Spectrophotometric identification of a mixed-valence cation-cation complex between aquadioxovanadium(V) and aquaoxovanadium(IV) ions in perchloric, sulfuric, and hydrochloric acid media

Leveraging visible and near-infrared spectroelectrochemistry to calibrate a robust model for Vanadium(IV/V) in varying nitric acid and temperature levels

Spectroelectrochemistry and optimal design of experiments can be used to rapidly build accurate models for species quantification and enable a greater level of process awareness. Optical spectroscopy can provide vital elemental and molecular information, but several hurdles must be overcome before it can become a widely adopted analytical method for remote analysis in the nuclear field. Analytes with varying oxidation state, acid concentration, and fluctuating temperature must be efficiently accounted for to minimize time and resources in restrictive hot cell environments. The classic one-factor-at-a-time approach is not suitable for frequent calibration/maintenance operations in this setting. Therefore, a novel alternative was developed to characterize a system containing vanadium(IV/V) (0.01-0.1 M), nitric acid (0.1-4 M), and varying temperatures (20-45 °C). Spectroelectrochemistry methods were used to acquire a sample set selected by optimal design of experiments. This new approach allows for the accurate analysis of vanadium and HNO₃ concentration by leveraging UV-Vis-NIR absorption spectroscopy with robust and accurate chemometric models. The top model's root mean squared error of prediction percent values were 3.47%, 4.06%, 3.40%, and 10.9% for V(IV), V(V), HNO₃, and temperature, respectively. These models, efficiently developed using the designed approach, exhibited strong predictive accuracy for vanadium and acid with varying oxidation states and temperature using only spectrophotometry, which advances current technology for real-world hot cell applications. Additionally, Nernstian analysis of the V(IV/V) standard potential was performed using traditional absorbance methods and multivariate curve resolution (MCR). The successful tests demonstrated that MCR Nernst tests may be valuable in highly convoluted spectral systems to better understand the redox processes' behavior.

Determination of the through-plane profile of vanadium species in hydrated Nafion studied with micro X-ray absorption near-edge structure spectroscopy - proof of concept

Vanadium-ion transport through the polymer membrane results in a significant decrease in the capacity of vanadium redox flow batteries. It is assumed that five vanadium species are involved in this process. Micro X-ray absorption near-edge structure spectroscopy (micro-XANES) is a potent method to study chemical reactions during vanadium transport inside the membrane. In this work, protocols for micro-XANES measurements were developed to enable through-plane characterization of the vanadium species in Nafion 117 on beamline P06 of the PETRA III synchrotron radiation facility (DESY, Hamburg, Germany). A Kapton tube diffusion cell with a diameter of 3 mm was constructed. The tube diameter was chosen in order to accommodate laminar flow for cryogenic cooling while allowing easy handling of the cell components by hand. A vertical step size of 2.5 µm and a horizontal step size of 5 µm provided sufficient resolution to resolve the profile and good statistics after summing up horizontal rows of scan points. The beam was confined in the horizontal plane to account for the waviness of the membrane. The diffusion of vanadium ions during measurement was inhibited by the cryogenic cooling. Vanadium oxidation, e.g. by water radiolysis (water percentage in the hydrated membrane ∼23 wt%), was mitigated by the cryogenic cooling and by minimizing the dwell time per pixel to 5 ms. Thus, the photo-induced oxidation of V3+ in the focused beam could be limited to 10%. In diffusion experiments, Nafion inside the diffusion cell was exposed on one side to V3+ electrolyte and on the other side to VO2+. The ions were allowed to diffuse across the through-plane orientation of the membrane during one of two short defrost times (200 s and 600 s). Subsequent micro-XANES measurements showed the formation of VO2+ from V3+ and VO2+ inside the water body of Nafion. This result proves the suitability of the experimental setup as a powerful tool for the determination of the profile of vanadium species in Nafion and other ionomeric membranes.

