On the Dominating Mechanism of the Hydrogen Evolution Reaction at Polycrystalline Pt Electrodes in Acidic Media

The hydrogen evolution reaction (HER) is of paramount importance for both fundamental and applied electrocatalysis. However, many issues related to understanding of this reaction remain unclear. These, for instance, include a surprising pH dependence of the electrode activities and non-Tafel dependency of the HER-associated current for various electrocatalysts. Even the dominating mechanism for this reaction at different potentials is often difficult to reveal. In this article, we use electrochemical impedance spectroscopy to estimate the relative contribution of the Volmer-Heyrovsky and Volmer-Tafel pathways to the overall hydrogen evolution process at polycrystalline Pt electrodes at pH = 0, 1, and 2 as a function of the electrode potential. Pt microelectrodes were used to facilitate impedance measurements at high current densities (up to ∼1 A·cm^-2 in 1 M HClO₄) and to overcome common complications due to the fast kinetics of this reaction. Our results show that it is possible to distinguish different reaction pathways experimentally at each electrode potential using impedance measurements and demonstrate that the relative contributions of the Volmer-Heyrovsky and Volmer-Tafel pathways are in most cases comparable. Both mechanisms contribute differently to the total current at different electrode potentials, and none of them can be considered as absolutely dominating at a given complex Pt surface. These findings can be particularly used for elaboration of theoretical models and interpretation of non-Tafel behavior of polarization HER curves in acidic media.