Diffusion Of Vanadium Ions Research Articles

The significance of charge and discharge current densities in the performance of vanadium redox flow battery

In this study, the effects of charge current density (CDChg), discharge current density (CDDchg), and the simultaneous change of both have been investigated on the performance parameters of the vanadium redox flow battery (VRFB). In addition, the crossover and ohmic polarization have been studied from a mechanism point of view to understand how the current density affects the characteristics of VRFB. Generally, increasing CDChg, CDDchg, or both of them simultaneously, leads to a decrease in the parameters of charge and discharge capacities, charge and discharge energies, as well as voltage efficiency. While the discharge current is constant, the energy efficiency does not change significantly with increasing CDChg. However, as CDDchg or both of CDChg and CDDchg increase, the energy efficiency decreases. In addition, increasing the current density generally increases the coulombic efficiency. Increasing CDChg has no major impact on the discharge voltage. Similarly, increasing CDDchg does not change the charge voltage significantly. From a mechanism view, low currents cause more crossover of vanadium ions because there are more opportunities for ions to diffuse across the membrane, which lowers the coulombic efficiency. The high charging current causes a reduction in the crossover of vanadium ions because there is not enough time for more diffusion of vanadium ions. On the other hand, because of the high current, electrons transfer more quickly while there are not enough vanadium species to react with all the electrons. This leads to a high ohmic polarization, which causes a reduction in the capacity and voltage efficiency.

3D Carbon Nanonetwork Coated Composite Electrode with Multi-Heteroatom Doping for High-Rate Vanadium Redox Flow Batteries.

With the advantages of benign mechanical property, electrochemical stability, and low cost, graphite fibers (GFs) have been widely used as electrodes for vanadium redox flow batteries (VRFBs). However, GFs usually possess inferior electrochemical activity and ion diffusion kinetics for electrode reaction, vastly limiting their application in VRFBs. Here, a 3D carbon nanonetwork coated GFs with multi-heteroatom doping was constructed for application in VRFBs via low temperature polymerization between linear polymer monomer and phytic acid, and subsequent carbonization (900 °C) on the GFs (GF@PCNs-900). Benefiting from the 3D structural features and multi-heteroatom doping (O, N and P), the composite electrode displayed sufficient diffusion of vanadium ions, rapid electron conduction, and highly enhanced electrochemical activity of reactive site on the electrodes. As a result, the GF@PCNs-900 delivered a high discharge capacity of 21 Ah L-1 and energy efficiency of above 70% with extraordinary stability during 200 cycles at 200 mA cm-2. Even at a huge current density of 400 mA cm-2, the GF@PCNs-900 still maintained a discharge capacity of 5.0 Ah L-1, indicating an excellent rate of performance for VRFBs. Such design strategy opens up a clear view for further development of energy storage field.

Diffusion of Vanadium Ions in Artificial Saliva and Its Elimination from the Oral Cavity by Pharmacological Compounds Present in Mouthwashes.

In this study, diffusion coefficients of ammonium vanadate at tracer concentrations in artificial saliva with and without sodium fluoride, at different pH values, were measured using an experimental model based on the Taylor dispersion technique. Ternary mutual diffusion coefficients (D11, D22, D12, and D21) for four aqueous systems {NH4VO3 (component 1) + β-cyclodextrin (β-CD) (component 2),} {NH4VO3 (component 1) + β-cyclodextrin (HP-β-CD) (component 2)}, {NH4VO3 (component 1) + sodium dodecyl sulphate (SDS) (component 2)} and {NH4VO3 (component 1) + sodium hyaluronate (NaHy) (component 2)} at 25.00 °C were also measured by using the same technique. These data showed that diffusion of ammonium vanadate was strongly affected in all aqueous media studied. Furthermore, a significant coupled diffusion of this salt and β-CD was observed through the non-zero values of the cross-diffusion coefficients, D12, allowing us to conclude that there is a strong interaction between these two components. This finding is very promising considering the removal, from the oral cavity, of vanadium resulting from tribocorrosion of Ti-6Al-4V prosthetic devices.

