A fuel cell is a device which takes stored chemical energy and converts it to electrical energy directly. Essentially it takes the chemical energy that is stored within whatever energy source you have such as hydrogen, gasoline or methane and then through two electrochemical reactions it converts it directly to electricity.
Since the conversion of the fuel to energy takes place via an electrochemical process and not combustion, the process is non polluting and efficient (three times more efficient than fuel burning).
Much like a battery, a fuel cell creates energy by converting chemical energy into electrical energy, but unlike a battery it produces electricity from external supplies of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.
The major components of the fuel cell are electrolytes, which is also reactor so it keeps the reagents from mixing together. The next pieces are electrodes, these are pieces that act as catalysts for electrical chemical reaction. Generally what happens is that the reactants flow in and the reaction products flow out while the electrolyte remains in the cell. Then there is a bi-polar plate which can is also called a separator, this is a way to collect the current and build voltage from cells.
Fuel cells can use a variety of fuels including hydrogen, ethanol, methanol, and various acids and alkalines (bases).
Fuel cells run best on hydrogen but hydrogen is not something you can dig out of the ground. You can dig out a fossil fuel and convert it into hydrogen rich stream. But to do that for a fuel cell, you need to reform the fuel cell and clean up the gases quite a bit before you can form it into a fuel cell. Hydrogen is abundant in water, so hydrogen fuels and hydrogen power will possibly form an important part of our energy mix in the future.
A single cell which means it contains one fuel cell which generates 0.8 volts and which means if you want large voltages you have to put them in series. Fuel cells can power anything from tiny microchips to buildings, to buses.
The problem with fuel cells is that they are expensive technology compared to what is already in terms of energy storage such as deep cycle batteries and they also take up a lot of space.
Fuel cells are theoretically much more efficient than conventional power generation.
Example of energy conversions for a coal fired power station:
Each conversion has its own inefficiencies, so the overall process is very inefficient.
A fuel cell converts chemical energy directly into electrical energy and is, in theory, much more efficient.