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Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of −2.2 V or lower vs. ferrocene+/0 (Fc+/0), but the 3‐CNPy⋅− radical anion forms a sigma‐dimer that is re‐oxidized at E≈−1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge‐discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu4][PF6] shows that 2‐CNPy and 4‐CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density‐functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E1/2 only about +0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3‐CN‐6‐PhPy and 3‐CN‐4‐PhPy are the most promising as anolytes. They exhibit reversible reductions at E1/2=−2.19 and −2.22 V vs. ferrocene+/0, respectively, and retain about half of their capacity after 30 bulk charge‐discharge cycles. An improved version of 3‐CN‐6‐PhPy with three methyl groups (3‐cyano‐4‐methyl‐6‐(3,5‐dimethylphenyl)pyridine) has an extremely low reduction potential of −2.50 V vs. Fc+/0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge‐discharge cycling in acetonitrile. [ABSTRACT FROM AUTHOR]

Polyoxometalates (POMs) are a class of metal‐oxygen cluster molecular materials formed by covalently linking transition metal atoms and oxygen atoms. Those molecular cluster materials also shows multiple electron reversible redox capabilities and high solubility in solution. Thus, POMs also recently present interesting applications as redox active electrolyte in redox flow batteries (RFBs). In this review, we mainly focus on the structural design of POMs and analysis factors affecting the electrochemical properties of POMs. Spectroscopic characterizations are introduced to deeply explore the structural changes of POMs during the redox process. Furthermore, we summarize the cases of POMs used as liquid‐phase redox couple in aqueous or non‐aqueous redox flow batteries and discuss the advantages and deficiency of POMs with various configurations. [ABSTRACT FROM AUTHOR]

Enhancing the electrochemical stability and reversibility of redox‐active organic molecules is crucial to improve the performance of non‐aqueous redox flow batteries (RFBs). Compared with the widely adopted strategy of molecular engineering, we show in this study that tuning the intermolecular interaction between the active material with the supporting electrolyte is another feasible way to address the performance of non‐aqueous organic RFBs. Combined with theoretical and experimental investigations, the influence of Lewis acidity of the supporting electrolyte cations and anions on the electrochemical stability and reversibility of bipyridine‐based anode material is revealed. As a demonstration, a redox flow cell based on 4,4′‐bipyridine anolyte and ferrocene catholyte shows greatly enhanced performance by using supporting electrolyte composed of soft Lewis acid and soft Lewis base. This study provides an alternative, yet highly effective way to addressing the cyclability of an organic compound for non‐aqueous RFBs. [ABSTRACT FROM AUTHOR]

We have developed a novel molecular design that enables six-electron redox activity in fused phenazine-based organic scaffolds. Combined electrochemical and spectroscopic tests successfully confirm the two-step 6e− redox mechanism. This work offers an opportunity for achieving energy-dense redox flow batteries, on condition that the solubility and stability issues are addressed. [ABSTRACT FROM AUTHOR]

Flow batteries possess several attractive features including long cycle life, flexible design, ease of scaling up, and high safety. They are considered an excellent choice for large-scale energy storage. Carbon felt (CF) electrodes are commonly used as porous electrodes in flow batteries. In vanadium flow batteries, both active materials and discharge products are in a liquid phase, thus leaving no trace on the electrode surface. However, zinc-based flow batteries involve zinc deposition/dissolution, structure and configuration of the electrode significantly determine stability and performance of the battery. Herein, fabrication of a compressed composite using CF with polyvinylidene fluoride (PVDF) is investigated in a Zn–Fe flow battery (ZFB). Graphene (G) is successfully introduced in order to improve its electrochemical activity towards zinc reactions on the negative side of the ZFB. A compressed composite CF electrode offers more uniform electric field and lower nucleation overpotential (NOP) of zinc than a pristine CF, resulting in higher zinc plating/stripping efficiency. Batteries with modified electrodes are seen to provide lower overpotential. Particularly, the G-PVDF-CF electrode demonstrates maximum discharge capacity of 39.6 mAh cm−2 with coulombic efficiency and energy efficiency over 96% and 61%, respectively. Finally, results lead to increased efficiency and cycling stability for flow batteries. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) are emerging energy storage technologies due to their high availability, low cost of organic compounds, and the use of eco-friendly water-based supporting electrolytes. In the present work, we demonstrate a unique phenazine-based material that shows redox reversibility in neutral, basic, and acidic conditions with the redox potentials of −0.85 V (1.0 M KOH), −0.67 V (1.0 M NaCl), −0.26 V, and 0.05 V (1.0 M H2SO4) vs. the Ag/AgCl reference electrode and two-electron transfer process at all pH values. High solubility of the phenazine compound in water-based electrolytes up to 1.3 M is achieved by introducing quaternary amonium-based substituents, leading to the outstanding theoretical volumetric capacity of 70 Ah L−1. Laboratory redox flow batteries in neutral and acidic electrolytes presented >100 cycles of stable operation with a capacity loss of 0.25 mAh L−1 and 1.29 mAh L−1 per cycle, respectively. The obtained results demonstrate a material with the potential for not only fundamental understanding but also the practical application of AORFBs in the development of new-generation energy storage technologies. [ABSTRACT FROM AUTHOR]

The hydrogen–bromine redox flow battery is a promising energy storage technology with the potential for capital costs as low as 220 $ kWh−1 and high operational power densities in excess of 1.4 W cm−2. In this work, enhanced surface area bromine electrodes incorporating carbon black (CB) and graphene nanoplatelets (GnPs) on carbon paper and carbon cloth substrates were investigated, and the effect of electrolyte concentration on performance of the electrodes was studied. Carbon-black-modified electrodes are found to possess the largest electrochemically active surface areas, i.e., up to 11 times that of unmodified materials, while GnP electrodes are shown to have superior kinetic activity towards the bromine electrode reaction. In terms of performance, lower electrolyte concentrations are found to favour the improved kinetic parameters associated with graphene nanoplatelet electrodes, while highly concentrated electrolytes favour the larger electrochemically active surface area of carbon black electrodes. The optimal performance was achieved on a carbon-black-modified carbon cloth electrode in a 6 M HBr/2 M Br2 electrolyte concentration, with polarisation current densities approaching 1.6 A cm−2 at overpotentials of ±400 mV, and mean overpotentials of 364 mV during oxidation and 343 mV during reduction, resulting from bromine oxidation/reduction cycling tests at ±1.5 A cm−2. [ABSTRACT FROM AUTHOR]

