Zinc-Air Fuel Cells (ZAFC)

Zinc-air fuel cells are actually nothing more than Zinc-air batteries in which the zinc is replaced rather than being recharged. Zinc batteries are commonly found in hearing aids and other small devices.

Function and Reactions of ZAFCs

The anode in a ZAFC exists as a bed of zinc particles that are immersed in concentrated potassium hydroxide. The anode is designed as a cassette that can be removed from the cathodes that surround it on two sides. When the fuel cell is in operation, oxygen is extracted from the air by reducing it at the cathode to form hydroxide ions. These ions then react with the zinc fuel to produce zinc oxide as well as electrons. The electrons are removed back to the cathode via an external circuit. Once the zinc is completely consumed, the cassette can be replaced.

Benefits of ZAFCs

Extremely high power density is one of the defining characteristics of zinc air fuel cells. Their power to weight ratio ranges from 200 to 600% more than other fuel cells. 20 kilowatt fuel cells are relatively simple to produce, providing enough energy for even the largest of passenger vehicles.

ZAFCs have one of the most unique configurations of any fuel cell due to the cassette design of the anode, which makes refueling a relatively simple process. This allows vehicles, even as large as buses, to be refueled within minutes. It is also possible to remove and replace the zinc as a liquid, making refueling a ZAFC similar to refueling a standard internal combustion engine. The current worldwide production of zinc is estimated to be large enough to power 1 billion vehicles. Zinc can be reformed from zinc oxide through electrolysis, so the product is recyclable. This means zinc could be recycled at the premises where it is pumped into and out of vehicles. This means there would be little or no additional need for transport once a station was set up.

Drawbacks of ZAFCs

While the conversion in ZAFCs is generally from hydroxide straight to zinc oxide, some hydrogen is produced. This has to be dealt with through controlled venting, which poses a problem for storing these vehicles as any build-up of hydrogen in a confined area could be dangerous.

While zinc can be reclaimed, it currently requires far more energy to recycle it than can be gained from its use in vehicles. Until electricity supplies are clean and renewable, the use of ZAFCs will actually increase harmful emissions.