View source for Flow battery

{{Short description|Type of electrochemical cell}}
{{Use dmy dates|date=November 2021}}
[[File:Redox Flow Battery.jpg|alt=Redox Flow Battery|thumb|upright=1.5|A typical flow battery consists of two tanks of liquids which are pumped past a membrane held between two electrodes.<ref name="Qi 040801">{{Cite journal|last1=Qi|first1=Zhaoxiang|last2=Koenig|first2=Gary M.|date=2017-05-12|title=Review Article: Flow battery systems with solid electroactive materials|journal=Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena|volume=35|issue=4|pages=040801|doi=10.1116/1.4983210|bibcode=2017JVSTB..35d0801Q|issn=2166-2746|doi-access=free}}</ref>]]

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 [[Solution (chemistry)|dissolved]] in liquids that are pumped through the system on separate sides of a membrane.<ref name="journal.frontiersin.org">{{cite journal |last1=Badwal |first1=Sukhvinder P. S. |last2=Giddey |first2=Sarbjit S. |last3=Munnings |first3=Christopher |last4=Bhatt |first4=Anand I. |last5=Hollenkamp |first5=Anthony F. |date=24 September 2014 |title=Emerging electrochemical energy conversion and storage technologies |journal=Frontiers in Chemistry |volume=2 |pages=79 |bibcode=2014FrCh....2...79B |doi=10.3389/fchem.2014.00079 |pmc=4174133 |pmid=25309898 |doi-access=free}}</ref><ref>{{Cite journal |last1=Alotto |first1=P. |last2=Guarnieri |first2=M. |last3=Moro |first3=F. |year=2014 |title=Redox Flow Batteries for the storage of renewable energy: a review |journal=Renewable & Sustainable Energy Reviews |volume=29 |pages=325–335 |doi=10.1016/j.rser.2013.08.001 |bibcode=2014RSERv..29..325A |hdl-access=free |hdl=11577/2682306}}</ref> [[Ionic transfer|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.

Various flow batteries have been demonstrated, including inorganic<ref>{{Cite book |author=Hu, B. |title=Redox Active Inorganic Materials for Redox Flow Batteries in Encyclopedia of Inorganic and Bioinorganic Chemistry: Inorganic Battery Materials |author2=Luo, J. |author3=DeBruler C. |author4=Hu, M |author5=Wu, W. |author6=Liu, T. L. |year=2019 |pages=1–25}}</ref> and organic forms.<ref>{{Cite journal |author=Luo, J. |author2=Hu, B. |author3=Hu, M. |author4=Liu, T. L. |date=13 September 2019 |title=Status and Prospects of Organic Redox Flow Batteries towards Renewable Energy Storage |url=https://pubs.acs.org/doi/abs/10.1021/acsenergylett.9b01332 |journal=ACS Energy Lett. |volume=2019, 4 |issue=9 |pages=2220–2240 |doi=10.1021/acsenergylett.9b01332 |s2cid=202210484|url-access=subscription }}</ref> Flow battery design can be further classified into full flow, semi-flow, and membraneless.

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]].

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).

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,<ref>{{cite web |last1=Clark |first1=Elliot |date=17 Nov 2023 |title=What is a Calendar-life? |url=https://energytheory.com/what-is-a-calendar-life/ |access-date=3 May 2024 |website=Energy Theory}}</ref> and potentially lower [[total cost of ownership]],. However, flow batteries suffer from low cycle [[Electrical efficiency|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 (Zn-Cl<sub>2</sub>, Zn-Br<sub>2</sub> and H<sub>2</sub>-LiBrO<sub>3</sub> 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.<ref>{{cite journal |author1=Yuriy V. Tolmachev |author2=Svetlana V. Starodubceva |year=2022 |title=Flow batteries with solid energy boosters |journal=Electrochemical Science and Engineering |volume=12 |issue=4 |pages=731–766 |doi=10.5599/jese.1363 |doi-access=free}}</ref>

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.{{Cn|date=June 2024}}

[[Electrode potential#Potential difference of a cell assembled of two electrodes|Cell voltage]] is chemically determined by the [[Nernst equation]] and ranges, in practical applications, from 1.0 to 2.43 [[Volt|volts]]. The energy capacity is a function of the electrolyte volume and the power is a function of the surface area of the [[Electrode|electrodes]].<ref name="auto2">{{Cite journal |last=Tolmachev |first=Yuriy V. |date=2023-03-01 |title=Review—Flow Batteries from 1879 to 2022 and Beyond |url=https://iopscience.iop.org/article/10.1149/1945-7111/acb8de |journal=Journal of the Electrochemical Society |volume=170 |issue=3 |pages=030505 |doi=10.1149/1945-7111/acb8de |bibcode=2023JElS..170c0505T |issn=0013-4651|url-access=subscription }}</ref>

== History ==
The [[zinc–bromine flow battery]] (Zn-Br2) was the original flow battery.<ref name="auto2"/> John Doyle file patent {{Patent|US|224404}} on September 29, 1879. Zn-Br2 batteries have relatively high specific energy, and were demonstrated in electric cars in the 1970s.<ref>{{Cite report |chapter-url=https://www.sae.org/content/730248/ |chapter=A Zinc-Chloride Battery - The Missing Link to a Practical Electric Car |last=Amato |first=C. J. |title=SAE Technical Paper Series |date=1973-02-01 |volume=1 |doi=10.4271/730248}}</ref>

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.<ref>W. Kangro Dr, 1949.; W. Kangro Dr, 1954.;W. Kangro and H. Pieper, Electrochim Acta, 7 (4), 435-448 (1962)</ref> 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 [[Maria_Skyllas-Kazacos|Skyllas-Kazacos]]<ref>E. 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)</ref> at the [[University of New South Wales]] (UNSW) in Australia demonstrated [[Vanadium redox battery|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.<ref>{{Cite journal |last1=Kear |first1=Gareth |last2=Shah |first2=Akeel A. |last3=Walsh |first3=Frank C. |date=September 2012 |title=Development of the all-vanadium redox flow battery for energy storage: a review of technological, financial and policy aspects: All-vanadium redox flow battery for energy storage |url=https://onlinelibrary.wiley.com/doi/10.1002/er.1863 |journal=International Journal of Energy Research |language=en |volume=36 |issue=11 |pages=1105–1120 |doi=10.1002/er.1863}}</ref>

Organic redox flow batteries emerged in 2009.<ref name=":1" />

In 2022, [[Dalian]], [[China]] began operating a 400 MWh, 100 MW vanadium flow battery, then the largest of its type.<ref>{{Cite web |date=2022-10-03 |title=World's largest flow battery connected to the grid in China |url=https://newatlas.com/energy/worlds-largest-flow-battery-grid-china/ |access-date=2022-10-12 |website=New Atlas |language=en-US}}</ref>

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.<ref>{{Cite web |last1=Rivero |first1=Nicolás |last2=Wright |first2=Emily |date=November 26, 2024 |title=These batteries could harness the wind and sun to replace coal and gas |url=https://www.washingtonpost.com/climate-solutions/interactive/2024/flow-batteries-renewable-energy-storage/ |website=The Washington Post}}</ref>

[[File:Log number of publications about electrochemical powersources by year.png|thumb|800px|center|The log number of publications related to electrochemical power sources by year. Also shown as the magenta line is the inflation-adjusted oil price in US$/liter in linear scale]]

== 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 [[electricity|electrical energy]]. Electroactive elements are "elements in solution that can take part in an electrode reaction or that can be [[Adsorption|adsorbed]] on the electrode."<ref name=Science-Dictionary /> 

