Alkaline fuel cell

The alkaline fuel cell ( engl. Alkaline Fuel Cell, AFC) is a low-temperature fuel cell.

Principle

Typically, an aqueous potassium hydroxide solution is used as an electrolyte, used to, for example, hydrazine. Since the solutions are alkaline, the cell then has received its name. As the fuel gas is hydrogen at the anode, is converted by oxidation with pure oxygen at the cathode. This produces at the anode as the reaction product of water, which must be continuously removed. This type of cell is operated at 60-120 ° C.

The supplied to the cathode air must be free of CO2, as this will react with the lye to a carbonate. This is then a solid ( white precipitate, not resolved ), which clogs the porous electrodes and leads to a drop in performance of the cell. Compared to the PEMFC, the AFC has a lower power density but slightly better efficiency due to a higher cell voltage. However, it does not reach the currently high current densities as the PEMFC. The electrolyte also serves to control the temperature of the stack, whereby no further cooling circuit is required.

Applications

The AFC is used in manned space flight, for example, in the Apollo spacecraft, the spacecraft LOC and the Space Shuttle. Siemens installed in AFC U- boats, but then finally turned to the PEMFC.

The AFC still has a future, because it represents the ideals submarine drive which is externally balanced flue and noise, as a waste product only is water, and the heat development in contrast to the nuclear submarine holds very limited in armor. Thus, the U- boat is difficult locatable.

In space AFCs do not play such a big role, as it is only special at the moment, which are accordingly expensive. The development lies here a whole is still in its infancy. The water and energy use plays a minor role and is only seen as an addition to the existing systems.

Reaction equations

The internal charge transport takes place by means of hydroxide ions. On the cathode side, the reaction requires only pure oxygen and produced water on the anode side, is required on only pure hydrogen.

Advantages and disadvantages compared to other fuel cells

Benefits of AFC are:

• Robust system
• Good dynamic performance
• Inexpensive catalysts ( nickel, silver )

Disadvantages are:

• Very sensitive to contamination, in particular by CO2
• Low life due to the corrosive electrolyte,

A good potential for further development have alkaline fuel cells. Research mainly handle different materials for membranes and electrodes. Particularly effective are here electrodes have proved of gold.