1. Fuel Cell
A fuel cell generates electricity from reactions between a fuel and an oxidant, in the presence of an electrolyte.
The fuel cell has three segments sandwiched together: an anode, electrolyte, and cathode. Two chemical reactions occur at the
interfaces of the segments. The result is that fuel is consumed, water or carbon dioxide is released, and an electrical current is
created, which can be used to power electrical devices. There are many types of fuel cells, but they all work by the same principle.
The most important parts of a fuel cell are the the fuel, the electrolyte, the anode catalyst which breaks the fuel into electrons and
ions, and the cathode catalyst, which turns the ions into waste chemicals such as water or carbon dioxide.
A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. To deliver a desired amount of energy, fuel cells can be
combined in a fuel cell “stack.”
The most common type is a hydrogen fuel cell, or “hydrogen–oxygen proton exchange membrane fuel cell” (PEMFC).
Is Fuel Cell Technology Ready for Commercial Use?
The short answer is “yes” – but with some qualifications. Fuel cells have been used for decades in specialized applications. However,
a number of technical issues need to be resolved before fuel cells can become practical for a wider range of uses.
Costs. In 2002, a typical fuel cell system cost $1000 per kilowatt of electric power output. In 2009, the Department of Energy reported
that 80-kW automotive fuel cell system costs in volume production (500,000 units per year) were $61 per kilowatt.
The goal is to reduce the cost below $35 per kilowatt, in order to compete with other current technologies, including gasoline internal
combustion engines. Many companies are working on techniques to reduce the costs, for example by reducing the amount of
platinum needed in the individual cell. Ballard Power Systems has experimented with carbon silk-enhanced catalyst that enables a
30% reduction in platinum without lowering performance.
The production costs of the proton exchange membrane. The Nafion membrane currently costs $565.92/m².
Water and air management. In a typical hydrogen fuel cell, the membrane must be continuously hydrated. This requires that water in
the cell be evaporated at precisely the same rate that it is produced. If the water is evaporated too quickly, the membrane dries,
resistance across it increases, and eventually it will crack, creating a gas “short circuit” where hydrogen and oxygen combine
directly, generating heat that will damage the fuel cell.
If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the
reaction. Various methods to manage fuel cell water are under development, such as electroosmotic pumps that focus on flow
control. As in a combustion engine, a steady ratio between the reactant and oxygen is also necessary to keep the fuel cell operating
efficiently.
Temperature management. A uniform temperature must be maintained throughout the cell to prevent destruction of the cell from
overheating. This is particularly challenging, as a large amount of heat is generated within a fuel cell.
Durability, service life, and special requirements for certain types of cells. Stationary fuel cell applications typically require more than
40,000 hours of reliable operation to be economical, while automotive fuel cells require a 5,000-hour lifespan (the equivalent of
150,000 miles), while operating under extreme temperatures.
Limited carbon monoxide tolerance of the cathode.
What Other Types of Fuel Cells Might Be Viable?
Solid Oxide fuel cells (SOFCs). Unlike fuel cells that can only use hydrogen, solid oxide fuel cells can run on hydrogen, butane,
methanol, and other petroleum products.
Each fuel has its own chemistry. For example, in a methanol fuel cell, a catalyst breaks methanol and water down to carbon dioxide,
hydrogen ions, and free electrons. The hydrogen ions move across the electrolyte to the cathode side, where they react with oxygen
to create water. A lead connected externally between the anode and cathode completes the electrical circuit.
A major disadvantage of the SOFC is that the electrolyte is made of a solid material called yttria stabilized zirconia (YSZ), which is a
good ion conductor but only works at very high temperatures. The standard operating temperature is about 950°C, placing
considerable constraints on the materials that can be used for interconnections. Another disadvantage of running the cell at such a
high temperature is that other unwanted reactions may occur inside the cell; for example, it is common for carbon dust to build up on
the anode, which prevents fuel from reaching the catalyst. Much research is currently being done to find alternatives to YSZ that will
carry ions at a lower temperature.
