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Unveiling the pseudocapacitive charge storage mechanisms of nanostructured vanadium nitrides using in-situ analyses
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Exploring electrolyte preference of vanadium nitride supercapacitor electrodes
Highlights: • Hierarchical VN nanostructures were prepared on graphite foam. • Electrolyte preference of VN supercapacitor electrodes was explored. • VN showed better capacitive property in organic and alkaline electrolytes than LiCl. - Abstract: Vanadium nitride hierarchical nanostructures were prepared through an ammonia annealing procedure utilizing vanadium pentoxide nanostructures grown on graphite foam. The electrochemical properties of hierarchical vanadium nitride was tested in aqueous and organic electrolytes. As a result, the vanadium nitride showed better capacitive energy storage property in organic and alkaline electrolytes. This work provides insight into the charge storage process of vanadium nitride and our findings can shed light on other transition metal nitride-based electrochemical energy storage systems.
Low temperature plasma-enhanced atomic layer deposition of thin vanadium nitride layers for copper diffusion barriers
Thin vanadium nitride (VN) layers were grown by atomic layer deposition using tetrakis(ethylmethylamino)vanadium and NH{sub 3} plasma at deposition temperatures between 70 Degree-Sign C and 150 Degree-Sign C on silicon substrates and polymer foil. X-ray photoelectron spectroscopy revealed a composition close to stoichiometric VN, while x-ray diffraction showed the {delta}-VN crystal structure. The resistivity was as low as 200 {mu}{Omega} cm for the as deposited films and further reduced to 143 {mu}{Omega} cm and 93 {mu}{Omega} cm by annealing in N{sub 2} and H{sub 2}/He/N{sub 2}, respectively. A 5 nm VN layer proved to be effective as a diffusion barrier for copper up to a temperature of 720 Degree-Sign C.
Preparation of Vanadium Nitride by Magnesiothermic Reduction of V{sub 2}O{sub 3} in Nitrogen Atmosphere
The development of a highly efficient and low-cost production process is instrumental for the wide application of vanadium nitride (VN). The strong affinity of vanadium with carbon and oxygen resulted in the significant difficulty to produce VN with a high purity. In this paper, a novel production route was proposed that involves the magnesiothermic reduction and nitration of vanadium (III) oxide (V{sub 2}O{sub 3}) in nitrogen atmosphere with the assistance of magnesium chloride (MgCl{sub 2}) in the temperature range of 948 K to 1073 K (675 °C to 800 °C). Compared with the traditional methods, there was no contamination of carbon. Meanwhile, oxygen content can be controlled to a low level. Furthermore, VN could be prepared at low temperatures within a short time, which was beneficial for decreasing the particle size of VN. The phase transition and morphological evolution of samples were analyzed by X-ray diffraction and Field-Emission Scanning Electron Microscopy, respectively. The experimental results showed that high temperature assisted in decreasing the residual oxygen content in VN but can decrease the specific surface area. Oxygen content of the obtained VN powder after reacting at 1073 K (800 °C) for 3 hours was 1.36 wt pct.
Freestanding vanadium nitride nanowire membrane as an efficient, carbon-free gas diffusion cathode for Li–CO 2 batteries
A freestanding vanadium nitride nanowire (VN-NW) membrane is employed as a carbon-free gas diffusion cathode (GDC) for high-performance Li–CO 2 batteries. Li–CO 2 cells built with the VN-NW GDC exhibit excellent electrochemical performance, achieving 100 discharge/charge cycles with an ultralow round-trip overpotential of <1 V. The VN-NW GDC also delivers vastly improved capacity and rate capability compared to a conventional multiwalled carbon nanotube GDC. The morphology of the discharge products (Li 2 CO 3 + C) is greatly improved with the VN-NW GDC compared to the carbon-based cathode, allowing for facile decomposition on charge. Furthermore, the absence of carbon in the VN-NW GDC allows for easy characterization of the discharge products with a variety of techniques, including transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The impressive performance of the hierarchically structured VN-NW GDC in Li–CO 2 batteries justifies further investigation into noble metal-free and carbon-free materials for gas diffusion cathodes.
