Enthalpy-Entropy Synergy Governs Cation-Mediated Interfacial Water Activation on Cu(111).

The reactivity of interfacial water at metal electrodes plays a decisive role in governing proton-coupled electrocatalytic kinetics, yet a quantitative understanding of how electrolyte cations modulate this reactivity remains elusive. Herein, metadynamics-biased ab initio molecular dynamics (AIMD) simulations reveal that the regulation of water activity on the Cu(111) surface by potassium ion (K+) is intrinsically governed by enthalpy-entropy synergy. Thermodynamic decomposition reveals that the interfacial electric field dominates the enthalpic contribution, whereas hydrogen-bonded network disordering governs the entropic gain. Theoretically calculated infrared spectra and statistical distributions of hydrogen-bond configurations of interfacial water molecules further support a cation-induced transition from rigid, hydrogen-bond confined interfacial water to a more flexible and disordered water environment. These findings establish an atomistic thermodynamic picture of cation-regulated water activation and provide a conceptual basis for enthalpy-entropy synergistic control in proton-coupled electrocatalysis.