Designer Ferrocene Catholyte for Aqueous Organic Flow Batteries

Area of Science:

• Electrochemistry
• Materials Science
• Renewable Energy Storage

Background:

• Aqueous organic flow batteries (AOFBs) are promising for renewable energy storage.
• Development is limited by the lack of suitable catholyte organic molecules.
• The relationship between catholyte structure and AOFB lifetime is not well understood.

Purpose of the Study:

• To investigate the impact of ferrocene derivative structure on catholyte stability and performance in AOFBs.
• To identify key molecular design principles for robust and high-capacity catholytes.
• To establish a predictive method for assessing the water solubility of ferrocene-based catholytes.

Main Methods:

• Synthesis and characterization of six ferrocene derivatives with tuned chemical structures.
• Evaluation of redox properties and stability in aqueous electrolytes using electrochemical techniques.
• Assessment of ferrocene derivative performance in full and symmetric AOF cells.
• Computational analysis using density functional theory (DFT) to correlate structure with stability.
• Development and validation of a method to predict water solubility.

Main Results:

• Ferrocene derivative redox potentials correlated inversely with their lowest unoccupied molecular orbital (LUMO) energy.
• Localized LUMO density on the iron center was identified as a cause of rapid capacity fading.
• BQH-Fc, exhibiting minimal LUMO density on iron, demonstrated superior stability.
• The BQH-Fc/BTMAP-Vi cell showed no capacity loss at 0.1 M and high retention (99.993% h⁻¹) at 1.5 M, with degradation attributed to electrolyte crossover.
• The developed method accurately predicted the water solubility of ferrocene molecules.

Conclusions:

• Molecular design, specifically minimizing LUMO density on the iron center, is crucial for enhancing ferrocene catholyte stability in AOFBs.
• BQH-Fc represents a highly stable catholyte candidate for AOFB applications.
• The established solubility prediction method can accelerate the discovery of novel, high-performance catholytes for AOFBs.
• Further research into mitigating electrolyte crossover is needed for long-term AOFB stability.