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A water‐soluble anthraquinone derived from alizarin, 3HAAQ, is introduced as the redox‐active material in a negative potential electrolyte (anolyte) for aqueous redox flow batteries operating at pH 14. The synthesis of 3HAAQ is carried out using the Mannich reaction, which significantly improves the solubility of the new compound, an important factor for its use in RFB. Pairing with potassium ferri/ferrocyanide positive electrolyte, this flow battery exhibits an open‐circuit voltage of 1.24 V and maintains nearly 80% of the theoretical capacity at 40 mA cm−2 current density. [ABSTRACT FROM AUTHOR]

Development and application of large‐scale energy storage systems are surging due to the increasing proportion of intermittent renewable energy sources in the global energy mix. Redox flow batteries are prime candidates for large‐scale energy storage due to their modular design and scalability, flexible operation, and ability to decouple energy and power. To date, several different redox couples are exploited in redox‐flow batteries; some are already commercialized. This battery technology is facing a lot of challenges in the science, engineering, and economic front. Issues plaguing flow batteries are low energy density, high overall cost, poor stability of electrolytes, shifting of solvent from anolyte to catholyte while using cation exchange membrane, reverse flow with anion exchange membrane, and corrosion of graphite felt in the catholyte side. Significant research efforts are ongoing to address these challenges. This comprehensive and critical review summarizes the recent progress in electrolyte technologies, including electrochemical performance and stability, strategies to enhance the energy and power densities and, in the end, the levelized and life‐cycle cost of these batteries analyzed. A comprehensive outlook on this technology with respect to practical energy storage applications is also provided. [ABSTRACT FROM AUTHOR]

This review exploits the crucial role of computational methods in discovering and optimizing materials for redox flow batteries (RFBs). Integration of high‐throughput computational screening (HTCS) and machine learning (ML) accelerates materials discovery, guided by algorithms categorizing RFBs. A collaborative exploration, spanning macroscopic to mesoscopic scales, combines quantum machine learning with reinforcement learning, transfer learning, time series analysis, Bayesian optimization, active learning and various generative models. The collaborative integration of ML with computational techniques and experimental methods, anchored in experimentally validated Density Functional Theory (DFT) calculations and molecular dynamics (MD) simulations, proves indispensable for cost‐effective RFBs. Data collection and feature engineering are explored, emphasizing the integration of optimization goals and precise data collection within the ML framework. Feature analysis importance is highlighted, utilizing methods such as the filter, embedded, wrapper and deep learning methods for efficient energy materials exploration. Computational perspectives on materials features and operating conditions encompass membrane characteristics, fluid dynamics, temperature dependence and pressure sensitivity. Time‐dependent features and ML‐generated insights are crucial for understanding cycling performance intricacies, providing a comprehensive understanding of RFB materials. [ABSTRACT FROM AUTHOR]

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

The 6-electron electrochemical reduction of IO3− to I− represents a breakthrough for the development of next-generation redox flow batteries, offering substantially higher energy densities for oxidizer storage. Our study reveals that on a glassy carbon (GC) electrode in acidic electrolytes, HIO3 undergoes an autocatalyzed electrochemical reduction to I−. This process is mediated by the formation of a thin iodine layer on the electrode, acting as an intermediate and a catalyst. Under steady-state conditions, the iodine layer forms via a comproportionation reaction (HIO3 + I− + 5H+ = I2 (s) + 3H2O). Initially, the iodine layer is generated through the slow direct electrochemical reduction of HIO3 on pristine GC. Once established, this layer significantly enhances the rate of iodate reduction. On voltammetry curves, it is clearly observable as a step-wise current surge to reach a plateau. The limiting current density on the GC seemingly aligns with the Levich equation, varying with the RDE rotation rate. Earlier, we demonstrated the electrochemical oxidation of I− back to HIO3 using an H2/HIO3 flow cell, showcasing a full cycle that underpins the feasibility of this approach for energy storage. This study advances the understanding of iodate electroreduction and underscores its role in enhancing the capacity of next-generation energy storage systems. [ABSTRACT FROM AUTHOR]

