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Master i samfunnsvitenskap med fordypning i samfunnsikkerhet og terrorismestudier - 202

Ultrahigh-current-density niobium disulfide catalysts for hydrogen evolution

Metallic transition metal dichalcogenides (TMDs)1???8 are good catalysts for the hydrogen evolution reaction (HER). The overpotential and Tafel slope values of metallic phases and edges9 of two-dimensional (2D) TMDs approach those of Pt. However, the overall current density of 2D TMD catalysts remains orders of magnitude lower (~10???100 mA cm???2) than industrial Pt and Ir electrolysers (>1,000 mA cm???2)10,11. Here, we report the synthesis of the metallic 2H phase of niobium disulfide with additional niobium (2H Nb1+xS2, where x is ~0.35)12 as a HER catalyst with current densities of >5,000 mA cm???2 at ~420 mV versus a reversible hydrogen electrode. We find the exchange current density at 0 V for 2H Nb1.35S2 to be ~0.8 mA cm???2, corresponding to a turnover frequency of ~0.2 s???1. We demonstrate an electrolyser based on a 2H Nb1+ xS2 cathode that can generate current densities of 1,000 mA cm???2. Our theoretical results reveal that 2H Nb1+ xS2 with Nb-terminated surface has free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER. Therefore, 2H Nb1+ xS2 could be a viable catalyst for practical electrolysers

Watching mesoporous metal films grow during templated electrodeposition with in situ SAXS

In this paper, we monitor the real-time growth of mesoporous platinum during electrodeposition using small-angle X-ray scattering (SAXS). Previously, we have demonstrated that platinum films featuring the ‘single diamond’ (Fd3m) morphology can be produced from ‘double diamond’ (Pn3m) lipid cubic phase templates; the difference in symmetry provides additional scattering signals unique to the metal. Taking advantage of this, we present simultaneous in situ SAXS/electrochemical measurement as the platinum nanostructures grow within the lipid template. This measurement allows us to correlate the nanostructure appearance with the deposition current density and to monitor the evolution of the orientational and lateral ordering of the lipid and platinum during deposition and after template removal. In other periodic metal nanomaterials deposited within any of the normal topology liquid crystal, mesoporous silica or block copolymer templates previously published, the template and emerging metal have the same symmetry, so such a study has not been possible previously

Unusual synergistic effect in layered Ruddlesden-Popper oxide enables ultrafast hydrogen evolution

Efficient electrocatalysts for hydrogen evolution reaction are key to realize clean hydrogen production through water splitting. As an important family of functional materials, transition metal oxides are generally believed inactive towards hydrogen evolution reaction, although many of them show high activity for oxygen evolution reaction. Here we report the remarkable electrocatalytic activity for hydrogen evolution reaction of a layered metal oxide, Ruddlesden-Popper-type Sr2RuO4 with alternative perovskite layer and rock-salt SrO layer, in an alkaline solution, which is comparable to those of the best electrocatalysts ever reported. By theoretical calculations, such excellent activity is attributed mainly to an unusual synergistic effect in the layered structure, whereby the (001) SrO-terminated surface cleaved in rock-salt layer facilitates a barrier-free water dissociation while the active apical oxygen site in perovskite layer promotes favorable hydrogen adsorption and evolution. Moreover, the activity of such layered oxide can be further improved by electrochemistry-induced activation

Balancing hydrogen adsorption/desorption by orbital modulation for efficient hydrogen evolution catalysis

Hydrogen adsorption/desorption behavior plays a key role in hydrogen evolution reaction (HER) catalysis. The HER reaction rate is a trade-off between hydrogen adsorption and desorption on the catalyst surface. Herein, we report the rational balancing of hydrogen adsorption/desorption by orbital modulation using introduced environmental electronegative carbon/nitrogen (C/N) atoms. Theoretical calculations reveal that the empty d orbitals of iridium (Ir) sites can be reduced by interactions between the environmental electronegative C/N and Ir atoms. This balances the hydrogen adsorption/ desorption around the Ir sites, accelerating the related HER process. Remarkably, by anchoring a small amount of Ir nanoparticles (7.16 wt%) in nitrogenated carbon matrixes, the resulting catalyst exhibits significantly enhanced HER performance. This includs the smallest reported overpotential at 10 mA cm(-2) (4.5 mV), the highest mass activity at 10 mV (1.12 A mg(Ir)(-1)) and turnover frequency at 25 mV (4.21 H2 s(-1)) by far, outperforming Ir nanoparticles and commercial Pt/C

The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO_2 Reduction Conditions: An Experimental and Theoretical Study

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO_2 reduction (CO_2R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO_2R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO_2R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO_2R. On Ni-MoS_2, a limitation is the kinetic barrier for the proton–electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO_2R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers

The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO₂ Reduction Conditions: An Experimental and Theoretical Study

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO₂ reduction (CO₂R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO₂R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO₂R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO₂R. On Ni-MoS₂, a limitation is the kinetic barrier for the proton–electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO₂R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers

Engineering grain boundaries at the 2D limit for the hydrogen evolution reaction

Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs

Structural shape optimization using Cartesian grids and automatic h-adaptive mesh projection

[EN] We present a novel approach to 3D structural shape optimization that leans on an Immersed Boundary Method. A boundary tracking strategy based on evaluating the intersections between a fixed Cartesian grid and the evolving geometry sorts elements as internal, external and intersected. The integration procedure used by the NURBS-Enhanced Finite Element Method accurately accounts for the nonconformity between the fixed embedding discretization and the evolving structural shape, avoiding the creation of a boundary-fitted mesh for each design iteration, yielding in very efficient mesh generation process. A Cartesian hierarchical data structure improves the efficiency of the analyzes, allowing for trivial data sharing between similar entities or for an optimal reordering of thematrices for the solution of the system of equations, among other benefits. Shape optimization requires the sufficiently accurate structural analysis of a large number of different designs, presenting the computational cost for each design as a critical issue. The information required to create 3D Cartesian h- adapted mesh for new geometries is projected from previously analyzed geometries using shape sensitivity results. Then, the refinement criterion permits one to directly build h-adapted mesh on the new designs with a specified and controlled error level. Several examples are presented to show how the techniques here proposed considerably improve the computational efficiency of the optimization process.The authors wish to thank the Spanish Ministerio de Economia y Competitividad for the financial support received through the project DPI2013-46317-R and the FPI program (BES-2011-044080), and the Generalitat Valenciana through the project PROMETEO/2016/007.Marco, O.; Ródenas, J.; Albelda Vitoria, J.; Nadal, E.; Tur Valiente, M. (2017). 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