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Manganese (Mn), possessing ample reserves on the earth, exhibits various oxidation states and garners significant attentions within the realm of battery technology. Mn-based flow batteries (MFBs) are recognized as viable contenders for energy storage owing to their environmentally sustainable nature, economic feasibility, and enhanced safety features. Nevertheless, the advancement of MFBs is hindered by contentious reaction mechanisms, suboptimal energy density, and inadequate cycling stability. This review offers a comprehensive analysis of various MFBs based on the specific redox couples utilized in the catholyte, including Mn3+/Mn2+, MnO2/Mn2+, and MnO4−/MnO42−. Moreover, recent advancements and concerns encountered by each type of MFBs are subsequently addressed and discussed in detail. Additionally, the current understanding of the mechanisms for different Mn-based pairs and their potentials for energy storage applications are introduced. Finally, challenges for the future development of MFBs, along with suggested improvement strategies are outlined. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ABSTRACT A new 1,2,3‐triazole functionalized ferrocene (1,2,3‐TAFc) produced by Cu(I)‐catalyzed click reaction was investigated as positive electrolyte for aqueous organic flow batteries (AOFBs). The molecule is highly soluble in 1 M hydrochloric acid and displays high electrochemical reversibility. 1,2,3‐TAFc demonstrated good stability during cycling with a low capacity decay (0.011%/cyc, 3.0%/day) and high Coulombic efficiency (99.4%) over 280 cycles when tested in a flow battery at low concentration. This low capacity decay was attributed to the instability of ferrocene. These findings indicate that a stable and water‐soluble catholyte for AOFBs can be obtained with structural modifications of 1,2,3‐TAFc. [ABSTRACT FROM AUTHOR]

Fluorenones are suitable candidates for negolytes in flow batteries, as they demonstrate the ability to store 2 electrons, and can achieve reversibility, solubility, and stability with appropriate molecular design. However, limitations persist such as the use of alkaline media, high redox potentials, and a limited scope for optimization. Herein, azoniafluorenones is reported as a novel class of negolytes. They can be readily accessed in a highly modular fashion from inexpensive commercially available materials (e.g., boronic acids). Variations in the substitution patterns reveal the 3‐substituted N‐alkylated AZON3, which demonstrates excellent solubility at neutral pH (1.64 m) with two low reversible redox potentials (−0.31 and −0.58 V vs Ag/AgCl). AZON3 exhibits high stability when evaluated at high concentration in a neutral supporting electrolyte (1 m in 3 m KCl), paired with BTMAP‐Fc on the positive side. Capacity retentions of 99.95% and 99.91% per cycle (99.35% and 99.21% per day) are achieved when cycling with 1 and 2 electrons, respectively, coupled with high volumetric capacity of 46.4 Ah L−1 (87% of capacity utilization). [ABSTRACT FROM AUTHOR]

Viologens are ideal anolytes for organic redox flow batteries due to their stable redox behaviour, but they face the challenge of poor solubility of mono-cation radical dimers formed during the oxidation/reduction process. In our study, we prepared four viologen derivatives: sulfonic acid (SV), carboxylic acid (VAV), quaternized ammonium salt (QV), and imidazole (IV). IV exhibited the most stable mono-cation radical dimer, followed by QV, VAV, and SV. The stability was assessed by recording EPR spectra during the battery operation and confirmed by 1H NMR after the battery operation. Intriguingly, the most stable dimer led to poor battery performance. With the least stable dimer, SV performed best using 0.5 M [Fe(CN)6]3−/4− catholyte in a 1 M NH4Cl solution with a polyethylene/styrene interpolymer cation exchange membrane. The battery achieved an impressive 99% Coulombic efficiency (CE) and 60.3% energy efficiency (EE) at an applied current density of 100 mA cm−2. Over 200 stable charge and discharge cycles, it reached 11.25 Ah L−1 capacity at 20 mA cm−2 current density. This study highlights the need to address the stability and solubility of mono-cation radical dimers and emphasizes intelligent molecular engineering to optimize viologen derivatives in organic redox flow batteries. [ABSTRACT FROM AUTHOR]

A water‐soluble anthraquinone derived from alizarin, 3HAAQ, is introduced as the redox‐active material in a negative potential electrolyte (anolyte) for aqueous redox flow batteries operating at pH 14. The synthesis of 3HAAQ is carried out using the Mannich reaction, which significantly improves the solubility of the new compound, an important factor for its use in RFB. Pairing with potassium ferri/ferrocyanide positive electrolyte, this flow battery exhibits an open‐circuit voltage of 1.24 V and maintains nearly 80% of the theoretical capacity at 40 mA cm−2 current density. [ABSTRACT FROM AUTHOR]

Recent advances in 3D printing have enabled the manufacture of porous electrodes which cannot be machined using traditional methods. With micron-scale precision, the pore structure of an electrode can now be designed for optimal energy efficiency, and a 3D printed electrode is not limited to a single uniform porosity. As these electrodes scale in size, however, the total number of possible pore designs can be intractable; choosing an appropriate pore distribution manually can be a complex task. To address this challenge, we adopt an inverse design approach. Using physics-based models, the electrode structure is optimized to minimize power losses in a flow reactor. The computer-generated structure is then printed and benchmarked against homogeneous porosity electrodes. We show how an optimized electrode decreases the power requirements by 16% compared to the best-case homogeneous porosity. Future work could apply this approach to flow batteries, electrolyzers, and fuel cells to accelerate their design and implementation. [ABSTRACT FROM AUTHOR]

