New organic flow battery for low-cost energy storage

Scientists have developed a new organic aqueous flow battery that offers low-cost energy storage and will be cheaper than such existing batteries.
The battery is expected to cost USD 180 per kilowatt-hour - 60 per cent less than today's standard flow batteries - once the technology is fully developed.
The lower cost is due to the battery's active materials being inexpensive organic molecules, compared to the commodity metals used in today's flow batteries.
"Moving from transition metal elements to synthesised molecules is a significant advancement because it links battery costs to manufacturing rather than commodity metals pricing," said Imre Gyuk, energy storage programme manager for the US Department of Energy's Office of Electricity Delivery and Energy Reliability (OE).

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

Flow batteries generate power by pumping liquids from external tanks into a central stack. The tanks contain liquid electrolytes that store energy.
Solid batteries, such as the lithium-ion batteries carry much more energy in a smaller space, making them ideal for mobile uses. However issues with performance, safety and lifespan can limit the technology's use for stationary energy storage.

Flow batteries store their active chemicals separately until power is needed, greatly reducing safety concerns.
The 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, 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. As a result, it can also make the renewable energy it stores and the power grid it supports greener," Wang said.
To test the new battery design, Wang and his colleagues created a small, 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 centimetre.
The test battery's optimal performance was between 40 and 50 milliAmperes per square centimetre, where about 70 per cent of the battery's original voltage was retained.
They also found the battery continued to operate well beyond 100 cycles.
The research was published in the journal Advanced Energy Materials.