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

Authors: 
Chen, L. D., SUNCAT Center for Interface Science and Catalysis, Stanford University and SLAC National Accelerator Laboratory
Norskov, J. K., SUNCAT Center for Interface Science and Catalysis, Stanford University and SLAC National Accelerator Laboratory
Bajdich, M., SLAC National Accelerator Laboratory
Chan, K., Stanford University
Luntz, A. C., IBM Almaden

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.