Correcting the Hydrogen Diffusion Limitation in Rotating Disk Electrode Measurements of Hydrogen Evolution Reaction Kinetics

Rotating disk electrode (RDE) method is widely employed in studies on the hydrogen oxidation/evolution reaction (HOR/HER) owing to its well-defined mass transport behaviors. While it is accepted that the measured HOR current is controlled by both the electrode kinetics and the diffusion of H2, HER is typically assumed to be free of diffusion limitation. Here we demonstrate that HER could also be diffusion limited when the electrode kinetics is fast, as evidenced by the rotation speed dependent HER current on Pt in acid (pH = 1) and the overlap of the HER polarization curve with the concentration overpotential curve. The HER diffusion limitation originates from the insufficient mass transport of produced H2 from the electrode surface to the bulk electrolyte and the highly reversible nature of HOR/HER. Kinetic analyses based on HER polarization curves on Pt in acid without correcting for the diffusion limitation could lead to inaccurate Tafel slopes and mechanistic interpretations, and significantly underestimated HER activities. A general data analysis protocol based on the reversible Koutecky-Levich equation is developed to obtain accurate kinetic information of HOR/HER even when electrode kinetics is facile. This new method is compared with other existing methods on Pt disk electrodes at different pHs and thin-film electrodes with different Pt loadings.