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The gradual capacity decrease of vanadium redox flow battery (VRFB) over long‐term charge‐discharge cycling is determined by electrolyte degradation. While it was initially believed that this degradation was solely caused by crossover, recent research suggests that oxidative imbalance induced by hydrogen evolution reaction (HER) also plays a significant role. In this work by using vanadium pentoxides with different impurities content, we prepared three grades of vanadium electrolyte. By measuring electrochemical properties on carbon felt electrode in three‐electrode cell and VRFB membrane‐electrode assembly we evaluate the influence of impurity content on battery polarization and rate of side reactions which is indicated by the increase of average oxidation state (AOS) during charge‐discharge tests and varies from 0.061 to 0.027 day−1 for electrolytes made from 99.1 and 99.9 wt % V2O5. We found that increase of AOS correlates with the increase of open‐circuit voltage of VRFB in the discharged state ranging from 9.6 to 14.9 mV day−1 for highest and lowest electrolyte purity levels, respectively. While AOS increase is significant, it does not solely determine capacity fade. It is demonstrated that the presence of vanadium crossover decreases capacity fade, i. e. levels the contribution of side reactions on capacity drop. [ABSTRACT FROM AUTHOR]

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This study introduces a novel approach through the design and creation of a composite electrode, uniquely made of three distinct layers of micro/mesoporous electrospun carbon nanofiber (CNF) mats, featuring a gradient in pore size. This innovative gradient pore structure merges the benefits of varying pore sizes, significantly enhancing redox flow battery (RFB) efficiency. The first layer, a microporous CNF mat situated near the membrane, offers an extensive reactive surface area, minimizing charge transfer resistance and speeding up electrochemical reactions—key factors in enhancing battery reaction efficiency. The next layer, a mesoporous CNF mat, fine‐tunes the flow properties of the electrolyte, lowering flow resistance while ensuring superior charge transfer capabilities. This structured gradient in pore size not only facilitates improved electrolyte penetration and even distribution but also harmonizes the balance between charge transfer efficiency and electrolyte flow, thus mitigating energy losses without compromising reaction velocity. Charge–discharge testing demonstrated notable performance gains: an energy efficiency of 82% at 100 mA cm−2 (surpassing traditional electrodes by 71.5%) and 69% at 200 mA cm−2, alongside a 77.4% increase in peak power density. This advancement not only enhances energy and power densities but also its lifespan, marking a significant step forward for RFB technologies. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries (VRFBs) depend on the separator membrane for their efficiency and cycle life. Herein, two amphoteric ion exchange membranes are synthesized, based on sulfonic acid group‐grafted poly(p‐terphenyl piperidinium), for VRFBs. Using ether‐free poly(p‐terphenyl piperidine) (PTP) as the polymer matrix, and sodium 2‐bromoethanesulphonate (ES) and 1,4‐butane sultone (BS) as grafting agents, We achieve quaternization of PTP through an environmentally friendly process without alkaline catalysts. PTP‐ES and PTP‐BS membranes exhibit low area resistance, high H+ permeability, and significantly reduced vanadium ion permeability, leading to exceptional ion selectivity, which is 3.06 × 106 S min cm−3 and 4.34 × 106 S min cm−3, respectively, three orders of magnitude higher than that of Nafion115 (0.27 × 104 S min cm−3). The VRFB with PTP‐BS achieves a self‐discharge duration of 190 h, compared to 86 h for Nafion 115. Additionally, under current densities of 40—160 mA cm−2, PTP‐BS shows coulombic efficiencies of 98.1–99.1% and energy efficiencies of 92.0–82.1%, outperforming Nafion 115. The VRFB with PTP‐BS also demonstrates excellent cycle stability and discharge capacity retention over 300 cycles at 100 mA cm−2. Therefore, the amphoteric PTP‐BS membrane shows remarkable performance, offering significant potential for VRFB applications. [ABSTRACT FROM AUTHOR]

The energy sector is currently exploring eco-friendly energy conversion and storage devices, as an alternative to traditional fossil fuel devices. A unique hybrid proton exchange membrane is developed for proton exchange membrane fuel cells (PEMFC) and vanadium redox flow battery (VRFB) application. Polymeric nanocomposite membranes are effective for the selective and controlled transportation of ions between the electrodes. This study utilizes various modifiers such as GO, SGO, SPES, and SiC, to modify the PES polymer matrix. Analysis of the functional groups confirmed the presence of modifiers and hydroxyl groups on the nanocomposite membrane. Additionally, the use of Field Emission Scanning Electron Microscopy (FESEM) validated the existence of honeycomb and sponge-like voids as well as the surface morphology on the membrane surface. The incorporation of modifiers into the nanocomposite membrane matrix serves as a reliable barrier for the transport of vanadium ions, leading to a significant reduction in vanadium ion permeability across the membrane. The mechanical properties and ion exchange capacity of nanocomposite membranes are enhanced, while the permeability of VO2+ decreases compared to that of the bare PES membrane. The enhancement of proton conductivity is further achieved with the utilization of modifiers in PES polymer. As compared to bare PES membrane, PES/SPES + SGO (5%) exhibits less than half the permeability for vanadium, and PES/SiC shows 44.03 % elongation means twice of bare PES membrane. [Display omitted] • Influences of GO, SGO, SPES, and SiC, and functional groups in PEMFCs and VRFB is also studied. • PES/SiC shows 44.03 % elongation means twice of bare PES membrane. • Enhanced the proton conductivity through the utilization of modifiers with PES polymer. • PES/SPES + SGO (5%) exhibits less than half the permeability for vanadium. [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 application of sulfonated poly (ether ether ketone) (SPEEK) membrane in vanadium redox flow batteries (VRFBs) is hindered by the trade-off between proton conductivity and ion selectivity. To optimize its performance, the metal-organic frameworks loaded with phytic acid (PA-UiO-66-NH2) are introduced into the SPEEK matrix to construct a proton-selective transport channel. Better proton conductivity and ion selectivity are achieved in composite membranes due to the unique porous structure and chemical characteristics of PA-UiO-66-NH2 fillers. When the PA-UiO-66-NH2 content is at 2 wt%, the S/PA-UiO-66-NH2-2 membrane exhibits higher proton conductivity (35.3 mS cm−1) and ion selectivity (41.0 × 103 S min cm−3) compared to other composite membranes, pristine SPEEK and commercial Nafion 212 membranes. As a result, the S/PA-UiO-66-NH2-2 membrane shows excellent energy efficiencies (88.1%-74.0%) at current densities ranging from 60 to 180 mA cm−2. The cell efficiencies maintain stability during the 300 times charge-discharge cycle, proving the outstanding stability and durability of the S/PA-UiO-66-NH2-2 membrane in a strong acidic and oxidizing environment. These results suggest that the combination of phytic acid and metal-organic framework can effectively improve the performance of the membranes, and it also can be further utilized to solve other challenges in the membrane separation field. [ABSTRACT FROM AUTHOR]

