Vanadium Redox Flow Battery: Chemistry, Electrolyte, Limits

A vanadium redox flow battery (VRFB, also called an all-vanadium redox flow battery) stores electricity in two liquid electrolytes, both made of dissolved vanadium in sulfuric acid, held in external tanks and pumped through an electrochemical cell. The negative side cycles between V(II) and V(III); the positive side cycles between V(IV) and V(V). Because both sides use the same element, ion crossover through the membrane lowers efficiency but does not permanently contaminate the electrolyte.

Short answer

An all-vanadium flow battery keeps its active material in tanks rather than in solid electrodes. During discharge, V(II) is oxidised to V(III) in the negative electrolyte while V(V) is reduced to V(IV) in the positive electrolyte, and protons move across an ion-exchange membrane to balance charge. The usual electrolyte is vanadium in sulfuric acid, kept below about 2 M vanadium and run between roughly 10 and 40 °C. Power (set by cell area and stack size) and energy (set by electrolyte volume and concentration) are sized independently. For general background on the wider family, see the CASRAI guide to the flow battery.

Definition and scope

The term covers the “all-vanadium” system in which both half-cells use vanadium at different oxidation states. A 2021 open-access review in Batteries by Ghimire and co-authors, including Maria Skyllas-Kazacos, describes it as one of several redox flow chemistries, alongside iron-chromium, vanadium-chromium, all-iron and zinc-bromine designs, and says the University of New South Wales group led by Skyllas-Kazacos carried out the first successful demonstration in the 1980s using vanadium in sulfuric acid in each half-cell. The review names stationary applications such as grid-connected and off-grid renewable storage, isolated power supply, load levelling and backup.

Two features set the VRFB apart from a conventional sealed battery. First, the electrolyte is stored outside the cell, so the amount of energy stored is a tank-size decision, not a cell-count decision. Second, the same element appears on both sides. Ghimire et al. state that nearly every flow chemistry suffers from cross-over of active ions through the membrane, causing cross-contamination and permanent capacity loss, and that the VRFB avoids the permanent part of that problem by using vanadium in both half-cells.

This page covers the chemistry, electrolyte design and operating limits of the VRFB itself. It does not cover cost, product selection or project economics.

How a vanadium redox flow battery works

The four oxidation states

Vanadium can exist in aqueous sulfuric acid at four oxidation states relevant here. According to Ghimire et al. and to Crothers et al. (Journal of the Electrochemical Society, 2020), the positive half-cell holds V(IV) as VO²⁺ and V(V) as VO₂⁺, and the negative half-cell holds V(II) as V²⁺ and V(III) as V³⁺. Each oxidation state has its own colour in solution, which is why optical methods can be used to follow state of charge (see below).

Half-cell and overall reactions

Ghimire et al. give the reactions, written for charging, as:

• Positive side: VO²⁺ + H₂O ⇌ VO₂⁺ + 2H⁺ + e⁻, E = 1.000 V
• Negative side: V³⁺ + e⁻ ⇌ V²⁺, E = −0.255 V
• Overall: V²⁺ + VO₂⁺ + 2H⁺ ⇌ VO²⁺ + V³⁺ + H₂O, E = 1.255 V

On discharge the reactions run in reverse: V²⁺ is oxidised to V³⁺ in the negative tank and VO₂⁺ is reduced to VO²⁺ in the positive tank, while H⁺ migrates across the membrane to maintain charge balance. Crothers et al. quote essentially the same couples (−0.26 V and 1.00 V versus the standard hydrogen electrode) but use a measured open-circuit potential of 1.42 V for their model cell, a reminder that a measured cell voltage depends on acid concentration and state of charge and is not the same quantity as a standard potential. The data table below lays the two papers’ values side by side.

Cell and stack hardware

According to Ghimire et al., a single cell consists of porous carbon or graphite felt electrodes, carbon bipolar plates, an ion-exchange membrane, flow frames, gaskets, current collectors and end plates. The felt supplies a large surface area for the redox couples to react on. Electrolyte is pumped from the two tanks through the felt on either side of the membrane. The same review notes that the electrolyte is usually prepared by electrochemical dissolution of vanadium pentoxide.

