Non-aqueous Redox Flow Batteries Research Articles

Anthraquinone-based electroactive ionic species as stable multi-redox anode active materials for high-performance nonaqueous redox flow batteries

Three AQ-based materials are designed by incorporating acetamide and tetraalkylammonium ionic groups. The solubility and stability are enhanced, and the NARFB with a two-electron transfer process shows high cycling performance.

Multicore Ferrocene Derivative as a Highly Soluble Cathode Material for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries (NARFBs) are considered promising electrochemical energy storage systems to overcome the limited electrochemical window of aqueous counterparts. However, the limited...

Insights into the Redox Chemistry of Organosulfides Towards Stable Molecule Design in Nonaqueous Energy Storage Systems

AbstractNonaqueous redox flow batteries (RFBs) have great potential to achieve high‐energy storage systems. However, they have been limited by low solubility and poor stability of active materials. Here we demonstrate organosulfides as a new‐type model material system to explore the rational design of redox‐active molecules in nonaqueous systems. The tetraethylthiuram disulfide (TETD) molecule shows high solubility in various common organic solvents and achieves a high reversible capacity of ca. 50 Ah L−1 at a high concentration of 1 M. The resonance structures in the reduced product endow the molecule with high electrochemical stability in different organic electrolytes. The underlying mechanism in redox chemistry of organodisulfides involving the cleavage and reformation of disulfide bonds is revealed by material/structural characterizations. This study provides a new perspective of molecule designs for the development of redox‐active materials for high‐performance nonaqueous RFBs.

Insights into the Redox Chemistry of Organosulfides Towards Stable Molecule Design in Nonaqueous Energy Storage Systems

AbstractNonaqueous redox flow batteries (RFBs) have great potential to achieve high‐energy storage systems. However, they have been limited by low solubility and poor stability of active materials. Here we demonstrate organosulfides as a new‐type model material system to explore the rational design of redox‐active molecules in nonaqueous systems. The tetraethylthiuram disulfide (TETD) molecule shows high solubility in various common organic solvents and achieves a high reversible capacity of ca. 50 Ah L−1 at a high concentration of 1 M. The resonance structures in the reduced product endow the molecule with high electrochemical stability in different organic electrolytes. The underlying mechanism in redox chemistry of organodisulfides involving the cleavage and reformation of disulfide bonds is revealed by material/structural characterizations. This study provides a new perspective of molecule designs for the development of redox‐active materials for high‐performance nonaqueous RFBs.

Diphenyl ditelluride as a low-potential and fast-kinetics anolyte for nonaqueous redox flow battery applications

Developing low-potential, fast-kinetics and highly concentrated anolyte materials is one of the major challenges for nonaqueous redox flow batteries (NRFBs). Organosulfides exhibit promising potentials for NRFBs owing to their high solubility and structure diversity. However, they suffer from poor reaction kinetics and intermediate reversible potential, leading to poor roundtrip efficiency and low full cell voltage. Herein, we report diphenyl ditelluride (PDTe) as a low-potential and fast-kinetics anolyte candidate. The reversible potential of PDTe (average 1.97 V vs. Li/Li+) is lower than that of its sulfur counterpart PDS (2.25 V), which offers higher full cell voltages. We demonstrated a Li/2.5 M PDTe half-cell with a high volumetric capacity (128 Ah L−1PDTe), and high coulombic and energy efficiency (>99.5% and 84.1%) over 100 cycles. A Li/0.1 M PDTe flow cell achieved a stable long cycling over 260 cycles (38 days) at 1.5 mA cm−2. Coupling with lithium iodide (LiI) catholyte, the PDTe/LiI full cells demonstrated stable cycling over 120 cycles. Density functional theory calculation reveals that the cleavage of ditelluride bond requires a much lower bond dissociation energy than that of disulfide bond, which significantly reduces battery voltage hysteresis (by 429 mV). This study demonstrates the promising potential of organotellurides as anolytes for energy storage applications.

