Search

As one of the latest research directions, membraneless batteries provide an economical solution to redox flow batteries. Advances in electrolyte and device design have promoted membraneless batteries from microfluidic demonstration to systems with stimulated modes and considerable electrochemical properties. However, the achieved cycling volumetric capacity is typically limited. Herein, a biphasic membraneless zinc‐iodine battery (Z|T‐I) is proposed, through optimized Zn growth, the Z|T‐I battery achieved a volumetric capacity of 8.93 Ah L−1 for 100 cycles with an average Coulombic efficiency (CE) of 98.49% (2300 h), the deep cycling exhibited a volumetric capacity of 21.7 Ah L−1 (50.43 Wh L−1 based on typical membraneless battery calculation, 1200 h); both values are the highest among single/biphasic membraneless batteries using liquid active materials with/out stimulations (<6.6 Ah L−1) reported so far based on the total electrolyte volume. A thorough analysis of Zn growth in Zn‐Br2/I2 electrolytes with various testing conditions provided direct evidence of the contrasting Zn electrodeposition morphologies at both macro and micro scales. The biphasic ZnI2 battery design strategy gives insights into optimizing material crossover/spatial utilization and electrolyte interfacial stability to realize a scalable membraneless energy storage system with a reduced cost. [ABSTRACT FROM AUTHOR]

In the pursuit of sustainable energy solutions, zinc‐based flow batteries stand out for their potential in large‐scale energy storage, offering a blend of cost efficiency and safety. Although the porous electrode provides an increased specific surface area for reaction, the non‐uniform deposition of zinc, attributed to uneven concentration distribution within the porous electrode, has been a pivotal issue in accelerating the formation of zinc dendrites, which hinders the enhancement of energy density. A composite electrode with a strategic hierarchical pore structure has been developed with aligned nitrogen‐doped carbon fibers and traditional carbon felt. This structure takes advantage of the large pores of the carbon felt for efficient through‐flow paths, ensuring higher flow rates, while the dual‐scale pores within the electrospun film enhance mass transfer and increase the specific surface area. At 320 mA cm−2, it achieved an ≈11.4% improvement in the battery's energy efficiency. Moreover, the nitrogen doping and the optimized reaction uniformity within the composite electrode have been instrumental in reducing the generation of zinc dendrites. The battery, integrated with the innovative composite electrode, maintained an energy efficiency of 59.2% at 320 mA cm−2 after 300 cycles, which is a substantial improvement in operational longevity. [ABSTRACT FROM AUTHOR]

Among multivalent redox flow batteries, the Zn‐based redox flow battery (RFB) has the advantages of high energy density, nontoxicity, and low cost. However, the severe dendrite issue hinders its deployment. Therefore, the Sn‐based RFB is reported as a prospective alternative, particularly in alkaline systems, due to its lower redox potential and four electrons transfer in redox reactions. However, the continuous growth of Sn during charge forms large Sn bulk, which increases the risk of “dead Sn” formation. By combining experiments and theoretical calculations, urea is proposed as an electrolyte additive to regulate Sn deposition on graphite felt. This is achieved through the enhanced adsorption and robust reaction of Sn(OH)62− with graphite felt. Consequently, a more uniform morphology ensues, reducing “dead Sn” formation and thus improving the average discharge voltage and areal capacity. Moreover, the Columbic efficiency becomes more stable, extending the cycle life to over 420 h (>200 cycles) without deep discharge during cycling. This study demonstrates the role of urea in achieving high‐performance Sn‐I flow battery with long cycle life, paving the way for further development of metal‐based hybrid RFBs. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries (VRFBs) hold great promise for large‐scale energy storage, but their performance requires further improvement. Herein, a design is proposed for vanadium colloid flow batteries (VCFBs) that integrates the redox chemistry of polyvalent vanadium‐based colloid suspensions with dispersed conductive agents into traditional vanadium electrolytes. The redox‐active colloids combine the advantages of nanoparticle suspensions and dissolved electrolytes, exhibiting good dispersibility, fluidity, conductivity, redox reversibility, and electrochemical kinetics. By leveraging a reversible dissolution/suspension process of high‐concentration vanadium‐based colloids, the VCFBs achieved an energy density of 48 Wh L−1, nearly double that of conventional VRFBs. This work presents a rational design for homologous active material colloids to enhance the energy density of aqueous redox flow batteries, thereby advancing the potential for grid‐scale and renewable energy storage. [ABSTRACT FROM AUTHOR]