The ever-increasing characteristics of microcomputers, sensors, actuators, and communication systems require more powerful and more compact autonomous power sources. Al/bromate and Al/iodate flow batteries are proposed as new power supply units for use in oxygen-deficient environments. The batteries employ a mechanically rechargeable aluminum anode flooded with aqueous salt electrolytes or seawater, a cation-exchange membrane, and a carbonaceous porous cathode, where acidified alkali metal bromate, or iodate, is reduced in a six-electron process. The theoretical energy density of an Al/bromate flow cell per reactants is 0.65 kWh kg−1. Seawater is assumed as an electrolyte for the anode compartment. Using a H2/iodate flow cell, it is shown that iodate–iodine–iodide electrochemical transformations can be realized in both directions in acidic media at carbonaceous electrodes. At 30 °C, the area-specific power of the single cells of the Al/bromate and Al/iodate flow batteries reaches 0.26 W cm−2 and 0.075 W cm−2, respectively. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries offer an environmentally benign, tunable, and safe route to large-scale energy storage. The energy density is one of the key performance parameters of organic redox flow batteries, which critically depends on the solubility of the redox-active molecule in water. Prediction of aqueous solubility remains a challenge in chemistry. Recently, machine learning models have been developed for molecular properties prediction in chemistry and material science. The fidelity of a machine learning model critically depends on the diversity, accuracy, and abundancy of the training datasets. We build a comprehensive open access organic molecular database "Solubility of Organic Molecules in Aqueous Solution" (SOMAS) containing about 12,000 molecules that covers wider chemical and solubility regimes suitable for aqueous organic redox flow battery development efforts. In addition to experimental solubility, we also provide eight distinctive quantum descriptors including optimized geometry derived from high-throughput density functional theory calculations along with six molecular descriptors for each molecule. SOMAS builds a critical foundation for future efforts in artificial intelligence-based solubility prediction models. Measurement(s) quantum descriptors • molecular descriptors • physical descriptors Technology Type(s) Computation • experiment [ABSTRACT FROM AUTHOR]

Tetrathiafulvalene (TTF) exhibits two reversible oxidation steps and is used as a novel multi‐electron catholyte for nonaqueous organic redox flow batteries. To increase solubility in polar organic solvents, TTF derivatives with polar side chains are synthesized. 4‐Methoxymethyltetrathiafulvalene emerges as a promising two‐electron catholyte because it is a liquid at room temperature and miscible with acetonitrile. Bulk‐electrolysis experiments and UV‐vis‐NIR absorption spectroscopy reveal excellent cycling stability for the first and second electrochemical oxidations. In the doubly oxidized state, the TTF derivatives show a reversible about 1 % loss of the state of charge per day, due to extrinsic effects. In a symmetric redox flow battery, 4‐methoxymethyltetrathiafulvalene shows a volumetric capacity loss of only 0.2 % per cycle with a Coulombic efficiency (CE) of 99.6 %. In an asymmetric redox flow battery with a pyromellitic diimide as two‐electron anolyte, the capacity loss is 0.8 % per cycle, with CE>99 % in each cycle. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) have been developed as safe and economical energy storage systems for renewable energy applications. Herein, the solubility of two‐electron‐transfer organic molecules, naphthalene diimides (NDIs), was improved by adding two propyl‐spaced ammonium functionalities. Under neutral conditions, the di‐ammonium‐functionalized NDIs exhibited a solubility of ∼0.7 M (1.4 M/e−) in water. Using 0.3 M NDIs as negolyte (negative electrolyte) and iodide/triiodide as posolyte (positive electrolyte), AORFBs achieved 300 galvanostatic cycles with approximately 100 % capacity retention. The post‐mortem analyses revealed negligible chemical decomposition with no crossover while using a Nafion membrane. This study presents a promising NDI negolyte that can achieve stable two‐electron transfer in AORFBs. [ABSTRACT FROM AUTHOR]

Cerium(III/IV) is a redox couple with promising applications in redox flow batteries and redox‐mediated water electrolyzers, but due to its high oxidation potential, parasitic oxygen evolution can occur and decrease the Faradaic efficiency (FE) of the oxidation reaction. Dynamic potential dosing can help balance the transport limitations and the electrooxidation rates, allowing for control over the cerium(III) concentration near the electrode, enhancing FE. Due to the large number of possible pulsing operating conditions, Bayesian optimization (BO) was used to rapidly identify the optimal potential pulses. With an active pulse voltage of 2.5 V vs. Ag/AgCl, the maximum FE achieved was 0.91 (active pulse time = 5 ms, resting pulse time = 136 ms), underscoring the need for long inactive times for cerium(III) diffusion to the electrode. Furthermore, a multiobjective BO was performed to identify optimal trade‐offs between FE and the partial current density to achieve improvements over constant current operation. [ABSTRACT FROM AUTHOR]

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An increasing number of electroactive compounds have recently been explored for their use in high-performance redox flow batteries for grid-scale energy storage. Given the vast and highly diverse chemical space of the candidate compounds, it is alluring to access their physicochemical properties in a speedy way. High-throughput virtual screening approaches, which use powerful combinatorial techniques for systematic enumerations of large virtual chemical libraries and respective property evaluations, are indispensable tools for an agile exploration of the designated chemical space. Herein, RedDB: a computational database that contains 31,618 molecules from two prominent classes of organic electroactive compounds, quinones and aza-aromatics, has been presented. RedDB incorporates miscellaneous physicochemical property information of the compounds that can potentially be employed as battery performance descriptors. RedDB's development steps, including: (i) chemical library generation, (ii) molecular property prediction based on quantum chemical calculations, (iii) aqueous solubility prediction using machine learning, and (iv) data processing and database creation, have been described. Measurement(s) Clean Database • Physicochemical properties of redox active molecules • aqueous solubility Technology Type(s) Calculation • Physics • machine learning [ABSTRACT FROM AUTHOR]

An anionic gamma-aminobutyric acid-functionalized naphthalene diimide (GABA-NDI) was investigated for its possible application in flow batteries. A GABA-NDI/ferrocyanide flow battery was stable over 200 cycles and demonstrated an average coulombic efficiency of 99.96% and an excellent energy efficiency of 80.9% at 60 mA cm−2 while accessing 95% of the theoretical capacity of GABA-NDI. An increase in solubility and stability was achieved compared to previous studies. [ABSTRACT FROM AUTHOR]

In recent years, the interest in redox flow batteries as stationary energy storage devices is growing, for which disordered three‐dimensional electrodes such as felts have been the state‐of‐the‐art due to their large surface area and improved mass transfer properties. However, such electrodes present a high pressure drop and operating cost. In this work, the performance of structured three‐dimensional electrodes in a redox flow battery was studied. Compared to the carbon felt electrode similar electrochemical behaviour was demonstrated with these structured electrodes, which were based on the most common static mixer designs, but at a power consumption that was two to three orders of magnitude lower. Moreover, due to its ordered electrode design unwanted side reactions were suppressed. Consequently, these results demonstrate the influence of structured static mixer based three‐dimensional electrodes in redox flow batteries and the opportunities they hold to surpass the current state‐of‐the‐art. [ABSTRACT FROM AUTHOR]

