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A review of vanadium electrolytes for vanadium redox flow batteries
Abstract
There is increasing interest in vanadium redox flow batteries (VRFBs) for large scale-energy storage systems. Vanadium electrolytes which function as both the electrolyte and active material are highly important in terms of cost and performance. Although vanadium electrolyte technologies have notably evolved during the last few decades, they should be improved further towards higher vanadium solubility, stability and electrochemical performance for the design of energy-dense, reliable and cost-effective VRFBs. This timely review summarizes the vanadium electrolyte technologies including their synthesis, electrochemical performances, thermal stabilities, and spectroscopic characterizations and highlights the current issues in VRFB electrolyte development. The challenges that must be confronted to further develop vanadium electrolytes may stimulate more researchers to push them forward.
... To address this prominent issue, various ions have been introduced into the electrolyte of electrolytic MnO 2 -Zn batteries to enhance their electrochemical performance [31][32][33][34][35][36] . For instance, the incorporation of Ni 2+ ions serves as a catalyst for the Mn 2+ /MnO 2 reaction kinetics, enabling charge and discharge at a rate of 50 C while maintaining a lifespan of over 450 cycles at an areal capacity of 1 mAh cm -2 [31] . ...
... For instance, the incorporation of Ni 2+ ions serves as a catalyst for the Mn 2+ /MnO 2 reaction kinetics, enabling charge and discharge at a rate of 50 C while maintaining a lifespan of over 450 cycles at an areal capacity of 1 mAh cm -2 [31] . Additionally, Al 3+ ions play a vital role in promoting MnO 2 dissolution by creating oxygen vacancies during deposition, leading to improved cycling stability over 2,000 cycles at an areal capacity of 2 mAh cm -2 [36] . Another approach involves the utilization of redox mediators such as Iand Brions [30,33] , which interact with MnO 2 to prevent its accumulation. ...
Electrolytic MnO2-Zn batteries possess high energy density due to the high reduction potential and capacity of the cathode Mn2+/MnO2. However, the low reversibility of the Mn2+/MnO2 conversion results in a limited lifespan. In this study, we propose the utilization of VOSO4 as a redox mediator in the MnO2-Zn battery to facilitate the dissolution of MnO2. Through various techniques such as electrochemical measurements, ex-situ UV-visible spectroscopy, X-ray diffraction, and scanning electron microscopes, we validate the interaction between VO2+ and MnO2, which effectively mitigates the accumulation of MnO2. The introduction of the redox mediator results in exceptional redox reversibility and outstanding cycling stability of the MnO2/VOSO4-Zn battery at high areal capacities, with 900 cycles at 5 mAh cm-2 and 500 cycles at 10 mAh cm-2. Notably, even in the flow battery device, the battery exhibits a stable cycling performance over 300 cycles at 20 mAh cm-2. These research findings shed light on the potential large-scale application of electrolytic MnO2-Zn batteries.
... The individual characteristics of VRFBs as well as the importance and development of the respective cell components have already been discussed in several review articles. [5] In addition to investigations on the electrolyte solutions [6] and the ion exchange membranes, [7] the used electrode materials are highly relevant, as they catalyse the above-mentioned electrochemical reactions (see equations 1 and 2). In particular, carbon-based electrodes are the established materials, as they are known for their good electrical conductivity, high chemical stability and reasonable cost. ...
This mini‐review summarises and discusses recent findings form the literature on the degradation of carbon‐based electrodes for vanadium redox flow batteries (VRFBs). It becomes evident that the focus of current investigations is on carbon paper, carbon felt and graphite felt electrodes, which is understandable from a practical point of view. However, the structural complexity of these materials often prohibits doubtless attribution of observed performance reduction (or increase) to changes in the electrode materials. Among the discussed major causes for degradation are formation or change of surface functional groups, changes in the carbon sp²/sp³ ratio, intercalation of ions as well as formation of inhibiting adsorbates. In order to gain deeper insight into the changes of carbon electrodes in VRFBs under relevant operation conditions, the authors suggest reducing complexity of the investigated materials and applying in situ‐studies under well‐defined and controllable conditions on model electrodes. These studies then should be extended towards more practical systems and may finally help to reduce degradation phenomena including enhanced overvoltages and thus could improve cycling and energy efficiency as well as long‐term stability of vanadium redox flow batteries.
... [1][2][3] However, renewable energy sources often suffer from intermittency and instability, thus it will cause grievous disturbances when directly integrating the electricity generated from renewable energy into the grid. [4][5][6] An effective solution to this problem is the adoption of large-scale energy storage systems, which store the electricity and deploy it into the grid in a controlled manner. [7][8][9] Since the invention of redox flow battery by Thaller in 1974, it has been one of the most promising energy storage technologies due to its high energy conversion efficiency, large storage capacity, high reliability and long cycle life. ...
Electrospinning technology has demonstrated excellent prospects in the preparation of structurally controllable functional carbon nanofibers for vanadium redox flow batteries. However, traditional electrospinning carbon nanofibers used for vanadium flow batteries still suffer from defects in electrochemical activity. Herein, a highly active carbon nanofiber electrode based on metal-organic framework materials has been prepared. The introduction and carbonization of the metal-organic framework UiO-66 in the fibers increase the mesoporous structure of the electrode surface. Additionally, the carbonized UiO-66 forms catalytic ZrO2, which enhances the catalytic activity of the carbon nanofibers. Compared to traditional electrospinning carbon nanofibers, the carbon nanofiber electrode based on metal-organic framework exhibits significantly improved wettability and electrochemical properties, which enhance the mass transfer performance and electrochemical activity. The vanadium flow battery adopting active carbon nanofibers achieves an energy efficiency of 83.33% at 200 mA cm-2, and possesses excellent durability performance with unobvious decay after 1000 charge-discharge cycles at 200 mA cm-2. This study provides guidance for further synthesis of high-performance electrodes for vanadium redox flow batteries.
The impurity ions have negative effects on the thermal stability and electrochemical performance of the electrolyte, limiting the cycling stability of vanadium redox flow battery (VRFB). Since the Ni ions are considered as one of the most common impurity ions in the electrolyte of VRFB, this study focuses on the effect of Ni ions on various aspects of battery performance. The results showed that Ni ions above 10 mg/L observably accelerated the precipitation of the V(V) electrolyte and deteriorated the reversibility of the V²⁺/V³⁺ couple. In addition, the Ni ions above 10 mg/L have greatly negative influences on the diffusion coefficient of vanadium ions, the coulombic efficiency, and energy efficiency of the battery. It was found that such unfavorable effects were mainly due to the Ni ions in the electrolyte masking the active sites of graphite felt for vanadium redox reaction.
This comprehensive review article explains the influence of various GO and GO-polymer membrane modifications for VRFB, which range from cation and anion exchange to amphoteric and zwitterionic membranes.
A vanadium redox flow battery (VRFB) is an intermittent energy storage device that is primarily used to store and manage energy produced using sustainable sources like solar and wind. In this work, we study the modeling and operation of a single-cell VRFB whose active cell area is 25 cm\(^2\). Initially, we operate the cell at multiple flow rates by varying the current densities to study the behavior of the cell at different flow rates. Based on the observations from this experimental study, a suitable flow rate is selected such that the power density of the cell is maximized. In order to accurately represent the dynamic behavior of the VRFB, an electrical equivalent circuit model (ECM) is introduced from existing models documented in the literature based on simplicity and accuracy based on the RMS value of the error in the output voltage. Additionally, the pulse charge–discharge test was conducted on the VRFB under the specified operating flow rate. The data obtained from this test were utilized to estimate the parameters of the dynamical equivalent circuit model. In our study, we have proposed a modified state of charge (SOC) equation considering the self-discharge current loss through a parallel resistor while identifying the SOC–OCV relationship. Analysis of VRFB charge–discharge data is employed to validate the ECM, and the accuracy of the model is determined based on RMSE between the terminal voltage predicted by the model and the actual experimental setup. The resulting RMSE is found to be 19.4 mV, describing the model accuracy of 98.73%.
