Beyond Dielectrics: Interfacial Water Orientational Polarization Governs Graphene-Based Electrochemical Interfaces

Water molecules are traditionally viewed as passive dielectric media in electrochemical systems. In this work, we challenge this conventional perspective through molecular dynamics simulations and theoretical analysis. We demonstrate that interfacial water exhibits a distinct orientational polarization compared to bulk water and excessively screens the electrostatic potential between ions and the surface, going beyond the classic electric double layer (EDL) model, which considered water merely a passive dielectric. This overscreening occurs because a significant portion of water polarization responds directly to the graphene surface in addition to screening the electrostatic interaction between ions and charged surfaces. Furthermore, we reveal that this surface-induced interfacial water polarization governs the electric potential distribution and the EDL capacitance and can even invert the electrode surface potential polarity, overriding the ion's contribution. These molecular-level insights underpin a revised EDL model that more accurately captures the electric and chemical potential distributions in the interfacial EDL regions.