Chloride supporting electrolytes for all-vanadium redox flow batteries

This paper examines vanadium chloride solutions as electrolytes for an all-vanadium redox flow battery. The chloride solutions were capable of dissolving more than 2.3 M vanadium at varied valence states and remained stable at 0-50 °C. The improved stability appeared due to the formation of a vanadium dinuclear [V(2)O(3)·4H(2)O](4+) or a dinuclear-chloro complex [V(2)O(3)Cl·3H(2)O](3+) in the solutions over a wide temperature range. The all-vanadium redox flow batteries with the chloride electrolytes demonstrated excellent reversibility and fairly high efficiencies. Only negligible, if any, gas evolution was observed. The improved energy capacity and good performance, along with the ease in heat management, would lead to substantial reduction in capital cost and life-cycle cost, making the vanadium chloride redox flow battery a promising candidate for stationary applications.     

Titrimetric determination of vanadium(IV) with cerium (IV)sulphate at room temperature using ferroin as indicator

A procedure has been developed for the visual titrimetric determination of vanadium(IV) with cerium(IV) sulphate, using ferroin is redox indicator. The method has been extended to the determination of iron(II) and vanadium(IV) in mixtures.     

High electro-catalytic graphite felt/Mn O2composite electrodes for vanadium redox flow batteries

A mild and simple synthesis process for large-scale vanadium redox flow batteries(VRFBs)energy storage systems is desirable.A graphite felt/Mn O_2(GF-MNO)composite electrode with excellent electrocatalytic activity towards VO~(2+)/VO_2~+redox couples in a VRFB was synthesized by a one-step hydrothermal process.The resulting GF-MNO electrodes possess improved electrochemical kinetic reversibility of the vanadium redox reactions compared to pristine GF electrodes,and the corresponding energy efficiency and discharge capacity at 150 m A cm~(-2)are increased by 12.5%and 40%,respectively.The discharge capacity is maintained at 4.8 A h L~(-1)at the ultrahigh current density of 250 m A cm~(-2).Above all,80%of the energy efficiency of the GF-MNO composite electrodes is retained after 120 charge-discharge cycles at 150 m A cm~(-2).Furthermore,these electrodes demonstrated that more evenly distributed catalytic active sites were obtained from the Mn O_2particles under acidic conditions.The proposed synthetic route is facile,and the raw materials are low cost and environmentally friendly.Therefore,these novel GF-MNO electrodes hold great promise in large-scale vanadium redox flow battery energy storage systems.     

全钒液流电池磺化石墨烯/Nafion复合膜的研究

本文通过磺化石墨烯对Nafion膜进行改性,研究了磺化石墨烯/Nafion复合膜（GRS-Nafion复合膜）的吸水率、电阻率和钒离子迁移数. 结果表明,经磺化石墨烯改性之后,GRS-Nafion复合膜的面电阻和钒离子渗透率显著降低. 全钒液流电池的测试结果表明,GRS-Nafion复合膜有着更加优异的电化学性能,展示出GRS-Nafion复合膜在液流电池中的应用潜力.     

Flow injection analysis for oxidation state speciation of vanadium(IV) and vanadium(V) in natural water.

A sensitive and simultaneous spectrophotometric flow injection method for the determination of vanadium(IV) and vanadium(V) is proposed. The method is based on the effect of ligands such as 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPTZ) and diphosphate on the conditional redox potential of iron(III)/iron(II) system. A four-channel flow system is assembled. In this flow system, diluted hydrochloric acid (1.0 x 10(-2) mol dm(-3)) as a carrier for standard/sample, acetate buffer (pH 5.5) as a carrier for diphosphate solution, an equimolar mixed solution of iron(III) and iron(II) and a TPTZ solution are delivered, so that the baseline absorbance can be established by forming a constant amount of iron(II)-TPTZ complex (lambda(max) = 593 nm). Vanadium(IV) and/or vanadium(V) (400 microL) and diphosphate (200 microL) solutions are simultaneously introduced into the flow system; in this system the diphosphate solution passes through a delay coil. The potential of the iron(III)/iron(II) system increases in the presence of TPTZ, and therefore vanadium(IV) is easily oxidized by iron(III) to vanadium(V) to produce an iron(II)-TPTZ complex (a positive peak for vanadium(IV) appears). On the other hand, the potential of the redox system decreases in the presence of diphosphate, so that vanadium(V) can be easily reduced by iron(II) to vanadium(IV). In this case, the amount of iron(II) decreases according to the amount of vanadium(V). As a result, the produced iron(II)-TPTZ complex decreases (a negative peak for vanadium(V) appears). In this manner, two peaks for vanadium(IV) and vanadium(V) can be alternately obtained. The limits of detection (S/N = 3) are 1.98 x 10(-7) and 2.97 x 10(-7) mol dm(-3) for vanadium(IV) and vanadium(V), respectively. The method is applied to the simultaneous determination of vanadium(IV) and vanadium(V) in commercial bottled mineral water samples.     

