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Iron electroplating is great interest for multiple large-scale industrial and emerging energy applications, such as all-iron redox flow batteries. However, the process efficiency and material lifetime are limited by the poorly understood plating process and the presence of competitive reactions. In this work, we propose a methodology to deconvolute the nucleation parameters of iron plating via a suite of electrochemical techniques, spectroscopy, and analytical models, coupled with microscopic and crystallographic techniques. We perform a systematic analysis with iron-based electrolytes to deconvolute the simultaneous plating and hydrogen evolution reactions, and investigate an array of additives to tune electroplating descriptors. We find that all additives studied are able to regulate the plating process and that highly stable iron-complexes based on buffers, such as iron-borates or -citrates deliver greater overall electroplating performance. These additives show superior selectivity, with improvements in faradaic efficiencies from 60 % to ∼90 % due to the balanced effects of enhanced nucleation and side reaction suppression. Herewith, the aim of this study is to bridge the knowledge gap between the role of additives, kinetics and efficiency of the electrodeposition reaction, and their interplay defining the quality of the resulting plated layers. [ABSTRACT FROM AUTHOR]

• The work proposes an electrode with three-dimensional gradient pore structure. • The electrode has continuous gradient nanofiber size prepared by electrospun. • Energy efficiency of this electrode improves 74.2 % at 10 mA cm−2 compared to GF. To boost the performance of non-aqueous redox flow batteries (RFBs), it is important to synergistically improve the flow/mass transfer efficiencies and the uniformity of overpotential distribution into the porous electrode. In this work, electrostatic spinning technology is developed to propose a novel porous electrode with gradient pore distribution both in the in-plane and through-plane directions, and applied in deep eutectic solvent (DES) electrolyte-based iron-vanadium RFB. On the one hand, the new in-plane gradient design modifies the distribution of reactive species of electrode near the membrane side, resulting in the decreasing polarization loss. On the other hand, the increasing porosity of electrodes from the flow field side to the membrane side attains a trade-off between the charge transfer and the electrolyte flow resistances. According to the experimental results, compared to the graphite felt electrode, the energy efficiency of this RFB with three-dimensional gradient electrode improves by 74.2 % at a current density of 10 mA·cm−2. Moreover, the numerical simulation reveals the reactive transfer behaviors of three-dimensional gradient porous electrode. The results show that the proposed three-dimensional gradient design can enhance the uniformity of overpotential distribution and achieve the decrease of polarization resistance, thus improving the performance of DES electrolyte-based iron-vanadium RFB effectively. [ABSTRACT FROM AUTHOR]

Zinc-based flow batteries hold great promise for grid-scale energy storage. However, the formation of zinc dendrites challenges their lifespan and safety. In this work, we devise a bimetallic organic framework material on carbon cloth (CuZn@MOF-CC) using a one-step dip coating method to address the dendrite issue. The prepared CuZn@MOF-CC promotes the creation of zincophilic sites on the electrode surface, facilitating smooth and uniform zinc deposition. Electrochemical csharacterizations and density functional theory (DFT) calculations reveal much stronger electronic interactions between CuZn@MOF and the zinc atoms compared to the commonly used anode material through an obvious electron interaction between the Zn atom and the Cu sites linked in the CuZn@MOF chain. By mitigating the zinc dendrite problem, this approach effectively improves the coulombic efficiency, cycle life, and safety of zinc-based flow batteries. The battery incorporating the prepared material demonstrates impressive stability, exceeding 450 cycles without dendrites at a high current density of 320 mA cm−2, paving the way for efficient and reliable energy storage solutions and a sustainable future. CuZn@MOF interfacial layer can improve the nucleation uniformity of zinc, inhibit dendrite formation and form a flat and uniform zinc coating. [Display omitted] [ABSTRACT FROM AUTHOR]

• Water optimizes deep eutectic solvent electrolyte viscosity and conductivity. • 75 % water in deep eutectic electrolyte boosts peak current up to 9.5-fold. • Tuned deep eutectic electrolyte matches all-vanadium battery performance. • Water-tuning enables cost-effective, efficient iron-vanadium flow batteries. Deep eutectic solvents (DES) are being recognized as a highly promising electrolyte option for redox flow batteries. This study examines the impact of modifying the molar ratio of water to a DES consisting of urea and choline chloride on important measures of electrolyte performance, such as viscosity, cyclic voltammetry, and impedance spectroscopy. According to the findings, when the molar ratio of water to DES is 1:3, the viscosity of the electrolyte falls by 75 %, the peak current density increases by 5–9.5 times, the ionic conductivity increases by 5–10 times, and the ohmic resistance decreases by 60–90 %. This approach greatly enhances the conductivity and diffusion coefficient of the electrolyte, resulting in a novel, cost-effective, and highly efficient electrolyte for iron-vanadium redox flow battery applications. This study enhances the efficiency of DESs at the molecular level, establishing the foundation for future extensive implementations. [ABSTRACT FROM AUTHOR]

The exceptional advantages of vanadium redox flow batteries (VRFBs) have garnered significant attention, establishing them as the preferred choice for large-scale and long-term energy storage solutions. However, side reactions such as hydrogen evolution reaction (HER) lead to suboptimal performance of VRFB parameters, resulting in an overall decrease in VRFB performance. Therefore, it is imperative to investigate the mechanism by which HER affects VRFB performance and identify effective methods to mitigate its impact. In light of this, critical parameters such as charge-discharge curve, internal resistance, pressure-drop, pump power consumption, efficiency, and capacity retention rate are selected to evaluate the impact of HER on VRFB performance. Experimental results demonstrate that HER causes an increases in voltage during charging and a decrease during discharging stages leading to voltage loss; additionally, bubble generation significantly increases pressure drop and pump power consumption; furthermore, blockage of flow channels and electrodes due to bubbles leads to an increase in ohmic resistance. Finally, HER has a substantial impact on efficiency and capacity with an average energy efficiency of 2.92 %. After 60 cycles, the capacity retention rate decreased by nearly 7.69 %. [ABSTRACT FROM AUTHOR]

• The N and S heteroatom doping can adjust the O vacancy percentage of Ti-WO 3 and Nb-h-WO 3. • Doped transition metal oxides greatly enhance the actual electrochemical activity of graphite felt. • Doped transition metal oxides are advantageous in reducing the capacity decay of AORFBs. In this study, a graphite felt (GF) electrode was modified with N and S heteroatoms before being loaded with Ti-doped or Nb-doped WO 3 to produce Ti-WO 3 /GF-N-S and Nb-WO 3 /GF-N-S electrodes. X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy were used to analyze the crystal structure, morphology, and elemental state of the modified GF electrodes and investigate the impact of nonmetallic and metallic element doping. The influence of metal doping on the electronic distribution of the oxide was determined using first-principle calculations. The electrochemical properties of the modified GF were studied using cyclic voltammetry, electrochemical impedance spectroscopy, and single-cell tests. The results showed that Ti or Nb doping changed the energy band structure, bond length, and bond angle in the WO 3 crystal and formed O vacancies. The chemical properties of Ti and Nb resulted in different numbers of O vacancies for the two doped-WO 3.The doping of N and S heteroatoms on the carbon fibres altered the O vacancy concentration of WO 3. The synergism between the two doping methods accelerated electron transfer in the anthraquinone redox reaction. In addition, the N-containing functional groups provide lone-pair electrons, which accelerate electron transfer and anthraquinone adsorption. Therefore, compared to the pristine GF, the discharge capacities of the Ti-WO 3 /GF-N-S and Nb-h-WO 3 /GF-N-S graphite GF electrodes increased by 58 % and 41 %, respectively, and the Nb-h-WO 3 /GF-N-S graphite GF electrodes exhibited a good capacity retention rate. This study broadens the practical applications of aqueous anthraquinone redox flow batteries. [ABSTRACT FROM AUTHOR]

