Synergistic ionic modification strategy enhances the stability of naphthalene diimide zwitterions for cost-effective aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold significant promise for energy storage due to their unique advantages and characteristics. However, their development is hindered by the lack of decomposition resistance and cycle stability over long periods. In this study, we synthesized naphthalene diimide (NDI) derivatives with zwitterions in their side chains via the atmospheric pressure method, namely (CBu)2NDI and (SPr)2NDI. The electrostatic repulsion between (CBu)2NDI precisely regulates π-π stacking into a parallel-staggered pattern. The synergistic zwitterions strategy effectively mitigates the positive charge of N+ in (CBu)2NDI compared with (NPr)2NDI and dex-NDI; this not only enhances the aromaticity of the naphthalene diimide core but also inhibits the side chain decomposition caused by the SN2 nucleophilic attack of hydroxyl ions (OH-) on the C=O. The calculation of the single point energy proves that during the charging processes of (CBu)2NDI, the K+ will be close to the naphthalene core to form dimers or monomers with lower energy configurations under electrostatic attraction. (CBu)2NDI achieved a water solubility up to 1.49 M, which can be paired with K4Fe(CN)6 under two-electron transfer with total electrolyte costs as low as $6.58 Ah-1. The 0.1 M battery maintains full capacity after 5070 cycles. Furthermore, the battery delivers an impressive 100% capacity retention under 2 M e- during 220 cycles.

Designs and Applications of Organic Redox Couples in Aqueous Flow Batteries

The increasing global climate change and the rising share of renewable energy sources have jointly driven the growing demand for grid-level energy storage systems. Aqueous organic redox flow batteries (AORFBs), due to their excellent energy density and long lifespan, have surfaced as a promising energy storage solution. Since 2007, researchers have been optimizing simple organic electrolytes, such as quinones and viologens, through molecular engineering to enhance their performance. This paper reviews the development of AORFB technology, focusing on the progress of organic electroactive materials. It discusses their electrochemical performance in terms of energy density and cycling stability, as well as limitations related to solubility and scalability. Additionally, the paper outlines potential directions for optimizing organic redox electrolytes in the future, aiming to advance the application and development of AORFBs for large-level energy storage.

Redox-Mediated Lithium Recovery From Spent LiFePO4 Stabilizes Ferricyanide Catholyte for Durable Zinc-Ferricyanide Flow Batteries

The scarcity of lithium resources and the increasing volume of spent lithium-ion batteries (LIBs) exacerbate the imbalance between lithium supply and demand. The development of efficient recovery strategies of valuable lithium ion (Li+) from spent LIBs and their subsequent utilization presents both significant opportunities and challenges. Here, we propose an innovative approach for Li+ recovery from spent lithium iron phosphate (LiFePO4) batteries (LFPs) and its subsequent utilization in alkaline zinc-ferricyanide flow batteries (AZFFBs). Utilizing a redox-mediated reaction, we achieve exceptional Li+ recovery efficiency from spent LFPs. Furthermore, the recovered Li+ in solution leads to the elevated ionic strength in the electrolyte, enhancing the concentration of [Fe(CN)6]4- to a remarkable level of 1.74 M. Utilizing the above catholyte, an AZFFB cell demonstrates the cycling life extending to 11 000 cycles with a degradation rate as low as 0.00019% per cycle and 0.09% per day at a current density of 120 mA cm-2. This study introduces a straightforward and efficient protocol that eliminates additional intermediate processes, achieving effective Li+ recovery from spent LFPs and subsequent utilization in flow batteries. The resulting AZFFB exhibits high energy density and long lifespan, positioning it as a promising candidate for large-scale energy storage solutions.

Leveraging Long-Life Alkaline Redox Flow Batteries Using Durable and High-Hydroxide Exchange N-Bridged Triazine Framework Membranes

Fluorine-free organic framework polyelectrolyte membranes showing near frictionless ionic conductivities are gaining cognitive insights. However, the co-precipitation of COFs in the membranes often brings trade-offs to commission long-life electrochemical energy storage solutions. Herein, a durable and ionically miscible dual-ion exchange membrane based on triazine organic framework (TOF) is designed for alkaline redox flow batteries (RFB). Bearing dual ion-exchange architectures, the all-hydrocarbon TOF-based PEMs (sTOF's) surpass fluorinated Nafion in terms of energy efficiency (>80%), energy density, and peak power densities. The fabricated sTOF's evidenced the highest net ion-exchange of >2.1 meq g-1 which encourages electrolyte utilization with ≈100% and offers excellent capacities. Moreover, >97% efficiencies are preserved, and rate capability studies illustrate that, with sTOF-5, the RFB can operate at reduced overpotentials (η ≤200 mV) and can uplift batteries life. The sTOF's supports successful demonstrations of batteries at higher redoxolyte concentrations thereby multiplying the energy densities. The afterlife performance of sTOF-5 revealed efficiencies equivalent to fresh Nafion-117 and surpassed bearing >50% capacity after ≈3000 continuous cycles. With sTOF-5, the cell delivered a peak power (Pmax) of 2.3 W which is ≈60% higher than that of Nafion-117 (Pmax = 1.45 W).

Artificial α-amino acid based on cysteine grafted natural aloe-emodin for aqueous organic redox flow batteries

Natural redox-active anthraquinone derivatives possess promising attributes for applications in aqueous organic redox flow batteries (AORFBs) due to their environmental friendliness and abundant sources. However, their limited aqueous solubility and electrochemical stability have posed significant challenges to their practical utilization. Herein, inspired by click chemistry, we report the synthesis of an artificial α-amino acid derived from cysteine-functionalized natural aloe-emodin (namely Cys-AE), which exhibits good water-solubility and redox-reversibility, particularly suited for alkaline AORFBs. The bio-inspired Cys-AE molecule exhibits a threefold increase in aqueous solubility compared to pristine aloe-emodin. Furthermore, the AORFB based Cys-AE negolyte with an electron concentration of 1.0 M demonstrates a low capacity fade rate of 0.000948% cycle-1 (equivalent to 0.0438% day-1) during 592 cycles, significantly outperforming the AORFB based on pristine aloe-emodin (0.00446% cycle-1, or 0.908% day-1) during 1564 cycles. Our investigation incorporates time-dependent density functional theory (TDDFT) simulations and detailed spectroscopic analyses reveal the essential role played by the asymmetric distribution of multiple solubilizing groups in enhancing the aqueous solubility and cycling stability of Cys-AE. This study highlights the potential of nature-inspired molecular engineering strategies in creating and crafting redox-reversible organic species poised to revolutionize large-scale and sustainable energy storage applications.