Molten carbonate fuel cells (MCFCs). Molten carbonate fuel cells operate in a similar manner, except that the electrolyte consists of
liquid (molten) carbonate. Because the electrolyte loses carbonate in the oxidation reaction, the carbonate must be replenished. This
is often done by recirculating the carbon dioxide from the oxidation products to the cathode, where it reacts with the incoming air and
reforms carbonate.
Unlike hydrogen fuel cells, the catalysts in SOFCs and MCFCs are not poisoned by carbon monoxide, due to their much higher
operating temperatures.
MCFCs can be used to reduce the CO2 emissions from coal-fired power plants, as well as gas turbine power plants.
Are Fuel Cells a Recent Discovery?
The principle of fuel cells was discovered by a German scientist, Christian Friedrich Schönbein, in 1838. Based on his work, in 1839
Welsh scientist Sir William Robert Grove built a fuel cell that used materials similar to today’s phosphoric-acid fuel cell.
In 1955, W. Thomas Grubb, a chemist at General Electric, modified the original fuel cell design by using a sulphonated polystyrene
ion-exchange membrane as the electrolyte.
Three years later, another GE chemist, Leonard Niedrach, devised a way to deposit platinum onto the membrane, which served as a
catalyst for the hydrogen oxidation and oxygen reduction reactions. This became known as the “Grubb-Niedrach fuel cell.”
GE developed this technology for NASA and McDonnell Aircraft, leading to its use in Project Gemini, NASA’s second human space
flight program, in 1965-66. This was the first commercial use of a fuel cell.
In 1959, British engineer Francis Thomas Bacon developed the first successful stationary fuel cell. In the same year, a team led by
Harry Ihrig built a 15 kW fuel cell-powered tractor for Allis-Chalmers, which was demonstrated at state fairs across the U.S. The
system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants.
Also in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. And in the
1960s, Pratt and Whitney licensed Bacon’s U.S. patents for use in the space program to supply electricity and drinking water
(hydrogen and oxygen being readily available from spacecraft tanks).
UTC Power was the first company to commercialize a large stationary fuel cell system for use as a co-generation power plant in
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hospitals, universities, and large office buildings. UTC Power continues to market its fuel cell as the 400-kW PureCell.
UTC Power remains the only supplier of fuel cells for NASA’s space vehicles, having supplied the Apollo missions and the Space
Shuttle program. UTC is also developing fuel cells for automobiles, buses, and cell phone towers. The company has demonstrated a
proton exchange automotive fuel cell, the first fuel cell capable of starting under freezing conditions.
How Efficient Are Fuel Cells?
A fuel cell’s efficiency depends on the amount of power drawn from it. As a general rule, the more power drawn, the lower the
efficiency. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the hydrogen energy is converted to
electrical energy, and the remaining 50% is converted to heat.
If propulsion is the goal, the electrical output of the fuel cell must be converted to mechanical power, with a corresponding loss of
efficiency. However, fuel cells can have very high efficiency in converting chemical energy to electrical energy, especially when they
are operated at low power density and use pure hydrogen and oxygen as reactants.
It should be underlined that fuel cells (especially high-temperature cells) can be used as a heat source in conventional heat engines
(e.g., gas turbines). In this case, ultra-high efficiency is predicted (above 70%).
Fuel cells that operate on air (rather than bottled oxygen) need to pressurize and dehumidify the air, which lowers their efficiency to
near that of a gasoline engine.
The tank-to-wheel efficiency of a fuel-cell vehicle is about 45% at low loads, and shows average values of about 36%. The comparable
value for a diesel vehicle is 22%. In 2008, Honda released its FCX Clarity model, powered by a fuel cell stack with a claimed 60%
tank-to-wheel efficiency.
Other losses in efficiency stem from fuel production, transportation, and storage. Fuel cell vehicles that run on compressed hydrogen
may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid
hydrogen.
Over 70% of U.S. electricity used for hydrogen production comes from thermal power, which has an efficiency of only 33% to 48%,
resulting in a net increase in carbon dioxide from using hydrogen in vehicles.
