Cage-Like Porous Carbon with Superhigh Activity and Br2 -Complex-Entrapping Capability for Bromine-Based Flow Batteries

Cation-driven phase transition and anion-enhanced kinetics for high energy efficiency zinc-interhalide complex batteries

Aqueous Zn-halogen batteries, valued for high safety, large capacity, and low cost, suffer from the polyhalide shuttle effect and chaotic zinc electrodeposition, reducing energy efficiency and lifespan. Here we show a cation-driven positive electrode phase transition to suppress the shuttle effect and achieve uniform zinc electrodeposition, along with an anion kinetic enhancement strategy to improve energy efficiency and lifespan. Taking tetramethylammonium halide (TMAX, X = F, Cl, Br) as a subject, TMA+ promotes oriented zinc (101) deposition on the negative electrode through electrostatic shielding, significantly extending cycling life. Concurrently, it captures I3- on the positive electrode, forming a stable solid-phase interhalide complex that enhances coulombic efficiency. Compared to I3- and TMAI3, X- anions lower the Gibbs free energy differences of I- → I2X- and I2X- → TMAI2X, accelerating I-/I2X-/TMAI2X conversions and improving voltage efficiency. In TMAF-modified electrolytes, zinc interhalide complex batteries achieve a high energy efficiency of 95.2% at 0.2 A g-1 with good reversibility, showing only 0.1% capacity decay per cycle over 1000 cycles. At 1 A g-1, they show a low decay rate of 0.1‰ per cycle across 10,000 cycles. This study provides insights into enhancing energy efficiency and long-term stability for sustainable energy storage.

Reaction Kinetics and Mass Transfer Synergistically Enhanced Electrodes for High-Performance Zinc-Bromine Flow Batteries

Zinc-bromine flow batteries (ZBFBs) hold great promise for grid-scale energy storage owing to their high theoretical energy density and cost-effectiveness. However, conventional ZBFBs suffer from inhomogeneous zinc deposition and sluggish Br2/Br- redox kinetics, resulting in a short cycle life and low power density. Herein, a multiscale porous electrode with abundant nitrogen-containing functional groups is developed by growing zeolitic imidazolate framework-8 in situ on graphite felts, followed by a facile carbonization process to simultaneously tackle both the challenges. Theoretical and experimental results reveal that nitrogen-containing functional groups exhibit a high adsorption energy toward zinc atoms, while the microstructures promote pore-level mass transport, thereby resulting in compact and uniform zinc deposition. In the meantime, the electrode boosts the Br2/Br- reaction kinetics due to its high catalytic activity and large surface area. As a result, the ZBFBs equipped with optimized electrodes at both negative and positive sides can operate at an ultrahigh current density of 250 mA cm-2 while maintaining an energy efficiency of 68.0%, far surpassing that with pristine graphite felts (50.7%). Remarkably, the battery exhibits excellent cycling stability over 2000 cycles without obvious decay. This study provides a simple yet effective method for developing high-performance electrodes to tackle the critical challenges in ZBFBs, thereby promoting the commercialization of this promising energy storage technology.

Rational Design Toward Advanced Non-Flow Aqueous Zinc-Bromine Systems Boosted by Alkaline-Neutral Decoupling Electrolytes

Non-flow aqueous zinc-bromine batteries (AZBBs) are highly attractive owing to their lightweight construction and largely reduced cost compared with the flow ones. Yet, their development is restricted by the sluggish reaction kinetics of Br2/Br-, the shuttle of soluble polybromide species (Brn -, n is odd), and the poor stability of Zn-based anode. Herein, an effective alkaline-neutral electrolyte decoupling system is constructed to mitigate these issues, where nitrogen-doped carbon felt with high catalytic activity to Br2/Br- reaction is developed for cathode, a cost-effective cation exchange membrane (CEM) of polyethersulfone/sulfonated polyether ether ketone (PES/SPEEK-M) that can stop Brn - is used as separator, and glucose that can inhibit dendrites is introduced as anolyte additive. The constructed flowless AZBB mainly consists of two separate redox couples, including Zn/Zn(OH)4 2- in alkaline anolyte and Br2/Br- in neutral media, where non-cations (e.g. OH-, Zn(OH)4 2-, H2O,  and Brn -) can be restricted to their respective chamber by the PES/SPEEK-M while cations can pass by. In the optimized system, good electrochemical performance is achieved, mainly including a surprising discharge voltage of 2.01 V, a high average Coulombic efficiency of 96.7%, and a good cycling life of ≈1000 cycles without obvious capacity decay at a fixed charge capacity of 2 mAh cm-2.

