Non-aqueous Redox Flow Batteries Research Articles

Persistent Radicals Derived from N-Heterocyclic Carbenes for Material Applications.

Persistent radicals are potential building blocks of novel materials in many fields. Recently, highly stable persistent radicals are considered to be within reach, thanks to several radical stabilization strategies such as spin delocalization and steric protection. N-Heterocyclic carbene (NHC)-derived substituents can be attached to a radical center for these purposes, as illustrated by numerous NHC-stabilized radicals reported in the last two decades.This Account describes our recent work on developing NHC-derived persistent radicals, as well as their prospective applications. Considering that NHCs not only stabilize radicals but also reversibly interact with gas molecules, in 2015 our group reported NHC-nitric oxide (NHC-NO) radicals produced by reversibly trapping nitric oxide (NO) radical gas in NHCs. The resultant compounds were loaded into biocompatible poly(ethylene glycol)-block-poly(caprolactone) (PEG-b-PCL) micelles and injected into tumor-bearing mice. Then, NO release was triggered by high-intensity focused ultrasound irradiation of the tumor tissue. Furthermore, the NHC-NO radicals could also serve as a platform to generate other organic radicals such as oxime ether or iminyl radicals. Apart from medicine-related applications, radicals stabilized by NHCs can be used as energy storage materials. In this context, the triazenyl radical containing two NHC units reported by our laboratory could be a cathode active material in batteries, as an organic alternative to LiCoO2. The subsequently prepared unsymmetrical triazenyl radical derivatives were applied as anolytes in nonaqueous all-organic redox flow batteries. In addition, a ferrocene-based redox flow battery anolyte was obtained by introducing NHC-derived substituents that effectively stabilize the ferrocenate derivatives previously reported only at low temperatures. The batteries containing NHC-supported radicals exhibited high energy efficiency and insignificant radical decomposition over multiple cycles. Finally, toward developing air-persistent organic radicals for flexible devices and MRI contrasting agents, we also highlight our recent air- and physiologically stable organic radicals derived from NHCs. Coordination of tris(pentafluorophenyl)borane to the NHC-NO radical produced a new radical cation that is stable in an organic solvent under air for several months. The readily accessible 1,2-dicarbonyl radical cations generated by the reaction of NHCs with oxalyl chloride are remarkably persistent even in an aqueous solution for several months. They are also highly stable even under physiological conditions, making them particularly attractive potential candidates for organic MRI contrast agents. We hope that this Account will serve as a guide for the future development of stable NHC-derived organic radicals and draw the attention of the synthetic community to their potential applications in material science.

A Critical Approach to Multi-Electron Materials for High Energy Density Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are very promising storage systems in the transition towards renewable energy sources. They can be broadly classified in aqueous and non-aqueous systems. Last-named operate with organic solvents, which allow for a much broader potential window (up to three times higher) compared to water. Combination of organic solvents with organic redox active materials could pave the way for all carbon-based RFBs with superior energy densities compared to aqueous systems.In this contribution, newly developed organic electrolytes will be presented, focusing on their performance as RFB materials in acetonitrile with quaternary ammonium electrolyte salts. To push the limits of energy density, we investigated multi-electron reduction and oxidation reactions on a single molecule.As catholyte, tetrathiafulvalene (TTF) as a core structure was used. The unsubstituted TTF exhibits two reversible oxidation events (-0.04 and +0.34 V vs Fc/Fc+) but a poor solubility in acetonitrile. To optimize this and to achieve a higher oxidation potential a series of molecules with different side chains was synthesized. The resulting solubilities led to volumetric capacities of up to 71 Ah/L for the newly designed compounds. Electrochemical cycling stability was evaluated in bulk electrolysis, UV-Vis-NIR and flow cycling experiments. Current state-of-the-art anolytes based on N-methylphthalimide compounds exhibit one reversible reduction pair at a desirable low reduction potential (-1.87 V vs Fc/Fc+) and good cycling stability in bulk electrolysis experiments.[1] Expanding on this structure, new derivatives with one or two additional functional imide groups per phthalimide core were synthesized. Consequently, molecules with a single (-1.87 V vs Fc/Fc+), double (-1.26 and -1.88 V vs Fc/Fc+) and triple reduction event (-1.02, -1.65 and -2.37 V vs Fc/Fc+) can be obtained. To optimize their solubility in acetonitrile, a series of molecules with different side chains was synthesized for each of the three core structures. Determination of the solubility led to volumetric capacities of up to 66 Ah/L for the newly developed compounds. Additionally, we tested the electrochemical cycling stability in bulk electrolysis, UV-Vis-NIR, coin cell and flow cycling experiments. In the final flow battery, a high energy density of 24 Wh/L was achieved (at 1 M of transferred electrons).[2] All of this led to a critical comparison between the different molecular designs, cumulating in design rules that will influence future designs of the organic redox active compounds for organic RFBs. [1] Wei, X.; Duan, W.; Huang, J.; Zhang, L.; Li, B.; Reed, D.; Xu, W.; Sprenkle, V.; Wang, W. A High-Current, Stable Nonaqueous Organic Redox Flow Battery. ACS Energy Lett. 2016, 1, 705−711 [2] Daub, N.; Janssen, R. A. J.; Hendriks, K. H. Imide-Based Multielectron Anolytes as High-Performance Materials in Nonaqueous Redox Flow Batteries. ACS Appl. Energy Mater. 2021, 4, 9, 9248–9257

