A flow battery, also called a redox flow battery, is a type of device that uses chemical reactions to produce electricity. It works by using two liquid solutions, each containing chemicals that are pumped through the system on opposite sides of a membrane. Ions move across the membrane during the process, while the liquids flow through their separate areas. This movement creates an electric current that can be used to power devices.

Flow batteries can function in two ways. Like a fuel cell, they can receive new charged liquids (called negolyte and posolyte) to keep producing electricity. Alternatively, they can act like rechargeable batteries by using an external power source to restore the chemicals inside.

The main difference between flow batteries and traditional batteries is where energy is stored. In traditional batteries, energy is stored in the materials of the battery itself. In flow batteries, energy is stored in the liquid electrolyte solutions. This design allows flow batteries to be easily scaled up for large energy storage needs, making them suitable for long-term use in stationary systems like power grids.

Flow batteries have some advantages over traditional batteries. They can be designed to have separate control over power (based on the size of the system) and energy storage (based on the amount of liquid stored). They also have a long lifespan and may cost less over time for applications that require power over many hours. However, they are less efficient, with energy efficiency between 50% and 80%. This is because they need to operate at high current levels to reduce costs and prevent chemical mixing through the membrane. Most flow batteries are heavier than lithium-ion batteries because they use water as a solvent to keep the chemicals in liquid form.

The voltage of a flow battery is determined by chemical reactions and typically ranges from 1.0 to 2.43 volts. Increasing the amount of liquid in the storage tanks raises the battery’s energy capacity, while increasing the size of the system raises its power output.

Flow batteries can be categorized in several ways:

1) Full-flow batteries use only liquid solutions (such as vanadium redox flow batteries), while semi-flow batteries use at least one solid material (like zinc-bromine batteries).

2) Inorganic flow batteries use non-organic chemicals, and organic flow batteries use organic chemicals. As of 2025, only inorganic flow batteries have been successfully used in commercial products. Organic flow batteries often break down more quickly.

3) Membrane flow batteries use a barrier to separate chemicals, while membraneless flow batteries do not.

As of 2021, patent classifications for flow batteries were not fully developed. Some systems group flow batteries under a category for regenerative fuel cells, even though flow batteries are a type of fuel cell.

History

The zinc–bromine flow battery (Zn–Br₂) was the first type of flow battery developed. John Doyle filed a patent (US 224404) on September 29, 1879. Zn–Br₂ batteries have high energy storage per unit weight and were tested in electric cars during the 1970s.

In the 1950s, Walther Kangro, an Estonian chemist working in Germany, first showed flow batteries using dissolved transition metal ions, such as Ti–Fe and Cr–Fe. Early experiments with Ti–Fe chemistry led to NASA and other groups in Japan and other countries choosing Cr–Fe chemistry for further study. Scientists mixed solutions containing both chromium and iron in the negative and positive electrolytes to reduce changes in concentration during battery use.

In the late 1980s, researchers Sum, Rychcik, and Skyllas-Kazacos at the University of New South Wales (UNSW) in Australia demonstrated vanadium flow battery chemistry. UNSW filed several patents related to vanadium flow batteries, which were later licensed to companies in Japan, Thailand, and Canada. These companies attempted to sell the technology, with mixed results.

Organic redox flow batteries were first introduced in 2009.

In 2022, Dalian, China, started operating a 400 MWh, 100 MW vanadium flow battery, which was the largest of its kind at the time.

Sumitomo Electric has built flow batteries for use in Taiwan, Belgium, Australia, Morocco, and California. In April 2022, Hokkaido’s flow battery farm became the largest in the world until China built a system eight times larger, capable of matching the power output of a natural gas plant.

Design

A flow battery is a type of rechargeable battery that uses a liquid called an electrolyte, which contains substances that can help create electricity. This liquid flows through a device called an electrochemical cell, which changes chemical energy into electrical energy and can also change it back. The substances in the electrolyte are called electroactive elements because they can take part in reactions on the battery's electrodes or stick to the electrodes.

The electrolyte is stored in tanks outside the battery and is usually pumped through the battery's cells. Flow batteries can be quickly recharged by replacing the used electrolyte liquid, similar to how a car is refilled with fuel. The spent liquid can also be reused for future charging. These batteries can also be recharged while still in the system. Many flow batteries use carbon felt electrodes because they are inexpensive and can conduct electricity well, even though they do not produce as much power as some other materials. The amount of electricity a flow battery can produce depends on how much electrolyte is used.

Flow batteries are designed based on the rules of electrochemical engineering.

Evaluation

Redox flow batteries, and some hybrid flow batteries, have several benefits:

• They can be designed separately for energy storage (tanks) and power output (stack), making them more efficient and cost-effective for different uses.
• They last a long time because they do not have parts that change from solid to solid, which can damage lithium-ion and similar batteries.
• They respond quickly to changes in energy demand.
• They do not require "equalisation" charging, which is when a battery is overcharged to ensure all parts have the same charge.
• They do not release harmful substances.
• They do not lose energy when not in use.
• Materials used in the battery can be recycled.