Stable Zinc Electrode Reaction Enabled by Combined Cationic and Anionic Electrolyte Additives for Non-Flow Aqueous Zn─Br2 Batteries

Aqueous zinc-bromine batteries hold immense promise for large-scale energy storage systems due to their inherent safety and high energy density. However, achieving a reliable zinc metal electrode reaction is challenging because zinc metal in the aqueous electrolyte inevitably leads to dendrite growth and related side reactions, resulting in rapid capacity fading. Here, it is reported that combined cationic and anionic additives in the electrolytes using CeCl3 can simultaneously address the multiple chronic issues of the zinc metal electrode. Trivalent Ce3+ forms an electrostatic shielding layer to prevent Zn2+ from concentrating at zinc metal protrusions, while the high electron-donating nature of Cl- mitigates H2O decomposition on the zinc metal surface by reducing the interaction between Zn2+ and H2O. These combined cationic and anionic effects significantly enhance the reversibility of the zinc metal reaction, allowing the non-flow aqueous Zn─Br2 full-cell to reliably cycle with exceptionally high capacity (>400 mAh after 5000 cycles) even in a large-scale battery configuration of 15 × 15 cm2.

Urchin-Like Mesoporous TiN Hollow Sphere Enabling Promoted Electrochemical Kinetics of Bromine-Based Flow Batteries

Bromine-based flow batteries (BFB) have always suffered from poor kinetics due to the sluggish Br3 -/Br- redox, hindering their practical applications. Developing cathode materials with high catalytic activity is critical to address this challenge. Herein, the in-depth investigation for the free energy of the bromine redox electrode is conducted initially through DFT calculations, establishing the posterior desorption during oxidation as the rate-determining step. An urchin-like titanium nitride hollow sphere (TNHS) composite is designed and synthesized as the catalyst for bromine redox. The large difference in Br- and Br3 - adsorption capability of TNHS promotes rapid desorption of generated Br3 - during the oxidation process, liberating active sites timely to enable smooth ongoing reactions. Besides, the urchin-like microporous/mesoporous structure of TNHS provides abundant active surface for bromine redox reactions, and ample cavities for the bromine accommodation. The inherently high conductivity of TNHS enables facile electron transfer through multiple channels. Consequently, zinc-bromide flow batteries with TNHS catalyst exhibit significantly enhanced kinetics, stably operating at 80 mA cm-2 with 82.78% energy efficiency. Overall, this study offers a solving strategy and catalyst design approach to the sluggish kinetics that has plagued bromine-based flow batteries.

The design engineering of nanocatalysts for high power redox flow batteries

Redox flow batteries (RFBs) are one of the most promising long-term energy storage technologies which utilize the redox reaction of active species to realize charge and discharge. With the decoupled power and energy components, RFBs exhibit high battery pile construction flexibility and long lifespan. However, the inherent slow electrochemical kinetics of the current widely applied redox active species severely impedes the power output of RFBs. Developing high performance electrocatalysts for these redox active species would boost the power output and energy efficiency of RFBs. Here, we present a critical review of nanoelectrocatalysts to improve the sluggish kinetics of different redox active species, mainly including the chemical components, structure and integration methods. The relationship between the physicochemical properties of nanoelectrocatalysts and the power output of RFBs is highlighted. Finally, the future design of nanoelectrocatalysts for commercial RFBs is proposed.

Achievement of Efficient and Stable Nonflow Zinc-Bromine Batteries Assisted by Rational Decoration upon the Two Electrodes

Aqueous zinc-bromine batteries (ZBBs) are highly promising because of the advantages of safety and cost. Compared with flow ZBBs, static ones without the assistance of pumping and tank components possess decreased cost and increased energy density and efficiency. Yet, the issues of Zn dendrites and shuttle effect of polybromide ions (Brn-) are more serious in nonflow ZBBs. Meanwhile, the hydrogen evolution reaction (HER) and the sluggish kinetics of the Br2/Br- couple are also in-negligible. Herein, a compressive approach, the cation-exchange membrane (CEM) coating on Zn anodes and N-defect decoration toward carbon felt cathodes, is developed. The CEM with cation-only function can inhibit the formation of Zn dendrites via tuning the Zn2+ flow at the interface, block the noncationic substances, and hence prevent the shuttle of Br2/Brn- and the water decomposition-concerned HER. The optimized nonflow ZBBs can deliver high Coulombic, voltage, and energy efficiencies of 94.1, 92.8, and 87.4%, respectively, which can be well remained in 1000 cycles. Meanwhile, the output voltage is as high as 1.7 V at 10 mA cm-2 with a high areal capacity of 2 mA h cm-2, and a LED with a rated voltage of 1.6 V can be powered successfully, exhibiting high application value.

Hollow Nitrogen-Doped Carbon Spheres as Zincophilic Sites for Zn Flow Battery

Severe dendrite growth on Zn anodes poses a significant challenge to the development of Zn-based batteries. An effective strategy for inhibiting the formation of Zn dendrites involves electrode modification. In this study, hollow nitrogen-doped carbon spheres (HNCS) are synthesized and used as electrodes to regulate Zn deposition in Zn-based flow batteries. The electrochemical performance of HNCS reveals that the pyrrole nitrogen of HNCS changes the electrode surface state. Therefore, HNCS can inhibit the hydrogen evolution reaction and achieve uniform Zn deposition. HNCS can effectively inhibit dendrite growth and improve the reversibility of the Zn plating/stripping process to regulate the reversibility of Zn-based batteries. The zinc-bromine redox flow battery assembled with HNCS significantly reduces the hydrogen evolution reaction and exhibits a coulombic efficiency of 90 % and energy efficiency of 73 % at a current density of 60 mA cm-2. Similarly, an alkaline zinc-iron flow battery can maintain high Coulombic efficiency and energy efficiency of 83 %.

