The organic aqueous flow battery is expected to cost £120 per kilowatt-hour 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 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," said Wei Wang, materials scientist at 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."
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.
Unlike lithium-ion batteries that have issues with performance, safety and lifespan, limiting the technology's use for stationary energy storage, flow batteries store their active chemicals separately until power is needed, reducing safety concerns.
Nearly 79% of the world's working flow batteries are vanadium-based, according to data from the Global Energy Storage Database. While vanadium chemistries are expected to be the standard for some time, future flow battery cost reductions will require less expensive alternatives such as organics.
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, 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, the researchers repeatedly charged and then discharged a 600mW battery at various electric current densities, ranging from 20 to 100mA/cm2. The test battery's optimal performance was between 40 and 50mA/cm2, where about 70% of the battery's original voltage was retained. They also found the battery continued to operate well beyond 100 cycles.
The team plans to make a larger version of its test battery that is able to store up to 5kW of electricity, which could support the peak load of a typical home. Other ongoing efforts include improving the battery's cycling so it can retain more of its storage capacity longer.