Electrolyte is stored externally, generally in tanks, and is typically pumped through the cell (or cells) of the reactor.<ref name="Fujii" /> Flow batteries can be rapidly "recharged" by replacing discharged electrolyte liquid (analogous to refueling [[internal combustion engine]]s) 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.<ref>{{cite journal |last1=Aaron |first1=Douglas |title=In Situ Kinetics Studies in All-Vanadium Redox Flow Batteries |journal=ECS Electrochemistry Letters |volume=2 |issue=3 |pages=A29–A31 |doi=10.1149/2.001303eel|year=2013 }}</ref><ref>{{Cite journal|last=McCreery|first=Richard L.|date=July 2008|title=Advanced Carbon Electrode Materials for Molecular Electrochemistry|journal=Chemical Reviews|language=EN|volume=108|issue=7|pages=2646–2687|doi=10.1021/cr068076m|pmid=18557655|issn=0009-2665}}</ref> 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]].<ref>{{cite journal|last1=Arenas|first1=L.F.|last2=Ponce de León|first2=C.|last3=Walsh|first3=F.C.|title=Engineering aspects of the design, construction and performance of modular redox flow batteries for energy storage|journal=Journal of Energy Storage|date=June 2017|volume=11|pages=119–153|doi=10.1016/j.est.2017.02.007|bibcode=2017JEnSt..11..119A |url=https://eprints.soton.ac.uk/406710/1/Eng_RFB_Review_20_Feb_2017_eprints.pdf}}</ref>

== 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 transition]]s, 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:<ref name="Tolmachev 2022">{{Cite journal |last=Tolmachev |first=Yuriy |date=2023-01-10 |title=Flow Batteries From 1879 To 2022 And Beyond |journal=Qeios |url=https://www.qeios.com/read/G6G4EA.3 |doi=10.32388/G6G4EA.3|doi-access=free }}</ref>

* 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]].<ref>{{cite journal|title="Evaluation of redox flow batteries goes beyond round-trip efficiency: A technical review"|year=2018|journal=Journal of Energy Storage|volume=16|pages=108–116|doi=10.1016/j.est.2018.01.005|last1=Xu |first1=Q. |last2=Ji |first2=Y.N. |last3=Qin |first3=L.Y. |last4=Leung |first4=P.K. |last5=Qiao |first5=F. |last6=Li |first6=Y.S. |last7=Su |first7=H.N. |bibcode=2018JEnSt..16..108X }}</ref>

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<sup>2</sup> was demonstrated for hydrogen–bromine flow batteries, and a high specific energy (530&nbsp;Wh/kg at the tank level) was shown for hydrogen–bromate flow batteries<ref>{{cite journal |last1=Cho |first1=Kyu Taek |last2=Tucker |first2=Michael C. |last3=Ding |first3=Markus |last4=Ridgway |first4=Paul |last5=Battaglia |first5=Vincent S. |last6=Srinivasan |first6=Venkat |last7=Weber |first7=Adam Z. |year=2015 |title=Cyclic Performance Analysis of Hydrogen/Bromine Flow Batteries for Grid-Scale Energy Storage |journal=ChemPlusChem |volume=80 |issue=2 |pages=402–411 |doi=10.1002/cplu.201402043 |s2cid=97168677}}</ref><ref>{{cite journal |last1=Yu |last2=Tolmachev |first2=V. |year=2013 |title=Hydrogen-halogen electrochemical cells: A review of applications and technologies |journal=Russian Journal of Electrochemistry |volume=50 |issue=4 |pages=301–316 |doi=10.1134/S1023193513120069 |s2cid=97464125}}</ref><ref>{{cite journal |last1=Tolmachev |first1=Yuriy V. |year=2015 |title=Energy cycle based on a high specific energy aqueous flow battery and its potential use for fully electric vehicles and for direct solar-to-chemical energy conversion |journal=Journal of Solid State Electrochemistry |volume=19 |issue=9 |pages=2711–2722 |doi=10.1007/s10008-015-2805-z |s2cid=97853351}}</ref>

== Traditional flow batteries ==
The [[redox]] cell uses redox-active species in fluid (liquid or gas) media. Redox flow batteries are rechargeable ([[secondary battery|secondary]]) cells.<ref name="LindenReddy" /> Because they employ [[Electron transfer#Heterogeneous electron transfer|heterogeneous electron transfer]] rather than [[Chemical reaction#Solid-state reactions|solid-state diffusion]] or [[Lithium ion#Charging and discharging|intercalation]] they are more similar to [[fuel cell]]s than to conventional batteries. 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. {{chem|H|2}}/{{chem|O|2}}, 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 Cr3+ 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 H<sub>2</sub> overvoltage and Au salts for catalyzing the chromium electrode reaction.<ref>{{Cite journal |last1=Sun |first1=Chuanyu |last2=Zhang |first2=Huan |date=2022-01-10 |title=Review of the Development of First-Generation Redox Flow Batteries: Iron-Chromium System |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202101798 |journal=ChemSusChem |language=en |volume=15 |issue=1 |pages=e202101798 |doi=10.1002/cssc.202101798 |pmid=34724346 |bibcode=2022ChSCh..15E1798S |issn=1864-5631|url-access=subscription }}</ref>

Traditional redox flow battery chemistries include iron-chromium, vanadium, [[polysulfide–bromide battery|polysulfide–bromide]] (Regenesys), and [[uranium]].<ref name="Shiokawa" /> Redox fuel cells are less common commercially although many have been proposed.<ref name="Borchers" /><ref name="Nernst" /><ref name="Keefer" /><ref name="Kummer_and_Oei_1982" />

=== 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, however, 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 H2 and O2 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.<ref name="Spagn">{{Cite journal|last1=Spagnuolo|first1=G.|last2=Petrone|first2=G.|last3=Mattavelli|first3=P.|last4=Guarnieri|first4=M.|year=2016|title= Vanadium Redox Flow Batteries: Potentials and Challenges of an Emerging Storage Technology |journal= IEEE Industrial Electronics Magazine|volume=10|issue=4|pages=20–31|doi=10.1109/MIE.2016.2611760|hdl=11577/3217695|s2cid=28206437|hdl-access=free}}</ref> Major challenges include: low abundance and high costs of V<sub>2</sub>O<sub>5</sub> (> $30 / Kg); parasitic reactions including hydrogen and oxygen evolution; and precipitation of V<sub>2</sub>O<sub>5</sub> during cycling.

== Hybrid ==
The hybrid flow battery (HFB) uses one or more electroactive components deposited as a solid layer.<ref name="Bartolozzi" /> 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. 

HFBs include [[zinc–bromine flow battery|zinc–bromine]], [[zinc–cerium redox flow battery|zinc–cerium]],<ref name="PKLeung" /> soluble [[lead–acid battery|lead–acid]],<ref>{{Cite journal|last1=Krishna|first1=M.|last2=Fraser|first2=E. J.|last3=Wills|first3=R. G. A.|last4=Walsh|first4=F. C.|date=2018-02-01|title=Developments in soluble lead flow batteries and remaining challenges: An illustrated review|journal=Journal of Energy Storage|volume=15|pages=69–90|doi=10.1016/j.est.2017.10.020|issn=2352-152X|doi-access=free|bibcode=2018JEnSt..15...69K }}</ref> and [[Iron redox flow battery|all-iron]] flow batteries. Weng et al. reported a vanadium–[[metal hydride]] hybrid flow battery with an experimental OCV of 1.93 V and operating voltage of 1.70 V, relatively high values. It consists of a graphite felt positive electrode operating in a mixed solution of {{Chem|VOSO|4}}and {{Chem|H|2|SO|4}}, and a metal hydride negative electrode in [[potassium hydroxide|KOH]] aqueous solution. The two electrolytes of different pH are separated by a bipolar membrane. The system demonstrated good reversibility and high efficiencies in coulomb (95%), energy (84%), and voltage (88%). They reported improvements with increased current density, inclusion of larger 100&nbsp;cm<sup>2</sup> electrodes, and series operation. Preliminary data using a fluctuating simulated power input tested the viability toward kWh scale storage.<ref>{{Cite journal |last1=Weng |first1=Guo-Ming |last2=Li |first2=Chi-Ying Vanessa |last3=Chan |first3=Kwong-Yu |last4=Lee |first4=Cheuk-Wing |last5=Zhong |first5=Jin |date=2016 |title=Investigations of High Voltage Vanadium-Metal Hydride Flow Battery toward kWh Scale Storage with 100 cm 2 Electrodes |url=https://iopscience.iop.org/article/10.1149/2.0271601jes |journal=Journal of the Electrochemical Society |language=en |volume=163 |issue=1 |pages=A5180–A5187 |doi=10.1149/2.0271601jes |issn=0013-4651|url-access=subscription }}</ref> In 2016, a high energy density Mn(VI)/Mn(VII)-Zn hybrid flow battery was proposed.<ref name="Colli2016" />