Soluble organic molecules with diverse and highly tunable structures are receiving more and more attention as redox‐active materials for aqueous organic flow batteries (AOFBs). However, the undesirable parasitic reactions of redox‐active molecules restrict their practical application. Herein, a general molecular stabilization strategy is proposed by introducing amino groups on 3,7‐positions of polycyclic azine derivatives (phenothiazine and phenoxazine compounds) to effectively enhance the redox activity and electrochemical stability of molecules. The theoretical calculation and a series of electrochemical analyses revealed that the 3,7‐substituted amino groups serving as the redox active sites can stabilize the intermediate radicals by delocalizing the electron density in the acidic solution, preventing nucleophilic/electrophilic attack. As a result, the designed 3,7‐bis‐((2‐hydroxyethyl)(methyl)amino)phenothiazin‐5‐ium bromide not only achieved the remarkable electrochemical cycling performance in the flow cell with an ultra‐low‐capacity fade rate of 0.00029% per cycle for 18000 cycles (62 days) and but also delivered a stable high capacity of 47 Ah L−1 (with capacity fade 0.0056% per cycle, 0.077% per day) under ambient condition. Such finding undoubtedly provides a guidance to design stable redox‐active molecules for AOFBs. [ABSTRACT FROM AUTHOR]

In this study, a low‐melting organic‐inorganic crystalline ionic liquid compound, N‐butyl pyridinium tetrachlorido ferrate (III) is described. The material can easily be synthesized using a one‐pot approach in an ionic liquid medium. Single‐crystal X‐ray diffraction confirms that the basic inorganic block is [FeCl4]−, which is counterbalanced by an N‐butyl pyridinium cation. The compound exhibits a melting point of 37.6 °C by differential scanning calorimetry, which is among the lowest values for a pyridinium‐based metal‐containing ionic liquid. The material shows promising electrochemical behavior at room temperature in both aqueous and nonaqueous solvents, and at elevated temperatures in its pure liquid state. Given its appreciable solubility in both water and acetonitrile, the compound can act as a redox‐active species in a supporting electrolyte for redox flow battery applications. These classes of low‐melting ionic solids with long‐range order and interesting electrochemical applications are potential candidates for a range of green energy storage and harvesting systems. [ABSTRACT FROM AUTHOR]

A novel iron‐based posolyte redox species are presented for an aqueous redox flow battery, (Tetrakis(2‐pyridylmethyl)ethylenediamine)iron(II) dichloride, which is obtained by a simple synthetic route, shows a high redox potential of 0.788 V versus SHE, and exhibits exceptional aqueous solubility of 1.46 M. Paired with bis(3‐trimethylammonio)propyl viologen tetrachloride at neutral pH, the battery demonstrates an open‐circuit voltage of 1.19 V and delivers good cycling performance, with a capacity fade rate of 0.28% per day and coulombic efficiency of 99.3%. Postmortem chemical and electrochemical analyses of the posolyte species suggest future routes for stabilization of the complex. Among all the iron complexes with a redox potential above 0.4 V versus SHE, this compound exhibits the highest solubility. These results offer valuable insights that can be applied to the development of future posolyte species for sustainable energy storage solutions. [ABSTRACT FROM AUTHOR]

Bipolar redox‐active organic molecule (BROM) is a feasible strategy to address the cross‐contamination issue of the electrolyte and, thus, improve the stability of the flow battery. Herein, a highly‐soluble BROM is developed by combining the 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) and viologen moieties, and extensive characterizations are performed to evaluate its applicability in flow battery. Salient findings are as follows. First, the compound, viz. 1‐(1‐oxyl‐2,2,6,6‐tetramethylpiperidin‐4‐yl)‐1′‐(3‐(trimethylammonio)propyl)‐4,4′‐bipyridinium trichloride ((TPABPy)Cl3), features highly hydrophilic groups and yields a high aqueous solubility of 1.76 m. Second, the electrochemical result reveals that the (TPABPy)Cl3 displays two pairs of highly reversible peaks at −0.56 and 0.76 V, which respectively correspond to the viologen and TEMPO moieties. The electronic structure during the redox reactions is identified by both the density functional theory calculation and the electron paramagnetic resonance. Third, the flow battery fed with the 1.0 m (TPABPy)Cl3 solution delivers a high capacity of 25 Ah L−1 and a superior stability over the non‐bipolar counterparts. More to the point, the capacity decay can be effectively recovered by applying the polarity‐inversion rebalance strategy on the BROM. In summary, this work provides a molecular engineering way to rationally design a BROM to improve the capacity and stability of aqueous organic redox flow batteries. [ABSTRACT FROM AUTHOR]