The efficient operation of vanadium redox flow batteries requires the half cell specific monitoring of the state of charge (SOC). Monitoring of the SOC is affected by temperature fluctuations, which need to be distinguished from the SOC signal. Several SOC monitoring approaches have been proposed in the literature and either been evaluated individually or compared to one other approach. The aim of this study is to contribute to SOC sensor development for VRFB by providing a first correlated assessment of several established and new SOC monitoring methods. We perform multi observable measurements of: half cell electrolyte potentials, electrolyte densities, electrolyte volumes, electrolyte viscosity related pressure drops, pH related potentials and UV/Vis absorbances. We determine SOC correlations of these observables in full charge/discharge cycles at constant temperature and derive temperature corrections in additional measurements over a range of 12 °C to 32 °C at 25 %, 50 % and 75 % SOC. We compare the SOC and temperature sensitivity as well as the accuracy of these monitoring methods at constant and varied temperature. We find that the established half cell electrolyte potentials and UV/Vis absorbances can estimate the SOC at constant temperature with measurement errors of 1.7% and 4%, respectively and with slightly higher errors at varying temperatures. The viscosity related pressure drop and electrolyte density are both suitable for SOC estimation if temperature corrections are included. The pH related potentials for the positive half cell and both half cell electrolyte volumes could be used for a rough SOC estimate, but need accurate temperature corrections and further long term stability evaluation. [ABSTRACT FROM AUTHOR]

• Carbon paper electrode shows poor mechanical stability, affecting the AORFB performance. • The mechanical robustness of carbon cloth electrode contributes to a reliable and durable AORFB performance. • Carbon cloth electrode showed high stability when used on quinone oxidation–reduction cycles. • Electrochemical active sites on the carbon cloth increase current densities and AORFB power density. • Carbon cloth electrode provides higher AORFB power density at high electrolyte flow rate. Aqueous organic redox flow batteries (AORFBs) are emerging as promising candidates for efficient energy storage in grid-scale applications. In this study, we present a comprehensive analysis comparing the performance of thermally treated carbon paper and chemically oxidized carbon cloth electrodes, which have been extensively examined for their potential suitability in AORFBs. Results indicate that carbon cloth electrodes exhibit a higher concentration of oxygenated functional groups, contributing to enhanced electrochemical kinetics and improved electrochemical stability. These attributes translate to an AORFB with 62 times higher power density when compared to paper-based electrodes used. Through repeated charge-discharge cycles utilizing the BQDS║AQS model as a representative AORFB prototype, superior performance in the carbon cloth electrodes is consistently observed. Our findings even suggest the potential for optimizing the flow rate of the electrolyte to further enhance performance. [ABSTRACT FROM AUTHOR]

• Ferrocene and viologen-based redox active polymers have been synthesized for aqueous all-polymer redox flow battery. • Catholyte-P have shown the dependence of electrochemical properties on molecular weight. • Correlation of HOMO energy of polymers with redox potential. • The RFB with size-exclusion membrane has a capacity retention of 99.93% per cycle at 10 mA cm−2. Aqueous all-polymer redox flow batteries (APRFBs) working with size exclusion membranes are safe, low-cost, scalable solutions for energy storage applications. However, their development is still limited owing to challenges in optimizing the redox potential and solution viscosity of the polymers to deliver optimal energy density. Herein, we demonstrate an APRFB composed of ferrocene and viologen-based polymers with tunable redox potential. The theoretical calculations and electrochemical measurements reveal the contribution of HOMO energy change and polyelectrolyte dynamics in the charge transfer process with an increase in the molecular weight of the Catholyte-P. The electrochemical experiments yielded reasonable activity of the Catholyte-P24 and Anolyte-P12 with an oxidation rate constant of 1.26 × 10−4 cm s−1 and a reduction rate constant of 5.38 × 10−3 cm s−1, respectively. The application of synthesized polymers with size exclusion membrane yielded a 0.85 V APRFB with energy efficiency up to 72%, capacity retention of 99.93% per cycle at 10 mA cm−2, demonstrated energy density of 2.6 W h L−1, and capacity retention of >82% over continuous 250 cycles. [Display omitted] [ABSTRACT FROM AUTHOR]

• The basic model of high C-rate is determined by the model configuration analysis. • Sampling frequency and time domain influences model parameters are analyzed. • A model parameter correction method based on the rule of different overpotentials is proposed. • A modified high C-rate model is innovatively established with current dependence and concentration modification. • The modified model is validated up to the 6C rate at different temperature. The current battery equivalent circuit models are mainly suitable for room temperature or low C-rate conditions. However, the polarization phenomenon is serious at high C-rate, which results in the obvious voltage drop and significantly reduces the model accuracy at high C-rate. In this paper, the characteristics analysis of different RC models is firstly studied. It is found that the first-order RC circuit model is more suitable for high C-rate conditions due to the coupling of model parameter extraction. Based on the first-order RC model, a basic equivalent circuit model adapted to high C-rate condition is then determined. Secondly, the influences of sampling frequency and the identified time domain on instantaneous impedance and polarization resistance are researched. Thirdly, the change rules of different polarizations at high C-rate are analyzed. Furthermore, the relationship between the change rule of the model parameters and that of different overpotentials are researched. Subsequently, a modified high C-rate model is innovatively established, which considers the current dependence and concentration/temperature modification of instantaneous impedance and polarization resistance. Finally, the accuracy of the modified high C-rate model is verified at 5 °C, 25 °C, and 55 °C, respectively. Results show that the modified high C-rate model can improve the simulation accuracy of the voltage at high C-rate (≤6C), and the mean absolute error (MAE) and Root Mean Squared Error (RMSE) are within 68 mV. Compared with the existing high C-rate model, the accuracy of the modified high C-rate model proposed in this paper is improved and the complexity is also significantly reduced. [ABSTRACT FROM AUTHOR]

• Semi-solid lithium slurry battery has attracted attention in energy storage. • Elucidating the heat generation under specific cycling protocols. • Clarified the safe charging cutoff voltage of the battery. • Explored the reasons for abnormal heat generation at high charging cutoff voltage. • Provided a clearer understanding of the safety of semi-solid lithium slurry battery. Semi-solid lithium slurry battery combines the advantages of the high energy density of lithium-ion battery and the flowability of flow battery electrodes and has attracted attention in energy storage. Elucidating the heat generation during cycling is crucial for evaluating the safety. However, there is a relative lack of research in this field of semi-solid lithium slurry battery. Herein, the heat generation of lithium iron phosphate (LiFePO 4) semi-solid lithium slurry battery during cycling under specific cycling protocols is investigated in this work. The results show that the battery has lower heat generation when cycling at an ambient temperature of 35–50 ℃ and a charging cutoff voltage below 4.0 V, meanwhile, it maintains considerable capacity. However, when the charging cutoff voltage exceeds 4.4 V, the cyclic heat generation of the battery increases significantly, and the capacity attenuates significantly. Meanwhile, the classical equation for calculating battery heat generation of lithium-ion battery is no longer reliable. This is mainly because the electrolyte content of the semi-solid lithium slurry battery is too high (over 80 %), and at high charging cutoff voltage, the side decomposition reactions of the electrolyte increase significantly. Subsequent characterization of electrode materials also confirms this inference. This study can provide a clearer understanding of the safety of semi-solid lithium slurry battery and has guiding implications for industrial applications. [ABSTRACT FROM AUTHOR]

There is a potential danger of battery internal short-circuiting and shortened life span in zinc-nickel single flow batteries which is usually caused by the formation of dendritic crystals as a result of non-uniform electrodeposition. For the purpose of restraining dendrite growth and prolonging the battery service life, it is necessary to investigate the mechanism of dendrite growth that occurs on the surface of zinc anode. Here, a model for two-dimensional zinc dendrite growth is established by the phase field-lattice Boltzmann method (PF-LBM) to simulate how the morphology of zinc dendrites evolves as well as the corresponding distribution of the concentration field, potential field and flow field, and to analyze the influences of the applied voltage, electrolyte flow rate, anisotropy strength and exchange current density on dendrite growth. As the results show, higher exchange current density and applied voltage can promote dendrite growth. By choosing a suitable anisotropic strength, it is possible to change the morphology of the dendrites and decrease the formation of dendrites. Enlarging the electrolyte flow rate may make the distribution of ion concentration more uniform, which can inhibit the dendrite growth as well as affect the morphology of dendrites. [ABSTRACT FROM AUTHOR]

