Chemistry Of Vanadium Research Articles

Air Plasma Modification of Graphite-Based Electrode for Improved Performance of Aqueous Redox Flow Batteries

Graphite felt is widely utilized as a porous carbon electrode in aqueous redox flow batteries (RFBs). However, its inherent hydrophobic nature and limited electrochemical activity present challenges. While the correlation between RFB performance and electrode properties has been extensively studied for vanadium chemistry and other inorganic redox active materials, it remains scarce in literature for organic systems. In this study, we employ air plasma treatment, known for its controllability, solvent-free nature, and short treatment duration, to modify commercially available graphite felt for RFB applications. A comprehensive analysis is conducted to establish correlations between plasma treatment, physical properties, electrochemical characteristics, and overall cell performance in aqueous RFBs. Comparative evaluation reveals a significant enhancement, with treated graphite felt exhibiting an 85% increase in capacity at 140 mA cm−2 compared to its pristine counterpart. By intentionally utilizing authentic RFB electrodes and employing state-of-the-art ferrocyanide posolyte, this study underscores the crucial role of the interface, even for rapid (reversible) redox-active materials utilized in AORFBs.

Long term performance evaluation of a commercial vanadium flow battery system

This paper describes the results of a performance review of a 10 kW/100 kWh commercial VFB system that has been commissioned and in operation for more than a decade. The evaluation focused on the system efficiencies, useable capacity, electrolyte stability and stack degradation. The analysis shows that the system has stable performance and very little capacity loss for over a decade since commissioning. Only very recently, a slight decrease in system discharge capacity (approximately 5 %) was observed and indicated by changes in the electrolyte valence state. The capacity was restored to the original level after conducting a rebalancing procedure. This demonstrates the advantage that the flow batteries employing vanadium chemistry have a very long cycle life. Furthermore, electrochemical impedance spectroscopy analysis was conducted on two of the battery stacks. Some degradation was observed in one of the stacks reflected by the increased charge transfer resistance. To further investigate the ageing of the stacks, reverse polarity tests were carried out. It is found that reversing the polarity both hydraulically and electrically can restore the stack performance. The study shows that the VFB is a reliable technology for large-scale energy storage applications.

Vanadium Chemistry in the 21st Century

Guest Editors Armando J. L. Pombeiro, Manas Sutradhar and Isabel Correia introduce the New Journal of Chemistry “Vanadium Chemistry in the 21st Century” themed collection.

Vanadium Compounds with Antidiabetic Potential.

Over the last four decades, vanadium compounds have been extensively studied as potential antidiabetic drugs. With the present review, we aim at presenting a general overview of the most promising compounds and the main results obtained with in vivo studies, reported from 1899-2023. The chemistry of vanadium is explored, discussing the importance of the structure and biochemistry of vanadate and the impact of its similarity with phosphate on the antidiabetic effect. The spectroscopic characterization of vanadium compounds is discussed, particularly magnetic resonance methodologies, emphasizing its relevance for understanding species activity, speciation, and interaction with biological membranes. Finally, the most relevant studies regarding the use of vanadium compounds to treat diabetes are summarized, considering both animal models and human clinical trials. An overview of the main hypotheses explaining the biological activity of these compounds is presented, particularly the most accepted pathway involving vanadium interaction with phosphatase and kinase enzymes involved in the insulin signaling cascade. From our point of view, the major discoveries regarding the pharmacological action of this family of compounds are not yet fully understood. Thus, we still believe that vanadium presents the potential to help in metabolic control and the clinical management of diabetes, either as an insulin-like drug or as an insulin adjuvant. We look forward to the next forty years of research in this field, aiming to discover a vanadium compound with the desired therapeutic properties.

