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Abstract Aqueous organic redox flow batteries (AORFBs) have been recognized as a promising technology for large‐scale, long‐duration energy storage of renewables (e.g., solar and wind) by overcoming their intermittence and fluctuation. However, simultaneous demonstration of high energy densities and stable cycling are still challenging for AORFBs. Herein, asymmetrically substituted sulfonate viologen molecular designs, e.g. (1‐[3‐sulfonatopropyl]‐1′‐[4‐sulfonatobutane]‐4,4′‐bipyridinium (3,4‐S2V), as capacity dense, chemically stable anolytes for cation exchange AORFBs are presented. The robust cycling performance of 3,4‐S2V is confirmed using half‐cell and full‐cell flow battery studies at pH neutral conditions. The 3,4‐S2V based AORFB is demonstrated with a discharge capacity of 23.2 Ah L−1for 1700 cycles or 100 days without observing chemical degradation. Furthermore, a 3,4‐S2V/(NH4)4[Fe(CN)6] AORFB with a discharge capacity of 259.9 mAh is demonstrated for 50 days of authentic energy storage for the first time with a total capacity retention of 97.77% or a temporal capacity retention rate of 99.955% per day, representing the most stable, longest cycled AORFB to date.
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Cycling Performance and Mechanistic Insights of Ferricyanide Electrolytes in Alkaline Redox Flow Batteries
Abstract 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 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.
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- Award ID(s):
- 1847674
- PAR ID:
- 10401829
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Energy Materials
- Volume:
- 13
- Issue:
- 15
- ISSN:
- 1614-6832
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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