The Effect of Weakly Adsorbed Cations on the Helmholtz Capacitance of Metal–Water Interfaces

High Resolution Image Download MS PowerPoint Slide The interface between a weakly adsorbed, solvated cation and a metal electrode is investigated by density functional theory using a hybrid implicit-explicit solvation model. The cations remain charged upon adsorption, the counter-charge resides on the metal surface. There is no diffuse double-layer in this system; experimentally this corresponds to adsorption from a low concentration of cations. The change Δϕ in electrode potential upon adsorption is determined from the change in the electrostatic potential at large distances from the electrode. The phenomenon of partial charge transfer leads to two separate operational definitions for the charge on the ions: the formal charge ze 0, where z is the valency, and the partial charge remaining after adsorption, which we quantify with the Bader charge. The measurable integral capacitance is determined by z e 0, and for all systems investigated it lies in a range which compares well with experimental data. The divalent ions Ca 2+ and Mg 2+ show a substantial partial charge transfer, which enhances the capacitance. In contrast, alkali ions K + and Na + keep their unit charge. Since there is no diffuse layer, the electrosorption valency equals the negative of the partial charge transfer. Interpreting our results within a simple parallel plate capacitor model allows us to calculate an effective dielectric constant, governed mainly by contributions from the polarizabilities of the metal surface and of the water molecules. Thus, our model provides important insights into the structure of the compact double-layer and its concomitant interactions.