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

Exploration of Vitamin B6-Based Redox-Active Pyridinium Salts Towards the Application in Aqueous Organic Flow Batteries

Pyridoxal hydrochloride, a vitamin B6 vitamer, was synthetically converted to a series of diverse redox-active benzoyl pyridinium salts. Cyclic voltammetry studies demonstrated redox reversibility under basic conditions, and two of the most promising salts were subjected to laboratory-scale flow battery tests involving galvanostatic cycling at 10 mM in 0.1 M NaOH. In these tests, the battery was charged completely, corresponding to the transfer of two electrons to the electrolyte, but no discharge was observed. Both CV analysis and electrochemical simulations confirmed that the redox wave observed in the experimental voltammograms corresponds to a two-electron process. To explain the irreversibility in the battery tests, we conducted bulk electrolysis with the benzoyl pyridinium salts, affording the corresponding benzylic secondary alcohols. Computational studies suggest that the reduction proceeds in three consecutive steps: first electron transfer (ET), then proton-coupled electron transfer (PCET) and finally proton transfer (PT) to give the secondary alcohol. 1H NMR deuterium exchange studies indicated that the last PT step is not reversible in 0.1 M NaOH, rendering the entire redox process irreversible. The apparent reversibility observed in CV at the basic media likely arises from the slow rate of the PT step at the timescale of the measurement.

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.

Recent Development of Electrolytes for Aqueous Organic Redox Flow Batteries (Aorfbs): Current Status, Challenges, and Prospects

In recent years, aqueous organic redox flow batteries (AORFBs) have attracted considerable attention due to advancements in grid-level energy storage capacity research. These batteries offer remarkable benefits, including outstanding capacity retention, excellent cell performance, high energy density, and cost-effectiveness. The organic electrolytes in AORFBs exhibit adjustable redox potentials and tunable solubilities in water. Previously, various types of organic electrolytes, such as quinones, organometallic complexes, viologens, redox-active polymers, and organic salts, were extensively investigated for their electrochemical performance and stability. This study presents an overview of recently published novel organic electrolytes for AORFBs in acidic, alkaline, and neutral environments. Furthermore, it delves into the current status, challenges, and prospects of AORFBs, highlighting different strategies to overcome these challenges, with special emphasis placed on their design, composition, functionalities, and cost. A brief techno-economic analysis of various aqueous RFBs is also outlined, considering their potential scalability and integration with renewable energy systems.

A Stable and Energy-Dense Polysulfide/Permanganate Flow Battery

Redox flow batteries (RFBs) as promising technologies for energy storage have attracted burgeoning efforts and have achieved many advances in the past decades. However, for practical applications, the exploration of high-performance RFB systems is still of significance. In this work, inspired by the high solubility and low cost of both polysulfides and permanganates, the S/Mn RFBs with S42-/S22- and MnO4-/MnO42- as negative and positive redox pairs are demonstrated. Moreover, to solve the poor cycling performance caused by the sluggish kinetics of polysulfide-involved redox reactions and instability of the carbon felt (CF) electrode in the strong oxidative and corrosive catholyte, both the anode and cathode are designed to obtain high performance. Herein, the NiSx/Ni foam exhibiting electrocatalysis activity toward polysulfide ions is prepared and works as the anode while the graphene-modified carbon felt (G/CF) with high stability is fabricated and utilized as the cathode. Additionally, NaMnO4 with a high solubility limit (3.92 M) in the alkaline supporting electrolyte is preferred to KMnO4 as the redox-active molecule in the catholyte. The resulting S/Mn RFB cells show outstanding cell performance, such as high energy density (67.8 Wh L-1), long cycling lifetime with a temporal capacity fade of 0.025% h-1, and low chemical cost of electrolytes (17.31 $ kWh-1). Moreover, a three-cell stack shows good cycling stability over 100 cycles (226.8 h) with high performance, verifying the good scalability of the proposed S/Mn RFB system. Therefore, the present strategy provides a reliable candidate for stable, energy-dense, and cost-effective devices for future energy storage applications.

