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Redox flow batteries (RFBs) are considered as promising candidates for large‐scale energy storage. However, traditional RFBs based on toxic metal ions have deficiencies in resource utilization and environmental protection. Considering the corrosiveness of acidic and alkaline electrolytes and sustainability of energy storage devices, neutral aqueous organic redox flow batteries (AORFBs) have more development prospects. Herein, an AORFB is reported, using 9,10‐anthraquinone‐2,7‐disulfonic salt (2,7‐AQDS) and 1,4‐dihydroxyphenylsulfonate potassium (HQS) as negative and positive electroactive materials, respectively. It is found that the anions in the neutral solution further affected the solubility and kinetics of the electroactive materials by affecting the hydrogen bond in the solution, and the Na2SO4 solution showed the optimal comprehensive performance. Therefore, an all‐quinone AORFB employing neutral Na2SO4 electrolytes with a cell voltage of 0.9 V is constructed, which presented a capacity utilization of 70.1% and delivered stable cycling performance at 60 mA cm−2. This work designs an innovative neutral all‐quinone AORFB and points out how anions affect the properties of the electrolyte by affecting hydrogen bonds within the solution. These findings open a new avenue for the design and application of neutral all‐organic aqueous RFBs. [ABSTRACT FROM AUTHOR]

Five-membered pyrroline nitroxides with high-potential is fascinating as catholyte for aqueous organic redox flow batteries (AORFBs), however, it suffers from a primary deficiency of insufficient stability due to ring-opening side reaction. Herein we report a spatial structure regulation strategy by host-guest chemistry, encapsulating 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-oxyl (CPL) into hydrosoluble cyclodextrins (CDs) with an inclusion structure of N-O· head towards cavity bottom, to boost the solubility and cyclability of pyrroline nitroxides significantly. The armor-clad CPL (CPL⊂HP-β-CD) catholyte in 0.05-0.5 M presents a battery capacity fade rate as low as 0.002 %/cycle (0.233 %/day) compared to the sole CPL in 0.05 M (0.039 %/cycle or 5.23 %/day) over 500 cycles in assembled AORFBs. The optimized reclining spatial structure with N-O· head towards CD cavity bottom effectively inhibits the attack of Lewis base species on the hydrogen abstraction site in pyrroline ring, and thus avoids the ring-opening side reaction of pyrroline nitroxides. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) represent innovative and sustainable systems featuring decoupled energy capacity and power density; storing energy within organic redox-active materials. This design facilitates straightforward scalability, holding the potential for an affordable energy storage solution. However, AORFBs face challenges of unsatisfied energy density and stability. Redox-targeting (RT) reaction is a promising way to resolve these problems, which involves a closed-loop electrochemical–chemical cycle between soluble redox mediators and solid materials. Among all these systems, the aqueous organic redox-targeting system is the most promising due to its greater sustainability, safety, low cost, and excellent tunability when compared to non-aqueous or all-vanadium systems, especially when it comes to energy storage on a large scale. Firstly, various types of AORFBs and their characteristics are discussed and analyzed, followed by introducing the concept and the evolution of RT. In addition, advanced characterization techniques to analyze RT-based AORFBs are summarized. Finally, the challenges lying in aqueous organic redox-targeting flow batteries are stated and corresponding recommendations are provided. It is anticipated that AORFBs with advanced solid materials will provide a promising solution for large-scale energy storage. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs), which exploit the reversible electrochemical reactions of water soluble organic electrolytes to store electricity, gain increasing momentum for the grid‐scale integration of renewable electricity. However, the crossover of organic electrolytes across the membrane and the limited capacity significantly impede the widespread adoption of AORFBs. Here we report a PEGylated viologen anolyte bearing four positive charges, namely BTMAE−Vi, for crossover‐free and high capacity pH‐neutral AORFBs. This anolyte is rationally designed by appending positively charged ethylene glycol side chains to raise water solubility and increase the molecular size. The solubility of BTMAE−Vi is increased to 3.4 M because of an improved molecular polarity and the existence of four positive charges, while membrane crossover rate is significantly reduced. A BTMAE−Vi‐based flow cell demonstrates an extremely high capacity retention rate of 99.996 % per cycle or 99.979 % per hour when the first reversible redox reaction is utilized, but the irreversible molecular decomposition of the doubly reduced BTMAE−Vi results in severe capacity fade. Our results and the proposed concept in mitigating membrane crossover and elevating capacity will pave the way for developing high‐performance AORFBs. [ABSTRACT FROM AUTHOR]

