Aqueous organic redox flow batteries (AORFBs) have been widely recognized as promising solutions for longduration, large-scale energy storage owing to their use of the earth-abundant active materials. However, the low stability of reported extended viologen-based anolytes limits the performance of AORFBs. Here, we report a stable extended viologen-based anolyte with electron-withdrawing imidazole group, N,N'-di [(3-methylimidazolium)ethyl]-4,4'- [1,4-bis(4-pyridyl)benzene] dichloride (DBPyImCl4), for use in AORFBs. This anolyte exhibits a low redox potential of -0.90 V (vs. Ag/AgCl) and an ultra-low permeability of 6.12 & times; 10-12 cm2 s- 1. The assembled AORFB with 0.1 M DBPyImCl4 as the anolyte maintains a capacity retention of as high as 93.3% after 1,216 cycles, resulting in a low decay rate of 0.0055% average per cycle. Post-cycling analysis, including nuclear magnetic resonance, cyclic voltammograms, and high-resolution mass spectrometry, confirms the anolyte's high stability, with its chemical structure fully preserved and crossover absent after long-term cycling. Results from the single crystal structure and theoretical calculations demonstrate that the high stability of the anolyte is attributed to the inductive effect of the electron-withdrawing imidazole group, which weakens the positive charge on the sp3 carbon bonded to the pyridinic nitrogen in DBPyImCl4, thereby suppressing its reactivity and enhancing overall stability. Moreover, a 0.5 M flow battery with 1.0 M electron concentration displays a capacity retention of 95.9% (0.0122% per cycle) after 330 cycles. This work demonstrates a highperformance anolyte for AORFBs and provides an effective strategy for designing highly stable AORFB electrolytes.

Aqueous organic redox flow batteries are promising for large-scale energy storage owing to their molecular tunability and potential cost advantages, yet their power performance is often constrained by sluggish reaction kinetics at conventional porous carbon electrodes. Here, we engineer the electrode-electrolyte interface by constructing an interconnected multi-walled carbon nanotube network on carbon felt (MWCNT@CF), enabling simultaneous enhancement of accessible reaction area, molecular affinity, and electronic transport. The MWCNT modification increases the specific surface area from 0.4811 to 1.7783 m2 g-1 (~3.7-fold) and approximately doubles the electrochemically active surface area. Density functional theory calculations further reveal substantially stronger interaction between BTMAP-Fc and the MWCNT-containing carbon interface, with an adsorption energy of -1.00 eV compared with -0.30 eV on pristine carbon. These coupled interfacial effects promote apparent charge-transfer and mass-transport kinetics, thereby suppressing polarization and improving discharge capacity, voltage efficiency, energy efficiency, and electrolyte utilization over a broad current-density range of 20–120 mA cm-2. Consequently, the BTMAP-Fc/Dex-Vi flow cell employing MWCNT@CF achieves a peak power density of 74.7 mW cm-2, approximately 24% higher than that using pristine carbon felt, while retaining 90.9% of its initial discharge capacity after 1,000 cycles at 60 mA cm-2. This work demonstrates that coordinated regulation of molecular affinity, accessible reaction interfaces, and electronic connectivity provides an effective electrode-design strategy for high-rate performance AORFBs.

Flow fields in redox flow batteries are pattern designed to achieve a maximized uniformity of electrolyte distributions with a minimum pump work. It is challenging to scale up a lab-scale flow field to a stack level due to the lack of effective scale-up methods. In this work, a split strategy by dividing the active area into subzones is proposed to scale up a high-performing convection-enhanced flow field. Two split patterns, which have large and small aspect ratios in terms of different flow channel orientations, are designed and evaluated under varying scale-up factors. It is revealed that the subzone flow field with a large aspect ratio reduces the convection distance in both channels and electrodes, thereby decreasing the system pressure drop while maintaining the enhanced mass transport. In the meanwhile, the bifurcated distribution/collection channels ensure the uniform electrolyte distribution among subzones and demonstrate good scalability. As a result, compared to the conventional flow field with a single channel, the application of the split pattern comprising 4 subzones to a 1142.4cm(2) vanadium redox flow battery enables a 91.7 % reduction in pump loss and improves the system efficiency from 75.7 % to 84.8 % at 0.8 mL min(-1) cm(-2) and 100 mA cm(-2). This work presents that the split strategy offers a promising solution for scaling-up flow fields which paves the way for further commercialization of stack-scale flow batteries.

