Density-Potential Functional Theory with Explicit Solvation and Desolvation for Electrical Double Layers

Understanding electrical double layers (EDLs) under realistic electrochemical conditions requires models that capture not only electrostatics but also the molecular mechanisms of solvation and desolvation. Here, we develop DPFTsol, an extended density-potential functional theory that incorporates explicit ion-solvent binding energies and configurational mixing entropy directly into the EDL grand potential. This framework distinguishes bound and free solvent molecules and allows solvation numbers to respond self-consistently to local electric fields, metal-solvent interaction, and specific ion-solvent binding energy. Using a single parameter set, DPFTsol quantitatively reproduces the differential capacitance curves of Ag(111)-KPF6 aqueous solutions across five concentrations. Microscopic analysis of the resulting EDL structures reveals potential-dependent ion desolvation, solvent layering, dielectric variations, and interfacial free-energy redistribution. Parametric studies show how maximum coordination number, ion-solvent binding energies, and solvent size modulate capacitance and local electric fields, including high-potential secondary peaks arising from solvation-mediated restructuring. Collectively, these findings underscore the necessity of explicitly accounting for solvation and desolvation to accurately predict EDL structure and capacitance, establishing DPFTsol as a robust theoretical framework for the design of advanced electrolytes and electrochemical interfaces.