Antimony-decorated graphite felt electrode of vanadium redox flow battery in mixed-acid electrolyte: Promoting electrocatalytic and gas-evolution inhibitory properties

• The shift of electrode gas-evolution potential by antimony. • Utilizing the electrocatalytic properties of antimony in stabilizing the electrode. • Improving vanadium redox flow battery efficiency by facilitating redox reactions. Facilitation of redox reactions and inhibition of gas evolution in the graphite felt electrode of vanadium redox flow battery (VRFB) is investigated by adding antimony ions to the electrolyte. The electrolyte is used as a mixture of sulfuric acid and hydrochloric acid to stabilize the vanadium ion. Antimony has quasi-reversible reactions prior to the electrochemical reactions of vanadium ion pairs. In general, as shown by the results of scanning electron microscopy, metallic antimony is decorated on the surface of graphite felt during charge–discharge cycles. The results of full-window cyclic voltammetry indicate an increase in the cyclic stability of samples containing antimony, especially at a concentration of 7.5 mM. This is due to the electrocatalytic properties of antimony and the facilitation of vanadium electrochemical reactions. Ease of reactions is also closely related to the improvement of vanadium ion diffusion coefficients in samples containing antimony. The reduction peak of V 3+ to V 2+ in the bare sample practically disappears after a few cycles, which is due to the superiority of hydrogen gas evolution. Linear sweep voltammograms confirm the effectiveness of antimony in shifting the potential position of hydrogen and oxygen evolution. Wettability and contact angle tests show more water adsorption by antimony-decorated graphite felts compared to the bare one which has a direct effect on improving the performance of the electrodes. Although antimony is added to anolyte in VRFB and has the most beneficial effect on the negative electrode, antimony cross-contamination does not have a bad effect on positive electrode performance and even it also stabilizes the reduction–oxidation currents ratio on positive electrode based on cyclic voltammetry results. In general, the presence of antimony increases all three types of coulombic, voltage, and energy efficiency in VRFB.

Reduced graphene oxide nanofluidic electrolyte with improved electrochemical properties for vanadium flow batteries

Development of the Vanadium Redox Flow Battery (VRFB) has been widely reported but typically only focuses on one part of the cell (e.g. electrode, electrolyte, or membrane). Improvement to a single part of the cell may cause side effects on other parts during long-term cycling leading to an overall drop in the performance of the battery. To avoid this, the use of nanofluidic electrolyte seems to be a promising approach to enhance the performance of both electrode and electrolyte simultaneously. This paper aims to investigate the electrochemical performance of a newly prepared reduced graphene oxide (rGO) nanofluidic vanadium electrolyte, applicable for Vanadium Redox Flow Batteries (VRFB). Herein, we report for the first time a stable rGO/vanadium nanofluidic electrolyte with improved electrochemical performance. Benefiting from the low degree of oxidation as compared to GO, the rGO can provide high electrical conduction due to the presence of sufficient functional groups, which can facilitate the redox reactions. The effect of various concentrations of rGO on the electrochemical performance is investigated. The current collector (carbon cloth (CC) electrode) was further characterized using different physico-chemical techniques to underpin the stability of rGO nanofluids. The results suggested that the electrochemical performance of vanadium electrolyte increases with the concentration of rGO. Improvements of approx. 15% to 20% were achieved in peak potential separation and current density rates, respectively. In addition, the incorporation of rGO in nanofluidic electrolyte significantly decreases the electrolyte and charge transfer resistance by ∼10% and ∼99%, respectively, and improves the vanadium ion diffusion process by ∼50%.

