Three‐dimensional Nitrogen‐Doped Reduced Graphene Oxide/Carbon Nanotube Composite Catalysts for Vanadium Flow Batteries

The development of vanadium redox flow battery is limited by the sluggish kinetics of the reaction, especially the cathodic VO₂⁺/VO²⁺ redox couples. Therefore, it is vital to develop new electrocatalysts with enhanced activity to improve the battery performance. Herein, we synthesized the hydrogel precursor by a facile hydrothermal method. After the following carbonization, nitrogen‐doped reduced graphene oxide/carbon nanotube composite was obtained. By virtue of the large surface area and good conductivity, which are ensured by the unique hybrid structure, as well as the proper nitrogen doping, the as‐prepared composite presents enhanced catalytic performance toward the VO₂⁺/VO²⁺ redox reaction. We also demonstrated the composite with carbon nanotube loading of 2 mg/mL exhibits the highest activity and remarkable stability in aqueous solution due to the strong synergy between reduced graphene oxide and carbon nanotubes, indicating that this composite might show promising applications in vanadium redox flow battery.     

Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery

Different modified multiwalled carbon nanotubes (MWCNTs) are prepared by heat treatments in the air and in the H₂SO₄?+?HNO₃ (1:1) mixed acids which are investigated by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Brunaur–Emmett–Teller, and cyclic voltammetry measurements. The results show the physicochemical properties of MWCNTs change significantly after these different modification processes, especially the electrochemical catalytic activity towards the VO₂ ⁺/VO²⁺ and V³⁺/V²⁺ redox pairs. The MWCNTs treated in the air at 600 °C for 30 min shows better electrochemical performances for the VO₂ ⁺/VO²⁺ redox reactions (58.8 and ?32.4 μA for the oxidation and reduction peaks at 10 mV?s^?1, respectively) than any other samples. Compared with the V³⁺/V²⁺ redox couple, the VO₂ ^+/VO²⁺ redox reactions are more easily affected by the physicochemical property changes of the MWCNTs. The enhanced electrochemical catalytic activity of the modified MWCNTs is not only related to the surface oxygen content, but also to the specific surface area, conductivity and the unique structure variations of the MWCNTs. The investigation demonstrated that the modified MWCNTs have a promising future application in the vanadium redox flow battery.     

The First Neutral Dinuclear Vanadium Complex Comprising VO and VO2 Cores: Synthesis,Structure, Electrochemical Properties,and Catalytic Activity

A neutral dinuclear vanadium complex containing both oxido and dioxidovanadium cores with hydrazone based ligand, [VO(OCH₃)(CH₃OH)(HL)VO₂] ( 1 ) {H₄L = bis[(E)‐N′‐(5‐bromo‐2‐hydroxybenzylidene)]‐carbohydrazide}, was synthesized and fully characterized by X‐ray crystallography and spectroscopic methods (IR, UV/Vis, NMR). The ligand acts as a trinegative hexadentate N₃O₃ donor ligand to form a dinuclear complex and during the reaction V⁴⁺ is oxidized to V⁵⁺. The coordination polyhedra are a VO₅N distorted octahedron for the mono‐oxidovanadium core and a VO₃N₂ trigonal bipyramid for the dioxidovanadium core. The results of catalytic reactions indicate that 1 is a highly active catalyst in the clean epoxidation reaction of cis‐cyclooctene using aqueous hydrogen peroxide in acetonitrile. Cyclic voltammetric experiments of 1 in DMSO reveal two quasi‐reversible peaks due to the VO³⁺–VO²⁺ and VO₂⁺–VO₂ couples.     

Direct measurement of electrochemical reaction kinetics in flow-through porous electrodes

This work demonstrates the feasibility of measuring electrochemical reaction rates on common flow-through porous electrodes by traditional Tafel analysis. A customized microfluidic channel electrode was designed and demonstrated by measuring the intrinsic kinetics of the V^2 +/V^3 + and VO^2 +/VO₂⁺ redox reactions in carbon paper electrodes under forced electrolyte flow. The exchange current density of the V^2 +/V^3 + reaction was found to be nearly two orders of magnitude slower than the VO^2 +/VO₂⁺ reaction, indicating that this may be the limiting reaction in vanadium redox flow batteries. The forced convection in this technique is found to generate reproducible exchange current densities which are consistently higher than for conventional electrochemical methods due to improved mass transport.     