Vanadium redox flow battery (VRFB) is a very promising solution for large‐scale energy storage, but some technical issues need to be addressed. Crossover, i.e., the undesired permeation of vanadium ions through the cell separator, causes capacity loss and self‐discharge. Low‐cost and highly selective separators are thus required to improve the competitiveness of this technology. This work investigates the use of silica nanoparticles in an innovative selective layer to improve membrane selectivity and reduce its thickness. 1.5 μm thick barrier layers composed of 1100EW Nafion ionomer with silica (≈3–13 nm diameter) and Vulcan XC‐72R (≈40 nm) nanoparticles in different proportions are directly deposited on 50 μm thick Nafion membranes. The barrier layer composed only of silica nanoparticles reduces the self‐discharge due to crossover by 5 times and increases the average efficiency of the battery. Finally, during more than 1000 h of operation, the barrier layer on a 25 μm Nafion membrane demonstrates excellent stability, working with a constant coulombic efficiency higher than 99% and a capacity decay rate comparable with a thicker Nafion membrane, thus enabling the use of thinner membranes in VRFB, allowing an estimated 8% stack costs reduction with respect to NR212. [ABSTRACT FROM AUTHOR]

This mini‐review summarises and discusses recent findings form the literature on the degradation of carbon‐based electrodes for vanadium redox flow batteries (VRFBs). It becomes evident that the focus of current investigations is on carbon paper, carbon felt and graphite felt electrodes, which is understandable from a practical point of view. However, the structural complexity of these materials often prohibits doubtless attribution of observed performance reduction (or increase) to changes in the electrode materials. Among the discussed major causes for degradation are formation or change of surface functional groups, changes in the carbon sp2/sp3 ratio, intercalation of ions as well as formation of inhibiting adsorbates. In order to gain deeper insight into the changes of carbon electrodes in VRFBs under relevant operation conditions, the authors suggest reducing complexity of the investigated materials and applying in situ‐studies under well‐defined and controllable conditions on model electrodes. These studies then should be extended towards more practical systems and may finally help to reduce degradation phenomena including enhanced overvoltages and thus could improve cycling and energy efficiency as well as long‐term stability of vanadium redox flow batteries. [ABSTRACT FROM AUTHOR]

[Display omitted] Heteroatom-doped electrodes offer promising applications for enhancing the longevity and efficiency of vanadium redox flow battery (VRFB). Herein, we controllably synthesized N, P co-doped graphite fiber electrodes with conductive network structure by introducing protonic acid and combining electrodeposition and high temperature carbonization. H 2 SO 4 and H 3 PO 4 act as auxiliary and dopant, respectively. The synergistic effect between N and P introduces additional defect structures and active sites on the electrodes, thereby enhancing the reaction rate, as confirmed by density functional theory calculations. Furthermore, the conductive network structure of carbon fibers improves electrode-to-electrode connectivity and reduces internal battery resistance. The optimized integration of these strategies enhances VRFB performance significantly. Consequently, the N, P co-doped carbon fiber modified graphite felt electrodes demonstrate remarkably high energy efficiency at 200 mA cm−2, surpassing that of the blank battery by 7.9 %. This integrated approach to in-situ controllable synthesis provides innovative insights for developing high-performance, stable electrodes, thereby contributing to advancements in the field of energy storage. [ABSTRACT FROM AUTHOR]

A vanadium redox flow battery (VRFB) is an intermittent energy storage device that is primarily used to store and manage energy produced using sustainable sources like solar and wind. In this work, we study the modeling and operation of a single-cell VRFB whose active cell area is 25 cm 2 . Initially, we operate the cell at multiple flow rates by varying the current densities to study the behavior of the cell at different flow rates. Based on the observations from this experimental study, a suitable flow rate is selected such that the power density of the cell is maximized. In order to accurately represent the dynamic behavior of the VRFB, an electrical equivalent circuit model (ECM) is introduced from existing models documented in the literature based on simplicity and accuracy based on the RMS value of the error in the output voltage. Additionally, the pulse charge–discharge test was conducted on the VRFB under the specified operating flow rate. The data obtained from this test were utilized to estimate the parameters of the dynamical equivalent circuit model. In our study, we have proposed a modified state of charge (SOC) equation considering the self-discharge current loss through a parallel resistor while identifying the SOC–OCV relationship. Analysis of VRFB charge–discharge data is employed to validate the ECM, and the accuracy of the model is determined based on RMSE between the terminal voltage predicted by the model and the actual experimental setup. The resulting RMSE is found to be 19.4 mV, describing the model accuracy of 98.73%. [ABSTRACT FROM AUTHOR]

