Flow fields are a key component in redox flow batteries, which is to distribute electrolytes onto electrodes at the maximum uniformity with the minimum pump work. Achieving this design goal requires accurate simulations of electrolyte flows and identification of the dead zones where the flows become weak or stagnant. However, conventional case-by-case numerical simulation requires significant computational resources. In this work, we use deep learning to predict the electrolyte flow in flow batteries with a neural network knows as U-Net. The UNet is well trained by learning the mapping between the input (flow field geometry) and output (velocity magnitude distribution). Results show that the pixel-wise comparison of the velocity magnitudes between the UNet-predicted results and finite element simulated results exhibits an average Euclidean distance of 442.6 and an average R2 of 0.979, indicating that the electrolyte distribution can be accurately simulated based on the geometric characteristics of flow fields. In addition, dead zones are precisely identified by labeling the regions with low velocity magnitudes. Modifying the channel depth in these regions substantially enhances the under-rib convection, thereby improving the system efficiency by 5.5 % at 200 mA cm-2. Furthermore, compared to the numerical simulation, the U-Net-assisted prediction significantly reduces the computational time by 99.9 %. It is anticipated that the U-Net-assisted simulation provides an accurate and efficient tool for obtaining the velocity distribution of flow fields that can assist the flow field design especially in large quantities and large scale.

Ion exchange membranes play a vital role in redox flow batteries. However, polymer membranes with a microscopic thickness of approximately 20-50 mu m are susceptible to micro defects, which substantially reduces the battery's energy efficiency and cycling stability. Hence, there is a need for an effective strategy to identify and resolve membrane imperfections, which is currently missing in the literature. In this work, a pressure- retention setup and hot-pressing method are proposed and show that defective membranes can be effectively identified and resolved. For instance, a membrane with around 100-mu m pinholes exhibits a low coulombic efficiency of 77.5 % at the current density of 100 mA cm(-2). However, the coulombic efficiency can be raised to 96.3 % by removing the defects, thus attaining the level of the undamaged pristine membrane (96.4 %). The capacity retention rate of the vanadium redox flow batteries with the repaired membrane is 71.1 % over 100 cycles at the current density of 200 mA cm(-2), close to that of the pristine membrane (72.2 %). In addition, the repaired membrane exhibits quite similar physicochemical properties to the pristine membrane from various characterizations. The proposed method represents a convenient, economical, and non-destructive membrane detecting and repairing strategy, demonstrating great potential for redox flow batteries.

Redox flow batteries are promising electrochemical systems for energy storage owing to their inherent safety, long cycle life, and the distinct scalability of power and capacity. This review focuses on the stack design and optimization, providing a detailed analysis of critical components design and the stack integration. The scope of the review includes electrolytes, flow fields, electrodes, and membranes, along with the uniformity issues, thermal management, and system integration. This review aims to bridge the gap between academic research and commercial application, promoting redox flow batteries as a more reliable system for large-scale, long-term energy storage applications.

Improving the power density of vanadium redox flow batteries (VRFBs) is the key to achieving cost reduction and efficiency increase, and it is necessary to carry out comprehensive collaborative optimization of electrode materials. How to regulate the activity, conductivity and mass transfer performance comprehensively must be the focus. Herein, a three-dimensional porous electrode (WC-ECNFs) with coordinated properties has been prepared by collecting the electrospun fibers in a water coagulation bath and then freeze-drying and heat-treating. In the freeze-drying process, ice crystals are used as the fiber support template to provide higher porosity. More importantly, a thin carbon layer with more orderly graphite microcrystalline structure has been formed on the fiber surface and endows the WC-ECNFs electrode with higher electrical conductivity. This unique macroscopic 3D porous and microscopic skin-core fiber structure give it coordinated mass- and electron- transfer properties. The energy efficiency of VRFB with WC-ECNFs as electrode is 4.33 % higher than that of the battery assembled with common ECNFs at 200 mAcm(-2). After 1000 cycles of continuous charging and discharging, the attenuation rate of energy efficiency for single-cycle is only 0.0053 %. The results demonstrate the efficiency of the coagulation bath-freeze drying method to realize the multi-dimensional control of carbon fibers electrode and achieve the coordination and matching of each pivotal properties.

