Function

• In a fuel cell, hydrogen is ionized and donates its electrons to a negative electrode. The naked protons cross a membrane to join with oxygen molecules. Meanwhile, the electrons from the negative electrode flow to the positive electrode, where they can recombine with the hydrogen ions and the oxygen atoms to form water. The chemical reaction is H2 + O2 --> H2O.
  
  

Features

• The reaction that takes place in a fuel cell in a hydrogen car is the reverse of the electrolysis reaction used to produce hydrogen. Because electrons flow from the negative electrode to the positive electrode during the course of the reaction, the fuel cell generates electricity, which the car can use to turn a motor and provide power.
  
  

Considerations

• The combustion reaction in a hydrogen car is exceptionally clean because the only byproduct of combustion is water. Greenhouse gases or carbon dioxide are not released. The reaction that takes place in the fuel cell will never generate more energy than that which was used to split the water to make hydrogen in the first place. Because the two reactions are essentially the reverse of each other, it follows from the laws of thermodynamics that the reverse reaction cannot generate more energy than the amount needed to drive the forward reaction.