Crosslinked poly(arylene ether ketone) membrane with high anion conductivity and selectivity for non-aqueous redox flow batteries

Anion exchange membranes (AEMs) consisting of poly(arylene ether ketone) (PAEK) were synthesized and crosslinked with alkyl chains containing quaternary ammonium ions for transporting anions. These crosslinked PAEK membranes (cPAEKs) enhance not only chemical/mechanical stability but also tetrafluoroborate (BF4−) conductivity for operating non-aqueous redox flow batteries (RFBs). Increased quaternary ammonium moieties of cPAEKs promote a high ion exchange capacity (IEC) and an aggregation of charge clusters that allowed the connection of ion transport channels. Moreover, the chemical structure of the polymer with butyl bridges can be expected to allow ion selectivity of the membrane for BF4−, for realizing AEMs for operating non-aqueous RFBs. The cell performance as a battery is investigated with improved cell efficiencies of 92%, 90% and 81% for coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) at a current density of 1 mA cm−1, respectively.

Exploring Redox Active and Electrochemically Stable Organic Molecules for > 3 V Non-Aqueous Redox Flow Batteries

Non-Aqueous Redox flow batteries (NARFBs) are emerging grid-scale energy storage systems that have the potential to overcome issues associated with aqueous RFBs, such as a limited voltage window due to water electrolysis and low energy densities. However, most non-aqueous electrolytes consist of metal coordination complexes that have significantly lower solubility due to their large molecular size compared to their aqueous counterparts and their cell voltages are typically limited to < 3 V.1 Besides, widespread acceptance of metal coordination complexes as the non-aqueous electrolyte is also restricted by the higher cost of the metal. Many of these metal coordination complexes undergo disproportionation; therefore, resulting flow batteries do not require selective membranes. However, cost targets outlined by Darling et al. and the U.S. Department of Energy require redox active materials to have > 3 V cell voltage and 4-5 M solubility and ~$5/Kg material cost.2 Therefore, there have been substantial efforts made to develop organic molecules either as anolyte or catholyte that meet many of these criteria for application in NARFBs. Some reported redox active organic molecules have solubility up to 5 M in common organic solvents.3-4 The lower molecular weights of organic molecules provide more room and freedom for molecular engineering to improve solubility, stability, and increase the potential window. However, deployment of these molecules in asymmetrical NARFBs raises the inherent issue of RFBs, i.e., irreversible capacity fade due to the absence of selective membranes.4 In this presentation, we will report a new class of sulfur and oxygen-containing redox active organic molecules for high voltage (> 3 V) NARFBs. Specifically, we will highlight the screening procedure and electrochemical properties of various anolytes and catholytes with higher redox potentials. We will also discuss strategies to minimize the capacity fade due to crossover. As proof of concept, anolyte and catholyte will be combined in a full cell, and cycling performance will also be discussed. Acknowledgements: The authors would like to thank Dr. Imre Gyuk and the U.S. Department of Energy, Office of Electricity for supporting this work.

Modeling the Impact of Sequential Two-Electron Transfer Compounds on Redox Flow Battery Performance

