The Electrochemical Interface at the Atomic-Scale

There is a greater need for catalysts than ever before; we are facing an inevitable transition from fossil fuels to renewable energy sources, where particularly ”Power-to-X” is expected to become crucial. X can represent a myriad of chemicals and synthetic fuels, traditionally extracted from oil, but X can also stand for hydrogen. Regardless of what X symbolizes, electrocatalysis is a critical component in the Power-to-X processes.

Electrocatalysis is necessary when CO or CO2 is converted into valuable chemicals, and electrocatalysis also plays a crucial role in storing energy in the chemical bond of the hydrogen molecule.

To ensure optimal electrochemical processes, it is important that the catalyst on which the reaction occurs is optimal. One method to fine-tune the catalyst involves the use of binary alloys. In this thesis, I investigate how these binary alloys can be modeled and optimized through calculations based on density functional theory and statistical models.

Traditionally, calculations are performed in gas-phase, but how do the molecules and ions in the electrolyte affect the electrochemical reactions? In this thesis, I also examine the interface between the catalyst and the surrounding electrolyte by combining electrochemical experiments with computer simulations.

A key electrochemical experiment is cyclic voltammetry, which provides a qualitative insight into the processes occurring at the interface. To quantify and study these processes, an accurate computer model is crucial. However, since the atoms in the interface are dynamic, and the number of atoms varies with the applied potential, modeling the system poses a challenge.

In Chapter 4, I present a model that allows for the modeling of the interface. Instead of letting the number and structure of atoms depend on the applied potential, the potential varies as a function of the number of atoms and their structure. In accordance with the variational principle, we can thus determine the ground state of the interface at a specific potential. To achieve different interface structures in a potential range, we employ ab initio molecular dynamics simulations. By using a generalization of the computer hydrogen electrode (see Chapter 3), the representation of the interface can also include cations and anions at a specific pH. The studies in Chapter 4 and 5 demonstrate how important these ”spectator” ions are for quantifying the signals in the experimental results and for understanding the contradictory trends observed in the electrocatalytic production of hydrogen.