We design the crossover-free N -heterocycle-linked TEMPO and symmetry-breaking bipyridinium redox pair for high-voltage, two-electron and stable aqueous organic redox flow batteries (AORFBs). This innovative work emphasizes the mild synthesis of new high water-soluble catholyte and anolyte materials based on TEMPO and bipyridinium cores, as well as the two-electron capability of bipyridinium for AORFBs. [Display omitted] • The neat N -heterocycle-substituted TEMPO and asymmetric bipyridinium redox pair are designed with minimal synthetic steps. • The high water-soluble redox pair display a crossover-free feature through a commercially available membrane. • The high-voltage, two-electron and stable aqueous organic flow batteries are achieved. • The X-ray single-crystal structure of N -heterocycle-substituted TEMPO is obtained. • The versatile design can extend to fabricating various amino-functionalized TEMPO and asymmetric bipyridinium derivatives. Aqueous organic redox flow batteries (AORFBs) are the brilliant technologies for safe and sustainable stationary energy storage. However, the cross-contamination, limited cell voltage, and inferior cycling stability remain challenges. Herein, the N -heterocycle-substituted TEMPO (TMP-TEMPO) and pyrrolidinium-/ammonium-grafted bipyridinium ([PyrTMAV]Cl 4) redox pair with multiple charges are designed for high-performance AORFBs. The whole material preparation is relatively simple and only needs two or three synthetic steps. The TMP-TEMPO and [PyrTMAV]Cl 4 show high aqueous solubility of 2.4 M and 1.71 M, respectively, excellent electrochemical reversibility, and fast redox kinetics. Notably, the introduction of multiple charges into the active materials can produce a strong Gibbs-Donnan effect with ion exchange membranes, thus preventing the permeability of molecules through the membrane and avoiding the cross-contamination of active molecules. By using the crossover-free TMP-TEMPO and two-electron [PyrTMAV]Cl 4 redox pair, the assembled two-electron AORFBs at an electron concentration of 0.2 M exhibit stable battery performance in environments with O 2 contents of both 10 ppm and 100 ppm with a capacity retention of over 99 % per day for hundreds of cycles (268 and 310 cycles), and a high energy efficiency of ∼89 % and Coulombic efficiency of ∼100 %. Furthermore, the AORFB achieves a high capacity of ∼37.4 Ah L−1 at an ultra-high electron concentration of 1.5 M, while maintaining a capacity retention rate of ∼99.7 % over 138 cycles and a theoretical capacity utilization rate of 93 %. The versatile molecular design for TMP-TEMPO and [PyrTMAV]Cl 4 can be easily extended to the synthesis of various amino-functionalized TEMPO and asymmetric bipyridinium derivatives. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) hold potential for large-scale energy storage, which are limited by poor chemical stability of organic compounds. To create stable organic active materials, systematic molecular engineering approaches involving a range of substitutions of functional moieties become imperative. Here, we introduce varying numbers of hydroxyl groups onto p -benzoquinone (p-BQ) core structure as stabilizing agents and then integrate the series of hydroxyl-functionalized BQs into the anolyte of alkaline RFBs to investigate how these substitutions affect cycle stability in a full cell operation. Comprehensive full cell cycling tests followed by ex-situ spectroscopic analyses confirm that an increased quantity of hydroxyl groups plays a crucial role in enhancing the electrochemical stability of molecules. Nevertheless, when BQ is completely substituted, the molecular structure becomes less stable due to increased electrostatic repulsion around the benzene ring. This observation suggests the existence of optimal ranges for functional group quantities to fine-tune molecular stability. Furthermore, our findings indicate that the principal contributor to instability in BQ derivatives stems from electrochemically irreversible redox reactions, rather than the alkaline environment itself. This empirical insight into structure-stability relationships has the potential to provide a robust design toolkit for highly stable organic molecules tailored for AORFB electrolytes. An experimental investigation of the relationship between the number of substituents and the cycle stability of benzoquinone-based redox-active molecules for aqueous redox flow batteries was conducted. [Display omitted] • A series of BQ derivatives with varied number of hydroxyl groups was investigated. • RFB full-cell operations and ex-situ spectroscopic analyses were conducted. • The substitution of hydroxyl groups on BQ improved electrochemical stability. • Electrochemical stability had a predominant impact compared to alkaline stability. • Full substitution rather led to molecular destabilization and inferior performance. [ABSTRACT FROM AUTHOR]

