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A flow battery, or redox flow battery (after reduction–oxidation), is a type of
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
where
chemical energy Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (2018). "How ...
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 flow of electric current through an external circuit) occurs through the membrane while both liquids circulate in their own respective space.
Cell voltage The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Definit ...
is chemically determined by the
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...
and ranges, in practical applications, from 1.0 to 2.43
volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...
s. The energy capacity is a function of the electrolyte volume and the power is a function of the surface area of the
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
s. A flow battery may be used like a
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
(where the spent fuel is extracted and new fuel is added to the system) or like a
rechargeable battery A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prima ...
(where an electric power source drives regeneration of the fuel). While 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, potentially lower
total cost of ownership Total cost of ownership (TCO) is a financial estimate intended to help buyers and owners determine the direct and indirect costs of a product or service. It is a management accounting concept that can be used in full cost accounting or even ecolog ...
, all flow batteries suffer from inferior cycle energy efficiency (50-80%) compared to
lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
. This inferior energy efficiency stems from the need to operate flow batteries at high (>= 100 mA/cm2) current densities to reduce the effect of the internal crossover (through the membrane/separator inside each cell) and to reduce the cost of power (size of stacks).


History

Zinc-bromine flow battery is the oldest flow battery chemistry, with John Doyle’s patent US224404 filed on September 29, 1879. Zn-Br2 flow batteries have relatively high specific energy, and they were demonstrated as power sources for electric cars in the 1970's. Walther Kangro, an Estonian chemist working in Germany, was the first to demonstrate in the 1950’s flow batteries based fully on dissolved transition metal ions: Ti-Fe and Cr-Fe. After some initial experimentations with Ti-Fe RFB NASA and some other groups in Japan and elsewhere selected Cr-Fe chemistry for further development. In order to reduce the effect of time-varying concentration during RFB cycling, mixed solutions (i.e. comprising both chromium and iron species in the negolyte and in the posolyte) were used. Among disadvantages of the Cr-Fe chemistry are: hydrate isomerism (i.e. the equilibrium between electrochemically active Cr3+ chloro-complexes and inactive hexa-aqua complex, which is can be alleviated by adding chelating amino-ligands) and hydrogen evolution on the negode (which is mitigated by adding Pb salts for increasing the H2 overvoltage and Au salts for catalyzing the chromium electrode reaction). In the late 1980’s Sum, Rychcik and Skyllas-Kazacos at the
University of New South Wales The University of New South Wales (UNSW), also known as UNSW Sydney, is a public research university based in Sydney, New South Wales, Australia. It is one of the founding members of Group of Eight, a coalition of Australian research-intensiv ...
(UNSW) in Australia demonstrated the advantages of all-vanadium RFB chemistry, such as the existence of 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. UNSW filed several patents related to VRFBs, that were later licensed to Japanese, Thai and Canadian corporations, which tried to commercialize this technology with variable success. In 2022,
Dalian Dalian () is a major sub-provincial port city in Liaoning province, People's Republic of China, and is Liaoning's second largest city (after the provincial capital Shenyang) and the third-most populous city of Northeast China. Located on ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
began operating a 400 MWh, 100 MW vanadium flow battery, then the largest of its type.


Construction principle

A flow battery is a rechargeable
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
in which an
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
containing one or more dissolved electroactive elements flows through an
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
that reversibly converts
chemical energy Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (2018). "How ...
directly to
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describe ...
. Electroactive elements are "elements in solution that can take part in an electrode reaction or that can be adsorbed on the electrode." 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 engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
s) while simultaneously recovering the spent material for recharging. Many flow batteries use carbon felt electrodes due to its low cost and adequate electrical conductivity, although these electrodes somewhat limit
power density Power density is the amount of power (time rate of energy transfer) per unit volume. In energy transformers including batteries, fuel cells, motors, power supply A power supply is an electrical device that supplies electric power to a ...
due to their low inherent activity toward many redox couples. In other words, a flow battery is an
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
, with the property that the ionic solution (
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
) is stored outside of the cell (instead of in the cell around the electrodes) and can be fed into the cell in order to generate electricity. The total amount of electricity that can be generated depends on the volume of electrolyte in the tanks. Flow batteries are governed by the design principles established by electrochemical engineering.


