The vanadium redox battery (VRB), also known as the vanadium flow battery (VFB) or vanadium redox flow battery (VRFB), is a type of rechargeable flow battery. It employs vanadium ions as charge carriers. The battery uses vanadium's ability to exist in a solution in four different oxidation states to make a battery with a single electroactive element instead of two. For several reasons, including their relative bulkiness, vanadium batteries are typically used for grid energy storage, i.e., attached to power plants/electrical grids. Numerous companies and organizations are involved in funding and developing vanadium redox batteries. History Pissoort explored the possibility of VRFB's in the 1930s. NASA researchers and Pellegri and Spaziante followed suit in the 1970s, but neither was successful. Maria Skyllas-Kazacos presented the first successful demonstration of dissolved vanadium in a solution of sulfuric acid in the 1980s. Her design used sulfuric acid electrolytes, and was patented by the University of New South Wales in Australia in 1986. Advantages and disadvantages Advantages VRFB's main advantages over other types of battery: no limit on energy capacity can remain discharged indefinitely without damage mixing electrolytes causes no permanent damage single charge state across the electrolytes avoids capacity degradation safe, non-flammable aqueous electrolyte; wide operating temperature range including passive cooling long charge/discharge cycle lives: 15,000-20,000 cycles and 10–20 years. low levelized cost: (a few tens of cents), approaching the 2016 $0.05 target stated by the US Department of Energy and the European Commission Strategic Energy Technology Plan €0.05 target. Disadvantages VRFB's main disadvantages compared to other types of battery: high and volatile prices of vanadium minerals (i.e. the cost of VRFB energy) ; relatively poor round trip efficiency (compared to lithium-ion batteries) ; heavy weight of aqueous electrolyte ; relatively poor energy-to-volume ratio compared to standard storage batteries ; toxicity of vanadium (V) compounds. Materials A vanadium redox battery consists of an assembly of power cells in which two electrolytes are separated by a proton exchange membrane. The electrodes in a VRB cell are carbon based. The most common types are carbon felt, carbon paper, carbon cloth, graphite felt, and carbon nanotubes. Both electrolytes are vanadium-based. The electrolyte in the positive half-cells contains VO2+ and VO2+ ions, while the electrolyte in the negative half-cells consists of V3+ and V2+ ions. The electrolytes can be prepared by several processes, including electrolytically dissolving vanadium pentoxide (V2O5) in sulfuric acid (H2SO4). The solution is strongly