Aqueous organic redox flow batteries (AORFBs) hold promise for safe, sustainable and cost-effective grid energy storage. However, developing catholyte redox molecules with the desired stability, power and energy density remains challenging. In this study, we synthesized a class of ionic liquid-mimicking (2,2,6,6-tetramethylpiperidin-1-yl)oxyl dimers (i-TEMPODs) through a building-block assembly platform. By systematically investigating 21 i-TEMPOD derivatives, we uncovered the optimal size and charge properties that prevent membrane crossover and allow formation of a water-in-salt state. Leveraging these advances, we realized substantial improvements in AORFB performance using the optimum i-TEMPOD catholyte at 2 M concentration. These enhancements encompass several crucial metrics showcased across multiple experiments, including robust cycling stability without apparent capacity decay during 96 days of cycling, facile electrochemical kinetics with a high maximum power density of 0.325 W m−2 and a high full-cell energy density of 47.3 Wh l−1 in a capacity-balanced configuration. These molecular designs pave the way towards low-cost and scalable AORFBs.
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D.F. acknowledges start-up funds from the University of Wisconsin–Madison, the WARF Accelerator Project (AP) and the Draper Technology Innovation Fund (TIF). P.T.S. thanks the Vice Chancellor for Research and Graduate Education (VCRGE) for support. D.F., S.J., W.L., C.-J.C. and H.-C.F. thank UW–Madison WARF Accelerator for an electrification challenge grant. C.-J.C. thanks the National Science and Technology Council (NSTC) in Taiwan (109-2917-I-564-010) for a postdoctoral fellowship. R.J. and D.M. were supported by the Department of Energy (DOE; award no. DE-SC0020419). The authors thank I. Guzei for assistance with the collection of single-crystal X-ray diffraction data. The Bruker Quazar APEX2 was purchased by the UW–Madison Department of Chemistry with a portion of a generous gift from P. J. Bender and M. M. Bender. The Bruker AVANCE 400 NMR spectrometer was supported by the NSF (grant no. CHE-1048642). The purchase of the Thermo Q Exactive Plus mass spectrometer in 2015 was funded by the NIH (award no. 1S10 OD020022-1) to the Department of Chemistry. We also thank J. Berry and his student, J. Harris, in the UW–Madison Department of Chemistry for equipment and support in FTIR measurements.
Wisconsin Alumni Research Foundation (WARF) has filed a US utility patent application (P210314US01) regarding the catholyte materials described in this paper with D.F., X.-L.L., P.T.S. and W.L. as inventors. D.F., X.-L.L., and P.T.S. have ownership stakes in Flux XII, which may profit from the results reported herein. The remaining authors declare no competing interests.
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Lv, XL., Sullivan, P.T., Li, W. et al. Modular dimerization of organic radicals for stable and dense flow battery catholyte. Nat Energy 8, 1109–1118 (2023). https://doi.org/10.1038/s41560-023-01320-w
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