Lignin is an abundant polyaromatic polymer with a wide range of potential future uses. However, the conversion of lignin into valuable products comes at a cost, and medium- to high-value applications are thus appropriate. Two examples of these are polymers (e.g., as fibers, plasticizers, or additives) and flow batteries (e.g., as redox species). Both of these areas would benefit from lignin-derived molecules with potentially low molecular weight and high (electro)chemical functionality. A promising route to obtain these molecules is oxidative lignin depolymerization, as it enables the formation of targeted compounds with multiple functionalities. An application with high potential in the production of plastics is the synthesis of new sustainable polymers. Employing organic molecules, such as quinones and heterocycles, would constitute an important step toward the sustainability of aqueous flow batteries, and lignin and its derivatives are emerging as redox species, mainly due to their low cost and renewability.
In this study, new fluorite high-entropy oxide (HEO), (BiZrMoWCeLa)O2, nanoparticles were produced using a surfactant-assisted hydrothermal technique followed by calcination and were used as novel catalytic materials for vanadium redox flow batteries (VRFBs). The HEO calcined at 750 °C (HEO-750) demonstrates superior electrocatalytic activity toward V³⁺/V²⁺ and VO²⁺/VO2⁺ redox couples compared to those of cells assembled with other samples. The charge–discharge tests further confirm that VRFBs using the HEO-750 catalyst demonstrate excellent Coulombic efficiency, voltage efficiency, and energy efficiency of 97.22, 87.47, and 85.04% at a current density of 80 mA cm–2 and 98.10, 74.76, and 73.34% at a higher current density of 160 mA cm–2, respectively. Moreover, with 500 charge–discharge cycles, there is no discernible degradation. These results are attributed to the calcination heat treatment, which induces the formation of a new single-phase fluorite structure, which facilitates the redox reactions of the vanadium redox couples. Furthermore, a high surface area, wettability, and plenty of oxygen vacancies can give more surface electroactive sites, improving the electrochemical performance, the charge transfer of the redox processes, and the stability of the VRFBs’ electrode. This is the first report on the development of fluorite structure HEO nanoparticles in VRFBs, and it opens the door to further research into other HEOs.
A novel high-efficiency industrialized clean production technology based on multi-stage gradient batching and smelting was proposed for the production of high-quality ferrovanadium. The thermodynamic mechanism of aluminothermic reduction equilibrium, alloy settlement and raw material impurity distribution were confirmed, and a multi-stage double-gradient aluminum addition pattern (DG-ADP), the highly efficient separation of molten slag and alloy, and typical impurity control standards of raw materials were achieved on the basis of a self-propagating high-temperature synthesis with an electric auxiliary heating (SHS-EAH) process. The reduction efficiency, separation efficiency and the comprehensive utilization rate of the secondary resources were significantly improved, as the whole total vanadium (T.V) content in the industrially produced residue slag reduced from 2.34 wt.% to 0.60 wt.%, while the corresponding smelting yield increased from 93.7 wt.% to 98.7 wt.% and the aluminum consumption decreased from 510 kg·t−1 to 400 kg·t−1. The multi-stage DG-ADP process enabled the internal circulation of vanadium-bearing materials in the ferrovanadium smelting system, as well as the external circulation of iron and residue slag in the same system, and finally achieved the zero discharge of solid and liquid waste from the ferrovanadium production line, which provides a brand-new perspective for the cleaner production of ferrovanadium alloy.
Vanadium redox flow batteries (VRFBs) hold great promise for large‐scale energy storage, but their performance requires further improvement. Herein, a design is proposed for vanadium colloid flow batteries (VCFBs) that integrates the redox chemistry of polyvalent vanadium‐based colloid suspensions with dispersed conductive agents into traditional vanadium electrolytes. The redox‐active colloids combine the advantages of nanoparticle suspensions and dissolved electrolytes, exhibiting good dispersibility, fluidity, conductivity, redox reversibility, and electrochemical kinetics. By leveraging a reversible dissolution/suspension process of high‐concentration vanadium‐based colloids, the VCFBs achieved an energy density of 48 Wh L⁻¹, nearly double that of conventional VRFBs. This work presents a rational design for homologous active material colloids to enhance the energy density of aqueous redox flow batteries, thereby advancing the potential for grid‐scale and renewable energy storage.
The all-vanadium redox flow battery is widely regarded as the most effective solution for mitigating the intermittent nature of renewable energy sources and simultaneously achieving “carbon neutrality goals”. Nevertheless, the battery’s overall performance is adversely affected by capacity loss resulting from side reactions, hence constraining its viability for usage in large-scale energy storage systems. This study proposes a novel approach that aims to minimize side reactions and mitigate capacity fade by employing an appropriate charge cut-off voltage mechanism. The experiment centers around the examination of performance indicators, including battery charge/discharge capabilities, internal resistance, capacity retention rate, and efficiency, for analytical purposes. The findings indicate that by setting the charge cut-off reference voltage at 1.65V, the battery’s capacity can be sustained at 61.76% after 60 cycles. When the charge cut-off voltage is raised by increments of 0.1 and 0.15 V, the corresponding reductions in battery capacity amount to 1.74% and 5.16% respectively. The aforementioned findings emphasize the significance of considering the effect of side reactions on battery capacity and implement that mechanism can significantly enhance the battery’s overall performance.
This article explores the novel application of a trained artificial neural network (ANN) in the prediction of vanadium redox flow battery behaviour and compares its performance with that of a two-dimensional numerical model. The aim is to evaluate the capability of two ANNs, one for predicting the cell potential and one for the overpotential under various operating conditions. The two-dimensional model, previously validated with experimental data, was used to generate data to train and test the ANNs. The results show that the first ANN precisely predicts the cell voltage under different states of charge and current density conditions in both the charge and discharge modes. The second ANN, which is responsible for the overpotential calculation, can accurately predict the overpotential across the cell domains, with the lowest confidence near high-gradient areas such as the electrode membrane and domain boundaries. Furthermore, the computational time is substantially reduced, making ANNs a suitable option for the fast understanding and optimisation of VRFBs.
Aqueous all‐iron flow batteries (AIFBs) are attractive for large‐scale and long‐term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2‐methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge‐discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe³⁺ and Fe²⁺, without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5 % at 80 mA cm⁻² and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100 % after 1,400 cycles. The capital cost of this AIFB is $ 33.2 kWh⁻¹ (e.g., 20 h duration), cheaper than Li‐ion battery and vanadium flow battery. This double‐ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries.
Aqueous all‐iron flow batteries (AIFBs) are attractive for large‐scale and long‐term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2‐methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge‐discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe3+ and Fe2+, without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5% at 80 mA cm‐2 and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100% after 1,400 cycles. The capital cost of this AIFB is $33.2 kWh‐1 (e.g., 20 h duration), cheaper than Li‐ion battery and vanadium flow battery. This double‐ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries.
The use of symmetrical cells is becoming popular for the search of new electroactive materials in redox flow batteries. Unfortunately, low‐cost battery cyclers, commonly used for electrochemical battery testing, are not compatible with symmetrical cells since they usually cannot apply negative bias voltages needed for symmetrical cells. The insertion of a Ni−Cd battery in the voltage sensing path is a simple and effective methodology to overcome this limitation for certain battery cyclers. Herein, the validity of this useful method is evaluated for other battery cyclers, realizing that the strategy is not universal. A modified methodology is developed for a battery cycler in which the previous method is not valid. The new strategy is based on inserting a Ni‐MH battery in the current path, and enables using a low‐cost Neware CT‐4008T‐5V6A‐S1 cycler for ferro‐ /ferricyanide symmetrical cells demonstrating proper operation for >19 days. This new method possesses advantages, e. g. direct reading of the cell voltage, and disadvantages, e. g. the Ni‐MH battery is charged/discharged during operation, which are discussed. The four battery cyclers evaluated show that, despite neither method is universal, both methods are complementary to each other. Thus, the decision of using either one method or the other must be reached on a case‐by‐case basis.
Vanadium redox flow batteries are recognized as well-developed flow batteries. The flow rate and current density of the electrolyte are important control mechanisms in the operation of this type of battery, which affect its energy power. The thermal behavior and performance of this battery during charging and discharging modes are also important. As a consequence, the aim of this investigation is to deeply study the impact of different working parameters on the temperature distribution and state of charge of these batteries. To achieve these goals, a single battery thermal model is established. The effects of various operating parameters, including working temperature, molar concentration, flow rate, and current density of the electrolyte, on the thermal behavior, state of charge, and performance of this type of battery are investigated. It is observed that the temperature distribution of high flow rate (90 mL min−1) is more uniform than that of other flow rates (30 and 60 mL min−1). In the end of the discharging mode, the battery voltage performance increases with the increase in the electrolyte flow rate. The temperature distribution of high current density (80 mA cm−2) is relatively uneven, and the local heating is produced at the battery outlet. The end time in the charging and discharging modes for the case of the high current density (80 mA cm−2) is faster than other current densities (20 and 40 mA cm−2).