Can Fuel Cells Store Energy?
Fuel cells cannot store energy, but in some applications, fuel cells are combined with electrolyzers and external storage systems to
store energy. The overall efficiency of such plants is between 30% and 50%. While a much cheaper lead-acid battery might return
about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore a better long-term storage
solution.
Solid-oxide fuel cells produce intense heat (up to 800°C). The heat can be captured and used to heat water, raising total efficiency to
80-90%. But this does not consider losses in efficiency during production and distribution.
What Other Applications are Fuel Cells Good For?
Power. Fuel cells are useful as power sources in remote locations, such as spacecraft, remote weather stations, parks, rural
locations, and for military applications.
A fuel cell system running on hydrogen can be compact and lightweight. Because fuel cells have no moving parts and do not involve
combustion, in ideal conditions they can achieve up to 99.9999% reliability, which equates to about one minute of downtime in two
years.
Cogeneration. Micro combined heat and power systems (MicroCHP) for office buildings and factories are now in mass production.
The system generates constant electric power and produces hot air and water from waste heat. Excess power can be sold to the grid.
MicroCHP systems usually produce under 5 kWe for a home or small business. Their low fuel-to-electricity efficiency is practical (e.g.,
15-20%), because most of the energy not converted into electricity is used as heat. Some heat is lost with exhaust gas, as in a normal
furnace; thus the combined heat and power efficiency is still lower than 100%, typically around 80%.
The overall efficiency of a MicroCHP system could be improved by generating maximum electricity and using it to drive a heat pump.
Phosphoric-acid fuel cells (PAFC), the largest segment of today’s CHP products, provide combined efficiencies close to 90% (35-50%
electric, and the remainder as thermal).
Emergency power systems are a type of fuel cell system that may include lighting, generators, and other apparatus to provide backup
resources in a crisis or when regular systems fail. They find uses in a wide variety of settings, from residential homes to hospitals,
scientific laboratories, data centers, for telecommunication equipment, and in modern naval ships.
Uninterrupted power supplies (UPSs) provide emergency power and, depending on the topology, can provide line regulation to
connected equipment by supplying power from a separate source when utility power is unavailable. Unlike a standby generator, the
UPS can provide instant protection.
What Cars Have Used Fuel Cells?
The GM 1966 Electrovan was the automakers’ first attempt at a hydrogen fuel cell-powered vehicle. The Electrovan weighed more
than twice as much as a normal van and could travel up to 70 mph for 30 seconds.
The 2001 Chrysler Natrium used an on-board hydrogen processor to create hydrogen for fuel cells by reacting sodium borohydride
fuel with borax, both of which Chrysler claimed were available in large quantities in the U.S.
The hydrogen produced electric power for near-silent operation with a range of 300 miles, without impinging on passenger space.
Chrysler also developed vehicles that separated hydrogen from gasoline in the vehicle, to reduce emissions without depending on a
nonexistent hydrogen infrastructure, and to avoid large storage tanks.
Are There Any Hydrogen “Gas Stations”?
The first public hydrogen refueling station was opened in Reykjavík, Iceland in April 2003. The station continues to serve three buses
built by DaimlerChrysler for Reykjavík’s public transport system. The station produces its own hydrogen with an electrolyzing unit
produced by Norsk Hydro, and thus does not need refilling; the only raw materials that enter the station are electricity and water.
Royal Dutch Shell is a partner in the project. The station has no roof, to allow any leaked hydrogen to escape into the atmosphere.
Are There Hydrogen Fuel Cells in Our Future?
In 2003, President George Bush proposed the Hydrogen Fuel Initiative (HFI), which was later implemented through the 2005 Energy
Policy Act and the 2006 Advanced Energy Initiative. These acts aimed aimed at developing hydrogen fuel cells and their infrastructure
technologies, with the ultimate goal of producing commercial fuel cell vehicles by 2020. By 2008, the U.S. had contributed $1 billion to
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the project.
