Alkaline fuel cells (AFCs) were one of the first fuel cell technologies to be developed and were originally used by NASA in the space programme to produce both electricity and water aboard spacecraft. AFCs continued to be used on NASA space shuttles throughout the programme, alongside a limited number of commercial applications. 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!
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.
Like 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-