An iron complex, tris(4,4′‐bis(hydroxymethyl)‐2,2′‐bipyridine) iron dichloride is reported, which operates at near‐neutral pH with a redox potential of 0.985 V versus SHE. This high potential compound is employed in the posolyte of an aqueous flow battery, paired with bis(3‐trimethylammonio)propyl viologen tetrachloride in the negolyte, exhibiting an open‐circuit voltage of 1.3 V at near‐neutral pH. It demonstrates excellent cycling performance with a low temporal capacity fade rate of 0.07% per day over 35 days of cycling. The extended cycling lifetime is the result of low permeability and improved structural stability of the newly developed iron complex compared to that of the iron tris(bipyridine) complex. The combination of high redox potential and low capacity fade rate compares favorably with those of all previously demonstrated organic and organometallic aqueous posolytes. Extensive investigation into the possible degradation mechanisms, including post‐mortem chemical and electrochemical analyses, indicates that stepwise ligand dissociations of the iron complex are responsible for the reported capacity loss during cell cycling. This investigation provides unprecedented insight to guide further improvements of such metalorganic compounds for energy storage and conversion applications. [ABSTRACT FROM AUTHOR]

Aqueous flow batteries (AFBs) that utilize inorganic or organic redox-active species dissolved in aqueous electrolytes to store electrical energy attract a great deal of attention in large-scale energy-storage applications because of their high efficiency, long cycling life, high design flexibility, excellent safety and environment friendliness. However, unsatisfactory energy density is one main challenge for AFBs, which originates from the relatively low solubility of redox-active substances in aqueous electrolytes. Herein, we synthesized a water-insoluble fused phenazine derivative, 1,2:3,4-dibenzophenazine (DBPZ), by a facile Schiff-base condensation reaction, featuring a very negative redox potential and highly reversible two-electron transfer kinetics in aqueous alkaline electrolytes. A DBPZ anode with high mass loading is prepared by integrating DBPZ into carbon felt via a deep eutectic solvent (DES)-assisted preparation approach. Aqueous hybrid flow batteries (AHFBs) employing this high-loading DBPZ anode achieve a high current density, a large areal capacity and remarkable cycling stability. Furthermore, using the high-loading DBPZ anode as a capacity booster, an aqueous organic flow battery (AOFB) with a benzo[a]hydroxyphenazine-7/8-carboxylic acid (BHPC) anolyte and a K4[Fe(CN)6] catholyte shows a significantly enhanced capacity and excellent cycling durability due to the rapid redox kinetics, similar redox potential and high stability of both DBPZ and BHPC, as well as a redox-targeting reaction occurring between DBPZ and BHPC. [ABSTRACT FROM AUTHOR]

Iron–chromium redox flow batteries (ICRFB) possess the advantage of low raw material cost, intrinsic safety, long charge–discharge cycle life, good life-cycle economy, and environmental friendliness, which has attracted attention from academia and industry over time. The proton exchange membrane (PEM) is an important part of the ICRFB system, impacting the efficiency and lifetime of the battery. Currently, the most widely used PEMs in the market are per-fluorinated sulfonic acid (PFSA) membranes, which possess high electrolyte stability and achieve the separation of positive and negative electrolytes. In addition, the complex preparation process and extremely high market price limited the usage of PEM in ICRFB. In this paper, we developed a remanufactured membrane (RM) strategy from waste PFSA resins. The RM has higher electrical conductivity and better proton transport ability than the commodity membrane N212. In the cell performance test, the RM exhibits similar coulombic efficiency (CE) as N212 at different current densities, which is stabilized at over 95%. Furthermore, the voltage efficiency (VE) and energy efficiency (EE) of the RM are improved compared to N212. At a current strength of 140 mA cm−2, the degree of energy loss is lower in the RM, and after 60 cycles, the capacity decay rate is lower by only 16.66%, leading to long-term battery life. It is a cost-effective method for membrane recovery and reformulation, which is suitable for large-scale application of ICRFB in the future. [ABSTRACT FROM AUTHOR]

The mismatch between power generation and load demand causes unwanted fluctuations in frequency and tie-line power, and load frequency control (LFC) is an inevitable mechanism to compensate the mismatch. For this issue, this paper explores the influence of energy storage device (ESD) on ameliorating the LFC performance for an interconnected dual-area thermal and solar photovoltaic (PV) power system. Initially, to alleviate the frequency and tie-line power deviations, a proportional-integral (PI) controller is chosen and utilized in the system due to its effectiveness and simplicity in practice. For achieving the highest performance from this controller, salp swarm algorithm (SSA) is employed to search for optimal controller parameters by using integral of time-multiplied absolute error (ITAE) criterion. To affirm the contribution of SSA optimized PI controller, it is contrasted with a recent approach utilizing PI controller optimized by genetic algorithm (GA) and firefly algorithm (FA). It is observed that the results acquired for SSA are better than for GA and FA. To improve the system performance further, ESD such as redox flow battery (RFB) famous for its excellent disturbance rejection capability is integrated with the thermal power unit for the first time in the literature. It is divulged from the results that the system performance with RFB has boosted considerably with regard to shorter settling time, less undershoot/overshoot and smaller ITAE value of the frequency and tie-line power fluctuations. According to the sensitivity analysis, our proposal is found robust against system parameters variations and different loading conditions. [ABSTRACT FROM AUTHOR]

The proposed anthraquinone-bromate cell combines the advantages of anthraquinone-bromine redox flow batteries and novel hybrid hydrogen-bromate flow batteries. The anthraquinone-2,7-disulfonic acid is of interest as a promising organic negolyte due its high solubility, rapid kinetics of electrode reactions and suitable redox potentials combined with a high chemical stability during redox reactions. Lithium or sodium bromates as posolytes provide an anomalously high discharge current density of order ~A cm−2 due to a novel autocatalytic mechanism. Combining these two systems, we developed a single cell of novel anthraquinone-bromate flow battery, which showed a power density of 1.08 W cm−2, energy density of 16.1 W h L−1 and energy efficiency of 72% after 10 charge–discharge cycles. [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]

Non‐aqueous redox flow batteries (RFBs) are emerging electrochemical technologies for grid energy storage. Non‐aqueous Mg RFBs that use Mg metal as the anode are especially promising due to various benefits of the Mg metal anode, including its low potential, high volumetric capacity, SEI‐free, highly reversible operation and low cost. Despite the potential, there are rarely any studies on developing non‐aqueous Mg RFBs. Herein, a non‐aqueous Mg redox flow battery using a polymer catholyte is reported. Through rational molecular engineering, a carbonyl‐based moiety is combined with a polyethylene glycol moiety to achieve a polymer with high voltage and high solubility in the ether‐based electrolyte. A series of polymers with different polyethylene glycol chain lengths are synthesized and their performances are measured first at the molecular level, and then at the device level in a Mg redox flow battery using a Mg foil as the anode, the polymer solution as the catholyte and a porous membrane as the separator. The flow battery delivers a voltage of 1.8 V, a maximum capacity of 475 mAh/L, an average Coulombic efficiency of 90.5%, an average voltage efficiency of 67.4%, an energy efficiency of 61.0%, and an energy density of 0.855 Wh/L. Systematic mechanistic studies are performed to understand the performance decay mechanism and possible strategies for future improvement are discussed. This work opens a new avenue for the development of energy storage technologies for grid electricity storage. [ABSTRACT FROM AUTHOR]