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 capacity fade in organic redox-flow batteries by combining spectroscopy with statistical inference techniques

Organic redox-active molecules are attractive as redox-flow battery (RFB) reactants because of their low anticipated costs and widely tunable properties. Unfortunately, many lab-scale flow cells experience rapid material degradation (from chemical and electrochemical decay mechanisms) and capacity fade during cycling (>0.1%/day) hindering their commercial deployment. In this work, we combine ultraviolet-visible spectrophotometry and statistical inference techniques to elucidate the Michael attack decay mechanism for 4,5-dihydroxy-1,3-benzenedisulfonic acid (BQDS), a once-promising positive electrolyte reactant for aqueous organic redox-flow batteries. We use Bayesian inference and multivariate curve resolution on the spectroscopic data to derive uncertainty-quantified reaction orders and rates for Michael attack, estimate the spectra of intermediate species and establish a quantitative connection between molecular decay and capacity fade. Our work illustrates the promise of using statistical inference to elucidate chemical and electrochemical mechanisms of capacity fade in organic redox-flow battery together with uncertainty quantification, in flow cell-based electrochemical systems.

Controlling π-π Interactions of Highly Soluble Naphthalene Diimide Derivatives for Neutral pH Aqueous Redox Flow Batteries

Organic redox-active molecules are a promising platform for designing sustainable, cheap, and safe charge carriers for redox flow batteries. However, radical formation during the electron-transfer process causes severe side reactions and reduces cyclability. This problem is mitigated by using naphthalene diimide (NDI) molecules and regulating their π-π interactions. The long-range π-stacking of NDI molecules, which leads to precipitation, is disrupted by tethering four ammonium functionalities, and the solubility approaches 1.5 m in water. The gentle π-π interactions induce clustering and disassembling of the NDI molecules during the two-electron transfer processes. When the radical anion forms, the antiferromagnetic coupling develops tetramer and dimer and nullifies the radical character. In addition, short-range-order NDI clusters at 1 m concentration are not precipitated but inhibit crossover. They are disassembled in the subsequent electron-transfer process, and the negatively charged NDI core strongly interacts with ammonium groups. These behaviors afford excellent RFB performance, demonstrating 98% capacity retention for 500 cycles at 25 mA cm^-2 and 99.5% Coulombic efficiency with 2 m electron storage capacity.

Gaseous Nitrogen Oxides Catholyte for Rechargeable Redox Flow Batteries

There is a strong interest in finding highly soluble redox compounds to improve the energy density of redox flow batteries (RFBs). However, the performance of electrolytes is often negatively influenced by high solute concentration. Herein, we designed a high-potential (0.5 V vs. Ag/Ag⁺ ) catholyte for RFBs, where the charged and discharged species are both gaseous nitrogen oxides (NO_x ). These species can be liberated from the liquid electrolyte and stored in a separate gas container, allowing scale-up of storage capacity without increasing the concentration and volume of the electrolyte. The oxidation of NO in the presence of NO₃ ^- affords N₂ O₃ , and the reduction of N₂ O₃ regenerates NO and NO₃ ^- , together affording the electrochemical reaction: NO₃ ^- +3 NO⇌2 N₂ O₃ +e^- with a low mass/charge ratio of 152 grams per mole of stored electron. A proof-of-concept NO_x symmetric H-cell shows 200 stable cycles over 400 hours with >97 % Coulombic efficiency and negligible capacity decay.

Stable Operation of Aqueous Organic Redox Flow Batteries in Air Atmosphere

As a green route for large-scale energy storage, aqueous organic redox flow batteries (AORFBs) are attracting extensive attention. However, most of the reported AORFBs were operated in an inert atmosphere. Herein, we clarify this issue by using the reported AORFB (i.e., 3, 3'-(9,10-anthraquinone-diyl)bis(3-methylbutanoicacid) (DPivOHAQ)||Ferrocyanide) as an example. We demonstrate that the dissolved O₂ can oxidize the discharged DPivOHAQ in anolyte, leading to capacity-imbalance between anolyte and catholyte. Therefore, this cell shows continuous capacity fading when operated in an air atmosphere. We propose a simple strategy for this challenge, in which the oxygen evolution reaction (OER) in catholyte is employed to balance oxygen reduction reaction (ORR) in anolyte. When using the Ni(OH)₂ -modifed carbon felt (CF) as a current collector for catholyte, this cell shows an excellent stability in air atmosphere because the Ni(OH)₂ -induced OER capacity in catholyte exactly balances the ORR capacity in anolyte. Such O₂ -balance strategy facilitates AORFBs' practical application.