Some types can easily show how much energy is stored by checking the voltage. They need less maintenance and can handle being overcharged or overused without damage.

They are safe because they usually do not use flammable materials. The materials can be kept separate from the main parts of the battery.

The main drawbacks include:

• They store less energy in a small space, requiring large tanks to hold enough energy.
• They charge and discharge energy slowly, which means they need bigger parts, increasing costs.
• They use energy less efficiently because they work with higher electricity flow to reduce losses and costs.

Flow batteries are more efficient than fuel cells but less efficient than lithium-ion batteries.

Traditional flow battery designs have low energy storage per weight (making them too heavy for electric vehicles) and low power output per weight (making them too expensive for stationary energy storage). However, some types, like hydrogen–bromine flow batteries, can produce a power level of 1.4 watts per square centimeter. Others, like hydrogen–bromate flow batteries, can store up to 530 watt-hours of energy per kilogram.

Traditional flow batteries

Redox cells use substances that can gain or lose electrons in liquid or gas environments. Redox flow batteries are rechargeable cells. They work by moving electrons between different phases, not through solid materials, making them more similar to fuel cells than traditional batteries. Fuel cells are not considered batteries because they were originally designed to produce electricity directly from fuels and air through a chemical process that does not involve burning. Later, in the 1960s and 1990s, rechargeable fuel cells, such as hydrogen-oxygen systems used in NASA’s Helios Prototype, were developed.

Cr–Fe chemistry has issues, such as hydrate isomerism, which refers to the balance between active chromium-chloride compounds and inactive water-based forms. This problem can be reduced by adding chelating amino-ligands. Hydrogen production on the negative electrode can be lessened by adding lead salts to increase resistance and gold salts to help chromium reactions occur more efficiently.

Common redox flow battery chemistries include iron-chromium, vanadium, polysulfide–bromide (Regenesys), and uranium. Redox fuel cells are less common in commercial use, though many designs have been proposed.

Vanadium redox flow batteries are the most widely used commercially. They use vanadium at both the positive and negative electrodes, which prevents contamination between the two sides. However, the limited solubility of vanadium salts slightly reduces this benefit. This chemistry has advantages, such as four possible oxidation states within the voltage range of the graphite-aqueous acid interface, which avoids dilution issues seen in Cr–Fe batteries. A key benefit is the close match between the voltage range of the carbon-aqueous acid interface and vanadium’s redox reactions. This match helps carbon electrodes last longer and reduces harmful side reactions, such as hydrogen and oxygen production. These features allow vanadium batteries to last for many years and endure up to 15,000 to 20,000 cycles, resulting in a very low cost per unit of energy (LCOE). The LCOE is about a few cents per kilowatt-hour, much lower than solid-state batteries and close to government targets of 5 cents per kilowatt-hour. Challenges include the high cost and limited availability of vanadium pentoxide (V₂O₅), reactions that produce hydrogen and oxygen, and the formation of vanadium pentoxide during use.

Hybrid

The hybrid flow battery (HFB) uses one or more materials that react to electricity, deposited as a solid layer. A major disadvantage is that this setup can reduce the ability to separate energy and power. Each cell has one battery electrode and one fuel cell electrode. This type of battery is limited in energy storage by the size of the electrode surface.

HFBs include types such as zinc–bromine, zinc–cerium, soluble lead–acid, and all-iron flow batteries. Weng et al. created a vanadium–metal hydride hybrid flow battery with an experimental open circuit voltage of 1.93 V and an operating voltage of 1.70 V, which are relatively high values. This battery uses a graphite felt positive electrode in a solution of VOSO₄ and H₂SO₄, and a metal hydride negative electrode in a KOH solution. Two electrolytes with different pH levels are separated by a bipolar membrane. The system showed good reversibility and high efficiencies: 95% for coulomb, 84% for energy, and 88% for voltage. Improvements were seen with higher current density, larger 100 cm electrodes, and series operation. Early tests with fluctuating power input showed potential for large-scale energy storage. In 2016, a high energy density Mn(VI)/Mn(VII)-Zn hybrid flow battery was introduced.

A prototype zinc–polyiodide flow battery achieved an energy density of 167 Wh/L. Older zinc–bromide batteries reached 70 Wh/L. For comparison, lithium iron phosphate batteries store 325 Wh/L. The zinc–polyiodide battery is considered safer because it does not use acidic electrolytes, is not flammable, and can operate in temperatures from -4 to 122 °F (-20 to 50 °C) without needing extensive cooling systems. One issue is zinc buildup on the negative electrode, which can pass through the membrane and lower efficiency. Zinc dendrite formation limits the ability of Zn-halide batteries to operate at high current density (>20 mA/cm²), reducing power density. Adding alcohol to the electrolyte of the ZnI battery may help. Challenges with Zn/I RFBs include the high cost of iodide salts (> $20/kg), limited capacity for zinc deposition, and zinc dendrite formation.

