A flow battery, or redox flow battery (after reduction–oxidation), is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids that are pumped through the system on separate sides of a membrane. Ion exchange (accompanied by flow of electric current) occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.43 volts.
A flow battery may be used like a fuel cell (where the spent fuel is extracted and new fuel is added to the system) or like a rechargeable battery (where an electric power source drives regeneration of the fuel). While it has technical advantages over conventional rechargeables, such as potentially separable liquid tanks and near unlimited longevity, current implementations are comparatively less powerful and require more sophisticated electronics.
The energy capacity is a function of the electrolyte volume and the power is a function of the surface area of the electrodes.
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 directly to electricity. 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 engines) 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 due to their low inherent activity towards many redox couples.
In other words, a flow battery is an electrochemical cell, with the property that the ionic solution (electrolyte) 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.
Various types of flow cells (batteries) have been developed, including redox, hybrid and membraneless. The fundamental difference between conventional batteries and flow cells is that energy is stored in the electrode material in conventional batteries, while in flow cells it is stored in the electrolyte.
The redox (reduction–oxidation) cell is a reversible cell in which electrochemical components are dissolved in the electrolyte. Redox flow batteries are rechargeable (secondary cells). Because they employ heterogeneous electron transfer rather than solid-state diffusion or intercalation they are more appropriately called fuel cells rather than batteries. In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H
2 system. The unitized regenerative fuel cell on NASA's Helios Prototype is another reversible fuel cell. The European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18). Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery. 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 a number of advantages they provide over other chemistries, despite their limited energy and power densities. Since they use vanadium at both electrodes, they do not suffer cross-contamination issues. For the same reason, they have unparalleled cycle lives (15,000–20,000 cycles), which in turns results in record levelized cost of energy (LCOE, i.e. the system cost divided by the usable energy, the cycle life, and round-trip efficiency), which are in the order of a few tens of $ cents or € cents per kWh, namely much lower than other solid-state batteries and not so far from the targets of $0.05 and €0.05, stated by US and EC government agencies.
A prototype zinc-polyiodide flow battery has been demonstrated with an energy density of 167 Wh/l (watt-hours per liter). 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 −4 to 122 °F (−20 to 50 °C) that does not require extensive cooling circuitry, which would add weight and occupy space. One unresolved issue is zinc build-up on the negative electrode that permeated the membrane, reducing efficiency. Because of the Zn dendrite formation, the 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 slightly control the problem.
When the battery is fully discharged, both tanks hold the same electrolyte solution: a mixture of positively charged zinc ions (Zn2+
) and negatively charged iodide ion, (I−
). When charged, one tank holds another negative ion, polyiodide, (I−
3). 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 (Br
–) are used as the complexing agent to stabilize the free iodine, forming iodine-bromide ions (I
) as a means to free up iodide ions for charge storage.
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/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
One system uses organic polymers and a saline solution with a cellulose membrane. The prototype withstood 10000 charging cycles while retaining substantial capacity. The energy density was 10 Wh/l. Current density reached 100 milliamperes/cm2.
The hybrid flow battery uses one or more electroactive components deposited as a solid layer. In this case, the electrochemical cell contains one battery electrode and one fuel cell electrode. This type is limited in energy by the surface area of the electrode. Hybrid flow batteries include the zinc-bromine, zinc–cerium, 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, very 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 VOSO4 and H2SO4, 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 new redox couple with achievements of increased current density, operation 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. Recently, a high energy density Mn(VI)/Mn(VII)-Zn hybrid flow battery has been proposed.
A membraneless battery 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 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 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 as other membraneless systems— and an order of magnitude higher than lithium-ion batteries.
Recently, 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.
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 redox-active components.
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.
Quinones 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/PbSO4 was the anolyte in a hybrid acid AORFB. The 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.
9,10-Anthraquinone-2,7-disulfonic acid (AQDS), also a quinone, has been evaluated. 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 Br
redox couple, yields a peak galvanic power density 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 although no data were published. While this total organic system appears robust, it has a low cell voltage (ca. 0.55 V) and a low energy density (< 4 Wh/L).
Hydrobromic acid used as an electrolyte has been replaced with a far less toxic alkaline solution (1 M KOH) and ferrocyanide. The higher pH is less corrosive, allowing the use of inexpensive polymer tanks. The increased electrical resistance in the membrane was compensated by increasing the voltage. The cell voltage was 1.2 V. The cell's efficiency exceeded 99%, while round-trip efficiency measured 84%. The battery has an expected lifetime of at least 1,000 cycles. Its theoretic energy density was 19 Wh/L. Ferrocyanide's chemical stability in high pH KOH solution without forming Fe(OH)2 or Fe(OH)3 needs to be verified before scale-up.