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.

Redox Flow Batteries: Electrolyte Chemistries Unlock the Thermodynamic Limits

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.

Bio-Derived and Cost-Effective Membranes with High Selectivity for Redox Flow Batteries Based on Host-Guest Chemistry

Redox flow batteries (RFBs) stand out as a promising energy storage system to solve the grid interconnection problems of renewable energy. Membranes play a critical role in regulating the performance of RFBs, and the selectivity is commonly controlled via either size exclusion or Donnan exclusion. Membranes typically account for 40% of the stack cost of RFBs, and it is essential to develop cost-effective membranes with high selectivity to achieve widespread application. Here, a type of membrane composed of highly abundant materials derived in nature, based on a scalable fabrication process, is reported. Moreover, high selectivity is achieved attributed to the host-guest interactions between membranes and redox species, which effectively alleviate the crossover of redox-active molecules. By incorporating starch into a chitosan matrix for zinc-iodine RFBs, the highly selective recognition of starch and chitosan (host) toward triiodide (guest) builds a "wall" to block the triiodide-based active materials, meanwhile, the conducting properties of such a membrane are not compromised. The proof-of-concept battery delivers a Coulombic efficiency of 98.6% and energy efficiency of 77.4% at a current density of 80 mA cm^-2 , showing the promise of such a novel and cost-effective membrane design beyond traditional selectivity chemistry.

Rechargeable Batteries for Grid Scale Energy Storage

Ever-increasing global energy consumption has driven the development of renewable energy technologies to reduce greenhouse gas emissions and air pollution. Battery energy storage systems (BESS) with high electrochemical performance are critical for enabling renewable yet intermittent sources of energy such as solar and wind. In recent years, numerous new battery technologies have been achieved and showed great potential for grid scale energy storage (GSES) applications. However, their practical applications have been greatly impeded due to the gap between the breakthroughs achieved in research laboratories and the industrial applications. In addition, various complex applications call for different battery performances. Matching of diverse batteries to various applications is required to promote practical energy storage research achievement. This review provides in-depth discussion and comprehensive consideration in the battery research field for GSES. The overall requirements of battery technologies for practical applications with key parameters are systematically analyzed by generating standards and measures for GSES. We also discuss recent progress and existing challenges for some representative battery technologies with great promise for GSES, including metal-ion batteries, lead-acid batteries, molten-salt batteries, alkaline batteries, redox-flow batteries, metal-air batteries, and hydrogen-gas batteries. Moreover, we emphasize the importance of bringing emerging battery technologies from academia to industry. Our perspectives on the future development of batteries for GSES applications are provided.

Conjugate-Driven Electron Density Delocalization of Piperidine Nitroxyl Radical for Stable Aqueous Zinc Hybrid Flow Batteries

Stable and soluble redox-active nitroxyl radicals are highly desired for high-capacity and long-life aqueous zinc hybrid flow batteries (AZHFBs). Here we report a "π-π" conjugated imidazolium and "p-π" conjugated acetylamino co-functionalized 2,2,6,6-tetramethylpiperidine-N-oxyl (MIAcNH-TEMPO) as stable catholyte for AZHFBs. The incorporation of double-conjugate substituents could delocalize the electron density of the N-O head and thus remarkably stabilize the radical and oxoammonium forms of TEMPO, avoiding the side reaction of ring-opening. Consequently, the applied MIAcNH-TEMPO/Zn AZHFB demonstrates the hardly time-dependent stability with a constant capacity retention of 99.95 % per day over 16.7 days at a high concentration catholyte of 1.5 M and high current density of 50 mA cm^-2 . This proposed molecular engineering strategy based on electron density regulation of redox-active structures displays an attractive efficacy and thus represents a remarkable advance in high-performance AZHFBs.