Uncovering the effect of ion exchange membrane on capacity decay and efficiency for all-vanadium redox flow battery by modeling analysis

ABSTRACT During the long-term charge–discharge cycle, the vanadium permeability of membrane has a significant impact on the capacity and efficiency of the all-vanadium redox flow battery (VFB). Fundamentally, the diffusion of vanadium ions will enhance the self-discharge reaction, resulting in an imbalance in the state of charge of the electrolytes of the two half-cells and a subsequent decrease in capacity. In order to predict the capacity decay and efficiencies of VFBs in the long-term cycle caused by the diffusion of vanadium ions, an electrochemical model of VFB is established on the basis of mass conservation. In this paper, VFBs with five commercialized membranes including cation exchange membrane and anion membrane were investigated at varied densities. Simulation results prove that the diffusion coefficient of vanadium ions has a significant effect on the capacity decay of VFB and the capacity decay can be to some extent alleviated by increasing the charge–discharge current. Therefore, this simulated model can be an excellent guidance for VFB practical operation.

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.

Polymer Membranes for All-Vanadium Redox Flow Batteries: A Review.

Redox flow batteries such as the all-vanadium redox flow battery (VRFB) are a technical solution for storing fluctuating renewable energies on a large scale. The optimization of cells regarding performance, cycle stability as well as cost reduction are the main areas of research which aim to enable more environmentally friendly energy conversion, especially for stationary applications. As a critical component of the electrochemical cell, the membrane influences battery performance, cycle stability, initial investment and maintenance costs. This review provides an overview about flow-battery targeted membranes in the past years (1995–2020). More than 200 membrane samples are sorted into fluoro-carbons, hydro-carbons or N-heterocycles according to the basic polymer used. Furthermore, the common description in membrane technology regarding the membrane structure is applied, whereby the samples are categorized as dense homogeneous, dense heterogeneous, symmetrical or asymmetrically porous. Moreover, these properties as well as the efficiencies achieved from VRFB cycling tests are discussed, e.g., membrane samples of fluoro-carbons, hydro-carbons and N-heterocycles as a function of current density. Membrane properties taken into consideration include membrane thickness, ion-exchange capacity, water uptake and vanadium-ion diffusion. The data on cycle stability and costs of commercial membranes, as well as membrane developments, are compared. Overall, this investigation shows that dense anion-exchange membranes (AEM) and N-heterocycle-based membranes, especially poly(benzimidazole) (PBI) membranes, are suitable for VRFB requiring low self-discharge. Symmetric and asymmetric porous membranes, as well as cation-exchange membranes (CEM) enable VRFB operation at high current densities. Amphoteric ion-exchange membranes (AIEM) and dense heterogeneous CEM are the choice for operation mode with the highest energy efficiency.

Single-layer graphene as a highly selective barrier for vanadium crossover with high proton selectivity

We report near-zero crossover for vanadium cross-permeation through single-layer graphene immobilized at the interface of two Nafion® polymer electrolyte membranes. Vanadium ion diffusion and migration, including proton mobility through membrane composites, were studied with and without graphene under diffusion and migration conditions. Single-layer graphene was found to effectively inhibit vanadium ion diffusion and migration under specific conditions. The single-layer graphene composites also enabled remarkable ion transmission selectivity improvements over pure Nafion® membranes, with proton transport being four orders of magnitude faster than vanadium ion transport. Resistivity values of 0.02 ± 0.005 Ω cm2 for proton and 223 ± 4 Ω cm2 for vanadium ion through single atomic layer graphene are reported. This high selectivity may have significant impact on flow battery applications or for other electrochemical devices where proton conductivity is required, and transport of other species is detrimental. Our results emphasize that crossover may be essentially completely eliminated in some cases, enabling for greatly improved operational viability.

