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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 is 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, a suite of battery tests and spectroscopic studies are presented 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 is found that the balanced K₄[Fe(CN)₆]/K₃[Fe(CN)₆] half‐cell experiences a fast capacity decay under dark conditions. The studies reveal that 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, decomposes into CO₃^2‐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.

Aqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid‐scale renewable energy storage. However, physicochemical properties of redox‐active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N,N′‐dibutyrate‐4,4′‐bipyridinium,(CBu)₂V, as a highly stable, high capacity anolyte material under near pH neutral conditions.(CBu)₂Vcan achieve solubility of 2.1 M and display a reversible, kinetically fast reduction at −0.43 V vs NHE at pH 9. DFT studies revealed that the high solubility of(CBu)₂Vis attributed to its high molecular polarity while its negative reduction potential is benefitted from electron‐donating carboxylate groups. A 0.89 V (CBu)₂V/(NH)₄Fe(CN)₆AORFB demonstrated exceptional energy storage performance, specifically, 100 % capacity retention with a discharge energy density of 9.5 Wh L⁻¹for 1000 cycles, power densities of up to 85 mW cm⁻², and an energy efficiency of 70 % at 60 mA cm⁻².(CBu)₂Vnot only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days) and the most stable flow battery performance to date.

Aqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid‐scale renewable energy storage. However, physicochemical properties of redox‐active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N,N′‐dibutyrate‐4,4′‐bipyridinium,(CBu)₂V, as a highly stable, high capacity anolyte material under near pH neutral conditions.(CBu)₂Vcan achieve solubility of 2.1 M and display a reversible, kinetically fast reduction at −0.43 V vs NHE at pH 9. DFT studies revealed that the high solubility of(CBu)₂Vis attributed to its high molecular polarity while its negative reduction potential is benefitted from electron‐donating carboxylate groups. A 0.89 V (CBu)₂V/(NH)₄Fe(CN)₆AORFB demonstrated exceptional energy storage performance, specifically, 100 % capacity retention with a discharge energy density of 9.5 Wh L⁻¹for 1000 cycles, power densities of up to 85 mW cm⁻², and an energy efficiency of 70 % at 60 mA cm⁻².(CBu)₂Vnot only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days) and the most stable flow battery performance to date.