Considering the costs, waste, and impact on the environment of current energy consumption, accurate, cost‐effective, and safely deployed energy storage systems are required. Lithium (Li)‐free redox flow batteries (RFBs) are a feasible solution. RFBs can store enormous amounts of energy effectively and are increasingly used for large‐scale applications. The use of RFBs has significantly enhanced the performance of energy storage systems and effectively reduced the costs and wastage of energy storage operations. Vanadium‐based RFBs are an emerging energy‐storage technology being explored for large‐scale deployment owing to their numerous benefits, including zero cross‐contamination, scalability, flexibility, extended life cycle, and nontoxic working state. This study describes the fundamental operating principles of redox flow battery‐based systems as well as the design considerations and constraints placed on each component. It discusses recent progress in the design and deployment of RFBs for energy‐related applications and the remaining obstacles and prospects. Finally, this study highlights the enormous potential of RFBs and suggests some solutions to scale up the use of RFBs in the near future. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are a promising option for long‐duration energy storage (LDES) due to their stability, scalability, and potential reversibility. However, solid‐state and non‐aqueous flow batteries have low safety and low conductivity, while aqueous systems using vanadium and zinc are expensive and have low power and energy densities, limiting their industrial application. An approach to lower capital cost and improve scalability is to utilize cheap Earth‐abundant metals such as iron (Fe). Nevertheless, all‐iron RFBs have many complications, involving voltage loss from ohmic resistance, side reactions such as hydrogen evolution, oxidation, and most significantly electrode plating, and dendrite growth. To address these issues, researchers have begun to examine the effects of various alterations to all‐iron RFBs, such as adding organic ligands to form Fe complexes and using a slurry electrode instead of common materials such as graphite or platinum rods. Overall, progress in improving aqueous all‐iron RFBs is at its infant stage, and new strategies must be introduced, such as the utilization of nanoparticles, which can limit dendrite growth while increasing storage capacity. This review provides an in‐depth overview of current research and offers perspectives on how to design the next generation of all‐iron aqueous RFBs. [ABSTRACT FROM AUTHOR]

Cerium, a unique rare earth element, possesses a relatively high abundance, low cost, and high redox voltage, making it an attractive candidate for redox flow batteries. However, the sluggish kinetics and corrosion nature of the Ce3+/Ce4+ electrolyte result in overpotential and degradation of carbon felt (CF) electrodes, which hinders the development of cerium‐based flow batteries. Therefore, it is essential to develop an electrode with high catalytic activity and corrosion resistance to the Ce3+/Ce4+ electrolyte. Herein, a TiC/TiO2 coated carbon felt (TiC/TiO2‐CF) electrode is proposed. Remarkably, the TiC/TiO2 coating effectively minimizes the exposure of the CF to the highly corrosive cerium electrolyte, consequently enhancing the electrode's corrosion resistance. Additionally, X‐ray photoelectron spectroscopy and high‐resolution transmission electron microscopy characterizations reveal the formation of a heterojunction between TiC and TiO2, which significantly enhances the redox reaction kinetics of the Ce3+/Ce4+ redox couple. Eventually, the practical application of TiC/TiO2‐CF catalytic electrode in a Ce–Fe flow battery is demonstrated. This study sheds light on the synthesis conditions of the TiC/TiO2‐CF electrode, elucidates its heterojunction structure, and presents a novel Ce–Fe flow battery system. [ABSTRACT FROM AUTHOR]

Pseudo‐nanophase separation enabled by supramolecular‐interaction‐grafted sidechains proves a promising alternative for constructing high‐performance commercially viable membranes with quick ion transport, excellent chemical stability, and simplified membrane manufacturing. Nonetheless, the concept of pseudo‐nanophase separation is still in nuce, and determinants for controlling pseudo‐nanophase separation remain somewhat opaque. In this contribution, supramolecular sidechain topology is found critical to engineering pseudo‐nanophase separation. Three supramolecular sidechain topological (viz. linear, branched, and cyclic) structures are investigated using experimental and theoretical protocols, and the underlying mechanisms by which supramolecular sidechain topology alters the microstructure and ion‐conducting behaviors of the membranes are proposed. Consequently, the cyclic sidechain‐mediated membrane achieves the highest proton conductivity with an area resistance as low as 0.10 Ω cm2. The resulting membrane endows an acidic aqueous redox flow battery with an energy efficiency of up to 80.7% even at high current densities of 220 mA cm−2, breaking the record set by the pseudo‐nanophase separation strategy constructed membranes and ranking among the highest values ever documented. This study advances the understanding of supramolecular sidechain topology for the design and preparation of high‐performance membranes via pseudo‐nanophase separation engineering for flow batteries and beyond. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are perceived to lead the large-scale energy storage technology by integrating with intermittent renewable energy resources such as wind and solar to overcome current challenges in conventional energy storage devices. Recently, several modifications have been employed in the development of RFBs to achieve efficient energy storage at an economically acceptable cost of the system. For large-scale deployment, further improvement in energy efficiency, longevity of the system, and reduction in cost are required. Most of the recent research activities are focused on the discovery of new materials to increase RFBs' energy storage-release performance with long-term stable operations suitable for implementation in mobile devices. Considering the growth in research interest, and the volume of literature generated over the past decade on the topic, a review article highlighting the key development and status of research is essential. Herein, we intend to provide the basics of the RFB system including their cell components, various types, and the current trends highlighting the study gaps that require extra effort. Moreover, we conducted an analysis of the cost of the RFBs, associated challenges, and mitigation strategies. The review also includes electrode/electrolyte materials in a table format for quick access for comparison purposes. [ABSTRACT FROM AUTHOR]

