Modeling Electrochemical Impedance Spectroscopy for Hydrogen Evolution Reaction on Single-Atom Catalysts

Single-atom catalysts (SACs) represent an emerging frontier in electrocatalysis, capable of bridging the gap between homogeneous and heterogeneous catalysis [1,2]. Their intrinsic nature of isolated metal atoms atomically dispersed on a supporting matrix gives them a highly active phase. Computational and theoretical chemistry provide powerful tools with which to investigate catalytic processes at an atomic level and to rationalize reaction mechanisms. Among electrochemical reactions, the hydrogen evolution reaction (HER) has attracted considerable attention for decades because of its pivotal role in energy conversion technologies, and it has been extensively studied through both experimental and computational approaches. HER generally proceeds via the Volmer-Heyrovsky-Tafel pathway, where the Volmer step involves hydrogen adsorption, followed by either an electrochemical (Heyrovsky) or chemical (Tafel) desorption step [3]. Computational studies have shown that hydrogen complexes may form on SACs, reflecting their analogy with homogeneous catalysts [4]. However, the experimental identification of such intermediates remains challenging. Electrochemical impedance spectroscopy (EIS) is a particularly promising approach for probing electrocatalytic mechanisms, as it provides access to kinetic information that cannot be obtained from standard steady-state measurements (e.g. polarization curves) [5]. In this study, we present a unified microkinetic and EIS modeling framework for HER on SACs that considers both the conventional Volmer–Heyrovsky pathway and an alternative Volmer–Heyrovsky mechanism mediated by hydrogen complex formation. Our results show that simulated Tafel plots do not always allow unambiguous identification of hydrogen complexes, whereas simulated impedance spectra display distinctive features that can serve as sensitive mechanistic fingerprints [6]. Overall, this work demonstrates how integrating microkinetic modeling and impedance analysis can elucidate electrocatalytic mechanisms at the atomic scale. This supports the rational design of next-generation catalysts for sustainable energy applications. In parallel, we are extending this EIS modeling strategy to other electrocatalyzed reactions and developing a first principles-based methodology that enables the direct simulation of impedance spectra. This establishes a consistent link between electronic-structure calculations and experimentally measurable electrochemical responses. References [1] X. F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Single-atom catalysts: a new frontier in heterogeneous catalysis, Accounts of chemical research, 46 (2013) 1740. [2] G. Di Liberto, G. Pacchioni, Modeling Single‐Atom Catalysis, Advanced Materials, 35 (2023) 2307150. [3] A. Lasia, Mechanism and kinetics of the hydrogen evolution reaction. international journal of hydrogen energy, 44 (2019) 19484. [4] G. Di Liberto, L. A. Cipriano, G. Pacchioni, Role of dihydride and dihydrogen complexes in hydrogen evolution reaction on single-atom catalysts, Journal of the American Chemical Society, 143 (2021) 20431. [5] D. Koster, A. R. Zeradjanin, A. Battistel, F. La Mantia, Extracting the kinetic parameters of the hydrogen evolution reaction at Pt in acidic media by means of dynamic multi-frequency analysis, Electrochimica Acta, 308 (2019) 328. [6] M. Spotti, N. Pianta, D. Brogioli, F. La Mantia, G. Di Liberto, Modeling electrochemical impedance spectroscopy of hydrogen complexes during hydrogen evolution on single-atom electrocatalysts, Electrochimica Acta, 545 (2025) 147758.