Thin-Layer Electrochemistry: Visualizing the Diffusion Layer During Chronoamperometry and Cyclic Voltammetry

The phenomena that occur in the diffusion layer play a central role in electrochemical processes but are often challenging for students to understand. Here, we present a demonstration and activity to directly observe and analyze the changes in diffusion layer during chronoamperometry and cyclic voltammetry. Methyl viologen (MV²⁺), a redox indicator with distinct color changes upon one-electron reduction, was used to generate an observable color change. The reaction was carried out in a homemade thin-layer cell, and the color changes were monitored and recorded using an optical microscope equipped with a digital camera. In chronoamperometry, students examined the time dependence of current decay and diffusion-layer growth, then calculated the diffusion coefficient of the methyl viologen radical cation (MV^+ •). In cyclic voltammetry, students visually tracked the appearance and disappearance of the colored diffusion layer during forward and reverse potential scans. Linking these visual observations to the electrochemical responses enhanced students' ability to comprehend the underlying processes. This activity offers a versatile and visually engaging tool for teaching electrochemistry in both introductory and advanced courses.