Types

Various types of flow cells (batteries) have been developed, including inorganic flow battery and organic flow batteries. Under each category, flow battery design can be further classified into full flow batteries, semi-flow batteries, and membraneless flow batteries. The fundamental difference between conventional batteries and flow cells is that energy is stored in the
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
material in conventional batteries, while in flow cells it is stored in the
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
. Patent Classifications for Flow Batteries have not been fully developed as of 2021.
Cooperative Patent Classification The Cooperative Patent Classification (CPC) is a patent classification system, which has been jointly developed by the European Patent Office (EPO) and the United States Patent and Trademark Office (USPTO).regenerative fuel cell (H01M8/18), even though it is more appropriate to consider fuel cells as a subclass of flow batteries.


Inorganic


Inorganic full-flow

The
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
(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 Secondary may refer to: Science and nature * Secondary emission, of particles ** Secondary electrons, electrons generated as ionization products * The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...
) cells. Because they employ heterogeneous electron transfer rather than solid-state diffusion or intercalation they are more similar to
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
s rather than to conventional batteries (such as lead-acid or
lithium-ion A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also ...
). The main reason
Fuel Cells A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
are not considered to be batteries, is because originally (in the 1800's) fuel cells emerged as a means to produce electricity directly from fuels (and air) via a non-combustion electrochemical process. Later, particularly in the 1960's and 1990's, rechargeable fuel cells (i.e. /, such as unitized regenerative fuel cells in
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
's Helios Prototype) were developed. Examples of redox flow batteries are the vanadium redox flow battery,
polysulfide bromide battery The polysulfide-bromine battery (PSB; sometimes ''polysulphide-polybromide'' or "bromine-sulfur"), is a type of rechargeable electric battery, which stores electric energy in liquids, such as water-based solutions of two salts: sodium bromide and s ...
(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 V2O5 (> $30 / Kg), the raw materials for VRFB; parasite reactions including hydrogen and oxygen evolution; and precipitation of V2O5 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 Energy storage is the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an accumulator or battery. Energy comes in ...
). 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 In 2022,
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the A ...
SBIR The Small Business Innovation Research (or SBIR) program is a U.S. government funding program, coordinated by the Small Business Administration, intended to help certain small businesses conduct research and development (R&D). Funding takes the f ...
-funded Influit Energy announced a nonflammable, surface-modified nanoelectrofuel made of a metal oxide suspended in an aqueous solution. The material does not settle out of the solution, even at high concentration and features and energy density higher than Lion batteries. Operating temperatures are -40 to 80 °C. They require no lithium,
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as ...
, or
rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...
s.


