Oxygen enters the fuel cell at the
cathode and, in some cell types (like the one illustrated above), it there combines
with electrons returning from the
electrical circuit and hydrogen ions that have traveled through the electrolyte from
the anode. In other cell types the oxygen picks up electrons and then travels through
the electrolyte to the anode, where it combines with hydrogen ions.
The electrolyte plays a key role. It must permit only the appropriate ions to pass
between the anode and cathode. If free electrons or other substances could travel
through the electrolyte, they would disrupt the chemical reaction.
Whether they
combine at anode or cathode, together hydrogen and oxygen form water, which drains
from the cell. As long as a fuel cell is supplied with hydrogen and oxygen, it will
generate electricity.
Even better, since fuel cells create electricity chemically, rather than by combustion,
they are not subject to the thermodynamic laws that limit a conventional power plant
(see "Carnot Limit" in the glossary). Therefore, fuel cells are more efficient in
extracting energy from a fuel. Waste heat from some cells can also be harnessed,
boosting system efficiency still further