Why power and energy scale independently

Ghimire et al. explain that energy capacity depends on reactant concentration and electrolyte volume, whereas power depends on the electrode area and the number of cells in a stack, so the two can be optimised separately. The Pacific Northwest National Laboratory (PNNL) makes the same point in its description of redox flow batteries for grid-scale storage: capacity (kWh) and power (kW) scale independently. Ghimire et al. add that, since capacity grows by adding electrolyte volume, cost per kWh falls markedly with longer storage durations, which is why they describe the VRFB as particularly attractive for storage durations above about four hours.

Electrolyte design: sulfate-only versus mixed-acid

The electrolyte is the part of a VRFB that researchers and lab staff handle most directly, and it is where the main design trade-offs sit.

The conventional sulfate electrolyte

PNNL describes the all-vanadium, sulfate-based electrolyte as historically the state of the art, and says that deployment has been limited by poor performance across a wide temperature range and by relatively high cost. Ghimire et al. say that vanadium concentration is held below 2 M because of the solubility limits of vanadium ions in sulfuric acid, and that the operating temperature of a VRFB usually runs from 10 °C to 40 °C. Above 40 °C the V(V) species in the positive electrolyte can precipitate thermally; below 10 °C a V(II) and V(III) sulfate can precipitate from the negative electrolyte. Because of this narrow window they note that heating and cooling systems are often needed in harsh climates.

The reference electrolyte in the Crothers model gives a concrete picture of composition. It was prepared from 1.5 mol dm⁻³ V(IV) sulfate in 2.6 mol dm⁻³ sulfuric acid and charged twice. At 50% state of charge the negative electrolyte holds 0.75 mol dm⁻³ each of V(II) and V(III) with 4.66 mol dm⁻³ total sulfate, and the positive electrolyte holds 0.75 mol dm⁻³ each of V(IV) and V(V) with 4.11 mol dm⁻³ total sulfate; the electrolyte density is 1.19 g cm⁻³. Crothers et al. also report that high sulfuric acid concentrations gave the best cell performance in their calculations.

The mixed sulfate-chloride electrolyte

Li and co-authors at PNNL reported a different electrolyte in Advanced Energy Materials (2011): a mixed sulfate-chloride solution that dissolves more than 2.5 M vanadium, is stable from −5 to 60 °C, and offers about 70% higher energy density than the vanadium sulfate system. PNNL’s technology page for redox flow batteries repeats the 70% energy-density figure and the broader temperature range.

Property	Sulfate-only (Ghimire et al., 2021)	Mixed sulfate-chloride (Li et al., 2011, via PNNL)
Vanadium concentration	Held below 2 M (solubility limit in sulfuric acid)	More than 2.5 M dissolved
Temperature range	Usually 10 to 40 °C	Stable from −5 to 60 °C
Failure at the edges	V(V) precipitation above 40 °C; V(II)/V(III) sulfate precipitation below 10 °C	Not described in the PNNL summary
Energy density	Reference system	About 70% higher than vanadium sulfate

The two columns come from different studies, so they describe the electrolytes as reported rather than a head-to-head test under identical conditions.

Membrane crossover, self-discharge and rebalancing

Crothers et al. state that transport through the membrane strongly influences VRFB performance. Vanadium ions partition into the membrane and move by diffusion and migration, and “vanadium crossover causes the cell to self-discharge.” In their model, crossover lowers the state of charge and, because charge is lost to chemical reactions inside the electrolyte, lowers Coulombic efficiency. Their Table I lists the chemical reactions that follow when crossed ions meet the other half-cell’s species, for example V²⁺ + 2VO₂⁺ + 2H⁺ → 3VO²⁺ + H₂O, and note that these happen in the electrode solutions rather than within the membrane because the vanadium concentration absorbed in the membrane is low.