The implementation of renewable energies into the electrical grid is one of our best options to mitigate the climate change. Redox flow batteries (RFB) are one of the most promising candidates for energy storage due to their scalability, durability and low cost. Despite this, just few studies have explained the basic concepts of RFBs and even fewer have reviewed the experimental conditions that are crucial for their development. This work aspired to be a helpful guide for beginner researchers who want to work in this exciting field. This guided tour aimed to clearly explain all the components and parameters of RFBs. Using a well-studied chemistry of anthraquinone (AQDS)-based anolyte and Na4[Fe(CN)6] catholyte, different techniques for the characterization of RFBs were described. The effects of some experimental parameters on battery performance such as electrolyte pH, O2 presence, membrane pretreatment and the capacity limiting side, were demonstrated. Furthermore, this analysis served to introduce different electrochemical techniques, i.e., load curve measurements, electrochemical impedance spectroscopy and charge–discharge cycling tests. This work aimed to be the nexus between the basic concepts and the first experimental steps in the RFB field merging theory and experimental data. [ABSTRACT FROM AUTHOR]

The drawbacks of current state‐of‐the‐art selective membranes, such as poor barrier properties, high cost, and poor recyclability, limit the large‐scale deployment of electrochemical energy devices such as redox flow batteries (RFBs) and fuel cells. In recent years, cellulosic nanomaterials have been proposed as a low‐cost and green raw material for such membranes, but their performance in RFBs and fuel cells is typically poorer than that of the sulfonated fluoropolymer ionomer membranes such as Nafion. Herein, sulfonated cellulose nanofibrils densely cross‐linked to form a compact sulfonated cellulose membrane with limited swelling and good stability in water are used. The membranes possess low porosity and excellent ionic transport properties. A model aqueous organic redox flow battery (AORFB) with alizarin red S as negolyte and tiron as posolyte is assembled with the sulfonated cellulose membrane. The performance of the nanocellulose‐based battery is superior in terms of cyclability in comparison to that displayed by the battery assembled with commercially available Nafion 115 due to the mitigation of crossover of the redox‐active components. This finding paves the way to new green organic materials for fully sustainable AORFB solutions.

Aqueous organic redox flow batteries (AORFBs) have become a promising electrochemical energy storage technology due to their low cost, high safety, and sustainability. As key components of emerging AORFBs, membranes face challenges such as scarcity of available membranes and high prices. Moreover, currently available membranes have low battery performance because these membranes are not optimized for specific AORFBs. In this work, a new cation exchange membrane sulfonated anthrone-containing poly(aryl ether ketone) (SAnPEK) membrane was prepared for the first time and achieved significantly improved performance in (SPr) 2 V/K 4 [Fe(CN) 6 ] AORFBs. SAnPEK membrane has excellent selectivity and high mechanical strength (>50 MPa). SAnPEK membranes exhibit high conductivity in 1 M NH 4 Cl solution. For example, SAnPEK-4 membranes exhibit low surface resistance (0.176 Ωcm2) in 1 M NH 4 Cl solution, which is significantly better than Nafion117 (1.02 Ωcm2) and Nafion212 membranes (0.276 Ωcm2). SAnPEK membrane exhibtis impressive performance in (SPr) 2 V/K 4 [Fe(CN) 6 ] AORFB. The SAnPEK-4 membrane exhibits high coulombic efficiency (CE) and excellent energy efficiency (EE) (CE = 99.1 %, EE = 83.4 % at 100 mAcm−2), and the EE for SAnPEK-4 membrane is significantly better than that for the Nafion117 membrane (CE = 99.3 %, EE = 62.4 %) and Nafion212 membrane (CE = 99.1 %, EE = 80.8 %, 100 mAcm−2). In addition, the SAnPEK-4 membrane exhibits excellent cycle stability over 3000 cycles in AORFB. As a high-performance, easy-to-prepare, low-cost cation exchange membrane, SAnPEK membranes have good application potential in AORFBs. [Display omitted] • New sulfonated anthrone-containing poly(aryl ether ketone) membranes are prepared. • SAnPEK membranes are first applied in AORFBs. • SAnPEK membranes exhibit low area resistance and high membrane selectivity. • SAnPEK membranes exhibit an impressive energy efficiency of 83.4 % at 100 mAcm−2. [ABSTRACT FROM AUTHOR]

