An ultramicroelectrode study of ferrocene diffusion and heterogeneous electron transfer rate in polyelectrolyte

An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces

The fundamental process in non-aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox-active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot-analysis method relies on the measurement of heterogeneous electron transfer rate constants (k_f or k_b ) as a function of applied potential (E-E⁰ '). As demonstrated by COMSOL simulations, this method enables the quantification of Butler-Volmer kinetic parameters, the standard heterogeneous rate constant, k⁰ , and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene-based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi-layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition-grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non-idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox-flow battery research.

Ultramicroelectrode Voltammetric Investigation of the Micellar Phase of Sodium Dodecyl Sulfate in Aqueous Solution

Ultramicroelectrode voltammetric measurements have been made on Sodium Dodecyl Sulfate (SDS) in aqueous solutions using K4Fe(CN)6 as the electroactive probe and H2SO4 as the supporting electrolyte. By extrapolation, the SDS micellar diffusion coefficient (D m), hydrodynamic radius (R m), aggregation number (n), and the molecular weight of SDS micellar in aqueous solutions at various surfactant concentrations are obtained. The partition coefficients, KD , of the electroactive probe between micelle and water continuous phase are also estimated. The results suggest that the probe is sensitive to the surfactant concentration and reflect the changes of the structure and the hydrophobic nature of micelles.