Energy storage system owners could see savings from a flow battery technology that is projected to cost 60% less than today's standard flow batteries.

The organic aqueous flow battery, developed by Pacific Northwest National Laboratory (PNNL) researchers, is expected to cost $180 per kilowatt hour once the technology is fully developed. The lower cost results from the battery's use of inexpensive organic molecules for its active materials rather than commodity metals used in today's flow batteries.

Xiaoliang Wei prepares a demonstration organic flow battery. Image credit: PNNL.Xiaoliang Wei prepares a demonstration organic flow battery. Image credit: PNNL."Moving from transition metal elements to synthesized molecules is a significant advancement because it links battery costs to manufacturing rather than commodity metals pricing," says Imre Gyuk, energy storage program manager for the Department of Energy's Office of Electricity Delivery and Energy Reliability, which funded the research.

The battery's water-based liquid electrolytes are designed to be a drop-in replacement for current flow battery systems, says PNNL materials scientist Wei Wang. "Current flow battery owners can keep their existing infrastructure, drain their more expensive electrolytes and replace them with PNNL's electrolytes."

Flow batteries generate power by pumping liquids from external tanks into a central stack. The tanks contain liquid electrolytes that store energy. When energy is needed, pumps move the electrolytes from both tanks into the stack, where electricity is produced by an electrochemical reaction.

PNNL's flow battery features two main electrolytes: a methyl viologen anolyte (negative electrolyte) and a 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, or 4-HO-TEMPO catholyte (positive electrolyte). A third supporting electrolyte carries sodium chloride, whose chloride ions enable the battery to discharge electricity by shuffling electrons in the central stack.

"Using readily available materials makes our all-organic aqueous flow battery more sustainable and environmentally friendly," Wang says. "As a result, it can also make the renewable energy it stores and the power grid it supports greener."

To test the battery design, Wang and his colleagues created a 600-milliwatt battery on a lab countertop. They repeatedly charged and then discharged the battery at various electric current densities, ranging from 20 to 100 milliAmperes per square centimeter. The test battery's optimal performance was between 40 and 50 milliAmperes per square centimeter, at which about 70% of the battery's original voltage was retained. They also found the battery continued to operate well beyond 100 cycles.

Next, the team plans to make a larger version of their test battery that is able to store up to 5 kilowatts of electricity, which could support the peak load of a typical U.S. home. Other ongoing efforts include improving the battery's cycling so it can retain more of its storage capacity longer.

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