• Carbon paper electrode shows poor mechanical stability, affecting the AORFB performance. • The mechanical robustness of carbon cloth electrode contributes to a reliable and durable AORFB performance. • Carbon cloth electrode showed high stability when used on quinone oxidation–reduction cycles. • Electrochemical active sites on the carbon cloth increase current densities and AORFB power density. • Carbon cloth electrode provides higher AORFB power density at high electrolyte flow rate. Aqueous organic redox flow batteries (AORFBs) are emerging as promising candidates for efficient energy storage in grid-scale applications. In this study, we present a comprehensive analysis comparing the performance of thermally treated carbon paper and chemically oxidized carbon cloth electrodes, which have been extensively examined for their potential suitability in AORFBs. Results indicate that carbon cloth electrodes exhibit a higher concentration of oxygenated functional groups, contributing to enhanced electrochemical kinetics and improved electrochemical stability. These attributes translate to an AORFB with 62 times higher power density when compared to paper-based electrodes used. Through repeated charge-discharge cycles utilizing the BQDS║AQS model as a representative AORFB prototype, superior performance in the carbon cloth electrodes is consistently observed. Our findings even suggest the potential for optimizing the flow rate of the electrolyte to further enhance performance. [ABSTRACT FROM AUTHOR]

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

The implementation of renewable energies into the electrical grid is one of our best options to mitigate the climate change. Redox flow batteries (RFB) are one of the most promising candidates for energy storage due to their scalability, durability and low cost. Despite this, just few studies have explained the basic concepts of RFBs and even fewer have reviewed the experimental conditions that are crucial for their development. This work aspired to be a helpful guide for beginner researchers who want to work in this exciting field. This guided tour aimed to clearly explain all the components and parameters of RFBs. Using a well-studied chemistry of anthraquinone (AQDS)-based anolyte and Na4[Fe(CN)6] catholyte, different techniques for the characterization of RFBs were described. The effects of some experimental parameters on battery performance such as electrolyte pH, O2 presence, membrane pretreatment and the capacity limiting side, were demonstrated. Furthermore, this analysis served to introduce different electrochemical techniques, i.e., load curve measurements, electrochemical impedance spectroscopy and charge–discharge cycling tests. This work aimed to be the nexus between the basic concepts and the first experimental steps in the RFB field merging theory and experimental data.

Aqueous organic redox flow batteries (AORFBs) operated at neutral pH are considered a promising energy storage technology for massive-scale renewable energy storage. The development of suitable electrolytes is of particular significance because they determine the performances and cost of AORFBs. Viologen based redox-active materials are typical negative electrolytes for pH-neutral AORFBs. However, the widespread adoption of most reported viologen derivatives is limited by their solubility in water and the complicated synthesis. Herein, we propose a densely hydroxylated viologen, which can be obtained readily through a one-step synthesis with a yield of 70%, promising an ideal negative electrolyte for pH-neutral AORFBs. When operated in a 0.1 M practical pH-neutral AORFB against NMe-TEMPO, the battery displays an open-circuit voltage of 1.32 V, and delivers a capacity retention rate of 99.979% per cycle (temporal capacity fade rate of 0.18% per hour) over 600 consecutive cycles. An energy-dense pH-neutral AORFB based on this densely hydroxylated viologen is demonstrated and the mechanism leading to battery capacity fade is analyzed. [Display omitted] • A densely hydroxylated viologen is proposed as a negolyte for pH-neutral AORFBs. • The highly water soluble BDiOH-Vi is obtained by a facile one-step synthesis. • The degradation mechanism of BDiOH-Vi during AORFB cycling is disclosed. [ABSTRACT FROM AUTHOR]