Fundamental Aspects of the Electron Transfer Processes in Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) have garnered increasing attention for energy storage applications with non-aqueous RFBs offering several advantages such as larger voltage windows and the prospect of high energy densities. To increase the efficacy of non-aqueous RFBs, much effort has gone into the design of redox moieties that are highly soluble, cheaper, and robust, with desired electrochemical properties. To evaluate the capability of these redox moieties for RFB applications, the kinetics and thermodynamics of electron transfer events need to be understood. These insights could help towards rational design of redox moieties and to predict RFB performance. Techniques such as cyclic voltammetry and rotating disk electrode are commonly used to study the fundamental parameters of electron transfer process and thermodynamic properties of the redox moieties in both the neutral and charged states.In this work, we investigate the effects of supporting electrolytes on the thermodynamics and kinetics of the redox moieties. Vanadium (III) acetylacetonate and iron complexes are the bipolar model redox moieties used as both negative and positive electrolyte in non-aqueous RFB applications.1 While many redox moieties are stable in the neutral state, it is during cycling that degradation of the complexes typically occurs. As shown previously, the supporting electrolyte composition plays a key role in the performance and stability of the redox species in RFBs.2 We found that ion pairing between the charged redox moieties and the supporting electrolyte resulted in lower diffusion coefficients, it also has a negative impact on the kinetics of charge transfer process. We propose that the observed ion pairing can explain the stabilizing effects of supporting electrolytes on redox moieties. The magnitude of this effect can be explained by hard soft acid base theory (HSAB) which can be further used to predict the extent of ionic interactions of the charged species in the RFB electrolytes. Acknowledgements: Los Alamos National Laboratory is operated by Triad, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (Contract No. 89233218NCA000001). Authors would like to thank Dr. Imre Gyuk and financial support from the U.S. Department of Energy’s Office of Electricity for the publication of this work is gratefully acknowledged. Sharma, G.A. Andrade, S. Maurya, I.A. Popov, E.R. Batista, B.L. Davis, R. Mukundan, N.C. Smythe, A.M. Tondreau, P. Yang, J.C. Gordon, Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications, Energy Storage Mater., 37 (2021), pp. 576-586Wei X, Xu W, Huang J, Zhang L, Walter E, Lawrence C, Vijayakumar M, Henderson WA, Liu T, Cosimbescu L, Li B, Sprenkle V, Wang W. “Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery” Angew Chem Int Ed Engl. 2015, 54 (30): 8684-7.

Novel Anolyte Redox Active Organic Molecules for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries (NARFBs) offer several advantages over traditional aqueous electrolyte-based redox flow batteries, such as higher cell voltage, potentially higher energy density, and flexible operating temperatures. However, the current aqueous chemistries use toxic metals such as Vanadium and Chromium and highly acidic and oxidative acid mixtures. The efforts to develop metal-ligand based chemistries to tap the benefits of NARFBs have met with mixed success and only V(acac)3 based symmetric NARFBs have shown potential for long term operations. Even so, the solubility of V(acac)3 in non-aqueous solvents is low to compete with their aqueous counterpart. Moreover, modification of V(acac)3 to enhance the solubility has resulted in poor redox cyclability. As a result, recent research trends have shifted from the development of Metal-ligand complexes to small redox-active organic molecules (ROMs), which could provide higher solubility. However, due to the developing field, a few ROMs have been reported as stable electron donors and acceptors in their oxidized and reduced states. Therefore, we have focused on developing highly soluble anolyte materials with stable redox states. This presentation will discuss the design principle, synthesis, and electrochemical analysis of novel anolyte ROMs. The additional information about the kinetics, bulk electrolysis, and the performance in the full cell will also be discussed. Acknowledgement This work is supported by Laboratory Directed Research & Development (20210680ECR), Los Alamos National Laboratory.