Interface Concentrated‐Confinement Suppressing Cathode Dissolution in Water‐in‐Salt Electrolyte

AbstractMass dissolution is one main problems for cathodes in aqueous electrolytes due to the strong polarity of water molecules. In principle, mass dissolution is a thermodynamically favorable process as determined by the Gibbs free energy. However, in real situations, dissolution kinetics, which include viscosity, dissolving mass mobility, and interface properties, are also a critical factor influencing the dissolution rate. Both thermodynamic and kinetic dissolving factors can be regulated by the ratio of salt to solvent in the electrolyte. In this study, concentration‐controlled cathode dissolution is investigated in a susceptible Na3V2(PO4)3 cathode whose time‐, cycle‐, and state‐of‐charge‐dependent dissolubility are evaluated by multiple electrochemical and chemical methods. It is verified that the super‐highly concentrated water‐in‐salt electrolyte has a high viscosity, low vanadium ion diffusion, low polarity of solvated water, and scarce solute−water dissolving surfaces. These factors significantly lower the thermodynamic‐controlled solubility and the dissolving kinetics via time and physical space local mass interfacial confinement, thereby inducing a new mechanism of interface concentrated‐confinement which improves the cycling stability in real aqueous rechargeable sodium‐ion batteries.

A model study on effects of vanadium ion diffusion through ion exchange membranes in a non-aqueous redox flow battery

A simple charge-discharge model of a nonaqueous vanadium redox flow battery (VRFB) was introduced in this study to analyze the diffusion coefficients of active materials permeating an electrolyte membrane. Using this model, the effects of diffusion coefficients on the long-term performance, efficiency, and capacity of the system have been studied. Based on a literature review, the diffusion coefficients of approximately 10−7–10−6 cm2 s−1 were applied and the results were analyzed to determine relevant parameters for this study. Through this study, it has been confirmed that the capacity of the system was maintained at 100% under ideal conditions in the long-term operation, whereas that of the system reduced by more than half when only one vanadium species moved and is 10 mAh when all vanadium species moved. In addition, the simulation clarified the effect of diffusion coefficients on coulombic efficiency. In most conditions of the various diffusion coefficients, coulombic efficiency enhanced through cycle had been operated. In the case of voltage efficiency, the value was not largely changed regardless of diffusion coefficients since it was affected more by other factors. One important factor revealed through this model study is that electrolyte membrane performance of the VRFB system was not only affected by the diffusion coefficient of a specific ion, but also by the relationship between the diffusion coefficients of all active ions involved in the reaction. This study provides guidance for the development of an electrolyte membrane to improve the performance of VRFB systems.

Effect of Fe(III) on the positive electrolyte for vanadium redox flow battery.

It is important to study the effect of Fe(III) on the positive electrolyte, in order to provide some practical guidance for the preparation and use of vanadium electrolyte. The effect of Fe(III) on the thermal stability and electrochemical behaviour of the positive electrolyte for the vanadium redox flow battery (VRFB) was investigated. When the Fe(III) concentration was above 0.0196 mol l−1, the thermal stability of V(V) electrolyte was impaired, the diffusion coefficient of V(IV) species decreased from (2.06–3.33) × 10−6 cm2 s−1 to (1.78–2.88) × 10−6 cm2 s−1, and the positive electrolyte exhibited a higher electrolyte resistance and a charge transfer resistance. Furthermore, Fe(III) could result in the side reaction and capacity fading, which would have a detrimental effect on battery application. With the increase of Fe(III), the collision probability of vanadium ions with Fe(III) and the competition with the redox reaction was aggravated, which would interfere with the electrode reaction, the diffusion of vanadium ions and the performance of VRFB. Therefore, this study provides some practical guidance that it is best to bring the impurity of Fe(III) below 0.0196 mol l−1 during the preparation and use of vanadium electrolyte.