Constructing Hydrophilic Polymer Membranes with Microporosity for Aqueous Redox Flow Batteries

Ion selective membranes (ISMs) are key components of aqueous redox flow batteries (ARFBs), and their property in selective ion transport largely determines the energy storage efficiency of ARFBs. Traditional ISMs are based on microphase-separated structures and have been advanced for many years, but most of them show poor performance as membrane separators in ARFBs due to their conductivity-selectivity. In recent years, using confined micropores instead of dense hydrophilic regions as ion channels has been demonstrated to effectively break this tradeoff. We here summarize the synthetic strategies for constructing hydrophilic polymer membranes with microporosity and highlight the performance of some typical microporous ISMs in ARFBs. We also propose fundamental issues that remain to be addressed for the further development of ISMs.

Mechanism guided two-electron energy storage for redox-flow batteries using nickel bis(diphosphine) complexes

The storage of multiple electrons per molecule can greatly enhance the energy density of redox-flow batteries (RFBs). Here, we show that nickel bis(diphosphine) complexes efficiently store multiple electrons through either sequential 1e- redox waves or a concerted 2e- redox wave, depending on their coordination environment. Mechanistic studies comparing ligand sterics (-Me vs. -Ph) and coordination of monodentate ligands (MeCN vs. Cl-) allow for selective control of the electron transfer pathway, steering electron storage toward the more favorable 2e- wave. Continuous charge-discharge cycling experiments show more negative charge-discharge potentials and improved capacity retention in the presence of Cl-, thus improving the energy storage of nickel bis(diphosphine) complexes as anolytes in RFBs. This work shows how mechanistic understanding of 2e- redox cycles for transition metal complexes can create new opportunities for multi-electron storage in RFBs.

Skeletal Rearrangement of Azo Compounds Enables Low-Potential, High-capacity Organic Anodes for Rechargeable Alkaline Batteries

The widespread adoption of rechargeable alkaline batteries is plagued by the performance-limiting metal anodes, which are prone to (electro)chemical corrosion and raise environmental or economic concerns. Organic redox-active materials offer a potential solution, but they typically struggle with dissolution-induced degradation and insufficiently negative reduction potentials. Herein, we introduce benzo[c]cinnoline and its derivatives (collectively referred to as BCCs), a class of aromatic azo compounds with fused-ring structure, as promising organic anode materials. BCCs exhibit pronounced aromaticity and Brønsted basicity, conferring low reduction potentials and intrinsic insolubility in alkaline solutions. Paired with the industrially established nickel hydroxide cathode, these batteries deliver excellent capacity (297 mAh g-1 anode), high output voltage of 1.3 V, and extended cycle life (≈16000 cycles). Notably, they also operate efficiently at extremely low temperatures down to -85 °C with an 8 M KOH electrolyte. We further explore the feasibility for all-organic alkaline batteries by paring BCCs with the dihydro form of 4,4'-azopyridine, utilizing azo compounds for both anode and cathodes. This chemistry harnesses the unique properties of small organic molecules to enable all-organic batteries with high capacity of 236 mAh g-1, fast charging at 1200 C, and easy recyclability.

Patterning the Pore Orientation of Nanoporous Metal via Self-Organization in Flow Cells

Nanoporous metals, a class of free-standing, high specific-area materials, evolve from interface-controlled self-organization in a selective dissolution (e.g., dealloying). The process creates randomly oriented pores, in which slow mass transport has limited the functional applications of nanoporous metals. Here the control of the pore orientation is demonstrated with a dealloying analogy, reduction-induced decomposition, achieved in flow cells. Via forced convection, the self-organization is placed under the control of sufficiently rapid mass transport to suppress pore branching and align 100 nm-wide ligaments and pores along the direction of reaction propagation, boosting the permeability by an order of magnitude while retaining the large surface area. The pore orientation can be further manipulated with a flow field for an orientation pattern akin to the expected fluid pattern, enabling a nanoporous silver electrode to deliver a peak power of 0.3 W cm-2 in a redox-flow battery, outperforming commercial carbon electrodes.

Manipulating Aggregate Electrochemistry for High-Performance Organic Redox Flow Batteries

Organic molecule in solutions is the energy storage unit in the organic redox flow batteries (ORFBs), of which the aggregation is acknowledged pivotal but has been rarely investigated. By establishing a pyridinium library, the manipulation over the aggregation in solutions is investigated at the molecular level. Both theoretical calculations and physiochemical methods are used to characterize the aggregate's structure, and salient findings are as follows. First, the singly-reduced monoradicals simultaneously aggregate in a concentrated solution, which is driven by the solvation effect, orbital overlap and dispersion interaction. Second, the aggregation can be manipulated by the molecular engineering strategy and counteracted by introducing either electrostatic repulsive force or twisted geometry. Third, the monoradical's aggregation yields a decrease in the molecular singly occupied molecular orbital energy level and a linear scaling relationship with its thermodynamic potential. As a result, the increase in the concentration lowers the battery's voltage, which counteracts its effort to increase the battery's energy density. The anti-aggregation is proven effective in breaking the scaling relationship and accordingly, a molecular strategy to manipulate aggregate electrochemistry is developed. This work provides physical insights into the electrolytic solution and chemical strategy for optimizing the flow battery.

Aqueous colloid flow batteries with nano Prussian blue

Flow battery is a safe and scalable energy storage technology in effectively utilizing clean power and mitigating carbon emissions from fossil fuel consumption. In the present work, we demonstrate an aqueous colloid flow battery (ACFB) with well-dispersed colloids based on nano-sized Prussian blue (PB) cubes, aiming at expanding the chosen area of various nano redox materials and lowering the cost of chemicals. Taking advantage of the two redox pairs of PB, the developed all-PB cell employing a low-cost dialysis membrane with the synthesized PB on both sides displays an open-circuit voltage (OCV) of 0.74 V. Moreover, when paired with an organic tetra pyridine macrocycle the cell with PB as positive electrolyte exhibits an OCV of 1.33 V and a capacity fade rate of 0.039 %/cycle (0.8 %/day). Redox-active colloids exhibit enduring physicochemical stability, with no evident structural or morphological changes after extensive cycling, highlighting their potential for cost-effective and reliable ACFB energy storage.

Adjusting Hirshfeld charge of TEMPO catholytes for stable all-organic aqueous redox flow batteries

2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives are typical catholytes in aqueous organic redox flow batteries (AORFBs), but reported lifetime of them is limited. We find that the increase of Hirshfeld charge decreases the Gibbs free energy change (ΔG) values of side reactions of TEMPO, a near-linear relationship, and then exacerbates their degradation. Here we predict and synthesize a TEMPO derivative, namely TPP-TEMPO, by analyzing the Hirshfeld charge. TPP-TEMPO, with the smallest Hirshfeld charge and highest ΔG, is an order of magnitude more stable than TMA-TEMPO (N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride, a control with the largest Hirshfeld charge and lowest ΔG). We further elaborate on their decomposition pathways, identify byproducts, and mitigate degradation by supporting electrolyte engineering. Finally, a TPP-TEMPO/BTMAP-Vi (1,1'-bis[3-(trimethylammonio)propyl]-4,4'-bipyridinium tetrachloride) cell achieves a capacity density of ~12 Ah L-1 and a low capacity fade rate of 0.0018% per cycle (or 0.0067% per hour).

