A Six-Electron Energy Storage Material for Ultra-Stable Aqueous Organic Redox Flow Batteries

Area of Science:

• Electrochemistry
• Materials Science
• Sustainable Energy

Background:

• Aqueous organic redox flow batteries (AORFBs) are promising for large-scale energy storage due to sustainable, earth-abundant organic materials.
• Existing AORFB materials primarily use single- or dual-electron transfer, limiting storage capacity.
• Stable multi-electron (n > 4) redox systems for AORFBs are underexplored.

Purpose of the Study:

• To engineer a novel six-electron (n=6) redox-active organic molecule for AORFBs.
• To investigate the solubility, stability, and electrochemical performance of the new material.
• To demonstrate the potential of multi-electron systems for advanced energy storage.

Main Methods:

• Molecular engineering of a phenazine-based molecule (PPA) with a π-extended core and hydrophilic side chains.
• Molecular dynamics simulations to predict and understand solubility-enhancing mechanisms.
• Electrochemical testing of the PPA-based negative electrolyte paired with ferrocyanide in an AORFB.

Main Results:

• Engineered PPA achieved unprecedented aqueous solubility (1.2 m) and six-electron storage capacity (193.0 Ah L⁻¹).
• The PPA/ferrocyanide cell operated at 1.31 V with >99.8% Coulombic efficiency and 85% six-electron utilization.
• Exceptional cycling stability was observed, with a capacity decay rate of 0.032% per cycle over 80 days.

Conclusions:

• A stable, high-solubility six-electron organic redox material (PPA) was successfully developed for AORFBs.
• This work demonstrates a scalable approach for designing multi-electron energy storage materials.
• The findings pave the way for next-generation AORFBs with significantly enhanced energy density.