Characterization of Dimeric Vanadium Uptake and Species in Nafion™ and Novel Membranes from Vanadium Redox Flow Batteries Electrolytes

A core component of energy storage systems like vanadium redox flow batteries (VRFB) is the polymer electrolyte membrane (PEM). In this work, the frequently used perfluorosulfonic-acid (PFSA) membrane Nafion™ 117 and a novel poly (vinylidene difluoride) (PVDF)-based membrane are investigated. A well-known problem in VRFBs is the vanadium permeation through the membrane. The consequence of this so-called vanadium crossover is a severe loss of capacity. For a better understanding of vanadium transport in membranes, the uptake of vanadium ions from electrolytes containing V_dimer(IV–V) and for comparison also V(II), V(III), V(IV), and V(V) by both membranes was studied. UV/VIS spectroscopy, X-ray absorption near edge structure spectroscopy (XANES), total reflection X-ray fluorescence spectroscopy (TXRF), inductively coupled plasma optical emission spectrometry (ICP-OES), and micro X-ray fluorescence spectroscopy (microXRF) were used to determine the vanadium concentrations and the species inside the membrane. The results strongly support that V_dimer(IV–V), a dimer formed from V(IV) and V(V), enters the nanoscopic water-body of Nafion™ 117 as such. This is interesting, because as of now, only the individual ions V(IV) and V(V) were considered to be transported through the membrane. Additionally, it was found that the V_dimer(IV–V) dimer partly dissociates to the individual ions in the novel PVDF-based membrane. The V_dimer(IV–V) dimer concentration in Nafion™ was determined and compared to those of the other species. After three days of equilibration time, the concentration of the dimer is the lowest compared to the monomeric vanadium species. The concentration of vanadium in terms of the relative uptake λ = n(V)/n(SO₃) are as follows: V(II) [λ = 0.155] > V(III) [λ = 0.137] > V(IV) [λ = 0.124] > V(V) [λ = 0.053] > V_dimer(IV–V) [λ = 0.039]. The results show that the V_dimer(IV–V) dimer needs to be considered in addition to the other monomeric species to properly describe the transport of vanadium through Nafion™ in VRFBs.

Photometrical Determination of the State-of-Charge in Vanadium Redox Flow Batteries Part II: In Combination with Open-Circuit-Voltage

By means of in-situ UV/Vis/NIR spectrometry, separately both in the anolyte as well as in the catholyte of a vanadium redox flow battery (single cell) partial state-of-charge values are determined online. The UV/Vis/NIR spectroscopic experimental set-up is calibrated using the state-of-charge value determined from measurements of the open-circuit-voltage (OCV) in the pristine state of the battery which is related to Nernst’s equation taking into account also H+ formation/consumption during the V4+/V5+ redox process. The comparison of both partial state-of-charge values indicates a possible imbalance of the battery, which can occur after long-term operation.

Photometrical Determination of the State-of-Charge in Vanadium Redox Flow Batteries Part I: In Combination with Potentiometric Titration

The stepwise oxidation of vanadium ions in electrolytes, as used in all vanadium redox flow batteries (VRFB), is studied offline by a combination of potentiometric titration and simultaneous UV/Vis/NIR spectroscopy. Eight different total vanadium concentrations between 0.2 mol L−1 and 1.6 mol L−1 have been investigated. The analyte (titrand, V2+ solution) is the anolyte (V2+/V3+ side) of a fully charged laboratory vanadium redox flow battery (VRFB). Absorption maxima are observed at λ = 850 nm for V2+ and at λ = 400 nm for V3+, the corresponding absorption coefficients are determined. In the former case an extrapolation procedure is necessary because during transfer from the VRFB to the titration cell, oxidation to V3+ by ambient oxygen cannot completely be avoided. Based on the knowledge of the absorption coefficients, via simultaneous photometry of V2+ and V3+, the state-of-charge of the anolyte of a VRFB can be determined. In the catholyte (V4+/V5+ side) of a VRFB the formation of an intermediate mixed valence VIV–VV complex at large vanadium concentration prevents a simple photometric SOC determination.

Vanadium Electrolyte Studies for the Vanadium Redox Battery-A Review

The electrolyte is one of the most important components of the vanadium redox flow battery and its properties will affect cell performance and behavior in addition to the overall battery cost. Vanadium exists in several oxidation states with significantly different half-cell potentials that can produce practical cell voltages. It is thus possible to use the same element in both half-cells and thereby eliminate problems of cross-contamination inherent in all other flow battery chemistries. Electrolyte properties vary with supporting electrolyte composition, state-of-charge, and temperature and this will impact on the characteristics, behavior, and performance of the vanadium battery in practical applications. This Review provides a broad overview of the physical properties and characteristics of the vanadium battery electrolyte under different conditions, together with a description of some of the processing methods that have been developed to produce vanadium electrolytes for vanadium redox flow battery applications.

