State-to-state Photoionization Dynamics of Vanadium Nitride by Two-color Laser Photoionization and Photoelectron Methods

We have conducted a two-color visible-ultraviolet （VIS-UV） resonance-enhanced laser pho- toionization and pulsed field ionization-photoelectron （PFI-PE） study of gaseous vana- dium mononitride （VN） in the total energy range of 56900-59020 cm-1. The VN molecules were selectively excited to single rotational levels of the intermediate VN（D3H0, v＇=0） state by using a VIS dye laser prior to photoionization by employing a UV laser. This two-color scheme allows the measurements of rovibronically selected and re- solved PFI-PE spectra for the VN＋（X2A; v＋=0, 1, and 2） ion vibrational bands. By simulating the rotationally resolved PFI-PE spectra, J＋=3/2 is determined to be the lowest rotational level of the ground electronic state, indicating that the symmetry of the ground VN＋ electronic state is 2A3/2. The analysis of the PFI-PE spectra for VN＋ also yields accurate values for the adiabatic ionization energy for the formation of VN＋（X2A3/2）, IE（VN）=56909.5＋0.8 cm-1 （7.05588±0.00010 eV）, the vibrational fre- quency wc＋=1068.0±0.8 cm-1, the anharmonicity constant wc＋Xe＋=5.8±0.8 cm-1, the rotational constants Be＋=0.6563±0.0005 cm-1 and ae＋=0.0069±0.0004 cm-1, and the equi-librium bond length, re＋=1.529A, for VN＋（X2A3/2）; along with the rotational constants Bc＋=0.6578i0.0028 cm-1 and a＋=0.0085±0.0028 cm-1, and the equilibrium bond length re＋=1.527A for VN＋（X2As/2）, and the spin-orbit coupling constant A=153.3±0.8 cm-1 for VN＋（X2/k5/2,3/2）. The highly precise energetic and spectroscopic data obtained in the present study are valuable for benchmarking the predictions based on state-of-the-art ab initio quantum calculations.

A Novel Hierarchical Porous 3D Structured Vanadium Nitride/Carbon Membranes for High-performance Supercapacitor Negative Electrodes

Transition-metal nitrides exhibit wide potential windows and good electrochemical performance, but usually experience imbalanced practical applications in the energy storage field due to aggregation, poor circulation stability, and complicated syntheses. In this study, a novel and simple multiphase polymeric strategy was developed to fabricate hybrid vanadium nitride/carbon(VN/C) membranes for supercapacitor negative electrodes, in which VN nanoparticles were uniformly distributed in the hierarchical porous carbon 3D networks. The supercapacitor negative electrode based on VN/C membranes exhibited a high specific capacitance of 392.0 F g^(-1) at 0.5 A g^(-1) and an excellent rate capability with capacitance retention of 50.5% at 30 A g^(-1). For the asymmetric device fabricated using Ni(OH)_2//VN/C membranes, a high energy density of 43.0 Wh kg^(-1) at a power density of800 W kg^(-1) was observed. Moreover, the device also showed good cycling stability of 82.9% at a current density of 1.0 A g^(-1) after 8000 cycles. This work may throw a light on simply the fabrication of other high-performance transition-metal nitridebased supercapacitor or other energy storage devices.

Synthesis of Vanadium Nitride by a One Step Method

Vanadium nitrides were prepared via one step method of carbothermal reduction and nitridation of vanadium trioxide. Thermalgravimetric analysis （TGA） and X-ray diffraction were used to determine the reaction paths of vanadium carbide, namely the following sequential reaction： V2O3→V8C7 in higher temperature stage, the rule of vanadium nitride synthesized was established, and defined conditions of temperature for the production of the carbides and nitrides were determined. Vanadium oxycarbide may consist in the front process of carbothermal reduction of vanadium trioxide. In one step method for vanadium nitride by carbothermal reduction and nitridation of vanadium trioxide, the nitridation process is simultaneous with the carbothermal reduction. A one-step mechanism of the carbothermal reduction with simultaneous nitridation leaded to a lower terminal temperature in nitridation process for vanadium nitride produced, compared with that of carbothermal reduction process without nitridation. The grain size and shape of vanadium nitride were uniform, and had the shape of a cube. The one step method combined vacuum carborization and nitridation （namely two step method） into one process. It simplified the technological process and decreased the costs.

