Volcano-Type Behavior in Spatially Resolved Electron Transfer and Hydrogen Evolution Reaction Mapping over 2D Electrocatalysts.

In situ mapping of interfacial electron transfer dynamics to reveal electrocatalytic activity with high spatial resolution is crucial for developing efficient electrochemical devices. While emerging 2D materials for catalysis have attracted substantial attention, there is still a notable lack of studies examining their electrochemical properties at the nanoscale, particularly in a layer-by-layer context. Here, both outer-sphere and inner-sphere electron transfer at a 2D semiconducting WSe electrode-electrolyte interface are spatially resolved and quantified in high resolution. The investigations reveal that WSe exhibits a volcano-type behavior in its electrochemical activity, with a peak performance observed at a specific thickness of four layers. This phenomenon is attributed to the interplay between the layer-specific electronic density of states and the tunneling of charge carriers across the interlayer regions. This observation is further exemplified by micro-electrochemical hydrogen evolution reaction measurements.