=== Zinc-polyiodide ===
A prototype [[zinc]]–[[polyiodide]] flow battery demonstrated an energy density of 167&nbsp;Wh/L. Older zinc–bromide cells reach 70&nbsp;Wh/L. For comparison, [[lithium iron phosphate battery|lithium iron phosphate batteries]] store 325&nbsp;Wh/L. The zinc–polyiodide battery is claimed to be safer than other flow batteries given its absence of acidic electrolytes, nonflammability and operating range of {{convert|-4|to|122|F|C}} that does not require extensive cooling circuitry, which would add weight and occupy space. One unresolved issue is zinc buildup on the negative electrode that can permeate the membrane, reducing efficiency. Because of the Zn dendrite formation, Zn-halide batteries cannot operate at high current density (>&nbsp;20 mA/cm<sup>2</sup>) and thus have limited power density. Adding alcohol to the electrolyte of the ZnI battery can help.<ref>{{cite news |title=High-performance flow battery could rival lithium-ions for EVs and grid storage |first=Dario |last=Borghino |date=27 February 2015 |publisher=Gizmag}}</ref> The drawbacks of Zn/I RFB lie are the high cost of Iodide salts (> $20 / Kg); limited area capacity of Zn deposition, reducing the decoupled energy and power; and Zn dendrite formation.

When the battery is fully discharged, both tanks hold the same electrolyte solution: a mixture of positively charged zinc ions ({{chem|Zn|2+}}) and negatively charged iodide ion, ({{chem|I|-}}). When charged, one tank holds another negative ion, polyiodide, ({{chem|I|3|-}}). The battery produces power by pumping liquid across the stack where the liquids mix. Inside the stack, zinc ions pass through a selective membrane and change into metallic zinc on the stack's negative side.<ref>{{cite news|title=New flow battery to keep big cities lit, green and safe |date=25 February 2015 |first=Frances |last=White |publisher=R&D |url=http://www.rdmag.com/videos/2015/02/new-flow-battery-keep-big-cities-lit-green-and-safe}}</ref> To increase energy density, bromide ions ({{chem|Br|–}}) are used as the complexing agent to stabilize the free iodine, forming iodine–bromide ions ({{chem|I|2|Br|-}}) as a means to free up iodide ions for charge storage.<ref>{{cite journal|last1=Weng|first1=Guo-Ming|title=Unlocking the capacity of iodide for high-energy-density zinc/polyiodide and lithium/polyiodide redox flow batteries|journal=Energy & Environmental Science|date=2017|volume=10|issue=3|pages=735–741|doi=10.1039/C6EE03554J}}</ref>

=== Proton flow ===
Proton flow batteries (PFB) integrate a [[metal hydride]] storage electrode into a reversible [[proton exchange membrane]] (PEM) [[fuel cell]]. During charging, PFB combines hydrogen ions produced from splitting water with electrons and metal particles in one electrode of a fuel cell. The energy is stored in the form of a metal hydride solid. Discharge produces electricity and water when the process is reversed and the protons are combined with ambient oxygen. Metals less expensive than lithium can be used and provide greater energy density than lithium cells.<ref name="Quick" /><ref name="Andrews" />

== Organic ==
Compared to inorganic redox flow batteries, such as vanadium and Zn-Br<sub>2</sub> batteries, organic redox flow batteries' advantage is the tunable redox properties of their active components. As of 2021, organic RFB experienced low durability (i.e. calendar or cycle life, or both) and have not been demonstrated on a commercial scale.<ref name=":1">{{Cite journal |last1=Kwabi |first1=David G. |last2=Ji |first2=Yunlong |last3=Aziz |first3=Michael J. |date=2020-07-22 |title=Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00599 |journal=Chemical Reviews |language=en |volume=120 |issue=14 |pages=6467–6489 |doi=10.1021/acs.chemrev.9b00599 |pmid=32053366 |osti=1799071 |issn=0009-2665}}</ref>

Organic redox flow batteries can be further classified into aqueous (AORFBs) and non-aqueous (NAORFBs).<ref>{{Cite journal|last1=Brushett|first1=Fikile|last2=Vaughey|first2=John|last3=Jansen|first3=Andrew|date=2012|title=An All-Organic Non-aqueous Lithium-Ion Redox Flow Battery|journal=Advanced Functional Materials|language=en|volume=2|issue=11|pages=1390–1396|doi=10.1002/aenm.201200322|bibcode=2012AdEnM...2.1390B |s2cid=97300070 }}</ref><ref>{{Cite journal|last1=Bamgbopa|first1=Musbaudeen O.|author2-link=Yang Shao-Horn|last2=Shao-Horn|first2=Yang|last3=Almheiri|first3=Saif|date=2017|title=The potential of non-aqueous redox flow batteries as fast-charging capable energy storage solutions: demonstration with an iron–chromium acetylacetonate chemistry|journal=Journal of Materials Chemistry A|language=en|volume=5|issue=26|pages=13457–13468|doi=10.1039/c7ta02022h|issn=2050-7488}}</ref> AORFBs use water as solvent for electrolyte materials while NAORFBs employ organic solvents. AORFBs and NAORFBs can be further divided into total and hybrid systems. The former use only organic electrode materials, while the latter use inorganic materials for either anode or cathode. In larger-scale energy storage, lower solvent cost and higher conductivity give AORFBs greater commercial potential, as well as offering the safety advantages of water-based electrolytes. NAORFBs instead provide a much larger voltage window and occupy less space.

=== pH neutral AORFBs ===
pH neutral AORFBs are operated at pH 7 conditions, typically using [[Sodium chloride|NaCl]] as a supporting electrolyte. At pH neutral conditions, organic and organometallic molecules are more stable than at corrosive acidic and alkaline conditions. For example, K4[Fe(CN)], a common catholyte used in AORFBs, is not stable in alkaline solutions but is at pH neutral conditions.<ref>{{Cite journal |author=Luo, J. |author2=Sam, A. |author3=Hu, B. |author4=DeBruler C. |author5=Liu, T. L. |title=Unraveling pH Dependent Cycling Stability of Ferricyanide / Ferrocyanide in Redox Flow Batteries |journal=Nano Energy|year=2017 |volume=2017, 42|pages=215–221|doi=10.1016/j.nanoen.2017.10.057 |bibcode=2017NEne...42..215L }}</ref>

AORFBs used [[Paraquat|methyl viologen]] as [[anolyte|an anolyte]] and [[4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl]] as a catholyte at pH neutral conditions, plus NaCl and a low-cost anion exchange membrane. This MV/TEMPO system has the highest cell voltage, 1.25{{nbsp}}V, and, possibly, lowest capital cost ($180/kWh) reported for AORFBs as of 2015. The aqueous liquid electrolytes were designed as a drop-in replacement without replacing infrastructure. A 600-milliwatt test battery was stable for 100 cycles with nearly 100 percent efficiency at current densities ranging from 20 to 100 mA/cm{{sup|2}}, with optimal performance rated at 40–50{{nbsp}}mA, at which about 70% of the battery's original voltage was retained.<ref>{{Cite web|title = New flow battery projected to cost 60% less than existing standard|url = http://www.gizmag.com/pnnl-low-cost-sustainable-flow-battery/41028|website = www.gizmag.com|access-date = 23 December 2015|date = 22 December 2015|last = Moss|first = Richard}}</ref><ref>{{Cite journal|title = A Total Organic Aqueous Redox Flow Battery Employing a Low Cost and Sustainable Methyl Viologen Anolyte and 4-HO-TEMPO Catholyte|journal = Advanced Energy Materials|date = 1 November 2015|issn = 1614-6840|pages = 1501449|doi = 10.1002/aenm.201501449|language = en|first1 = Tianbiao|last1 = Liu|first2 = Xiaoliang|last2 = Wei|first3 = Zimin|last3 = Nie|first4 = Vincent|last4 = Sprenkle|first5 = Wei|last5 = Wang|volume=6|issue = 3| s2cid=97838438 }}</ref> Neutral AORFBs can be more environmentally friendly than acid or alkaline alternatives, while showing electrochemical performance comparable to corrosive RFBs. The MV/TEMPO AORFB has an energy density of 8.4{{nbsp}}Wh/L with the limitation on the TEMPO side. In 2019[[Viologen]]-based flow batteries using an ultralight [[sulfonate]]–viologen/[[ferrocyanide]] AORFB were reported to be stable for 1000 cycles at an energy density of 10 Wh/L, the most stable, energy-dense AORFB to that date.<ref>{{Cite journal |author=Luo, J. |author2=Hu, B. |author3=DeBruler C. |author4=Zhao, Y. |author5=Yuan B. |author6=Hu, M. |author7=Wu, W. |author8=Liu, T. L.|title=Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries|url=https://www.cell.com/joule/fulltext/S2542-4351(18)30472-0|journal=Joule|date=2019 |volume=4|issue=1 |pages=149–163|doi=10.1016/j.joule.2018.10.010 |bibcode=2019Joule...3..149L |url-access=subscription |doi-access=free }}</ref>