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

Redox flow batteries (RFBs) with high energy densities are essential for efficient and sustainable long‐term energy storage on a grid scale. To advance the development of nonaqueous RFBs with high energy densities, a new organic RFB system employing a molecularly engineered tetrathiafulvalene derivative ((PEG3/PerF)‐TTF) as a high energy density catholyte was developed. A synergistic approach to the molecular design of tetrathiafulvalene (TTF) was applied, with the incorporation of polyethylene glycol (PEG) chains, which enhance its solubility in organic carbonate electrolytes, and a perfluoro (PerF) group to increase its redox potential. When paired with a lithium metal anode, the two‐electron‐active (PEG3/PerF)‐TTF catholyte produced a cell voltage of 3.56 V for the first redox process and 3.92 V for the second redox process. In cyclic voltammetry and flow cell tests, the redox chemistry exhibited excellent cycling stability. The Li|(PEG3/PerF)‐TTF batteries, with concentrations of 0.1 M and 0.5 M, demonstrated capacity retention rates of ~94 % (99.87 % per cycle, 97.52 % per day) and 90 % (99.93 % per cycle, 99.16 % per day), and the average Coulombic efficiencies of 99.38 % and 98.35 %, respectively. The flow cell achieved a high power density of 129 mW/cm2. Furthermore, owing to the high redox potential and solubility of (PEG3/PerF)‐TTF, the flow cell attained a high operational energy density of 72 Wh/L (100 Wh/L theoretical). A 0.75 M flow cell exhibited an even higher operational energy density of 96 Wh/L (150 Wh/L theoretical). [ABSTRACT FROM AUTHOR]

Ion exchange membranes (IEMs) play a critical role in aqueous organic redox flow batteries (AORFBs). Traditional IEMs that feature microphase‐separated microstructures are well‐developed and easily available but suffer from the conductivity/selectivity tradeoff. The emerging charged microporous polymer membranes show the potential to overcome this tradeoff, yet their commercialization is still hindered by tedious syntheses and demanding conditions. We herein combine the advantages of these two types of membrane materials via simple in situ hypercrosslinking of conventional IEMs into microporous ones. Such a concept is exemplified by the very cheap commercial quaternized polyphenylene oxide membrane. The hypercrosslinking treatment turns poor‐performance membranes into high‐performance ones, as demonstrated by the above 10‐fold selectivity enhancement and much‐improved conductivities that more than doubled. This turn is also confirmed by the effective and stable pH‐neutral AORFB with decreased membrane resistance and at least an order of magnitude lower capacity loss rate. This battery shows advantages over other reported AORFBs in terms of a low capacity loss rate (0.0017 % per cycle) at high current density. This work provides an economically feasible method for designing AORFB‐oriented membranes with microporosity. [ABSTRACT FROM AUTHOR]

Hybrid flow chemical power source (Pt–C)H2|Nafion|VO2+(C) in which the membrane–electrode assembly combines gas-diffusion anode of hydrogen–air fuel cell and cathode of vanadium redox flow battery is studied. Concept of such a hydrogen–vanadium flow battery had been proposed earlier (2013) as an alternative to the vanadium redox flow battery, also designed for large-scale electrical energy storage but its practical implementation has so far been limited to single cells having the active area within several tens of cm2. The goal of this work is the establishing of the factors limiting the discharge power density of such hybrid. hydrogen–vanadium flow battery cells which is inferior to both hydrogen–air fuel cell and vanadium redox flow batteries, even though the hydrogen–vanadium flow battery cell represents a combination of their more reversible half-cells. The object of the study is a cell with a 2 × 2 cm membrane–electrode assembly equipped with Luggin capillary on the vanadium electrolyte side. Measurements of the current–voltage characteristics of the entire cell, as well as the polarization characteristics of its half-cells, are performed using a six-electrode scheme of the cell connection with varied vanadium electrolyte circulation rate and different cathode materials (carbon felts, 4.6 or 2.5 mm thick, as well as carbon paper). The contribution of the hydrogen gas diffusion electrode to the total dc resistance of the hydrogen–vanadium flow battery cell is shown being twice that of the flow-through vanadium cathode. A record high discharge power density has been achieved: 0.75 W cm–2, for the cell based on the commercially available material, Sigracell GFD 2.5 EA-TA carbon felt as the cathode material, without its special surface modification. [ABSTRACT FROM AUTHOR]

For proton exchange membranes (PEM) used in vanadium redox batteries (VRBs), doping metal-organic framework (MOF) materials to enhance the proton permeability and vanadium ion barrier property of PEM has become a research focus. In synthesizing MOFs, conventional hydrothermal method is hindered by prolonged reaction time and suboptimal material structures. This study utilizes microwave assisted heating for the preparation of UIO-66-NH2 crystals, which are then combined with Nafion to fabricate composite membranes. The results indicate that microwave heating enables a more efficient and rapid synthesis of MOF crystals (MU-NH2), with enhanced uniformity and higher yield than the conventional hydrothermal process (CU-NH2). Compared to MU-NH2/Nf-0(vanadium ion permeability: 4.4 × 10−7 cm2·min−1; ion selectivity: 5 × 104 S·min·cm−3), the vanadium ion permeability of MU-NH2/Nf-3 and CU-NH2/Nf-3 membranes exhibits a similar level of reduction, approximately 82%. Additionally, the former demonstrates an enhanced ion selectivity by 33.3%, while the latter shows an increase of 30.9%. VRBs equipped with MU-NH2/Nf membranes exhibit higher energy efficiency (83.8% at 100 mA·cm−2) and better capacity retention than those with CU-NH2/Nf membranes (82.7% at 100 mA·cm−2). Therefore, UIO-66-NH2 synthesized via the microwave method can effectively enhance the comprehensive properties of proton exchange membranes and shows promising prospects in optimizing the performance of VRBs. [ABSTRACT FROM AUTHOR]

This work presents the first proof‐of‐concept of a membraneless micro redox flow battery with an automated closed‐loop control. Using micro actuators and micro sensors, charge and discharge is achieved in continuous operation in recirculation. A maximum value of 60 % State of Charge with commercial Vanadium electrolyte, monitored via online spectrometry, is delivered, with a maximum discharge current density of 60 mA/cm2, and a discharge volumetric capacity of 21.5 Ah/L. Next, cycling tests show a decrease in coulombic efficiency of 0.5 % per cycle, and capacity loss. To overcome some drawbacks encountered with Vanadium, an alkaline quinone electrolyte is used instead in the membraneless micro redox flow battery. Using this new electrolyte, sixty continuous cycles are performed showing a decrease in coulombic efficiency of 0.6 % per cycle, and a capacity loss of only 1.2 % per cycle. The performances obtained outshine previous literature results. The highest energy efficiency ever obtained for a membraneless micro redox flow battery is presented here with alkaline quinone having an efficiency of 28.9 %. The cycling of a membraneless micro redox flow battery is successfully performed for the first time. This work also includes performance improvement suggestions for future work, with this landmark opening a promising path towards achieving the metrics of conventional redox flow batteries. [ABSTRACT FROM AUTHOR]