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

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

Ion‐selective membranes are widely used in various energy storage applications. However, conventional polymer‐based ion‐selective membranes, such as Nafion, are not perfectly ion‐selective, leading to a significant permeation of undesirable ionic species. As the imperfect ion‐selectivity of membranes leads to the failure of energy storage devices, much effort is devoted to improving membrane ion‐selectivity. Herein, an immiscible liquid‐state membrane is introduced for aqueous redox flow batteries. As hydrophobic ionic liquids are immiscible with aqueous catholyte and anolyte solutions, they separate the two without crossover. This property renders them suitable for use as a membrane for aqueous redox flow batteries. In addition, ionic liquids with long side alkyl chains, such as 1‐hexyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM‐TFSI), are miscible with sulfuric acid, whereas transition metal sulfates remain insoluble in HMIM‐TFSI. For this reason, HMIM‐TFSI is selectively permeable to protons and remains impermeable to transition metal cations. As a result, the HMIM‐TFSI membrane is almost perfectly proton‐selective, leading to negligible permeability of unfavorable ionic species, such as transition metal cations. Eventually, the HMIM‐TFSI membrane shows the excellent electrochemical performance of vanadium redox flow batteries, such as negligible self‐discharge over 2800 h, high Coulombic efficiency (≈99%), and stable capacity retention over 100 cycles. [ABSTRACT FROM AUTHOR]

Transition metal single atoms anchored on nitrogen‐doped carbon (M‐N‐C) matrix with M‐N‐C active sites have shown to be promising catalysts for both hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Herein, a hybrid catalyst with low‐level loading of atomic Pt and Co species encapsulated in nitrogen‐doped graphene (Pt@CoN4‐G) is developed. The Pt@CoN4‐G shows low overpotential for HER in wide‐pH electrolyte and manifests improved mass activity with almost eight times greater than that of Pt/C at an overpotential of 50 mV. The Pt@CoN4‐G also exhibits a top‐level ORR activity (half‐wave potential, E1/2 = 0.893 V) and robust stability (>200 h) in alkaline medium. Using theoretical calculations and comprehensive characterizations , the strong metal–support interactions between Pt species and CoN4‐G support and synergistical cooperation of multiple active sites are clarified. A flow alkali‐Al/acid hybrid fuel cell using Pt@CoN4‐G as cathode catalyst delivers a large power density of 222 mW cm−2 with excellent stability to achieve simultaneously hydrogen evolution and electricity generation. In addition, Pt@CoN4‐G endows a flow Zn‐air battery with high power density (316 mW cm−2), good stability under large current density (>100 h at 100 mA cm−2), and long cycle life (over 600 h at 5 mA cm−2). [ABSTRACT FROM AUTHOR]

Aluminum (Al) is an ideal anode material in low‐cost battery system for energy storage, with high theoretical capacities. However, the sluggish Al3+‐involved kinetics challenges the selection of common cathode materials (Al3+ intercalation or conversion). Herein, a redox‐active Fe–Cl complex serves as the liquid‐state cathode to couple with a low‐cost Al anode, which synergizes the advantages of redox flow batteries and Al rechargeable batteries. The interplay of Fe‐Cl coordinated formula and electrochemical properties are revealed for the first time. It is found that [Fe2Cl7]− molecule has a high voltage versus Al anode (1.3 V), and the novel Fe‐Al hybrid battery fulfills a capacity of 1.6 mAh cm−2 (20 Ah L−1) record high in a coin cell among Al‐based batteries. Furthermore, the energy efficiency, which is a vital parameter to evaluate the energy cost of the energy storage technology, reaches 85% (superior to most Al‐based batteries) and an average of 70% over ≈900 h cycling. Particularly, the unique air‐stable character enables normal operation of the battery assembled in ambient air. This work establishes a new application scenario for Al anode toward low‐cost large‐scale energy storage. [ABSTRACT FROM AUTHOR]

Zinc‐based flow batteries are promising for distributed energy storage due to their low‐cost and high‐energy density advantages. One of the most critical issues for their practical application is the reliability that results from the heterogeneous zinc deposition and dead zinc from falling off the electrode. Herein, nitrogen‐doped carbon nanotubes (N‐CNTs)‐based composite membrane through a facilely partially embedded method is reported to enable a dendrite‐free alkaline zinc‐based flow battery. The results indicate that the electrically conductive N‐CNTs functional layer can enhance the transport dynamics of charge carriers and homogenize electric field distribution in membrane–electrode interface, which induces the initial nucleation of metallic zinc from the carbon felt electrode to N‐CNTs functional layer and further achieve a uniform and dense plating of metallic zinc in alkaline media. Thus, the engineered membrane enables a stable alkaline zinc–iron flow battery performance for more than 350 h at a current density of 80 mA cm−2. Moreover, an energy efficiency of over 80% can be afforded at a current density of 200 mA cm−2. The scientific finding of this study provides a new strategy on composite membranes design and their capability to adjust the plating of metallic zinc in alkaline media. [ABSTRACT FROM AUTHOR]

