A zinc–air battery is a metal–air electrochemical cell powered by the oxidation of zinc with oxygen from the air. During discharge, a mass of zinc particles forms a porous anode, which is saturated with an electrolyte. Oxygen from the air reacts at the cathode and forms hydroxyl ions which migrate into the zinc paste and form zincate (Zn(OH)
4), releasing electrons to travel to the cathode. The zincate decays into zinc oxide and water returns to the electrolyte. The water and hydroxyl from the anode are recycled at the cathode, so the water is not consumed. The reactions produce a theoretical voltage of 1.65 volts, but is reduced to 1.35–1.4 V in available cells.
These batteries have high energy densities and are relatively inexpensive to produce. Zinc–air batteries have some properties of fuel cells as well as batteries: the zinc is the fuel, the reaction rate can be controlled by varying the air flow, and oxidized zinc/electrolyte paste can be replaced with fresh paste.