Zn Anode Surviving Extremely Corrosive Polybromide Environment with Alginate-Graphene Oxide Hydrogel Coating

Zinc-bromine (Zn-Br) redox provides a high energy density and low-cost option for next-generation energy storage systems, and polybromide diffusion remains a major issue leading to Zn anode corrosion, dendrite growth, battery self-discharge and limited electrochemical performance. A dual-functional Alginate-Graphene Oxide (AGO) hydrogel coating is proposed to prevent polybromide corrosion and suppress dendrite growth in Zn-Br batteries through negatively charged carboxyl groups and enhanced mechanical properties. The battery with anode of plain zinc coated with AGO (Zn]AGO) survives a severely corrosive environment with higher polybromide concentration than usual without a membrane, and achieves 80 cycles with 100% Coulombic and 80.65% energy efficiencies, four times compared to plain Zn anode. The promising performance is comparable to typical Zn-Br batteries using physical membranes, and the AGO coating concept can be well adapted to various Zn-Br systems to promote their applications.

Rechargeable Aqueous Zinc-Halogen Batteries: Fundamental Mechanisms, Research Issues, and Future Perspectives

Aqueous zinc-halogen batteries (AZHBs) have emerged as promising candidates for energy storage applications due to their high security features and low cost. However, several challenges including natural subliming, sluggish reaction kinetics, and shuttle effect of halogens, as well as dendrite growth of the zinc (Zn) anode, have hindered their large-scale commercialization. In this review, first the fundamental mechanisms and scientific issues associated with AZHBs are summarized. Then the research issues and progresses related to the cathode, separator, anode, and electrolyte are discussed. Additionally, emerging research opportunities in this field is explored. Finally, ideas and prospects for the future development of AZHBs are presented. The objective of this review is to stimulate further exploration, foster the advancement of AZHBs, and contribute to the diversified development of electrochemical energy storage.

Zinc-Bromine Batteries: Challenges, Prospective Solutions, and Future

Zinc-bromine batteries (ZBBs) have recently gained significant attention as inexpensive and safer alternatives to potentially flammable lithium-ion batteries. Zn metal is relatively stable in aqueous electrolytes, making ZBBs safer and easier to handle. However, Zn metal anodes are still affected by several issues, including dendrite growth, Zn dissolution, and the crossover of Br species from cathodes to corrode anodes, resulting in self-discharge and fast performance fading. Similarly, Br2 undergoes sluggish redox reactions on cathodes, which brings several issues such as poor reaction kinetics, the highly corrosive nature of Br species leading to corrosion of separators and poisoning of anodes, and the volatile nature of Br species causing increased internal pressures, etc. These issues are compounded in flowless ZBB configuration as no fresh electrolyte is available to provide extra/fresh reaction species. In this review, the factors controlling the performance of ZBBs in flow and flowless configurations are thoroughly reviewed, along with the status of ZBBs in the commercial sector. The review also summarizes various novel methodologies to mitigate these challenges and presents research areas for future studies. In summary, this review will offer a perspective on the historical evolution, recent advancements, and prospects of ZBBs.

Multi-Ion Engineering Strategies toward High Performance Aqueous Zinc-Based Batteries

As alternatives to batteries with organic electrolytes, aqueous zinc-based batteries (AZBs) have been intensively studied. However, the sluggish kinetics, side reactions, structural collapse, and dissolution of the cathode severely compromise the commercialization of AZBs. Among various strategies to accelerate their practical applications, multi-ion engineering shows great feasibility to maintain the original structure of the cathode and provide sufficient energy density for high-performance AZBs. Though multi-ion engineering strategies could solve most of the problems encountered by AZBs and show great potential in achieving practical AZBs, the comprehensive summaries of the batteries undergo electrochemical reactions involving more than one charge carrier is still in deficiency. The ambiguous nomenclature and classification are becoming the fountainhead of confusion and chaos. In this circumstance, this review overviews all the battery configurations and the corresponding reaction mechanisms are investigated in the multi-ion engineering of aqueous zinc-based batteries. By combing through all the reported works, this is the first to nomenclate the different configurations according to the reaction mechanisms of the additional ions, laying the foundation for future unified discussions. The performance enhancement, fundamental challenges, and future developing direction of multi-ion strategies are accordingly proposed, aiming to further accelerate the pace to achieve the commercialization of AZBs with high performance.

Scientific issues of zinc-bromine flow batteries and mitigation strategies

Zinc-bromine flow batteries (ZBFBs) are promising candidates for the large-scale stationary energy storage application due to their inherent scalability and flexibility, low cost, green, and environmentally friendly characteristics. ZBFBs have been commercially available for several years in both grid scale and residential energy storage applications. Nevertheless, their continued development still presents challenges associated with electrodes, separators, electrolyte, as well as their operational chemistry. Therefore, rational design of these components in ZBFBs is of utmost importance to further improve the overall device performance. In this review, the focus is on the scientific understanding of the fundamental electrochemistry and functional components of ZBFBs, with an emphasis on the technical challenges of reaction chemistry, development of functional materials, and their application in ZBFBs. Current limitations of ZBFBs with future research directions in the development of high performance ZBFBs are suggested.