Deciphering the structural origins of high sulfur solubility in vanadium-containing borosilicate glasses

The addition of V2O5 has been long known to increase the sulfur (as SO42-) solubility in borosilicate glasses. However, the mechanism governing this effect is still unknown. Although several studies have been published in the past two decades attempting to decipher the structural origins of increasing sulfur solubility as a function of V2O5 in borosilicate glasses, most of these studies remain inconclusive. The work presented in this paper attempts to answer the question, “Why does V2O5 increase sulfur solubility in borosilicate glasses?” Accordingly, a series of melt-quenched glasses in the system [30 Na2O – 5 Al2O3 – 15 B2O3 –50 SiO2](100-x) – xV2O5, where x varies between 0 – 9 mol.%, have been characterized for their short-to-intermediate range structure and the redox chemistry of vanadium using 11B, 27Al, 51V MAS NMR, Raman, and XPS spectroscopy. The impact of V2O5 on sulfur solubility in glasses has been followed by ICP-OES. The addition of ≤ 5 mol.% V2O5 results in a linear increase in sulfur solubility in the investigated glass system. Based on the results, we hypothesize that adding vanadium to the glasses increases their network connectivity, but reduces the network rigidity by replacing stronger Si–O–Si linkages with weaker Si–O–V linkages and forming (VO3)n-single chains. These modifications to the glass structure increase the flexibility of the network, thus making it possible to accommodate SO42− in their voids/open spaces.

Reversible Electrodeposition of Potassium-bridged Molecular Vanadium Oxides: A New Approach Towards Multi-Electron Storage.

Molecular metal oxides, so-called polyoxometalates (POMs), have shown outstanding performance as catalyst and lately attracted interest as materials in energy conversion and storage systems due to their capability of storing and exchanging multiple electrons. Here, we report the first example of redox-driven reversible electrodeposition of molecular vanadium oxide clusters, leading to the formation of thin films. The detailed investigation of the deposition mechanism reveals that the reversibility is dependent on the reduction potential. Correlating electrochemical quartz microbalance studies with X-ray photoelectron spectroscopy (XPS) data gave insight into the redox chemistry and oxidation states of vanadium in the deposited films in dependence on the potential window. A multi-electron reduction of the polyoxovanadate cluster, which facilitates the K+ cation assisted reversible formation of potassium vanadium oxide thin films was confirmed. At anodic potentials, re-oxidation of the polyoxovanadate and complete stripping of the thin film is observed for films deposited at potentials more positive than -500 mV vs. Ag/Ag+, while electrodeposition at more negative cathodic potential reduces the electrochemical reversibility of the process and increases the stripping overpotential. As proof of principle, we demonstrate the electrochemical performance of the deposited films for potential use in potassium-ion batteries.

Reversible elektrochemische Abscheidung Kalium‐verbrückter molekularer Vanadiumoxide: Ein neuer Ansatz zur Multielektronenspeicherung

AbstractMolekulare Metalloxide, so genannte Polyoxometallate (POMs), sind ausgezeichnete Katalysatoren, die kürzlich Beachtung als Materialien für Energieumwandlungs‐ und ‐speichersysteme gefunden haben, aufgrund ihrer Eigenschaft, mehrere Elektronen speichern und austauschen zu können. In dieser Studie berichten wir erstmals über die redox‐getriebene, reversible Elektrodeposition von molekularen Vanadiumoxid‐Clustern, die zur Abscheidung von dünnen Filmen führt. Die eingehende Untersuchung des Abscheidemechanismus zeigt eine Abhängigkeit der Reversibilität vom Reduktionspotential. Untersuchungen mittels der elektrochemischen Quarzkristall‐Mikrowaage und Studien mittels Röntgen‐Photoelektronenspektroskopie (XPS) geben Aufschluss über das Redoxverhalten und die Oxidationsstufen des Vanadiums in den abgeschiedenen Filmen in Abhängigkeit vom gewählten Potential während der Abscheidung. Es konnte eine Multielektronenreduktion des Polyoxovanadat‐Clusters bestätigt werden, welche eine durch K+‐Ionen unterstützte reversible Bildung von Kaliumvanadiumoxid‐Dünnschichten ermöglicht. Im positiven (anodischen) Potentialbereich wird eine Re‐oxidierung des Polyoxovanadats und ein vollständiges Entfernen (Stripping) des dünnen Films beobachtet für Filme, die bei einem Potential von höher als −500 mV vs. Ag/Ag+ abgeschieden wurden, während die Elektroabscheidung bei einem negativeren kathodischen Potential die elektrochemische Reversibilität des Prozesses verringert und das Potential für das Stripping erhöht. Das Potential dieser Filme und der mögliche Einsatz der abgeschiedenen Filme für die Anwendung in Kalium‐Ionen‐Batterien wird gezeigt.