The integration of energy storage batteries into renewable energy stations is a crucial development in the quest for sustainable and reliable energy solutions. This review provides a comprehensive analysis of this integration, detailing the types of energy storage batteries, including lithium-ion, lead-acid, and flow batteries, as well as their respective benefits and limitations. The study addresses significant challenges such as the intermittency of renewable energy sources, battery degradation and lifetime, cost and efficiency issues, and technical challenges. It also explores potential solutions to these challenges. Additionally, future directions are discussed, focusing on emerging battery technologies and smart grid system integration. This paper highlights the essential role of energy storage batteries in overcoming the intermittency of renewable sources and ensuring a stable and efficient energy supply. [ABSTRACT FROM AUTHOR]

Nonaqueous flow batteries hold promise given their high cell voltage and energy density, but their performance is often plagued by the crossover of redox compounds. In this study, we used permselective lithium superionic conducting (LiSICON) ceramic membranes to enable reliable long-term use of organic redox molecules in nonaqueous flow cells. With different solvents on each side, enhanced cell voltages were obtained for a flow battery using viologen-based negolyte and TEMPO-based posolyte molecules. The thermoplastic assembly of the LiSICON membrane realized leakless cell sealing, thus overcoming the mechanical brittleness challenge. As a result, stable cycling was achieved in the flow cells, which showed good capacity retention over an extended test time. [ABSTRACT FROM AUTHOR]

Solar redox flow batteries (SRFB) have attracted increasing interest for simultaneous capture and storage of solar energy by integrating a photoelectrochemical cell with a redox flow battery. Herein, a scalable, nanostructured α‐Fe2O3 photoanode exhibiting a high photovoltage of 0.68 V in a fully integrated Na4Fe(CN)6/AQDS SRFB is demonstrated. Thanks to its optimal band alignment, it uniquely enables stable, unassisted photocharging of the SRFB up to a state‐of‐charge (SOC) higher than 50%. Concurrently, its improved charge transfer results in a record unbiased photocurrent density of 0.22 mA cm−2, with a sixfold increase at zero SOC compared to α‐Fe2O3 film. Through an in‐depth optical and photoelectrochemical characterization of different α‐Fe2O3 morphologies, the impact of nanostructuring on charge transfer is quantified. Most interestingly, an increase in unbiased photocurrent is observed at 10% SOC (0.31 mA cm−2) and attributed to adsorption of ferricyanide, which enhances charge transfer. Importantly, it is demonstrated that the superior performance is retained after device scale‐up to 5.72 cm2. Overall, the demonstrated unassisted device is on par with previously reported dye‐sensitized solar cell‐assisted hematite‐based SRFBs. More broadly, this work contributes to the real‐world deployment of cost‐effective SRFBs based on Earth‐abundant materials. [ABSTRACT FROM AUTHOR]

This study examines the need for bismuth as a catalyst for the Cr2+/Cr3+ redox couple in an iron–chrome redox flow battery (ICRFB) using 1) open‐circuit voltage (OCV) periods to understand the impact of bismuth and the mechanism of hydrogen production with and without electrolyte flow, and 2) charge/discharge cycles to evaluate how bismuth influences ICRFB performance. The OCV study finds that the capacity decay in the ICRFB cycling is not solely due to the hydrogen evolution reaction, suggesting an alternative oxidation reaction is involved, likely catalyzed by metallic carbides like bismuth carbide. Both the OCV and the ICRFB confirm that the presence of bismuth negatively influences the battery performance due to increased H2 production. Further research is ongoing to validate the decay mechanism and confirm these results on a larger scale. [ABSTRACT FROM AUTHOR]

At present, the energy consumption and carbon emissions of maritime transportation have raised concerns about environmental issues. A potential way to reduce carbon emissions from vessels is the use of chemical-based carbon capture and storage (CCS) technology. However, this technology faces challenges such as high energy consumption, large space occupation, and high processing costs. Therefore, the development of a technology with low energy consumption and compact CO2 storage is crucial to promote the advancement of CCS technology. This paper introduces a magnesium CO2 battery system that converts CO2 into new energy, in the form of hydrogen, while storing CO2. By preparing highly efficient catalytic electrodes and testing the electrolyte and CO2 flow rate on the battery performance, the optimal process parameters were determined to be Pd/CeO2-oct for the electrodes, a 0.5 mol/L NaOH solution for the electrolyte, and a CO2 flow rate of 1 L/h. The battery system demonstrated high cycling stability and conversion efficiency at a current density of 8 mA·cm−2, with a stable cycling time of 600 min (20 cycles), a cathode hydrogen production of 10.135 mL, and a Faraday efficiency of 97.03%. [ABSTRACT FROM AUTHOR]

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

Development and application of large‐scale energy storage systems are surging due to the increasing proportion of intermittent renewable energy sources in the global energy mix. Redox flow batteries are prime candidates for large‐scale energy storage due to their modular design and scalability, flexible operation, and ability to decouple energy and power. To date, several different redox couples are exploited in redox‐flow batteries; some are already commercialized. This battery technology is facing a lot of challenges in the science, engineering, and economic front. Issues plaguing flow batteries are low energy density, high overall cost, poor stability of electrolytes, shifting of solvent from anolyte to catholyte while using cation exchange membrane, reverse flow with anion exchange membrane, and corrosion of graphite felt in the catholyte side. Significant research efforts are ongoing to address these challenges. This comprehensive and critical review summarizes the recent progress in electrolyte technologies, including electrochemical performance and stability, strategies to enhance the energy and power densities and, in the end, the levelized and life‐cycle cost of these batteries analyzed. A comprehensive outlook on this technology with respect to practical energy storage applications is also provided. [ABSTRACT FROM AUTHOR]