Redox flow batteries (RFBs) are promising candidates for grid-scale energy storage, but further cost reductions are needed for widespread use.1 As such, redox-active organic molecules and metal-coordination complexes are emerging as charge storage materials considering their redox potential, solubility, and stability can be tailored through molecular functionalization.2,3 Moreover, a number of these compounds can support multiple electron transfers, affording dramatic increases in energy storage capacity.4,5 However, the nature of the electron transfer events, concerted or sequential, plays a key role in determining whether the added capacity can be accessed efficiently. Specifically, depending on molecular features and cell operation, voltaic inefficiencies may be incurred which, in turn, offset any benefits of increased capacity. Therefore, we sought to quantify these losses and their relationship to molecular- and cell-level parameters to better understand the feasibility of multi-electron transfer in RFBs.In this presentation, we use a zero-dimensional electrochemical model, accounting for thermodynamic, kinetic, and mass transfer, to simulate the performance of two-electron compounds in redox flow cells. First, using a single half-cell, we show that voltaic efficiency is significantly influenced by the average redox potential and potential difference between the redox events. We also discuss the effects of different mass transfer coefficients and comproportionation reaction rates. Second, using a full cell, we consider the effects of utilizing two-electron compounds in both half-cells and subsequent effects of charge imbalance, both resulting in further inefficiencies. By understanding the operational tradeoffs of multi-electron compounds, we can develop design criterion to guide molecular engineering efforts towards more efficient high energy density electrolytes for RFBs.AcknowledgementsThis work was funded by the National Science Foundation (NSF) under Award Number 1805566. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.References(1) Chalamala, B. R.; Soundappan, T.; Fisher, G. R.; Anstey, M. R.; Viswanathan, V. V.; Perry, M. L. Redox Flow Batteries: An Engineering Perspective. Proceedings of the IEEE 2014, 102 (6), 976–999.(2) Hogue, R. W.; Toghill, K. E. Metal Coordination Complexes in Nonaqueous Redox Flow Batteries. Current Opinion in Electrochemistry 2019.(3) Ding, Y.; Zhang, C.; Zhang, L.; Zhou, Y.; Yu, G. Molecular Engineering of Organic Electroactive Materials for Redox Flow Batteries. Chem. Soc. Rev. 2018, 47 (1), 69–103.(4) Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries. J. Mater. Chem. A 2017, 5 (46), 24371–24379.(5) Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. Mechanism-Based Development of a Low-Potential, Soluble, and Cyclable Multielectron Anolyte for Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2016, 138 (47), 15378–15384.

Modeling and Understanding Variations in Redoxmer Cyclability

Non-aqueous redox flow batteries (RFBs) offer several benefits over aqueous RFBs including higher cell voltages and energy densities. Among candidate active materials for nonaqueous RFBs, redox active organics, or redoxmers, offer several advantages including low cost and high solubility in non-aqueous solvents. However, they currently lack the stability and cyclability to be used beyond laboratory scale. One challenge complicating redoxmer performance characterization is dependence on electrolyte choice and experimental conditions. We performed bulk electrolysis on three common catholyte materials while varying concentration, cycling rate, and electrolyte to identify trends in cyclability and recovery. In this talk, we report the results of these comprehensive cycling experiments, the ensuing trends, and preliminary modeling to help identify ideal conditions for cycling these redoxmers.

High Energy Density Non-Aqueous Organic Redox Flow Batteries

Renewable energy sources such as wind and solar are replacing fossil fuels for electricity generation. However, intermittency of wind and solar limits their wide-spread adoptions. The energy fed into the power grid must be matched with the consumer energy demand to prevent blackouts and destabilization of the grid [1, 2]. Recently, redox flow batteries (RFBs) have gained practical interest among the other energy storage technologies in light of their long lifetime, independent sizing of power and energy, high round-trip efficiency, scalability and design flexibility, fast response, and low environmental impact [3-5]. Organic redox-active materials have recently received attention as they provide competitive electrochemical characteristics, flexible design, and they are abundant in nature [6]. Aqueous designs face commercial difficulty because RFBs have low energy and power density due to the limited cell voltage of 1.23 V. The limited voltage is due to the evolutions of hydrogen and oxygen in the water electrolysis [7]. Solvent substitution is one solution to enable higher energy densities in RFBs, using non-aqueous solution also provides a large design space for enhancement of material solubility, cell potential and the number of electrons stored in the redox species [7-8].In this study, a new organic redox molecule, tetra amino anthraquinone (Disperse Blue 1: DB-1), is evaluated and compared with other organic systems reported in the literature [5, 7] such as benzoquinone (BQ), naphthoquinone (NQ), anthraquinone (AQ), tetramethyl piperidinyloxyl (Tempo), and phenylenediamine (PD) in non-aqueous solvent by means of cyclic voltammetry. A three-electrode system was utilized to conduct cyclic voltammetry (CV) experiments using glassy carbon working electrodes. The battery performance was evaluated using a flow cell with an electrode area of 2.5 cm2. The electrolytic solution, containing 40 mM DB-1 solution in dimethyl sulfoxide solvent (DMSO) and 1 M Bis (trifluoromethane) sulfonimide lithium salt, was circulated through the cell at a flow rate of 10 cm3 min-1. Graphite felt and Nafion 115 were used as the electrode and membrane, respectively.In addition, density functional theory (DFT) calculations were used to better understand the electrochemical behavior of the active quinone molecules at different oxidation states. Figure 1 shows the molecular orbital energy levels (HOMO and LUMO) of the DB-1 organic dye and other similar organic molecules obtained by DFT calculations in DMSO. A relatively small HOMO-LUMO gap means a lower overpotential required for the oxidation and reduction processes [8]. The DB-1 had narrower bandgaps (<3 eV) than other quinone molecules (> 3.9 eV), suggesting that the selected molecule has better kinetics than other organic molecules. The result describes a systematic evaluation of DB-1 as a redox active material for energy storage applications by means of electronic structures, electrochemical properties, and flow cell studies. Voltammetric studies and DFT calculations (in DMSO solvent) demonstrated that this was capable of forming both anions and cations at electrode potentials ranging from 1.7 to 4.4 V vs. Li, involving up to 6 electron-transfers and exhibiting one of the highest electrode potentials (up to 4.4 V vs. Li) in the literature. Since these quinone derivatives can be extracted from biomass materials, the proposed organic RFB system provides a greener and more sustainable option for grid-scale energy storage applications than metal based redox systems. The proposed chemistry in this work showed high energy efficiency (ca. 68 %) at a current density (20 mA cm-2) with close to 100% capacity retention. These results reveal that the molecular tuning of quinone structures is promising for RFBs applications. Figure 1