To improve the industrial applicability of aqueous organic redox flow batteries (AORFB), inexpensive redox compounds highly soluble in aqueous medium and stable during cycling are required. Thus, a water-soluble quinizarin derivative, 1,4-dihydroxy-2-carboxymethyl-9,10-anthraquinone (1,4-CDHAQ) prepared by a one-pot synthesis from relatively low-cost leucoquinizarin (widely used as intermediate in the dye industry) is evaluated in AORFB. It exhibits a good solubility in basic medium (> 0.4 M) and a low potential (-0.64 V vs Ag/AgCl) leading to a cell voltage of about 1 V when paired with ferri/ferrocyanide couple. Long-cycling AORFB of 2.5 months performed at a concentration of 0.4 M gives good performance in terms of energy efficiency and power density. The influence of the temperature on ASR values and so on the power density underlines the difficulty to obtain reproducible results at room temperature. A capacity fade rate of 0.008 %/cycle (0.28 %/day) is obtained after 2500 cycles, which is close to other values reported for anthraquinones in strong basic medium for long-cycling tests. Interestingly, the discharge cut-off voltage clearly affects the stability of the battery, leading to the formation of different degradation products as shown by 1H NMR. These results highlight the importance of the cycling conditions to improve the battery performance. [ABSTRACT FROM AUTHOR]

• In this work we have successfully implemented two-step synthesis of redox-active PNIPAM-APMA nanogels grafted with 4-(3-carboxypropanamido)-TEMPO in mild aqueous conditions. • We suggested a universal approach to evaluate the "effective" concentration and diffusion coefficient of redox-active groups grafted to nanogel particles by eliminating the contribution from adsorbed layer of nanogels. • We provided evidence that electromigration of nanogels is significant even at high concentration of supporting electrolyte used in the electrochemical experiments. • Our work will promote further investigations in redox-active micro/nanogels electrochemistry and the role of their structure on the electrochemical properties, forwarding the basis for the rational design of redox-active gels for utilization as redox flow battery electrolytes. In this paper, we provide a thorough electrochemical study of redox-active nanosized cationic gels which are promising materials for redox flow battery electrolytes. We use two-step synthesis under mild aqueous conditions: precipitation polymerization of nanogels based on poly-(N-isporopylacrylamide- co -N-(3-aminopropyl)methacrylamide hydrochloride) (PNIPAM-co-APMA), and grafting of redox-active 4-(3-carboxypropanamido)-TEMPO units to the nanogels. We demonstrate stable reduction–oxidation behavior of such nanogels and suggest a universal approach to evaluate the "effective" concentration and diffusion coefficient of redox-active groups grafted to nanogel particles. For the TEMPO-grafted PNIPAM-co-APMA nanogels we find the "effective" concentration of TEMPO-groups to be approximately 50 % of their total concentration and demonstrate an increase of the "effective" concentration upon electrode rotation. Also, we investigate electron transfer kinetics of redox-active nanogels and provide an evidence that the adsorbed layer of nanogels facilitates electron transfer. [Display omitted] [ABSTRACT FROM AUTHOR]

Eutectic solvents, with high salt concentrations and suppressed volatility, are promising alternatives to aqueous and volatile organic electrolytes for grid-scale energy storage devices such as redox flow batteries (RFBs). However, it is not known a priori which hydrogen bond acceptors and donors would form a eutectic solvent and at which compositions of these mixtures would yield desirable properties for RFBs. Specifically, the known deep eutectic solvents generally have high viscosities and low ionic conductivities, especially in the landscape of RFB electrolytes. Here, a high throughput experimental (HTE) study was carried out to assess the properties of eutectic solvents based on choline chloride as the hydrogen bond acceptor and ethylene glycol and aniline as the hydrogen bond donors. In addition, water, acetonitrile, and dimethyl sulfoxide as co-solvents were examined in terms of tuning the viscosity, conductivity, and solubility of two redox-active species: methyl viologen dichloride hydrate (MVCl 2 ·H 2 O, or MV for simplicity) and 2,1,3-benzothiadiazole (BTZ) that are relevant to RFBs. The HTE framework enabled the development of a rapid and comprehensive understanding of the design of new eutectic electrolytes incorporating co-solvents to improve the physicochemical and electrochemical properties for RFBs. [Display omitted] [ABSTRACT FROM AUTHOR]

All-vanadium redox flow battery (VRFB) with high power density is urgent in energy storage area. This study investigated the impact of Ti 3 C 2 T X /Bi as catalyst on VRFB performance at high current density. The Ti 3 C 2 T X /Bi decorated electrode was prepared based on a facile dropping method. Owing to the synergistic effect between Bi and Ti 3 C 2 T X , the hydrogen evolution reaction (HER) is effectively inhibited and the electrochemical activity towards V2+/V3+ redox reaction is enhanced. With the current densities ranging from 100 mA·cm−2 to 220 mA·cm−2, VRFB equipped with Ti 3 C 2 T X /Bi modified graphite felt shows the best rate performance. Furthermore, at a current density of 200 mA·cm−2, the battery with Bi and Ti 3 C 2 T X modified electrode shows the highest capacity retention rate of 70.3 % after 100 charge-discharge cycles and the highest energy efficiency (EE) of 80.1 %. [ABSTRACT FROM AUTHOR]

• Fabricate a parallel double chain fiber electrode using electrospinning technique. • Single concentration ratio electrodes show improved battery performance with increasing concentration. • Single concentration ratio electrodes outperform dual concentration ratio electrode in terms of battery performance. • The structure enhances mass transport, provides sufficient active sites. • At 200 mA cm−2, achieves 80.5% energy efficiency. This article proposes a parallel double-chain fibrous electrode structure with dual fibers of different diameters by electrospinning technique for vanadium redox flow battery to improve the battery performance. The larger diameter fibers contribute to the formation of larger pores, enhancing electrolyte flow and mass transfer. Meanwhile, the smaller diameter fibers provide abundant reaction sites. Different fiber diameters are fabricated by using precursor solutions with different concentration ratios. For a single concentration ratio electrode, as the concentration of PAN (Polyacrylonitrile) solution in syringe A increases, the performance of the battery significantly improves. The best performance is achieved by employing a concentration ratio of 20%:10%, i.e., ECF (Electrospun Carbon Fibers)-20%/10% electrode, at various current densities due to largest pore diameter and enough reaction sites. At current densities of 100 mA cm−2 and 200 mA cm−2, the ECF-20%/10% electrode achieves energy efficiencies of 88% and 80.5%, which are twice the energy efficiency of traditional carbon felt electrodes. Moreover, after 200 cycles, the battery capacity decay of ECF-20%/10% electrode only is 27.5%, which is 20.3% lower than that of the carbon felt electrode after 100 cycles. Dual concentration ratio electrode is also fabricated and found to show lower performance than the best single concentration ratio electrode for the reason that the second fabrication stage decreases the pore sizes. [ABSTRACT FROM AUTHOR]

• Modification with battery's own electrolyte eliminates contamination by impurities. • Defect and oxygen-containing group are crucial influence on electrochemical kinetic. • The electrochemical performance of the battery was significantly improved. • A novel idea for electrode modification. In the past, non-aqueous redox flow batteries usually used organic solvents or ionic liquids as electrolytes. However, these two liquids are not suitable as electrolytes in terms of safety and production costs. Herein, we chose a deep eutectic solvent (DES) for non-aqueous redox flow battery. DES has been applied to play two functions in Fe/V non-aqueous redox flow battery: commercial graphite felt electrode is treated with DES and heat treatment for surface modification; DES also acts as a main electrolyte for the non-aqueous redox flow battery. While ensuring no external substances are introduced to produce side reactions, the electrode reaction kinetics are enhanced, and better cyclability is achieved. The modified treatment increases the number of oxygen-containing groups and defect sites on electrode surface. Electrode exhibits higher electrochemical activity and lower charge transfer resistance. In the full battery test, the maximum power density with modified electrode reaches 13.26 mW·cm−2, which is 114% higher than commercial graphite felt electrode. The energy efficiency reaches 90.50% and battery life is extended by 70.58% compared with commercial graphite felt electrode. This study has successfully improved the electrochemical performance of Fe/V DES redox flow battery and provides a novel idea for electrode modification. [Display omitted] [ABSTRACT FROM AUTHOR]

