Noncovalent Metal-Water Polarization Governs Helmholtz Capacitance

Electrified metal-water interfaces govern charge storage and electrochemical reactivity, yet the microscopic origin of their metal-dependent Helmholtz capacitance has remained unresolved. Here, we use extensive density functional theory-based ab initio molecular dynamics simulations to reproduce and explain the long-established experimental capacitance trend at fcc(111) metal-water interfaces. We show that this trend is governed primarily by the electrode-side electronic response of the explicit metal-water interface, which controls the effective electron spillover length and, hence, the bias capacitance. This relation remains predictive for cross-geometry Pt/Au systems. Our findings clarify the microscopic origin of metal-dependent Helmholtz capacitance and suggest a physically grounded design principle for electrode materials in sustainable-energy technologies.