Non-aqueous Redox Flow Batteries Research Articles

An electrospinning carbon nanofiber composite electrode with gradient porous structure for deep eutectic solvent electrolyte-based iron‑vanadium redox flow battery

This work designs and prepares a composite electrode that is made of triple-layer micropore/mesopore electrospinning carbon nanofiber (ECNF) felts with gradient pore distributions and graphite felt (GF) as backing layer. The micropore ECNF felt can provide sufficient reactive surface area as well as reduce the charge transfer resistance near the membrane side. Meanwhile, the other two mesopore ECNF felts prepared by two nozzle vertical same-side blending spinning method are sandwiched between GF backing layer and micropore ECNF felt, thus making a trade-off between the reducing charge transfer resistance and the decreasing electrolyte flow resistance. Experimental results indicate that the deep eutectic solvent electrolyte-based vanadium‑iron redox flow battery (RFB) assembled with this gradient porous composite electrodes has the energy efficiency of 82.02 % at 5 mA·cm−2, which is 42.57 % higher than that of conventional RFB with mono-layer GF electrode. Even at 10 mA·cm−2, its energy efficiency can still be above 70 %, which is 70.62 % higher than that of a conventional RFB. In addition, the peak power density is also 77.44 % higher than that of the GF electrode. It has been demonstrated that the performance of non-aqueous RFB can be greatly improved by using carbon nanofiber composite electrodes with gradient pore structure.

Molecular size exclusion effect extending the cycling stability of a non-aqueous redox flow battery

Non-aqueous organic redox flow batteries (NAORFBs) suffer from rapid capacity fading mainly due to the crossover of redox-active species across the membrane. Minimizing the crossover of redox-active species through ion exchange membranes remains a complex challenge in NAORFBs. To address the crossover issue, we approached the problem through a molecular size exclusion principle designing a dimer of viologen derivative as an anode material. Coupled with N-hexyl phenothiazine as a catholyte, a static cell was demonstrated, which exhibits an excellent cycling stability (100 cycles) with an average Coulombic efficiency of 90% at 10 mA cm−2 current density.

N-Isobutylphenothiazine as a reversible and stable catholyte in non-aqueous organic redox flow batteries

Nonaqueous organic redox flow batteries (NAORFBs) are the focus of current research because redox-active organic materials can be synthesized from abundant elements, and nonaqueous electrolytes offer higher open circuit voltage (OCV). The development of NAORFBs requires stable redox active materials in neutral forms and radical ions. N-substituted phenothiazines are organic catholytes with high electrochemical reversibility and stability. In this study, N-isobutylphenothiazine (iBuPT) was successfully synthesized in a single step from inexpensive precursors of 10H-phenothiazine and 1-bromo-2-methylpropane and used as catholyte material for NAORFB. iBuPT, a member of the N-substituted phenothiazine family, has a half-wave redox potential of 0.432 V, higher than most N-substituted phenothiazines. When iBuPT is coupled with anolyte 2,1,3-benzothiadiazole (BTD) with a half-wave potential of −1.82 V, the OCV value reaches 2.25 V. The half-wave potential of phenothiazine increases by 115 mV when an isobutyl group replaces the N‑hydrogen, and the stability of the radical cation formed during electrochemical oxidation improves. The steric hindrance can be used to tune redox potential and stability. The stability of the iBuPT radical cation is attributed to the steric shielding and resonance stabilization provided by the one isobutyl group and two phenyl rings attached to the redox-active center, N, from which electron transfer occurs. iBuPT is a promising catholyte candidate for NAORFBs.

Molecular Engineering of Redox Couples for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) have attracted significant attention as a promising electrochemical energy storage technology, offering various advantages such as grid-scale electricity production with variable intermittent electricity delivery, enhanced safety compared to metal-ion batteries, decoupled energy and power density, and simplified manufacturing processes. For this review, we exclusively focus on organic, non-aqueous redox flow batteries. Specifically, we address the most recent progress and the major challenges related to the design and synthesis of robust redox-active organic compounds. An extensive examination of the synthesis and characterization of a wide spectrum of redox-active molecules, focusing particularly on derivatives of posolytes such as quinone, nitroxyl radicals, dialkoxybenzenes, and phenothiazine and negolytes such as viologen and pyridiniums, is provided. We explore the incorporation of various functional groups as documented in the references, aiming to enhance the chemical and electrochemical stability, as well as the solubility, of both the neutral and radical states of redox-active molecules. Additionally, we offer a comprehensive assessment of the cell-cycling performance exhibited by these redox-active molecules.