Mechanistic Insights into Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride Nanoparticles
Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, a selective electrocatalyst is lacking. Here we show that vanadium nitride (VN) nanoparticles are active, selective, and stable ENRR catalysts with an ENRR rate and a Faradaic efficiency (FE) of 3.3 × 10 –10 mol s –1 cm –2 and 6.0% at -0.1 V within 1 h, respectively. ENRR with 15 N 2 as the feed produces both 14 NH 3 and 15 NH 3 , which indicates that the reaction follows a Mars–van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified VN0.7O0.45 as the active phase in the ENRR. Operando X-ray absorption spectroscopy and catalyst durability test results corroborate this hypothesis and indicate that the conversion of VN 0.7 O0.45 to the VN phase leads to catalyst deactivation. We hypothesize that only the surface N sites adjacent to a surface O are active in the ENRR. Finally, an ammonia production rate of 1.1 × 10 –10 mol s –1 cm –2 can be maintained for 116 h, with a steady-state turnover number of 431.
Evolution dependence of vanadium nitride nanoprecipitates on directionality of ion irradiation
The influence of the directionality of Fe 2+ ion irradiation on the evolution of vanadium nitride platelet–shaped nanoprecipitates at 500 °C was investigated in this paper in a ferritic alloy using transmission electron microscopy. When the ion-irradiation direction was approximately aligned with the initial particle length, particles grew longer and sectioned into shorter lengths at higher doses, resulting in increased particle densities. As ion-irradiation direction deviated from particle-length direction, some particles sectioned lengthwise and then dissolved, resulting in decreased particle densities. Surviving particles were transformed into parallelograms with a different orientation relationship with the matrix. Finally, nanoprecipitate evolution dependence on beam-nanoprecipitate orientation is a process that may be different from reactor irradiation.
Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride
Abstract Despite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady‐state active sites on ENRR catalysts that follow the Mars–van Krevelen mechanism via quantitative isotope‐exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active‐site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate‐limiting step in the ENRR is likely the initial N≡N bond activation via the addition of a proton and an electron to the adsorbed N 2 on the N vacancies to form N 2 H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR.
2D molybdenum and vanadium nitrides synthesized by ammoniation of 2D transition metal carbides (MXenes)
MXenes are a rapidly growing class of 2D transition metal carbides and nitrides, finding applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while more than 20 carbide MXenes have already been synthesized, Ti 4 N 3 and Ti 2 N are the only nitride MXenes reported so far. Here by ammoniation of Mo 2 CT x and V 2 CT x MXenes at 600 °C, we report on their transformation to 2D metal nitrides. Carbon atoms in the precursor MXenes are replaced with N atoms, resulting from the decomposition of ammonia molecules. The crystal structures of the resulting Mo 2 N and V 2 N were determined with transmission electron microscopy and X-ray pair distribution function analysis. Our results indicate that Mo 2 N retains the MXene structure and V 2 C transforms to a mixed layered structure of trigonal V 2 N and cubic VN. Temperature-dependent resistivity measurements of the nitrides reveal that they exhibit metallic conductivity, as opposed to semiconductor-like behavior of their parent carbides. As important, room-temperature electrical conductivity values of Mo2N and V2N are three and one order of magnitude larger than those of the Mo 2 CT x and V 2 CT x precursors, respectively. In conclusion, this study shows how gas treatment synthesis such as ammoniation can transform carbide MXenes into 2D nitrides with higher electrical conductivities and metallic behavior, opening a new avenue in 2D materials synthesis.
