Elucidating Ligand Exchange Dynamics of Hexacyanochromate-Based Redox Mediators in Aqueous Iron-Chromium Redox Flow Batteries

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.