Non-aqueous Redox Flow Batteries Research Articles

Transport properties of ethylene glycol functionalized membranes exposed to nonaqueous electrolytes

Non-aqueous redox flow batteries (RFBs) are a promising technology to meet growing demand for grid-scale energy storage. Membrane separators, designed specifically for use with organic solvents, are necessary to advance non-aqueous RFBs. Herein, we report the development of a series of poly(phenylene oxide) (PPO) membranes functionalized with poly(ethylene glycol) (PEG) side chains to investigate the influence of PEG side chain length and degree of PEGylation on membrane transport properties. Increasing the degree of PEGylation generally led to increases in electrolyte uptake, hydroxy TEMPO permeability, and ionic conductivity likely caused by an increase in overall PEG content, as opposed to specific interactions caused by changing the degree of PEGylation. For membranes with similar PEG content, increasing the length of the PEG side chain resulted in decreases in electrolyte uptake, permeability, and conductivity possibly due to differences in the solvation behavior of the PEG chains with different lengths.

Ferrocene/naphthalimide bi-redox molecule for enhancing the cycling stability of symmetric nonaqueous redox flow battery

Derived from N-(ferrocenylmethyl)-N-(butylphthalimide)-N,N-dimethylammonium bis(trifluoromethane-sulfonyl)imide (FcPI-TFSI), ferrocene/naphthalimide (FcNI-TFSI) based ionic bipolar redox-active organic molecule is constructed via nucleophilic substitution between amines and halogenated hydrocarbons. Due to the elevated delocalized negative charge density in the radical anion, the FcNI-TFSI based nonaqueous redox flow batteries (NARFBs) exhibits substantially improved cycling stability; 764 and 233 cycles are successfully performed before 50% of the initial discharge capacity is lost at 0.01 M and 0.1 M, respectively, whereas 82 cycles for FcPI-TFSI at 0.01 M. Similar to capacity diving phenomenon in lithium-ion batteries, sudden accelerated capacity loss is observed in the high-concentration battery. The possible degradation mechanism is discussed, involving crossover issue and parasitic reactions.

Stabilization of Naphthalene Diimide Anions by Ion Pair Formation in Nonaqueous Organic Redox Flow Batteries.

In redox flow batteries, a compelling strategy for enhancing the charge capacity of redox-active organic molecules involves storing multiple electrons within a single molecule. However, this approach poses unique challenges such as chemical instability by forming radicals, elevated energy requirements, and unsustainable charge concentration. Ion pairing is a possible solution to achieve charge neutrality and engineer redox potential shifts but has received limited attention. In this study, we demonstrate that Li+ can stabilize naphthalene diimide (NDI) anions dissolved in acetonitrile and significantly shift the second cathodic potential close to the first. Our findings, supported by density functional theory calculations and Fourier transform infrared spectroscopy, indicate that dimeric NDI species form stable ion pairs with Li+. Conversely, K+ ions exhibit weak interactions, and cyclic voltammograms confirm significant potential shifts when stronger Lewis acids and solvents with lower donor numbers are employed. Galvanostatic examinations reveal a single voltage plateau with Li+, which indicates a rapid redox process involving doubly charged NDI2- with Li+. These aggregated ion pairs offer the additional benefits of hindering crossover events, contributing to excellent cyclability, and suppressing undesirable side reactions even after 1000 redox cycles.

4]Helicenium Ion as Bipolar Redox Material for Symmetrical Fully Organic Pole‐less Redox Flow Battery

AbstractLong duration storage batteries such as Redox Flow Batteries (RFBs) are promising storage system to address the energy storage requirement that our society will require in the years to come. Recent effort has been focused on the development of metal free and high energy density system such as all‐organic non‐aqueous redox flow batteries (NAORFBs). However high‐voltage NAORFBs currently use distinct anolytes and catholytes, which are separated by a membrane sensitive to osmotic pressure, resulting in rapid capacity and energy density degradation over time. To address this issue, symmetric organic redox flow batteries (SORFBs) have been proposed as an elegant solution. We have introduced dimethoxyquinacridiniums (DMQA+) ions as efficient bipolar redox molecules (BRMs) in static H‐cell conditions. In this study, we present the first application of DMQA+ ions in a complete flow RFB prototype, showcasing their ability to operate with polarity reversal. Key kinetic properties were evaluated through cyclic voltammetry and DFT calculations. While coulombic and energy efficiency metrics were moderate, pegylated DMQA+ demonstrated impressive capacity retention of over 99.99 % and the capability to operate under polarity inversion, making it a highly attractive choice for grid‐scale, long‐lifespan energy storage applications.

Improved solubility of titanium-doped polyoxovanadate charge carriers for symmetric non-aqueous redox flow batteries.