Functional materials for aqueous redox flow batteries: merits and applications

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, offering vast potential for large-scale applications. Their unique configuration allows energy and power to be decoupled, making them highly scalable and flexible in design. Aqueous RFBs stand out as the most promising technologies, primarily due to their inexpensive supporting electrolytes and high safety. For aqueous RFBs, there has been a skyrocketing increase in studies focusing on the development of advanced functional materials that offer exceptional merits. They include redox-active materials with high solubility and stability, electrodes with excellent mechanical and chemical stability, and membranes with high ion selectivity and conductivity. This review summarizes the types of aqueous RFBs currently studied, providing an outline of the merits needed for functional materials from a practical perspective. We discuss design principles for redox-active candidates that can exhibit excellent performance, ranging from inorganic to organic active materials, and summarize the development of and need for electrode and membrane materials. Additionally, we analyze the mechanisms that cause battery performance decay from intrinsic features to external influences. We also describe current research priorities and development trends, concluding with a summary of future development directions for functional materials with valuable insights for practical applications.

Effective Enhancement of Energy Density of Zinc-Polyiodide Flow Batteries by Organic/Penta-iodide Complexation

Based on the ambipolar characteristics and high solubility of ZnI2, zinc-polyiodide flow batteries (ZIFB) have attracted attention as high-energy density flow batteries. However, due to the various oxidation products of iodide (I-) and the formation of iodine (I2) solid precipitates at the positive electrode, the limiting state-of-charge (SoC) of ZIFB has not been clearly defined. Herein, a clear definition of SoC in ZIFBs is given based on the thermodynamic relationship among I-(aq), I3-(aq), I5-(aq), and I2(aq) in the electrolyte. Conventional ZIFBs are limited by their maximum attainable SoC of 87%, at which the fully charged catholyte includes I-, I3-, and I5- ions at molar ratios of 49.6, 32.2, and 18.1%, respectively. Furthermore, two effective strategies to extend the maximum SoC are suggested: (1) increasing the formation constant (Keq) of I3- can raise the availability of I- for electrooxidation by suppressing I2 precipitation, and (2) promoting the production of higher-order polyiodides such as I5- can increase the oxidation state of the charged electrolyte. The addition of 5 vol % triethylene glycol (tri-EG) to the electrolyte increased Keq from 710 to 1123 L mol-1; this increase was confirmed spectrophotometrically. Tri-EG stabilized I5- ions in the form of the I5-/tri-EG complex, thereby converting the main oxidation product from I3- to I5-. The preferred electrochemical production of I5- in the tri-EG electrolyte was observed by electrochemical and computational analyses. As a result, the maximum attainable SoC was enhanced remarkably to 116%, yielding molar ratios of I-, I3-, and I5- ions of 9.1, 11.2, and 79.7%, respectively. This SoC extension effect was confirmed in the ZIFB flow cell with stable charge-discharge cycling at the SoC 120% limit, demonstrating the highest energy density, 249.9 Wh L-1, among all reported ZIFBs.

A Gram Scale Soft-Template Synthesis of Heteroatom Doped Nanoporous Hollow Carbon Spheres for Oxygen Reduction Reaction

Heteroatom-doped nanoporous carbon materials with unique hierarchical structures have been shown to be promising supports and catalysts for energy conversion; however, hard-template methods are limited by their inflexibility and time-consuming process. Soft-template methods have been suggested as an alternative, but they are limited by their picky requirements for stable reactions and the few known precursors for small-batch synthesis. In this study, a gram-scale soft-template-based silica-assisted method was investigated for producing nitrogen-doped hollow nanoporous carbon spheres (N-HNCS). Nitrogen doping is accomplished during preparation with enhanced electrocatalytic activity without complicating the methodology. To investigate the effect of the unique structural characteristics of N-HNCS (specific surface area: 1250 m2 g-1; pore volume: 1.2 cm3 g-1), cobalt was introduced as an active center for the oxygen reduction reaction. Finely tuned reaction conditions resulted in well-dispersed cobalt particles with minimal agglomeration. This sheds light on the advancement of new experimental procedures for developing more active and promising non-noble catalysts in large and stable batches.

Zinc-Bromine Rechargeable Batteries: From Device Configuration, Electrochemistry, Material to Performance Evaluation

Zinc-bromine rechargeable batteries (ZBRBs) are one of the most powerful candidates for next-generation energy storage due to their potentially lower material cost, deep discharge capability, non-flammable electrolytes, relatively long lifetime and good reversibility. However, many opportunities remain to improve the efficiency and stability of these batteries for long-life operation. Here, we discuss the device configurations, working mechanisms and performance evaluation of ZBRBs. Both non-flow (static) and flow-type cells are highlighted in detail in this review. The fundamental electrochemical aspects, including the key challenges and promising solutions, are discussed, with particular attention paid to zinc and bromine half-cells, as their performance plays a critical role in determining the electrochemical performance of the battery system. The following sections examine the key performance metrics of ZBRBs and assessment methods using various ex situ and in situ/operando techniques. The review concludes with insights into future developments and prospects for high-performance ZBRBs.

