As part of an effort to overcome the challenge of long-term energy storage, engineers at the University of Wisconsin-Madison have invented a water-soluble chemical additive that improves the performance of a type of electrochemical storage called a bromide water flow battery.

“Bromine-based water flow batteries are a promising solution, but there are a lot of messy electrochemical problems with them. That’s why there are no real successful bromide-based products today,” said Patrick Sullivan, who graduated from UW-Madison with a PhD in chemistry in 2023. “Yet our single additive can solve so many different problems.”

Sullivan, graduate student Gyohun Choi, and Dawei Feng, assistant professor of materials science and engineering at UW-Madison, developed the additive. The research was published on October 23, 2024 by the journal Nature.

Currently, giant tractor-trailer-sized lithium-ion batteries store energy for the grid – but with technical limitations. Lithium batteries have safety concerns due to the risk of fires and explosions and a complicated international supply chain.

However, water flow batteries can make grid-scale storage safer and cheaper. In these batteries, positive and negative liquid electrolytes circulate over electrodes separated by a membrane. Because the batteries use ions dissolved in a liquid – water – they can be scalable, durable and safe.

The most commercially mature flow batteries are based on vanadium ions, which, like lithium, are expensive and difficult to procure. But another version of these flow batteries relies on bromide, a cheap, widely available ion that performs similar to vanadium—at least on paper.

In practice, however, small bromide ions cause all sorts of problems in flow batteries. They can pass through the membrane that separates the electrodes, reducing the battery’s efficiency. Sometimes the ions fall out of the electrolyte and form a cloudy oil that “sinks” to the bottom of the solution. Sometimes the ions also form toxic bromine gas. These problems hinder practical performance and reliability.

An additive called a complexing agent can help. Choi set out to find an additive that improves battery performance for aqueous bromide. The researchers used molecular design to construct over 500 candidate organic molecules that they call “soft-hard zwitterionic traps.” They synthesized and tested 13 of these representative molecules as potential additives for the bromide batteries.

The resulting multifunctional additives solve the flow battery’s main problem. It encapsulates the bromide ions while allowing them to remain water-soluble, and since the resulting complex is now larger, they cannot pass through the membrane. The ions are also “phase stable”, meaning they do not separate out of the water electrolyte or create toxic bromine gas.

Importantly, the additives dramatically improve flow battery performance, increasing the efficiency and longevity of the chemical system. “Our devices with the additive worked without failure for nearly two months compared to those without it, which typically fail within a day,” says Feng. “This is important because for green energy storage you want to use it for 10 or 20 years.”

The team plans to continue refining the work. Choi will study the basic science behind additives for bromide and iodide flow batteries, while Sullivan, who is CEO of Flux XII – a renewable energy spinoff company he co-founded with Feng – will explore the commercial viability of the additive, which has already been successfully produced in industrial reactions on a ton scale.

Dawei Feng is the Y. Austin Chang Assistant Professor of Materials Science and Engineering. Other UW-Madison authors include Xiu-Liang Lv, Wenjie Li, Kwanpyung Lee, Haoyu Kong, Sam Gessler, and JR Schmidt.