Flow Battery Research Articles

Experimental Set‐Up for Measurement of Half‐Cell‐ and Over‐Potentials of Flow Batteries during Operation

The study of flow batteries (FBs) requires the development of tools able to evaluate their performance during operation in a reliable and simple way. In this work, we present an experimental set‐up that allows the on‐line monitoring of the half‐cells state of charge and apparent overpotentials on the positive and negative electrodes during battery operation. These measurements are feasible by using additional flow cells that include a reference electrode on each side. We used the experimental set‐up to study the performance of the vanadium system as well as a previously reported stable organic couple. The studies consisted on short cycling operation at different current densities and polarization curves at different flow rates and states of charge. By confirming previous results obtained for vanadium‐FBs and extending the analysis to further systems, we demonstrated that this approach provides a reliable deeper insight into the battery performance and the processes taking place during operation.

Comparative Analysis of Thermal Activation on Felts and Continuous Carbon Filament Electrodes for Vanadium Redox Flow Batteries

AbstractThermal treatments are commonly used to improve electrode kinetics in vanadium redox flow batteries (VRFB). The impact of the widely adopted thermal treatment—400 °C for least 24 hours—was investigated on polyacrylonitrile (PAN)‐based continuous carbon filaments (tows) and compared to PAN‐based graphite felts. Surface properties were assessed with scanning electron microscopy (SEM), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS) and wettability measurements. The electrode activity was investigated via electrochemical impedance spectroscopy (EIS). Charge‐transfer resistances and the constant phase element parameters related to the electric double layer were determined, revealing a correlation between enhanced electrode activity and increased double layer across all electrodes. An 8‐hour 400 °C thermal treatment was sufficient to improve electrode activity for tows, whereas felts required longer durations, up to 24 hours, attributed to differences in the carbonization process employed for each material, with the tows undergoing continuous processing and the felts following a batch process. Three‐electrode half‐cell EIS measurements were conducted to elucidate positive and negative electrode contributions. Activated continuous carbon filament electrodes exhibited consistent electrode activities in both the catholyte (VO2+/VO2+) and anolyte (V3+/V2+), whereas the electrochemical activity of felts was limited by the electrode deactivation in the anolyte.

Development of new cathode materials for all-iron redox flow batteries using hard carbon-based composites containing niobium pentoxide

We report in this study new cathodes for all-Fe RFBs synthesized using carbon flakes dispersed in plasticizing and hardening agents to obtain hard and dense carbon (HC) materials in the absence and presence (HC-Nb) of niobium pentoxide (Nb2O5). These cathode materials exhibited excellent resistance to wear in acidic solutions and larger potential intervals for water stability (e.g., 1200–1300 mV) compared to conventional graphite (e.g., 800 mV). These characteristics decreased the parasitic occurrence of hydrogen evolution reaction (HER) using a laboratory-made battery cell during the charging process. The latter was based on the Fe0/Fe2+ and Fe2+/Fe3+ redox couples present in the anode and cathode compartments, respectively. The different ex-situ and in-situ characterization studies evidenced that the presence of Nb2O5 in HCs substantially changed their physicochemical properties. The highest heterogenous kinetic rate constant (k0) representing the electrocatalytic activity for electron transfer was verified for the cathode containing 10 wt.% Nb2O5. An electromotive force (EMF) of 1.053 V was verified for the fully charged all-Fe RFB. Galvanostatic charge–discharge (GCD) studies revealed excellent coulombic efficiency (> 88 %) after 300 cycles. A pH control by acid addition as a function of the battery operation is necessary to avoid iron hydroxide formation on the anode’s surface.

Accelerating the Reduction Kinetics of V4+ to V3+ on Atomically Fe─N4 Decorated Carbon Nanotubes for Vanadium Electrolyte Preparation.

