The Critical Analysis of Membranes toward Sustainable and Efficient Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFB) are a promising technology for large-scale storage of electrical energy, combining safety, high capacity, ease of scalability, and prolonged durability; features which have triggered their early commercial implementation. Furthering the deployment of VRFB technologies requires addressing challenges associated to a pivotal component: the membrane. Examples include vanadium crossover, insufficient conductivity, escalated costs, and sustainability concerns related to the widespread adoption of perfluoroalkyl-based membranes, e.g., perfluorosulfonic acid (PFSA). Herein, recent advances in high-performance and sustainable membranes for VRFB, offering insights into prospective research directions to overcome these challenges, are reviewed. The analysis reveals the disparities and trade-offs between performance advances enabled by PFSA membranes and composites, and the lack of sustainability in their final applications. The potential of PFSA-free membranes and present strategies to enhance their performance are discussed. This study delves into vital membrane parameters to enhance battery performance, suggesting protocols and design strategies to achieve high-performance and sustainable VRFB membranes.

Realizing one-step two-electron transfer of naphthalene diimides via a regional charge buffering strategy for aqueous organic redox flow batteries

Naphthalene diimide derivatives show great potential for application in neutral aqueous organic redox flow batteries (AORFBs) due to their highly conjugated molecular structure and stable two-electron storage capacity. However, the two-electron redox process of naphthalene diimides typically occurs via two separate steps with the transfer of one electron per step ("two-step two-electron" transfer process), which leads to an inevitable loss of voltage and energy. Herein, we report a novel regional charge buffering strategy that utilizes the core-substituted electron-donating group to adjust the redox properties of naphthalene diimides, realizing two electron transfer via a single-step redox process ("one-step two-electron" transfer process). The symmetrical battery testing of NDI-DEtOH revealed exceptional intrinsic stability lasting for 11 days with a daily decay rate of only 0.11%. Meanwhile, AORFBs with NDI-DMe/FcNCl and NDI-DEtOH/FcNCl exhibited a remarkable 40% improvement in peak power density at 50% state of charge (SOC) in comparison to NDI/FcNCl-based AORFBs. In addition, the battery's energy efficiency has increased by 24%, resulting in much more stable output power and significantly improved energy efficiency. These results are of great significance to practical applications of AORFBs.

SIS-Based Electrostatic Spinning High-Safety Lithium-Ion Battery Separators

As an important component, the properties of separators directly affect the capacity, life, and safety performance of lithium-ion batteries (LIBs). The high thermal stability and safety application value of the thermoplastic elastomer poly(styrene-b-isoprene-b-styrene) block copolymer (SIS) with different block ratios were explored to enhance the thermal stability and mechanical strength of the cross-linked polyacrylonitrile (PAN) membranes by vulcanization cross-linking and heat treatment. Among these membranes, the sample named the S/PAN/SIS-4019 separator was confirmed to be a self-closing separator that can cope with the thermal runaway, attributing to the continued fusion of the SIS soft and hard segments in the cross-linked structure under high-temperature heat treatment. Moreover, the tensile strength of S/PAN/SIS-4019 separator increased to 17.49 MPa, which was better than that of Celgard 2400, PAN, and other inlay separators. Using S/PAN/SIS-4019 as a battery separator, lithium-ion batteries showed a superior electrochemical performance compared to the usage of Celgard 2400. Owing to the stable pore structure and thermally protected self-shutdown mechanism, the overall properties of the obtained cross-linked separator were improved in terms of higher thermal stability, high ionic conductivity, and electrochemical properties.

Benzidine Derivatives: A Class of High Redox Potential Molecules for Aqueous Organic Flow Batteries

The development of water-soluble redox-active molecules with high potentials is one of the effective ways to enhance the energy density of aqueous organic flow batteries (AOFBs). Herein, a series of promising N-substituted benzidine analogues as water-soluble catholyte candidates with controllable redox potentials (0.78-1.01 V vs. standard hydrogen electrode (SHE)) were obtained by the molecular engineering of aqueous irreversible benzidines. Theoretical calculations reveal that the redox potentials of these benzidine derivatives in acidic solution are determined by their electronic structure and alkalinity. Among these benzidine derivatives, N,N,N',N'-tetraethylbenzidine(TEB) shows both high redox potential (0.82 V vs. SHE) and good solubility (1.1 M). Pairing with H4 [Si(W3 O10 )4 ] anolyte, the cell displayed discharge capacity retention of 99.4 % per cycle and a high coulombic efficiency (CE) of ∼100 % over 1200 cycles. The stable discharge capacity of 41.8 Ah L-1 was achieved at the 1.0 M TEB catholyte with a CE of 97.2 % and energy efficiency (EE) of 91.2 %, demonstrating that N-substituted benzidines could be promising for AOFBs.

