Halogen ligand-enhanced single atom catalysts for superior HER performance in Pd-anchored MoS2 monolayer

Single-atom catalysts (SACs) have attracted ever-growing interest due to their high atom-utilization efficiency and potential for cost-effective of hydrogen production. However, key challenges still remain in developing high-performance SACs for hydrogen evolution reaction (HER) technology. Herein, innovatively, the effects of surface ligands (F, Cl, Br, I) on the HER performance and mechanism of single-atom (Pd or Cu)-anchored MoS₂ monolayer are detailly investigated using first-principles calculation. The results indicate that the relative Gibbs free energy for the adsorbed hydrogen atom in the I–Pd@MoS₂ system is an exceptionally low value of −0.13 eV, which is not only comparable to that of Pt-based catalysts but also significantly more favorable than the calculated 0.84 eV for Pd@MoS₂. The ligand restructures the local chemical environment around SAC Pd, creating impurity bands near the Fermi level that couple with H atom s states, thus yielding numerous highly-active sites to enhance catalytic performance. Comparatively, the ligands around SAC Cu cause impurity bands far below the Fermi level, which are raised to Fermi energy in H-absorbed systems. The molecular dynamics results exhibit X–Pd@MoS₂ are more thermally stable than X–Cu@MoS₂ at room temperature, and the impurity bands near the Fermi level in the pure electrocatalysts rather than in the systems after hydrogen adsorption, enhance the activity and stability. Furthermore, the climbing-image nudged elastic band method (CI-NEB) elucidates that the enhanced HER mechanism for the I–Pd@MoS₂ catalyst should belong to the coexistence of the Volmer-Tafel and Volmer-Heyrovsky reactions. This investigation provides a valuable framework for experimental design and development of innovative single-atom catalysts.