Near-electrode anion dehydration and field-dependent dielectric response govern double-layer capacitance

Predicting electrical double-layer (EDL) capacitance from continuum theories without fitting parameters has remained a long-standing challenge due to an incomplete understanding of interfacial electrostatics. In this work, we construct a classical framework that achieves favorable agreement with experiments without fitting for three distinct electrolytes over a range of concentrations. Our model reveals near-electrode ion-specific hydration to be the most important physical mechanism in determining EDL capacitance, followed by the field-dependent dielectric response of water and ion-specific dielectric decrement. It also uncovers the modest role of excluded-volume electrostatics in determining EDL capacitance, although this factor significantly affects the EDL structure. Our classical first-principles framework demystifies electrochemical phenomena, enables predictive modeling of EDLs, and could be incorporated into implicit solvation models in quantum-mechanical calculations.