Modeling dynamic electrochemical impedance spectroscopy for hydrogen evolution reaction on single-atom catalysts

Single-Atom Catalysts (SACs) are an emerging frontier in heterogeneous electrocatalysis. They are made of metal atoms atomically dispersed on a matrix. Computational and theoretical chemistry allow us to study catalytic processes with atomic detail and to rationalize reaction mechanisms. A lot of attention has been dedicated to the study of Hydrogen Evolution Reaction (HER) mechanism, due to its relevance in energy conversion technologies, both with computational and experimental methods. HER typically follows the Volmer-Heyrovsky-Tafel pathway: the Volmer step involves hydrogen adsorption, followed by either an electrochemical (Heyrovsky) or chemical (Tafel) desorption [1]. It has been computationally demonstrated that hydrogen complexes can form on SACs due to their analogy with homogeneous catalysts [2]. Unfortunately, it is hard to “visualize” these species experimentally. A recently developed experimental technique, dynamic Electrochemical Impedance Spectroscopy (dEIS) could be the most promising approach to study electrocatalytic mechanisms [3]. In this work, we present Microkinetic and dynamic Electrochemical Impedance Spectroscopy models for HER on SACs, describing Volmer-Heyrovsky and an unconventional Volmer–Heyrovsky mechanism mediated by the formation of hydrogen complexes. Our models demonstrate that simulated Tafel plots cannot help in the visualization of hydrogen complexes formation unlike simulated electrochemical impedance spectra. This study highlights the potential of combining advanced spectroscopy with theoretical modeling to unravel electrocatalytic mechanisms at the atomic scale, contributing to the rational design of next-generation catalysts for sustainable energy applications. [1] Lasia, A., Int J Hydrogen Energy 44, 19484-19518 (2019) [2] Di Liberto, G. et al., J Am Chem Soc 143, 20431–20441 (2021) [3] Koster, D. et al., Electrochim Acta 308, 328–336 (2019)