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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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

As one of the latest research directions, membraneless batteries provide an economical solution to redox flow batteries. Advances in electrolyte and device design have promoted membraneless batteries from microfluidic demonstration to systems with stimulated modes and considerable electrochemical properties. However, the achieved cycling volumetric capacity is typically limited. Herein, a biphasic membraneless zinc‐iodine battery (Z|T‐I) is proposed, through optimized Zn growth, the Z|T‐I battery achieved a volumetric capacity of 8.93 Ah L−1 for 100 cycles with an average Coulombic efficiency (CE) of 98.49% (2300 h), the deep cycling exhibited a volumetric capacity of 21.7 Ah L−1 (50.43 Wh L−1 based on typical membraneless battery calculation, 1200 h); both values are the highest among single/biphasic membraneless batteries using liquid active materials with/out stimulations (<6.6 Ah L−1) reported so far based on the total electrolyte volume. A thorough analysis of Zn growth in Zn‐Br2/I2 electrolytes with various testing conditions provided direct evidence of the contrasting Zn electrodeposition morphologies at both macro and micro scales. The biphasic ZnI2 battery design strategy gives insights into optimizing material crossover/spatial utilization and electrolyte interfacial stability to realize a scalable membraneless energy storage system with a reduced cost. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are considered as promising candidates for large‐scale energy storage. However, traditional RFBs based on toxic metal ions have deficiencies in resource utilization and environmental protection. Considering the corrosiveness of acidic and alkaline electrolytes and sustainability of energy storage devices, neutral aqueous organic redox flow batteries (AORFBs) have more development prospects. Herein, an AORFB is reported, using 9,10‐anthraquinone‐2,7‐disulfonic salt (2,7‐AQDS) and 1,4‐dihydroxyphenylsulfonate potassium (HQS) as negative and positive electroactive materials, respectively. It is found that the anions in the neutral solution further affected the solubility and kinetics of the electroactive materials by affecting the hydrogen bond in the solution, and the Na2SO4 solution showed the optimal comprehensive performance. Therefore, an all‐quinone AORFB employing neutral Na2SO4 electrolytes with a cell voltage of 0.9 V is constructed, which presented a capacity utilization of 70.1% and delivered stable cycling performance at 60 mA cm−2. This work designs an innovative neutral all‐quinone AORFB and points out how anions affect the properties of the electrolyte by affecting hydrogen bonds within the solution. These findings open a new avenue for the design and application of neutral all‐organic aqueous RFBs. [ABSTRACT FROM AUTHOR]

Alkaline zinc‐ferricyanide flow batteries are efficiency and economical as energy storage solutions. However, they suffer from low energy density and short calendar life. The strongly alkaline conditions (3 mol L−1 OH−) reduce the solubility of ferri/ferro‐cyanide (normally only 0.4 mol L−1 at 25 °C) and induce the formation of zinc dendrites at the anode. Here, we report a new zinc‐ferricyanide flow battery based on a mild alkalescent (pH 12) electrolyte. Using a chelating agent to rearrange ferri/ferro‐cyanide ion‐solvent interactions and improve salt dissociation, we increased the solubility of ferri/ferro‐cyanide to 1.7 mol L−1 and prevented zinc dendrites. Our battery has an energy density of ~74 Wh L−1catholyte at 60 °C and remains stable for 1800 cycles (1800 hours) at 0 °C and for >1400 cycles (2300 hours) at 25 °C. An alkalescent zinc‐ferricyanide cell stack built using this alkalescent electrolyte stably delivers 608 W of power for ~40 days. [ABSTRACT FROM AUTHOR]

Most investigations on flow batteries (FBs) make the assumption of perfectly mixed electrolytes inside the tanks without estimating their likelihood, while specific analyses are missing in the literature. This paper presents a pioneering investigation of the electrolyte flow dynamics inside FB tanks. This study considers the Open Circuit Voltage (OCV) measured at the stack of a 9 kW/27 kWh Vanadium FB with 500 L tanks. Order-of-magnitude estimates of the measured dynamics suggest that differences in densities and viscosities of the active species drive gradients of concentrations with different patterns in the positive and negative tanks and in charge and discharge, affected by current and flow rate, which result in significant deviation from homogeneity, affecting the State of Charge (SoC) of the electrolytes flowed into the stack and thus the FB performance. In particular, stratifications of the inlet electrolytes may appear which are responsible for delays in reaching the outlets, with initial plateau and following step (s) in the SoC at the stack. These events can have a major impact in the performance of industrial FBs with large tanks and suggest that specific tank designs may improve the overall dynamics, calling for further analysis. [ABSTRACT FROM AUTHOR]

Flow batteries are promising for large‐scale energy storage in intermittent renewable energy technologies. While the iron–chromium redox flow battery (ICRFB) is a low‐cost flow battery, it has a lower storage capacity and a higher capacity decay rate than the all‐vanadium RFB. Herein, the effect of electrolyte composition (active species and supporting electrolyte concentrations), Fe/Cr molar ratio, and supporting electrolyte type (HCl and H2SO4) on the performance (current efficiency (CE), voltage efficiency (VE), energy efficiency, discharge capacity, and capacity decay) of an ICRFB is investigated. The storage capacity of the optimum electrolyte (1.3 m FeCl2, 1.4 m CrCl3, 5.0 mm Bi2O3 in 1.0 m HCl) is 40% higher (from 17.5 to 24.4 Ah L−1), while the capacity decay rate is tenfold lower (from 3.0 to 0.3% h−1) than the performance of the previously used 1.0 m FeCl2, 1.0 m CrCl3 in 3.0 m HCl. At the optimum Fe and Cr concentrations and ratio in 0.5 m HCl, a near constant CE (92.3%), VE (78.7%), and EE (72.6%) are obtained over 50 cycles. The significantly higher capacity decay when using 1.0 m H2SO4 (1.6% h−1) compared to 1.0 m HCl (0.3% h−1) confirms that HCl is the more suitable supporting electrolyte. [ABSTRACT FROM AUTHOR]

