Zwitterionic Ferrocenes: An Approach for Redox Flow Battery (RFB) Catholytes

Area of Science:

• Electrochemistry
• Materials Science
• Organic Chemistry

Background:

• Redox flow batteries (RFBs) are promising for large-scale energy storage.
• Development of stable and highly soluble redox-active materials is crucial for advancing RFB technology.
• Ferrocene derivatives offer tunable electrochemical properties for potential RFB applications.

Purpose of the Study:

• To synthesize and characterize novel ferrocene compounds with zwitterionic side chains for RFB applications.
• To evaluate the electrochemical performance and stability of these new compounds in neutral aqueous electrolytes.
• To investigate the impact of side chain length and conformation on battery performance.

Main Methods:

• Synthesis of ferrocene compounds Fc3 (propyl zwitterionic chain) and Fc4 (butyl zwitterionic chain).
• Electrochemical evaluation in high-concentration neutral aqueous redox flow batteries using anthraquinone-2,7-disulfonate as the anolyte.
• Stability testing over 20 cycles and capacity retention analysis.
• Density functional theory (DFT) calculations to study conformational properties.

Main Results:

• Fc3 and Fc4 exhibit high water solubility (0.66 M and 2.01 M, respectively).
• Both compounds achieved excellent performance: ~88% voltage and energy efficiency, >99% Coulombic efficiency.
• Fc4 demonstrated higher stability than Fc3, with capacity decreasing by only ~5% over 20 cycles.
• DFT calculations revealed Fc4's linear side chain conformation contributes to its enhanced stability.

Conclusions:

• Zwitterionic ferrocenes Fc3 and Fc4 are highly soluble and effective redox-active materials for neutral aqueous RFBs.
• The butyl zwitterionic chain (Fc4) provides superior electrochemical stability compared to the propyl chain (Fc3).
• Side chain conformation, influenced by length, plays a critical role in the stability of ferrocene-based RFBs.