Electrolytes For Redox Flow Batteries Research Articles

Unraveling the coordination behavior and transformation mechanism of Cr3+ in Fe–Cr redox flow battery electrolytes

Currently, the iron chromium redox flow battery (ICRFB) has become a research hotspot in the energy storage field owing to its low cost and easily-scaled-up. However, the activity of electrolyte is still ambiguous due to its complicated solution environment. Herein, we performed a pioneering investigation on the coordination behavior and transformation mechanism of Cr3+ in electrolyte and prediction of impurity ions impact through quantum chemistry computations. Based on the structure and symmetry of electrostatic potential distribution, the activity of different Cr3+ complex ions is confirmed as [Cr(H2O)5Cl]2+ > [Cr(H2O)4Cl2]+ > [Cr(H2O)6]3+. The transformation mechanism between [Cr(H2O)6]3+ and [Cr(H2O)5Cl]2+ is revealed. We find the metal impurity ions (especially Mg2+) can exacerbate the electrolyte deactivation by reducing the transformation energy barrier from [Cr(H2O)5Cl]2+ (24.38 kcal mol−1) to [Cr(H2O)6]3+ (16.23 kcal mol−1). The solvent radial distribution and mean square displacement in different solvent environments are discussed and we conclude that the coordination configuration limits the diffusivity of Cr3+. This work provides new insights into the activity of electrolyte, laying a fundamental sense for the electrolyte in ICRFB.

Hydrated eutectic electrolyte as catholyte enables high-performance redox flow batteries

Compared with conventional electrolytes, eutectic electrolytes are recognized as an ideal electrolyte for the next-generation electrochemical energy storage system characterized by facile synthesis and composition tunability. However, the eutectic electrolytes still suffer from sluggish kinetics, which inhibit their widespread application in redox flow batteries. Here, a well-designed hydrated eutectic electrolyte system (HEE) composed of iron chloride hexahydrate, urea and water with stoichiometric ratio at 2:1:6 is developed as catholyte to improve the cycling performance of hybrid redox flow batteries. The HEEs with precisely controlled water content could inhibit the water-induced side reactions, while expand the electrochemical stability windows. Moreover, the water molecules gradually replace the anions to participate in the solvation sheath of Fe3+ ions, forming a solvation structure of [FeClx(H2O)y]3−x, which result in an improved ion dissociation degree, ionic conductivity and redox kinetics. As a result, hybrid iron-based redox flow battery based on the optimal HEE exhibits a highly reversibly redox reaction of Fe3+/Fe2+ and reduced overpotentials. Furthermore, the battery could stably cycle over 120 cycles with a capacity retention of 87.75 % at a relatively high current density of 10 mA cm−2, delivering a maximum power density of ∼50 mW cm−2, which is much higher than reported eutectic-based flow batteries. This research offers new insights into rationally designing of advanced eutectic electrolytes for high-efficient redox flow batteries.

Screening of First-Row Transition Metal Complexes for Aqueous Redox Flow Batteries: Experimental and Density Functional Theory Approaches

The development of redox flow batteries took new directions in the last decades. From initial metal-based systems, it evolved towards organic and hybrid approaches in aqueous media. Furthermore, redox targeting flow batteries are an emerging alternative to the traditional redox flow battery architecture which offer improved energy density via an added electroactive solid ‘booster’. Whatever the system, there is a need of soluble electroactive species with potentials allowing large cell potentials within the water stability window or imperative redox potentials matching between soluble redox mediator and the solid for redox targeting. Transition metal complexes are a promising class of redox electroactive centers due to the tunability of their solubility and electrochemical potential, and the ability to take into account sustainability1. Recent reports underlined for instance that either the modification of the ligand2 or using heteroleptic complexes3, can lead to iron complexes with solubilities higher than 1 M and potentials higher than 1 V vs SHE. Furthermore, reliable and time-efficient prediction tools for the redox potentials of transition metal complexes are needed to help the search for new potential systems. While density functional theory usually provides a good trade-off between computational cost and accuracy, calculations on transition metal complexes often result in large errors. We thus developed a procedure and compared different solvation methods and levels of theory using an initial experimental data set based on aqueous iron complexes with bidentate ligands (Figure 1).In the present contribution, we will illustrate this coordination approach through a screening of new complexes based on non-toxic and affordable transition metal complexes. In this perspective, electrochemical characterization at different pH, coupled with NMR and UV-Visible studies will be presented, to screen for new candidates as electrolyte in redox flow batteries. Finally, we will show how the calculations corrected using a simple linear regression using training data yields a good prediction of redox potentials (mean average error =0.09 V).

