Alkaline fuel cells
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The alkaline fuel cell (AFC) is one of the most developed fuel cell technologies and is the cell that flew Man to the Moon. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. AFCs consume hydrogen and pure oxygen producing potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.
The fuel cell produces power through a redox reaction between hydrogen and oxygen. At the anode, hydrogen is oxidized according to the reaction:
producing water and releasing two electrons. The electrons flow through an external circuit and return to the cathode, reducing oxygen in the reaction:
producing hydroxide ions. The net reaction consumes one oxygen molecule and two hydrogen atoms in the production of two water molecules. Electricity and heat are formed as by-products of this reaction.
The two electrodes are separated by a porous matrix saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH). Aqueous alkaline solutions do not reject carbon dioxide (CO2) so the fuel cell is easily poisoned. Because of this, the fuel cell requires pure oxygen, or at least purified air. These processes are relatively expensive, so not many continual developments are being made on AFC technology. NASA has made the decision to shift to Proton-exchange fuel cells for the next generation of Space Shuttles.
AFCs are, however, the cheapest of fuel cells to manufacture. The catalyst required for the electrodes can be any of a number of different chemicals that are relatively inexpensive compared to those required for other types of fuel cells.