The limited photoelectric conversion efficiency poses one of the critical constraints on commercializing solar flow batteries (SFBs). This study compares the chemical and photoelectrochemical properties of three commonly used redox couples. Additionally, magnetic Fe3O4 nanoparticles, for the first time, are introduced to optimize the electrolyte, and they are compared with the original electrolyte. Across different redox couples, the variations in semiconductor flat-band potentials and carrier concentrations result in changes in photoelectric current density. Notably, FeCl2/FeCl3 redox coupled with TiO2 photoelectrodes exhibits the highest photoelectric current density, reaching 75.7 µA cm−2. However, the trade-off of this electrolyte, i.e., providing high photocurrent while being unable to supply sufficient open-circuit voltage, imposes limitations on the practical application of SFBs. Alternatively, for TEMPO and 4-OH-TEMPO electrolytes, which can provide a higher open-circuit voltage, the electrochemical activity is enhanced, and the solution ohmic resistance is reduced by introducing magnetic nanoparticles to form a magnetic nanofluid. As a result, the photoanode's photocurrent density increases by 36.6% and 17.0%, respectively, in the two electrolytes. The work reported here effectively addresses the current issue of low photocurrent density in SFBs and presents new optimization strategies for SFBs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The ever‐increasing threat of climate change and the depletion of fossil fuel resources necessitate the use of solar‐ and wind‐based renewable energy sources. Large‐scale energy storage technologies, such as redox flow batteries (RFBs), offer a continuous supply of energy. Depending on the nature of the electrolytes used, RFBs are broadly categorized into aqueous redox flow batteries (ARFBs) and non‐aqueous redox flow batteries (NARFBs). ARFBs suffer from various problems, including low conductivity of electrolytes, inferior charge/discharge current densities, high‐capacity fading, and lower energy densities. NARFBs offer a wider potential window and range of operating temperatures, faster electron transfer kinetics, and higher energy densities. In this review article, a critical analysis is provided on the design of organic electroactive molecules, their physiochemical/electrochemical properties, and various organic solvents used in NARFBs. Furthermore, various redox‐active organic materials, such as metal‐based coordination complexes, quinones, radicals, polymers, and miscellaneous electroactive species, explored for NARFBs during 2012–2023 are discussed. Finally, the current challenges and prospects of NARFBs are summarized. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents an electrochemical–thermal–hydraulic–mechanical (ETHM) coupling model by introducing the electrolyte flow field into the model of lithium-ion batteries (LIBs). First, the ETHM coupling model is established on the basis of the electrochemical–thermal–mechanical (ETM) coupling model and poroelasticity model. Then, the ETHM coupling model for the discharge of LIBs is numerically resolved and verified against the ETM coupling model using the commercial 11.5 Ah LiMn2O4 battery as an example. Subsequently, the flow and deformation characteristics (such as the Darcy's velocity, the pore pressure, the Péclet number, and the volumetric strain) are analyzed. The results suggest that the electrolyte does flow during the operation of LIBs. Furthermore, the electrolyte flow is governed by the internal pore pressure gradient induced by the poroelastic effect. The electrolyte flow behavior is also influenced by the boundary conditions. The findings in this study are of benefit to obtain in-depth understanding of the coupling mechanisms among the electrolyte flow field and the stress (strain) field, the temperature field, and the electrochemical field. [ABSTRACT FROM AUTHOR]

Due to its adaptability in scaling up, a redox flow battery (RFB) is seen to be one of the finest options for large-scale electrical backup systems. As a result, it is feasible to create RFB systems that are both cost and performance effective. Recently, a zinc-manganese RFB that relies on Zn(s)/Zn2+(aq) and Mn2+(aq)/MnO2 redox couples has gained attention since both zinc and manganese are cheap, abundant, and eco-friendly. However, the reversibility of Mn2+(aq)/MnO2 at the positive electrode is limited by the formation of Mn3+ species upon charge/discharge (CD) cycling, resulting in severe capacity fading. Herein, this study examines the use of reducing agents as electrolyte additives to enhance the reversibility of the Mn2+(aq)/MnO2 reaction. Experimental results indicate that sulfuric acid and oxalic acid as additives can significantly improve the reversibility of the Mn2+(aq)/MnO2 reaction and the cycling stability of zinc-manganese RFBs. The acetate-based system demonstrates better reversible reaction than the sulfate-based system having more than 100 CD cycles at a current density of 10 mA/cm2. Coulombic efficiency (CE) is also seen to be higher than 90%. Overall, results lead to increased efficiency and cycling stability for zinc-manganese RFBs. [ABSTRACT FROM AUTHOR]

There are a number of critical requirements for electrolytes in aqueous redox flow batteries. This paper reviews organic molecules that have been used as the redox‐active electrolyte for the positive cell reaction in aqueous redox flow batteries. These organic compounds are centred around different organic redox‐active moieties such as the aminoxyl radical (TEMPO and N‐hydroxyphthalimide), carbonyl (quinones and biphenols), amine (e. g., indigo carmine), ether and thioether (e. g., thianthrene) groups. We consider the key metrics that can be used to assess their performance: redox potential, operating pH, solubility, redox kinetics, diffusivity, stability, and cost. We develop a new figure of merit – the theoretical intrinsic power density – which combines the first four of the aforementioned metrics to allow ranking of different redox couples on just one side of the battery. The organic electrolytes show theoretical intrinsic power densities which are 2–100 times larger than that of the VO2+/VO2+ couple, with TEMPO‐derivatives showing the highest performance. Finally, we survey organic positive electrolytes in the literature on the basis of their redox‐active moieties and the aforementioned figure of merit. [ABSTRACT FROM AUTHOR]

The electrochemical performance of iron chromium flow batteries was improved by optimizing the electrolyte with high-entropy oxides. The influence of high-entropy oxide content in the electrolyte on the electrochemical performance of iron chromium flow batteries was studied and analyzed. Due to the synergistic effect of conductivity and electrochemical activity, the flow batteries exhibit better electrochemical performance when the electrolyte doping amount is 1 wt%. When the current density is 160 mAcm-2, the capacity of the assembled flow battery after optimizing the electrolyte is 210 mAhL-1, and the capacity after 100 cycles is 187 mAhL-1. From the calculation of adsorption energy and Gibbs free energy, it can be seen that the adsorption free energy of CuMn 2 O 4 for iron ions is -151.256 J/mol, which greatly promotes the rate of redox reaction in the iron chromium flow batteries. This work effectively improves the electrochemical performance of iron chromium flow batteries, and further provides theoretical guidance and data support for the application of high-entropy oxides in flow batteries. [ABSTRACT FROM AUTHOR]

In recent years, there has been considerable interest in the potential of quinones as a promising category of electroactive species for use in aqueous organic redox flow batteries. These materials offer tunable properties and the ability to function as both positive and negative electrolytes, making them highly versatile and suitable for a range of applications. Ongoing research has focused on improving the stability, solubility, and performance of quinones, with a particular emphasis on the creation of stable negolytes. The pairing of these advancements with alternate chemistries has created new prospects for commercial applications. However, challenges persist regarding the stability of quinones in high‐potential electrolytes and the limited number of viable quinones available. Despite these obstacles, significant strides have been made, and the potential for quinones to revolutionize energy storage technology is vast. This review article provides a comprehensive overview of recent progress in this area, with a specific focus on redox potential, solubility, and stability, and offers valuable insights into the future of quinone‐based aqueous organic redox flow batteries. [ABSTRACT FROM AUTHOR]