Inorganic semi-flow RFBs

The hybrid flow battery uses one or more electroactive components deposited as a solid layer. The major disadvantage is the loss decoupled energy and power as seen in full flow batteries from using a solid state electrode. The cell contains one battery electrode and one fuel cell electrode. This type is limited in energy by the electrode surface area. Hybrid flow batteries include the zinc-bromine, zinc–cerium, soluble lead–acid, and iron-salt flow batteries. Weng et al. reported a vanadium- metal hydride rechargeable hybrid flow battery with an experimental OCV of 1.93 V and operating voltage of 1.70 V, relatively high values among rechargeable flow batteries with aqueous electrolytes. This hybrid battery consists of a graphite felt positive electrode operating in a mixed solution of and , and a metal hydride negative electrode in 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 further improvements of this redox couple with achievements of increased current density, inclusion of larger 100 cm2 electrodes, and the operation of 10 large cells in series. Preliminary data using a fluctuating simulated power input tested the viability toward kWh scale storage. In 2016, a high energy density Mn(VI)/Mn(VII)-Zn hybrid flow battery was proposed. A prototype
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
-
polyiodide The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms. The most common and simplest member is the triiodide ion, . Other known larger polyiodides include 4sup>2−, 5sup>−, 6sup>2−, 7sup>−, 8sup>2−, 9su ...
flow battery demonstrated an energy density of 167 Wh/L ( watt-hours per
liter The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3) ...
). Older zinc-bromide cells reach 70 Wh/L. For comparison, lithium iron phosphate batteries store 233 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 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 (> 20 mA/cm2) and thus have limited power density. Adding alcohol to the electrolyte of the ZnI battery can help with the problem. The drawbacks of Zn/I RFB lie at the high cost of Iodide salts (> $20 / Kg); limited area capacity of Zn deposition also losing 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 () and negatively charged iodide ion, (). When charged, one tank holds another negative ion, polyiodide, (). The battery produces power by pumping liquid from external tanks into the battery's stack area where the liquids are mixed. Inside the stack, zinc ions pass through a selective membrane and change into metallic zinc on the stack's negative side. To further increase the energy density of the zinc-iodide flow battery, bromide ions () are used as the complexing agent to stabilize the free iodine, forming iodine-bromide ions () as a means to free up iodide ions for charge storage. Proton flow batteries (PFB) integrate a metal hydride storage electrode into a reversible
proton exchange membrane A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen g ...
(PEM)
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
. 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 solid-state metal hydride. 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.


Organic

Compared to redox flow batteries that are inorganic, such as vanadium redox flow batteries and Zn-Br2 batteries, which have been developed for decades, organic redox flow batteries emerged in 2009. The primary appeal of organic redox flow batteries lies in the tunable redox properties of the active components. As of 2021, organic RFB experience low durability (i.e. calendar or cycle life, or both). For this reason, only inorganic RFB have been demonstrated on a commercial scale. Organic redox flow batteries can be further classified into aqueous (AORFBs) and non-aqueous (NAORFBs). AORFBs use water as solvent for electrolyte materials while NAORFBs employ organic solvents. AORFBs and NAORFBs can be further divided into total and hybrid organic systems. The former use only organic electrode materials, while the latter use inorganic materials for anode or cathode. In larger-scale energy storage, lower solvent cost and higher conductivity give AORFBs greater commercial potential, as well as offering safety advantages from water-based electrolytes. NAORFBs instead provide a much larger voltage window and occupy less physical space.


pH neutral AORFBs

pH neutral AORFBs are operated at pH7 conditions, typically using
NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
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 e(CN) a common catholyte used in AORFBs, is not stable in alkaline solutions but is at pH neutral conditions. AORFBs used methyl viologen as 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, 1.25V, and, possibly, lowest capital cost ($180/kWh) reported for AORFBs. The aqueous liquid electrolytes were designed as a drop-in replacement for current systems without replacing existing 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, with optimal performance rated at 40–50mA, at which about 70% of the battery's original voltage was retained. Neutral AORFBs can be more environmentally friendly than acid or alkaline AORFBs while showing electrochemical performance comparable to corrosive RFBs. The MV/TEMPO AORFB has an energy density of 8.4Wh/L with the limitation on the TEMPO side.
Viologen Viologens are organic compounds with the formula (C5H4NR)2n+. In some viologens, the pyridyl groups are further modified. Viologens are called so, because these compounds produce violet color on reduction iolet + Latin ''gen'', generator of T ...
-based flow batteries have been mainly developed by Liu’s group at
Utah State University Utah State University (USU or Utah State) is a public land-grant research university in Logan, Utah. It is accredited by the Northwest Commission on Colleges and Universities. With nearly 20,000 students living on or near campus, USU is Utah ...
. In 2019, the group reported an ultralight sulfonate-viologen/
ferrocyanide Ferrocyanide is the name of the anion CN)6">cyanide.html" ;"title="e(cyanide">CN)6sup>4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe(CN)6. e ...
AORFB stable for 1000 cycles at an energy density of 10 Wh/L, so far the most stable, energy dense AORFB.


