Water Structures at Metal Electrodes Studied by Ab Initio Molecular Dynamics Simulations

Abstract

The structure of water on metal electrodes is addressed based on first-principles calculations. Special emphasis is paid on the competition between water-metal and water-water interaction as the structure determining factors. Thus the question will be discussed whether water at metal surfaces is ice-or rather liquid-like. The proper description of liquid phases requires to perform thermal averages. This has been done by combining first-principles electronic structure calculations with molecular dynamics simulations. After reviewing recent studies about water on flat, stepped and pre-covered metal electrodes, some new results will be presented.

Water adsorption on bimetallic PtRu/Pt(111) surface alloys

“…Interestingly enough, it is 37 meV per water molecule more stable to have the oxygen-bounded water molecules above the Ru site, but the H-down water molecules above the Pt sites ( figure 2b). Apparently, the water-water binding is stronger when the other water molecules are above the less strongly interacting Pt sites [21]. Still, the adsorption is weaker than on Pt 2 Ru 1 /Pt(111) although the same local adsorption geometries are realized.…”

“…the boundary between an ordered and a disordered phase. This importance has motivated a multitude of experimental [13,14] and theoretical [15][16][17][18][19][20][21][22] studies addressing the structure of metal-water interfaces. As far as closed-packed hexagonal metal electrodes are concerned, ab initio molecular dynamics simulations indicate that at room temperature water does not remain in an ice-like hexagonal structure but rather becomes disordered, in particular, with respect to the orientation of the water molecules [18].…”

Structural characterization of water-metal interfaces

“…Hydrogen production through the splitting of water at a metal surface is of great interest for solar cell devices [1], and generating hydrogen and oxygen gas has been proposed as a means to store energy [2]. Despite significant efforts both computationally [3][4][5][6][7][8][9][10][11][12][13][14][15][16] and experimentally [14,[17][18][19] to study electrode-water interfaces, to-date, there has yet to be an explicit treatment of liquid water next to a realistic, catalytic surface computed at the level of accurate, first principles molecular dynamics. Almost 10 years ago, Cicero et al [3] studied several liquid water-graphene interfaces as well as water confined in carbon nanotubes with differing radii using DFT with the PBE exchange correlation functional [20].…”

Some challenges in the first-principles modeling of structures and processes in electrochemical energy storage and transfer

“…Such a high halide coverage has a significant impact on the properties of the electrode/ electrolyte interface as it for example displaces water layers away from the metal surface [40]. For aqueous electrolytes, the pH value of the electrolyte plays an important role, as the concentration of protons in the electrolyte can lead to a hydrogen layer on the electrode.…”

“…The electrolyte can be described explicitly in an atomistic model or implicitly through a polarizable medium [44]. We are currently testing an implicit solvent model [45] implemented into the VASP code and compare it with results obtained for an explicit representation of an aqueous electrolyte [40]. Table 1 compares calculated results for standard electrode potentials in water with experimental values [46].…”