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

An adsorbed hydroxide (OH_ad) 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 OH_ad layer is strongly governed by the hydrophilicity of the cation interacting with OH_ad 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, OH_ad, and Li were investigated using infrared spectroscopy and density functional theory calculations. By optimizing the coverage of OH_ad (θ_OH) and Li (θ_Li), the bending mode of the PtOH (δ_PtOH) 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 OH_ad species interacting with hydrated Li⁺ on Pt(111) was successfully modeled under UHV conditions. According to the interfacial structure modeled under UHV conditions, θ_OH = 0.3, and Li interacts directly with OH_ad at a stoichiometric ratio (θ_OH/θ_Li) of 2. Modeling of the EDL, including the outer Helmholtz plane, is beneficial for identifying the microscopic details of the EDL under electrochemical conditions.