Water transport study across commercial ion exchange membranes in the vanadium redox flow battery

The water transfer behaviour of Selemion CMV, AMV and DMV membranes (Asahi Glass, Japan) has been studied in the vanadium redox cell, as was the water transfer across Nafion 117 membrane (E.I. Du Pont, USA). The earlier water transport studies of a variety of commercial ion exchange membranes and non-ionic separators in the vanadium redox cell have shown that the net water transport through anion exchange membranes and non-ionic separators in the vanadium redox cell is from the positive half cell (+ve) to the negative half cell (−ve), while for cation exchange membranes the net water transport is in the opposite direction. In the present study, it was found that a significant amount of water is transferred across cation exchange membranes from the −ve vanadium half cell electrolyte to the +ve vanadium half cell electrolyte by the hydration shells of V²⁺ and V³⁺ ions which carry a large amount of water and can easily permeate through cation exchange membranes due to their relatively high charge numbers. The net amount of water of hydration which is transferred across anion exchange membranes from the −ve half cell electrolyte, however, is almost equal to the net amount of water of hydration which is transferred from the +ve half cell electrolyte. Thus, the net amount of water which is transferred across anion exchange membranes is in the same direction as the osmotic water transfer.     

Mn2+浓度对钒液流电池正极液的电化学性能影响

电解液中金属离子会影响钒液流电池的电化学性能。本文采用循环伏安法和电化学阻抗谱研究了正极液中Mn2+浓度对V髨/V(Ⅳ)电对的氧化还原过程影响规律,发现Mn2+在正极液中没有发生副反应,但严重影响V髨/V(Ⅳ)的反应活性、电极反应可逆性、离子扩散与电荷转移反应等电化学性能。循环伏安测试结果表明Mn2+浓度为0.04-0.13 g.L-1时,V髨/V(Ⅳ)电对电极反应可逆性和反应活性较高,钒离子扩散系数由参照溶液中的8.89×10-7-1.098×10-6增大至1.302×10-6-1.800×10-6 cm2.s-1,提高了-60%;电化学阻抗测试结果表明Mn2+浓度为0-0.04 g.L-1时,V髨/V(Ⅳ)电对电极反应阻抗和界面阻抗均较参照溶液中的增加不明显,但当Mn2+浓度增至0.07 g.L-1时,上述阻抗值较参照溶液增大了25%-28%。基于二者结果,Mn2+对电极反应有不同程度的负面影响,但是适当的Mn2+浓度有利于钒离子的扩散。     

Determination of iron, nickel, chromium and vanadium by means of redox and ion-exchange columns

Redox and anion-exchange columns have been used to separate and determine iron, nickel, chromium and vanadium in solution. The anion-exchange columns provide some of the separations, and the redox columns are used for the determination of the iron, chromium and vanadium. The chromium and vanadium may be determined in the presence of the iron by adjustment of the acidity in the redox column. By using a column "memory" technique, titration of the actual metal solution has been avoided. The method shows some advantages over conventional methods.     