Co3V2O8/石墨烯复合负极材料的合成及其储锂性能

以石墨烯为基底,CoCl2·2H2O和NH4VO3为原料,采用水热结合热处理方法合成了Co3V2O8/石墨烯复合电极材料;采用XRD、Raman、XPS、SEM、(HR-)TEM和恒电流充放电等对材料进行了结构表征与电化学性能测试。结果表明:Co3V2O8/rGO复合材料表现出优异的放电比容量、优秀的倍率性能和稳定的循环性能(当电流密度为200 mA·g^-1,经过100次循环后,可逆放电比容量为1208 mAh·g^-1);Co3V2O8/rGO电极材料表现出优异的倍率和循环性能可以归因于:独特的石墨烯包覆结构可以有效地提高材料的导电性和增强结构的稳定性、缓解Co3V2O8粒子在循环过程中的聚结和膨胀现象;此外,Co3V2O8纳米颗粒均匀地嵌在石墨烯层间防止了石墨烯片层间的堆叠。     

Microwave‐Assisted Solvothermal Synthesis of VO2 Hollow Spheres and Their Conversion into V2O5 Hollow Spheres with Improved Lithium Storage Capability

Monodispersed hierarchically structured V₂O₅ hollow spheres were successfully obtained from orthorhombic VO₂ hollow spheres, which are in turn synthesized by a simple template‐free microwave‐assisted solvothermal method. The structural evolution of VO₂ hollow spheres has been studied and explained by a chemically induced self‐transformation process. The reaction time and water content in the reaction solution have a great influence on the morphology and phase structure of the resulting products in the solvothermal reaction. The diameter of the VO₂ hollow spheres can be regulated simply by changing vanadium ion content in the reaction solution. The VO₂ hollow spheres can be transformed into V₂O₅ hollow spheres with nearly no morphological change by annealing in air. The nanorods composed of V₂O₅ hollow spheres have an average length of about 70 nm and width of about 19 nm. When used as a cathode material for lithium‐ion batteries, the V₂O₅ hollow spheres display a diameter‐dependent electrochemical performance, and the 440 nm hollow spheres show the highest specific discharge capacity of 377.5 mAhg^?1 at a current density of 50 mAg^?1, and are better than the corresponding solid spheres and nanorod assemblies.     

A Vanadium(V) Oxide Nanorod Promoted Platinum/Reduced Graphene Oxide Electrocatalyst for Alcohol Oxidation under Acidic Conditions

A Pt‐V₂O₅/rGO ternary hybrid electrocatalyst was designed by using active vanadium(V) oxide (V₂O₅) nanorods and reduced graphene oxide (rGO) components. The V₂O₅ nanorods were synthesized by a simple polyol‐assisted solvothermal method and were incorporated uniformly onto rGO sheets by intermittent microwave heating. Subsequently, Pt nanoparticles (2–3 nm in size) were deposited over the V₂O₅/rGO composite by the conventional polyol reflux method. The electrocatalytic performance of the Pt‐V₂O₅/rGO ternary hybrid and bare Pt/rGO catalysts towards the oxidation of simple alcohols was evaluated in acidic media. The ternary hybrid catalyst exhibited higher electrocatalytic activity than bare Pt/rGO and also showed good stability. The higher electrocatalytic activity of the Pt‐V₂O₅/rGO ternary hybrid was attributed to a synergistic effect among the Pt, V₂O₅, and rGO components. In addition, oxygen‐containing species, such as OH groups, were generated on V₂O₅ at lower potentials. These groups were able to scavenge intermediate species such as CO_ads on the Pt surfaces and helped to regenerate the active sites on the Pt surface more effectively for the routine alcohol oxidation reaction.     

