Operando Time-Resolved Spectroelectrochemical Microscopy Reveals Distinct Electron Depletion Pathways in Electrocatalysis

Understanding electrocatalysis requires direct knowledge of how charge carriers are distributed and consumed at active sites under the operating conditions. However, the spatiotemporal dynamics of injected electrons in working catalysts remain largely inaccessible, hindering a mechanistic understanding of catalytic heterogeneity. Here, we develop an operando time-resolved spectroelectrochemical microscopy that enables direct and quantitative mapping of electron density with microsecond temporal resolution and micrometer spatial resolution. Using monolayer MoS2 as a model system, we directly visualize the spatial redistribution and depletion of electrochemically injected electrons during the hydrogen evolution reaction. We observe pronounced site-specific electron depletion that correlates with catalytic activity and further identify two distinct mechanisms governing charge loss: reaction-limited depletion at highly active sites, including edges and Pt-functionalized sites, and injection-limited depletion in bilayer regions. These results establish a quantitative link between local charge carrier dynamics and catalytic activity and provide a general framework for probing nanoscale carrier dynamics in electrocatalysts.