Secondary batteries, or rechargeable batteries, have revolutionized various industries by offering the ability to be reused after depletion. Membranes in secondary batteries act as separators, preventing direct contact between electrodes while facilitating ion transport, crucial for energy storage and preventing short circuits. Despite their theoretical ability to be used infinitely, real-time applications face challenges, including the inefficiency of available membranes. This review focuses on the use of chitosan, a biopolymer derived from chitin, in three promising secondary batteries: vanadium redox flow batteries, Aq. zinc batteries, and lithium-ion batteries due to their wide range of applications and promising future scope. For lithium-ion batteries, N-succinyl chitosan-chitosan and chitosan–lithium membranes offer potential improvements in ionic conductivity and mechanical strength, while in Aq. zinc battery chitosan-based carbon membrane and phosphorylcholine zwitterionic protective layer reduces the dendrite formation and alleviates side reactions and for vanadium redox flow battery chitosan modified batteries aim to reduce vanadium ion crossover. Using a chitosan-based membrane increases the energy efficiency of the vanadium redox flow battery to 88.6% from 60% and sustains an Aq. zinc ion battery for up to 2000 cycles. Comprehensively, this review also imparts a roadmap leading to the future prospects of chitosan biopolymer-based secondary batteries to ameliorate the energy density, and overall electrochemical performance of chitosan-derived batteries by modifying the electrode material, for heading toward a green, and sustainable energy storage system. [ABSTRACT FROM AUTHOR]

The high penetration of Renewable Energy Sources (RESs) in the modern power system poses a challenge to power system stability. This stability is affected by the stochastic, fluctuating output of RESs, which is influenced by weather conditions, and a lack of inertia resulting from reduced rotating mass. To address this issue, a new controller, referred to as Proportional-Fractional Integrator Plus Proportional-Derivative with Filter, P I λ (1+PDF), is designed for Load Frequency Control (LFC) with the support of a Hybrid Energy Storage System (HESS) for power systems with high-RES penetration. The HESS comprises a Superconducting Magnetic Energy Storage System (SMES) and a Vanadium Redox Flow Battery (VRFB) coupled with an Interline Power Flow Controller Flexible AC Transmission Systems (IPFC-FACTs) controller. The HESS, working in conjunction with the proposed LFC, injects virtual inertia and maintains power flow to expedite the frequency stability process. These systems are also integrated with Alternating Current (AC) and High Voltage Direct Current (HVDC) transmission lines to collectively enhance both the system's stability and the capacity of its transmission lines. To optimize the P I λ (1+PDF) controller parameters, Zebra Optimization Algorithm (ZOA) is employed utilizing an Integral Time Absolute Error (ITAE) objective function. The proposed controller is tested on a four-area power system integrated with a wind turbine, photovoltaic (PV) panels, a biodiesel generator, and a hydrogen aqua electrolyzer fuel cell, representing a high penetration of RESs in modern power systems. The results are compared with those obtained using Proportional-Integral-Derivative (PID) and Fractional Order Proportional Integral Derivative (FOPID) controllers. Sensitivity analysis and robustness tests are also performed to verify the stability of the power network by changing system parameters and under randomly chosen loading conditions. The proposed P I λ (1+PDF) controller tuned with ZOA outperforms PID and FOPID controllers by minimizing settling time for frequency changes by 62 %, eliminating overshoot, and reducing undershoots for frequency and tie-line power changes by 73 % and 55 %, respectively. Simulation results demonstrate that the proposed controller outperforms PID and FOPID controllers by effectively damping frequency and tie-line deviations, resulting in reduced frequency overshoots, undershoots, and shorter settling times. [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]

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]

Vanadium redox flow batteries (VRFBs) are of considerable importance in large-scale energy storage systems due to their high efficiency, long cycle life and easy scalability. In this work, chemical vapor deposition (CVD) grown carbon nanotubes (CNTs)-modified electrodes and Nafion 117 membrane are utilised for formulating a vanadium redox flow battery (VRFB). In a CVD chamber, the growth of CNTs is carried out on an acid-treated graphite felt surface. Cyclic voltammetry of CNT-modified electrode and acid-treated electrode revealed that CNTs presence improve the reaction kinetics of V3+/V2+ and VO2+/VO2+ redox pairs. Battery performance is recorded for analysing, the effect of modified electrodes, varying electrolyte flow rates, varying current densities and effect of removing the current collector plates. CNTs presence enhance the battery performance and offered 96.30% of Coulombic efficiency, 79.33% of voltage efficiency and 76.39% of energy efficiency. In comparison with pristine electrodes, a battery consisting CNTs grown electrodes shows a 14% and 15% increase in voltage efficiency and energy efficiency, respectively. Battery configured without current collector plates performs better as compared to with current collector plates which is possibly due to decrease in battery resistance. [ABSTRACT FROM AUTHOR]