• Improvement of fluidity, conductivity, reaction rate constant, and diffusivity of Fe2+/3+ - ethaline by systematic addition of water while still maintaining 2.0 v electrochemical stability window. • This approach is based on all non-toxic and earth-abundant materials that can be sustainably resourced. • Calculation of anodic and cathodic charge transfer coefficients, and the exchange current density using the ex-BV method. • Calculation of diffusion coefficient using Levich equation. Deep eutectic solvents (DESs) have recently attracted much attention as potential green electrolyte solvents for redox flow batteries. DESs are considered not only as environmentally sustainable but also economically attractive electrolytes because they can be resourced from biological feedstock (alcohols, urea, choline) and are earth-abundant and of low toxicity. Despite these advantages, DESs still have limitations in important aspects such as reactant and ion transport, which is inhibited due to hydrogen-bonding-induced viscosity. Thus, improving the transport properties of redox species in DESs is essential. In addition, we explore the quantitative addition of water to ethaline (a 1:2 choline chloride: ethylene glycol mixture) in order to understand its influence on the kinetics and mass transport properties of DESs. In this work, we show that DESs can be made more fluid and less dense, while avoiding most of the electrochemical instabilities of water. Herein, we investigate the effects of gradually increasing amounts of water to the redox system of Fe2+/3+in ethaline. Our study shows that systematic addition of water leads to a three-fold increase in ionic conductivity and decrease in viscosity that enhances the mass transport and kinetics of DES-based electrolytes while still maintaining an electrochemical window of approximately 1.90 V. The use of environmentally benign electrolyte components together with the observed increase in conductivity will result in a more efficient redox flow battery (RFB) that operates at higher power density without relying on harmful solvents and fossil fuel-based processes. [ABSTRACT FROM AUTHOR]

A polyiodide complex has been newly obtained during the cathodic oxidation of iodide ions (I−) in the presence of propylene carbonate (PC). This complex demonstrates features of both ionicity and hydrophobicity. The properties and structure of this complex were investigated and discussed in the light of electrochemical investigations, instrumental analyses, and theoretical calculations using quantum chemistry. The oxygen atom of the C O bond in PC was considered to play an essential role in forming this complex, and a large dipole moment structure between I 2 and PC was suggested by the theoretical calculations. This PC/I 2 structure was considered to play an essential role in enhancing the dissolution rate of I 2 films on a cathode and in accelerating the cathodic oxidation of I−. A model for the structure of this complex is demonstrated. A zinc/iodine redox flow battery of an aqueous-based single electrolyte-type was examined as an application of this complex to a battery without a cation-exchange membrane. Over 90% Coulombic efficiency was observed through charge and discharge cycles. [ABSTRACT FROM AUTHOR]

The low power density, due primarily to the sluggish reaction kinetic of Br 2 /Br−, is one of the main barriers that hinder the widespread application of zinc-bromine flow batteries (ZBFBs). Here, N-doped graphene nanoplatelets are synthesized by a facile method and applied as a catalyst for the Br 2 /Br− redox reactions. Electrochemical characterizations reveal that N-doped graphene nanoplatelets exhibit a remarkable catalytic activity toward Br 2 /Br− reactions, thus enabling the ZBFB to achieve an energy efficiency of as high as 84.2% at 80 mA cm−2, far surpassing those with the non-doped counterpart and pristine graphite-felt electrodes. More strikingly, even when the current density is raised up to 120 mA cm−2, the battery can still maintain an energy efficiency of 78.8%, which represents the highest performance for the ZBFBs reported in the open literature. Additionally, the ZBFB with the N-doped graphene nanoplatelets catalyst shows no degradation after 100 cycles. These superior results demonstrate that N-doped graphene nanoplatelets are an efficient and promising catalyst for high-performance bromine-based flow batteries. • N-GnP are synthesized and applied as a catalyst for Br 2 /Br− redox reactions. • N-GnP exhibits a remarkable catalytic activity toward Br 2 /Br− reactions. • ZBFB equipped with N-GnP presents an energy efficiency of 78.8% at 120 mA cm−2. • The battery shows no degradation after 100 cycles. [ABSTRACT FROM AUTHOR]

The redox flow battery is a leading candidate to supply society with large-scale energy storage. In the category of aqueous organic redox flow batteries, 9,10-anthraquinone-2,7-disulfonic acid (AQDS) is considered a top-performing molecule and shows ideal electrochemical behavior at low concentrations around 1 mM. However, its behavior at higher concentrations was rarely studied. In this work, the correlation between concentration and electrochemical capacity is evaluated for AQDS in 1 M H 2 SO 4 and 1 M alkaline sodium carbonate buffer, employing cyclic voltammetry, rotating disk electrode voltammetry and diffusion NMR. It was found that an electrochemically inactive dimer was formed by the unreduced AQDS molecules, with dimerization equilibrium constants determined to be K = 5 M−1 for the acidic electrolyte and 8 M−1 for the alkaline one, significantly inhibiting full electrochemical utilization of the system. However, upon reduction of the bulk material, the dimerization will shift in favor of monomer or quinhydrone formation, possibly alleviating the impediment to reduction. Finally, it was found that the dimerization could be decreased slightly by increasing the temperature of the system. [ABSTRACT FROM AUTHOR]

Carbon-based electrodes are promising for flow batteries. While cerium redox reactions on carbon materials have been investigated, further studies are of necessity to improve the fundamental understanding of the impact of surface functionalization on reaction kinetics. An electrochemical study of the cerium redox reaction on glassy carbon (GC) electrode, as a standard carbon-based material, provides insights into the mechanism and underlying factors. Thus, an in situ electrochemical treatment is proposed to activate GC, and its influence on the kinetic parameters is demonstrated. Furthermore, a comprehensive investigation on reaction kinetics of cerium is conducted on activated GC, where the impact of sulfuric acid and methanesulfonic acid supporting electrolytes is studied. Moreover, the results reflect the contributions of different cerium complexes on electron transfer and diffusion processes. Image 105265 • An in situ electrochemical treatment is proposed to activate glassy carbon. • Impact of electrochemical measurements on reaction kinetics is investigated. • Charge transfer of cerium redox reaction is accelerated on oxidized glassy carbon. • Cerium complexes affect electron transfer and diffusion. [ABSTRACT FROM AUTHOR]

A two-dimensional and steady-state model of the vanadium redox flow battery with an interdigitated channel and a thin active electrode was developed to visualize the flow velocity, concentrations of the reactants, and local current distribution in the through-plane direction of the electrode sheets. The concentration overpotential was increased in the low flow speed region. The low mass transport coefficient by a slow flow speed resulted in a higher concentration overpotential. The mass flux distributions at the state of charge of 20% were strongly dependent on the flow speeds due to low bulk concentration of the reactants. Based on the visualized data, not only the reactivity improvement of the electrode material but also the advanced design of the porous electrode structure for a high flow permeability of the electrolyte solution are essential to improve the cell voltage during discharge. • A 2D & steady state VRFB model was developed to visualize the cell characteristics. • Distribution of flow velocity, concentrations, and current density were visualized. • The interdigitated channel and thin active electrode were modeled. • The concentration overpotential increased in the low flow speed region. • The distribution of mass flux in the boundary film was changed by the SoC. [ABSTRACT FROM AUTHOR]