Redox Flow Batteries (RFBs) are a versatile and durable type of electrochemical storage and a promising option for large-scale stationary energy storage. Aqueous Organic Redox Flow Batteries (AORFBs) are an innovative category of RFBs that utilize organic species as active molecules in aqueous electrolytes. These species allow for customization of their properties to achieve high technical performance and reduce battery cost. This study presents a comprehensive techno-economic analysis of AORFBs, evaluating their cost metrics and their associated uncertainties. The work modeled both capital cost and Levelized Cost of Storage (LCOS) for RFBs. The model was validated on the Vanadium Redox Flow Battery (VRFB), and it was employed to evaluate the costs for a generic AORFB, using a Monte Carlo technique to incorporate the uncertainty related to the value of critical parameters. Through stochastic analysis, AORFBs are estimated to have an average specific capital cost of 674 €/kWh for 4 h, and 398 €/kWh for 8 h batteries, and probabilities between 16.9% and 29.6% of having lower capital costs compared to VRFBs. AORFBs are estimated to have a current levelized cost, calculated including only the cost of energy lost in the storage due to irreversibility, of about 530 €/MWh for 4 h, and 411 €/MWh for 8 h batteries. The levelized costs of storage, calculated including the total cost of energy charged into the storage, have average values of 663 €/MWh for 4 h, and 543 €/MWh for 8 h batteries. AORFBs have less than 1% probability of having lower LCOS than VRFBs. Current AORFB systems have higher costs compared to state-of-the-art VRFBs, even assuming a low fabrication cost for available organic molecules. This is caused primarily by the AORFBs' low energy and power densities and high degradation rates. To ensure cost competitiveness with VRFBs, it is essential to identify better-performing organic redox pairs, which should exhibit high open circuit voltage (≥ 1.1 V), should maintain reasonable round-trip efficiency (≥ 71%) while operating at a high current density (≥ 55 mA/cm 2 ). Furthermore, new organic species should have low degradation rates (≤ 0.4 %/day). • Redox Flow Batteries' (RFB) capital and levelized cost of storage (LCOS) models. • Cyclic and calendar capacity fade rates included in the levelized cost model. • Stochastic analysis based on available experimental data for organic redox species. • AORFBs exhibit average capital costs of 674 €/kWh for 4 h and 398 €/kWh for 8 h. • AORFBs exhibit average levelized costs of 530 €/MWh for 4 h and 411 €/MWh for 8 h. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) are a promising electrochemical technology for large‐scale energy storage. We report a biomimetic, ultra‐stable AORFB utilizing an amino acid functionalized phenazine (AFP). A series of AFPs with various commercial amino acids at different substituted positions were synthesized and studied. 1,6‐AFPs display much higher stability during cycling when compared to 2,7‐ and 1,8‐AFPs. Mechanism investigations reveal that the reduced 2,7‐ and 1,8‐AFPs tend to tautomerize and lose their reversible redox activities, while 1,6‐AFPs possess ultra‐high stability both in their oxidized and reduced states. By pairing 3,3′‐(phenazine‐1,6‐diylbis(azanediyl))dipropionic acid (1,6‐DPAP) with ferrocyanide at pH 8 with 1.0 M electron concentration, this flow battery exhibits an OCV of 1.15 V and an extremely low capacity fade rate of 0.5 % per year. These results show the importance of molecular engineering of redox‐active organics for robust redox‐flow batteries. [ABSTRACT FROM AUTHOR]

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.

