Functioning Water-Insoluble Ferrocenes for Aqueous Organic Flow Battery via Host-Guest Inclusion

Area of Science:

• Electrochemistry
• Materials Science
• Renewable Energy Storage

Background:

• Ferrocene (Fc) derivatives are limited catholyte options for aqueous organic flow batteries (AOFBs) due to poor water solubility.
• Existing methods for enhancing Fc solubility are inefficient, time-consuming, and yield low amounts of product.
• Aqueous organic flow batteries are crucial for addressing the intermittency of renewable energy sources.

Purpose of the Study:

• To develop an efficient method for creating water-soluble ferrocene-based catholytes.
• To investigate the stability and electrochemical properties of ferrocene-beta-cyclodextrin inclusion complexes.
• To identify and mitigate capacity loss mechanisms in neutral pH AOFBs utilizing these complexes.

Main Methods:

• A host-guest inclusion strategy involving mixing ferrocene derivatives with beta-cyclodextrins (β-CDs) in water.
• Electrochemical characterization of the resulting inclusion complexes in a neutral pH AOFB.
• Analysis of capacity fade mechanisms through identification of chemical degradation pathways.

Main Results:

• A facile method was established to produce water-soluble ferrocene catholytes by forming inclusion complexes with β-CDs.
• Key factors influencing the stability and electrochemical performance of these complexes were identified.
• The primary cause of capacity loss was determined to be nucleophilic attack on the oxidized ferrocene center.
• A low capacity fade rate of 0.0073% h⁻¹ was achieved by limiting the state of charge.

Conclusions:

• Host-guest inclusion with β-CDs offers a practical route to water-soluble ferrocene catholytes for AOFBs.
• Understanding degradation mechanisms is key to improving the long-term stability of ferrocene-based AOFBs.
• This strategy holds potential for broader application to other water-insoluble organic electroactive compounds for AOFBs.