Insights into the Electrocatalytic Hydrogen Evolution Reaction of MoS2 Probed by Electron Spin Resonance Spectroscopy

Nanocrystalline (NC) transition-metal dichalcogenides have received significant attention as earth-abundant electrocatalysts for the hydrogen evolution reaction (HER). MoS2 is the most extensively studied alternative to Pt-group catalysts; however, atomistic insight into its size-dependent catalytic activity remains limited. Here, we investigate HER performance in NC MoS2 with controlled crystallite sizes (6, 11, and 48 nm). The smallest crystallites exhibit markedly enhanced HER activity and higher H2 production compared to larger counterparts, highlighting a strong size–activity relationship. To elucidate the underlying mechanism, we combine electrochemical measurements with in situ and ex situ electron spin resonance (ESR), hyperfine sublevel correlation spectroscopy (HYSCORE), electron–nuclear double resonance (ENDOR), and first-principles calculations. In situ ESR shows that potential-dependent sulfur vacancies in small crystallites actively drive HER. Advanced pulsed ESR techniques (HYSCORE and ENDOR), supported by theoretical modeling, further demonstrate that these sulfur vacancies are preferentially passivated by OH groups rather than hydrogen atoms, forming MoS2-(OH)S adsorbate–defect complexes. These results provide direct spectroscopic evidence linking crystallite size, defect chemistry, and electrocatalytic activity in MoS2. More broadly, this work establishes in situ ESR and advanced magnetic resonance methods as powerful tools for resolving active sites and reaction intermediates in electrochemical energy conversion processes.