Another organic AORFB used methyl viologen as anolyte and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl as catholyte, plus sodium chloride and a low-cost anion exchange membrane. This MV/TEMPO system has the highest cell voltage, 1.25 V, and, possibly, lowest capital cost ($180/kWh) reported for AORFBs. The water-based 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/cm2, with optimal performance rated at 40–50 mA, 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 8.4 Wh/L with the limitation on the TEMPO side.
One flow-battery concept is based on redox active, organic polymers. It employs viologen and TEMPO with dialysis membranes. The polymer-based redox-flow battery (pRFB) uses functionalized macromolecules (similar to acrylic glass or Styrofoam) dissolved in water as active material for the electrodes. Thereby, simple dialysis membranes can be employed. The 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 the small counterions to pass. The concept may solve the high cost of traditional Nafion membrane, but the design and synthesis of redox active polymer with high water solubility is not trivial.
Aligned with the tunability of the redox-active components as the main advantage of organic redox flow batteries, the idea of integrating both anolyte and catholyte in the same molecule has been developed. Those so called, bifunctional analytes or combi-molecules allow the same material to be used in both tanks, which has relevant advantages such as diminishing the effect of crossover. Thus, diaminoanthraquinone, also a quinone, and indigo-based molecules as well as TEMPO/phenazine combining molecules have been presented as potential electrolytes for the development of symmetric redox-flow batteries (SRFB).
Another anolyte candidate is fluorenone, reengineered to increase its water solubility. A reversible ketone (de)hydrogenation demonstration cell operated continuously for 120 days over 1,111 charging cycles at room temperature without the use of a catalyst, retaining 97% percent of its capacity. The cell offers more than double the energy density of vanadium-based systems.
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 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.
Metal-organic flow batteries use organic ligands to provide more favorable properties to redox-active metals. The ligands can be chelates like EDTA, and can enable electrolyte to be in neutral or alkaline pH, conditions under which metal aquo complexes would otherwise precipitate. By blocking the coordination of water to the metal, organic ligands can also inhibit metal-catalyzed water-splitting reactions, resulting in some of the highest voltage all-aqueous systems ever reported. 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. Metal-organic flow batteries are sometimes known as coordination chemistry flow batteries, which represents the technology behind Lockheed Martin's Gridstar Flow technology.
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 flow-type batteriesEdit
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.
A wide range of chemistries have been tried for flow batteries.
|Couple||Max. cell voltage (V)||Average electrode power density (W/m2)||Average fluid energy density||Cycles|
|Hydrogen–lithium bromate||1.1||15,000||750 Wh/kg|
|Hydrogen–lithium chlorate||1.4||10,000||1400 Wh/kg|
|Organic (2013)||0.8||13,000||21.4 Wh/L||10|
|Organic (2015)||1.2||7.1 Wh/L||100|
|Vanadium–vanadium (sulphate)||1.4||~800||25 Wh/L|
|Vanadium–vanadium (bromide)||50 Wh/L||2000|
|Zinc–bromine||1.85||~1,000||75 Wh/kg||> 2000|
|Zinc–cerium (methanesulfonate)||2.43||< 1,200–2,500|
Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of
- flexible layout (due to separation of the power and energy components)
- long cycle life (because there are no solid-to-solid phase transitions)
- quick response times
- no need for "equalisation" charging (the overcharging of a battery to ensure all cells have an equal charge)
- no harmful emissions.
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.
The two main disadvantages are
- 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 costs.
Compared to non-reversible fuel cells or electrolyzers using similar electrolytic chemistries, flow batteries generally have somewhat lower efficiency.
Flow batteries are normally considered for relatively large (1 kWh – 10 MWh) stationary applications. These are for:
- Load balancing – where the battery is attached to an electrical grid 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 or solar for discharge during periods of peak demand.
- Peak shaving, where spikes of demand are met by the battery.
- UPS, where the battery is used if the main power fails to provide an uninterrupted supply.
- Power conversion – because all cells share the same electrolyte(s). Therefore, the electrolyte(s) may be charged using a given number of cells and discharged with a different number. Because the voltage of the battery is proportional to the number of cells used the battery can therefore act as a very powerful DC–DC converter. In addition, if the number of cells is continuously changed (on the input and/or output side) power conversion can also be AC/DC, AC/AC, or DC–AC with the frequency limited by that of the switching gear.
- Electric vehicles – Because flow batteries can be rapidly "recharged" by replacing the electrolyte, they can be used for applications where the vehicle needs to take on energy as fast as a combustion engined vehicle. A common problem found with most RFB chemistries in the EV applications is their low energy density which translated into a short driving range. Flow batteries based on highly soluble halates are a notable exception.
- 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 make the most efficient use of it to save fuel. Currently, flow batteries are being used in solar micro grid applications throughout the Caribbean.
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