A new technology has emerged to boost the performance of iron-chromium redox flow batteries, a next-generation large-scale energy storage system (ESS) that is both explosion-free and inexpensive. Experts say this development lays the groundwork for providing safe and affordable backup power to facilities consuming massive amounts of electricity, such as AI data centers.
A research team led by Professor Lee Hyun-wook of the Department of Energy and Chemical Engineering at the Ulsan National Institute of Science and Technology (UNIST) announced on the 2nd that they improved battery energy efficiency by coating electrode surfaces with bismuth (Bi), increasing chromium's reaction rate more than tenfold while simultaneously suppressing parasitic side reactions.
Iron-chromium redox flow batteries are next-generation batteries that charge and discharge by storing aqueous solutions containing iron and chromium—the electricity storage materials—in separate tanks and flowing them to electrodes when needed. Using water instead of volatile electrolytes reduces explosion risk, and since iron and chromium are inexpensive and abundant, these batteries have better cost competitiveness than other metal-based redox flow batteries.
The problems have been chromium's low reactivity and side reactions. Due to chromium's low reactivity, higher voltages were required to charge the battery, and hydrogen-producing side reactions consumed electrons that should have been stored during charging. This caused the ratio of actually usable energy to stored energy to decrease with repeated charge-discharge cycles.
The research team solved both problems simultaneously by coating electrodes with bismuth, improving energy efficiency. This was achieved because bismuth acts as a "selective reaction regulator," accelerating chromium's oxidation-reduction reactions while suppressing hydrogen generation reactions.
In actual experiments, batteries with bismuth coating maintained an average energy efficiency of 75.22% even after more than 500 charge-discharge cycles. Conventional iron-chromium redox flow batteries often see energy efficiency drop to the 40% range before completing several hundred charge-discharge cycles. The reaction rate constant for chromium also increased approximately tenfold compared to conventional electrodes. Furthermore, hydrogen generation side reactions were significantly reduced, achieving a coulombic efficiency—the ratio of electrons used for actual battery reactions to electrons input during charging—of 99.29%.
The research team explained that they first selected bismuth, indium (In), and tin (Sn) as candidate metal groups capable of serving as selective reaction regulators, then coated actual electrodes with these metals for comparative analysis. Among them, bismuth showed the most balanced performance in terms of reaction rate improvement and side reaction suppression.
"We have demonstrated that the low reactivity and side reaction problems that had blocked commercialization of iron-chromium flow batteries can be solved through a simple electrode coating process," Professor Lee said. "This will contribute to building low-cost, high-efficiency large-scale ESS capable of meeting rapidly increasing power demand from data centers and other facilities."
The research was conducted with support from the Ministry of Science and ICT's National Research Foundation of Korea Nano and Future Materials Core Technology Development Program, Individual Research Program, and the National Research Council of Science and Technology's Global TOP Strategic Research Group Support Program.
The research results were published online on January 7 in the international academic journal "Journal of Materials Chemistry A."