Front Cover: The Chemistry of Vanadium bis‐Phenolate NHC Complexes in Three Oxidation States (Eur. J. Inorg. Chem. 28/2023)

Front Cover: The Chemistry of Vanadium bis‐Phenolate NHC Complexes in Three Oxidation States (Eur. J. Inorg. Chem. 28/2023)

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

AbstractTwenty‐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.

Ultrafast 3D Hybrid‐Ion Transport in Porous V2O5 Cathodes for Superior‐Rate Rechargeable Aqueous Zinc Batteries

AbstractLayered V2O5 is a star cathode material of rechargeable aqueous zinc‐based batteries (RAZBs) owing to the rich redox chemistry of vanadium, which commonly exhibits the 2D ion‐diffusion mechanism through Zn2+ (de)intercalation at edge sites but is plagued by the inert basal planes. Here, hierarchically porous V2O5 nanosheets vertically grown on carbon cloth (V2O5/C) are innovatively prepared, where the porous structure with lattice defects successfully unlocks the V2O5 basal plane to provide additional ion‐diffusion channels and abundant active sites. Thus, highly efficient and ultrafast 3D Li+/Zn2+ co‐insertion/extraction behaviors along both the c‐axis and ab plane of V2O5 are realized for the first time in the formulated 15 m LiTFSI + 1 m Zn(CF3SO3)2 aqueous electrolyte, as elucidated by systematic ex situ analyses, multiple electrochemical measurements, and theoretical computations. As a result, the porous V2O5/C electrode delivers an exceptional high‐rate capability (up to 100 A g−1) and an ultralong cycling durability (15 000 cycles) in RAZBs. Finally, quasi‐solid‐state wearable rechargeable zinc batteries employing the porous V2O5/C cathode demonstrate respectable performance even under severe deformations and low temperatures. This work achieves a conceptual breakthrough represented by an upgrading of the traditional 2D ion transportation in layered cathodes to the more facile 3D diffusion for designing high‐performance battery electrochemistry.

Biological Consequences of Vanadium Effects on Formation of Reactive Oxygen Species and Lipid Peroxidation

Lipid peroxidation (LPO), a process that affects human health, can be induced by exposure to vanadium salts and compounds. LPO is often exacerbated by oxidation stress, with some forms of vanadium providing protective effects. The LPO reaction involves the oxidation of the alkene bonds, primarily in polyunsaturated fatty acids, in a chain reaction to form radical and reactive oxygen species (ROS). LPO reactions typically affect cellular membranes through direct effects on membrane structure and function as well as impacting other cellular functions due to increases in ROS. Although LPO effects on mitochondrial function have been studied in detail, other cellular components and organelles are affected. Because vanadium salts and complexes can induce ROS formation both directly and indirectly, the study of LPO arising from increased ROS should include investigations of both processes. This is made more challenging by the range of vanadium species that exist under physiological conditions and the diverse effects of these species. Thus, complex vanadium chemistry requires speciation studies of vanadium to evaluate the direct and indirect effects of the various species that are present during vanadium exposure. Undoubtedly, speciation is important in assessing how vanadium exerts effects in biological systems and is likely the underlying cause for some of the beneficial effects reported in cancerous, diabetic, neurodegenerative conditions and other diseased tissues impacted by LPO processes. Speciation of vanadium, together with investigations of ROS and LPO, should be considered in future biological studies evaluating vanadium effects on the formation of ROS and on LPO in cells, tissues, and organisms as discussed in this review.

Molybdenum, Vanadium, and Tungsten-Based Catalysts for Sustainable (ep)Oxidation.

This article gives an overview of the research activity of the LAC2 team at LCC developed at Castres in the field of sustainable chemistry with an emphasis on the collaboration with a research team from the University of Zagreb, Faculty of Science, Croatia. The work is situated within the context of sustainable chemistry for the development of catalytic processes. Those processes imply molecular complexes containing oxido-molybdenum, -vanadium, -tungsten or simple polyoxometalates (POMs) as catalysts for organic solvent-free epoxidation. The studies considered first the influence of the nature of complexes (and related ligands) on the reactivity (assessing mechanisms through DFT calculations) with model substrates. From those model processes, the work has been enlarged to the valorization of biomass resources. A part concerns the activity on vanadium chemistry and the final part concerns the use of POMs as catalysts, from molecular to grafted catalysts, (ep)oxidizing substrates from fossil and biomass resources.