Development of flow battery technologies using the principles of sustainable chemistry

Realizing decarbonization and sustainable energy supply by the integration of variable renewable energies has become an important direction for energy development. Flow batteries (FBs) are currently one of the most promising technologies for large-scale energy storage. This review aims to provide a comprehensive analysis of the state-of-the-art progress in FBs from the new perspectives of technological and environmental sustainability, thus guiding the future development of FB technologies. More importantly, we evaluate the current situation and future development of key materials with key aspects of green economy and decarbonization to promote sustainable development and improve the novel energy framework. Finally, we present an analysis of the current challenges and prospects on how to effectively construct low-carbon and sustainable FB materials in the future.

Conversion-Type Cathode Materials for Aqueous Zn Metal Batteries in Nonalkaline Aqueous Electrolytes: Progress, Challenges, and Solutions

Aqueous Zn metal batteries are attractive as safe and low-cost energy storage systems. At present, due to the narrow window of the aqueous electrolyte and the strong reliance of the Zn2+ ion intercalated reaction on the host structure, the current intercalated cathode materials exhibit restricted energy densities. In contrast, cathode materials with conversion reactions can promise higher energy densities. Especially, the recently reported conversion-type cathode materials that function in nonalkaline electrolytes have garnered increasing attention. This is because the use of nonalkaline electrolytes can prevent the occurrence of side reactions encountered in alkaline electrolytes and thereby enhance cycling stability. However, there is a lack of comprehensive review on the reaction mechanisms, progress, challenges, and solutions to these cathode materials. In this review, four kinds of conversion-type cathode materials including MnO2 , halogen materials (Br2 and I2 ), chalcogenide materials (O2 , S, Se, and Te), and Cu-based compounds (CuI, Cu2 O, Cu2 S, CuO, CuS, and CuSe) are reviewed. First, the reaction mechanisms and battery structures of these materials are introduced. Second, the fundamental problems and their corresponding solutions are discussed in detail in each material. Finally, future directions and efforts for the development of conversion-type cathode materials for aqueous Zn batteries are proposed.

Application of Poly(ether sulfone)-Based Membranes in Clean Energy Technology

Poly(ether sulfone) (PES) is a kind of polymer materials with excellent electrical insulation and acid/alkali stability. PES can be operated at high temperature continuously for a long time and still maintain excellent property stability in the environments with rapidly changed temperature, namely, great thermostability. Moreover, PES has low molding shrinkage, good dimensional stability and excellent film-forming characteristics. Compared with inorganic membranes, PES-based membranes have lower cost, which have received more attention and wide recognition in the field of clean energy technologies in recent years, such as flow batteries, fuel cells, water treatment, and gas separation. Therefore, this review summarizes the research status and prospect of the utilization of PES-based membranes in clean energy fields, in order to further promote their development and application.

A Zinc-Bromine Battery with Deep Eutectic Electrolytes

A deep eutectic solvent (DES) is an ionic liquid-analog electrolyte, newly emerging due to its low cost, easy preparation, and tunable properties. Herein, a zinc-bromine battery (ZBB) with a Zn-halide-based DES electrolyte prepared by mixing ZnBr2 , ZnCl2 , and a bromine-capturing agent is reported. The water-free DES electrolyte allows a closed-cell configuration for the ZBB owing to the prevention of Br2 evaporation and H2 evolution. It is found that the Cl- anion changes the structure of the zinc-halide complex anions and demonstrated that it improves the ion mobility and electrode reaction kinetics. The DES electrolyte with the optimized ZnCl2 composition shows much higher rate capability and a cycle life 90 times longer than that of a ZnCl2 -free DES electrolyte. A pouch-type flexible ZBB battery based on the DES electrolyte exhibits swelling-free operation for more than 120 cycles and stable operation under a folding test, suggesting its potential in consumer applications such as wearable electronics.

Machine learning for flow batteries: opportunities and challenges

With increased computational ability of modern computers, the rapid development of mathematical algorithms and the continuous establishment of material databases, artificial intelligence (AI) has shown tremendous potential in chemistry. Machine learning (ML), as one of the most important branches of AI, plays an important role in accelerating the discovery and design of key materials for flow batteries (FBs), and the optimization of FB systems. In this perspective, we first provide a fundamental understanding of the workflow of ML in FBs. Moreover, recent progress on applications of the state-of-art ML in both organic FBs and vanadium FBs are discussed. Finally, the challenges and future directions of ML research in FBs are proposed.

