Operando Optical Imaging Reveals Beyond-Meniscus Nanoelectrochemistry Mediated by a Nanometric Wetting Film.

Recent studies have highlighted the role of micrometer-scale, three-phase interfacesbetween an electrode, an electrolyte, and a gas phasein confining and activating chemical reactions. Here, we combine scanning electrochemical cell microscopy (SECCM) to create a localized air|water|indium tin oxide (ITO) meniscus and interference reflection microscopy (IRM) to image nanoscale electrochemical processes occurring in its vicinity. This approach reveals that, under the conditions used here (20 μm diameter KCl electrolyte meniscus, ca. 40-60% relative humidity, ITO partial wetting properties, and several tens of seconds of contact time), a nanometric electrolyte film extends beyond the visible meniscus, over several micrometers. This expands the three-phase region, enabling nanocatalytic sites located outside the meniscus-confined electrochemical cell to react. During cathodic polarization, IRM monitors these beyond-meniscus events through the appearance of emerging optical features associated with the formation of localized electrolyte microdroplets and dendritic metallic structures. These events are quantified at the single-entity level, particularly in the framework of the capillary work associated with microdroplet growth and relaxation. This analysis shows that fA-level electrochemical activity can be partly converted into mechanical capillary work via electrolyte transport through the nanometric electrolyte film. We finally discuss how such thin-film-mediated peripheral activity can potentially bias meniscus-confined electrochemical measurements.