The urgent need to address global warming and the energy crisis, caused by dependence on fossil fuels, has led to enhanced research for sustainable energy sources. The adoption of renewable energy alternatives has been swift, but the intermittent nature of these sources makes consistent power production challenging. To address this, various techniques are used to store energy from renewable sources, which can then be used in a controlled manner to meet rising energy demands while reducing global CO2 emissions. This article analyzes the state of the art of energy storage technologies, focusing on their characteristics, classifications, applications, comparisons, and limitations. The study also includes recent research on new energy storage types, as well as significant advances and developments.
In this work, a complete beneficiation technical route combining physical and chemical methods, namely a roasting-flotation-leaching scheme, is proposed to produce a qualified grade Ti-concentrate from altered Vanadium titanomagnetite (VTM) ore. Based on the character of the ore sample, it is recommended to recover the Ti-bearing minerals, ilmenite and anatase, as composite mineral. Pretreatment experiments indicate that the oxidation roasting (800 °C) and acid washing methods increase the flotation indexes significantly. Flotation condition tests show that the optimal conditions are a grinding fineness of −0.045 mm 83%, sulfuric acid dosage of 2000 g/t, water glass dosage of 1500 g/t, oxalic acid dosage of 200 g/t, and EM328 dosage of 1500 g/t. An open flotation circuit test obtains a flotation concentrate with a TiO2 grade and recovery of 38.30% and 25.99%, respectively. A leaching exploration test shows that the TiO2 grade of the flotation concentrate can be improved to 53.90%. XRD analyses reveal that the ilmenite in the VTM ore is converted into anatase and rutile during the roasting process at 600–800 °C, but pseudobrookite begins to form at 900 °C. Compared to the flotation concentrate, it is confirmed that the content of Ti-bearing minerals is increased significantly in the leaching residue.
The interest in flow batteries as energy storage devices is growing due to the rising share of intermittent renewable energy sources. In this work, the performance of a vanadium flow battery is improved, without altering the electrochemical cell, by applying a pulsating flow. This novel flow battery flow regime aims to enhance performance by improving its mass transfer properties. Both the pulse volume and frequency have an influence on the battery performance, and lead to a 38.7 % increase in accessible discharge capacity compared to the conventional steady flow regime at values of 0.40 cm³ and 2.4 Hz in this cell, respectively. Furthermore, at these parameter settings a reduction of the mass transfer resistance by 71.4 % is achieved at 0.2 cm³/min ⋅ cm². These results show that a pulsating flow can be beneficial in certain conditions, opening new possibilities for boosting performance in flow batteries.
Chlorate (ClO3–) is a toxic oxyanion pollutant from industrial wastes, agricultural applications, drinking water disinfection, and wastewater treatment. Catalytic reduction of ClO3– using palladium (Pd) nanoparticle catalysts exhibited sluggish kinetics. This work demonstrates an 18-fold activity enhancement by integrating earth-abundant vanadium (V) into the common Pd/C catalyst. X-ray photoelectron spectroscopy and electrochemical studies indicated that VV and VIV precursors are reduced to VIII in the aqueous phase (rather than immobilized on the carbon support) by Pd-activated H2. The VIII/IV redox cycle is the predominant mechanism for the ClO3– reduction. Further reduction of chlorine intermediates to Cl– could proceed via VIII/IV and VIV/V redox cycles or direct reduction by Pd/C. To capture the potentially toxic V metal from the treated solution, we adjusted the pH from 3 to 8 after the reaction, which completely immobilized VIII onto Pd/C for catalyst recycling. The enhanced performance of reductive catalysis using a Group 5 metal adds to the diversity of transition metals (e.g., Cr, Mo, Re, Fe, and Ru in Groups 6–8) for water pollutant treatment via various unique mechanisms.
Redox flow batteries (RFBs) are promising electrochemical energy storage systems, offering vast potential for large-scale applications. Their unique configuration allows energy and power to be decoupled, making them highly scalable and flexible in design. Aqueous RFBs stand out as the most promising technologies, primarily due to their inexpensive supporting electrolytes and high safety. For aqueous RFBs, there has been a skyrocketing increase in studies focusing on the development of advanced functional materials that offer exceptional merits. They include redox-active materials with high solubility and stability, electrodes with excellent mechanical and chemical stability, and membranes with high ion selectivity and conductivity. This review summarizes the types of aqueous RFBs currently studied, providing an outline of the merits needed for functional materials from a practical perspective. We discuss design principles for redox-active candidates that can exhibit excellent performance, ranging from inorganic to organic active materials, and summarize the development of and need for electrode and membrane materials. Additionally, we analyze the mechanisms that cause battery performance decay from intrinsic features to external influences. We also describe current research priorities and development trends, concluding with a summary of future development directions for functional materials with valuable insights for practical applications.
To achieve carbon neutrality, integrating intermittent renewable energy sources, such as solar and wind energy, necessitates the use of large-scale energy storage. Among various emerging energy storage technologies, redox flow batteries are particularly promising due to their good safety, scalability, and long cycle life. In order to meet the ever-growing market demand, it is essential to enhance the power density of battery stacks to lower the capital cost. One of the key components that impact the battery performance is the flow field, which is to distribute electrolytes onto electrodes. The design principle of flow fields is to maximize the distribution uniformity of electrolytes at a minimum pumping work. This review provides an overview of the progress and perspectives in flow field design and optimization, with an emphasis on the scale-up process. The methods used to evaluate the performance of flow fields, including both experimental and numerical techniques, are summarized, and the benefits of combining diverse methods are highlighted. The review then investigates the pattern design and structure optimization of serpentine- and interdigitated-based flow fields before discussing challenges and strategies for scaling up these flow fields. Finally, the remaining challenges and the prospects for designing highly efficient flow fields for battery stacks are outlined.
The traditional roasting technique using sodium salts in vanadium production has been disadvantageous due to the large consumption of energy and the emission of harmful gases. A modified process using molten salt roasting and water leaching to extract vanadium and titanium from domestic titanomagnetite concentrate was investigated. The roasting process was performed under optimal conditions: the weight ratio between the sample and NaOH of 1:1, the temperature of 400 °C, and the experiment time 90 min, and the conversion of vanadium could be maximized to 90%. The optimization of water leaching (at 60 °C for 90 min with a pulp density of 0.05 g/mL) could extract 98% of the vanadium from the roasted products into the solution, leaving titanium and iron remaining in the residue. Further purification of vanadium and titanium using the precipitation/ hydrolysis process followed by calcination obtained the final products V2O5 and TiO2 with high purities of 90% and 96%, respectively. A potential approach with modification of the roasting stage using NaOH was proposed, which was not only efficient to selectively extract the value metals from the titanomagnetite but also eco-friendly based on the reduction in energy consumption and emission of harmful gases.
Constructing active sites with enhanced intrinsic activity and accessibility in a confined microenvironment is critical for simultaneously upgrading the round-trip efficiency and lifespan of all-vanadium redox flow battery (VRFB) yet remains under-explored. Here, we present nanointerfacial electric fields (E-fields) featuring outstanding intrinsic activity embodied by binary Mo2C–Mo2N sublattice. The asymmetric chemical potential on both sides of the reconstructed heterogeneous interface imposes the charge movement and accumulation near the atomic-scale N–Mo–C binding region, eliciting the configuration of an accelerator-like E-field from Mo2N to Mo2C sublattice. Supported with theoretical calculations and intrinsic activity tests, the improved vanadium ion adsorption behavior and charge-transfer process at the nanointerfacial sites were further substantiated, hence expediting the electrochemical kinetics. Accordingly, the pronounced promotion is achieved in the resultant flow battery, yielding an energy efficiency of 77.7% and an extended lifespan of 1000 cycles at 300 mA cm–2, outperforming flow cells with conventional single catalysts in most previous reports.
