AC Generator (or Alternator)
An electric device that produces an electric current that reverses direction many times per second. Also called a synchronous generator.


The adhesion of the molecules of gases, dissolved substances, or liquids to the surface of the solids or liquids with which they are in contact.


The mixture of oxygen, nitrogen, and other gases that, with varying amounts of water vapor, forms the atmosphere of the earth.

Alkaline Fuel Cell (AFC)

A type of hydrogen/oxygen fuel cell in which the electrolyte is concentrated potassium hydroxide (KOH) and the hydroxide ions (OH-) are transported from the cathode to the anode.


Mixture containing mostly metals. For example, brass is an alloy of copper and zinc. Steel contains iron and other metals but also carbon.

Alternating Current (AC)

A type of current that flows from positive to negative and from negative to positive in the same conductor.

Alternative Fuel

An alternative to gasoline or diesel fuel that is not produced in a conventional way from crude oil. Examples include compressed natural gas (CNG), liquefied petroleum gas (LPG), liquefied natural gas (LNG), ethanol, methanol, and hydrogen.

Ambient Air

The air surrounding a given object or system.

Ambient Temperature

The temperature of the surrounding medium, usually used to refer to the temperature of the air in which a structure is situation or a device operates.


A negatively charged ion; an ion that is attracted to the anode.


The electrode at which oxidation (a loss of electrons) takes place. For fuel cells and other galvanic cells, the anode is the negative terminal; for electrolytic cells (where electrolysis occurs), the anode is the positive terminal.

Atmospheric Pressure

The force exerted by the movement of air in the atmosphere, usually measured in units of force per area. For fuel cells, atmospheric pressure is usually used to describe a system where the only pressure acting on the system is from the atmosphere; no external pressure is applied.


The smallest physical unit of a chemical element that can still retain all the physical and chemical properties of that element. Atoms combine to form molecules, and they themselves contain several kinds of smaller particles. An atom has a dense central core (the nucleus) consisting of positively charged particles (protons) and uncharged particles (neutrons). Negatively charged particles (electrons) are scattered in a relatively large space around this nucleus and move about it in orbital patterns at extremely high speeds. An atom contains the same number of protons as electrons and thus is electrically neutral (uncharged) and stable under most conditions.



An energy storage device that produces electricity by means of chemical action. It consists of one of more electric cells each of which has all the chemicals and parts needed to produce an electric current.

Bipolar Plates

The Conductive plate in a fuel cell stack that acts as an anode for one cell and a cathode for the adjacent cell. The plate may be made of metal of a conductive polymer (which may be a carbon-filled composite). The plate usually incorporates flow channels for the fluid feeds and may also contain conduits for heat transfer.

British Thermal Unit (Btu)

The mean British thermal unit is 1/180 of the heat required to raise the temperature of one pound (1 lb) of water from 32*F to 212*F at a constant atmospheric pressure. The Btu is equal to the quantity of heat required to raise one pound (1 lb) of water 1*F.


Carbon (C)

An atom and primary constituent of hydrocarbon fuels. Carbon is routinely left as a black deposit on engine part, such as pistons, rings, and valves, by the combustion of fuel.

Carbon Dioxide (CO2)

A colorless, odorless, noncombustible gas that is slightly more than 1.5 times as dense as air and becomes a solid (dry ice) below -78.5*C. It is present in the atmosphere as a result of the decay of organic material and the respiration of living organisms. It is produced by the burning of wood, coal, coke, oil, natural gas, or other fuels containing carbon.

Carbon Monoxide

A colorless, odorless, tasteless, poisonous gas that results from incomplete combustion of carbon with oxygen.


A chemical substance that increases the rate of a reaction without being consumed; after the reaction, it can potentially be recovered from the reaction mixture and is chemically unchanged. The catalyst lowers the activation energy required, allowing the reaction to proceed more quickly or at a lower temperature. In a fuel cell, the catalyst facilitates the reactions of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the membrane in the fuel cell.

Catalyst Poisoning

The process of impurities binding to a fuel cell’s catalyst, lowering the catalyst’s ability to facilitate the desired chemical reaction. See also: fuel cell poisoning.


The electrode at which reduction (a gain of electrons) occurs. For fuel cells and other galvanic cells, the cathode is the positive terminal; for electrolytic cells (where electrolysis occurs), the cathode is the negative terminal.


A positively charged ion.