Dicationic Heteroaryl Pyridinium As a Highly Stable, Soluble, and Crossover-Resistant Anolyte for Nonaqueous Redox Flow Batteries

Recently, redox flow batteries (RFBs) have emerged as one of the promising candidates for safe and economical grid-scale energy storage system (ESS) that store the intermittent energies such as wind or solar energy. Unlike the conventional aqueous vanadium RFBs, the design of new molecules in nonaqueous RFB (NRFB) has been in the spotlight for the high energy density, cycling stability, and low cost. Among the redox active materials, in particular, organic molecule has various structures through functionalization, and thus its redox potential, solubility, and chemical stability can be controlled. However, there are only a few viable negative-side redox electrolyte, called a negolyte (or anolyte) to date, and even the reported negolytes have suffered from low solubility and stability, and severe crossover problems for long-term cycling.Here we show the systematic design strategies for pyridinium-based organic redox molecule to enhance the its stability and solubility and suppress the crossover rate for NRFB. A benzo[d]thiazole ring, which provides an electron-withdrawing effect, was introduced at the C4 position of pyridinium core by Pd catalyzed C-H arylation. The addition of the π-conjugation system to the pyridinium redox core was key to enhance chemical and electrochemical stability, resulting in negatively low redox potential of -1.19 ~ 1.21 V vs. Fc/Fc+. The solubility of pyridinium derivatives was significantly enhanced from 0.26 M to 1.00 M in acetonitrile by simple anion exchange from tetrafluoroborate (BF4 -) or hexafluorophosphate (PF6 -) anion to bis(trifluoromethanesulfonyl)imide (TFSI-) anion. Moreover, exchanging the functional group on N of pyridinium from methyl group to cationic 3-(trimethylammonio)propyl (TMAP) group suppressed the crossover rate with an anion-exchange membrane in the NRFB. 4-(benzo[d]thaizol-2-yl)-1-(3-(trimethylammonio)propyl)pyridin-1-ium (TMAP-BTP) led to electrochemical stability in symmetric cell, showing a capacity decay rate of 0.0083% per cycle. Contrary to the results of only 60% capacity retention after 100 cycle in full cell with 4-(benzo[d]thaizol-2-yl)-1-methylpyridin-1-ium (BTP) as negolyte, in the case of full cell with TMAP-BTP as negolyte, the capacity retention was significantly increased, showing 89.8 % after 100 cycle, which is ~0.08% capacity decay rate per cycle. In this presentation, I will demonstrate the more detailed design strategies, cycling performances, and electrolyte analysis. (Figure 1).1 References Ahn, J. H. Jang, J. Kang, M. Na, J. Seo, V. Singh, J. M. Joo and H. R. Byon, ACS Energy Lett., 6, 3390 (2021). Figure 1

(Digital Presentation) Novel Organic Materials for Non-Aqueous Redox Flow Batteries: Implementation of Triarylamine and Phenazine Core Structures

The rapid growth of the role of renewable energy sources dictates new requirements for the efficiency, stability and scales of electrochemical energy storage devices for stationary applications [1]. Among the storage systems, redox flow batteries (RFBs) are regarded as a promising technology, since their advantages of excellent scalability, low cost, easy fabrication and operation, long lifetime, and safety. Today inorganic RFBs are penetrating the market, however, low specific capacity in conjunction with low electrochemical stability window of aqueous electrolytes (≈1.5 V) and safety issues, hinders their wide-scale commercialization. [2]. Replacing the inorganic materials with environment-friendly organic redox-active molecules may solve the capacity problems and safety issues. Moreover, the application of non-aqueous electrolytes provides a wide electrochemical stability window (e.g., up to 5 V for acetonitrile) enabling high-voltage batteries with increased energy density [3].Within the framework of the current project, we implemented a comprehensive study for a large group of novel highly soluble organic materials based on aromatic amines with general formulas of NPh3RnBrm (M1-M4) and N2Ph5RnBrm (M5-M7) where R=-(OCH2CH2)2-OCH3 (Fig. 1a). All the compounds demonstrated high solubility in MeCN (from >2.2 M up to complete miscibility), which can potentially enable outstanding specific capacities of organic RFBs approaching 134 Ah L-1 [4]. Compounds demonstrated one or two quasi-reversible electron transition processes with redox potential up to 0.6 V vs. Ag/AgNO3 electrode, which makes them perspective for the investigation in the RFBs as catholyte materials.For the RFB investigation butylviologen perchlorate (-0.75V vs. Ag/AgNO3, ~1.15 V battery voltage) was chosen as the redox pair (Fig. 1b, d). On the first step, the selection of the most appropriate electrolyte was performed: it was shown that the usage of electrolytes that contained lithium cations (Li+) and hexafluorophosphate anions (PF6 -) leads to fast decreasing of all the parameters of the RFBs, whereas the usage of the tetrabutylammonium tetrafluoroborate (TBABF4) and NaClO4 produces the stable characteristics (Fig. 1e). Final RFB tests proved that the most promising systems are capable to exhibit 65% of maximum capacities and more than 95% coulombic efficiency after 50 cycles [4] (Fig. 1f).In the next step, we focused on the creation of low-voltage anolyte material: thus, we synthesized and investigated novel phenazine derivative with oligomeric ethylene glycol ether substituents as promising anolyte material for non-aqueous organic RFBs (Fig. 1c) [5]. The designed compound undergoes a reversible and stable reduction at -1.72 V vs. Ag/AgNO3 and demonstrates excellent (>2.5 M) solubility in MeCN. A non-aqueous organic redox flow battery assembled using novel phenazine derivative as anolyte and substituted triarylamine derivative as a catholyte exhibited high specific capacity (~93% from the theoretical value), >95% coulombic efficiency, 65% utilization of active materials and good charge-discharge cycling stability (Fig. 1g).To summarize, triarylamine-based and phenazine-based materials establish themselves attractive for future research: obtained redox potentials, high solubility, fast diffusion and kinetics opens promising future directions for their usage as organic cathodic and anodic materials for non-aqueous RFBs.

