) results in a drop in cell voltage of almost 15%. The
reduction in cell voltage is due to increased ionic and electrical resistance
and a reduction in electrode kinetics. 
Molten
Carbonate Fuel Cells (MCFCs) are currently being developed for natural gas
and coal-based power plants for electrical utility, industrial, and military
applications. Molten Carbonate Fuel
Cells are high-temperature fuel cells that use an electrolyte composed of
a molten carbonate salt mixture suspended in a porous, chemically inert
ceramic lithium aluminum oxide (LiAlO2) matrix. Since they operate
at extremely high temperatures of 650°C (roughly 1,200°F) and above,
non-precious metals can be used as catalysts at the anode and cathode,
reducing costs.
Improved
efficiency is another reason Molten
Carbonate Fuel Cells offer significant cost reductions over Phosphoric
Acid Fuel Cells (PAFCs). Molten
Carbonate Fuel Cells can reach efficiencies approaching 60 percent,
considerably higher than the 37-42 percent efficiencies of a phosphoric acid
fuel cell plant. When the waste heat is captured and used, overall fuel
efficiencies can be as high as 85 percent.
Unlike
alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, Molten
Carbonate Fuel Cells don't require an external reformer to convert more
energy-dense fuels to hydrogen. Due to the high temperatures at which Molten
Carbonate Fuel Cells operate, these fuels are converted to hydrogen within
the fuel cell itself by a process called internal reforming, which also
reduces cost.
Molten
Carbonate Fuel Cells are not prone to carbon monoxide or carbon dioxide
"poisoning" —they can even use carbon oxides as fuel—making them
more attractive for fueling with gases made from coal. Because they are more
resistant to impurities than other fuel cell types, scientists believe that
they could even be capable of internal reforming of coal, assuming they can be
made resistant to impurities such as sulfur and particulates that result from
converting coal, a dirtier fossil fuel source than many others, into hydrogen.
The
primary disadvantage of current Molten
Carbonate Fuel Cell technology is durability. The high temperatures at
which these cells operate and the corrosive electrolyte used accelerate
component breakdown and corrosion, decreasing cell life. Scientists are
currently exploring corrosion-resistant materials for components as well as
fuel cell designs that increase cell life without decreasing performance.
Phosphoric
Acid Fuel Cells
Phosphoric
Acid Fuel Cells use liquid phosphoric acid as an electrolyte—the acid is
contained in a Teflon-bonded silicon carbide matrix—and porous carbon
electrodes containing a platinum catalyst. The chemical reactions that take
place in the cell are shown in the diagram to the right.
The
Phosphoric Acid Fuel Cell
(PAFC)
is considered the "first generation" of modern fuel cells. It is one
of the most mature cell types and the first to be used commercially, with over
200 units currently in use. This type of fuel cell is typically used for
stationary power generation, but some phosphoric
acid fuel cells have been used to power large vehicles such as city buses.
Phosphoric
Acid Fuel Cells are more tolerant of impurities in fossil fuels that have
been reformed into hydrogen than Proton
Exchange Membrane Fuel Cells, which are easily "poisoned" by
carbon monoxide—carbon monoxide binds to the platinum catalyst at the anode,
decreasing the fuel cell's efficiency. They are 85 percent efficient when used
for the co-generation of electricity and heat, but less efficient at
generating electricity alone (37 to 42 percent). This is only slightly more
efficient than combustion-based power plants, which typically operate at 33 to
35 percent efficiency. Phosphoric
acid fuel cells are also less powerful than other fuel cells, given the
same weight and volume. As a result, these fuel cells are typically large and
heavy. Phosphoric acid fuel cells
are also expensive. Like Proton
Exchange Membrane Fuel Cells, Phosphoric
acid fuel cells require an expensive platinum catalyst, which raises the
cost of the fuel cell. A typical phosphoric acid fuel cell costs between
$4,000 and $4,500 per kilowatt to operate.