Reduction of alkali or alkaline earth metal chlorates to chlorides is of great interest for its use as the positive-electrode process of a redox flow battery in view of very large theoretical estimates of its specific charge per solution unit mass or volume owing to high solubilities of the reagent and the product as well as to the six-electron transfer per one chlorate ion. Such use of this oxidizing agent requires overcoming a fundamental difficulty: the non-electroactivity of the chlorate anion at electrodes in the required potential range. Promising approach to the implementation of this process is using a mediator catalysis based on an Ox/Red redox couple which has a high positive potential and a fairly large exchange current, while its Red form is able reacting chemically in solution with the chlorate anion, reducing it to the chloride anion, with regeneration of the Ox form. Such a mediator cycle can be implemented in the positive part of the flow battery based on multiple pumping of the solution from the reservoir through the discharging device for the electrochemical conversion of the Ox form of the redox couple into the Red component, with the electricity generation. Meanwhile, the chemical stage, i.e., the reaction of the chlorate anion with the Red form takes place inside the reservoir. Theoretical analysis of the functioning of such a system under galvanostatic mode has been performed in this study. Time-variation of the component concentrations and the electrode potential has been predicted. Two different scenarios for the system evolution have been revealed, depending on the relationship between the passing current and its critical value. Treatment of experimental data for the variation of the electrode potential and the redox-couple component (Ox or Red) concentration are proposed, in order to establish the values of the system parameters including the critical current value and the rate constant of the chemical stage of the process. [ABSTRACT FROM AUTHOR]

Charge carriers have been studied for use in applications such as fuel cells, redox flow batteries, and electrochemical CO2 reactors for conversion to value-added products. Here, transient-based equilibrium models are developed for two well-known charge carriers: methyl viologen (MV) and ethyl viologen (EV). The models are simulated using Butler-Volmer kinetics until steady-state is reached. EV is favored over MV due to lower dimerization, and enabling over 2 × production of reduced EV+ over MV+. MV and EV products do not appear to significantly change, except only under sufficiently acidic conditions (pH < 4). Charge and energy input requirement are used to assess total system efficiency and potential for system scale-up via chronoamperometry. The charge and energy analysis performed with EV as the charge carrier reveals that optimal charging voltage is around − 0.8 to − 0.85 V vs. Ag/AgCl, which is above the minimum reduction voltage (around − 0.6 to − 0.7 V vs. Ag/AgCl) and suggest more favorable conditions for performing such charge carrier reductions. [ABSTRACT FROM AUTHOR]

We present a novel laboratory setup for studying the fluid dynamics in liquid metal batteries (LMBs). LMBs are a promising technology suited for grid-scale energy storage, but flows remain a confounding factor in determining their viability. Two important drivers of flow are thermal gradients, caused by internal heating during operation, and electrovortex flow (EVF), induced by diverging current densities. Our setup explores, for the first time, electrovortex flow combined with both adverse and stabilizing thermal gradients in a cylindrical layer of liquid gallium, simulating the behavior in a single layer of an LMB. In this work, we discuss the design principles underlying our choices of materials, thermal control, and current control. We also detail our diagnostic tools—thermocouple measurements for temperature and Ultrasonic Doppler Velocimetry probes for velocities—and the design principles which go into choosing their placement on the setup. We also include a discussion of our post-processing tools for quantifying and visualizing the flow. Finally, we validate convection and EVF in our setup: we show that scaling relationships between the nondimensional parameters produced by our data agree well with theory and previous studies. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) have been recognized as a promising technology for large‐scale, long‐duration energy storage of renewables (e.g., solar and wind) by overcoming their intermittence and fluctuation. However, simultaneous demonstration of high energy densities and stable cycling are still challenging for AORFBs. Herein, asymmetrically substituted sulfonate viologen molecular designs, e.g. (1‐[3‐sulfonatopropyl]‐1′‐[4‐sulfonatobutane]‐4,4′‐bipyridinium (3,4‐S2V), as capacity dense, chemically stable anolytes for cation exchange AORFBs are presented. The robust cycling performance of 3,4‐S2V is confirmed using half‐cell and full‐cell flow battery studies at pH neutral conditions. The 3,4‐S2V based AORFB is demonstrated with a discharge capacity of 23.2 Ah L−1 for 1700 cycles or 100 days without observing chemical degradation. Furthermore, a 3,4‐S2V/(NH4)4[Fe(CN)6] AORFB with a discharge capacity of 259.9 mAh is demonstrated for 50 days of authentic energy storage for the first time with a total capacity retention of 97.77% or a temporal capacity retention rate of 99.955% per day, representing the most stable, longest cycled AORFB to date. [ABSTRACT FROM AUTHOR]

The focus of the paper is on the renewable energy-based rural electrification system. Two concepts have been used in finding solutions to the issues related to this micro-grid system. The first is the best fitted model of hybrid configuration system, which utilizes village-owned resources, such as abundant biomass and solar irradiation. The second is that observed improved performance in terms of system sizing, techno-economic performance, and environmental stability of the hybrid model over different power storage backup media. The practical aspects and suggestion over the hybrid village electrification system have been presented through a case study of Korkadu village, Puducherry state, India. A comparative analysis of the hybrid model has been made with different power storage backup systems, namely the solid-state battery (lead acid-ASM battery, Li-ion-ASM battery) and flow battery (Redflow ZBM2 battery) arrangement. The result revealed hybrid rural electrification system with LI battery as the most favourable choice of the electrification system to the village, considering that its economic factor includes the total net present cost, cost of energy are found to be lowest. The technical parameters of power production capacity surface and battery state of charge were observed to be optimum with reduction in the environmental pollution parameter (GHS emission) value. [ABSTRACT FROM AUTHOR]

This article proposes a high-gain transformerless three-port converter (TPC) for standalone photovoltaic (PV) systems. The TPC is designed and developed based on a dual-inductor high-gain two-port converter by utilizing one of the buffer capacitors to derive the third part for PV input. A hybrid pulse-frequency modulation scheme unified with a pulsewidth modulation control strategy is adopted, which realizes the maximum power point tracking control, load voltage regulation, and bidirectional energy flow at the battery port. The proposed TPC offers the unique advantages of high voltage gain, continuous battery port current, reduced power semiconductors, lower voltage stresses, common ground shared by all the ports, low-cost gate driver, and small size of the rear-end inductor. The working principle, steady-state characteristics, small-signal models, and the control method, including design conditions, are comprehensively analyzed. The correctness of the theoretical analysis is verified by developing a 300-W experimental prototype, which shows that the maximum efficiency is 97.7%. [ABSTRACT FROM AUTHOR]