The global rapid transition from fossil fuels to renewable energy resources necessitates the implementation of long‐duration energy storage technologies owing to the intermittent nature of renewable energy sources. Therefore, the deployment of grid‐scale energy storage systems is inevitable. Sulfur‐based batteries can be exploited as excellent energy storage devices owing to their intrinsic safety, low cost of raw materials, low risk of environmental hazards, and highest theoretical capacities (gravimetric: 2600 Wh/kg and volumetric: 2800 Wh/L). However, sulfur‐based batteries exhibit certain scientific limitations, such as polysulfide crossover, which causes rapid capacity decay and low Coulombic efficiency, thereby hindering their implementation at a commercial scale. In this review article, we focus on the latest research developments between 2012–2023 to improve the separators/membranes and overcome the shuttle effect associated with them. Various categories of ion exchange membranes (IEMs) used in redox batteries, particularly polysulfide redox flow batteries and lithium‐sulfur batteries, are discussed in detail. Furthermore, advances in IEM constituents are summarized to gain insights into different fundamental strategies for attaining targeted characteristics, and a critical analysis is proposed to highlight their efficiency in mitigating sulfur cross‐shuttling issues. Finally, future prospects and recommendations are suggested for future research toward the fabrication of more effective membranes with desired properties. [ABSTRACT FROM AUTHOR]

The use of graphene oxide (GO) has shown potential in improving the performance of redox flow lithium‐ion batteries (RFLIBs). These types of batteries use a liquid electrolyte containing redox‐active species to store and release energy. Despite being scalable, RFLIBs face limitations, namely, low energy density of the electrolyte and reduced cycling stability of the electrodes. However, GO's unique properties, such as its high conductivity as well as large surface area, create an attractive option for enhancing the electrochemical properties of both the electrolyte and electrodes in RFLIBs. When used as an electrode, GO improves the kinetic reversibility reactions, leading to increased electrochemical activity towards redox couples. Charge transfer resistances of positive and negative reactions are reduced, leading to increased voltage energy and efficiency of lithium batteries in terms of energy usage. As redox flow batteries made of lithium ions are an established subsystem and a growing research and development field, there is potential to enhance their performance and reduce costs through the use of GO. The objective of this review is to provide an overview of the chemistry of GO as it pertains to RFLIBs use, covering topics such as its surface chemistry, functionalization, and interactions with redox‐active species, as well as its potential for enhancing high‐density storage of electricity in batteries. Specifically, it will discuss the impact of GO on redox reactions in the electrolyte, including its ability to raise the redox‐active species concentration as well as enhance their stability. The review will also examine how GO impacts the electrodes, including its potential to increase their surface area and conductivity and promote cycling stability. Additionally, the review will address the importance of optimizing the quantity and distribution of GO in both the electrolyte and electrodes of RFLIBs. [ABSTRACT FROM AUTHOR]

High solubility of active materials is crucial for achieving a high‐energy‐density catholyte/anolyte in redox flow batteries. However, solubility largely depends on the compatibility with electrolyte, limiting the types of redox‐active materials that can be used in aqueous electrolytes. Herein, a universal strategy is introduced to attain a high solubility of active materials regardless of the compatibility with aqueous electrolytes while preserving their intrinsic redox activity via micellar solubilization. Leveraging the amphiphilic nature of surfactant molecules, insoluble redox‐active materials are encapsulated by surfactants to be dissolvable with significant solubility. As a demonstration, it is showed that an order‐of‐magnitude solubility enhancement can be achieved for (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) in aqueous catholyte (≈0.8 m). Consequently, the catholyte performance of TEMPO is fully harnessed, leading to an energy density enhancement of more than ten times compared to that in bare electrolyte. It is also observed that micellar solubilization unexpectedly improves the cycle stability, attributed to the mitigation of intermolecular side reactions and reduced crossover. Finally, the fundamental electrochemical reaction mechanism of micelle‐encapsulated TEMPO is discussed. This strategy offers a new insight regarding the solubility and stability of the catholyte/anolyte, and is expected to be applicable to other redox‐active molecules, opening up an unexplored micellar chemistry in redox flow batteries. [ABSTRACT FROM AUTHOR]

To achieve carbon neutrality, integrating intermittent renewable energy sources, such as solar and wind energy, necessitates the use of large-scale energy storage. Among various emerging energy storage technologies, redox flow batteries are particularly promising due to their good safety, scalability, and long cycle life. In order to meet the ever-growing market demand, it is essential to enhance the power density of battery stacks to lower the capital cost. One of the key components that impact the battery performance is the flow field, which is to distribute electrolytes onto electrodes. The design principle of flow fields is to maximize the distribution uniformity of electrolytes at a minimum pumping work. This review provides an overview of the progress and perspectives in flow field design and optimization, with an emphasis on the scale-up process. The methods used to evaluate the performance of flow fields, including both experimental and numerical techniques, are summarized, and the benefits of combining diverse methods are highlighted. The review then investigates the pattern design and structure optimization of serpentine- and interdigitated-based flow fields before discussing challenges and strategies for scaling up these flow fields. Finally, the remaining challenges and the prospects for designing highly efficient flow fields for battery stacks are outlined. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) have attracted significant attention as a promising electrochemical energy storage technology, offering various advantages such as grid-scale electricity production with variable intermittent electricity delivery, enhanced safety compared to metal-ion batteries, decoupled energy and power density, and simplified manufacturing processes. For this review, we exclusively focus on organic, non-aqueous redox flow batteries. Specifically, we address the most recent progress and the major challenges related to the design and synthesis of robust redox-active organic compounds. An extensive examination of the synthesis and characterization of a wide spectrum of redox-active molecules, focusing particularly on derivatives of posolytes such as quinone, nitroxyl radicals, dialkoxybenzenes, and phenothiazine and negolytes such as viologen and pyridiniums, is provided. We explore the incorporation of various functional groups as documented in the references, aiming to enhance the chemical and electrochemical stability, as well as the solubility, of both the neutral and radical states of redox-active molecules. Additionally, we offer a comprehensive assessment of the cell-cycling performance exhibited by these redox-active molecules. [ABSTRACT FROM AUTHOR]