Acidic AORFBs

Quinone The quinones are a class of organic compounds that are formally "derived from aromatic compounds uch as benzene or naphthalene">benzene.html" ;"title="uch as benzene">uch as benzene or naphthalene] by conversion of an even number of –CH= group ...
s and their derivatives are the basis of many organic redox systems. 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, PbSO4 was the anolyte in a hybrid acid AORFB. Quinones accept two units of electrical charge, compared with one in conventional catholyte, implying that such a battery could store twice as much energy in a given volume. Another quinone 9,10-Anthraquinone-2,7-disulfonic acid (AQDS), has been evaluated. AQDS undergoes rapid, reversible two-electron/two-proton reduction on a glassy carbon
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
in
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the /
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
couple, yields a peak galvanic
power density Power density is the amount of power (time rate of energy transfer) per unit volume. In energy transformers including batteries, fuel cells, motors, power supply A power supply is an electrical device that supplies electric power to a ...
exceeding 6,000 W/m2 at 13,000 A/m2. Cycling showed > 99% storage capacity retention per cycle. Volumetric energy density was over 20 Wh/L. 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 Br2. The battery was claimed to last for 1,000 cycles without degradation. While this system appears robust, it has a low cell voltage (ca. 0.55V) and a low energy density (< 4Wh/L).
Hydrobromic acid Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...
used as an electrolyte has been replaced with a less toxic alkaline solution (1M KOH) and
ferrocyanide Ferrocyanide is the name of the anion CN)6">cyanide.html" ;"title="e(cyanide">CN)6sup>4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe(CN)6. e ...
. The higher pH is less corrosive, allowing the use of inexpensive
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
tanks. The increased electrical resistance in the membrane was compensated increased voltage. The cell voltage was 1.2V. The cell's 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 19Wh/L.
Ferrocyanide Ferrocyanide is the name of the anion CN)6">cyanide.html" ;"title="e(cyanide">CN)6sup>4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe(CN)6. e ...
's chemical stability in high pH KOH solution without forming Fe(OH) or Fe(OH) needs to be verified before scale-up. Integrating both anolyte and catholyte in the same molecule has been examined. Such bifunctional analytes or combi-molecules 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 relevant advantages such as diminishing the effect of crossover. Thus, quinone diaminoanthraquinone and indigo-based molecules as well as TEMPO/
phenazine Phenazine is an organic compound with the formula (C6H4)2N2. It is a dibenzo annulated pyrazine, and the parent substance of many dyestuffs, such as the toluylene red, indulines, and safranines (and the closely related eurhodines). Phenazine c ...
combining molecules are potential electrolytes for the development of 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.


Alkaline

Quinone molecules have been used as anolytes in alkaline AROFBs. Another anolyte candidate is fluorenone, reengineered to increase its water solubility. A reversible
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...
(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 offers more than double the energy density of vanadium-based systems. The major challenge for alkaline AORFBs is the lack of a stable catholyte, holding their energy densities below 5 Wh/L. All reported alkaline AORFBs use excess
potassium ferrocyanide Potassium ferrocyanide is the inorganic compound with formula K4 e(CN)6�3H2O. It is the potassium salt of the coordination complex e(CN)6sup>4−. This salt forms lemon-yellow monoclinic crystals. Synthesis In 1752, the French chemist Pie ...
catholyte because of the stability issue of
ferrocyanide Ferrocyanide is the name of the anion CN)6">cyanide.html" ;"title="e(cyanide">CN)6sup>4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe(CN)6. e ...
in alkaline solutions. Metal-organic flow batteries use organic
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s to improve the properties of redox-active metals. The ligands can be
chelates Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands ar ...
like
EDTA Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid with the formula H2N(CH2CO2H)2sub>2. This white, water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes ev ...
, and can enable the electrolyte to be in neutral or alkaline conditions under which
metal aquo complex In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...
es would otherwise precipitate. By blocking the coordination of water to the metal, organic ligands can inhibit metal-catalyzed water-splitting reactions, resulting in higher voltage all-aqueous systems. For example, the use of
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...
coordinated to 1,3-propanediaminetetraacetate (PDTA), gave cell potentials of 1.62 V vs. ferrocyanide and a record 2.13 V vs.
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
. Metal-organic flow batteries may be known as coordination chemistry flow batteries, which represents the technology behind
Lockheed Martin The Lockheed Martin Corporation is an American aerospace, arms, defense, information security, and technology corporation with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It ...
's Gridstar Flow technology.


