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

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Cost-effective redox-active materials are critical for the advancement of redox flow batteries (RFBs).
• Iron is an abundant and suitable element for redox couples, but achieving stable and fast reactions in aqueous RFBs is challenging.

Purpose of the Study:

• To develop a stable and high-performance iron-based negolyte for aqueous redox flow batteries.
• To investigate the role of ligand design and intermolecular interactions in enhancing the stability and kinetics of iron redox couples.

Main Methods:

• Synthesis and characterization of a hexadentate ligand-stabilized iron complex.
• Electrochemical evaluation of the Fe complex in near-neutral aqueous electrolytes.
• Operando electrochemical Raman spectroscopy and density functional theory (DFT) calculations to elucidate stabilization mechanisms.

Main Results:

• The sulfonate-substituted Fe complex demonstrated a formal potential of -0.44 V vs. Ag/AgCl and a high rate constant of 0.69 cm s⁻¹.
• Redox flow batteries using 0.5 M Fe complex exhibited excellent cycling stability, with no capacity fading over 300 cycles.
• Intermolecular hydrogen bonds and Fe(II) π-backdonation to the ligand were identified as key stabilization factors.

Conclusions:

• Ligand design incorporating sulfonate groups and promoting intermolecular hydrogen bonding significantly enhances the stability and performance of iron-based negolytes.
• The study highlights the critical role of intermolecular interactions and electronic effects (π-backdonation) in stabilizing redox-active species for aqueous RFBs.
• These findings offer a pathway for developing cost-effective, high-performance redox-active materials for next-generation aqueous redox flow batteries.