AFCs were used on Apollo space missions to provide electricity for the
on-board needs of the shuttle. In addition to this, the major advantage
of AFCs is that pure hot water is produced as the ‘waste’ product. Therefore,
on the shuttle in addition to providing electricity the AFCs provided Heat,
Cooling, Hot water, and ultimately – drinking water for the astronauts!
How it Works
Although there are fewer
companies developing AFC technology than PEM technology, the possibility
to use less expensive materials, such as non-platinum catalysts and other
plastic injected components makes them a commercially attractive technology
option. Alkaline fuel cells (AFC’s) have an alkaline electrolyte - commonly
a liquid such as Potassium Hydroxide (KOH). In an AFC, hydroxide ions (OH-)
travel from cathode to anode. This differentiates them from PEM Fuel Cells
which have a solid polymer electrolyte which conducts protons.
PEM fuel cells, AFC's operate on pure hydrogen but have a lower power density.
Theoretical efficiency of Alkaline fuel cells is better than that of PEM.
A typical Alkaline fuel cell will have an electrical efficiency of ~50%,
though 60% has been achieved by UTC Power, who supply Alkaline fuel cells
to NASA’s Space Shuttle Fleet.
There are both low temperature and high temperature AFC’s. Low temperature
AFC’s operate at temperatures as low as 25°C up to 75°C. High temperature
AFC’s operate at 100°C up to 250°C.
Anode Reaction: 2H2 + 4OH- »» 4H2O + 4e-
Cathode Reaction: O2 + 2H2O + 4e- »» 4OH-
Image source - http://www.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html