Ion conductive membranes (ICMs) with robust sub‐2‐nm channels show high proton transport rate in flow battery, but it remains a great challenge to precisely control the ion sieving of the membranes. Herein, as a promising proton‐selective carrier, sulfonated piperazine covalent triazine framework (s‐pCTF) with the channel size of ≈1.5 nm and abundant fast proton hopping sites is introduced into sulfonated poly(ether ether ketone) (SPEEK) to fabricate advanced ICM for vanadium flow battery (VFB) application. The interior protoplasmic channels of s‐pCTF demonstrate significant Donnan exclusion effect, resulting in a high proton/vanadium ion selectivity in theory (6.22 × 105). Meanwhile, the nitrogen‐rich sub‐2‐nm channels yield fast proton highway, and exterior‐grafted sulfonic acid groups further facilitate the proton transfer. By regulating the ion sieving and proton conductivity, the optimal hybrid membrane exhibits synchronously improved battery performance with an enhanced energy efficiency (92.41% to 78.53% at 40–200 mA cm−2) and long‐term stability for 900 cycles over 400 h (EE: 87.2–85% at 120 mA cm−2), outperforming pure SPEEK and Nafion212 membranes. This study validates the applicability of organic porous CTF with sub‐2‐nm channels and desired functionality in ICMs for high‐performance VFB application. [ABSTRACT FROM AUTHOR]

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

Ion exchange membranes (IEMs) that can selectively transport ions are crucial to a variety of applications, such as ion extraction/separation, fuel cells, redox flow batteries, and water electrolysis. IEM performance, in terms of membrane permeability/conductivity and selectivity, relies heavily on the formation of effective ion transport channels within the membranes, and there exists a tradeoff between permeability/conductivity and selectivity, which obstructs the development and widespread adoption of IEM‐based processes. To overcome this tradeoff and to advance IEM‐related applications, extensive research efforts are devoted to the construction and tuning of ion transport channels, and various strategies are proposed. These strategies mainly include 1) inducing microphase separation by properly regulating the polymer architecture, 2) introducing a third phase/region for ion transfer, 3) realizing a high free volume with polyrotaxanes or polymers with sterically bulky groups, and 4) constructing ion transport channels with nanoporous materials. These strategies are outlined and summarized in this review. Perspectives and future research directions are also discussed. [ABSTRACT FROM AUTHOR]

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

Covalent organic frameworks (COFs) have permanent, nanometer‐scale pores endowed with high porosity and thermal and chemical stability, which is widely used as composite membranes and applied in the field of separation. Conventional methods for preparing composite membranes, such as spray coating, spin coating, and pressure‐assisted or vacuum‐assisted filtration, have problems such as the selective layer can easily fall off. Here, an integrated casting–precipitation–evaporation method is proposed to prepare flexible COF composite membranes (DPBI/HTpBD). The DPBI/HTpBD composite membranes constructed with hollow spherical shell COF selective layer (HTpBD) and porous supporting layer are designed and applied in a vanadium flow battery. The excellent coulombic efficiency (CE, 99.5%) and voltage efficiency (VE, 81.1%) of batteries (180 mA cm−2) are achieved by the nanometre‐scale pores of the hollow spherical shell COF selective layer. In addition, the rigid pore structure inside COF bonded by covalent bonds endows the DPBI/HTpBD composite membranes with outstandingly high temperature stability. Compared to that of the Nafion 212 and DPBI pristine membranes (CE reduces by 5.2%, 3.3%), the CE of DPBI/HTpBD‐9 composite membrane‐based cell only reduces by 0.9% as the temperature increases from 25 to 55 °C. This work provides a new preparation method for high ion‐selective and high temperature stable battery membranes. [ABSTRACT FROM AUTHOR]

Viologen and its derivatives have been widely investigated as the electroactive materials in aqueous flow batteries (AFBs). The high redox potential of viologen poses a challenge to the AFB's energy density when used as the negative electrolyte. A molecular engineering strategy is developed by inserting an electron‐rich π‐bridge unit (viz. thiophene, furan) between the two pyridiniums to lower the redox potential. The resultant 1,1′‐bis[3‐(trimethylamonium)propyl]‐4,4′‐(2,5‐thiophenediyl)bispyridinium tetrachloride ((ATBPy)Cl4) and 1,1′‐bis[3‐(trimethylamonium)propyl]‐4,4′‐(2,5‐furandiyl)bispyridinium tetrachloride ((AFBPy)Cl4) are investigated by a variety of physiochemical methods to track the structure of the molecule and its evolution during the electrochemical reaction. A flow battery is assembled to verify the applicability of (ATBPy)Cl4 with 1‐(1‐oxyl‐2,2,6,6‐tetramethylpiperidin‐4‐yl)‐1′‐(3‐(trimethylammonio)propyl)‐4,4′‐bipyridinium trichloride as the positive electrolyte. Run at the concentration of 1.0 m, the battery achieves a standard cell voltage of 1.51 V, capacity of 23.6 Ah L–1, energy efficiency of 85.7% at 60 mA cm–2, and a record peak power density of 302 mW cm–2, which outperform the reported viologen‐AFBs in literature. This work provides an effective strategy to develop electrochemically active compounds with tunable redox potential and superior chemical stability by molecule engineering. [ABSTRACT FROM AUTHOR]

