Publication
Electrochemical reactions involving electron/proton transfer occur in several technologically relevant environments, such as solar cells, energy conversion, and storage devices. In these systems, the electron/proton transfer is governed by the applied bias potential and leads to the occurrence of complex electrochemical reactions. While experiments provide access to most of the macroscopic electrochemical properties of these reactions, they are unable to describe the electrochemical processes occurring at the atomic scale. At this level, modelling electrochemical processes either in solution or at metal interfaces is of paramount importance towards a full control and understanding of reaction mechanisms occurring under applied bias potential. Several techniques have been developed to study systems under bias potential in combination with ab initio molecular dynamics and advanced electronic structure techniques. Among them, constant Fermi level ab initio molecular dynamics (CFL-MD) is emerging as a promising and efficient scheme. In this Chapter, we focus on the constant Fermi level molecular dynamics technique and present some applications to electrochemistry. In the methods section, we provide a detailed review of the constant Fermi level ab initio molecular dynamics technique. Then, we illustrate the performance of CFL-MD in determining redox levels of half reactions in solution. In particular, we show that redox half-reactions can be driven by varying the Fermi level within the electronic gap of liquid water, thereby revealing reaction mechanisms without any prior knowledge of the reaction products. On the basis of Janak’s theorem, we derive a scheme for determining redox potentials from the evolution of single-particle energy levels upon charging (or discharging) the system. For the considered redox couples, the calculated redox levels are found to agree closely with those obtained via the thermodynamic integration method. The agreement with experiment is consistent with the accuracy of the adopted level of theory. In the following section, we then consider the case of metal-water interfaces with a particular focus on the spontaneous occurrence of chemical reactions. Specifically, we show how chemical reactions (e.g. the Volmer reaction) can be induced using the CFL-MD technique and discuss the subsequent reaction pathway. In addition to chemical reactions, the CFL-MD offers the possibility of studying electrochemical properties of metal/water interfaces under applied bias potential. However, this requires a proper alignment of the electrode potential to the standard hydrogen electrode (SHE) level. To achieve this, we introduce an alignment procedure, which includes a correction based on classical electrostatics to account for the charge compensation in the simulation cell. As a consequence of this alignment, we extract macroscopic properties of the Pt(111)/water interface under variable bias potential. Our scheme yields a potential of zero c