The objective of this work is to carry out a bibliographic review, leading to the evaluation and comparison of alternatives for internal components of a vanadium redox flow battery, which can improve efficient and economically viable energy storage. This review leads to the design of a theoretical BRFV on a laboratory scale (100 Wh and 12 V.), handling the following parameters: electrolyte 3.0 M in V2O5 and 6.0 M HCl and 3.0 M H2SO4, felt electrodes of graphite treated with polyacrylonitrile and NafionTM 117 membrane. In parallel, it is suggested to use 9 cells, 2 liters of electrolyte, flow rate of 373 mL/min and charging time of 13.69 hours. Finally, an Excel file is generated with the operating parameters for the proposed battery. [ABSTRACT FROM AUTHOR]

All vanadium flow batteries (VFBs) are considered one of the most promising large‐scale energy storage technology, but restricts by the high manufacturing cost of V3.5+ electrolytes using the current electrolysis method. Here, a bifunctional liquid fuel cell is designed and proposed to produce V3.5+ electrolytes and generate power energy by using formic acid as fuels and V4+ as oxidants. Compared with the traditional electrolysis method, this method not only does not consume additional electric energy, but also can output electric energy. Therefore, the process cost of producing V3.5+ electrolytes is reduced by 16.3%. This fuel cell has a maximum power of 0.276 mW cm−2 at an operating current of 1.75 mA cm−2. Ultraviolet–visible spectrum and potentiometric titration identify the oxidation state of prepared vanadium electrolytes is 3.48 ± 0.06, close to the ideal 3.5. VFBs with prepared V3.5+ electrolytes deliver similar energy conversion efficiency and superior capacity retention to that with commercial V3.5+ electrolytes. This work proposes a simple and practical strategy to prepare V3.5+ electrolytes. [ABSTRACT FROM AUTHOR]

High energy density and cost‐effective zinc‐iodide flow battery (ZIFB) offers great promise for future grid‐scale energy storage. However, its practical performance is hindered by poor cyclability, because of irreversible zinc plating/stripping, slow kinetics of redox reactions, and solid I2 precipitation. Herein, we report NaCl‐supported electrolyte chemistry to address these issues simultaneously. The formation of soluble chloride anions by coordination interactions between Zn2+ and Cl−, are key factor for the improvement of Zn/Zn2+ redox reversibility. While formation of soluble I2Cl− complex stabilizes ZIFB from I2 precipitation, participation of Na+ and K+ as dominant migrating carriers passing through Nafion, restricts Zn2+ migration, thus, blocking electrolyte crossover. A ZIFB with this improved electrolyte stably cycled 100 times with excellent capacity retention and energy efficiency at 20 mA cm−2, while conventional ZIFB shows the trend of capacity fade from 10th cycle onwards. These encouraging results of NaCl‐added electrolyte chemistry enlighten great prospects for high‐performance ZIFB applications. [ABSTRACT FROM AUTHOR]

Semi-solid lithium-ion flow battery (SSLFB) is a promising candidate in the field of large-scale energy storage. However, as a key component of SSLFB, the slurry presents a great fire hazard due to the extremely flammable electrolyte content in the slurry as high as 70 wt%–95 wt%. To evaluate the fire risk of SSFLB, the combustion experiments of electrolyte and slurry were conducted using cone calorimeter, and the critical fire parameters such as heat release rate (HRR), mass loss rate (MLR), and gas production were analyzed. This study firstly compared the combustion characteristics of electrolytes with the addition of different lithium salts (LiPF6 and LiTFSI). The results showed that the peak HRR (pHRR) and peak MLR (pMLR) of LiTFSI-based electrolyte were reduced by 30.3% and 33.2%, respectively, compared with LiPF6-based electrolyte. Also, LiTFSI-based electrolyte possessed a relatively lower toxic hazard. Overall, LiTFSI could reduce the fire hazard of the electrolyte compared with LiPF6. Then, the combustion behaviour of slurries containing different electrode materials (Li4Ti5O12, LiNi0.8Co0.1Mn0.1O2, LiFePO4, and Graphite) was investigated. It was observed that the splashing occurred in the early stage combustion of slurries. The splashing of S-LTO and S-LFP was relatively violent, while only sporadic splashing occurred for S-NCM and S-graphite. Based on the pHRR and pMLR test results, the order of fire risk of the four slurries is determined as S-LTO > S-LFP > S-NCM > S-Graphite. The pHRR and pMLR of slurries other than S-LTO are lower than that of electrolyte, thus their fire risk is lower than electrolyte. The results of this study can provide a reference for the fire hazard evaluation and safety improvement of the SSFLB system. [ABSTRACT FROM AUTHOR]

This study concerns the development of sustainable design strategies of aqueous electrolytes for redox flow batteries using redox‐active organic materials. A green spontaneous grafting reaction occurs between a redox‐active organic radical and an electrochemically activated structural modifier at room temperature through a simple mixing step. Then, a physical mixing method is used to formulate a structured aqueous electrolyte and enables aqueous solubilization of the organic solute from below 0.5 to 1.5 m beyond the conventional dissolution limit. The as‐obtained concentrated mixture can be readily used as catholyte for a redox flow battery. A record high discharge cell voltage (1.6 V onset output voltage) in aqueous non‐hybrid flow cell is attained by using the studied electrolytes. [ABSTRACT FROM AUTHOR]

Ferrocyanide, such as K4[Fe(CN)6], is one of the most popular cathode electrolyte (catholyte) materials in redox flow batteries. However, its chemical stability in alkaline redox flow batteries is debated. Mechanistic understandings at the molecular level are necessary to elucidate the cycling stability of K4[Fe(CN)6] and its oxidized state (K3[Fe(CN)6]) based electrolytes and guide their proper use in flow batteries for energy storage. Herein, a suite of battery tests and spectroscopic studies are presented to understand the chemical stability of K4[Fe(CN)6] and its charged state, K3[Fe(CN)6], at a variety of conditions. In a strong alkaline solution (pH 14), it is found that the balanced K4[Fe(CN)6]/K3[Fe(CN)6] half‐cell experiences a fast capacity decay under dark conditions. The studies reveal that the chemical reduction of K3[Fe(CN)6] by a graphite electrode leads to the charge imbalance in the half‐cell cycling and is the major cause of the observed capacity decay. In addition, at pH 14, K3[Fe(CN)6] undergoes a slow CN‐/OH‐ exchange reaction. The dissociated CN‐ ligand can chemically reduce K3[Fe(CN)6] to K4[Fe(CN)6] and it is converted to cyanate (OCN‐) and further, decomposes into CO32‐ and NH3. Ultimately, the irreversible chemical conversion of CN‐ to OCN‐ leads to the irreversible decomposition of K4/K3[Fe(CN)6] at pH 14. [ABSTRACT FROM AUTHOR]

