Flat Ni Surface Research Articles

Ultrashort pulse laser-structured nickel surfaces as hydrogen evolution electrodes for alkaline water electrolysis

In this study, we report on micro- and nanostructured Ni surfaces produced by an ultrashort pulse laser process as cathode materials for the alkaline electrolysis of water. We studied the influence of the laser-induced microstructure and surface morphology as well as a cyclic voltammetric activation process on the electrochemical activity of the hydrogen evolution reaction. Galvanostatic techniques, steady-state polarization curves to attain Tafel parameters and capacitance calculations via electrochemical impedance spectroscopy were used to analyze the electrodes. The analyses reveal that the ultrashort pulse laser process increases the specific surface on formerly flat Ni surfaces. Further, the cyclic voltammetric activation process gives rise to an increased intrinsic activity. Both effects lead to a strongly reduced overpotential value. This work demonstrates that different processes can be combined to dramatically boost the activity of Ni electrodes for the hydrogen evolution reaction.

Theoretical study of methane adsorption on perfect and defective Ni(1 1 1) surfaces

We employ periodic density functional theory (DFT-GGA-PW91) calculations to study the adsorption of CH 4, on a perfect and defective (1 1 1) face of nickel, at a coverage of 0.25 monolayer (ML). As a surface defect, we consider a Ni adatom. We investigate systematically the site preference for CH 4, for various molecular orientations with 1, 2 or 3 H pointing toward the surface. Whatever the CH 4 adsorption site could be, the most stable configurations are obtained when 2 H atoms are directed to the surface. CH 4 stabilises weakly on the flat Ni surface, the adsorption energies being at best in the 50 meV range. However, beside a dominating physical interaction, some features are indicative of a chemical interaction through the Ni d-band. In presence of a Ni adatom, the chemical nature of the interaction manifests plainly, with sizeable adsorption energies up to 0.37 eV. The molecular restructuring and the mechanism of the interaction are examined.

Chemisorption and diffusion of hydrogen on surface and subsurface sites of flat and stepped nickel surfaces

Plane-wave density functional theory calculations were performed to investigate the binding and diffusion of hydrogen on three flat Ni surfaces, Ni(100), Ni(110), and Ni(111), and two stepped Ni surfaces, Ni(210) and Ni(531). On each surface, the favored adsorption sites were identified by considering the energy and stability of various binding sites and zero-point energy corrections were computed. Binding energies are compared with experimental and theoretical results from the literature. Good agreement with experimental and previous theoretical data is found. At surface coverages where adsorbate-adsorbate interactions are relatively weak, the binding energy of H is similar on the five Ni surfaces studied. Favorable binding energies are observed for stable surface sites, while subsurface sites have unfavorable values relative to the gas phase molecular hydrogen. Minimum energy paths for hydrogen diffusion on Ni surfaces and into subsurface sites were constructed.

Atomic-scale study of friction and energy dissipation

This paper presents an analysis of the interaction energy and various forces between two surfaces, and the microscopic study of friction. Atomic-scale simulations of dry sliding friction and boundary lubrication are based on the classical molecular dynamics (CMD) calculations using realistic empirical potentials. The dry sliding of a single metal asperity on an incommensurate substrate surface exhibits a quasi-periodic variation of the lateral force with two different stick–slip stage involving two structural transformation followed by a wear. The contact area of the asperity increases discontinuously with increasing normal force. Xe atoms placed between two atomically flat Ni surfaces screen the Ni–Ni interaction, decrease the corrugation of the potential energy as well as the friction force at submonolayer coverage. We present a phononic model of energy dissipation from an asperity to the substrates.

Effects of Hydrogen on the Three-Dimensional Epitaxial Growth of Ni(100), (110), and (111)

The effects of hydrogen on Ni homoepitaxy have been investigated by examining rate constants for mobility of Ni during the Ni-growth process with and without the presence of a small amount of hydrogen impurity which is found to act primarily as a catalyst. The rate constant activation energies are determined by classical-potential total-energy calculations with semiclassical zero-point energy corrections for the hydrogen atom. This work extends previous work in which we have examined the effects of H on submonolayer (two-dimensional) Ni(100) growth in Haug, K.; Zhang, Z.; John, D.; Walters, C. F.; Zehner, D. M.; Plummer, W. E. Phys. Rev. B. 1997, 55, R10233−R10236 and Haug, K.; Do, N. K. N. Phys. Rev. B. 1999, 60, 11095. We find that fast diffusion of H atoms occurs on the flat Ni surfaces and the presence of these highly mobile H atoms is found to have significant effects on the mobility of lone Ni adatoms on Ni(100), but not on the Ni(110) and Ni(111) surfaces. The H atoms are also found to have significant catalytic effects lowering the step edge (Ehrlich−Schwoebel descent) barrier and therefore accelerating breakdown of three-dimensional Ni islands which form during the epitaxial growth on the Ni(100) surfaces, and to a very modest extent on the Ni(111) surface, but this net catalytic effect is not found on the Ni(110) surfaces. While the overall H atom effects on the Ni mobility in these cases are primarily catalytic, the kinetically determined Ni island morphologies may differ substantially over time periods which are long on the deposition time scale, and therefore the morphology differences can become frozen in place.

Application of discrete transform methods to reactive scattering in curvilinear coordinates Dissociative adsorption of H2 on Ni

The dissociative adsorption of H 2 over a flat Ni surface is modelled quantum mechanically via a time-dependent pseudo-spectralapproach which uses transform methods in curvilinear coordinates. These discrete angular (Legendre) and radial (Bessel) transforms, which have not previously been applied to reactive scattering problems, are briefly reviewed. Results are presented for the dissociation probability of H 2 as a function of the molecule's initial translational energy and rotational state. Our approach is compared with other formulations of reactive scattering, and the inclusion of additional degrees of freedom is discussed. The application of these methods to Eley–Rideal reactions is examined.