(Digital Presentation) Development of a Bio-Inspired Non-Aqueous Redox Flow Battery Utilizing Anionic Active Materials

Energy storage is an indispensable part of future electrical grids as they increasingly shift toward renewable energy sources.1 Although many other energy storage technologies have shown potential, redox flow batteries are especially promising because of advantages arising from their decoupled power and energy capacity, wide range of possible electroactive materials, wider potential window (in non-aqueous solvents) and safety. 2 However, the nascent non-aqueous redox flow battery (NRFB) technology currently faces key challenges that need to be addressed to ensure their reliability and widespread implementation. Low stability of active material during deep cycling is one such challenge that needs to be addressed while low solubility of active material in organic solvent is another main hurdle that greatly limits energy density.Herein, we present a cost-effective, experimental-theoretical approach 3 , 4 to improve the solubility of a vanadium-based, highly stable, anionic active material known as vanadium-bis-hydroxyiminodiacetate (VBH). This is accomplished by tuning the key thermodynamic quantities of free energy of solvation and free energy of the lattice. We also demonstrate that the lattice free energy, which has been largely ignored in theoretical solubility models, is vital to obtain a meaningful prediction of solubility, and cannot be overlooked. Finally, we report the full cell cycling of the highly soluble VBH, coupled with an anthraquinone, exhibiting excellent cyclability and a reliable spectroscopic method to characterize pre- and post-cycled electrolytes.References(1) IEA. Renewable Energy Market Update 2021 ; Paris, 2021.(2) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334 (6058), 928-935. DOI: 10.1126/science.1212741.(3) Pahari, S. K.; Gokoglan, T. C.; Visayas, B. R. B.; Woehl, J.; Golen, J. A.; Howland, R.; Mayes, M. L.; Agar, E.; Cappillino, P. J. Designing high energy density flow batteries by tuning active-material thermodynamics. RSC Advances 2021, 11 (10), 5432-5443, 10.1039/D0RA10913D. DOI: 10.1039/D0RA10913D.(4) Visayas, B. R. B.; Pahari, S. K.; Gokoglan, T. C.; Golen, J. A.; Agar, E.; Cappillino, P. J.; Mayes, M. L. Computational and experimental investigation of the effect of cation structure on the solubility of anionic flow battery active-materials. Chemical Science 2021, 12 (48), 15892-15907, 10.1039/D1SC04990A. DOI: 10.1039/D1SC04990A.

Temperature Dependent Study of AcFc-FeIII(acac)3 Redox Couple for Non-Aqueous Redox Flow Battery

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale. Non-aqueous redox flow battery (NARFB) is one of the primary response to fulfill the next generation energy demand.1–6 This study describes the temperature dependence electrochemical analysis of acetyl ferrocene (AcFc) and iron(III) acetyl acetonate ([Fe(acac)3]) for NARFBs. An electrochemical cell consisting of AcFc and ([Fe(acac)3]) as catholyte and anolyte species, respectively, was constructed with a cell voltage of 1.41 V and Coulombic efficiencies >99% for up to 50 total cycles at 25o C (RT) and 0o C. Rotating ring disk electrode (RRDE) experiments suggest that diffusion coefficient decreases as the temperature decreases but overall storing capacity far better than aqueous redox flow battery (ARFBs). The electrochemical kinetic rate constant (k0) of AcFc found larger than Fe(acac)3 but remains similar at both temperatures. NMR study shows no structural change in changing temperature and after battery experiments. References (1) Mazumder, M. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron Vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. ECS Meet. Abstr. 2021, MA2021-01 (1), 52–52. https://doi.org/10.1149/ma2021-01152mtgabs.(2) Islam, R.; Mazumder, M. M. R.; Farnum, B. H. Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage. ECS Meet. Abstr. 2021, MA2021-01 (1), 54–54. https://doi.org/10.1149/ma2021-01154mtgabs.(3) Saha, S.; Sahil, S. T.; Mazumder, Md. M. R.; Stephens, A. M.; Cronin, B.; Duin, E. C.; Jurss, J. W.; Farnum, B. H. Synthesis, Characterization, and Electrocatalytic Activity of Bis(Pyridylimino)Isoindoline Cu( ii ) and Ni( ii ) Complexes. Dalton Trans. 2021, 10.1039.D0DT03030A. https://doi.org/10.1039/D0DT03030A.(4) Mazumder, M. M. R.; Farnum, B. H. Two One Electron vs One Two-Electron Redox System of Nickel Diethyldithiocarbamate for Redox Flow Battery; ACS, 2021. https://doi.org/10.1021/scimeetings.1c00951.(5) Mazumder, M. M. R.; Farnum, B. H; Burton Andricus. Mechanistic Study of Multi-Electron Redox Cycle of Nickel Dithiocarbamate Complex for Redox Flow Battery Application; ACS, 2021. https://doi.org/10.1021/scimeetings.1c00387.(6) Mazumder, M. M. R. Ni(IV/II) Redox Couple 2e- Reversibility Improvement for Redox Flow Battery. ECS Meet. Abstr. 2021, MA2021-02 (5), 1855–1855. https://doi.org/10.1149/ma2021-0251855mtgabs. Figure 1

