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Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery


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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Fig. 1: Rational design strategy of an asymmetric iron complex.
Fig. 2: Characterization of metal complexes.
Fig. 3: Electrochemical characterization of metal complexes.
Fig. 4: AORFB testing results with Na4[FeII(Dcbpy)2(CN)2] as catholyte.
Fig. 5: Characterization of electrolytes after cycling.
Fig. 6: Flow battery testing results of high-concentration cells.

Data availability

All the relevant data are included in the paper and its Supplementary Information. Source data are provided with this paper.


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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.

Author information




X.L, P.G. and Y.-Y.L. contributed equally to the work. X.L. and Y.Z. conceived the idea. W.W. and Y.Z. directed the project. X.L. designed the experiments, performed the synthesis, material characterizations, 1H NMR and 13C NMR spectroscopy, electrochemical measurements and capacity-balanced flow battery tests. P.G. performed the DFT calculations, AIMD simulations and classical MD simulations. Y.-Y.L. fabricated and tested all the catholyte capacity-limiting cells and assisted with materials characterization. J.D.B. performed the 23Na NMR and 17O NMR spectroscopy. A.H. and R.F. verified the solubility and prepared the NMR samples, Y-T.L. and H.-Y.L. assisted with the flow battery fabrication and tests. V.M. oversaw the NMR work. C.-F.C. and R.F. assisted with the electrochemical tests. W.-Y.T., J.W. and S.Z. helped with the materials synthesis and characterization. C.-L.W. helped with the materials design. All the authors discussed and analysed the data. X.L., W.W. and Y.Z. wrote and revised the manuscript.

Corresponding authors

Correspondence to Wei Wang or Yu Zhu.

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Competing interests

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.

Additional information

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.

Supplementary information

Supplementary Information

Supplementary Methods, Figs. 1–29 and Tables 1 and 2.

Source data

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).

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