Observation of H 2 Evolution and Electrolyte Diffusion on MoS 2 Monolayer by In Situ Liquid-Phase Transmission Electron Microscopy.

Unit-cell-thick MoS 2 is a promising electrocatalyst for the hydrogen evolution reaction (HER) owing to its tunable catalytic activity, which is determined based on the energetics and molecular interactions of different types of HER active sites. Kinetic responses of MoS 2 active sites, including the reaction onset, diffusion of the electrolyte and H 2 bubbles, and continuation of these processes, are important factors affecting the catalytic activity of MoS 2 . Investigating these factors requires a direct real-time analysis of the HER occurring on spatially independent active sites. Herein, the H 2 evolution and electrolyte diffusion on the surface of MoS 2 are observed in real time by in situ electrochemical liquid-phase transmission electron microscopy (LPTEM). Time-dependent LPTEM observations reveal that different types of active sites are sequentially activated under the same conditions. Furthermore, the electrolyte flow to these sites is influenced by the reduction potential and site geometry, which affects the bubble detachment and overall HER activity of MoS 2 .