Alkaline Fuel Cells (AFC)

Alkaline fuel cells (AFC) are one of the most developed technologies and have been used since the mid-1960s by NASA in the Apollo and Space Shuttle programs. The fuel cells on board these spacecraft provide electrical power for on-board systems, as well as drinking water. AFCs are among the most efficient in generating electricity at nearly 70%.

Alkaline fuel cells use an electrolyte that is an aqueous (water-based) solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The concentration of KOH can be varied with the fuel cell operating temperature, which ranges from 65°C to 220°C. The charge carrier for an AFC is the hydroxyl ion (OH-) that migrates from the cathode to the anode where they react with hydrogen to produce water and electrons. Water formed at the anode migrates back to the cathode to regenerate hydroxyl ions. Therefore, the chemical reactions at the anode and cathode in an AFC are shown below. This set of reactions in the fuel cell produces electricity and by-product heat.

Anode Reaction:	2 H2 + 4 OH- => 4 H2O + 4 e-
Cathode Reaction:	O2 + 2 H2O + 4 e- => 4 OH-
Overall Net Reaction:	2 H2 + O2 => 2 H2O

One characteristic of AFCs is that they are very sensitive to CO₂ that may be present in the fuel or air. The CO₂ reacts with the electrolyte, poisoning it rapidly, and severely degrading the fuel cell performance. Therefore, AFCs are limited to closed environments, such as space and undersea vehicles, and must be run on pure hydrogen and oxygen. Furthermore, molecules such as CO, H₂O and CH₄, which are harmless or even work as fuels to other fuel cells, are poisons to an AFC.

On the positive side, AFCs are the cheapest fuel cells to manufacture. This is because the catalyst that is required on the electrodes can be any of a number of different materials that are relatively inexpensive compared to the catalysts required for other types of fuel cells.

AFCs are not being considered for automobile applications. Their sensitivity to poisoning, which requires use of pure or cleansed hydrogen and oxygen, is an insurmountable obstacle at the present time. Conversely, AFCs operate at relatively low temperatures and are among the most efficient fuel cells, characteristics that would enable a quick starting power source and high fuel efficiency, respectively.