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 (catholyte) 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 K4[Fe(CN)6] and its oxidized state (K3[Fe(CN)6]) based electrolytes and guide their 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. In a strong alkaline solution (pH 14), it was found that the balanced K4[Fe(CN)6]/K3[Fe(CN)6] half-cell experienced a fast capacity decay under dark conditions. Our studies revealed the chemical reduction of K3[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, K3[Fe(CN)6] undergoes a slow CN‒/OH‒ exchange reaction. The dissociated CN‒ ligand can chemically reduce K3[Fe(CN)6] to K4[Fe(CN)6], and it is converted to cyanate (OCN‒) and further, decompose into CO32‒ and NH3. Ultimately, the irreversible chemical conversion of CN‒ to OCN‒ leads to the irreversible decomposition of K4/K3[Fe(CN)6] at pH 14.