Development of high-voltage and high-energy membrane-free nonaqueous lithium-based organic redox flow batteries

Lithium-based nonaqueous redox flow batteries (LRFBs) are alternative systems to conventional aqueous redox flow batteries because of their higher operating voltage and theoretical energy density. However, the use of ion-selective membranes limits the large-scale applicability of LRFBs. Here, we report high-voltage membrane-free LRFBs based on an all-organic biphasic system that uses Li metal anode and 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine (Tri-TEMPO), N-propyl phenothiazine (C3-PTZ), and tris(dialkylamino)cyclopropenium (CP) cathodes. Under static conditions, the Li||Tri-TEMPO, Li||C3-PTZ, and Li||CP batteries with 0.5 M redox-active material deliver capacity retentions of 98%, 98%, and 92%, respectively, for 100 cycles over ~55 days at the current density of 1 mA/cm2 and a temperature of 27 °C. Moreover, the Li||Tri-TEMPO (0.5 M) flow battery delivers an initial average cell discharge voltage of 3.45 V and an energy density of ~33 Wh/L. This flow battery also demonstrates 81% of capacity for 100 cycles over ~45 days with average Coulombic efficiency of 96% and energy efficiency of 82% at the current density of 1.5 mA/cm2 and at a temperature of 27 °C.

Organic Electroactive Materials for Aqueous Redox Flow Batteries

Organic electroactive materials take advantage of potentially sustainable production and structural tunability compared to present commercial inorganic materials. Unfortunately, traditional redox flow batteries based on toxic redox-active metal ions have certain deficiencies in resource utilization and environmental protection. In comparison, organic electroactive materials in aqueous redox flow batteries (ARFBs) have received extensive attention in recent years for low-cost and sustainable energy storage systems due to their inherent safety. This review aims to provide the recent progress in organic electroactive materials for ARFBs. The main reaction types of organic electroactive materials are classified in ARFBs to provide an overview of how to regulate their solubility, potential, stability, and viscosity. Then, the organic anolyte and catholyte in ARFBs are summarized according to the types of quinones, viologens, nitroxide radicals, hydroquinones, etc, and how to increase the solubility by designing various functional groups is emphasized. The research advances are presented next in the characterization of organic electroactive materials for ARFBs. Future efforts are finally suggested to focus on building neutral ARFBs, designing advanced electroactive materials through molecular engineering, and resolving problems of commercial applications.

Understanding Electron Transfer Reactions Using Constrained Density Functional Theory: Complications Due to Surface Interactions

The kinetic rates of electrochemical reactions depend on electrodes and molecules in question. In a flow battery, where the electrolyte molecules are charged and discharged on the electrodes, the efficiency of the electron transfer is of crucial importance for the performance of the device. The purpose of this work is to present a systematic atomic-level computational protocol for studying electron transfer between electrolyte and electrode. The computations are done by using constrained density functional theory (CDFT) to ensure that the electron is either on the electrode or in the electrolyte. The ab initio molecular dynamics (AIMD) is used to simulate the movement of the atoms. We use the Marcus theory to predict electron transfer rates and the combined CDFT-AIMD approach to compute the parameters for the Marcus theory where it is needed. We model the electrode with a single layer of graphene and methylviologen, 4,4'-dimethyldiquat, desalted basic red 5, 2-hydroxy-1,4-naphthaquinone, and 1,1-di(2-ethanol)-4,4-bipyridinium were selected for the electrolyte molecules. All of these molecules undergo consecutive electrochemical reactions with one electron being transferred at each stage. Because of significant electrode-molecule interactions, it is not possible to evaluate outer-sphere ET. This theoretical study contributes toward the development of a realistic-level prediction of electron transfer kinetics suitable for energy storage applications.

