Fuel Cell Vehicles (FCV)

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Figure 1 In NECar-5 by Daimler-Benz methanol is reformed to produce hydrogen to run a PEM fuel cell.
Figure 1 In NECar-5 by Daimler-Benz methanol is reformed to produce hydrogen to run a PEM fuel cell.

Fuel cells produce electricity through a chemical reaction between hydrogen and oxygen. The fuel cell was invented in 1839 by the British physicist Sir William Grove but, because of inherent problems associated with its operation, was soon forgotten. It was only in the 1960s that the first fuel cell was first employed aboard Gemini and Apollo capsules to replace heavy batteries used in previous space missions. Ever since, fuel cells have been used in a number of applications such as in airplanes, stationary power generators, and now automobiles.

In 2002, the Honda FCX and Toyota FCHV-4 became the first fuel cell vehicles to enter the market. Daimler-Chrysler has also introduced a limited number of their newest fuel cell model in 2006 (Figure 1).


Principle of Operation

Fuel cell operation can be accurately described as reverse electrolysis. In electrolysis, an electric current is passed across two electrodes immersed in an electrolyte solution, splitting water into its constituents – hydrogen and oxygen. The fuel cell works in the opposite direction; hydrogen and oxygen (usually from air) recombine in an electrochemical reaction to form water and an electric current is produced. The reaction is different from combustion, as it usually occurs at lower temperatures and electricity (instead of heat) is the output. Since fuel cells are not heat engines, their efficiencies can be quite high and are not constrained by Carnot principles. Efficiencies of around 50% are typical.

Figure 2 Principle of operation of a PEM Fuel Cell.
Figure 2 Principle of operation of a PEM Fuel Cell.

The most common fuel cell for passenger cars and light-duty trucks is Proton Exchange Membrane (PEM). It consists of a positive and a negative electrode separated by a membrane or electrolyte (Figure 2). Hydrogen gas is introduced at the negative electrode (anode) where a catalyst strips electrons from the hydrogen atom. Hydrogen ions migrate through the membrane toward the positive polymer electrode (cathode) and react with oxygen to form water. Without the membrane, hydrogen and oxygen mix and react directly in a chemical reaction, generating heat instead of electricity. The membrane works by allowing the hydrogen ions (protons) to pass through while blocking electrons. Electrons are then diverted through an external wire before arriving on the other side where they are recombined with protons and oxygen to complete the reaction. As long as the flow of hydrogen gas is not interrupted, the electrical current is maintained, which can turn a light on or be used to power an electric motor. The PEM operates best at temperatures of around 80o C.

To summarize, the reaction at each electrode and the overall reaction can be written as:

Anode: 2 H2 -> 4 H+ + 4 e-

Cathode: O2 + 4 e- + 4 H+ -> 2 H2O

Overall Reaction: 2 H2 + O2 -> 2 H2O

Besides PEM fuel cells, three other technologies show promise for various transportation applications. The Phosphoric Acid Fuel Cell (PAFC) is most appropriate for locomotives, heavy-duty trucks, and urban transit buses. These fuel cells are already used for stationary power generation. Two other technologies, Solid Oxide and Direct Methanol fuel cells, are mainly of interest for on-board fuel processing and reformer technologies. The major problem with all these fuel cells is that they operate at much higher temperatures and at very high costs.


Fuel cells suitable for a car must be small, have a high power density, refuel in a short time, store sufficient hydrogen for a range of 500 kilometers or better, and be available at a cost comparable to conventional internal combustion engines. When fuel cells use pure hydrogen as fuel, there are no emissions except for the pollutants produced in generating the hydrogen. Producing hydrogen in large quantities and storing it in convenient containers is not always practical, however. Hydrogen has a low volumetric energy density, and storing it requires very high pressures or cryogenic temperatures. Hydrogen can also be stored as metal or chemical hydrides. Compressed gas occupies the largest volume, while hydrides are the heaviest of all options. Storing hydrogen in liquid form is impractical and not an option at this time. To travel 500 kilometers, a fuel-cell car requires three kilograms of hydrogen. If stored at 20 MPa (3000 psia) pressure tanks, it occupies a volume of 190 liters. Furthermore, the current lack of convenient refueling stations is a big impediment in marketing fuel cell vehicles using pure hydrogen.

Rather than relying on storing hydrogen and using it directly, a number of car companies are concentrating on developing on-board reformers. Reformers use a variety of fuels to produce hydrogen by reformation, where it is subsequently used in the fuel cell. Gasoline, methanol, or any other fuel can be used. Reforming natural gas does not seem to be the best option as natural gas is most suitable to be used in combined cycle turbines to generate electricity relatively cleanly and at a high efficiency. Methanol is considered to be a favorite because it is liquid at room temperature and can be produced from biomass. Although methanol has lower energy content than gasoline, the higher efficiency of methanol reformation can compensate for lower energy density and vehicle range remains relatively the same.

