Electron Spillover into Water Layers: A Quantum Leap in Understanding Capacitance Behavior

We investigate the electronic and molecular properties of the electrified Pt(111)-water interface using molecular dynamics simulations, leveraging electronic-structure-aware density-functional theory (DFT) and classical force field approaches. Electrification is induced by introducing excess electrons with homogeneously distributed, non-ionic counter-charges, allowing for a targeted analysis of electronic and water density responses without interference from electrolyte ions. Our results reveal that, within the DFT framework, the Pt(111)-water interface deviates from the classical picture, where excess electronic charge remains localized at the metallic surface. Instead, approximately 30--40\% of the electronic excess charge density penetrates into the interfacial water region -— a behavior that is absent in vacuum conditions or when using classical force fields. This redistribution of charge provides a compelling explanation for long-standing discrepancies in the modeling of this interface, including the stabilization of partially charged interfacial species such as H$^+$ and most importantly the severe underestimation -- by an order of magnitude -- of the interfacial capacitance in force-field-based methods. Our findings highlight the crucial role of electronic charge spillover in defining interfacial behavior which provides critical insights about the approximations in classical descriptions and for the development of more accurate computational models of electrochemical systems.