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

Alkali
A chemical "base" (loosely, the opposite of an acid). Certain types of alkalis (especially potassium hydroxide) have been used as fuel cell electrolytes.

Alkaline Fuel Cell (AFC)
A type of hydrogen/oxygen fuel cell in which the electrolyte is concentrated potassium hydroxide (KOH), and hydroxide ions (OH -) are transported from the cathode to the anode. Temperature of operation can vary from <120°C to approximately 250°C depending upon electrolyte concentration.

Anion
A negative ion. Alkali, molten carbonate and solid oxide fuel cells are "anion-mobile" cells – anions migrate through the electrolyte toward the anode.

Anode
One of two electrodes in a fuel cell or battery. In a fuel cell it is where the fuel reacts or "oxidizes", and releases electrons.

Atom
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.

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

Bipolar Plates
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 or 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.

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

Carbon Dioxide (CO2)
Carbon dixoide is a colorless, odorless, noncombustible gas that is slightly more than 1.5 times as dense as air and becomes a solid (dry ice) below –78.5ºC. It is present in the atmosphere as a result of the decay of organic material and the respiration of living organisms, and it represents about 0.033% of the air. Carbon dioxide is produced by the burning of wood, coal, coke, oil, natural gas, or other fuels containing carbon, by the action of an acid on a carbonate, or naturally from springs and wells.

Carbon Monoxide (CO)
A pollutant from engine exhaust that is a colorless, odorless, tasteless, poisonous gas that results from incomplete combustion of carbon with oxygen.

Carnot Limit
A theoretical limit on the efficiency of an engine based on the flow of heat between two reservoirs. Named for its discoverer Sadi Carnot (1796-1832) of France, this limit stems from basic physical laws and applies to all steam engines. Fossil fuel and nuclear power plants are bound by this limit, but most fuel cells are not.

Catalyst
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 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 reaction 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 that 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.

Cathode
One of two electrodes in a fuel cell or battery. In a fuel cell, it is where oxygen (usually taken from the air) "reduction" occurs.

Cation
A positive ion. Phosphoric acid and PEM fuel cells are "cation-mobile" cells – the cation migrates through the electrolyte toward the cathode.

Current Collector
Term used to describe 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 for fluid flow through them) and lie in between the catalyst/electrolyte surfaces and the bipolar plates.

Direct Fuel Cell
A type of fuel cell in which a hydrocarbon fuel is fed directly to the fuel cell stack, without requiring an external "reformer" to generate hydrogen.

Electrode
An electrical terminal that conducts an electric current into or out of a fuel cell.

Electrolysis
The process where an electric current is passed through an electrolytic solution or other appropriate medium, causing a chemical reaction. The process of driving a redox reaction in the reverse direction by passage of an electric current through the reaction mixture.

Electrolyte
A chemical compound that conducts ions from one electrode to the other inside a fuel cell.

Electron
A sub-atomic particle carrying a negative charge.

Fuel Cell
A device for generating electricity by the chemical combination a fuel and oxygen.

Generating Capacity
The maximum amount of electric power produced by a generator.

Hydrogen (H2)
A chemical element consisting of one proton and one electron. Two hydrogen atoms combine with one oxygen atom to form a molecule of water. Hydrogen serves as the fuel for most fuel cells. Hydrogen forms water when combusted, or when otherwise joined with air, as within a fuel cell.

Inverter
A device used to convert direct current electricity produced by a fuel cell (or other source) to alternating current.

Ion
An atom that carries a positive or negative charge due to the loss or gain of an electron.

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