Application of EPR spectroscopy for elucidation of vanadium speciation in VO x /ZrO2 catalysts subject to redox treatment

Application of EPR spectroscopy corroborated by spectra simulation in speciation studies of the tetravalent vanadium in supported VO _x /ZrO₂ catalyst has been discussed. Implementation of genetic algorithms into automated analysis of the EPR spectra has greatly improved the simulation efficiency. The performance of the new procedure has been benchmarked against common simplex method using the multi-component model and real EPR spectra of tetravalent vanadium in VO _x /ZrO₂ catalysts. The analysis has revealed speciation of vanadium into surface isolated and clustered vanadyl entities and isolated bulk V _Zr ^x ions due to formation of Zr_1?x V _x O₂ solid solution in the near to surface region. The structural heterogeneity of vanadium can be controlled by the calcination temperature and the redox treatment.     

Direct Oxidation of Benzene to Phenol by Dioxygen over Nano-vanadium Oxide

Reducing regents, such as ascorbic acid, are needed for vanadium‐containing catalysts to catalyze the direct oxidation of benzene to phenol by dioxygen. Quadrivalent vanadium species, reduced from quinquevalent vanadium species, can activate dioxygen to produce active oxygen species, which is important for the reaction. The key step is to prepare more V⁴⁺‐containing catalysts. Here, VO_X‐C₁₆‐A was prepared in an acidic medium to produce more V⁴⁺ species. The results of XPS and XRD studies confirmed that the vanadium species in VO_X‐C₁₆‐A was mainly V⁴⁺ ions. The results of XRD and electron diffraction patterns revealed that VO_X‐C₁₆‐A consisted of tetragonal VO₂ and monoclinic VO₂. Morphology observations display that the VO_X‐C₁₆‐A is made of nanorod. Investigations into the performances of the catalysts showed that VO_X‐C₁₆‐A was reusable, producing a 1.9% conversion of benzene without reducing agent.     

Systematic Exploration of the Synthetic Parameters for the Production of Dynamic VO2(M1)

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO₂(M1) to tetragonal VO₂(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO₂(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO₂(B), obtained via a hydrothermal route, was converted into the stable VO₂(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior.     

SnO2 Quantum Dots Distributed along V2O5 Nanobelts for Utilization as a High-Capacity Storage Hybrid Material in Li-Ion Batteries

In this study, the facile synthesis of SnO₂ quantum dot (QD)-garnished V₂O₅ nanobelts exhibiting significantly enhanced reversible capacity and outstanding cyclic stability for Li⁺ storage was achieved. Electrochemical impedance analysis revealed strong charge transfer kinetics related to that of V₂O₅ nanobelts. The SnO₂ QD-garnished V₂O₅ nanobelts exhibited the highest discharge capacity of ca. 760 mAhg⁻¹ at a density of 441 mAg⁻¹ between the voltage ranges of 0.0 to 3.0 V, while the pristine V₂O₅ nanobelts samples recorded a discharge capacity of ca. 403 mAhg⁻¹. The high capacity of QD-garnished nanobelts was achieved as an outcome of their huge surface area of 50.49 m²g⁻¹ and improved electronic conductivity. Therefore, the as-presented SnO₂ QD-garnished V₂O₅ nanobelts synthesis strategy could produce an ideal material for application in high-performance Li-ion batteries.     

Ein neuer Zugang zu Alkalivanadaten(IV,V) Die Kristallstruktur von Rb2V3O8

A New Access to Alkali Vanadates(IV,V) Crystal Structure of Rb₂V₃O₈ By heating vanadium(V) oxide with rubidium iodide to 500°C, the vanadium experiences partial reduction and Rb₂V₃O₈ is obtained. It has the fresnoite structure. Crystal data: a = 892.29(7), c = 554.49(9) pm at 20°C, tetragonal, space group P4bm, Z = 2. X-ray crystal structure determination with 620 observed reflexions, R = 0.027. V₂O₇ units share vertices with VO₅ square pyramids, forming layers; a layer can be regarded as association product of VO²⁺ and V₂O₇^4? ions. The Rb⁺ ions between the layers have pentagonal-antiprismatic coordination.     