Zinc–iron redox flow batteries (ZIRFBs) possess intrinsic safety and stability and have been the research focus of electrochemical energy storage technology due to their low electrolyte cost. This review introduces the characteristics of ZIRFBs which can be operated within a wide pH range, including the acidic ZIRFB taking advantage of Fen+ with high solubility, the alkaline ZIRFB operating at a relatively high open-circuit potential and current densities, and the neutral ZIRFB providing a non-toxic, harmless, and mild environment. No matter what kind of ZIRFB, there are always zinc dendrites limiting areal capacity on the anode, which has become an obstacle that must be considered in zinc-based RFBs. Therefore, we focus on the current research progress, especially the summarizing and analysis of zinc dendrites, Fe(III) hydrolysis, and electrolytes. Given these challenges, this review reports the optimization of the electrolyte, electrode, membrane/separator, battery structure, and numerical simulations, aiming to promote the performance and development of ZIRFBs as a practical application technology. Based on these investigations, we also provide the prospects and development direction of ZIRFBs. [ABSTRACT FROM AUTHOR]

There are increasing numbers of scientific articles dedicated to developments in the field of redox flow batteries. To date it is most common to provide efficiency values as a measure of performance. However, there are no agreed standard experimental conditions for these measurements, and so their merit as a tool for comparing different innovations among research groups is put into question. In the following manuscript, various experimental precautions are outlined to reduce experimental artefacts. Original experimental measurements on vanadium flow cells, together with data from the literature, are examined to explore efficiencies and two alternative benchmarking metrics: resistivity and self-discharge current density. The sensitivity of these parameters to current density, temperature, flow rate and state-of-charge range are examined, from which it is concluded that resistivity and self-discharge current density exhibit superior properties to efficiencies for quantifying flow battery improvements. [ABSTRACT FROM AUTHOR]

Energy management strategy (EMS) works as an exchange where the allotment of power is decided between different sources in the hybrid energy storage system (HESS). While designing EMS, the performance indicators of the HESS, like voltage and state-of-charge of sources, dc-link voltage, and battery power delivering rate, should be considered for extended battery operation to enhance the vehicle performance effectively. The rate limiter restricts the rate of power flow from the battery and thus protects the battery from a high current rate, ensuring extended operation. This paper proposes a modified topology and EMS for controlling performance indicators with rate limiter operation. The HESS consists of one battery and two ultracapacitor banks. The auxiliary ultracapacitor is used to counter the effect of the rate limiter on vehicle dynamics. The auxiliary battery with reserve capacity is considered to run the vehicle in an emergency condition. This auxiliary battery storage is integrated with renewable (solar) as a standby provision. The proposed schemes are capable of providing supervisory control over performance indicators. It is evident from the simulation results that the proposed scheme saves 11.79% of battery energy for a designed load torque as compared with a battery-alone electric vehicle. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFB) have emerged as one of the most promising technologies for large‐scale energy storage owing to their high safety, long operation life, and decoupled design of energy and power. However, the problems of high cost and low energy density restrict their further development. The cost merit and tunable structure of organic redox‐active materials have prompted the development of organic RFBs. The solubility of the redoxmer is recognized as a parameter that contributes directly to the energy density. Herein, we focus on strategies for enhancing the solubility of organic redoxmers in aqueous RFBs. The effects of incorporating different hydrophilic functional groups on the solubility of the redoxmer and its effect on the performance of other batteries are systematically and exhaustively described. Other strategies, such as molecular symmetry tuning and employing more soluble counterions and cosolvents, are also summarized. The development trends and prospects for organic RFBs are also discussed. [ABSTRACT FROM AUTHOR]

The rational design of ion exchange membranes (IEMs) is becoming more pertinent as their usage becomes broader and as their staple applications (i.e., electrodialysis, flow batteries, and fuel cells) improve in commercial viability. Such efforts would be catalyzed by an improved fundamental understanding of ion transport in IEMs. This review discusses recent progress in modeling ion partitioning and diffusion in IEMs in an effort to relate IEM performance metrics to fundamental membrane properties over which researchers and membrane manufacturers possess direct and sometimes precise control. Central focus is given to the Donnan‐Manning model for ion partitioning and the Manning‐Meares model for ion diffusion in IEMs. These two frameworks, which are derived from Manning's counter‐ion condensation theory for polyelectrolyte solutions, have been widely used within the IEM literature since their recent introduction. To explore this topic, the mathematical derivation of both models is revisited, followed by a survey of experimental and computational discussions of counter‐ion condensation in IEMs. Alternative models which fulfill similar roles in predicting IEM transport properties are compared. This review concludes by highlighting the uniquely favorable positions of the Donnan‐Manning and Manning‐Meares models and discussing their prospects as leading predictors of IEM partitioning and diffusive properties. [ABSTRACT FROM AUTHOR]

The integrated design of solar energy conversion and storage systems has attracted increasing attention, and non-spontaneous redox reactions driven by dual photoelectrodes provide a potential solution to this issue. This study presents a solar rechargeable flow battery (SRFB) that combines dual photoelectrodes (BiVO 4 or Mo–BiVO 4 as photoanode, polyterthiophene (pTTh) as photocathode) with cost-effective redox pairs (Fe3+/Fe2+ and Br 3 −/Br−). The system charges under simulated solar illumination (100 mW∙cm−2, AM 1.5G) and releases stored energy controllably as electrical energy. Research indicates the Mo–BiVO 4 photoanode and pTTh photocathode achieve an open-circuit voltage of 0.34 V and a short-circuit current density of 0.38 mA cm−2. Using a specially designed Fe3+/Fe2+-Br 3 −/Br− (Fe–Br) SRFB device made via 3D printing, a charging photocurrent of approximately 1.9 mA cm−2 is attained. In constant current discharge tests, an initial discharge voltage of 0.23 V is observed at 0.1 mA∙cm−2 within the 10 discharge cycles, demonstrating system stability and offering a viable solution for low-cost, large-scale solar energy storage and conversion. • A novel all-in-one solar rechargeable flow battery was designed. • Mo–BiVO 4 and pTTh dual photoelectrodes enables solar-charging of Fe–Br flow battery. • The proposed SRFB system achieved a photocharging current of 1.9 mA cm−2. • The SRFB exhibits stable charge-discharge performance in multiple cycles. • The construction of SRFB provides cost-effective promise for the utilization of solar energy. [ABSTRACT FROM AUTHOR]