A novel polybenzimidazole (PBI)‐based trilayer membrane assembly is developed for application in vanadium redox flow battery (VRFB). The membrane comprises a 1 µm thin cross‐linked poly[2,2′‐(p‐oxydiphenylene)−5,5′‐bibenzimidazole] (OPBI) sandwiched between two 20 µm thick porous OPBI membranes (p‐OPBI) without further lamination steps. The trilayer membrane demonstrates exceptional properties, such as high conductivity and low area‐specific resistance (ASR) of 51 mS cm−1 and 81 mΩ cm2, respectively. Contact with vanadium electrolyte increases the ASR of trilayer membrane only to 158 mΩ cm2, while that of Nafion is 193 mΩ cm2. VO2+ permeability is 2.73 × 10−9 cm2 min−1, about 150 times lower than that of Nafion NR212. In addition, the membrane has high mechanical strength and high chemical stability against VO2+. In VRFB, the combination of low resistance and low vanadium permeability results in excellent performance, revealing high Coulombic efficiency (>99%), high energy efficiency (EE; 90.8% at current density of 80 mA cm−2), and long‐term durability. The EE is one of the best reported to date. [ABSTRACT FROM AUTHOR]

This paper presents an extensive study on the electrochemical, shunt currents, and hydraulic modeling of a vanadium redox flow battery of m stacks and n cells per stack. The shunt currents model of the battery has been developed through the use of Kirchoff's laws, taking into account the different design cases that can occur and enumerating the equations of nodes and meshes specifying them so that the software implementation can be performed in a direct way. The hydraulic model has been developed by numerical methods. These models are put to work simultaneously in order to simulate the behavior of a VRFB battery during charging and discharging, obtaining the pressure losses and shunt currents that occur in the battery. Using these models, and by using a PSO-type optimization algorithm, specifically designed for discrete variables, the battery design is optimized in order to minimize the round-trip efficiency losses due to pressure losses and shunt currents. In the optimization of the battery design, value is given to the number of stacks in which the total number of cells in the battery is distributed and the dimensions of the piping relative to both the stacks and the cells. [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]

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]

Incorporating renewables in the power grid presents challenges for stability, reliability, and operational efficiency. Integrating energy storage systems (ESSs) offers a solution by managing unpredictable loads, enhancing reliability, and serving the grid. Hybrid storage solutions have gained attention for specific applications, suggesting higher performance in some respects. This article compares the performance of hybrid energy storage systems (HESSs) to a single battery, evaluating their energy supply cost and environmental impact through optimization problems. The optimization model is based on a MILP incorporating the energy and degradation terms. It generates an optimized dispatch, minimizing cost or environmental impact of supplying energy to a generic load. Seven technologies are assessed, with an example applied to an industrial site combining a vanadium redox flow battery (VRFB) and lithium battery considering the demand of a local load (building). The results indicate that efficiency and degradation curves have the highest impact in the final costs and environmental functions on the various storage technologies assessed. For the simulations of the example case, a single system only outperforms the hybrid system in cases where lithium efficiency is higher than approximately 87% and vanadium is lower approximately 82%. [ABSTRACT FROM AUTHOR]

In order to develop intermittent renewable energy sources, the development of energy storage systems (ESSs) has become a research hotspot, but high capital and operating costs remain their main drawbacks. Vanadium redox flow batteries (VRFBs) have emerged as promising large-scale electrochemical EESs due to their environmental friendliness, persistent durability, and commercial value advantages. Significant efforts have been devoted to VRFB electrode modification to improve their economic applicability and electrochemical performance while retaining environmental friendliness. In this review, the progress of VRFBs’ electrode treatment is summarized from the practical perspective. Considering the commercial prospects, this review provides an overview of these treatments for VRFB electrodes in terms of environmental friendliness, economic applicability, and electrochemical performance, which can be referred to as the “3Es”. The advantages and disadvantages of each processing method are analyzed with “3Es” as the standard, which provides a reference for the large-scale commercialization process of VRFBs. In the end, practical recommendations are put forward on the relationship between the development plan and the objectives of VRFB. [ABSTRACT FROM AUTHOR]

The temperature is a very important parameter for an operating vanadium redox flow battery (VRFB). During charging and discharging, the temperature of VRFB is constantly changing. In this paper, a self‐made 35 kW vanadium stack was charged & discharged at the current density of 100 and 120 mA cm−2 to investigate the change trend of real‐time operating temperature. The results show that the temperature decreases during charging and increases during discharging. And the average temperature of each cycle increases with continuous charge‐discharge cycles. The influences of real‐time temperature on performance of columbic efficiency (CE), voltage efficient (VE), over‐potential, capacity, energy, state of charge (SOC), and pressure loss were further investigated. The results show that the temperature fluctuation largely affects the electrochemical properties of VRFB. With the real‐time average temperature increased from 16.02 °C to 29.91 °C, the VE, capacity, energy and the SOC range of the battery increased, while the CE, the over‐potential and the average pressure loss decreased. This work not only gives data support to temperature management of VFB, but also provides direction to the engineering application of all flow batteries. [ABSTRACT FROM AUTHOR]

Herein, a zinc‐air flow battery (ZAFB) as an environmentally friendly and inexpensive energy storage system is investigated. For this purpose, an optimized ZAFB for households is designed based on the most recent publications, and an economic and ecological analysis of the system is carried out. The results show that the customer price of the designed ZAFB is estimated at ≈5000 € and is thus about one‐third lower than the quoted price for a comparable vanadium flow battery (VFB). The carbon footprint (CF) analysis shows that especially the alkaline electrolyte and the silver‐containing gas diffusion electrode have a high share of the greenhouse gas emissions of the ZAFB system. However, the CF is 41% lower than that of the VFB, highlighting the great potential of the ZAFB as an alternative home storage system. [ABSTRACT FROM AUTHOR]