Advantages and Disadvantages of Fuel Cell Cars

Figure 3 Fuel leak simulation. Hydrogen-powered vehicle is on the left, and gasoline-powered vehicle is on the right. Notice that after 1 minute, hydrogen flame is subsiding, whereas gasoline flame is engulfing the entire vehicle.
Figure 3 Fuel leak simulation. Hydrogen-powered vehicle is on the left, and gasoline-powered vehicle is on the right. Notice that after 1 minute, hydrogen flame is subsiding, whereas gasoline flame is engulfing the entire vehicle.

The advantages of fuel cells are many. They are efficient, operate virtually vibration-free and silently, have a high energy density, have no moving parts, and produce little or no emissions. Furthermore, fuel cells have no exhaust and thus have a low thermal signature. They can be customized to any shape and for any power demand, which makes these devices ideal for numerous applications. They will eventually replace a major portion of internal combustion engines and power future cell phones, computers, camcorders, and other cordless devices. The major barriers to commercial development of fuel cell cars are the public’s concern over safety and the cost. If hydrogen is used directly as the fuel instead of being produced by an on-board reformer, then other issues such as hydrogen distribution, delivery, and storage would also become a problem.

The issues of hydrogen storage were briefly discussed above, namely, the low density of hydrogen even at very high pressures or at liquid temperatures. At present, hydrogen can be stored at 1-2% by weight, at best. To make fuel cell cars practical, the Department of Energy has set a goal of 62 kg hydrogen per cubic meter for storage space. Metal hydride and carbon nanofibers are two promising technologies.

Concern about hydrogen safety is partially justified, but mostly over-exaggerated. Hydrogen gas is highly explosive, and its flame is invisible. Images of the catastrophic explosion of the hydrogen-propelled “Hindenburg Zeppelin” in 1937 are still alive in many people’s minds, (a) even though some recent studies have shown that the cause of explosion was not the hydrogen, but the electrostatic charge accumulated by an oily substance used to treat the cotton skin of the airship. The lack of flame visibility can be easily resolved with certain additives. New advances in composites allow the design of hydrogen tanks that can withstand major crashes and falls from buildings several stories tall. In addition, hydrogen sensors are available that can detect and shut down hydrogen flow as the smallest leaks are detected. Finally, since hydrogen is the lightest of all gases, hydrogen flames are highly buoyant and spread in the form of jets diffusing upward and away from the body of the car and its occupants, as demonstrated by several crash and fire simulations (See Figure 3) (1).

Lack of a hydrogen distribution and delivery system is another important factor delaying the introduction of fuel cell equipped passenger cars. To make fuel cells viable, a sizable investment in hydrogen generation and distribution infrastructure is required. This is an uphill battle for hydrogen enthusiasts, who have to compete with already established gasoline infrastructure. The United States now has more than a dozen hydrogen stations, mostly in California, used only for demonstration and research purposes.

Fuel cells produce power at considerably higher costs than internal combustion engines-- at around $4 a watt compared to $0.05 a watt. To make fuel cells a viable alternative to batteries and internal combustion engines, their cost must be brought down, they must work virtually maintenance free, and their safety must be improved. This cannot happen unless fuel cell-operated devices can be produced at a fraction of what it costs today. The prices cannot fall unless fuel cells can be mass-produced, which happens only when prices are already low and sufficient infrastructure (such as hydrogen filling stations) is in place - a classic chicken and egg problem (2).


(1) Swain, M. R., “Fuel Leak Simulation”, Proceedings of the 2001 DoE Hydrogen Program Review, NREL/CP-570-30535.

(2) “The Ultimate Incinerators,” Scientific American, September 1995, p. 180.

(3) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Comments

(a) Hindenburg Zeppelin was a giant hydrogen-filled airship (at 245 meters, it was the largest ever to fly) that operated regularly in 1930s taking transatlantic passengers between Germany and both North and South America before it was destroyed by a fire in 1937.

Further Reading

Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004.

Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005.

Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001.

Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003.

Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002.

Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004.

Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science.

Transportation Research Part C: Emerging Technologies, Direct Science Elsevier Publishing Company; this journal focuses on scholarly research on development, application, and implications in the fields of transportation, control systems, and telecommunications, among others.

Fuel Cells Bulletin, Direct Science Elsevier Publishing Company, Fuel Cells Bulletin is the leading source of technical and business news for the fuel cells sector.

International Journal of Hydrogen Energy, Direct Science Elsevier Publishing Company, Quarterly journal covering various aspects of hydrogen energy, including production, storage, transmission, and utilization, as well as economical and environmental aspects.

External Links

US Department of Transportation (http://www.dot.gov).

US Department of Energy (http://www.doe.gov).

US Environmental Protection Agency (http://www.epa.gov).

National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (http://www.nrel.gov/vehiclesandfuels/hev).

FreedomCar (http://www.eere.energy.gov/vehiclesandfuels).