Molecular Engineering of Organic Species for Aqueous Redox Flow Batteries

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.

Aggregation of Electrochemically Active Conjugated Organic Molecules and Its Impact on Aqueous Organic Redox Flow Batteries

Molecule aggregation in solution is acknowledged to be universal and can regulate the molecule's physiochemical properties, which however has been rarely investigated in electrochemistry. Herein, an electrochemical method is developed to quantitatively study the aggregation behavior of the target molecule methyl viologen dichloride. It is found that the oxidation state dicationic ions stay discrete, while the singly-reduced state monoradicals yield a concentration-dependent aggregation behavior. As a result, the molecule's energy level and its redox potential can be effectively regulated. This work does not only provide a method to investigate the molecular aggregation, but also demonstrates the feasibility to tune redox flow battery's performance by regulating the aggregation behavior.

Electrochemical investigation of structurally varied azinium scaffolds

Inspired by the successful utilization of aziniums as anolytes in redox-flow batteries, we have designed and prepared a systematically extended series of (di)azinium compounds based on pyrazine, bipyridine, 1,5-naphthyridine, 3,8-phenanthroline, (E)-4,4'-diazastilbene and 1,2-bis(pyridin-4-yl)acetylene. It has been revealed that the fundamental electrochemical properties are affected mostly by the water-solubility and chemical stability of the particular redox forms. Based on the systematically evolved azinium structure and gathered electrochemical data, structure-property relationships were thoroughly elucidated. Further investigation on flow battery cells identified that the known 4,4'-bipyridinium decorated with two peripheral N-propyl-3-sulfonato pendants allows utilizing both redox steps with good cycling stability, while the naphthyridine scaffold turned out to be a new and promising scaffold for redox-flow batteries.

Spatial Structure Regulation: A Rod-Shaped Viologen Enables Long Lifetime in Aqueous Redox Flow Batteries

A stable rod-like sulfonated viologen (R-Vi) derivative is developed through a spatial-structure-adjustment strategy for neutral aqueous organic redox flow batteries (AORFBs). The obtained R-Vi features four individual methyl groups on the 2,2',6,6'-positions of the 4,4'-bipyridine core ring. The tethered methyls confine the movement of the alkyl chain as well as the sulfonic anion, thus driving the spatial structure from sigmoid to rod shape. The R-Vi with weak charge attraction and large molecular dimension displays an ultralow membrane permeability that is only 14.7 % of that of typical sigmoid viologen. Moreover, the electron-donating effect of methyls endows R-Vi with the lowest redox potential of -0.55 V vs. SHE among one-electron-storage viologen-based AORFBs. The AORFB with the R-Vi anolyte and a K₄ Fe(CN)₆ catholyte exhibits an energy efficiency up to 87 % and extremely low capacity-fade rate of 0.007 % per cycle in 3200 continuous cycles.

Radical Stabilization of a Tripyridinium-Triazine Molecule Enables Reversible Storage of Multiple Electrons

A novel organic molecule, 2,4,6-tris[1-(trimethylamonium)propyl-4-pyridiniumyl]-1,3,5-triazine hexachloride, was developed as a reversible six-electron storage electrolyte for use in an aqueous redox flow battery (ARFB). Physicochemical characterization reveals that the molecule evolves from a radical to a biradical and finally to a quinoid structure upon accepting four electrons. Both the diffusion coefficient and the rate constant were sufficiently high to run a flow battery with low concentration and kinetics polarization losses. In a demonstration unit, the assembled flow battery affords a high specific capacity of 33.0 Ah L^-1 and a peak power density of 273 mW cm^-2 . This work highlights the rational design of electroactive organics that can manipulate multi-electron transfer in a reversible way, which will pave the way to development of energy-dense, manageable and low-cost ARFBs.

A high potential biphenol derivative cathode: toward a highly stable air-insensitive aqueous organic flow battery

Aqueous organic flow batteries have attracted dramatic attention for stationary energy storage due to their resource sustainability and low cost. However, the current reported systems can normally be operated stably under a nitrogen or argon atmosphere due to their poor stability. Herein a stable air-insensitive biphenol derivative cathode, 3,3',5,5'-tetramethylaminemethylene-4,4'-biphenol (TABP), with high solubility (>1.5 mol L^-1) and redox potential (0.91 V vs. SHE) is designed and synthesized by a scalable one-step method. Paring with silicotungstic acid (SWO), an SWO/TABP flow battery shows a stable performance of zero capacity decay over 900 cycles under the air atmosphere. Further, an SWO/TABP flow battery manifests a high rate performance with an energy efficiency of 85% at a current density of 60 mA cm^-2 and a very high volumetric capacity of more than 47 Ah L^-1. This work provides a new and practical option for next-generation practical large-scale energy storage.