Due to decoupled energy and power, the aqueous organic redox flow battery (AORFB) represents a promising energy storage technology that stores energy in redox‐active organic compounds dissolved in aqueous electrolytes. The organic compounds are composed of earth‐abundant elements and are therefore potentially cost‐effective. Their structures are diverse and highly tunable, rendering it possible to regulate the redox potential, water solubility, and cycling lifespan. Therefore, the adoption of redox‐active organics greatly expands the space for enabling exceeding performance merits compared to their inorganic counterparts. Herein, this work reviews the molecular design and engineering scheme of organic electrolytes, focusing exclusively on pH‐neutral AORFB that uses non‐corrosive and nonflammable pH‐neutral aqueous electrolytes with inexpensive inorganic salts as supporting electrolytes and features high safety, low corrosivity, low cost, and environmental benignity. Some comments on the present challenges and prospects of this ascendant domain are also presented. [ABSTRACT FROM AUTHOR]

Membrane with ordered channels is the key to controlling ion sieving and proton conductivity in flow batteries. However, it remains a great challenge for finely controlling the nanochannels of polymeric membranes. Herein, two types of acid-stable Zr-metal organic framework (MOF-801 and MOF- 808) with variable pore structures and channel properties are introduced as fillers into a non-fluorinated sulfonated poly (ether ether ketone) (SPEEK). The membrane incorporated with MOF-801 of a smaller triangular window (≈3.5 Å) successfully translates the molecular sieving property into the flow battery membrane, resulting in enhanced coulombic efficiency (98.5-99.2%) at 40-120 mA cm-2 compared with the pristine SPEEK membrane (97.1-98.5%). In contrast, more protophilic internal interconnected channels of MOF-808 yield faster proton highway, leading to a significant increase of voltage efficiency (93.7-84.1%) at 40-120 mA cm-2 compared with the pristine SPEEK membrane (91.7-78.9%). By regulating the ion sieving and proton conductivity, MOF-801/MOF-808 binary composite membrane exhibits synchronously improved performance in the vanadium redox flow battery system. The revealed structure -- property relationship in the Zr-MOFs-based membranes provides a general guideline to design new proton exchange membranes with ordered channels for flow battery application. [ABSTRACT FROM AUTHOR]

The utilization of redox‐active and stable cyclic nitroxide radicals (CNRs) holds a great promise in neutral aqueous organic redox flow batteries (AORFBs) for large‐scale energy storage. Herein, a new class of CNRs with five‐membered ring pyrrolidine and pyrroline motifs for AORFBs is reported. By rational molecular engineering of introducing CC double bond into the pyrrolidine‐based molecule, 3‐carbamoyl‐2,2,5,5‐tetramethylpyrroline‐1‐oxyl (CPL) with a high redox potential of 0.76 V (vs Ag/AgCl) is demonstrated, which is 160 mV higher than the common 2,2,6,6‐tetramethylpiperidine 1‐oxyl derivatives with a six‐membered ring as the core structure. Density functional theory calculations reveal that the much enhanced redox potential for CPL is largely contributed by lowered standard free energy in reduction reaction and charge population sum of NO radical head. When paired with the BTMAP‐viologen anolyte, the CPL‐based AORFB delivers constant capacity retention of up to 99.96%/cycle over 500 cycles. [ABSTRACT FROM AUTHOR]

Rechargeable aqueous zinc‐based flow batteries (ZFBs) are promising candidates for large scale energy storage devices. However, the challenges from zinc dendrites and limited areal capacity considerably impede their wide application. Here, an in situ vertical growth of layered double hydroxide membrane (LDH‐G) is constructed to enable long‐life ZFBs. Owing to the high hydroxide ion conductivity and ion selectivity nature of LDH nanosheets, specifically, the precise control of directional ion transport in vertical arrangement LDHs, a superior battery performance can be realized. Moreover, the defect‐free LDHs layer serves as a buffer layer to enable a uniform Zn deposition, which effectively enhances the areal capacity of the battery. As a result, the designed membrane endows an alkaline zinc‐iron flow battery with excellent rate performance and cycling stability, maintaining an energy efficiency of 80% at 260 mA cm−2 for 800 cycles, which is the highest performance ever reported. Most importantly, the LDHs layer enables the battery for 1200 h long‐cycle stability with a uniform Zn deposition and high areal capacity of 240 mAh cm−2. This work realizes an in situ growth of 3D LDHs arrays on the polymer substrate, which provides a strategy toward high areal capacity and dendrite‐free Zn deposition for ZFBs. [ABSTRACT FROM AUTHOR]