Unusual phenomena in the reduction process of vanadium(v) on a graphite electrode at high overpotentials

Two unusual phenomena are observed at a transition potential (E_K) by steady-state potentiodynamic polarization and impedance spectroscopy measurements. The possible vanadium complexes such as V₂O₃³⁺ might change the reduction mechanism at high overpotentials.

Fe(III) coordination properties of two new saccharide-based enterobactin analogues: methyl 2,3,4-tris-O-[N-[2,3-di(hydroxy)benzoyl-glycyl]-aminopropyl]-alpha-D-glucopyranoside and methyl 2,3,4-tris-O-[N-[2,3-di-(hydroxy)-benzoyl]-aminopropyl]-alpha-D-glucopyranoside

The synthesis of two saccharide-based enterobactin analogues, methyl 2,3,4-tris-O[-N[2,3-di(hydroxy)benzoyl-glycyl]-aminopropyl]-alpha-D-glucopyranoside (H(6)L(A)) and methyl 2,3,4-tris-O-[N-[2,3-di(hydroxy)benzoyl]-aminopropyl]-alpha-D-glucopyranoside (H(6)L(B)), are reported along with their pK(a) values, Fe(III) binding constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titration techniques. Use of a saccharide platform to synthesize a hexadentate triscatechol chelator provides some advantages over other approaches to enterobactin models, including significant water solubility, resistance to hydrolysis, and backbone chirality which may provide favorable recognition and availability to cells. The protonation constants for the catechol ligand hydroxyl moieties were determined for both ligands and found to be significantly different, which is attributed to the differences in the spacer chain of the two triscatechols. Proton dependent Fe(III)-ligand equilibrium constants were determined using a model involving the sequential protonation of the Fe(III)-ligand complex. These results were used to calculate the formation constants, log beta(110) = 41.38 for Fe(III)-H(6)L(A) and log beta(110) = 46.38 for Fe(III)-H(6)L(B). The calculated pM values of 28.6 for H(6)L(A) and 28.3 for H(6)L(B) indicate that these ligands possess Fe(III) affinities comparable to or greater than other enterobactin models and are thermodynamically capable of removing Fe(III) from transferrin.

Synthesis and aqueous solution properties of multinuclear oxo-bridged vanadium(IV/V) complexes

Reaction of the multifunctional phenolic ligands 2,5-bis[N,N-bis(carboxymethyl)aminomethyl]hydroquinone (H6cahq), 2,2'-bis[N,N-bis(carboxymethyl)aminomethyl]-4,4'-isopropylidenediphen ol(H6capd),2,2',2''-tris[N,N-bis(carboxymethyl)aminomethyl]-1,1 ,1-tris(4-hydroxyphenyl)ethane (H9catp) and the monofunctional 2-[N,N-bis(carboxymethyl)aminomethyl]-4-carboxyphenol (H3cacp), with VOSO4 and NaVO3 affords the oxo-bridged mixed-valence vanadium(IV/V) Na6[(VO)4(mu-O)2(mu-cahq)2] x Na2SO4 x 20H2O (1), HnNa(3-n)[(VO)2(mu-O)(mu-cacp)2] (2), HnNa(3-n)[(VO)4(mu-O)2(mu-capd)2] (3), HnNa(9-n)[(VO)6(mu-O)3(mu3-catp)2] (4). In addition to the synthesis, we report the infrared, magnetic, optical and electrochemical properties of these complexes. The hydrolytic stability at different pH values was also investigated using visible spectroscopy.