A quantitative evaluation of computational methods to accelerate the study of alloxazine-derived electroactive compounds for energy storage

Alloxazines are a promising class of organic electroactive compounds for application in aqueous redox flow batteries (ARFBs), whose redox properties need to be tuned further for higher performance. High-throughput computational screening (HTCS) enables rational and time-efficient study of energy storage compounds. We compared the performance of computational chemistry methods, including the force field based molecular mechanics, semi-empirical quantum mechanics, density functional tight binding, and density functional theory, on the basis of their accuracy and computational cost in predicting the redox potentials of alloxazines. Various energy-based descriptors, including the redox reaction energies and the frontier orbital energies of the reactant and product molecules, were considered. We found that the lowest unoccupied molecular orbital (LUMO) energy of the reactant molecules is the best performing chemical descriptor for alloxazines, which is in contrast to other classes of energy storage compounds, such as quinones that we reported earlier. Notably, we present a flexible in silico approach to accelerate both the singly and the HTCS studies, therewithal considering the level of accuracy versus measured electrochemical data, which is readily applicable for the discovery of alloxazine-derived organic compounds for energy storage in ARFBs.

Phenylene-Bridged Bispyridinium with High Capacity and Stability for Aqueous Flow Batteries

A rotating phenyl ring is introduced between the two pyridinium rings, namely, 1,1'-bis[3-(trimethylamonium)propyl]-4,4'-(1,4-phenylene)bispyridinium tetrachloride ((APBPy)Cl₄ ), to form a switchable conjugation. In this design, the conjugation is switched "off" in the oxidized state and the two pyridinium rings behave independently during the redox process, yielding a concomitant transfer of two electrons at the same potential and, thus, simplifying the battery management. The conjugation is switched "on" in the reduced state and the charge can be effectively delocalized, lowering the Lewis basicity and improving its chemical stability. By pairing 0.50 m (APBPy)Cl₄ with a 2,2,6,6-tetramethylpiperidin-1-yl oxyl derivative as the positive electrolyte, a flow battery delivers a high standard cell voltage of 1.730 V and a high specific capacity of 20.0 Ah L^-1 . The battery also shows an exceptionally high energy efficiency of 80.8% and a superior cycling stability at 80 mA cm^-2 . This strategy proves itself a great success in engineering viologen as a two-electron storage mediator with high capacity and stability.

Interrogation of 2,2'-Bipyrimidines as Low-Potential Two-Electron Electrolytes

As utilization of renewable energy sources continues to expand, the need for new grid energy storage technologies such as redox flow batteries (RFBs) will be vital. Ultimately, the energy density of a RFB will be dependent on the redox potentials of the respective electrolytes, their solubility, and the number of electrons stored per molecule. With prior literature reports demonstrating the propensity of nitrogen-containing heterocycles to undergo multielectron reduction at low potentials, we focused on the development of a novel electrolyte scaffold based upon a 2,2'-bipyrimidine skeleton. This scaffold is capable of storing two electrons per molecule while also exhibiting a low (∼-2.0 V vs Fc/Fc+) reduction potential. A library of 24 potential bipyrimidine anolytes were synthesized and systematically evaluated to unveil structure-function relationships through computational evaluation. Through analysis of these relationships, it was unveiled that steric interactions disrupting the planarity of the system in the reduced state could be responsible for higher levels of degradation in certain anolytes. The major decomposition pathway was ultimately determined to be protonation of the dianion by solvent, which could be reversed by electrochemical or chemical oxidation. To validate the hypothesis of strain-induced decomposition, two new electrolytes with minimal steric encumbrance were synthesized, evaluated, and found to indeed exhibit higher stability than their sterically hindered counterparts.