Carbon Materials as Positive Electrodes in Bromine-Based Flow Batteries

Bromine based redox flow batteries (RFBs) can provide sustainable energy storage due to the abundance of bromine. Such devices pair Br₂ /Br^- at the positive electrode with complementary redox couples at the negative electrode. Due to the highly corrosive nature of bromine, electrode materials need to be corrosion resistant and durable. The positive electrode requires good electrochemical activity and reversibility for the Br₂ /Br^- couple. Carbon materials enjoy the advantages of low cost, excellent electrical conductivity, chemical resistance, wide operational potential ranges, modifiable surface properties, and high surface area. Here carbon based materials for bromine electrodes are reviewed, with a focus on application in zinc-bromine, hydrogen-bromine, and polysulphide-bromine RFB systems, aiming to provide an overview of carbon materials to be used for design and development of bromine electrodes with improved performance. Aspects deserving further R&D are highlighted.

Rechargeable aqueous zinc-bromine batteries: an overview and future perspectives

Zinc-bromine batteries (ZBBs) receive wide attention in distributed energy storage because of the advantages of high theoretical energy density and low cost. However, their large-scale application is still confronted with some obstacles. Therefore, in-depth research and advancement on the structure, electrolyte, anode, cathode and membrane are of great significance and impendency. Herein, we review the past and present investigations on ZBBs, discuss the key problems and technical challenges, and propose perspectives for the future, with the focus on materials and chemistry. This perspective would provide valuable information on further development of ZBBs.

The Emerging of Aqueous Zinc-Based Dual Electrolytic Batteries

As high performance and safety alternatives to the batteries with organic electrolytes, aqueous zinc-based batteries are still far from satisfactory in practical use because of the limitation of the intercalation reaction mechanism and the strict requirements for the cathodes. Very recently, zinc-based dual electrolytic batteries (DEBs), where the cathode and anode are both based on reversible electrolytic reactions, are emerging. It features with electrode-free configuration, thus avoiding the preliminary active materials or electrode fabrication procedures. Meanwhile, the new battery chemistry typically possesses a high specific capacity, output voltage, faster reaction rates, and long cycling life. Herein, the advances of the development of various zinc-based DEBs, including Zn-MnO₂ , Zn-Br₂ , and Zn-I₂ DEBs, are systematically summarized. This review will focus on the working mechanisms of these batteries and how the decoupling catholyte and anolyte affect their output voltages. The perspectives of the opportunities and challenges are also suggested in the aspects of protecting zinc anode, enhancing volumetric energy density, suppressing fast self-discharge, and developing multifunctional integrated zinc-based DEBs.

A Dendrite-Free Tin Anode for High-Energy Aqueous Redox Flow Batteries

Metal-based aqueous redox flow batteries (ARFBs) such as zinc-based ARFBs have attracted remarkable attention owing to their intrinsic high energy density. However, severe dendrite issues limit their efficiency and lifespan. Here an aqueous metal anode operating between Sn(OH)₆ ^2- (stannate) and metal Sn is presented, providing a reversible four-electron transfer at -0.921 V vs standard hydrogen electrode. In strong contrast to severe Zn dendrites, the Sn(OH)₆ ^2- /Sn electrode shows smooth and dendrite-free morphology, which can be attributed to its intrinsic low-surface-energy anisotropy which facilitates isotropic crystal growth of Sn metal. By coupling with iodide/tri-iodide (I^- /I₃ ^- ), the static Sn-I cell demonstrates a stable cycling for 500 cycles (more than 2 months). In contrast, the state-of-the-art Zn anode suffers from serious dendrites and lasts less than 45 cycles (190 h) in Zn-I cells. A stable continuous flow cycling of Sn-I cell achieves a Sn areal capacity of 73.07 mAh cm^-2 at an average discharge voltage of 1.3 V for 350 h. The alkaline Sn electrode demonstrates dendrite-free morphology and superior performance in cycle life and areal capacity compared to state-of-the-art Zn metal anodes, offering a promising metal anode for high-energy ARFBs and other metal-based rechargeable aqueous batteries.

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.

Effect of Electrolyte Additives on the Water Transfer Behavior for Alkaline Zinc-Iron Flow Batteries

Alkaline zinc-iron flow batteries (AZIFBs) are a very promising candidate for electrochemical energy storage. The electrolyte plays an important role in determining the energy density and reliability of a battery. The substantial water migration through a membrane during cycling is one of the critical issues that affect the reliability and performance of an AZIFB. In this work, it has been proven that the reason for water migration in AZIFBs is the synergetic effect of concentration gradient, different ionic strengths of negolyte and posolyte, and the electric field. To address the issue of water migration in AZIFBs, a series of additives are employed and the effects of additives on the water transfer behavior and electrochemical performance of AZIFBs are investigated in detail. The results indicate that all investigated additives can suppress water migration through a polybenzimidazole membrane because of the shrunken gap of osmotic pressure and ionic strength between negolyte and posolyte. Moreover, organic additives such as glucose can decrease battery performance because of the increased polarizability of the electrode, whereas inorganic additives such as Na₂SO₄ demonstrate no distinct effect on battery performance. Specifically, an AZIFB that employs Na₂SO₄ as an additive in the negative electrolyte can afford a Coulombic efficiency of ∼99% and a voltage efficiency of ∼88% for 120 cycles at 80 mA cm^-2, together with a good effect for inhibiting water migration behavior. This paper presents an effective way to suppress water migration and increase the reliability of AZIFBs.