[Display omitted] • Uniform and ultrathin two-dimensional bismuth nanosheets are decorated on graphite felt (BiNS/GF). • Bismuth nanosheets can inhibit HER and induce homogeneous zinc deposition. • Total internal reflection imaging reveals the instability of zinc deposition caused by micro-bubbles. • BiNS/GF exhibits an average CE of 99.2 % in 300 cycles in aqueous neutral zinc-iron flow battery. • BiNS/GF achieves a peak power density of 295.2 mW cm−2 in zinc-iron flow battery. The poor cycling durability and low Coulombic efficiency (CE) of zinc (Zn) anodes substantially restrict their further development, which should be attributed to the severe uneven plating and hydrogen evolution reaction (HER) across the repeated plating/stripping processes. Herein, bismuth nanosheets decorated graphite felt (BiNS/GF) is constructed for Zn anode via galvanic replacement reaction. BiNS/GF affords more robust Zn nucleation sites, lower Zn nucleation overpotential and higher HER overpotential to guide even and dense Zn deposition. In addition, a total internal reflection imaging method is employed to detect micro-bubbles generated by HER on the electrode surface, revealing that BiNS prevents the formation of micro-bubbles and thus avoids dead Zn caused by unstable Zn deposition. As a proof of concept, BiNS/GF exhibits an average CE of 99.2 % in 300 cycles and a peak power density of 295.2 mW cm−2 at a current density of 380 mA cm−2 in aqueous neutral zinc-iron flow batteries. Therefore, it is reasonably anticipated that the BiNS/GF may emerge as suitable electrode for other aqueous zinc-based flow batteries. [ABSTRACT FROM AUTHOR]

• Add three different shapes of obstruction in the main channel of flow field. • Achieved the highest concentration uniformity of the active species (0.903). • The efficiency based on output power experienced a maximum improvement of 2.5%. • Exhibit the lowest charging voltage and the highest discharging voltage. All-vanadium redox flow batteries (VRFBs) are pivotal for achieving large-scale, long-term energy storage. A critical factor in the overall performance of VRFBs is the design of the flow field. Drawing inspiration from biomimetic leaf veins, this study proposes three flow fields incorporating differently shaped obstacles in the main flow channel. These designs aim to enhance the electrolyte diversion into branches, achieving a more uniform electrolyte distribution. Among these, the structure featuring circular obstacles demonstrated superior performance, exhibiting the lowest charging voltage and the highest discharging voltage. It also achieved the highest concentration uniformity of the active species (0.903). In comparison, at the flow rate of 3ml/s and the current density of 40mA cm−2, the efficiency based on pump power saw a maximum increase of 1.7 % and the efficiency based on output power experienced a maximum improvement of 2.5 %. This work offers a new direction in studying biomimetic flow field design and holds considerable potential for practical engineering applications. [ABSTRACT FROM AUTHOR]

Poly(thiophene-3‑boronic acid) (PTBA) was studied as a promising active material for aqueous environments in this paper. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), the solubility and electrochemical behavior of it was studied in a range of aqueous solutions. Fourier Transform Infrared Spectrometry (FTIR) results verify the successful synthesis. PTBA shows promising solubility qualities in certain pH ranges, especially in alkaline solutions. However, among alkaline, neutral, and acidic environments the best environment for redox properties of aqueous 1 mM PTBA is the neutral one. The peak current (i p) of 1 mM PTBA for 100 m V /s in the neutral environment is 0.01 mA and half peak potential (E p/2) is −0.1 V (vs Ag/AgCl). Diffusion coefficient of PTBA is found as 4.97 × 10−8 cm2/s. The impedance tests also confirm that the neutral solvent decreases the charge transfer resistance. [Display omitted] • Electrochemical properties of Poly(thiophene-3-boronic acid) (PTBA) in aqueous environment was studied. • Three different aqueous solutions and molarities were prepared (1 M HCl, 1 M NaCl, 1 M NaOH; 1 mM, 5 mM, 10 mM). • Nine different solutions were subjected electrochemical testing using CV and EIS. • The ideal scenario for electrochemical tests was found to be 1 mM PTBA with 1 M NaCl. • The best solubility was observed when using with 1 M NaOH. [ABSTRACT FROM AUTHOR]

Composite bipolar plates (BPP) composed of conductive carbon and insulating resin are promising for electrochemical systems due to their combined mechanical strength, electrical conductivity, and chemical stability. However, the optimal BPP characteristics vary significantly. While both metallic and composite BPP are commonly used in fuel cells, redox flow batteries (RFB) preferred composite BPP that prioritize corrosion resistance and minimal leakage. Conversely, electrolysers may require alternative BPP materials due to potential limitations with carbon-based options. The current obstacles are also examined, and other potential areas for further research are suggested. Currently, there has not been a comprehensive review study published on the topic of BPP technology specifically for the fuel cell, RFB, and electrolyser system. By critically evaluating BPP technology across these diverse electrochemical systems, this review provide guideline for selection of appropriate strategies for optimizing the efficiency of each device type. [Display omitted] • Characteristic and performance of composite BPP in electrochemical device. • PEMFC and RFB system favoured composite BPP. • Composite BPP in electrolyser have some limitations that effect the performance. • Challenges and innovation associated with BPP technology. [ABSTRACT FROM AUTHOR]

Several obstacles impede renewable energy penetration into the global energy sector. Amongst the apparent challenges which require credible research attention, is the selection of appropriate energy storage technologies in the context of intermittent renewable resources and frequent grid outages. Given these, the present study investigated the technical, economic, and environmental effects of an erratic national grid on four distinct battery technologies in hybrid wind, solar, and diesel energy systems. The study was conducted at Baze University Abuja, Nigeria using the Hybrid Optimization Model for Electric Renewables software. The battery types considered are vanadium redox flow battery (VRB), lead-acid battery, nickel-iron battery, and lithium-ion battery (LIB). The VRB-based hybrid energy systems demonstrated superior performances in meeting the electricity demands of the university at the lowest net present cost, levelized cost of energy, and carbon dioxide emissions of $6,328,003.00, $0.0722/kWh, and 21,754 kg/year respectively. This battery technology is characterized by storage depletion, throughput, and losses of 796 kWh/year, 406,570 kWh/year, and 182,757 kWh/year respectively. The sensitivity analysis showed that the frequency and duration of main grid outages affect the optimal systems' economics, component sizes, battery energy losses, battery energy storage depletion, and renewable energy penetration. • Performance of HRES utilizing different battery technologies has been analyzed. • The analysis considered grid outage frequency and mean repair time. • Frequent grid outage impacts battery losses, storage depletion, and state of charge. • Compared to other batteries, VRB is superior in complementing HRES operation. • HRES performance can be improved by selecting an appropriate battery technology. [ABSTRACT FROM AUTHOR]

