Researchers at the Department of Energy’s Pacific Northwest National Laboratory have developed a pathway to safe, water-based flow batteries made with Earth-abundant materials.
Researchers have repurposed a commonplace chemical used in water treatment facilities for large-scale energy storage in a new flow battery design.
The new design provides a pathway to incorporating intermittent energy sources such as wind and solar energy into the nation’s electrical grid.
Published in Nature Communications, the iron-based battery exhibited remarkable cycling stability over 1,000 consecutive charging cycles, whilst maintaining 98.7% of its maximum capacity.
In comparison, studies of similar iron-based batteries reported degradation of the charge capacity two orders of magnitude higher over fewer charging cycles.
What is a flow battery?
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. This can power electrical devices.
Flow batteries have two external supply tanks of liquid constantly circulating through them to supply the electrolyte.
The larger the electrolyte supply tank, the more energy the flow battery can store.
Flow batteries can act as backup generators for the electric grid and are one of the key pillars of a decarbonisation strategy. They can be built at any scale, from the lab bench scale to the size of a city block.
What are iron-based flow batteries?
Designed for large-scale energy storage, iron-based flow batteries have been around since the 1980s.
This battery is different from other batteries because it stores energy in a unique liquid chemical formula that combines charged iron with a neutral-pH phosphate-based energy carrier.
The chemical, called nitrogenous triphosphonate, nitrilotri-methylphosphonic acid or NTMPA, is commercially available in industrial quantities. This is because it is typically used to inhibit corrosion in water treatment plants.
Phosphonates are a broad chemical family based on the element phosphorus. Many phosphonates dissolve well in water and are non-toxic chemicals used in fertilisers 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, 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.”
The new battery could alleviate safety concerns
Grid operators are currently looking to locate battery energy storage systems in urban or suburban areas near energy consumers.
Often, city planners must overcome consumer safety concerns. This type of aqueous flow battery could help alleviate safety concerns.
“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 benefits of a system built with Earth-abundant materials
The team reported that their initial flow battery design can reach energy density, a key design feature of up to nine watt-hours per litre.
In comparison, commercialised 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. However, a system built with Earth-abundant materials could be scaled to provide the same energy output.
Accelerating the development of future flow battery technology
“Our next step is to improve battery performance by focusing on aspects such as voltage output and electrolyte concentration, which will help 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.”
The team plans to scale up this and other new battery technologies at the Grid Storage Launchpad opening at PNNL in 2024.
Funded by the Department of Energy’s Office of Electricity, the Grid Storage Launchpad will help accelerate the development of flow batteries as well as strategies so that new energy storage systems can be safely deployed.