Expanding the Scanning Electrochemical Microscopy Toolset for Evaluating Electron Transfer Kinetics of Concentrated and Specialized Redox Electrolytes on Carbon Surfaces

Performing quantitative measurements of heterogeneous electron transfer reactions at electrodes (e.g., determining k0 and a) under conditions of high redox active molecule concentration in solution, such as those encountered in electrolytes for redox flow batteries (RFBs), presents significant technical challenges. For example, attempting to use transient voltammetry methodology quickly leads to limitations imposed by iR drop, as currents can be large and resistance, especially in non-aqueous systems, can be high. Additionally, using steady-state techniques based on convective principles, such as the rotating disk electrode (RDE), involve the use of relatively large equipment, with moving parts, and usually with requirements of tens to hundreds of mL of solution, which may be limiting for exploring new compounds at high concentration.To address these issues, here we introduce the use of scanning electrochemical microscopy (SECM) for the determining the rates of electron transfer of redox active species at high concentration in non-aqueous [1] and other unconventional systems, such as deep eutectic solvents. SECM enables the resolution of higher rates of electron transfer compared to transient voltammetry and RDE, and it also enables experiments with spatial resolution, enabling the exploration of reactivity on various sites of a heterogeneous electrode. In this presentation, we will describe theoretical and practical aspects of these measurements, and we will demonstrate their utility using two model systems. The first one consists on the observation of inherent rate asymmetries observed for ferrocene derivatives, ubiquitous redox probes, at the graphite/carbonate solvent interface. These asymmetries are not present on aqueous systems or metallic electrodes, demonstrating the ability of the SECM technique to capture unique behaviors. In the second case, we will discuss how SECM measurements can be used on deep eutectic solvents such as ethaline to address how interactions between redox actives and the electrode or the solvent systems modulate the rate of electron transfer. By systematically modifying the strength of interactions by means of molecule choice, solvent composition, and electrode type and structure, we can gain insight into the processes that determine reactivity at redox flow battery electrodes. This insight is critical for exploring the interfacial aspects such as adsorption, passivation, and double-layer effects on kinetics of electron transfer.[1] Gaddam, R.; Sarbapalli, D.; Howard, J.; Curtiss, L.A.; Assary, R.S.; Rodríguez-López, J. An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces Chem: Asian J., 2023, 18, 2, e202201120. Figure 1

