September 2001

Zinc-Air Batteries

How zinc-air batteries work.

By Technology Review

In a world gone portable, batteries are key. without those unassuming little power supplies to run our laptops, cell phones and personal digital assistants, we might as well return to the days of paper and pigeons. For as much as batteries offer, though, they still annoy us with their frequent need for recharging or replacement altogether. Lately, the zinc-air battery has been turning up as a new choice of power for handheld electronics, providing up to three times the energy of common alkaline batteries in a more compact package.

Zinc-air cells work like conventional batteries in that they generate electrical power from chemical reactions. But instead of packing the necessary ingredients inside the cell, zinc-air batteries get one of their main reactants-oxygen-from the outside air. Oxygen molecules enter the cell through tiny holes in the top and then come into contact with a positively charged electrode (cathode) made of porous carbon. Water and other molecules already present in the pores of the electrode react with the oxygen to produce hydroxyl. These molecules, and other preexisting hydroxyls, migrate through an air separator to a negatively charged electrode (anode) that consists of a zinc gel. The hydroxyls bond to a zinc molecule to form zincate, which immediately splits into two hydroxyls, a water molecule and zinc oxide, and releases two electrons that travel through a circuit to power a device-usually a cell phone or hearing aid.

Using a reactant from the air saves on space, reducing the size and weight of the battery. And unlike some batteries used in wireless devices, zinc-air cells contain no toxic compounds and are neither highly reactive nor flammable. In fact, they can be recycled, safely disposed of, or in some cases, recharged with new zinc. Their only downside is that constant contact with ambient air can either dry up the zinc gel or, if conditions are too humid, flood it with water vapor. Both render the battery less potent. AER Energy Resources of Smyrna, GA, has found a way to diffuse the air (bottom inset). And another company, Electric Fuel, is developing zinc-air battery technology for automobiles. Instead of sucking gas, our cars could one day be breathing air.