Performance Of Redox Flow Battery Research Articles

A Comparative Study of Nafion 212 and Sulfonated Poly(Ether Ether Ketone) Membranes with Different Degrees of Sulfonation on the Performance of Iron-Chromium Redox Flow Battery.

For an iron-chromium redox flow battery (ICRFB), sulfonated poly(ether ether ketone) (SPEEK) membranes with five various degrees of sulfonation (DSs) are studied. To select the SPEEK membrane with the ideal DS for ICRFB applications, the physicochemical characteristics and single-cell performance are taken into consideration. Following all the investigations, it has been determined that the SPEEK membrane, which has a DS of 57% and a thin thickness of 25 μm, is the best option for replacing commercial Nafion 212 in ICRFB. Firstly, it exhibits a better cell performance according to energy efficiency (EE) and coulombic efficiency (CE) at the current density range between 40 mA cm-2 and 80 mA cm-2. Additionally, it has a more stable EE (79.25-81.64%) and lower discharge capacity decay rate (50%) than the Nafion 212 (EE: 76.74-81.45%, discharge capacity decay: 76%) after 50 charge-discharge cycles, which proves its better oxidation stability as well. In addition, the longer self-discharge time during the open-circuit voltage test further demonstrates that this SPEEK membrane could be employed for large-scale ICRFB applications.

Leveraging sulfonated poly(ether ether ketone) for superior performance in zinc iodine redox flow batteries

Sulfonated poly (ether ether ketone) SPEEK membranes with a degree of sulfonation of approximately 73 % were synthesized and investigated as the cation exchange membrane (CEM) for the zinc iodine (ZI) redox flow battery (RFB). Specifically, SPEEK was used in ZI RFB with 1 mol L−1 electrolyte and its performance was compared to the current benchmark CEM, Nafion™. Notably, columbic and energy efficiencies of 92.9 % and 75.9 %, respectively, were measured for the ZI battery with SPEEK at 17 mA cm−2, which was a 370 % increase in energy density compared to those with Nafion 212 membranes. RFBs with SPEEK exbibit a peak power density of 122 mW cm−2 at a current density of 166 mA cm−2 (98 mW/cm2 at 133 mA/cm2 with Nafion 212) because of their lower overpotential and higher discharge voltage. These results demonstrate the high cation selectivity of the SPEEK membranes in neutral, salt-based electrolytes, and provide the groundwork to develop a suitable replacement to Nafion for low-cost RFBs.

Enhancing Vanadium redox flow batteries performance through local compression ratio adjustment using stiffness gradient carbon felt electrodes

Vanadium redox flow batteries (VRFBs) are assembled by compressing the electrodes to reduce the contact resistance between electrodes and bipolar plates and prevent liquid electrolyte leakage. However, an excessive compression ratio can impede the mass transfer and cause high-pressure drops in the electrolyte, ultimately reducing energy and system efficiencies. Accordingly, conventional stacks require a compromised compression ratio of 20 to 30%, resulting in substantial contact resistance and lower cell performance. To address this issue, we proposed a new approach to adjust the local compression ratio of carbon felt electrodes by creating a gradient in compressive stiffness along the through-thickness direction. We achieved this by selectively treating the carbon felt electrode to increase the interaction of carbon fibers in a treated region. This approach deforms the electrode non-uniformly, increasing the electrical contact point between the electrode and bipolar plate in regions with higher deformation and improving the convection and diffusion of ions at the interface between the electrode and membrane in regions with lower deformation, where porosity is relatively high. In a VRFB single-cell test, the electrode with the stiffness gradient demonstrated the highest energy efficiency (86.5%) under a current density of 100 mA∙cm−2, 10% higher than that of neat CF.

Contrasting Miscibility of Ionic Liquid Membranes for Nearly Perfect Proton Selectivity in Aqueous Redox Flow Batteries

