The Intricacies of Computational Electrochemistry

Abstract

Computational electrochemistry is hardanybody who has ever tried will know. We argue that the reasons for its complexity lie not only in the multiscale nature of electrochemical processes but also in the rapid, ongoing method development in the field. This has resulted in a lack of clear guidelines and many open discussions in the community. These issues were also the topic of a recent Lorentz Center workshop, the key take-away messages of which are highlighted in this Perspective. In particular, we discuss why the choice between constant potential and constant charge simulations is less trivial than it may seem, why interpreting electrochemical reaction free energy diagrams can be challenging, why the Poisson-Nernst-Planck equation is not all there is, and why we desperately need more benchmarking in the field.

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Schematic of the various simulation types (atomistic, microkinetic, and transport modeling) with increasing length and time scales needed to study electrochemical processes.

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An overview of the advantages and limitations of the various constant potential and constant charge approaches available to compute electrochemical barriers. “fully explicit” and “implicit” refer to an atomistic and mean-field description of the electrolyte, respectively. See text for details.

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a) Schematic of possible causes for pseudo-initial states in simulations. The true initial state is characterized by the ion residing in a region where it does not interact with the electrode and the electric field is zero. For particles residing (too) close to the interface, charge transfer and electrostatic energy gradients can affect the reactant state and its energy. b) Sketch of a free energy diagram when using a correct initial state (green) vs a pseudo-initial state (red). The use of a pseudo-initial state will lead to incorrect barrier predictions and incorrect reaction free energies.

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A sample free energy profile for a two-step electrocatalytic oxidation of A_(g) to C_(g) with the effective barrier ΔG _eff ^‡ approximated as the largest free energy difference between the transition state and the resting state (A*). The exergonic adsorption of A_(g) results in a sizable coverage of A*, making it the resting state of the catalyst. The free energy of B* is irrelevant to the rate of the reaction as long as ΔG _A→B > 0 and its barrier is not as high as that of the next step. Finally, the free energy of C* also does not affect r, as it is located after rate-limiting step of the reaction.

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Dependence of the negative of the workfunction, W _e, of Pt(100) on (a) the number of atoms, N _A, in a N _A × N _A atom unit cell and (b) as a function of the number of layers l in the slab when only the Γ-point is considered during k-point averaging. The workfunction can change dramatically (several hundreds of meV) depending on the exact setup and can hence differ strongly from the true value. (l = 4 in panel a, and N _A = 4 in panel b.) This figure is reproduced from the Supporting Information of ref .