Hydrogen-bond-networks control redox-coupled reactivity in ionic liquid electrolytes

Area of Science:

• Electrochemistry
• Materials Science
• Chemical Engineering

Background:

• Electrochemical process selectivity relies on controlling electrogenerated intermediates.
• Electrolyte properties are often optimized for bulk transport, neglecting molecular-level interactions.
• Ionic liquids (ILs) offer tunable properties but their structural impact on electrochemistry is underexplored.

Purpose of the Study:

• Investigate how hydrogen-bond network organization in a tunable ionic liquid electrolyte platform influences electrochemical behavior.
• Determine the role of electrolyte structure versus bulk properties in controlling the fate of electrogenerated intermediates.
• Establish electrolyte structure as a design handle for selective electrochemical processes.

Main Methods:

• Utilized a choline-geranate-geranic acid ionic liquid (IL) system with varying geranic acid content.
• Characterized hydrogen-bond network changes using infrared spectroscopy and variable-temperature 1H NMR.
• Assessed electrochemical behavior using cyclic voltammetry and rotating-disk electrode with ferrocene/ferrocenium probe.
• Investigated reaction kinetics and intermediate fate via time-resolved UV-Vis spectroelectrochemistry.

Main Results:

• Increasing geranic acid content reorganized the IL's hydrogen-bond network while maintaining ionic character.
• Electrochemical reversibility of the ferrocene/ferrocenium couple improved with higher geranic acid content, despite increased viscosity.
• Irreversibility at low geranic acid content was linked to kinetically coupled chemical reactions, not transport limitations.
• Ferrocenium intermediate stability was dependent on the local solvation environment, being consumed in carboxylate-rich and persistent in hydrogen-bond-rich media.

Conclusions:

• Electrolyte structure and local solvation environment are critical for controlling electrogenerated intermediate reactivity and electrochemical behavior.
• Electrolyte design based on molecular-level structure, particularly hydrogen-bonding, offers a powerful strategy for tuning electrochemical selectivity.
• This structure-centric approach advances the design of ionic liquids, deep eutectic solvents, and hybrid electrolytes for targeted applications.