Treated carbon felt as electrode material in vanadium redox flow batteries: a study of the use of carbon nanotubes as electrocatalyst

Electrodes for large-scale usage in vanadium redox flow battery are usually fabricated without any electrocatalyst due to the lack of good, viable options. The best performance is achieved of carbon-based materials. Recently, some researchers have been reported regarding the use of carbon nanotube as the electrocatalyst in the vanadium redox flow batteries. However, these researches have been carried out without making any comparison between the performance of the traditional method and the carbon nanotube electrocatalyst. In the present study, the loading of multi-walled carbon nanotube, the acid–heat treatment, and their combination were used to modify the carbon felt electrode to be applied in the vanadium redox flow battery. The obtained results showed better electrochemical properties for acid–heat-treated carbon felt electrode compared to the carbon nanotube-loaded one. The best electrode was obtained for using in a vanadium redox flow battery in terms of electrochemical and surface properties after applying a combination of two modification strategies. Applying this proposed method in modification of the carbon felt electrode increased its hydrophilicity more than 17 times and its capability to absorb VOSO₄ solution more than eight times. Also, the charge transfer resistance of a modified electrode, by the combination of the carbon nanotube and the acid–heat treatment, significantly decreased in both positive and negative poles of vanadium redox flow battery. Consequently, the exchange current density enhanced more than 100- and 175-fold in positive and negative poles, respectively, in comparison with carbon felt electrode.     

A membrane based on sulfonated polystyrene for a vanadium solid-salt battery

An ion exchange membrane (IEM) usually serves as a separator between the two half-cells and provides an ionic conduction path in redox flow batteries. The new vanadium solid-salt battery (VSSB) presents higher energy density than the traditional vanadium redox flow batteries (VRFBs). However, present IEMs are based on very expensive Nafion® membranes. In pursuit of lower cost, a membrane from sulfonated polystyrene (PE-01) is used for VSSB. In comparison with the traditional Nafion® 1135, PE-01 shows high energy efficiency with good cycling performance at current densities less than 10 mA cm^?2. This suggests that sulfonated polystyrene membrane is a promising candidate as separator for VSSB.     

Micellar Catalysis on the Redox Reaction of Ascorbic Acid-Vanadium(V) System

Stoichiometry of the redox reaction of vanadium(V) by ascorbic acid (H₂A) has been experimentally determined to be H₂A + 2V(V) → A + 2V(IV) + 2H ⁺ . Evidence of induced polymerization of acrylonitrile and the reduction of mercuric chloride indicates that a free-radical mechanism operates during the course of reaction. Vanadium(V) is only reduced to vanadium(IV). The kinetics of this redox reaction have been investigated spectrophotometrically at 35°C in acidic media of H₂SO₄. In this kinetic study we have observed the nature of vanadium(V)-H₂A interaction in presence of anionic surfactant of SDS. In V(V)-H₂A system, the addition of anionic surfactant (SDS) enhanced the reaction rate and shows catalytic effect. This trend was explained by the incorporation/solubilization of vanadium(V) and ascorbic acid in the Stern layer.     

Synergistic Catalysis of SnO2/Reduced Graphene Oxide for VO2+/VO2+ and V2+/V3+ Redox Reactions

In spite of their low cost, high activity, and diversity, metal oxide catalysts have not been widely applied in vanadium redox reactions due to their poor conductivity and low surface area. Herein, SnO₂/reduced graphene oxide (SnO₂/rGO) composite was prepared by a sol–gel method followed by high-temperature carbonization. SnO₂/rGO shows better electrochemical catalysis for both redox reactions of VO²⁺/VO₂⁺ and V²⁺/V³⁺ couples as compared to SnO₂ and graphene oxide. This is attributed to the fact that reduced graphene oxide is employed as carbon support featuring excellent conductivity and a large surface area, which offers fast electron transfer and a large reaction place towards vanadium redox reaction. Moreover, SnO₂ has excellent electrochemical activity and wettability, which also boost the electrochemical kinetics of redox reaction. In brief, the electrochemical properties for vanadium redox reactions are boosted in terms of diffusion, charge transfer, and electron transport processes systematically. Next, SnO₂/rGO can increase the energy storage performance of cells, including higher discharge electrolyte utilization and lower electrochemical polarization. At 150 mA cm⁻², the energy efficiency of a modified cell is 69.8%, which is increased by 5.7% compared with a pristine one. This work provides a promising method to develop composite catalysts of carbon materials and metal oxide for vanadium redox reactions.     