Preparation and properties of quaternized poly(phthalazinone ether ketone ketone) anion-exchange membrane for all-vanadium redox flow battery

As one of stationary energy storage device, all-vanadium redox flow battery (VRB) could effectively balance the relationship of electricity generation and demand side, in which interests are based on having features of high efficiency, flexible design, low cost and so on. Ion exchange membrane is one of the key components of VRB, separating redox-active ions and allowing the transfer of non-reaction species. Nafion series from Dupont company could ideally satisfy the requirements of high conductivity and good stability but facing the disadvantages of poor vanadium blocking ability in VRB. Modified Nafion membranes partially conquered the problem of poor ion selectivity but still suffering from high cost. Cationic exchange membranes prepared from non-fluorinated polymers have lower vanadium ion permeability and cost while cationic exchange groups hinder the further improvement of vanadium ion blocking. On the contrary, anionic exchange membranes (AEMs) exhibit low vanadium ion diffusion coefficient because of Donnan exclusive effection. It is much of importance to explore new AEMs. And the lifetime of the VRB cell depends on the selectivity and stability of the membranes to some degree. However, most of ion exchange membranes for VRB application showed low selectivity or stability. In order to improve the selectivity of ion exchange membranes, poly(phthalazinone ether ketone ketone) containing bromomethyl moieties (BPPEKK) with varied content of bromomethyl were dissolved to prepare BPPEKK base membranes using solution casting method. Quaternized poly(phthalazinone ether ketone ketone) (QBPPEKK) anion-exchange membranes were prepared from the amination of BPPEKK membranes in trimethylamine aqueous solutions. The measured ion exchange capacity (IEC) values of QBPPEKK membranes were in the range of 1.15–1.51 mmol/g, which were lower than the theoretical IEC (IECT) values. The result indicated the incomplete amination reaction of bromomethyl groups with trimethylamine. Properties of QBPPEKK membranes including water uptake, swelling ratio, area resistance and vanadium ion permeability were investigated. Water uptake and swelling ratio of QBPPEKK membranes increased with the increase in content of bromomethyl in BPPEKK, while area resistance and vanadium ion permeability decreased. The vanadium ion mass transfer coefficient of QBPPEKK membranes was in the range of 2.3×10-5 –5.0×10-5 cm/min, less than that of Nafion 117 membrane (11.9×10-5 cm/min). QBPPEKK membranes showed much lower vanadium ion permeability than Nafion 117. VRB with each QBPPEKK membrane showed higher columbic efficiency (CE) than the battery with Nafion117 membrane. QBPPEKK membrane with higher IEC exhibited higher voltage efficiency and energy efficiency (EE). When the content of bromomethyl reached 0.99, the VRB with QBPPEKK90 membrane had EE of 87.7% (current density: 40 mA/cm2), higher than that of Nafion117 membrane (86.0%). When the charge-discharge current density increased from 20 to 80 mA/cm2, the CE of VRB with QBPPEKK90 membrane increased from 97.1% to 99.0%, and EE of VRB cell with QBPPEKK90 decreased from 91.1% to 80.2%. After QBPPEKK membranes were immersed in VO2+ solution for 60 d, the tensile strength of the membranes was more than 36 MPa, which was lower than that of the original QBPPEKK90 membrane. There were no significant changes on microstructure of QBPPEKK membrane after the membrane had been immersed into 1.5 mol/L VO2+ solutions for 60 d. Compared with the original QBPPEKK90 membrane, CE changed slightly and EE decreased 1.4% at the charge-discharge current density ranging from 20 to 60 mA/cm2 after the membrane was immersed in VO2+ solution for 60 d. The results suggested that QBPPEKK membranes showed good chemical stability.