Highly Durable Inverted Inorganic Perovskite/Organic Tandem Solar Cells Enabled by Multifunctional Additives

Inverted perovskite/organic tandem solar cells (P/O TSCs) suffer from poor long-term device stability due to halide segregation in organic-inorganic hybrid wide-band gap (WBG) perovskites, which hinders their practical deployment. Therefore, developing all-inorganic WBG perovskites for incorporation into P/O TSCs is a promising strategy because of their superior stability under continuous illumination. However, these inorganic WBG perovskites also face some critical issues, including rapid crystallization, phase instability, and large energy loss, etc. To tackle these issues, two multifunctional additives based on 9,10-anthraquinone-2-sulfonic acid (AQS) are developed to regulate the perovskite crystallization by mediating the intermediate phases and suppress the halide segregation through the redox-shuttle effect. By coupling with organic cations having the desirable functional groups and dipole moments, these additives can effectively passivate the defects and adjust the alignment of interface energy levels. Consequently, a record Voc approaching 1.3 V with high power conversion efficiency (PCE) of 18.59 % could be achieved in a 1.78 eV band gap single-junction inverted all-inorganic PSC. More importantly, the P/O TSC derived from this cell demonstrates a T90 lifetime of 1000 h under continuous operation, presenting the most stable P/O TSCs reported so far.

Rational Design of Hierarchical Structure Electrodes to Suppress Shuttle Diffusion in Redox-Enhanced Supercapacitors

In carbon-based supercapacitors, redox couples can effectively improve the energy density of supercapacitors; however, most redox couples still suffer from serious shuttle diffusion. Currently, there is no universal strategy to effectively constrain their shuttle diffusion. Therefore, developing a simple, effective, and universal method to suppress shuttle diffusion remains a great challenge. Herein, we designed and prepared a hierarchical structure electrode composed of three functional layers (inner conductive layer, intermediate storage layer, and outer confinement layer) for redox-enhanced supercapacitors. The hierarchical electrode can be fabricated on a large scale in three simple steps based on commercial carbon felt. The rationally designed three functional layers endow the hierarchical structure electrode with excellent conductivity, adsorption capacity, and confinement ability. Long-term charge-discharge cycle results prove that the hierarchical structure electrode exhibits an outstanding restriction effectiveness on three redox media with different molecular sizes (anthraquinone (AQ), naphthoquinone (NQ), and p-benzoquinone (PQ)). Specifically, for AQ, NQ, and PQ, the hierarchical structure electrodes showed CV peak current attenuation rates of only 5.64%, 3.54%, and 10.34% after 5,000 cycles of charge-discharge, while common electrodes exhibited attenuation rates of 10.44%, 15.02%, and 20.20%, respectively. Moreover, the soft-pack supercapacitors composed of hierarchical electrodes still had a capacitance retention of 86.6% after 5,000 cycles of charge-discharge. All the results demonstrate that the structure design is reasonable and the hierarchical structure electrode has a great potential to be the universal electrode for suppressing the shuttle diffusion of redox media in supercapacitors and other energy storage devices.

Zinc-Ferricyanide Flow Batteries Operating Stably under -10 °C

Alkaline ferri/ferro-cyanide-based flow batteries are well suited for energy storage because of their features of high electrochemical activity, good kinetics and low material cost. However, they suffer from low energy density and poor temperature adaptability. The ferri/ferro-cyanide catholyte exhibits low solubility (~0.4 M at 25 °C) in NaOH- or KOH-based supporting electrolyte and can easily form precipitates below room temperature. Here we report a lithium-based supporting electrolyte that significantly enhances the solubility of ferrocyanide. The use of LiOH intensifies the ion-dipole interaction between water molecules and solutes and cripples polarization among ferrocyanide ions. Thus, we have achieved a ferrocyanide-based catholyte of 1.7 M at 25 °C and of 0.8 M at -10 °C. A zinc-ferricyanide flow battery based on the lithium-based supporting electrolyte demonstrates a steady charge energy of ~72 Wh L-1 catholyte at 25 °C, and maintains stable for ~4200 cycles (~4200 hours). Furthermore, it remains stable for ~800 cycles (~800 hours) at -10 °C.

Flow field design and visualization for flow-through type aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs), which exploit the reversible redox reactions of water-soluble organic electrolytes to store electricity, have emerged as a promising electrochemical energy storage technology. Organic electrolytes possess fast electron-transfer rates that are two or three orders of magnitude faster than those of their inorganic or organometallic counterparts; therefore, their performance at the electrode is limited by mass transport. Direct adoption of conventional cell stacks with flow fields designed for inorganic electrolytes may compromise AORFB performance owing to severe cell polarization. Here, we report the design of a flow field for flow-through type AORFBs based on three-dimensional multiphysics simulation, to realize the uniform distribution of electrolyte flow and flow enhancements within a porous electrode. The electrolyte flow is visualized by operando imaging. Our results show that multistep distributive flow channels at the inlet and point-contact blocks at the outlet are crucial geometrical merits of the flow field, significantly reducing local concentration overpotentials. The prototype pH-neutral TEMPTMA/MV cell at 1.5 M assembled with the optimized flow field exhibits a peak power density of 267.3 mW cm-2. The flow field design enables charging of the cell at current densities up to 300 mA cm-2, which is unachievable with the conventional serpentine flow field, where immediate voltage cutoff of the cell occurs. Our results highlight the importance of AORFB cell stack engineering and provide a method to visualize electrolyte flow, which will be appealing to the field of aqueous flow batteries.

Zero-Shot Discovery of High-Performance, Low-Cost Organic Battery Materials Using Machine Learning

Organic electrode materials (OEMs), composed of abundant elements such as carbon, nitrogen, and oxygen, offer sustainable alternatives to conventional electrode materials that depend on finite metal resources. The vast structural diversity of organic compounds provides a virtually unlimited design space; however, exploring this space through Edisonian trial-and-error approaches is costly and time-consuming. In this work, we develop a new framework, SPARKLE, that combines computational chemistry, molecular generation, and machine learning to achieve zero-shot predictions of OEMs that simultaneously balance reward (specific energy), risk (solubility), and cost (synthesizability). We demonstrate that SPARKLE significantly outperforms alternative black-box machine learning algorithms on interpolation and extrapolation tasks. By deploying SPARKLE over a design space of more than 670,000 organic compounds, we identified ≈5000 novel OEM candidates. Twenty-seven of them were synthesized and fabricated into coin-cell batteries for experimental testing. Among SPARKLE-discovered OEMs, 62.9% exceeded benchmark performance metrics, representing a 3-fold improvement over OEMs selected by human intuition alone (20.8% based on six years of prior lab experience). The top-performing OEMs among the 27 candidates exhibit specific energy and cycling stability that surpass the state-of-the-art while being synthesizable at a fraction of the cost.