High electro-catalytic graphite felt/MnO<Subscript>2</Subscript> composite electrodes for vanadium redox flow batteries

A mild and simple synthesis process for large-scale vanadium redox flow batteries (VRFBs) energy storage systems is desirable. A graphite felt/MnO₂ (GF-MNO) composite electrode with excellent electrocatalytic activity towards VO²⁺/VO₂⁺ redox couples in a VRFB was synthesized by a one-step hydrothermal process. The resulting GF-MNO electrodes possess improved electrochemical kinetic reversibility of the vanadium redox reactions compared to pristine GF electrodes, and the corresponding energy efficiency and discharge capacity at 150 mA cm^?2 are increased by 12.5% and 40%, respectively. The discharge capacity is maintained at 4.8 A h L^?1 at the ultrahigh current density of 250 mA cm^?2. Above all, 80% of the energy efficiency of the GFMNO composite electrodes is retained after 120 charge-discharge cycles at 150 mA cm^?2. Furthermore, these electrodes demonstrated that more evenly distributed catalytic active sites were obtained from the MnO₂ particles under acidic conditions. The proposed synthetic route is facile, and the raw materials are low cost and environmentally friendly. Therefore, these novel GFMNO electrodes hold great promise in large-scale vanadium redox flow battery energy storage systems.     

The Chemistry of Vanadium bis-Phenolate NHC Complexes in Three Oxidation States

Twenty-one vanadium bis-phenolate benzimidazolylidene complexes, spanning three oxidation states, have been investigated. Special emphasis is placed on their salt metathesis reactivity and the accessibility of the +IV oxidation state by reductive or oxidative routes, starting from vanadium(V) or vanadium(III) respectively. While the reductive route is highly dependent on the reducing agent and starting material used, the oxidative route gives clean access to vanadium(IV) dihalide complexes. The low-valent vanadium(III) complexes are excellent precursors for salt metathesis reactions which lead to the isolation of a rare vanadium(III) NHC alkyl complex. All new complexes have been characterized by (paramagnetic) ¹H NMR and ⁵¹V NMR, UV–VIS, IR and EPR spectroscopy as well as elemental analysis. Cyclic voltammetry has been performed in selected cases to study the influence of imido or phenolate supporting ligands towards the redox-potential of the vanadium(V/IV) redox couple compared to the parent oxo-chlorido complex A .     

Indirect Spectrophotometric Determination of Vanadium(IV) by Flow Injection Analysis Based on the Redox Reaction with Copper(II) in the Presence of Neocuproine

Abstract

An indirect photometric method with a continuous-flow analysis is presented for the determination of trace amounts of vanadium(IV). It is based on the redox reaction of copper(II) with vanadium(1V) in the presence of neocuproine. In the presence of neocuproine, copper(I1) is reduced easily by vanadium(I V) to a copper(1)-neocuproine complex, which shows a n absorption maximum at 454 nm. By measuring t h e absorbance of the complex at this wavelength, vanadium(1V) in t h e range 2×10^?6 - 8 × mol dm^?5 mol dm^?3 can be determined at a rate of 120 samples h^?1. The fractional determination of vanadium(1V) and iron(I1) is also studied.     

Detrimental role of hydrogen evolution and its temperature-dependent impact on the performance of vanadium redox flow batteries

This paper addresses the damaging role of the parasitic hydrogen evolution reaction(HER) in the negative half-cell of a vanadium redox flow battery(VRFB) on state-of-the-art carbon felt electrodes at different temperatures. It was found that increasing the temperature resulted in a better catalytic performance for both the positive and negative half-cell reactions. In addition, increasing the temperature significantly enhanced the undesired HER at the negative side. Operating the VRFB cell at higher temperature led to a decrease in the coulombic efficiency attributed to the higher hydrogen production. More pronounced hydrogen production caused an oxidation on the surface of the carbon fibers and a degradation of the electrode as indicated from scanning electron microscopy and X-ray photoelectron spectroscopy measurements. This observed degradation results in fading of the overall performance of the vanadium redox flow battery over time.     

双核钒酰腙配合物[VO(C14H1oN202]2(μ-O)的合成与表征

The dinuclear complex [VO (L)]2 (μ-O) (HL = salicylaldehyde benzoylhydrazone) was synthesized and characterized by elemental analysis, IR, UV, cyclic voltammetry. The results showed that two oxygen atoms and one nitrogen atom of the tridentate hydrazone ligand are coordinated to vanadium(Ⅴ) atom. The vanadium atom approximates to a distorted square pyramidal coordination sphere. The redox potentials of the title complex in different solvents follow the order:CH2Cl2＜CH3CN＜DMF.     

