The redox (reduction–oxidation) cell is a reversible cell in which redox-active species are in fluid (liquid or gas) media. Redox flow batteries are rechargeable (secondary) cells. Because they employ heterogeneous electron transfer rather than solid-state diffusion or intercalation they are more similar to fuel cells rather than to conventional batteries (such as lead–acid or lithium-ion). The main reason fuel cells are not considered to be batteries, is because originally (in the 1800s) fuel cells emerged as a means to produce electricity directly from fuels (and air) via a non-combustion electrochemical process. Later, particularly in the 1960s and 1990s, rechargeable fuel cells (i.e. /, such as unitized regenerative fuel cells in NASA's Helios Prototype) were developed.
Examples of redox flow batteries are the vanadium redox flow battery, polysulfide–bromide battery (Regenesys),  iron redox flow battery (IRFB), and uranium redox flow battery. Redox fuel cells are less common commercially although many systems have been proposed.
Vanadium redox flow batteries are the most marketed flow batteries at present, due to the advantages they provide over other chemistries, despite limited energy and power densities. Since they use vanadium at both electrodes, they do not suffer cross-contamination. The limited solubility of vanadium salts, however, offsets this advantage in practice.  More importantly for the commercial success of VRFBs is actually an almost perfect match of the voltage window of carbon/aqueous acid interface with the working voltage range of the vanadium redox-couples. This assures the durability of the low-cost carbon electrodes and low-impact of side reactions, such as H2 and O2 evolutions, resulting in record-long calendar (many years) and cycle(15,000–20,000 cycles) lives, which in turn results in a record low levelized cost of energy (LCOE, i.e. the system cost divided by the usable energy, the cycle life, and round-trip efficiency). The long lifetimes of flow batteries allow for the amortization of their relatively high capital cost (due to vanadium, carbon felts, bipolar plates, membranes). The levelized cost of energy for VRFBs is in the order of a few tens of $ cents or € cents per kWh, much lower than of solid-state batteries and not so far from the targets of $0.05 and €0.05, stated by US and EC government agencies. The major challenges for the broad implementation include: low abundance and high costs of VO (> $30 / Kg), the raw materials for VRFB; parasite reactions including hydrogen and oxygen evolution; and precipitation of VO during cycling. It is the major driving force to develop alternative flow battery technologies.
Traditional flow battery chemistries have both low specific energy (which makes them too heavy for fully electric vehicles) and low specific power (which makes them too expensive for stationary energy storage). However a high power of 1.4 W/cm was demonstrated for hydrogen–bromine flow batteries, and a high specific energy (530 Wh/kg at the tank level) was shown for hydrogen–bromate flow batteries