Combining Electrochemical, Fluid Dynamic, and Imaging Analyses to Understand the Effect of Electrode Microstructure and Electrolyte on Redox Flow Batteries

Understanding the interplay between electrode microstructure and cell performance of electrochemical devices is important both for modelling and experimental design. Redox Flow Batteries (RFBs) are an electrochemical energy storage technology with potential for grid-scale energy storage applications, although costs need to be further reduced to be competitive. One pathway to lowering the costs involves increasing the power density of each cell, such that fewer cells are required. This can be achieved in a variety of ways, including by improving the design of the electrode microstructures. The aim of this work is to better understand the relationship between electrode microstructure and RFB performance and, ultimately, to make inferences about electrode utility.The performances of a variety of commercially available carbon electrodes are examined via a series of commonly used microstructural and electrochemical analyses (Figure a-c). [1] We present a comprehensive study of pore-scale mass-transport processes occurring in each of the electrodes and rationalize their effect on the overall cell performance. A matrix of electrochemical tests were carried out in a flow-through RFB cell using incremental flow rates (Figure b-c) and two non-aqueous TEMPO electrolytes with distinct viscosity and diffusivity properties. Scanning electron microscopy (SEM) was used to image the electrodes and large 3 mm samples of each were scanned using X-ray computed tomography (Figure a). A customized segmentation technique was subsequently developed that resamples the image data to ensure the fiber dimensions agree with SEM images, improving the validity of the various extracted metrics. From these images, calculations and electrochemical tests, several microstructural parameters were extracted and a pore network model was used to calculate the permeabilities of the electrodes. A 1D model was developed across half the symmetric membrane electrode assembly and, using the parameters extracted from XCT, fit to galvanostatic polarization measurements to obtain the mass transfer coefficients at each unique operating condition – these were found to be in the range of and m s-1. Transport processes and distributions through the cell are analysed and correlations between dimensionless Reynolds and Sherwood numbers are subsequently discussed, as demonstrated in previous work. [2, 3]Given a ‘good performance’ is associated with a system being able to reach a high current with a low overpotential, the following findings were made. Firstly, while volume-specific surface area, porosity and tortuosity are useful descriptors for evaluating porous media, they are not good indicators of performance in these systems. Instead the permeability and, by extension, the ease with which convective transport can occur is found to be directly correlated with performance in this system. It is asserted that systems using solvents with higher viscosity have a reduced performance due, in part, due to regions in the electrode being starved of new electrolyte. Consistently, across all electrodes and in each electrolyte a better performance is observed when a higher flow rate is used. This effect is, in part, attributed to the reduction in size of the boundary layer at the electrode surface. Finally, in agreement with previous works, [1, 4] the carbon cloths tested in this work are shown to have higher mass-transfer coefficients and permeabilities than the carbon papers and we conclude that the best performing electrode is the thicker carbon cloth, which has the lowest resistance to convective flow, highest permeability, and highest mass-transfer coefficients in both solvents.[1]: A. Forner-Cuenca, E. E. Penn, A. M. Oliveira and F. R. Brushett, Exploring the Role of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries. J. Electrochem. Soc., 166, 2230-2241 (2019).[2]: Y. A. Gandomi, D. S. Aaron, T. A. Zawodzinski and M. M. Mench, In Situ Potential Distribution Measurement and Validated Model for All-Vanadium Redox Flow Battery. J. Electrochem. Soc., 163, A5188-A5201 (2016).[3]: K. M. Tenny, A. Forner-Cuenca, Y.-M. Chiang and F. R. Brushett, Comparing Physical and Electrochemical Properties of Different Weave Patterns for Carbon Cloth Electrodes in Redox Flow Batteries. J. Electrochem. En. Conv. Stor., 17, 041108 (2020).[4]: M. A. Sadeghi, M. Aganou, M. D. R. Kok, M. Aghighi, G. Merle, J. Barralet and J. Gostick, Exploring the Impact of Electrode Microstructure on Redox Flow Battery Performance Using a Multiphysics Pore Network Model. J. Electrochem. Soc., 166, A2121-A2130 (2019). Figure 1