Oligomer

Oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relati ...
redox-species RFB have been proposed to reduce the crossover of the electroactive species, while using low cost membranes. Such redox-active oligomers are known as redoxymers. One system uses organic polymers and a saline solution with a
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
membrane. The prototype underwent 10,000 charging cycles while retaining substantial capacity. The energy density was 10 Wh/L. Current density reached 100 milliamperes/cm2. Another oligomer RFB employs viologen and TEMPO redoxymers in combination with low-cost dialysis membranes. Functionalized
macromolecules A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
(similar to acrylic glass or
Styrofoam Styrofoam is a trademarked brand of closed-cell extruded polystyrene foam (XPS), commonly called "Blue Board", manufactured as foam continuous building insulation board used in walls, roofs, and foundations as thermal insulation and water barrie ...
) dissolved in water are the active electrode material. The size-selective nanoporous membrane works like a strainer and is produced much more easily and at lower cost than conventional ion-selective membranes. It retains the big "spaghetti"-like polymer molecules, while allowing small counterions to pass. The concept may solve the high cost of traditional
Nafion Nafion is a brand name for a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Dr. Walther Grot of DuPont. Nafion is a brand of the Chemours company. It is the first of a class of synthetic polymers with ...
membrane, but the design and synthesis of redox active polymer with high water solubility is not trivial. So far, RFBs with
oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relati ...
redox-species have not demonstrated competitive area-specific power. It is not clear whether low operating current density is an intrinsic feature of large redox-molecules or not.


Membraneless

A membraneless battery relies on
laminar flow In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mi ...
in which two liquids are pumped through a channel, where they undergo electrochemical reactions to store or release energy. The solutions stream through in parallel, with little mixing. The flow naturally separates the liquids, eliminating the need for a membrane. Membranes are often the most costly and least reliable components of batteries, as they can be corroded 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 Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...
that can destroy the membrane. Both materials are available at low cost. 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 any membraneless design. One such membraneless flow battery published in August 2013 produced a maximum power density of 0.795 mW/cm2, three times as much
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
as other membraneless systems— and an order of magnitude higher than lithium-ion batteries. In 2018, a macroscale membraneless redox flow battery capable of recharging and recirculation of the same electrolyte streams for multiple cycles has been demonstrated. The battery is based on immiscible organic catholyte and aqueous anolyte liquids, which exhibits high capacity retention and Coulombic efficiency during cycling.


Nano-network

Lithium–sulfur system arranged in a network of nanoparticles 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.


Other chemistries

Other flow-type batteries include the zinc–cerium hybrid flow battery, the zinc–bromine hybrid flow battery, and the hydrogen bromine battery.


