Separating Convective from Diffusive Mass Transport Mechanisms in Ionic Liquids by Redox Pro-fluorescence Microscopy

Area of Science:

• Electrochemistry
• Analytical Chemistry
• Physical Chemistry

Background:

• Electrochemical reactivity studies require methods to probe reactant and product diffusion at electrified interfaces.
• Current transient and cyclic voltammetry data modeling provide indirect diffusion coefficient information but lack spatial resolution and assume negligible convection.
• Detecting adventitious convection in viscous solvents like ionic liquids is challenging.

Purpose of the Study:

• To develop a direct, spatiotemporally resolved optical tracking method for diffusion fronts.
• To detect and resolve convective disturbances to linear diffusion.
• To accurately measure diffusion coefficients in electrochemical systems, particularly in ionic liquids.

Main Methods:

• Development of a direct optical tracking technique for diffusion fronts.
• Spatiotemporal resolution of diffusion processes.
• Tracking of an electrode-generated fluorophore to monitor diffusion and convection.
• Analysis of current transients and cyclic voltammetry data in conjunction with optical tracking.

Main Results:

• The developed optical tracking method successfully detects and resolves convective disturbances.
• Parasitic gas evolving reactions were shown to cause up to 10-fold overestimates of macroscopic diffusion coefficients.
• A correlation between large inner-sphere redox reaction barriers (e.g., hydrogen gas evolution) and double layer structures in ionic liquids was hypothesized.

Conclusions:

• Direct optical tracking provides a robust method for assessing diffusion and detecting convection in electrochemical systems.
• Accurate diffusion coefficient determination requires accounting for parasitic reactions and convective effects, especially in ionic liquids.
• Gas evolution reactions in imidazolium-based ionic liquids may be linked to specific double layer structures.