Commercial electrolyte for vanadium flow batteries is modified by dilution with sulfuric and phosphoric acid so that series of electrolytes with total vanadium, total sulfate, and phosphate concentrations in the range from 1.4 to 1.7 m , 3.8 to 4.7 m , and 0.05 to 0.1 m , respectively, are prepared. The electrolyte samples of the series for positive and negative half‐cells at various state‐of‐charges are produced by electrolysis and are investigated for stability in the range of temperatures from −20 to +65 °C. It is attempted to reveal a correlation between initial electrolyte formulation in terms of total vanadium and total sulfate concentrations, which are measurable parameters in practice, and electrolyte thermal stability properties. The study of negative electrolyte samples by headspace online mass spectrometry enables to detect hydrogen gas, which evolves by chemical reaction of vanadium(II) species with protons during thermally induced aging. The battery with vanadium electrolyte at 1.4 m total vanadium, 4.7 m total sulfate, and 0.1 m phosphate concentrations displays more stable operation in terms of capacity decay during galvanostatic charge–discharge cycles than the battery with electrolyte at 1.7 m vanadium, 3.8 m sulfate, and 0.05 m phosphate concentrations under the same conditions.
Metal ions in the electrolyte will affect the electrochemical performance of vanadium redox flow batteries (vRBs). The influence of Mn 2+ concentration in the anolyte on the redox process of V(V)/V(IV) couple has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is found that Mn 2+ doesn't cause a side reaction, but greatly affects the electrochemical performance of V (V)/V(IV) redox reaction, including reaction activity, reversibility of electrode reaction, vanadium ion diffusion, and charge transfer reaction. The results from Cv show that the reversibility and the reaction activity of V(V)/V(IV) couple can be improved as the Mn 2+ concentration ranges from 0.04 to 0.13 g-L -1. The vanadium ion diffusion coefficient increases from 8.89×10 -7~1.098×10 -6 in the reference anolyte to 1.302×10 -6~1.800×10 -6 cm 2•s -1, an increase of ~ 60%. EIS investigation indicates that the electrode reaction resistance and interfacial resistance of V (V)/V(IV) couple have a slight increase when the Mn 2+ concentration is no more than 0.04 g •L -1. When the Mn 2+ concentration reaches 0.07 g• L -1, the values of these resistances are 25%~28% higher than those in the reference anolyte. Considering the results from CV and EIS, Mn 2+ has different adverse effects on the electrode reaction, but Mn 2+ with proper concentration is beneficial to improve the diffusion of vanadium ions.
In this paper, a high energy density vanadium redox battery employing a 3 M vanadium electrolyte is reported. To stabilise the highly supersaturated vanadium solutions, several additives were evaluated as possible stabilizing agents for the thermal precipitation of supersaturated V(V) solutions at elevated temperatures. The Blank 3 M V(V) solution in a sulfuric acid supporting electrolyte containing 5 M total sulfates, showed thermal precipitation after 3 days, while the solution containing 1 wt% H3PO4 additive increased the induction time for precipitation to over 47 days at 30°C. After 32 days, 3 M V(V) solutions containing 1 wt% sodium pentapoIyphosphate, 1 wt% K3PO4 and 2 wt% (NH4)2SO4 + 1 wt% H3PO4 showed final V(V) concentrations of 2.7, 2.7 and 2.6 M respectively, compared to 2.4 M V(V) in the Blank solution. From the screening tests, selected additives were used in vanadium redox flow cell cycling studies employing a 3 M vanadium electrolyte. The cell was subjected to 90 charge-discharge cycles and no precipitation or capacity loss was observed in the presence of 1wt% H3PO4 + 2 wt% ammonium sulfate. These results demonstrate that a significant enhancement in the energy density of the VRB can be achieved in the presence of additives that act as precipitation inhibitors for the vanadium ions.
The vanadium redox flow battery, which was first suggested by Skyllas-Kazacos and co-workers in 1985, is an electrochemical storage system which allows energy to be stored in two solutions containing different redox couples. Unlike commercially available batteries, all vanadium redox flow batteries have unique configurations, determined by the size of the electrolyte tanks. This technology has been proven to be an economically attractive and low-maintenance solution, with significant benefits over the other types of batteries. Moreover, the soaring demand for large-scale energy storage has, in turn, increased demands for unlimited capacity, design flexibility, and good safety systems. This work reviews and discusses the progress on electrodes and their reaction mechanisms as key components of the vanadium redox flow battery over the past 30 years. In terms of future outlook, we also provide practical guidelines for the further development of self-sustaining electrodes for vanadium redox flow batteries as an attractive energy storage system.
This paper presents a 2-D transient, isothermal model of a vanadium redox flow battery that can predict the species crossover and related capacity loss during operation. The model incorporates the species transport across the membrane due to convection, diffusion, and migration, and accounts for the transfer of water between the half-cells to capture the change in electrolyte volume. The model also accounts for the side reactions and associated changes in species concentration in each half-cell due to vanadium crossover. A set of boundary conditions based on the conservations of flux and current are incorporated at the electrolyte| membrane interfaces to account for the steep gradients in concentration and potential at these interfaces. In addition, the present model further improves upon the accuracy of existing models by incorporating a more complete version of the Nernst equation, which enables accurate prediction of the cell potential without the use of a fitting voltage. A direct comparison of the model predictions with experimental data shows that the model accurately predicts the measured voltage of a single charge/discharge cycle with an average error of 1.83%, and estimates the capacity loss of a 45 cycle experiment with an average error of 4.2%.
In this study, the oxidation of the V(II) by dissolved oxygen was examined quantitatively using UV-visible spectrophotometry. UV-visible spectrophotometry is an accurate method of determining the concentration of vanadium ions at both the negative and positive half-cell electrolytes. To apply Beer's law, the concentration should be diluted below 0.15 M to achieve a linear relationship between the absorbance and concentration. UV-visible spectrophotometry revealed that the concentration of V(II) in the negative half-cell electrolyte decreases continuously with cycling due to the rapid oxidation of the V(II) by dissolved oxygen. This decrease gives rise to an imbalance between the positive and negative half-cell electrolytes, which results in a significant capacity loss.
In this study, the concentration of vanadium ions in a carbon felt electrode was examined quantitatively using UV-visible spectrophotometry with respect to the current density and flow rate to examine the effect of the concentration overpotential on the performance of a vanadium redox flow battery (VRB). Experimental results revealed that the concentration of vanadium ions in the positive electrode is different from that observed in the positive electrolyte tank, and the deviation increases with increasing current density and decreasing flow rate. The concentration of V(IV) is higher in the positive electrode compared to the positive electrolyte tank after the discharging process, which leads to a premature cut-off in the discharge of the VRB. Therefore, this result suggests that it is important to improve electrolyte transport through the electrode for better performance of VRBs, and the measurement of the concentration of vanadium ions in the electrode could be useful for studying the concentration overpotential.
In-depth evaluation of the electrochemical performance of all-vanadium redox flow batteries (VRFBs) under operando conditions requires the insertion of a reliable reference electrode in the battery cell. In this work, an easy-to-make reference electrode based on silver–silver sulfate is proposed and described for VRFBs. The relevance and feasibility of the information obtained by inserting the reference electrode is illustrated with the study of ammoxidized graphite felts. In this case, we show that the kinetic of the electrochemical reaction VO2+/VO2+ is slower than that of V2+/V3+ at the electrode. While the slow kinetics at the positive electrode limits the voltage efficiency, the operating potential of the negative electrode, which is outside the stability widow of water, reduces the coulombic efficiency due to the hydrogen evolution.
Cyclic voltammetry (CV) and electrochemical impedance spectroscopies (EIS) of 2M V(IV) electrolyte in CH 3 SO 3 H and H 2 SO 4 mixed acid solution (MAS sample) and in H 2 SO 4 (Pristine sample) as the positive electrolyte in vanadium redox flow battery (VRFB) were investigated. CV results revealed that the MAS sample had a higher reversible redox reaction than Prisinte sample. EIS showed that. The performance of a VRFB with MAS sample as positive electrolyte was investigated with the charge-discharge technique. At 120 mA/cm 2 , VRFB with MAS sample exhibited the higher energy density (39.87 Wh/L) in comparison of Pristine sample. Cycling tests indicated that VRFB with the mixed acid system possessed a stable cycling performance during 30 cycles.
