In Situ\nQuantification of a Wetted Surface Area during\nScanning Electrochemical Cell Microscopy Using Retraction Curves

This work presents a new methodology to estimate the\nsurface area\nof the working electrode during scanning electrochemical cell microscopy\n(SECCM) <i>in situ</i> by utilizing retraction curves. In\nthis approach, the current is measured as a function of pipet displacement\nin the <i>z</i>-direction. When the current drops to zero,\nit is indicative of droplet detachment from the surface, allowing\nfor the estimation of the droplet contact diameter based on the pipet\ndisplacement. This enables real-time estimations of surface areas\nof the wetted electrode at each point of measurement, rather than\nperforming time-consuming measurements using <i>ex situ</i> correlative image analysis or estimating an average working electrode\nsize from the pipet aperture. Notably, during SECCM measurements on\ncopper in nitric acid, the working electrode diameter estimated using\nretraction curves was significantly smaller than the droplet footprint\ndiameter observed post experiment using <i>ex situ</i> correlative\nimage analysis. This discrepancy is attributed to droplet spreading\nafter pipet retraction, as confirmed by goniometer and silanized pipet\nmeasurements. Upon cleaning the surface, the true wetted surface areas\nduring SECCM measurements were found to be in good agreement with\nvalues estimated using retraction curves yet were larger than the\npipet aperture. Additionally, the effects of approach separation,\nretraction rates, and probe diameter on the droplet contact size were\nanalyzed using retraction curves. These findings were compared to <i>ex situ</i> methods to assess the reliability of the retraction\ncurves for determining the working electrode surface area. This study\ndemonstrates the potential of retraction curves to provide a higher\naccuracy in the quantitative analysis of local current density values\nextracted using SECCM.