This work reports the preparation and characterization of composite membranes with potential applications in flow battery devices. A polymer solution of polyvinylidene fluoride (PVDF), sulfonated polyether ether sulfone (SPEES), and polyether sulfone (PES) was used to prepare proton exchange membranes with low permeation of cationic species, which is a desirable feature in energy conversion devices, such as vanadium flow batteries (VRFB). Vanadyl ion (VO2+) acidic solution was selected as a model to reveal the permeation capacity through the membranes as an affordable alternative to the Nafion® 117 commercial membrane. The effects of the chemical composition and the thickness of the membranes on the morphology and proton exchange properties were evaluated. Characterization of the membranes was performed using physicochemical techniques including water uptake, swelling ratio, thermogravimetric analysis, scanning electron microscopy, Fourier transform infrared, surface charge density, atomic force microscopy, dynamic mechanical analysis, electrochemical impedance spectroscopy for proton conductivity, proton exchange rate, and the permeability of VO2+ ions. With VO2+ ion permeability of the order of 0.62 mM and H+ exchange rates of up to 498.5 × 1018 H+ cm−2*µm−1*min−1, the PVDF/PES/SPEES composite membranes emerge as an option with potential application for VRFB compared to the Nafion® 117 membrane, which presented a VO2+ ion permeation value of 2.72 mM and H+ exchange rates of 445.2 × 1018 H+ cm−2*µm−1*min−1. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries are promising energy storage devices and are already ahead of lead–acid batteries in terms of installed capacity in energy systems due to their long service life and possibility of recycling. One of the crucial tasks today is the development of models for assessing battery performance and its residual resource based on the battery's present state. A promising method for estimating battery capacity is based on analyzing present voltage and current values under various load conditions. This paper analyzes the discharge characteristics of a 10 kW all-vanadium redox flow battery at fixed load powers from 6 to 12 kW. A linear dependence of operating voltage and initial discharge voltage on load power is established. It is also determined that the slope of the discharge curve linear section does not increase linearly in absolute value, and the Box–Lucas model can be used to describe it. Models for predicting current VRFB capacity based on different curve fitting functions are proposed. These models can be used to roughly estimate battery capacity at different load powers. [ABSTRACT FROM AUTHOR]

In this research, the performance of vanadium redox flow batteries (VRFBs) in grid-connected energy storage systems centering on frequency and power sharing using voltage source inverters was evaluated. VRFBs are increasingly promising due to their scalability and long lifespan. We explore the impact of voltage source inverters on the frequency of the power supply when there is a change in load and power sharing between the grid and VRFB through MATLAB simulations. The test system demonstrates a storage system at an 11-kV substation and considers a load profile commencing from residential and commercial consumers. The outcome of this study reveals that VRFBs provide the required power to maintain load demand. Due to fast response time, VRFBs also regulate the frequency efficiently during the change in load demand and maintaining current total harmonic distortions (THDs). This research confirms the design and operation of VRFB-based energy storage systems, contributing to resilient grid infrastructure and reliable power distribution systems. [ABSTRACT FROM AUTHOR]

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

In this paper, bismuth (Bi) was successfully deposited on graphite felts to improve the electrochemical performances of vanadium redox flow batteries. Modified graphite felts with different Bi particle loadings were obtained through electrochemical deposition at voltages of 0.8 V, 1.2 V and 1.6 V in 0.1 M BiCl3 solution for 10 min. The optimal Bi particle loading was confirmed by scanning electron microscopy (SEM), single cells and electrochemical tests. The SEM images revealed the deposition of granular Bi particles on the fiber surface. The Bi-modified felts which were electro-chemically deposited at 1.2 V (Bi/TGF-1.2V) showed excellent electrochemical performances in cyclic voltammetry curves and impedance spectroscopy. Meanwhile, the single cells assembled with Bi/TGF-1.2V as negative electrodes exhibited higher voltage efficiencies than the others. The optimized Bi particle loading induced better catalysis of the V3+/V2+ reaction and hence significantly improved the cell performances. In addition, the prepared Bi-modified felts showed stable cell performances and slower charge–discharge capacity declines than the other electrodes at current densities between 20 mA/cm2 and 80 mA/cm2. Compared with the pristine felt, the voltage efficiency of the vanadium redox flow battery assembled with Bi/TGF-1.2V graphite felt was 9.47% higher at the current density of 80 mA/cm2. The proposed method has considerable potential and guiding significance for the future modification of graphite felt for redox flow batteries. [ABSTRACT FROM AUTHOR]

