Elucidation of alkaline electrolyte-surface interaction in SECCM using a pH-independent redox probe

Scanning electrochemical cell microscopy (SECCM) has been used to elucidate the interaction between aqueous alkaline electrolyte and a glassy carbon electrode surface using a free-diffusing Os complex, Os(2,2’-bipyridine)₂(N,N’-dimethyl-2,2 ́-biimidazole), as a pH-independent redox probe (Os³⁺/Os²⁺). The voltammetric response of the complex showed a significant component of surface-confined behavior, rather than the expected purely sigmoidal nanoelectrode voltammogram. To elucidate and understand the response, we complemented the SECCM experiments with numerical simulations to investigate the impact of the droplet geometry and size on the voltammetric signature. Comparison of experimental and simulated voltammograms reveals a surprisingly large wetted area over the glassy carbon substrate, as confirmed by scanning electron microscopy of droplet residues. By investigating different tip sizes, tip approach rates and pH values, we elucidated the key factors controlling the wetting behavior. Simulations further indicated that the analysis of the peak to limiting current and charge passed provides a route to elucidating the extent of wetting in-situ, although quantitative analysis requires further understanding of evaporation and convective flow. In general, knowledge about droplet geometry and size through this study provides an improved understanding of droplet-surface interactions which is essential for a quantitative interpretation of SECCM measurements.