When the battery is fully discharged, both tanks contain the same electrolyte solution: a mix of positively charged zinc ions (Zn²⁺) and negatively charged iodide ions (I⁻). When charged, one tank holds polyiodide ions (I₃⁻). The battery generates power by moving liquid across the stack, where the liquids mix. Inside the stack, zinc ions pass through a selective membrane and become metallic zinc on the negative side. To increase energy density, bromide ions (Br⁻) are used to stabilize free iodine, forming iodine–bromide ions (I₂Br) to release iodide ions for charge storage.

Proton-flow batteries (PFBs) combine a metal hydride storage electrode with a reversible proton-exchange membrane (PEM) fuel cell. During charging, PFBs join hydrogen ions from splitting water with electrons and metal particles in one fuel cell electrode. Energy is stored as a metal hydride solid. During discharge, electricity and water are produced when the process is reversed, combining protons with oxygen. Metals less expensive than lithium can be used, offering greater energy density than lithium batteries.

Organic

Compared to inorganic redox flow batteries, such as vanadium and Zn-Br₂ batteries, organic redox flow batteries have an advantage because their active components can be adjusted to have different chemical properties. As of 2021, organic RFBs had low durability (how long they last or how many times they can be used) and had not been used in real-world applications.

Organic redox flow batteries can be divided into two types: aqueous (AORFBs) and non-aqueous (NAORFBs). AORFBs use water as the solvent for their electrolyte materials, while NAORFBs use organic solvents. Both AORFBs and NAORFBs can be further split into total systems and hybrid systems. Total systems use only organic materials for both the anode and cathode, while hybrid systems use inorganic materials for either the anode or cathode. For large-scale energy storage, AORFBs have lower solvent costs and better conductivity, making them more likely to be used in real-world applications. They also offer safety benefits because they use water-based electrolytes. NAORFBs, however, can operate at higher voltages and take up less space.

pH neutral AORFBs operate at pH 7, often using NaCl as a supporting electrolyte. At this pH level, organic and organometallic molecules are more stable than in acidic or alkaline conditions. For example, K₄[Fe(CN)₆], a common catholyte in AORFBs, is unstable in alkaline solutions but stable at pH 7.

AORFBs using methyl viologen as the anolyte and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl as the catholyte, along with NaCl and a low-cost anion exchange membrane, achieved the highest cell voltage (1.25 V) and lowest capital cost ($180/kWh) as of 2015. These electrolytes were designed to work without replacing existing infrastructure. A 600-milliwatt test battery was stable for 100 cycles with nearly 100% efficiency at current densities from 20 to 100 mA/cm², with best performance at 40–50 mA, where about 70% of the battery’s original voltage was retained. Neutral AORFBs are more environmentally friendly than acidic or alkaline alternatives and perform similarly to corrosive RFBs. The MV/TEMPO AORFB had an energy density of 8.4 Wh/L, limited by the TEMPO side. In 2019, a viologen-based AORFB using an ultralight sulfonate-viologen/ferrocyanide system was reported to be stable for 1,000 cycles with an energy density of 10 Wh/L, the most stable and energy-dense AORFB at that time.

Quinones and their derivatives are used in many organic redox systems. In one study, 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) and 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) were used as cathodes, while conventional Pb/PbSO₄ was the anolyte in a hybrid acid AORFB. Quinones can accept two units of electrical charge, compared to one in conventional catholytes, meaning they can store twice as much energy in the same volume.

Another quinone, 9,10-anthraquinone-2,7-disulfonic acid (AQDS), was tested. AQDS rapidly and reversibly reduces by two electrons and two protons on a glassy carbon electrode in sulfuric acid. An AORFB using inexpensive carbon electrodes and combining the quinone/hydroquinone couple with the Br₂/Br⁻ redox couple achieved a peak power density over 6,000 W/m² at 13,000 A/m². Cycling showed over 99% storage capacity retention per cycle. Volumetric energy density was over 20 Wh/L. Using 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 avoided the use of hazardous Br₂. The battery lasted 1,000 cycles without degradation, had a low cell voltage (about 0.55 V), and a low energy density (<4 Wh/L).

Replacing hydrobromic acid with a less toxic alkaline solution (1 M KOH) and using ferrocyanide reduced corrosion, allowing inexpensive polymer tanks. Increased membrane resistance was offset by raising the voltage to 1.2 V. Cell efficiency exceeded 99%, while round-trip efficiency measured 84%. The battery had an expected lifetime of at least 1,000 cycles and a theoretical energy density of 19 Wh/L. Ferrocyanide’s stability in high pH KOH solution was not confirmed.