To improve the stability of 2M V(V) in 3M H 2 SO 4 electrolyte in charge-discharge cycles, Trishydroxymethyl aminomethane (Tris) was used as an additive of the electrolyte and first investigated in VRB applicaton. Thermal stability of the V(V) electrolyte at 40 °C was improved by 2-4% Tris additive. The cyclic voltammetry results of 2 M V(IV) in 3M H 2 SO 4 electrolyte with various additive amounts showed the electrochemical activity of the electrolyte with 3% Tris additive was improved compared to blank electrolyte, and electrochemical impedance spectroscopy results indicated the electrolyte solution resistance (0.43 ohm) was less than blank sample (0.91 ohm). The VRB employing the vanadium solution with the Tris additive as positive electrolyte exhibited higher voltage efficiency and energy efficiency at the same current densities compared with the blank electrolyte system.
In this study, the operation of a vanadium redox flow battery (VRFB)
under asymmetric current conditions (i.e., different current densities
during charge and discharge) was investigated as a technique to reduce
its capacity loss. Two different membrane types (a convection-dominated
membrane and a diffusion-dominated membrane) were analyzed. In these
analyses, the charging current density was varied while the discharging
current was held constant. For both membranes, it was found that
increasing the charging current decreases the net convective crossover
of vanadium ions, which reduces the capacity loss of the battery. When
the tested membranes were compared, the improvement in capacity
retention was found to be larger for the diffusion-dominated membrane
(12.4%) as compared to the convection-dominated membrane (7.1%). The
higher capacity retention in the diffusion-dominated membrane was
attributed to the reduction in the cycling time (and hence, suppressed
contribution of diffusion) due to the increased charging current. While
asymmetric current operation helps reduce capacity loss, it comes at the
expense of a reduction in the voltage efficiencies. Increasing the
charging current was found to increase the ohmic losses, which lead to a
decrease of 6% and 4.3% in the voltage efficiencies of the
convection-dominated and diffusion-dominated membranes, respectively.
The composition of electrolyte affects to a great extent the electrochemical performance of vanadium redox flow batteries (VRB). The effects of Cr3+ concentration in the anolyte on the electrode process of V(V)/V(IV) couple have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that Cr3+ causes no side reactions, but affects the electrochemical performance of V(V)/V(IV) redox reaction, including the reaction activity, the reversibility of electrode reaction, the diffusivity of vanadium ions, the interface film impedance, and the electrode reaction impedance. The experimental results show that Cr3+ within a certain concentration range can improve the reversibility of electrode reaction and the diffusion of vanadium ions. With the Cr3+ concentration increasing from 0 to 0.30 g L−1, the reversibility of V(V)/V(IV) reaction increases, while the diffusion resistance decreases. Correspondingly, the diffusion coefficient of vanadium ions increases from (5.48–6.77) × 10−7 to (6.82–8.44) × 10−7 cm2 s−1, an increase of ∼24%. However, the diffusion resistance increases and the diffusion coefficient decreases when Cr3+ concentration is over 0.30 g L−1, while the impedances of the interface, the film as well as the charge transfer increase continuously. As a result, Cr3+ with a certain concentration improves the diffusion and mass transfer process, but the resistances of the film, the interface, and the charge transfer rise. Furthermore, Cr3+ concentration of no more than 0.10 g L−1 has few effect on the electrode reaction process, and that of no more than 0.30 g L−1 is favorable to the diffusion of vanadium ions.
A major issue with the existing vanadium redox flow battery (VRFB) models is the inaccurate prediction of the open circuit voltage (OCV), which results in a discrepancy of 131 to 140mV in predicted cell voltages when compared to experimental data. This deviation is shown to be caused by the incomplete description of the electrochemical double layers within the cell when calculating the OCV using the Nernst equation adopted from fuel cell literature. Here, we propose a more complete description of the Nernst equation, which accounts for two additional electrochemical mechanisms that exist in a VRFB, namely: i) the proton activity at the positive electrode due to the involvement of the protons in the redox reaction, and ii) the Donnan potential due to the proton concentration differences across the membrane. The complete form of the Nernst equation proposed herein accurately predicts the reported experimental data with an average error of 1.2%, showing a significant improvement over the incomplete Nernst equation (8.1% average error) currently used in VRFB models.
Temperature-dependent ⁵¹V nuclear magnetic resonance (NMR) spectroscopy is used to study the high temperature stability of the VO2⁺ positive electrolyte of vanadium redox flow batteries (VRFBs). The NMR spectra at high temperatures feature significant line broadening of the VO2⁺ signal and a narrow line from VO(OH)3. The temperature, acid concentration, and VO2⁺ concentration dependencies of the line broadening collectively indicate the formation of paramagnetic VO²⁺ with increasing temperature and consequent paramagnetic dipolar broadening. In order to more clearly monitor the signal from VO(OH)3, which is indicative of the thermal instability of the VO2⁺ electrolyte, paramagnetic dipolar broadening of the VO2⁺ signal is intentionally induced by adding an appropriate amount of VO²⁺. This new analysis shows that, contrary to the previous perception, VO(OH)3 exists even at room temperature. The induced paramagnetic dipolar broadening can be utilized to assess approaches to improve temperature stability of the vanadium electrolyte.
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.
We present a review of the spectroscopy of post-transition metal ions. We have illustrated it with some examples: the s2 ground state configuration ions (T1+, In+, Ga+) doped the alkali halide hosts orBi3+ ions doped crystals or the d10 ground state configuration ions (Cu+). These kinds of systems have been chosen because they give rise to reference works with the Jahn-Teller effect in the excited states and because a lot of data are examined to get a better knowledge of the kinetic of their fluorescence. With Bi3+ ions there are also some strange variations of the luminescence properties not yet very well understood and it is worthwhile to give a synthesis of the main results. We will show how important is the role played by the metastable level 3Alu (3Po) for T1+- like ions or T1g for Cu+-like ions. Particular attention will be devoted to certain application such as the scintillator Bi4Ge3O12 and to the development of solid-state lasers.
Several inorganic additives are investigated as potential precipitation inhibitors for the positive half-cell electrolyte of the vanadium redox flow battery (VRB) at elevated temperatures. Electrolyte stability tests on the more concentrated 2M vanadium electrolyte show that of the additives tested, the best results are obtained with the addition of 1% phosphoric acid that give induction times of 40, 22 and 18 days for 80, 90 and 95% state of charge (SOC) solutions at 50°C, compared with 5, 2 and 1 days respectively for the corresponding blank solution. Several additives are also evaluated at 45°C in a vanadium redox flow cell operating with a 2M vanadium electrolyte. In the case of the cell employing the blank solution, a sudden drop in capacity, followed by pump failure is observed after around 70 cycles, while the cells with the additives continue to cycle without failure for 120 and 150 cycles for ammonium sulphate and ammonium phosphate additives respectively. A similar stabilising effect has previously been observed with these additives at low temperatures in the negative half-cell electrolyte of the VRB, confirming that these stabilising agents have the potential to extend the upper and lower operating temperature limits of a 2M vanadium electrolyte in the VRB to 45 and 5°C respectively, while also significantly enhancing its energy density without the need for potentially hazardous chloride additions.
An extensive screening of additives was undertaken to identify precipitation inhibitors to reduce the lower temperature limit of the negative half-cell electrolyte of the vanadium redox battery. In contrast to the 2 M VII blank solution that precipitated within 16 h at 5.0 °C, in a supporting electrolyte of sulfuric acid with 5 M total sulfates, the induction time for precipitation was more than 46 h in the presence of ammonium phosphate, ammonium sulfate, Flucon-100, sodium pentapolyphosphate and glycerol. In the case of the 2 M VIII solutions, considerable precipitation was observed after 4 days with 6.5 M total sulfates and the induction time increased to more than 40 days in the presence of ammonium phosphate, ammonium sulfate, sodium pentapolyphosphate and Orotan 1124. Cell cycling experiments confirmed that ammonium phosphate can dramatically extend the cell operation of a 2 M vanadium solution without precipitation over more than 250 cycles at 5 °C, compared with less than 15 cycles for a blank solution without additives.
