Iron-Water Interface at Different Electrochemical Conditions

Understanding the interaction between water and electrocatalyst surfaces, as well as the structure of the interface at different applied potentials, is of great importance for improving the efficiency of electrochemical processes, such as hydrogen evolution, COreduction, and nitrogen fixation. In this study, density functional theory (DFT) simulations are employed to explore the interaction of water with different crystalline facets of body-centered cubic iron─namely, Fe(100), Fe(110), and Fe(111)─under different electrochemical conditions. For that, we considered several water coverages, from an isolated molecule to a full monolayer. Simulations reveal that at low coverages, water dissociates twice, forming *O and 2 *H on the three surfaces. However, increasing coverage reduces the availability of favorable adsorption sites, lowering the energy benefits of dissociation. The hydrogen bonding between adsorbed and nonadsorbed water molecules does not depend on water coverage; thus, for the full HO monolayer, the preferred structure involves 50% dissociation for the three facets, normally without the presence of *O species on the surface. Results also show that each facet responds differently to the applied potential. According to the computed Pourbaix diagrams, under reducing potentials typical of Nor COelectroreduction, Fe(110) and Fe(111) are covered with a full hydrogen monolayer, while Fe(100) presents a lower H-coverage at the usual cathodic potentials.