POM Anolyte for All-Anion Redox Flow Batteries with High Capacity Retention and Coulombic Efficiency at Mild pH

A highly soluble Li₅ BW₁₂ O₄₀ cluster delivers 2 e^- redox reaction with fast electron transfer rates (2.5 × 10^-2  cm s^-1 ) and high diffusion coefficients (≈2.08 × 10^-6 cm² s^-1 ) at mild pH ranging from 3 to 8. In-operando aqueous-flowing Raman spectroscopy and density functional theory calculations reveal that Raman shift changing of {BW12} clusters is due to the bond length changing between W-O_b -W and W-O_c -W at different redox states. The structure changing and redox chemistry of Li₅ BW₁₂ O₄₀ are highly reversible, which makes the Li₅ BW₁₂ O₄₀ cluster versatile to construct all-anion aqueous redox flow batteries (RFBs). The cation-exchange Nafion membrane will also repel the cross permeability of the anion redox couples. Consequently, by coupling with Li₃ K[Fe(CN)₆ ] catholyte, the aqueous RFB can be operated at pH 8 with a capacity retention up to 95% and an average Coulombic efficiency more than 99.79% over 300 cycles within 0 to 1.2 V. Meanwhile, Li₅ BW₁₂ O₄₀ cluster can also be paired with LiI catholyte to form aqueous RFBs at pH 7 and pH 3, the capacity retention of 94% and 90% can be realized over 300 cycles within 0 to 1.3 V.

Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review

Redox flow batteries (RFBs) are an increasingly attractive option for renewable energy storage, thus providing flexibility for the supply of electrical energy. In recent years, research in this type of battery storage has been shifted from metal-ion based electrolytes to soluble organic redox-active compounds. Aqueous-based organic electrolytes are considered as more promising electrolytes to achieve "green", safe, and low-cost energy storage. Many organic compounds and their derivatives have recently been intensively examined for application to redox flow batteries. This work presents an up-to-date overview of the redox organic compound groups tested for application in aqueous RFB. In the initial part, the most relevant requirements for technical electrolytes are described and discussed. The importance of supporting electrolytes selection, the limits for the aqueous system, and potential synthetic strategies for redox molecules are highlighted. The different organic redox couples described in the literature are grouped in a "family tree" for organic redox couples. This article is designed to be an introduction to the field of organic redox flow batteries and aims to provide an overview of current achievements as well as helping synthetic chemists to understand the basic concepts of the technical requirements for next-generation energy storage materials.

A neutral polysulfide/ferricyanide redox flow battery

Energy storage systems are crucial in the deployment of renewable energies. As one of the most promising solutions, redox flow batteries (RFBs) are still hindered for practical applications by low energy density, high cost, and environmental concerns. To breakthrough the fundamental solubility limit that restricts boosting energy density of the cell, we here demonstrate a new RFB system employing polysulfide and high concentrated ferricyanide (up to 1.6 M) species as reactants. The RFB cell exhibits high cell performances with capacity retention of 96.9% after 1,500 cycles and low reactant cost of $32.47/kWh. Moreover, neutral aqueous electrolytes are environmentally benign and cost-effective. A cell stack is assembled and exhibits low capacity fade rate of 0.021% per cycle over 642 charging-discharging steps (spans 60 days). This neutral polysulfide/ferricyanide RFB technology with high safety, long-duration, low cost, and feasibility of scale-up is an innovative design for storing massive energy.

The Dual Role of Bridging Phenylene in an Extended Bipyridine System for High-Voltage and Stable Two-Electron Storage in Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) are regarded as a promising solution for grid-scale and sustainable energy storage, but some long-standing problems such as low energy density and cycling stability should be resolved. Herein, a highly soluble bipyridine modified with a bridging phenylene group and two quaternary ammonium terminals, namely, [(NPr)₂PV]·4Cl, was synthesized and used as an ultralow-potential and two-electron storage anolyte for AORFBs. The phenylene group, which is linked but not coplanar with the two pyridinium redox centers, can thus prevent their communication and result in an exceptionally low redox potential (-0.77 V vs standard hydrogen electrode, 2e^-). Moreover, the introduction of a phenylene group can warrant a certain degree of large π-conjugation effects and mitigate the intramolecular Coulombic repulsion between the two positively charged pyridinium centers, thus helping to enhance the electrochemical stability. When paired with 4-trimethylammonium-TEMPO as the catholyte, [(NPr)₂PV]·4Cl enabled an exceptionally high cell voltage up to 1.71 V. The AORFB delivers outstanding battery performances, specifically, ∼89% energy efficiency, ∼100% Coulombic efficiency, and ∼99.94% capacity retention per cycle during a long-term cycling process. The two overlapped single-electron reductions of [(NPr)₂PV]·4Cl from the initial cationic form to the monoradical form and then to the quinoid form during the charging process were clearly verified by a series of spectroscopic techniques, including no-deuterium nuclear magnetic resonance and electron paramagnetic resonance. This work presents a significant improvement for the construction of high-voltage AORFBs by virtue of the designability, diversity, and tunability of multiredox organic molecules.