The influence of d-shell occupation on the structure of the first hydration sphere for a series of hydrated transition metal ions has been evaluated by using theoretical calculations to optimize the geometry and find the conformations with lowest energy. General trends in the properties reflecting the calculated metal-oxygen bond strength and energy are discussed and compared with experimental values. The metal-oxygen distances, calculated by ab initio SCF methods using large basis sets for high-spin hexaaqua complexes in the TH symmetry of the di- and trivalent 3d metal ions, follow closely the trends found in crystal structure determinations of the isomorphous series of the 3d hexaaqua metal ions in Tutton and alum salts. The variation of the binding energies in a hexaaqua complex shows the double-humped features expected for a splitting of the d orbitals in an octahedral ligand field with the largest stabilization at formal d3 and d8 electron configurations of the metal ions. In cases with degenerate d-orbitals, an additional splitting of the energy levels by a lowering of the symmetry of the hexahydrated ion will in some cases significantly increase the binding energy. For the largest d1 and d6 metal ions an "all-vertical", and for some large d2 ions an "all-horizontal", conformation of the hydrogen atoms of the planarly coordinated water ligands around the trifold axis in the D3d symmetry is favored. For the hexahydrated d9 ion Ag2+, the first-order Jahn-Teller effect leading to a tetragonal elongation of the octahedral coordination has been studied. The possibility that the nondistorted d4 [Cr(H2O)6]2+ complex is forced into a low-spin state in its hexafluorosilicate salt due to compression in the lattice has been considered. The electrostatically dominated binding in the hexaaqua complexes shows an increasing covalent contribution to the right in the transition rows, especially for the trivalent ions. The ligand field effects are generally larger for the trivalent than for the divalent ions and also larger for the 4dn than for the 3dn elements, although the overall bond strength is lower due to the larger size of the 4dn ions. The high ligand field strength results in low-spin ground states for the hexahydrated Co3+, Ru2+, Rh3+ (d6), and Ru3+ (d5) ions in solution but, except for Rh3+, these high- to low-spin transitions proved to be difficult to reproduce by computations for the isolated hexaaqua clusters. Electron correlation was introduced, but the effect of the surroundings, which also may provide an additional important contribution to the stabilization of the low-spin state, was not accounted for. The low-spin square-planar configurations of the d8 complexes [Pd(H2O)4]3+ and [Ag(H2O)4]3+ are discussed in terms of ligand field effects.
The vanadium redox battery currently employs solutions of up to 2 M V(II)/V(III) and 2 M V(IV)/V(V) as the negative and positive half‐cell electrolytes. This concentration is limited by the solubility of the different vanadium ions in the temperature range of 10 to 40°C. Generally, the solubility of V(II), V(III), and V(IV) increases with an increase in temperature; however, the V(V) electrolyte suffers from the effect of thermal precipitation at temperatures of 40°C and above. While thermal precipitation is a serious problem in solutions of V(V) concentrations between 1.5 and 2.0 M, a surprising result was observed at concentrations above 3.0 M. As the results presented here show, at higher vanadium concentrations the V(V) solution demonstrated increased stability and there was no evidence of thermal precipitation over a 30 day period at temperatures above 40°C.
Studies have shown that the different vanadium ions in the all-vanadium redox battery (VRB) exhibit unequal rates of diffusion across the ion exchange membrane that gives rise to capacity loss due to a build up of vanadium ions in one half-cell with a corresponding decrease in the other. In this paper concentration profiles of the different vanadium ions have been modeled as a function of time under different charge and discharge currents in order to predict expected capacity loss over extended charge-discharge cycling. In the case of cation exchange membranes, a build up in total vanadium ion concentration in the positive half-cell is predicted, in agreement with observed trends in laboratory prototype VRB systems. Effects of factors other than diffusion are discussed in detail, along with electrolyte maintenance strategies to restore capacity during long-term VRB operation. (c) 2012 Elsevier B.V. All rights reserved.
We report results of polarization measurements resolved for the negative and positive electrodes of vanadium redox batteries (VRBs) using a dynamic hydrogen electrode in an operating battery cell. Electrochemical experiments with symmetric electrolyte feeds were also performed. Greater kinetic polarization is observed at the negative (V3/2+) electrode compared to the positive electrode (V5/4+), in contrast with previously reported ex situ measurements. For the positive electrode, the polarization in the low-current regime was modest and was not kinetically controlled. The relative rates of reaction are a surprise since it might be expected that the V3/2+ redox reaction is a simple outer-sphere electron transfer.
Electrolyte imbalance in vanadium redox flow batteries is an important problem for its long-term operation as it leads to loss of energy. To address this problem, a modified open circuit voltage (OCV) cell is developed by adding a middle half cell between the negative and positive half cells of a conventional OCV cell and used to predict the oxidation state of vanadium in the electrolyte solution from the measured voltage in each side of the electrolyte (positive and negative). The correlation between the oxidation state of vanadium and cell voltage is explained by a basic electrochemical principle and the Nernst equation. The experimental results show that at different oxidation states of vanadium, the predicted OCV agrees reasonably with the experimental data. In addition, the effect of the state of charge (SOC) and electrolyte imbalance on the energy capacity of a cell is discussed.
Sb3+ ions are introduced into the negative electrolyte of vanadium redox flow batteries (VRFB), and their influence on the electrochemical performance of VRFB are investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results show that the electrochemical activity and sluggish kinetics of V(II)/V(III) redox couple can be improved by the addition of Sb3+ ions, and the optimal concentration of Sb3+ ions is found to be 5 mM. Meanwhile, Sb3+ ions can lead to an increase of the diffusion coefficient of V(III) species and a decrease of charge transfer resistance. Moreover, the VRFB cell using negative electrolyte with Sb3+ ions exhibits excellent cycling stability and high average energy efficiency, especially under high power operation. The energy efficiency (67.1%) of the VRFB employing electrolyte with 5 mM Sb3+ ions is increased by 9.6% at a current density of 120 mA · cm−2, compared to the pristine one (57.5%). The improved electrochemical performance should be ascribed to the prominent catalytic effect of Sb particles, which are simultaneously electrodeposited onto the surface of graphite felts during operation of the flow cell and facilitate charge transfer process.
A general electrolyte model for calculation of the liquid electrolyte transport in fuel cells is presented. A 2-D formulation is used to describe the transport in an alkaline fuel cell. Numerical results were obtained by using commercial CFD software, in conjunction with the user defined functions that calculate the source terms of the transport equations. An order of magnitude analysis is conducted of the energy transport in the separator and electrode regions. The numerical calculation also examines the local primary current at the anode and cathode electrodes. The calculated current flux showed a higher value near the separator entrance and it decreased along the stream-wise direction. The non-uniformity of the local primary current is caused mainly by the species transport resistance between the electrodes instead of the temperature difference. The effects of four different electrolytes were also studied. The results suggested that the cell voltage differences were due to the competing effects of electrolyte conductance and species diffusion. Numerical calculation also captured the presence of shunt current. A net shunt current as high as 0.1 A/cm2 is calculated at the separator inlet and exit. Provisions to reduce shunt currents seem to be warranted for AFC operated at a condition similar to that examined in this paper.
Trishydroxymethyl aminomethane (Tris) was used as an additive of the positive electrolyte for all vanadium redox battery (VRB) and cycling and electrochemical stabilities of the positive electrolyte were investigated. The 50 cyclic voltammetry (CV) cycles suggested that the positive electrolyte with the Tri additive after charge-discharge cycles has good cycle stability compared to that before the charge-discharge cycles. The VRB employing the vanadium electrolyte with the Tris additive as positive electrolyte exhibited better charge-discharge behavior and less discharge capacity fade rate with cycles compared with the blank electrolyte system. The UV/visible spectroscopy showed that the vanadium concentration in the positive electrolyte containing Tri additive during 40 charge-discharge cycles remains unchanged. The X-ray photoelectron spectroscopy (XPS) verified that the positive electrolyte with the Tri additive has no etching and oxidation effect on the carbon felt electrode.
The present work seeks to understand the chemical structure and solvation thermodynamics of aqueous vanadium cations commonly employed in vanadium redox flow batteries. First principles based electronic structure calculations were performed to obtain the hydration structure and partial charges distributions of the cations in bulk water. The thermodynamics of solvation was determined within the context of a quasi-chemical theory of solution. The free energies of solvation for the V2+, V3+, VO2+, and View the MathML source cations were computed to be: −440.0, −1018.8, −457.2, and −184.4 kcal/mol, respectively, which agree well with available experimental values.