=== Acidic AORFBs ===
[[Quinone]]s and their derivatives are the basis of many organic redox systems.<ref>{{Cite journal|last1=Gong|first1=K|last2=Fang|first2=Q|last3=Gu|first3=S|last4=Li|first4=F.S.Y.|last5=Yan|first5=Y|date=2015|title=Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs|journal=Energy and Environmental Science|language=en|volume=8|issue=12|pages=3515–3530|doi=10.1039/C5EE02341F|doi-access=free}}</ref><ref>{{Cite book |last1=Xu |first1=Yan |last2=Wen |first2=Yuehua |last3=Cheng |first3=Jie |last4=Yanga |first4=Yusheng |last5=Xie |first5=Zili |last6=Cao |first6=Gaoping |chapter=Novel organic redox flow batteries using soluble quinonoid compounds as positive materials |date=September 2009 |title=2009 World Non-Grid-Connected Wind Power and Energy Conference |chapter-url=http://dx.doi.org/10.1109/wnwec.2009.5335870 |publisher=IEEE |pages=1–4 |doi=10.1109/wnwec.2009.5335870|isbn=978-1-4244-4702-2 }}</ref><ref>{{cite journal|last1=Xu|first1=Yan|last2=Wen|first2=Yue-Hua|last3=Cheng|first3=Jie|last4=Cao|first4=Gao-Ping|last5=Yang|first5=Yu-Sheng|year=2010|title=A study of tiron in aqueous solutions for redox flow battery application|journal=Electrochimica Acta|volume=55|issue=3|pages=715–720|doi=10.1016/j.electacta.2009.09.031|issn=0013-4686}}</ref> In one study, 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) and 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) were employed as cathodes, and conventional Pb/[[Lead(II) sulfate|PbSO<sub>4</sub>]] was the anolyte in a hybrid acid AORFB. Quinones accept two units of electrical charge, compared with one in conventional catholyte, implying twice as much energy in a given volume.

Another quinone 9,10-Anthraquinone-2,7-disulfonic acid (AQDS), was evaluated.<ref name="Wald" /> AQDS undergoes rapid, reversible two-electron/two-proton reduction on a glassy carbon [[electrode]] in [[sulfuric acid]]. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the {{chem|Br|2}}/{{chem|Br|−}} [[redox]] couple, yields a peak galvanic [[power density]] exceeding 6,000&nbsp;W/m<sup>2</sup> at 13,000&nbsp;A/m<sup>2</sup>. Cycling showed >&nbsp;99% storage capacity retention per cycle. Volumetric energy density was over 20&nbsp;Wh/L.<ref name="greencar" /> Anthraquinone-2-sulfonic acid and anthraquinone-2,6-disulfonic acid on the negative side and 1,2-dihydrobenzoquinone- 3,5-disulfonic acid on the positive side avoids the use of hazardous Br<sub>2</sub>. The battery was claimed to last 1,000 cycles without degradation.<ref>{{Cite web|last=Szondy|first=David|date=29 June 2014|title=New water-based organic battery is cheap, rechargeable and eco-friendly|url=http://www.gizmag.com/organic-redox-flow-battery/32739|publisher=Gizmag}}</ref> It has a low cell voltage (ca. 0.55{{nbsp}}V) and a low energy density (< 4{{nbsp}}Wh/L).

Replacing [[hydrobromic acid]] with a less toxic alkaline solution (1{{nbsp}}M [[potassium hydroxide|KOH]]) and [[ferrocyanide]]<ref>{{Cite web|title=A rechargeable battery to power a home from rooftop solar panels|url=https://phys.org/news/2015-09-rechargeable-battery-power-home-rooftop.html|website=phys.org}}</ref> was less corrosive, allowing the use of inexpensive [[polymer]] tanks. The increased electrical resistance in the membrane was compensated increased voltage to 1.2{{nbsp}}V.<ref>{{cite web|author=Matthew Gunther, ChemistryWorld|title=Flow Battery Could Smooth Irregular Wind and Solar Energy Supply|url=http://www.scientificamerican.com/article/flow-battery-could-smooth-irregular-wind-and-solar-energy-supply/|work=Scientific American}}</ref><ref>{{Cite journal |last1=Lin |first1=Kaixiang |last2=Chen |first2=Qing |last3=Gerhardt |first3=Michael R. |last4=Tong |first4=Liuchuan |last5=Kim |first5=Sang Bok |last6=Eisenach |first6=Louise |last7=Valle |first7=Alvaro W. |last8=Hardee |first8=David |last9=Gordon |first9=Roy G. |last10=Aziz |first10=Michael J. |last11=Marshak |first11=Michael P. |date=2015-09-25 |title=Alkaline quinone flow battery |url=https://www.science.org/doi/10.1126/science.aab3033 |journal=Science |language=en |volume=349 |issue=6255 |pages=1529–1532 |doi=10.1126/science.aab3033 |pmid=26404834 |bibcode=2015Sci...349.1529L |issn=0036-8075|url-access=subscription }}</ref> Cell efficiency exceeded 99%, while round-trip efficiency measured 84%. The battery offered an expected lifetime of at least 1,000 cycles. Its theoretic energy density was 19{{nbsp}}Wh/L.<ref>{{Cite web|last=Borghino|first=Dario|date=30 September 2015|title=Greener, safer flow battery could store renewable energy on the cheap|url=http://www.gizmag.com/alkaline-flow-battery/39606/|access-date=8 December 2015|website=www.gizmag.com}}</ref> [[Ferrocyanide]]'s chemical stability in high pH KOH solution was not verified.

Integrating both anolyte and catholyte in the same molecule, i.e., bifunctional [[analytes]]<ref name="Carretero-González 2016 3521–3530">{{Cite journal|last1=Carretero-González|first1=Javier|last2=Castillo-Martínez|first2=Elizabeth|last3=Armand|first3=Michel|date=2016|title=Highly water-soluble three-redox state organic dyes as bifunctional analytes|journal=Energy & Environmental Science|language=en|volume=9|issue=11|pages=3521–3530|doi=10.1039/C6EE01883A|issn=1754-5692}}</ref> or combi-molecules<ref name="Winsberg 976–980">{{Cite journal|last1=Winsberg|first1=Jan|last2=Stolze|first2=Christian|last3=Muench|first3=Simon|last4=Liedl|first4=Ferenc|last5=Hager|first5=Martin D.|last6=Schubert|first6=Ulrich S.|date=2016-11-11|title=TEMPO/Phenazine Combi-Molecule: A Redox-Active Material for Symmetric Aqueous Redox-Flow Batteries|journal=ACS Energy Letters|language=en|volume=1|issue=5|pages=976–980|doi=10.1021/acsenergylett.6b00413|issn=2380-8195}}</ref> allow the same material to be used in both tanks. In one tank it is an electron donor, while in the other it is an electron recipient. This has advantages such as diminishing crossover effects.<ref name="Potash 2016 A338–A344">{{Cite journal|last1=Potash|first1=Rebecca A.|last2=McKone|first2=James R.|last3=Conte|first3=Sean|last4=Abruña|first4=Héctor D.|date=2016|title=On the Benefits of a Symmetric Redox Flow Battery|journal=Journal of the Electrochemical Society|language=en|volume=163|issue=3|pages=A338–A344|doi=10.1149/2.0971602jes|osti=1370440|s2cid=101469730 |issn=0013-4651|doi-access=free}}</ref> Thus, quinone diaminoanthraquinone<ref name="Potash 2016 A338–A344" /> and indigo-based<ref name="Carretero-González 2016 3521–3530" /> molecules as well as TEMPO/[[phenazine]]<ref name="Winsberg 976–980" /> are potential electrolytes for such symmetric redox-flow batteries (SRFB).

