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Insufficient electrochemical activity and mass transport have been a longstanding issue hindering the development of high‐performance electrode for vanadium redox flow batteries. In this work, vertical‐aligned bismuth‐nanosheets that unlock interfacial dynamics with enhanced catalytic and transport properties are decorated on the electrode surface. The vertical‐aligned structures form highways to guide reactants transport from bulk solution to electrode surface, reducing transport resistance and optimizing active sites utilization. Meanwhile, the bismuth‐nanosheets, dominated by (012) planes with large amounts of unsaturated bismuth atoms, boost the number of electroactive sites and catalytic activity. Full‐cell EIS tests show that the charge transfer resistance and the finite diffusion resistance with prepared electrode are 49.4 and 47.7 mΩ cm−2, 71.3%, and 6.7% lower than those with traditional electrode. In full‐cell tests, the battery delivers a peak power density of 1340 mW cm−2 and achieves an energy efficiency of 85.8% at 200 mA cm−2, which is 4.4% higher than that with traditional electrodes. Remarkably, the battery can maintain an energy efficiency of 82.4% at 300 mA cm−2 and be stably cycled for over 2000 cycles with an only 0.0016% efficiency decay per cycle. This work paves a feasible platform to fabricate advanced electrode for vanadium redox flow batteries. [ABSTRACT FROM AUTHOR]

Accurate prediction of battery temperature rise is very essential for designing efficient thermal management scheme. In this paper, machine learning (ML)‐based prediction of vanadium redox flow battery (VRFB) thermal behavior during charge–discharge operation has been demonstrated for the first time. Considering different currents with a specified electrolyte flow rate, the temperature of a kW scale VRFB system is studied through experiments. Three different ML algorithms; linear regression (LR), support vector regression (SVR), and extreme gradient boost (XGBoost) have been used for prediction. The training and validation of ML algorithms have been done by the practical dataset of a 1 kW 6 kWh VRFB storage under 40 , 45, 50, and 60 A charge–discharge currents and 10 L min−1 of flow rate. A comparative analysis among ML algorithms is done by performance metrics such as correlation coefficient (R2), mean absolute error (MAE), and root mean square error (RMSE). XGBoost shows the highest R2 value of around 0.99, which indicates its higher prediction accuracy compared to other ML algorithms used. The ML‐based prediction results obtained in this work can be very useful for controlling the VRFB temperature rise during operation and act as an indicator toward further development of an optimized thermal management system. [ABSTRACT FROM AUTHOR]

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]

Nafion has gained widespread recognition as the predominant membrane due to its good proton conductivity, robust chemical resistance, and commendable mechanical stability. However, due to its well-developed water channels, it has poor barrier properties toward vanadium ions. Herein, to reduce vanadium ions permeability across the membranes without compromising the proton conductivity, graphene nanofiber (Herringbone type, GNF-H) as a filler has been incorporated into the Nafion matrix to fabricate the composite membrane. The membranes were subjected to physiochemical characterization, vanadium ion permeability, electrochemical impedance spectroscopy, and galvanostatic charge-discharge at different current densities. Vanadium permeability has significantly reduced in the 0.75% and 1% GNF-H composite membranes. Composite membranes (0.5%, 0.75%, and 1% GNF-H) showed a capacity of ~18.2, ~18.9, and ~16.8 Ah L−1 at 100 mA cm−2, respectively, whereas NafionTM 117 exhibited only ~16.3 Ah L−1 capacity at the same current density. The peak power of the cells consisted of 0.5, 0.75, and 1% GNF-H composite membrane and NafionTM 117 is ~538, ~507, ~465 and 388 mW cm−2, respectively. The present study concludes that applying Nafion/GNF-H in the VRFB system can be a promising strategy to reduce the vanadium ion permeation, cost-cutting and improve the VRFB performance. GNF-H serves as a physical barrier to vanadium ion movement within the Nafion matrix, potentially lengthening the transport path of vanadium ions through the membrane. This reduces crossover and enhances membrane selectivity while not impeding proton transport, thereby enhancing the performance of VRFB. [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]

