Subparticle Operando Imaging for Probing Electrocatalytic Intermediates and Cation Effects

Surface-bound metal-H* intermediates derived from interfacial water play a central role in many electrocatalytic transformations, yet their nanoscale behavior remains difficult to resolve because of their transient nature and the ensemble heterogeneity of conventional measurements. Here, we employ operando single-molecule super-resolution imaging to directly probe electrocatalytic intermediates' dynamics on individual Cu nanoplates with nanometer precision while systematically varying electrolyte cation identity and concentration. Single- and subparticle analyses reveal pronounced structure-dependent kinetics, with activity following the order corner > edge > basal, reflecting coordination-dependent interfacial solvation environments. Across 85 nanoplates spanning a wide range, the reactivity exhibits an unexpected biphasic dependence on particle size, indicating a crossover between kinetically controlled and transport-influenced regimes. Spatial correlation analysis further uncovers negative catalytic cooperativity between corners and adjacent edge segments, suggesting that surface intermediates propagate through local competition for proton supply and interfacial water accessibility. Electrolyte composition strongly regulates these dynamics. Increasing cation size (Li⁺ → Cs⁺) monotonically suppresses activity, consistent with the disruption of structured interfacial water, whereas ionic strength produces a biphasic promotion-inhibition behavior with a critical concentration near 0.5 M. Our imaging approach converts Cu-H* into an experimentally measurable descriptor linking nanoscale structure and interfacial solvation, offering key mechanistic insights for rational electrolyte design in aqueous electrocatalysis, readily extensible to other intermediates (e.g., OH*, CO*) and various electrocatalytic reactions.