Neutron Radiography As a Powerful Method to Visualize Reactive Flows in Redox Flow Batteries

Spatial and temporal gradients in reactant concentration, influenced by local microstructure and surface properties, govern the performance and durability of various advanced electrochemical systems. The cell and stack performance is typically assessed using traditional electrochemical diagnostics (e.g. polarization curves, electrochemical impedance spectroscopy) and the influence of materials is macroscopically evaluated based on empirical comparison of novel materials with the current state-of-the-art. While this is a valid approach to identify promising candidates, valuable information is lost due to the difficulty of identifying performance-limiting factors. Operando imaging of electrochemical systems, in tandem with complementary electrochemical diagnostics, has been instrumental in the development of advanced polymer electrolyte fuel cells1,2 and, more recently, lithium-ion batteries3,4. Over the past few years, several groups have developed novel imaging and spectroscopic techniques for operando characterization of redox flow batteries, which is the focus of this work. Wong et al. employed fluorescence microscopy and particle velocimetry to image concentration and velocity distributions near the electrode-flow field interface5. Tanaka et al. visualized flow distribution in redox flow batteries with infrared thermography6. Zhao et al. employed in-situ nuclear magnetic resonance to track reaction mechanisms occurring within the electrolyte7. Finally, several groups recently employed X-ray tomographic microscopy to visualize gas pockets within the liquid electrolyte imbibed porous electrode8–10. While these methods have provided important insights, an approach that enables quantitative mapping of species concentrations, in a non-invasive fashion and within an operating cell, has remained elusive.In this presentation, I will discuss our recent efforts to develop neutron radiography as an operando characterization method for non-aqueous redox flow batteries. We leverage the high attenuation of organic materials (i.e., high hydrogen content) in solution and, combined with isotopic labelling, we perform subtractive neutron imaging to quantify the concentration of active species and supporting electrolytes. To demonstrate the potential of this diagnostic tool, we characterize active species concentration distribution within a redox flow cell in a single electrolyte configuration with a non-aqueous electrolyte containing a TEMPO/TEMPO+ redox couple and study the influence of electrode microstructure, membrane type (e.g. porous or dense), and flow field design. To resolve the concentration profiles across the different layers, we employ the in-plane imaging configuration11 and correlate these concentration profiles to cell performance via polarization measurements under different operating conditions. In the final part of the talk, I will discuss our latest experimental campaign in which we investigated the use of energy-selective neutron radiography to deconvolute concentrations of active species and supporting electrodes during operation. References 1 P. Boillat, E. H. Lehmann, P. Trtik and M. Cochet, Curr. Opin. Electrochem., , DOI:https://doi.org/10.1016/j.coelec.2017.07.012.2 J. Eller, T. Rosén, F. Marone, M. Stampanoni, A. Wokaun and F. N. Büchi, J. Electrochem. Soc., 2011, 158, B963.3 B. Michalak, H. Sommer, D. Mannes, A. Kaestner, T. Brezesinski and J. Janek, Sci. Rep., 2015, 5, 15627.4 D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett and P. R. Shearing, Nat. Commun., 2015, 6, 6924.5 A. A. Wong, M. J. Aziz and S. Rubinstein, ECS Trans. , 2017, 77, 153–161.6 H. Tanaka, Y. Miyafuji, J. Fukushima, T. Tayama, T. Sugita, M. Takezawa and T. Muta, J. Energy Storage, 2018, 19, 67–72.7 E. W. Zhao, T. Liu, E. Jónsson, J. Lee, I. Temprano, R. B. Jethwa, A. Wang, H. Smith, J. Carretero-González, Q. Song and C. P. Grey, Nature, 2020, 579, 224–228.8 R. Jervis, L. D. Brown, T. P. Neville, J. Millichamp, D. P. Finegan, T. M. M. Heenan, D. J. L. Brett and P. R. Shearing, J. Phys. D. Appl. Phys., , DOI:10.1088/0022-3727/49/43/434002.9 F. Tariq, J. Rubio-Garcia, V. Yufit, A. Bertei, B. K. Chakrabarti, A. Kucernak and N. Brandon, Sustain. Energy Fuels, 2018, 2, 2068–2080.10 K. Köble, L. Eifert, N. Bevilacqua, K. F. Fahy, A. Bazylak and R. Zeis, J. Power Sources, , DOI:10.1016/j.jpowsour.2021.229660.11 P. Boillat, D. Kramer, B. C. Seyfang, G. Frei, E. Lehmann, G. G. Scherer, A. Wokaun, Y. Ichikawa, Y. Tasaki and K. Shinohara, Electrochem. commun., 2008, 10, 546–550.