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 vanadium redox flow battery (VRFB) holds promise for large‐scale energy storage applications, despite its lower energy and power densities compared to advanced secondary batteries available today. Carbon materials are considered suitable catalyst electrodes for improving many aspects of the VRFB. However, pristine graphite structures in carbon materials are catalytically inert and require modification to activate their catalytic activity. Among the various strategies developed so far, O‐functionalization and chemical doping of carbon materials are considered some of the most promising pathways to regulate their electronic structures. Building on the catalytic mechanisms involved in the VRFB, this concise review discusses recent advancements in the O‐functionalization and chemical doping of carbon materials. Furthermore, it explores how these materials can be tailored and highlights future directions for developing more promising VRFBs to guide future research. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries (VRFBs) have emerged as a promising energy storage solution for stabilizing power grids integrated with renewable energy sources. In this study, we synthesized and evaluated a series of zeolitic imidazolate framework-67 (ZIF-67) derivatives as electrode materials for VRFBs, aiming to enhance electrochemical performance. Four materials—Co/NC-700, Co/NC-800, Co3O4-350, and Co3O4-450—were prepared through thermal decomposition under different conditions and coated onto graphite felt (GF) electrodes. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses confirmed the structural integrity and distribution of the active materials. Electrochemical evaluations revealed that electrodes with ZIF-67-derived coatings exhibited significantly lower charge transfer resistance (Rct) and higher energy efficiency (EE) compared to uncoated GF electrodes. Co/NC-800//GF delivered the highest energy efficiency and discharge capacity among the tested configurations, maintaining stable performance over 100 charge–discharge cycles. These results indicate that Co/NC-800 holds great potential for use in VRFBs due to its superior electrochemical activity, stability, and scalability. [ABSTRACT FROM AUTHOR]

The electronics panels and controllers used in electric vehicle (EV) charging stations need to function securely and optimally in countries such as India, where the external temperature remains high for a significant part of the year. This study is the first to use a solar-battery storage integrated switchable glazing architecture to supply HVAC (heating, ventilation, and air conditioning) in the control rooms of electric vehicle charging stations throughout the day. The EV charging station has been equipped with a rooftop-mounted solar PV source as part of an initiative to promote renewable energy and sustainable forms of mobility. The utilization of a vanadium redox flow battery (VRFB) by the system ensures energy security through the provision of a durable solution for storing energy over an extended period of time. A smart scheduling system utilizing IoT technology was demonstrated to effectively meet the glazing load need of buildings in real-time, taking into account dynamic climatic conditions. This system incorporates solar PV, VRFB storage, and the neighborhood power grid. The efficiency of the suggested method was validated using four separate transitory scenarios: sunny, intermittently cloudy, prolongedly cloudy, and low solar radiation with frequent grid outages. Due to its versatility to be applied in various situations, the suggested method has the potential to greatly enhance capacity. [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]

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]

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]

Carbon electrodes are one of the key components of vanadium redox flow batteries (VRFBs), and their wetting behavior, electrochemical performance, and tendency to side reactions are crucial for cell efficiency. Herein, we demonstrate three different types of electrode modifications: poly(o-toluidine) (POT), Vulcan XC 72R, and an iron-doped carbon-nitrogen base material (Fe-N-C + carbon nanotube (CNT)). By combining synchrotron X-ray imaging with traditional characterization approaches, we give thorough insights into changes caused by each modification in terms of the electrochemical performance in both half-cell reactions, wettability and permeability, and tendency toward the hydrogen evolution side reaction. The limiting performance of POT and Vulcan XC 72R could mainly be ascribed to hindered electrolyte transport through the electrode. Fe-N-C + CNT displayed promising potential in the positive half-cell with improved electrochemical performance and wetting behavior but catalyzed the hydrogen evolution side reaction in the negative half-cell.

Abstract In this study, Li4Ti5O12 (LTO) and TiO2 nanocomposites uniformly were synthesized on the heat‐treated graphite felt through (HGF) hydrothermal and heat treatment methods, denoted by LTO/TiO2@HGF, which LTO/TiO2@HGF acts as effective electrocatalysts to enhance the electrochemical activity in vanadium redox flow battery (VRFB) systems. The cyclic voltammetry (CV) curves of the LTO/TiO2@HGF show higher peak current densities and smaller peak separation than TiO2@HGF, HGF, and pristine graphite felt (PGF) for catalyzing V2+/V3+ and VO₂+/VO2+, indicating superior electrochemical activity of LTO/TiO2@HGF. The VRFB using LTO/TiO2@HGF as the positive and negative electrodes demonstrates an energy efficiency of 82.89 % at 80 mA cm−2. When the VRFB using LTO/TiO2@HGF is applied by a high current density of 200 mA cm−2, it still shows an energy efficiency of 62.22 %. However, the VRFB using PGF cannot perform any performance, and the VRFB using HGF only performs 51.94 %. This improvement can be attributed to the uniform distribution of LTO/TiO2 nanowires on the surface of the graphite felt and the presence of oxygen vacancies on LTO/TiO2, which increased the number of active sites for vanadium ion absorption.

