Electrolytes For Redox Flow Batteries Research Articles

In Situ Characterization of Kinetics, Mass Transfer, and Active Electrode Surface Area for Vanadium Redox Flow Batteries

Engineering the electrochemical reactor of a vanadium redox flow battery (VRFB) is critical to deliver sufficiently high power densities to achieve cost-effective, grid-scale energy storage. Understanding and ultimately alleviating the cell-level resistive losses in VRFBs fundamentally depend on the ability to accurately measure the electron and mass transfer rates as a function of applied potential and interpret the results in the context of VRFB operation. In this study, an in situ electroanalytical technique of electrochemical reaction in porous electrodes is proposed by a symmetrical cell design for VRFB. For both V2+/V3+ and VO2+/VO2 + redox couples, the polarization curves at different flow rates are acquired on the symmetrical flow cell. The high-frequency resistance is also obtained by electrochemical impedance spectroscopy at open circuit. The ohmic, kinetic, and mass transfer resistance are obtained by deconvoluting the total polarization curve. Corresponding key parameters (i.e., membrane conductivity, reaction rates, and mass transfer coefficients) are obtained along with the specific surface area of porous electrode. The full-cell simulations using extracted key parameters are in excellent agreement with experimental full-cell tests at different applied currents. This novel in situ electroanalytical technique provides an invaluable approach to characterize the performance of electrolyte and electrode in redox flow batteries.

Grafting and Solubilization of Redox-Active Organic Materials for Aqueous Redox Flow Batteries.

This study concerns the development of sustainable design strategies of aqueous electrolytes for redox flow batteries using redox-active organic materials. A green spontaneous grafting reaction occurs between a redox-active organic radical and an electrochemically activated structural modifier at room temperature through a simple mixing step. Then, a physical mixing method is used to formulate a structured aqueous electrolyte and enables aqueous solubilization of the organic solute from below 0.5 to 1.5 m beyond the conventional dissolution limit. The as-obtained concentrated mixture can be readily used as catholyte for a redox flow battery. A record high discharge cell voltage (1.6 V onset output voltage) in aqueous non-hybrid flow cell is attained by using the studied electrolytes.

Towards cobalt-containing polyoxometalate-based electrolytes for redox flow batteries operating under mild conditions

Sustainability and safety of Redox Flow Batteries (RFBs) are some of the limitations that hinder the further implementation of this technology. To overcome the disadvantages of conventional RFBs, new electrolyte formulations are being proposed. Herein we report on two cobalt(II)-containing Keggin-type polyoxometalates (POMs) working as electroactive species. These are the plenary [α-CoW12O40]6− (CoW12) anion and the cobalt(II)-monosubstituted [α-Co(H2O)SiW11O39]6− (CoSiW11) tungstosilicate. During the battery performance, cobalt-containing POMs play the role of both positive and negative electrolyte, because the ability of Co (CoIIWVI/CoIIIWVI) and W (CoIIWVI/CoIIWV) centres to be oxidized and reduced, respectively. These redox processes are separated by 1.7 V (CoW12) and 1.9 V (CoSiW11) thus, both POMs supply with a suitable operational voltage for RFBs. The reversibility of the reactions was examined by a kinetic study. Among the formulations tested, those accomplished in aqueous 1 M HAc/LiAc and 1 M LiAc revealed high stability while operating under mild pH conditions. Moreover, the nature of the selected supporting electrolytes (SEs) avoids the occurrence of gassing side reactions. In addition, the physicochemical characterization of the electrolytes showed high conductivity (ca. 60 mS cm−1) and low viscosity (ca. 1.4 mPa s) for highly concentrated electrolytes (up to 120 mM). Therefore, the developed electrolytes exhibit promising properties for their direct incorporation in RFBs, including high energy density (21.86 and 24.44 W h L−1 for CoW12 and CoSiW11 respectively) and enhanced safety.

XAFS Analysis of Sulfuric Acid Solution Related to the Effect of Bi in Suppressing the Disproportionation Reaction of Mn<sup>3+</sup> in Mn/Ti Redox Flow Cells

In sulfuric acid (H2SO4) aqueous solutions containing manganese (Mn) and titanium (Ti) ions, which are used for the positive electrolyte in redox flow (RF) batteries, the addition of a small quantity of bismuth (Bi) has been found to be very effective in suppressing the MnO2 precipitation in the charging process from Mn2+ to Mn3+. The effect of the addition of Bi on the structural change in these solutions has been analyzed using x-ray absorption fine structure (XAFS) techniques. X-ray absorption near edge structure (XANES) spectra indicated that some Mn4+ ions were generated only when Bi was not added. It was also confirmed that the Mn-O octahedral coordination was maintained at a high state of charge (SOC). On the other hand, no change was observed in the valence of Ti and Bi ions even at high SOCs. Extended x-ray absorption fine structure (EXAFS) measurements combined with fitting analysis clarified that the addition of Bi resulted in the generation of Mn-O-Bi coordination at the SOCs of 70 and 90 %. Bi-O-Mn coordination may suppress the MnO2 precipitation at a high SOC.