Novel Hybrid Nanoparticles of Vanadium Nitride/Porous Carbon as an Anode Material for Symmetrical Supercapacitor

Hybrid materials of vanadium nitride and porous carbon nanoparticles(VN/PCNPs) were fabricated by a facile pyrolysis process of vanadium pentoxide(V_2O_5) xerogel and melamine at relatively low temperature of 800 °C for supercapacitor application. The effects of the feed ratio of V_2O_5 to melamine(r), and nitrogen flow rate on the microstructure and electrochemical performance were also investigated. It was found that the size of the as-synthesized nanoparticles is about 20 nm. Both r value and N_2 flow rate have enormous impacts on morphology and microstructure of the nanoparticle, which correspondingly determined the electrochemical performance of the material. The VN/C hybrid nanoparticles exhibited high capacitive properties, and a maximum specific capacitance of 255.0 Fg^(-1) was achieved at a current density of 1.0 Ag^(-1) in 2 M KOH aqueous electrolyte and the potential range from 0 to -1.15 V. In addition,symmetrical supercapacitor fabricated with the as-synthesized VN/PCNPs presents a high specific capacitance of 43.5 F g^(-1) at 0.5 A g^(-1) based on the entire cell, and an energy density of 8.0 Wh kg^(-1) when the power density was 575 W kg^(-1). Even when the power density increased to 2831.5 W kg^(-1), the energy density still remained 6.1 Wh kg^(-1).

Preparation of high purity vanadium nitride by magnesiothermic reduction of V2O3 followed by nitriding in N2 atmosphere

High purity vanadium nitride(VN)powders were prepared via a two-step process using vanadium trioxide(V2 O3)as the raw material.The V2 O3 was firstly reduced at 873 K in Ar atmosphere via magnesiothermic reduction reaction to get the mixture of V and MgO,and then the products were further nitrided at 1473 K in N2 atmosphere.Finally,the as-prepared samples were acid-leached to obtain pure VN powders.X-ray diffractometry and field-emission scanning electron microscopy were used to analyze the phase transition and morphological evolution of the samples.The results reveal that the overall morphology of the obtained VN powder retains the morphology of the initial V2 O3 powders.After removing MgO by acidic leaching,the porous VN particles can be obtained,with the oxygen content of 0.178 wt.%.Compared with the traditional methods,high purity VN powders with a small amount of oxygen and no carbon can be obtained.

Elastic and thermodynamic properties of vanadium nitride under pressure and the effect of metallic bonding on its hardness

By the particle-swarm optimization method, it is predicted that tetragonal P42mc, 141md, and orthorhombic Amm2 phases of vanadium nitride （VN） are energetically more stable than NaCl-type structure at 0 K. The enthalpies of the predicted three new VN phases, along with WC, NaC1, AsNi, CsCl type structures, are calculated each as a function of pressure. It is found that VN exhibits the WC-to-CsCl type phase transition at 256 GPa. For the considered seven crystal- lographic VN phases, the structures, elastic constants, bulk moduli, shear moduli, and Debye temperatures are investigated. Our calculated equilibrium structural parameters are in very good agreement with the available experimental results and the previous theoretical results for the NaC1 phase. The Debye temperatures of VN predicted three novel phases, which are all higher than those of the remaining structures. The elastic constants, thermodynamic properties, and elastic anisotropies of VN under pressure are obtained and the mechanical stabilities are analyzed in detail based on the mechanical stability criteria. Moreover, the effect of metallic bonding on the hardness of VN is also investigated, which shows that VNs in P42mc, 141md, and Amm2 phases are potential superhard phases. Further investigation on the experimental level is highly recommended to confirm our calculations presented in this paper.