Another approach adopted a Blatter radical as the donor/recipient. It endured 275 charge and discharge cycles in tests, although it was not water-soluble.<ref>{{Cite web |last=Lavars |first=Nick |date=2022-03-17 |title=Symmetrical flow battery may strike right balance for grid-scale storage |url=https://newatlas.com/energy/symmetrical-flow-battery-grid-scale-storage/ |access-date=2022-03-18 |website=New Atlas |language=en-US}}</ref>

=== Alkaline ===
Quinone and [[fluorenone]] molecules can be reengineered to increase water solubility. In 2021 a reversible [[ketone]] (de)hydrogenation demonstration cell operated continuously for 120 days over 1,111 charging cycles at room temperature without a catalyst, retaining 97% percent capacity. The cell offered more than double the energy density of vanadium-based systems.<ref>{{Cite web|last=Lavars|first=Nick|date=21 May 2021|title=Candle compound brings high density to grid-scale battery technology|url=https://newatlas.com/energy/redox-flow-battery-fluorenone-high-density-grid-scale-pnnl/|access-date=2021-05-26|website=New Atlas|language=en-US}}</ref><ref>{{Cite journal|last1=Feng|first1=Ruozhu|last2=Zhang|first2=Xin|last3=Murugesan|first3=Vijayakumar|last4=Hollas|first4=Aaron|last5=Chen|first5=Ying|last6=Shao|first6=Yuyan|last7=Walter|first7=Eric|last8=Wellala|first8=Nadeesha P. N.|last9=Yan|first9=Litao|last10=Rosso|first10=Kevin M.|last11=Wang|first11=Wei|date=2021-05-21|title=Reversible ketone hydrogenation and dehydrogenation for aqueous organic redox flow batteries|url=https://www.science.org/doi/10.1126/science.abd9795|journal=Science|language=en|volume=372|issue=6544|pages=836–840|doi=10.1126/science.abd9795|issn=0036-8075|pmid=34016776|bibcode=2021Sci...372..836F|s2cid=234794555|url-access=subscription}}</ref> The major challenge was the lack of a stable catholyte, holding energy densities below 5 Wh/L. Alkaline AORFBs use excess [[potassium ferrocyanide]] catholyte because of the stability issue of [[ferrocyanide]] in alkaline solutions.

Metal-organic flow batteries use organic [[ligand]]s to improve redox properties. The ligands can be [[Chelation|chelates]] such as [[Ethylenediaminetetraacetic acid|EDTA]], and can enable the electrolyte to be in neutral or alkaline conditions under which [[metal aquo complex]]es would otherwise precipitate. By blocking the coordination of water to the metal, organic ligands can inhibit metal-catalyzed [[Water splitting|water-splitting reactions]], resulting in higher voltage aqueous systems. For example, the use of [[chromium]] coordinated to 1,3-propanediaminetetraacetate (PDTA), gave cell potentials of 1.62 V vs. ferrocyanide and a record 2.13 V vs. [[bromine]].<ref name=":0">{{Cite journal|last1=Robb|first1=Brian H.|last2=Farrell|first2=Jason M.|last3=Marshak|first3=Michael P.|date=2019|title=Chelated Chromium Electrolyte Enabling High-Voltage Aqueous Flow Batteries|journal=Joule|language=en|volume=3|issue=10|pages=2503–2512|doi=10.1016/j.joule.2019.07.002|doi-access=free|bibcode=2019Joule...3.2503R }}</ref> Metal-organic flow batteries may be known as coordination chemistry flow batteries, such as [[Lockheed Martin]]'s Gridstar Flow technology.<ref>{{Cite web|title=Energy Storage: GridStar Flow|url=https://www.lockheedmartin.com/en-us/capabilities/energy/energy-storage.html|access-date=2020-07-27|website=Lockheed Martin}}</ref>

=== Oligomer ===
[[Oligomer]] redox-species were proposed to reduce crossover, while allowing low-cost membranes. Such redox-active oligomers are known as redoxymers. One system uses organic polymers and a saline solution with a [[cellulose]] membrane. A prototype underwent 10,000 charging cycles while retaining substantial capacity. The energy density was 10 Wh/L.<ref>{{Cite web|date=21 October 2015|title=Chemists present an innovative redox-flow battery based on organic polymers and water|url=http://phys.org/news/2015-10-chemists-redox-flow-battery-based-polymers.html|access-date=6 December 2015|website=phys.org}}</ref> Current density reached ,1 amperes/cm<sup>2</sup>.<ref>{{Cite journal|last1=Janoschka|first1=Tobias|last2=Martin|first2=Norbert|last3=Martin|first3=Udo|last4=Friebe|first4=Christian|last5=Morgenstern|first5=Sabine|last6=Hiller|first6=Hannes|last7=Hager|first7=Martin D.|last8=Schubert|first8=Ulrich S.|year=2015|title=An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials|journal=Nature|volume=527|issue=7576|pages=78–81|bibcode=2015Natur.527...78J|doi=10.1038/nature15746|pmid=26503039|s2cid=4393601}}</ref>

Another oligomer RFB employed viologen and TEMPO redoxymers in combination with low-cost [[dialysis (chemistry)|dialysis]] membranes. Functionalized [[macromolecules]] (similar to acrylic glass or [[styrofoam]]) dissolved in water were the active electrode material. The size-selective nanoporous membrane worked like a strainer and is produced much more easily and at lower cost than conventional ion-selective membranes. It block the big "spaghetti"-like polymer molecules, while allowing small counterions to pass.<ref>{{cite journal | last1 = Janoschka | first1 = Tobias | last2 = Martin | first2 = Norbert | last3 = Martin | first3 = Udo | last4 = Friebe | first4 = Christian | last5 = Morgenstern | first5 = Sabine | last6 = Hiller | first6 = Hannes | last7 = Hager | first7 = Martin D. | last8 = Schubert | first8 = Ulrich S. | year = 2015 | title = An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials | journal = Nature | volume =  527| issue = 7576| pages =  78–81| doi = 10.1038/nature15746 | pmid = 26503039 | bibcode = 2015Natur.527...78J | s2cid = 4393601 }}</ref> The concept may solve the high cost of traditional [[Nafion]] membrane. RFBs with [[oligomer]] redox-species have not demonstrated competitive area-specific power. Low operating current density may be an intrinsic feature of large redox-molecules.{{Cn|date=June 2024}}

== Other types ==
Other flow-type batteries include the [[zinc–cerium battery]], the [[zinc–bromine battery]], and the [[hydrogen–bromine battery]].

=== Membraneless ===
A membraneless battery<ref>{{Cite journal|last1=Bamgbopa|first1=Musbaudeen O.|last2=Almheiri|first2=Saif|last3=Sun|first3=Hong|date=2017|title=Prospects of recently developed membraneless cell designs for redox flow batteries|journal=Renewable and Sustainable Energy Reviews|volume=70|pages=506–518|doi=10.1016/j.rser.2016.11.234|bibcode=2017RSERv..70..506B |issn=1364-0321}}</ref> relies on [[laminar flow]] in which two liquids are pumped through a channel, where they undergo electrochemical reactions to store or release energy. The solutions pass in parallel, with little mixing. The flow naturally separates the liquids, without requiring a membrane.<ref name="Kurzweil" />

Membranes are often the most costly and least reliable battery components, as they are subject to corrosion by repeated exposure to certain reactants. The absence of a membrane enables the use of a liquid bromine solution and hydrogen: this combination is problematic when membranes are used, because they form [[hydrobromic acid]] that can destroy the membrane. Both materials are available at low cost.<ref name="Braff" /> The design uses a small channel between two electrodes. Liquid bromine flows through the channel over a graphite cathode and hydrobromic acid flows under a porous anode. At the same time, hydrogen gas flows across the anode. The chemical reaction can be reversed to recharge the battery – a first for a membraneless design.<ref name="Braff" /> One such membraneless flow battery announced in August 2013 produced a maximum power density of 0.795&nbsp;W/cm<sup>2</sup>, three times more than other membraneless systems—and an order of magnitude higher than lithium-ion batteries.<ref name="Braff" />

