Although today's high-tech fuel cells hardly resemble their forefathers, the process upon which fuel cells are based has been known to science for more than 100 years. The first fuel cell (or "gas battery" as he called it), was invented by William Robert Grove in 1839, only 39 years after Alessandro Volto invented the battery (the voltaic cell).
Unfortunately, the materials that Grove used were unstable and public interest dwindled. It wasn't until the 1960s that fuel cells were revived for use on manned space flights. Because fuel cells are quiet, reliable, and clean, and produce water as a by-product, NASA developed fuels cells as the ideal supply of both power and drinking water for the astronauts. And when used in electrolysis mode, fuel cells had the added benefit of producing breathable oxygen and hydrogen for rocket fuel.
Hydrogen (H2) is the most plentiful, renewable element in the universe. We will never run out of hydrogen. It is has excellent electrochemical reactivity and it is clean burning and efficient.
When coupled with a fuel reformer (which separates hydrogen from a hydrocarbon fuel), fuel cells can also run on a wide variety of fuels, such as methanol, natural gas, propane, gasoline and coal gas, or even coal powder. Some fuel cells designs can run on renewable fuels such as biogas and alcohol. When using other hydrocarbons as fuel, the reformer can produce some small amounts of pollutants, such as carbon dioxide or impurities present in the fuel, but the levels are much smaller than from traditional combustion generators.
If hydrogen and oxygen were simply mixed as gasses at room temperature, nothing would happen. Exposing these gases to a spark would cause them to combust. Fuel cells, however, can control the release of the energy contained in these gases by virtue of regulated electrochemical reactions. Because there is no combustion, there is no emission of pollutants such as nitrous oxides, sulfur oxides, or particulates into the air.
Individual fuel cells are inherently low voltage in nature. System voltage is increased by stacking individual fuel cells together in series, forming a system capable of providing voltages necessary for commercial use.
One form of fuel cell is represented below. In the fuel cell, a semi-porous electrolyte separates the hydrogen and oxygen and keeps them from combining, but allows the gas ions to pass through so that electrochemical actions can occur. These actions separate the electrons from the hydrogen atoms on one side of the fuel cell. The severed electrons are forced to travel a separate circuit to rejoin the oxygen on the other side of the fuel cell, forming water or water vapor. While being drawn through this separate circuit, the electrons create electrical power.