[Display omitted] • AORFB using Lawsone and FeCN as redox couple is suggested. • Composite supporting electrolyte containing hydroxide and sulfite anions is rationally designed. • Solubility and chemical stability of Lawsone are enhanced with sulfite anions. • Hydroxide anions improve redox kinetics of Lawsone. • AORFB short stack shows high power of 12.4 W with excellent capacity retention of 90%. Although aqueous organic redox flow batteries (AORFBs) have many benefits, their organic active materials have the limitations in solubility and chemical instability. To overcome the problems, efforts to develop new organic active materials is usually made. However, these efforts have still limitations in some prospects. Thus, in this study, instead of developing new organic active materials, a rationally designed composite supporting electrolyte is suggested to improve energy density, power density, life-cycle, and efficiency of AORFB using 2-hydroxy-1,4-naphthoquinone (Lawsone) and potassium ferrocyanide as redox couple. The composite supporting electrolyte consists of potassium hydroxide (KOH) and potassium sulfite and is optimized by density functional theory (DFT). DFT predicts that sulfite anions help the solubility and stability of Lawson to improve, while hydroxide anions promote reactivity. The predictions are proved by electrochemical evaluations. With that, when the optimal composite supporting electrolyte is used, high capacity and excellent retention of AORFB (90 % at 100th cycle (from 20.27 to 18.42 Ah·L−1)) are achieved, while high power of 12.4 W is produced by AORFB short stack. [ABSTRACT FROM AUTHOR]

Herein, a computational predictive tool for redox flow batteries based on NBO and ADCH charge distribution studies is presented and supported by experimental evidence. Using highly water soluble (>2 M) non-planar 2,2′-bipyridinium salts as a case of study, this work presents a DFT protocol that successfully predicts the stability and forecasts their potential application as active materials for Aqueous Organic Redox Flow Batteries (AORFB). An initial theoretical-experimental characterization of selected bipyridines served to determine the effect of the ring size, geometry, and electron density on the physico-chemical properties of the materials. Nonetheless, the NBO and ADCH charge analyses were essential tools to understand the stability of the reduced species in terms of electronic delocalization and the importance of the molecular design on the stability of electrolyte for AORFB. Based on these results, the cell performance of seven-membered 2,2′-bypiridinium salt, (2), and m -Me substituted homologous, (4), were compared. The significantly lower capacity decay rendered by compound 4 based electrolyte, (0.35%/cycle) compared with compound 2 based electrolyte, (0.71%/cycle) was in good agreement with the predicted stability. The aim of this work is to highlight the powerful synergy between DFT calculations and organic chemistry in predicting the behaviour of different negolytes. [Display omitted] • 2,2′-Bipyridinium salts have been selected as negolytes for this study. • DFT calculation let correlate structural parameters with different redox properties. • NBO and ADCH charges shed light on the stability of reduced species. • Double capacity retention of selected negolyte shows the predicting ability of DFT. [ABSTRACT FROM AUTHOR]

• Aqueous organic redox flow battery(AORFB) with Alizarin Red & potassium ferrocyanide. • The performance of the AORFB was studied in alkaline media. • ARS (negolyte) exhibits good performance without significant losses of efficiencies. • The AORFB was found stable with an average storage capacity of 7 mWh.L-1. • The AORFB displays coulombic, voltaic and energy efficiencies of 90%, 75% and 68% An aqueous organic redox flow battery (AORFB) based on Alizarin Red S, 3,4-dihydroxy-9,10-anthraquinone-2-sulfonic acid (ARS) and potassium ferrocyanide (PF) was studied in alkaline medium. Charge-discharge processes and potential-current response behaviour in a full cell combining ARS, as the negolyte, and PF, as the posolyte, were investigated. The diffusion coefficient (D) and kinetic rate constant (k 0) of ARS were obtained from electrochemical measurements indicating a two-electron reduction reaction of ARS. The electrochemical stability test of the redox pair showed that the negolyte (ARS), even when subjected to extreme potential and current conditions, does not lose its redox activity and exhibits good performance without significant losses of efficiencies in a single cell test. The AORFB was found stable with an average storage capacity of 5.9 mWh L-1 and capacity retention of 89 and 91.5% for a single cell test before and after stability test, respectively. Additionally, the photophysics of ARS was investigated aiming to rationalize the stability of the compound as negolyte. The system showed to have potential as an organic aqueous flow battery closely related to the electrochemical stability and efficiencies associated to ARS. [ABSTRACT FROM AUTHOR]

