The Crucial Role of Rotation Speed on the Determination of Tafel Slopes of Electrocatalysts in Rotating Disk Electrode Experiments
Felix Hiege, Luca Marie Sicking, Kannasoot Kanokkanchana, Paolo Cignoni, Victor Dudarev, Alfred Ludwig, Kristina Tschulik
As Tafel slopes are widely used in the electrocatalysis community to evaluate the intrinsic activity of catalysts or to identify rate-determining steps, we conducted an in-depth study on how to conduct Tafel analysis accurately that fulfills the restrictions of the Butler-Volmer (BV) theory. We used Rotating Disk Electrodes (RDE) since they are widely used in literature reports. Our work shows the crucial influence of the rotation speed on the determined Tafel slope, which decreases with increasing rotation speed. Furthermore, we showed that potentiodynamic experiments (e.g., cyclic voltammetry) do not provide sufficient steady-state conditions for the extraction of kinetic parameters (e.g., Tafel slope, exchange current density, or kinetic rate constant). Thus, we emphasize to use of chronoamperometric experiments at the highest possible rotation rate, which still guarantees a laminar flow, to determine accurate Tafel slope values for an electrocatalytic reaction (herein we used nickel selenides-based films for OER as an example system). However, the determination of Tafel slopes from potentiodynamic experiments can still be useful when observing activation trends of a catalyst with ongoing reaction time. Worth noting, it should be focused on the qualitative trend of the change in Tafel slopes rather than on the absolute Tafel slope values in those experiments.
Our study contains decent information and suggestions of experimental and macroscopic (e.g., geometry of the working electrode) parameters that should be used for appropriate Tafel analysis in electrocatalysis.