AbstractIon‐selective membranes are widely used in various energy storage applications. However, conventional polymer‐based ion‐selective membranes, such as Nafion, are not perfectly ion‐selective, leading to a significant permeation of undesirable ionic species. As the imperfect ion‐selectivity of membranes leads to the failure of energy storage devices, much effort is devoted to improving membrane ion‐selectivity. Herein, an immiscible liquid‐state membrane is introduced for aqueous redox flow batteries. As hydrophobic ionic liquids are immiscible with aqueous catholyte and anolyte solutions, they separate the two without crossover. This property renders them suitable for use as a membrane for aqueous redox flow batteries. In addition, ionic liquids with long side alkyl chains, such as 1‐hexyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM‐TFSI), are miscible with sulfuric acid, whereas transition metal sulfates remain insoluble in HMIM‐TFSI. For this reason, HMIM‐TFSI is selectively permeable to protons and remains impermeable to transition metal cations. As a result, the HMIM‐TFSI membrane is almost perfectly proton‐selective, leading to negligible permeability of unfavorable ionic species, such as transition metal cations. Eventually, the HMIM‐TFSI membrane shows the excellent electrochemical performance of vanadium redox flow batteries, such as negligible self‐discharge over 2800 h, high Coulombic efficiency (≈99%), and stable capacity retention over 100 cycles.

Physics-informed machine learning of redox flow battery based on a two-dimensional unit cell model

In this paper, we present a physics-informed neural network (PINN) approach for predicting the performance of an all-vanadium redox flow battery, with its physics constraints enforced by a two-dimensional (2D) mathematical model. The 2D model, which includes 6 governing equations and 24 boundary conditions, provides a detailed representation of the electrochemical reactions, mass transport and hydrodynamics occurring inside the redox flow battery. To solve the 2D model with the PINN approach, a composite neural network is employed to approximate species concentration and potentials; the input and output are normalized according to prior knowledge of the battery system; the governing equations and boundary conditions are first scaled to an order of magnitude around 1, and then further balanced with a self-weighting method. Our numerical results show that the PINN is able to predict cell voltage correctly, but the prediction of potentials shows a constant-like shift. To fix the shift, the PINN is enhanced by further constrains derived from the current collector boundary. Finally, we show that the enhanced PINN can be even further improved if a small number of labeled data is available.

Modified Membranes for Redox Flow Batteries-A Review.

In this review, the state of the art of modified membranes developed and applied for the improved performance of redox flow batteries (RFBs) is presented and critically discussed. The review begins with an introduction to the energy-storing chemical principles and the potential of using RFBs in the energy transition in industrial and transport-related sectors. Commonly used membrane modification techniques are briefly presented and compared next. The recent progress in applying modified membranes in different RFB chemistries is then critically discussed. The relationship between a given membrane modification strategy, corresponding ex situ properties and their impact on battery performance are outlined. It has been demonstrated that further dedicated studies are necessary in order to develop an optimal modification technique, since a modification generally reduces the crossover of redox-active species but, at the same time, leads to an increase in membrane electrical resistance. The feasibility of using alternative advanced modification methods, similar to those employed in water purification applications, needs yet to be evaluated. Additionally, the long-term stability and durability of the modified membranes during cycling in RFBs still must be investigated. The remaining challenges and potential solutions, as well as promising future perspectives, are finally highlighted.

A 3D modelling study on all vanadium redox flow battery at various operating temperatures

To understand whether the optimization of the operating/electrode structural parameters are temperature dependent, a 3D numerical model is developed and validated to gain insight into the impact of practical operating temperature (273.15 K–323.15 K) on vanadium redox flow battery (VRFB) performance, in which the property parameters are from published experimental data. The operating temperature is found significantly influence the optimal design of VRFBs. Increasing the inlet flow rate and state of charge (SOC), decreasing the electrode porosity and fibre diameter can all improve the battery performance with interdigitated flow channels, and the improvement increases with increasing temperature. In contrast, decreasing the fibre diameter or porosity increases the flow resistance and costs higher pump consumption, which is more pronounced at a lower temperature due to higher electrolyte viscosity. The effect of electrode thickness is also different at various temperatures. The gradient porosity electrode is applied in VRFB with interdigitated flow channels. The electrochemical performance of VRFB with gradient electrode (porosity increases from 0.8 at channel side to 0.93 at membrane side) performs similarly with the VRFB with 0.8 porosity electrode, while the pressure drop is reduced by 40% at all temperature. This model provides a deep understanding of effects of a wide range of working temperature on the optimization of operating/electrode parameters and on the VRFBs’ performance.