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

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

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

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

(Invited) New Membranes for Preventing Crossover in Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries have been considered as an alternative battery technology for grid storage due to the ability to operate at higher potentials with organic solvents. However, there are no commercial ion exchange membranes capable to handling these non-aqueous redox flow batteries, because they swell or degrade over time. This paper will discuss strategies for synthesizing stable anion exchange membrane separators for non-aqueous redox flow batteries to prevent crossover and improve long term stability.

Sterically Bulky Asymmetric Benzophenone Derivatives as Negolytes in a Non-Aqueous Redox Flow Battery

To increase renewable energy generation to the electrical grid, intermittent energy sources, such as wind and solar energy, need a long-term, large scale energy storage method. Redox flow batteries (RFB) are an attractive solution as they allow for long term storage in various redox states of an organic or inorganic compound. The ideal redox-active compound for a RFB should have a high energy density, which can be achieved through a large potential difference in redox events, high concentration of charge carrier, or by increasing the number of electrons transferred during a given redox event. In particular, nonaqueous RFB (NRFB) are promising for building a system with a large potential difference for redox events due the large electrochemical solvent window (~5 V for acetonitrile) as compared to water (~2 V). While metal complexes featuring redox active ligands have been thoroughly investigated as carriers in NRFBs, they are often too expensive for widespread use. This has led to increasing interest in organic carriers. Benzophenone is a promising candidate for an organic negolyte due to its high solubility in acetonitrile and the presence of a reversible reduction at -2.1 V vs. Ag/Ag+. Unfortunately, the durability of benzophenone during cycling is low. Here, we discuss the degradation products of benzophenone during cycling and show how the incorporation of sterically bulky groups ortho to the benzophenone carbonyl improves durability. We then evaluate the utility of these benzophenone derivatives as negolytes in a NRFB flow cell.

Parametric Modelling of a Non-Aqueous Redox Flow Battery with Solid Booster Materials

With an increasing use of renewable energy resources, such as wind and solar, one of the major challenges that need to be addressed is the energy reliability associated with the intermittent nature of these resources. Redox flow batteries (RFBs), which are considered as a promising large-scale energy storage technology, have the potential to address this issue due to their enhanced capacity retention, ease of scalability, and cost-effectiveness in long-duration energy storage [1]. Despite the promise, the widespread implementation of RFBs is currently hindered by their lower energy density compared to lithium-ion batteries and hydrogen energy storage [2].Non-aqueous redox flow batteries (NRFBs) offer volumetric energy density improvement by increasing the electrochemical potential window. While the idea of non-aqueous systems is not new in concept, they have become more prevalent and very few models have been described in literature [3]. In this study, a mathematical model of a NRFB using a bio-inspired active material, vanadium(iv/v) bis-hydroxyiminodiacetate (VBH) [4] is developed. To further increase the energy density of the system, a solid capacity booster material, cobalt hexacyanoferrate, is introduced into the electrolyte tanks. The solid booster increases the electrolyte capacity by chemically reacting with the redox active material in the tank [5]. Since the inclusion of solid booster materials is a relatively novel concept for RFBs, few resources are available, and to the best of our knowledge, no model has yet to be presented for a solid booster system [6].The mathematical model is developed with charge and species conservation using the Nernst-Planck equation. Then, the model is validated with an experimental charge/discharge curve without solid booster material. The solid booster concept is included by incorporating the shrinking core model and Gibbs free energy [7]. Using the model developed, sensitivity and correlation analyses are conducted to identify independent parameters associated with the solid booster material. Lastly, the charge/discharge experimental data with the solid booster material is fitted to the model prediction to identify the thermodynamic and kinetic parameters of the indirect chemical reaction that occurs in the tanks. The development of this model can provide insights for future studies utilizing solid boosters in redox flow batteries and help identify key parameters affecting the performance of these systems.