Fe 2 (SO 4) 3 was investigated as an additive to improve the thermal stability of the positive electrolyte for an all-vanadium flow battery (VFB). The effects of the Fe 2 (SO 4) 3 addition on the electrochemical activity, capacity, and charge/discharge cycle performance were also studied. The addition of Fe 2 (SO 4) 3 significantly enhanced the thermal stability of V(V) in the positive electrolyte. For a positive electrolyte consisting of 2.0 M vanadium ions, the addition of Fe 2 (SO 4) 3 within the concentration range of 0.5–0.7 M enabled the electrolyte to remain stable for more than 168 h at 50 °C. Cyclic voltammetry (CV) measurements showed that the peak positions of the Fe2+/Fe3+ and VO2+/VO 2 + couples did not cross each other, indicating that the Fe 2 (SO 4) 3 addition did not lead to any side reactions. Electrochemical impedance spectroscopy (EIS) analysis revealed that the addition of Fe 2 (SO 4) 3 led to a slight increase of the electrolyte ohmic resistance and reduction of the charge-transfer resistance on the carbon electrode. Both the CV and EIS results indicate that Fe 2 (SO 4) 3 had no adverse effects on the reversibility or electrochemical activity of VO2+/VO 2 +. The capacity for battery charge/discharge cycles hardly changed compared with that of the pristine electrolyte during operation for over 360 h at 50 °C. The improved vanadium positive electrolyte with the Fe 2 (SO 4) 3 addition is expected to be used for practical applications. The Fe 2 (SO 4) 3 addition can extend the VFB operation temperature to 50 °C, thus reducing the cost of thermal management of the energy storage system and improving the technical and economic performance of VFBs. Schematic diagram of a vanadium flow battery using the improved vanadium positive electrolyte, in which Fe 2 (SO 4) 3 was used as stabilizer to enhance the thermal stability of V(V) in the positive electrolyte, thus leading to VFB reliable operation at 50 °C. Image 1 • High-concentration Fe 2 (SO 4) 3 was used as an additive for VFB positive electrolyte first time. • The addition of Fe 2 (SO 4) 3 stabilizes the vanadium electrolyte at 50 °C. • No adverse effects were found on reversibility and electrochemical activity of VO2+/VO 2 +. [ABSTRACT FROM AUTHOR]

Tailoring the energy density of Ni(II)-chelated complex cation is accomplished by substituting various counter anions. Larger-sized anions are favored for increasing both the solubility and working voltage. The former advantage with larger-sized anion can be readily assumed since the binding energy is smaller to allow a higher solubility in solvents. The latter comes from the fact that Ni(II)(cyclam) complex cation in square-planar geometry shows a higher working voltage. Furthermore, the solubility of redox couple is enhanced by increasing the dielectric constant of used solvent. Consequently, the largest anion (TFSI−) demonstrates the most promising physicochemical/electrochemical properties; the energy density of redox couple reaches 27.3 W h L−1 with the working voltage, 2.55 V. The Ni(cyclam)[TFSI] 2 shows very reversible electrochemistry in 1.0 M TEABF 4 in EC/PC (1:1 = v/v) electrolyte and presents stable cycleability at non-flowing coin type static cell. [ABSTRACT FROM AUTHOR]

Abstract A two-dimensional transient model for the study of zinc-nickel single flow battery was developed. The model is based on a comprehensive description of mass, momentum and charge transport and conservation, combining with a global kinetic model for reactions involving ions and proton. The model is validated against the experimental date and is used to study the effects of variations in concentration, flow rate, and applied current density. The model results indicate that increasing the initial concentration of hydroxide ions can increase the discharge voltage of the battery without increasing the ions concentration polarization, and higher zinc ion concentration can slightly decrease the discharge voltage. An increase in flow rate leads to a more rapid generation or consumption of the reactant concentration, a faster mass transfer and a more uniform distribution of the ion concentration. However, the effect of flow rate on the discharge voltage of the battery is small and limited. Bulk reaction rate depends on the applied current density. As the applied current density increases by one or two time, ion concentration polarization in reaction zone and the transfer current density almost double or triple and the over-potential increased a fixed amount each time. [ABSTRACT FROM AUTHOR]

Abstract A semi-solid flow battery, consisting in a single channel with millimetric width, was operated in flow conditions using Li 4 Ti 5 O 12 (LTO) or LiNi 1/3 Co 1/3 Mn 1/3 O 2 (LNCM) based suspensions versus Li metal. Cell voltages were measured experimentally for various values of charge/discharge current. A model of the electrochemical cell was developed and validated by comparing calculated results with experimental data. The model consists in solving simultaneously the suspension rheology, potential and current within the cell (secondary distribution) and state of charge evolution between the inlet and outlet. Modelling results indicate low pressure drop (<50 kPa), nearly-homogeneous velocity profile along the channel and potential and current gradients in the cell. Comparison of experimental and calculated results indicate that the model takes into account the influence of applied current density on the cell potential but with significant differences on their absolute value, which are discussed in terms of electrochemical stability for the semi-solid suspensions during cycling. Results detailed in this article can, therefore, be applied to larger electrochemical cells or systems based on alternative chemistries, operated in flow conditions. [ABSTRACT FROM AUTHOR]

Abstract H 2 SO 4 concentration has an important influence on the performance of vanadium electrolytes and flow batteries. However, the comprehensive research is still inadequate. In this work, a series of electrolytes with 1.5 M vanadium ions (V2+, V3+, V3.5+, VO2+ and VO 2 +) in various concentrations of H 2 SO 4 (1.5 M, 2.0 M, 2.5 M, 3.0 M, 3.5 M and 4.0 M) are prepared and their physicochemical properties, electrochemical activities, single-cell performances, and static stability are studied in detail. The conductivity and viscosity of the electrolytes increase with increasing H 2 SO 4 concentration. Besides, the electrochemical properties are improved in higher concentration of H 2 SO 4 except for the charge transfer resistance and hydrogen evaluation reaction. Meanwhile, the single-cell performances with 3.0–4.0 M of H 2 SO 4 are superior to those with 1.5–2.5 M of H 2 SO 4. However, V3+ in 4.0 M H 2 SO 4 is not stable enough to store for a long time at room temperature. The above results indicate that 3.0 M and 3.5 M of H 2 SO 4 should be used as supporting electrolytes to achieve efficient and stable vanadium flow batteries. This work may also provide a guide for study and practical application of vanadium flow batteries. Graphical abstract Image 1 Highlights • The effect of H 2 SO 4 concentration on the VFB electrolyte is studied. • Both electrolyte properties and cell performances are evaluated synthetically. • Electrolytes with 3.0–4.0 M H 2 SO 4 show better electrochemical and cell performance. • V3+ in 4.0 M H 2 SO 4 and VO 2 + in 1.5 M H 2 SO 4 are unstable at room temperature. [ABSTRACT FROM AUTHOR]

Abstract Fe-triethanolamine is a promising candidate as anolyte for redox flow batteries (RFBs), owning to its low potential, high solubility and low cost. We report here a new dinuclear structure of this complex at solid state when prepared with a stoichiometric amount of triethanolamine and iron in basic medium, whereas more than two equivalents of ligands are usually used to prepare Fe-triethanolamine for RFBs application. We achieve a calibration curve to estimate Fe(III) concentration in solution and coulometric experiments highlight a one-electron reduction process per iron atom, corresponding to the reduction of the two Fe(III) atoms of the dinuclear complex into Fe(II). A solubility higher than 1.2 mol dm−3 can be reached for Fe-triethanolamine with the new synthesis proposed in this work. All-Fe alkaline RFBs implemented with Fe-triethanolamine exhibit good performances in terms of coulombic, voltage and energy efficiencies, and is stable over a hundred of cycles. A power density around 80–120 mW cm−2 is reached and the high solubility of Fe-triethanolamine allows the achievement of high volumetric capacities, superior to 10 Ah dm−3. [ABSTRACT FROM AUTHOR]