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

Non‐precious metal‐based nitrogen‐doped carbon (M‐Nx/C) shows great potential as a substitute for precious metal Pt‐based catalysts. However, the conventional pyrolytic methods for forming M‐Nx/C active sites are prone to issues such as the lack of synergistic interactions among bimetallic atoms and the potential encasement of active sites, leading to compromised catalytic efficiency and hindered mass transfer. In this work, a highly active FeCo‐N‐C@U‐AC electrocatalyst is developed with a high density of active sites, adequate exposure of catalytic sites, and robust mass transfer capability using the chemical vapor‐phase deposition (CVD) technique. The resulting catalyst demonstrates impressive oxygen reduction reaction (ORR) catalytic performance and stability, with half‐wave potentials of 0.820 V (0.1 M HClO4) and 0.911 V (0.1 M KOH), respectively. It also exhibits significantly enhanced stability, retaining 93.25% and 98.38% of current after continuous 50 000 s of durability testing, surpassing the retention rates of Pt/C (80.31% in HClO4 and 84.96% in KOH electrolytes). Notably, when employed as a cathode catalyst in proton exchange membrane fuel cells (PEMFCs) and zinc‐air flow batteries (ZAFBs), the FeCo‐N‐C@U‐AC catalyst delivers peak power densities of 859 and 162 mW·cm−2, respectively, showcasing competitive performance comparable to benchmark Pt/C. [ABSTRACT FROM AUTHOR]

This research investigates the interactions between Carbopol® and zinc microparticles and their role in the stabilization of zinc within the framework of zinc slurry–air redox flow batteries (Zn-air RFBs). The study explores the potential of yield stress fluids, particularly PAA (Carbopol®) microgels, as stabilizers for zinc particles during battery operation, and regarding sedimentation in particular. Two effects that can limit the effectiveness of PAA yield stress in stabilizing the suspension are examined in this study. First is the ionic strength and pH that can evolve during the slurry formulation. The second effect is associated to zinc polymer interactions that can develop during the suspension preparation. Using methodologies based on rheometry and UV–Vis spectroscopy, the research identifies that the primary stabilization challenge is the adsorption of Carbopol® microgels onto zinc surfaces, which significantly influences the gel's yield stress. These insights contribute to an enhanced understanding of the physical chemistry of the suspending fluid, facilitating the development of more efficient and stable Zn-Air RFBs. [ABSTRACT FROM AUTHOR]

When considering the frequency stability issues brought on by load shifts in a microgrid (μ G) due to a significant integration of fluctuating renewable energy source-based generators and an inherent low inertia, energy storage units (ESUs) are unavoidable. The ESUs can immediately charge or discharge to make up for any shifts in load in the μ G since they react more quickly. This research investigates how a redox flow battery unit (RFBU) impacts the dynamic responses of a standalone μ G (S μ G) in this respect. As the secondary controller, an optimal intelligent model predictive control (iMPC) approach is presented. The cyclical parthenogenesis algorithm is implemented to improve the tuning parameter ( τ w ) in the iMPC cost function to demonstrate an optimal performance of the recommended control approach. The effectiveness of the iMPC approach is contrasted with that of other well-known control approaches. The simulation findings clearly demonstrate the impact of RFBU integration in the S μ G and the capability of the recommended iMPC approach in terms of improved dynamic responses and their transient characteristics. As a consequence, the peak value of the S μ G dynamic response improves by 70.53%, and the settling time improves by 82.26%. The suggested control approach's sensitivity to the S μ G parametric uncertainties is also justified. The closed-loop stability of the suggested approach is also established. The effectiveness of the proposed iMPC approach and the impact of the RFBU integration are then confirmed by statistical analysis. [ABSTRACT FROM AUTHOR]

Aqueous Organic Redox Flow Batteries are a family of promising energy storage systems. However, they face various challenges related to their lifetime, such as the Faradaic imbalance due to the occurrence of parasitic reaction leading to the fading of its energy storage capacity. Herein, automatization of a rebalancing system to reverse the detrimental effects of Faradaic imbalance due to the unavoidable presence of small quantities of oxygen in the negative reservoir or hydrogen evolution reaction is developed and implemented in an alkaline flow battery. A membrane‐free rebalancing cell is proposed to promote the oxygen evolution reaction and reverse the accumulated charge in the catholyte showing a 100 % coulombic efficiency. The programmable logic controller monitors the open circuit voltage to calculate the charge stored in each charge/discharge step and closes a circuit so a fixed voltage is applied to the rebalancing cell when the battery needs to be rebalanced. The system is tested using an alkaline flow battery consisting of ferrocyanide and 2,6‐dihydroxyanthraquinone, improving the energy capacity retention from 0.27 % cycle‐1 and 0.47 % h‐1 without rebalancing system to 100 % retention after >850 cycles and >24 days (without Ar‐filled glovebox), demonstrating the feasibility of this proposed system to address the Faradaic imbalance. [ABSTRACT FROM AUTHOR]

This review exploits the crucial role of computational methods in discovering and optimizing materials for redox flow batteries (RFBs). Integration of high‐throughput computational screening (HTCS) and machine learning (ML) accelerates materials discovery, guided by algorithms categorizing RFBs. A collaborative exploration, spanning macroscopic to mesoscopic scales, combines quantum machine learning with reinforcement learning, transfer learning, time series analysis, Bayesian optimization, active learning and various generative models. The collaborative integration of ML with computational techniques and experimental methods, anchored in experimentally validated Density Functional Theory (DFT) calculations and molecular dynamics (MD) simulations, proves indispensable for cost‐effective RFBs. Data collection and feature engineering are explored, emphasizing the integration of optimization goals and precise data collection within the ML framework. Feature analysis importance is highlighted, utilizing methods such as the filter, embedded, wrapper and deep learning methods for efficient energy materials exploration. Computational perspectives on materials features and operating conditions encompass membrane characteristics, fluid dynamics, temperature dependence and pressure sensitivity. Time‐dependent features and ML‐generated insights are crucial for understanding cycling performance intricacies, providing a comprehensive understanding of RFB materials. [ABSTRACT FROM AUTHOR]

