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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Mg(VS2)4 by Materials Project

Mg(VS2)4 crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent S2- atoms to form MgS6 octahedra that share corners with six equivalent VS6 octahedra and edges with six equivalent VS6 octahedra. The corner-sharing octahedral tilt angles are 2°. All Mg–S bond lengths are 2.51 Å. There are two inequivalent V+3.50+ sites. In the first V+3.50+ site, V+3.50+ is bonded to six equivalent S2- atoms to form VS6 octahedra that share corners with six equivalent MgS6 octahedra and edges with six equivalent VS6 octahedra. The corner-sharing octahedral tilt angles are 2°. All V–S bond lengths are 2.40 Å. In the second V+3.50+ site, V+3.50+ is bonded to six S2- atoms to form VS6 octahedra that share edges with two equivalent MgS6 octahedra and edges with six VS6 octahedra. There are two shorter (2.40 Å) and four longer (2.42 Å) V–S bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three V+3.50+ atoms. In the second S2- site, S2- is bonded in a distorted T-shaped geometry to three equivalent V+3.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(VS2)4 by Materials Project

Mg(VS2)4 crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent S2- atoms to form MgS6 octahedra that share corners with six equivalent VS6 octahedra and edges with six equivalent VS6 octahedra. The corner-sharing octahedral tilt angles are 5°. All Mg–S bond lengths are 2.57 Å. There are two inequivalent V+3.50+ sites. In the first V+3.50+ site, V+3.50+ is bonded to six S2- atoms to form VS6 octahedra that share edges with two equivalent MgS6 octahedra and edges with six VS6 octahedra. There are two shorter (2.37 Å) and four longer (2.39 Å) V–S bond lengths. In the second V+3.50+ site, V+3.50+ is bonded to six equivalent S2- atoms to form VS6 octahedra that share corners with six equivalent MgS6 octahedra and edges with six equivalent VS6 octahedra. The corner-sharing octahedral tilt angles are 5°. All V–S bond lengths are 2.38 Å. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three V+3.50+ atoms. In the second S2- site, S2- is bonded in a distorted T-shaped geometry to three equivalent V+3.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Nitrate-To-Ammonia Electroconversion at Neutral pH on Polycrystalline Vanadium Sulfide Derived from Vanadium Disulfide

The electrochemical nitrate reduction reaction (NO3RR) offers a pathway to produce NH3 for fuel and fertilizer from waste NO3-. In this work, a polycrystalline vanadium sulfide (VSx), which is derived from solvothermally grown and annealed VS2, is shown to exhibit excellent NO3RR activity (2.3 +- 0.6 mg.cm-2 geo..h-1 @ -0.92 VRHE) and Faradaic efficiency to NH4+ (69 +- 6% at -0.69 VRHE) in buffered neutral pH electrolyte containing 0.1 M NO3-. A variety of characterization techniques are leveraged to support the VSx assignment, including X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, selected area electron diffraction, and X-ray diffraction measurements. The VS2 annealing step reduces the oxide character and generates VSx, which, based on the improved NO3RR activity, results in the creation of active sites for NO3- binding. To help shed light on NO3RR on VSx, VS2 is used as a model system, and a grand-canonical density functional theory (GC-DFT) investigation of VS2 shows strong evidence that S vacancies are active sites for NO3RR, where NO3- outcompetes H+ for adsorption at the S-vacancy sites. Moreover, GC-DFT results highlight a thermodynamically favorable reaction to generate NH4+ in an aqueous electrolyte at relevant cathodic potentials. As an annealed material, VSx may contain undersaturated V sites, which show an electronic structure similar to the theoretically calculated S-vacancy site of VS2, and these sites may contribute to the observed increase in NO3RR activity and selectivity for NH4+ on VSx versus unannealed VS2. Finally, kinetic isotope effect measurements suggest that the kinetic rate-limiting step of the NO3RR on VSx is not proton-coupled, indicating it may be the first electron transfer to adsorbed NO3*.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