The Oxygen Evolution Reaction at MoS2 Edge Sites: The Role of a Solvent Environment in DFT-Based Molecular Simulations.

Density functional theory (DFT) calculations are employed to study the oxygen evolution reaction (OER) on the edges of stripes of monolayer molybdenum disulfide. Experimentally, this material has been shown to evolve oxygen, albeit with low efficiency. Previous DFT studies have traced this low catalytic performance to the unfavourable adsorption energies of some reaction intermediates on the MoS2 edge sites. In this work, we study the effects of the aqueous liquid surrounding the active sites. A computational approach is used, where the solvent is modeled as a continuous medium providing a dielectric embedding of the catalyst and the reaction intermediates. A description at this level of theory can have a profound impact on the studied reactions: the calculated overpotential for the OER is lowered from 1.15 eV to 0.77 eV. It is shown that such variations in the reaction energetics are linked to the polar nature of the adsorbed intermediates, which leads to changes in the calculated electronic charge density when surrounded by water. These results underline the necessity to computationally account for solvation effects, especially in aqueous environments and when highly polar intermediates are present.

Reactivity of Cobalt-Fullerene Complexes towards Deuterium.

The adsorption of molecular deuterium (D2 ) onto charged cobalt-fullerene-complexes Con C60 + (n=1-8) is measured experimentally in a few-collision reaction cell. The reactivity is strongly size-dependent, hinting at clustering of the transition metal atoms on the fullerenes. Formation and desorption rate constants are obtained from the pressure-dependent deuterogenation curves. DFT calculations indeed find that this transition metal clustering is energetically more favorable than decorating the fullerene. For n=1, D2 is predicted to bind molecularly and for n=2 dissociative and molecular configurations are quasi-isoenergetic. For n=3-8, dissociation of D2 is thermodynamically preferred. However, reaching the ground state configuration with dissociated deuterium on the timescale of the experiment may be hindered by dissociation barriers.

Nanoalloys of Metals Which Do Not Form Bulk Alloys: The Case of Ag-Co.

Ag and Co metals do not form macroscopic solid or liquid alloys. However, AgmCon clusters have been produced in dual-target dual-laser vaporization experiments, and the same occurs for other pairs of immiscible metals. We have performed density functional calculations to shed light on this phenomenon. The main result, obtained for clusters with sizes m + n not larger than 11, is that the cohesive energies justify that those clusters can be formed starting from free Ag and Co atoms, which is the case in the vaporization experiments. At the same time, mixing of Ag and Co is difficult even at the nanoscale. This is revealed by the application of several miscibility criteria. Those two features become, nevertheless, compatible in the clusters. Even if the cluster sizes considered are small, the emerging trend becomes clear: Co atoms form a core in the inner part, surrounded by a shell of Ag atoms on the surface. A consequence is that core-shell clusters can be formed from pairs of metals that do not form macroscopic alloys. The magnetic moments µ of the AgmCon clusters are due mainly to the Co atoms, and the presence of Ag induces a reduction in the magnitude of µ.

Hydrogen divacancy diffusion: a new perspective on H migration in MgH2 materials for energy storage.

The formation and diffusion of pairs of hydrogen vacancies (divacancies) in magnesium hydride is modeled using density functional theory. Compared to the commonly studied case of single hydrogen vacancies, it is found that divacancies are energetically favored over two isolated vacancies. Also, as a function of the diffusion axis considered, the calculated diffusion barriers of divacancies are either smaller or of comparable magnitude with respect to the diffusion barriers of a single vacancy. These findings shed new light on hydrogen transport in MgH2, which is of crucial importance to understand the kinetics of hydrogen take-up and release in this storage material.