Comparison of computational chemistry methods for the discovery of quinone-based electroactive compounds for energy storage

High-throughput computational screening (HTCS) is a powerful approach for the rational and time-efficient design of electroactive compounds. The effectiveness of HTCS is dependent on accuracy and speed at which the performance descriptors can be estimated for possibly millions of candidate compounds. Here, a systematic evaluation of computational methods, including force field (FF), semi-empirical quantum mechanics (SEQM), density functional based tight binding (DFTB), and density functional theory (DFT), is performed on the basis of their accuracy in predicting the redox potentials of redox-active organic compounds. Geometry optimizations at low-level theories followed by single point energy (SPE) DFT calculations that include an implicit solvation model are found to offer equipollent accuracy as the high-level DFT methods, albeit at significantly lower computational costs. Effects of implicit solvation on molecular geometries and SPEs, and their overall effects on the prediction accuracy of redox potentials are analyzed in view of computational cost versus prediction accuracy, which outlines the best choice of methods corresponding to a desired level of accuracy. The modular computational approach is applicable for accelerating the virtual studies on functional quinones and the respective discovery of candidate compounds for energy storage.

Low-Cost Titanium-Bromine Flow Battery with Ultrahigh Cycle Stability for Grid-Scale Energy Storage

Flow batteries are one of the most promising large-scale energy-storage systems. However, the currently used flow batteries have low operation-cost-effectiveness and exhibit low energy density, which limits their commercialization. Herein, a titanium-bromine flow battery (TBFB) featuring very low operation cost and outstanding stability is reported. In this battery, a novel complexing agent, 3-chloro-2-hydroxypropyltrimethyl ammonium chloride, is employed to stabilize bromine/polybromides and suppress Br diffusion. The results reveal that the complexing agent effectively inhibits Br crossover and reduces Br-induced corrosivity, which in turn significantly improves the reliability of the TBFB system. The novel TBFB demonstrates 95% coulombic efficiency and 83% energy efficiency at 40 mA cm^-2 current density. Moreover, it can run smoothly for more than 1000 cycles without any capacity decay. Furthermore, an assembled 300 W TBFB stack can be continuously operated for more than 500 cycles, thereby confirming the practical applicability of the proposed TBFB. Because the TBFB utilizes an ultralow-cost electrolyte (41.29 $ kWh^-1 ) and porous polyolefin membranes, it serves as a reliable and low-cost energy-storage device. Therefore, considering its ultrahigh stability and low cost, the TBFB can be used as a large-scale energy-storage device.

Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies

Redox flow batteries (RFBs) are critical enablers for next-generation grid-scale energy-storage systems, due to their scalability and flexibility in decoupling power and energy. Aqueous RFBs (ARFBs) using nonflammable electrolytes are intrinsically safe. However, their development has been limited by their low energy density and high cost. Developing ARFBs with higher energy density, lower cost, and longer lifespan than the current standard is of significant interest to academic and industrial research communities. Here, a critical review of the latest progress on advanced electrolyte material designs of ARFBs is presented, including a fundamental overview of their physicochemical properties, major challenges, and design strategies. Assessment methodologies and metrics for the evaluation of RFB stability are discussed. Finally, future directions for material design to realize practical applications and achieve the commercialization of ARFB energy-storage systems are highlighted.

Recent Development in Composite Membranes for Flow Batteries

Flow batteries (FBs) are one of the most attractive candidates for stationary energy storage and vital in realizing the wide application of renewable energies. Membranes play an important role in isolating redox couples while transporting ions to close the internal electrical circuit. Therefore, membranes with high selectivity and conductivity are highly important. Among different membranes, a composite membrane with independent design of support layer and thin selective top layer becomes one of the most promising candidates to break the trade-off between selectivity and conductivity. In this Review, recent studies on composite membranes for FBs and the principles of membrane design in different systems are discussed and summarized. Finally, the future direction on membrane design for different FBs is presented, which will provide an extensive, comprehensive reference to design and construct high-performance composite membranes for FBs.

Screening Viologen Derivatives for Neutral Aqueous Organic Redox Flow Batteries

Viologen derivatives have been developed as negative electrolyte for neutral aqueous organic redox flow batteries (AOFBs), but the structure-performance relationship remains unclear. Here, it was investigated how the structure of viologens impacts their electrochemical behavior and thereby the battery performance, by taking hydroxylated viologens as examples. Calculations of frontier molecular orbital energy and molecular configuration promise to be an effective tool in predicting potential, kinetics, and stability, and may be broadly applicable. Specifically, a modified viologen derivative, BHOP-Vi, was proved to be the most favorable structure, enabling a concentrated 2 m battery to exhibit a power density of 110.87 mW cm^-2 and an excellent capacity retention rate of 99.953 % h^-1 .

