Why Diffusive Ions Do Not Block Adsorption: The Dominant Role of the Stern Layer in Electrocatalysis

The intrinsic complexity of the electric double layer (EDL) remains a major challenge in the theoretical description of electrocatalysis. Although ab initio molecular dynamics simulations have delivered valuable insights, their computational cost and limited statistical convergence hinder a complete characterization of the interfacial electrostatics. Here, by integrating the continuum Stern layer model with classical density functional theory (cDFT), we systematically investigate how electrolyte composition and electrode potential shape the electrostatic profile. We identify a Stern-layer-limited regime in which both interfacial charge and potential are rapidly saturated such that the ionic distributions are determined by a concentration-controlled state, rendering negligible effects due to the steric blocking at the Outer Helmholtz plane (OHP). Accordingly, molecular adsorption and electron transfer at the electrode surface are dictated by the strong interfacial electric field and the distinct solvent structure within the Stern layer. These findings establish a unified mechanistic framework linking electrolyte properties, applied electrode potential, and interfacial electrostatics, offering useful guidelines for electrolyte-oriented electrocatalytic design.