Toward High Energy Density Redox Targeting Flow Batteries With a Mushroom-Derived Electrolyte

Abstract Among several types of redox flow batteries (RFBs) under development, non-aqueous redox flow batteries (NRFBs) have the potential to approach the energy density of lithium-ion batteries, while maintaining the advantages of flow systems, including ability to decouple power and energy ratings, and thermal stability. Despite their promise, NRFBs suffer from low energy densities because the solubility limitation of redox species in non-aqueous solvents remains relatively lower compared to water. One promising concept for drastically improving the energy density of NRFBs is the utilization of solid charge storage materials, which are reversibly oxidized or reduced in the electrolyte tanks upon interaction with the redox active species (mediators) dissolved in electrolyte (i.e., redox-targeting flow battery (RTFB)). Herein, we demonstrate a RTFB using a highly stable, bio-inspired mediator, vanadium(IV/V)bis-hydroxyiminodiacetate (VBH), coupled with cobalt hexacyanoferrate (CoHCF) as the solid charge storage material. Based on the charge/discharge cycling experiments, the energy capacity was found to be enhanced by ∼5x when CoHCF pellets were added into the tank compared to the case without CoHCF. With the pellet approach, up to ∼70% of the theoretical capacity of CoHCF were utilized at 10 mA cm−2 current density. Sufficient evidence has indicated that this concept utilizing redox-targeting reactions makes it possible to surpass the solubility limitations of the active material, allowing for unprecedented improvements to the energy density of RFBs.

Fluorination Enables Simultaneous Improvements of a Dialkoxybenzene-Based Redoxmer for Nonaqueous Redox Flow Batteries.

Redoxmers or redox-active organic materials, are one critical component for nonaqueous redox flow batteries (RFBs), which hold high promise in enabling the time domain of the grid. While tuning redox potentials of redoxmers is a very effective way to enhance energy densities of NRFBs, those improvements often accompany accelerated kinetics of the charged species, undermining stability and cycling performance. Herein, a strategy for designing redoxmers with simultaneous improvements in redox potential and stability is proposed. Specifically, the redoxmer 1,4-di-tert-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene (ANL-C46) is developed by incorporating fluorinated substitutions into the dialkoxybenzene-based platform. Compared to the non-fluorinated analogue, ANL-C46 demonstrates not only an increased (∼0.41 V) redox potential but also much enhanced stability (1.6 times) and cyclability (4 times) evidenced by electron paramagnetic resonance kinetic study, H-cell and flow cell cycling. In fact, the cycling performance of ANL-C46 is among the best of high potential (>1.0 V vs Ag/Ag+) redoxmers ever reported. Density functional theory calculations suggest that while the introduced fluorine substitutions elevate the redox potentials, they also help to depress the decomposition reactions of the charged redoxmers, affording excellent stability. The findings represent an interesting strategy for simultaneously improving energy density and stability, which could further prompt the development of high-performance redoxmers.