Investigating Transport through Membranes in Non-Aqueous Redox Flow Batteries with Neutron Radiography

Electrochemical technologies offer a strong platform to decarbonize and electrify our energy economy and thus are poised to play an increasingly important role. Nevertheless, all electrochemical technologies suffer from thermodynamic, kinetic, and other operational losses that can drastically limit the performance of the device. Current macroscopic characterization techniques used to assess those losses (i.e., polarization, electrochemical impedance spectroscopy) fail to assess local inefficiencies challenging complete understanding of the system being investigated. Operando imaging of electrochemical systems, in tandem with electrochemical diagnostics offers a viable option to resolve local information and correlate it with macroscopic electrochemical performance. This dual characterization strategy has been successfully applied for the development of other electrochemical technologies (i.e., polymer electrolyte fuel cells[1,2], lithium-ion batteries[3,4]). In the past years, several groups developed and investigated imaging and spectroscopic techniques for operando characterization of redox flow batteries (RFBs) such as fluorescence microscopy[5], nuclear magnetic resonance spectroscopy[6,7] and infrared thermography[8]. Only recently, Clement et al. reported for the first time the use of neutron radiography to perform through-plane imaging of the gas evolution during operation of an all-vanadium RFB[9]. However, an approach enabling non-invasive operando concentration mapping of the species of interest within the reactor area has remained elusive.In this work, we develop two novel neutron radiography operando methodologies to quantify concentration distributions in RFBs. We first perform cuvette experiments to calibrate the neutron attenuation response of the redox active materials (i.e., TEMPO and TEMPO+) and supporting salts (i.e., BF4 -) used in this study. We find a linear response between the neutron absorption and the concentrations of the species of interest in the electrolyte in the studied range (0.0 to 0.5 M). Next, we performed experiments with redox flow cells under operation and use the ex-situ calibrations together with subtractive neutron imaging to quantify species concentration in the electrolyte. By employing an in-plane optical imaging setup[10], we can resolve species concentration across the porous electrodes (Figure 1). We observe strong variations in the concentration profiles within the reactor area upon changes in the operating conditions and link those differences to material properties (i.e., membrane electrostatic charge). Finally, I will discuss our latest results obtained using time-of-flight neutron imaging to deconvolute concentration maps for TEMPO/TEMPO+ and BF4 - species. Using this methodology, we study mass transport across a porous separator and a dense anion exchange membrane. We find that, while TEMPO/TEMPO+ and BF4 - show almost symmetrical concentration changes in the case of a porous separator, asymmetric transport occurs when a charge selective membrane is used where only BF4 - is allowed to crossover the anion exchange polymer. The developed methodology offers a non-invasive diagnostic tool to image concentration distributions in electrochemical cells and can be applied to support theoretical and experimental efforts aiming to understand the role of materials and reactor designs on complex mass transport phenomena.[1] P. Boillat, E. H. Lehmann, P. Trtik, M. Cochet, Current Opinion in Electrochemistry 2017, 5, 3.[2] J. Eller, T. Rosén, F. Marone, M. Stampanoni, A. Wokaun, F. N. Büchi, J. Electrochem. Soc. 2011, 158, B963.[3] B. Michalak, H. Sommer, D. Mannes, A. Kaestner, T. Brezesinski, 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, P. R. Shearing, Nat Commun 2015, 6, 6924.[5] A. A. Wong, M. J. Aziz, S. Rubinstein, ECS Trans. 2017, 77, 153.[6] E. W. Zhao, T. Liu, E. Jónsson, J. Lee, I. Temprano, R. B. Jethwa, A. Wang, H. Smith, J. Carretero-González, Q. Song, C. P. Grey, Nature 2020, 579, 224.[7] E. W. Zhao, E. Jónsson, R. B. Jethwa, D. Hey, D. Lyu, A. Brookfield, P. A. A. Klusener, D. Collison, C. P. Grey, J. Am. Chem. Soc. 2021, 143, 1885.[8] H. Tanaka, Y. Miyafuji, J. Fukushima, T. Tayama, T. Sugita, M. Takezawa, T. Muta, Journal of Energy Storage 2018, 19, 67.[9] J. T. Clement, Investigation of Localized Performance and Gas Evolution in All-Vanadium Redox Flow Batteries via in-Situ Distributed Diagnostic Techniques, Ph.D. thesis, University of Tennessee, 2016.[10] P. Boillat, G. Frei, E. H. Lehmann, G. G. Scherer, A. Wokaun, Electrochem. Solid-State Lett. 2010, 13, B25. Figure 1

