They are very clean sources of energy. Remember, most power plants burn coal, oil or gas to boil water to turn the turbine. When you burn any fossil fuel, you release emissions into the air. This is not unlike your car. When your car's engine burns gasoline, it releases emissions through the tailpipe.
Fuel cells are electrochemical devices that convert a fuel's energy directly to electrical energy.
But unlike a battery, a fuel cell does not run down or require recharging. It will produce energy in the form of electricity and heat as long as fuel is supplied. Encouraged by a catalyst, the hydrogen atom splits into a proton and an electron, which take different paths to the cathode. The proton passes through the electrolyte. The electrons create a separate current before they return to the cathode, to be reunited with the hydrogen and oxygen in a molecule of water.