The limited availability of cycling-stable anolytes has significantly hindered the progress of aqueous organic redox flow batteries for reliable, large-scale energy storage applications. Here, we design and synthesize a class of viologen derivatives, i.e., 1,1'-bis(2-(1-methyl-imidazolium)ethyl)-4,4'-bipyridinium chloride (C2ViIm), 1,1'-bis(3-(1-methyl-imidazolium)propyl)-4,4'-bipyridinium chloride (C3ViIm), and 1,1'-bis(4-(1-methyl-imidazolium)butyl)-4,4'-bipyridinium chloride (C4ViIm). These compounds exhibit distinctive electron donor‒acceptor interactions between the bipyridine core and the imidazolium ion, validated through a range of spectral analyses and theoretical calculations. We investigate how these interactions influence the chemical and electrochemical stabilities of the compounds. Our findings indicate that stronger donor–acceptor interactions enhance chemical stability through electron delocalization, facilitating ultrastable cycling. Evaluation of symmetric flow batteries reveals a stability trend of C2ViIm > C3ViIm > C4ViIm, which aligns with the strength of donor–acceptor interactions. Notably, full batteries utilizing 0.1 M C2ViIm as the anolyte demonstrate operation over 10,230 cycles (exceeding 49 days) with an ultralow capacity fading rate of 0.00069% per cycle (0.142% on average per day). Even at a high concentration of 1.0 M C2ViIm, the flow battery maintains a low capacity fading rate of 0.018% per cycle (0.20% on average per day) over 500 cycles (more than 43 days). This work provides essential insights into the rational design of electrolytes, enabling control of electron density through tuning donor–acceptor interactions, and thus supports the development of ultrastable electrolytes.

Aqueous pH-neutral organic redox flow batteries are emerging as viable solutions for sustainable grid-scale energy storage. However, their advancement is hindered by the limited availability of catholytes that maintain stable cyclability in both ambient air and inert environments, as well as challenges related to low-cost synthesis and achieving high volumetric capacity. Here, we present the design and synthesis of a series of imidazolium-decorated ferrocene derivatives, culminating in the identification of an ionic liquid-like catholyte that sustains cycling stability in air for over 2000 cycles. At a concentration of 0.5 M, this catholyte exhibits remarkable cycling stability, with no capacity loss observed over a 68-day period in an inert atomosphere. Theoretical calculations and spectroscopic analyses reveal that the methyl imidazolium pendant group enhances compound stability and reduces hydrophilic attacks on ferrocenium through hydrogen-bonding interactions, thereby improving cycling stability. Notably, we have successfully constructed a pH-neutral stacked flow battery that achieves a peak power of 140 W and maintains an unprecedented capacity retention of 95% at 60 mA cm-2 after 20,000 cycles. These findings not only introduce a novel pathway for developing highly stable catholytes but also facilitate assembling high-power-density stacked pH-neutral aqueous organic redox flow batteries.

We report the design and synthesis of a highly water-soluble sulfonic acid anion-functionalized phenothiazine (PTZS), which exhibits a solubility of 1.95 M in mixed acids (2.7 M H2SO4 and 1.8 M AA) and 1.7 M in water, resulting in a theoretical capacity of 52.26 Ah L−1. Due to the strong electron-withdrawing effect of the sulfur atom in the skeleton, PTZS achieves a high potential of 0.5 V (vs. Ag/AgCl). When paired with an anthraquinone anolyte, the assembled aqueous organic redox flow battery (AORFB) demonstrates a capacity retention of 62 % over 1000 cycles at a current density of 30 mA cm−2 with a capacity decay of as low as 0.038 % per cycle. We also investigated the mechanism of capacity decay by conducting a series of measurements, including cyclic voltammetry (CV), nuclear magnetic resonance spectroscopy (NMR), and high-resolution mass spectrometry (HR-MS). Results indicate that although PTZS does generate certain sulfoxide byproducts, the capacity decay of the flow battery is primarily caused by the crossover of PTZS.