The metric temperature scale and unit of temperature (*C). Named for Swedish astronomer Anders Celsius (1701-1744) even though the thermometer first advocated by him in 1743 has 100* as the freezing point of water and 0* as the boiling point, the reverse of the modern Celsius scale. Also called the Centrigrade scale (Latin for “hundred degrees”).

Centimeter (cm)

A metric unit of linear measure. One centimeter equals about 0.4 inch, and one inch equals about 2.5 centimeters. One foot is equal to approximately 30 centimeters.


The burning fire produced by the proper combination of fuel, heat, and oxygen. In the engine, the rapid burning of the air-fuel mixture that occurs in the combustion chamber.

Combustion Chamber

In an internal combustion engine, the space between the top of the piston and the cylinder head in which the air-fuel mixture is burned.


Material created by combining materials differing in composition or form on a macroscale to obtain specific characteristics and properties. The constituents retain their identity; they can be physically identified, and they exhibit an interface among one another.

Compressed Hydrogen Gas (CHG)

Hydrogen gas compressed to a high pressure and stored at ambient temperature.

Compressed Natural Gas (CNG)

Mixtures of hydrocarbon gases and vapors, consisting principally of methane in gaseous form that has been compressed.


A device used for increasing the pressure and density of gas. Also see Turbocharger.

Cryogenic Liquefaction

The process through which gases such as nitrogen, hydrogen, helium, and natural gas are liquefied under pressure at very low temperatures.

Current Collector

The conductive material in a fuel cell that collects electrons (on the anode side) or disburses electrons (on the cathode side). Current collectors are microporous (to allow fluid to flow through them) and lie in between the catalyst/electrolyte surfaces and the bipolar plates.



The amount of mass in a unit volume. Density varies with temperature and pressure.

Direct Methanol Fuel Cell (DMFC)

A type of fuel cell in which the fuel is methanol (CH3OH) in gaseous or liquid form. The methanol is oxidized directly at the anode instead of first being reformed to produce hydrogen. The electrolyte is typically a PEM.


The spatial property of being scattered over an area or volume.



A conductor through which electrons enter or leave an electrolyte. Batteries and fuel cells have a negative electrode (the anode) and a positive electrode (the cathode).


A process that uses electricity, passing through an electrolytic solution or other appropriate medium, to cause a reaction that breaks chemical bonds (e.g., electrolysis of water to produce hydrogen and oxygen).


A substance that conducts charged ions from one electrode to the other in a fuel cell, battery, or electrolyzer.


A stable atomic particle that has a negative charge; the flow of electrons through a substance constitutes electricity.

Emission Standards

Regulatory standards that govern the amount of a given pollutant that can be discharged into the air from a given source.


A chemical reaction that absorbs or requires energy (usually in the form of heat).


The quantity of work a system or substance is capable of doing, usually measured in British thermal units (Btu) or Joules (J).

Energy Content

Amount of energy for a given weight of fuel.

Energy Density

Amount of potential energy in a given measurement of fuel. See Gravimetric Energy Density and Volumetric Energy Density.


A machine that converts heat energy into mechanical energy.

Ethanol (CH3CH2OH)

An alcohol containing two carbon atoms. Ethanol is a clear, colorless liquid and is the same alcohol found in beer, wine, and whiskey. Ethanol can be produced from cellulosic materials or by fermenting a sugar solution with yeast.

Exhaust Emissions

Materials emitted into the atmosphere through any opening downstream of the exhaust ports of an engine, including water, particulates, and pollutants.


A chemical reaction that gives off heat.



A temperature scale and unit of temperature (°F) named for German physicist Gabriel Daniel Fahrenheit (1686–1736), who was the first to use mercury as a thermometric fluid in 1714.

Flammability Limits

The flammability range of a gas is defined in terms of its lower flammability limit (LFL) and its upper flammability limit (UFL). Between the two limits is the flammable range in which the gas and air are in the right proportions to burn when ignited. Below the lower flammability limit, there is not enough fuel to burn. Above the higher flammability limit, there is not enough air to support combustion.


The lowest temperature under very specific conditions at which a substance will begin to burn.

Flexible Fuel Vehicle

A vehicle that can operate on a wide range of fuel blends (e.g., blends of gasoline and alcohol) that can be put in the same fuel tank.


A material used to create heat or power through conversion in such processes as combustion or electrochemistry.

