Kinetic and Diffusion Control with Rotating Electrodes (RDE and RRDE)

This article provides a structured, didactic introduction to hydrodynamic electrochemistry using rotating disk (RDE) and ring-disk (RRDE) electrodes. The Fe²⁺/Fe³⁺ redox couple is used as a model system to illustrate the experimental separation of kinetic- and diffusion-controlled current contributions. In diffusion- and kinetic-controlled reactions, the reaction velocity step is diffusion- or kinetic-controlled, respectively. Two additional systems supplement this one: the copper system and the organic hydroquinone/quinone (HQ/Q) system. At first, Cu²⁺ is reduced to Cu⁺, followed by subsequent reduction to Cu. This is an electrochemical three-component system. The RDE results of the HQ/Q system are compared by fitting the cyclic voltammogram. Then, the calculated diffusion coefficient and velocity constant are compared to the RDE measurement results. The quantitative evaluation using Levich analysis is illustrated step by step. We also introduce the RRDE technique as a mechanistic tool that enables direct correlation between charge transfer and product formation. The Koutecky–Levich formalism provides a more rigorous treatment of mixed kinetic and mass-transport control. In this approach, the experimentally measured reciprocal total current is expressed as the sum of the kinetic- and diffusion-limited currents according to the serial resistance of the electrode and solution. From a didactic perspective, Koutecky–Levich analysis is powerful because it enables students to visualize the transition from kinetic to diffusion control using experimental data. All calculations can be found in the Supporting Information. This manuscript is intended for advanced undergraduate and graduate education in electrochemistry.