Biphasic self-stratified batteries (BSBs) provide a new direction in battery philosophy for large-scale energy storage, which successfully reduces the cost and simplifies the architecture of redox flow batteries. However, current aqueous BSBs have intrinsic limits on the selection range of electrode materials and energy density due to the narrow electrochemical window of water. Thus, herein, we develop nonaqueous BSBs based on Li-S chemistry, which deliver an almost quadruple increase in energy density of 88.5 Wh L−1 as compared with the existing aqueous BSBs systems. In situ spectral characterization and molecular dynamics simulations jointly elucidate that while ensuring the mass transfer of Li+, the positive redox species are strictly confined to the bottom-phase electrolyte. This proof-of-concept of Li-S BSBs pushes the energy densities of BSBs and provides an idea to realize massive-scale energy storage with large capacitance. The use of energy-dense materials is inherently limited in biphasic self-stratified batteries due to the aqueous electrolyte environment. Here, the authors extended the concept of biphasic self-stratified batteries to non-aqueous systems, resulting in increased energy density and output voltage. [ABSTRACT FROM AUTHOR]

A comparative study of electrochemical performance and hydrodynamic effects on a single cell for all vanadium redox flow batteries has been investigated in the present study. Electrochemical performance of a cell has been conducted with an active area of 414 cm2 fitted with serpentine, interdigitated, and enhanced cross‐flow split serpentine (ECFSS) flow fields. The effects of electrolyte circulation rates on the electrochemical performance have been investigated for each flow field, and stable energy efficiency was achieved in 20 charge/discharge cycles for all three flow fields. Higher coulombic, voltage, and energy efficiencies in the serpentine flow field were achieved to be 96%, 82%, and 79%, respectively. The maximum peak power density values for the serpentine, ECFSS, and interdigitated flow field were found to be 158.2, 152.5, and 136.8 mW·cm−2, respectively, at a flow rate of 345 mL·min−1. Furthermore, detailed flow analysis was simulated by using computational fluid dynamics, and these studies postulated the reasons for the higher performance of the serpentine flow field compared to other flow fields. [ABSTRACT FROM AUTHOR]

There is a strong interest in finding highly soluble redox compounds to improve the energy density of redox flow batteries (RFBs). However, the performance of electrolytes is often negatively influenced by high solute concentration. Herein, we designed a high‐potential (0.5 V vs. Ag/Ag+) catholyte for RFBs, where the charged and discharged species are both gaseous nitrogen oxides (NOx). These species can be liberated from the liquid electrolyte and stored in a separate gas container, allowing scale‐up of storage capacity without increasing the concentration and volume of the electrolyte. The oxidation of NO in the presence of NO3− affords N2O3, and the reduction of N2O3 regenerates NO and NO3−, together affording the electrochemical reaction: NO3−+3 NO⇌2 N2O3+e− with a low mass/charge ratio of 152 grams per mole of stored electron. A proof‐of‐concept NOx symmetric H‐cell shows 200 stable cycles over 400 hours with >97 % Coulombic efficiency and negligible capacity decay. [ABSTRACT FROM AUTHOR]

Redox flow battery (RFB) electrolyte degradation is a common failure mechanism in RFBs. We report an RFB using genetically engineered, phenazine-producing Escherichia coli to serve as an anolyte regeneration system capable of repairing the degraded/decomposed redox-active phenazines. This work represents a new strategy for improving the stability of RFB systems because, under the influence of genetically engineered microbes, the anolyte species does not display degradation after battery cycling. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) have gained increasing attention for large‐scale storage due to the advantages of decoupled energy and power, safe and sustainable chemistry, and tunability of the redox‐active species. Here, we report the development of a neutral‐pH AORFB assembled with a highly water‐soluble ferrocene 1,1‐disulfonic disodium salt (DS−Fc) and two viologen derivatives, 1,1'‐bis(3‐sulfonatopropyl)‐viologen (BSP−Vi) and Bis(3‐trimethylammonium)propyl viologen tetrachloride (BTMAP−Vi). Synthesized electrolytes showed excellent redox potential, good diffusion coefficient, and a good transfer rate constant. In particular, BSP−Vi has a more negative redox potential (‐0.4 V) than BTMAP−Vi (−0.3 V) and faster kinetics; therefore, it was selected to be assembled in an AORFB as anolyte, coupled with DS−Fc as catholyte.The resulting AORFB based on BTMAP−Vi/DS−Fc and BSP−Vi/DS−Fc redox couple had a high cell voltage (1.2 V and 1.3 V, respectively) and theoretical energy density (13 WhL−1 and 14 WhL−1 respectively) and was able to sustain 70 charge‐discharge cycles with energy efficiency as high as 97 %. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) represent a promising approach to enabling the widespread integration of intermittent renewable energy. Rapid developments in RFB materials and electrolyte chemistries are needed to meet the cost and performance targets. In this review, special emphasis is given to the recent advances how electrolyte design could circumvent the main thermodynamic restrictions of aqueous electrolytes. The recent success of aqueous electrolyte chemistries has been demonstrated by extending the electrochemical stability window of water beyond the thermodynamic limit, the operating temperature window beyond the thermodynamic freezing temperature of water and crystallization of redox‐active materials, and the aqueous solubility beyond the thermodynamic solubility limit. They would open new avenues towards enhanced energy storage and all‐climate adaptability. Depending on the constituent, concentration and condition of electrolytes, the performance gain has been correlated to the specific solvation environment, interactions among species and ion association at a molecular level. [ABSTRACT FROM AUTHOR]

