Quantifying and Decoupling Molecular Interactions of Ionic Liquids with Gold Electrodes

Area of Science:

• Surface Science
• Materials Chemistry
• Electrochemistry

Background:

• Ionic liquids (ILs) are promising electrolytes for energy storage but their surface interactions require precise quantification.
• Understanding molecular interactions at electrode-electrolyte interfaces is crucial for designing efficient energy storage devices.
• Fluorine-free phosphonium-based ILs offer potential advantages, necessitating characterization of their interfacial behavior.

Purpose of the Study:

• To accurately quantify the molecular interactions between fluorine-free phosphonium-based ionic liquids (ILs) and gold electrode surfaces.
• To establish a methodology combining atomic force microscopy (AFM) and quartz crystal microbalance (QCM) for interfacial interaction analysis.
• To determine the relationship between IL composition and the strength of molecular interactions with gold.

Main Methods:

• Utilized gold colloid probe atomic force microscopy (AFM) to measure force-distance curves and determine IL interactions per unit area (N/m²).
• Employed quartz crystal microbalance (QCM) to quantify the adsorbed amount of IL molecules per unit area (Num/m²).
• Combined AFM and QCM data to estimate the quantified molecular interaction force (F₀, nN/Num) between ILs and the gold electrode.

Main Results:

• Quantified molecular interaction forces (F₀) were found to be dependent on IL composition, with longer phosphonium cations leading to stronger interactions.
• Experimental results align with predictions from the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.
• Van der Waals forces were identified as the dominant contribution to the overall IL-gold interaction.

Conclusions:

• The study successfully quantified IL-gold molecular interactions, providing experimental data for molecular simulations.
• Findings enable the virtual design of novel ionic liquids with tailored properties for enhanced energy storage applications.
• The developed methodology offers a pathway for precise interfacial characterization of ILs in electrochemical systems.