Low-Temperature Solid State Synthesis and Characterization of Superconducting Vanadium Nitride

Superconducting vanadium nitride （VN） is successfully synthesized by a solid-state reaction of vanadium pentox- ide, sodium amide and sulfur in an autoclave at a relatively low temperature （240-400℃）. The obtained samples are characterized by x-ray diffraction, x-ray photoelectron spectroscopy and transmission electron microscopy. The result of the magnetization of the obtained VN product as a function of temperature indicates that the onset superconducting transition temperature is about 8.4K. Furthermore, the possible reaction mechanism is also discussed.

Energy Storage Mechanism of Vanadium Nitride via Intercalating Different Atomic Radius for Expanding Interplanar Spacing

As a promising anode material in supercapacitors,vanadium nitride has been widely concerned due to its ultra-high theoretical specific capacitance.However,its routine test capacitance value is still far from the theoretical value and its energy storage mechanism is controversial.In order to solve these two key problems,here we prepare interplanar spacing expanded vanadium nitride materials with different impurity atoms intercalation from two anionic precursors of vanadium-based metal organic frameworks with different functional groups.The obtained vanadium nitride reaches a higher specific capacitance;and further,through ex situ X-Ray diffraction and in situ Raman,the charge storage of vanadium nitride is contributed by two processes:the first benefit is from the K^(+) de/intercalation in the interplanar spacing,and the other one is derived from the redox reaction with OH−by adsorption on surface.Furthermore,both of the first principle calculation and extended experiments support this idea.We believe that such detailed research on the energy storage mechanism can provide a clear idea for the application of metal nitrides in supercapacitors and other energy storage devices.

Vanadium Nitride Nanoparticles as Anode Material for Lithium Ion Hybrid Capacitor Applications

A high production efficiency synthesis method was used to produce a stacked vanadium nitride nanoparticle structure with an inexpensive raw material as an anode material and high surface area polystyrene was used the cathode material for lithium ion hybrid capacitors. The Li-HCs cell displayed an excellent specific capacitance of 64.2 F·g^-1 at a current density of 0.25 A·g^-1 and a wide potential window of 0.01 to 3.5 V. Furthermore, the device exhibited a high energy density of 109.3 W·h·kg^-1 at a power density of 512.3 W·kg^-1 and retained an energy density of 69.2 W·h·kg^-1 at a high power density of 3 498.9 W· kg^-1 at 2 A·g^-1. Due to the short synthesis time and simple raw materials, this method is suitable for industrial production.

Hydrogel-derived nitrogen-doped porous carbon framework with vanadium nitride decoration for supercapacitors with superior cycling performance

Transition metal nitrides(TMNs)and their composites with carbon materials hold tremendous potential for supercapacitor(SC)electrodes because of their excellent electronic conductivity and electrochemical activity.However,realizing cycling stable TMN/carbon-based supercapacitors with economically viable and environmentally-friendly approaches remains a significant challenge.Significantly,polyacrylamide(PAM)hydrogel,as a water-soluble linear polymer electrolyte,is expected to be a remarkable candidate precursor for preparing N-doped porous carbon(NPC)due to the high contents of carbon and nitrogen elements.In this study,vanadium nitride(VN)embedded in PAM hydrogel-derived NPC was fabricated successfully via an ammonia-free process.The VN/NPC delivers a high specific capacitance of 198.3 F g^(−1)at a current density of 1 A g^(−1),with a remarkable cycling stability of 107%after 16,000 cycles.The electrochemical performances of VN/NPC compared to bare VN nanoparticles are strongly improved due to the composite structure.Additionally,the VN/NPC-based solid-state symmetric device delivers an excellent energy density of 21.97µWh cm^(−2)at a power density of 0.5 mW cm^(−2),and an outstanding cycling durability of 90.9%after 18,000 cycles.This work paves the way to design metal nitride/porous carbon materials,which also opens up unique horizons for the recovery of hydrogel electrolyte.