The introduction of the CoHCF redox mediator accelerated the reaction kinetics of iodine and increased its utilization. The Zn-I aqueous flow battery employing the redox mediator-modified electrode achieved a high capacity reaching 95.59% of the theoretical amount. [Display omitted] • A redox mediator was designed to support enhancing the utilization of iodine. • The redox-mediating process accelerates reaction kinetics of iodine with reduced activation energies. • AIRFB applying modified electrode reaches 95.59% of the theoretical capacity. Aqueous iodine redox flow batteries (AIRFBs) have been identified as a promising technology for large-scale energy storage. However, practical capacity of AIRFBs is limited by the relatively low utilization of iodine caused by the low reaction kinetics. Here, we report an electrode modified by cobalt hexacyanoferrate (CoHCF), which supports a redox-mediating process developing alternative redox reaction pathways of iodine with reduced redox activation energies. Contributed by the strong adsorption of iodine by CoHCF, redox reaction kinetics of iodine species is accelerated, and the utilization of iodine in batteries is increased. Applying the CoHCF modified carbon felt as cathode electrode, the constructed zinc-iodine redox flow battery exhibits a high iodine utilization reaching 95.59% of the theoretical capacity at a current density of 20 mA cm−2, which enhances 139.89% comparing to the battery without electrode modification. The redox mediator-based electrode enabling fast reaction kinetics and high utilization of iodine species offers a promising strategy enhancing the battery performance thereby advancing the development of AIRFBs. [ABSTRACT FROM AUTHOR]

• Introduced the pioneering 3D electrochemical flow coupling model for ICRFBs. • Optimal VE pump achieved with a 4 mm spacing between interdigitated flow channels. • Employed machine learning for enhanced analysis of ICRFBs performance. • Impact on VE pump : current density > channel spacing > flow rate. This study explores the impact of interdigitated flow channel spacing on electrolyte distribution and flow velocity in porous electrodes, thereby affecting pump consumption and system efficiency. Utilizing a 3D electrochemical flow-coupled model, the research investigates the electrochemical performance and energy efficiency across various channel spacings, specific flow rates, and current densities. It elucidates the mechanisms by which interdigitated channel spacing influences battery performance. Through controlled trial-and-error, the optimal spacing for flow channels in Iron-Chromium Redox Flow Batteries (ICRFBs) was determined to be 4 mm. At a current density of 140 mA/cm2, the voltage efficiency reached 86.3 %, and the pump-based voltage efficiency achieved 85.9 %. To quickly and efficiently explore the impact of each individual parameter on battery efficiency and identify the optimal operating conditions, this study further developed a multi-task machine learning (ML) model. Initially trained on simulation data, the model achieved high predictive accuracy (R2 > 0.88) and effectively linked the interdigitated flow field design of ICRFBs with current density, specific flow rate, and channel spacing. The ML model was then validated through further investigation to ensure its accuracy and to achieve optimum performance efficiency as recommended by the model. This integrated approach offers a comprehensive understanding of RFBs performance under varying conditions, driving advancements in RFBs technology. [ABSTRACT FROM AUTHOR]

• Stable derivatives of DiPhenOx are synthesized and characterized as anolytes. • Cyano and ester groups improve electrochemical stability of parent molecule. • Adding an ester group does not always increase solubility in non-aqueous solutions. • DiPhenOx derivatives paired with catholytes enable >3 V non-aqueous redox flow battery. Effective utilization of energy from renewable sources such as wind and solar requires the development of long duration energy storage (LDES) systems that can accommodate intermittent energy accrual. One option under investigation is the use of a redox flow battery (RFB). A significant amount of work has explored aqueous RFB systems with a variety of inorganic and organic carriers. However, moving to a nonaqueous solvent such as acetonitrile (MeCN) for RFB provides a much larger electrochemical window, which could lead to increased energy density if properly utilized. In this work, we investigate a series of 2,5-diphenyl-1,3,4-oxadiazole (DiPhenOx) derivatives as anolytes for a redox flow battery. DiPhenOx has a low voltage redox event that, while reversible by cyclic voltammetry, was determined to be irreversible during bulk electrolysis. To improve cycling performance, we introduced various ester and cyano groups to the phenyl rings of DiPhenOx using molecular engineering. We characterized these derivatives spectroscopically and electrochemically to assess their feasibility for flow battery applications. The ester derivatives with the best cycling performance were tested in a flow cell vs. ferrocene, 2,5-di- tert -butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB) and thianthrene, which resulted in ∼2 V, ∼3 V and ∼3 V redox flow batteries, respectively. [ABSTRACT FROM AUTHOR]

We design the crossover-free N -heterocycle-linked TEMPO and symmetry-breaking bipyridinium redox pair for high-voltage, two-electron and stable aqueous organic redox flow batteries (AORFBs). This innovative work emphasizes the mild synthesis of new high water-soluble catholyte and anolyte materials based on TEMPO and bipyridinium cores, as well as the two-electron capability of bipyridinium for AORFBs. [Display omitted] • The neat N -heterocycle-substituted TEMPO and asymmetric bipyridinium redox pair are designed with minimal synthetic steps. • The high water-soluble redox pair display a crossover-free feature through a commercially available membrane. • The high-voltage, two-electron and stable aqueous organic flow batteries are achieved. • The X-ray single-crystal structure of N -heterocycle-substituted TEMPO is obtained. • The versatile design can extend to fabricating various amino-functionalized TEMPO and asymmetric bipyridinium derivatives. Aqueous organic redox flow batteries (AORFBs) are the brilliant technologies for safe and sustainable stationary energy storage. However, the cross-contamination, limited cell voltage, and inferior cycling stability remain challenges. Herein, the N -heterocycle-substituted TEMPO (TMP-TEMPO) and pyrrolidinium-/ammonium-grafted bipyridinium ([PyrTMAV]Cl 4) redox pair with multiple charges are designed for high-performance AORFBs. The whole material preparation is relatively simple and only needs two or three synthetic steps. The TMP-TEMPO and [PyrTMAV]Cl 4 show high aqueous solubility of 2.4 M and 1.71 M, respectively, excellent electrochemical reversibility, and fast redox kinetics. Notably, the introduction of multiple charges into the active materials can produce a strong Gibbs-Donnan effect with ion exchange membranes, thus preventing the permeability of molecules through the membrane and avoiding the cross-contamination of active molecules. By using the crossover-free TMP-TEMPO and two-electron [PyrTMAV]Cl 4 redox pair, the assembled two-electron AORFBs at an electron concentration of 0.2 M exhibit stable battery performance in environments with O 2 contents of both 10 ppm and 100 ppm with a capacity retention of over 99 % per day for hundreds of cycles (268 and 310 cycles), and a high energy efficiency of ∼89 % and Coulombic efficiency of ∼100 %. Furthermore, the AORFB achieves a high capacity of ∼37.4 Ah L−1 at an ultra-high electron concentration of 1.5 M, while maintaining a capacity retention rate of ∼99.7 % over 138 cycles and a theoretical capacity utilization rate of 93 %. The versatile molecular design for TMP-TEMPO and [PyrTMAV]Cl 4 can be easily extended to the synthesis of various amino-functionalized TEMPO and asymmetric bipyridinium derivatives. [ABSTRACT FROM AUTHOR]