Ultrathin free-standing electrospun carbon nanofibers web as the electrode of the vanadium flow batteries

Ultrathin free-standing electrospun carbon nanofiber web(ECNFW) used for the electrodes of the vanadium flow battery(VFB) has been fabricated by the electrospinning technique followed by the carbonization process in this study to reduce the ohmic polarization of the VFB. The microstructure, surface chemistry and electrochemical performance of ECNFW carbonized at various temperatures from 800 to 1400 °C have been investigated. The results show that ECNFW carbonized at 1100 °C exhibits the highest electrocatalytic activity toward the V~(2+)/V~(3+)redox reaction, and its electrocatalytic activity decreases along with the increase of carbonization temperature due to the drooping of the surface functional groups.While for the VO~(2+)/VO_2~+redox couple, the electrocatalytic activity of ECNFW carbonized above 1100 °C barely changes as the carbonization temperature rises. It indicates that the surface functional groups could function as the reaction sites for the V~(2+)/V~(3+)redox couple, but have not any catalytic effect for the VO~(2+)/VO_2~+redox couple. And the single cell test result suggests that ECNFW carbonized at 1100 °C is a promising material as the VFB electrode and the VFB with ECNFW electrodes obtains a super low internal resistance of 250 mΩ cm~2.     

EPR-Untersuchungen an V3O7 (VO2,33)

E.S.R. Investigation of V₃O₇(VO_2,33) The oxides of vanadium in the range from VO_2,17 to VO_2,5 have been investigated by the e.s.r. method. The e.s.r. signal is caused mainly by the vanadiumoxide V₃O₇ which belongs to the homolognes V_nO_2n+1 series. The results are discussed by means of the crystallographic investigations described by several authors.     

Improvement of lithium-ion conductivity in A-site-disordered lithium lanthanum titanates by V5+/Nb5+ substitution

The V⁵⁺/Nb⁵⁺-substituted lithium lanthaum titanates are synthesized by a conventional solid-state reaction method at high temperature in air. The structural and conductivity studies of the obtained perovskite oxide samples are investigated by x-ray diffraction (XRD), SEM, and impedance spectroscopy. From the powder XRD patterns, it is clearly observed that the synthesized samples exhibit a well-defined cubic structure with the Pm3m (Z = 1) space group. The lattice parameter is decreased with increasing vanadium content in Li_0.5?x La_0.5Ti_1?x V _x O₃, but increased with the increasing niobium content in Li_0.5?x La_0.5Ti_1?x Nb _x O₃. The scanning electron microscope measurements confirmed that these materials consist of fairly ordered grains throughout the surface area. The conductivity variations with the substitution of vanadium/niobium are also reported. The bulk ionic conductivity measured in the temperature range from room temperature to 150 °C is about the same as reported earlier for the related lithium lanthanum titanate. However, the low activation energies for ionic conduction observed for these samples encourage further investigations for better conductors in this system.     

Formation of gels and gel-glasses in the VO2(V2O5)-SiO2 systems

Blue-coloured gels have been prepared in the VO₂-SiO₂ system up to 80 mol% VO₂ by sol-gel technology using TEOS and aqueous solutions of VOSO₄·5H₂O. It is established by means of VIS and ESR spectra that at low temperatures VO²⁺ complexes are formed. An oxidation of V⁴⁺ has taken place with increasing temperature, and V₂O₅ and cristobalite have been separated. Silica gel glasses stable up to 800°C have been obtained from gels containing 1–3 mol% VO₂.     

