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Short answer: A flow battery (redox flow battery, RFB) stores energy in liquid electrolytes held in tanks outside the cell. Pumps move the electrolyte through a cell stack, where charging and discharging take place. The US Department of Energy describes the dominant chemistries today as vanadium and hybrid zinc-bromine, and treats flow batteries as candidates for long-duration energy storage.
Definition and scope
The US DOE Office of Electricity defines redox-flow batteries as systems that operate by “dissolving active species into electrolytes which are then stored in containers away from the cell’s electrodes.” During operation, electrolyte is moved from the storage tanks to a cell stack where charge and discharge occur (DOE Office of Electricity, Energy Storage Safety Strategy, April 2024, section 2.3; read 2026-11-08). The defining feature is physical: the active material is in the liquid, and the liquid is stored apart from the electrodes.
This page covers the concept and the research and facility-safety questions it raises for laboratories and storage facilities. It does not cover product selection or pricing.
How a redox flow system is used in practice
The same DOE report describes electrolyte storage containers, a cell stack, and mechanical pumps that move the electrolyte. Three points follow from the report’s text.
- Long-duration role. The report says flow batteries “can provide long-duration storage services” but are “slow to ramp up, or are inefficient under small loads due to the parasitic losses of the mechanical pumps” (section 2.6).
- Chemistry in use. “The dominant flow battery chemistries are currently all vanadium and hybrid Zn-Br batteries.” Fe-flow, slurry flow and organic charge-carrier systems are described as under consideration.
- Deployment scale. The report cites an estimate of roughly 340 MW / 1200 MWh of redox-flow capacity deployed globally in 2020.
The Oak Ridge National Laboratory overview presented at the DOE Office of Electricity 2024 Peer Review states that flow batteries “can play a transformative role for Long-Duration Energy Storage (LDES) Systems,” while noting that current commercialized systems are based on vanadium flow battery technology and “suffer from cost competitiveness” (M. Balasubramanian, ORNL, August 2024; read 2026-11-08).
Types and limitations by chemistry
The ORNL overview gives a non-exhaustive table of redox flow systems and their limitations. It is reproduced below in condensed form; energy density is as stated on the slide.
| System (ORNL slide) | Electrolyte type | Energy density (Wh/kg) | Limitations stated |
|---|---|---|---|
| Vanadium | Aqueous, H+ conducting membrane | 25-30 | Low energy density, high acid concentration, toxic |
| Quinone-HBr/Br2 | Aqueous, H+ conducting membrane | 30-35 | Low energy density, toxic |
| Lithium metal / aqueous cathode | Mixed solvents / solid electrolyte separator | 150 | Lithium metal passivation, dendrites, safety issues; new membranes needed |
| Non-aqueous RFBs | Non-aqueous | >200 | Anode passivation, dendrites, safety, new membranes |
The slide also lists slurry-based systems at about 100 Wh/kg, with rheology control, low power density and cost as limitations. The pattern is the research problem in miniature: the commercial chemistries are aqueous with low energy density per kilogram, while the higher-energy-density options carry passivation, dendrite or safety limitations and need new membranes.
Where it matters for labs and facilities
Electrolyte hazards
The DOE report states that “RFB electrolytes are often strong acids and bases carrying high concentrations of metal ions,” and that hazard mitigation “typically involves additional containment for any electrolyte that spills out of the primary system containment.” For laboratories, this places flow-battery work alongside other corrosive-liquid work; see corrosive chemicals in the lab and secondary containment requirements. Chemical labelling duties are covered in the Hazard Communication Standard for laboratories.
Gas generation
The report calls gas generation “a ubiquitous issue in aqueous redox flow batteries.” The stable voltage window of water is about 1.2 V, shifting with pH; outside it, hydrogen and oxygen can evolve, along with toxic gases depending on the chemistry. Generated hydrogen and oxygen can often be recombined into water or vented through an engineered exhaust, and toxic gases may need other mitigation.
What federally funded research is working on
The ORNL peer-review overview lists DOE Office of Electricity projects on non-PFAS ion-selective membranes (ORNL) and on hydroxyphenazine derivatives for aqueous flow-battery anolytes (PNNL), the latter reported at under 1% capacity loss per year for a hydroxyphenazine-derivative anolyte. These are research results presented at a peer review, not commercial specifications.
For a contrasting storage chemistry see zinc-air batteries, and for transport rules on a different battery class see lithium battery shipping regulations.
Common misunderstandings
- “Flow means better in every respect.” The sources above describe slow ramp-up, pump parasitic losses at small loads, and low energy density in the commercial aqueous chemistries.
- “Water-based means harmless.” The DOE report notes strong acid or base electrolytes with high metal-ion concentrations and possible toxic gas evolution.
- “Commercial means cost-competitive.” ORNL states that current commercialized vanadium systems suffer from cost competitiveness.
FAQ
What is a flow battery?
A battery in which dissolved active species sit in electrolyte tanks outside the cell and are pumped through a cell stack for charge and discharge (DOE Office of Electricity, 2024).
Which flow battery chemistries are dominant?
According to the DOE report, vanadium and hybrid zinc-bromine; iron-flow, slurry and organic charge-carrier systems are being considered.
What are the main hazards?
Corrosive electrolytes needing secondary containment, and gas evolution in aqueous systems, including toxic gases depending on chemistry.
Sources
- US Department of Energy, Office of Electricity, Energy Storage Safety Strategy (April 2024), sections 2.3 and 2.6. energy.gov PDF. Read 2026-11-08.
- M. Balasubramanian (ORNL), Overview of Flow Batteries, DOE Office of Electricity 2024 Peer Review, August 2024. Peer review PDF. Read 2026-11-08.
