The limited availability of a high-performance catholyte has hindered the development of aqueous organic redox flow batteries (AORFB) for large-scale energy storage. Here we report a symmetry-breaking design of iron complexes with 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid and cyanide ligands. By introducing two ligands to the metal centre, the complex compounds (M4[FeII(Dcbpy)2(CN)2], M = Na, K) exhibited up to a 4.2 times higher solubility (1.22 M) than that of M4[FeII(Dcbpy)3] and a 50% increase in potential compared with that of ferrocyanide. The AORFBs with 0.1 M Na4[FeII(Dcbpy)2(CN)2] as the catholyte were demonstrated for 6,000 cycles with a capacity fading rate of 0.00158% per cycle (0.217% per day). Even at a concentration near the solubility limit (1 M Na4[FeII(Dcbpy)2(CN)2]), the flow battery exhibited a capacity fading rate of 0.008% per cycle (0.25% per day) in the first 400 cycles. The AORFB cell with a nearly 1:1 catholyte:anolyte electron ratio achieved a cell voltage of 1.2 V and an energy density of 12.5 Wh l–1.
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Y.Z., X.L., Y.-Y.L., W.-Y.T., C.-F.C. Y.-T.L. and H.-Y.L. acknowledge the support of the University of Akron. P.G, J.D.B, A.H., V.M., R.F. and W.W. acknowledge joint financial support from the US Department of Energy (DOE) Office of Electricity (OE) Energy Storage Program (under Contract no. 57558) and Energy Storage Materials Initiative, which is a Laboratory Directed Research and Development Project at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated by Battelle for the US DOE under contract DE-AC05-76RL01830.
The metal complex materials disclosed in this work have been filed as US Provisional Patent Application USPTO: 63/080,374 with Y.Z. as the applicant, and Y.Z and X.L. as inventors. The status of the patent application is pending.
Peer review information Nature Energy thanks Michael Aziz and Michael Marshak for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Source Data Fig. 2
NMR source data.
Source Data Fig. 3
Source data for CV, redox potential, RDE and Levich plot.
Source Data Fig. 4
Source data for Ratability test, low concentration cell.
Source Data Fig. 5
Source data for NMR and CV.
Source Data Fig. 6
Source data for high concentration cell.
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Li, X., Gao, P., Lai, YY. et al. Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery. Nat Energy 6, 873–881 (2021). https://doi.org/10.1038/s41560-021-00879-6
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