Porous electrodes govern the electrochemical performance and pumping requirements in redox flow batteries, yet conventional carbon‐fiber‐based porous electrodes have not been tailored to sustain the requirements of liquid‐phase electrochemistry. 3D printing is an effective approach to manufacturing deterministic architectures, enabling the tuning of electrochemical performance and pressure drop. In this work, model grid structures are manufactured with stereolithography 3D printing followed by carbonization and tested as flow battery electrode materials. Microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics are employed to investigate the resulting electrode properties, mass transport, and pressure drop of ordered lattice structures. The influence of the printing direction, pillar geometry, and flow field type on the cell performance is investigated and mass transfer vs. electrode structure correlations are elucidated. It is found that the printing direction impacts the electrode performance through a change in morphology, resulting in enhanced performance for diagonally printed electrodes. Furthermore, mass transfer rates within the electrode are improved by helical or triangular pillar shapes or by using interdigitated flow field designs. This study shows the potential of stereolithography 3D printing to manufacture customized electrode scaffolds, which could enable multiscale structures with superior electrochemical performance and low pumping losses. [ABSTRACT FROM AUTHOR]

Transition metal complexes are a promising class of redox mediators for targeting redox flow batteries due to the tunability of their electrochemical potentials. However, reliable time‐efficient tools for the prediction of their reduction potentials are needed. In this work, we establish a suitable density functional theory protocol for their prediction using an initial experimental data set of aqueous iron complexes with bidentate ligands. The approach is then cross‐validated using different complexes found in the redox‐flow literature. We find that the solvation model affects the prediction accuracy more than the functional or basis set. The smallest errors are obtained using the COSMO‐RS solvation model (mean average error (MAE)=0.24 V). With implicit solvation models, a general deviation from experimental results is observed. For a set of similar ligands, they can be corrected using simple linear regression (MAE=0.051 V for the initial set of iron complexes). [ABSTRACT FROM AUTHOR]

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

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction‐limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non‐solvent‐induced phase‐separation (NIPS) technique that enables the reproducible synthesis of macrovoid‐free electrodes with well‐defined across‐thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity‐gradient electrodes are compared with macrovoid‐containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity‐gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid‐containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity‐gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems. [ABSTRACT FROM AUTHOR]

Renewable energy systems are essential for carbon neutrality and energy savings in industrial facilities. Factories use a lot of electrical and thermal energy to manufacture products, but only a small percentage is recycled. Utilizing energy storage systems in industrial facilities is being applied as a way to cut energy costs and reduce carbon emissions. However, lithium-based batteries, which are predominantly used in traditional industries, face challenges in terms of affordability and reliability. Redox flow batteries, on the other hand, offer high power output and reliability, and are economical to manufacture for installations with high capacity. Although redox flow batteries are difficult to use in general electrical systems due to their small volume-to-capacity ratio, they can be easily utilized as energy storage devices in industrial parks or renewable energy parks with relatively little space constraints. In addition, since factories use a lot of heat energy in addition to electricity, utilizing combined heat and power can further reduce heat energy. In this study, we analyzed the cost estimation and economic feasibility of utilizing photovoltaics, redox flow cells, and combined heat and power to save energy in a factory's energy management system. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are promising for large‐scale long‐duration energy storage owing to their inherent safety, decoupled power and energy, high efficiency, and longevity. Membranes constitute an important component that affects mass transport processes in RFBs, including ion transport, redox‐species crossover, and the net volumetric transfer of supporting electrolytes. Hydrophilic microporous polymers, such as polymers of intrinsic microporosity (PIM), are demonstrated as next‐generation ion‐selective membranes in RFBs. However, the crossover of redox species and water migration through membranes are remaining challenges for battery longevity. Here, a facile strategy is reported for regulating mass transport and enhancing battery cycling stability by employing thin film composite (TFC) membranes prepared from a PIM polymer with optimized selective‐layer thickness. Integration of these PIM‐based TFC membranes with a variety of redox chemistries allows for the screening of suitable RFB systems that display high compatibility between membrane and redox couples, affording long‐life operation with minimal capacity fade. Thickness optimization of TFC membranes further improves cycling performance and significantly restricts water transfer in selected RFB systems. [ABSTRACT FROM AUTHOR]

