A schematic illustration of the double layer is presented in Figure (a). The metal electrode is assumed to be planarly uniform. Surface roughness is not considered here. The first-layer water molecules are usually highly polarized due to the high interfacial electric field and chemical interactions with the metal, see a recent review by Groß and Sakong. Adsorbed chemisorbates, if any, are located at the IHP. It is assumed that the first-layer water molecules and adsorbed chemisorbates line up rigidly at the IHP, neglecting actual unevenness due to thermal fluctuation and size difference. Ions with their solvation shell reside in the diffuse layer stretching from the OHP towards the solution bulk. The space between the metal and the IHP, that between the IHP and the OHP, and the diffuse layer are described as dielectric continua. An electrolyte solution with monovalent ions is considered, in accordance with experimental conditions in the work of Ojha et al. Figure (left) Schematic illustration of the electrocatalytic double layer with partially charged chemisorbates and water molecules in the inner Helmholtz layer. (right) Electric potential profile from the bulk metal to the bulk solution. There are three potential differences (p.d.) in the space between the metal and the IHP, including one due to the adsorbed water dipoles, one due to the partially charged chemisorbates, and one due to the surface charge.