To address the problem of suboptimal performance in deep eutectic solvents displayed by traditional TiO2 photoelectrodes and Cu2O photoelectrodes that have undergone simplistic modifications that result in a mismatch with battery discharge capacity, a method combining hydrothermal and dip-coating techniques was developed to create a Fe2O3-CuO heterojunction structure on the FTO surface. Then, the impact of the heterojunction structure on the performance of solar flow batteries was investigate in this study. The experimental findings reveal that the formation of the heterojunction structure effectively mitigates the recombination rate of photogenerated carriers within the photoelectrode. Furthermore, by meticulously adjusting the CuO loading, the harmonious balance between charging and discharging currents was achieved, thereby enhancing the overall performance of the solar redox flow batteries. In comparison to standalone Fe2O3 photoelectrodes, this innovative approach significantly broadens the spectrum of sunlight utilization. Notably, the fabricated Fe2O3/CuO-2 photoelectrode demonstrates a remarkable photocharging performance, far surpassing both Fe2O3 photoelectrodes and commercial TiO2 photoelectrodes. Specifically, the Fe2O3/CuO-2 photoelectrode boosts an average current density of 598.68 μA∙cm−2, with its charging current density being 2.74 and 5.15 larger, respectively, than that of the Fe2O3 and commercial TiO2 photoelectrodes. [ABSTRACT FROM AUTHOR]

Abstract Recent advances in 3D printing have enabled the manufacture of porous electrodes which cannot be machined using traditional methods. With micron-scale precision, the pore structure of an electrode can now be designed for optimal energy efficiency, and a 3D printed electrode is not limited to a single uniform porosity. As these electrodes scale in size, however, the total number of possible pore designs can be intractable; choosing an appropriate pore distribution manually can be a complex task. To address this challenge, we adopt an inverse design approach. Using physics-based models, the electrode structure is optimized to minimize power losses in a flow reactor. The computer-generated structure is then printed and benchmarked against homogeneous porosity electrodes. We show how an optimized electrode decreases the power requirements by 16% compared to the best-case homogeneous porosity. Future work could apply this approach to flow batteries, electrolyzers, and fuel cells to accelerate their design and implementation.

[Display omitted] Zinc-based flow batteries hold potential promise for extensive energy storage on a large scale owing to their high energy density and low cost. However, their widespread implementation is impeded by challenges associated with zinc (Zn) dendrites and side reactions like the hydrogen evolution reaction on the anode. Theoretical calculations have confirmed that glycine (Gly) has the ability to coordinate with Zn2+, displacing H 2 O molecules in the solvation shell, thereby restoring the solvation structure of Zn2+ and promoting the release of reactive Zn2+ during plating/stripping processes. As a result, the incorporation of Gly into the anolyte of a neutral zinc-ferricyanide (Zn/Fe) flow battery (ZIFB) effectively inhibits the formation of Zn dendrites and impedes side reactions, leading to highly reversible and stable Zn plating/stripping reactions. A Zn||Zn symmetric flow battery utilizing Gly in the anolyte demonstrated extended cycling durability, lasting over 550 h at a current density of 30 mA cm−2, in contrast to the failure of a Gly-free anolyte system after 150 h. Notably, this approach facilitates a neutral ZIFB achieving an impressive energy efficiency exceeding 70 %, even at a high current density of 70 mA cm−2, with a cycle lifespan exceeding 800 h (33 days) at a current density of 30 mA cm−2. Conversely, the neutral ZIFB lacking Gly showed a significantly shorter cycle life of only 260 h under identical operational conditions (30 mA cm−2). Due to the economic benefits of Gly and the proposed user-friendly route, this strategy demonstrates great potential for promoting the widespread adoption of zinc-based flow batteries with improved performance for practical use. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) have pioneered new routes for large-scale energy storage. The tunable nature of redox-active organic molecules provides a robust foundation for creating innovative AORFBs with exceptional performance. Molecular engineering endows various organic molecules with considerable advantages in solubility, stability, and redox potential. Advanced characterizations have enabled a comprehensive understanding of the redox reaction and degradation mechanisms of these organic molecules. Computational chemistry and machine learning have guided the development of new organic molecules. The practical application of AORFBs will depend on the complementary efforts of multiple parties. This paper consolidates the current design principles of molecular engineering, degradation mechanisms, characterization techniques, and the utilization of computational chemistry. It also offers perspectives and forecasts the necessary attributes and strategic efforts for the next-generation AORFBs, aiming to provide the research community with a deeper understanding. [ABSTRACT FROM AUTHOR]

As a promising large‐scale energy storage technology, all‐vanadium redox flow battery has garnered considerable attention. However, the issue of capacity decay significantly hinders its further development, and thus the problem remains to be systematically sorted out and further explored. This review provides comprehensive insights into the multiple factors contributing to capacity decay, encompassing vanadium cross‐over, self‐discharge reactions, water molecules migration, gas evolution reactions, and vanadium precipitation. Subsequently, it analyzes the impact of various battery parameters on capacity. Based on this foundation, the article expounds upon the significance of battery internal state estimation technology. Additionally, the review also summarizes domestic and international mathematical models utilized for simulating capacity decay, serving as a valuable reference for future research endeavors. Finally, through the comparison of traditional experimental methods and mathematical modeling methods, this article offers effective guidance for the future development direction of battery state monitoring. This review generally overview the problems related to the capacity attenuation of all‐vanadium flow batteries, which is of great significance for understanding the mechanism behind capacity decay and state monitoring technology of all‐vanadium redox flow battery. [ABSTRACT FROM AUTHOR]

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]

Ion conducting membranes play a crucial role in redox flow batteries, separating anolyte and catholyte while allowing proton transport to complete the circuit. However, most membranes are trapped in a trade‐off relation and show either low conductivity or high vanadium crossover. This study investigates the use of dense sulfonated para‐polybenzimidazole membranes for vanadium redox flow batteries (VRFBs), and analyzes the effects of membrane preparation process, membrane thickness and operating temperature on the VRFB performance. The results demonstrate superior performance of VRFBs utilizing fluorine‐free sulfonated para‐polybenzimidazole membranes compared to other types. Under optimal conditions, the VRFB exhibits high coulombic efficiency (>99%) and high energy efficiency (EE, 92.2% at a current density of 80 mA cm−2), and durability. The achieved EE represents one of the highest reported in the literature for VRFBs. In addition, it is shown that operation at 35 °C has benefits at high current densities (EE at 300 mA cm−2 is over 80% at 35 °C but 72% at 25 °C), while the operation at 80 mA cm−2 only shows a small temperature effect (91.8 and 92.2%, respectively). [ABSTRACT FROM AUTHOR]