Abstract Sulfur and nitrogen containing groups have been bonded to carbon in graphite felt electrodes using a hydrothermal treatment with ammonium persulfate (APS) for use in an aqueous all-vanadium redox flow battery (VRB). Our results reveal that the hydrothermal treatment with APS is a very effective means to attach both sulfur and nitrogen groups to the surface of graphite felt, and as a result, the inclusion of these groups significantly improved the voltaic efficiencies (∼95%), discharge capacities (>400 mAh), and capacity retention (∼83%) of these batteries at current density of 50 mA/cm2. The direct addition of reactive sulfur and nitrogen groups also substantially extended the range of current densities available to the battery (in excess of 200 mA/cm2) without sacrificing capacity and capacity retention. Linear sweep voltammetry indicated that APS treated graphite felts showed an electrochemical enhancement factor over double that of untreated pristine graphite felts. These results suggest that the direct addition of sulfur and nitrogen containing groups on the graphite felt as a simple and effective method for improving the performance of all-vanadium redox flow batteries. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Water crossover through the membrane of a vanadium redox flow battery system is not desirable because it floods one half-cell, diluting the vanadium solution on one side and consequently increasing the concentration of vanadium in the other half-cell. To analyze the effect of water crossover and the resultant electrolyte imbalance issue in the vanadium redox flow battery, herein we newly develop a water transport model and incorporate it into our previously developed 3D vanadium redox flow battery model. The model rigorously accounts for water production/consumption by the redox reaction of VO2+/VO 2 and side reactions as well as various mechanisms of water crossover through the membrane arising from diffusion, electro-osmotic drag (EOD), and vanadium crossover. The numerical model is successfully validated against in situ data collected during experiments in which the electrolyte volumes and cell voltages are measured during charge–discharge cycles carried out under various current densities. The detailed simulation results clearly elucidate water crossover behaviors at different stages of charging and discharging, and further reveal the individual contributions of water crossover mechanisms to the overall electrolyte imbalance between the negative and positive sides of the VRFB system. Highlights • The effect of water crossover and resultant electrolyte imbalance are considered. • A water transport model is developed by extending a previous 3D VRFB model. • The numerical model is validated against in situ experimental data. • Water crossover flux via EOD was symmetrical between charging and discharging. • Water diffusion through the membrane was unidirectional, accompanying proton transport. [ABSTRACT FROM AUTHOR]

Abstract Compared with conventional aqueous electrolyte, due to unique merits of deep eutectic solvents including a wide electrochemical window, ease in preparation, low vapor pressure and low cost, they have been used as electrolyte in the non-aqueous flow battery. This work reports a reline deep eutectic solvent, which is prepared as the anolyte and catholyte of a vanadium-iron redox flow battery. Since the solubility of the vanadium and iron species in deep eutectic solvent has been improved, an increased specific capacity can be obtained, up to 54.6% compared with the aqueous electrolyte one. However, the viscosity of this electrolyte is much larger than that of aqueous electrolyte, which causes more pumping losses. With the increase of operating temperature, the viscosity experiences a huge drop. The open-circuit voltage of such an non-aqueous system begins near 0.9 V with a decrease rate of 0.005 V h−1, suggesting a better capacity retention ability than does the aqueous flow battery. The cycling performance of this redox flow battery is evaluated at a current density of 2.0 mA cm−2 under the temperature of 30 °C. The coulombic efficiency is up to 94.8%, while voltage efficiency and energy efficiency are just 68.3% and 64.7%, respectively. The low voltage efficiency stems from the sluggish redox kinetics of the vanadium ions, and large internal ohmic resistance. With an improved thermal stability, this vanadium-iron redox flow battery is preferred to be operated at elevated temperature, which helps to greatly reduce the ohmic loss and pumping loss, such that to increase the system energy efficiency. Graphical abstract A vanadium-iron redox flow battery works with deep eutectic solvent electrolyte. Image 1 Highlights • A V-Fe redox flow battery (RFB) with DES electrolyte is set up and tested. • Enhanced solubility of ions results in an increased capacity up to 54.6% compared with the aqueous electrolyte. • This DES-based RFB has a better capacity retention ability than aqueous one. • Low voltage efficiency stems from both sluggish kinetics and large ohmic resistance. • Improved stability helps to operate at elevated temperature with higher energy efficiency. [ABSTRACT FROM AUTHOR]

Abstract A combination of analytical techniques and molecular modelling methods was used to study the influence of chloride-based supporting electrolytes on zinc electrodeposit surfaces in the zinc half-cell of zinc/bromine flow batteries. Scanning electron microscopy and X-ray diffraction analysis of zinc electrodeposits obtained during charging show that: (a) the primary preferred orientation is planar, and (b) increasing the duration of exposure to the Cl-containing electrolyte influences crystallinity and changes the preferred secondary orientations. Analysis using periodic density functional methods indicates that the binding energy of Cl to the Zn(001) surface depends more strongly on surface coverage than on binding site location. Solvation increases the stability of Cl binding to the surface. The binding energies of Cl with and without water solvation are between 288 and 327 kJ mol−1 and 249–288 kJ mol−1, respectively, for coverages between 0.11 and 0.33 ML. The binding of Cl leads to Zn surface buckling and decreases surface uniformity. Binding at 0.33 ML and above results in surface instabilities, leading to the formation of surface cavities due to significant vertical displacement of surface Zn sites. [ABSTRACT FROM AUTHOR]

Abstract In this article, we report that electrochemically generated quaternary ammonium polybromide (QBr 2n+1) droplets can act not only as electrochemical reactors for the electro-oxidation of Br−, but also as tiny reductants for Br 2 dissolved in an aqueous phase. We suggest two different theoretic models: Cloud and Droplet. In the Cloud model, we consider a cloud composed of small droplets located in the vicinity of a Pt ultramicroelectrode (UME). The positive feedback loop of the redox reaction is derived in the gap between the Cloud and the Pt UME, which leads to catalytic current enhancement, like the positive feedback mode of scanning electrochemical microscopy (SECM). In the Droplet model, a droplet adsorbed on the center of a Pt UME drives the catalytic feedback loop of the redox reaction. Next, we adopted the two theoretical models to explain the current amplification by QBr 2n+1 observed in our experimental systems. In the early potential region for electro-oxidation of Br−, we found the QBr 2n+1 droplets- Cloud model was a more reliable scenario for the catalytic current amplification. As the potential became more positively biased, stochastic collisions of QBr 2n+1 droplets occurred on the Pt UME, and in this stage, we determined that the QBr 2n+1 - Droplet model was the main catalytic mechanism for Br− electro-oxidation in the presence of QBr in the solution. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

All-vanadium redox flow battery (VRFB) is a promising large-scale and long-term energy storage technology. However, the actual efficiency of the battery is much lower than the theoretical efficiency, primarily because of the self-discharge reaction caused by vanadium ion crossover, hydrogen and oxygen evolution side reactions, vanadium metal precipitation and other operational factors. Based on the above analysis, this paper proposes effective strategies and methods for improving the efficiency of the VRFB. The experimental results indicate that the voltage efficiency and system efficiency increased by 1.86% and 0.48%, respectively, when constant flow rate and variable current density charge/discharge methods are used. Meanwhile, when variable flow rate and current density charge/discharge methods are employed, the energy efficiency and system efficiency increased by 9.07% and 8.34%, respectively, resulting in significant improvement in energy storage capacity. The experiment has verified that this method can improve the efficiency and performance of VRFB, and can promote the development of VRFB. [ABSTRACT FROM AUTHOR]

In this research, the flow behavior through a commercial electrode used in Redox Flow Batteries (RFB) was characterized by means of Planar Laser Induced Fluoresce imaging (PLIF) for different cell configurations, without channels or with channels machined either in the back plate or in the electrode. Measurements of the pressure drop were obtained for each case, and the evolution of the electrode soaking process was quantified defining an electrode surface wetting rate. Using the pressure drop measurements, equivalent Darcy and intrinsic permeability coefficients were calculated for the different configurations. It was observed that the tight fitting of the electrode within its housing appears to be crucial for an adequate distribution of the electrolyte within the porous media. In general, when channels are available, the electrolyte tends to circulate along them, without crossing through the electrode felt, and lower pressure losses are accompanied by lower wetting rates. The flow field with serpentine channels in the back plate could be considered the most suitable configuration in terms of energy consumption, nearly 87% less than in a case without any channels, but this same percentage of electrolyte will exit the cell without traversing the electrode, and thus not contributing to the electrochemical reactions. [ABSTRACT FROM AUTHOR]

