Project

# Electric double layer

Goal: Atomic modeling of the electric double layer using static bilayer configurations, implicit solvent methods, and ab initio molecular dynamics simulations and establishing concrete understandings of the double layer based on the model electrolyte/electrode interfaces.

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## Project log

This article reviews recent forays in theoretical modeling of the double layer structure at electrode/electrolyte interfaces by current atomistic and continuum approaches. We will briefly discuss progress in both approaches and present a perspective on how to better describe the electric double layer by combining the unique advantages of each method. First-principles atomistic approaches provide the most detailed insights into the electronic and geometric structure of electrode/electrolyte interfaces. However, they are numerically too demanding to allow for a systematic investigation of the electric double layers over a wide range of electrochemical conditions. Yet, they can provide valuable input for continuum approaches that can capture the influence of the electrochemical environment on a larger length and time scale due to their numerical efficiency. However, continuum approaches rely on reliable input parameters. Conversely, continuum methods can provide a preselection of interface structures and conditions to be further studied on the atomistic level.
Structures and processes at water/metal interfaces play an important technological role in electrochemical energy conversion and storage, photoconversion, sensors, and corrosion, just to name a few. However, they are also of fundamental significance as a model system for the study of solid−liquid interfaces, which requires combining concepts from the chemistry and physics of crystalline materials and liquids. Particularly interesting is the fact that the water−water and water−metal interactions are of similar strength so that the structures at water/metal interfaces result from a competition between these comparable interactions. Because water is a polar molecule and water and metal surfaces are both polarizable, explicit consideration of the electronic degrees of freedom at water/metal interfaces is mandatory. In principle, ab initio molecular dynamics simulations are thus the method of choice to model water/metal interfaces, but they are computationally still rather demanding. Here, ab initio simulations of water/metal interfaces will be reviewed, starting from static systems such as the adsorption of single water molecules, water clusters, and icelike layers, followed by the properties of liquid water layers at metal surfaces. Technical issues such as the appropriate first-principles description of the water−water and water−metal interactions will be discussed, and electrochemical aspects will be addressed. Finally, more approximate but numerically less demanding approaches to treat water at metal surfaces from first-principles will be briefly discussed.
The theoretical modeling of the double layer structure at electrode/electrolyte interfaces by current atomistic and continuum approaches is reviewed. We will briefly discuss recent progress in both approaches and present a perspective on how to better describe the electric double layer by exchanging the unique advantages of each method. First-principles atomistic approaches provide detailed insights into the electronic and geometric structure of electrode/electrolyte interfaces. However, they are numerically too demanding to allow a systematic study of the properties of electric double layers for a wide range of electrochemical conditions. Still they can provide valuable input for continuum approaches which due to their numerical efficiency can capture the influence of the electrochemical environment on larger length and time scale. Still, these methods rely on reliable input parameters. Conversely, continuum methods can provide a preselection of interface structures and conditions to be further studied on the atomistic level.
Structures and processes at water/metal interfaces play an important technological role in electro-chemical energy conversion and storage, photoconversion, sensors or corrosion, just to name a few. However, they are also of fundamental significance as a model system for the study of solid-liquid interfaces which requires to combine concepts from chemistry and physics of crystalline materials and of liquids. Particularly interesting is the fact that the water-water and the water-metal interaction are of similar strength so that the structures at water/metal interfaces result from a competition between these comparable interactions. As water is a polar molecule and water and metal surfaces are both polarizable, furthermore the explicit consideration of the electronic degrees of freedom at water/metal interfaces is mandatory. In principle, ab initio molecular dynamics simulations are thus the method of choice to model water/metal interfaces, but they are computationally still rather demanding. Here, ab initio simulations of water/metal interfaces will be reviewed, starting from static systems such as the adsorption of single water molecules, water clusters and ice-like layers, followed by the properties of liquid water layers at metal surfaces. Technical issues such as the appropriate first-principles description of the water-water and the water-metal interaction will be discussed, also electrochemical aspects will addressed. Finally, more approximate, but numerically less demanding approaches to treat water at metal surfaces from first principles will be briefly discussed.
Semi-tutorial talk about theoretical methods to describe electrode/electrolyte interfaces
We have performed density functional theory calculations to explore the possibility to overcome the linear scaling relations in the oxygen reduction reaction (ORR) using local inhomogeneities on Pt-based surface alloys, supported Pt monolayers, and Pt islands. We demonstrate that invoking inequivalent neighboring reaction sites allows overcoming the restrictions of one-dimensional linear scaling relations. As a consequence, the ORR activity at a (111) edge site of a Pt island on a Ru(0001) substrate outperforms the one on flat Pt(111). Furthermore, we show that it is critical to appropriately take the electrochemical environment into account, including the proper surface coverage and the presence of the electrolyte, which leads to microscopically modified ORR reaction schemes and a redetermination of the rate-limiting step.
The influence of steps and island edges on the local electronic structure of a (bi-)metallic single crystalline electrode surface and on the local, site-specific adsorption energy of adsorbed species, the so-called structural effects, was studied by periodic density functional theory based calculations, focusing on longer-range effects. Using hydrogen adsorption energies as a local probe, calculations were performed both for partly Pt monolayer covered planar Ru(0001) surfaces and for a stepped Ru(1019\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$10\bar {19}$\end{document}) surface decorated with a row of Pt atoms. The calculations demonstrate that the steps/island edges affect not only the nearest neighbor adsorption sites but also more distant ones with the extent depending on the particular structure. This longer-range effect is in excellent agreement with recent temperature-programmed desorption and spectroscopy experiments (Hartmann et al. Phys. Chem. Chem. Phys. 14, 10919, 2012). For the interaction of water molecules with partly Pt monolayer covered Ru(0001), similar trends as in the hydrogen adsorption have been found. In addition, hydrogen adsorption energies as a function of coverage have been used to derive the hydrogen coverage as a function of the electrode potential, exhibiting a broad range of stable hydrogen adsorption structures. Graphical AbstractLocal adsorption properties of Pt monolayer island modified Ru(0001) electrodes are studied by first-principles calculations