Electrochemical energy conversion and storage are playing an increasingly important role in shaping the sustainable future. Differential electrochemical mass spectrometry (DEMS) offers an operando and cost-effective tool to monitor the evolution of gaseous/volatile intermediates and products during these processes. It can deliver potential-, time-, mass- and space-resolved signals which facilitate the understanding of reaction kinetics. In this review, we show the latest developments and applications of DEMS in various energy-related electrochemical reactions from three distinct perspectives. (I) What is DEMS addresses the working principles and key components of DEMS, highlighting the new and distinct instrumental configurations for different applications. (II) How to use DEMS tackles practical matters including the electrochemical test protocols, quantification of both potential and mass signals, and error analysis. (III) Where to apply DEMS is the focus of this review, dealing with concrete examples and unique values of DEMS studies in both energy conversion applications (CO2 reduction, water electrolysis, carbon corrosion, N-related catalysis, electrosynthesis, fuel cells, photo-electrocatalysis and beyond) and energy storage applications (Li-ion batteries and beyond, metal–air batteries, supercapacitors and flow batteries). The recent development of DEMS-hyphenated techniques and the outlook of the DEMS technique are discussed at the end. As DEMS celebrates its 40th anniversary in 2024, we hope this review can offer electrochemistry researchers a comprehensive understanding of the latest developments of DEMS and will inspire them to tackle emerging scientific questions using DEMS. [ABSTRACT FROM AUTHOR]

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−1catholyte 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. [ABSTRACT FROM AUTHOR]

Tin‐based hybrid flow batteries have demonstrated dendrite‐free morphology and superior performance in terms of cycle life and energy density. However, the quick accumulation of electrodeposits near the electrode/membrane interface blocks the ion transport pathway during the charging of the battery, resulting to a very limited areal capacity (especially at high current density) that significantly hinders its deployment in long‐duration storage applications. Herein, a conductivity‐activity dual‐gradient design is disclosed by electrically passivating the carbon felt near the membrane/electrode interface and chemically activating the carbon felt near the electrode/current collector interface. In consequence, the tin metals are preferentially plated at the region near electrode/current collector, preventing the ion transport pathway from being easily blocked. The resultant gradient electrode demonstrated an unprecedentedly high areal capacity of 268 mAh cm−2 at a current density of as high as 80 mA cm−2. Numerical modeling and experimental characterizations show that the dual‐gradient electrode differs from conventional electrodes with regard to their reaction current density distribution and electrodeposit distribution during charging. This work demonstrates a new design strategy of 3D electrodes for hybrid flow batteries to induce a desirable distribution of electrodeposits and achieve a high areal capacity at commercially relevant current densities. [ABSTRACT FROM AUTHOR]

The 6-electron electrochemical reduction of IO3− to I− represents a breakthrough for the development of next-generation redox flow batteries, offering substantially higher energy densities for oxidizer storage. Our study reveals that on a glassy carbon (GC) electrode in acidic electrolytes, HIO3 undergoes an autocatalyzed electrochemical reduction to I−. This process is mediated by the formation of a thin iodine layer on the electrode, acting as an intermediate and a catalyst. Under steady-state conditions, the iodine layer forms via a comproportionation reaction (HIO3 + I− + 5H+ = I2 (s) + 3H2O). Initially, the iodine layer is generated through the slow direct electrochemical reduction of HIO3 on pristine GC. Once established, this layer significantly enhances the rate of iodate reduction. On voltammetry curves, it is clearly observable as a step-wise current surge to reach a plateau. The limiting current density on the GC seemingly aligns with the Levich equation, varying with the RDE rotation rate. Earlier, we demonstrated the electrochemical oxidation of I− back to HIO3 using an H2/HIO3 flow cell, showcasing a full cycle that underpins the feasibility of this approach for energy storage. This study advances the understanding of iodate electroreduction and underscores its role in enhancing the capacity of next-generation energy storage systems. [ABSTRACT FROM AUTHOR]

In today's electric power network, fast changes are the norm, and auxiliary services such as automatic generation control (AGC) play a crucial role in maintaining the quality of the power supply. AGC ensures a balance between power generation, demand, and losses to sustain frequency stability and variation in tie‐line power within a set limit, even when the load changes. To accomplish this, it is always beneficial to consider new approaches to controlling the situation. This paper introduces the design of new AGC regulators that consider deviation in DC tie‐line power as an extra control variable for the turbine controller. In this study, three different deregulated power systems (containing two control areas), namely three thermal generations, the combination of one thermal and two hydro generations, and three hydro generations, including two power distribution companies (DISCOs) in each control area. These optimal AGC regulators are implemented in the proposed thermal–thermal–thermal system to carry out the various power contracts. The results have shown that the dynamic outcomes meet the AGC standards. To improve further dynamic results and the proposed systems' stability margins by incorporating redox flow batteries (RFB). In actual operating conditions, the system parameters do not remain constant due to the aging effect, assumptions made in simplifying the mathematical model, etc. Thus, ±50% deviations in the nominal value of system parameters to assess how well the optimal AGC regulators perform in the system under investigation. The suggested realistic AGC system incorporates many parameter fluctuations and works well with the optimal AGC regulators developed for the proposed plans. This study expanded to include a two‐area, thermal‐hydro‐hydro (THH) system under a deregulated framework connected via an asynchronous transmission link with or without a thyristor‐controlled phase shifter (TCPS) and RFB. The genetic algorithm (GA) can solve many issues and has global search, flexibility to varied problem types, intrinsic parallelism, and the ability to handle massive, complex search spaces. Thus, a two‐area deregulated hydro‐hydro‐hydro system with parallel tie‐lines utilizes GA‐PID, GA‐FOPID, and GA‐(1 + PI)‐FOPID controllers. Moreover, a random load disturbance (RLD) is employed in each section of the offered approach to show the resilience and elite performance of the proposed control strategy. To produce various dynamic reactions in the plans, MATLAB software version R2013a is employed. [ABSTRACT FROM AUTHOR]