Investigating Mass Transfer Relationships in Stereolithography-Based 3D Printed Electrodes for Redox Flow Batteries

Redox flow batteries are a promising option for large-scale energy storage but remain too costly for widespread deployment. One approach to increase cost competitiveness is to improve the power density and efficiency of the electrochemical cell. Porous electrodes determine the performance of the cell, and their three-dimensional structure determines the electrolyte transport, fluid pressure drop, and the available surface area for electrochemical reactions [1]. Conventional porous electrodes are fibrous mats assembled in coherent structures [2]; however, they are repurposed from fuel cell gas diffusion electrodes and have not been tailored for redox flow batteries. Hence, an effective means to improve the performance of redox flow batteries is by engineering porous electrode structures to sustain the requirements of liquid-phase electrochemistry. To this purpose, there is a need to develop new manufacturing techniques affording a higher degree of control over the electrode microstructure and resulting properties. Additive manufacturing, or 3D printing, can be employed to manufacture customized, controlled, and deterministic architectures, enabling the fine-tuning of the electrical and hydraulic performance of porous electrodes [3].In this study, we manufacture model grid structures using stereolithography 3D printing followed by carbonization (Figure 1a) to tune the physiochemical properties of electrodes to be used in redox flow batteries. We employ a suite of microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics to understand the impact of the electrode structure on the mass transport and hydraulic performance of ordered lattice structures in flow cells. Here, the influence of the printing direction, pillar geometry (Figure 1b), and flow field type on mass transport is investigated using an organic electrolyte. We elucidate correlations between the electrode structure and performance metrics, namely surface area, pressure drop, and mass transfer correlations. We find that the printing orientation impacts the electrode performance through a change in electrode morphology caused by resin spreading and surface roughness, affecting the shrinking direction upon carbonization, internal surface area, and thus the charge transfer, mass transfer, and hydraulic resistances. Furthermore, we find that mass transfer rates within the electrode are enhanced by using an interdigitated flow field or by altering the pillar shape to a helical or triangular design, which we hypothesize improves mixing. Compared to commercial carbon-fiber electrodes, the pressure drop is significantly reduced (Figure 1c) as expected due to larger pore sizes (~500 μm for the grids vs. 2-100 μm for the Freudenberg H23 paper electrode and 2-300 μm for the ELAT Cloth electrode). Even though the commercial electrodes feature a superior internal surface area compared to the 3D printed electrodes, their area normalized mass transfer coefficients are lower (Figure 1d). Going forward, the use of printing technologies enabling finer features in combination with carbonization at elevated temperatures can be used to manufacture multiscale electrodes simultaneously providing low hydraulic resistance and excellent electrochemical performance. Additive manufacturing in combination with emerging computational approaches in topology optimization [4] might enable the bottom-up design and manufacturing of advanced electrode materials.

Optimizing the Electrolyte Mixing in Industrial Tanks of Vanadium Redox Flow Batteries

Redox flow batteries (RFB) are an emerging technology for large-scale grid energy storage. Different approaches have been adopted to investigate the operation of RFBs, such as experimental testing and mathematical modelling of the electrochemical cells [1]. However, the effect of the flow and mixing in the electrolyte tanks on the performance of RFBs has been largely overlooked and only recently has started to receive some attention [2]. In a recent work, the authors used numerical simulations to show how the fluid dynamics of the electrolytes within small lab-scale tanks can lead to different flow behaviors depending on whether forced or natural convection is dominant. The irregular transient mixing of the electrolyte in the tanks modifies both the cell potential and battery capacity with respect to those predicted under the widely used perfect mixing assumption (i.e., continuous stirred-tank reactor) [3]. However, this first attempt relied on several simplifying assumptions, such as two-dimensional geometries under small-cell (i.e., laminar) flow conditions, and thus provided only a qualitatively understanding of the flow in the tanks.This work exploits the idea of using CFD simulations to investigate the effect of operating conditions on the electrolyte flow in the tanks and its impact on battery response, focusing on high capacity industrial tanks with high (i.e., turbulent) flow rates and less restrictive simplifying assumptions. Our numerical simulations show how turbulence improves mixing, but also produces quasi-steady flows where a turbulent jet crosses the tank leaving dead electrolyte regions. Different tank solutions are presented aiming to improve the mixing and increase the capacity of the battery, playing both with the tank geometry and the operating conditions. Results on the charging and discharging operations, cell potential and electrolyte mixing index are presented and discussed. Acknowledgments This work has been partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación Projects PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033, and by Grant IND2019/AMB-17273 of the Comunidad de Madrid.

