Electrical Double Layer on the Pt(111) Electrode Modeled under Ultrahigh Vacuum Conditions

Abstract

An adsorbed hydroxide (OHad) layer is formed on platinum electrodes at positive potentials, which affects many important electrochemical reactions, such as the oxygen reduction reaction. The stability of the OHad layer is strongly governed by the hydrophilicity of the cation interacting with OHad in the electrical double layer (EDL). Therefore, elucidating the detailed hydration structure and role of cations in the EDL is a key research goal. In this study, the structures of H2O, OHad, and Li were investigated using infrared spectroscopy and density functional theory calculations. By optimizing the coverage of OHad (θOH) and Li (θLi), the bending mode of the PtOH (δPtOH) band on the Pt(111) surface under ultrahigh vacuum (UHV) conditions matches with that on the Pt(111) electrode at 0.8–0.9 V versus reversible hydrogen electrode (RHE) in LiOH solution, indicating that a quasi-EDL composed of OHad species interacting with hydrated Li+ on Pt(111) was successfully modeled under UHV conditions. According to the interfacial structure modeled under UHV conditions, θOH = 0.3, and Li interacts directly with OHad at a stoichiometric ratio (θOH/θLi) of 2. Modeling of the EDL, including the outer Helmholtz plane, is beneficial for identifying the microscopic details of the EDL under electrochemical conditions.