Serpentine flow fields with flat ribs can effectively improve the performance of redox flow batteries. However, the marginal pressure difference between adjacent channels near the end side of ribs in conventional designs leads to weak convection in the regions, increasing the local concentration overpotential. This problem will be exacerbated in scaled-up redox flow battery stacks due to enlarged under-rib regions near the end side of ribs. In this work, two modified serpentine flow fields with height-changing ribs are proposed to adjust the electrode compression ratio at the under-rib region, thereby enhancing the convection in the electrode near the end side of ribs. Therefore, the active species are distributed more uniformly, resulting in an improved battery performance. Specifically, the uniformity of V2+, the energy efficiency and the system efficiency of the vanadium redox flow batteries with the ramp ribs are improved by 21 %, 2.7 %, and 2.7 % respectively compared with that of the conventional flat ribs, at the current density of 200 mA cm(-2), the flow rate of 3.4 mL min(-1) cm(-2), and the active area of 117 cm(2).

Abstract Electrodes, which offer sites for mass transfer and redox reactions, play a crucial role in determining the energy efficiencies and power densities of redox flow batteries. This review focuses on various approaches to enhancing electrode performance, particularly the methods of surface etching and catalyst deposition, as well as some other advanced strategies for regulating electrode surface properties. These approaches aim to increase active sites and enhance kinetics for the redox reactions, which are crucial for elevating power density and electrolyte utilization, eventually determining the performance of the flow battery. Highlighting the need for interdisciplinary research, this mini‐review suggests that future advancements in electrode design will significantly impact the commercial viability and adoption of redox flow batteries in sustainable energy storage solutions.

The operating temperature of vanadium redox flow batteries (VRFBs) affects their performance and reliability. However, previous studies focused on evaluating the effects on the performance of lab-scale single cells, in which electrolyte flow rates and current densities are different from those in stack-scale VRFBs, leading to the lack of guidance for the design of stacks. In this work, we investigate thermal effects on the performance of stack-scale VRFBs. It is found that as the operating temperature increases from 25 to 50 °C, the discharge capacity increases by 42%, whereas the energy efficiency increases by 10%, implying that the temperature has greater effects on the discharge capacity than that on the energy efficiency. Additionally, the enhancement effect of temperature on the energy efficiency is gradually weakened with increasing flow rate, while that on the discharge capacity is almost unchanged. Furthermore, the enhancement effect of temperature on energy efficiency increases with the operating current density. Notably, an optimum operating condition of the stack-scale VRFBs is identified with a critical flow rate (2.88 ml min −1 cm −2 ) at 40 °C to achieve a high system efficiency. This work provides guidance for the design of stack-scale VRFBs with high performance and safety.

Designing flow fields with enhanced convection is crucial to achieve a uniform electrolyte distribution and thus to improve the battery performance. In this work, we numerically model a new type of convection-enhanced flow field, which is designed by repatterning the flow path of serpentine flow field to strengthen the mass transport between neighboring channels. Key geometric parameters and flowing patterns are investigated. It is revealed that decreasing the channel fraction and increasing the channel number result in a more uniform reactants distribution, but lead to an obvious increase of pumping work. Additionally, by tailoring rotary methods with two criteria of the path number and path sequence, seven novel patterns with rationally designed convection-enhanced flow path are proposed. Results show that when the number of paths is five and the outflow path is in the middle, the most uniform reactants distribution and the lowest pressure drop between inlet and outlet can be achieved. More impressively, the vanadium redox flow battery with the optimized flow field achieves a higher pump-based voltage efficiency than that with the serpentine flow field (87.1% vs. 82.8%) at 150 mA cm-2, indicating that the convection-enhanced pattern shows great promise for the application in high-performance flow batteries.

The uniform and dense distribution of highly active electrocatalysts onto graphite fibers of scale-up anodes is essential for the engineered vanadium redox flow batteries to be operated efficiently and stably. In this work, a new in-situ electrodeposition strategy of bismuth is devised by using a catholyte with a low concentration of vanadium ions (33 mM V3+). Compared with the conventional method using a catholyte with a high concentration of vanadium ions (1700 mM VO2+), the present strategy renders Bi nanoparticles not being oxidized by VO2+ transported across the membrane from the catholyte, enabling bismuth nanoparticles with sizes of about 58 nm being uniformly and densely electrodeposited onto graphite fibers of an anode (117 cm-2) at an extremely low electrodeposition current density (2 mA cm-2). The vanadium redox flow battery with the present scale-up anode achieves an energy efficiency of 76.3% even at a current density of 300 mA cm-2, which is higher than that of batteries with a conventional method treated anode (74.9%), and an untreated (73.3%). Furthermore, the battery with the present scale-up anode shows an energy efficiency of 82.5% and stable operation for 200 cycles at a current density of 200 mA cm-2.

