Cycling Performance and Mechanistic Insights of Ferricyanide Electrolytes in Alkaline Redox Flow Batteries

Abstract Ferrocyanide, such as K ₄ [Fe(CN) ₆ ], is one of the most popular cathode electrolyte (catholyte) materials in redox flow batteries. However, its chemical stability in alkaline redox flow batteries is debated. Mechanistic understandings at the molecular level are necessary to elucidate the cycling stability of K ₄ [Fe(CN) ₆ ] and its oxidized state (K ₃ [Fe(CN) ₆ ]) based electrolytes and guide their proper use in flow batteries for energy storage. Herein, a suite of battery tests and spectroscopic studies are presented to understand the chemical stability of K ₄ [Fe(CN) ₆ ] and its charged state, K ₃ [Fe(CN) ₆ ], at a variety of conditions. In a strong alkaline solution (pH 14), it is found that the balanced K ₄ [Fe(CN) ₆ ]/K ₃ [Fe(CN) ₆ ] half‐cell experiences a fast capacity decay under dark conditions. The studies reveal that the chemical reduction of K ₃ [Fe(CN) ₆ ] by a graphite electrode leads to the charge imbalance in the half‐cell cycling and is the major cause of the observed capacity decay. In addition, at pH 14, K ₃ [Fe(CN) ₆ ] undergoes a slow CN ^‐ /OH ^‐ exchange reaction. The dissociated CN ^‐ ligand can chemically reduce K ₃ [Fe(CN) ₆ ] to K ₄ [Fe(CN) ₆ ] and it is converted to cyanate (OCN ^‐ ) and further, decomposes into CO ₃ ^2‐ and NH ₃ . Ultimately, the irreversible chemical conversion of CN ^‐ to OCN ^‐ leads to the irreversible decomposition of K ₄ /K ₃ [Fe(CN) ₆ ] at pH 14.