Investigating Transport through Membranes in Non-Aqueous Redox Flow Batteries with Neutron Radiography

Electrochemical technologies offer a strong platform to decarbonize and electrify our energy economy and thus are poised to play an increasingly important role. Nevertheless, all electrochemical technologies suffer from thermodynamic, kinetic, and other operational losses that can drastically limit the performance of the device. Current macroscopic characterization techniques used to assess those losses (i.e., polarization, electrochemical impedance spectroscopy) fail to assess local inefficiencies challenging complete understanding of the system being investigated. Operando imaging of electrochemical systems, in tandem with electrochemical diagnostics offers a viable option to resolve local information and correlate it with macroscopic electrochemical performance. This dual characterization strategy has been successfully applied for the development of other electrochemical technologies (i.e., polymer electrolyte fuel cells[1,2], lithium-ion batteries[3,4]). In the past years, several groups developed and investigated imaging and spectroscopic techniques for operando characterization of redox flow batteries (RFBs) such as fluorescence microscopy[5], nuclear magnetic resonance spectroscopy[6,7] and infrared thermography[8]. Only recently, Clement et al. reported for the first time the use of neutron radiography to perform through-plane imaging of the gas evolution during operation of an all-vanadium RFB[9]. However, an approach enabling non-invasive operando concentration mapping of the species of interest within the reactor area has remained elusive.In this work, we develop two novel neutron radiography operando methodologies to quantify concentration distributions in RFBs. We first perform cuvette experiments to calibrate the neutron attenuation response of the redox active materials (i.e., TEMPO and TEMPO+) and supporting salts (i.e., BF4 -) used in this study. We find a linear response between the neutron absorption and the concentrations of the species of interest in the electrolyte in the studied range (0.0 to 0.5 M). Next, we performed experiments with redox flow cells under operation and use the ex-situ calibrations together with subtractive neutron imaging to quantify species concentration in the electrolyte. By employing an in-plane optical imaging setup[10], we can resolve species concentration across the porous electrodes (Figure 1). We observe strong variations in the concentration profiles within the reactor area upon changes in the operating conditions and link those differences to material properties (i.e., membrane electrostatic charge). Finally, I will discuss our latest results obtained using time-of-flight neutron imaging to deconvolute concentration maps for TEMPO/TEMPO+ and BF4 - species. Using this methodology, we study mass transport across a porous separator and a dense anion exchange membrane. We find that, while TEMPO/TEMPO+ and BF4 - show almost symmetrical concentration changes in the case of a porous separator, asymmetric transport occurs when a charge selective membrane is used where only BF4 - is allowed to crossover the anion exchange polymer. The developed methodology offers a non-invasive diagnostic tool to image concentration distributions in electrochemical cells and can be applied to support theoretical and experimental efforts aiming to understand the role of materials and reactor designs on complex mass transport phenomena.[1] P. Boillat, E. H. Lehmann, P. Trtik, M. Cochet, Current Opinion in Electrochemistry 2017, 5, 3.[2] J. Eller, T. Rosén, F. Marone, M. Stampanoni, A. Wokaun, F. N. Büchi, J. Electrochem. Soc. 2011, 158, B963.[3] B. Michalak, H. Sommer, D. Mannes, A. Kaestner, T. Brezesinski, J. Janek, Sci Rep 2015, 5, 15627.[4] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett, P. R. Shearing, Nat Commun 2015, 6, 6924.[5] A. A. Wong, M. J. Aziz, S. Rubinstein, ECS Trans. 2017, 77, 153.[6] E. W. Zhao, T. Liu, E. Jónsson, J. Lee, I. Temprano, R. B. Jethwa, A. Wang, H. Smith, J. Carretero-González, Q. Song, C. P. Grey, Nature 2020, 579, 224.[7] E. W. Zhao, E. Jónsson, R. B. Jethwa, D. Hey, D. Lyu, A. Brookfield, P. A. A. Klusener, D. Collison, C. P. Grey, J. Am. Chem. Soc. 2021, 143, 1885.[8] H. Tanaka, Y. Miyafuji, J. Fukushima, T. Tayama, T. Sugita, M. Takezawa, T. Muta, Journal of Energy Storage 2018, 19, 67.[9] J. T. Clement, Investigation of Localized Performance and Gas Evolution in All-Vanadium Redox Flow Batteries via in-Situ Distributed Diagnostic Techniques, Ph.D. thesis, University of Tennessee, 2016.[10] P. Boillat, G. Frei, E. H. Lehmann, G. G. Scherer, A. Wokaun, Electrochem. Solid-State Lett. 2010, 13, B25. Figure 1