Effect of Glycine on a Fe-Based Electrolyte for Redox Flow Batteries

Among the rising stationary energy storage technologies, Redox Flow Batteries (RFBs) emerge as a promising electrochemical device capable of high flexibility in terms of power and energy combined with potential low cost and low environmental impact. Alternative chemistries can employ cheap and earth-abundant materials, possibly alleviating the demand for critical raw materials, which are extensively employed in the field of energy production and storage. Iron-based chemistries are among the best candidates for aqueous pH-neutral catholytes, offering fast reaction kinetics and a well-placed standard reduction potential of 0.771 V, that, coupled with a Zinc-based anolyte, would give a nominal cell voltage of 1.53 V [1]. Unfortunately, the dissolution of Iron salts in a neutral aqueous solution triggers hydrolysis, resulting in the formation of Iron hydroxides in the form of insoluble deposits. This causes the loss of electroactive species from the electrolyte, and consequently decreases the battery capacity. Thus far, the conventional solution is the electrolyte acidification, which, however, significantly increases the costs of materials for stacks and electrolyte handling components and aggravates the already critical problem of Hydrogen evolution at the Zinc side. As an alternative, several complexing agents have been proposed in the literature, with preliminary studies on their complexation mechanisms [2]. Glycine is often considered as one of the most promising candidates, even though its suppressing effect on the Fe(II)/Fe(III) kinetics at high concentrations is widely reported [3].This research investigates an optimized pH-neutral catholyte for Zinc-Iron RFBs, focusing on the use of Glycine to avoid Iron-induced hydrolysis. The study is carried out on highly concentrated solutions of Iron chlorides, with the addition of Glycine as ligand at various concentrations. The aim is to preserve the performances of Fe(II)/Fe(III) kinetics and to avoid insoluble hydroxides formation by an accurate choice of the Gly-Fe ratio. The produced electrolytes have been characterized by measuring the solutions conductivity, kinematic viscosity and density. Then, an electrochemical characterization is performed evaluating the diffusion coefficients (𝐷) of the dissolved species, the equilibrium potential (𝐸0) and the exchange current density (𝑗0) as well as the rate constant of the reactions (𝑘0) by means of cyclic voltammetries with a glassy carbon rotating disc electrode. The diffusion coefficients are obtained from Randles-Sevcik equation while the equilibrium potential, the exchange current density and the reaction rate constants are evaluated with a Koutecky-Levich analysis. An optimization of the electrolyte preparation method was also performed, evaluating the effects of pH and temperature and monitoring the complexation time, with the aim of accelerating the Gly-Fe complexation reaction. The effects of KCl and CaCl2 on the electrolyte conductivity are also investigated, to identify suitable supporting electrolytes.The research results show how the addition of Glycine successfully prevents the formation of precipitates in equimolar solutions even at low Gly-Fe ratios. However, Glycine suppresses Fe(II)/Fe(III) kinetics and lowers the electrolyte equilibrium potential, consequently reducing both energy density and the maximum current for its use in RFBs. It is thus possible to identify an optimum for the Gly-Fe ratio by setting a compromise between chemical stability and electrochemical performances. We propose the range 0.5:1 to 1:1 as a convenient Gly-Fe ratio for the 1M FeCl2 and 1M FeCl3 system, reaching a potential higher than 0.721 V vs. SHE, making it a very attractive catholyte with excellent suppression of Iron hydrolysis as no precipitation is observed after more than three months.This work is funded by the Italian Ministry of Enterprises and Made in Italy in the framework of the Important Project of Common European Interest (IPCEI) European Battery Innovation (project IPCEI Batterie 2 - CUP: B62C22000010001). The IPCEI European Battery Innovation is also funded by public authorities from Austria, Belgium, Croatia, Finland, France, Germany, Greece, Poland, Slovakia, Spain and Sweden.[1] Zhang, Huan, Chuanyu Sun, and Mingming Ge. "Review of the research status of cost-effective zinc–iron redox flow batteries." Batteries 8.11 (2022): 202.[2] Hawthorne, Krista L., Jesse S. Wainright, and Robert F. Savinell. "Studies of iron-ligand complexes for an all-iron flow battery application." Journal of The Electrochemical Society 161.10 (2014): A1662.[3] Xie, Congxin, et al. "A Low‐Cost Neutral Zinc–Iron Flow Battery with High Energy Density for Stationary Energy Storage." Angewandte Chemie International Edition 56.47 (2017): 14953-14957. Figure 1

Electrochemical Evaluation of Manganese Electrolytes for Redox Flow Batteries

Redox flow batteries (RFBs) are promising candidates for long-duration energy storage mainly due to the lower cost of improving the energy density compared to lithium-ion batteries. However, the full sustainability of RFBs has not been achieved yet, and there is a lot of room for further improvement. One of the main parameters showing a strong influence on the sustainability of RFBs is redox active materials. In this work, manganese was selected for electrochemical evaluation as it fulfills the sustainability requirements. Manganese is a non-toxic, low-cost and abundant material with sufficient aqueous solubility[1]. Moreover, the standard potential of manganese (1.51 V) is higher than that of vanadium (1.26 V).For electrochemical parameters, we have, through cyclic voltammetry (CVs) and polarization curves, estimated the diffusion coefficient (D0 ) and heterogeneous standard electron transfer rate (k 0) of manganese in an acidic electrolyte in the presence of titanium as an additive. In order to investigate the performance of an RFB system using a manganese electrolyte, a hybrid system was designed that uses hydrogen gas on the negative side and an acidic manganese electrolyte the positive side. We have studied the resistance behavior of the hybrid RFB through electrochemical impedance spectroscopy at different states of charge. From EIS data, cell ohmic resistance, and a general idea of the different processes taking place at different state of charge were obtained. Moreover, we studied the performance of the hybrid RFB during 70 cycles, obtaining an average coulombic efficiency of 98.1%, voltage efficiency of 77.8% and capacity retention of 97.7%. Carney, J.R., B.R. Dillon, and S.P. Thomas, Recent Advances of Manganese Catalysis for Organic Synthesis. European Journal of Organic Chemistry, 2016. 2016(23): p. 3912-3929.

Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte and the Study of Ion Crossover in Zn-Ce Redox Flow Battery

The development of a highly efficient and inexpensive energy storage technology is central to the operation of a sustainable smart grid and widespread utilization of intermittent renewable energy sources. An emerging and promising grid-scale technology for electrochemical energy storage is the redox flow battery (RFB). A unique and particularly valuable aspect of the design of RFBs is that their energy and power capacities can be scaled independently. Some of their other advantages include short response time, long life-cycles and no detrimental effects associated with being fully charged or discharged [1]. The Zn-Ce RFB, first developed by Plurion Inc (UK) in 2005 [2], has an open-circuit voltage as high as 2.4 V which is one of the highest among the various RBFs.To further improve the efficiency of Zn-Ce RBFs, the effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce RFB has been studied. The effectiveness of mixed methanesulfonic/sulfuric acid, mixed methanesulfonic/nitric acid and pure methanesulfonic acid has been assessed and compared on the basis of the cyclic voltammetry response for the Ce(III)/Ce(IV) redox couple and galvanic charge-discharge of a bench-scale Zn-Ce RFB. We have observed the Ce(III)/Ce(IV) reaction to exhibit faster kinetics and the battery to attain higher coulombic efficiency and long-term performance over ~40 charge-discharge cycles in the mixed 2 mol/L MSA–0.5 mol/L H2SO4 electrolyte compared to that achieved in the commonly used 4 mol/L pure MSA electrolyte due to lower H+ crossover and the prevention of CeO2 precipitation. The coulombic efficiency fade rate in the mixed MSA-H2SO4 electrolyte is 0.55% per cycle over 40 charge-discharge cycles, while the fade rate is 1.26% in the case of 4 mol/L MSA. Furthermore, the positive electrode reaction is no longer the limiting half-cell reaction even at the end of long-term battery charge-discharge operation [3]. Our results show that a mixed MSA–H2SO4 acid electrolyte may be a better option for the positive side of a Zn-Ce RFB as a large-scale energy storage device.At the same time, undesired ion crossover is a practical issue that cannot be neglected since it decreases the concentrations of electroactive species in both negative and positive electrolytes and can cause battery self-discharge and thereby lower battery efficiency and longevity. The concentrations of electroactive species in both the positive and negative electrolytes after each charge-discharge cycle have been monitored as a function of cycle number. We have found that as much as 48.6% of the Zn(II) ions are transported through the Nafion 117 membrane from the negative to the positive electrolyte and 11.1% of Ce(III) ions are transported from the positive to the negative electrolyte by the end of 30 cycles (~55 hours of battery operation). A simple and efficient method to mitigate this problem is to add a certain amount of Zn(II) to the positive electrolyte and Ce(III) to the negative electrolyte at the outset of operation in order to reduce the concentration gradients between the two sides. From our cyclic voltammetry results, the addition of Zn(II) ions into the positive electrolyte has no deleterious effect on the Ce(III)/Ce(IV) redox couple and in fact appears to slightly reduce the separation between the reduction and oxidation peaks, suggesting better reversibility of the positive electrode reaction. In addition, experiments involving 30 cycles of battery charge-discharge have been conducted using Zn(II)-containing pure MSA positive electrolyte as well as Zn(II)-containing MSA-H2SO4 mixed acid electrolytes to analyze battery efficiency. Our results show that both the coulombic and voltage efficiencies are increased compared to that attained using the original electrolytes without Zn(II) initially added to the positive electrolytes. The energy efficiency increases by 19.7% in the case of the Zn(II)-containing pure MSA electrolyte and 6.1% for the Zn(II)-containing H2SO4-MSA mixed acid electrolyte (see Figure 1). Another benefit of adding Zn(II) ions to the MSA pure positive electrolyte is that it prevents the precipitation of CeO2 routinely observed to occur in MSA-only electrolytes after 10 – 15 charge-discharge cycles, which further enhances the energy density of the Zn-Ce RFB.References Nguyen, T. and R.F. Savinell, Flow Batteries. Electrochemical Society Interface, 2010. 19(3), 54.Clarke, R., B. Dougherty, S. Harrison, P. Millington, and S. Mohanta, US Patent 2004/0202925 A1. Cerium Batteries, 2004.Yu, H., M. Pritzker, and J. Gostick, Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte in Zn-Ce Redox Flow Battery. J. Electrochemical Society, 2023. 170(2), 020536. Figure 1

X-Ray Absorption Spectroscopy Studies of Ti-Mn Redox Flow Battery to Clarify the Redox Reaction

