Zinc-Air Fuel Cells (ZAFC)

Zinc-Air Fuel Cells (ZAFCs) share characteristics with a number of the other types of fuel cells as well as some characteristics of batteries.

The electrolyte for a ZAFC is a ceramic solid that employs the hydroxyl ion, OH-, as the charge carrier. To achieve the high electrical fuel efficiency with hydrocarbon fuels and a high electrolyte conductivity for the charge carrier, the ZAFC operates at 700ºC. The anode is composed of zinc and is supplied with hydrogen or even hydrocarbons. The cathode is separated from the air supply with a gas diffusion electrode (GDE), a permeable membrane that allows atmospheric oxygen to pass through. At the cathode, the oxygen reacts with hydrogen to form hydroxyl ions and water.

Anode Reaction:	CH4 + H2O => CO2 + 6H+ + 6e-
	Zn + OH- => ZnO + H + e-
Cathode Reaction:	O2 + 2H+ + 2e- => 2 OH-
	O2 + 4H+ + 4e- => 2 H2O
Overall Cell Reaction:	CH4 + 2 O2 => CO2 + 2 H2O

The high operating temperature of the ZAFC enables internal reforming of hydrocarbons, eliminating the need for an external reformer to generate hydrogen. An additional advantage of the high operating temperature is that the by-product heat can be used to generate high-pressure steam that is useful in many industrial and commercial applications.

The electrolyte for the ZAFC has some advantages over other electrolytes. It does not require water saturation as does the polymer membrane of the PEMFC and therefore, cannot dry out, eliminating the need to carefully monitor and control anode and cathode moisture levels. Additionally, as a solid, no leakage of the electrolyte will occur as can with liquid electrolytes.

Due to the consumption of the zinc anode, this component requires replacing at intervals.