Vanadium redox flow batteries (VRFBs) are one of the most promising technologies for renewable en-ergy storage. However, complex thermal issues caused by excessive heat generation during high-rate op-erations and various heat transfer behaviors in diverse climates dramatically affect the efficiency and stability of VRFBs. In this review, we summarize the thermal issues of VRFBs reported in the litera-ture. First, the fundamental mechanisms of heat generation and heat transfer are elaborated. Thermal effects on electrochemical reactions and critical components in VRFBs are then presented. An operating temperature range of 10 similar to 40 degrees C for VRFBs with high efficiency, weak side reactions, high electrolyte sta-bility, and low crossover is suggested. Furthermore, thermal models including two-dimensional models, three-dimensional models, and lumped models are summarized. Moreover, existing thermal management methods are analyzed. Employing titanium heat exchangers with anti-corrosive properties to adjust the temperature of electrolytes is recommended. Finally, the remaining challenges to enhance the efficiency and stability of VRFBs under harsh thermal conditions are provided. This review offers an in-depth in-sight into the thermal issues of VRFBs, facilitating the design of next-generation VRFBs with high-power density.(c) 2022 Elsevier Ltd. All rights reserved.

Vanadium flow battery(VFB)is one of the most promising energy storage technologies because of its superior safety,reliability and cycle life,but the poor electrochemical performance at high cur-rent density limits its commercial application.Herein,an advanced design of the dual-gradient carbon nanofibers/graphite felt(DG-CNFs/GF)composite electrode is firstly proposed for the next-generation VFB with high power density.Specifically,there is a macro gradient distribution of CNFs along the thickness direction of the electrode,meanwhile a micro gradient distribution of CNFs is also existed along the ra-dial direction of a single fiber,and both the macro and micro gradient structure are verified through the physicochemical characterizations.In addition,the DG-CNFs/GF with a dual-gradient structure exhibits an excellent electrocatalytic activity and a fast mass transfer characteristic.It is worth noting that the energy conversion efficiencies,cycling stability in addition to power density of VFB with DG-CNFs/GF are much better than those with commercial GF,which make the dual-gradient DG-CNFs/GF to be a promis-ing alternative.Most importantly,the accomplishment of this work will provide a promising development direction of the highly efficient electrode for the next-generation VFB with high power density.

The polarization of redox flow batteries (RFBs) consists of activation polarization, ohmic polarization, and concentration polarization. However, the three types of polarizations are coupled in practice, making it difficult to quantify the respective attributions to the total voltage loss and to compare the reported performance of RFBs under different working conditions. Here, we propose a method to operando decouple the polarizations of RFBs based on the different response times of the three kinds of polarizations. The decoupled polarizations in RFBs under different working conditions are presented with specific voltage losses, which clarifies the limit parameters of battery performance and makes the reports of RFBs comparable even with similar battery performances. This work opens up a method to quantitatively analyze activation polarization and concentration polarization separately in RFBs, which provides significant guidance for improving battery performance effectively.

Nafion series membranes are widely used in vanadium redox flow batteries (VRFBs). However, the poor ion selectivity of the membranes to vanadium ions, especially for V2+, results in a rapid capacity decay during cycling. Although tremendous efforts have been made to improve the membrane's ion selectivity, increasing the ion selectivity without sacrificing the proton conductivity is still a challenging issue. In this work, instead of focusing on enhancing the membranes' ion selectivity, we develop an efficient valence regulation strategy to suppress the capacity decay caused by the crossover of V2+ in VRFBs. Despite the discharge capacity of the VRFB with the elevated average valence electrolytes (V3.68+) being slightly lower than that with commercial electrolytes (V3.50+) in the first 35 cycles, the accumulated discharge capacity in 400 cycles is improved by 52.33%. Moreover, this method is efficient, is easy to scale up, and provides deep insights into the capacity decay mechanism of VRFBs.

Flow fields play a crucial role in determining the electrochemical performance and pumping consump-tion of flow batteries. In this work, a three-dimensional model coupling fluid flow, mass transport and electrochemical reactions is developed to numerically optimize the interdigitated flow field for high-performance flow batteries. Key design parameters including the aspect ratio (length to width ratio) and the channel fraction are systematically investigated. It is revealed that large aspect ratios and small chan-nel fractions reduce the stagnant zone within the porous electrodes, leading to superior battery perfor-mance. However, when the aspect ratio exceeds a critical value, the extremely long and narrow structure leads to uneven distribution of reactants along the channel, thereby resulting in remarkable concentra-tion loss. By taking both the electrochemical loss and pumping consumption into consideration, the flow fields with unit widths from 1.5 to 2.5 mm and channel fractions from 0.125 to 0.375 enable the vana-dium redox flow battery to deliver the lowest total power loss. Furthermore, the systematic simulation is also applied to optimize a larger unit with an active area of 540 mm2 for identifying the design principles in the scale-up. It is shown that larger aspect ratios are desirable to alleviate the dramatically increasing pumping loss.(c) 2022 Elsevier Ltd. All rights reserved.