Unexpected Role of the Interlayer “Dead Zn2+” in Strengthening the Nanostructures of VS2 Cathodes for High‐Performance Aqueous Zn‐Ion Storage

AbstractLayered VS2 holds great potential as a cathode material for aqueous Zn‐ion batteries owing to its large interlayer spacing, high electrical conductivity, and the rich redox chemistry of vanadium. Nevertheless, structural instability during charge/discharge severely hinders the further development of VS2 cathodes. Herein, distinctive hierarchitectures of 1T‐VS2 nanospheres assembled by nanosheets, which feature abundant active sites, superior electron/ion transport property, and robust structure, are developed. More intriguingly, Zn2+ “pillars” residing in VS2 interlayers, achieved by controlling the charge cut‐off voltage are first proven to reinforce the layered structure of VS2 upon repeated Zn2+ insertion/extraction, redefining the commonly perceived “dead Zn2+”. Hence, exceptional rate performance (212.9 and 102.1 mA h g−1 at 0.1 and 5 A g−1, respectively) and ultralong cycling life (86.7% capacity retention over 2000 cycles at 2 A g−1) are obtained. The rapid and highly reversible Zn‐ion (de) intercalation behavior within the VS2 nanospheres is verified by first‐principles computations and multiple ex‐situ characterizations. Finally, the flexible quasi‐solid‐state rechargeable Zn battery employing the tailored VS2 cathode demonstrates great application prospects in wearable devices. This work provides new perspectives for prolonging the lifespan of layered Zn‐storage materials by simply modulating the charge/discharge processes.

Investigation on Vanadium Chemistry in Basic-Oxygen-Furnace (BOF) Slags—A First Approach

Basic oxygen furnace (BOF) slag accounts for the majority of all residual materials produced during steelmaking and may typically contain certain transition metals. Vanadium, in particular, came into focus in recent years because of its potential environmental toxicity as well as its economic value. This study addresses the vanadium chemistry in BOF slags to better understand its recovery and save handling of the waste stream. The experimental results from the electron probe microanalysis (EPMA) study show that vanadium is preferably incorporated in calcium orthosilicate-like compounds (COS), with two variations occurring, a low vanadium COS (COS-Si) (approx. 1 wt.%), and a high vanadium COS (COS-V) (up to 18 wt.%). Additionally, vanadium is incorporated in dicalcium ferrite-like compounds (DFS) with an average amount of 3 wt.%. Using powder x-ray diffraction analysis (PXRD), EPMA, and virtual component models, stoichiometric formulas of the main vanadium-bearing phases were postulated. The stoichiometries give an estimate of the oxidation states of vanadium in the respective hosts. According to these results, trivalent vanadium is incorporated on the Fe-position in dicalcium ferrite solid solution (DFS), and V4+ and V5+ are incorporated on the Si-position of the COS.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry.

We report the synthesis of vanadium(V) oxo complex 1 with a pincer-type dianionic mesoionic carbene (MIC) ligand L1 and the general formula [VOCl(L1)]. A comparison of the structural (SC-XRD), electronic (UV–vis), and electrochemical (cyclic voltammetry) properties of 1 with the benzimidazolinylidene congener 2 (general formula [VOCl(L2)]) shows that the MIC is a stronger donor also for early transition metals with low d-electron population. Since electrochemical studies revealed both complexes to be reversibly reduced, the stronger donor character of MICs was not only demonstrated for the vanadium(V) but also for the vanadium(IV) oxidation state by isolating the reduced vanadium(IV) complexes [Co(Cp*)2][1] and [Co(Cp*)2][2] ([Co(Cp*)2] = decamethylcobaltocenium). The electronic structures of the compounds were investigated by computational methods. Complex 1 was found to be a moderate precursor for salt metathesis reactions, showing selective reactivity toward phenolates or secondary amides, but not toward primary amides and phosphides, thiophenols, or aryls/alkyls donors. Deoxygenation with electron-rich phosphines failed to give the desired vanadium(III) complex. However, treatment of the deprotonated ligand precursor with vanadium(III) trichloride resulted in the clean formation of the corresponding MIC vanadium(III) complex 6, which undergoes a clean two-electron oxidation with organic azides yielding the corresponding imido complexes. The reaction with TMS-N3 did not afford a nitrido complex, but instead the imido complex 10. This study reveals that, contrary to popular belief, MICs are capable of supporting early transition-metal complexes in a variety of oxidation states, thus making them promising candidates for the activation of small molecules and redox catalysis.