Selective ion transport through hydrated micropores in polymer membranes

Ion-conducting polymer membranes are essential in many separation processes and electrochemical devices, including electrodialysis1, redox flow batteries2, fuel cells3 and electrolysers4,5. Controlling ion transport and selectivity in these membranes largely hinges on the manipulation of pore size. Although membrane pore structures can be designed in the dry state6, they are redefined upon hydration owing to swelling in electrolyte solutions. Strategies to control pore hydration and a deeper understanding of pore structure evolution are vital for accurate pore size tuning. Here we report polymer membranes containing pendant groups of varying hydrophobicity, strategically positioned near charged groups to regulate their hydration capacity and pore swelling. Modulation of the hydrated micropore size (less than two nanometres) enables direct control over water and ion transport across broad length scales, as quantified by spectroscopic and computational methods. Ion selectivity improves in hydration-restrained pores created by more hydrophobic pendant groups. These highly interconnected ion transport channels, with tuned pore gate sizes, show higher ionic conductivity and orders-of-magnitude lower permeation rates of redox-active species compared with conventional membranes, enabling stable cycling of energy-dense aqueous organic redox flow batteries. This pore size tailoring approach provides a promising avenue to membranes with precisely controlled ionic and molecular transport functions.

A Multielectron and High-Potential Spirobifluorene-Based Posolyte for Aqueous Redox Flow Batteries

The rising energy demand driven by human activity has posed pressing challenges in embracing renewable energy, necessitating advances in energy storage technologies to maximize their utilization efficiency. Recent studies in aqueous organic redox flow batteries have focused primarily on the development of organic negative electrolytes, while the progress in organic positive electrolytes remains constrained by limitations in their redox potentials and effective electron concentrations. Herein, we report a spatially twisted chlorinated spirobifluorene ammonium salts (CSFAs), created through an unexpected green chlorination-protection pathway during the initial cycling in the flow battery cell, utilizing chloride ions from counterions in aqueous solution. The chlorinated, nonplanar spiral structure of CSFAs possesses a one-step four-electron transfer electrochemical property and offers exceptional resistance to nucleophilic attacks, exhibiting an unprecedented redox potential as high as 1.05 V (vs. SHE). A full redox flow battery based on CSFA-Cl (chloride ions as the counter ions) with 1.4 M electron concentration achieved an average coulombic efficiency exceeding 99.4 % and a capacity utilization reaching 95 % of the four-electron capacity for a stable cycling over 250 cycles (~22 days). The present work exemplifies the use of side reactions to develop new redox species, which can be extended to create more structurally versatile energy storage materials.

Phenomenological observations of quinone-mediated zinc oxidation in an alkaline environment

Redox-mediated electrochemistry is an area of growing interest, particularly in the context of energy storage. The development of such systems requires knowledge of underlying reaction mechanisms, which bear similarities to the processes that underpin corrosion and semiconductor electrochemistry. Herein we discuss an example system, quinone-mediated zinc oxidation in an alkaline environment, using knowledge from the corrosion and semiconductor fields to understand the phenomenological aspects of the reaction.

200-Fold Lifetime Extension of 2,6- Dihydroxyanthraquinone Electrolyte during Flow Battery Operation

We study the capacity fade rate of a flow battery utilizing 2,6-dihydroxyanthraquinone (DHAQ) and its dependence on hydroxide concentration, state of charge, cutoff voltages for the discharge step and for the electrochemical regeneration (oxidation of decomposition compounds back to active species) step, and the period of performing the electrochemical regeneration events. Our observations confirm that the first decomposition product, 2,6-dihydroxyanthrone (DHA), is stable, but after electro-oxidative dimerization, the anthrone dimer decomposes. We identify conditions for which there is little time after dimerization until the dimer is rapidly reoxidized electrochemically to form DHAQ. Combining these approaches, we decrease the fade rate to 0.02%/day, which is 18 times lower than the lowest rate reported previously of 0.38%/day, and over 200 times lower than the value under standard cycling conditions of 4.3%/day. The findings and their mechanistic interpretation are expected to extend the lifetime and enhance the effectiveness of in situ electrochemical regeneration for other electroactive species with finite lifetimes.

Ferrocene-based metal-organic frameworks with dual synergistic active sites for selectively electrochemical removal of arsenic from contaminated water

The effective removal of trace levels of the highly toxic arsenite (As(Ⅲ)) from groundwater is crucial to address the threat to drinking water supply. Herein, we developed an electrochemical separation system utilizing redox-active ferrocene-based metal-organic frameworks (termed Fe-DFc) for selective removal of As(III). This system leveraged 1,1'-ferrocenedicarboxylic acid as a ligand coordinated with iron, enabling the highly selective capture and conversion of As(III) from groundwater. The Fe-DFc electrode-based electrochemical system not only effectively removed As(III) even in the presence of a 1250-fold excess of competing electrolytes, but also converted about 96 % of the adsorbed As(III) into the less toxic As(V), surpassing the results of those documented in the current literature. X-ray absorption fine structure analysis and density functional theory calculations demonstrated that the high selectivity of Fe-O6 moiety and the exceptional redox activity of Fc synergistically contributed to the efficient removal of As(III). Moreover, the electrochemical separation system enabled the remediation of arsenic-contaminated groundwater at a low energy cost of 0.033 kWh m-3 during long-term operation, highlighting the application potential of the electrochemical technology for arsenic removal from contaminated water.

High-voltage and dendrite-free zinc-iodine flow battery

Zn-I2 flow batteries, with a standard voltage of 1.29 V based on the redox potential gap between the Zn2+-negolyte (-0.76 vs. SHE) and I2-posolyte (0.53 vs. SHE), are gaining attention for their safety, sustainability, and environmental-friendliness. However, the significant growth of Zn dendrites and the formation of dead Zn generally prevent them from being cycled at high current density (>80 mA cm-2). In addition, the crossover of Zn2+ across cation-exchange-membrane also limits their cycle stability. Herein, we propose a chelated Zn(P2O7)26- (donated as Zn(PPi)26-) negolyte, which facilitates dendrite-free Zn plating and effectively prevents Zn2+ crossover. Remarkably, the utilization of chelated Zn(PPi)26- as a negolyte shifts the Zn2+/Zn plating/stripping potential to -1.08 V (vs. SHE), increasing cell voltage to 1.61 V. Such high voltage Zn-I2 flow battery shows a promising stability over 250 cycles at a high current density of 200 mA cm-2, and a high power density up to 606.5 mW cm-2.

Fast Outer-Sphere Electron Transfer and High Specific Capacitance at Covalently Modified Carbon Electrodes

Carbon electrodes typically display sluggish electron transfer kinetics due to the adsorption of adventitious molecules that effectively insulate the surface. Here, we describe a method for rendering graphitic carbon electrodes permanently hydrophilic by functionalization with 4-(diazonium)benzenesulfonic acid. In aqueous electrolytes, these hydrophilic carbon electrodes exhibit metal-like specific capacitance (∼40 μF/cm2) as measured by cyclic voltammetry, suggesting a change in the double-layer structure at the carbon surface. Additionally, the modified electrodes show fast charge transfer kinetics to outer-sphere one-electron redox couples such as ferro-/ferricyanide as well as improved electron transfer kinetics in alkaline aqueous redox flow batteries.