(Invited) Advanced Redox Flow Batteries: Process and Design Innovation Opportunities

Large-scale electric-energy-storage (EES) systems have the potential to transform the electric grid, especially long-duration energy storage (LDES) solutions [1]. However, the economic requirements for LDES systems are more challenging than those for EES applications with shorter discharge durations, since the value proposition for most LDES applications tends to be significantly lower on a normalized basis (i.e., $/kWh of EES capacity). EES systems with independent power and energy components, e.g., pumped-storage hydro (PSH), redox flow batteries (RFBs), and reversible fuel cells (RFCs) have a major inherent cost advantage, since one can readily increase the energy/power ratio by selectively adding more energy capacity, without the added cost of excessive power components. RFBs also have other unique attributes that make this battery architecture ideally suited for grid-scale LDES, namely: long cycle life even with deep cycles, inherently superior safety [2], good round-trip energy efficiency (i.e., compared to RFCs), and simplified recyclability due to the separated components. A rigorous techno-economic analysis (TEA) model has shown that RFBs can enable LDES systems with lower capital cost than other types of batteries at comparable production volumes [3]. However, production volumes of RFBs are currently substantially lower than some other types of batteries, especially lithium-ion batteries. This same TEA model also projects that the capital cost of all-vanadium RFBs (VRFBs) at low production volumes is > $500/kWh. Therefore, what is needed to accelerate the commercialization of VRFBs are innovations that can potentially improve capital costs at low production volumes [4]. This invited talk will focus on some potentially near-term possibilities; namely, those that do not necessarily require new materials. This includes: 1) high performance RFB cells [5], 2) a variety of advanced RFB manufacturing processes, including innovative electrolyte-purification [6] and electrolyte-shipping processes [7], 3) alternative state-of-charge (SOC) measurement methods [8], 4) simple RFB performance diagnostics [9], 5) novel RFB system architectures (e.g., [10]), and 6) business-model innovations [11]. References P. Denholm, et.al., “Storage Futures Study, The Four Phases of Storage Deployment” National Renewable Energy Laboratory, NREL/TP-6A20-77480 (2021).R. Wittman, M. L. Perry, et.al, “Perspective: On the Need for Reliability and Safety of Grid-Scale Aqueous Batteries,” JES, 167 (2020). DOI: 10.1149/1945-7111/ab9406R. M. Darling, et.al., “Pathways to Low-Cost Electrochemical Energy Storage,” Energy & Environ. Science (2014). DOI: 10.1039/c4ee02158dM. L. Perry, “The Pathway to Widespread Commercialization of Redox Flow Batteries,” A03-0475 (Invited), 241st ECS Meeting (2022).M. L. Perry, et.al., “High Power Density Redox Flow Battery Cells,” ECS Transactions, 53 (2013). DOI: 10.1149/05307.0007ecstC. Tai-Chieh Wan, K. E. Rodby, M. L. Perry, Y.-M. Chiang, F. R. Brushett, “Hydrogen evolution mitigation in iron-chromium redox flow batteries via electrochemical purification of the electrolyte,” J. of Power Sources, 554 (2023). DOI: 10.1016/j.jpowsour.2022.232248T. V. Nguyen, Y. Li, and M. L. Perry, “Densification, Precipitation, and Dissolution of Vanadium Electrolyte for Vanadium Redox Flow Battery Systems,” Submitted for 2023 ECS Meeting, Symposia A03.E. J. Alexandrescu and M. L. Perry, “Measuring Electrochemical Potentials for Health Monitoring of RFBs,” Submitted for 2023 ECS Meeting, Symposia A03.M. L. Perry, “Development of a Simple and Rapid Diagnostic for Redox Flow-Battery Cells,” Symposium I02, 242nd ECS Meeting, (2022).M. L. Perry, “Flow battery with two-phase storage,” U.S. Patent 9,350,039 (2016).Largo Physical Vanadium, Investor Presentation, lpvanadium.com (2022).

