Renewable energy is ramping up quickly; in 2022, wind and solar power accounted for 12 percent of global energy output according to climate think tank Ember. But for these intermittent renewable energy sources to reach their full potential, they need be paired with grid-scale batteries that can store the energy to continue delivering power when the sun isn’t shining and the breeze isn’t blowing.
Such batteries are on the horizon. Materials engineers at the University of Wisconsin-Madison are developing an inexpensive, safe and sustainable grid-scale device called an aqueous organic redox flow battery (AORFB). In research published August 3, 2023 in the journal Nature Energy, they demonstrate a platform to design and synthesize molecules for the cathode (positive) side of the battery. That’s a big step forward in making AORFBs commercially viable.
“A battery is only as good as its weakest link. You need both sides of a battery, the anode and the cathode, to be high performance,” says Patrick Sullivan, who earned his PhD in materials chemistry from UW-Madison in May, 2023 and is now CEO of Flux XII, a spinoff company he co-founded to advance commercial-scale flow batteries. “What this paper is about is developing a library of molecules with extra high capacity and stability for the flow battery cathode, which has been especially problematic for AORFB systems.”
Currently, giant lithium-ion batteries—supersized versions of the batteries found in laptop computers and electric cars—store some renewable energy. But that technology poses a fire risk, and lithium is an expensive metal with a complicated supply chain.
Enter flow batteries. In this alternative technology, the positive and negative sides of the battery are liquids—usually metal ions dissolved in water—which eliminates the fire risk, makes the battery easier to scale up and enables new types of uses.
However, most commercially mature flow batteries use vanadium, another scarce and expensive metal. Other types of flow batteries utilize cheaper metals like iron, zinc, bromine, and chromium but sacrifice performance and scalability.
New flow battery chemistries are needed to overcome each of these problems. Instead of metals, organic compounds crafted from earth’s most abundant elements offer more tunability in chemistry along with the potential for sustainable and secure supply chains. Researchers have been looking for organic molecules with ideal designs for nearly a decade but have been unable to find versions that are energy efficient, energy dense and stable while remaining scalable and low-cost.
Another issue is that while other researchers have found some high-performing organics, they often can’t produce them in useful quantities, limiting research on the structures.
Using a newly developed building block assembly synthetic platform developed by Dawei Feng, an assistant professor of materials science and engineering at UW-Madison, the researchers designed over 100 types of organic molecules called ionic oligomers, dubbed i-TEMPODs, that are ideally suited for flow batteries.
Xiu Liang Lyu, a postdoctoral scholar in Feng’s group, used a high-throughput, modular approach to synthesize 50 of these compounds. In the lab, Lyu was able to produce several grams of 21 molecules and even synthesized kilograms of some compounds; compared to previous efforts, that’s like a chef going from preparing a tiny sample for a photo shoot to preparing enough for a whole football team.
Lyu and Sullivan then fully investigated the i-TEMPODs, screening for physical, electrochemical, chemical, and battery properties to figure out the optimal structures. What they found were several molecules that performed much better than previous liquid cathodes, or catholytes, pointing them in new directions.
“We were able to see some general trends,” says Sullivan. “But one molecule is likely not going to be the best at every single thing. Maybe the best performing one is a little bit trickier to make, whereas others may be a little bit easier to make, but they’re lower performing. The point of the paper is looking at ways to optimize all of these parameters at once so that we end up with something practical to make a real-world impact.”
The researchers are now thinking bigger. Late in 2022, they developed a prototype 1-kilowatt aqueous organic redox flow battery, which stores roughly enough energy to power a house. They used that system to validate the performance of some of their newly synthesized molecules, which have been produced at the ton-scale. Using these results and previous research, Flux XII now plans to scale up to a 20- or 50-kilowatt system over the next year, continuing to fine-tune the battery chemistry along the way. “Once we show that our 20-kilowatt plus system works, building it out into a real grid-scale storage system will be just like stacking together Legos,” says Sullivan.
Feng is the Y. Austin Chang Assistant Professor in materials science and engineering
Other UW-Madison authors include Wenjie Lib, Hui-Chun Fub, Ryan Jacobs, Chih-Jung Chen,
Harvey D. Spangler Professor Dane Morgan, and Chemistry Professor Song Jin.
The authors acknowledge support from the University of Wisconsin-Madison, WARF
Accelerator Project via 197500-135-AAJ4936; the Draper Technology Innovation Fund
under 197500-135-AAJ4236; the Vice Chancellor for Research and Graduate Education via 1975XX-135-AAI2755; The King Abdullah University of Science and Technology Office of Sponsored Research under award no. OSR-2017-CRG6-3453.02; and the Ministry of Science and Technology in Taiwan via 109-2917-I-564-010.
Featured image caption: Working with Assistant Professor Dawei Feng, Patrick Sullivan (left) and Xiu Liang Lyu designed and synthesized new organic molecules that could make aqueous organic redox flow batteries a commercially viable way of storing renewable energy for the grid. Credit: Flux XII