A flow battery, or redox flow battery (after reduction–oxidation), is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids that are pumped through the system on separate sides of a membrane. Ion transfer inside the cell (accompanied by current flow through an external circuit) occurs across the membrane while the liquids circulate in their respective spaces.
A flow battery may be used like a fuel cell (where new charged negolyte (a.k.a. reducer or fuel) and charged posolyte (a.k.a. oxidant) are added to the system) or like a rechargeable battery (where an electric power source drives regeneration of the reducer and oxidant).
The fundamental difference between conventional and flow batteries is that energy is stored in the electrode material in conventional batteries, while in flow batteries it is stored in the electrolyte. According to Battery Council International, this provides flow batteries with advantages for scalability and long-duration energy storage capabilities, making them ideal for stationary applications that demand consistent and reliable power.
Flow batteries have certain technical advantages over conventional rechargeable batteries with solid electroactive materials, such as independent scaling of power (determined by the size of the stack) and of energy (determined by the size of the tanks), long cycle and calendar life, and potentially lower total cost of ownership at multi-hour half cycle design times. However, flow batteries suffer from a lower cycle energy efficiency (50–80%). This drawback stems from the need to operate flow batteries at high (>= 100 mA/cm2) current densities to reduce the effect of internal crossover (through the membrane/separator) and to reduce the cost of power (size of stacks). Also, most flow batteries (, and are exceptions) have lower specific energy (heavier weight) than lithium-ion batteries. The heavier weight results mostly from the need to use a solvent (usually water) to maintain the redox active species in the liquid phase.
Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.43 volts. The energy capacity increased with the volume of the fluids in the tanks, and the power increases with the size of the stack.
Flow batteries can be classified using different schemes:
1) Full-flow (where all reagents are in fluid phases: gases, liquids, or liquid solutions), such as vanadium redox flow battery vs semi-flow, where one or more electroactive phases are solid, such as zinc-bromine battery.
2) Type of reagents: inorganic vs. organic and organic forms. As of 2025, only inorganic RFBs have been demonstrated on a commercial scale. Organic RFBs often suffer from poor durability.Yuriy V. Tolmachev. 2023. Review—Flow Batteries from 1879 to 2022 and Beyond. J. Electrochem. Soc. 170 030505. DOI:10.1149/1945-7111/acb8de
3) By cell separator design: Membrane RFBs vs membraneless RFB.
Patent Classifications for flow batteries had not been fully developed as of 2021. Cooperative Patent Classification considers flow batteries as a subclass of regenerative fuel cell (H01M8/18), even though it is more appropriate to consider fuel cells as a subclass of flow batteries.
History The zinc–bromine flow battery () was the original flow battery. John Doyle file patent on September 29, 1879. Zn-Br2 batteries have relatively high specific energy, and were demonstrated in electric cars in the 1970s.
Walther Kangro, an Estonian chemist working in Germany in the 1950s, was the first to demonstrate flow batteries based on dissolved transition metal ions: Ti–Fe and Cr–Fe.W. Kangro Dr, 1949.; W. Kangro Dr, 1954.;W. Kangro and H. Pieper, Electrochim Acta, 7 (4), 435–448 (1962) After initial experimentations with Ti–Fe redox flow battery (RFB) chemistry, NASA and groups in Japan and elsewhere selected Cr–Fe chemistry for further development. Mixed solutions (i.e. comprising both chromium and iron species in the negolyte and in the posolyte) were used in order to reduce the effect of time-varying concentration during cycling.
In the late 1980s, Sum, Rychcik and Skyllas-KazacosE. R. Sum, M.; Skyllas-Kazacos, M., J Power Sources, 16 (2), 85-95 (1985); E. S.-K. Sum, M., J Power Sources, 15 (2-3), 179-190 (1985); M. Rychcik and M. Skyllas-Kazacos, J Power Sources, 19 (1), 45-54 (1987); M. Rychcik and M. Skyllas-Kazacos, J Power Sources, 22 (1), 59-67 (1988) at the University of New South Wales (UNSW) in Australia demonstrated vanadium RFB chemistry UNSW filed several patents related to VRFBs, that were later licensed to Japanese, Thai and Canadian companies, which tried to commercialize this technology with varying success.
Organic redox flow batteries emerged in 2009.
In 2022, Dalian, China began operating a 400 MWh, 100 MW vanadium flow battery, then the largest of its type.