Fuel Cell

A device that produces electricity through an electrochemical process, usually from hydrogen and oxygen.

Fuel Cell Poisoning

The lowering of a fuel cell’s efficiency due to impurities in the fuel binding to the catalyst.

Fuel Cell Stack

Individual fuel cells connected in a series. Fuel cells are stacked to increase voltage.

Fuel Processor

Device used to generate hydrogen from fuels such as natural gas, propane, gasoline, methanol, and ethanol for use in fuel cells.



Fuel gas such as natural gas, undiluted liquefied petroleum gases (vapor phase only), liquefied petroleum gas-air mixtures, or mixtures of these gases.

  • Natural Gas—Mixtures of hydrocarbon gases and vapors consisting principally of methane(CH4) in gaseous form.
  • Liquefied Petroleum Gases (LPG)—Any material composed predominantly of any of the following hydrocarbons or mixtures of them: propane, propylene, butanes (normal butane or isobutane) and butylenes.
  • Liquefied Petroleum Gas-Air Mixture—Liquefied petroleum gases distributed at relatively low pressures and normal atmospheric temperatures that have been diluted with air to produce desired heating value and utilization characteristics.

Gas Diffusion

Mixing of two gases caused by random molecular motions. Gases diffuse very quickly, liquids diffuse much more slowly, and solids diffuse at very slow (but often measurable) rates. Molecular collisions make diffusion slower in liquids and solids.


Mineral consisting of a form of carbon that is soft, black, and lustrous and has a greasy feeling. Graphite is used in pencils, crucibles, lubricants, paints, and polishes.

Gravimetric Energy Density

Potential energy in a given weight of fuel.

Greenhouse Effect

Warming of the Earth’s atmosphere due to gases in the atmosphere that allow solar radiation (visible, ultraviolet) to reach the Earth’s atmosphere but do not allow the emitted infrared radiation to pass back out of the Earth’s atmosphere.

Greenhouse Gas (GHG)

Gases in the Earth’s atmosphere that contribute to the greenhouse effect.


Heat Exchanger

Device (e.g., a radiator) that is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.

Heating Value (TOTAL)

The number of British thermal units (Btu) produced by the combustion of one cubic foot of gas at constant pressure when the products of combustion are cooled to the initial temperature of the gas and air, when the water vapor formed during combustion is condensed, and when all the necessary corrections have been applied.

  • Lower (LHV)—The value of the heat of combustion of a fuel measured by allowing all products of combustion to remain in the gaseous state. This method of measure does not take into account the heat energy put into the vaporization of water (heat of vaporization).
  • Higher (HHV)—The value of the heat of combustion of a fuel measured by reducing all of the products of combustion back to their original temperature and condensing all water vapor formed by combustion. This value takes into account the heat of vaporization of water.

Higher Heating Value (HHV)

See heating value.

Hybrid Electric Vehicle (HEV)

A vehicle combining a battery-powered electric motor with a traditional internal combustion engine. The vehicle can run on either the battery or the engine or both simultaneously, depending on the performance objectives for the vehicle.


Chemical compounds formed when hydrogen gas reacts with metals. Used for storing hydrogen gas.

Hydrocarbon (HC)

An organic compound containing carbon and hydrogen, usually derived from fossil fuels, such as petroleum, natural gas, and coal.

Hydrogen (H2)

Hydrogen (H) is the most abundant element in the universe, but it is generally bonded to another element. Hydrogen gas (H2) is a diatomic gas composed of two hydrogen atoms and is colorless and odorless. Hydrogen is flammable when mixed with oxygen over a wide range of concentrations.

Hydrogen-Rich Fuel

A fuel that contains a significant amount of hydrogen, such as gasoline, diesel fuel, methanol (CH3OH), ethanol (CH3CH2OH), natural gas, and coal.



Undesirable foreign material(s) in a pure substance or mixture.

Internal Combustion Engine (ICE)

An engine that converts the energy contained in a fuel inside the engine into motion by combusting the fuel. Combustion engines use the pressure created by the expansion of combustion product gases to do mechanical work.


Atom or molecule that carries a positive or negative charge because of the loss or gain of electrons.


Kilogram (kg)

Metric unit of weight or mass equal to approximately 2.2 lb. Related units are the milligram (mg) at 1,000,000 per kg and the metric tonne at 1,000 kg.

Kilowatt (kW)

A unit of power equal to about 1.34 horsepower or 1,000 watts.