High-rate sodium-ion storage of vanadium nitride via surface-redox pseudocapacitance

Vanadium nitride(VN)electrode displays high-rate,pseudocapacitive responses in aqueous electrolytes,however,it remains largely unclear in nonaqueous,Na+-based electrolytes.The traditional view supposes a conversion-type mechanism for Na+storage in VN anodes but does not explain the phenomena of their size-dependent specific capacities and underlying causes of pseudocapacitive charge storage behaviors.Herein,we insightfully reveal the VN anode exhibits a surface-redox pseudocapacitive mechanism in nonaqueous,Na+-based electrolytes,as demonstrated by kinetics analysis,experimental observations,and first-principles calculations.Through ex situ X-ray photoelectron spectroscopy and semiquantitative analyses,the Na+storage is characterized by redox reactions occurring with the V5+/V4+to V3+at the surface of VN particles,which is different from the well-known conversion reaction mechanism.The pseudocapacitive performance is enhanced through nanoarchitecture design via oxidized vanadium states at the surface.The optimized VN-10 nm anode delivers a sodium-ion storage capability of 106 mAh g−1 at the high specific current of 20 A g−1,and excellent cycling performance of 5000 cycles with negligible capacity losses.This work demonstrates the emerging opportunities of utilizing pseudocapacitive charge storage for realizing high-rate sodium-ion storage applications.

Metallic mesocrystal nanosheets of vanadium nitride for high-performance all-solid-state pseudocapacitors

Transition metal nitrides （TMNs） are of particular interest by virtue of their synergic advantages of superior electrical conductivity, excellent environmental durability and high reaction selectivity, yet it is difficult to achieve flexible design and operation. Herein, mesocrystal nanosheets （MCNSs） of vanadium nitride （VN） are synthesized via a confined-growth route from thermally stable layered vanadium bronze, representing the first two-dimensional （2D） metallic mesocrystal in inorganic compounds. Benefiting from their single-crystalline-like long-range electronic connectivity, VN MCNSs deliver an electrical conductivity of 1.44×10^5 S/m at room temperature, among the highest values observed for 2D nanosheets. Coupled with their unique pseudocapacitance, VN MCNS-based flexible supercapacitors afford a superior volumetric capacitance of 1,937 mF/cm3. Nitride MCNSs should have wide applications in the energy storage and conversion fields because their intrinsic high conductivity is coupled with the reactivity of inorganic lattices.

Vanadium nitride nanoparticles embedded in carbon matrix with pseudocapacitive behavior for high performance lithium-ion capacitors

Lithium-ion capacitors(LICs)have attracted wide attention due to their potential of achieving merits of high-power output as well as high energy density.How-ever,the key issue of kinetics mismatch between anode and cathode hinders the electrochemical performance of LICs.Therefore,a vanadium nitride composite with nanoparti-cles embedded in carbon matrix(VN-C)was prepared as an efficiently pseudocapacitive anode material with high electronic conductivity and fast Li-ion diffusion rate.The VN-C composites were synthesized through one-step ammonia heating treatment at different temperatures among which the sample annealed at 600℃exhibits high specific capacity(513 mAh·g^(-1)at 0.1 A·g^(-1)),outstanding rate performance(~300 mAh·g^(-1)at 10 A·g^(-1)),and excellent cyclic steadiness(negligible capacity decay over 2000 cycles)in half-cell devices.A high-performance lithium-ion capacitor device was also fabricated by using VN-C-600 as the anode and activated carbon as the cath-ode,delivering a maximum energy density of 112.6 Wh·kg^(-1)and an extreme power density of 10 kW·kg^(-1).

Non-isothermal kinetics of synthesizing vanadium nitride by one-step method

Vanadium nitride was synthesized by one-step method using V2O5 and carbon black as raw materials in nitrogen atmosphere. The phases of different reaction products prepared in different reaction temperatures were analyzed by X-ray diffraction(XRD), and the dynamic behavior of the process of synthesizing vanadium nitride(VN) by one-step method was studied with non-isothermal thermogravimetry. The mechanism function and kinetic parameters of reaction process were calculated by thermal gravimetric analyses(TGA), and the reaction rate equation was established. The XRD results show that for the samples tested with minimal VN after holding for 4 h at 1273 K, the main phase of products is VN at 1476 K, while some vanadium nitrides transform into vanadium carbides again over 1573 K. It is found that N2 is beneficial to stimulate reduction and proceed carbonization reaction,and the reduction and nitridation reaction can occur simultaneously. The activation energy of preparing VN by one-step method is 104.005 kJ·mol-1, and the frequency factor is 470.52 at 1280–1358 K, and 150.052 kJ·mol-1 and 2.35 9 104 at 1358–1426 K, respectively.

