Zinc–air batteries (non-rechargeable), and zinc–air fuel cells (mechanically rechargeable) are metal–air batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. Sizes range from very small button cells for hearing aids, larger batteries used in film cameras that previously used mercury batteries, to very large batteries used for electric vehicle propulsion and grid-scale energy storage. 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 (), 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. 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. Zinc–air batteries can be used to replace now discontinued 1.35 V mercury batteries (although with a significantly shorter operating life), which in the 1970s through 1980s were commonly used in photo cameras and hearing aids. Possible future applications of this battery include its deployment as an electric vehicle battery and as a utility-scale energy storage system. History The effect of oxygen was known early in the 19th century when wet-cell Leclanche batteries absorbed atmospheric oxygen into the carbon cathode current collector. In 1878, a porous platinized carbon air electrode was found to work as well as the manganese dioxide () of the Leclanche cell. Commercial products began to be made on this principle in 1932 when George W. Heise and Erwin A. Schumacher of the National Carbon Company built cells, treating the carbon electrodes with wax to prevent flooding. This type is still used for large zinc–air cells for navigation aids and rail transportation. However, the current capacity is low and the cells are bulky. Large primary zinc–air cells such as the Thomas A. Edison Industries Carbonaire type were used for railway signaling, remote communication sites, and navigation buoys. These were long-duration, low-rate applications. Development in the 1970s of thin electrodes