In 2018, a macroscale membraneless RFB capable of recharging and recirculation of the electrolyte streams was demonstrated. The battery was based on immiscible organic catholyte and aqueous anolyte liquids, which exhibited high capacity retention and Coulombic efficiency during cycling.<ref>{{Cite journal|last1=Bamgbopa|first1=Musbaudeen O.|last2=Shao-Horn|first2=Yang|author2-link=Yang Shao-Horn|last3=Hashaikeh|first3=Raed|last4=Almheiri|first4=Saif|date=2018|title=Cyclable membraneless redox flow batteries based on immiscible liquid electrolytes: Demonstration with all-iron redox chemistry|journal=Electrochimica Acta|volume=267|pages=41–50|doi=10.1016/j.electacta.2018.02.063|issn=0013-4686}}</ref>

=== Suspension-based ===
[[File:Semi-Solid Flow Battery.png|thumb|Semi-solid flow battery<ref>{{Cite journal|last1=Qi|first1=Zhaoxiang|last2=Koenig|first2=Gary M.|date=2017|title=Review Article: Flow battery systems with solid electroactive materials|journal=Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena|language=en|volume=35|issue=4|pages=040801|doi=10.1116/1.4983210|bibcode=2017JVSTB..35d0801Q|issn=2166-2746|doi-access=free}}</ref>]]

A lithium–sulfur system arranged in a network of [[nanoparticle]]s eliminates the requirement that charge moves in and out of particles that are in direct contact with a conducting plate. Instead, the nanoparticle network allows electricity to flow throughout the liquid. This allows more energy to be extracted.<ref name=MITTech />

In a [[semi-solid flow battery]], positive and negative electrode particles are suspended in a carrier liquid. The suspensions are flow through a stack of reaction chambers, separated by a barrier such as a thin, porous membrane. The approach combines the basic structure of aqueous-flow batteries, which use electrode material suspended in a liquid electrolyte, with the chemistry of lithium-ion batteries in both carbon-free suspensions and slurries with a conductive carbon network.<ref name="Qi 040801"/><ref>{{Cite journal|last=Duduta|first=Mihai|date=May 2011|title=Semi-Solid Lithium Rechargeable Flow Battery|journal=Advanced Energy Materials|volume=1|issue=4|pages=511–516|doi=10.1002/aenm.201100152|bibcode=2011AdEnM...1..511D |s2cid=97634258 }}</ref><ref>{{Cite journal|last1=Qi|first1=Zhaoxiang|last2=Koenig Jr.|first2=Gary M.|date=2016-08-15|title=A carbon-free lithium-ion solid dispersion redox couple with low viscosity for redox flow batteries|journal=Journal of Power Sources|volume=323|pages=97–106|doi=10.1016/j.jpowsour.2016.05.033|bibcode=2016JPS...323...97Q|doi-access=free}}</ref> The carbon-free semi-solid RFB is also referred to as [[Solid dispersion redox flow battery|solid dispersion redox flow batteries]].<ref>{{Cite journal|last1=Qi|first1=Zhaoxiang|last2=Liu|first2=Aaron L.|last3=Koenig Jr|first3=Gary M.|date=2017-02-20|title=Carbon-free Solid Dispersion LiCoO2 Redox Couple Characterization and Electrochemical Evaluation for All Solid Dispersion Redox Flow Batteries|journal=Electrochimica Acta|volume=228|pages=91–99|doi=10.1016/j.electacta.2017.01.061}}</ref> Dissolving a material changes its chemical behavior significantly. However, suspending bits of solid material preserves the solid's characteristics. The result is a viscous suspension.<ref>{{Cite news|url = http://www.technologyreview.com/article/425107/go-with-the-flow/|title = Go with the Flow – Cambridge Crude|last = Chandler|first = David L.|date = 23 August 2011|work = [[Technology Review]]}}</ref>

In 2022, Influit Energy announced a flow battery electrolyte consisting of a metal oxide suspended in an aqueous solution.<ref>{{Citation |title=Darpa Nanoelectrofuel Flow Battery | date=18 March 2022 |url=https://www.youtube.com/watch?v=8333g246i3o |access-date=2022-08-09 |language=en}}</ref><ref>{{Cite web |last=Blain |first=Loz |date=2022-08-09 |title=Influit moves to commercialize its ultra-high density liquid batteries |url=https://newatlas.com/energy/influit-flow-battery-density/ |access-date=2022-08-09 |website=New Atlas |language=en-US}}</ref>

Flow batteries with redox-targeted solids (ROTS), also known as solid energy boosters (SEBs)<ref>{{Cite journal |last1=Yan |first1=Ruiting |last2=Wang |first2=Qing |date=November 2018 |title=Redox-Targeting-Based Flow Batteries for Large-Scale Energy Storage |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.201802406 |journal=Advanced Materials |language=en |volume=30 |issue=47 |pages=e1802406 |doi=10.1002/adma.201802406 |pmid=30118550 |bibcode=2018AdM....3002406Y |issn=0935-9648|url-access=subscription }}</ref><ref>{{Cite journal |last1=Ye |first1=Jiaye |last2=Xia |first2=Lu |last3=Wu |first3=Chun |last4=Ding |first4=Mei |last5=Jia |first5=Chuankun |last6=Wang |first6=Qing |date=2019-10-30 |title=Redox targeting-based flow batteries |url=https://iopscience.iop.org/article/10.1088/1361-6463/ab3251 |journal=Journal of Physics D: Applied Physics |volume=52 |issue=44 |pages=443001 |doi=10.1088/1361-6463/ab3251 |bibcode=2019JPhD...52R3001Y |issn=0022-3727|url-access=subscription }}</ref><ref>{{Cite journal |last=Li |first=Xianfeng |date=September 2019 |title=Redox Targeting Improves Flow Batteries |url=https://linkinghub.elsevier.com/retrieve/pii/S2542435119304234 |journal=Joule |language=en |volume=3 |issue=9 |pages=2066–2067 |doi=10.1016/j.joule.2019.08.020|bibcode=2019Joule...3.2066L }}</ref><ref>{{Cite journal |last1=Zhou |first1=Mingyue |last2=Chen |first2=Yan |last3=Salla |first3=Manohar |last4=Zhang |first4=Hang |last5=Wang |first5=Xun |last6=Mothe |first6=Srinivasa Reddy |last7=Wang |first7=Qing |date=2020-08-17 |title=Single-Molecule Redox-Targeting Reactions for a pH-Neutral Aqueous Organic Redox Flow Battery |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.202004603 |journal=Angewandte Chemie International Edition |language=en |volume=59 |issue=34 |pages=14286–14291 |doi=10.1002/anie.202004603 |pmid=32510721 |issn=1433-7851|url-access=subscription }}</ref><ref>{{Cite journal |last1=Wang |first1=Xun |last2=Zhou |first2=Mingyue |last3=Zhang |first3=Feifei |last4=Zhang |first4=Hang |last5=Wang |first5=Qing |date=October 2021 |title=Redox targeting of energy materials |url=https://linkinghub.elsevier.com/retrieve/pii/S2451910321000570 |journal=Current Opinion in Electrochemistry |language=en |volume=29 |pages=100743 |doi=10.1016/j.coelec.2021.100743|url-access=subscription }}</ref><ref>{{Cite journal |last1=Zhang |first1=Feifei |last2=Gao |first2=Mengqi |last3=Huang |first3=Shiqiang |last4=Zhang |first4=Hang |last5=Wang |first5=Xun |last6=Liu |first6=Lijun |last7=Han |first7=Ming |last8=Wang |first8=Qing |date=June 2022 |title=Redox Targeting of Energy Materials for Energy Storage and Conversion |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.202104562 |journal=Advanced Materials |language=en |volume=34 |issue=25 |pages=e2104562 |doi=10.1002/adma.202104562 |pmid=34595770 |bibcode=2022AdM....3404562Z |issn=0935-9648|url-access=subscription }}</ref><ref>{{Cite web|url=https://www.researchgate.net/publication/362509981|title=130+ million publications organized by topic on ResearchGate|access-date=21 May 2023}}</ref> either the posolyte or negolyte or both (a.k.a. redox fluids), come in contact with one or more solid electroactive materials (SEM). The fluids comprise one or more redox couples, with redox potentials flanking the redox potential of the SEM. Such SEB/RFBs combine the high specific energy advantage of conventional batteries (such as [[lithium-ion]]) with the decoupled energy-power advantage of flow batteries. SEB(ROTS) RFBs have advantages compared to semi-solid RFBs, such as no need to pump viscous slurries, no precipitation/clogging, higher area-specific power, longer durability, and wider chemical design space. However, because of double energy losses (one in the stack and another in the tank between the SEB(ROTS) and a mediator), such batteries suffer from poor energy efficiency. On a system-level, the practical specific energy of traditional [[lithium-ion batteries]] is larger than that of SEB(ROTS)-flow versions of [[lithium-ion batteries]].<ref>{{Cite journal |last1=Tolmachev |first1=Yuriy |last2=Starodubceva |first2=Svetlana V. |date=2022-09-18 |title=Flow batteries with solid energy boosters: Review Paper |url=https://www.pub.iapchem.org/ojs/index.php/JESE/article/view/1363 |journal=Journal of Electrochemical Science and Engineering |volume=12 |issue=4 |pages=731–766 |doi=10.5599/jese.1363 |issn=1847-9286}}</ref>