Alkaline
Fuel Cells
Alkaline
Fuel Cells (AFCs) were one of the first fuel cell technologies developed,
and they were the first type widely used in the U.S. space program to produce
electrical energy and water onboard spacecraft. These fuel cells use a
solution of potassium hydroxide in water as the electrolyte and can use a
variety of non-precious metals as a catalyst at the anode and cathode.
High-temperature Alkaline Fuel Cells
operate at temperatures between 100°C and 250°C (212°F and 482°F).
However, newer AFC designs operate at lower temperatures of roughly 23°C to
70°C (74°F to 158°F)
Alkaline
Fuel Cells' high performance is due to the rate at which chemical
reactions take place in the cell. They have also demonstrated efficiencies
near 60 percent in space applications.
The
disadvantage of this fuel cell type is that it is easily poisoned by carbon
dioxide.
In fact, even the small amount of CO2 in the air can affect this
cell's operation, making it necessary to purify both the hydrogen and oxygen
used in the cell. This purification process is costly. Susceptibility to
poisoning also affects the cell's lifetime (the amount of time before it must
be replaced), further adding to cost.
Cost
is less of a factor for remote locations such as space or under the sea.
However, to effectively compete in most mainstream commercial markets, these
fuel cells will have to become more cost-effective. Alkaline
Fuel Cells have been shown to maintain sufficiently stable operation for
more than 8,000 operating hours. To be economically viable in large-scale
utility applications, these fuel cells need to reach operating times exceeding
40,000 hours, something that has not yet been achieved due to material
durability issues. This is possibly the most significant obstacle in
commercializing this fuel cell technology.
Direct
Methanol Fuel Cells
Most
fuel cells are powered by hydrogen, which can be fed to the fuel cell system
directly or can be generated within the fuel cell system by reforming
hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct
Methanol Fuel Cells (DMFCs), however, are powered by pure methanol, which
is mixed with steam and fed directly to the fuel cell anode.
Direct
Methanol Fuel Cells do not have many of the fuel storage problems typical
of some fuel cells since methanol has a higher energy density than
hydrogen—though less than gasoline or diesel fuel. Methanol is also easier
to transport and supply to the public using our current infrastructure since
it is a liquid, like gasoline.
Direct
Methanol Fuel Cell technology is relatively new compared to that of fuel
cells powered by pure hydrogen, and Direct
Methanol Fuel Cell research and development are roughly 3-4 years behind
that for other fuel cell types.
Proton
Exchange Membrane Fuel Cells - sometime called a 
Polymer
Electrolyte Membrane Fuel Cell — deliver high power density and
offer the advantages of low weight and volume, compared to other fuel cells. Proton
Exchange Membrane Fuel Cells use a solid polymer as an electrolyte and
porous carbon electrodes containing a platinum catalyst. They need only
hydrogen, oxygen from the air, and water to operate and do not require
corrosive fluids like some fuel cells. They are typically fueled with pure
hydrogen supplied from storage tanks or onboard reformers.
Proton
Exchange Membrane Fuel Cells operate at relatively low temperatures,
around 80°C (176°F). Low temperature operation allows them to start quickly
(less warm-up time) and results in less wear on system components, resulting
in better durability. However, it requires that a noble-metal catalyst
(typically platinum) be used to separate the hydrogen's electrons and protons,
adding to system cost. The platinum catalyst is also extremely sensitive to CO
poisoning, making it necessary to employ an additional reactor to reduce CO in
the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel.
This also adds cost. Developers are currently exploring platinum/ruthenium
catalysts that are more resistant to CO.
Proton
Exchange Membrane Fuel Cells are used primarily for transportation
applications and some stationary applications. Due to their fast startup time,
low sensitivity to orientation, and favorable power-to-weight ratio, Proton
Exchange Membrane Fuel Cells are particularly suitable for use in
passenger vehicles, such as cars and buses.
A
significant barrier to using these fuel cells in vehicles is hydrogen storage.
Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the
hydrogen onboard as a compressed gas in pressurized tanks. Due to the low
energy density of hydrogen, it is difficult to store enough hydrogen onboard
to allow vehicles to travel the same distance as gasoline-powered vehicles
before refueling, typically 300-400 miles. Higher-density liquid fuels such as
methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline can be
used for fuel, but the vehicles must have an onboard fuel processor to reform
the methanol to hydrogen. This increases costs and maintenance requirements.
The reformer also releases carbon dioxide (a greenhouse gas), though less than
that emitted from current gasoline-powered engines.
Protonic
Ceramic Fuel Cells
Protonic
Ceramic Fuel Cells (PCFC) are a relatively new type of fuel cell is based
on a ceramic electrolyte material that exhibits high protonic conductivity at
elevated temperatures.
Protonic
Ceramic Fuel Cells share the thermal and kinetic advantages of high
temperature operation at 700 degrees Celsius with molten carbonate and solid
oxide fuel cells, while exhibiting all of the intrinsic benefits of proton
conduction in Proton
Exchange Membrane Fuel Cells and Phosphoric
Acid Fuel Cells.
The
high operating temperature is necessary to achieve very high electrical fuel
efficiency with hydrocarbon fuels. Protonic
Ceramic Fuel Cells can operate at high temperatures and electrochemically
oxidize fossil fuels directly to the anode. This eliminates the intermediate
step of producing hydrogen through the costly reforming process. Gaseous
molecules of the hydrocarbon fuel are absorbed on the surface of the anode in
the presence of water vapor, and hydrogen atoms are efficiently stripped off
to be absorbed into the electrolyte, with carbon dioxide as the primary
reaction product. Additionally, Protonic
Ceramic Fuel Cells have a solid electrolyte so the membrane cannot dry out
as with Proton Exchange
Membrane Fuel Cells, or liquid can't leak out as with Phosphoric
Acid Fuel Cells.
Solid Oxide Fuel Cells
Solid
Oxide Fuel Cells (SOFCs) use a hard, non-porous ceramic compound as the
electrolyte. Since the electrolyte is a solid, the cells do not have to be
constructed in the plate-like configuration typical of other fuel cell types. Solid
Oxide Fuel Cells are expected to be around 50-60 percent efficient at
converting fuel to electricity. In applications designed to capture and
utilize the system's waste heat (co-generation), overall fuel use efficiencies
could top 80-85 percent.
Solid
Oxide Fuel Cells operate at very high temperatures—around 1,000°C
(1,830°F). High temperature operation removes the need for precious-metal
catalyst, thereby reducing cost. It also allows Solid
Oxide Fuel Cells to reform fuels internally, which enables the use of a
variety of fuels and reduces the cost associated with adding a reformer to the
system.
Solid
Oxide Fuel Cells are also the most sulfur-resistant fuel cell type; they
can tolerate several orders of magnitude more sulfur than other cell types. In
addition, they are not poisoned by carbon monoxide (CO), which can even be
used as fuel. This allows Solid Oxide
Fuel Cells to use gases made from coal.
High-temperature
operation has disadvantages. It results in a slow startup and requires
significant thermal shielding to retain heat and protect personnel, which may
be acceptable for utility applications but not for transportation and small
portable applications. The high operating temperatures also place stringent
durability requirements on materials. The development of low-cost materials
with high durability at cell operating temperatures is the key technical
challenge facing this technology.
Scientists
are currently exploring the potential for developing lower-temperature Solid
Oxide Fuel Cells operating at or below 800°C that have fewer durability
problems and cost less. Lower-temperature Solid
Oxide Fuel Cells produce less electrical power, however, and stack
materials that will function in this lower temperature range have not been
identified.
What
are Regenerative Fuel Cells?
Regenerative
Fuel Cells produce electricity from hydrogen and oxygen and generate heat
and water as byproducts, just like other fuel cells. However, Regenerative
Fuel Cells can also use electricity from solar power or some other source
to divide the excess water into oxygen and hydrogen fuel—this process is
called "electrolysis." This is a comparatively young fuel cell
technology being developed by NASA and others.
*
Some of the above information from the Department of Energy website with
permission.
www.MoltenCarbonateFuelCell.com
Renewable Energy Institute
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