Electrodes based on nano-tree-like vanadium nitride and carbon nanotubes for micro-supercapacitors

Vanadium nitride （VN） was deposited by DC-sputtering on a vertically aligned carbon nanotube （CNTs） template for the purpose of nano-structuration. This led to the fabrication of hierarchically composite electrodes consisting of porous and nanostructured VN grown on vertically aligned CNTs in a nano-treelike configuration for micro-supercapacitor application. The electrodes show excellent performance with an areal capacitance as high as 37.5 mF cm^-2 at a scan rate of 2 mV s^-1 in a 0.5 M K2504 mild electrolyte solution. Furthermore, the capacitance decay was only 15% after 20,000 consecutive cycles. Moreover, the capacitance was found to increase with VN deposit thickness. The X-ray photoelectron spectroscopy analyses of the electrodes before and after cycling suggest that the oxide layers that form at the VN surface is the responsible for the redox energy storage in this material. Such electrodes can compete with other transition metal nitride based electrodes for micro-supercapacitors.

Covalent interfacial coupling of vanadium nitride with nitrogen-rich carbon textile boosting its lithium storage performance as binder-free anode

Eliminating the usage of metal current collectors and binders in traditional battery electrode configuration is an effective strategy to significantly improve the capacities of lithium ion batteries (LIBs). Herein, we demonstrate the construction of porous vanadium nitride (VN) nanosheet network in situ grown on nitrogen-rich (N-rich) carbon textile (N-C@P-VN) as lightweight and binder-free anode for LIBs. The N-rich carbon textile is used both as the current collector and host to store Li^(+), thus improving the specific capacities of binder-free VN anode and meanwhile reducing the inert mass of the whole cell. Moreover, the open spaces in carbon textile and vertically aligned pores in VN nanosheet network can not only provide an expressway for Li+ and e− transport, but also afford more active sites. As a result, the binder-free N-C@P-VN anode maintains a specific capacity of 1,040 mAh·g^(−1) (or an areal capacity of 2.6 mAh·cm^(−2)) after 100 cycles at 0.1 mA·cm^(−2) in half cell. Moreover, in an assembled N-C@P-VN//LiFePO4 full cell, it exhibits an areal capacity of 1.7 mAh·cm^(−2) after 300 cycles at 0.1 C. The synergistic strategy of N-C substrate and porous VN network could be applied to guide rational design of similar N-C@nitride or sulfide hybrid systems with corresponding sulfur-doped carbon textile as the substrate.

Multilayer Porous Vanadium Nitride Microsheets Anodes for Highly Stable Na-ion Batteries

Sodiumion batteries(SIBs)have attracted intensive attention as promising alternative to lithium-ionbatteries(LIBs)for large scale energy storage systems because of low cost of sodium,similar energy storage mechanism and the reasonable performance.However,it is still a great challenge to search and design a robust structure of anode materials with excellent cycling stability and high rate capability for SIBs.Herein,multilayer porous vanadium nitride(VN)microsheets are synthesized through a facile and scalable hydrothermal synthesis-nitrogenization strategy as an effective anode material for SIBs.The multilayer porous VN microsheets not only offer more active sites for fast Na+insertion/extraction process and short diffusion pathway,but also effectively buffer the volume change of anode due to more space in the multilayer porous structure.The large proportions of capacitive behavior imply that the Na+charge storage depends on the intercalation pseudocapacitive mechanism.The multilayer porous VN microsheets electrodes manifest excellent cycling stability and rate capability,delivering a discharge capacity of 156.1 mA·h/g at 200 mA/g after 100 cycles,and a discharge capacity of 111.9 mA·h/g at 1.0 A/g even after 2300 cycles with the Coulombic efficiency of nearly 100%.