• A 3D model integrating fluid flow, mass transport, and electrochemistry is established. • The relationship between geometric parameters and under-rib convection is studied. • The optimized channel fraction and channel depth are identified. • The battery can deliver a pump-based efficiency of 91.4 % at 150 mA cm−2. The under-rib convection, driven by the pressure differences between neighboring channels, plays a crucial role in determining the performance of flow-field structured vanadium redox flow batteries. However, the correlation between key geometric characteristics and under-rib convection is unclear, limiting the exploration of flow-related transport mechanisms and the development of high-performance flow fields. In this work, a three-dimensional model integrating fluid flow, mass transport, and electrochemical reactions is developed, and the under-rib convection is enhanced by tailoring the critical geometric characteristics adopting a rotary serpentine flow field as an example. Results show that variations in channel fraction and channel depth can mitigate concentration polarization by influencing the active material transport velocity within a localized region (i.e., under-rib convection intensity) without deteriorating the distribution of active material transport. More remarkably, the battery with the optimal geometric characteristics is able to deliver a preferable total pump-based efficiency (91.4 %) at a current density of 150 mA cm−2 and a flow rate of 40 ml min−1. This study offers a comprehensive analysis of the correlation between flow field geometric characteristics and under-rib convection to serve as guidance for the design of high-performance vanadium redox flow batteries. [Display omitted] [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) can potentially revolutionize large-scale energy-storage technologies for both conventional (fossil fuel) and modern (renewable) electric power systems. This has stimulated the development of new methods for reducing the costs of such batteries by lowering material costs and increasing the energy density. Driven by the abundance and low costs of sulfur and bromine salts, this study investigates the viability of an aqueous flow battery system, in which sodium bromide (NaBr) is used as a catholyte, and a novel electrolyte called elemental added sulfur sodium polysulfide (EASSP) is utilized as an anolyte. The molar ratio of elemental sulfur to sodium (S/Na) in the sodium polysulfide solution is maintained at 1:4. Various concentrations of the EASSP and NaBr electrolytes are examined, and their optimal values corresponding to the minimum overpotential are determined. A bromine–polysulfide redox flow battery (BPRFB) containing a 1.4 M EASSP anolyte and 8 M NaBr catholyte is charged and discharged for 5 min, which results in a capacity of 17 mAh (3.4 mAh cm−2) at a current density of 40 mA cm−2. The cyclic performance of the BPRFB is characterized by a capacity of 17 mAh at 40 mA cm−2, which is retained over 150 cycles with a 98 % coulombic efficiency. The assembled BPRFB cell with the optimized electrolyte composition (1.4 M EASSP: 8 M NaBr) exhibits constant current (without leakage) and potential. The findings of this work demonstrate a high application potential of the developed anolyte for BPRFBs, which is highly soluble in water, inexpensive, and can be scaled up for energy storage applications. [Display omitted] • Novel anolyte of elemental added sulfur sodium polysulfide (EASSP) for aq. RFBs. • Reduction in overpotential observed for optimized conc. of anolytes and catholytes. • Reused thermally activated GF shows charge/discharge capacities of 10 and 7.5 mAh • Assembled RFB with optimized electrolytes shows consistent current and voltage. [ABSTRACT FROM AUTHOR]

The practical implementation of Zn-based flow batteries encounters the challenges associated with uneven deposition of Zn ions and undesirable side reactions. Here, a hybrid Zn-based electrolyte system composed of ZnBr 2 , ethylene glycol (EG), H 2 O and potassium gluconate (KGlu) is developed as anolyte to modulate the solvation structure and interface engineering for a sustainable Zn-based flow battery. The EG molecules could exclude water molecules outside the solvation structure, inhibiting the water-induced side reactions and Zn corrosion. Moreover, the incorporation of potassium gluconate constructs an artificial stable anionic interface for dendrite-free Zn deposition. Chemical stability and hydrogen evolution potential tests demonstrate that the activity of water molecules could be suppressed in the EG-containing hybrid electrolyte. Deposition morphologies and Zn//Zn symmetric flow battery tests also reveal that gluconate anions preferentially adsorbed on zinc anode could effectively facilitate uniform zinc deposition. As a result, the Zn-based flow battery with the proposed hybrid electrolyte delivers a stable cycling performance over 200 cycles and high reversibility with an average CE of over 97.4 % at 20 mA cm−2, exhibiting a peak power density of 103.2 mW cm−2. This work provides a universal electrolyte design strategy for realizing a sustainable Zn-based flow battery. • A hybrid electrolyte based on ZnBr 2 , EG, H 2 O and potassium gluconate is developed. • EG molecules exclude water molecules and inhibit water-induced side reactions. • Adsorbed gluconate anions facilitate stable Zn plating/stripping reactions. • Zn//Zn symmetric cell exhibits a stable cycling over 130 h at 20 mA cm−2. • Zinc-iron flow battery displays a relatively high cycling stability over 200 cycles. [ABSTRACT FROM AUTHOR]

Crossover of vanadium ions and consequent capacity loss during the cycling of the vanadium redox flow batteries remain an issue for its long-term operation. This work describes novel methods to study the diffusion of vanadium ions through cation (FS-940) and anion (FAP-450) exchange membranes to understand the vanadium crossover processes during charging and resting of the battery. The proposed methods are implemented for ∼0 % SoC and ∼100 % SoC of the battery. The experimental setup do not require any additional technique besides a potentiostat/battery tester and therefore can be widely used in flow battery studies. The self-discharge method provided a facile determination of vanadium ions diffusion coefficients from the linear relation of capacity change, ΔQ, over the charging and resting period of the battery. The steady-state current method described the effect of applied constant voltage on the diffusion of vanadium ions during cycling. The electrochemical titration method described the diffusion of vanadium ions based on the linear relation between ΔQ during cycling of the battery with enriched and deficient half-cells. The results of these methods are in good agreement with each other with good accuracy as well as with the literature. [Display omitted] • Novel methods to evaluate V ions diffusion and migration intensities are proposed. • Requiring no additional devices, they can be incorporated into standard VRFB tests. • The methods give close results and are validated for 0 % and 100 % SoC of the battery. • The diffusion coefficients for VO 2 + and VO2+ ions are determined. • Transport properties of FS-940 and FAP-450 membranes are compared. [ABSTRACT FROM AUTHOR]

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]

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]