四核环状钒氧酸三邻菲罗啉合锌合成, 单晶结构及电化学行为

本文用VOSO4在水溶液中合成了四核环状钒氧阴离子和三邻菲罗啉合锌阳离子组成的盐晶体[Zn(Phen3]2[V4O12].16H2O,晶体属P1空间群, a=1.3523(6), b=1.4803(5),c=1.2587(7)nm, α=112.37(3), β=105.89(4), γ=83.75(4)°, R=0.068。单晶结构解析表明, 标题化合物中钒氧阴离子的空间结构与以往用V2O5合成所得的阴离子相同。此外, 还用循环伏安法研究了钒氧阴离子[V4O12]^4^-在反应中的氧化还原性能。     

表面修饰模式对Nafion离子选择性影响及在VRFB中的应用

本文采用壳聚糖-磷钨酸层对Nafion膜表面分别进行单面和双面修饰改性,研究了修饰模式对Nafion膜钒离子渗透率、电导率及离子选择性的影响. 结果表明,单面、双面修饰改性均会使Nafion膜的钒离子渗透率显著降低,最高降幅分别达到89.9% (单面修饰) 和92.7% (双面修饰);单面、双面修饰改性均会使Nafion膜的电导率下降,但存在明显差异,在相同修饰厚度条件下,双面修饰改性对Nafion膜电导率的影响比单面修饰改性更小。因此,双面修饰复合膜展示出了比单面修饰复合膜更高的离子选择性,并且在修饰层厚度为17 μm时达到最大值（1.12×10⁵ S•min•cm^-3）. 基于优化的双面修饰Nafion膜的全钒液流电池,在充放电流密度30 mA•cm^-2 时,库伦效率和能量效率分别达到93.5%和 80.7%, 并且在测试时间内展示出良好的循环稳定性.     

Vereinfachung der kombinierten direkten ma\analytischen Bestimmung der Oxydationsstufe des Vanadins und des Vanadingehalts in Oxidsystemen

Zusammenfassung Es wird ein vereinfachtes ma\analytisches Verfahren beschrieben, das die kombinierte direkte Bestimmung der Oxydationsstufe des Vanadins und des Vanadingehalts in Vanadinoxidsystemen gestattet. Es beruht darauf, da\ die Substanz in PhosphorsÄure mit einer eingestellten KMnO₄-Lösung oxydierend gelöst und gegen Ferroin bis zum Äquivalenzpunkt von Vanadin(V) titriert wird. Nach genügender Pufferung, die zur Bildung farbloser Polyvanadat(V)-Ionen führt, ist es möglich, den Endpunkt der manganometrischen Titration auch ohne Verwendung eines Redoxindicators visuell mit hinreichender Genauigkeit zu erkennen. Anschlie\end wird die Lösung zur Bestimmung des Vanadingehalts mit einer FeSO₄-Lösung gegen Natrium-N-methyldiphenylamin-p-sulfonat bis zum Äquivalenzpunkt von Vanadin(IV) titriert. Aus dem VerhÄltnis zwischen KMnO₄- und FeSO₄-Verbrauch wird die Oxydationsstufe des Vanadins berechnet.

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Summary A simple method is described for the combined direct titration of the oxidation number of vanadium and of the vanadium content in vanadium oxide systems. The sample is dissolved in phosphoric acid using a KMnO₄ solution of known content as oxidizing agent. The titration with KMnO₄ is continued until the equivalence point of vanadium(V) is reached, indicated by the redox indicator ferroin. The equivalence point may also be observed visually, without a redox indicator, after buffering the solution until colourless polyvanadate(V) ions are formed. Then the vanadium content is determined by titration of the vanadium(V) solution with FeSO₄ solution to the equivalence point of vanadium(IV), indicated by the colour change of sodium N-methyldiphenylamine-p-sulphonate. From the KMnO₄/FeSO₄ ratio the oxidation number of vanadium can be calculated.