Ferrocene/anthraquinone based bi-redox molecule for symmetric nonaqueous redox flow battery

The crossover of anolyte and catholyte during cycling places a serious obstacle for the performance of non-aqueous redox flow batteries (NARFBs). Employing symmetric RFB is an effective strategy to alleviate this issue. A new bi-redox material, 1-(ferrocenylmethyl-amino)-anthraquinone (FcMeAAQ) composed of ferrocene and anthraquinone two moieties, is designed for symmetrical NARFB. Both experimental and density functional theory calculational results show the relatively good stability of FcMeAAQ when one-electron redox reaction is involved. The assembled symmetric NARFB with the Daramic AA-250 porous membrane delivers an open circuit voltage of 1.42 V, Coulombic efficiency of 90.8%, and energy efficiency of 81.8% over 100 cycles at the current density of 2 mA cm−2. Furthermore, a peak power density of ~16 mW cm−2 is achieved at the state of charge of 100%.

High-Energy, Single-Ion-Mediated Nonaqueous Zinc-TEMPO Redox Flow Battery.

Two distinct advantages of nonaqueous redox flow batteries (RFBs) are the feasibility of building a high cell voltage (without a constraint of the water-splitting potential) and the operability at low temperatures (without a concern of freezing below 0 °C). However, electrochemically active organic redox couples are usually selectively soluble in specific nonaqueous solvents, and their solubility is relatively low (in contrast to that in aqueous solutions). The selective and low solubility of redox couples seriously constrict the practical energy density of nonaqueous RFBs. Herein, we present a hybrid nonaqueous RFB with a solid zinc anode and a liquid (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) cathode. Toward accessing a high solubility of the TEMPO cathode and to sufficiently accommodate the discharge products of a Zn anode, asymmetric electrolyte solvents, viz., propylene carbonate (PC) and acetonitrile (ACN), have, respectively, been employed at the cathode and anode. To prevent a mixing of the two asymmetric electrolyte solvents, a NASICON-type Na+-ion conductive solid-state electrolyte (SSE, Na3Zr2Si2PO12) is employed to serve as a mediator-ion separator. The shuttling of Na+ ions through the Na3Zr2Si2PO12 SSE sustains the ionic charge balance between the two electrodes. The Zn-TEMPO nonaqueous cell with a stable energy density of ca. 12-18 Wh L-1 over 50 cycles was demonstrated.

