Surprising Sugar: New Catalysis for Flow Battery

By Battery Power Online Staff

July 14, 2023 | Thanks to a common sugar often used as a food and medicine additive, a research team from the Department of Energy’s Pacific Northwest National Laboratory reports a surprising catalysis step that maintained a flow battery’s capacity to store and release energy for more than a year of continuous charge and discharge.

The study, published in Joule (DOI: 10.1016/j.joule.2023.06.013), details the first use of a dissolved simple sugar called β-cyclodextrin, a derivative of starch, to boost battery longevity and capacity. In a series of experiments, the scientists optimized the ratio of chemicals in the system until it achieved 60% more peak power. Then they cycled the battery over and over for more than a year, only stopping the experiment when the plastic tubing failed. During all that time, the flow battery barely lost any of its activity to recharge. This is the first laboratory-scale flow battery experiment to report more than a year of continuous use with minimal loss of capacity.

“This is a brand new approach to developing flow battery electrolyte,” said Wei Wang, a long-time PNNL battery researcher and the principal investigator of the study in a press release. “We showed that you can use a totally different type of catalyst designed to accelerate the energy conversion. And further, because it is dissolved in the liquid electrolyte it eliminates the possibility of a solid dislodging and fouling the system.”

Accidental Catalysis

The study is the next generation of a PNNL-patented flow battery design first described in the journal Science in 2021. There, the researchers showed that another common chemical, called fluorenone, is an effective flow battery component. But that initial breakthrough needed improvement because the process was slow compared with commercialized flow battery technology.

“We were looking for a simple way to dissolve more fluorenol in our water-based electrolyte,” said Ruozhu Feng, the first author of the study. “The β-cyclodextrin helped do that, modestly, but it’s real benefit was this surprising catalytic ability.”

The researchers then worked with co-author Sharon Hammes-Schiffer of Yale University, a leading authority on the chemical reaction underlying the catalytic boost, to explain how it works. Basically, the sugar additive accepts positively charged protons, which helps balance out the movement of negative electrons as the battery discharges.

Now the team is working to further improve the system by experimenting with other compounds that are similar to β-cyclodextrin but smaller. Like honey, β-cyclodextrin addition also makes the liquid thicker, which is less than ideal for a flowing system. Nonetheless, the researchers found its benefits outweighed its drawbacks.

Flow Battery Advantages

Flow batteries are all-liquid systems consisting of two chambers, each filled with a different liquid. The batteries charge through an electrochemical reaction and store energy in chemical bonds. When connected to an external circuit, they release that energy, which can power electrical devices. Flow batteries differ from solid-state batteries in that they have two external supply tanks of liquid constantly circulating through them to supply the electrolyte. The β-cyclodextrin dissolves in solution, so it didn’t add any solids to the battery.

Flow batteries are also extremely scalable. The larger the electrolyte supply tank, the more energy the flow battery can store. So if they are scaled up to the size of a football field or more, flow batteries can serve as backup generators for the electric grid, but they can be built at any scale, from the lab-bench scale, as in the PNNL study, to the size of a city block.

This scalability makes the flow battery particularly useful for large-scale energy storage when relying on intermittent energy sources, such as wind, solar, or hydroelectric power. When severe weather or high demand hobble the ability to supply electricity to homes and businesses, energy stored in large-scale flow battery facilities can help minimize disruption or restore service.