A Zinc-Dual-Halogen Battery with a Molten Hydrate Electrolyte

Halogen redox couples offer several advantages for energy storage such as low cost, high solubility in water, and high redox potential. However, the operational complexity of storing halogens at the oxidation state via liquid-phase media hampers their widespread application in energy-storage devices. Herein, an aqueous zinc-dual-halogen battery system taking the advantages of redox flow batteries (inherent scalability) and intercalation chemistry (high capacity) is designed and fabricated. To enhance specific energy, the designed cell exploits both bromine and chlorine as the cathode redox couples that are present as halozinc complexes in a newly developed molten hydrate electrolyte, which is distinctive to the conventional zinc-bromine batteries. Benefiting from the reversible uptake of halogens at the graphite cathode, exclusive reliance on earth-abundant elements, and membrane-free and possible flow-through configuration, the proposed battery can potentially realize high-performance massive electric energy storage at a reasonable cost.

A High-Performance Aqueous Zinc-Bromine Static Battery

The highly reversible zinc-bromine redox couple has been successfully applied in the zinc-bromine flow batteries; however, non-electroactive pump/pipe/reservoir parts and ion-selective membranes are essential to suppress the bromine diffusion. This work demonstrates a zinc-bromine static (non-flow) battery without these auxiliary parts and utilizing glass fiber separator, which overcomes the high self-discharge rate and low energy efficiency while the advantages of the zinc-bromine chemistry are well preserved. It is achieved by a multifunctional additive, tetrapropylammonium bromide (TPABr), which not only mitigates the bromine cross-diffusion by regulating the fluidic bromine to a condensed solid phase but also provides a favorable interface for zinc electrodeposition toward non-dendritic growth. The proposed zinc-bromine static battery demonstrates a high specific energy of 142 Wh kg-1 with a high energy efficiency up to 94%. By optimizing the porous electrode architecture, the battery shows an ultra-stable cycling life for over 11,000 cycles with controlled self-discharge rate.

Fluorine Doped Cagelike Carbon Electrocatalyst: An Insight into the Structure-Enhanced CO Selectivity for CO2 Reduction at High Overpotential

The critical bottleneck of electrocatalytic CO₂ reduction reaction (CO₂RR) lies in its low efficiency at high overpotential caused by competitive hydrogen evolution. It is challenging to develop an efficient catalyst achieving both high current density and high Faradaic efficiency (FE) for CO₂RR. Herein, we synthesized fluorine-doped cagelike porous carbon (F-CPC) by purposely tailoring its structural properties. The optimized F-CPC possesses large surface area with moderate mesopore and abundant micropores as well as high electrical conductivity. When used as catalyst for CO₂RR, F-CPC exhibits FE of 88.3% for CO at -1.0 V vs RHE with a current density of 37.5 mA·cm ^-2. Experimental results and finite element simulations demonstrate that the excellent CO₂RR performance of F-CPC at high overpotential should be attributed to its structure-enhanced electrocatalytic process stemming from its cagelike morphology.

Dendrite-Free Zinc Deposition Induced by Tin-Modified Multifunctional 3D Host for Stable Zinc-Based Flow Battery

Zinc (Zn) plating/stripping is a promising anodic electrochemical reaction for aqueous batteries because of its high safety, low cost, two electron transfer, and rapid reaction kinetics. However, the notorious dendrite growth of Zn has become one of the biggest obstacles hindering its further commercialization. A multifunctional binder-free tin (Sn)-modified 3D carbon felt anodic host (SH) is constructed for aqueous zinc-based flow batteries (ZFB) via a facile and scalable strategy. Compared with the pristine carbon felt host (PH), the as-fabricated SH affords more robust Zn nucleation sites, lower hydrogen evolution reaction potential and lower nucleation overpotential of Zn and thus better induces uniform Zn plating/stripping with very high Coulombic efficiency (CE). Based on such an SH, a symmetrical flow battery exhibits superior CE (290 cycles with average CE of 99.4%) and a zinc-bromine flow battery demonstrates a longer cycle life (142 cycles with average CE of 97.2%), much better than pristine PH. This is a simple, novel, and effective way to suppress Zn dendrites and improve the performance of ZFBs.

Enhanced Electrosorption Ability of Carbon Nanocages as an Advanced Electrode Material for Capacitive Deionization

The structure of an electrode material has an important impact on the performance of a capacitive deionization (CDI) device. However, it is still a challenge to design and synthesize electrode materials with a rational structure based on deep understanding of their structure-dependent CDI performance. Herein, we report the preparation of carbon nanocages (CNCs) with regulated shell thickness and a rich pore structure as an advanced material for high-performance CDI electrodes. The as-prepared CNC has a considerable specific capacitance of 149 F g^-1 at a scan rate of 5 mV s^-1. When used as CDI electrodes, the CNC shows an outstanding electrosorption ability of 17.5 mg g^-1 at 1.4 V at an initial concentration of 250 mg L^-1 NaCl solution. Furthermore, the CNC electrode displays high salt adsorption rate and good cyclic stability. Finite element simulations reveal that the superior structure of the CNC substantially promotes the ion transfer rate by shortening ion diffusion paths in the cavity of the electrode material. Also, both inner and outer walls of the CNC provide sufficient active sites for fast adsorption and desorption of salty ions. This work not only demonstrates that the CNC is a potential electrode material for CDI applications but also paves a way to design and prepare high-performance electrode materials based on a new perspective on their structure-performance relationship.