Considering the costs, waste, and impact on the environment of current energy consumption, accurate, cost‐effective, and safely deployed energy storage systems are required. Lithium (Li)‐free redox flow batteries (RFBs) are a feasible solution. RFBs can store enormous amounts of energy effectively and are increasingly used for large‐scale applications. The use of RFBs has significantly enhanced the performance of energy storage systems and effectively reduced the costs and wastage of energy storage operations. Vanadium‐based RFBs are an emerging energy‐storage technology being explored for large‐scale deployment owing to their numerous benefits, including zero cross‐contamination, scalability, flexibility, extended life cycle, and nontoxic working state. This study describes the fundamental operating principles of redox flow battery‐based systems as well as the design considerations and constraints placed on each component. It discusses recent progress in the design and deployment of RFBs for energy‐related applications and the remaining obstacles and prospects. Finally, this study highlights the enormous potential of RFBs and suggests some solutions to scale up the use of RFBs in the near future. [ABSTRACT FROM AUTHOR]

This research focuses on the improvement of porosity distribution within the electrode of an all‐vanadium redox flow battery (VRFB) and on optimizing novel cell designs. A half‐cell model, coupled with topology and shape optimization framework, is introduced. The multiobjective functional in both cases aims to minimize pressure drop while maximizing reaction rate within the cell. Topology optimization results reveal dependencies on initial value, porosity constraint, and flow rate. The distribution with lower porosity is preferred downstream of the inlet manifold. This design enhances active surface area, thus facilitating more effective conversion of incoming educts and improving mass transport of products. Compared to homogeneous electrodes, two‐part design demonstrates superior performance at specific porosity values. For combined porosities of 0.7 and 0.95, optimized distribution results in 81 % reduction in pressure drop, while conversion rate decreases by 7%. As regards various cell designs, optimization suggests a need to reconsider the vertical format of a rectangular cell. Horizontal cells are favored for nearly all porosities and flow rates. Trapezoidal and radial designs characterized by reduced downstream cross sections lead to higher pressure drops and are not preferred. This work provides further valuable insight into optimizing VRFB electrodes and challenges conventional cell design assumptions. [ABSTRACT FROM AUTHOR]

In the literature, the hierarchy of value retention strategies (R-strategies) is utilized to describe the impacts on various circular economy (CE) factors. However, this approach is not suitable for batteries, such as the vanadium flow battery (VFB), due to its technical complexity. The presented model primarily focuses on VFBs, as a deep technical understanding is identified as a fundamental prerequisite for a comprehensive CE analysis. Based on the R-strategies, a new model called the dynamic multi-dimensional value retention strategy model (DDS) is developed accordingly. The DDS divides the R-strategies into three dimensions, as changes in the studied object each have a unilateral influence on the underlying dimensions. In addition, interactions among the R-strategies within the dimensions are observed. Moreover, the model enables the transparent and comprehensible examination of various CE objective factors. Through the model, future adjustments to CE for batteries can be analyzed and quantified. In particular, the analysis yields new insights into individual end-of-life (EoL) strategies, based on new findings regarding the VFB. Consequently, important new perspectives on the VFB are also illuminated. The DDS model is applicable to other complex technologies as well as simple product systems. [ABSTRACT FROM AUTHOR]

Global environmental concerns and resource scarcity are driving the growth in sales of electric vehicles (EVs). Reusing and recycling retired batteries from EVs has significant economic value and reduces the environmental burden. Rising raw material prices have intensified competition among recyclers; in particular, recyclers without corporate social responsibility (CSR) have been added. These observations lead to a game-theoretical model consisting of three players: a battery manufacturer, a recycler with CSR and a recycler without CSR (non-CSR). The non-CSR recycler enjoys a cost advantage over the CSR recycler, but may not be considered by the consumers with high environmental awareness (CEA). We explore the incentive strategies for CSR recyclers outperform, and how the equilibrium is affected by the recyclers' Stackelberg game. Results show that (1) the deposit- refund is the most profitable strategy for all members and the whole supply chain if raw material price rises high enough; otherwise, a contract strategy should be adopted. (2) Improving CEA and echelon utilization ratio is more conducive to the implementation of revenue-sharing contract. In addition, increasing CEA contributed to CSR recycler collects more retired batteries instead of non-CSR recycler. (3) Stackelberg game between recyclers may hurt supply chain. However, CSR recycler may benefit from the non-CSR recycler-led Stackelberg game. Our work provides the basis of incentive strategies for different participants in the closed-loop supply chain of retired batteries, in particular, to encourage retired batteries flow to CSR recyclers. [ABSTRACT FROM AUTHOR]

Manufacturers of electric cars provide their vehicles with many technical data that are important for the user. This includes information on dimensions, mass, performance, consumption, battery capacity, range, payload, etc. However, some interesting parameters are usually withheld from the end user. These parameters include, for example, the loss in the energy flow from the battery to the driving wheels or the rolling resistance of the vehicle. However, since these loss parameters have a significant influence on the vehicle's consumption, it is of interest to know them. This article presents a method for determining these two parameters. The basis for this are simple driving tests that can be carried out by anyone on public roads. [ABSTRACT FROM AUTHOR]

Redox Flow Batteries (RFBs) offer a promising solution for energy storage due to their scalability and long lifespan, making them particularly attractive for integrating renewable energy sources with fluctuating power output. This study investigates the performance of a prototype Zinc-Chlorine Flow Battery (ZCFB) designed for low-cost and readily available electrolytes. The ZCFB utilizes a saltwater electrolyte containing ZnCl2 and NaCl, paired with a mineral spirits catholyte. The electrolyte consists of a 4 M ZnCl2 and a 2 M NaCl solution, both with a pH of 4.55. The anode was a zinc metal electrode, while the cathode comprised a porous carbon electrode on a titanium grid current collector. The cell volume was approximately 4.0 mL, with separate reservoirs for the NaCl/H2O and mineral spirits electrolytes. Experiments were conducted under constant current conditions, with a 0.2 A charging current and a 5 mA discharge current chosen for optimal cell voltage. The study analyzed the relationship between voltage, current, power, and capacity during both charging and discharging cycles. Results from multiple charge/discharge cycles found that the current density of the battery is around 62.658 mA/cm2 with an energy capacity average of 1.2 Wh. These findings can contribute to the development of more efficient and practical ZCFBs, particularly for applications requiring low-cost and readily available electrolytes. [ABSTRACT FROM AUTHOR]

