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## International Workshop on Computational Electrochemistry

#### Location: CECAM-FI

#### Organisers

*For up-to-date information on this workshop, please visit the official website:* iwce2018.computational-electrochemistry.org

Plenty of work has been done on atomistic calculations of surfaces, interfaces and molecular adsorption. Even local and semilocal exchange-correlation density functionals can yield quantitatively accurate adsorption energies for a number of systems. However, this good description does not extend to surfaces with a net charge or where the position of the Fermi level is a tunable parameter. This imposes a serious drawback to modeling electrochemical interfaces where the effect of the potential is explicitly taken into account. In practice, one needs to resort to workarounds. Seminal work was done in the 2000s, most notably by the Nørskov group in Denmark. Several approaches have been explored, for instance reviewed early on by Rossmeisl et al. [1] for water splitting on metal electrodes. There is a very recent review by Skúlason and Jónsson [2] which summarizes the state of the art in the field.

The simplest solution is to include potential differences only implicitly as an energy shift, neglecting changes in the electronic structure of the system induced by the potential. Another way is through the inclusion of a constant external electric field within the electrode-water interface, neglecting local variations of the electric field. One can also introduce an explicit excess charge by changing the number of electrons in the calculation, which spontaneously accumulate on the metal surface. While in principle this would represent the solution closest to experiment, in practice the excess charge needs to be compensated with a homogeneous background of the opposite sign, leading to numerical artifacts and other problems. A different approach is to charge the electrode indirectly by introducing hydrogen atoms adsorbed on the surface metal sites. This approach requires, however, that the adsorbed species be able to provide the charge necessary to shift the potential. Work on this topic using thermodynamic integration has also been done by the Sprik group [3], who rely on a "computational hydrogen electrode", which in principle allows direct comparison of redox potentials with experiment.

All of these approaches have their own advantages and inconsistencies: the definition of computational electrochemical potential itself and the precise link to experiment is somewhat blurry. A main task of this workshop will be to gather together these different views and achieve a consensus on how each of them

does or does not provide a satisfactory connection with experimental electrochemical potentials.

The explicit description of charge transfer deserves an aside mention. Much work in recent years focused on constrained-DFT and its application to study electron transfer reactions, for instance via DFT-based parametrization of Marcus theory. Here the challenges are usually related to implementation and the (in)ability of local and semi-local density functionals to accurately describe charged molecules. The possibility to apply methodologies based on non-equilibrium Green's functions to treat electrode/solution interfaces in a similar way to semiconductor/metal interfaces also emerges as an attractive prospect. However, this possibility has not been explored yet. This workshop will provide an outstanding opportunity to have a good look at these problems.

The topics in computational electrochemistry that will be covered during the workshop are the following:

• Electron transfer, including new theoretical methodologies (Green’s functions, constrained DFT, ...). We will discuss about new implementations of constrained DFT (CP2K, GPAW codes) and their applicability to, for example, parametrize Marcus theory. We will also discuss the applicability of transport methodology, such as non-equilibrium Green’s function formalism (e.g., the QuantumWise implementation), to be extended from the semiconductor/metal framework to the electrolyte/electrode framework.

• Solute/electrode interaction. We will look at the theoretical and computational description of molecular adsorption (inner sphere redox reactions), how the electrode morphology affects the adsorption energies (e.g., different crystal facets), the role of electrode potential on adsorption, developments in transition state theory to explain catalytic properties of surfaces, and the ability of new methodologies, such as machine learning, to efficiently optimize structures within the huge parameter space of molecular adsorption.

• Solvent/electrode interface. We will tackle the problem of accurately describing the electrical double layer under different electrode potentials and electrode morphologies. The need for and domain of applicability of different methodological frameworks (e.g., molecular dynamics vs. optimized geometries) will be discussed.

• Solvent effects. We will look at the effect of solvation sheets and solvation free energies on redox potentials for outer-sphere complexes. In particular, we will debate under which conditions simple continuum solvation models are enough and under which conditions explicit solvation with atomistic resolution is required. QM/MM methodologies will be explored here, on how to accurately account for the first solvation layers and incorporate the "bulk" solvent effects at a lower level of theory.

• Effect of pH and solute concentration. Some effects cannot be directly simulated due to the scales involved, these are for instance incorporation of pH values when the proton concentration is lower than 1/100 or 1/1000 (which is the case for pH > 1). Indirect methods are therefore required to estimate pH effects on phase-stability and coexisting phases, compute Pourbaix diagrams, or calculate pKa values. Such indirect strategies and their degree of accuracy will be discussed. How to compute the effect of solute concentration on the reaction kinetics will be another topic of discussion.

• Surface charge and potential tunability. We will explore the ability and limitations of different ways to incorporate surface charge and Fermi level changes into atomistic simulations of the electrode/electrolyte interface. We will look at indirect methods of charging the surface such as counter-ions/adsorbates and novel direct methods such as modified pseudopotentials and Green's functions approaches.

• Electrochemical scales. Direct and indirect ways to align computed redox levels with reference electrochemical scales will be discussed. For example, the applicability and reliability of "computational hydrogen electrode" (indirect) and the use of electrostatic potentials and vacuum offsets (direct) will be discussed.

• Multiscale approaches. We will look at how QM/MM and other methods to couple different length scales can be applied to study the electrode/electrolyte interface, to model the band bending in semiconductor electrodes, etc.

• Non-aqueous electrochemistry, liquid/liquid interfaces and other exotic electrochemistry problems. Electrochemistry in solvents other than water (e.g., acetonitrile) and exotic electrochemical problems which need radically different approaches, such as water/organic solvent electrochemical interfaces, will also be discussed during the workshop.

## References

**Finland**

Miguel Caro (Aalto University) - Organiser

Kari Laasonen (Aalto University) - Organiser

Tomi Laurila (Aalto University) - Organiser

**Iceland**

Hannes Jónsson (University of Iceland) - Organiser