Non-aqueous redox flow batteries constitute a promising solution for grid-scale energy storage due to the ability to achieve larger cell voltages than can be readily accessed in water. However, their widespread application is limited by low solubility of the electroactive species in organic solvents. In this work, we demonstrate that organic functionalization of titanium-substituted polyoxovanadate-alkoxide clusters increases the solubility of these assemblies over that of their homoleptic congeners by a factor of >10 in acetonitrile. Cyclic voltammetry, chronoamperometry, and charge-discharge cycling experiments are reported, assessing the electrochemical properties of these clusters relevant to their ability to serve as multielectron charge carriers for energy storage. The kinetic implications of ligand variation are assessed, demonstrating the role of ligand structure on the diffusivity and heterogeneous rates of electron transfer in mixed-metal charge carriers. Our results offer new insights into the impact of structural modifications on the physicochemical properties of these assemblies.

Potassium supporting electrolyte enhances stability of Ti-substituted polyoxovanadates for nonaqueous redox flow batteries

“Goldilocks” affinity of K+ for the POV surface stabilizes reduced form of assembly for improved cycling stability!

Thin Reinforced Anion-Exchange Membranes for Non-Aqueous Redox Flow Battery Employing Fe/Co-Metal Complex Redox Species

Non-aqueous redox flow batteries (NARFBs) have been attracting much attention because they can significantly increase power and energy density compared to conventional RFBs. In this study, novel pore-filled anion-exchange membranes (PFAEMs) for application to a NAPFB employing metal polypyridyl complexes (i.e., Fe(bpy)32+/Fe(bpy)33+ and Co(bpy)32+/Co(bpy)33+) as the redox species are successfully developed. A porous polyethylene support with excellent solvent resistance and mechanical strength is used for membrane fabrication. The PFAEMs are prepared by filling an ionic liquid monomer containing an imidazolium group and a crosslinking agent into the pores of the support film and then performing in situ photopolymerization. As a result, the prepared membranes exhibit excellent mechanical strength and stability in a non-aqueous medium as well as high ion conductivity. In addition, a low crossover rate for redox ion species is observed for the prepared membranes because they have relatively low swelling characteristics in non-aqueous electrolyte solutions and low affinity for the metal-complex redox species compared to a commercial membrane. Consequently, the PFAEM is revealed to possess superior battery performance than a commercial membrane in the NARFB tests, showing high energy efficiency of about 85% and stable operation for 100 cycles.

Selective Ion Conducting Polymers for Non-Aqueous Redox Flow Battery Applications

Providing sustainable supplies of clean energy is a critical global challenge for the future. As intermittent renewable energy sources are increasingly deployed, grid-scale energy storage solutions are increasingly needed. One approach to addressing this challenge is to use flow battery technology to store and deliver grid-scale amounts of energy. Non-aqueous redox flow batteries can be operated at higher voltages and energy densities compared to aqueous systems and, as such, may offer high volumetric energy density compared to other flow batteries. A significant challenge facing non-aqueous flow batteries, however, is the lack of selective membrane separators engineered for the unique challenges of non-aqueous electrochemical systems. Suitable membranes for non-aqueous redox flow battery applications must be stable in aggressive solvent environments, offer high conductivity, and provide selectivity to prevent cross-over of redox active molecules. Here, we report the synthesis and characterization of a series of negatively charged ion conductive polymeric membranes for non-aqueous flow battery applications. Fixed negative charges were added to a poly(phenylene oxide) backbone via a custom sulfonate group-based side chain. Lithium ion conductivity and ferrocene permeability properties were characterized to evaluate the selectivity of the membrane for ion transport relative to redox active molecule cross-over (as ferrocene is a representative redox active molecule). The polymers exhibited combinations of conductivity and selectivity that are favorable compared to other Nafion-based materials, and the materials appear to be dimensionally stable in a non-aqueous electrolyte over a period of several months. Redox active molecule cross-over was analyzed within a thermodynamic regular solution framework to highlight how thermodynamic factors contribute to cross-over properties. Ultimately, the data suggest a decoupling of ion and redox active molecule transport that could inform future efforts to develop advanced non-aqueous redox flow battery membrane separators. Altogether, this presentation discusses the transport properties and stability of these materials that show promise for non-aqueous flow battery applications.

