Metal–air electrochemical cell

Metal–air electrochemical cells (or metal–air batteries) are electrochemical cells that use an anode made from pure metal and an external cathode of ambient air, typically with an aqueous or aprotic electrolyte. They function by the electrochemical oxidation of the metal at the anode and the reduction of oxygen from the air at the cathode. Because the cathode active material (oxygen) is not stored within the battery but is instead absorbed from the environment, metal–air batteries have a much higher theoretical specific energy than traditional lithium-ion batteries and can effectively approach the energy density of gasoline.

The technology has seen commercial success in specific applications, most notably zinc–air batteries, which are widely used in hearing aids and railway signaling due to their high energy density and safety. Other chemistries, such as lithium–air, aluminium–air, and iron–air, are the subject of intense research for grid-scale storage and electric vehicle applications.

Despite their high potential, the development of rechargeable (secondary) metal–air cells has been hindered by significant technical challenges.

These include the sluggish kinetics of the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charge, which necessitate efficient electrocatalysts. Additionally, metal anodes suffer from corrosion, passivation, and dendritic growth during cycling, while the open cell architecture exposes the electrolyte to evaporation and contamination from atmospheric carbon dioxide.