Electrochemical properties of electrolyte for lead fluoroborate redox flow battery prepared with recovered lead

The electrochemical properties of electrolyte for soluble lead flow battery(SLFB) are a key factor for the improvement of its performance. Pb(BF4)2 in which Pb2+ ions are highly soluble is cheaper and more stable than conventional Pb(CH3SO3)2. This paper presents the electrochemical properties of HBF4 and Pb(BF4)2 prepared with recovered lead for the electrolytes of SLFB. The density difference of reagent grade and recovered lead electrolytes is less than 0.05 g/cm3 and their viscosities increase up to 132 %, 120 %, respectively when concentration of Pb(BF4)2 changes from zero to 2.0 mol·dm−3. The conductivity of 1.5 mol·dm−3 electrolyte prepared with recovered lead has a peak when concentration of HBF4 is less than 1.0 mol·dm−3. The results of cyclic voltammetry, linear sweep voltammetry and constant current charge/discharge test showed that impurities of recovered lead exert insignificant influences on the Pb2+/Pb and Pb2+/PbO2 reactions. These results are promising to apply recovered lead to SLFB.

Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte in Zn-Ce Redox Flow Battery

The effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce redox flow battery (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 voltammetric response for the Ce(III)/Ce(IV) redox couple and galvanic charge-discharge of a bench-scale Zn-Ce RFB. The Ce(III)/Ce(IV) reaction exhibits faster kinetics and the RFB exhibits higher coulombic efficiency and long-term performance over ∼40 charge-discharge cycles in the mixed 2 mol l−1 MSA-0.5 mol l−1 H2SO4 electrolyte compared to that achieved in the commonly used 4 mol l−1 MSA electrolyte due to lower H+ crossover and higher Ce(IV) solubility. 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−1 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. This work shows 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.

MXene-based electrolyte for vanadium-bromide redox flow battery for green energy

MXene-based electrolyte for vanadium-bromide redox flow battery for green energy

Computational design of quinone electrolytes for redox flow batteries using high-throughput machine learning and theoretical calculations

Molecular design of redox-active materials with higher solubility and greater redox potential windows is instrumental in enhancing the performance of redox flow batteries Here we propose a computational procedure for a systematic evaluation of organic redox-active species by combining machine learning, quantum-mechanical, and classical density functional theory calculations. 1,517 small quinone molecules were generated from the building blocks of benzoquinone, naphthoquinone, and anthraquinone with different substituent groups. The physics-based methods were used to predict HOMO-LUMO gaps and solvation free energies that account for the redox potential differences and aqueous solubility, respectively. The high-throughput calculations were augmented with the quantitative structure-property relationship analyses and machine learning/graph network modeling to evaluate the materials’ overall behavior. The computational procedure was able to reproduce high-performance cathode electrolyte materials consistent with experimental observations and identify new electrolytes for RFBs by screening 100,000 di-substituted quinone molecules, the largest library of redox-active quinone molecules ever investigated. The efficient computational platform may facilitate a better understanding of the structure-function relationship of quinone molecules and advance the design and application of all-organic active materials for RFBs.

Ferrocene to functionalized ferrocene: a versatile redox-active electrolyte for high-performance aqueous and non-aqueous organic redox flow batteries

This review article covers various synthesis aspects of functionalized ferrocene as redox-active electrolytes for the development of aqueous and nonaqueous redox flow batteries (RFBs).

Biological anolyte regeneration system for redox flow batteries.

Redox flow battery (RFB) electrolyte degradation is a common failure mechanism in RFBs. We report an RFB using genetically engineered, phenazine-producing Escherichia coli to serve as an anolyte regeneration system capable of repairing the degraded/decomposed redox-active phenazines. This work represents a new strategy for improving the stability of RFB systems because, under the influence of genetically engineered microbes, the anolyte species does not display degradation after battery cycling.