Zinc‐bromine flow batteries (ZBFBs) are regarded as one of the most promising technologies for energy storage owing to high energy density and low cost. However, the sluggish reaction kinetics of Br2/Br− couples and zinc dendrite issue lead to low power density and poor cycle stability. Herein, a multifunctional carbon felt‐based electrode (NTCF) with N‐rich defects is fabricated for ZBFBs. The defects with abundant N‐containing groups on carbon fibers of NTCF provide high catalytic activity on Br2/Br− reactions. Simultaneously, the lower energy barrier of N‐rich defects to adsorb zinc atoms, and more deposition sites on NTCF induce more uniform zinc deposition. Thus, a ZBFB using NTCF as both the anode and cathode can stably operate at an unprecedentedly high current density of 180 mA cm−2 with a coulombic efficiency of 97.25%. Moreover, a long cycle life of over 140 cycles with a coulombic efficiency of 98.93% for a Zn symmetric flow battery at 80 mA cm−2 is achieved under a high areal capacity of 40 mAh cm−2. This current density and areal capacity are by far the highest values ever reported for Zn symmetry flow batteries. Therefore, this work provides an available approach to improve the power density and cycle life of ZBFBs. [ABSTRACT FROM AUTHOR]

Bromine‐based flow batteries (Br‐FBs) are considered one of the most promising energy storage systems due to their features of high energy density and low cost. However, they generally suffer from uncontrolled diffusion of corrosive bromine particularly at high temperatures. That is because the interaction between polybromide anions and the commonly used complexing agent (N–methyl–N–ethyl–pyrrolidinium bromide [MEP]) decreases with increasing temperatures, which causes serious self‐discharge and capacity fade. Herein, a novel bromine complexing agent, 1–ethyl–2–methyl–pyridinium bromide (BCA), is introduced in Br‐FBs to solve the above problems. It is proven that BCA can combine with polybromide anions very well even at a high temperature of 60 °C. Moreover, the BCA contributes to decreasing the electrochemical polarization of Br−/Br2 couple, which in turn improves their power density. As a result, a zinc–bromine flow battery with BCA as the complexing agent can achieve a high energy efficiency of 84% at 40 mA cm−2, even at high temperature of 60 °C and it can stably run for more than 400 cycles without obvious performance decay. This paper provides an effective complexing agent to enable a wide temperature range Br‐FB. [ABSTRACT FROM AUTHOR]

The selective ion transport characteristics of a conducting polymer electrode, based on poly(3,4‐ethylenedioxythiophene) (PEDOT), is evaluated with respect to its electrocatalytic performance, specifically targeting redox switching of quinone couples. Employing model organic redox quinones, here, the novel phenomenon of ion‐selective electrocatalysis (ISEC) is conceptualized. The effect of ISEC is studied and evaluated using two forms of PEDOT electrodes, which differ in their ion‐exchange characteristics, by comparing the redox transformations of catechol and tiron. It is rationalized that the choice of the specific redox couple and the ion selectivity characteristics of the conducting polymer electrode impacts the activation losses in aqueous organic redox‐flow batteries. By carefully selecting and designing the conducting polymer electrodes, high conversion efficiency on acid‐resistant electrodes is obtained. As far as it is known, this is the first redox flow battery to include conducting polymer electrodes operating in both the posolyte and negolyte configurations, thus the first "all‐organic" RFBs. [ABSTRACT FROM AUTHOR]

Here, a pH neutral aqueous organic redox flow battery (AORFB) consisting of three electrolytes channels (i.e., an anolyte channel, a catholyte channel, and a central salt water channel) to achieve integrated energy storage and desalination is reported. Employing a low cost, chemically stable methyl viologen (MV) anolyte, and sodium ferrocyanide catholyte, this desalination AORFB is capable of desalinating simulated seawater (0.56 m NaCl) down to 0.023 m salt concentration at an energy cost of 2.4 W h L−1 of fresh water—competitive with current reverse osmosis technologies. Simultaneously, the cell delivers stored energy at 79.7% efficiency with a cell voltage of 0.85 V. Furthermore, the cell is also capable of higher current operation up to 15 mA cm−2, providing 4.55 mL of fresh water per hour. Combining energy storage and water desalination into such a bifunctional device offers the opportunity to address two growing global issues from one hardware installation. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) are one of the promising technologies for large‐scale energy storage applications. For practical implementation of RFBs, it is of great interest to improve their efficiency and reduce their cost. One of the key components of RFBs that can greatly influence the efficiency and final cost is the electrode. The chemical and structural nature of electrodes can modify the kinetics of redox reactions and the accessibility of the electroactive species to available active sites. The ideal electrocatalyst for RFBs must have good activity for the desirable redox reaction, provide a high surface area, and exhibit sufficient conductivity and durability over repeated use. One strategy is to coat the electrode with metal and metal oxide electrocatalysts. Metal electrocatalysts have the advantage of high conductivity, while metal oxide catalysts are usually less expensive and so more economically attractive. In order to gain a better understanding of the performance of the electrocatalysts in RFBs, a comprehensive review of the progress in the development of metal and metal oxide electrocatalysts for RFBs is provided and a critical comparison of the latest developments is presented. Finally, practical recommendations for advancement of electrocatalysts and effective transfer of knowledge in this field are provided. [ABSTRACT FROM AUTHOR]