Cross-Linked Anion Exchange Membranes for Non-Aqueous Redox Flow Battery

The development of redox flow batteries (RFBs) has been one of the primary responses to the need to store harvested renewable energy for improving grid reliability and utilization.1–6 In this study, we synthesized and characterized different Bromobutyl Norbornene backbone-based anion-exchange membranes (AEM) with different Tetramethyl hexadiamine crosslinker amounts and explored the performance of AEMs in non-aqueous redox flow batteries under ideal conditions. Performance was compared based on high stability in a non-aqueous solvent, high permeability to the charge-carrying ion, low electric cell resistance, low cross-over of the redox-active molecules, high mechanical property, and low cost. 100 cycles battery data indicates that Bromobutyl Norbornene-based AEM performance is far better than the commercial best membrane Fuma-375. The best membrane will be found which will show no cross-over of redox-active electrolytes after 1000 battery cycle experiments and exhibit low resistance and high structural/chemical stability in MeCN solvent. References (1) Mazumder, Md. 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. Inorg. Chem. 2021, 60 (17), 13388–13399. https://doi.org/10.1021/acs.inorgchem.1c01699.(2) 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.(3) 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.(4) 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.(5) Rahaman Mazumder, M. M.; Islam, R.; Khan, M. A. R.; Anis‐Ul‐Haque, K. M.; Rahman, M. M. Efficient AcFc‐[Fe III (Acac) 3 ] Redox Couple for Non‐aqueous Redox Flow Battery at Low Temperature. Chem. – Asian J. 2023, 18 (1). https://doi.org/10.1002/asia.202201025.(6) Mazumder, Md. M. R.; Dalpati, N.; Pokkuluri, P. R.; Farnum, B. H. Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries. Inorg. Chem. 2022, 61 (48), 19039–19048. https://doi.org/10.1021/acs.inorgchem.2c03124.

Exploring the Impact of Oligomerization on Redox Flow Cell Performance

The redox flow battery (RFB) is a promising stationary energy storage technology due to its inherent decoupling of energy and power, long service lifetime, and potential for low-cost manufacturing.1 While current embodiments are constrained by high upfront cost, nonaqueous RFBs may offer a pathway to cost-competitive energy storage systems through higher energy densities;2 however, this concept is at an early stage and several challenges must be overcome. In particular, the development of stable, high-voltage redox chemistries in conjunction with low-cost, selective membranes is needed to mitigate difficult-to-reverse capacity fade and reduced efficiencies. As such, a range of redox-active organic macrostructures, including oligomers (RAOs),3 polymers (RAPs),4 and colloids (RACs),5 have been described in the peer-reviewed literature. Of specific interest are RAOs, which, like the other macrostructures, can be paired with size-exclusion membranes but exhibit electrochemical and fluid dynamic properties that more closely align with monomers. While this proof-of-concept has been demonstrated,6,7 to our knowledge, few studies have systematically evaluated how the molecular properties of RAOs influence their performance in flow cells.In this presentation, we will describe our efforts to understand how the molecular properties of different oligomers influence their behavior in redox flow cells. To this end, we synthesize and characterize a set of model compounds, ranging from monomers to tetramers, that utilize TEMPO as the redox-active unit. We then assess the thermodynamic, kinetic, and transport properties of these compounds using a suite of voltammetric techniques. Subsequently, we characterize the performance and durability of these compounds in symmetric flow cells via electrochemical impedance spectroscopy, DC polarization, and durational cycling. We perform post-test characterization after the cycling experiments to disaggregate molecular- and cell-level sources of capacity fade. We find that, in general, oligomers with increased size exhibit lower diffusivities while maintaining the facile charge transfer characteristics afforded by the appended monomer. These size-dependent variations in mass transport rates directly translate to differences in cell polarization and cycling performance. Time-dependent capacity fade observed during cycling is confirmed by post-mortem analysis. Broadly, these results align with previously developed relationships between molecular size, electrochemical properties, and flow cell performance; we posit that the principles for functionalization and testing utilized here can be applied to other RAOs with improved stability. Acknowledgements This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. 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. The authors thank Jeffrey Kowalski and Katharine Greco of the Brushett Group as well as Yu Cao and Jeffrey Moore of the Moore Group at the University of Illinois for their contributions to conceptualizing the framework and preliminary testing for this work.