Efficient in Situ One-Pot Synthesis of Water-Soluble Hydroxynaphthoquinones for Redox Flow Batteries

Given the importance of energy storage and its hybridization with renewable technologies for the energy transition, the development of redox flow batteries (RFB) is receiving particular attention. Among the various emerging technologies, aqueous organic redox flow batteries (AORFBs) are of particular interest, as the objectives in terms of durability, cost, and safety can be achieved thanks to the possibilities offered by molecular engineering. While anthraquinones have been widely explored as negolytes, few works report the use of naphthoquinones. This work aims to exploit an innovative in situ and cost-effective method for the one-pot synthesis of water-soluble naphthoquinones for application as a negolyte in redox flow batteries. As exemplified with alizarin, the energy of the naphthoquinone synthetic reaction in fuel cell mode can be recovered and the electrolyte solution used directly in redox flow batteries without purification. A 0.3 M naphthoquinone solution paired with 0.6 M ferrocyanide demonstrated good stability compared with other naphthoquinones, with a capacity fade rate of 0.017%/cycle (0.84%/day) over 320 cycles. Additionally, the system exhibited one of the highest energy efficiencies (82%) and a power density of 80-105 mW cm-2 at 50% SOC. These first results are promising for further exploration of new water-soluble naphthoquinones efficiently synthesized from hydroxyanthraquinones for application in AORFBs.

New Alkalescent Electrolyte Chemistry for Zinc-Ferricyanide Flow Battery

Alkaline zinc-ferricyanide flow batteries are efficiency and economical as energy storage solutions. However, they suffer from low energy density and short calendar life. The strongly alkaline conditions (3 mol L-1 OH-) reduce the solubility of ferri/ferro-cyanide (normally only 0.4 mol L-1 at 25 °C) and induce the formation of zinc dendrites at the anode. Here, we report a new zinc-ferricyanide flow battery based on a mild alkalescent (pH 12) electrolyte. Using a chelating agent to rearrange ferri/ferro-cyanide ion-solvent interactions and improve salt dissociation, we increased the solubility of ferri/ferro-cyanide to 1.7 mol L-1 and prevented zinc dendrites. Our battery has an energy density of ~74 Wh L-1 catholyte at 60 °C and remains stable for 1800 cycles (1800 hours) at 0 °C and for >1400 cycles (2300 hours) at 25 °C. An alkalescent zinc-ferricyanide cell stack built using this alkalescent electrolyte stably delivers 608 W of power for ~40 days.

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

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

Commercializable Naphthalene Diimide Anolytes for Neutral Aqueous Organic Redox Flow Batteries

Neutral aqueous organic redox flow batteries (AORFBs) hold the potential to facilitate the transition of renewable energy sources from auxiliary to primary energy, the commercial production of anolyte materials still suffers from insufficient performance of high-concentration and the high cost of the preparation problem. To overcome these challenges, this study provides a hydrothermal synthesis methodology and introduces the charged functional groups into hydrophobic naphthalene diimide cores, and prepares a series of high-performance naphthalene diimide anolytes. Under the synergistic effect of π-π stacking and H-bonding networks, the naphthalene diimide exhibits excellent structural stability and the highest water solubility (1.85 M for dex-NDI) reported to date. By employing the hydrothermal method, low-cost naphthalene diimides are successfully synthesized on a hundred-gram scale of $0.16 g-1 ($2.43 Ah-1), which is also the lowest price reported to date. The constructed full battery achieves a high electron concentration of 2.4 M, a high capacity of 54.4 Ah L-1, and a power density of 318 mW cm-2 with no significant capacity decay observed during long-duration cycling. These findings provide crucial support for the commercialization of AORFBs and pave the way for revolutionary developments in neutral AORFBs.

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.

Directional molecular transport in iron redox flow batteries by interfacial electrostatic forces

The mounting global energy demand urges surplus electricity generation. Due to dwindling fossil resources and environmental concerns, shifting from carbon-based fuels to renewables is vital. Though renewables are affordable, their intermittent nature poses supply challenges. In these contexts, aqueous flow batteries (AFBs), are a viable energy storage solution. This study tackles AFBs' energy density and efficiency challenges. Conventional strategies focus on altering molecule's solubility but overlook interface's transport kinetics. We show that triggering electrostatic forces at the interface can significantly enhance the mass transport kinetics of redox active molecules by introducing a powerful electrostatic flux over the diffusional flux, thereby exerting a precise directionality on the molecular transport. This approach of controlling the directionality of molecular flux in an all iron redox flow battery amplifies the current and power rating with approximately 140 % enhancement in the energy density.

Starch-mediated colloidal chemistry for highly reversible zinc-based polyiodide redox flow batteries

Aqueous Zn-I flow batteries utilizing low-cost porous membranes are promising candidates for high-power-density large-scale energy storage. However, capacity loss and low Coulombic efficiency resulting from polyiodide cross-over hinder the grid-level battery performance. Here, we develop colloidal chemistry for iodine-starch catholytes, endowing enlarged-sized active materials by strong chemisorption-induced colloidal aggregation. The size-sieving effect effectively suppresses polyiodide cross-over, enabling the utilization of porous membranes with high ionic conductivity. The developed flow battery achieves a high-power density of 42 mW cm-2 at 37.5 mA cm-2 with a Coulombic efficiency of over 98% and prolonged cycling for 200 cycles at 32.4 Ah L-1posolyte (50% state of charge), even at 50 °C. Furthermore, the scaled-up flow battery module integrating with photovoltaic packs demonstrates practical renewable energy storage capabilities. Cost analysis reveals a 14.3 times reduction in the installed cost due to the applicability of cheap porous membranes, indicating its potential competitiveness for grid energy storage.

Bifunctional Potential Structure Design Breaks Electrolyte Limitations of Zinc Ion Battery

Aqueous zinc-ion batteries (ZIBs) are safe and economical for grid applications. However, current ZIBs have limitations in terms of inferior capacity and low output voltage, which are hampered by the electrolyte applicability of the Zn2+ hosts. In this study, we propose a novel organic cathode design strategy with a bifunctional potential region. This polymeric Zn2+ host combines the conjugated polyaniline backbone to tune the molecular surface pH and [Fe(CN)6]3-/4- redox couple for high output voltage and capacity. The polyaniline doped with ferricyanide (PAF) electrode exhibits two forms of charge storage in ZIBs: proton-assisted Zn2+ doping below 1.2 V (mechanism I), and [Fe(CN)6]3-/4- redox pair above 1.8 V (mechanism II). Density functional theory calculations and in situ pH experiments demonstrated that the H+ doping process of mechanism I forms a localized pH regulation on the molecular chain surface, providing a favorable reaction environment for mechanism II. The Zn-polymer battery delivered an outstanding discharge capacity (405.2 mAh g-1) and high output voltage (1.8 V) in the Zn(CF3SO3)2 electrolyte. This study provides a new route for enhancing the structural stability of electrodes and overcoming the electrolyte limitations of ferricyanide in weakly acidic electrolytes.