To close the feedback loop between artificial intellegence‐controlled materials synthesis and characterization, material functionality must be rapidly tested. A platform for high‐throughput multifunctional materials characterization is developed using a quartz crystal microbalance with auxiliary in‐plane electrodes and a custom gas/vapor flow cell, enabling simultaneous scanning probe microscopy and electrical, optical, gravimetric, and viscoelastic characterization on the same film under controlled environment. The lab‐on‐a‐crystal in situ multifunctional output allows direct correlations between the gravimetric/viscoelastic, electrical, and optical responses of polymer film in response to environment. When multiple film properties are used to augment the training set for machine learning regression, prediction of material response to the environment improves by a factor of 13 when <5% of the total dataset is used for model training. [ABSTRACT FROM AUTHOR]

Metal–CO2 batteries represent an economical and efficient CO2 utilization technique, which provides a mechanism combining CO2 reduction with electricity generation instead of electricity input. Existing metal–CO2 batteries generally work in a closed system by recycling CO2. In this study, a flow battery is designed with a hollow fiber of carbon nanotubes (cathode), Zn wire (anode), and 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (electrolyte). The battery can continuously consume CO2 to produce CH4 under ambient conditions and promptly output the gaseous product through the hollow fiber, with a Faradaic efficiency up to 94%. Simultaneously, the battery generates electricity, with an energy density of 288.3 Wh kg−1 (based on the zinc mass) and a stability up to 8 days. The high selectivity and efficiency of the battery is attributed to a water‐shuttling assisted proton mechanism and delicate electrode–electrolyte interplay. Moreover, the Zn anode is electrochemically renewed and the battery assembled with the regenerated Zn anode restores battery performances to the former level. The renewable characteristic implies that, if the regeneration of Zn anode is coupled to excessive renewable energy sources, then the Zn–CO2 flow battery will be promising to accomplish a net reduction of CO2 emission. [ABSTRACT FROM AUTHOR]

Abstract: Lithium–sulfur (Li–S) batteries have advantages in terms of their high specific capacity, natural abundance, and low cost of elementary sulfur on the basis of the multielectron conversion reactions in organic electrolytes. Despite their potential as next‐generation batteries, Li–S batteries are still limited by critical challenges such as redox shuttling and the parasitic reaction of polysulfides arising from intrinsic electrochemistry as well as a low electrical conductivity of sulfur and the insolubility of Li2S associated with the materials' properties. The unique redox electrochemistry of sulfur in aqueous electrolytes, which is completely different from that in organic electrolytes, provides a rational strategy to resolve the aforementioned problems by the design of new materials and cell constructions. Furthermore, this system enables to achieve significant benefits of aqueous systems in terms of safety, chemical tractability, environmental friendliness, low cost, and high ionic conductivity. Here, at first materials and cell constructions for aqueous Li–S batteries are reviewed, covering the fundamental electrochemistry of sulfur in aqueous electrolytes, the advances in the host materials and aqueous electrolytes, and the cell design of flow‐type aqueous Li–S batteries. Additionally, the current impediments and perspectives into the future direction of this field are provided. [ABSTRACT FROM AUTHOR]

Porous membranes with critically hydrophobic/hydrophilic phase-separated-like structures for use in vanadium flow battery application are first realized by solvent-induced reassembly of a polymer blend system. Porous poly(ether sulfone) (PES)/sufonated poly(ether ether ketone) (SPEEK) blend membranes with tunable pore size are prepared via the phase inversion method. After solidification, isopropanol (IPA) is introduced to induce the reassembly of sulfonated groups and further form ion-transport channels by using the interaction between IPA and functional groups in SPEEK. As a result, a highly phase separated membrane structure is created, composed of a highly stable hydrophobic porous PES matrix and hydrophilic interconnected small pores. The charged pore walls are highly beneficial to improving proton conductivity, while pores are simultaneously shrunk during the IPA treatment. Therefore, the resultant membranes show an excellent battery performance with a coulombic efficiency exceeding 99%, along with an energy efficiency over 91%, which is among the highest values ever reported. This article supplies an ease-to-operate and efficient method to create membranes with controlled ion-transport channels. [ABSTRACT FROM AUTHOR]

Advanced charged sponge-like porous membranes with ultrahigh stability and selectivity are designed and fabricated for vanadium flow battery (VFB) applications. The designed porous membranes are fabricated via constructing positively charged cross-linked networks on the pore walls of polysulfone membranes. The charge density of the pore walls can be tuned by changing the crosslinking time. The positively charged pore walls can effectively retain vanadium ions via Donnan exclusion, hence keeping extremely high selectivity, while the crosslinked network effectively increases the membrane stability. As a result, the designed membranes exhibit an outstanding performance, combining extremely high selectivity and stability. The single cell assembled with the prepared porous membrane shows a columbic efficiency of 99% and an energy efficiency of 86% at a current density of 80 mA cm−2, which is much higher than Nafion 115 (93.5%; 82.3%). A battery assembled with the prepared membrane shows a stable battery performance over more than 6000 cycles, which is by far the longest record for porous membranes ever reported. These results indicate that advanced, charged, sponge-like, porous membranes with a crosslinked pore-wall structure are highly promising for VFB applications. [ABSTRACT FROM AUTHOR]

