Demystifying the Stern layer at metal-electrolyte interface: Local dielectric constant, ion adsorption and partial charge transfer

Electric double layer (EDL) models are commonly used in electrochemical sciences and diverse applications. Whereas phenomenological models have been long established to describe ionic distributions, a faithful description of the electrode-electrolyte interface remains a daunting challenge. In this work, we study the charging behavior of Ag (111) electrode in NaF aqueous solutions by a combination of experimental results with theoretical calculations based on the Gouy-Chapman–Stern (GCS) model, the classical density functional theory (cDFT), and the joint density functional theory (JDFT). When the electrode is negatively charged, the ionic distribution can be described by the GCS/cDFT model with the dielectric constant of the Stern layer depending on the local electric field. F^- adsorption in the Stern layer takes place when the electrode is positively charged and can be attributed to both physical and chemical interactions. The chemisorption exhibits partial charge transfer from F^- to the Ag that depends on the applied voltage. Qualitatively, F^- binding and partial charge transfer are supported by the JDFT calculations. Our findings shed insights on the characteristics of the Stern layer and the charge behavior of adsorbed species not specified by conventional EDL models.