First-principles study of phase stability of stoichiometric vanadium nitrides
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Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium Nitride Having a V 3 N 4 Double-Diamond Core
Here, transmetallation of [VCl 3 (THF) 3 ] and [TlTp tBu,Me ] afforded [(Tp tBu,Me )VCl 2 ] (1, Tp tBu,Me = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC 8 to form a $C_{3v}$ symmetric V II complex, [(Tp tBu,Me )VCl] (2). Complex 1 has a high-spin ($\textit{S}$ = 1) ground state and displays rhombic high-frequency and -field electron paramagnetic resonance (HFEPR) spectra, while complex 2 has an $\textit{S}$ = 3/2 4 A 2 ground state observable by conventional EPR spectroscopy. Complex 1 reacts with NaN 3 to form the V V nitride-azide complex [(Tp tBu,Me )V≡N(N 3 )] (3). A likely V III azide intermediate en route to 3, [(Tp tBu,Me )VCl(N 3 )] (4), was isolated by reacting 1 with N 3 SiMe 3 . Complex 4 is thermally stable but reacts with NaN3 to form 3, implying a bis-azide intermediate, [(Tp tBu,Me )V(N 3 ) 2 ] (A), leading to 3. Reduction of 3 with KC 8 furnishes a trinuclear and mixed-valent nitride, [{(Tp tBu,Me )V} 2 ($μ_{4-}$VN 4 )] (5), conforming to a Robin–Day class I description. Complex 5 features a central vanadium ion supported only by bridging nitride ligands. Contrary to 1, complex 2 reacts with NaN 3 to produce an azide-bridged dimer, [{(Tp tBu,Me )V} 2 (1,3-$μ_2$-N 3 ) 2 ] (6), with two antiferromagnetically coupled high-spin V II ions. Complex 5 could be independently produced along with [($κ_2$-Tp tBu,Me ) 2 V] upon photolysis of 6 in arene solvents. The putative {V IV ≡N} intermediate, [(Tp tBu,Me )V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(Tp tBu,Me )V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [($κ_2$-Tp tBu,Me ) 2 V]. A more stable adduct (B-OPPh 3 ) was prepared by reacting B-THF with OPPh 3 . These adducts of B are the first neutral and mononuclear V IV nitride complexes to be isolated.
Iron-based alloy and nitridation treatment for PEM fuel cell bipolar plates
A corrosion resistant electrically conductive component that can be used as a bipolar plate in a PEM fuel cell application is composed of an alloy substrate which has 10-30 wt. % Cr, 0.5 to 7 wt. % V, and base metal being Fe, and a continuous surface layer of chromium nitride and vanadium nitride essentially free of base metal. A oxide layer of chromium vanadium oxide can be disposed between the alloy substrate and the continuous surface nitride layer. A method to prepare the corrosion resistant electrically conductive component involves a two-step nitridization sequence by exposing the alloy to a oxygen containing gas at an elevated temperature, and subsequently exposing the alloy to an oxygen free nitrogen containing gas at an elevated temperature to yield a component where a continuous chromium nitride layer free of iron has formed at the surface.
Characterization and electrochemical activities of nanostructured transition metal nitrides as cathode materials for lithium sulfur batteries
The Lithium Sulfur (Li–S) battery system is one of the most promising candidates for electric vehicle applications due to its higher energy density when compared to conventional lithium ion batteries. However, there are some challenges facing Li-S battery commercialization, such as: low active material utilization, high self-discharge rate, and high rate of capacity fade. In this work, a series of transition metal nitrides: Tungsten nitride (WN), Molybdenum Nitride (Mo 2 N), and Vanadium Nitride (VN) was investigated as cathode materials for lithium polysulfide conversion reactions. Capacities of 697, 569, and 264 mAh g -1 were observed for WN, Mo 2 N, VN, respectively, with 8 mg cm -2 loading, after 100 cycles at a 0.1 C rate. WN higher electrochemical performance may be attributed to a strong reversible reaction between nitrides and polysulfide, which retains the sulfur species on the electrode surface, and minimizes the active material and surface area loss. X-ray photoelectron spectroscopy (XPS) analysis was performed to gain a better understanding of the mechanism underlying each metal nitride redox reactions.
