Iron-based redox-active materials can have cost and environmental advantages when used in redox flow batteries.1 In the past few years, considerable progress has been made on the development of posolytes composed of iron complexes,2,3,4,5,6,7 but it is challenging to achieve a high redox potential without compromising molecular lifetime for an iron complex.8 In this study, we develop an aqueous organic−metalorganic redox flow battery utilizing a new high-potential posolyte species, tris(2,2'-bipyridine-4,4'-diyldimethanol) iron dichloride, operating at near-neutral pH, paired with bis(3-trimethylammonio)propyl viologen tetrachloride in the negolyte. This near-neutral aqueous flow battery exhibits an open-circuit voltage of 1.3 V with a power density over 120 mW cm-2, and it demonstrates stable cycling performance with a low temporal capacity fade rate of 0.07%/day after 35 days of cycling. The extended cycling lifetime is the result of lower permeability and improved structural stability of the newly developed iron-complex. Post-mortem chemical and electrochemical analyses of the posolyte indicate that stepwise ligand dissociations of the iron complex are responsible for the irreversible capacity loss during cell cycling. The findings provide metalorganic complex design guidance to develop stable metalorganics with high potential in future.
References
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2Gong, K., et al. (2016). "All-Soluble All-Iron Aqueous Redox-Flow Battery." ACS Energy Letters 1(1): 89-93.
3Hawthorne, K. L., et al. (2014). "Studies of Iron-Ligand Complexes for an All-Iron Flow Battery Application." Journal of The Electrochemical Society 161(10): A1662-A1671.
4Beh, E. S., et al. (2017). "A Neutral pH Aqueous Organic–Organometallic Redox Flow Battery with Extremely High Capacity Retention." ACS Energy Letters 2(3): 639-644.
5Zhao, Z., et al. (2020). "Investigations Into Aqueous Redox Flow Batteries Based on Ferrocene Bisulfonate." ACS Applied Energy Materials 3(10): 10270-10277.
6Hu, B., et al. (2017). "Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) toward Sustainable and Safe Energy Storage." J Am Chem Soc 139(3): 1207-1214.
7Li, X., et al. (2021). "Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery." Nature Energy 6(9): 873-881.
8Ruan, W., et al. (2020). "Communication—Tris(bipyridyl)iron Complexes for High-Voltage Aqueous Redox Flow Batteries." Journal of The Electrochemical Society 167(10).
