Fuel cells are electrochemical devices that convert chemical energy in hydrogen and oxygen into electricity and heat without combustion and have a wide range of applications in the stationary and automotive markets. Fuel cells have several advantages over traditional sources of power, including combustion engines, small scale back-up power generators and batteries. Fuel cells are fuel-efficient and rely on a broader range of fuels and generate fewer harmful emissions than combustion engines and small scale back-up power generators. Fuel cells can produce more power than conventional batteries of equivalent volume and weight, generally have a longer shelf life and can be disposed with less harm to the environment. In addition, unlike batteries which generally have limited runtimes because finite amounts of chemicals are stored inside, fuel cells can continue to produce electricity as long as there is a constant inflow of hydrogen and oxygen.
BASIC REACTIONS
Fuel cell designs differ in the components and materials they use to generate electricity. However, the basic reactions are more or less the same across designs. The main components in fuel cell reactions are the anode and cathode. The anode conducts electrons freed from the hydrogen molecules introduced to the cell so that they can be used in an external circuit. The anode is the negative post of the fuel cell. The cathode half of the cell is where oxygen is introduced. The cathode conducts the electrons back from the external circuit, where they can recombine with the hydrogen ions and oxygen to form water. This is the positive post of the fuel cell.
The chemical reaction that takes place is fairly simple. As the hydrogen gas is introduced into the fuel cell it comes into contact with the anode and separates into electrons (electricity) and H+ ions (protons) as follows:
2H2 --> 4H+ + 4e-
The electrons are routed out of the cell and used in an external circuit, and then back into the cell to the cathode. At the cathode, oxygen is introduced into the cell where it reacts with the electrons and hydrogen ions to produce water as follows:
O2 + 4H+ + 4e- --> 2H2O
In order for the stack to produce energy, electrons produced at the anode must pass through an electrical circuit to the cathode. In addition, protons must pass through the electrolyte or proton exchange membrane (PEM). It is important that this membrane allow protons to pass through but not electrical charges. Otherwise electrons would go through the electrolyte rather than through the external circuit. The figure below depicts these same basic reactions in a PEM fuel cell. PEM fuel cells are currently one of the leading fuel cell designs in the stationary, automotive and micro markets.
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