Redox flow battery (RFB) is a promising candidate of large-scale stationary energy storage which is necessary to utilize renewable energy. For RFBs, the capability to easily increase the capacity by enlarging the electrolyte tank, high charge-discharge cycle performance, and wide flexibility to high- and low-frequency fluctuations are highly advantageous.Nowadays vanadium RFBs are used for practical use, but the material cost of vanadium is problematic to further spread them. An alternative candidate for commercial use is titanium-manganese (Ti-Mn) RFBs [1] whose material cost should be lower. On the other hand, the stability of the electrolyte for positive electrode (catholyte) need to be improved, because precipitates are created by a charge disproportionation reaction of Mn ions in the charged catholyte [2]. The formation of precipitates decreases the energy density and degrades the cyclability. To improve the characteristics, the chemical state and redox reaction of the electrolytes of Ti-Mn RFBs should be clarified by precise analyses.We performed synchrotron radiation X-ray absorption spectroscopy (XAS) for the electrolytes of a Ti-Mn RFB to directly observe the redox reaction [3]. In this study, the same Ti-Mn RFB electrolyte was used for both positive and negative electrodes. The Ti K-edge XAS of the electrolyte for negative electrode (anolyte) revealed gradual reduction reaction of Ti from Ti4+ on the charge process. The Ti L 2,3-edge XAS spectrum for the anolyte at state of charge (SOC) of 80% was mostly attributed to Ti3+ state. The results for Ti are consistent with electrochemical views in previous reports [2]. The Mn K-edge XAS of the catholyte gradually shifted to higher energy-side up to SOC of 50%, indicating a slight oxidation reaction of Mn from Mn2+. However, the Mn L 2,3-edge XAS spectrum for the catholyte solution at SOC of 80% was of Mn2+ state that was not changed from the initial state. Instead, scanning transmission X-ray microscopy (STXM; XAS mapping with a high spatial resolution) at the Mn L 3-edge unveiled that the precipitates mostly consist of Mn4+. Thus, the charge disproportionation reaction of 2Mn3+ -> Mn2+ (solution) + Mn4+ (precipitates) in the charged catholyte was confirmed. In the presentation, the electronic-structure change of the Ti and Mn ions in both catholyte and anolyte will be discussed in detail.

Theoretical Study of Microgel Functional Groups' Mobility.

Polymer microgels, micrometer-sized cross-linked polymer particles, are considered to be a promising type of advanced materials for a wide range of applications. To enhance the microgels' applicability, it is essential to incorporate various functional groups into a microparticle polymer network. Yet, the availability of functional groups for the interaction with surroundings depends strongly on the properties of the polymer network and has a great impact on further effective usage. In this theoretical study, we address this question and, with the help of coarse-grained molecular dynamics computer simulations, assess the segmental mobility and accessibility of functional groups bound to polymer network depending on microgel architecture and solvent quality. Additionally, we evaluate the minimum number of functional groups needed to facilitate the hopping mechanism between the functional groups (i.e., charge transfer). As an example of practical implementation of the obtained results, we estimate the optimal network topology for redox-active microgels to provide the maximum charge capacity for the dispersion electrolyte in redox-flow batteries.

Associative pyridinium electrolytes for air-tolerant redox flow batteries

Pyridinium electrolytes are promising candidates for flow-battery-based energy storage1–4. However, the mechanisms underlying both their charge–discharge processes and overall cycling stability remain poorly understood. Here we probe the redox behaviour of pyridinium electrolytes under representative flow battery conditions, offering insights into air tolerance of batteries containing these electrolytes while providing a universal physico-chemical descriptor of their reversibility. Leveraging a synthetic library of extended bispyridinium compounds, we track their performance over a wide range of potentials and identify the singlet–triplet free energy gap as a descriptor that successfully predicts the onset of previously unidentified capacity fade mechanisms. Using coupled operando nuclear magnetic resonance and electron paramagnetic resonance spectroscopies5,6, we explain the redox behaviour of these electrolytes and determine the presence of two distinct regimes (narrow and wide energy gaps) of electrochemical performance. In both regimes, we tie capacity fade to the formation of free radical species, and further show that π-dimerization plays a decisive role in suppressing reactivity between these radicals and trace impurities such as dissolved oxygen. Our findings stand in direct contrast to prevailing views surrounding the role of π-dimers in redox flow batteries1,4,7–11 and enable us to efficiently mitigate capacity fade from oxygen even on prolonged (days) exposure to air. These insights pave the way to new electrolyte systems, in which reactivity of reduced species is controlled by their propensity for intra- and intermolecular pairing of free radicals, enabling operation in air.

Recent Development of Electrolytes for Aqueous Organic Redox Flow Batteries (Aorfbs): Current Status, Challenges, and Prospects.