A selective composite membrane for vanadium redox flow battery (VRFB) was successfully prepared by layer-by-layer (LbL) technique using a perfluorosulfonic sulfonic acid or Nafion 117 (N117). The composite membrane referred as N117-(PEI/GO)n, was obtained by depositing alternating layers of positively charged polyethylenimine (PEI) and negatively charged graphene oxide (GO) as polyelectrolytes. The physicochemical properties and performance of the pristine and composite membranes were investigated. The membrane showed an enhancement in proton conductivity and simultaneously exhibited a notable 90% reduction in vanadium permeability. This, in turn, results in a well-balanced ratio of proton conductivity to vanadium permeability, leading to high selectivity. The highest selectivity of the LbL membranes was found to be 19.2 × 104 S.min/cm3, which is 13 times higher than the N117 membrane (n = 0). This was translated into an improvement in the battery performance, with the n = 1 membrane showing a 4–6% improvement in coulombic efficiency and a 7–15% improvement in voltage efficiency at current densities ranging from 40 to 80 mA/cm2. Furthermore, the membrane displays stable operation over a long-term stability at around 88% at a current density of 40 mA/cm2, making it an attractive option for VRFB applications using the LbL technique. The use of PEI/GO bilayers maintains high proton conductivity and VE of the battery, opening up possibilities for further optimization and improvement of VRFBs.

Vanadium redox flow batteries (VRFBs) have received significant attention for use in large-scale energy storage systems (ESSs) because of their long cycle life, flexible capacity, power design, and safety. However, the poor electrochemical activity of the conventionally used carbon felt electrode results in low energy efficiency of the VRFBs and consequently impedes their commercialization. In this study, a carbon felt (CF) electrode with numerous nanopores and robust oxygen-containing functional groups at its edge sites is designed to improve the electrochemical activity of a carbon felt electrode. To achieve this, Ni metal nanoparticles were initially precipitated on the surface of the CF electrode, followed by etching of the precipitated Ni nanoparticles on the CF electrode using sulfuric acid. The resulting CF electrode had a specific surface area eight times larger than that of the pristine CF electrode. In addition, the oxygen-containing functional groups anchored at the graphite edge sites of the nanopores can act as robust electrocatalysts for VO2+/VO2+ and V2+/V3+ redox reactions. Consequently, the VRFB cell with the resulting carbon felt electrode can deliver a high energy efficiency of 86.2% at the current density of 60 mA cm−2, which is 20% higher than that of the VRFB cell with the conventionally heat-treated CF electrode. Furthermore, the VRFB cell with the resultant carbon felt electrodes showed stable cycling performance with no considerable energy efficiency loss over 200 charge-discharge cycles. In addition, even at a high current density of 160 mA cm−2 , the developed carbon felt electrode can achieve an energy efficiency of 70.1%.

The present study aimed to investigate the effect of Nafion/TiO2 nanoparticles as an electrocatalyst on the electrochemical behavior of graphite felt (GF) electrodes in a vanadium redox flow battery. Various electrochemical tests, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV), were conducted to assess the performance of these electrodes at different concentrations of nanoparticles (2–4 g l-1). Additionally, X-ray diffraction, Fourier transform infrared spectroscopy, and field emission scanning electron microscopy were employed to analyze the chemical composition, bonding, and morphology of the decorated electrodes, respectively. The CV results revealed several effects of TiO2 nanoparticles on the GF electrode, such as increased peak intensity and shifted peak sites. As the concentration of nanoparticles increased, the peak intensity rose by up to 44%. Moreover, the potentials of the decorated electrodes shifted towards the favorable side compared to the GF electrode, with changes ranging from 18 to 84%. Overall, the optimal concentration of TiO2 nanoparticles (3 g l-1) exhibited excellent electrode performance, characterized by the highest calculated diffusion coefficient, greater reversibility, enhanced electron transfer kinetics, improved stability, lower over-potential, and the lowest activation energy for redox reactions. EIS results demonstrated a significant decrease in polarization resistance (67.2–70.8%) for the decorated electrodes. Furthermore, LSV measurements indicated that the utilization of nano-electrocatalysts effectively inhibited hydrogen evolution at negative potentials.