Trust is good, control is better: a review on monitoring and characterization techniques for flow battery electrolytes

Flow batteries (FBs) currently are one of the most promising large-scale energy storage technologies for energy grids with a large share of renewable electricity generation. Among the main technological challenges for the economic operation of a large-scale battery technology is its calendar lifetime, which ideally has to cover a few decades without significant loss of performance. This requirement can only be met if the key parameters representing the performance losses of the system are continuously monitored and optimized during the operation. Nearly all performance parameters of a FB are related to the two electrolytes as the electrochemical storage media and we therefore focus on them in this review. We first survey the literature on the available characterization methods for the key FB electrolyte parameters. Based on these, we comprehensively review the currently available approaches for assessing the most important electrolyte state variables: the state-of-charge (SOC) and the state-of-health (SOH). We furthermore discuss how monitoring and operation strategies are commonly implemented as online tools to optimize the electrolyte performance and recover lost battery capacity as well as how their automation is realized via battery management systems (BMSs). Our key findings on the current state of this research field are finally highlighted and the potential for further progress is identified.

Sulfonated tryptanthrin anolyte increases performance in pH neutral aqueous redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold great promise as low-cost, environmentally friendly and safe alternative energy storage media. Here we present aqueous organometallic and all-organic active materials for RFBs with a water-soluble active material, sulfonated tryptanthrin (TRYP-SO₃H), working at a neutral pH and showing long-term stability. Electrochemical measurements show that TRYP-SO₃H displays reversible peaks at neutral pH values, allowing its use as an anolyte combined with potassium ferrocyanide or 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt monohydrate as catholytes. Single cell tests show reproducible charge-discharge cycles for both catholytes, with significantly improved results for the aqueous all-organic RFB reaching high cell voltage (0.94 V) and high energy efficiencies, stabilized during at least 50 working cycles.

Evaluation of Electrochemical Stability of Sulfonated Anthraquinone-Based Acidic Electrolyte for Redox Flow Battery Application

Despite intense research in the field of aqueous organic redox flow batteries, low molecular stability of electroactive compounds limits further commercialization. Additionally, currently used methods typically cannot differentiate between individual capacity fade mechanisms, such as degradation of electroactive compound and its cross-over through the membrane. We present a more complex method for in situ evaluation of (electro)chemical stability of electrolytes using a flow electrolyser and a double half-cell including permeation measurements of electrolyte cross-over through a membrane by a UV–VIS spectrometer. The method is employed to study (electro)chemical stability of acidic negolyte based on an anthraquinone sulfonation mixture containing mainly 2,6- and 2,7-anthraquinone disulfonic acid isomers, which can be directly used as an RFB negolyte. The effect of electrolyte state of charge (SoC), current load and operating temperature on electrolyte stability is tested. The results show enhanced capacity decay for fully charged electrolyte (0.9 and 2.45% per day at 20 °C and 40 °C, respectively) while very good stability is observed at 50% SoC and lower, even at 40 °C and under current load (0.02% per day). HPLC analysis conformed deep degradation of AQ derivatives connected with the loss of aromaticity. The developed method can be adopted for stability evaluation of electrolytes of various organic and inorganic RFB chemistries.

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.

Organic Electrode Materials for Non-aqueous K-Ion Batteries

The demands for high-performance and low-cost batteries make K-ion batteries (KIBs) considered as promising supplements or alternatives for Li-ion batteries (LIBs). Nevertheless, there are only a small amount of conventional inorganic electrode materials that can be used in KIBs, due to the large radius of K+ ions. Differently, organic electrode materials (OEMs) generally own sufficiently interstitial space and good structure flexibility, which can maintain superior performance in K-ion systems. Therefore, in recent years, more and more investigations have been focused on OEMs for KIBs. This review will comprehensively cover the researches on OEMs in KIBs in order to accelerate the research and development of KIBs. The reaction mechanism, electrochemical behavior, etc., of OEMs will all be summarized in detail and deeply. Emphasis is placed to overview the performance improvement strategies of OEMs and the characteristic superiority of OEMs in KIBs compared with LIBs and Na-ion batteries.

