Research opens a ‘new universe’ of organic molecules that can store energy in flow batteries
Harvard researchers have identified a whole new class of high-performing organic molecules, inspired by vitamin B2, that can safely store electricity from intermittent energy sources like solar and wind power in large batteries.
The development builds on previous work in which the team developed a high-capacity flow battery that stored energy in organic molecules called quinones and a food additive called ferrocyanide. That advance was a game-changer, delivering the first high-performance, non-flammable, non-toxic, non-corrosive, and low-cost chemicals that could enable large-scale, inexpensive electricity storage.
While the versatile quinones show great promise for flow batteries, Harvard researchers continued to explore other organic molecules in pursuit of even better performance. But finding that same versatility in other organic systems has been challenging.
“Now, after considering about a million different quinones, we have developed a new class of battery electrolyte material that expands the possibilities of what we can do,” said Kaixiang Lin, a Ph.D. student at Harvard and first author of the paper. “Its simple synthesis means it should be manufacturable on a large scale at a very low cost, which is an important goal of this project.”
Pump-free design for flow battery could offer advantages in cost and simplicity.
A new approach to the design of a liquid battery, using a passive, gravity-fed arrangement similar to an old-fashioned hourglass, could offer great advantages due to the system’s low cost and the simplicity of its design and operation, says a team of MIT researchers who have made a demonstration version of the new battery.
Liquid flow batteries — in which the positive and negative electrodes are each in liquid form and separated by a membrane — are not a new concept, and some members of this research team unveiled an earlier concept three years ago. The basic technology can use a variety of chemical formulations, including the same chemical compounds found in today’s lithium-ion batteries. In this case, key components are not solid slabs that remain in place for the life of the battery, but rather tiny particles that can be carried along in a liquid slurry. Increasing storage capacity simply requires bigger tanks to hold the slurry.
But all previous versions of liquid batteries have relied on complex systems of tanks, valves, and pumps, adding to the cost and providing multiple opportunities for possible leaks and failures.
The new version, which substitutes a simple gravity feed for the pump system, eliminates that complexity. The rate of energy production can be adjusted simply by changing the angle of the device, thus speeding up or slowing down the rate of flow. The concept is described in a paper in the journal Energy and Environmental Science, co-authored by Kyocera Professor of Ceramics Yet-Ming Chiang, Pappalardo Professor of Mechanical Engineering Alexander Slocum, School of Engineering Professor of Teaching Innovation Gareth McKinley, and POSCO Professor of Materials Science and Engineering W. Craig Carter, as well as postdoc Xinwei Chen, graduate student Brandon Hopkins, and four others.
Energy storage system owners could see significant savings from a new flow battery technology that is projected to cost 60 percent less than today’s standard flow batteries.
The organic aqueous flow battery, described in a paper published in the journal Advanced Energy Materials, is expected to cost $180 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 synthesized molecules is a significant advancement because it links battery costs to manufacturing rather than commodity metals pricing” said 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,” said PNNL materials scientist Wei Wang, one of the paper’s corresponding 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.
Both flow and solid batteries, such as the lithium-ion batteries that power most electric vehicles and smartphones today, were invented in the 1970s. Lithium-ion batteries can carry much more energy in a smaller space, making them ideal for mobile uses. The technology gained market acceptance quickly, for both mobile uses like cell phones and larger, stationary uses like supporting the power grid.
Lithium-ion batteries now make up about 70 percent of the world’s working, grid-connected batteries, according to data from DOE-OE’s Global Energy Storage Database. However issues with performance, safety and lifespan can limit the technology’s use for stationary energy storage.
Flow batteries, on the other hand, store their active chemicals separately until power is needed, greatly reducing safety concerns. Vanadium-based flow batteries have become more popular in recent years, especially after PNNL developed a new vanadium battery design in 2011 that increased storage capacity by 70 percent. Three different companies have licensed the technology behind PNNL’s vanadium design.
Nearly 79 percent 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.
Tried & tested
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,” Wei 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 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 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.