Influence of In3+ ions on electrochemical performance of positive electrolyte for vanadium redox flow battery was investigated in this paper. The electrochemical activity and kinetics of V(IV)/V(V) redox couple can be enhanced by the addition of In3+ ions, and the optimal concentration of In3+ ions was found at 10 mM. At this condition, the oxidation peak current with 10 mM In3+ ions is 46.6 mA at a scan rate of 20 mV s−1, larger than that of pristine electrolyte (41.8 mA), and the standard rate constant is 6.53 × 10−5 cm s−1, 42 % larger than that of the pristine electrolyte (4.58 × 10−5 cm s−1). The cell using electrolyte with 10 mM In3+ ions was assembled, and the charge–discharge performance was evaluated, and the average energy efficiency increases by 1.9 % compared with the pristine cell. The improved electrochemical performance may be ascribed to that In3+ ions change the hydration state of vanadium ions in electrolyte and promote charge transfer process.
This study reports important observations regarding the convective transport through the membrane and related effects on the species crossover and the capacity loss in vanadium redox flow batteries (VRFBs). A 2-dimensional, isothermal, transient model is utilized to simulate several extended charge/discharge cycles with varying flow rates and electrolyte viscosities. The simulations indicate that osmotic and electro-osmotic convections in the membrane are major mechanisms contributing to species crossover. In addition, variations in electrolyte viscosity are observed to have a significant impact on the direction and magnitude of species crossover during VRFB operation. Finally, the simulations suggest that one potential approach to minimize the capacity loss in VRFBs would be to operate the system at constant pressure condition through the utilization of asymmetric flow rates (i.e. different flow rates in the ‘+’ and ‘−’ half-cells) to reduce the impact of osmotic convection.
A quaternary ammonium functionalized poly(fluorenyl ether) anion exchange membrane (AEM) with extremely low VO2 + permeation was characterized for vanadium redox flow battery (VRFB) application. One hundred percent coulombic efficiency (CE) was achieved for the AEM-based VRFB at all the current densities tested. Comparatively, the CE of a N212 membrane-based VRFB was lower than 94% and varied with charge/discharge current density. At current densities lower than 60 mA cm− 2, the energy effiency of the AEM-based VRFB was higher than that of a device with N212. The cycling performance demonstrated that the AEM-based VRFB was free of capacity fade, which is a consequence of its low VO2 + permeability. These observations are of significant importance for flow batteries that operate intermittently or at moderate current densities.
In vanadium redox flow batteries (VFB), the power of the battery is determined by the number of cells in the stack. Serial and parallel layouts are commonly adopted interactively to suit the designed power demand. The bipolar stack design inevitably introduces shunt currents bypassing into the common manifolds in the stack and thereby resulting in a parasitic loss of power and energy. During standby, shunt current and its associated internal discharge reactions can generate heat and increase stack temperature, potentially leading to thermal precipitation in the positive half-cell. This study aims to investigate the effect of shunt current on stack efficiency and temperature variation during standby periods for a 40-cell stack. Dynamic models based on mass balance, energy balance and electrical circuit are developed for simulations and the results provide an insight into stack performance that will aid in optimising stack design and suitable cooling strategies for the VFB.
In this study, a 2-D, transient vanadium redox flow battery (VRFB) model was used to investigate and compare the ion transport mechanisms responsible for vanadium crossover in Nafion® 117 and sulfonated Radel (s-Radel) membranes. Specifically, the model was used to distinguish the relative contribution of diffusion, migration, osmotic and electro-osmotic convection to the net vanadium crossover in Nafion® and s-Radel. Model simulations indicate that diffusion is the dominant mode of vanadium transport in Nafion®, whereas convection dominates the vanadium transport through s-Radel due to the lower vanadium permeability, and thus diffusivity of s-Radel. Among the convective transport modes, electro-osmotic convection (i.e., electro-osmotic drag) is found to govern the species crossover in s-Radel due to its higher fixed acid concentration and corresponding free ions in the membrane. Simulations also show that vanadium crossover in s-Radel changes direction during charge and discharge due to the change in the direction of electro-osmotic convection. This reversal in the direction of crossover during charge and discharge is found to result in significantly lower “net” crossover for s-Radel when compared to Nafion®. Comparison of these two membranes also provides guidance for minimizing crossover in VRFB systems and underscores the importance of measuring the hydraulic and the electro-kinetic permeability of a membrane in addition to vanadium diffusion characteristics, when evaluating new membranes for VRFB applications.
The vanadium redox flow battery (VRB) is one of the most promising electrochemical energy storage systems deemed suitable for a wide range of renewable energy applications that are emerging rapidly to reduce the carbon footprint of electricity generation. Though the Generation 1 Vanadium redox flow battery (G1 VRB) has been successfully implemented in a number of field trials and demonstration projects around the world, it suffers from low energy density that limits its use to stationary applications. Extensive research is thus being carried out to improve its energy density and enhance its performance to enable mobile applications while simultaneously trying to minimize the cost by employing cost effective stack materials and effectively controlling the current operating procedures. The vast bulk of this research was conducted at the University of New South Wales (UNSW) in Sydney during the period 1985–2005, with a large number of other research groups contributing to novel membrane and electrode material development since then. This paper presents a historical overview of materials research and development for the VRB at UNSW, highlighting some of the significant findings that have contributed to improving the battery's performance over the years. Relevant work in this field by other research groups in recent times has also been reviewed and discussed.
Coulter dispersants were investigated as the additive into the positive electrolyte (more than 1.8 M vanadium ions) of vanadium redox flow battery (VRB). The electrolyte stability tests showed that, at 45, 50 and 60 °C, the addition of 0.050–0.10 w/w Coulter dispersant IIIA (mainly containing coconut oil amine adduct with 15 ethylene oxide groups) into the positive electrolyte of VRB could significantly delay the time of precipitate formation from 1.8–12.3 h to 30.3 h ∼ 19.3 days. Moreover, the trace amount of Coulter dispersant IIIA as the additive can enhance the electrolyte stability without changing the valence state of vanadium ions, reducing the reversibility of the redox reactions and incurring other side reactions at the electrode. Using the Coulter IIIA dispersant as the additive also improved the energy efficiency of the VRB. The UV–vis spectra confirmed that the trace amount of Coulter IIIA dispersant did not chemically react with V(V) to form new substances. The synergy of Coulombic repulsion and steric hindrance between the macromolecular cationic surfactant additive and the solution reduced the aggregation of vanadium ions into V2O5 and increased the supersaturation of V2O5 crystal in the solution.
The performance of a 2 kW, 10 kW h zincbromine battery is reported. The battery uses new carbon/PVDF bipolar electrodes and a circulating polybromide/aqueous zincbromine electrolyte. A turn-around efficiency of 65–70% is achieved. Disclosure is also given of an innovative non-flowing-electrolyte cell. This system is less complex and, hence, gives increased reliability and a higher return efficiency. A 25 A h single cell has completed over 400 cycles (100% depth-of-discharge) with a total return energy efficiency of over 75%. Such technology is extremely attractive for remote-area power-supply applications.
With the increase need to seamlessly integrate the renewable energy with the current grid which itself is evolving into a more intelligent, efficient, and capable electrical power system, it is envisioned that the energy storage system will play a more prominent role in bridging the gap between the current technology and a clean sustainable future in grid reliability and utilization. Redox flow battery technology is leading the way in this perspective in providing a well balanced approach for current challenges. Recent progress in the research and development of redox flow battery technology is reviewed here with a focus on new chemistries and systems.
The shunt current loss is one of main factors to affect the performance of the vanadium redox flow battery, which will shorten the cycle life and decrease the energy transfer efficiency. In this paper, a stack-level model based on the circuit analog method is proposed to research the shunt current loss of the vanadium redox flow battery, in which the SOC (state of charge) of electrolyte is introduced. The distribution of shunt current is described in detail. The sensitive analysis of shunt current is reported. The shunt current loss in charge/discharge cycle is predicted with the given experimental data. The effect of charge/discharge pattern on the shunt current loss is studied. The result shows that the reduction of the number of single cells in series, the decrease of the resistances of manifold and channel and the increase of the power of single cell will be the further development for the VRFB stack.