Iron-Tungsten Redox Flow Battery

Redox flow batteries are hailed as a panacea for grid level storage. Since the successful commercialization of vanadium redox batteries, a lot of efforts are being made to develop alternative chemistries. Most of them are based on metal ions which can exist in multiple oxidation states. One of them being, Fe based batteries developed first by R. F. Savinell.[1] Iron based batteries offer significantly cheaper and abundant alternatives to vanadium chemistry and is crucial from a strategic viewpoint. We present here initial development of an iron-tungsten based redox flow battery. One of the key advantages of the battery system presented here is the determination of state of charge by colorimetry. The iron salt as ferrous sulphate was used as an electrolyte for the positive electrode and the tungsten salt as phosphotungstic acid was used as the negative electrode. Although the Fe reaction involves 1 electron only, the reaction of phosphotungstic acid 4 electrons, thus the battery is expected to provide higher capacity compared to a normal Fe battery. Moreover, the maximum potential that can be achieved by using the highest reduced form of phosphotungstic acid is comparable to a vanadium or an all-iron battery.A flow cell was constructed and assembled in house using normal single cell fuel cell setup complete with gold-plated copper current collectors, graphite plates engraved with a serpentine flow field, porous graphite felt as electrode or reaction surface and nafion 117 as the selective proton conducting membrane. The schematic of the flow cell is presented in Figure 1(a). The electrolyte used for the positive electrode was iron(II) sulphate heptahydrate and for the negative electrode was phosphotungstic acid. Prior to the flow cell tests, CV experiments were conducted in a traditional 3 electrode setup. A usual CV (see Figure 1(b)) for Fe2+ and Fe3+ was obtained while the CV of phosphotungstic acid showed 3 oxidation and reduction waves corresponding to the 3 reactions. At very low negative potential, the hydrogen evolution reaction severely interfered with the 3rd reduction wave. An analysis of the peak potential with scan rate revealed that the rate constant for the Fe reaction to be 3x10-4 cm/s close to the reported value of 3.3x10-5 cm/s.[2] Meanwhile, an analysis of the peak separation of the 3 peaks for the phosphotungstic acid revealed that the reactions are reversible with a peak separation of ~ 59 mV and invariant with respect to scan rate.Meanwhile, for the flow cell experiments, the solution concentrations were similar to that for the CV and initially the battery was charged by applying a potential of 1.3 V for 30 minutes. It may be mentioned here that the volume of each of the electrolytes was same and equal to 100 mL. The flow rate of the electrolyte through the cell was kept constant at 120 mL/min. The discharge was performed by drawing current from 10 mA to 1000 mA for 10 s and the potential was recorded to ensure pseudo steady state. Finally the current and the potential values were rearranged to obtain the IV (current-potential) plot along with the power density curve (see Figure 1(c)). The maximum power obtained was 50 mW/cm2 and the maximum current that could be drawn was 160 mA/cm2, which is slightly low compared to the all-iron battery reported by R. F. Savinell.[1] The results obtained here suggest that the Iron-PTA battery demonstrated here could indeed prove to be a viable alternative to the vanadium flow battery. The electrochemical reactions for the iron-phosphotungstic acid redox flow battery are as follows:Negative:[PW12O40]3- + e - ↔ [PW12O40]4- Eo ' = +0.222V vs. RHE[PW12O40]4- + e - ↔ [PW12O40]5- Eo ' = -0.033V vs. RHE[PW12O40]5- + 2e - + H+ ↔ H[PW12O40]6- Eo ' = -0.348V vs. RHEPositive:Fe2+ ↔ Fe3+ + e - Eo ' = +0.705V vs. RHEOverall:4Fe2+ + [PW12O40]3- ↔ 4Fe3+ + H[PW12O40]6- Eo ' = +1.053V vs. RHEReferences[1] Hruska, L. W., & Savinell, R. F. (1981). Investigation of Factors Affecting Performance of the Iron‐Redox Battery. Journal of The Electrochemical Society, 128(1), 18–25.[2] Tanimoto, S., & Ichimura, A. (2013). Discrimination of Inner- and Outer-Sphere Electrode Reactions by Cyclic Voltammetry Experiments. Journal of Chemical Education, 90(6), 778–781. Figure 1

