In this contribution, we examine solvated hydronium and hydroxide ions in multiple explicit water bilayers outside a Pt(111) surface using density functional theory (DFT), simulating an electrochemical interface. In the outer Helmholtz plane, the Bader charge of ions has been observed to be fractional at the general gradient approximation (GGA) level of theory, and this fractional charge has been previously attributed to well-known delocalization errors in DFT. We use exact-exchange calculations in this study to benchmark the Bader charge on these solvated ions and show that the fractional nature of the charge is due to (1) a physical spillover of electron density from the metal in the case of hydronium, and (2) unphysical alignment of the water molecular orbitals with the metal Fermi level that leads to charge transfer between the metal and solvent in the case of hydroxide. These results suggest that GGA provides an accurate description of the electrochemical interface and also have significant implications on how we treat the energetics of transporting a charged species in bulk solution to the outer Helmholtz plane.

Charge of an ion in the outer Helmholtz plane

253rd National Meeting (2017)

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