The diffusion of vanadium ions across the membrane along with side reactions can have a significant impact on the capacity of the vanadium redox flow battery (VFB) over long-term charge–discharge cycling. Differential rates of diffusion of the vanadium ions from one half-cell into the other will facilitate self-discharge reactions, leading to an imbalance between the state-of-charge of the two half-cell electrolytes and a subsequent drop in capacity. Meanwhile side reactions as a result of evolution of hydrogen or air oxidation of V2+ can further affect the capacity of the VFB. In this paper, a dynamic model is developed based on mass balances for each of the four vanadium ions in the VFB electrolytes in conjunction with the Nernst Equation. This model can predict the capacity as a function of time and thus can be used to determine when periodic electrolyte remixing or rebalancing should take place to restore cell capacity. Furthermore, the dynamic model can be potentially incorporated in the control system of the VFB to achieve long term optimal operation. The performance of three different types of membranes is studied on the basis of the above model and the simulation results together with potential operational issues are analysed and discussed.
Fructose, mannitol, glucose, d-sorbitol are explored as additives in electrolyte for vanadium redox battery (VRB), respectively. The effects of additives on electrolyte are studied by cyclic voltammetry (CV), charge–discharge technique, electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. The results indicate that the vanadium redox cell using the electrolyte with the additive of d-sorbitol exhibits the best electrochemical performance (the energy efficiency 81.8%). The EIS results indicate that the electrochemical activity of the electrolyte is improved by adding d-sorbitol, which can be interpreted as the increase of available (–OH) groups providing active sites for electron transfer. The Raman spectra show that VO2+ ions take part in forming a complex with the d-sorbitol, which not only improve solubility of V(V) electrolyte, but also provide more activity sites for the V(IV)/V(V) redox reaction.
The vanadium redox battery currently employs solutions of up to 2 MV(II)/V(III) and 2 MV(IV)/V(V) as the negative and positive half-cell electrolytes. This concentration is limited by the solubility of the different vanadium ions in the temperature range of 10 to 40 degrees C. Generally, the solubility of V(II), V(III), and V(IV) increases with an increase in temperature; however, the V(V) electrolyte suffers from the effect of thermal precipitation at temperatures of 40 degrees C and above. While thermal precipitation is a serious problem in solutions of V(V) concentrations between 1.5 and 2.0 M a surprising result was observed at concentrations above 3.0 M. As the results presented here show, at higher vanadium concentrations the V(V) solution demonstrated increased stability and there was no evidence of thermal precipitation over a 30 day period at temperatures above 40 degrees C.
Diffusion coefficients of the vanadium ions across Nafion 115 (Dupont) in a vanadium redox flow battery (VRFB) are measured and found to be in the order of V2+>VO2+>VO2+>V3+. It is found that both in self-discharge process and charge–discharge cycles, the concentration difference of vanadium ions between the positive electrolyte (+ve) and negative electrolyte (−ve) is the main reason causing the transfer of vanadium ions across the membrane. In self-discharge process, the transfer of water includes the transfer of vanadium ions with the bound water and the corresponding transfer of protons with the dragged water to balance the charges, and the transfer of water driven by osmosis. In this case, about 75% of the net transfer of water is caused by osmosis. In charge–discharge cycles, except those as mentioned in the case of self-discharge, the transfer of protons with the dragged water across the membrane during the electrode reaction for the formation of internal electric circuit plays the key role in the water transfer. But in the long-term cycles of charge–discharge, the net transfer of water towards +ve is caused by the transfer of vanadium ions with the bound water and the transfer of water driven by osmosis.
The specific energy of the vanadium redox battery is determined by the solubility of the four vanadium oxidation states in sulfuric acid. While recent studies have shown that a higher vanadium concentration than that initially proposed might be feasible, further reliable solubility data for the various vanadium ions is required if the electrolyte composition is to be properly optimized. This study describes the results of a solubility study of vanadyl sulfate in sulfuric acid. VOSO4 is the species which exists in the discharged positive half-cell of the vanadium redox cell. The solubility data have been generated in sulfuric acid concentrations that range from 0 to 9 mol/l and at temperatures between 10 and 50°C. The solubility of VOSO4 is found to decrease continuously with increasing H2SO4 concentration and decreasing temperature. At 20°C, the solubility of VOSO4 in distilled water is 3.280 mol/l whereas in 9 M H2SO4 it is 0.260 mol/l. The drop in solubility with increasing H2SO4 concentration is significant and is more pronounced at lower concentrations. A multivariable solubility prediction model has been developed as a function of temperature and total sulfate/bisulfate (SO42− and HSO4−) concentration using the extended Debye–Huckel functional form. The average absolute deviation of the predicted solubility values from experimental data is 4.5% with a maximum deviation of about 12% over the abovementioned temperature and sulfuric acid concentration range. When solubility data in the more useful H2SO4 concentration range of 3–7 M is considered, the solubility correlation improved with an average absolute deviation of only 3.0% and a maximum deviation of about 7%.
Conducting polyethylene (PE) composite material is fabricated by mixing polyethylene with conducting fillers. Electrical, mechanical, permeation and electrochemical studies show that the PE composite is a good electrode matrix material for the vanadium redox battery. Chemical treatment studies on two kinds of PE composites show that the material with a high graphite-fibre content has good electrochemical activity and stability after treatment. Further cyclic voltammetry and SEM investigations indicate that chemical treatment increases the active surface area of the PE composite.
Although the vanadium redox battery (VRB) has recently attracted considerable interest as an energy storage technology, it has a relatively poor energy-to-volume ratio and a system complexity compared with other technologies; however, modelling can assist in optimizing cell and stack design. This paper analyzes a 2D time-dependent single-phase isothermal model for the operation of a single cell in a VRB. Unlike in all previous work, asymptotic methods are used to determine the characteristic current density scale in terms of operating conditions and cell component properties. Also, the analysis reveals that the fluid mechanics decouples from the electrochemistry, at leading order; an asymptotically reduced model is then proposed which preserves the original geometrical resolution. This approach is recommended for accurate and computationally efficient VRB stack models, as has been achieved for polymer electrolyte fuel cells; this will be a prerequisite for the use of modelling in stack design and thence large-scale commercialization of the VRB. Finite-element methods are used to compute results for the 1D steady state high-stoichiometry limit; although an idealized case, it is recommended for the in-situ experimental acquisition of VRB electrokinetic data that can then be used for the model when applied under more general operating conditions.
As part of a project aimed at increasing the energy density of the vanadium redox battery for mobile applications, a number of additives was evaluated as precipitation inhibitors to enhance the stability of supersaturated vanadyl electrolytes in . While the saturation solubility of vanadyl sulfate in is less than at , supersaturated vanadyl sulfate solutions could readily be prepared. A blank solution of vanadyl sulfate in began to precipitate after 22 days at , reaching an equilibrium concentration of after 80 days. In the presence of or sodium hexametaphosphate, however, there was no sign of precipitation after 80 days at . ©1999 The Electrochemical Society
Water transfer behaviour across commercially available cation exchange membranes in the vanadium redox battery (VRB) was investigated. Evaluations showed that the direction of preferential water transfer was dependent on the state of charge (SOC) of the vanadium electrolytes employed. When the vanadium electrolytes are at an initial SOC of 100 and 50% the direction of water transfer is toward the positive half-cell. As the electrolyte discharges from 50 to 0% SOC the direction of water transfer reverses toward the negative half-cell. It appears that the most significant level of water transfer occurs as the cell goes into over-discharge where the V(III) and V(IV) electrolytes become fully mixed below the 0% SOC state. The membranes evaluated displayed comparable water transfer properties.
Recent studies on anion exchange membranes and electrodialysis methods to permeate specific anions through the membranes are reviewed. The studies are classified: (1) to increase cross-linkage of the anion exchange membranes, (2) to form tight surface layers on the anion exchange membranes, (3) to decrease hydrophilicity of the anion exchange membranes or their surfaces by introducing specific anion exchange groups in the membranes, (4) to impregnate hydrophilic compounds in the anion exchange membranes to increase hydrophilicity of the membranes, (5) to control permselectivity of anions by photoirradiation using membranes with a photoresponsive group and (6) to control permselectivity of anions through thermally responsive anion exchange membranes with temperature. Permselectivity of specific anions through the anion exchange membranes is governed mainly by the balance of hydration energy of anions with hydrophilicity of the membranes, partially by hydrated ionic size of the anions, except the membranes having an oppositely charged layer on the membrane surface.