The Crucial Role of Rotation Speed on the Determination of Tafel Slopes of Electrocatalysts in Rotating Disk Electrode Experiments

This article highlights the crucial influence of rotation speeds in determining the Tafel slopes of electrocatalysts in rotating disk electrode (RDE) experiments. As an exemplary system, we investigate catalytically active nickel oxide/oxyhydroxide films that were generated from the transformation of electrodeposited nickel selenide at Au-RDEs, under oxygen evolution reaction (OER) conditions in alkaline electrolytes at three different rotation rates (1600, 2500, and 4000 rpm). Tafel slopes were extracted using cyclic voltammetry (CV) and chronoamperometry (CA). Our results underline the benefit of higher rotation speed in overcoming undesired mass transport limitations. Additionally, we point out that using CA instead of CV methods is preferable for the extraction of kinetic parameters since CA ensures steady state conditions and simplifies data analysis. Furthermore, we highlight that rigorous determination of reaction mechanisms based on Tafel slopes is only appropriate if the boundary conditions of the Butler-Volmer formalism are fulfilled, and we recommend not using Tafel slopes as the single parameter to compare the activity of electrocatalysts if these are obtained at different experimental conditions. Instead, we suggest reporting relevant measurement parameters such as the rotation speed of the experiment, way of Ohmic drop compensation, electrode geometry, and the electrochemical method used for Tafel analysis alongside the calculated Tafel slopes. Doing so may allow comparison to literature-reported Tafel slopes if the data were determined under similar experimental conditions. Rather than focusing on individual values, trends between systematically modified catalysts or at different aging stages should be considered meaningful.