A new organic aqueous flow battery technology promises to drastically lower the cost and sustainability of running energy storage systems. The technology, which was developed at the Pacific Northwest National Laboratory, uses low-cost and sustainable synthesized molecules rather than the usual commodity metals, and could be retrofitted to existing batteries.
Flow batteries differ in both form and function to the more common lithium-ion batteries. They store their active chemicals – liquid electrolytes – in two external tanks. To generate power, these liquids are pumped into a central stack with two electrodes that are separated by a membrane. They exchange ions across this membrane, and this generates electricity. To store electricity, the process is reversed.
Flow batteries are safer and more scalable than lithium-ion batteries, and they can withstand greater temperatures or periods of idleness, which makes them well suited to storing and releasing the energy produced by renewable energy such as solar panels and wind turbines – particularly in the home.
The caveat is that the standard electrolyte materials are vanadium and bromide, which are expensive, dangerous, and toxic commodity metals. Nearly 79 percent of existing flow batteries adhere to this standard.
But the PNNL researchers believe their new organic electrolytes (methyl viologen as anolyte and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl as catholyte, plus sodium chloride to enable discharging) could become a new standard. They expect a flow battery with the new design to cost US$180 per kilowatt-hour, which is around 60 percent less than a typical vanadium-based flow battery. The new water-based liquid electrolytes have also been designed as a drop-in replacement for current systems, so there'll be no need to replace existing infrastructure.
The researchers tested the tech in a small, 600-milliwatt battery. It was stable for 100 cycles with "nearly" 100 percent efficiency at current densities ranging from 20 to 100 mA per square centimeter, with optimal performance rated at 40-50 MA per square centimeter – at which about 70 percent of the battery's original voltage was retained.
They now plan to make a larger test version that could support the peak load of a typical American home (up to around five kilowatts). The researchers are also trying to increase the cycling so that the battery can retain its storage capacity for a longer time without replacing the electrolytes.
The new design will face competition from another new kid on the block, too. Harvard scientists recently developed a similarly-sustainable flow battery that uses quinones – naturally-occurring chemicals involved in photosynthesis – and ferrocyanide – commonly used as a food additive or fertilizer. The Harvard battery has comparable efficiency and a much longer cycling lifetime than the new PNNL design, but it has a lower energy density.
The new technology is described in more detail in a paper published in the journal Advanced Energy Materials.