Electroactive Covalent Organic Frameworks: Design, Synthesis, and Applications

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with tailorable compositions, porosities, functionalities, and intrinsic chemical stability. The incorporation of electroactive moieties in the structure transforms COFs into electroactive materials with great potential for energy-related applications. Herein, the recent advances in the design and use of electroactive COFs as capacitors, batteries, conductors, fuel cells, water-splitting, and electrocatalysis are addressed. Their remarkable performance is discussed and compared with other porous materials; hence, perspectives in the development of electroactive COFs are presented.

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.

Radical Charge Population and Energy: Critical Role in Redox Potential and Cycling Life of Piperidine Nitroxyl Radical Cathodes in Aqueous Zinc Hybrid Flow Batteries

Redox-active 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have recently been investigated to expand the choice of catholyte for aqueous flow batteries (AFBs). However, the effects of substituent R in 4-position on redox potential and corresponding capacity fading mechanism are still unclear. Here, we conduct comparative studies of four R-TEMPO with R = -OH, -NH₂, -COOH, and -NHCOCH₃ in zinc hybrid AFBs. Experimental and theoretical analyses reveal that low-radical head charge population sum and radical energy, depending on R in 4-position, play a critical role in enhancing redox potential and cycling life of R-TEMPO. The electronic effect brought along by N-acetyl could redistribute the charge and lower systematic energy, making the ring-opening joint sturdy and therefore suppress the side reactions. Accordingly, the 4-NHCOCH₃-TEMPO/Zn battery achieves a high capacity retention of >99.65%/day and an open-circuit voltage of 1.71 V. Our findings on the effects of substituent are greatly anticipated to boost the high-energy density, long-life, and eco-friendly TEMPO-based AFBs.

A pH-Neutral, Aqueous Redox Flow Battery with a 3600-Cycle Lifetime: Micellization-Enabled High Stability and Crossover Suppression

Redox-flow batteries (RFBs) are a highly promising large-scale energy storage technology for mitigating the intermittent nature of renewable energy sources. Here, the design and implementation of a micellization strategy in an anthraquinone-based, pH-neutral, nontoxic, and metal-free aqueous RFB is reported. The micellization strategy (1) improves stability by protecting the redox-active anthraquinone core with a hydrophilic poly(ethylene glycol) shell and (2) increases the overall size to mitigate the crossover issue through a physical blocking mechanism. Paired with a well-established potassium ferrocyanide catholyte, the micelle-based RFB displayed an excellent capacity retention of 90.7 % after 3600 charge/discharge cycles (28.3 days), corresponding to a capacity retention of 99.67 % per day and 99.998 % per cycle. The mechanistic studies of redox-active materials were also conducted and indicated the absence of side reactions commonly observed in other anthraquinone-based RFBs. The outstanding performance of the RFB demonstrates the effectiveness of the micellization strategy for enhancing the performance of organic material-based aqueous RFBs.

Progress of Organic Electrodes in Aqueous Electrolyte for Energy Storage and Conversion

Aqueous batteries using inorganic compounds as electrode materials are considered a promising solution for grid-scale energy storage, while wide application is limited by the short life and/or high cost of electrodes. Organics with carbonyl groups are being investigated as the alternative to inorganic electrode materials because they offer the advantages of tunable structures, renewability, and they are environmentally benign. Furthermore, the wide internal space of such organic materials enables flexible storage of various charged ions (for example, H⁺ , Li⁺ , Na⁺ , K⁺ , Zn²⁺ , Mg²⁺ , and Ca²⁺ , and so on). We offer a comprehensive overview of the progress of organics containing carbonyls for energy storage and conversion in aqueous electrolytes, including applications in aqueous batteries as solid-state electrodes, in flow batteries as soluble redox species, and in water electrolysis as redox buffer electrodes. The advantages of organic electrodes are summarized, with a discussion of the challenges remaining for their practical application.

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 .