Alkaline fuel cell

Alkaline fuel cells consume hydrogen and pure oxygen, to produce potable water, heat, and electricity.

NASA has used alkaline fuel cells since the mid-1960s, in the Apollo-series missions and on the Space Shuttle.

[1] The fuel cell produces power through a redox reaction between hydrogen and oxygen.

The electrons flow through an external circuit and return to the cathode, reducing oxygen in the reaction:

The two electrodes are separated by a porous matrix saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH).

[2] Because of this, alkaline fuel cells typically operate on pure oxygen, or at least purified air and would incorporate a 'scrubber' into the design to clean out as much of the carbon dioxide as is possible.

[1] Because the generation and storage requirements of oxygen make pure-oxygen AFCs expensive, there are few companies engaged in active development of the technology.

An alternate method involves simply replacing the KOH which returns the cell back to its original output.

These fuel cells typically use platinum catalysts to achieve maximum volumetric and specific efficiencies.

More space is required between electrodes to enable this flow, and this translates into an increase in cell resistance, decreasing power output compared to immobilized electrolyte designs.

A further challenge for the technology is how severe the problem of permanent blocking of the cathode is by K2CO3; some published reports have indicated thousands of hours of operation on air.

The active layer consists of an organic mixture which is ground and then rolled at room temperature to form a crosslinked self-supporting sheet.

The hydrophobic structure prevents the electrolyte from leaking into the reactant gas flow channels and ensures diffusion of the gases to the reaction site.

Because of the alkaline chemistry, oxygen reduction reaction (ORR) kinetics at the cathode are much more facile than in acidic cells, allowing use of non-noble metals, such as iron, cobalt, nickel, manganese, or carbon-based nanomaterial at the anode (where fuel is oxidized); and cheaper catalysts such as silver at the cathode,[2] due to the low overpotentials associated with electrochemical reactions at high pH.

An alkaline medium also accelerates oxidation of fuels like methanol, making them more attractive.

The world's first fuel-cell ship, the Hydra, used an AFC system with 5 kW net output.

Another recent development is the solid-state alkaline fuel cell, utilizing a solid anion-exchange membrane instead of a liquid electrolyte.

This resolves the problem of poisoning and allows the development of alkaline fuel cells capable of running on safer hydrogen-rich carriers such as liquid urea solutions or metal amine complexes.