Theoretical analysis of the nature of hydrogen at the electrochemical interface between water and a Ni(111) single-crystal electrode

The adsorption, absorption, and diffusion of hydrogen on and into metals are elementary processes that are important to a wide variety of electrochemical and corrosion processes. Ab initio gradient corrected density functional theoretical calculations were carried out in order to probe these processes for hydrogen at the aqueous/Ni (111) interface under well-defined electrochemical conditions. The binding of hydrogen on Ni(111) in the presence of water is considered using a fully atomistic model of the solution environment. We calculate the changes in hydrogen binding energy due to the presence of water at the interface, as well as due to changes in applied potential and surface charge. Binding energies for hydrogen at the hexagonally close-packed and octahedral sites shifted endothermically as the potential was made more anodic, indicating that reductive partial charge transfer occurs. Conversely, hydrogen binding at the tetrahedral site was found to be partially oxidizing. The calculation of vibrational modes allowed the extrapolation of ab initio results to room temperature conditions. Surface Pourbaix diagrams were constructed to predict the chemical states of hydrogen obtained over a Ni (111) single-crystal surface as a function of pH and potential. These calculations indicate that the Tafel recombination mechanism is not active for Ni(111) at 300 K, but rather the hydrogen evolution reaction proceeds by the Heyrovsky mechanism following a very short potential span of underpotential deposited hydrogen as one scans the surface cathodically. These results apply to an ideal (111) single-crystal surface in the absence of surface impurities and adsorbed ions (other than hydrogen). (c) 2007 The Electrochemical Society.