Surface morphology-optical properties relationship in thermochromic VO_(2) thin films obtained by air oxidation of vanadium nitride

In this paper,vanadium nitride(VN)thin films have been deposited on Al substrates by reactive magnetron sputtering.Thermochromic VO_(2) films have been obtained by air oxidation of VN samples performed at three temperatures(450,525 and 550℃)at various durations(lower than 50 min).X-ray diffraction and Raman spectrometry of the VN oxidized films indicate that VO_(2) and V2O5 are the only phases produced during the oxidation process.Vanadium dioxide is the first oxide formed.It coexists with VN in a long period at 450℃ or suddenly disappears at 525 and 550℃.Whatever the temperature,V2O5 is exclusively detected after the total oxidation of VN.This oxide is detrimental to the thermochromic performance of films.The emissivity-switching properties of the oxidized films were analyzed by infrared camera in the 7.5e13 mm range.The comparison among all the samples exhibiting a thermochromic behavior shows that the maximum emissivity switch is independent of the oxidation temperature and the surface morphology of the samples.These results could open a new strategy in the investigation of VN oxidation as a method to obtain VO_(2),along with an insight into the correlation between surface morphology and optical properties.

Dual redox catalysis of VN/nitrogen-doped graphene nanocomposites for high-performance lithium-sulfur batteries

Lithium-sulfur(Li-S)batteries are regarded as one of the promising candidates for the next-generation energy storage system owing to their high capacity and energy density.However,the durable operation for the batteries is blocked by the shuttle behavior of soluble lithium polysulfides and the sluggish kinetics in the redox process.Here,VN nanoparticles on nitrogen-doped graphene(VN/NG)composite is synthesized by simple calcining method to modify the separators,which can not only chemically trap polysulfides,but also catalyze the conversion reaction between the polysulfides and the insoluble Li2S during the charge/discharge process.The catalytic effects of VN/NG are verified by the calculated activation energy(E_(a)),which is smaller than the counterpart with NG toward both directions of redox.Because of the synergistic adsorption-catalysis of VN/NG,the cells with VN/NG-modified separators deliver a superior rate performance(791 mAh g^(-1) at 5C)and cycling stability(863 mAh g^(-1) after 300 cycles with a low decaying rate of 0.068%per loop at 1C).This work provides a simple preparation strategy and fundamental understanding of the bifunctional catalyst for high-performance Li-S batteries.

Ultrafine VN nanodots induced generation of abundant cobalt single-atom active sites on nitrogen-doped carbon nanotube for efficient hydrogen evolution

Development of highly active and stable non-noble electrocatalysts with well-defined nanostructures is crucial for efficient hydrogen evolution reaction(HER). Herein, a novel three-dimensional(3D) selfsupported electrode consists of vanadium nitride(VN) nanodots and Co nanoparticles co-embedded and highly active single Co atoms anchored in N-doped carbon nanotubes supported on carbon cloth(VN-Co@CoSAs-NCNTs/CC) is fabricated via a one-step in situ nanoconfined pyrolysis strategy, which shows remarkable enhanced HER electrocatalytic activity in acidic medium. During pyrolysis, the formed VN nanodots induce the generation of atomic Co Nxsites in NCNTs, contributing to superior electrocatalytic activity. Experimental and density functional theory(DFT) calculation results reveal that the electrode has multiple accessible active sites, fast reaction kinetics, low charge/mass transfer resistances,high conductivity, as well as downshifted d-band center with a thermodynamically favorable hydrogen adsorption free energy(△G_(H·)), all of which greatly boost the HER performance. As a result, the VNCo@CoSAs-NCNTs/CC electrode displays superb catalytic performance toward HER with a low overpotential of 29 mV at 10 mA cm^(-2) in acidic medium, which could maintain for at least 60 h of stable performance. This work opens a facile avenue to explore low-cost, high performance, but inexpensive metals/nitrogen-doped carbon composite electrocatalysts for HER.