In May 2009, the Obama Administration announced that it would “cut off funds” for the development of fuel cell hydrogen vehicles,
since other technologies would lead to a quicker reduction in emissions. The U.S. Secretary of Energy explained that hydrogen
vehicles “will not be practical over the next 10 to 20 years,” citing the challenges in developing a hydrogen distribution infrastructure.
Nevertheless, the U.S. government will continue to fund research related to stationary fuel cells.
In 2005, the Intelligent Energy company in England produced the first working hydrogen-run motorcycle, the ENV (Emission Neutral
Vehicle). The motorcycle can hold enough fuel to run four hours and travel 100 miles in an urban area, at a top speed of 50 mph. In
2004, Honda developed a fuel-cell motorcycle based on the Honda FC Stack.
The Type 212 submarines of the German and Italian navies use fuel cells to enable them to remain submerged for weeks without
surfacing. Boeing and its partners in Europe conducted experimental flight tests in February 2008 of a manned airplane powered only
by a fuel cell and lightweight batteries. The Fuel Cell Demonstrator Airplane used a Proton Exchange Membrane (PEM) fuel
cell/lithium-ion battery hybrid system to power an electric motor, which was coupled to a conventional propeller.
In 2007, the Revolve Eco-Rally demonstrated several fuel cell vehicles on British roads.
Fuel cell-powered race vehicles, designed and built by university students from around the world, competed in the world’s first
hydrogen race series, the 2008 Formula Zero Championship in Rotterdam.
In 2003, the first propeller-driven airplane to be powered entirely by a fuel cell was flown. (The first fuel cell-powered aircraft was the
Space Shuttle.) The fuel cell was a unique FlatStacktm stack design that allowed the fuel cell to be integrated with the aerodynamic
surfaces of the plane.
As part of the California Hydrogen Highway initiative, a series of hydrogen refueling stations were built, and by July 2007 California
had 179 fuel cell vehicles and 25 stations, with 10 more stations planned for assembly. However, three hydrogen fueling stations have
since been decommissioned.
Japan has a hydrogen highway with 12 hydrogen fueling stations in 11 cities, and Canada, Sweden, and Norway also have hydrogen
highways.
Numerous prototype or production cars and buses based on fuel cell technology are being researched or manufactured by auto
manufacturers, including the Honda FCX Clarity, mentioned above.
In January 2007, Daimler AG completed a successful three-year trial in 11 cities with 36 experimental buses powered by Ballard
Power Systems fuel cells. A fleet of Thor buses with UTC Power fuel cells is operated in California by SunLine Transit Agency.
The first Brazilan hydrogen fuel cell bus prototype began operation in São Paulo in 2009. The program will include four buses in all.
What Technical Developments Will Be Needed to Make Fuel Cells Economical?
Low-temperature fuel cell stacks such as proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs), and
phosphoric acid fuel cells (PAFCs) use a platinum catalyst. Hydrogen impurities can create catalyst poisoning in these cells, reducing
activity and efficiency in these low-temperature cells. Thus, high hydrogen purity or higher catalyst densities are required.
Research at Brookhaven National Laboratory may lead to replacement of platinum by a gold-palladium coating that may be less
susceptible to poisoning, improving fuel cell lifespan.
Another method would use iron and sulphur instead of platinum. (This is possible through an intermediate conversion by bacteria.)
This would lower the cost of a fuel cell substantially, as the platinum in a regular fuel cell costs around $1500, and the same amount
of iron costs about $1.50. The concept is being developed by a coalition of the John Innes Centre and the University of Milan-Bicocca.
Meanwhile, recycling fuel cell components, including platinum, will conserve supplies. High-temperature fuel cells, e.g., molten
carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs), do use cheaper materials as catalysts, such as nickel and nickel
oxide. They also do not experience catalyst poisoning by carbon monoxide, so they do not require high-purity hydrogen. They can
use fuels with an existing and extensive infrastructure, such as natural gas, without having to reform it to hydrogen and CO, and then
remove CO.
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