Optimization of electrochemical time of flight measurements for precise determinations of diffusion coefficients over a wide range in various media

Abstract

Diffusion coefficients are an important physical property to both physical and analytical electrochemistry and the role they play in sensors, batteries, and catalysis. In electrochemical time of flight (ETOF), the time needed for an electrochemically generated species to travel to an adjacent electrode is measured. Previously relegated to verifying diffusion modeling and theoretical work, ETOF is a powerful and elegant technique whose broad applicability has been overlooked. In the determination of diffusion coefficients, ETOF does not require setting delicate hydrodynamic conditions required by rotating ring disk (RDE) methods. In addition, no foreknowledge about the complete electrochemical mechanistic system, nor about its electron stoichiometry is required. ETOF is ideal for determining the diffusion coefficients for non-elucidated systems, such as for short-lived intermediates assuming they survive ETOF travel time. Here we report the construction of a universal empirical calibration curve for the determination of diffusion coefficients using experimental parameters optimized by computer modeling. Using a platinum micro electrode array, diffusion coefficients were determined for a variety of species including ferrocenium, ruthenium (III) bisbipyridine dichloride, and vanadium (III) acetylacetonate, some of which being previously unreported. The elegance of the method is that the calibration curve constructed with a few well-established species in a given electrolyte can be used for any species in any electrolyte.