Which Is Better for Hydrogen Evolution on Metal@MoS₂ Heterostructures from a Theoretical Perspective: Single Atom or...

Single atom (SA)- and monolayer (ML)-supported catalysts are two main technical routines to increase electrochemical catalytic performance and reduce cost. To date, it is still a debate which one is better for catalysis in experiments as both routines face a puzzling problem of searching for balance between stability and catalytic activity. Here, hydrogen evolution on two-dimensional 2H-MoS₂ with SA- and ML-adsorbed metal atoms (23 kinds in total) is taken as an example to solve this question by first-principles calculations. The thermodynamic stability during synthesis, in vacuum, and in electrochemical reaction conditions is determined to access the stability of MoS₂ loaded with single (M_S@MoS₂) and monolayer metal atoms (M_M@MoS₂). The realistic catalytic surfaces determined by surface Pourbaix diagrams, the free energy changes of hydrogen atoms at different coverages, and the exchange current densities are applied to determine hydrogen evolution reaction (HER) activity. The results show that all M_M@MoS₂ are much more stable than the corresponding M_S@MoS₂ as the metal-metal interaction in MLs could make the former structures more stable. In general, M_M@MoS₂ show higher hydrogen evolution activities than those of M_S@MoS₂. In detail, the exchange current densities of MoS₂ loaded by Pd ML and Au ML are 6.208, and 1.109 mA/cm^-2, respectively, which are comparable to Pt(111). Combining with small binding energies, the Pd and Au MLs are the most promising catalysts for hydrogen evolution. The purpose of this work is to highlight the advantages and disadvantages of SA- and ML-supported surfaces as HER catalysts and provide a fundamental standard for studying them.