Other flow-type batteries


Semi-solid

In a semi-solid flow cell, the positive and negative electrodes are composed of particles suspended in a carrier liquid. The positive and negative suspensions are stored in separate tanks and pumped through separate pipes into a stack of adjacent reaction chambers, where they are 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 conductive carbon network. The carbon free semi-solid redox flow battery is also sometimes referred to as Solid Dispersion Redox Flow Battery. 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 that flows like
molasses Molasses () is a viscous substance resulting from refining sugarcane or sugar beets into sugar. Molasses varies in the amount of sugar, method of extraction and age of the plant. Sugarcane molasses is primarily used to sweeten and flavour foods ...
.


Redox-targeted solids

Flow batteries with redox-targeted solids (ROTS), also known as solid energy boosters (SEBs), is another recent development. In these batteries either posolyte or negolyte or both (a.k.a. redox fluids), come in contact with a one or more solid electroactive materials, stored in tanks outside the power stack. The redox fluids comprise one or more redox couples, with redox potentials flanking the redox potential of the solid electroactive material. Such RFBs with Solid Energy Boosters (SEBs) combine the high specific energy advantage of conventional batteries (such as
lithium-ion A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also ...
) with the decoupled energy-power advantage of flow batteries. SEB(ROTS) RFBs have several advantages compared to semi-solid RFBs, such as no need to pump viscous slurries, no precipitation /clogging , higher area-specific power, longer durability, wider chemical design space. Zinc-bromine flow batteries are the oldest, their origin going back to 1880s.Tolmachev, Yuriy. "Flow batteries from 1879 to 2022 and beyond." Journal of Electrochemical Science and Engineering (2022)
preprint
.


Advantages

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 In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...
s, that cause degradation of lithiuim-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 to no self-discharge during standing * full recycling of electroactive materials Some types also 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 * electrolytes can be stored away from the power stack. These technical merits make redox flow batteries a well-suited option for large-scale energy storage.


Disadvantages

Flow batteries have three main disadvantages compared to batteries with solid electroactive materials * low energy density (you need large tanks of electrolyte to store useful amounts of energy) * low charge and discharge rates (compared to other industrial electrode processes). This means that the electrodes and membrane separators need to be large, which increases the cost of power. * flow batteries have a lower energy efficiency, because they operate at higher current densities to minimize the effects of the cross-over (internal self-discharge) and to reduce the cost of power. Flow batteries typically have a higher energy efficiency than
fuel cells A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
, but lower than
lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
.


Applications

Flow batteries are normally considered for relatively large (1 kWh – 10 MWh) stationary applications with multi-hour charge-discharge cycles. Flow batteries are not cost-efficient for shorter charge/discharge times. Some are examples of flow battery market niches are: * Load balancing – where the battery is attached to an
electrical grid An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids vary in size and can cover whole countries or continents. It consists of:Kaplan, S. M. (2009). Smart Grid. Electrical Power ...
to store excess electrical power during off-peak hours and release electrical power during peak demand periods. The common problem limiting the use of most flow battery chemistries in this application is their low areal power (operating current density) which translates into a high cost of power. * Storing energy from renewable sources such as
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
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), the electrolytes may be charged using a given number of cells and discharged with a different number. As battery
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
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 vehicle An electric vehicle (EV) is a vehicle that uses one or more electric motors for propulsion. It can be powered by a collector system, with electricity from extravehicular sources, or it can be powered autonomously by a battery (sometimes ch ...
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 combustion engined vehicle. A common problem found with most RFB chemistries in the EV applications is their low energy density which translated into a short driving range. Zinc-chlorine batteries and batteries with highly soluble halates are a notable exception. *
Stand-alone power system A stand-alone power system (SAPS or SPS), also known as remote area power supply (RAPS), is an off-the-grid electricity system for locations that are not fitted with an electricity distribution system. Typical SAPS include one or more methods of e ...
– 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 make the most efficient use of it to save fuel.


See also

*
Glossary of fuel cell terms The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to ...
* Hydrogen technologies *
Redox electrode The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode sys ...


References


External links


Electropaedia on Flow Batteries


*
South Australian Flow Battery Project
{{Authority control Electrochemistry Fuel cells Battery types