S.O.S: shape, orientation and size tune solvation in electrocatalysis

Current models to understand the reactivity of metal/aqueous interfaces in electrochemistry are based on the adsorption free energies of reactants and products, e.g. volcano plots. Theory, in particular electronic calculations, played a major role in the quantification and comprehension of these free energies in terms of the interactions that the reactive species form with the surface. However, also solvation free energies come into play in two ways: (i) by modulating the adsorption free energy together with solute-surface interactions, as the solute has to penetrate the water adlayer in contact with the surface and get partially desolvated (which costs free energy); (ii) by regulating transport across the interface, i.e. the free energy profile from the bulk to the interface, which is strongly non-monotonic due to the unique nature of metal/aqueous interfaces. Here, we use constant potential Molecular Dynamics to study the solvation contributions and we uncover huge effects of the shape and orientation (on top of the already known size effect) of the solute on its adsorption free energy. We propose a minimal theoretical model, the S.O.S. model, that accounts for size, orientation and shape effects. These novel aspects are rationalized by recasting the concepts at the base of the Lum- Chandler-Weeks theory of solvation into a layer-by-layer form, where the properties of each interfacial region close to the metal are explicitly taken into account.