(537b) Saddle Point Searches in Electrochemical Reactions

Electrochemical reactions have gained considerable interest as potential solutions for renewable fuels and energy storage. An accurate computational model of electrochemical systems remains challenging, due in part to the presence of a solvent and the need for charged interfaces. Moreover, electrode potentials are governed by an external circuit, which keeps the electrochemical potential of the electrode constant throughout the reaction. Conventional density functional theory (DFT) is carried out within a canonical framework, where the number of electrons is fixed. This constant-charge approach results in a potential bias in the course of an electrochemical reaction since the surface charge of the electrode is depleted [1], and practical models to overcome this constant-electron situation have no obvious energy-referencing scheme.

The nudged elastic band (NEB) method has proven useful in evaluating minimum energy pathways (MEP) in chemical reactions. One can devise methods that continuously vary the number of electrons or the local field in the unit cell to keep the chemical potential of the electrode constant. This presents another difficulty: on-site energies of systems with different numbers of electrons cannot be directly compared since the energy reference varies between images. To circumvent this issue, we introduce a force integration method within the NEB method. This method relies solely on canonical force components parallel to the MEP in each image in the band. As the resolution of images increases, the potential energy surface approaches that of a grand-canonical system, and the path-independence of the potential energy means that the interpolated images need not lie on the minimum energy pathway. We employ this method to study the kinetics of the proton discharge step of the hydrogen evolution reaction.

[1] Skúlason, E.; Tripkovic, V.; Björketun, M. E.; Gudmundsdóttir, S.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jónsson, H.; Nørskov, J. K. J. Phys. Chem. C 2010, 114 (42), 18182–18197.