The Zn/Br battery was developed by Exxon in the early 1970s. In each cell of a Zn/Br battery two different electrolytes flow past carbon-plastic composite electrodes in two compartments separated by a micro porous polyolefin membrane. During discharge zinc is converted to zinc ion (with the release of two electrons) and bromine to bromide which then combine to form zinc bromide. The chemical process used to generate the electric current increases the zinc-ion and bromide-ion concentration in both electrolyte tanks. During charge metallic zinc is deposited as a thin film on one side of the carbon-plastic composite electrode. Meanwhile, bromine is evolved as a dilute solution on the other side of the membrane, reacting with other compounds in solution (organic amines) to form viscous, dense bromine-adduct oil that sinks to the bottom of the tank. The bromine oil is allowed to mix with the rest of the electrolyte during discharge. The net efficiency of this battery is about 75%.
Integrated Zn/Br energy storage systems have been tested on transportable trailers (up to 1 MW/3 MWh) for utility-scale applications. These systems could be connected in parallel for use in much larger applications. They can also be used in 5-kW/20-kWh Community Energy Storage (CES) systems that are now being tested by utilities.