A study of microstructure of Fe-Cu-Ni-Sn and Fe-Cu-Ni-Sn-VN metal matrix for diamond containing composites
Highlights: • A nanocrystalline sample of coarse-grained powders of iron, copper, nickel, tin was obtained. • The structure of the sample consists of a supersaturated solid solution of nitrogen and vanadium in α-Fe. • Grain refinement occurs as a result of the decomposition of supersaturated solid solution of nitrogen and vanadium in α-Fe. To obtain bulk of 51Fe–32Cu–9Ni–8Sn and 49.47Fe–31.04Cu–8.73Ni–7.76Sn–3VN nanocomposites, a pressing method at room temperature and subsequent hot pressing in a vacuum was used. The microstructure of the sintered composites by X-ray diffraction and transmission electron microscopy was investigated. It is shown that in the 49.47Fe–31.04Cu–8.73Ni–7.76Sn–3VN composite there was a dissolution of vanadium nitride in γ‑iron and formation of a supersaturated solid solution of nitrogen and vanadium in the α‑iron. As a result, there was a substantial grinding of ferrite grains (from 5 to 50 μm to 20–400 nm) as compared to a sample without vanadium nitride (from 5 to 50 μm to 400–800 nm). Nanocomposites are considered as promising materials to create a new generation of diamond containing composites.
Assessing Stability of Transition Metal Nitrides in Aqueous Environments: The Case of Molybdenum, Iron, Vanadium and Nickel Nitride
Analysis of the MoN, Mo 2 N, VN, Fe 2 N, Fe 4 N, Ni 3 N stability in the aqueous media was performed based on the thermodynamic data available in HSC Chemistry software. Our analysis show that among all the studied nitrides, cubic Mo 2 N is the most resistant while cubic VN is the least resistant to oxidation and decomposition. The change in temperature up to 100 °C does not significantly influence the formation potentials of transition metal oxides from nitrides; however, pH significantly affects the stability of the studied transition metal nitrides. In the case of Mo 2 N, MoN, and VN changing pH from acidic to neutral and alkaline decreases the stability of the nitrides and the oxide formation potentials shift to more negative values. Additionally, based on thermodynamics, orthorhombic Fe 2 N should be stable only in alkaline conditions, while cubic Fe 4 N and hexagonal Ni 3 N should be the most stable at pH ~ 7 and pH ~ 9. Changing pH from these values decreases the stability of both Fe 4 N and Ni 3 N and the formation of oxy-, hydroxy- species becomes thermodynamically more favorable. Overall, the limited stability of the metal nitrides places severe constraints for their use in aqueous electrocatalytic applications.
Surface Characteristics and Hardness Variations in Electrical Discharge Machining of Enhanced Nitrogen in Vanadium Steels
Enhanced nitrogen in steels containing vanadium causes fine precipitates of vanadium nitride, which improve the hardenability properties. However, understanding the response of such steels to electrical discharge machining (EDM) is incomplete and needs further investigation. Therefore, EDM of enhanced nitrogen in steels containing vanadium was compared with a similar compositional steel that was free of nitrogen. The surface morphology, microstructural alterations, microhardness variation, and compositional depth profiling of the samples machined by EDM in oil and deionized water dielectric liquids were examined. Microscopic studies were carried out using optical and scanning electron microscopy. Phases were identified by X-ray diffractometry, and elemental depth profiling was performed using glow discharge optical emission spectroscopy. The hardness of the resolidified and heat-affected layers was measured using a Vickers type microhardness tester. The results of this study revealed that the dissolved nitrogen in steel decreased the probability of surface cracks and resulted in a softer resolidified layer structure when machining in the oil dielectric liquid. Thus, the presence of nitrogen reduced the formation of tension-induced martensite in the resolidified layer. Moreover, the heat-affected zone below the resolidified layer exhibited a uniform and harder structure, compared with the steel without nitrogen content, indicating the fast nature of the thermal cycles in EDM. The free nitrogen in steel did not dissociate during sparking; therefore, precipitation strengthening occurred in the heat-affected zone.