Redox flow batteries consist of two liquid electrolytes. They contain metal ions which flow through electrodes made of porous graphite fleece. A membrane separates the electrodes and therefore only protons get through. During this exchange of charge electricity flows and can be used.
Wikipedia definition (similar term):
The vanadium redox battery (VRB) (or Vanadium flow battery) is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy. The vanadium redox battery exploits the ability of vanadium to exist in solution in four different oxidation states, and uses this property to make a battery that has just one electroactive element instead of two. For several reasons, including their relatively bulky size, most vanadium batteries are currently used for grid energy storage, such as being attached to power plants or electrical grids. The first successful demonstration of the all-vanadium redox flow battery which employed vanadium in a solution of sulfuric acid in each half was by Maria Skyllas-Kazacos at the University of New South Wales in the 1980s. Her design used sulfuric acid electrolytes, and was patented by the University of New South Wales in Australia in 1986. The possibility of creating a vanadium flow battery was explored variously by Pissoort in 1930's, NASA researchers in the 1970s, and Pellegri and Spaziante in the 1970s, but none of them were successful in demonstrating the technology. Numerous companies and organizations involved in funding and developing vanadium redox batteries including Imergy (formerly Deeya), Vionx (formerly Premium Power), UniEnergy Technologies, and Ashlawn Energy in the United States; Renewable Energy Dynamics Technology in Ireland; Gildemeister AG (formerly Cellstrom GmbH in Austria) in Germany; Cellennium in Thailand Rongke Power; Prudent Energy in China; Sumitomo in Japan; H2, Inc. in South Korea; and redT in Britain. The main advantages of the vanadium redox battery are: * It can offer almost unlimited energy capacity simply by using larger electrolyte storage tanks. * It can be left completely discharged for long periods with no ill effects. * If the electrolytes are accidentally mixed, the battery suffers no permanent damage. * A single state of charge between the two electrolytes avoids the capacity degradation due to a single cell in non-flow batteries. * The electrolyte is aqueous and inherently safe and non-flammable. * The generation 3 formulation using a mixed acid solution developed by the Pacific Northwest National Laboratory operates at a high temperature allowing for passive cooling The main disadvantages with vanadium redox technology are a relatively poor energy-to-volume ratio, although the Generation 3 formulation has doubled the energy density of the system complexity in comparison with standard storage batteries, and the aqueous electrolyte makes the battery heavy and therefore only useful for stationary applications.