Cyclic synthesis of lignin anthraquinone electrolytes for aqueous redox flow batteries

Lignin, which is rich in phenolic hydroxyl/methoxy groups as redox active groups, is a potential electrolyte material for aqueous redox flow batteries (ARFBs). This work demonstrated to the synthesis of lignin-derived electrolytes via cyclization with 1,4-dihydroxyanthraquinone (1,4-DHAQ), in the absence of hazardous or noble metal catalysts in mild conditions (0 °C, 1 atm). The structure of lignin anthraquinone derivatives (LAQDs) cyclized in basis alkaline solution was experimentally determined. An exhaustive comparative study was conducted with respect to the electrochemical properties, charging-discharging tests and cycling performances. The initially volumetric capacitance, the capacity retention rate and coulombic efficiency of two LAQDs were determined to be 148.0 mAh.L−1, 89.3 % and 99.0 % for coniferaldehyde-anthraquinone derivative [LAQD(G)], and 132.1 mAh.L−1, 81.2 % and 99.0 % for sinusaldehyde-anthraquinone derivative [LAQD(S)], respectively. The theoretical value calculated by DFT is consistent with the actual value. Such LAQDs can be used as organic electrolyte materials, which can overcome poor chemical stability of anthraquinone, while improving the electrochemical activity of lignin-based electrolyte materials. This technology provides a pathway to prepare organic electrolyte for the development of environment friendly and better energy storage performance electrolytes for ARFBs.

Positronium chemistry of a Fe2+/3+ solution under electrochemical control.

The positronium chemistry of a Fe2+/3+ solution is studied under full electrochemical control. For this novel approach to positronium electrochemistry, a suitable cell setup is used, which allows simultaneously both electrochemical measurements and positron annihilation spectroscopy. For the Fe2+/3+ redox couple, positronium serves as an ideally suited atomic probe owing to the rather different positronium chemistry of Fe2+ (spin conversion) and Fe3+ (total positronium inhibition and oxidation). This enabled the precise in situ monitoring of oxidation and reduction by means of positron lifetime upon slow cycling voltammetry or galvanostatic charging. The variation of the mean positron lifetime with the Fe2+/3+ concentration ratio could be quantitatively described by a reaction rate model for positronium formation and annihilation. An asymmetric behavior of the variation of the mean positron lifetime with applied potential, as compared to the simultaneously recorded symmetric current-potential curve, could be explained by the stronger influence of Fe3+ on the characteristics of positronium formation and annihilation. The highly reversible galvanostatic charging behavior monitored by positron lifetime underlines the attractive application potentials of positronium electrochemistry for in situ studies of iron-based redox-flow battery electrolytes.

The Cluster Design and Redox Behavior Characterization of Polyoxometalates for Redox Flow Batteries.

Polyoxometalates (POMs) are a class of metal-oxygen cluster molecular materials formed by covalently linking transition metal atoms and oxygen atoms. Those molecular cluster materials also shows multiple electron reversible redox capabilities and high solubility in solution. Thus, POMs also recently present interesting applications as redox active electrolyte in redox flow batteries (RFBs). In this review, we mainly focus on the structural design of POMs and analysis factors affecting the electrochemical properties of POMs. Spectroscopic characterizations are introduced to deeply explore the structural changes of POMs during the redox process. Furthermore, we summarize the cases of POMs used as liquid-phase redox couple in aqueous or non-aqueous redox flow batteries and discuss the advantages and deficiency of POMs with various configurations.

Clean preparation of mixed trivalent and quadrivalent vanadium electrolyte for vanadium redox flow batteries by catalytic reduction with hydrogen

The vanadium redox flow battery (VRFB) is a promising technology for large-scale stationary energy storage systems. However, the high preparation cost of mixed valent vanadium electrolyte hinders the large-scale commercial application of VRFB. In this work, a simple, green and low-cost method is proposed to prepare the mixed valent vanadium electrolyte for VRFB. The clean hydrogen is chosen as the reducing agent to obtain trivalent vanadium ions from quadrivalent vanadium ions. The Pt/C material is used as the catalyst to accelerate the reduction rate at atmospheric pressure. The impurity-free mixed valent vanadium electrolyte, with a mean valence number of 3.5, prepared by the catalytic reduction process exhibits excellent battery performance with CE of 93% and EE of 85%. Furthermore, a catalytic reactor using Pt/C decorated graphite felt is designed and used to continuously produce the mixed valent vanadium electrolyte. According to the result of simple cost analysis, the proposed catalytic hydrogen reduction process can reduce theoretically the manufacturing cost by approximately 22.6% compared with the present industrial electrolytic process. As a consequence, the simple, clean and low-cost manufacture method of the mixed valent vanadium electrolyte exhibits remarkable competitiveness and promising potential for the practical production application of VRFB.