Polysulfides aqueous redox flow batteries (PS‐ARFBs) with large theoretical capacity and low cost are one of the most promising solutions for large‐scale energy storage technology. However, sluggish electrochemical redox kinetics and nonnegligible crossover of aqueous polysulfides restrict the battery performances. Herein, it is found that the Co, Zn dual‐doped N‐C complex have enhanced electrochemical adsorption behaviors for Na2S2. It exhibits significantly electrochemical redox activity compared to the bare glassy carbon electrode. And the redox reversibility is also improved from ΔV = 210 mV on Zn‐doped N‐C complex to ΔV = 164 mV on Co, Zn‐doped N‐C complex. Furthermore, membrane‐electrode assembly (MEA) based on Co, Zn‐doped N‐C complex is firstly proposed to enhance the redox performances and relieve the crossover in PS‐ARFBs. Thus, an impressively high and reversible capacity of 157.5 Ah L−1 for Na2S2 with a high capacity utilization of 97.9% could be achieved. Moreover, a full cell PS‐ARFB with Na2S2 anolyte and Na4[Fe(CN)6] catholyte exhibits high energy efficiency ≈88.4% at 10 mA cm−2. A very low capacity decay rate of 0.0025% per cycle is also achieved at 60 mA cm−2 over 200 cycles. [ABSTRACT FROM AUTHOR]

To preserve frequency and tie-line power flow within a tolerable range in modern intricate and renewable energy sources (RES) integrated power system, an intelligent, expert, and robust load frequency control (LFC) scheme is requisite. This paper proposes a new load frequency control (LFC) approach using a dual-stage controller composed of fractional-order tilted-integral-derivative (TID) and integer-order proportional-derivative (PD) controllers. The recently developed salp swarm algorithm has tuned the parameters of the controller. The proposed scheme exhibits robustness and fast disturbance rejection performance because the tilted-integral-derivative controller is structured in a fractional-order framework, and transient response is improved due to the proportional-derivative controller. The overall activity of the LFC has been further upgraded by incorporating the flexible AC transmission system (FACTS) device namely thyristor-controlled phase shifter (TCPS) unit and energy storage system called redox flow battery (RFB) units. The proposed control approach is applied to diverse types of multi-area multi-source power systems with thermal, hydro, and wind turbine units along with nonlinear functions namely generation rate constraint (GRC), governor dead band (GDB), and communication delay (CD). The simulation results yield improved frequency regulation activity with and without TCPS and RFB using the proposed control approach when compared with the recently developed LFC approaches. Sensitivity analysis also established the robustness of the proposed control approach. [ABSTRACT FROM AUTHOR]

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

Phenazines are an emerging class of organic compounds that have been recently utilized in aqueous redox flow batteries, a promising technology for large‐scale energy storage. A virtual screening based on density functional theory calculations is used to investigate the redox potentials of around 100 phenazine derivatives in aqueous media containing various electron‐donating or electron‐withdrawing groups at different positions. The calculations identify the crucial positions that should be functionalized with multiple hydroxy groups to design new anolytes. The combined experimental–computational methodology reported herein guides the development of a new molecule with a record low reversible redox potential as a potential anolyte for aqueous redox flow batteries. [ABSTRACT FROM AUTHOR]

Providing sustainable energy storage is a challenge that must be overcome to replace fossil‐based fuels. Redox flow batteries are a promising storage option that can compensate for fluctuations in energy generation from renewable energy production, as their main asset is their design flexibility in terms of storage capacity. Current commercial options for flow batteries are mostly limited to inorganic materials such as vanadium, zinc, and bromine. As environmental aspects are one of the main drivers for developing flow batteries, assessing their environmental performance is crucial. However, this topic is still underexplored, as researchers have mostly focused on single systems with defined use cases and system boundaries, making the assessments of the overall technology inaccurate. This review was conducted to summarize the main findings of life cycle assessment studies on flow batteries with respect to environmental hotspots and their performance as compared to that of other battery systems. [ABSTRACT FROM AUTHOR]

