As part of an initiative to tackle the challenge of long-term energy storage, engineers from the University of Wisconsin-Madison have developed a water-soluble chemical additive that enhances the efficiency of bromide aqueous flow batteries, a type of electrochemical storage.
With the rapid growth of solar and wind energy, it’s crucial to have safe and affordable batteries for storing this renewable energy, which can be intermittent.
The engineers from UW-Madison have made strides in addressing the long-term storage issue by creating an innovative water-soluble additive that boosts the functionality of bromide aqueous flow batteries.
According to Patrick Sullivan, a recent PhD graduate in chemistry, “Bromide-based aqueous flow batteries are a potentially effective solution, but they face several complex electrochemical challenges. That’s why we see very few successful bromide-based products currently available. However, our additive can address numerous issues at once.”
Alongside PhD student Gyohun Choi and assistant professor Dawei Feng from materials science and engineering, Sullivan was involved in developing the additive, with their research published in the journal Nature on October 23, 2024.
At the moment, large lithium-ion battery packs, often the size of trailer trucks, are being used to store energy for the power grid, but they come with technical limitations. Lithium batteries introduce safety risks, including the possibility of fires and explosions, along with a complicated global supply chain.
In contrast, aqueous flow batteries may offer a safer and more cost-effective alternative for large-scale energy storage. These batteries work by having positive and negative liquid electrolytes flow over electrodes that are separated by a membrane. Utilizing ions dissolved in water allows for scalability, sustainability, and safety.
The most advanced flow batteries currently in use rely on vanadium ions, which, similar to lithium, are pricey and difficult to source. Alternatively, another type of flow battery that uses bromide offers a cheaper and more readily available option, performing similarly to vanadium in theory.
In practice, however, small bromide ions present numerous challenges in flow batteries. They can migrate through the membrane separating the electrodes, thereby diminishing battery efficiency. In some cases, these ions can crystallize out of the electrolyte, forming a messy residue that settles at the bottom. Additionally, they can generate harmful bromine gas. These factors significantly impair practical performance and reliability.
A complexing agent added to the battery system could provide a solution. Choi embarked on a quest to discover an additive that would enhance the performance of bromide aqueous flow batteries. The researchers successfully designed and constructed over 500 candidate organic molecules known as “soft-hard zwitterionic trappers” and examined 13 of them to identify effective additives.
The resulting multi-functional additives effectively address key issues faced by flow batteries. They encapsulate the bromide ions while keeping them water-soluble and, due to their increased size, the ions can no longer pass through the membrane. Furthermore, these “phase-stable” ions do not crystallize from the electrolyte or release toxic bromine gas.
Significantly, these additives greatly enhance battery performance, extending both efficiency and lifespan. “Our devices with the additive remained functional for nearly two months, whereas those without it commonly fail within a day,” notes Feng. “This is critical for green energy storage, which ideally needs to last 10 to 20 years.”
The research team intends to further improve their work. Choi will delve into the fundamental science governing additives for bromide and iodide flow batteries, while Sullivan, who is also the CEO of the renewable energy startup Flux XII that he co-founded with Feng, will investigate the commercial prospects of the additive, which has already been produced at an industrial scale.
Dawei Feng holds the Y. Austin Chang Assistant Professorship in materials science and engineering. Other contributing authors from UW-Madison include Xiu-Liang Lv, Wenjie Li, Kwanpyung Lee, Haoyu Kong, Sam Gessler, and JR Schmidt.