Perspective on Radiolytic Charging for Redox Flow Battery Electrolytes Using the Nuclear Decay Energy of Spent Nuclear Fuel/Radionuclides

The conversion of nuclear energy into electricity is facilitated by chemical intermediates and molecular products formed during the radiolysis of water. In this work, we hypothesize a novel radiolytic charging approach for redox flow battery electrolytes. Radiolytically produced ionic intermediates and molecular products oxidize or reduce the metal ion solutes in the electrolytes. A qualitative study for choice of redox couples based on electrochemical principles followed by a feasibility study of radiolytic conversion of active material is presented. A framework for an empirical investigation of radioactive source, equipment and dose, and product characterization techniques is discussed. The proposed method finds application in the utilization of spent nuclear fuel (specifically gamma emissions from fission products in early activity stages) as a radiolysis radiation source for electrolyte charging. We present a perspective on future investigations that are required to harness nuclear energy for charging electrolytes and developing a self-operating RFB system.

Influence of Several Phosphate-Containing Additives on the Stability and Electrochemical Behavior of Positive Electrolytes for Vanadium Redox Flow Battery

The poor operational stability of electrolytes is a persistent impediment in building redox flow battery technology; choosing suitable stability additives is usually the research direction to solve this problem. The effects of five phosphate containing additives (including 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), hexamethylene diamine tetramethylene phosphonic acid (HDTMPA), amino trimethylene phosphonic acid (ATMPA), sodium ethylenediamine tetramethylene phosphonate (EDTMPS), and diethyl triamine pentamethylene phosphonic acid (DTPMP)) on the thermal stability and electrochemical performance of the positive electrolyte of vanadium redox flow battery were investigated. With 0.5 wt% addition, most of the selected additives were able to improve the thermal stability of the electrolyte. HEDP and HDTMPA extended the stability time of the pentavalent vanadium electrolyte at 50 °C from 5 days (blank sample) to 30 days and 15 days, respectively. The electrochemical performance of the electrolyte was further investigated by cyclic voltammetry, steady state polarization, and electrochemical impedance spectroscopy tests. It was found that most of the additives enhanced the electrochemical activity of the positive electrolyte, and the diffusion coefficients, exchange current densities, and reaction rate constants of V(IV) species became larger with the addition of these additives. It is verified that the thermal stability and electrochemical stability of the electrolyte are significantly improved by the combination of ATMPA + HEDP or ATMPA + HDTMPA. This study provides a new approach to improve the stability of the positive electrolyte for vanadium redox flow battery.

Determination of the Formation Potential of Solid-Electrolyte Interphase in Some Amide-Type Ionic Liquids Containing Metal Salts

Amide-type ionic liquids (ILs) have been investigated as the electrolyte for secondary batteries because they have such favorable properties as non-flammability, low-volatility, and the electrochemical stability.1,2 However, the ILs have been reported to decompose cathodically at the potentials more positive than the deposition potential of Li and form solid-electrolyte interphase (SEI) on the electrode.3 Although the formation of the SEI is favorable for lithium secondary batteries, the SEI may hinder the electrode reactions of the active materials when the ILs are used as the electrolytes for redox flow batteries. In the present study, the formation potentials of the SEI in two amide-type ILs, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing metal salts were investigated in the presence of a redox species.BMPBr was prepared by the reaction of 1-methylpyrrolidine and 1-bromobutane in acetonitrile. BMPFSA was obtained by the interaction of BMPBr with LiFSA in distilled water. BMPFSA was dried under vacuum at 80 °C for 48 hours before use. BMPTFSA and the metal salts were used as supplied. The water content in BMPFSA and BMPTFSA was less than 10 ppm. The electrochemical measurement was carried out with a three-electrode cell in an argon-filled globe box. A Pt disk and wire were used as a working and counter electrode, respectively. Ag wire immersed in BMPFSA or BMPTFSA containing 0.10 M AgCF3SO3 was used as a reference electrode, which is denoted by Ag|Ag(I).The anodic peak potentials in the cyclic voltammograms in the ILs containing the redox species were almost constant after holding the potentials more positive than the cathodic decomposition potentials. On the other hand, the anodic peak potentials changed by holding the potential at the negative potential region in the presence of the metal species, suggesting that the potential of the SEI formation can be determined more sensitively by monitoring the redox reaction.