Effect of electrolyte circulation rate in flow-through mode on the performance of vanadium redox flow battery

Large-size redox flow battery stacks require flow channels for uniform flow circulation of electrolyte over the electrode without incurring too high a pressure drop penalty. In the present work, the use of thin graphite sheets in a flow-through mode, i.e., with the electrolyte being pumped directly into the electrode is explored. In order to reduce the pressure drop when such designs are scaled to large size cells, use of relatively thick polypropylene frames to hold carbon felt electrodes is studied. Experiments have been conducted in cells of nominal electrode areas of 120 and 440 cm2 having a 0.6 mm thick graphite sheet as the bipolar plate. Pressure drop and electrochemical characterization studies show that sufficient convection velocities of electrolyte in the carbon felt, in the range of 3–5 mm s−1, are needed to obtain high round-trip energy efficiency as well as discharge energy density. This observation is supported by estimates of mass transport limited current density for the cell. Under these conditions, use of thin graphite sheets yields a weight reduction of over 75% when compared to cells with thick graphite plates while giving comparable hydrodynamic and electrochemical performance.

Cost, Performance, and Sustainability of Redox Flow Batteries Using 4,5-Dihydroxy-1,3-benzenedisulfonic Acid and Vanadium Enhanced by Cross Compensation of the Activation Process

Cost, Performance, and Sustainability of Redox Flow Batteries Using 4,5-Dihydroxy-1,3-benzenedisulfonic Acid and Vanadium Enhanced by Cross Compensation of the Activation Process

Investigating Mass Transfer Relationships in Stereolithography 3D Printed Electrodes for Redox Flow Batteries

AbstractPorous electrodes govern the electrochemical performance and pumping requirements in redox flow batteries, yet conventional carbon‐fiber‐based porous electrodes have not been tailored to sustain the requirements of liquid‐phase electrochemistry. 3D printing is an effective approach to manufacturing deterministic architectures, enabling the tuning of electrochemical performance and pressure drop. In this work, model grid structures are manufactured with stereolithography 3D printing followed by carbonization and tested as flow battery electrode materials. Microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics are employed to investigate the resulting electrode properties, mass transport, and pressure drop of ordered lattice structures. The influence of the printing direction, pillar geometry, and flow field type on the cell performance is investigated and mass transfer vs. electrode structure correlations are elucidated. It is found that the printing direction impacts the electrode performance through a change in morphology, resulting in enhanced performance for diagonally printed electrodes. Furthermore, mass transfer rates within the electrode are improved by helical or triangular pillar shapes or by using interdigitated flow field designs. This study shows the potential of stereolithography 3D printing to manufacture customized electrode scaffolds, which could enable multiscale structures with superior electrochemical performance and low pumping losses.

Enhancement in vanadium redox flow battery performance using reduced graphene oxide nanofluid electrolyte

Low energy storage performance of aqueous flow batteries is the main limitation in commercialization and worldwide implementation. The effect of nanofluids on the electrochemical behaviour of electrolytes to alleviate this problem has been rarely studied in contrast with the extensive heat-transfer-related literature on nanofluids from the field of thermal science. Therefore, this study aims to experimentally investigate the long-term effect of using reduced graphene oxide (rGO) nanofluidic electrolyte on vanadium redox flow battery performance, placing it amongst the very few studies on the effectiveness of nanofluids in flow batteries. Scanning Electron Microscopy (SEM), EDS elemental mapping, X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) characterizations reveal changes in morphology, structural and elemental properties of the carbon felt electrodes due to the presence of suspended nano catalysts in the electrolyte. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) exhibited significant improvements of 36 % and 26 % in oxidation/reduction peaks, ~ 95 % of reduction in charge transfer resistance, and enhanced redox reactions reversibility of the optimum concentration of nanofluidic electrolyte (0.1 wt%). Moreover, active surface area of the electrodes were determined to be increased by ~130 % and ~ 135 % for positive and negative 1.0 wt% nanofluid-affected electrode samples compared to pure electrolyte-affected electrode. In addition, considering the optimum nanoparticle concentration of 0.1 wt%, charge, and discharge capacities of the flow battery outperformed where pure electrolyte was applied in different current densities. These enhancements show considerable increase in energy storage of the VRFB. Furthermore, energy efficiency and voltage efficiency of the flow cell were observed to be increased respectively by 15 % to 24 % and 8 % to ~21 % for the current densities from 25 to 150 mAcm−2 when the optimum concentration of nanoparticles was applied. This study gives new insight into the rarely studied applicability of nanofluids in vanadium flow batteries.

