First-Principles Calculations of the Electrochemical Reactions of Water at an Immersed Ni„111.../H2O Interface

Electrochemical processes occurring in aqueous solutions are critically dependent upon the interaction between the metal electrode and the solvent. In this work, density functional theory is used to calculate the potentials for which molecular water and its activation products adsorbed hydrogen and hydroxide are stable when in contact with an immersed Ni 111 electrode. The adsorption geometries of water and its dissociation products are also determined as functions of potential. At zero kelvin, water activates to form a surface hydroxide overlayer at potentials anodic of −0.5 V vs a normal hydrogen electrode NHE . The cathodic activation of water to form a surface hydride occurs at potentials negative of −0.3 V NHE. There is a potential range at which both H and OH form on the surface, in agreement with inferences made from the experimental literature. The surface hydroxide/oxide phase transition occurs at 0.2 V NHE. The increased binding of oxygen to the surface at progressively anodic potentials correlates with weakening nickel-nickel interactions and the lifting of a metal atom above the surface plane. Thermodynamic extrapolations are made to ambient 300 K and elevated 600 K temperatures and Pourbaix diagrams calculated for the inert and activated surface phases of water on Ni 111 and compared with experiment. © 2006 The Electrochemical Society. DOI: 10.1149/1.2357721 All rights reserved.