In recent years, aqueous organic redox flow batteries (AORFBs) have attracted considerable attention due to advancements in grid-level energy storage capacity research. These batteries offer remarkable benefits, including outstanding capacity retention, excellent cell performance, high energy density, and cost-effectiveness. The organic electrolytes in AORFBs exhibit adjustable redox potentials and tunable solubilities in water. Previously, various types of organic electrolytes, such as quinones, organometallic complexes, viologens, redox-active polymers, and organic salts, were extensively investigated for their electrochemical performance and stability. This study presents an overview of recently published novel organic electrolytes for AORFBs in acidic, alkaline, and neutral environments. Furthermore, it delves into the current status, challenges, and prospects of AORFBs, highlighting different strategies to overcome these challenges, with special emphasis placed on their design, composition, functionalities, and cost. A brief techno-economic analysis of various aqueous RFBs is also outlined, considering their potential scalability and integration with renewable energy systems.

Nanoarchitectonics and electrochemical properties of redox-active nanogels for redox flow battery electrolytes

In this paper, we provide a thorough electrochemical study of redox-active nanosized cationic gels which are promising materials for redox flow battery electrolytes. We use two-step synthesis under mild aqueous conditions: precipitation polymerization of nanogels based on poly-(N-isporopylacrylamide-co-N-(3-aminopropyl)methacrylamide hydrochloride) (PNIPAM-co-APMA), and grafting of redox-active 4-(3-carboxypropanamido)-TEMPO units to the nanogels. We demonstrate stable reduction–oxidation behavior of such nanogels and suggest a universal approach to evaluate the “effective” concentration and diffusion coefficient of redox-active groups grafted to nanogel particles. For the TEMPO-grafted PNIPAM-co-APMA nanogels we find the “effective” concentration of TEMPO-groups to be approximately 50 % of their total concentration and demonstrate an increase of the “effective” concentration upon electrode rotation. Also, we investigate electron transfer kinetics of redox-active nanogels and provide an evidence that the adsorbed layer of nanogels facilitates electron transfer.

Comparative analysis of single-acid and mixed-acid systems as supporting electrolyte for vanadium redox flow battery

Comparative analysis of single-acid and mixed-acid systems as supporting electrolyte for vanadium redox flow battery

Development of highly stable and active intermetallic Pt-alloyed catalysts for the production of V3.5+ electrolyte in the vanadium redox flow batteries

Vanadium based battery is considered one of the most promising candidates to overcome the limitations of lithium ion battery in large-scale energy storage systems. However, the high manufacturing cost of the vanadium electrolyte has been hindered to commercialize. Although the catalytic production of vanadium electrolyte was reported, with an organic reducing agent such as formic acid is considered as a promising method, the continuous production of vanadium electrolyte is still difficult owing to the limited catalyst durability. Herein, we present the highly durable and active intermetallic Pt-alloyed catalysts, i.e., L12-Fe0.25Pt0.75/C and L12Co0.25Pt0.75/C, for the catalytic production of V3.5+ electrolyte. In the catalytic test of electrolyte production, the intermetallic L12-Fe0.25Pt0.75/C showed nearly 3 times higher activity than the commercial Pt/C catalyst. In the catalyst durability test, on the other hand, the intermetallic L12Co0.25Pt0.75/C showed only 3 % decrease of initial activity whereas the intermetallic L12-Fe0.25Pt0.75/C and commercial Pt/C exhibited ca. 9 % and ca. 20 % decrease of initial activity, respectively, for one hundred times continuous electrolyte production test. Among the catalysts tested, the intermetallic L12-Fe0.25Pt0.75/C showed the highest initial catalytic activity while the L12Co0.25Pt0.75/C exhibited the strongest catalyst durability. In terms of catalytic activity and stability, the intermetallic L12Co0.25Pt0.75/C was found to be an efficient catalyst for the practical production of V3.5+ electrolyte.

Two birds, one stone: Deep eutectic solvent (DES) modified graphite felt electrode significantly improves performance of DES-electrolyte non-aqueous flow battery

In the past, non-aqueous redox flow batteries usually used organic solvents or ionic liquids as electrolytes. However, these two liquids are not suitable as electrolytes in terms of safety and production costs. Herein, we chose a deep eutectic solvent (DES) for non-aqueous redox flow battery. DES has been applied to play two functions in Fe/V non-aqueous redox flow battery: commercial graphite felt electrode is treated with DES and heat treatment for surface modification; DES also acts as a main electrolyte for the non-aqueous redox flow battery. While ensuring no external substances are introduced to produce side reactions, the electrode reaction kinetics are enhanced, and better cyclability is achieved. The modified treatment increases the number of oxygen-containing groups and defect sites on electrode surface. Electrode exhibits higher electrochemical activity and lower charge transfer resistance. In the full battery test, the maximum power density with modified electrode reaches 13.26 mW·cm−2, which is 114% higher than commercial graphite felt electrode. The energy efficiency reaches 90.50% and battery life is extended by 70.58% compared with commercial graphite felt electrode. This study has successfully improved the electrochemical performance of Fe/V DES redox flow battery and provides a novel idea for electrode modification.