Combining anolyte and catholyte in the same molecule, called bifunctional analytes or combi-molecules, allows the same material to be used in both tanks. In one tank, it acts as an electron donor, and in the other, it acts as an electron recipient. This reduces crossover effects. Quinone diaminoanthraquinone, indigo-based molecules, and TEMPO/phenazine are potential electrolytes for symmetric redox-flow batteries (SRFB).

Another approach used a Blatter radical as the donor/recipient.

Other types

Other types of flow batteries include the zinc–cerium battery, the zinc–bromine battery, and the hydrogen–bromine battery.

A membraneless battery uses laminar flow, where two liquids are pumped through a channel. These liquids undergo chemical reactions to store or release energy. The liquids flow in parallel with little mixing, and the flow naturally separates them without needing a membrane.

Membranes are often the most expensive and least reliable parts of batteries. This is because they can corrode when exposed to certain chemicals. Without a membrane, a liquid bromine solution and hydrogen can be used. These materials are less expensive and work well together. The design uses a small channel between two electrodes. Liquid bromine flows 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, which is a new feature for membraneless designs. A membraneless flow battery announced in August 2013 produced a maximum power density of 0.795 W/cm², three times more than other membraneless systems—and ten times higher than lithium-ion batteries.

In 2018, a large-scale membraneless redox flow battery capable of recharging and recirculating electrolyte streams was demonstrated. The battery used immiscible organic catholyte and aqueous anolyte liquids. These materials showed high capacity retention and efficient energy use during repeated use.

A lithium–sulfur system arranged in a network of nanoparticles removes the need for charge to move through particles directly connected to a conducting plate. Instead, the nanoparticle network allows electricity to flow throughout the liquid, enabling more energy to be extracted.

In a semi-solid flow battery, positive and negative electrode particles are suspended in a carrier liquid. These suspensions flow through a stack of reaction chambers, separated by a barrier such as a thin, porous membrane. This design combines the structure of aqueous-flow batteries, which use electrode material suspended in a liquid electrolyte, with the chemistry of lithium-ion batteries. The carbon-free semi-solid redox flow battery is also called a solid dispersion redox flow battery. Dissolving a material changes its chemical properties, but suspending solid pieces keeps their original characteristics. This creates a thick, viscous suspension.

In 2022, Influit Energy announced a flow battery electrolyte made of a metal oxide suspended in an aqueous solution.

Flow batteries with redox-targeted solids (ROTS), also called solid energy boosters (SEBs), use either the posolyte, negolyte, or both (also called redox fluids) to interact with one or more solid electroactive materials (SEM). The fluids contain redox couples with redox potentials near those of the SEM. These SEB/RFB systems combine the high energy density of traditional batteries, like lithium-ion batteries, with the ability to separate energy storage and power delivery found in flow batteries. Compared to semi-solid RFBs, SEB(ROTS) RFBs have advantages such as no need to pump thick suspensions, no clogging, higher power output, longer durability, and more chemical flexibility. However, these batteries lose energy twice—once in the reaction stack and once in the tank between the SEB(ROTS) and a mediator—leading to lower energy efficiency. On a system level, traditional lithium-ion batteries have higher practical energy storage than SEB(ROTS)-based lithium-ion flow batteries.

Applications

Redox flow batteries have advantages that make them good for storing large amounts of energy. These batteries are usually used in large, stationary systems (1 kWh to 10 MWh) that require energy to be stored and released over many hours. They are not efficient for short-term energy use. Key uses include:

• Grid storage: Storing energy for the power grid, either for short or long periods. This helps balance energy supply and demand by storing power during times of low use (off-peak hours) and releasing it during high demand (peak periods). A common challenge is that most flow battery designs have low power output per unit area, which increases costs. These batteries also help store energy from unpredictable sources like wind or solar for use during high-demand times. They can reduce sudden spikes in energy demand (peak shaving).
• Uninterruptible power supplies: Providing backup power when the main electricity source fails.
• Power conversion: Since all battery cells share the same electrolyte, the number of cells used for charging and discharging can differ. This allows the battery to adjust voltage levels, acting as a DC–DC converter. If the number of cells changes continuously, the battery can also convert energy between AC and DC, or between different AC and DC forms, limited by the switching equipment’s frequency.
• Electric vehicles: Flow batteries can be quickly recharged by replacing the electrolyte, making them suitable for vehicles that need rapid energy refills, similar to gas vehicles. However, most flow battery designs have low energy density, which limits driving range. Exceptions include zinc-chlorine batteries and those using highly soluble halates.
• Stand-alone power systems: Used in locations without grid access, such as cellphone base stations. These batteries work with solar or wind power to manage inconsistent energy supply and with generators to reduce fuel use.