The high manufacturing cost of vanadium electrolytes is caused by the sluggish kinetics of V4+ to V3+, which restricts the commercialization of all vanadium flow batteries (VFBs). Here, density functional theory calculations first reveal the detailed reaction pathway and point out the rate-determined step by the desorption of the end product [V(H2O)6]3+. Catalytic site engineering at the molecular level can optimize the adsorption energy of [V(H2O)6]3+ to boost the kinetics. Furthermore, iron single-atoms embedded nitrogen-doped carbon nanotubes (FeSA/NCNT) are designed to decrease the adsorption energy of [V(H2O)6]3+. The reaction rate constant of FeSA/NCNT toward V4+ to V3+ is 1.62 × 10-7cm s-1, 37.5 times that of the commercial carbon catalyst. Therefore, the energy consumption is reduced by 22.5%. Meanwhile, the prepared vanadium electrolyte is of high quality with the ideal oxidation state of + 3.5 without impurities. This work reveals the catalytic mechanism of V4+ to V3+ and proposes a simple but practical strategy to reduce the preparation cost of V3.5+ electrolyte.

Air-Stable Membrane-Free Magnesium Redox Flow Batteries.

Membrane-free biphasic self-stratified batteries (MBSBs) utilizing aqueous/nonaqueous electrolyte systems have garnered significant attention owing to their flexible manufacturing and cost-effectiveness. In this study, we present an ultrastable high-voltage Mg MBSB based on an aqueous/nonaqueous electrolyte system. The engineered aqueous electrolyte had a wide electrochemical stability window of 3.24 V. The Mg metal anode features a Mg2+-conductive protective coating. Two metal-free redox compounds, 2,2,6,6-tetramethylpiperdinyl oxy (TEMPO) and N-propyl phenothiazine (C3-PTZ), were used as catholytes. The Mg||TEMPO and Mg||C3-PTZ MBSBs exhibited high cell voltages of 2.07 and 2.12 V, respectively, and were studied under static, stirred, and flow conditions. The Mg MBSBs were initially evaluated at different catholyte concentrations (0.1, 0.3, and 0.5 M) under static conditions. Notably, the Mg||TEMPO (0.5 M) and Mg||C3-PTZ (0.5 M) static batteries maintained exceptional performances over 500 cycles at 8 mA/cm2, with capacity retention rates of 97.84% and 98.87%, Coulombic efficiencies of 99.17% and 99.12%, and capacity utilization of 70.2% and 71.3%, respectively. Under stirred and flow conditions, the Mg||TEMPO (0.5 M) and Mg||C3-PTZ (0.5 M) batteries cycled 500 times at 12 mA/cm2 demonstrated capacity retention rates of 99.82% and 99.88% (stirred), 93.58% and 92.16% (flow), respectively. Under flow conditions, the Mg||TEMPO (0.5 M) and Mg||C3-PTZ (0.5 M) batteries demonstrated power densities of 195 and 191 mW/cm2, respectively, surpassing those of 139 and 144 mW/cm2 under static conditions. These cost-effective Mg MBSBs exhibit remarkable performance and advance the application of Mg chemistry in organic flow batteries.

Tuning the composition of mixed anthraquinone derivatives towards an affordable flow battery negolyte

Having a long lifespan and being capable of scaling capacity and power independently, redox flow batteries (RFB) offer great opportunities for energy storage. However, the challenge lies in finding an ideal electrolyte. The most mature version of RFB utilizes vanadium solutions and suffers from rising and highly volatile prices of this metal. To address this, organic electrolytes are gaining attention, as they can be obtained from abundant feedstocks. Among those, Anthraquinone-2,7-disulfonic acid (2,7-AQDS) solutions are particularly prominent, demonstrating reversible and fast redox kinetics coupled with reasonable solubility. This paper explores the possibility of synthesizing 2,7-AQDS together with other electroactive compounds (2,6-AQDS, 2-AQS) through the reaction of anthraquinone sulfonation. It shows that obtained mixtures act as electrolytes without any purification or separation, while synthesis conditions can adjust mixture composition and hence their redox behavior. Although the performance of anthraquinone-bromine RFB utilizing these mixtures exhibits a trade-off between power and stability, the best of them are comparable or even superior to 2,7-AQDS. For instance, RFB with a mixture free of 2-AQS demonstrates an energy efficiency of 76.4 % and a capacity fade rate of 0.04 %/cycle at a current density of 75 mA cm−2. The specific capacity of such mixtures can reach 70 Ah L−1, which makes them promising and affordable RFB negolyte.