On the Tunability of Toxicity for Viologen-Derivatives as Anolyte for Neutral Aqueous Organic Redox Flow Batteries.

Viologen-derivatives are the most widely used redox organic molecules for neutral pH negative electrolyte of redox flow batteries. However, the long-established toxicity of the herbicide methyl-viologen raises concern for deployment of viologen-derivatives at large scale in flow batteries. Herein, we demonstrate the radically different cytotoxicity and toxicology of a series of viologen-derivatives in in vitro assays using model organisms representative of human and environmental exposure, namely human lung carcinoma epithelial cell line (A549) and the yeast Saccharomyces cerevisiae. The results show that safe viologen derivatives can be molecularly engineered, representing a promising family of negolyte materials for neutral redox flow batteries.

Recent Developments on Electroactive Organic Electrolytes for Non-Aqueous Redox Flow Batteries: Current Status, Challenges, and Prospects.

The ever-increasing threat of climate change and the depletion of fossil fuel resources necessitate the use of solar- and wind-based renewable energy sources. Large-scale energy storage technologies, such as redox flow batteries (RFBs), offer a continuous supply of energy. Depending on the nature of the electrolytes used, RFBs are broadly categorized into aqueous redox flow batteries (ARFBs) and non-aqueous redox flow batteries (NARFBs). ARFBs suffer from various problems, including low conductivity of electrolytes, inferior charge/discharge current densities, high-capacity fading, and lower energy densities. NARFBs offer a wider potential window and range of operating temperatures, faster electron transfer kinetics, and higher energy densities. In this review article, a critical analysis is provided on the design of organic electroactive molecules, their physiochemical/electrochemical properties, and various organic solvents used in NARFBs. Furthermore, various redox-active organic materials, such as metal-based coordination complexes, quinones, radicals, polymers, and miscellaneous electroactive species, explored for NARFBs during 2012-2023 are discussed. Finally, the current challenges and prospects of NARFBs are summarized.

Analysis of vanadium species(V(IV)/V(III)) in the electrolyte manufacturing process for vanadium redox flow battery using digital image

It is important to analyze concentrations of vanadium species with different valences in the electrolyte for vanadium redox flow battery (VRFB) for production of electrolytes and stable operation of the batteries. Herein, we proposed an analytical method by digital imaging to determine the vanadium species (V(IV)/V(III)) concentrations in the vanadium electrolyte manufacturing process. Standard electrolytes with different V(IV):V(III) ratios underwent digital imaging and the RGB values were extracted from the obtained images, and then the calibration curve was plotted based on these values. The data obtained from the blue channel were used as the analytical signal because it showed the best sensitivity and linearity (R2 = 0.9836, 0.9909). Precisions were determined using electrolyte samples with 50% and 100 % V(III), respectively, and their RSDs were 2.18% and 1.01%, respectively. The accuracy of the digital imaging analysis method was evaluated by comparison with potentiometric titration, and the agreement was well made within the error limit. The results showed that this simple analytical method might be very suitable for real-time analysis of vanadium electrolyte.

Exploring Surfactant Electrolyte-Porous Electrode Interactions

Microemulsions are an alternative to aqueous organic electrolytes for redox flow batteries (RFB) that can be modified without synthesis. Water, oil, and emulsifiers, when combined using optimized chemistries and compositions, produce stable mixtures with aggregate structures (microemulsions). Design limitations imposed by the interdependency of redox solubility and electrolyte conductivity are circumvented by solubilizing redox active molecules in the oil phase and supporting ions in the aqueous phase. While macroscopically homogeneous, microemulsions are biphasic at the nanometer scale, effectively decoupling solubility from conductivity. Recently, the first microemulsion RFB was demonstrated using ferrocene solubilized in an anionic surfactant system. Polarization curve analysis revealed performance limitations seemingly arising from electrolyte interactions with the cell components including porous electrodes. Further optimization requires a deeper understanding of microemulsion-porous electrode interactions.Due to the complexity of microemulsion electrochemistry, a first step towards understanding these interactions is made by investigating the relationship between surfactants and porous electrodes. Vanadium redox flow battery (VRFB) electrolytes with surfactant additives are used as a model system to explore fundamental interactions, in situ, between a surfactant-containing electrolyte and a porous electrode. Transport and kinetics are evaluated as a function of electrode, electrolyte, and surfactant properties. Redox species transport is quantified through mass transfer coefficients, using a generalization of an existing porous electrode model fit to experimental polarization curve data. Electrochemical impedance spectroscopy, interpreted using existing kinetic models, is also employed to gain insight into the effects of surfactant on electron transfer rates.

