March 25, 2024
News Release

New All-Liquid Iron Flow Battery for Grid Energy Storage

A new recipe provides a pathway to a safe, economical, water-based, flow battery made with Earth-abundant materials

Lead author and battery researcher Gabriel Nambafu assembles a test flow battery apparatus

Lead author and battery researcher Gabriel Nambafu assembles a test flow battery apparatus.

(Photo by Andrea Starr | Pacific Northwest National Laboratory)

RICHLAND, Wash.— A commonplace chemical used in water treatment facilities has been repurposed for large-scale energy storage in a new battery design by researchers at the Department of Energy’s Pacific Northwest National Laboratory. The design provides a pathway to a safe, economical, water-based, flow battery made with Earth-abundant materials. It provides another pathway in the quest to incorporate intermittent energy sources such as wind and solar energy into the nation’s electric grid.

Electric grid transmission lines
New flow battery technologies are needed to help modernize the U.S. electric grid and provide a pathway for energy from renewable sources such as wind and solar power to be stored. (Photo by Andrea Starr | Pacific Northwest National Laboratory)

The researchers report in Nature Communications that their lab-scale, iron-based battery exhibited remarkable cycling stability over one thousand consecutive charging cycles, while maintaining 98.7 percent of its maximum capacity. For comparison, previous studies of similar iron-based batteries reported degradation of the charge capacity two orders of magnitude higher, over fewer charging cycles.

Iron-based flow batteries designed for large-scale energy storage have been around since the 1980s, and some are now commercially available. What makes this battery different is that it stores energy in a unique liquid chemical formula that combines charged iron with a neutral-pH phosphate-based liquid electrolyte, or energy carrier. Crucially, the chemical, called nitrogenous triphosphonate, nitrilotri-methylphosphonic acid or NTMPA, is commercially available in industrial quantities because it is typically used to inhibit corrosion in water treatment plants.

Phosphonates, including NTMPA, are a broad chemical family based on the element phosphorus. Many phosphonates dissolve well in water and are nontoxic chemicals used in fertilizers and detergents, among other uses.

“We were looking for an electrolyte that could bind and store charged iron in a liquid complex at room temperature and mild operating conditions with neutral pH,” said senior author Guosheng Li, a senior scientist at PNNL who leads materials development for rechargeable energy storage devices. “We are motivated to develop battery materials that are Earth-abundant and can be sourced domestically.”

What is a flow battery?

As their name suggests, flow batteries consist of two chambers, each filled with a different liquid. The batteries charge through an electrochemical reaction and store energy in chemical bonds. When connected to an external circuit, they release that energy, which can power electrical devices. Unlike other conventional batteries, flow batteries feature two external supply tanks of liquid constantly circulating through them to supply the electrolyte, serving as the battery system’s “blood supply.” The larger the electrolyte supply tank, the more energy the flow battery can store.

Animation of an energy storage and release cycle using a new flow battery design from PNNL researchers. Small circles representing energy (electrons) are shown traveling through an electrolyte solution containing aqueous iron. When the stored energy is needed, the iron releases the charge to supply energy to the electric grid.
The aqueous iron (Fe) redox flow battery here captures energy in the form of electrons (e-) from renewable energy sources and stores it by changing the charge of iron in the flowing liquid electrolyte. When the stored energy is needed, the iron can release the charge to supply energy (electrons) to the electric grid. (Animation by Sara Levine | Pacific Northwest National Laboratory)

Flow batteries can serve as backup generators for the electric grid. Flow batteries are one of the key pillars of a decarbonization strategy to store energy from renewable energy resources. Their advantage is that they can be built at any scale, from the lab-bench scale, as in the PNNL study, to the size of a city block.

In the near term, grid operators are looking to locate battery energy storage systems (BESS) in urban or suburban areas near energy consumers. Often, city planners must grapple with consumer safety concerns. The type of aqueous flow battery reported here could help alleviate safety concerns.

[Learn more about how communities grapple with BESS siting.]

“A BESS facility using the chemistry similar to what we have developed here would have the advantage of operating in water at neutral pH,” said Aaron Hollas, a study author and team leader in PNNL’s Battery Materials and Systems Group. “In addition, our system uses commercially available reagents that haven’t been previously investigated for use in flow batteries.”

The research team reported that their initial design can reach energy density, a key design feature, up to 9 watt-hours per liter (Wh/L). In comparison, commercialized vanadium-based systems are more than twice as energy dense, at 25 Wh/L. Higher energy density batteries can store more energy in a smaller square footage, but a system built with Earth-abundant materials could be scaled to provide the same energy output.

Future development of aqueous redox flow batteries

“Our next step is to improve battery performance by focusing on aspects such as voltage output and electrolyte concentration, which will help to increase the energy density,” said Li. “Our voltage output is lower than the typical vanadium flow battery output. We are working on ways to improve that.”

PNNL researchers plan to scale-up this and other new battery technologies at a new facility called the Grid Storage Launchpad (GSL) opening at PNNL in 2024. The GSL, funded by the Department of Energy’s Office of Electricity, which also funded the current study, will help accelerate the development of future flow battery technology and strategies so that new energy storage systems can be deployed safely.

Grid Storage Launchpad building
The Grid Storage Launchpad, opening on the Richland, Washington, campus of Pacific Northwest National Laboratory in 2024, will help evaluate new grid-scale battery technology. (Photo by Andrea Starr | Pacific Northwest National Laboratory)

Study contributors included co-lead authors Gabriel S. Nambafu and Hollas, as well as Peter S. Rice, Daria Boglaienko, John L. Fulton, Miller Li, Qian Huang, David M. Reed, Vincent L. Sprenkle, and G. Li from PNNL. Shuyuan Zhang and Yu Zhu from the University of Akron in Akron, Ohio, also participated in the research.


About PNNL

Pacific Northwest National Laboratory draws on its distinguishing strengths in chemistry, Earth sciences, biology and data science to advance scientific knowledge and address challenges in sustainable energy and national security. Founded in 1965, PNNL is operated by Battelle for the Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States. DOE’s Office of Science is working to address some of the most pressing challenges of our time. For more information, visit For more information on PNNL, visit PNNL's News Center. Follow us on Twitter, Facebook, LinkedIn and Instagram.