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

• The nickel-coated porous polyethylene membrane (SB-PE-Ni-x) is developed. • SB-PE-Ni-x is optimized to achieve superior stability and hydrophilicity in alkaline media. • SB-PE-Ni-50 shows the best stability and hydrophilicity. • AORFB using NQSO/FeCN redox couple and SB-PE-Ni-50 shows the best performance. • The capacity loss rate of AORFB using SB-PE-Ni-50 was 0.0235 Ah∙L−1 per cycle. The surface of porous polyethylene (SB-PE) membrane is modified with nickel coating using a hydrothermal method (SB-PE-Ni-x membrane). To optimize effects of pore size and hydrophilicity on properties of SB-PE-Ni-x, the amount of included nickel is varied. According to evaluations, structure of SB-PE-Ni-x is well preserved even under strong alkaline due to the formation of nickel silicate shell. The nickel silicate shell induces the preservation of stability and hydrophilicity and increase in ionic conductivity, while porosity and pore size of SB-PE-Ni-x are reduced from 67.4% and 0.819 µm (SB-PE) to 57.9% and 0.557 µm (SB-PE-Ni-100). Chemical characterizations show that when nickel is coated onto SB-PE, porous structure and chemical bonding of SB-PE-Ni-x remain unchanged with high water uptake (32.5%) even after alkali treatment. This is evidence of high ionic conductivity. The performances of aqueous organic redox flow batteries (AORFBs) using mixed naphthoquinone and potassium ferrocyanide dissolved in potassium hydroxide are then investigated with SB-PE and SB-PE-Ni-x membranes. As a result, AORFB using SB-PE-Ni-50 shows high energy efficiency (80%) and low capacity loss rate (0.0235 Ah∙L−1 per cycle). This is because SB-PE-Ni-50 has optimal pore size showing less cross-over of active materials and high ionic conductivity. [ABSTRACT FROM AUTHOR]

Amino acids, aqueous organic redox flow batteries, energy storage, molecular engineering Titelbild: Biomimetic Amino Acid Functionalized Phenazine Flow Batteries with Long Lifetime at Near-Neutral pH (Angew. Keywords: amino acids; aqueous organic redox flow batteries; energy storage; molecular engineering EN amino acids aqueous organic redox flow batteries energy storage molecular engineering 5005 5005 1 02/24/21 20210301 NES 210301 B Wässrig-organische Redox-Flow-Batterien b (AORFB) sind eine vielversprechende elektrochemische Technologie für die Energiespeicherung im großen Maßstab. [Extracted from the article]

All redox flow batteries suffer from low energy storage density in comparison with conventional Li-ion batteries. However, this issue can be mitigated by utilization of solid energy storage materials to enhance the energy storage capacity. In this paper we demonstrate the utilization of copper hexacyanoferrate (CuHCF) Prussian blue analogue for this purpose, coupled with N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TEMPTMA) as a soluble redox mediator to target the redox transitions of the solid material. In this case, indirect charging and discharging of CuHCF suspended in the electrolyte by electrochemically oxidized/reduced TEMPTMA was observed by chronoamperometry. Secondly, electrochemistry of different CuHCF composites with carbon black and multi-walled carbon nanotubes were investigated, highlighting that the high conductivity of the solid energy storage materials is crucial to access the maximal charge storage capacity. Finally, a CuHCF-TEMPTMA/Zn aqueous redox flow battery achieved stable cycling performances with high coulombic efficiency of 95% and volumetric capacity of 350 C mL−1. • A redox solid charge-storage for the positive electrolyte of an aqueous organic redox flow battery. • Study and simulation of K+-diffusion process inside a Prussian blue analogue (CuHCF), where electrical conductance between CuHCF particles was enhanced by MWCNT. • Enhancement of the gravimetric capacity by 3-fold (up to 70 mAh g−1) to the pristine CuHCF material (2 mAh g−1). [ABSTRACT FROM AUTHOR]