Iron-Chromium Flow Batteries Boost Energy Storage Lifespan

Abstract

Aqueous redox flow batteries (AQRFBs) are revolutionizing energy storage by integrating sustainability with cutting-edge innovation. Among them, Iron-Chromium RFBs (Fe-Cr RFBs), which utilize aqueous-based electrolytes, effectively address critical challenges in renewable energy integration while offering unparalleled safety, low-cost scalability and environmental compatibility. Potassium hexacyanochromate (K3[Cr(CN)6]) has emerged as a promising negolyte material in Fe-Cr RFBs due to its favorable electrochemical properties. However, enhancing its long-term stability and elucidating its structural transformations remain crucial for optimized performance. Investigations into ligand exchange mechanism reveal connections to detrimental side reactions, notably hydrogen evolution reaction (HER) and hexacyanochromate instability, highlighting pathways for targeted improvement. Density functional theory (DFT) calculations illuminate the effects of ligand exchange dynamics and structural variations on redox stability, providing mechanistic insights into electrolyte behavior. By strategically incorporating sodium hydroxide with sodium cyanide as supporting electrolytes, our study demonstrates significantly improved stability of the redox couple, achieving a stable cycling performance over 250 cycles with an energy density of 13.91 Wh L−1 and energy efficiencies exceeding 76%-77%. This research provides valuable insights into the degradation pathways of hexacyanochromate-based negolyte and emphasizes the importance of optimized electrolyte design for advancing sustainable energy storage technologies.

Researchers, affiliated with UNIST have achieved a significant breakthrough in prolonging the lifespan of iron-chromium redox flow batteries (Fe-Cr RFBs), large-capacity and explosion-proof energy storage systems (ESS). This advancement enhances the safety and reliability of storing renewable energy sources, such as wind and solar, which often produce electricity intermittently, enabling secure storage and on-demand retrieval.

Professor Hyun-Wook Lee from the School of Energy Chemical Engineering at UNIST, in collaboration with Professor Dong-Hwa Seo of KAIST and Professor Guihua Yu from the University of Texas at Austin, the team identified the causes of performance degradation in iron-chromium flow batteries. They also developed an optimized electrolyte formulation that maintains capacity through repeated charge and discharge cycles.

Unlike conventional batteries, flow batteries store energy in liquid electrolytes that act as liquid electrodes. The electrolytes are circulated via pumps during charging and discharging. Using water instead of volatile chemicals makes them inherently safer with no explosion risk. Additionally, their capacity can be easily adjusted by controlling the electrolyte volume, making them suitable for large-scale energy storage from variable renewable sources.

The team discovered that the primary cause of capacity decline is a ligand exchange process involving hexacyanochromate ([Cr(CN)6]4-/3-). Although adding hexacyanochromate improves output and charging speed, cycling induces a side reaction where cyanide (CN⁻) ions surrounding chromium ions are replaced by hydroxide (OH⁻) ions. This exchange destabilizes the electrolyte structure, leading to rapid capacity loss.

To address this, the researchers optimized the ratio of cyanide to hydroxide ions within the electrolyte, effectively suppressing the unwanted reaction. The new electrolyte formulation reliably maintained stable capacity and efficiency over more than 250 cycles.

Professor Lee emphasized, "This work demonstrates the potential to develop high-performance, long-lasting flow batteries using cost-effective iron-chromium electrolytes. Such technology is especially promising for countries with abundant renewable resources and large land areas, like China and European nations, seeking scalable energy storage solutions."

While vanadium flow batteries are currently closer to commercial deployment, their high costs and limited regional resource availability present challenges. This breakthrough offers an alternative approach toward more affordable and scalable large-capacity energy storage.

Supported by the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT (MSIT), these findings were published online on July 2 in Angewandte Chemie International Edition, a leading journal in chemistry.

Journal Reference

Ji-Eun Jang, Vithiya Muralidharan, Yoon Seong Kim, et al., "Elucidating Ligand Exchange Dynamics of Hexacyanochromate-Based Redox Mediators in Aqueous Iron-Chromium Redox Flow Batteries," Angew. Chem. Int. Ed., (2025).