Enhancement of vanadium redox flow battery performance with nitrogen-functionalized graphite felt electrodes etched by K2FeO4

Enhancement of vanadium redox flow battery performance with nitrogen-functionalized graphite felt electrodes etched by K2FeO4

Effect of variable cross-section electrode on the battery performance of all-vanadium redox flow battery

The global warming can be relieved by the application of the renewable energy, which should be supported by the large-scale energy storage such as the vanadium redox flow battery (VRFB). The charge-discharge reactions mainly take place in the porous electrodes, which can influence the battery performance. In the paper, a variable cross-section electrode with interdigital flow field was proposed to enhance the mass transfer in the porous electrode. The charge-discharge voltage, overpotential, concentration distribution and uniformity factors are predicted by a 3-D numerical model to evaluate the battery performance of different electrodes (the Conventional electrode; Variable coss-section electrode #1: the variable coss-section electrode with the interdigital flow field distributed on one side; Variable coss-section electrode #2: the variable coss-section electrode with the interdigital flow field distributed on both sides). The discharging voltage of the variable coss-section electrode #2 was 1.25% higher than that of the variable coss-section electrode #1. The uniformity factor of the variable coss-section electrode #1 and the variable coss-section electrode #2 was 14.24% and 19.56% higher than that of the conventional electrode, respectively. Considering the machining process and promotion of the battery performance, the variable coss-section electrode#1 was the best design of the three battery models mentioned in this article.

Influence of strong bromine binding complexing agent in electrolytes on the performance of hydrogen/bromine redox flow batteries

1-n-Hexylpyridin-1-ium bromide [C6Py]Br is investigated in this work as bromine complexing agent (BCA) in aqueous bromine electrolytes on its influence on hydrogen bromine redox flow battery (H2/Br2-RFB) performance. [C6Py]+-cations bind bromine of aqueous polybromide solutions safely in an additional fused salt phase limiting the vapor pressure of Br2. Dissolved in aqueous electrolyte solutions, however, [BCA]+ cations drastically lower PFSA membranes' conductivity in the H2/Br2-RFB. In this work the combination of the very strong bromine-binding [C6Py]+cation and an excess of bromine in the electrolyte lead to an almost complete absorption of 99.6 mol% [C6Py]+ into the fused salt within the electrolyte's operation range. In comparison to similar application of short side chain 1-ethylpyridinium bromide, adverse effects are stronger compensated by use of [C6Py]Br. Increases in membrane resistance of the membrane in contact with [C6Py]+ cations are reversible in contrast for contact with [C2Py]+ cations. Electrolytes are cycled for at least 12 cycles and offer a stable useable capacity of 176.7 Ah L−1 and reach and stable discharge energy densities of 138.91 Wh L−1. Strong bromine binding properties between [C6Py]+ cations and polybromides are confirmed by DFT studies. In contrast to [C2Py]+ electrolytes, the long-chain [C6Py]+ cation in electrolytes has a higher cycling stability or a low decrease in capacity and energy density.

Engineering Redox Flow Battery Electrodes with Spatially Varying Porosity Using Non‐Solvent‐Induced Phase Separation

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction‐limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non‐solvent‐induced phase‐separation (NIPS) technique that enables the reproducible synthesis of macrovoid‐free electrodes with well‐defined across‐thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity‐gradient electrodes are compared with macrovoid‐containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity‐gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid‐containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity‐gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems.

A hybrid analytical and numerical model for cross-over and performance decay in a unit cell vanadium redox flow battery

Developing an accurate and efficient model for cross-over is critical for improving the long-term performance of redox flow batteries (RFBs). This work presents a hybrid analytical and numerical model that combines a two-dimensional analytical solution to the active species, a one-dimensional analytical model for cross-over mechanisms, and a zero-dimensional numerical model for outlet concentrations of reactants. By comparing with experiment over 41 cycles (ca. 144 h), the model reported a mean voltage difference of 0.0089 V, mean time difference of 14 s per cycle (of 3.5 h), maximum relative difference for capacity and energy of 1.34% and 1.63%, respectively. The predicted mean concentrations for V2+, V3+, and VO2+ in membrane are 65% ∼ 77% of measured values from the literature. Upon validation, the model reproduced behaviors in electrolyte imbalance similar to those observed in experiments and numerical models, and revealed the control of cross-over, self-discharge, stoichiometry of side reactions, and Coulombic efficiency on electrolyte imbalance. The model also demonstrates excellent computational efficiency for simulating 41 cycles (around 37,000 points) within 3 ∼ 4 s. The demonstrated efficiency and accuracy for predicting cross-over and its impacts on voltage, capacity&energy decay, membrane concentrations, and electrolyte imbalance makes it a reliable tool for optimizing RFBs’ long-term performance.