The poor operational stability of electrolytes is a persistent impediment in building redox flow battery technology; choosing suitable stability additives is usually the research direction to solve this problem. The effects of five phosphate containing additives (including 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), hexamethylene diamine tetramethylene phosphonic acid (HDTMPA), amino trimethylene phosphonic acid (ATMPA), sodium ethylenediamine tetramethylene phosphonate (EDTMPS), and diethyl triamine pentamethylene phosphonic acid (DTPMP)) on the thermal stability and electrochemical performance of the positive electrolyte of vanadium redox flow battery were investigated. With 0.5 wt% addition, most of the selected additives were able to improve the thermal stability of the electrolyte. HEDP and HDTMPA extended the stability time of the pentavalent vanadium electrolyte at 50 °C from 5 days (blank sample) to 30 days and 15 days, respectively. The electrochemical performance of the electrolyte was further investigated by cyclic voltammetry, steady state polarization, and electrochemical impedance spectroscopy tests. It was found that most of the additives enhanced the electrochemical activity of the positive electrolyte, and the diffusion coefficients, exchange current densities, and reaction rate constants of V(IV) species became larger with the addition of these additives. It is verified that the thermal stability and electrochemical stability of the electrolyte are significantly improved by the combination of ATMPA + HEDP or ATMPA + HDTMPA. This study provides a new approach to improve the stability of the positive electrolyte for vanadium redox flow battery. [ABSTRACT FROM AUTHOR]

In this study, the crossover of the electroactive species Zn(II), Ce(III), Ce(IV), and H+ across a Nafion 117 membrane was measured experimentally during the operation of a bench-scale hybrid Zn–Ce redox flow battery. For the conditions considered in this study, as much as 36% of the initial Zn(II) ions transferred from the negative to the positive electrolyte and 42.5% of the H+ in the positive electrolyte crossed over to the negative electrolyte after 30 charge–discharge cycles. Both of these phenomena contributed to the steady fade in battery performance over the course of operation. Based on these findings, additional experiments were conducted in which different amounts of Zn(II) were intentionally added to the positive electrolytes. This action was shown to reduce the crossover of Zn(II) from the negative side to the positive side, improve both the battery coulombic and voltage efficiencies and reduce the decay of battery performance over the 30 charge–discharge cycles. The average energy efficiency over 30 charge–discharge cycles was increased by 19.7% by adding 0.6 mol L−1 Zn(II) to 4 mol L−1 MSA positive supporting electrolyte and 6.4% by adding 0.4 mol L−1 Zn(II) to 2 mol L−1 MSA–0.5 mol L−1 H2SO4.Graphical abstract: In this study, the crossover of the electroactive species Zn(II), Ce(III), Ce(IV), and H+ across a Nafion 117 membrane was measured experimentally during the operation of a bench-scale hybrid Zn–Ce redox flow battery. For the conditions considered in this study, as much as 36% of the initial Zn(II) ions transferred from the negative to the positive electrolyte and 42.5% of the H+ in the positive electrolyte crossed over to the negative electrolyte after 30 charge–discharge cycles. Both of these phenomena contributed to the steady fade in battery performance over the course of operation. Based on these findings, additional experiments were conducted in which different amounts of Zn(II) were intentionally added to the positive electrolytes. This action was shown to reduce the crossover of Zn(II) from the negative side to the positive side, improve both the battery coulombic and voltage efficiencies and reduce the decay of battery performance over the 30 charge–discharge cycles. The average energy efficiency over 30 charge–discharge cycles was increased by 19.7% by adding 0.6 mol L−1 Zn(II) to 4 mol L−1 MSA positive supporting electrolyte and 6.4% by adding 0.4 mol L−1 Zn(II) to 2 mol L−1 MSA–0.5 mol L−1 H2SO4. [ABSTRACT FROM AUTHOR]

Developing large-scale storage of intermittent renewable energy to meet growing energy demands is a pressing current need. Multiphase single flow batteries are a promising solution for such grid-scale energy storage, demonstrating an affordable redox flow battery design that reduces both cell and balance of plant costs. However, their major limitation is the considerable variance in electrolyte conductivity under different battery flow conditions and electrolyte properties, with no current predictive model to comprehensively understand and optimize it. Here, we develop an analytical model for such emulsion electrolytes with a continuous aqueous-based phase and dispersed reactant-rich phase, which enables electrolyte resistance prediction. We show that a key mechanism affecting electrolyte conductivity is the formation of a sedimented layer along the flow channel, revealing the critical effect of non-aqueous phase sedimentation. Experimental validation using a zinc-bromine single flow battery demonstrates excellent agreement with theoretical results during both transient and steady operations, allowing extraction of challenging-to-measure parameters, such as the in-situ size of dispersed phase droplets. This foundational model is essential in minimizing power losses, improving electrolyte and cell designs, and holds broad applicability across diverse chemistries for single-flow batteries. [Display omitted] • Electrolyte resistance limits the performance of single flow batteries. • Sedimentation greatly affects electrolyte resistance, reducing power output. • A model is provided for the electrolyte separation in a multiphase flow battery. • Adjusting conditions reduces sedimentation thickness and electrolyte resistance. • Experiments validate theory for both transient and steady operations. [ABSTRACT FROM AUTHOR]

High‐concentration operation of redox flow batteries (RFBs) is essential for increasing their energy‐storage capacity, but non‐acidic electrolytes struggle to achieve the high concentrations of metal ions dissolved in acid, limiting the development of energy‐dense neutral pH electrolytes. We report neutral pH RFB operation of chromium 1,3‐propylenediaminetetraacetate (CrPDTA) at concentrations of 1.2 M at room temperature and 1.6 M at 40 °C, demonstrating 60% higher negolyte capacity, up to 42.9 Ah L−1, than previously reported for non‐additive‐utilizing solutions of this promising material. With extended full cell cycling, we demonstrate the importance of buffer selection and pH when using the Fumasep E‐620(K) membrane. Finally, we expand the pH operation range of CrPDTA to pH 7, which when cycled at 100 mA cm−2 against a ferrocyanide posolyte demonstrated excellent coulombic efficiencies >99.7% and energy efficiencies >87%, while operating at almost 700 mV more negative than the thermodynamic hydrogen evolution window. [ABSTRACT FROM AUTHOR]

In this article, an electrolyte flow rate control approach is developed for an all-vanadium redox flow battery (VRB) system based on the linear parameter varying (LPV) framework. The electrolyte flow rate is regulated to provide a trade-off between stack voltage efficiency and pumping energy losses, so as to achieve an energy efficient battery operation. The nonlinear process model is embedded in a linear parameter varying state–space description and a set of state feedback controllers are designed to handle fluctuations in current during both charging and discharging. Simulation studies have been conducted under different operating conditions to demonstrate the performance of the proposed approach. This control approach was further implemented on a laboratory scale VRB system. • A novel electrolyte flow rate control approach is developed for vanadium redox batteries (VRB). • The nonlinear battery system model is embedded into a Linear Parameter Varying (LPV) description. • A computationally efficient controller is designed to optimise the operational efficiency. • The proposed control design is successfully implemented on a lab-scale VRB system. [ABSTRACT FROM AUTHOR]

