Subparticle Operando Imaging for Probing Electrocatalytic Intermediates and Cation Effects

) 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.