Field Effects at Protruding Defect Sites in Electrocatalysis at Metal Electrodes?

Potential dependencies of adsorption energies at metal electrodes are often interpreted in terms of a dipole-field interaction picture. Classical electrostatics would then suggest particularly strong dependencies at protruding defect sites like steps and kinks due to a local field enhancement. Here, we use first-principles density functional theory calculations that capture the capacitive charging of the electric double layer to analyze these dependencies for prototypical adsorbates at vicinal Pt(111) surfaces in an implicit aqueous electrolyte. This analysis confirms effective dipole-field-type dependencies but rationalizes why they can neither be estimated on the basis of accessible molecular dipoles nor the mere atomic-scale geometry of the adsorption site. The observed magnitude questions mechanistic analyses of surface catalytic reactions made on the basis of the prevalent computational hydrogen electrode approach that is agnostic to these potential-induced adsorption energy variations.