Serpentine flow fields with flat ribs can effectively improve the performance of redox flow batteries. However, the marginal pressure difference between adjacent channels near the end side of ribs in conventional designs leads to weak convection in the regions, increasing the local concentration overpotential. This problem will be exacerbated in scaled-up redox flow battery stacks due to enlarged under-rib regions near the end side of ribs. In this work, two modified serpentine flow fields with height-changing ribs are proposed to adjust the electrode compression ratio at the under-rib region, thereby enhancing the convection in the electrode near the end side of ribs. Therefore, the active species are distributed more uniformly, resulting in an improved battery performance. Specifically, the uniformity of V2+, the energy efficiency and the system efficiency of the vanadium redox flow batteries with the ramp ribs are improved by 21 %, 2.7 %, and 2.7 % respectively compared with that of the conventional flat ribs, at the current density of 200 mA cm(-2), the flow rate of 3.4 mL min(-1) cm(-2), and the active area of 117 cm(2).

Advanced energy storage technologies are widely needed to efficiently utilize renewable energies such as solar and wind energy and satisfy burgeoning clean energy needs while mitigating the environmental impact of fossil fuels[1-4].Aqueous organic redox flow batteries(AORFBs)have drawn substantial attention in this area because of their decoupled energy and power,excellent scalability,and use of highly structurally tunable organic redox active species[5].As a result,a wide range of organic and organometallic redox active species have been extensively applied in acidic,neutral,and alkaline AORFBs[6].Compared to AORFBs,which operate under highly corrosive acidic or basic conditions and exhibit side reactions such as hydrogen and oxygen evolution,pH-neutral AORFBs are noncorrosive,safer,and exhibit high cycling stability due to the absence of these reactions[7].

Abstract Electrodes, which offer sites for mass transfer and redox reactions, play a crucial role in determining the energy efficiencies and power densities of redox flow batteries. This review focuses on various approaches to enhancing electrode performance, particularly the methods of surface etching and catalyst deposition, as well as some other advanced strategies for regulating electrode surface properties. These approaches aim to increase active sites and enhance kinetics for the redox reactions, which are crucial for elevating power density and electrolyte utilization, eventually determining the performance of the flow battery. Highlighting the need for interdisciplinary research, this mini‐review suggests that future advancements in electrode design will significantly impact the commercial viability and adoption of redox flow batteries in sustainable energy storage solutions.

Redox flow batteries (RFBs) that employ sustainable, abundant, and structure-tunable redox-active species are of great interest for large-scale energy storage. As a vital class of redox-active species, metal coordination complexes (MCCs) possessing the properties of both the organic ligands and transition metal ion centers are attracting increasing attention due to the advantages of multielectron charge transfer, high structural tailorability, and reduced material crossover. Herein, we present a critical overview of RFBs that employ MCCs as redox-active materials in both aqueous and nonaqueous mediums. The progress is comprehensively summarized, including the design strategies, solubility characteristics, electrochemical properties, and battery cycling performance of MCCs. Emphasis is placed on the ligand selection and modification strategies used to tune the critical properties of MCCs, including their redox potential, solubility, cycling stability, and electron transfer redox reactions, to achieve stable cycled RFBs with a high energy density. Furthermore, we discuss the current challenges and perspectives related to the development of MCC-based RFBs for large-scale energy storage implementations.

The operating temperature of vanadium redox flow batteries (VRFBs) affects their performance and reliability. However, previous studies focused on evaluating the effects on the performance of lab-scale single cells, in which electrolyte flow rates and current densities are different from those in stack-scale VRFBs, leading to the lack of guidance for the design of stacks. In this work, we investigate thermal effects on the performance of stack-scale VRFBs. It is found that as the operating temperature increases from 25 to 50 °C, the discharge capacity increases by 42%, whereas the energy efficiency increases by 10%, implying that the temperature has greater effects on the discharge capacity than that on the energy efficiency. Additionally, the enhancement effect of temperature on the energy efficiency is gradually weakened with increasing flow rate, while that on the discharge capacity is almost unchanged. Furthermore, the enhancement effect of temperature on energy efficiency increases with the operating current density. Notably, an optimum operating condition of the stack-scale VRFBs is identified with a critical flow rate (2.88 ml min −1 cm −2 ) at 40 °C to achieve a high system efficiency. This work provides guidance for the design of stack-scale VRFBs with high performance and safety.

