A team of scientists from the Pacific Northwest National Laboratory have developed an organic aqueous flow battery expected to cost $180 per kilowatt-hour, a projected savings of about 60% as compared to standard flow batteries. The researchers were able to reduce the cost of the battery by using active materials that are made of inexpensive organic molecules, compared to the metals used in today's flow batteries.
"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 (OE), which funded this research.
"The battery's water-based liquid electrolytes are also designed to be a drop-in replacement for current flow battery systems," says Wei Wang, a PNNL materials scientist and one of the paper's authors. "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.
Flow batteries are different from Lithium-ion batteries, which make up about 70% of the world’s working batteries but come with lifespan and safety issues. Flow batteries store their active chemicals separately until power is needed, which reduces safety concerns.
PNNL's new 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 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. As a result, it can also make the renewable energy it stores and the power grid it supports greener," says Wei.
To test the new battery design, the team created a small, 600-milliwatt battery in its lab. 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, where about 70 percent of the battery's original voltage was retained. They also discovered that the battery continued to operate well beyond 100 cycles.
Going forward the team plans to make a larger version of the test battery that can store up to 5 kilowatts of electricity (enough to support the peak load of a typical U.S. home). It will also focus on improving the battery's cycling so it can retain more of its storage capacity longer.