A fuel cell is an electrochemical device that exploits the reaction of a fuel source with oxygen in order to produce electricity. The fuel may be hydrocarbon in nature (e.g. natural gas, propane, methanol), or it may be hydrogen gas. Fuel cells generated electrical power on NASA's Apollo space capsules and also today on the Space Shuttle.
A fuel cell is not a battery in that it is not self-contained. Fuel is supplied from an external source, either a fixed amount from within a storage reservoir or in a continuous supply from the natural gas infrastructure. Oxygen for the device can be supplied either from storage tanks or from the atmosphere. Rather than allowing the fuel and oxidant to react with one another directly in a combustion reaction, the fuel is oxidized electrochemically at the anode to generate electrons, which then recombine with oxygen at the cathode to form product water vapor.