Efficiency in this literature is split into three numbers. Voltage efficiency falls as current density rises because of ohmic and other overpotentials. Coulombic efficiency tends to rise with current density because the charge extracted grows relative to the charge lost to crossover reactions. Crothers et al. treat energy efficiency as the product of the two.

Membrane choice is a trade-off. The reference membrane in the Crothers study is Nafion 212, a perfluorosulfonic acid cation-exchange membrane, 51 µm dry, with an equivalent weight of 1100 g per mole of sulfonic acid groups. They find lower-equivalent-weight membranes perform better, and that at high equivalent weight a low membrane modulus is preferable. Separately, Ghimire et al. note that electrolyte transfer through the membrane can also occur over long operation, with net movement from negative to positive half-cell for cation-exchange membranes and the reverse for anion-exchange membranes, and this induces an electrolyte imbalance.

Ghimire et al. explain why this is manageable in a VRFB: because the same element is on both sides, most capacity loss from imbalance can be restored by periodic remixing of the two half-cell solutions, or by partial electrolyte replacement. Side reactions such as gas evolution, by contrast, cause imbalance that can only be recovered by chemical or electrochemical rebalancing. They also note that uneven electrolyte distribution within a large cell can cause local polarisation, gas evolution and degradation of the bipolar plates over long-term operation.

Measuring state of charge in a laboratory VRFB

Ghimire et al. describe the most widely used method for flow batteries as an open-circuit cell, a method first proposed by NASA for its iron-chromium flow battery programme in the 1980s. It assumes the two half-cells are balanced, which is not guaranteed once side reactions and uneven crossover have acted on the electrolytes. For that reason, Skyllas-Kazacos and co-workers proposed in the late 1980s methods that measure each half-cell separately: UV-visible spectroscopy and conductivity measurement. The review says each vanadium oxidation state has a specific colour, and that spectroscopy allows each half-cell electrolyte’s state of charge to be measured independently so that imbalance can be detected and corrected by partial or complete remixing.

The review also lists the limits of each approach. Half-cell potential monitoring is subject to reference-electrode drift in long-term operation; conductivity is affected by species transferring across the membrane and changing composition over time; and absorption spectroscopy of the positive couple does not obey Beer’s law linearly at high concentrations because VO₂⁺ and VO²⁺ interfere. These are the practical reasons a laboratory may run more than one measurement in parallel when characterising a cell.

Where it matters for labs and facilities

Storage duration and application fit

Ghimire et al. describe the VRFB as suited to long-duration storage, peak shaving and load levelling, and note a fast response that allows use as an uninterruptible power supply. They also say the cell can be discharged completely to a very low state of charge and recharged after a long discharge, and that capacity loss during extended standby is negligible because the electrolyte sits in external tanks. They state that, with periodic remixing or rebalancing, the electrolyte can in theory perform an unlimited number of cycles; “in theory” is their qualifier.

Known limits

• Low specific energy: the review gives about 25 Wh kg⁻¹, compared with Li-ion and nickel-metal hydride, limited by vanadium solubility over the operating temperature range. They say this is acceptable for most stationary uses but limits mobile ones.
• A narrow temperature window, as described above.
• Cost: vanadium is a relatively expensive transition metal, which drives electrolyte and system cost. The same authors add that the long life and re-usable electrolyte can give competitive levelised cost over the life of a system, and that the electrolyte retains residual value because it can be reprocessed to recover vanadium or redeployed in other VRFB systems.

Handling and containment

The electrolyte is a strongly acidic vanadium solution. Ghimire et al. note that a VRFB can harm the environment in the event of spillage and should be contained in double-layered tanks; they also say VRFBs are much safer than Li-ion batteries and are not prone to catching fire. Anyone preparing or handling vanadium sulfate electrolyte in a lab is working with sulfuric acid; the CASRAI pages on sulfuric acid safety data sheets, the corrosive hazard symbol and chemical storage cabinets cover the general hazard communication and storage questions. Follow the safety data sheet for the specific electrolyte formulation in use, because the mixed-acid variants contain chloride as well as sulfate.