Electrodeposition of Zinc for Redox Flow Batteries

The redox flow batteries (RFBs) have received great attention due to their attractive features for large-scale energy storage applications. A RFB is a type of flow-based energy storage device capable of providing reversible conversion between electrical and chemical energy through two redox half-cell reactions. Nowadays the most common RFBs exploit the vanadium chemistry, followed by the zinc-bromine chemistries. Though RFB technology has advantage for energy storage and has successful demonstration of systems up to MWh levels, they have not enjoyed broad market penetration. Some of the reasons are relatively high capital and life-cycle costs. New applications are envisaged for RFBs, including the electric mobility which is becoming increasingly important. Zinc based redox flow batteries (Zn-RFBs) based on new chemistries with respect to the chlorine and bromine ones can be the answer to new market success devices, not only in the large storage field but also in the medium and mobility sector.In this presentation, the electrodeposition of zinc will be discussed in light of its exploitation in the new generation of zinc based RFBs. Alkaline and acidic baths will be presented in combination with proper chemistries for the other half cells. The role of additives and pulse plating will be discussed in terms of nucleation and growth of the metallic layer and its dissolution during charging/discharging cycles. A zinc-iron RFB with low cost and high energy density will be presented. In particular, inorganic electrolytes based on high soluble salts have been developed, achieving a charge density of 30-70 Wh/l. Moreover, in order to enhance the capacity of Zn-Fe flow battery, the use of slurry dispersed electrodes to decuple capacity from the power rating have been studied. As another example, zinc electrodeposition and its optimization will be introduced in combination with the iodine chemistry for high energy efficiency Zn-I RFBs, in the order of 70% at 20 mA cm-2, with high energy density ranging from 25 to 60 Wh/l.

Facile One-Step Hydrothermal Synthesis of the rGO@Ni3V2O8 Interconnected Hollow Microspheres Composite for Lithium-Ion Batteries

Low-cost, vanadium-based mixed metal oxides mostly have a layered crystal structure with excellent kinetics for lithium-ion batteries, providing high energy density. The existence of multiple oxidation states and the coordination chemistry of vanadium require cost-effective, robust techniques to synthesize the scaling up of their morphology and surface properties. Hydrothermal synthesis is one of the most suitable techniques to achieve pure phase and multiple morphologies under various conditions of temperature and pressure. We attained a simple one-step hydrothermal approach to synthesize the reduced graphene oxide coated Nickel Vanadate (rGO@Ni3V2O8) composite with interconnected hollow microspheres. The self-assembly route produced microspheres, which were interconnected under hydrothermal treatment. Cyclic performance determined the initial discharge/charge capacities of 1209.76/839.85 mAh g−1 at the current density of 200 mA g−1 with a columbic efficiency of 69.42%, which improved to 99.64% after 100 cycles. High electrochemical performance was observed due to high surface area, the porous nature of the interconnected hollow microspheres, and rGO induction. These properties increased the contact area between electrode and electrolyte, the active surface of the electrodes, and enhanced electrolyte penetration, which improved Li-ion diffusivity and electronic conductivity.

