Electrode-liquid interfaces under voltage bias demonstrate unique electrostatic and structural properties that are influential in their many technological applications. However, mean-field and empirical models of the structure and electrostatics of the electrical double layer (EDL) that forms in response to the applied bias do not capture the instantaneous inhomogeneities that polarizable electrolyte and solvent molecules engender. Emerging methods for controlling the electrode potential within density functional theory calculations have made in situ simulations of these interfaces accessible. Born-Oppenheimer ab initio molecular dynamics calculations of electrified Au(111) slabs interfaced with multiple monolayers of liquid water were performed using a combined explicit-implicit solvent approach. The excess charges localized on the metal electrodes were fixed and the electrode potentials were computed on the fly, elucidating the equilibrium fluctuations of the measured potentials and recovering the system’s differential capacitance. The effects of modest positive and negative applied voltages on the interfacial structure, dynamics, and vibrational spectroscopic properties of interfacial water were quantified and compared to experiment where applicable. The computed dipoles and polarizabilities of water molecules at each voltage from representative snapshots provided further insights into the inhomogeneity of this simple electrochemical system. These controlled-potential simulations provide a framework for further investigation of more complex or reactive species in the EDL and broadly for characterizing electrochemical interfaces in situ.

Effects of applied voltage on water at a gold electrode interface from ab initio simulations

ACS Fall 2021

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