Designing flow fields that can uniformly distribute electrolytes while maintain a low pumping work is chal-lenging for high-performance redox flow batteries, especially for scaled-up battery stacks. In this work, we propose a bifurcate interdigitated flow field, which hierarchically splits the electrolyte and transports it into branch channels, thereby lowering the pumping loss and enhancing the uniformity of electrolyte distribution. The application of the flow field to a vanadium redox flow battery reduces the pressure drop by 45%, thereby increasing the pump-based voltage efficiency from 69.95% to 73.10% compared to that with conventional interdigitated flow field at a flow rate of 2.4 mL min-1 cm-2 and current density of 250 mA cm-2. Furthermore, three-dimensional modeling reveals that the superiority of reducing the pumping loss becomes more extraor-dinary with the enlargement of active area, indicating that the bifurcate interdigitated flow field shows great potential for the scale-up of high-performance flow batteries with a low pumping work.

Polybenzimidazole (PBI) membrane is one of the most promising proton exchange membranes for vanadium redox flow batteries (VRFBs) due to its excellent ion selectivity and stability. However, in this work, we first found the acid-doped PBI membrane conducts both H+ and SO24- in VRFBs. Besides, the capacity decay features of the VRFBs with acid-doped PBI membrane also present the same trend as that of anion exchange membrane, exhibiting an opposite direction of net electrolyte flux after long-term cycling than that of Nafion 212 (N212). Inspired by this finding, we build a VRFB two-cell stack consisting of an acid-doped PBI membrane and an N212 membrane to mitigate capacity decay. This method breaks the trade-off between voltage efficiency and capacity retention rate and significantly increases the accumulated discharge capacity by 119.77% compared to that with 2 pieces of N212 in 1000 cycles. This work deepens the understanding of the capacity decay mechanism of VRFB with different types of membranes and provides a simple yet highly effective strategy for mitigating the capacity decay of VRFB stacks.

To achieve carbon neutrality, integrating intermittent renewable energy sources, such as solar and wind energy, necessitates the use of large-scale energy storage. Among various emerging energy storage technologies, redox flow batteries are particularly promising due to their good safety, scalability, and long cycle life. In order to meet the ever-growing market demand, it is essential to enhance the power density of battery stacks to lower the capital cost. One of the key components that impact the battery performance is the flow field, which is to distribute electrolytes onto electrodes. The design principle of flow fields is to maximize the distribution uniformity of electrolytes at a minimum pumping work. This review provides an overview of the progress and perspectives in flow field design and optimization, with an emphasis on the scale-up process. The methods used to evaluate the performance of flow fields, including both experimental and numerical techniques, are summarized, and the benefits of combining diverse methods are highlighted. The review then investigates the pattern design and structure optimization of serpentine- and interdigitated-based flow fields before discussing challenges and strategies for scaling up these flow fields. Finally, the remaining challenges and the prospects for designing highly efficient flow fields for battery stacks are outlined.

Fabricating fiber‐based electrodes with a large specific surface area while maintaining high flow permeability is a challenging issue in developing high‐performance redox flow batteries. Here, a sponge‐like microfiber carbon electrode is reported with a specific surface area of as large as 853.6 m2 g−1 while maintaining a fiber diameter in the range of 5–7 µm and a macropore size of ≈26.8 µm. The electrode is developed by electrospinning cross‐linked poly(vinyl alcohol)‐lignin‐polytetrafluoroethylene precursors, followed by oxidation and pyrolysis. Applying the as‐synthesized electrodes to a vanadium redox flow battery enables the battery to achieve an energy efficiency of 79.1% at the current density of 400 mA cm−2 and a capacity retention rate of 99.94% over 2000 cycles, representing one of the best battery performances in the open literature. The strategy to fabricate sponge‐like porous carbon microfibers holds great promise for versatile applications in redox flow batteries and other energy storage systems.

Conventionally, electrodes for vanadium redox flow batteries (VRFBs) are characterized in lab-scale single cells. However, electrodes selected in such a manner may not provide the best performance in practical large-scale battery stacks, as the flow rates and electrode compression ratios in large stacks are usually much lower than those in single cells. In this work, we study the essential effects of flow rates and electrode compression ratios on the characterizations and selections of electrodes for VRFBs. It is found that the working conditions-induced performance reversal is the reason that leads to the different results in the optimal electrodes between large-scale battery stacks and lab-scale single cells. Hence, the working condition should be considered when evaluating electrodes for VRFBs with lab-scale single cells, especially under low flow rate and compression ratio. This work provides valuable insight and guidance for electrode evaluation with lab-scale single cells, which promotes lab studies to engineering applications.