Engineering potential electrocatalysts for both AEM Electrolyzers and Redox Flow Batteries: Design of experiments at the different scales

Both water electrolysis and electrochemical energy storage are promising technologies for dealing with the intermittency of green electricity generation. In this work we investigated the possibility of using the same electro-catalyst for hydrogen production by Anion Exchange Membrane Water Electrolysis (AEMWE) and energy storage by Redox Flow Batteries (RFB). A complete experimental campaign was designed to investigate the effects of the catalyst nanostructure and morphology on the electrode performance at different scales. Focus was devoted to the choice of the main factors influencing the catalyst structure, its morphology and electrochemical activity; taking further into account the technique to deposit the catalyst on the Gas Diffusion Layer (GDL) and its integration inside the Membrane Electrode Assembly (MEA). Some results are reported for a spinel-based NiCo2O4 catalyst, chosen as a base nanocomposite to develop more complex materials for both the Oxygen Evolution Reaction (OER) in AEMWE and the positive electrode in Vanadium RFB in the future.

High‐performance Porous Electrodes for Flow Batteries: Improvements of Specific Surface Areas and Reaction Kinetics

AbstractElectrodes, 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.

Enhancement in the performance of a vanadium-manganese redox flow battery using electrospun carbon metal-based electrode catalysts

This study investigates the performance of both a vanadium/manganese redox flow battery (V/Mn RFB) and an all-vanadium redox flow battery (VRFB), employing carbon metal fabrics (CMFs) prepared through electrospinning followed by carbonization. Noteworthy advancements are observed in both systems upon coupling CMFs with thermally treated graphite felt (GF) electrodes. Nearly doubled peak power density and 50 % higher capacity utilization over 150 charge/discharge cycles at 75 mA cm−2 are achieved for the V/Mn RFB with the incorporation of CMFs alongside graphite felt as catalysts. The VRFB demonstrates notable enhancements too, achieving approximately 200 cycles at a current density of 80 mA cm−2, with high efficiencies (85 %) and electrolyte utilization (79 %) when CMFs are used in combination with graphite felts. These advancements may facilitate pilot-scale testing and integration of the V/Mn RFB for the employment in the renewable energy storage sector and grid-balancing studies.

State of Charge (SoC) of the Vanadium and Other Redox Flow Batteries: Identification of the Electrode and Bipolar Plate Contributions

In this paper attention is directed toward one of the key measures of performance, State of Charge (SoC) for vanadium and other battery chemistries. Explored are its simple definition, as well as a more complete examination of just what is meant by this terminology. Developed is a more sophisticated exposition of how to calculate it properly, including spatial distributions, and what consequences arise from various simplifications. Open Circuit voltage through the Nernst equation, is related to the SoC. Non-equilibrium overpotential is also related to the concentrations when treated as averaged field variables. Other chemistries examined include iron-chromium, all-iron, and hydrogen-bromine. For the first time, a rigorous decomposition of the spatial variation of the electrode and bipolar plate contributions is identified, allowing new formulas to be obtained. This may allow optimization of redox flow battery performance. Finally, an expression is found for the open circuit voltage cost going between no bipolar plates and their incorporation into the redox flow battery.

Novel flow field design for redox flow battery using algorithmic channel generation and self-adaptive network model and experimental verification

Flow fields play a vital role in redox flow batteries (RFBs) to increase the power density. Traditional design for flow field usually employs CFD simulations with substantial computational costs and adopts enumeration and trial-and-error methods based on designers’ subjectivity, which have become a bottleneck in flow field design. In this study, a novel research paradigm is employed for flow field design in vanadium redox flow battery (VRFB), i.e., an adaptive three-dimensional equivalent network model is developed and a self-developed flow channel generation algorithm is employed to obtain optimum flow field through comparison and screening. Firstly, a flow channel library consisting of 18,135 flow fields is generated by self-developed flow channel generation algorithm. Secondly, the electrolyte flow, species transport and charge transport with various flow fields is researched by the self-adaptive equivalent network model with low computational costs due to eliminated meshing. Thirdly, pressure drop and voltage efficiency are employed to evaluate different flow fields for the aim of obtaining high-performance flow field structures through comparison and screening. The obtained high-performance flow field is fabricated and its high performance is confirmed by comprehensive experimental validation. The energy efficiency of the high-performance flow field is 4 % higher than that of the serpentine flow field investigated at 100 mA cm−2. This work employs adaptive equivalent network model to generate and optimize flow channel for the first time, which provides a reference for flow channel selection and design.