Impedance-Resolved Electrochemical Performance of Treated Carbon Felt Electrodes in Vanadium Redox Flow Batteries: Model Error Estimation

Vanadium redox flow batteries (VRFBs) have been targeted as a potential technology for integration of renewable energy sources onto grids as well as improving grid-level power generation efficiency. The primary reasons VRFBs are well-suited for this role are on account of their modularity, scalability, and relatively high performance [1]. There are still, however, many design parameters that can be more highly optimized to improve and specialize aspects of this technology. The kinetics at the anode of VRFBS utilizing graphite felt electrodes on are generally more limiting than at the cathode [2] as well as showing more rapid signs of performance degradation and are as such an area where sources of losses can be improved significantly.Electrochemical impedance spectroscopy is a commonly used method for characterizing sources of losses in VRFB systems. Impedance spectra are often analyzed in the context of the theory of Randles circuits, where electrochemical phenomena are treated analogously to circuit elements. Data reduction equations can then quantify these circuit elements and subsequently determine the values of physical parameters of interest in the system. The Randles circuit, however, has no geometric framework, nor any term which can capture the physical behavior of electrolyte resistance that is distributed throughout a porous medium. A model which was developed by Paasch and co-workers [3], referred to as the macrohomogeneous porous electrode (MHPE) model, addresses this phenomenon and has been used by others to characterize EIS data in systems with porous electrode where electrolyte resistance is non-negligible. Nguyen [4] et. al furthered this model in the context of inhomogeneous porous electrodes and membranes, Sun and co-workers [5] were the first to demonstrate the single MHPE model in the context of the all vanadium chemistry and Pezeshki and co-workers applied the model to quantify the effects of cell architecture in VRFBs [6].Much work on improving carbon felt-based electrodes in VRFBs does so by attempting to make charge transfer more facile [7]. As the charge transfer becomes smaller the effect of the distributed ohmic (pore) resistance begins to account for a larger portion of the cell losses and neglecting to account for it can misrepresent the series ohmic resistance (primarily membrane) and the charge transfer resistance.This work looks at the impact of using three different EIS models on different electrodes (seven treatments and one control) and the impact on the fitted parameters based on the utilization of a specific model. The first two models are both Randles circuits, which differ in the portions of the frequency spectrum which they are using. The first Randles circuit utilizes the full frequency spectrum referred to as RFS model (Randles Full Spectrum), the second Randles circuit truncates the spectrum, referred to as RTS model (Randles Truncated Spectrum), and excludes the distributed ohmic (pore) resistance, and third model is the aforementioned MHPE model. The electrode treatments include a nitric acid soak, heat treatment in 3 different gaseous environments (Oxygen, Nitrogen and Ammonia), and hydrothermal treatments in a nitric acid | sulfuric acid environment for 3 different durations. Electrodes were tested in-situ and cycled to characterize beginning-of-life stability.The results show that there are non-trivial tradeoffs between utilizing the different models. The serial ohmic (membrane) resistance cannot be predicted accurately by either of the Randles circuit models (RFS and RTS) for all of the electrode treatments where it is overestimated as seen in Figure 1. Further, if the RFS model is used, it can yield kinetics parameters that differ from the MHPE model by over 50%. However, the RTS model is in relatively high agreement with the MHPE model for kinetics parameters and mass transfer parameters that can be extracted from EIS data. Over the course of cycling the electrodes treated in the ammonia atmosphere demonstrated both the highest stability and the lowest charge transfer resistance of all of the tested electrodes (see Figure 2). Figure 1

Effect of Molecular Structure and Coordinating Ions on the Solubility and Electrochemical Behavior of Quinone Derivatives for Aqueous Redox Flow Batteries

Water soluble organic redox-species have been studied in redox flow batteries as promising alternatives to overcome the limitation of current vanadium chemistry such as low energy density and high cost. Herein, a comparative physicochemical and electrochemical study of several structurally similar quinones in different molalities of imidazolium-based aqueous electrolytes highlights the importance of the molecular structure of organic solutes and their coordination with the imidazolium cations in electrolytes. A quinone derivative of 2-methoxyl-hydroquinone with a record solubility of 7.9 M at room temperature is obtained in the aqueous imidazolium-based supporting electrolyte. This is close to a maximum value of 8.13 M in its molten state, suggesting a new approach to dissolving organic-active materials. In addition, strong coordination imposes a significant effect on the chemical/electrochemical stability and redox potential of the organic quinones. The reaction kinetics and cycling performance of the 2-methoxyl-hydroquinone as catholyte in a redox flow battery have been investigated by pairing it with a vanadium anolyte (V3+/V2+ redox pair), showing a high cycling efficiency and structural stability.