Alkaline Fuel Cells (AFC)

Alkaline Fuel Cells or AFCs are also known as Bacon fuel cells after their British inventor. Along with phosphoric acid fuel cells, they were one of the earliest FCs developed and have been used by NASA. They consume hydrogen and pure oxygen, to produce water, heat, and electricity. These products make AFCs ideal for use in space where carbon dioxide production can pose a major threat.

Function and Reaction of AFCs

Alkaline fuel cells are the most similar to batteries of all fuel cells. Their basic construction involves two electrodes that are separated by a porous matrix that is saturated with an aqueous alkaline solution. This construction is very similar to that of batteries, as are the metals and chemicals used. AFC’s rely on both hydrogen and pure oxygen to operate.

On the cathode side of the fuel cell, oxygen enters and combines with water to produce Hydroxyl ions. These ions are capable of permeating the electrolyte layer and of traveling to the anode where they interact with hydrogen to produce electrons and water. The water is recycled while the electrons are returned to the cathode through an external circuit. This is similar in some respects to how solid oxide fuel cells operate.

Benefits of AFCs

The cathode and anode of an alkaline fuel cell can be made of a variety of substances, as can the alkaline electrolyte (thought it is usually potassium hydroxide). This frees alkaline Fuel Cells to be produced from any number of inexpensive materials, making them the cheapest fuel cells to manufacture.

AFCs have efficiencies of near 60% in most applications, making them one of the few normal temperature (100 C to 250 C) fuel cells to achieve high efficiency without precious metal catalysts.

Drawbacks of AFCs

The major drawback of AFCs is their exquisite sensitivity to carbon dioxide. Even trace amounts of carbon dioxide can affect the cell’s operation substantially by converting the potassium hydroxide electrolyte into potassium carbonate. Potassium carbonate is a solid that blocks pores in the cathode. This reduces the ionic conductivity of fuel cell and diminishes the speed with which the reaction can proceed. This means that while manufacturing costs are low, the cost of refining hydrogen and oxygen for fuel is quite high. There is currently some effort in research to produce an AFC in which the potassium hydroxide is replaced at various intervals to maintain operation.

In addition to the need for pure fuel, the sensitivity to carbon dioxide reduces the lifespan of these cells and results in the need to replace them more frequently. Alkaline Fuel Cells have life spans of approximately 8,000 hours, roughly one fifth of the 40,000 hours they need to last to be economically viable.

AFC Subtypes

Name

Fuel

Operating Characteristics

Metal Hydride

Oxygen

< 40 C

Direct Borohydride

Sodium Borohydride

70 C

Zinc-air

Zinc

< 40 C with most producing 20 kW