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

An extremely stable, energy‐dense (53.6 Ah L−1, 2 m transferrable electrons), low crossover (permeability of <1 × 10−13 cm2 s−1 using Nafion 212 (Nafion is a trademark polymer from DuPont)), and potentially inexpensive anthraquinone with 2‐2‐propionate ether anthraquinone structure (abbreviated 2‐2PEAQ) is synthesized and extensively evaluated under practically relevant conditions for use in the negolyte of an aqueous redox flow battery. 2‐2PEAQ shows a high stability with a fade rate of 0.03–0.05% per day at different applied current densities, cut‐off voltage windows, and concentrations (0.1 and 1.0 m) in both a full cell paired with a ferro/ferricyanide posolyte as well as a symmetric cell. 2‐2PEAQ is further shown to have extreme long‐term stability, losing only ≈0.01% per day when an electrochemical rejuvenation strategy is employed. From post‐mortem analysis (nuclear magnetic resonance (NMR), liquid chromatography–mass spectrometry (LC‐MS), and cyclic voltammetry (CV)) two degradation mechanisms are deduced: side chain loss and anthrone formation. 2‐2PEAQ with the ether linkages attached on carbons non‐adjacent to the central ring is found to have three times lower fade rate compared to its isomer with ether linkages on the carbon adjacent to the central quinone ring. The present study introduces a viable negolyte candidate for grid‐scale aqueous organic redox flow batteries. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are promising candidates for large‐scale energy storage systems (ESSs) due to their unique architecture that can decouple energy and power. Aqueous RFBs based on organic molecules (AORFBs) work with a non‐flammable and intrinsically safe aqueous electrolyte, and organic compounds are performed as redox couples. The application of redox‐active organics tremendously expands the development space of RFBs owing to the highly tunable molecule structure. Molecular engineering enables the exceptional merits in solubility, stability, and redox potential of different organic molecules. Herein, this review summarizes the application of molecular engineering to several organic compounds, focusing on the fundamental overview of their physicochemical properties and design strategies. We discuss the electrochemical merits and performances along with the intrinsic properties of the designed organic components. Finally, we outline the requirements for rational design of innovative organics to motivate more valuable research and present the prospect of molecule engineering used in AORFBs. [ABSTRACT FROM AUTHOR]

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

Redox flow batteries are a promising technology to enable the middle term storage of fluctuating renewable electricity production. The membrane is a key component in the battery system and to further develop and improve the battery systems, detailed understanding of the membrane aging and degradation mechanisms are required. This review gives a comprehensive overview about the various membrane degradation mechanisms in the most relevant redox flow battery systems. We discuss different testing approaches for membranes and compare the influence of different battery chemistries, testing protocols and degradation mechanisms. Based on the current state of the art, an outlook on the greatest challenges for developing novel and more stable membrane materials is given. [ABSTRACT FROM AUTHOR]

The use of flow batteries for energy storage has attracted considerable attention with the increased use of renewable resources. It is well known that the performance of a flow battery depends, among other factors, on the properties of the electrodes, which are generally composed of graphite felt (GF). In this work, thermal, chemical and plasma treatments have been employed to modify the surface of the graphite felt to improve the electrochemical activity of the redox flow cell. The influence of the variables of each of these processes on the generation of surface functional groups and on changes in the obtained surface area have been examined. In this work, the kinetics of redox reactions relevant to the VO2+/VO2+ reaction have been studied with these treated electrodes and the relationship between the nature of the surface and electrochemical activity of the GF is discussed. As a result, an enhanced electrochemical performance (reduction over 200 mV of the separation between anodic and cathodic peaks and 110 mV of the onset potential) in comparison to the untreated GF is obtained for those GF treatments with low oxygenated groups concentration. [ABSTRACT FROM AUTHOR]

In redox flow batteries (RFBs), energy density mainly depends on the volume and concentration of active species in electrolyte. However, the poor solubility of organic molecules limits the further development of aqueous organic redox flow batteries (AORFBs). Herein, the viologen core structure is modified with asymmetric functional groups such as propyl and triethyl ammonium propyl for enhancing the solubility up to 1.82 m in aqueous medium. Further, the modified viologen structure lowers the redox potential (−0.58 V vs Ag/AgCl) and significantly reduces radical dimerization. Interestingly, synthesized [1‐propyl‐1'‐(3‐triethylammonio)propyl‐4,4'‐bipyridinium tribromide] (PV3+) coupled with bromide/bromine (2Br−/Br2) redox species shows cell voltage (1.66 V). The tested PV3+/PV2+•||2Br−/Br2 RFB having Nafion‐117 membrane exhibits a round‐trip efficiency of 99% over 100 cycles. The self‐discharge behavior of PV3+/PV2+•||2Br−/Br2 RFB system retains 85% of the cell voltage over 20 h, indicating the lowest permeability of bromine in RFB. Thus, the present approach of viologen modification with asymmetric functionalities drastically improves the RFB performance in terms of energy density and cycle life of the system. [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]

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]

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]

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]

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]

• 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]

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]

