Researchers at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) have repurposed a commonplace chemical used in water treatment facilities to create a new, large-scale energy storage solution. This innovative battery design, which utilizes Earth-abundant materials, offers a safe, economical, water-based flow battery that could significantly enhance the integration of intermittent energy sources like wind and solar into the nation’s electric grid.

Published in Nature Communications, the study reports that the iron-based battery demonstrated exceptional cycling stability, maintaining 98.7 percent of its maximum capacity over 1,000 consecutive charging cycles. This performance marks a substantial improvement over previous iron-based batteries, which exhibited significantly higher charge capacity degradation over fewer cycles.

Iron-based flow batteries have been in use since the 1980s and are commercially available. However, this new battery stores energy in a unique liquid chemical formula that combines charged iron with a neutral-pH phosphate-based liquid electrolyte, known as nitrogenous triphosphonate (nitrilotri-methylphosphonic acid or NTMPA). This chemical, typically used to inhibit corrosion in water treatment plants, is commercially available in industrial quantities.

“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.”

Flow batteries, which can serve as backup generators for the electric grid, are essential for a decarbonization strategy that includes energy storage from renewable resources. They can be built at various scales, from lab-bench models to city-block sizes, making them versatile for different applications.

Grid operators are increasingly looking to install battery energy storage systems (BESS) in urban or suburban areas near consumers, where safety is a significant concern. The aqueous flow battery developed by PNNL could help address these concerns due to its operation in water at neutral pH.

“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 co-author Aaron Hollas. “In addition, our system uses commercially available reagents that haven’t been previously investigated for use in flow batteries.”

The initial design of the PNNL battery achieves an energy density of up to nine watt-hours per liter (Wh/L). While this is less than the 25 Wh/L energy density of commercialized vanadium-based systems, the new design’s use of Earth-abundant materials allows for scalable construction to match energy output needs.

“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 the Grid Storage Launchpad (GSL), set to open at PNNL in 2024.

By Impact Lab