Cell-level comparison with other storage routes

For readers weighing storage routes for a grid-scale project, flow batteries are one of several options; the CASRAI guides to pumped storage and the lithium-sulfur battery cover two others, and the flow battery overview covers the wider chemistry family.

Common misunderstandings

• “No crossover happens in a VRFB.” Crossover does happen. Ghimire et al. say it is unavoidable because no ion-exchange membrane blocks ion movement completely. What the single-element design changes is the consequence: the loss can be reversed by remixing, and Crothers et al. show it still costs efficiency by driving self-discharge.
• “The electrolyte never degrades.” The review’s wording is that the electrolyte can theoretically cycle indefinitely given periodic remixing or rebalancing. Gas-evolution side reactions and precipitation outside the 10 to 40 °C window are real failure routes that need active management.
• “More sulfuric acid is always better.” Crothers et al. found that high acid concentrations gave the best cell performance for their modelled membrane, but they also warn dilute-solution assumptions can mislead; their finding is about a specific model system. Precipitation limits the vanadium that the acid system can hold.
• “VRFB energy density is comparable to Li-ion.” The reviewed figure is about 25 Wh kg⁻¹ for the conventional system, which is why the technology is aimed at stationary storage.
• “The mixed-acid electrolyte replaces sulfate-only systems.” PNNL calls sulfate-based electrolyte historically the state of the art and presents the mixed-acid electrolyte as an improvement reported in research; the sources read for this page do not state that it has displaced the sulfate system.

Frequently asked questions

What is a vanadium redox flow battery?

It is a rechargeable flow battery whose two electrolytes both contain vanadium in sulfuric acid, in different oxidation states: V(II)/V(III) on the negative side and V(IV)/V(V) on the positive side. The electrolytes are stored in tanks and pumped through a cell separated by an ion-exchange membrane.

Why does the battery use vanadium on both sides?

Using one element on both sides means crossover through the membrane is not a permanent contamination. Ghimire et al. say this does not affect life and performance, and that capacity lost to imbalance can be largely restored by remixing the half-cell solutions.

What cell voltage does a VRFB produce?

Ghimire et al. give a standard overall potential of 1.255 V from the two half-cell potentials of 1.000 V and −0.255 V. Crothers et al. used a measured open-circuit potential of 1.42 V for their model cell at 295 K.

What temperature range can it run in?

Conventional sulfate electrolyte usually runs between 10 °C and 40 °C. PNNL’s mixed sulfate-chloride electrolyte was reported stable from −5 to 60 °C.

Can power and energy be sized separately?

Yes. Energy depends on electrolyte volume and concentration; power depends on electrode area and the number of cells in the stack.

Why is its energy density low?

The review gives about 25 Wh kg⁻¹ and ties the limit to the solubility of vanadium ions in the electrolyte over the required temperature range. The mixed-acid electrolyte raises solubility and was reported to raise energy density by about 70%.

Sources

• Ghimire, P.C., Bhattarai, A., Lim, T.M., Wai, N., Skyllas-Kazacos, M., Yan, Q. “In-Situ Tools Used in Vanadium Redox Flow Battery Research—Review.” Batteries 7(3), 53 (2021), open access, doi:10.3390/batteries7030053. Read 2026-11-12.
• Crothers, A.R. et al. “Theory of Multicomponent Phenomena in Cation-Exchange Membranes: Part III.” Journal of the Electrochemical Society 167, 013549 (2020), doi:10.1149/1945-7111/ab6725. Read 2026-11-12.
• Li, L. et al. “A Stable Vanadium Redox-Flow Battery with High Energy Density for Large-scale Energy Storage.” Advanced Energy Materials 1(3), 394-400 (2011), via the PNNL publication page. Read 2026-11-12.
• Pacific Northwest National Laboratory, Redox Flow Batteries for Grid-scale Energy Storage. Read 2026-11-12.