Fabricating fiber‐based electrodes with a large specific surface area while maintaining high flow permeability is a challenging issue in developing high‐performance redox flow batteries. Here, a sponge‐like microfiber carbon electrode is reported with a specific surface area of as large as 853.6 m2 g−1 while maintaining a fiber diameter in the range of 5–7 µm and a macropore size of ≈26.8 µm. The electrode is developed by electrospinning cross‐linked poly(vinyl alcohol)‐lignin‐polytetrafluoroethylene precursors, followed by oxidation and pyrolysis. Applying the as‐synthesized electrodes to a vanadium redox flow battery enables the battery to achieve an energy efficiency of 79.1% at the current density of 400 mA cm−2 and a capacity retention rate of 99.94% over 2000 cycles, representing one of the best battery performances in the open literature. The strategy to fabricate sponge‐like porous carbon microfibers holds great promise for versatile applications in redox flow batteries and other energy storage systems. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) based on aqueous organic electrolytes are a promising technology for safe and cost‐effective large‐scale electrical energy storage. Membrane separators are a key component in RFBs, allowing fast conduction of charge‐carrier ions but minimizing the cross‐over of redox‐active species. Here, we report the molecular engineering of amidoxime‐functionalized Polymers of Intrinsic Microporosity (AO‐PIMs) by tuning their polymer chain topology and pore architecture to optimize membrane ion transport functions. AO‐PIM membranes are integrated with three emerging aqueous organic flow battery chemistries, and the synergetic integration of ion‐selective membranes with molecular engineered organic molecules in neutral‐pH electrolytes leads to significantly enhanced cycling stability. [ABSTRACT FROM AUTHOR]

The rapid evolution of electrical devices and the increasing demand for the supply of sustainable energy necessitate the development of high‐performance energy storage systems such as rechargeable and redox flow batteries. However, these batteries typically contain inorganic active materials, which exhibit several critical drawbacks hindering further development. In this regard, azo compounds are promising alternatives, offering the benefits of fast kinetics, multi‐electron redox reactions, and tunable (via structural adjustment) battery performance. Herein, we review the use of azo compounds as the active materials of rechargeable and redox flow batteries, discuss certain aspects of material design and electrochemical reaction mechanisms, and summarize the corresponding perspectives and research directions to facilitate further progress in this field. [ABSTRACT FROM AUTHOR]

Redox‐active organics are highly attractive in aqueous organic redox flow batteries (AORFBs). However, the lack of capacity dense, stable organic catholytes remains a challenge to develop energy‐dense, long cycle‐life AORFBs. Herein, a stable organic catholyte, 4‐[3‐(trimethylammonium)acetylamino]‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl chloride (TMAAcNHTEMPO) is developed through rational molecular engineering using connective acetamido and trimethylammonium groups. Paired with bis‐(trimethylammonium) propyl viologen tetrachloride anolyte, stable AORFBs (up to 1500 cycles) with a low capacity fade rate of ca. 0.0144% h−1 are achieved. Experimental characterizations and theoretical simulations revealed that TMAAcNH‐TEMPO is largely stabilized by the reduced reactivity of the nitroxyl radical moiety that mitigates a ring‐opening side reaction. [ABSTRACT FROM AUTHOR]

The design of new organic materials for redox‐flow batteries is an actively developing topic in the field of energy storage. Herein, two novel redox‐active organic molecules were presented, based on benzoxadiazole and benzothiadiazole cores bearing ethylene glycol substituents, which were first synthesized and evaluated as anolytes in nonaqueous all organic redox flow batteries. These two molecules were compared with their unsubstituted analogs in terms of electrochemistry, solubility and RFB cycling behavior. Substituted benzoxadiazole and benzothiadiazole possess low redox potentials near −2.1 V vs. Fc/Fc+, increased solubility in organic solvents, reduced permeability through the membrane and higher stability under charge‐discharge cycling in laboratory RFB cells than unsubstituted counterparts. [ABSTRACT FROM AUTHOR]

Compressing porous carbon electrodes is a common approach to improve flow battery performance, but the resulting impact on electrode structure, fluid dynamics, and cell performance is not well understood. Herein, microtomographic imaging, load cell testing, and flow cell diagnostics are employed to characterize how compression‐induced changes impact pressure drop, polarization, and mass‐transfer scaling. Five different compressions are tested, spanning ranges typically used in literature, for AvCarb 1071 cloth (0%, 9%, 20%, 25%, 32%) and Freudenberg H23 paper (0%, 8%, 12%, 17%, 22%). It is found that the two electrode structures have distinct responses to compression, resulting in differing optimal conditions identified for each material; specifically, the Freudenberg H23 exhibits lower combined ohmic, charge‐transfer, and mass‐transport values at 8% compression, resulting in improved electrochemical performance across all compressive values, as compared to the optimal AvCarb 1071 compression (20%). Overall, Freudenberg H23 exhibits a greater sensitivity to compression with peak electrochemical activity correlating with increased permeability, whereas AvCarb 1071 is insensitive to compressive loads but produces lower electrochemical performance. Herein, the trade‐offs of mechanical robustness on fluid‐dynamic and electrochemical performance between the two electrodes are demonstrated by the aforementioned findings, suggesting each could be used for specific operating environments. [ABSTRACT FROM AUTHOR]