Untangling dendrite growth dynamics in hybrid flow batteries

Despite advancements, dendrite growth at the anode continues to be a persistent roadblock in accelerating the widespread deployment of hybrid flow batteries as the next-generation energy storage solution, due to the significant impact of dendrites on cycling performance and the potential for battery failure. The ability to analyze energy storage systems at micro-to-macro levels offers unprecedented insights into their behavior and performance. Herein, we develop a multiscale model utilizing phase-field method to investigate dendrite formation, growth, and stripping under operational conditions. The Zn-I system is employed to unravel the intricacies of dendrite evolution and its mitigation through strategic utilization of critical battery parameters. Our findings not only uncover precise zinc morphologies but also provide valuable insights into battery performance toward developing a strategy for mitigating dendrite growth and enhancing battery efficiency at high current density. To our knowledge, this is the first work to comprehensively untangle electrodeposition dynamics at multiscale in the field of flow battery and related research. The findings revolutionize our understanding of deposition behavior, driving transformative advancements in hybrid flow battery design and development, with potential applicability to other battery systems.

Meta-substituted thienoviologen with enhanced radical stability via π-π interaction modulation for neutral aqueous organic flow batteries

The aqueous organic redox flow batteries (AORFBs) are recognized as the most promising large-scale storage technology for long-duration energy storage (LDES). Although viologen derivatives are widely used as anolyte materials for AORFBs, their practical application is strongly limited by weak conjugation and unstable radicals. Here, we present a novel thienoviologen derivative, [(NPr)2SV]Cl4, achieved utilizing a meta-substitution approach based on the pristine viologen derivatives ( [(NPr)2V]Cl4). Compared to other ortho- and para-substitution methods, this strategy features a locked plane configuration (0.14°), effectively regulating π-π interactions and suppressing reactivity between radical and oxygen, which is confirmed via X-ray single-crystal structural analyses, spectroscopy techniques, molecular dynamics simulation accompanied by linear ion trap mass spectrometry experiments. Additionally, the meta-substituted [(NPr)2SV]Cl4 significantly improves water solubility (2.39 M), enhances aromaticity, and extends radical lifetime compared with para-substituted [(NPr)2TV]Cl4. Consequently, the cycling stability of the [(NPr)2SV]Cl4-based AORFB over 2500 cycles is 4.1 times higher than that of [(NPr)2TV]Cl4. Furthermore, the 0.5 M battery delivers an impressive 99.83% capacity retention during 200 cycles and a power density of 147 mW cm-2.

Low-cost and durable polyvinyl alcohol modified polyethylene separator for vanadium redox flow battery

Porous separators are considered a viable alternative to the high cost Nafion membrane for vanadium redox flow battery (VRFB) commercialization. However, the porous separators suffer from high vanadium ion crossover due to the presence of large micron size pores, which leads to decay in capacity of the VRFB system. To tackle the same, a composite separator is synthesized wherein a cross-linked polyvinyl alcohol (PVA) layer is coated on to the porous polyethylene silica separator (PE separator) to mitigate the vanadium ion crossover rate and employed in the VRFB application. Scanning electron microscopy (SEM) analysis confirmed the successful coating of the PVA film on to the PE separator. Synthesized PE-PVA separator showed a significant decrease in the vanadium ion crossover by 31.44 times compared to the pristine PE separator. Consequently, the self-discharge time increases by 8 and 1.66 times compared to the pristine PE separator and the Nafion117 membrane, respectively. VRFB cell equipped with the synthesized separator showed better capacity retention with a capacity fade rate of 0.2 % Ah·cycle−1 than the Nafion117 membrane (0.5 % Ah·cycle−1). In-situ and ex-situ oxidative stability of the separator is explored in detail and a degradation mechanism of the cross-linked PVA membrane is proposed accordingly. The synthesized single side coated PE-PVA´ separator exhibits higher coulombic efficiency of 99 % and comparable energy efficiency of 71 % at a current density of 120 mA·cm−2 and demonstrated excellent lifetime durability tested up to 1600 charge-discharge cycles at 80 mA·cm−2.