Pore engineering of graphene aerogels for vanadium redox flow batteries

The all-vanadium redox flow battery is considered to be a dispersive and non-perennial energy source due to its grid reliability, high efficiency, standalone modular design, and excellent cycling stability. However, the large vanadium ionic size and relatively high viscosity lead to poor compatibility with most carbon-based microporous electrodes, resulting in sluggish mass diffusion and unsatisfied capacitance retention. Herein, a novel cross-coupled porous graphene aerogel is proposed via the NaNO3-template pore engineering strategy. The microscopic observations and N2 adsorption-desorption isotherms validate the successful regulation of the surface area and porous structure, with the addition of different porogen contents (6.25-25 g L-1). The vanadium redox flow battery delivers a specific capacity of 163.4 mA h g-1 (5.6 A h L-1) at a current density of 25 mA cm-2, surpassing most previously reported batteries with a similar reactor volume. This method holds great promise for the better design and preparation of porous electrodes, and potential suitable applications.

Water-soluble pH-switchable cobalt complexes for aqueous symmetric redox flow batteries

A water soluble cobalt complex with two redox couples that fall within the water splitting window can be applied as both the posolyte and negolyte in an aqueous symmetric redox flow battery.

A TiN Nanorod Array 3D Hierarchical Composite Electrode for Ultrahigh-Power-Density Bromine-Based Flow Batteries

Bromine-based flow batteries are well suited for stationary energy storage due to attractive features of high energy density and low cost. However, the bromine-based flow battery suffers from low power density and large materials consumption due to the relatively high polarization of the Br₂ /Br^- couple on the electrodes. Herein, a self-supporting 3D hierarchical composite electrode based on a TiN nanorod array is designed to improve the activity of the Br₂ /Br^- couple and increase the power density of the bromine-based flow battery. In this design, a carbon felt provides a composite electrode with a 3D electron conductive framework to guarantee high electronic conductivity, while the TiN nanorods possess excellent catalytic activity for the Br₂ /Br^- electrochemical reaction to reduce the electrochemical polarization. Moreover, the 3D micro-nano hierarchical nanorod-array alignment structure contributes to a high electrolyte penetration and a high ion-transfer rate to reduce diffusion polarization. As a result, a zinc-bromine flow battery with the designed composite electrode can be operated at a current density of up to 160 mA cm^-2 , which is the highest current density ever reported. These results exhibit a promising strategy to fabricate electrodes for ultrahigh-power-density bromine-based flow batteries and accelerate the development of bromine-based flow batteries.

A pH-Neutral, Metal-Free Aqueous Organic Redox Flow Battery Employing an Ammonium Anthraquinone Anolyte

Redox-active anthraquinone molecules represent promising anolyte materials in aqueous organic redox flow batteries (AORFBs). However, the chemical stability issue and corrosion nature of anthraquinone-based anolytes in reported acidic and alkaline AORFBs constitute a roadblock for their practical applications in energy storage. A feasible strategy to overcome these issues is migrating to pH-neutral conditions and employing soluble AQDS salts. Herein, we report the 9,10-anthraquinone-2,7-disulfonic diammonium salt AQDS(NH₄ )₂ , as an anolyte material for pH-neutral AORFBs with solubility of 1.9 m in water, which is more than 3 times that of the corresponding sodium salt. Paired with an NH₄ I catholyte, the resulting pH-neutral AORFB with an energy density of 12.5 Wh L^-1 displayed outstanding cycling stability over 300 cycles. Even at the pH-neutral condition, the AQDS(NH₄ )₂  /NH₄ I AORFB delivered an impressive energy efficiency of 70.6 % at 60 mA cm^-2 and a high power density of 91.5 mW cm^-2 at 100 % SOC. The present AQDS(NH₄ )₂ flow battery chemistry opens a new avenue to apply anthraquinone molecules in developing low-cost and benign pH-neutral flow batteries for scalable energy storage.