Catalytic CO Oxidation by O2 Mediated by Noble‐Metal‐Free Cluster Anions Cu2VO3–5−

Catalytic CO oxidation by molecular O₂ is an important model reaction in both the condensed phase and gas‐phase studies. Available gas‐phase studies indicate that noble metal is indispensable in catalytic CO oxidation by O₂ under thermal collision conditions. Herein, we identified the first example of noble‐metal‐free heteronuclear oxide cluster catalysts, the copper–vanadium bimetallic oxide clusters Cu₂VO_3–5^? for CO oxidation by O₂. The reactions were characterized by mass spectrometry, photoelectron spectroscopy, and density functional calculations. The dynamic nature of the Cu?Cu unit in terms of the electron storage and release is the driving force to promote CO oxidation and O₂ activation during the catalysis.     

CO Oxidation Promoted by the Gold Dimer in Au2VO3− and Au2VO4− Clusters

Investigations on the reactivity of atomic clusters have led to the identification of the elementary steps involved in catalytic CO oxidation, a prototypical reaction in heterogeneous catalysis. The atomic oxygen species O^.? and O^2? bonded to early‐transition‐metal oxide clusters have been shown to oxidize CO. This study reports that when an Au₂ dimer is incorporated within the cluster, the molecular oxygen species O₂^2? bonded to vanadium can be activated to oxidize CO under thermal collision conditions. The gold dimer was doped into Au₂VO₄^? cluster ions which then reacted with CO in an ion‐trap reactor to produce Au₂VO₃^? and then Au₂VO₂^?. The dynamic nature of gold in terms of electron storage and release promotes CO oxidation and O? O bond reduction. The oxidation of CO by atomic clusters in this study parallels similar behavior reported for the oxidation of CO by supported gold catalysts.     

Physicochemical properties of V5+ doped LiCoPO4 as cathode materials for Li-ion batteries

Phase pure olivine type V⁵⁺ doped and un-doped LiCoPO₄ (LiCo_1?xV_xPO₄ & LiCoP_1?xV_xO₄; x = 0.02, 0.04 and 0.06) were synthesized by combustion method. Compound formation temperature and thermal stability of the materials were studied through thermal analysis. X-ray diffraction pattern shows the prepared material possesses an orthorhombic structure with Pnmb space group. Further the functional group and vibrational analysis were carried out by Fourier Transform Infra-red and Raman spectroscopy techniques. The Scanning Electron Micrographs depicts the irregular shaped morphology with particle agglomeration of the pristine and doped LiCoPO₄ materials. The structural variation on addition of dopant on both sites Co²⁺ & P⁵⁺ were revealed from XPS spectra. The electrochemical aspects of these materials were investigated by cyclic voltammetry studies in conjunction with electrochemical impedance spectroscopy and chronoamperommetry measurements to understand the redox reactions and their capacity contribution at higher voltages. The EIS analysis shows that the conductance value was decreased for the vanadium doped samples for both the Co site and P site, which infers that the V⁵⁺ addition doesn’t make any significant enhancement in the electrochemical performance of the LiCoPO₄.     

In-situ Grown SnO2 Nanospheres on Reduced GO Nanosheets as Advanced Anodes for Lithium-ion Batteries

Nanostructured tin dioxide (SnO₂) has emerged as a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity (1494 mA h g⁻¹) and excellent stability. Unfortunately, the rapid capacity fading and poor electrical conductivity of bulk SnO₂ material restrict its practical application. Here, SnO₂ nanospheres/reduced graphene oxide nanosheets (SRG) are fabricated through in-situ growth of carbon-coated SnO₂ using template-based approach. The nanosheet structure with the external layer of about several nanometers thickness can not only accommodate the volume change of Sn lattice during cycling but also enhance the electrical conductivity effectively. Benefited from such design, the SRG composites could deliver an initial discharge capacity of 1212.3 mA h g⁻¹ at 0.1 A g⁻¹, outstanding cycling performance of 1335.6 mA h g⁻¹ after 500 cycles at 1 A g⁻¹, and superior rate capability of 502.1 mA h g⁻¹ at 5 A g⁻¹ after 10 cycles. Finally, it is believed that this method could provide a versatile and effective process to prepare other metal-oxide/reduced graphene oxide (rGO) 2D nanocomposites.