Influence of Metal Impurities or Additives in the Electrolyte of a Vanadium Redox Flow Battery

As one of the most attractive large-capacity energy storage systems, vanadium redox flow batteries have many advantages such as a rapid response, long service life, environment-friendliness, and non-flammability. However, the high cost of the vanadium electrolyte has hindered the broad market penetration. The price of the vanadium electrolyte can be significantly reduced if a cost-competitive technology for producing the electrolyte from recycled V2O5 may be used. Accordingly, the effects of the impurities in the recycled V2O5 on the performance of vanadium redox flow batteries (VRFBs) must be understood. However, there have been very few published studies on the effects of these electrolyte impurities. In order to establish a reliable electrolyte specification for VRFB developers, we analyzed the influence of a wide range of potential metal impurities or additives on the electrochemical activity, diffusion of vanadium ions, and battery performance of these systems. The investigated impurities include Li+, Na+, K+, Mg2+, Cr3+, Mn2+, Fe2+/F3+, Co2+, Ni2+, Cu2+, Zn2+, and Mo6+ ions. Most of the metal ions affected the V2+/V3+ redox reaction rather than the VO2+/VO2+ redox reaction. On the basis of our findings, we suggest detailed specifications that may be used to manage the quality of the vanadium electrolyte.

Electrochemical and Spectroscopic Measurements of Diffusion of Vanadium Species in Ionomer Membranes

The vanadium redox flow battery (VRFB) is a tool for energy storage that has been promising in applications of large grid storage. Two cells are separated by an ion exchange membrane, which allows proton conduction to preserve electrical balance during the charge/discharge cycles1. The battery is composed of positive and negative cells operating with VO2+/VO2 + and V(II)/V(III) redox couples, respectively. The membrane also allows water, vanadium and acid to crossover from one cell to the other, this can lead to concentration imbalances and self-discharge, ultimately decreasing the efficiency of the battery.2 , 3 The focus of this investigation is on measurements of diffusive properties of ionomer films. Perfluorinated ionomers (e.g. Nafion®) have been extensively studied owing to the application of Nafion in fuel cells and electrolyzers. Electron paramagnetic resonance (EPR) enables the measurement of diffusion in of a spin probe or EPR active metal. Rotational diffusion, R, is obtained from analyzing the EPR line shape of the probe.4 , 5 This illustrates the local viscosity in the diffusion pathway, i.e. the hydrated channels. In turn, Rmay be related to the effective local viscosity by the Stokes-Einstein equation (see attached image), where a is an effective hydrodynamic radius of the probe, s is an anisotropy factor, and i labels the axis. These values are probe dependent, for the Tempone spin probe, a = 4.2 Å, s || = 0.402 and s^ = 0.543. EPR has also previously been used to further understand the environment of absorbed species in the membrane, including alternative hydrocarbon membranes.6 , 7 Previous studies have shown the VO2+ ion permeability to have a dependence on acid concentration in solution.8 In addition, translational diffusion in hydrated membranes can be measured with UV/vis or EPR by allowing the vanadium permeants to diffuse through the membrane in a cell designed for this purpose where concentrated solution is in contact with one side of the membrane and concentration increase is monitored in a blank solution in contact with the opposite side. A recent paper has shown a more significant discrepancy in comparisons of rotational diffusion and translational diffusion in aromatic hydrocarbon membranes than in perfluorinated ionomer membranes, suggesting a different diffusion pathway in the hydrocarbon based membranes. 9 To further characterize the diffusion of vanadium ions in ionomer membranes electrochemical methods are be used. Two separate measurements determine the diffusion of the redox active species through a membrane cast onto a glassy carbon rotating disc electrode (RDE). By taking detailed voltammetric RDE and chronoamperometric measurements at varying film thicknesses and diffusant concentrations, considerable information about the diffusion of different species in the membrane. The effects of backbone architecture and ionization level on the diffusion and solubility of migrant species can be further elucidated. References (1) Rychcik, M.; Skyllas-Kazacos, M. Journal of Power Sources 1988, 22, 59. (2) Xi, J.; Wu, Z.; Teng, X.; Zhao, Y.; Chen, L.; Qiu, X. Journal of Materials Chemistry 2008, 18, 1232. (3) Weidmann, E.; Heintz, E.; Lichtenthaler, R. N. J. Mem. Sci 1998, 141, 207. (4) Lawton, J.; Smotkin, E.; Budil, D. J. Phys. Chem. B 2008, 112, 8549. (5) Lawton, J.; Budil, D. J. Phys. Chem. B 2009, 10679. (6) Lawton, J.; Budil, D. J. Mem. Sci 2010, 357, 47. (7) Lawton, J.; Budil, D. Macromolecules 2010, 43, 652. (8) Lawton, J. S.; Jones, A.; Zawodzinski, T. Journal of The Electrochemical Society 2013, 160, A697. (9) Lawton, J.; Jones, A.; Tang, Z.; Lindsey, M.; Fujimoto, C.; Zawodzinski, T. J. Electrochem. Soc. 2016, 163, A5229. Figure 1