Understanding the behavior of bipolar membranes under forward and reverse bias in multi-ion solutions is essential for their application in acid-base flow batteries (ABFB). To this end, a two-dimensional steady-state model of the bipolar membrane is proposed based on the Nernst-Plank and Poisson equations coupled with water dissociation and acid-base neutralization reactions. The second Wien effect and catalytic activity are included in the water dissociation rate model at the bipolar junction. Experimental current-voltage curves were obtained using linear sweep voltammetry to validate the model. A commercial bipolar membrane was used in NaCl solutions at 0.25 and 0.5 mol L−1 concentrations, a mixture of NaCl-HCl and NaCl-NaOH solutions at different concentrations, and solutions of HCl and NaOH at 0.5 mol L−1 concentration. The catalytic activity coefficient was the only model parameter fitted to the experimental data; all other properties and parameters were obtained from the literature or calculated from the membrane supplier data. The numerical solution of the bipolar membrane model complements the experimental results. It shows a full view of conductivity, ion concentration, and potentials across the bipolar membrane as a function of its polarization. [ABSTRACT FROM AUTHOR]

Solar redox flow batteries are a relatively new type of redox flow battery technology that uses solar energy to directly store chemical energy. Here we present a solar redox flow battery that uses a MoS 2 @TiO 2 thin film with a Nafion protection layer supported on FTO glass substrate as photoanode, employing VO2+/VO 2 + and AQDS/AQDS2− as redox active species. When the solar radiation strikes the photoelectrode, the photogenerated holes oxidize VO2+ to VO 2 +, while the photogenerated electrons reduce AQDS to AQDS2− at the counter electrode. The oxidized form of V5+ and reduced form of AQDS2− thus retain the chemical energy and can release the stored charged via the reverse electrochemical reaction. The addition of NH 4 Cl to the electrolyte was found to have a positive impact on the electrochemical performance of the redox flow cell. This effect was more evident for the VOSO 4 electrolyte, leading to an enhancement of the voltaic and energy efficiencies of more than 17.5%. The results suggest that NH 4 Cl promotes both mass transport of the vanadium redox species and charge transfer of the AQDS in the electrolyte. The solar-to-output energy conversion efficiency (SOEE) of the solar redox flow battery using 1.6 g L−1 NH 4 Cl in both anolyte and catholyte reached 9.73%, and an energy density of 87.45% after 10 consecutive one-hour photocharging cycles. Additionally, the use of Nafion to protect the MoS 2 @TiO 2 photoanode from photocorrosion was explored. The Nafion layer ensured an increased stability of MoS 2 @TiO 2 against the strong acidic environment while maintaining effective light response, which translated into enhanced photon and mass transport. An energy storage capacity of ∼60 mAh L−1 after 1-hour photocharging was observed. [ABSTRACT FROM AUTHOR]

Nowadays, redox flow batteries (RFBs) are considered the prospective solution for large-scale energy storage. All-vanadium systems are penetrating the market; however, they still can't be named a mass technology. The application of organic materials in the RFBs could sufficiently speed up the commercialization process by being more affordable, cheap and eco-friendly. In the current review, small organic molecules are considered redox-active species for the RFBs: we present the main organic core structures which are usually applied as anolytes and catholytes and discuss their typical physical and electrochemical properties. Then, different solubilizing substituents for the creation of water- or organic-soluble compounds are analyzed, and their effect on the electrochemical properties of the molecules is demonstrated. This review provides accumulated knowledge about organic anolytes and catholytes for the redox flow batteries and impetus for the creation of new generations of highly-soluble electrochemically stable organic redox-active materials. [ABSTRACT FROM AUTHOR]

• Hybrid flow batteries requires asymmetric electrolyte formulation. • Molar balancing of electrolytes is proposed for suppressing water shuttle. • Zinc crossover should be restricted to avoid water migration. • This strategy facilitates future upscaling of ZIFB. Zinc-iodide flow battery (ZIFB) is under research for the last years due to its suitability as a potential candidate for future electrochemical energy storage. During cycling, one of the biggest challenges that affect the reliable performance of ZIFB is the substantial water migration through the membrane because of differential molar concentrations between anolyte and catholyte that imbalance the osmotic pressures in each compartment. Considering the mass balances, herein we propose to equalize the total ionic concentration of electrolytes by the addition of extra solute into the compartment of lower ion concentration as a way to restrict the water crossover. Experimental validation of this electrolyte concentrations balancing strategy has been carried out by assessing the post-cycled electrolytes, and half-cell charged electrolytes, which confirms the efficient suppression of water migration from catholyte to anolyte. Besides, in-depth analysis of ions and water transport mechanism through Nafion 117 membrane confirms that solvated K+ ions of lower ionic radius compared to solvated Zn2+ ions, are the dominant migrating carrier. Therefore, the addition of extra KI solute is beneficial to suppress the major transport of large hydrated Zn2+ ions along with the higher amount of water. Finally, the improved ZIFB cell behaviour with enhanced electrical conductivity, discharge capacity, and voltage efficiency in the cell assembled with the electrolytes of balanced molar concentrations concludes our present study, proving that tuning the electrolytes concentrations is an effective way to suppress water migration as an appealing method in the prospect of upscaling ZIFB application. [Display omitted] [ABSTRACT FROM AUTHOR]

Modelling and simulation is a powerful tool to support the development of novel flow cells such as electrolysers and flow batteries. Electrolytes employed in such cells often consist of aqueous solutions of highly concentrated solutes at elevated temperatures. Such conditions pose numerous challenges in conventional model parametrisation because of non-ideal behaviour of the electrolytes. The aim of this work is to study mass transport of electroactive species in highly-concentrated media. We selected the hydrogen–bromine flow battery posolyte, HBr (aq) and Br 2 , as an exemplary flow battery electrolyte and we leveraged chronoamperometric techniques involving ultramicroelectrodes to study diffusion and migration of bromide and bromine at high concentration and temperature. We successfully simulated the current densities of HBr/Br 2 redox reactions in solutions up to 8 mol L–1 using advanced mass transport theory which agreed well with the results obtained with ultramicroelectrodes. While uncharged species transport (Br 2) can be credibly modelled using conventional theories such as Fick's law, charged species (Br–) require special treatment as the diffusion coefficient vary with concentration up to 50 % with respect to the limiting value at infinite dilution. The transport of charged species without added supporting electrolyte occurs via both migration and diffusion and the contribution of migration current may be up to 50 % of the total current. At HBr concentration > 0.6 mol L–1 migration appears to be suppressed due to the "self-screening" effect of the electrolyte. Proper experimental electrolyte characterisation under operating conditions similar to the actual flow cell applications is indispensable to establish predictive models and digital twins of electrochemical devices. Straightforward transfer of concepts known in electro-analytical chemistry to flow cells modelling may lead to erroneous simulations or model overfitting. [Display omitted] • Study on ion diffusion and migration in concentrated solutions. • Bromine and bromide redox limiting currents measured using ultramicroelectrodes. • Results interpreted using advanced theories of migration and diffusion. • Fick's law for charged species is inaccurate for flow battery models. • Ultramicroelectrodes are well-suited for measurements in highly concentrated media. [ABSTRACT FROM AUTHOR]