Phosphonate-based iron complex for a cost-effective and long cycling aqueous iron redox flow battery

A promising metal-organic complex, iron (Fe)-NTMPA2, consisting of Fe(III) chloride and nitrilotri-(methylphosphonic acid) (NTMPA), is designed for use in aqueous iron redox flow batteries. A full-cell testing, where a concentrated Fe-NTMPA2 anolyte (0.67 M) is paired with a Fe-CN catholyte, demonstrates exceptional cycling stability over 1000 charge/discharge cycles, and noteworthy performances, including 96% capacity utilization, a minimal capacity fade rate of 0.0013% per cycle (1.3% over 1,000 cycles), high Coulombic efficiency and energy efficiency near 100% and 87%, respectively, all achieved under a current density of 20 mA·cm-². Furthermore, density functional theory unveils two potential coordination structures for Fe-NTMPA2 complexes, improving the understanding between the ligand coordination environment and electron transfer kinetics. When paired with a high redox potential Fe-Dcbpy/CN catholyte, 2,2'-bipyridine-4,4'-dicarboxylic (Dcbpy) acid and cyanide (CN) ligands, Fe-NTMPA2 demonstrates a notably elevated cell voltage of 1 V, enabling a practical energy density of up to 9 Wh/L.

Stabilization of Naphthalene Diimide Anions by Ion Pair Formation in Nonaqueous Organic Redox Flow Batteries

In redox flow batteries, a compelling strategy for enhancing the charge capacity of redox-active organic molecules involves storing multiple electrons within a single molecule. However, this approach poses unique challenges such as chemical instability by forming radicals, elevated energy requirements, and unsustainable charge concentration. Ion pairing is a possible solution to achieve charge neutrality and engineer redox potential shifts but has received limited attention. In this study, we demonstrate that Li+ can stabilize naphthalene diimide (NDI) anions dissolved in acetonitrile and significantly shift the second cathodic potential close to the first. Our findings, supported by density functional theory calculations and Fourier transform infrared spectroscopy, indicate that dimeric NDI species form stable ion pairs with Li+. Conversely, K+ ions exhibit weak interactions, and cyclic voltammograms confirm significant potential shifts when stronger Lewis acids and solvents with lower donor numbers are employed. Galvanostatic examinations reveal a single voltage plateau with Li+, which indicates a rapid redox process involving doubly charged NDI2- with Li+. These aggregated ion pairs offer the additional benefits of hindering crossover events, contributing to excellent cyclability, and suppressing undesirable side reactions even after 1000 redox cycles.

Charge-Dependent Crossover in Aqueous Organic Redox Flow Batteries Revealed Using Online NMR Spectroscopy

Aqueous organic redox-flow batteries (AORFBs) are promising candidates for low-cost grid-level energy storage. However, their wide-scale deployment is limited by crossover of redox-active material through the separator membrane, which causes capacity decay. Traditional membrane permeability measurements do not capture all contributions to crossover in working batteries, including migration and changes in ion size and charge. Here we present a new method for characterizing crossover in operating AORFBs using online 1H NMR spectroscopy. By the introduction of a separate pump to decouple NMR and battery flow rates, this method opens a route to quantitative time-resolved monitoring of redox-flow batteries under real operating conditions. In this proof-of-concept study of a 2,6-dihydroxyanthraquinone (2,6-DHAQ)/ferrocyanide model system, we observed a doubling of the 2,6-DHAQ crossover during battery charging, which we attribute to migration effects. This new membrane testing methodology will advance our understanding of crossover and accelerate the development of improved redox-flow batteries.

Screening Ultra-Stable (Phenazine)dioxyalkanocic Acids with Varied Water-Solubilizing Chain Lengths for High-Capacity Aqueous Redox Flow Batteries

Aqueous redox flow batteries (ARFBs) hold great potential for large-scale energy storage. Recently, research on aqueous flow batteries has shifted toward water-soluble organic molecules with redox capabilities to reduce the use of mineral resources. The chemical and electrochemical stabilities of organic compounds are heavily influenced by their functional groups and reaction sites. In this study, we present a low-cost synthesis of the O-alkyl-carboxylate-functionalized derivatives of 2,3-dihydroxyphenazine, namely, phenazine-(2,3-diyl) dioxy dibutyric acid (DBEP) and phenazine-(2,3-diyl)dioxy diacetic acid (DAEP), which serve as negolytes and exhibit good reversibility and high redox kinetics. The evidence is provided to clarify the capacity degradation mechanisms of DAEP and DBEP by a series of comprehensive characterizations. Similar to anthraquinones functionalized with alkyl chains, the main degradation mechanism of DAEP modified with acetic acid is due to side chain loss. Longer side chains are more stable and can withstand long-term electrochemical reactions. DBEP modified with butyric acid exhibits superior chemical and electrochemical stability. Our results demonstrate that rational molecular design and suitable membranes, such as the alkaline ARFBs based on DBEP negolyte, potassium ferrocyanide (K4Fe(CN)6) posolyte, and custom sulfonated poly(ether ether ketone) membrane, can deliver a high open-circuit voltage of 1.17 V and high capacity retention of 99.997% per cycle for over 1000 cycles at 50 mA cm-2. This study highlights the importance of not only considering the modification position of the molecules but also focusing on the influence of various side chains on the redox core's stability toward sustainable grid-scale energy storage applications.

Exploring the Landscape of Heterocyclic Quinones for Redox Flow Batteries

Redox flow batteries (RFBs) rely on the development of cheap, highly soluble, and high-energy-density electrolytes. Several candidate quinones have already been investigated in the literature as two-electron anolytes or catholytes, benefiting from fast kinetics, high tunability, and low cost. Here, an investigation of nitrogen-rich fused heteroaromatic quinones was carried out to explore avenues for electrolyte development. These quinones were synthesized and screened by using electrochemical techniques. The most promising candidate, 4,8-dioxo-4,8-dihydrobenzo[1,2-d:4,5-d']bis([1,2,3]triazole)-1,5-diide (-0.68 V(SHE)), was tested in both an asymmetric and symmetric full-cell setup resulting in capacity fade rates of 0.35% per cycle and 0.0124% per cycle, respectively. In situ ultraviolet-visible spectroscopy (UV-Vis), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopies were used to investigate the electrochemical stability of the charged species during operation. UV-Vis spectroscopy, supported by density functional theory (DFT) modeling, reaffirmed that the two-step charging mechanism observed during battery operation consisted of two, single-electron transfers. The radical concentration during battery operation and the degree of delocalization of the unpaired electron were quantified with NMR and EPR spectroscopy.

