Time-resolved impact electrochemistry - A new method to determine diffusion coefficients of ions in solution

Abstract

Diffusion is often the rate-limiting factor of reactions in condensed phase. Thus, knowing the diffusion coefficient is key in numerous aspects ranging from drug release to steering of reactions in both homogeneous liquid phase and electrochemical reactions. Cyclic voltammetry at macro electrodes and chronoamperometry at micro electrodes are well-established methods to determine the diffusion coefficients of redox-active species dissolved in a solution. However, if the formal potentials of the redox species are outside of the potential window of the solvent, then these methods cannot be readily applied. Here we demonstrate a new concept to determine the diffusion coefficient of ions to overcome this limitation. We use their reaction with a well-defined amount of a redox-active indicator substance, which is confined in a nanoparticle suspended in a solution containing the species of interest. Employing transformative nanoparticle impact analysis, the diffusion-limited reaction of an indicator nanoparticle with the species of interest is initiated and followed by chronoamperometry. Measuring the time it takes to fully convert the indicator particle enables the determination of the diffusion coefficient of interest. This concept is demonstrated for variety of (pseudo-)halides in aqueous solution using Ag nanoparticles as redox indicator. Using chloride as an example, is further shown that this new methodology can be applied to study effects of temperature and viscosity on the diffusion coefficients. Given the multitude of nanoparticles that may serve as electrochemical redox indicator, this approach can be used to determine the diffusion coefficients for a large variety of species in different liquid environments.