(530c) Ab Inio Insights into the Electrochemical Double Layer

An ongoing challenge in computational electrochemistry
is the accurate determination of electrochemical barriers due to the large cost
associated with using adequately-sized cells to eliminate any artifacts
contributing to the final result. A recent publication from our group provides
a simple method to obtain these barriers more efficiently by employing a
capacitor model.¹ DFT calculations using
this capacitor model have demonstrated, contrary to accepted knowledge, that
the charge of a hydronium (H₃O⁺) ion in the outer
Helmholtz plane is not +1, but closer to +0.6. Thus, it is the goal of the
current project to understand whether this is a physical phenomenon and the
implications thereof on how we fundamentally treat electrochemical barriers.

Figure 1. Cumulative
charge difference between the protonated system (Pt slab + water + H) and the
system with one fewer H (Pt slab + water) as a function of the z-coordinate
of the unit cell, calculated with various functionals. Including various
amounts of exact exchange only increases the electron density very slightly on
the metal compared to GGA-level DFT. At the metal surface on the side of the
solvent, there is a difference of 0.6 electrons, implying that the solvated H₃O⁺
has a +0.6 charge instead of +1 in the first Helmholtz layer.

(1)       Chan, K.; Norskov,
J. K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett. 2015,
6 (14), 2663-2668.