Powder extrusion molding is proposed to fabricate ceramic LiFePO4 layers (0.5–1.0 mm thickness) as solid booster for ferricyanide electrolyte in semisolid redox flow battery. In some extruded layers, the binder is partially decomposed, while in others it is completely removed and, afterwards, the material is sintered, so materials with different porosity and dimensions are obtained. After characterizing the materials, the kinetics for the reaction with ferricyanide is evaluated, being the binder‐less materials the ones which react faster and reach larger degrees of oxidation. For the material with 1.0 mm thick comparable results to the ones already published are obtained (69 % capacity for LiFePO4 compared to the theoretical value). In the case of the 0.5 mm thick sintered solid, an outstanding performance is achieved, reaching almost the theoretical capacity (94 %) with a very high coulombic efficiency (>99 %) at 1 mA cm−2, results that were only obtained at much lower current densities in previous works. [ABSTRACT FROM AUTHOR]

Redox flow batteries are the most promising large‐scale energy storage technologies for solving intermittency issues of renewable energy sources such as wind, solar, etc. They have favorable features over other battery technologies like high energy efficiency, intrinsic safety, independent scaling, and a long lifetime. Among various RFBs, all‐Iron redox flow batteries are an attractive choice because iron is the second most abundant metal in earth's crust, is cheap and ecofriendly. However, low charging efficiency, parasitic hydrogen evolution reaction (HER) at the negative Fe/FeII electrode, self‐discharge by electrolyte cross‐over, and poor cycle‐life (due to ferric hydroxide precipitation) are the major technical challenges to be overcome for the successful commercialization of all‐iron redox flow batteries. Herein, we report an all‐iron redox flow battery containing Fe/FeII and FeIII/FeII redox couples separated by a self‐made anion exchange membrane. We also examined the impact of adding ZnII ions on the electrochemical performance of the Fe/FeII redox couple. The coulombic efficiency, voltage efficiency and energy efficiency of the cell with 0.03 m ZnCl2 was found to be greater (90 %, 70.96 % and 63.86 %) than those of the cell without ZnCl2 (80 %, 62.06 % and 49.64 %). The results reveal that the addition of small amounts of ZnII ions to the Fe/FeII electrode suppresses the hydrogen evolution reaction and increase the cell performance. [ABSTRACT FROM AUTHOR]

Quinone‐based aqueous redox flow batteries (RFBs) have drawn much attention due to their high safety, two‐electron involvement, rapid reaction kinetics, property tunability, and potentially low cost. RFBs operating in neutral solution feature a wide electrochemical window of around 2.5 V, which provides much more space for the design of redox‐active materials (RAMs) to realize a high voltage. However, it is still challenging to achieve low potential in the neutral condition for quinone‐based RAMs, owing to inherently pH‐dependent behaviors and deep LUMO (the lowest unoccupied molecular orbital) energy level. Herein, we report three low‐potential quinone‐based RAMs (1,4‐BDPAQCl2, 1,5‐BDPAQCl2, and 1,8‐BDPAQCl2) by bis‐dimethylamino substitution. The half‐wave potential of the quinones in 0.5 M KCl is approximately −0.55 to −0.57 V versus a normal hydrogen electrode. The low potential is ascribed to the introduced functional groups with two effects. First, the intramolecular hydrogen bonds formed between C=O and H−N can weaken the association between protons and dianion Q2−, resulting in a favorable distribution of products. Second, the functional groups can effectively increase the LUMO over 0.22 eV, compared with anthraquinone. Paired with Fe(glycine)2Cl2, the theoretical open‐circuit voltage of full RFBs is achieved at 1.27–1.29 V. We test full batteries using these quinones as negative RAMs at a lower concentration (0.1 M). The results show that 1,8‐BDPAQCl2 displays stability during 300 charge‐discharge cycles. In contrast, the other two quinones exhibit poor cycling stability due to side reactions. We further execute a higher concentration (0.4 M) for 1,8‐BDPAQCl2. The cycling stability of the quinone−iron RFBs is outstanding, with 0.048 % capacity decay per cycle and 0.88 % capacity decay per day. Our finding offers a feasible strategy to design low‐potential quinone molecules for the neutral RFBs. [ABSTRACT FROM AUTHOR]

Organodisulfides (RSSR) are a class of promising active materials for redox flow batteries (RFBs). However, their sluggish kinetics and poor cyclic stability remain a formidable challenge. Here, we propose carbon disulfide (CS2) as a unique redox mediator involving reversible C-S bond formation/breakage to facilitate the reduction reaction of organodisulfides in RFBs. In the discharge of RSSR, CS2 interacts with the negatively charged RSSR-• to promote cleavage of the S-S bond by reducing about one-third of the energy barrier, forming RSCS2Li. In the recharge, CS2 is unbonded from RSCS2Li while RSSR is regenerated. Meanwhile, the redox mediator can also be inserted into the molecular structure of RSSR to form RSCS2SR/RSCS2CS2SR, and these new active materials with lower energy barriers can further accelerate the reaction kinetics of RSSR. With CS2, phenyl disulfide exhibits an exceptional rate capability and cyclability of 500 cycles. An average energy efficiency of >90% is achieved. This strategy provides a unique redox-mediating pathway involving C-S bond formation/breakage with the active species, which is different from those used in lithium-oxygen or other batteries. [ABSTRACT FROM AUTHOR]