Conjugated Bipolar Redox-Active Electrolyte for Symmetric Redox Flow Battery

Organic non-aqueous redox-flow batteries (O-NRFBs) are gaining traction as viable alternatives for long-term, low-cost stationary energy storage with a wide potential window.[1] The symmetric O-NRFBs with single bipolar electrochemically active molecule as both anolyte and catholyte (i.e. bipolar redoxmer) helps to mitigate permanent cross-contamination and capacity fading.[2] However, symmetric O-NRFBs with bipolar molecules are hampered by the scarcity of redox active molecules capable of serving as a stable bipolar redoxmer with high cell potential and unpredictable side reactions.[3] This study proposes the π-conjugation of electrochemical active electron donors and electron acceptor which leads to the possible control over direct electronic perturbation between acceptor-donor to form new type of redox molecules. This study aims designing of benzothiadiazole based conjugated bipolar redox-active molecule with high potential window and high stability.[1]. M. Li, S. A. Odom, A. R. Pancoast, L. A. Robertson, T. P. Vaid, G. Agarwal, H. A. Doan, Y. Wang, T. M. Suduwella, S. R. Bheemireddy, R. H. Ewoldt, R. S. Assary, L. Zhang, M. S. Sigman, and S. D. Minteer, “Experimental protocols for studying organic non-aqueous redox flow batteries,” ACS Energy Lett., 2021, 6, 11, 3932–3943.[2]. M. Li, J. Case, and S. D. Minteer, “Bipolar redox-active molecules in non-aqueous organic redox flow batteries: status and challenges,” ChemElectroChem, 2021, 8, 1215–123.[3] M. Li, G. Agarwal, I. A. Shkrob, R. T. VanderLinden, J. Case, M. Prater, Z. Rhodes, R. S. Assary and S. D. Minteer, “Critical role of structural order in bipolar redox-active molecules for organic redox flow batteries,” J. Mater. Chem. A, 2021, 9, 23563-23573.

Evaluating the Stability and Performance of Nasicon in Low-Cost High Charge Density Redox Flow Battery Electrolytes

Solid superionic conductor membranes are being considered as alternatives to polymer-based membranes for use in redox flow batteries (RFBs) due to their superior abilities to mitigate reactant crossover,1 and enable the deployment of aqueous electrolytes comprising small, inorganic earth-abundant reactants.2,3 Much however remains to be understood about the evolution of the electrochemical performance and microstructural stability of these membranes while they are immersed in aqueous electrolytes.In this work, we evaluate the suitability of von Alpen sodium superionic conductor (NaSICON) as a prospective RFB membrane material by examining its resistance, permeability and interfacial morphology over time as a function of electrolyte pH and composition, as well as temperature. NaSICON is found to have a stable resistance profile for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes. Its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. Its area-specific resistance falls dramatically with increasing temperature and decreasing membrane thickness; we project that a membrane with a thickness of 100 μm or lower, if operated slightly above ambient temperature (~ 40 °C), can enable power densities comparable to or better than those of conventional polymer membrane-containing RFBs. Nevertheless, the glassy phase of the NaSICON microstructure at the membrane-electrolyte interface was found to undergo small amounts of etching while in contact with aqueous electrolytes containing sodium ions; this etching became significantly more extensive when potassium ions were present in the electrolyte, leading in certain instances to complete disintegration of the membrane.The extraordinary high stability of NaSICON in strongly alkaline electrolyte permits the construction of flow cells containing a positive electrolyte based on permanganate, a high-potential, inexpensive reactant with high volumetric capacity (> 100 Ah/L). The flow cells had open-circuit voltages 1.2 V and greater, along with negligible reactant crossover and very low capacity fade (< 0.02 %/day). This work highlights the promise of ceramic membranes as effective separators in RFBs operating under neutral pH to strongly alkaline pH conditions. It also points to the need for further research on the long-term stability of the membrane and its interface with the electrolyte in solid-state membranes under investigation as separators in aqueous RFBs. (1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.

Mass Transfer Characteristic Analysis and Optimization of Flow Field for Neutral Organic Redox Flow Battery Based on 1‐DPAQCl

AbstractA novel negative electrolyte for organic redox flow battery has been synthesized, which has higher solubility in neutral solution. In order to promote the commercialization of the novel electrolyte(1‐DPAQCl), a series of optimization should be carried out, in which the first is the operating conditions and mass transfer. A 3D steady state finite element model was established, which was compared with a lot of experimental data. The effects of only increasing the positive concentrations and only increasing the negative concentrations on the battery behaviors have been analyzed and the results showed that the setting of only increasing the positive concentrations can improve the discharge voltage up to 1.8 % and decrease the charging voltage by 1.21 %. The battery performances under four flow fields were investigated based on this model and the concentration uniformity factor of the four flow design were obtained. The results presented that the charge‐discharge voltages of the interdigital flow field (IFF) was the best, which was followed by the rotary flow field (RFF) and serpentine flow field (SFF), the parallel flow field (PFF) was worst. The uniformity factors of IFF were 23.5 % higher than that of PFF and 12.5 % higher than that of SFF.