Mesoporous graphite felt electrode prepared via thermal oxidative etching on all-vanadium redox flow batteries

Mesoporous graphite felt electrode prepared via thermal oxidative etching on all-vanadium redox flow batteries

Heat Management of Redox Flow Batteries and Its Progress

Heat Management of Redox Flow Batteries and Its Progress

Directional regulation on single-molecule redox-targeting reaction in neutral zinc-iron flow batteries

Directional regulation on single-molecule redox-targeting reaction in neutral zinc-iron flow batteries

3D printed optimized electrodes for electrochemical flow reactors

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

A Comprehensive Flow–Mass–Thermal–Electrochemical Coupling Model for a VRFB Stack and Its Application in a Stack Temperature Control Strategy

In this work, a comprehensive multi-physics electrochemical hybrid stack model is developed for a vanadium redox flow battery (VRFB) stack considering electrolyte flow, mass transport, electrochemical reactions, shunt currents, and as heat generation and transfer simultaneously. Compared with other VRFB stack models, this model is more comprehensive in considering the influence of multiple factors. Based on the established model, the electrolyte flow rate distribution across cells in the stack is investigated. The distribution and variation in shunt currents, single-cell current and single-cell voltage are analyzed. The distribution and variation in temperature and heat generation and heat transfer are also researched. It can be found that the VRFB stack temperature will exceed 40 °C when operating at 60 A and 100 mA cm−2 at an ambient temperature of 30 °C, which will lead to electrolyte ion precipitation, affecting the performance and safety of the battery. To control the stack temperature below 40 °C, a new tank cooling control strategy is proposed, and the suitable starting cooling point and the controlled temperature are specified. Compared with the common room cooling strategy, the new tank cooling strategy reduces energy consumption by 27.18% during 20 charge–discharge cycles.

Dibenzodioxin-Based Polymers of Intrinsic Microporosity with Enhanced Transport Properties for Lithium Ions in Aqueous Media.

Boosting the transport and selectivity properties of membranes based on polymers of intrinsic microporosity (PIMs) toward one specific working analyte of interest is challenging. In this work, a novel family of PIM membranes, prepared by casting and exhibiting optima mechanical properties and high thermal stability, was synthesized from 4,4'-(2,2,2-trifluoro-1-phenylethane-1,1-diyl) bis(benzene-1,2-diol) and two tetrafluoro-nitrile derivatives. Gas permeability measurements evidenced a CO2/CH4 selectivity up to 170% relative to the reference polymer, PIM-1, in agreement with their calculated fractional free volume and the analysis of the textural properties by N2 and CO2 gas adsorption. Besides, the chemical modification by acid hydrolysis of the PIM membranes favored the permeability for lithium ions (LiCl 2M, 6 × 10-9 cm2·s-1) compared to other alkali metal analogs such as sodium (NaCl 2M, 7.38 × 10-10 cm2·s-1) and potassium (KCl 2M, 1.05 × 10-9 cm2·s-1). Moreover, the complete mitigation of the crossover of redox species with higher molecular sizes than the ions from alkali metal salts was confirmed by using in-line benchtop NMR methods. Additionally, the modified PIM membranes were measured in a symmetric electrochemical flow cell using an aqueous electrolyte by combining lithium ferro/ferricyanide redox compounds and lithium chloride. The electrochemical tests showed low polarization, high-rate capability, and capacity retention values of 99% when cycled at 10 mA·cm-2 for over 50 cycles. Based on these results, these polymers could be used as highly selective and conducting membranes in electrodialysis for lithium separation and lithium-based redox flow batteries and as a protective layer in high-energy density lithium metal batteries.