Decoupling DynamicEffects from Adsorption in HydrogenElectrocatalysis: Frequency of Effective Collisions as Activity Determinant

Tuning the intrinsic rate of the electrocatalytic hydrogen evolution reaction (HER) requires an in-depth understanding of the interplay between the energetics of intermediate formation (i.e., M–Had) and the kinetics of each elementary step underpinning the reaction mechanism. Utilizing high-temperature hydrodynamic voltammetry on 16 metals, detailed data sets on rate constants of the elementary steps of HER were obtained under identical experimental conditions. Activity trends as well as the shape of HER “volcano”-plot, that is, HER rate at equilibrium potential vs free energy of intermediate adsorption, are strongly dependent on the ratio between the rate constants of the recombination step and the proton adsorption step. The rate constant of the adsorption step for the most active metals is 2–3 orders of magnitude larger than that of the recombination step. The M–Had bond strength has a systematic impact on activation energy implying that the Brønsted–Evans–Polanyi (BEP) relation holds; however, the BEP coefficient is low (i.e., ß = 0.25). Therefore, the activation process, besides the energetics of proton adsorption, strongly depends on metal–solvent interactions that are entropic in nature, specifically, the structural transformation of the interfacial water layer. Preexponential factors of the different elementary steps exhibit orders of magnitude differences. We propose that the reason for this is different interfacial dynamics, including proton–water and M–Had bond oscillations and the effective collision frequency of the protons with the active site. High frequency of metal–proton collisions and destabilization of proton solvation shells allow enhanced access to the electrode surface, even when the M–Had bond is not significantly strong. This scenario implies that despite weak M–Had bonds, very high Had coverage is reached due to the high frequency of metal–proton collisions, allowing more effective Had recombination reactions to occur. These are significantly important findings that unveil a more comprehensive perspective on the origins of HER electrocatalytic activity.