Sumitomo Electric has built flow batteries for use in Taiwan, Belgium, Australia, Morocco and California. Hokkaido's flow battery farm was the biggest in the world when it opened in April 2022—until China deployed one eight times larger that can match the output of a natural gas plant.
Design A flow battery is a rechargeable fuel cell in which an electrolyte containing one or more dissolved electroactive elements flows through an electrochemical cell that reversibly converts chemical energy to electrical energy. Electroactive elements are "elements in solution that can take part in an electrode reaction or that can be adsorbed on the electrode."
Electrolyte is stored externally, generally in tanks, and is typically pumped through the cell (or cells) of the reactor. Flow batteries can be rapidly "recharged" by replacing discharged electrolyte liquid (analogous to refueling internal combustion engines) while recovering the spent material for recharging. They can also be recharged in situ. Many flow batteries use carbon felt electrodes due to its low cost and adequate electrical conductivity, despite their limited power density due to their low inherent activity toward many redox couples. The amount of electricity that can be generated depends on the volume of electrolyte.
Flow batteries are governed by the design principles of electrochemical engineering.
Evaluation
Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of:
Independent scaling of energy (tanks) and power (stack), which allows for a cost/weight/etc. optimization for each application Long cycle and calendar lives (because there are no solid-to-solid phase transitions, which degrade lithium-ion and related batteries) Quick response times No need for "equalisation" charging (the overcharging of a battery to ensure all cells have an equal charge) No harmful emissions Little/no self-discharge during idle periods Recycling of electroactive materials
Some types offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/overdischarge.
They are safe because they typically do not contain flammable electrolytes, and electrolytes can be stored away from the power stack.
The main disadvantages are:
Low energy density (large tanks are required to store useful amounts of energy) Low charge and discharge rates. This implies large electrodes and membrane separators, increasing cost. Lower energy efficiency, because they operate at higher current densities to minimize the effects of cross-over (internal self-discharge) and to reduce cost.
Flow batteries typically have a higher energy efficiency than fuel cells, but lower than lithium-ion batteries.
Conventional 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/cm2 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
Conventional flow batteries The redox cell uses redox-active species 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 than to conventional batteries. The main reason fuel cells are not considered to be batteries is that 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.
Cr–Fe chemistry has disadvantages, including hydrate isomerism (i.e. the equilibrium between electrochemically active chloro-complexes and inactive hexa-aqua complex and hydrogen evolution on the negode. Hydrate isomerism can be alleviated by adding chelating amino-ligands, while hydrogen evolution can be mitigated by adding Pb salts to increase the overvoltage and Au salts for catalyzing the chromium electrode reaction.
Conventional redox flow battery chemistries include iron-chromium, vanadium, polysulfide–bromide (Regenesys), and uranium. Redox fuel cells are less common commercially although many have been proposed.
Vanadium Vanadium redox flow batteries are the commercial leaders. They use vanadium at both electrodes, so they do not suffer cross-contamination. The limited solubility of vanadium salts, somewhat offsets this advantage in practice. This chemistry's advantages include four oxidation states within the electrochemical voltage window of the graphite-aqueous acid interface, and thus the elimination of the mixing dilution, detrimental in Cr–Fe RFBs. More importantly for commercial success is the near-perfect match of the voltage window of carbon/aqueous acid interface with that of vanadium redox-couples. This extends the life of the low-cost carbon electrodes and reduces the impact of side reactions, such as and evolutions, resulting in many year durability and many cycle (15,000–20,000) lives, which in turn results in a record low levelized cost of energy (LCOE, system cost divided by usable energy, cycle life, and round-trip efficiency). These long lifetimes allow for the amortization of their relatively high capital cost (driven by vanadium, carbon felts, bipolar plates, and membranes). The LCOE is on the order of a few tens cents per kWh, much lower than of solid-state batteries and near the targets of 5 cents stated by US and EC government agencies. Major challenges include: low abundance and high costs of (> $30 / kg); parasitic reactions including hydrogen and oxygen evolution; and precipitation of during cycling.
Hybrid The hybrid flow battery (HFB) uses one or more electroactive components deposited as a solid layer. The major disadvantage is that this reduces decoupled energy and power. The cell contains one battery electrode and one fuel cell electrode. This type is limited in energy by the electrode surface area.