A benzoquinone-imidazole hybrid organic anolyte for aqueous redox flow batteries

A benzoquinone derivative annelated by two imidazole rings was investigated as an organic anolyte of aqueous redox flow batteries. The anolyte showed a high solubility of 0.18 M in 1 M KOH aqueous solution and exhibited a one-step two-electron reversible redox wave with a half-wave potential of -0.59 V VS. SHE. An aqueous redox flow cell comprising the benzoquinone-imidazole hybrid as the anolyte and potassium ferrocyanide as the catholyte showed an operating voltage of ∼1.1 V and minimum capacity fading of over 220 cycles.

Enabling Long-Life Aqueous Organic Redox Flow Batteries with a Highly Stable, Low Redox Potential Phenazine Anolyte

Aqueous organic redox flow batteries (AORFBs) are considered a promising energy storage technology due to the sustainability and designability of organic active molecules. Despite this, most of AORFBs suffer from limited stability and low voltage because of the chemical instability and high redox potential of organic molecules in anolyte. Herein, we propose a new phenazine derivative, 4,4'-(phenazine-2,3-diylbis(oxy))dibutyric acid (2,3-O-DBAP), as a water-soluble and chemically stable anodic active molecules. By combining calculations and experiments, we demonstrate that 2,3-O-DBAP exhibits a higher solubility, a lower redox potential (-0.699 V vs SHE), and greater chemical stability than other O-DBAP isomers. Then, we demonstrate a long-lasting flow cell with an average discharge voltage of 1.12 V, a low fade rate of 0.0127%, and a lifespan of 62 days at pH 14 using 2,3-O-DBAP paired with ferri/ferrocyanide. The negligible self-discharge behavior also verifies the high stability of 2,3-O-DBAP. These results highlight the importance of molecular engineering for AORFBs.

Electrode Treatments for Redox Flow Batteries: Translating Our Understanding from Vanadium to Aqueous-Organic

Redox flow batteries (RFBs) are a promising technology for long-duration energy storage; but they suffer from inefficiencies in part due to the overvoltages at the electrode surface. In this work, more than 70 electrode treatments are reviewed that are previously shown to reduce the overvoltages and improve performance for vanadium RFBs (VRFBs), the most commercialized RFB technology. However, identifying treatments that improve performance the most and whether they are industrially implementable is challenging. This study attempts to address this challenge by comparing treatments under similar operating conditions and accounting for the treatment process complexity. The different treatments are compared at laboratory and industrial scale based on criteria for VRFB performance, treatment stability, economic feasibility, and ease of industrial implementation. Thermal, plasma, electrochemical oxidation, CO2 treatments, as well as Bi, Ag, and Cu catalysts loaded on electrodes are identified as the most promising for adoption in large scale VRFBs. The similarity in electrode treatments for aqueous-organic RFBs (AORFBs) and VRFBs is also identified. The need of standardization in RFBs testing along with fundamental studies to understand charge transfer reactions in redox active species used in RFBs moving forward is emphasized.

Searching High-Potential Dihydroxynaphthalene Cathode for Rocking-Chair All-Organic Aqueous Proton Batteries

The lack of acid-proof high-potential cathode largely limits the development and competitiveness of proton batteries. Herein, the authors systematically investigated six dihydroxynaphthalenes (DHNs) and found that 2,6-DHN delivered the best cathode performance in proton battery with the highest redox potential (0.84 V, vs SHE) and a specific capacity of 91.6 mAh g-1 at 1 A g-1 . In situ solid-state electropolymerization of DHNs is responsible for the voltage and capacity fading of DHNs, and 2,6-DHN's excellent electrochemical performance is derived from its high polymerization energy barrier. By compounding with rGO, the 2,6-DHN/rGO electrode can maintain a specific capacity of 89 mAh g-1 even after 12 000 cycles at 5 A g-1 . When it is paired with the 2,6-dihydroxyanthraquinone (DHAQ) anode, the assembled rocking-chair all-organic proton battery exhibited a high cell voltage of 0.85 V, and excellent energy/power densities (70.8 Wh kg-1 /850 W kg-1 ). This study showcases a new-type high-potential proton-containing organic cathode and paves the way for constructing a high-voltage rocking-chair proton battery. Also, in situ solid-state electropolymerization will inspire the further study of phenol-based small-molecule electrodes.

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

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

Active Learning Guided Computational Discovery of Plant-Based Redoxmers for Organic Nonaqueous Redox Flow Batteries

Organic nonaqueous redox flow batteries (O-NRFBs) are promising energy storage devices due to their scalability and reliance on sourceable materials. However, finding suitable redox-active organic molecules (redoxmers) for these batteries remains a challenge. Using plant-based compounds as precursors for these redoxmers can decrease their costs and environmental toxicity. In this computational study, flavonoid molecules have been examined as potential redoxmers for O-NRFBs. Flavone and isoflavone derivatives were selected as catholyte (positive charge carrier) and anolyte (negative charge carrier) molecules, respectively. To drive their redox potentials to the opposite extremes, in silico derivatization was performed using a novel algorithm to generate a library of > 40000 candidate molecules that penalizes overly complex structures. A multiobjective Bayesian optimization based active learning algorithm was then used to identify best redoxmer candidates in these search spaces. Our study provides methodologies for molecular design and optimization of natural scaffolds and highlights the need of incorporating expert chemistry awareness of the natural products and the basic rules of synthetic chemistry in machine learning.

Membrane-Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Alkaline Flow Cells

Membrane transport properties are crucial for electrochemical devices, and these properties are influenced by the composition and concentration of the electrolyte in contact with the membrane. We apply this general membrane-electrolyte system approach to alkaline flow batteries, studying the conductivity and ferricyanide crossover of Nafion and E-620. We report undetectable crossover for as-received Nafion and E-620 after both sodium and potassium exchange but high ferricyanide permeability of 10-7 to 10-8 cm2 s-1 for Nafion subjected to pretreatment prevalent in the flow battery literature. We show how the electrolyte mass fraction in hydrated membranes regulates the influence of ion concentration on membrane conductivity, identifying that increasing electrolyte concentration may not increase membrane conductivity even when it increases electrolyte conductivity. To illustrate this behavior, we introduce a new metric, the membrane penalty, as the ratio of the conductivity of the electrolyte to that of the membrane equilibrated with the electrolyte. We discuss the trade-off between flow battery volumetric capacity and areal power density that arises from these findings. Finally, we apply insights from this approach to provide recommendations for use of membranes in alkaline flow cells and electrochemical reactors in general.

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.

