title: Development of an ab initio electrochemical cell: Understanding the dielectric properties of interfacial water and Mg dissolution from first principles
title: Development of an ab initio electrochemical cell: Understanding the dielectric properties of interfacial water and Mg dissolution from first principles
titleType: AlternativeTitle
Creators: name: Deißenbeck, Florian
Publisher: name: Philipps-Universität Marburg
Year: 2024
Subjects: DFT
Physics
Physik
Hydrogen Evolution
Water
Magnesium Dissolution
Molekulardynamik
Fully Solvated Electrode
ab initio
Magnesium
Wasserstoffentwicklung
Erste Prinzipien
Dielectric Constant
Molecular Dynamics
Dielektrizitätskonstante
Electrochemistry
Elektrochemie
Simulation
Thermopotentiostat
Wasser
First Principles
Nanoeingeschlossenes Wasser
Magnesiumauflösung
Potentiostat
Nanoconfined Water

description: Electrochemical process will play an important role in sustainable energy conversion and storage solutions. Examples are new generations of supercapacitors, metal-air-batteries, transient electronics or new concepts in sustainable metallurgy. In order to realize such disruptive innovations it is imperative to develop accurate modelling techniques and achieve an atomistic scale understanding of electrochemical processes. First principles techniques, which are free of empirical parameters, would be the optimum method.
While experiments are routinely performed at constant electrode potential, realizing these conditions in ab initio simulations has remained very challenging. At present, none of the 40+ commonly used DFT codes provides a potentiostat scheme to explore charge transfer and reactions at electrified interfaces via ab initio molecular dynamics. A viable potentiostat technique requires two constituents: (i) a robust method to either apply an electric field or charge the electrode and (ii) an algorithm to treat the electrode charge as a thermodynamic degree of freedom in order to drive the system to the desired electrode potential.
In this thesis, I introduce a novel method to include electric fields in electronic structure simulations, the fully solvated electrode (FSE). It allows us to realize applied electric fields of any distribution, shape and magnitude inside an ab initio simulation cell. In the spirit of the Helmholtz layer that is comprised of solvated ions, the FSE is able to place a fully solvated counter charge directly into the liquid phase. This counter charge compensates for the electrode charge, so that the simulation cell is charge-neutral in total, in order to enable a straightforward implementation into existing electronic structure codes.
In order to derive a potential control algorithm, the electrode charge has to be treated as a thermodynamic degree of freedom. I discuss the challenges towards achieving potential control in electronic structure simulations and derive a canonical thermopotentiostat algorithm in order to determine the flow of charge into or out of the simulation cell at constant electrode potential conditions.
Both the FSE and the thermopotentiostat can be straightforwardly included into standard ab initio DFT packages via a flexible, extendable python-interface which exposes the internal variables of standard DFT implementations to external python routines. The presented computational setup enables flexible user-defined workflows that use the DFT package as a "quantum engine". Moreover, the python interface provides a platform for future developments, such as, e.g., advanced implicit solvent models, QM/MM approaches and pH-control algorithms. In order to highlight the opportunities provided by this new approach and demonstrate its performance, I study the structure and dynamics of interfacial water at electrified surfaces at the atomistic-scale. Recent experiments demonstrated that nano-confined water features dielectric constants that are lowered by up to a factor 40, compared to bulk water. Because interfacial water serves as the environment for electrochemical reactions, understanding its screening properties is essential to study electron-transfer reactions. I reveal the origin of this strong reduction of the dielectric constant of nano-confined water via combined empirical and ab initio molecular dynamics at constant electrode potential: Interfacial water features a dielectric dead layer, within which most of the interfacial potential drop occurs and where the field is essentially unscreened. As a consequence, for sufficiently large fields water will screen the electric field via dissociation into H+ and OH-. In studies of electrochemical reactions with implicit solvents, the water dissociation effect is usually not considered, although it may considerably affect the mechanisms of electrochemical reactions.
Combining the FSE and the thermopotentiostat, I studied the dissolution of Magnesium in water under externally applied fields. Contrary to existing models, the simulations demonstrate that water is not only a spectator, but acts as an active reactant. Moreover, the identified mechanism provides new insights towards the origin of the anomalous hydrogen-evolution reaction (HER) on Magnesium under anodic conditions.
The developments presented in this thesis provide a ready-to-use simulation package. They open the door towards simulations of electrochemical and electro-catalytic processes from first principles to the broad community of computational scientists.