Phenazine derivatives represent an important class of emerging redox-active organic electrolyte materials in aqueous flow batteries for sustainable energy storage applications. But when serving as the anolyte or catholyte, the working voltage is relatively low in comparison to their inorganic counterparts. Moreover, most of the reported phenazine-based electrolyte materials have insufficient water-solubility, which plague their application to aqueous flow battery systems. Here, we investigated the redox potentials and solvation free energies for phenazine derivatives containing various electron-donating or electron-withdrawing groups at different substitution positions by applying density functional theory calculations combined with a thermodynamic Born–Haber cycle. The calculation results are validated with experimental data. On the basis of our calculations, we identified several promising anolyte and catholyte candidates for alkaline aqueous organic flow batteries. The information obtained from this study can provide useful clues for designing high-performance redox-active electrolyte materials to promote the practical application of aqueous organic flow batteries. [ABSTRACT FROM AUTHOR]

Aqueous‐organic redox flow batteries (RFBs) have gained considerable interest in recent years, given their potential for an economically viable energy storage at large scale. This, however, strongly depends on both the robustness of the underlying electrolyte chemistry against molecular decomposition reactions as well as the device's operation. With regard to this, the presented study focuses on the use of in situ IR spectroscopy in combination with a multivariate curve resolution approach to gain insight into both the molecular structures of the active materials present within the electrolyte as well as crucial electrolyte state parameters, represented by the electrolyte's state of charge (SOC) and state of health (SOH). To demonstrate the general applicability of the approach, methyl viologen (MV) and bis(3‐trimethylammonium)propyl viologen (BTMAPV) are chosen, as viologens are frequently used as negolytes in aqueous‐organic RFBs. The study's findings highlight the impact of in situ spectroscopy and spectral deconvolution tools on the precision of the obtainable SOC and SOH values. Furthermore, the study indicates the occurrence of multiple viologen dimers, which possibly influence the electrolyte lifetime and charging characteristics. [ABSTRACT FROM AUTHOR]

Summary: Aiming to improve electrolyte distribution in flow batteries at lower pumping cost, a sinusoidal wave‐type serpentine (SWS) flow pattern was designed and fabricated on graphite plate. The performance of the channel with graphite felt electrode, employing in vanadium redox flow battery, was analyzed by polarization curves and electrochemical impedance spectroscopy (EIS) measurements, obtained by varying flow rates of electrolyte. The pressure drop of the half‐cell assembly was measured by varying water flow rates. Results show that increasing flow rate reduces potential loss. Mass transfer overpotential is low even at 30 mL/min, and at 120 mL/min, the overpotential is negligible even at very high current density of 500 mA/cm2. The iR‐free power density monotonically increases with current density. The EIS measurements show that charge transfer resistance decreases with increasing flow rates and is lower than that for conventional serpentine flow pattern. The pressure drop across the cell is also lower. These results clearly indicate that better electrolyte distribution is achieved using SWS channels at lower pumping cost. [ABSTRACT FROM AUTHOR]

Summary: In this study, long‐term stability of all‐iron aqueous redox flow batteries (all‐iron ARFBs) using iron‐3‐[bis(2‐hydroxyethyl)amino]‐2‐hydroxypropanesulfonic acid complex (Fe[DIPSO]) and ferrocyanide as redox couple is evaluated. In this system, there are two problems to address. First, stability issue of catholyte including ferrocyanide that is worsened under alkali electrolyte and second, unbalanced pH of catholyte and anolyte occurring by the crossover of water molecules during cycling of ARFB, and these degrade the performance of ARFB steadily. To enhance the stability of the catholyte, the stability of ferrocyanide is investigated in various electrolyte conditions. Furthermore, to alleviate the unbalanced pH effects of both electrolytes (catholyte and anolyte), their electrolyte conditions are set differently to quantify the crossover of water molecules and optimize electrolytes. When electrolyte condition applied in both electrolytes is not appropriate, the capacity decay rate of ARFB is 50% for cycling of 150 hours. In contrast, when pH and concentration conditions of catholyte and anolyte are optimally designed, the capacity of ARFB is well preserved for the entire cycling. More specifically, in catholyte, optimal pH and concentration are 12 and 0.25 M, while they are 14 and 0.5 M in anolyte. Eventually, using the optimized electrolyte condition, its performances are well maintained for 23 days, guaranteeing its long‐term stability. These prove that the stability of ARFB using Fe(DIPSO) and ferrocyanide can be enhanced by maneuvering the pH and concentration conditions of its electrolytes. [ABSTRACT FROM AUTHOR]

Metal‐air batteries are emerging as a preferable choice among different battery technologies explored as an alternative to existing lithium‐ion batteries. These batteries can outperform the current lithium batteries in terms of specific energy, energy cost, and ability for alternative metal choice. Metal‐air flow batteries are being investigated in place of conventional metal‐air batteries to reduce metal passivation and dendrite formation which are considered to be the major limitations in conventional metal‐air batteries. For emerging battery chemistry, a parametric study is necessary to identify the batteries performance variations to identify the optimal technology design. As far as flow batteries are concerned, electrolyte parameters play a notable role in the performance. In this study, we investigate the influence of electrolyte parameters namely, electrolyte flow rate, electrolyte volume (volume stored in the container), and electrolyte concentration to understand the effect of each parameter on the nominal voltage of a planar zinc‐air flow battery utilising a polymer gel electrolyte separator. We followed a 2n factorial design approach to determine the percentage contribution of each parameter. From this experimental study, we found the percentage contribution of electrolyte concentration to be most significant and the percentage contribution of electrolyte flow rate as reasonably significant. The percentage contribution of electrolyte volume appeared least significant among the chosen parameters. This study will help researchers to perceive the influence of electrolyte parameters on the performance of zinc‐air flow batteries. [ABSTRACT FROM AUTHOR]

This perspective provides a brief overview of the recent work on electrolytes with hydrogen (H)-bonding network, specifically the deep eutectic solvents (DESs), and outlines important factors to consider when adapting these electrolytes in redox flow batteries (RFBs). The redox behavior, solubility, and stability of several redox molecules of relevance to RFBs in DESs are presented, including some of our work within the Breakthrough Electrolytes for Energy Storage (BEES)--an Energy Frontier Research Center of the United States Department of Energy. Particularly, the challenges and opportunities for further development of DESs for energy storage are discussed. [ABSTRACT FROM AUTHOR]