Misreported non-aqueous reference potentials: The battery research endemic

Beth E. Murdock is a PhD student working across the Chemistry Department and Materials Science Institute at Lancaster University, under the supervision of Dr. Nuria Tapia-Ruiz and Dr. Kathryn Toghill. Her current research is focused on understanding the structure-property relationships in high-voltage substituted spinel cathode materials for lithium-ion batteries using X-ray and neutron diffraction techniques. Craig G. Armstrong joined Lancaster University in 2011 as an undergraduate student in natural sciences and later graduated in 2015 with a first-class MS degree with chemistry and physics study themes. He then began a PhD in chemistry at LU with Dr. Kathryn Toghill as her first doctoral student. In 2019 he was awarded his doctorate in electrochemistry regarding non-aqueous redox flow batteries with five original research articles. He then continued research with Dr. Kathryn Toghill as a postdoctoral researcher in carbon dioxide reduction, and after a decade at LU, he is now a postdoc at Swansea University. Daniel E. Smith is a recent PhD graduate from the University of Nottingham's School of Chemistry under the supervision of Dr. Darren A. Walsh. His PhD research concerned electrocatalytic reactions in ionic liquid electrolytes: specifically, the oxygen-reduction, hydrogen-oxidation, and formic acid-oxidation reactions. Published work contains fundamental insights into proton shuttling in protic ionic liquid electrolyte-based fuel cells. Daniel joins the group of Dr. Kathryn Toghill studying the electro-reduction of carbon dioxide on a European Research Council grant. In 2021, Daniel was awarded the Sir Fraser and Norma Stoddart Prize for Outstanding Citizenship and Academic Excellence in Postgraduate Research from the University of Nottingham School of Chemistry. Nuria Tapia-Ruiz obtained her PhD from the University of Glasgow in 2013. She then worked as a research fellow in the team of Prof. Bruce at the University of St. Andrews and the University of Oxford from 2013 to 2016. Currently, she is a senior lecturer in the Department of Chemistry at Lancaster University. Her research interests include the understanding of the structure-property-performance relationships in materials for energy storage such as monovalent and multivalent batteries and supercapacitors. Kathryn E. Toghill joined Lancaster University, UK in 2014. Prior to this, she worked at EPFL in Switzerland following a PhD and a postdoctoral position at Oxford University. As a senior lecturer in the Department of Chemistry, Kathryn teaches on electrochemistry and electrochemical energy storage. Her research interests include electrochemical energy storage, electrochemical CO₂ reduction, catalysis, and electroanalysis, spanning fundamental understanding to applied systems with a focus on sustainability.

Techno-economic analysis of non-aqueous hybrid redox flow batteries

Renewable energy has become indispensable to improving human life, but its growth is hampered by a lack of cost-effective energy storage systems to solve the intermittency problem. Non-aqueous hybrid redox flow batteries (NAqHRFBs), based on lithium metal anode and organic redox molecules (redoxmers), have been investigated as an attractive energy storage option because of their high cell voltages and energy densities compared to other redox flow battery candidates. However, little is known about the economic potential of NAqHRFBs, as well as the operational and materials impacts. This work establishes a techno-economic model to analyze the capital costs of NAqHRFBs with selected organic redoxmers, including 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Sensitivity analyses for current density, area-specific resistance, cell voltage, electrolyte composition, redoxmer price, and equivalent molecular weight indicate the key factors in controlling NAqHRFB capital cost. To make the current NAqHRFB cost-effective, the first priority is to increase the operation current density over 10 times of those used in lab-scale tests, followed by adjusting redoxmer-related characteristics to afford more cost reduction space such as decreasing the unit price by ∼20 fold. The results have shed light on potential material development and system engineering directions to make NAqHRFBs viable for renewable energy storage.

Coupling Tetraalkylammonium and Ethylene Glycol Ether Side Chain To Enable Highly Soluble Anthraquinone-Based Ionic Species for Nonaqueous Redox Flow Battery.

Nonaqueous redox flow batteries (NARFBs) have promise for large-scale energy storage with high energy density. Developing advanced active materials is of paramount importance to achieve high stability and energy density. Herein, we adopt the molecular engineering strategy by coupling tetraalkylammonium and an ethylene glycol ether side chain to design anthraquinone-based ionic active species. By adjusting the length of the ethylene glycol ether chain, an ionic active species 2-((9,10-dioxo-9,10-dihydroanthracen-1-yl)amino)-N-(2-(2-methoxyethoxy)ethyl)-(N,N-dimethylethan-1-aminium)-bis(trifluoromethylsulfonyl)imide (AQEG2TFSI) with high solubility and stability is obtained. Paired with a FcNTFSI cathode, the full battery provides an impressive cycling performance with discharge capacity retentions of 99.96% and 99.74% per cycle over 100 cycles with 0.1 and 0.4 M AQEG2TFSI, respectively.

Highly selective metal-organic framework-based (MOF-5) separator for non-aqueous redox flow battery

• A facile and scalable process is developed to fabricate separator for NARFB. • The separator achieves 100% materials utilization and excellent repairability. • The separator shows a remarkable ionic selectivity of 26.6 and delivers 99.7% CE. • 0.04% per cycle capacity decay over 200 cycles of the NARFB. Non-aqueous redox flow batteries (NARFBs) are promising in the grid-scale energy storage for the harvesting of the intermittent renewable power sources. However, the lack of efficient separator is still impeding their further development. A flexible nanoporous separator is prepared through a “rolling dough” strategy, with zinc metal-organic framework (MOF-5) and polytetrafluoroethylene as the substrate. The prepared separator shows a remarkable ionic selectivity of Li + over N-(ferrocenylmethyl)-N,N-dimethyl-N-ethylammonium ions (Fc1N112 + ) at a high ratio 26.6. A Li-based hybrid NARFB constructed with the separator exhibits substantially high Coulombic efficiency 99.7% and average discharge capacity 2.26 Ah L −1 with capacity retention 99.96% per cycle over 200 cycles at 4 mA cm −2 . The excellent performance is attributed to the MOF nanoparticles and the designed composite structure, which achieves both high ionic selectivity and chemical stability simultaneously. This work provides a facile and efficient strategy to fabricate high-performance separator for NARFBs.