A comparison study was conducted for various supporting electrolytes of sulfuric acid (H2SO4), hydrochloric acid (HCl), and mixed acids (H2SO4 + HCl) in a vanadium redox flow battery (VRFB). The cyclic voltammetry (CV) results show that the highest value of − Ipc/Ipa (cathodic to anodic peak current ratio) and the lowest value of ΔEp (difference between oxidation and reduction peaks potential) are for the H2SO4 + HCl sample. Therefore, the V(IV)/V(V) redox reaction in the mixed system is more electrochemically reversible. According to the results of electrochemical impedance spectroscopy (EIS), the H2SO4 + HCl electrolyte also exhibits the lowest charge-transfer resistance, indicating more practicability in the transfer of vanadium ions and electrons for the mixed electrolyte. The electrolyte absorption measurements represent that the mixed-acid sample further penetrates the electrode porosity and increases the reaction rate of vanadium species. According to the rate capability test of VRFB, at various current densities of 60, 80, and 100 mA cm−2, the capacity has improved by approximately 10% because of the addition of HCl to the H2SO4 supporting electrolyte. The discharge polarization curves of VRFBs depict that adding HCl to the supporting electrolyte lowered the internal resistance of VRFB from 10 to 9.5 Ω cm2. The peak power density for H2SO4 supporting electrolyte is 496.4 mW cm−2, whereas this value is 530.1 mW cm−2 for mixed-acid supporting electrolyte. [ABSTRACT FROM AUTHOR]

The vanadium redox flow battery (VRFB) is a highly favorable tool for storing renewable energy, and the catalytic activity of electrode materials is crucial for its development. Taurine-functionalized carbon nanotubes (CNTs) were prepared with the aim of augmenting the redox process of vanadium ions and enhancing the efficiency of the VRFB. Sulfonated CNTs were synthesized through a simple modification process in a taurine solution and used as electrocatalysts for redox reactions involving VO2+/VO2+ and V2+/V3+. The SO3H-CNTs modified at 60 °C for 2 h exhibit the best electrocatalytic activity, showing higher redox peak current values compared to pristine carboxylated CNTs (COOH-CNTs). Sulfonic acid groups added to the surface of CNTs increase active sites for redox reactions and act as carriers for mass transfer and bridges for charge transfer, accelerating the rate of the electrode reactions. A battery consisting of SO3H-CNTs as catalysts demonstrates the outstanding charge–discharge performance at a current density of 300 mA∙cm−2. This configuration displays voltage and energy efficiencies of 81.46% and 78.83%, respectively, representing enhancements of 6.15% and 6.12% compared to that equipped with conventional graphite felts (75.31%, 72.71%). This study illustrates that taurine-functionalized carbon nanotubes serve as an efficient and promising catalyst for both the anode and cathode, leading to the improved performance of the VRFB. [ABSTRACT FROM AUTHOR]

The development of electrodes with high performance and long‐term stability is crucial for commercial application of vanadium redox flow batteries (VRFBs). This study compared the performance of VRFB with thermal‐treated and MXene‐modified carbon paper. To prepare the MXene, a modified‐etching process with ammonium−bifluoride (NH4HF2) led to a mild and efficient conversion of the MAX‐phase to MXene compared to etching process with hydrofluoric‐acid (HF). Electron microscopy and X‐ray diffraction studies revealed that the etching process with NH4HF2 led to MXene nanostructures with a large interlayer spacing. The results show that at a current density of 60 mA cm−2, the energy efficiency increased by 25.5 % when using a NH4HF2 ‐etched MXene‐modified negative electrode, by 12.5 % with a thermal‐treated MXene‐modified electrode, and by 4 % with an HF‐etched MXene‐modified electrode, in comparison to the pristine electrode. The maximum power density of the battery was increased by more than 40 %. In long‐term cycling experiments the MXene modified electrode exhibited excellent stability over 1000 cycles of charge‐discharge, with 0.05 % discharge capacity decay per cycle, amongst the lowest values reported to date and four times lower than for thermally‐treated electrode. The superior performance was linked to the improved electrical conductivity and wettability, higher interlayer spacing, and lower charge transfer resistance for the V2+/V3+ redox reaction. [ABSTRACT FROM AUTHOR]

Vanadium electrolyte is one of the most critical materials for vanadium redox batteries (VRB). Reducing the cost of vanadium electrolyte and improving its performance are ongoing research priorities for VRB. Currently, the control of the cost of vanadium electrolyte mainly relies on the development of new processes and optimization of traditional processes. Improving the performance of electrolytes mainly involves two aspects: mass transfer and charge transfer, such as introducing additives, optimizing supporting electrolytes, and developing new electrode catalysts. This article reviews the progress in improving the performance of VRB in the past 10 years. It focuses on three main aspects: the preparation of electrolytes, the influence of mass transfer on battery performance, and the influence of charge transfer on battery performance. It also further discusses the impact of different factors on the improvement of VRB performance. Finally, it summarizes the challenges faced by VRB in performance improvement and commercial applications, and made suggestions for future research and development of VRB. [ABSTRACT FROM AUTHOR]

An innovative reactor concept is reported that allows for efficient mass transfer from the liquid phase to the base material and compensates for the growth of the material throughout the synthesis of polymer electrolyte membranes (PEM). The novel reactor allows for the synthesis of PEMs with high reproducibility of their dimensions and properties. PEMs are synthesized via graft copolymerization of the monomers acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid on poly(vinylidene fluoride) films serving as base material, which was activated by electron beam treatment. Both monomers are already containing protogenic groups; thus, follow-up functionalization reactions are avoided. The PEMs were characterized with respect to their electrochemical properties (area specific resistance, recharge current, and ion exchange capacity) relevant for application in vanadium flow batteries and compared to commercially available PEMs. [ABSTRACT FROM AUTHOR]

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-the-rib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution. [ABSTRACT FROM AUTHOR]

Aqueous all‐iron flow batteries (AIFBs) are attractive for large‐scale and long‐term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2‐methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge‐discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe3+ and Fe2+, without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5 % at 80 mA cm−2 and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100 % after 1,400 cycles. The capital cost of this AIFB is $ 33.2 kWh−1 (e.g. 20 h duration), cheaper than Li‐ion battery and vanadium flow battery. This double‐ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries. [ABSTRACT FROM AUTHOR]