The redox-active electrolyte materials are critical components in redox flow batteries (RFBs). Quaternary bipyridinium derivatives, such as viologens (4,4′-bipyridine double quaternary salts), represent an important class of organic anolyte materials that can be well-matched with cheap and non-corrosive pH-neutral aqueous electrolytes in RFB applications. Herein, to search for promising quaternary bipyridinium compounds, we investigated the redox potentials and kinetic rate constants of 228 C-substituted quaternary bipyridinium derivatives that either contain paraquat or diquat parent structures by applying density functional theory (DFT) calculations. The analysis of the results reveals the effects of the terminal functional groups, number of methylene units, substitution sites and parent structure on the electrochemical properties. On this basis, we identify several promising candidates for the anolyte materials of neutral aqueous organic RFBs and further assess their aqueous solubility and structural stability through DFT theoretical calculations. [ABSTRACT FROM AUTHOR]

Pyridoxal hydrochloride, a vitamin B6 vitamer, was synthetically converted to a series of diverse redox‐active benzoyl pyridinium salts. Cyclic voltammetry studies demonstrated redox reversibility under basic conditions, and two of the most promising salts were subjected to laboratory‐scale flow battery tests involving galvanostatic cycling at 10 mM in 0.1 M NaOH. In these tests, the battery was charged completely, corresponding to the transfer of two electrons to the electrolyte, but no discharge was observed. Both CV analysis and electrochemical simulations confirmed that the redox wave observed in the experimental voltammograms corresponds to a two‐electron process. To explain the irreversibility in the battery tests, we conducted bulk electrolysis with the benzoyl pyridinium salts, affording the corresponding benzylic secondary alcohols. Computational studies suggest that the reduction proceeds in three consecutive steps: first electron transfer (ET), then proton‐coupled electron transfer (PCET) and finally proton transfer (PT) to give the secondary alcohol. 1H NMR deuterium exchange studies indicated that the last PT step is not reversible in 0.1 M NaOH, rendering the entire redox process irreversible. The apparent reversibility observed in CV at the basic media likely arises from the slow rate of the PT step at the timescale of the measurement. [ABSTRACT FROM AUTHOR]

Carbon-based electrodes are used in flow batteries to provide active centers for vanadium redox reactions. However, strong controversy exists about the exact origin of these centers. This study systematically explores the influence of structural and functional groups on the vanadium redox reactions at carbon surfaces. Pyridine, phenol and butyl containing groups are attached to carbon felt electrodes. To establish a unique comparison between the model and real-world behavior, both non-activated and commercially used thermally activated felts serve as a substrate. Results reveal enhanced half-cell performance in non-activated felt with introduced hydrophilic functionalities. However, this cannot be transferred to the thermally activated felt. Beyond a decrease in electrochemical activity, a reduced long-term stability can be observed. This work indicates that thermal treatment generates active sites that surpass the effect of functional groups and are even impeded by their introduction. [ABSTRACT FROM AUTHOR]

The development of membranes with rapid and selective ionic transport is imperative for diverse electrochemical energy conversion and storage systems, including fuel cells and flow batteries. However, the practical application of membranes is significantly hindered by their limited conductivity and stability under strong alkaline conditions. Herein, a unique composite membrane decorated with functional Cu2+ cross‐linked chitosan (Cts‐Cu‐M) is reported and their high hydroxide ion conductivity and stability in alkaline flow batteries are demonstrated. The underlying hydroxide ions transport of the membrane through Cu2+ coordinated nano‐confined channels with abundant hydrogen bonding network via Grotthuss (proton hopping) mechanism is proposed. Consequently, the Cts‐Cu‐M membrane achieves high hydroxide ion conductivity with an area resistance of 0.17 Ω cm2 and enables an alkaline zinc‐based flow battery to operate at 320 mA cm−2, along with an energy efficiency of ≈80%. Furthermore, the membrane enables the battery for 200 cycles of long‐cycle stability at a current density of 200 mA cm−2. This study offers an in‐depth understanding of ion transport for the design and preparation of high‐performance membranes for energy storage devices and beyond. [ABSTRACT FROM AUTHOR]

From micro to macro level, the performance of zinc-air flow battery (ZAFB) hinges on two crucial factors: the effective oxygen reduction reaction (ORR) electrocatalysts, and the durable triple-phase electrodes. However, sluggish kinetic process always occurs due to insufficient active sites and hysteretic mass transfer. In this work, a well-confined iron phthalocyanine (FePc) structure onto carbonized metal-organic framework (C-MOF5) electrocatalyst is constructed through a pyrolysis-free strategy. The macrocyclic skeleton of MOF-5 has been confirmed to offer a stronger and more abundant environment for hosting FePc molecules. Attributing to the hierarchical pore engineering of MOF-5, the FePc@C-MOF5 electrocatalyst reveals a high content of Fe (5.68 wt%) with multiple exposed active sites. Notably, the FePc@C-MOF5 exhibits a remarkably high half-wave potential of 0.915 V for ORR in 0.1 M KOH, surpassing that of commercial Pt/C by nearly 60 mV. From a higher-level device perspective, the highest oxygen diffusion velocity and local current density of the FePc@C-MOF5 electrode is verified by computational fluid dynamics (CFD) simulations. As a result, the as-assembled ZAFB demonstrates long-term stability with 200 h' cycling. This work not only presents the hierarchical pore tailoring strategy for designing ORR electrocatalysts but also offers a deeper understanding of the essence of interfacial mass transfer. A novel FePc@C-MOF5 electrocatalyst for zinc-air flow batteries was proposed, enhancing ORR performance and stability while advancing understanding of mass transfer dynamics. [Display omitted] • A well-confined MOF-5-derived ORR catalyst was prepared with a high loading of Fe. • The unique pore structure accelerates both oxygen diffusion and charge transfer. • The FePc@C-MOF5 reveals good activity at both microscale for three-electrode characterization and macroscale for ZAFB test. • A numerical three-dimensional model combining transfer theory with reaction kinetics for ZAFB is established. [ABSTRACT FROM AUTHOR]