Membrane Permeability Rates of Vanadium Ions and Their Effects on Temperature Variation in Vanadium Redox Batteries

The inevitable diffusion of vanadium ions across the membrane can cause considerable capacity loss and temperature increase in vanadium redox flow batteries (VRFBs) over long term operation. Reliable experimental data of the permeability rates of vanadium ions are needed for membrane selection and for use in mathematical models to predict long-term behavior. In this paper a number of ion exchange membranes were selected for detailed evaluation using a modified approach to obtain more accurate permeation rates of V2+, V3+, VO2+ and VO2+ ions. Three commercial ion exchange membranes—FAP450, VB2 and F930—are investigated. The obtained diffusion coefficients are then employed in dynamic models to predict the thermal behavior under specific operating conditions. The simulation results prove that smaller and more balanced permeability rates of V2+ and VO2+ ions are more important to avoid large temperature increases in the cell stack during stand-by periods at high states-of-charge with pumps off.

Hydrocarbon Blended Membrane for Redox Flow Battery

With the expanding need for large electrical energy storage systems in connection with renewable energy sources, flow batteries, have been enormously considered due to their high flexibility in upgrade and low cost associated with scale-up. Of all the flow batteries, vanadium redox flow battery (VRFB) use of same element in both half-cell solutions that overcomes the inherent issue of cross contamination by diffusion of different ions across the membrane. Together with the absence of any toxic emissions, the vanadium redox flow battery has demonstrated its uniqueness in terms of safety and long life cycle. Typical charge-discharge reactions of a VRFB involve two vanadium redox couples, V(II)/V(III) and V(IV)/V(V), in the negative and positive half-cells, respectively. In a fashion similar to most batteries, electrons are transferred between the two electrodes through the external circuit during the charge and discharge processes. In a VRFB, the ion exchange membrane is a key component as an ionic conductor and separator: it not only provides an ionic conduction pathway between the two electrolytes but also prevents mixing of the negative and positive electrolytes. The crossover of ions through the membrane, with the diffusion of vanadium ions from one half-cell to the other due to the concentration gradients between the two electrolytes, will result in self-discharge and thus the loss of the chemical energy. In this study, the blended membrane of hydrocarbon polymer with perfluorinated organic membrane were fabricated and characterized in terms of ionic conductivity and permeability. The ionic conductivity was measured with four point probe method and the permeability was measured with UV spectroscopy. The blended membrane exhibited comparable ionic conductivity with perfluorinated organic membrane and 30 – 40 % lower permeability than perfluorinated organic membrane. With performances of VRFBs, the energy efficiencies of VRFBs will be discussed.

