Flow battery

The zinc–bromine flow battery (Zn–Br2) was the original flow battery. 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. After initial experimentations with Ti–Fe redox flow battery (RFB) chemistry, NASA and groups in Japan and elsewhere selected Cr–Fe chemistry for further development. Mixed solutions (i.e. comprising both chromium and iron species in the negolyte and in the posolyte) were used in order to reduce the effect of time-varying concentration during cycling. In the late 1980s, Sum, Rychcik and Skyllas-Kazacos at the University of New South Wales (UNSW) in Australia demonstrated vanadium RFB chemistry UNSW filed several patents related to VRFBs, that were later licensed to Japanese, Thai and Canadian companies, which tried to commercialize this technology with varying success. Organic redox flow batteries emerged in 2009. Sumitomo Electric has built flow batteries for use in Taiwan, Belgium, Australia, Morocco and California. Hokkaido's flow battery farm was the biggest in the world when it opened in April 2022—until China deployed one eight times larger that can match the output of a natural gas plant. == Design ==

A flow battery is a rechargeable fuel cell in which an electrolyte containing one or more dissolved electroactive elements flows through an electrochemical cell that reversibly converts chemical energy to electrical energy. Electroactive elements are "elements in solution that can take part in an electrode reaction or that can be adsorbed on the electrode." The amount of electricity that can be generated depends on the volume of electrolyte. Flow batteries are governed by the design principles of electrochemical engineering. == Evaluation ==

Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of: • Independent scaling of energy (tanks) and power (stack), which allows for a cost/weight/etc. optimization for each application • Long cycle and calendar lives (because there are no solid-to-solid phase transitions, which degrade lithium-ion and related batteries) • Quick response times • No need for "equalisation" charging (the overcharging of a battery to ensure all cells have an equal charge) • No harmful emissions • Little/no self-discharge during idle periods • Recycling of electroactive materials Some types offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/overdischarge. They are safe because they typically do not contain flammable electrolytes, and electrolytes can be stored away from the power stack. The main disadvantages are: • Low energy density (large tanks are required to store useful amounts of energy) • Low charge and discharge rates. This implies large electrodes and membrane separators, increasing cost. • Lower energy efficiency, because they operate at higher current densities to minimize the effects of cross-over (internal self-discharge) and to reduce cost. Flow batteries typically have a higher energy efficiency than fuel cells, but lower than lithium-ion batteries. 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 == Traditional flow batteries ==

The redox cell uses redox-active species in fluid (liquid or gas) media. Redox flow batteries are rechargeable (secondary) cells. Traditional redox flow battery chemistries include iron-chromium, vanadium, polysulfide–bromide (Regenesys), and uranium. Major challenges include: low abundance and high costs of V2O5 (> $30 / Kg); parasitic reactions including hydrogen and oxygen evolution; and precipitation of V2O5 during cycling. == Hybrid ==

The hybrid flow battery (HFB) uses one or more electroactive components deposited as a solid layer. and 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 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 improvements with increased current density, inclusion of larger 100 cm2 electrodes, and series operation. 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. 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 () and negatively charged iodide ion, (). When charged, one tank holds another negative ion, polyiodide, (). 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. To increase energy density, 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 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. == Organic ==

Compared to inorganic redox flow batteries, such as vanadium and Zn-Br2 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. 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 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 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)6], 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 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, 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 alternatives, 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. In 2019Viologen-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. Acidic AORFBs 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. 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. It has a low cell voltage (ca. 0.55V) and a low energy density (2. Another oligomer RFB employed viologen and TEMPO redoxymers in combination with low-cost 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. 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. == Other types ==

Other flow-type batteries include the zinc–cerium battery, the zinc–bromine battery, and the hydrogen–bromine battery. Membraneless 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 pass in parallel, with little mixing. The flow naturally separates the liquids, without requiring a membrane. Suspension-based A 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. The carbon-free semi-solid RFB is also referred to as solid dispersion redox flow batteries. 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. In 2022, Influit Energy announced a flow battery electrolyte consisting of a metal oxide suspended in an aqueous solution. Flow batteries with redox-targeted solids (ROTS), also known as solid energy boosters (SEBs) 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. == Comparison ==

Technical merits make redox flow batteries well-suited for large-scale energy storage. 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. Market niches include: • Grid storage: short and/or long-term energy storage for use by the grid • Load balancing: the battery is attached to the 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 or solar for use during periods of peak demand. and batteries with 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 save fuel. == See also ==