Bis(diisopropylamino)cyclopropenium-arene Cations as High Oxidation Potential and High Stability Catholytes for Non-aqueous Redox Flow Batteries.

This Article describes the development of 1,2-bis(diisopropylamino)-3-cyclopropenylium-functionalized (DAC-functionalized) benzene derivatives as high-potential catholytes for non-aqueous redox flow batteries. Density functional theory (DFT) calculations predict that the oxidation potentials (in CH3CN) of various DAC-benzene derivatives will range from +0.96 to +1.64 V vs Fc+/0, depending upon the substituents on the benzene ring. To test these predictions, a set of eight DAC-arene derivatives were synthesized and evaluated electrochemically. The molecule 1-DAC-4-tert-butyl-2-methoxy-5-pentafluoropropoxybenzene was found to offer the optimal balance of high redox potential (E1/2 = +1.19 V vs Fc+/0) and charge-discharge cycling stability (with 92% capacity retention over 116 h of cycling at 0.3 M concentration in a symmetrical flow cell). This optimal derivative was successfully deployed as a catholyte in a non-aqueous redox flow cell with butyl viologen as the anolyte to yield a 2.0 V battery.

Exploring the electrochemical properties of mixed ligand Fe(II) complexes as redox couples

Redox flow batteries are a promising technology for large scale energy storage systems. Although some types of the battery have been commercialized, the narrow working voltage ranges in the aqueous electrolytes limit their further applications. Organic solvents have shown to allow the use of a wider range of redox couples and the idea of non-aqueous redox flow batteries has been proposed. However, the low solubilities and poor electrochemical properties including stability and reversibility of the redox couples in the organic electrolytes remain a significant issue. In this work, a series of heteroleptic (mixed ligand) iron(II) complexes based on a combination of 2,2′-bipyridine, 1,10-phenanthroline and 4,4′-dimethyl-2,2′-bipyridine ligands were synthesized and their chemical and electrochemical properties investigated. For Fe (II) complexes, it was confirmed that ligand equilibration occurs in solution. The electrochemical properties of the complexes were studied by cyclic voltammogram and chronoamperometric methods. The mechanism of the redox processes of these complexes are proposed and how the mixed ligands will affect the redox potentials of the Fe (II) complexes is discussed. Furthermore, we show that the mixed ligand system can improve the total solubility of redox active species. The mixed ligand approach will also be beneficial for designing other new high solubility redox materials for non- aqueous flow batteries.

A Stable Organo-Aluminum Analyte Enables Multielectron Storage for a Nonaqueous Redox Flow Battery.

Redox flow batteries (RFBs) operate by storing electrons on soluble molecular anolytes and catholytes, and large increases in the energy density of RFBs could be achieved if multiple electrons could be stored in each molecular analyte. Here, we report an organoaluminum analyte, [(I2P-)2Al]+, in which four electrons can be stored on organic ligands, and for which charging and discharging cycles performed in a symmetric nonaqueous RFB configuration remain stable for over 100 cycles at 70% state of charge and 97% Coulombic efficiency (I2P is a bis(imino)pyridine ligand). The stability of the analyte is promoted by the kinetic inertness of the anolyte to trace water in solvents and by the redox inertness of the Al(III) ion to the applied current. The solubility of the analyte was optimized by exchanging the counteranion for trifluoromethanesulfonate (triflate), and the cell was further optimized using graphite rods as electrodes which, in comparison with glassy carbon and reticulated vitreous carbon, eliminated deposition of analyte on the electrode. Proof-of-principle experiments performed with an asymmetric NRFB configuration further demonstrate that up to four electrons can be stored in the cell with no degradation of the analyte over multiple cycles that show 96% Coulombic efficiency.

