Abrupt Mechanistic Variation of Hydrogen Evolution Reaction on Metal Electrode Surfaces in the Potential-pH Space

Understanding the mechanisms of multi-electron and multi-proton electrochemical reactions, particularly in the context of renewable electrochemical energy conversion, is an outstanding challenge. The electrocatalytic hydrogen evolution (2H + + 2e - ...> H 2 ) is the half reaction of water splitting and is heavily studied. However, the kinetic variation of this reaction under various pH conditions has not been fully understood. In this work, we studied hydrogen evolution reaction on different electrodes (Pt, Cu, FeS 2 , Ni, and Ti) over a wide range of neutral (pH=6.5) to acidic (pH=1.5) solutions. Despite variations in the properties of these electrodes, we observe a discontinuous change in the onset potential as a function of current in the pH range of 2.5 to 3.5 in all the electrodes. We also represent two fundamental kinetic parameters, empirical electron transfer coefficient α and the reaction order with respect to proton ρ, as a function of pH and potential. An important observation is that α is constant (α≈0.2) at pH values higher than the discontinuous jump, while at lower pH values alpha increases with decreasing pH. The maximum value of alpha varies for different electrodes and is increasing from Ti < Ni< Cu < FeS 2 < Pt. The proton reaction order is non-zero in the acidic side, whereas it is zero in the neutral limit and this trend is common for all the electrodes. To our knowledge, for the first time, we are presenting the kinetic parameters in two-dimensional plots as a function of both pH and potential. This representation allows us to clearly see global trends in the kinetics of the reaction. We will propose possible explanations for such common behavior based on a kinetic model that accounts for both concerted and step-wise electron and proton transfer. We hope that this study sheds light on the fundamentals of electron-proton transfer during hydrogen evolution. Figure 1