All-Liquid Iron Flow Battery Is Safe, Economical

Andrew Corselli

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.

The researchers reported 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.

“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. “We are motivated to develop battery materials that are Earth-abundant and can be sourced domestically.”

Here is an exclusive Tech Briefs interview — edited for length and clarity — with Li.

Tech Briefs: What was the biggest technical challenge you faced while developing this battery?

Li: The primary technical challenge in developing this battery lies in identifying a novel Fe-complex with electrochemical properties suitable for use as an anolyte. Discovering an appropriate Fe complex with a nitrogenous phosphonate is nontrivial, as most of Fe complexes reported to have the appropriate redox potential are also prone to rapid degradation.

Tech Briefs: Can you explain in simple terms how it works?

Li: Similar to conventional flow batteries, the reported all-soluble Fe redox flow battery employs liquid electrolytes containing two different Fe complexes dissolved within, serving as both catholyte and anolyte. While circulating the liquid electrolytes through the battery stack separated by an ion-selective membrane, the battery will be charged or discharged by altering the oxidation state of the Fe complex in both the anolyte and catholyte.

Tech Briefs: You’re quoted as saying, “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. Our voltage output is lower than the typical vanadium flow battery output. We are working on ways to improve that.” Can you please share any updates on it?

Li: Yes, the discovery of the nitrogenous phosphonate as a ligand has indeed opened the door to new possibilities for Fe-based flow battery technologies. Fortunately, we have advanced our knowledge by strategically modifying the ligand to enhance the battery's output voltage. We can confidently anticipate an improvement in output voltage by approximately 20 percent compared to that reported in the initial findings and those results will be reported in a subsequent publication.

Tech Briefs: It’s been said 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.” How is that plan progressing?

Li: The GSL will provide testing and validation capabilities to explore various new battery technologies up to 100 kW/400 kWh, and multiple testing spots will be available at a scale of 10 kW/40 kWh to expedite the new battery developments. The facility will be open and operational in mid-2024, but a formal opening date has not yet been set.

Tech Briefs: What are your next steps? Do you have any plans for further research?

Li: We are planning to expand our expertise and capabilities to enhance battery performance, aiming for longer cycling life, lower costs, and higher energy density. This will make the all-soluble Fe flow battery technology more suitable for longer-duration energy storage applications, which are crucial for achieving net-zero emissions and decarbonizing the power grid.

Tech Briefs: Do you have any advice for engineers/researchers aiming to bring their ideas to fruition?

Li: As Pacific Northwest National Laboratory researchers, we consider ourselves fortunate to have the opportunity to pursue our research by integrating both fundamental and applied approaches. This combination involves conducting fundamental research activities such as complex synthesis, theoretical calculations, and spectroscopic characterization, alongside applied testing methods like electrochemical characterization and battery testing.

We believe that the synergy created by combining these diverse multidisciplinary understandings significantly enhances our battery research and development endeavors. By leveraging both fundamental understanding and practical application, we gain deeper insights into battery performance and advance the field more effectively.

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