Characterisation of the ferrocene/ferrocenium ion redox couple as a model chemistry for non-aqueous redox flow battery research

The simple ferrocene/ferrocenium ion (Fc/FcBF4) redox couple was examined as a model chemistry for non-aqueous redox flow battery research. Its properties were fully characterised using voltammetry, flow-cell battery cycling, and UV–vis spectroscopy to validate flow-cell performance. Fc demonstrates facile kinetics and high stability of its oxidation states, making the Fc/FcBF4 redox couple a useful low-cost model chemistry, despite its limited 0.16 M solubility in acetonitrile. By use of ‘single redox couple cycling’, in which only the Fc/FcBF4 redox couple is battery cycled, the high capacity retention of Fc at 10 mM concentration was demonstrated; 80% capacity retention after 200 cycles (7.8 days). The mechanism for the capacity loss was investigated and diagnosed to occur via FcBF4 decomposition in the electrolyte, which proceeds irrespective of battery cycling.

Evaluation of ionic liquids as electrolytes for vanadium redox flow batteries

Non-aqueous redox flow batteries (NARFBs) are promising electrochemical energy storage devices due to their wide electrochemical potential windows, generally >2 V of organic solvents. This study aims to investigate the suitability of ionic liquids (ILs) as electrolytes for NARFBs containing a vanadium metal complex. The electrochemistry of a single-component NARFBs employing vanadium (III) acetylacetonate (V(acac)3) was studied in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C4mim][NTF2], and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, [C4mpyr][NTF2], electrolytes. The electrochemical kinetics of the anodic and cathodic reactions was measured using cyclic voltammetry. The VII/VIII and VIII/VIV couples were quasi-reversible and together yielded a cell potential of 2.2 V in both ILs. Charge/discharge characteristics show that a coulombic efficiency for cycles 1–50 ranged from 88 to 92% using a V(acac)3/[C4mpyr][NTF2] cell.

Viscous flow properties and hydrodynamic diameter of phenothiazine-based redox-active molecules in different supporting salt environments

We report viscous flow properties of a redox-active organic molecule, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), a candidate for non-aqueous redox flow batteries, and two of its radical cation salts. A microfluidic viscometer enabled the use of small sample volumes in determining viscosity as a function of shear rate and concentration in the non-aqueous solvent, acetonitrile, both with and without supporting salts. All solutions tested show Newtonian behavior over shear rates of up to 30 000 s−1, which was rationalized by scaling arguments for the diffusion-based relaxation time of a single MEEPT molecule without aggregation. Neat MEEPT is flowable but with a large viscosity (412 mPa⋅s at room temperature), which is ∼1000 times larger than that of acetonitrile. MEEPT solutions in acetonitrile have low viscosities; at concentrations up to 0.5 M, the viscosity increases by less than a factor of two. From concentration-dependent viscosity measurements, molecular information was inferred from intrinsic viscosity (hydrodynamic diameter) and the Huggins coefficient (interactions). Model fit credibility was assessed using the Bayesian Information Criterion. It is found that the MEEPT and its charged cations are “flowable” and do not flocculate at concentrations up to 0.5 M. MEEPT has a hydrodynamic diameter of around 8.5 Å, which is almost insensitive to supporting salt and state of charge. This size is comparable to molecular dimensions of single molecules obtained from optimized structures using density functional theory calculations. The results suggest that MEEPT is a promising candidate for redox flow batteries in terms of its viscous flow properties.

Reversible redox chemistry in azobenzene-based organic molecules for high-capacity and long-life nonaqueous redox flow batteries

Redox-active organic molecules have drawn extensive interests in redox flow batteries (RFBs) as promising active materials, but employing them in nonaqueous systems is far limited in terms of useable capacity and cycling stability. Here we introduce azobenzene-based organic compounds as new active materials to realize high-performance nonaqueous RFBs with long cycling life and high capacity. It is capable to achieve a stable long cycling with a low capacity decay of 0.014% per cycle and 0.16% per day over 1000 cycles. The stable cycling under a high concentration of 1 M is also realized, delivering a high reversible capacity of ~46 Ah L−1. The unique lithium-coupled redox chemistry accompanied with a voltage increase is observed and revealed by experimental characterization and theoretical simulation. With the reversible redox activity of azo group in π-conjugated structures, azobenzene-based molecules represent a class of promising redox-active organics for potential grid-scale energy storage systems.