Understanding the apparent fractional charge of protons in the aqueous electrochemical double layer

Abstract

A detailed at.-scale description of the electrochem. interface is essential to the understanding of electrochem. Energy transformations. In this work, we investigate the charge of solvated protons at the Pt(111) | H_2O and Al(111) | H_2O interfaces. Using semi-local d.-functional theory as well as hybrid functionals and embedded correlated wavefunction methods as higher-level benchmarks, we show that the effective charge of a solvated proton in the electrochem. double layer or outer Helmholtz plane at all levels of theory is fractional, when the solvated proton and solvent band edges are aligned correctly with the Fermi level of the metal (E_F). The obsd. fractional charge in the absence of frontier band misalignment arises from a significant overlap between the proton and the electron d. from the metal surface, and results in an energetic difference between protons in bulk soln. and those in the outer Helmholtz plane.