• Electrochemical behavior of electrolytes with different cu-cl complexes distributions. • Hysteresis in voltabsorbometries linked to the cu-cl complexes stability. • Quantum-chemical calculations get insights into the structure of the complexes. • Highly coordinated cu-cl complexes recommended in concentrated electrolytes. Redox Flow Batteries (RFB) are an ideal choice for large stationary applications. Among the different chemistries that can be exploited, all-copper aqueous RFB (CuRFB) use low-cost, earth-abundant raw materials with a well-defined European supply chain. The CuRFB takes advantage of the three oxidation states of copper. As Cu(I) is not stable in aqueous media, the system is based on the chlorocomplexation of the copper cations. We demonstrated that it is possible to evaluate the complexation characteristic of the concentrated solutions used in CuRFB by investigating the speciation of copper (II) in electrolytes with increasing Cu(II) concentration. Spectro-electrochemical tests in diluted solution give information on the electrochemical behavior of electrolytes with a different chloro‑complexes distribution. Quantum chemical calculations elucidate the molecular structure and electronic transitions of water solvated copper chloro‑complexes, thus complementing the experimental picture. [ABSTRACT FROM AUTHOR]

Zinc-bromine flow batteries have been regarded as one of the most promising technologies for large-scale energy storage. However, the widespread application of this technology is still hindered by its low power density, primarily resulting from a large polarization in the Br 2 /Br − electrode. In this work, carbonized tubular polypyrrole with abundant nitrogen- and oxygen-containing functional groups is synthesized and, for the first time, applied to a zinc-bromine flow battery as a positive electrode material. Experimental results unravel that carbonized tubular polypyrrole exhibits a high activity toward the Br 2 /Br − redox reaction, and enables the zinc-bromine flow battery to be operated at a current density of 80 mA cm −2 with a high energy efficiency of 76.0%. In contrast, the battery with the pristine graphite felt electrode can only output an energy efficiency of 69.4% at the same current density. In addition, no degradation is observed for the zinc-bromine flow battery equipped with carbonized tubular polypyrrole for 100 cycles. These superior results suggest that carbonized tubular polypyrrole is a promising positive electrode material for zinc-bromine flow batteries. [ABSTRACT FROM AUTHOR]

The vanadium redox flow battery (VRFB) has emerged as a promising technology for large-scale storage of intermittent power generated from renewable energy sources due to its advantages such as scalability, high energy efficiency and low cost. In the current study, a three-dimensional(3D) Lattice Boltzmann model is developed to simulate the transport mechanisms of electrolyte flow, species and charge in the vanadium redox flow battery at the micro pore scale. An electrochemical model using the Butler-Volmer equation is used to provide species and charge coupling at the surface of active electrode. The detailed structure of the carbon paper electrode is obtained using X-ray Computed Tomography(CT). The new model developed in the paper is able to predict the local concentration of different species, over-potential and current density profiles under charge/discharge conditions. The simulated capillary pressure as a function of electrolyte volume fraction for electrolyte wetting process in carbon paper electrode is presented. Different wet surface area of carbon paper electrode correspond to different electrolyte volume fraction in pore space of electrode. The model is then used to investigate the effect of wetting area in carbon paper electrode on the performance of vanadium redox flow battery. It is found that the electrochemical performance of positive half cell is reduced with air bubbles trapped inside the electrode. [ABSTRACT FROM AUTHOR]

With the consideration of understanding the interplays between the electrolyte and electrode for the VO 2 + /VO 2+ redox reactions at various electrodes and developing high-activity electrode materials for an all-vanadium redox flow battery, the reduction reaction kinetics of vanadium(V) ions on a platinum (Pt) electrode in comparison with that on carbon electrodes is investigated by steady-state potentiodynamic polarization and impedance spectroscopy measurements in sulfuric acid solutions with various pH and vanadium concentrations. No abnormal increase is observed neither in the cathodic Tafel slope nor the charge transfer resistance ( R ct ) at a transition potential ( E K ) on the Pt electrode. However, an abrupt change in the Tafel slope and R ct of vanadium(V) ions is observed on carbon electrodes. The above results indicate that the reduction reaction kinetics of vanadium(V) ions in acidic solutions on the Pt electrode show some unusual difference with that on carbon electrodes, and this is the first study to report such a new phenomenon. Additionally, possible mechanism pathways for the reduction reaction of vanadium(V) ions in acidic solutions are proposed for the above-mentioned carbon and Pt electrodes in this work. [ABSTRACT FROM AUTHOR]

Cu 2+ /Cu + –[Fe(CN) 6 ] 3- /[Fe(CN) 6 ] 4- redox couple is investigated for energy storage applications. Analogous to Pourbaix diagram, Potential – Time plot indicating regions such as Nernstian, Steady state and Redox were analyzed. Appropriate electrochemical reactions associated with these regions were also attempted. The redox couple provides power continuously for 3312 h with good stability. The coulombic, voltage and energy efficiencies obtained were 96%, 72.13% and 69.3% respectively. The maximum voltage attained by the cell is 0.88 V. The baseline voltage of the cell is 0.3 V and never reaches zero voltage output even after the inactivity of anolyte. Hence by replenishing the anolyte and anode, the device can be made functional for next 3312 h without replacing catholyte. Thus Cu 2+ /Cu + –[Fe(CN) 6 ] 3- /[Fe(CN) 6 ] 4- redox couple can be a low cost alternative or partial substitution redox couple to improve the performance of flow batteries. [ABSTRACT FROM AUTHOR]

Membraneless redox flow batteries reported to date are microscale designs that have shown poor capacity retention and cyclability due to reactant crossover. Here, we present a new design of macroscale membraneless redox flow battery capable of recharging and recirculation of the same electrolyte streams for multiple cycles and maintains the advantages of the decoupled power and energy densities. The battery is based on immiscible aqueous anolyte and organic catholyte liquids, which exhibits high capacity retention and columbic efficiency during cycling. The aqueous anolyte consists of iron(II) sulfate (FeSO 4 ) as active species, which is immiscible with the catholyte, having iron(III) acetylacetonate (Fe(acac) 3 ) as active species dissolved in water-immiscible ethyl acetate, supported by ionic liquid (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). Over 60% of initial discharge capacity was retained and over 80% of columbic efficiency was sustained after 25 cycles in a test flow cell, which is among the highest reported thus far. [ABSTRACT FROM AUTHOR]

A series of sulfonated poly(ether sulfone) copolymers (SPES-Xs) with varying degrees of sulfonation were prepared and investigated as ion-exchange membranes for vanadium redox flow battery (VRFB) applications. Sulfonated poly(thioether ether sulfone) copolymers (SPTES-Xs) were initially synthesized via polycondensation, and the SPES-Xs were then obtained by oxidation of the corresponding SPTES-Xs. The SPES-X membranes showed reduced vanadium-ion permeability, low area resistance, and, thereby, much superior selectivity compared with the parent SPTES-X membranes and a Nafion115 membrane. In single-cell VRFB performance tests, a SPES-50 membrane with an ion-exchange capacity of 1.80 meq/g exhibited a higher coulombic efficiency (>99%) and energy efficiency (76–89%) than the Nafion115 membrane over a wide range of current densities from 40 to 100 mA/cm 2 and a significantly larger capacity retention (>62%) during 200 charge-discharge cycles. The SPES-X materials, in which every benzene ring is deactivated by the presence of electron-withdrawing sulfone linkages, showed much better chemical stability during ex situ and in situ tests than the SPTES-X materials, which contain an electron-donating thioether linkage in their repeat unit. [ABSTRACT FROM AUTHOR]

A two-dimensional quasi-steady-state model is applied to investigate charge/discharge behavior and performance of a VRFB. Emphasis is focused on exploring the influences of various strategies of soc-dependent operating current density on battery performance. For constant-current operation, with the increase of current density, although the system mean power density is significantly boosted, the system capacity and net discharge energy in a cycle are obviously decreased. It is quite difficult to give consideration to both the system capacity and system power simultaneously such that overall battery performance can be improved. Applying the strategy of soc-dependent operating current (i.e., low current at the end of charge/discharge process while high current for the rest time), the system capacity, power density and net discharge energy in a cycle can be simultaneously boosted while comparable efficiencies can be achieved. Optimally, at the mean operating current density of about 70 mA cm −2 , the system capacity is maximally improved by 9.3% and the net discharge energy is maximally increased by 7.9% as compared with that under constant-current operation. As the same time, the energy-based system efficiency can be maintained over 78%, which is comparable with that for constant-current operation. [ABSTRACT FROM AUTHOR]