Mixed-Oxidation Divanadium(IV,V) Compound with Ligand Asymmetry: Electronic and Molecular Structure in Solution and in the Solid State

Reaction of [V(IV)OL(1)(Im)] (H(2)L(1) = S-methyl-3-((2-hydroxyphenyl)methyl)dithiocarbazate) with [V(V)OL(OCH(3))] allows isolation of (ImH)[L(1)OV-(&mgr;-O)-VOL] complexes 2 (H(2)L = H(2)L(2) = S-methyl-3-((5-bromo-2-hydroxyphenyl)methyl)dithiocarbazate) and 3 (H(2)L = H(2)L(1)), one of which (2) has ligand asymmetry not previously known in this type of complex. In the solid state, (ImH)[L(1)OV-(&mgr;-O)-VOL(2)] (2) provides an example of a divanadium(IV,V) compound with a syn angular [V(2)O(3)](3+) core structure that exhibits crystallographically imposed mirror symmetry due to static disorder. Crystals of 2 are orthorhombic, space group Pnma, with a = 10.740(2) Å, b = 18.912(4) Å, c = 17.163(4) Å, and Z = 4. In toluene at room temperature, both 2 and 3 have 8-line EPR spectra, characteristic of trapped-valence structure. When acetonitrile is added to these solutions, the spectra reveal 15-line features with asymmetric distortions that smooth out with the lowering of temperature. This probably has its origin in a solvent-dependent equilibrium involving two magnetically inequivalent structural forms of the divanadium(IV,V) compound, with syn angular and anti linear structures of the [V(2)O(3)](3+) core. Variable temperatures (298-220 K) (51)V NMR spectroscopic studies in solution also support this view. In acetonitrile, both 2 and 3 exhibit an intervalence transfer band in the near-IR region at ca. 970 nm (epsilon, 1600 and 1480 M(-)(1) cm(-)(1) for 2 and 3, respectively) and they undergo one-electron reversible oxidation at ca. 0.40 V (vs SCE) due to the V(IV)V(V)/V(V)V(V) couple.

Solution and Solid State Properties of [N-(2-Hydroxyethyl)iminodiacetato]vanadium(IV), -(V), and -(IV/V) Complexes(1)

A mononuclear vanadium(IV), a mononuclear vanadium(V), and a binuclear mixed valence vanadium(IV/V) complex with the ligand N-(2-hydroxyethyl)iminodiacetic acid (H(3)hida) have been structurally characterized. Crystal data for [VO(Hhida)(H(2)O)].CH(3)OH (1): orthorhombic; P2(1)2(1)2(1); a= 6.940(2), b = 9.745(3), c= 18.539(4) Å; Z = 4. Crystal data for Na[V(O)(2)(Hhida)(2)].4H(2)O (2): monoclinic; P2(1)/c; a = 6.333(2), b = 18.796(2), c = 11.5040(10) Å; beta = 102.53(2) degrees; Z = 4. Crystal data for (NH(4))[V(2)(O)(2)(&mgr;-O)(Hhida)(2)].H(2)O (3): monoclinic; C2/c; a = 18.880(2), b= 7.395(2), c = 16.010(2) Å; beta = 106.33(2) degrees; Z = 4. The mononuclear vanadium(IV) and vanadium(V) complexes are formed from the monoprotonated Hhida(2)(-) ligand, and their structural and magnetic characteristics are as expected for six-coordinate vanadium complexes. An interesting structural feature in these complexes is the fact that the two carboxylate moieties are coordinated trans to one another, whereas the carboxylate moieties are coordinated in a cis fashion in previously characterized complexes. The aqueous solution properties of the vanadium(IV) and -(V) complexes are consistent with their structures. The vanadium(V) complex was previously characterized; in the current study structural characterization in the solid state is provided. X-ray crystallography and magnetic methods show that the mixed valence complex contains two indistinguishable vanadium atoms; the thermal ellipsoid of the bridging oxygen atom suggests a type III complex in the solid state. Magnetic methods show that the mixed valence complex contains a free electron. Characterization of aqueous solutions of the mixed valence complex by UV/vis and EPR spectroscopies suggests that the complex may be described as a type II complex. The Hhida(2)(-) complexes have some similarities, but also some significant differences, with complexes of related ligands, such as nitrilotriacetate (nta), N-(2-pyridylmethyl)iminodiacetate (pmida), and N-(S)-[1-(2-pyridyl)ethyl]iminodiacetate (s-peida). Perhaps most importantly, the mixed valence Hhida(2)(-) complex is significantly less stable than the corresponding pmida and s-peida complexes of similar overall charge but very similar in stability to the nta and V(2)O(3)(3+) complexes with higher charges. Thus, there is the potential for designing stable mixed valence dimers.