Advanced Materials for Zinc-Based Flow Battery: Development and Challenge

Zinc-based flow batteries (ZFBs) are well suitable for stationary energy storage applications because of their high energy density and low-cost advantages. Nevertheless, their wide application is still confronted with challenges, which are mainly from advanced materials. Therefore, research on advanced materials for ZFBs in terms of electrodes, membranes, and electrolytes as well as their chemistries are of the utmost importance. Herein, the focus is on the scientific understandings of the fundamental design of these advanced materials and their chemistries in relation to the battery performance. The principles of using different materials in different ZFB technologies, the functions and structure of the materials, and further material improvements are discussed in detail. Finally, the challenges and prospects of ZFBs are summarized as well. This review provides valuable instruction on how to design and develop new materials as well as new chemistries for ZFBs.

A highly stable neutral viologen/bromine aqueous flow battery with high energy and power density

A neutral viologen/Br₂ flow battery with high power density and energy density was designed and presented.

Inhibition of Zinc Dendrite Growth in Zinc-Based Batteries

Zinc deposition and dissolution is a significant process in zinc-based batteries. During this process, the formation of zinc dendrites is pervasive, which leads to the loss of efficiency and capacity of batteries. The continually growing dendrites will finally pierce the separator and cause the batteries to short circuit. Thus, employing effective methods to inhibit the formation and growth of zinc dendrites is vital for the practical application of zinc-based batteries. This Minireview first clarifies the formation and growth principles of zinc dendrites. Then, the research and development of methods to solve the problem of zinc dendrites are reviewed, including ways to suppress the further formation and growth of dendrites as far as possible, to minimize the adverse effects of dendrites, along with ways to produce dendrite-free deposition processes. The mechanisms, advantages, drawbacks, and perspectives of these methods are illustrated. Thus, this overview of these methods will aid understanding of the formation process of zinc dendrites and provide an extensive, comprehensive, and professional reference to resolve the problem of zinc dendrites completely.

Self-assembled heteropolyacid on nitrogen-enriched carbon nanofiber for vanadium flow batteries

A novel polyoxometalate-based electrode was developed by incorporating phosphotungstic acid (PWA) in nylon-6,6 nanofiber, followed by carbonization. The developed PWA-carbon nanofiber (PWA-CNF) showed the characteristics of the dual-scale porosity of micro- and mesoporous substrate with surface area of around 684 m2 g-1. The compound exhibited excellent stability in vanadium electrolyte and battery cycling. Evaluation of electrocatalytic properties toward V2+/V3+ and VO2+/VO2+ redox couples indicated promising advantages in electron transfer kinetics and increasing energy efficiency, particularly for the VO2+/VO2+ couple. Moreover, the developed electrode exhibited substantially improved energy efficiency (14% higher than that of pristine carbon felt) in the single cell vanadium redox flow battery. This outstanding performance was attributed to high surface area and abundant oxygen-containing linkages in the developed electrode.

Neutral Red and Ferroin as Reversible and Rapid Redox Materials for Redox Flow Batteries

Neutral red and ferroin are used as redox indicators (RINs) in potentiometric titrations. The rapid response and reversibility that are prerequisites for RINs are also desirable properties for the active materials in redox flow batteries (RFBs). This study describes the electrochemical properties of ferroin and neutral red as a redox pair. The rapid reaction rates of the RINs allow a cell to run at a rate of 4 C with 89 % capacity retention after the 100^th  cycle. The diffusion coefficients, electrode reaction rates, and solubilities of the RINs were determined. The electron-transfer rate constants of ferroin and neutral red are 0.11 and 0.027 cm s^-1 , respectively, which are greater than those of the components of all-vanadium and Zn/Br₂ cells.

A Dual-Stimuli-Responsive Sodium-Bromine Battery with Ultrahigh Energy Density

Stimuli-responsive energy storage devices have emerged for the fast-growing popularity of intelligent electronics. However, all previously reported stimuli-responsive energy storage devices have rather low energy densities (<250 Wh kg^-1 ) and single stimuli-response, which seriously limit their application scopes in intelligent electronics. Herein, a dual-stimuli-responsive sodium-bromine (Na//Br₂ ) battery featuring ultrahigh energy density, electrochromic effect, and fast thermal response is demonstrated. Remarkably, the fabricated Na//Br₂ battery exhibits a large operating voltage of 3.3 V and an energy density up to 760 Wh kg^-1 , which outperforms those for the state-of-the-art stimuli-responsive electrochemical energy storage devices. This work offers a promising approach for designing multi-stimuli-responsive and high-energy rechargeable batteries without sacrificing the electrochemical performance.