Synthesis-Driven Computational Discovery of Organic Redoxmers for Non-Aqueous Redox Flow Batteries

Organic non-aqueous redox flow batteries (RFBs) are promising grid-scale energy storage systems for storing intermittent renewable energy in molecules. For practical applications, low-cost and stable redox active molecules (redoxmers) that display a large redox potential window and long electrochemical cycling stability are required to deliver a high-energy-density RFB with a long battery cycle life. To accelerate the discovery of redoxmer molecules, previous studies have utilized machine learning (ML) methods along with first-principles simulations. Although many ML-suggested molecules show promising electrochemical properties, they are generally difficult to synthesize because of their highly complex structure. To address this challenge, in this work, we propose a new synthesis-driven computational approach to discovering less-complex molecules with a large redox potential window that can deliver high-energy-density RFBs. Specifically, we devised an algorithm to generate an engineered molecular search space and applied a Bayesian optimization-based active learning algorithm to discover promising molecules from a large molecular search space while using a limited number of expensive density functional theory (DFT) calculations. To increase the energy density of RFBs, we focus on maximizing the redox potential window of redoxmer molecules (or cell voltage) by minimizing the reduction potential of anolyte molecules. Precisely, we studied the 2,1,3 Benzothiadiazole (BTZ) molecule, a promising anolyte (redoxmer) candidate for RFBs, and with our synthesis-driven approach discovered new BTZ-based molecules that show 15-43% smaller reduction potential than the BTZ and in principle can deliver organic RFBs with a cell voltage ≈ 3 V.

Benzylviologen/N-hexyl phenothiazine based non-aqueous organic redox flow battery in inert condition

Generally, non-aqueous organic redox flow batteries (NAORFBs) operate in glove box conditions to stabilize the charged species generate during energy storage. Even a trace amount of moisture entering the system destabilizes the capacity of the battery. Such systems may not be commercially viable due to stringent ambient requirements. To address this issue, we demonstrate a NAORFB capable of running in inert condition (without glovebox) using a mixed electrolyte of 0.2 M benzylviologen hexafluorophosphate [BV(PF6)2] and N-hexyl phenothiazine (HPT) at 40 mA cm−2 current density. This system exhibits cycling stability up to 100 cycles with 0.0051 % capacity fade per cycle and 6.09 % capacity fade per day.

Oxygen-less electrodes for stability enhancement of vanadium non-aqueous redox flow batteries

A meticulous study was conducted to investigate the capacity loss and degradation of non-aqueous vanadium flow batteries. It was observed that the presence of moisture and oxygen in the system triggers a parasitic reaction leading to the formation of irreversible VO(acac)2, which hampers high-performance operation. The main source of this issue was identified as oxygen-rich electrodes commonly used in these batteries. To address this challenge, a novel approach was employed to modify the surface of carbon felt electrodes through hydrogen reduction thermal treatment, effectively mitigating the concentration of oxygen functional sites. The electrodes of carbon felt were treated at 700 °C with hydrogen greatly reduced the oxygen content in the electrodes, leading to increased reaction rate constants and reduced overall system impedance. The thermal treatment under a reducing atmosphere successfully mitigated the degradation mechanisms and extended the device's lifespan. The battery equipped with these modified electrodes demonstrated a storage capacity of 521 mAh l−1 over 45 charge/discharge cycles, representing the highest reported stability. Moreover, the maximum current density was improved by 60 % compared to the system using pristine electrodes. Morphological analysis was conducted alongside electrochemical testing to evaluate the performance of the modified electrodes. This technique paves the way to a closer level toward pilot-scale commercialization.

Indolo[2,3-b]quinoxaline as a Low Reduction Potential and High Stability Anolyte Scaffold for Nonaqueous Redox Flow Batteries.

Redox flow batteries (RFBs) are a promising stationary energy storage technology for leveling power supply from intermittent renewable energy sources with demand. A central objective for the development of practical, scalable RFBs is to identify affordable and high-performance redox-active molecules as storage materials. Herein, we report the design, synthesis, and evaluation of a new organic scaffold, indolo[2,3-b]quinoxaline, for highly stable, low-reduction potential, and high-solubility anolytes for nonaqueous redox flow batteries (NARFBs). The mixture of 2- and 3-(tert-butyl)-6-(2-methoxyethyl)-6H-indolo[2,3-b]quinoxaline exhibits a low reduction potential (-2.01 V vs Fc/Fc+), high solubility (>2.7 M in acetonitrile), and remarkable stability (99.86% capacity retention over 49.5 h (202 cycles) of H-cell cycling). This anolyte was paired with N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT) to achieve a 2.3 V all-organic NARFB exhibiting 95.8% capacity retention over 75.1 h (120 cycles) of cycling.

