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

Ferrocyanide such as K4[Fe(CN)6] is one of the most popular cathode electrolyte materials in redox flow batteries. However, its chemical stability in alkaline redox flow batteries has been debated. Mechanistic understandings at the molecular level are necessary to elucidate the cycling stability of the K4[Fe(CN)6] electrolyte and guide its proper use in flow batteries for energy storage. Herein, we presented a suite of battery tests and spectroscopic studies to understand the chemical stability of K4[Fe(CN)6] and its charged state, K3[Fe(CN)6], at a variety of conditions. It was found that the cycling stability of an unbalanced K4[Fe(CN)6]/K3[Fe(CN)6] half-cell is limited by the stability of K3[Fe(CN)6]. In a strong alkaline solution (pH 14), K3[Fe(CN)6] undergoes a CN-/OH- exchange reaction, ultimately leading to Fe(OH)3 precipitate. In the meanwhile, the dissociated CN- ligand can chemically reduce K3[Fe(CN)6] to K4[Fe(CN)6] and is converted to cyanate (OCN-). The resulting charge imbalance of the K4[Fe(CN)6]/K3[Fe(CN)6] half-cell is primarily responsible for the observed rapid capacity decay at pH 14.