Alkali: A chemical "base" (loosely, the opposite of an acid). Certain types of alkalis (especially potassium hydroxide) have been used as fuel cell electrolytes.
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.
Capital Cost: The purchase price of a power generating system.
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 substance that causes or speeds a chemical reaction without itself being affected.
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.
Cogeneration: The use of waste heat from industrial processing, a steam turbine, or a fuel cell to generate electricity. Harnessing otherwise wasted heat boosts the efficiency of power-generating systems.
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.
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.
Graphite: A soft form of the element carbon. It is used for pencil leads, as a lubricant, as a moderator in nuclear reactors, and for other products.
It does not burn easily or fuse at high temperatures, and is an important material in the construction of phosphoric acid fuel cells.
Hydrogen: 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.
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.
kW: Kilowatt (1,000 watts). A measure of electric power.
kWh: Kilowatt-hour (1,000 watts for one hour). A measure of electric power consumption.
Matrix: A framework within a fuel cell that supports an electrolyte.
Molten Carbonate: A type of fuel cell electrolyte that contains carbon, oxygen and another element. Solid at room temperature, it must be melted in order to function.
MW: Megawatt (1,000,000 watts). A measure of electric power.
MWh: Megawatt-hour (1,000,000 watts for one hour). A measure of electric power consumption.
Nafion: A sulfuric acid in a solid polymer form. It is usually the electrolyte of PEM fuel cells.
O&M Costs: Operations and Maintenance. The cost of keeping a power plant running and in good repair.
Oxygen: A chemical element consisting of eight protons, eight neutrons and eight electrons. Two hydrogen atoms combine with one oxygen atom to form a molecule of water.
Peak Load: The maximum demand for electricity from an electrical system in a given period of time.
Phosphoric Acid: A solution of the elements phosphorus, hydrogen, and oxygen that serves as the electrolyte for one type of fuel cell. Chemically: 4H3PO4.
Polymer: A natural or synthetic compound composed of repeated links of simple molecules.
Potassium Hydroxide: A solution of the elements potassium, hydrogen, and oxygen that serves as the electrolyte for one type of fuel cell. Chemically: KOH.
Proton Exchange Membrane (PEM): A polymer sheet that serves as the electrolyte in one type of fuel cell.
Reformer: A device that extracts pure hydrogen from hydrocarbons.
Regenerative Fuel Cells: Several fuel cell types in which fuel and, in some types, the oxidant are regenerated from the oxidation product.
Solid Oxide: A solid combination of oxygen and another element (often zirconium) that serves as the electrolyte for one type of fuel cell.
Stack: Individual fuel cells connected in series within a generating assembly.
©2001 Smithsonian Institution