Due to decoupled energy and power, the aqueous organic redox flow battery (AORFB) represents a promising energy storage technology that stores energy in redox‐active organic compounds dissolved in aqueous electrolytes. The organic compounds are composed of earth‐abundant elements and are therefore potentially cost‐effective. Their structures are diverse and highly tunable, rendering it possible to regulate the redox potential, water solubility, and cycling lifespan. Therefore, the adoption of redox‐active organics greatly expands the space for enabling exceeding performance merits compared to their inorganic counterparts. Herein, this work reviews the molecular design and engineering scheme of organic electrolytes, focusing exclusively on pH‐neutral AORFB that uses non‐corrosive and nonflammable pH‐neutral aqueous electrolytes with inexpensive inorganic salts as supporting electrolytes and features high safety, low corrosivity, low cost, and environmental benignity. Some comments on the present challenges and prospects of this ascendant domain are also presented. [ABSTRACT FROM AUTHOR]

The iron‐chromium redox flow battery (ICRFB) is considered the first true RFB and utilizes low‐cost, abundant iron and chromium chlorides as redox‐active materials, making it one of the most cost‐effective energy storage systems. ICRFBs were pioneered and studied extensively by NASA and Mitsui in Japan in the 1970–1980s, and extensive studies on ICRFBs have been carried out over the past few decades. In addition, ICRFB is considered to be one of the most promising directions for cost‐effective and large‐scale energy storage applications, as its cost can theoretically be lower than that of zinc‐bromine and all‐vanadium RFBs, giving it the potential for large‐scale promotion. With the resolution of problems such as hydrogen evolution and electrolyte intermixing, the ICRFB technology is moving out of the laboratory and striving for greater power and more stable industrialization requirements. This Review summarizes the history, development, and research status of key components (carbon‐based electrode, electrolyte, and membranes) in the ICRFB system, aiming to give a brief guide to researchers who are involved in the related subject. [ABSTRACT FROM AUTHOR]

• Modelling of a RFB that uses two immiscible electrolytes. • Fluid-fluid interface is tracked using the Arbitrary Lagrangian-Eulerian method. • Maximum power densities between 1-1.75 mW/cm 2 in discharge and 1.5-4 mW/cm 2 in charge. • The limiting current density increases with the flow as j lim ∼ R e 0.3 . • The battery performance is limited by the availability of reactant near the electrodes. We present a mathematical model to study the steady-state performance of a membrane-less reversible redox flow battery formed by two immiscible electrolytes that spontaneously form a liquid-liquid system separated by a well defined interface. The model assumes a two-dimensional battery with two coflowing electrolytes and flat electrodes at the channel walls. In this configuration, the analysis of the far downstream solution indicates that the interface remains stable in all the parameter range covered by this study. To simplify the description of the problem, we use the dilute solution theory to decouple the calculation of the velocity and species concentration fields. Once the velocity field is known, we obtain the distribution of the mobile ionic species along with the current and the electric potential field of the flowing electrolyte solution. The numerical integration of the problem provides the variation of the battery current density I app with the State of Charge (SoC) for different applied cell voltages V cell. A detailed analysis of the concentration density plots indicates that the normal operation of the battery is interrupted when reactant depletion is achieved near the negative electrode both during charge and discharge. The effect of the electrolyte flow on the performance of the system is studied by varying the Reynolds, R e , and Péclet, P e , numbers. As expected, the flow velocity only affects the polarization curve in the concentration polarization region, when V cell is well below the equilibrium potential, resulting in limiting current densities that grow with R e as j lim ∼ R e 0.3 . In addition, both the single-pass conversion efficiency ψ and the product ψ j lim decrease with R e. Concerning the later, the decay rate with R e exhibits a power law with an exponent that almost doubles previous theoretical predictions obtained by imposing a prescribed velocity profile for the electrolyte in a membrane-less laminar flow battery with a liquid oxidant and gaseous fuel. The present work constitutes the first modelling attempt that simultaneously solves the fluid dynamical system formed by the two immiscible electrolytes and the electrochemical problem that determines the response of the membrane-less battery. The proposed model could be used as a valuable tool to optimize future flow battery designs based on immiscible electrolytes. [ABSTRACT FROM AUTHOR]

Redox molecules (RMs) as electron carriers have been widely used in electrochemical energy‐storage devices (ESDs), such as lithium redox flow batteries and lithium‐O2 batteries. Unfortunately, migration of RMs to the lithium (Li) anode leads to side reactions, resulting in reduced coulombic efficiency and early cell death. Our proof‐of‐concept study utilizes a biphasic organic electrolyte to resolve this issue, in which nonafluoro‐1,1,2,2‐tetrahydrohexyl‐trimethoxysilane (NFTOS) and ether (or sulfone) with lithium bis(trifluoromethane)sulfonimide (LiTFSI) can be separated to form the immiscible anolyte and catholyte. RMs are extracted to the catholyte due to the enormous solubility coefficients in the biphasic electrolytes with high and low polarity, resulting in inhibition of the shuttle effect. When coupled with a lithium anode, the Li‐Li symmetric, Li redox flow and Li‐O2 batteries can achieve considerably prolonged cycle life with biphasic electrolytes. This concept provides a promising strategy to suppress the shuttle effect of RMs in ESDs. [ABSTRACT FROM AUTHOR]

Electrolyte is an important component in redox flow batteries (RFBs) that determines the current capability, potential window, and safety, but both aqueous and nonaqueous electrolytes have their intrinsic limits. Here, we develop the proof‐of‐concept hybrid electrolyte chemistry to enable the design of safe and wide‐temperature RFBs. In addition to the non‐flammable characteristics, the hybrid electrolyte also inherits the high electrochemical stability and wide operational temperature range. It can show a potential window of 2.5 V and maintain high ion conductivities at low temperatures. It also enables LiI to achieve high Coulombic efficiencies of >99.9 %, showing long cycling stability over 800 cycles. Moreover, it enables the successful operation of Zn/LiI RFBs at −20 °C for 150 cycles with nearly no capacity loss. This study highlights the great potential of hybrid electrolyte chemistry for the approach of safe and high‐performance large‐scale energy storage systems in wide temperature ranges. [ABSTRACT FROM AUTHOR]