See liquefied hydrogen.

Liquefied Hydrogen (LH2)

Hydrogen in liquid form. Hydrogen can exist in a liquid state but only at extremely cold temperatures. Liquid hydrogen typically has to be stored at -253°C (-423°F). The temperature requirements for liquid hydrogen storage necessitate expending energy to compress and chill the hydrogen into its liquid state.

Liquefied Natural Gas (LNG)

Natural gas in liquid form. Natural gas is a liquid at -162°C (-259°F) at ambient pressure.

Liquefied Petroleum Gas (LPG)

Any material that consists predominantly of any of the following hydrocarbons or mixtures of hydrocarbons: propane, propylene, normal butane, isobutylene, and butylenes. LPG is usually stored under pressure to maintain the mixture in the liquid state.


A substance that, unlike a solid, flows readily but, unlike a gas, does not tend to expand indefinitely.

Lower Heating Value

See heating value.


Mechanical Energy

Energy in a mechanical form.

Megawatt (MW)

A unit of power equal to one million watts or 1,000 kilowatts.


The separating layer in a fuel cell that acts as electrolyte (an ion-exchanger) as well as a barrier film separating the gases in the anode and cathode compartments of the fuel cell.

Meter (m)

Basic metric unit of length equal to 3.28 feet, 1.09 yards, or 39.37 inches. Related units are the decimeter (dm) at 10 per meter, the centimeter (cm) at 100 per meter, the millimeter (mm) at 1,000 per meter, and the kilometer (km) at 1,000 meters.

Methane (CH4)

See natural gas.

Methanol (CH3OH)

An alcohol containing one carbon atom. It has been used, together with some of the higher alcohols, as a high-octane gasoline component and is a useful automotive fuel.

Miles Per Gallon Equivalent (MPGE)

Energy content equivalent to that of a gallon of gasoline (114,320 Btu).

Millimeter (mm)

Metric unit of length equal to 0.04 inches. There are 25 millimeters in an inch and 1,000 millimeters in a meter.

Milliwatt (mW)

A unit of power equal to one-thousandth of a watt.

Molten Carbonate Fuel Cell (MCFC)

A type of fuel cell that contains a molten carbonate electrolyte. Carbonate ions (CO3-2) are transported from the cathode to the anode. Operating temperatures are typically near 650°C.



Sulfonic acid in a solid polymer form that is usually the electrolyte of PEM fuel cells.

Natural Gas

A naturally occurring gaseous mixture of simple hydrocarbon components (primarily methane) used as a fuel.

Nitrogen (N2)

A diatomic colorless, tasteless, odorless gas that constitutes 78% of the atmosphere by volume.

Nitrogen Oxides (NOx)

Any chemical compound of nitrogen and oxygen. Nitrogen oxides result from high temperature and pressure in the combustion chambers of automobile engines and other power plants during the combustion process. When combined with hydrocarbons in the presence of sunlight, nitrogen oxides form smog. Nitrogen oxides are basic air pollutants; automotive exhaust emission levels of nitrogen oxides are regulated by law.



A chemical, such as oxygen, that consumes electrons in an electrochemical reaction.


Loss of one or more electrons by an atom, molecule, or ion.

Oxygen (O2)

A diatomic colorless, tasteless, odorless, gas that makes up about 21% of air.


Partial Oxidation

Fuel reforming reaction where the fuel is oxidized partially to carbon monoxide and hydrogen rather than fully oxidized to carbon dioxide and water. This is accomplished by injecting air with the fuel stream prior to the reformer. The advantage of partial oxidation over steam reforming of the fuel is that it is an exothermic reaction rather than an endothermic reaction and therefore generates its own heat.

Pascal (Pa)

The Pascal is the International System of Units (SI)-derived unit of pressure or stress. It is a measure of perpendicular force per unit area. It is equivalent to one newton per square meter. A megapascal equals 1,000,000 Pascals.


Ability of a membrane or other material to permit a substance to pass through it.

Phosphoric Acid Fuel Cell (PAFC)

A type of fuel cell in which the electrolyte consists of concentrated phosphoric acid (H3PO4). Protons (H+) are transported from the anode to the cathode. The operating temperature range is generally 160°C–220°C.


Natural or synthetic compound composed of repeated links of simple molecules.

