In Situ Quantification of a Wetted Surface Area during Scanning Electrochemical Cell Microscopy Using Retraction Curves

This work presents a new methodology to estimate the surface area of the working electrode during scanning electrochemical cell microscopy (SECCM) in situ by utilizing retraction curves. In this approach, the current is measured as a function of pipet displacement in the z-direction. When the current drops to zero, it is indicative of droplet detachment from the surface, allowing for the estimation of the droplet contact diameter based on the pipet displacement. This enables real-time estimations of surface areas of the wetted electrode at each point of measurement, rather than performing time-consuming measurements using ex situ correlative image analysis or estimating an average working electrode size from the pipet aperture. Notably, during SECCM measurements on copper in nitric acid, the working electrode diameter estimated using retraction curves was significantly smaller than the droplet footprint diameter observed post experiment using ex situ correlative image analysis. This discrepancy is attributed to droplet spreading after pipet retraction, as confirmed by goniometer and silanized pipet measurements. Upon cleaning the surface, the true wetted surface areas during SECCM measurements were found to be in good agreement with values estimated using retraction curves yet were larger than the pipet aperture. Additionally, the effects of approach separation, retraction rates, and probe diameter on the droplet contact size were analyzed using retraction curves. These findings were compared to ex situ methods to assess the reliability of the retraction curves for determining the working electrode surface area. This study demonstrates the potential of retraction curves to provide a higher accuracy in the quantitative analysis of local current density values extracted using SECCM.