Spin‐State and Clustering Effects in Fe‐Complex Negolytes for Near‐Neutral Aqueous Redox Flow Batteries

Ko, Donghwi; Kwon, Seongyeon; Nedsaengtip, Jantakan; Kim, Yohan; Choi, Yunseop; Kim, Dongwook; Lyu, Xingyi; Dixit, Ruchi; Zhang, Yugang; Li, Tao; Seo, Jongcheol; Baik, Mu‐Hyun; Byon, Hye Ryung

doi:10.1002/anie.202505747

Spin‐State and Clustering Effects in Fe‐Complex Negolytes for Near‐Neutral Aqueous Redox Flow Batteries

Journal Article · Mon Jun 02 00:00:00 EDT 2025 · Angewandte Chemie (International Edition)

DOI:https://doi.org/10.1002/anie.202505747· OSTI ID:2572407

^[1]; Kwon, Seongyeon ^[2]; Nedsaengtip, Jantakan ^[1]; Kim, Yohan ^[1]; Choi, Yunseop ^[3]; Kim, Dongwook ^[2]; Lyu, Xingyi ^[4]; Dixit, Ruchi ^[2]; ^[5]; ^[4]; ^[3]; ^[2]; ^[1]

Korea Advanced Inst. Science and Technology (KAIST), Daejeon (Korea, Republic of)
Korea Advanced Inst. Science and Technology (KAIST), Daejeon (Korea, Republic of); Institute of Basic Science (IBS), Daejeon (Korea, Republic of)
Pohang Univ. of Science and Technology (POSTECH) (Korea, Republic of)
Northern Illinois Univ., DeKalb, IL (United States)
Brookhaven National Laboratory (BNL), Upton, NY (United States). Center for Functional Nanomaterials (CFN)

Cost-effective redox-active materials are essential for advancing redox flow batteries (RFBs). Iron, with its abundance and suitability as a redox couple, is a promising candidate; however, achieving stable and fast redox reactions in aqueous RFBs remains a challenge. Here, this study presents an Fe-based negolyte stabilized by a hexadentate ligand, where Fe–ligand bonds are enhanced through intermolecular interactions. The sulfonate-substituted Fe complex exhibits a formal potential of −0.44 V versus Ag/AgCl and an exceptionally high rate constant of 0.69 cm s⁻¹. Near-neutral RFBs incorporating 0.5 M Fe complex show excellent cycling stability, with no discernible capacity fading over 300 cycles. This performance is attributed to intermolecular hydrogen bonds that reinforce Fe–ligand coordination and promote the formation of stable trimeric clusters. Operando electrochemical Raman spectroscopy and density functional theory reveal that π-backdonation from Fe(II) to the imino-phenolate moiety further stabilizes the complex after reduction. In contrast, the hydroxyl-substituted complex exhibits inferior stability due to weaker hydrogen bonding and less pronounced π-backdonation. These findings underscore the importance of ligand design and intermolecular interactions in developing cost-effective, high-performance redox-active materials for aqueous RFBs.

View Accepted Manuscript (DOE)

Research Organization:: Brookhaven National Laboratory (BNL), Upton, NY (United States)

Sponsoring Organization:: National Research Foundation of Korea (NRF); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)

Grant/Contract Number:: SC0012704

OSTI ID:: 2572407

Report Number(s):: BNL--228403-2025-JAAM

Journal Information:: Angewandte Chemie (International Edition), Journal Name: Angewandte Chemie (International Edition) Journal Issue: 32 Vol. 64; ISSN 1521-3773; ISSN 1433-7851

Publisher:: WileyCopyright Statement

Country of Publication:: United States

Language:: English

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