Low-cost and high-safety aqueous zinc batteries are promising for large-scale energy storage applications. However, the actual performance of aqueous zinc batteries is hampered by the irreversibility of the zinc metal anode originating from the dendrite growth and side reactions, which are associated closely with the hydrated solvation sheath of Zn2+ ions in aqueous electrolytes. Here, dimethylacetamide is employed as a water dragger agent in low-concentration ZnSO4 electrolytes to reshape the solvation structure of Zn2+ ions. Theoretical cal-culations and experimental investigations reveal that a low fractional addition of dimethylacetamide in aqueous ZnSO4 electrolytes performs strong interactions with water molecules and Zn2+ ions, which inhibits hydrated Zn2+-H2O solvation, facilitates Zn2+-SO42-association and captures free water molecules via H-bonds formation. The coulombic efficiency of Zn plating/stripping is improved from 94.3% to 98.1% and can be stabilized 99.5% over 200 cycles with the hybrid electrolyte. The cycle lifespan of the Zn symmetric cell is prolonged over 1000 h at 1 mA cm-2 and over 450 h at 5 mA cm-2 with the dimethylacetamide-hybrid electrolyte, which are over fourfold higher than that of the pristine ZnSO4 electrolyte. Moreover, when the designed electrolyte incorporated in full cells, the Zn-MnO2 cell exhibits an improved rate performance and an excellent long-term cyclability with a capacity-retention rate of 81% over 2,000 cycles at 1 A g-1. This work provides a feasible strategy for the development of highly stable aqueous zinc batteries.

The uniform and dense distribution of highly active electrocatalysts onto graphite fibers of scale-up anodes is essential for the engineered vanadium redox flow batteries to be operated efficiently and stably. In this work, a new in-situ electrodeposition strategy of bismuth is devised by using a catholyte with a low concentration of vanadium ions (33 mM V3+). Compared with the conventional method using a catholyte with a high concentration of vanadium ions (1700 mM VO2+), the present strategy renders Bi nanoparticles not being oxidized by VO2+ transported across the membrane from the catholyte, enabling bismuth nanoparticles with sizes of about 58 nm being uniformly and densely electrodeposited onto graphite fibers of an anode (117 cm-2) at an extremely low electrodeposition current density (2 mA cm-2). The vanadium redox flow battery with the present scale-up anode achieves an energy efficiency of 76.3% even at a current density of 300 mA cm-2, which is higher than that of batteries with a conventional method treated anode (74.9%), and an untreated (73.3%). Furthermore, the battery with the present scale-up anode shows an energy efficiency of 82.5% and stable operation for 200 cycles at a current density of 200 mA cm-2.

This paper describes the design and synthesis of a series of terpyridine-based complexes of the first-row transition metals Cr, Mn, Fe, and Co for non-aqueous redox flow batteries (NARFBs). Electrochemical studies reveal that these complexes can undergo multi-electron transfer redox reactions. In particular, the Mn and Fe-based com-plexes exhibit both low negative redox potentials and high positive redox potential, permitting them to serve as a bipolar electrolyte for symmetric RFBs with a cell voltage of more than 2 V. The solubility of these complexes can be effectively improved by incorporating a polyether substituent on the terpyridine ligand and counter anion optimization. The iron complex [Fe(tpy-O(CH2CH2O)3CH3)2][TFSI]2 shows a high solubility of 0.76 M in MeCN. The fabricated iron-based symmetric NARFB demonstrates a superior battery performance with a high cell voltage of 2.3 V, columbic efficiency of 97%, energy efficiency as high as 88%, and stable charge-discharge capacity retention of 60% after 160 cycles, corresponding to 99.75 % capacity retention per cycle. The post-cycling cyclic voltammetry (CV), UV-Vis, and 1H-NMR characterizations indicate only minor chemical decom-position of the cycled complex, confirming its good charging-discharging stability.

The polarization of redox flow batteries (RFBs) consists of activation polarization, ohmic polarization, and concentration polarization. However, the three types of polarizations are coupled in practice, making it difficult to quantify the respective attributions to the total voltage loss and to compare the reported performance of RFBs under different working conditions. Here, we propose a method to operando decouple the polarizations of RFBs based on the different response times of the three kinds of polarizations. The decoupled polarizations in RFBs under different working conditions are presented with specific voltage losses, which clarifies the limit parameters of battery performance and makes the reports of RFBs comparable even with similar battery performances. This work opens up a method to quantitatively analyze activation polarization and concentration polarization separately in RFBs, which provides significant guidance for improving battery performance effectively.