Hydrocarbon Membrane for Vanadium Redox Flow Batteries

With the expanding need for large electrical energy storage systems in connection with renewable energy sources, flow batteries, have been enormously considered due to their high flexibility in upgrade and low cost associated with scale-up. Of all the flow batteries, vanadium redox flow battery (VRFB) use of same element in both half-cell solutions that overcomes the inherent issue of cross contamination by diffusion of different ions across the membrane. Together with the absence of any toxic emissions, the vanadium redox flow battery has demonstrated its uniqueness in terms of safety and long life cycle. Typical charge-discharge reactions of a VRFB involve two vanadium redox couples, V(II)/V(III) and V(IV)/V(V), in the negative and positive half-cells, respectively. In a fashion similar to most batteries, electrons are transferred between the two electrodes through the external circuit during the charge and discharge processes. In a VRFB, the ion exchange membrane is a key component as an ionic conductor and separator: it not only provides an ionic conduction pathway between the two electrolytes but also prevents mixing of the negative and positive electrolytes. The crossover of ions through the membrane, with the diffusion of vanadium ions from one half-cell to the other due to the concentration gradients between the two electrolytes, will result in self-discharge and thus the loss of the chemical energy. In this study, in order to improve membrane selectivity, hydrocarbon based polymer membranes were synthesized and fabricated for VRFBs. Sulfonated polysulfone membranes (PATS-XX) were characterized in terms of ionic conductivity and permeability with degree of sulfonation. PATS-35 exhibited the ionic conductivity of 0.06 S/cm, which showed 40% lower permeability than Nafion 115 membrane. VRFB cell with PATS-35 exhibited higher coulombic and energy efficiency than that with nafion 115 membrane. The results of VRFBs with blended PATS-xx membrane will be discussed and compared with that with Nafion membrane.

Effect of Sulfuric and Triflic Acids on the Hydration of Vanadium Cations: An ab Initio Study.

Vanadium redox flow batteries (VRFBs) may be a promising solution for large-scale energy storage applications, but the crossover of any of the redox active species V(2+), V(3+), VO(2+), and VO2(+) through the ion exchange membrane will result in self-discharge of the battery. Hence, a molecular level understanding of the states of vanadium cations in the highly acidic environment of a VRFB is needed. We examine the effects of sulfuric and triflic (CF3SO3H) acids on the hydration of vanadium species as they mimic the electrolyte and functional group of perfluorosulfonic acid (PFSA) membranes. Hybrid density functional theory in conjunction with a continuum solvation model was utilized to obtain the local structures of the hydrated vanadium cations in proximity to H2SO4, CF3SO3H, and their conjugate anions. The results indicate that none of these species covalently bond to the vanadium cations. The hydration structure of V(3+) is more distorted than that of V(2+) in an acidic medium. The oxo-group of VO2(+) is protonated by either acid, in contrast to VO(2+) which is not protonated. The atomic partial charge of the four oxidation states of vanadium varies from +1.7 to +2.0. These results provide the local solvation structures of vanadium cations in the VRFBs environment that are directly related to the electrolytes stability and diffusion of vanadium ions into the membrane.

Ultra-low vanadium ion diffusion amphoteric ion-exchange membranes for all-vanadium redox flow batteries

An amphoteric ion-exchange membrane (AIEM) from fluoro-methyl sulfonated poly(arylene ether ketone) bearing content-controlled benzimidazole moiety, was firstly fabricated for vanadium redox flow battery (VRB). The AIEM and its covalently cross-linked membrane (AIEM-c) behave the highly suppressed vanadium-ion crossover and their tested VO2+ permeability are about 638 and 1117 times lower than that of Nafion117, respectively. This is further typically verified by the lower VO2+ concentration inside AIEM that is less than half of that inside Nafion117 detected by energy dispersive X-ray spectrometry, in addition of the nearly 3 times longer battery self-discharge time. The ultra-low vanadium ion diffusion could be ascribed to the narrower ion transporting channel originated from the acid-base interactions and the rebelling effect between the positively-charged benzimidazole structure and VO2+ ions. It is found that, VRB assembled with AIEM exhibits the equal or higher Coulombic efficiency (99.0% vs. 96.4%), voltage efficiency (90.7% vs. 90.7%) and energy efficiency (89.8% vs. 87.4%) than that with Nafion117 and keeps continuous 220 charge–discharge cycles for over 25 days, confirming that the AIEM of this type is a potentially suitable separator for VRB application.