== Comparison ==
{| class="wikitable sortable"
|+ Comparison of flow battery compositions
|-
! Couple
! Max. cell voltage (V)
! Average electrode power density (W/m<sup>2</sup>)
! Average fluid energy density
! Cycles
|-
| [[Hydrogen]]–[[lithium bromate]] || 1.1 || 15,000 || 750{{nbsp}}Wh/kg || 
|-
| [[Hydrogen]]–[[lithium chlorate]] || 1.4 || 10,000 || 1400{{nbsp}}Wh/kg ||
|-
| [[Bromine]]–[[hydrogen]] || 1.07 || 7,950 || ||
|-
| Iron–[[tin]] || 0.62 || <&nbsp;200 || || 
|-
| Iron–[[titanium]] || 0.43 || <&nbsp;200 || ||
|-
| Iron–[[chromium]] || 1.07 || <&nbsp;200 || ||
|-
| Iron–iron || 1.21 || <&nbsp;1000 || 20{{nbsp}}Wh/L || 10,000
|-
| Organic (2013) || 0.8 || 13,000 || 21.4{{nbsp}}Wh/L || 10
|-
| Organic (2015) || 1.2 || || 7.1{{nbsp}}Wh/L || 100
|-
| MV-TEMPO
| 1.25 
|
| 8.4{{nbsp}}Wh/L
| 100
|-
| Sulfonate viologen (NH4)4[Fe(CN)6]
| 0.9 
| > 500
| 10 Wh/L
| 1,000
|-
| Metal-organic[[Vanadium redox battery|–]]<nowiki/>ferrocyanide<ref name=":0" />
| 1.62
| 2,000
| 21.7{{nbsp}}Wh/L
| 75
|-
| Metal-organic[[Vanadium redox battery|–]]<nowiki/>bromine<ref name=":0" />
| 2.13
| 3,000
| 35{{nbsp}}Wh/L
| 10
|-
| [[Vanadium redox battery|Vanadium–vanadium (sulphate)]] || 1.4 || ~800 || 25 {{nbsp}}Wh/L ||
|-
| [[Vanadium redox battery|Vanadium–vanadium (bromide)]] ||  ||  || 50{{nbsp}}Wh/L || 2,000<ref name="journal.frontiersin.org"/>
|-
| [[Polysulfide–bromide battery|Sodium–bromine polysulfide]] || 1.54 || ~800 || ||
|-
| [[Sodium–potassium battery|Sodium–potassium]]<ref>{{Cite web|url=https://www.electronicsweekly.com/news/research-news/flow-battery-uses-liquid-sodium-potassium-alloy-2018-07/|title=Room-temperature flow battery uses liquid sodium-potassium alloy|first=Steve|last=Bush|date=20 July 2018}}</ref> || ||  || ||
|-
| Sulfur–oxygen-salt<ref>{{cite journal|last1=Li|first1=Zheng|last2=Sam Pan|first2=Menghsuan|last3=Su|first3=Liang|last4=Tsai|first4=Ping-Chun|last5=Badel|first5=Andres F.|last6=Valle|first6=Joseph M.|last7=Eiler|first7=Stephanie L.|last8=Xiang|first8=Kai|last9=Brushett|first9=Fikile R.|date=2017-10-11|title=Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage|journal=Joule|volume=1|issue=2|pages=306–327|doi=10.1016/j.joule.2017.08.007|last10=Chiang|first10=Yet-Ming|doi-access=free|bibcode=2017Joule...1..306L }}</ref>
|
|
|
|
|-
| [[Zinc–bromine battery|Zinc–bromine]] || 1.85 || ~1,000 || 75{{nbsp}}Wh/kg || > 2,000
|-
| [[Lead–acid battery|Lead–acid (methanesulfonate)]] || 1.82 || ~1,000 ||
|-
| [[Zinc–cerium redox flow battery|Zinc–cerium (methanesulfonate)]] || 2.43 || <&nbsp;1,200–2,500<!-- WTF is this supposed to mean? --> ||  ||
|-
| Zinc–permanganate redox flow battery|Zn-Mn(VI)/Mn(VII) || 1.2 ||   || 60{{nbsp}}Wh/L<ref name=Colli2016 />|| 
|}

== Applications ==
Technical merits make redox flow batteries well-suited for large-scale energy storage. Flow batteries are normally considered for relatively large (1&nbsp;kWh – 10&nbsp;MWh) stationary applications with multi-hour charge-discharge cycles.<ref>{{ cite journal | last = Service | first = R.F. | title = Advances in flow batteries promise cheap backup power | date = 2 November 2018 | journal = Science | volume = 362 | issue = 6414 | pages = 508–509 | doi = 10.1126/science.362.6414.508 | pmid = 30385552 | bibcode = 2018Sci...362..508S | s2cid = 53218660 }}</ref> Flow batteries are not cost-efficient for shorter charge/discharge times. Market niches include:

* [[Grid energy storage|Grid storage]]: short and/or long-term energy storage for use by the grid
** [[Load balancing (electrical power)|Load balancing]]: the battery is attached to the [[electrical grid|grid]] to store power during off-peak hours and release it during peak demand periods. The common problem limiting this use of most flow battery chemistries is their low areal power (operating current density) which translates into high cost.
** Shifting energy from intermittent sources such as [[wind farm|wind]] or [[Photovoltaic power station|solar]] for use during periods of peak demand.<ref name="REDT" />
** Peak shaving, where demand spikes are met by the battery.<ref name="redflow" />{{Better source needed|reason=old|date=January 2022}}
* [[Uninterruptible power supply|Uninterruptible power supplies]], where the battery is used if the main power is interrupted.
* [[Electric power conversion|Power conversion]]: Because all cells share the same electrolyte(s), the electrolytes may be charged using a given number of cells and discharged with a different number. As battery [[voltage]] is proportional to the number of cells used, the battery can act as a powerful [[DC–DC converter]]. In addition, if the number of cells is continuously changed (on the input and/or output side) [[Electric power conversion|power conversion]] can also be AC/DC, AC/AC, or DC–AC with the frequency limited by that of the switching gear.<ref name=Spaziente />
* [[Electric vehicle]]s: 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 gas vehicle.<ref name=EVRS /><ref name=EVRSfox /> A common problem with most RFB chemistries in EV applications is their low energy density which translated into a short driving range. Zinc-chlorine batteries<ref>{{Cite book|chapter-url=https://www.sae.org/publications/technical-papers/content/730248/|chapter=A Zinc-Chloride Battery - The Missing Link to a Practical Electric Car|first=C. J.|last=Amato|title=SAE Technical Paper Series |date=1 February 1973|volume=1 |doi=10.4271/730248 |via=www.sae.org}}</ref> and batteries with highly soluble halates are a notable exception.<ref name=sse />{{Better source needed|reason=old|date=January 2022}}
* [[Stand-alone power system]]: An example of this is in cellphone base stations where no grid power is available. The battery can be used alongside solar or wind power sources to compensate for their fluctuating power levels and alongside a generator to save fuel.<ref name=deeya /><ref name=sandia />{{Better source needed|reason=old|date=January 2022}}