Batteries dissolving active materials in liquids possess safety and size advantages compared to solid‐based batteries, yet the intrinsic liquid properties lead to material cross‐over induced self‐discharge both during cycling and idle when the electrolytes are in contact, thus highly efficient and cost‐effective solutions to minimize cross‐over are in high demand. An ultra‐low self‐discharge aqueous|organic membraneless battery using dichloromethane (CH2Cl2) and tetrabutylammonium bromide (TBABr) added to a zinc bromide (ZnBr2) solution as the electrolyte is demonstrated. The polybromide is confined in the organic phase, and bromine (Br2) diffusion‐induced self‐discharge is minimized. At 90% state of charge (SOC), the membraneless ZnBr2|TBABr (Z|T) battery shows an open circuit voltage (OCV) drop of only 42 mV after 120 days, 152 times longer than the ZnBr2 battery, and superior to 102 previous reports from all types of liquid active material batteries. The 120‐day capacity retention of 95.5% is higher than commercial zinc‐nickel (Zn–Ni) batteries and vanadium redox flow batteries (VRFB, electrolytes stored separately) and close to lithium‐ion (Li‐ion) batteries. Z|T achieves >500 cycles (2670 h, 0.5 m electrolyte, 250 folds of membraneless ZnBr2 battery) with ≈100% Coulombic efficiency (CE). The simple and cost‐effective design of Z|T provides a conceptual inspiration to regulate material cross‐over in liquid‐based batteries to realize extended operation. [ABSTRACT FROM AUTHOR]

In recent years, energy challenges such as grid congestion and imbalances have emerged from conventional electric grids. Furthermore, the unpredictable nature of these systems poses many challenges in meeting various users' demands. The Battery Energy Storage System is a potential key for grid instability with improved power quality. The present study investigates the global trend towards integrating battery technology as an energy storage system with renewable energy production and utility grid systems. An extensive review of battery systems such as Lithium-Ion, Lead–Acid, Zinc–Bromide, Nickel–Cadmium, Sodium–Sulphur, and the Vanadium redox flow battery is conducted. Furthermore, a comparative analysis of their working principles, control strategies, optimizations, and technical characteristics is presented. The review findings show that Lead–Acid, Lithium-Ion, Sodium-based, and flow redox batteries have seen increased breakthroughs in the energy storage market. Furthermore, the use of the BESS as an ancillary service and control technique enhances the performance of microgrids and utility grid systems. These control techniques provide potential solutions such as peak load shaving, the smoothing of photovoltaic ramp rates, voltage fluctuation reduction, a large grid, power supply backup, microgrids, renewable energy sources time shift, spinning reserve for industrial consumers, and frequency regulation. Conclusively, a cost summary of the various battery technologies is presented. [ABSTRACT FROM AUTHOR]

A unique functionalized organosilane (FOSi) is synthesized and prepared as a hybrid organic-inorganic composite membrane (OICM) using short-side-chain perfluorosulfonic acid (SSC-PFSA) at a large scale for vanadium redox flow battery (VRFB). The OICM solution is fabricated by using 75 wt % of SSC-PFSA and 25 wt % of FOSi and cast with the help of Roll to Roll (R2R) film casting equipment. The OICM's proton conductivity and performance are comparable to the Nafion membrane, owing to additional hydrophilic fields in the hybrid membrane. The OICM possesses proton conductivity of 0.13 S/cm and an ion exchange capacity of 1.44 meq/g, even though the water uptake has been lowered than the PFSA owing to better dispersion of FOSi. OICM has a low vanadium permeability (1.52 × 10−6 cm2/min) and remarkable selectivity, which is a significant benefit. Consequently, over 45th cycles, the OICM performs comparably to Nafion in energy efficiency combined with coulombic and voltage efficiency. The unit cell-test with OICM shows a normalized coulombic efficiency due to low permeability than PFSA and high voltage efficiency due to high proton conductivity. Hence OICM divulges a promising path to developing an alternative membrane for energy devices. [Display omitted] • Short-side-chain PFSA is mixed with Functionalized Organosilane (75:25). • Large-scale production of the composite membrane using roll-to-roll equipment. • Functionalized silane addition effectively increased ionic conductivity. • The composite membrane's vanadium ion permeability is substantially decreased. • Cell performance of composite membranes is comparable to that of Nafion. [ABSTRACT FROM AUTHOR]

This review examines the role of defective carbon‐based electrodes in sodium‐ion and vanadium flow batteries. Methods for introducing defects into carbon structures are explored and their effectiveness in improving electrode performance is demonstrated. In sodium‐based systems, research focuses primarily on various precursor materials and heteroatom doping to optimise hard carbon electrodes. Defect engineering increases interlayer spacing, porosity, and changes the surface chemistry, which improves sodium intercalation and reversible capacities. Heteroatom functionalisation and surface modification affect solid electrolyte interface formation and coulombic efficiencies. For flow batteries, post‐fabrication electrode enhancement methods produce defects to improve electrode kinetics, although these methods often introduce oxygen functional groups as well, making isolation of defect effects difficult. Continued research efforts are key to developing carbon‐based electrodes that can meet the unique challenges of future battery systems. [ABSTRACT FROM AUTHOR]