Rationally Designed Ternary Deep Eutectic Solvent Enabling Higher Performance for Non-Aqueous Redox Flow Batteries

Redox flow batteries hold promise as large-scale energy storage systems for off-grid electrification. The electrolyte is one of the key components of redox batteries. Inspired by the mechanism involved in solvents for extraction, a ternary deep eutectic solvent (DES) is demonstrated, in which glycerol is introduced into the original binary ethaline DES. Redox pairs (active substance) dissolved in the solvent have low charge transfer resistance. The results show that the viscosity of the solvent with the ratio of choline chloride to ethylene glycol to glycerol of 1:2:0.5 decreases from 51.2 mPa·s to 40.3 mPa·s after adding the redox pair, implying that the mass transfer resistance of redox pairs in this solvent is reduced. Subsequent cyclic voltammetry and impedance tests show that the electrochemical performance with the ternary DES as the electrolyte in redox flow batteries is improved. When the ratio of 1:2:0.5 ternary DES is used as the electrolyte, the power density of the battery (9.01 mW·cm−2) is 38.2% higher than that of the binary one (6.52 mW·cm−2). Fourier transform infrared spectroscopy further indicates that the introduction of glycerol breaks the hydrogen bond network of the solvent environment where the redox pair is located, unraveling the hydrogen bond supramolecular complex. Rational solvent design is an effective strategy to enhance the electrochemical performance of redox batteries.

Targeted Optimization of Phenoxazine Redox Center for Nonaqueous Redox Flow Batteries

Targeted Optimization of Phenoxazine Redox Center for Nonaqueous Redox Flow Batteries

Porous poly(vinylidene fluoride) (PVDF) membrane with 2D vermiculite nanosheets modification for non-aqueous redox flow batteries

Lack of high-performance membrane seriously limits the performance of non-aqueous redox flow batteries (NARFBs). Here, a porous poly(vinylidene fluoride) (PVDF) membrane with 2D vermiculite nanosheets modification is proposed and prepared. The vermiculite nanosheets act as barriers to alleviate the crossover of active species. The hydrogen bonding between vermiculite and PVDF skeleton effectively mitigates the swelling rate and increases the mechanical strength of the membrane. The resultant battery constructed with the new porous membrane exhibits higher Coulombic efficiency (97.9% vs 84.6%) and large average discharge capacity (0.132 vs 0.088 Ah L−1) compared with the one with pristine PVDF membrane at 2 mA cm−2. The vermiculite nanosheets have little effect on the voltage efficiency (90.6% vs 92.7%).

Molecular engineering the naphthalimide compounds as High-Capacity anolyte for nonaqueous redox flow batteries

Nonaqueous redox flow batteries (NARFBs) hold great promise to offer high energy density due to the broader electrochemical window. However, the achieved energy density is relatively low due to the poor stability and low solubility of redox active materials. Herein, we demonstrate that N-substituted naphthalimides can be employed as stable anolyte material for NARFBs. N-(naphthalimidoethyl)-N,N-dimethyl-N-ethylammonium bis(trifluoromethane-sulfonyl)imide (NI-TFSI) was designed via an ionic modification strategy, resulting in enhanced solubility from 0.31 M to 1.22 M. The NARFB based on NI-TFSI exhibits stable cycling with 90.3% discharge capacity retention over 100 cycles and high-rate capability with the average discharge capacity of 1.27 Ah L-1 even at 120 mA cm−2. Benefiting from the high solubility of the redox active materials, capacity of 16.3 Ah L-1 and energy density of 35.6 Wh L−1 can be achieved at 1.0 M, which represent the benchmark of high-energy–density NARFBs.

Microemulsions: Breakthrough Electrolytes for Redox Flow Batteries.

Aqueous and non-aqueous redox flow batteries (RFBs) have limited energy and current densities, respectively, due to the nature of the electrolytes. New approaches to electrolyte design are needed to improve the performance of RFBs. In this work, we combined a highly conductive aqueous phase and an organic redox-active phase in a microemulsion to formulate a novel RFB electrolyte. As a proof-of-concept, we demonstrate an RFB using this microemulsion electrolyte with maximum current density of 17.5 mA·cm−2 with a 0.19 M posolyte and 0.09 M negolyte at a flow rate of only ∼2.5 ml·min−1, comparable to early vanadium electrolyte RFBs at similar flow rates on a per molar basis. The novel active negolyte component is an inexpensive oil-soluble vitamin (K3). By combining aqueous and organic phases, the solvent potential window and energy density may be increased without sacrificing current density and new redox couples may be accessed. Microemulsion electrolytes show great promise for improved performance and increased energy densities in aqueous RFBs but the path forward is complex. We end with discussion of areas that need work to achieve the potential of these electrolytes.