Modeling electrochemical impedance spectroscopy of hydrogen complexes during hydrogen evolution on single-atom electrocatalysts

Single Atom Catalysts (SACs) are an emerging frontier in heterogeneous electrocatalysis. They are made of metal atoms atomically dispersed on a matrix. 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. The classical HER mechanism can be described by a Volmer–Heyrovsky–Tafel mechanism, where the two desorption steps are competitive. The Volmer-Heyrovsky mechanism is conventionally proposed for single-atom catalysts. It has been computationally demonstrated that hydrogen complexes can form on SACs due to their analogy with homogeneous catalysts. Unfortunately, it is hard to “visualize” these species experimentally. Electrochemical Impedance Spectroscopy (EIS) could be the most promising approach to study electrocatalytic mechanisms. In this work, we present microkinetic and Electrochemical Impedance Spectroscopy models for HER on SACs, describing Volmer-Heyrovsky and a mechanism mediated by the formation of hydrogen complexes. Our simulated data, applied to a case study based on Pd@TiN, show that Tafel plots will not suffice in the visualization of hydrogen complexes formation and will need the support of electrochemical impedance spectra in order to clarify the correct mechanism.