||This article may be too technical for most readers to understand. (April 2013)|
A flow battery is a rechargeable fuel cell in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that reversibly converts chemical energy directly to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known. Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material for re-energization.
Classes of flow batteries 
Various classes of flow batteries exist including the redox (reduction-oxidation) flow battery, a reversible fuel cell in which all electroactive components are dissolved in the electrolyte. If one or more electroactive components are deposited as a solid layer the system is known as a hybrid flow battery; that is, the electrochemical cell contains one battery electrode and one fuel cell electrode. The main difference between these two types of flow batteries is that the energy of the redox flow battery, as with other fuel cells, is fully decoupled from the power, because the energy is related to the electrolyte volume (tank size) and the power to the electrode area (reactor size). The hybrid flow battery, similar to typical batteries, is limited in energy by the size of the battery electrode (reactor size).
Energy producing electrochemical cells are generally divided into two categories. Cells that can be discharged only, with irreversible electrochemical reactions, are termed primary cells, while rechargeable cells with reversible reactions are termed secondary cells (see also primary and secondary batteries). Using this historical convention, a redox flow battery is better described as a secondary fuel cell or regenerative fuel cell, with the fundamental difference between batteries and fuel cells being whether energy is stored in a solid state electrode material (batteries) or in the electrolyte (fuel cells). This difference leads to the decoupling of energy and power in a fuel cell described above.
The misnomer of "redox flow battery" rather than "reversible fuel cell" has led to a great deal of confusion in understanding and terminology. For example, the processes of solid-state diffusion and intercalation in a Lithium Ion Battery do not apply to redox flow batteries, but the heterogeneous electron transfer in a fuel cell does. In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H2 / O2 system, with limited examples of reversible systems (i.e., the unitized regenerative fuel cell on NASA's Helios Prototype). The European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).
Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery. Hybrid flow batteries include the zinc-bromine, zinc-cerium  and lead-acid flow batteries. Redox fuel cells are less common commercially although many systems have been proposed.
|Couple||Max. cell voltage (V)||Average electrode power density (W/m2)||Average fluid energy density (W·h/kg)|
Advantages and disadvantages 
||This article contains a pro and con list. (November 2012)|
Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of flexible layout (due to separation of the power and energy components), long cycle life (because there are no solid-solid phase transitions), quick response times, no need for "equalisation" charging (the over charging of a battery to ensure all cells have an equal charge) and no harmful emissions. Some types also offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/ overdischarge.
On the negative side, flow batteries are rather complicated in comparison with standard batteries as they may require pumps, sensors, control units and secondary containment vessels. The energy densities vary considerably but are, in general, rather low compared to portable batteries, such as the Li-ion.
Flow batteries are normally considered for relatively large (1 kW·h – 10 MW·h) stationary applications. These are for
- Load balancing - where the battery is connected to an electrical grid to store excess electrical power during off-peak hours and release electrical power during peak demand periods.
- Storing energy from renewable sources such as wind or solar for discharge during periods of peak demand.
- Peak shaving, where spikes of demand are met by the battery.
- UPS, where the battery is used if the main power fails to provide an uninterrupted supply.
- power conversion - because all cells share the same electrolyte/s. Therefore, the electrolyte/s may be charged using a given number of cells and discharged with a different number. Because the voltage of the battery is proportional to the number of cells used the battery can therefore act as a very powerful DC/DC converter. In addition, if the number of cells is continuously changed (on the input and/or output side) power conversion can also be AC/DC, AC/AC, or DC/AC with the frequency limited by that of the switching gear.
- Electric vehicles - Because flow batteries can be rapidly "recharged" by replacing the electrolyte, they can be used for applications where the vehicle needs to take on energy as fast as a combustion engined vehicle.
- Stand-alone power system - An example of this is the telecomms industry for use in cellphone base stations where there is no grid power available. The battery can be used alongside a solar or a wind power to compensate for their fluctuating power levels and alongside a generator to make the most efficient use of it to save fuel.
See also 
- Glossary of fuel cell terms
- Hydrogen technologies
- Load balancing
- Polysulfide bromide battery
- Redox electrode
- Vanadium redox flow battery
- Zinc-cerium hybrid flow battery
- Zinc-bromine hybrid flow battery
- ^ T. Fujii, T. Hirose, and N. Kondou, in JP Patent 55096569 (1979), to Meidensha Electric Mfg. Co. Ltd.
- ^ M. Bartolozzi, "Development of redox flow batteries. A historical Bibliography," J. Power Sources, vol. 27, pp. 219–234, 1989.
- ^ Linden, D.; Reddy, T.B. (2002). Handbook of Batteries (Eds.). McGraw-Hill.
- ^ L. H. Cutler, in US Patent 3607420 (1969), to E.I. du Pont de Nemours and Co.
- ^ Y. Shiokawa, H. Yamana, and H. Moriyama, "An application of actinide elements for a redox flow battery," J. Nucl. Sci. Tech., vol. 37, pp. 253–256, 2000.
- ^ P. K. Leung, C. Ponce de Leon, A.A. Shah, F.C. Walsh, "Characterisation of a zinc-cerium flow battery", J. Power Sources, 2011.
- ^ W. Borchers, in US Patent 567959 (1894)
- ^ W. Nernst, in DE Patent 264026 (1912)
- ^ R. M. Keefer, in US Patent 3682704 (1970), to Electrocell Ltd.
- ^ J. T. Kummer and D.-G. Oei, "A chemically regenerative redox fuel cell," J. Appl. Electrochem., vol. 12, pp. 87–100, 1982
- ^ http://www.redflow.com.au/DNSP.htm
- ^ P. M. Spaziante, K. Kampanatsanyakorn, and A. Zocchi, in WO Patent 03043170 (2001), to Squirrel Holdings Ltd.
- ^ Talk by John Davis of Deeya energy about their flow battery's use in the telecomms industry
- Electropaedia on Flow Batteries
- Research on the uranium redox flow battery
- Improved redox flow batteries for electric cars