Advanced charged sponge-like porous membranes with ultrahigh stability and selectivity are designed and fabricated for vanadium flow battery (VFB) applications. The designed porous membranes are fabricated via constructing positively charged cross-linked networks on the pore walls of polysulfone membranes. The charge density of the pore walls can be tuned by changing the crosslinking time. The positively charged pore walls can effectively retain vanadium ions via Donnan exclusion, hence keeping extremely high selectivity, while the crosslinked network effectively increases the membrane stability. As a result, the designed membranes exhibit an outstanding performance, combining extremely high selectivity and stability. The single cell assembled with the prepared porous membrane shows a columbic efficiency of 99% and an energy efficiency of 86% at a current density of 80 mA cm-2, which is much higher than Nafion 115 (93.5%; 82.3%). A battery assembled with the prepared membrane shows a stable battery performance over more than 6000 cycles, which is by far the longest record for porous membranes ever reported. These results indicate that advanced, charged, spongelike, porous membranes with a crosslinked pore-wall structure are highly promising for VFB applications. [ABSTRACT FROM AUTHOR]

In article number 1903192, Hongli Zhu and co-workers design a cost-effective and scalable surface modification method using graphite felt to enhance the mass transfer and reaction kinetics in aqueous flow batteries. This treatment increases the electrode surface area, improves the electrode hydrophilicity, and provides a sufficient number of oxygen groups, resulting in enhanced kinetics and electrolyte accessibility at the electrode-electrolyte interface. Electrochemical exfoliation, hydrophilicity, hydrophilicity, kinetic, redox flow battery, surface treatment. [Extracted from the article]

A scalable and efficient process to modify electrodes with enhanced mass transfer and reaction kinetics is critical for redox flow batteries (RFBs). For the first time, this work introduces electrochemical exfoliation as a surface modification method of graphite felt (GF) to enhance the mass transfer and reaction kinetics in RFBs. Anion intercalation and subsequent gas evolutions at room temperature for one minute expand the graphite layers that increase the electrode surface area. Meanwhile, sufficient oxygen functional groups are introduced to the electrode, resulting in enhanced reaction kinetics and improved hydrophilicity. Further, spin‐polarized density functional theory is employed to reveal the role of oxygen functional groups in accelerating the vanadium redox reaction. Benefitting from sufficient oxygen groups, larger surface area, and superior wettability, the as‐prepared exfoliated GF (E‐GF) shows exceptional electrocatalytic activity with minimized overpotential, higher volumetric capacity, and improved energy efficiency. The redox flow battery assembled with the E‐GF electrode delivers voltage and energy efficiencies of 89.72% and 86.41% at the current density of 100 mA cm−2, respectively. Remarkably, compared to the traditional GF treatment method, the elimination of the high temperature and long‐time treatment processes make this approach much more energy and time efficient, scalable, and affordable for large‐scale manufacturing. [ABSTRACT FROM AUTHOR]

Highlights from the article: Mixed Matrix Membranes: A Cost-Effective Mixed Matrix Polyethylene Porous Membrane for Long-Cycle High Power Density Alkaline Zinc-Based Flow Batteries (Adv. Funct. Keywords: alkaline; anti-alkali hollow nanospheres; mixed matrix membranes; zinc-based flow batteries Alkaline, anti-alkali hollow nanospheres, mixed matrix membranes, zinc-based flow batteries.

Alkaline zinc‐based flow batteries (ZFBs) have received considerable interest for renewable energy storage due to their attractive features of low cost and high energy density. However, a membrane with high stability, high selectivity, and high ion conductivity is in urgent need. Herein, an economical mixed matrix membrane with highly anti‐alkali microporous hollow spheres (denoted as DM‐HM) is developed in this work. With excellent chemical and mechanical stability, DM‐HM can achieve a high area capacity of 100 mA h cm−2 for carbon felt (CF)||Zn@CF symmetrical flow battery, and thereby exhibits 500 stable cycles with a coulombic efficiency of 98.6% and an energy efficiency of 88.3% at 80 mA cm−2 for alkaline zinc–iron flow battery. Additionally, with 44 wt% of hollow spheres inside matrix, DM‐HM can dramatically shorten the ion transport pathway and results in a very high power density battery. A kilowatt stack assembled with DM‐HM shows a very impressive performance, further confirming the practicability of this scalable mixed matrix membrane for alkaline ZFBs. [ABSTRACT FROM AUTHOR]