Polymer Electrolyte Membrane (PEM)

A fuel cell incorporating a solid polymer membrane used as its electrolyte. Protons (H+) are transported from the anode to the cathode. The operating temperature range is generally 60°C–100°C.

Polymer Electrolyte Membrane Fuel Cell (PEMFC or PEFC)

A type of acid-based fuel cell in which the transport of protons (H+) from the anode to the cathode is through a solid, aqueous membrane impregnated with an appropriate acid. The electrolyte is a called a polymer electrolyte membrane (PEM). The fuel cells typically run at low temperatures (<100°C).

Propane (C3H8)

See LPG.


A subatomic particle in the nucleus of an atom that carries a positive electric charge and is not movable by electrical means.

Proton Exchange Membrane (PEM)

See polymer electrolyte membrane.



A chemical substance that is present at the start of a chemical reaction.


Device or process vessel in which chemical reactions (e.g., catalysis in fuel cells) take place.


Hydrocarbon fuel that has been processed into hydrogen and other products for use in fuel cells.


Device used to generate hydrogen from fuels such as natural gas, propane, gasoline, methanol, and ethanol for use in fuel cells.


A chemical process in which hydrogen-containing fuels react with steam, oxygen, or both to produce a hydrogen-rich gas stream.

Reformulated Gasoline

Gasoline that is blended so that, on average, it reduces volatile organic compounds and air toxics emissions significantly relative to conventional gasolines.

Regenerative Fuel Cell

A fuel cell that produces electricity from hydrogen and oxygen and can use electricity from solar power or some other source to divide the excess water into oxygen and hydrogen fuel to be re-used by the fuel cell.

Renewable Energy

A form of energy that is never exhausted because it is renewed by nature (within short time scales; e.g., wind, solar radiation, hydro power).

Reversible Fuel Cell

See regenerative fuel cell.


Solid Oxide Fuel Cell (SOFC)

A type of fuel cell in which the electrolyte is a solid, nonporous metal oxide, typically zirconium oxide (ZrO2) treated with Y2O3, and O-2 is transported from the cathode to the anode. Any CO in the reformate gas is oxidized to CO2 at the anode. Temperatures of operation are typically 800°C–1,000°C.


Material that sorbs another (i.e., has the capacity or tendency to take it up either by adsorption or absorption).


Process by which one substance takes up or holds another.


See fuel cell stack.

Steam Reforming

The process for reacting a hydrocarbon fuel, such as natural gas, with steam to produce hydrogen as a product. This is a common method for bulk hydrogen generation.


Technology Validation

Confirming that technical targets for a given technology have been met.


A measure of thermal content. See also ambient temperature.


Machine for generating rotary mechanical power from the energy in a stream of fluid. The energy, originally in the form of head or pressure energy, is converted to velocity energy by passing through a system for stationary and moving blades in the turbine.


A device used for increasing the pressure and density of a fluid entering a fuel cell power plant using a compressor driven by a turbine that extracts energy from the exhaust gas.


Machine for compressing air or other fluids (reactant if supplied to a fuel cell system) in order to increase the reactant pressure and concentration.


Volumetric Energy Density

Potential energy in a given volume of fuel.


Water (H20)

A colorless, transparent, odorless, tasteless liquid compound of hydrogen and oxygen. The liquid form of steam and ice. Fresh water at atmospheric pressure is used as a standard for describing the relative density of liquids, the standard for liquid capacity, and the standard for fluid flow. The melting and boiling points of water are the basis for the Celsius temperature system. Water is the only byproduct of the combination of hydrogen and oxygen and is produced during the burning of any hydrocarbon. Water is the only substance that expands on freezing as well as by heating and has a maximum density at 4*C.

Watt (W)

A unity of power equal to one Joule of work performed per second; 746 watts is the equivalent of one horsepower. The watt is named for James Watt, Scottish engineer (1736-1819) and pioneer in engine design.


The term wt.%  (abbreviation for weight percent) is widely used in hydrogen storage research to denote the amount of hydrogen stored on a weight basis, and the term mass % is also occasionally used. The term can be used for materials that store hydrogen or for the entire storage system (e.g., material or compressed/liquid hydrogen as well as the tank and other equipment required to contain the hydrogen such as insulation, valves, regulators, etc.). For example, 6 wt.% on a system-basis means that 6% of the entire system by weight is hydrogen. On a material basis, the wt.% is the mass of hydrogen divided by the mass of material plus hydrogen.