== See also ==
{{Portal|Energy|Renewable energy}}
* [[Glossary of fuel cell terms]]
* [[Hydrogen technologies]]
* [[List of battery types]]
* [[Microtubular membrane]]
* [[Redox electrode]]

== References ==
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<ref name=Nernst>{{cite patent|country=DE|status=patent|number=264026|title=Brennstoffelement mit unangreifbaren Elektroden [Fuel cell with impregnable electrodes]|pubdate=1912-06-15|inventor1-last=Nernst|inventor1-first=Walther|inventorlink1=Walther_Nernst}}</ref>
<ref name=Keefer>{{cite patent|country=US|status=patent|number=3682704|title=Redox fuel cell regenerated with sugar|pubdate=1972-08-08|assign=Electrocell Ltd.|inventor1-last=Keefer|inventor1-first= Richard Mackay}}</ref>
<ref name=Kummer_and_Oei_1982>{{Cite journal | last1 = Kummer | first1 = J. T. | last2 = Oei | first2 = D. -G. | doi = 10.1007/BF01059304 | title = A chemically regenerative redox fuel cell. II | journal = Journal of Applied Electrochemistry | volume = 15 | issue = 4 | pages = 619–629 | year = 1985 | s2cid = 96195780 }}</ref><ref name=redflow>{{Cite web|url=https://redflow.com/|archiveurl=https://web.archive.org/web/20100209050109/http://www.redflow.com.au/DNSP.htm|url-status=dead|title=Redflow – Sustainable Energy Storage|archivedate=9 February 2010}}</ref>
<ref name=Spaziente>in {{cite patent|country=WO|status=patent|number=03043170|title=System for storing and/or transforming energy from sources at variable voltage and frequency|pubdate=2003-05-22|assign=Squirrel Holdings Ltd.|inventor1-last=Spaziante|inventor1-first=Placido Maria|inventor2-last=Kampanatsanyakorn|inventor2-first=Krisada|inventor3-last=Zocchi|inventor3-first=Andrea}}</ref>
<ref name=deeya>{{youTube|id=GbLRKN3srUs|title=Talk by John Davis of Deeya energy about their flow battery's use in the telecomms industry}}</ref><ref name=Kurzweil>{{cite journal|url=http://www.kurzweilai.net/new-rechargeable-flow-battery-enables-cheaper-large-scale-energy-storage |title=New rechargeable flow battery enables cheaper, large-scale energy storage |journal=Nature Communications |volume=4 |pages=2346 |doi=10.1038/ncomms3346 |pmid=23949161 |access-date=20 August 2013|year=2013 |last1=Braff |first1=William A. |last2=Bazant |first2=Martin Z. |last3=Buie |first3=Cullen R. |arxiv=1404.0917 |bibcode=2013NatCo...4.2346B |s2cid=14719469 }}</ref><ref name=greencar>{{cite web|url=http://www.greencarcongress.com/2014/01/20140111-qbfb.html|title=Harvard team demonstrates new metal-free organic–inorganic aqueous flow battery; potential breakthrough for low-cost grid-scale storage|date=11 January 2014}}</ref><ref name=Braff>{{Cite journal | last1 = Braff | first1 = W. A. | last2 = Bazant | first2 = M. Z. | last3 = Buie | first3 = C. R. | title = Membrane-less hydrogen bromine flow battery | doi = 10.1038/ncomms3346 | journal = Nature Communications | volume = 4 | year = 2013 | pmid =  23949161| page=2346| arxiv = 1404.0917 | bibcode = 2013NatCo...4.2346B | s2cid = 14719469 }}</ref><ref name=sandia>{{Cite web|url=http://www.sandia.gov/ess/publications/SAND2013-2818C.pdf|title=Performance Testing of Zinc–Bromine Flow Batteries for Remote Telecom Sites|access-date=21 May 2023}}</ref><ref name=Wald>{{cite news |title=From Harvard, a Cheaper Storage Battery |url=https://www.nytimes.com/2014/01/09/business/energy-environment/from-harvard-a-cheaper-storage-battery.html|first1=MATTHEW L. |last1=WALD |date=8 January 2014 |access-date=10 January 2014 |work=New York Times}}</ref><ref name=MITTech>{{cite web|author1=Kevin Bullis|title=Nanoparticle Networks Promise Cheaper Batteries for Storing Renewable Energy|url=https://www.technologyreview.com/2014/04/24/13447/nanoparticle-networks-promise-cheaper-batteries-for-storing-renewable-energy/|website=MIT Technology Review|access-date=24 September 2014|date=24 April 2014}}</ref><ref name=EVRS>{{cite web|title=Electric Vehicle Refuelling System (EVRS) used in conjunction with Vanadium Redox Flow Technology|url=http://www.poweringnow.com/applications/electric-vehicles|work=REDT Energy Storage|archive-url=https://web.archive.org/web/20111210063628/http://www.poweringnow.com:80/applications/electric-vehicles|archive-date=2011-12-10|url-status=usurped}}</ref><ref name=EVRSfox>{{cite web|url=http://www.foxnews.com/auto/nanoflowcell-powered-quant-e-limo-approved-for-german-road-trials|title=nanoFLOWCELL-powered Quant e-Limo approved for german road trials|author=Antony Ingram|work=Fox News|date=11 October 2016 }}</ref><ref name=REDT>{{cite web|last=REDT Energy|title=Storing Renewable Energy|url=http://www.redtenergy.com/applications/renewable-energy|access-date=27 January 2014|archive-url=https://web.archive.org/web/20140201171818/http://www.redtenergy.com/applications/renewable-energy|archive-date=1 February 2014}}</ref>
<ref name=Quick>{{cite web|url=http://www.gizmag.com/proton-flow-battery-hydrogen-electricity-rmit/30818 |title=Proton flow battery simplifies hydrogen power |date=13 February 2014 |publisher=Gizmag.com |access-date=13 February 2014}}</ref><ref name=Andrews>{{Cite journal | doi = 10.1016/j.ijhydene.2013.11.010| title = Towards a 'proton flow battery': Investigation of a reversible PEM fuel cell with integrated metal-hydride hydrogen storage| journal = International Journal of Hydrogen Energy| volume = 39| issue = 4| pages = 1740–1751| year = 2014| last1 = Andrews | first1 = J. | last2 = Seif Mohammadi | first2 = S. | bibcode = 2014IJHE...39.1740A}}</ref>
<ref name=Colli2016>{{cite journal | doi = 10.1039/C6CC08070G | volume=52 | title=High energy density MnO4−/MnO42− redox couple for alkaline redox flow batteries | year=2016 | journal=Chemical Communications | pages=14039–14042 | last1 = Colli | first1 = Alejandro N. | last2 = Peljo | first2 = Pekka | last3 = Girault | first3 = Hubert H.| issue=97 | pmid=27853767 | url=http://infoscience.epfl.ch/record/222916/files/c6cc08070g.pdf }}</ref>
<ref name=sse>{{cite journal|last1=Tolmachev|first1=Yuriy V.|last2=Piatkivskyi|first2=Andrii|last3=Ryzhov|first3=Victor V.|last4=Konev|first4=Dmitry V.|last5=Vorotyntsev|first5=Mikhail A.|title=Energy cycle based on a high specific energy aqueous flow battery and its potential use for fully electric vehicles and for direct solar-to-chemical energy conversion|journal=Journal of Solid State Electrochemistry|date=2015|volume=19|issue= 9|pages=2711–2722|doi=10.1007/s10008-015-2805-z|s2cid=97853351}}</ref>
}}

== External links ==
* [http://www.mpoweruk.com/flow.htm Electropaedia on Flow Batteries]
* [https://web.archive.org/web/20050220030937/http://www.imr-oarai.jp/en/research/research4-5.html Research on the uranium redox flow battery]
* {{youTube|id=lAW-mMsCNlU|title=How flow batteries work}}
* [https://arena.gov.au/blog/south-australia-goes-with-the-flow-battery/ South Australian Flow Battery Project]

{{Galvanic cells}}
{{Fuel cells}}
{{Authority control}}

[[Category:Flow batteries| ]]
[[Category:Battery types]]
[[Category:Electrochemistry]]
[[Category:Fuel cells]]