Development of high-voltage and high-energy membrane-free nonaqueous lithium-based organic redox flow batteries

Lithium-based nonaqueous redox flow batteries (LRFBs) are alternative systems to conventional aqueous redox flow batteries because of their higher operating voltage and theoretical energy density. However, the use of ion-selective membranes limits the large-scale applicability of LRFBs. Here, we report high-voltage membrane-free LRFBs based on an all-organic biphasic system that uses Li metal anode and 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine (Tri-TEMPO), N-propyl phenothiazine (C3-PTZ), and tris(dialkylamino)cyclopropenium (CP) cathodes. Under static conditions, the Li||Tri-TEMPO, Li||C3-PTZ, and Li||CP batteries with 0.5 M redox-active material deliver capacity retentions of 98%, 98%, and 92%, respectively, for 100 cycles over ~55 days at the current density of 1 mA/cm2 and a temperature of 27 °C. Moreover, the Li||Tri-TEMPO (0.5 M) flow battery delivers an initial average cell discharge voltage of 3.45 V and an energy density of ~33 Wh/L. This flow battery also demonstrates 81% of capacity for 100 cycles over ~45 days with average Coulombic efficiency of 96% and energy efficiency of 82% at the current density of 1.5 mA/cm2 and at a temperature of 27 °C.

Composite Membrane for Sodium Polysulfide Hybrid Redox Flow Batteries.

Non-aqueous redox flow batteries (NARFBs) using earth-abundant materials, such as sodium and sulfur, are promising long-duration energy storage technologies. NARFBs utilize organic solvents, which enable higher operating voltages and potentially higher energy densities compared with their aqueous counterparts. Despite exciting progress throughout the past decade, the lack of low-cost membranes with adequate ionic conductivity and selectivity remains as one of the major bottlenecks of NARFBs. Here, we developed a composite membrane composed of a thin (<25 µm) Na+-Nafion coating on a porous polypropylene scaffold. The composite membrane significantly improves the electrochemical stability of Na+-Nafion against sodium metal, exhibiting stable Na symmetric cell performance for over 2300 h, while Na+-Nafion shorted by 445 h. Additionally, the composite membrane demonstrates a higher room temperature storage modulus than the porous polypropylene scaffold and Na+-Nafion separately while maintaining high Na+ conductivity (0.24 mS/cm at 20 °C). Our method shows that a composite membrane utilizing Na+-Nafion is a promising approach for sodium-based hybrid redox flow batteries.

Phenyl Acrylate-Based Cross-Linked Anion Exchange Membranes for Non-aqueous Redox Flow Batteries

Phenyl Acrylate-Based Cross-Linked Anion Exchange Membranes for Non-aqueous Redox Flow Batteries

Highly Soluble Dimethoxymethyl Tetrathiafulvalene with Excellent Stability for Non-Aqueous Redox Flow Batteries.

Non-aqueous redox flow batteries (RFBs) are highly attractive for grid-scale energy storage applications because of their independent design of energy and power, high energy density and efficiency, easy maintenance, and potentially low cost. In order to develop active molecules with large solubility, excellent electrochemical stability, and high redox potential for a non-aqueous RFB catholyte, herein, two flexible methoxymethyl groups had been attached to a famous redox-active tetrathiafulvalene (TTF) core. The strong intermolecular packing of the rigid TTF unit was effectively depressed, leading to a dramatically improved solubility of up to 3.1 M in conventional carbonate solvents. The performance of the obtained dimethoxymethyl TTF (DMM-TTF) was studied in a semi-solid RFB system with Li foil as the counter electrode. When using porous Celgard as the separator, the hybrid RFB with 0.1 M DMM-TTF had two high discharge plateaus at 3.20 and 3.52 V and a low capacity retention of 30.7% after 100 cycles at 5 mA cm-2. Replacing Celgard with a permselective membrane, the capacity retention was increased to 85.4%. Further increasing the concentration of DMM-TTF to 1.0 M and current density to 20 mA cm-2, the hybrid RFB exhibited a high volumetric discharge capacity of 48.5 A h L-1 and an energy density of 154 W h L-1. The capacity was maintained at 72.2% after 100 cycles (10.7 days). The great redox stability of DMM-TTF was revealed by UV-vis and 1H NMR tests and verified by density functional theory calculations. Therefore, the methoxymethyl group is an excellent group to increase the solubility while maintaining the redox capability of TTF for high-performance non-aqueous RFBs.