Nafion series membranes are widely used in vanadium redox flow batteries (VRFBs). However, the poor ion selectivity of the membranes to vanadium ions, especially for V2+, results in a rapid capacity decay during cycling. Although tremendous efforts have been made to improve the membrane's ion selectivity, increasing the ion selectivity without sacrificing the proton conductivity is still a challenging issue. In this work, instead of focusing on enhancing the membranes' ion selectivity, we develop an efficient valence regulation strategy to suppress the capacity decay caused by the crossover of V2+ in VRFBs. Despite the discharge capacity of the VRFB with the elevated average valence electrolytes (V3.68+) being slightly lower than that with commercial electrolytes (V3.50+) in the first 35 cycles, the accumulated discharge capacity in 400 cycles is improved by 52.33%. Moreover, this method is efficient, is easy to scale up, and provides deep insights into the capacity decay mechanism of VRFBs.

Critical issues of Zn anodes including undesirable dendrites formation and parasitic reactions severely limit the reversibility and cyclability of Zn anodes. To address these issues, a functional Janus separator with the structure of a mechanically strong sulfonated poly(arylene ether sulfone) (SPAES) dense layer composited on a porous glass fiber (GF) substrate is designed. The SPAES dense layer that faces the Zn anode containing abundant sulfonic acid groups effectively promotes the desolvation process of hydrated Zn ions, guides uniform Zn ion transfer, and blocks anions and water, contributing to dendrite-free and highly reversible Zn plating/stripping cycles, while the porous GF substrate retains high electrolyte uptake. As a result, the Zn symmetric cell with the Janus separator demonstrates an ultralong cycling lifespan of over 2000 h at the areal capacity of 1 mA h cm-2, which is 23-fold superior to that with a pristine glass fiber separator (<90 h). More impressively, the as-prepared Janus separator enables outstanding rate performance and excellent cycling stability of full Zn ion batteries with diverse cathode materials. For instance, when paired with the V2O5 cathode, the full battery with a Janus separator attains an ultrahigh initial specific capacity of 416.3 mA h g-1 and capacity retention of 60% over 450 cycles at 1 A g-1, exceeding that with a glass fiber separator. Hence, this work provides a facile yet effective approach to mitigating the dendrites formation and ameliorating the parasitic reactions of Zn metal anodes for high-performance Zn ion batteries.(c) 2023 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Designing flow fields that can uniformly distribute electrolytes while maintain a low pumping work is chal-lenging for high-performance redox flow batteries, especially for scaled-up battery stacks. In this work, we propose a bifurcate interdigitated flow field, which hierarchically splits the electrolyte and transports it into branch channels, thereby lowering the pumping loss and enhancing the uniformity of electrolyte distribution. The application of the flow field to a vanadium redox flow battery reduces the pressure drop by 45%, thereby increasing the pump-based voltage efficiency from 69.95% to 73.10% compared to that with conventional interdigitated flow field at a flow rate of 2.4 mL min-1 cm-2 and current density of 250 mA cm-2. Furthermore, three-dimensional modeling reveals that the superiority of reducing the pumping loss becomes more extraor-dinary with the enlargement of active area, indicating that the bifurcate interdigitated flow field shows great potential for the scale-up of high-performance flow batteries with a low pumping work.

Polybenzimidazole (PBI) membrane is one of the most promising proton exchange membranes for vanadium redox flow batteries (VRFBs) due to its excellent ion selectivity and stability. However, in this work, we first found the acid-doped PBI membrane conducts both H+ and SO24- in VRFBs. Besides, the capacity decay features of the VRFBs with acid-doped PBI membrane also present the same trend as that of anion exchange membrane, exhibiting an opposite direction of net electrolyte flux after long-term cycling than that of Nafion 212 (N212). Inspired by this finding, we build a VRFB two-cell stack consisting of an acid-doped PBI membrane and an N212 membrane to mitigate capacity decay. This method breaks the trade-off between voltage efficiency and capacity retention rate and significantly increases the accumulated discharge capacity by 119.77% compared to that with 2 pieces of N212 in 1000 cycles. This work deepens the understanding of the capacity decay mechanism of VRFB with different types of membranes and provides a simple yet highly effective strategy for mitigating the capacity decay of VRFB stacks.