Considerations for Ionic Diffusion in Slurry Electrolytes for Redox Flow Batteries

Slurry electrodes have been proposed as a means to enhance the scalability of hybrid redox flow battery (RFB) chemistries for better usability in utility scale energy storage applications1–3. In conventional hybrid RFB’s, scalability is limited due the spatial constraints of the flow cell and the metal deposited by the negative half-reaction on charge1. By using a slurry electrode, the solid metal can be deposited onto electrically conductive particles dispersed in the electrolyte instead of on the stationary electrode within the flow cell. In this way, hybrid RFB chemistries can achieve the same scalability as more commonly studied true RFB chemistries, such as all-vanadium. Due to the high abundance, low cost, and low toxicity of iron electrolytes, the all-iron RFB chemistry is of particular interest for use with a slurry electrode2,4. The usefulness of the slurry electrode depends on the current distribution of the plating reaction. To successfully decouple the storage and power capacities of the RFB and thus enhance its scalability5, the faradaic current of the plating reaction must occur predominantly on the mobile slurry particles, as opposed to on the stationary current collector1. This current distribution is dependent on a variety of factors, such as the applied overpotential, the electrical conductivity of the slurry, the ionic conductivity of the electrolyte, the kinetics of the reaction, and the rate of ionic mass transport to reaction sites. Ionic mass transport in electrolytes containing slurry electrodes may differ from ionic transport in neat electrolyte in interesting and important ways. Due to the volume fraction of the electrolyte occupied by solid particles, the effective concentration of the ionic species may be lower than in neat electrolyte. Further, the solid particle volume fraction hinders ionic diffusion by introducing diffusion path tortuosity. This effect is more severe in higher slurry particle loadings.In this work, the effect of varying dispersed solid particle loading on ionic diffusivity is investigated via voltammetry using a rotating disk electrode. The diffusivities of ionic iron species are measured as a function of the volume fraction of solids dispersed in the electrolyte. Comparisons with the Bruggeman correlation6,7 are made and amendments to the Levich equation are considered.(1) Petek, T. J.; Hoyt, N. C.; Savinell, R. F.; Wainright, J. S. Slurry Electrodes for Iron Plating in an All-Iron Flow Battery. J. Power Sources 2015, 294, 620–626. https://doi.org/10.1016/j.jpowsour.2015.06.050.(2) Petek, T. J. Enhancing the Capacity of All-Iron Flow Batteries: Understanding Crossover and Slurry Electrodes. Ph.D. Thesis 2015, No. May.(3) Narayanan, T. M.; Zhu, Y. G.; Gençer, E.; McKinley, G.; Shao-Horn, Y. Low-Cost Manganese Dioxide Semi-Solid Electrode for Flow Batteries. Joule 2021, 5 (11), 2934–2954. https://doi.org/10.1016/j.joule.2021.07.010.(4) Dinesh, A.; Olivera, S.; Venkatesh, K.; Santosh, M. S.; Priya, M. G.; Inamuddin; Asiri, A. M.; Muralidhara, H. B. Iron-Based Flow Batteries to Store Renewable Energies. Environ. Chem. Lett. 2018, 16 (3), 683–694. https://doi.org/10.1007/s10311-018-0709-8.(5) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Jeffrey, T.; Liu, Q. Redox Flow Batteries , a Review Environmental Energy Technologies Division , Lawrence Berkeley National Laboratory , Department of Mechanical , Aerospace and Biomedical Engineering , University of Tennessee , Department of Chemical Engineering , McGill Un. 1–72.(6) Tjaden, B.; Cooper, S. J.; Brett, D. J.; Kramer, D.; Shearing, P. R. On the Origin and Application of the Bruggeman Correlation for Analysing Transport Phenomena in Electrochemical Systems. Curr. Opin. Chem. Eng. 2016, 12, 44–51. https://doi.org/10.1016/j.coche.2016.02.006.(7) Chung, D. W.; Ebner, M.; Ely, D. R.; Wood, V.; Edwin García, R. Validity of the Bruggeman Relation for Porous Electrodes. Model. Simul. Mater. Sci. Eng. 2013, 21 (7). https://doi.org/10.1088/0965-0393/21/7/074009.