Bio-Based Polyhydroxyanthraquinones as High-Voltage Organic Electrode Materials for Batteries

Organic materials have gained much attention as sustainable electrode materials for batteries. Especially bio-based organic electrode materials (OEMs) are very interesting due to their geographical independency and low environmental impact. However, bio-based OEMs for high-voltage batteries remain scarce. Therefore, in this work, a family of bio-based polyhydroxyanthraquinones (PHAQs)-namely 1,2,3,4,5,6,7,8-octahydroxyanthraquinone (OHAQ), 1,2,3,5,6,7-hexahydroxyanthraquinone (HHAQ), and 2,3,6,7-tetrahydroxyanthraquinone (THAQ)-and their redox polymers were synthesized. These PHAQs were synthesized from plant-based precursors and exhibit both a high-potential polyphenolic redox couple (3.5-4.0 V vs Li/Li+) and an anthraquinone redox moiety (2.2-2.8 V vs Li/Li+), while also showing initial charging capacities of up to 381 mAh g-1. To counteract the rapid fading caused by dissolution into the electrolyte, a facile polymerization method was established to synthesize PHAQ polymers. For this, the polymerization of HHAQ served as a model reaction where formaldehyde, glyoxal, and glutaraldehyde were tested as linkers. The resulting polymers were investigated as cathode materials in lithium metal batteries. PHAQ polymer composites synthesized using formaldehyde as linker and 10 wt % multiwalled carbon nanotubes (MWCNTs), namely poly(THAQ-formaldehyde)-10 wt % MWCNTs and poly(HHAQ-formaldehyde)-10 wt % MWCNTs, exhibited the best cycling performance in the lithium metal cells, displaying a high-voltage discharge starting at 4.0 V (vs Li/Li+) and retaining 81.6 and 77.3 mAh g-1, respectively, after 100 cycles.

Benchmarking organic active materials for aqueous redox flow batteries in terms of lifetime and cost

Flow batteries are one option for future, low-cost stationary energy storage. We present a perspective overview of the potential cost of organic active materials for aqueous flow batteries based on a comprehensive mathematical model. The battery capital costs for 38 different organic active materials, as well as the state-of-the-art vanadium system are elucidated. We reveal that only a small number of organic molecules would result in costs close to the vanadium reference system. We identify the most promising candidate as the phenazine 3,3'-(phenazine-1,6-diylbis(azanediyl))dipropionic acid) [1,6-DPAP], suggesting costs even below that of the vanadium reference. Additional cost-saving potential can be expected by mass production of these active materials; major benefits lie in the reduced electrolyte costs as well as power costs, although plant maintenance is a major challenge when applying organic materials. Moreover, this work is designed to be expandable. The developed calculation tool (ReFlowLab) accompanying this publication is open for updates with new data.

Redox Targeting-based Neutral Aqueous Flow Battery with High Energy Density and Low Cost

Neutral aqueous flow batteries with common traits of the redox flow batteries, such as the independence of energy and power, scalability and operational flexibility, and additional merits of outstanding safety and low corrosivity show great promise for storing massive electrical energy from solar and wind energy. Particularly, the ferricyanide/ferrocyanide ([Fe(CN)6 ]3-/4- ) couple has been intensively employed as redox mediator to store energy in the catholyte ascribed to its abundance, low corrosivity, remarkable redox reversibility and stability. However, the low energy density arising from poor solubility of [Fe(CN)6 ]3-/4- restricts their commercial applications for energy storage systems. In this study, the practical energy density of a [Fe(CN)6 ]3-/4- -based catholyte is significantly boosted from 10.5 to 92.8 Wh L-1 by combining the counter-ion effect and the single-molecule redox-targeting (SMRT) reactions between [Fe(CN)6 ]3-/4- and Prussian blue (Fe4 [Fe(CN)6 ]3 , PB)/Prussian white (PW). Paired with concentrated K2 S anolyte, we demonstrate a neutral aqueous SMRT-based PB-Fe/S flow battery with ultra-long lifespan over 7000 cycles (4500 h) and ultra-low chemical cost of electrolytes in the cell as 19.26 $ kWh-1 . Remarkably, under the influences of SMRT reactions in the presence of PB granules in the catholyte, the capacity after 7000 cycles of the PB-Fe/S flow battery is 181.8 % of the initial capacity without PB.

Homogenizing Zn Deposition in Hierarchical Nanoporous Cu for a High-Current, High Areal-Capacity Zn Flow Battery

A Zn anode can offset the low energy density of a flow battery for a balanced approach toward electricity storage. Yet, when targeting inexpensive, long-duration storage, the battery demands a thick Zn deposit in a porous framework, whose heterogeneity triggers frequent dendrite formation and jeopardizes the stability of the battery. Here, Cu foam is transferred into a hierarchical nanoporous electrode to homogenize the deposition. It begins with alloying the foam with Zn to form Cu5 Zn8 , whose depth is controlled to retain the large pores for a hydraulic permeability ≈10-11  m2 . Dealloying follows to create nanoscale pores and abundant fine pits below 10 nm, where Zn can nucleate preferentially due to the Gibbs-Thomson effect, as supported by a density functional theory simulation. Morphological evolution monitored by in situ microscopy confirms uniform Zn deposition. The electrode delivers 200 h of stable cycles in a Zn-I2 flow battery at 60 mAh cm-2 and 60 mA cm-2 , performance that meets practical demands.

Sulfonated Poly(2,6-dimethyl-1,4-phenylene ether)-Modified Mixed-Matrix Bifunctional Polyelectrolyte Membranes for Long-Run Anthrarufin-Based Redox Flow Batteries

The resurgence in designing polyelectrolyte membrane (PEM) materials has propound grid-scale electrochemical energy storage devices. Herein, we report on studies corroborating the synergistic influence of ionic domain microstructure modification and intercalation of telechelic bis-piperidinium-functionalized graphene oxide (GO) to fabricate stable bifunctional membranes from sulfonated poly(2,6-dimethyl-1,4-phenylene ether) (sPPE) for efficient anthrarufin-based alkaline redox flow batteries. A critically long-lasting quest on alkaline stability and -OH conductivity dilemma in hydrocarbon-based PEMs is meticulously resolved via a bifunctional ion-conducting matrix. Preferential studies on hydrophilic domain distribution in sPPE suggest that, with high microphase homogeneity, higher specific capacity retentions are achievable during galvanostatic charge-discharge (GCD) analysis. Moreover, the low-capacity issues were overcome by improving the redoxolyte-membrane interface affinities incorporating bis-piperidinium-bearing graphene oxide (bis-QGO). Consequently, at 1.0 and 2.0 wt % intercalation of bis-QGO, the bifunctional polyelectrolyte membranes (BFPMs) impart lowest overpotentials of 93 mV (for BFPM-1.0) and ∼100 mV (for BFPM-2.0) which are ∼43 and 40% lower than that of Nafion-117 (i.e., ∼164 mV). Furthermore, the efficiency of BFPMs, viz., the Coulombic, voltage, and energy efficiencies, was ∼95-98%, ∼85%, and ≥80% at 20 mA cm-2, respectively. In long-cycling operations, the GCD profile evidenced ∼99% efficiency retention over 450 cycles and illustrated reproducible rate capability. Finally, the polarization studies of BFPMs revealed ∼54% higher peak power density (87.5 mW cm-2) delivery than Nafion-117 (∼57 mW cm-2). We believe that this strategic designing approach could offer newer and simple avenues to avail high-performance BFPMs at low intercalation loads for alkaline electrochemical energy storage and related applications.