The membrane is a central component in the commercialization of vanadium redox flow batteries (VRFB), with Nafion being the preferred material for those membranes. Nafion suffers however from high vanadium crossover and high cost, thus limiting its wider commercial application in VRFB. To address this, studies have focused on sulfonated polyether ether ketone (SPEEK) as an alternative material. This work presents an economic study on the production costs of 3D printed SPEEK (3D‐SPEEK) membranes for VRFB, with multiple production scenarios evaluated. This analysis relies on the performances of 3D‐SPEEK membranes prepared in our laboratory, which have shown higher Coulombic efficiency and lower vanadium ions diffusion than Nafion membranes. Results from the economic analysis revealed that the cost of the 3D‐SPEEK membrane varies from 567.77 to 79.35 €/m2 depending on the production scheme and scale used. Production costs lower than commercial Nafion membranes are achieved when a large production scheme is used. Further analysis shows that 3D‐SPEEK membranes might even be cheaper than conventional SPEEK membranes if their printing time is reduced. [ABSTRACT FROM AUTHOR]

The interest in flow batteries as energy storage devices is growing due to the rising share of intermittent renewable energy sources. In this work, the performance of a vanadium flow battery is improved, without altering the electrochemical cell, by applying a pulsating flow. This novel flow battery flow regime aims to enhance performance by improving its mass transfer properties. Both the pulse volume and frequency have an influence on the battery performance, and lead to a 38.7 % increase in accessible discharge capacity compared to the conventional steady flow regime at values of 0.40 cm3 and 2.4 Hz in this cell, respectively. Furthermore, at these parameter settings a reduction of the mass transfer resistance by 71.4 % is achieved at 0.2 cm3/min ⋅ cm2. These results show that a pulsating flow can be beneficial in certain conditions, opening new possibilities for boosting performance in flow batteries. [ABSTRACT FROM AUTHOR]

In an industrial‐scale vanadium redox flow battery system, cells are stacked to form a compact module that serves as the power system's most fundamental unit in its scale‐up. Considering the narrow temperature range restricted by the precipitation of vanadium ions in the electrolyte and the relatively strong current employed, heat generation and dissipation determine the stack's temperature, significantly affecting the performance. Using the mass, momentum, energy conservation, electrochemical kinetics in each cell, and circuit analysis in the manifolds and channels through which electrolyte flows and is distributed to each cell, a 2D thermal model is developed in this study to investigate the temperature distribution and variation within the cell stack. Benefit from the integrated simulation of a 2D model and analytical circuit in flow frames in the stack, the temperature distribution and hotspots within the stack during the dynamic charge–discharge cycles can be precisely determined. In the results, it is indicated that the global consideration of transport and electrochemical processes demonstrate a more systematic understanding of heat generation and transfer throughout the stack. [ABSTRACT FROM AUTHOR]

The scarcity of wettability, insufficient active sites, and low surface area of graphite felt (GF) have long been suppressing the performance of vanadium redox flow batteries (VRFBs). Herein, an ultra‐homogeneous multiple‐dimensioned defect, including nano‐scale etching and atomic‐scale N, O co‐doping, was used to modify GF by the molten salt system. NH4Cl and KClO3 were added simultaneously to the system to obtain porous N/O co‐doped electrode (GF/ON), where KClO3 was used to ultra‐homogeneously etch, and O‐functionalize electrode, and NH4Cl was used as N dopant, respectively. GF/ON presents better electrochemical catalysis for VO2+/VO2+ and V3+/V2+ reactions than only O‐functionalized electrodes (GF/O) and GF. The enhanced electrochemical properties are attributed to an increase in active sites, surface area, and wettability, as well as the synergistic effect of N and O, which is also supported by the density functional theory calculations. Further, the cell using GF/ON shows higher discharge capacity, energy efficiency, and stability for cycling performance than the pristine cell at 140 mA cm−2 for 200 cycles. Moreover, the energy efficiency of the modified cell is increased by 9.7% from 55.2% for the pristine cell at 260 mA cm−2. Such an ultra‐homogeneous etching with N and O co‐doping through "boiling" molten salt medium provides an effective and practical application potential way to prepare superior electrodes for VRFB. [ABSTRACT FROM AUTHOR]

The poor electrochemical activity and low wettability of graphite felt (GF) electrodes significantly limit the energy efficiency and power of vanadium redox flow batteries (VRFBs). To solve these problems, we developed an N-doped porous carbon-coated electrode by the carbonization of zeolitic imidazolate framework-8- (ZIF-8-) coated GF. By controlling the carbonization temperature, the effects of temperature on the structural morphology, pore structure, and amount and type of functional groups of the electrode were confirmed. In addition, the effect of the change in the pore structure and the amount and type of functional groups on the electrocatalytic activity for the vanadium redox reaction was demonstrated. The synthesized N-doped porous carbon-coated electrode, which showed the best electrochemical activity, had a sufficient amount of nitrogen functional groups with a high ratio of graphitic N and the highest surface area and pore volume among all the samples, providing abundant active sites for the VO2+/VO2+ redox reaction. The VRFB fabricated using prepared electrode exhibited energy and voltage efficiencies of 70.44% and 73.44%, respectively, at 200 mA cm-2, which were 11.77% and 11.08% higher than those of the bare GF electrode. Furthermore, the modified battery operated effectively at a high current density of 350 mA cm-2. The energy efficiency exhibited excellent stability over 500 charge/discharge cycles at 250 mA cm-2. Hence, this study presents a promising method for the development of high-performance VRFB electrodes. [ABSTRACT FROM AUTHOR]