Role of Dihydride and Dihydrogen Complexes in Hydrogen Evolution Reaction on Single-Atom Catalysts

Abstract

The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.

Universal Principles for the Rational Design of Single Atom Electrocatalysts? Handle with Care

“…It should be mentioned, however, that the model has been introduced for metal surfaces, and its use for SACs may be questionable due to the possible formation of another stable intermediate where two H atoms, and not only one, can bind to the active center (see section ). Nevertheless, here we concentrate on the hypothesis that the reaction proceeds with formation of a single TM-H intermediate, the condition that has been used so far. − The Δ G H values are obtained from Δ E H by adding a zero-point energy (ZPE) and an entropy term that can be approximated in the same way for all systems; this results in a correction of 0.24 eV (Δ G H = Δ E H + 0.24 eV).…”

Machine learning for design principles for single atom catalysts towards electrochemical reactions

“…To some extent, the black-box nature of ML techniques occasionally makes a physical interpretation of descriptors, such as the d-band center and enthalpy of vaporization, non-trivial. In particular, the d-band center is widely adopted as an efficient descriptor, 211 typically with high feature importance to describe the reactivity of SACs. However, the d levels of atomically dispersed metal atoms on a graphene substrate may not form a band that makes evaluating the position of the d-band center impossible.…”

Single Atom Catalysts: What Matters Most, the Active Site or The Surrounding?

“…It has been widely employed in its original formulation for the study of HER on SACs over the past few years. Recently, we have shown that SACs can form also dihydride and dihydrogen complexes (HMH), [37] in analogy with organometallic chemistry, and that the kinetics needs to include the possible formation of the HMH intermediates. This implies computing the free energy of adsorption of the second H atom, ΔG H (2) .…”