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

摘要

Ferrocyanide, such as K_4[Fe(CN)_6], is one of the most popular cathodeelectrolyte (catholyte) materials in redox flow batteries. However, itschemical stability in alkaline redox flow batteries is debated. Mechanisticunderstandings at the molecular level are necessary to elucidate thecycling stability of K_4[Fe(CN)_6] and its oxidized state (K_3[Fe(CN)_6]) basedelectrolytes and guide their proper use in flow batteries for energy storage.Herein, a suite of battery tests and spectroscopic studies are presentedto 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 (pH14), it is found that the balanced K4[Fe(CN)6]/K3[Fe(CN)6] half-cell experiencesa fast capacity decay under dark conditions. The studies reveal thatthe chemical reduction of K_3[Fe(CN)_6] by a graphite electrode leads to thecharge imbalance in the half-cell cycling and is the major cause of theobserved capacity decay. In addition, at pH 14, K_3[Fe(CN)_6] undergoes aslow CN-/OH- exchange reaction. The dissociated CN- ligand can chemicallyreduce 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, theirreversible chemical conversion of CN- to OCN- leads to the irreversibledecomposition of K4/K_3[Fe(CN)_6] at pH 14.