Non-aqueous redox flow batteries (RFBs) are a promising technology to meet growing demand for grid-scale energy storage. Membrane separators, designed specifically for use with organic solvents, are necessary to advance non-aqueous RFBs. Herein, we report the development of a series of poly(phenylene oxide) (PPO) membranes functionalized with poly(ethylene glycol) (PEG) side chains to investigate the influence of PEG side chain length and degree of PEGylation on membrane transport properties. Increasing the degree of PEGylation generally led to increases in electrolyte uptake, hydroxy TEMPO permeability, and ionic conductivity likely caused by an increase in overall PEG content, as opposed to specific interactions caused by changing the degree of PEGylation. For membranes with similar PEG content, increasing the length of the PEG side chain resulted in decreases in electrolyte uptake, permeability, and conductivity possibly due to differences in the solvation behavior of the PEG chains with different lengths. [Display omitted] • Increasing PEG content led to increases in permeability and conductivity. • Hydroxy TEMPO permeability was consistent with free volume theory. • Longer chain length PEG materials retained more selectivity. [ABSTRACT FROM AUTHOR]

Steady-state amperometry with microelectrodes based on potential step experiments arguably yields the best-in-class accuracy for the state-of-charge monitoring in redox flow battery (RFB) electrolytes. In fact, only a fraction of the current response obtained during the potential step was used in experiments for the state-of-charge (SOC) and state-of-health (SOH) assessment so far. This study explores the analysis of the transient chronoamperometric signal. It is demonstrated that the physicochemical information contained in the transient current signal enables both the replacement of microelectrodes with macroelectrodes for ex situ as well as in operando SOC measurements and for an ex situ concentration- and (theoretically) temperature-independent assessment of the electrolyte SOH with microelectrodes. The results demonstrate that chronoamperometry enables accurate, fast, and low-cost measurements of an electrolyte's SOC. Unlike previous macroelectrode-based approaches, which required a limitation of electrolyte flow and reactant transport to the working electrode surface, the rapid chronoamperometric potential steps presented in this study enable an electrode placement directly in the free-flowing electrolyte. Additionally, the developed ex situ microelectrode-based chronoamperometric SOH measurement method allows for the investigation of the electrolyte lifetime in isolated vessels. Consequently, the presented SOH assessment technique holds a promise for establishing high-throughput, ex situ electrolyte stability measurements. • Steady-state amperometry for SOC and SOH only uses part of the current signal. • Chronoamperometry enables fast (100 ms) SOC measurements at macroelectrodes (>1 mm). • Macroelectrode-based measurements within an error margin of ± 2.2% SOC (0.9% RMSD). • Robust, macroelectrode-based flow-through SOC sensors become feasible. • Temperature- & concentration-independent SOH measurement with ± 4.5% SOH (3.1% RMSD). [ABSTRACT FROM AUTHOR]

Iron‑chromium flow battery (ICFB) is the one of the most promising flow batteries due to its low cost. However, the serious capacity loss of ICFBs limit its further development. Herein, we analyze the capacity loss mechanism of ICFBs. The capacity loss is due to inactive Cr(H 2 O) 6 3+ ions result in the mismatched content of active ions in catholyte and anolyte. Thus, a simple method is proposed to raise the content of active Cr3+ species (Cr(H 2 O) 5 Cl2+ and Cr(H 2 O) 4 Cl 2 +), achieving the match of Fe2+/Fe3+ and Cr3+/Cr2+ redox couples and reducing the capacity loss of ICFBs. Meanwhile, the increasing content of active Cr3+ ions was proved by the quantitative analysis of the UV–vis absorption spectra. As a result, the ICFB with the improved electrolyte of 1 M FeCl 2 + 1.3 M CrCl 3 + 3 M HCl (E-1.3Cr) exhibits an energy efficiency (EE) of 84.51%, which is much higher than the original one of 1 M FeCl 2 + 1 M CrCl 3 + 3 M HCl (E-1Cr) (82.32%) at a current density of 80 mA cm−2. Most importantly, the ICFB with E-1.3Cr (7.44 mAh/cycle) shows the much slower capacity decay rate than that with E-1Cr (9.16 mAh/cycle). This work provides an effective way to accelerate engineering applications of ICFBs. [Display omitted] • The capacity loss mechanism of ICFBs is discussed in detail. • The ICFBs achieved the low capacity loss by the matched active materials. • The amount of Cr3+ species was quantitatively analyzed by UV–vis absorption spectra. • The work proposed a simple and effective method to improve the capacity of ICFB. [ABSTRACT FROM AUTHOR]

A H 2 /Br 2 flow battery operating with redox-mediated bromate anions (BrO 3 −) has shown several beneficial features induced by the bromate-bromide chemical reaction, particularly in terms of cell performance and safety issues. In this study, a kinetic model of the chemical reaction is developed by thoroughly accounting for detailed reaction steps and by incorporating a three-dimensional, transient H 2 /Br 2 flow battery model. First, the H 2 /Br 2 flow battery model with BrO 3 − is validated against experimental data where cell voltage recovery is observed until the BrO 3 − is completely consumed. The model successfully reproduces the measured voltage evolution data during discharge, thus accurately capturing the transient behavior of an H 2 /Br 2 cell influenced by the bromate-bromide chemical reaction. In addition, H 2 /Br 2 cells are simulated with and without BrO 3 − and model predictions reveal that the reversible voltage, ohmic overpotential in the membrane, and activation overpotential of the Br 2 reduction reaction are all improved due to the use of BrO 3 − in the catholyte. The present model for the H 2 - Br 2 - BrO 3 − system enables the quantitative analysis of the beneficial effects for using redox-mediated bromate anion and thus becomes a useful tool for designing and optimization of H 2 /Br 2 cell with BrO 3 −. • Hydrogen Bromate redox flow battery model was applied to a single cell geometry. • Simulation result showed good agreement with experimentally measured data. • Voltage recovery observed due to rigorous regeneration of Br 2 ensuing high efficiency. • Longer discharge time is achieved with using BrO 3 as electrolyte. • Membrane dehydration by Br− is reduced as Br− is controlled by the chemical reaction. [ABSTRACT FROM AUTHOR]

For the application in Fe/Fe-Redox-Flow-Batteries some important factors concerning the composition of the electrolyte and the influence of temperature on the properties of half-cell reactions were investigated. In contrast to previous investigations, the measurements were performed more realistically on deposited iron and by means of linear sweep voltammetry. Since the distinction between cathodic iron deposition and hydrogen generation is not possible by convention, with quantitative stripping analysis on a rotating disk electrode, partly a method was used to distinguish between these two reactions. The investigations were carried out at temperatures up to 80 °C, with addition of 10 mM of chlorides of Bi, Cu, In, Pb, Sn, Tl, Cd, Sb and Hg and different supporting salts of NH4+, Li+, K+. Na+, Cs+, Mg2+ and Al3+. [ABSTRACT FROM AUTHOR]