Novel Approaches for Improving Solubility of Pyridinium Anolytes in Non-Aqueous Redox Flow Batteries

Organic redox flow batteries (ORFBs) offer significant promise as a means of efficient grid-scale energy storage to compliment the intermittent power supply of renewable energy sources such as solar and wind power. ORFBs efficiently store energy in the form of redox-active organic molecules (ROMs) by passing them over two polarized electrodes to generate an oxidized (catholyte) and a reduced (anolyte) species, which are held in separate tanks until energy is needed. Structural diversity of anolytes and catholytes for ORFBs has expanded dramatically over the past decade as researchers have raced to discover or design ROMs suitable for use in grid-scale energy storage devices; however, a lack of correlations between ROM structure and a desired chemical or physical property has led to a “guess and check” approach that can overlook possibly transformative electrolyte structures. Recent work has demonstrated the ability to employ multivariate regression analysis to correlate computationally derived structural descriptors of ROMs to their corresponding stability. Based on this, we considered the possibility of employing a similar approach to identify quantitative structure-property relationships (QSPRs) for critical physical properties of ORFB electrolytes, such as solubility.Electrolyte solubility is an essential design characteristic in ORFBs as it directly impacts the corresponding battery’s energy storage capacity. Nevertheless, the fundamental question, “What makes an electrolyte soluble?” (especially in organic solvents) still lacks a concise answer despite decades of progress. As part of a collaboration with the Guarr group at the Michigan State University Bioeconomy Institute, our group has identified a novel class of pyridinium anolytes with exceptionally low redox potentials and highly persistent charged states. We will discuss our group’s recent progress to combine experimental and computational techniques to develop QSPR models to accurately predict solubility of pyridinium anolytes. The resulting models have provided new insights into the role that self-interaction between anolyte species in solution plays in dictating their corresponding solubility. Using these models, we have successfully expanded the solubility range for a class of pyridinium derivatives by >200% in acetonitrile.

Fabrication and Performance of Symmetric Flow Battery Systems Incorporating Novel Pyridinium Anolytes

The push to bring more renewable energy sources onto the grid has spawned a flurry of work to develop viable options for long duration energy storage. Redox flow batteries (RFBs) represent one of the most attractive possibilities, and research toward employing redox-active organic materials to improve or replace the incumbent commercially available vanadium-based RFB technology is burgeoning. In particular, numerous pyridinium-based salts have been extensively studied as anolytes in both aqueous and nonaqueous RFBs. We have recently reported a series of readily synthetically accessible 2,6-dimethyl-4-arylpyridinium compounds that display quite negative reduction potentials (ca. -1.55 to -1.8 V vs Fc/Fc+), along with high solubility and excellent electrochemical kinetics. Some of these materials are also extremely persistent in the reduced state, suggesting that they may be good candidates for RFB applications.In this presentation, we will describe the preparation and properties of new pyridinium compounds from the corresponding pyrylium precursors, as well as the fabrication of symmetric RFB systems in which these materials are covalently linked to phenothiazine or ferrocene catholyte moieties. Synthesis, solubility, and electrochemical performance (including preliminary RFB cycling data) of these materials will also be discussed.

Dithiolate-Based Scorpion Arm Ligand Synthesis and Characterization

Redox flow batteries (RFBs) play a vital role in the efficient and reliable utilization of renewable energy sources by providing a means to capture and store excess energy for later use.1–6 The synthesis and characterization of a novel scorpionate arm ligand, i-mntPy- (2,2-dicyano-1-pyridin-2-ylmethyl-1,1-dithiolate), are presented in this study. The ligand i-mntPy- was synthesized through a multistep synthetic route, starting from commercially available precursors. The chemical structure of the ligand was confirmed using various spectroscopic techniques, including NMR spectroscopy, ESI-MS, and Elemental analysis. Furthermore, the coordination ability of i-mntPy- to facilitate 2e- transfer in M(II/IV) coordination environment was investigated by the computational study and addition of Ni(II) and Pd(II) ions in the i-mntPy- solution. The results demonstrate the successful synthesis and characterization of the novel scorpionate arm ligand i-mntPy-, which coordinates with d8 metal centers and holds promise for future applications in coordination chemistry and energy storage. References (1) Mazumder, Md. M. R.; Jadhav, R. G.; Minteer, S. D. Phenyl Acrylate-Based Cross-Linked Anion Exchange Membranes for Non-Aqueous Redox Flow Batteries. ACS Mater. Au 2023. https://doi.org/10.1021/acsmaterialsau.3c00044.(2) 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.(3) 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.(4) 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, acs.inorgchem.1c01699. https://doi.org/10.1021/acs.inorgchem.1c01699.(5) MAZUMDER MDMOTIURRAHAMAN, Islam R. Temperature-dependent study of AcFc-FeIII(acac)3 redox couple for non-aqueous redox flow battery. ChemRxiv. Cambridge: Cambridge Open Engage; 2022; https://doi.org/10.26434/chemrxiv-2022-3177p(6) 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. Figure 1

Organic Bulk Liquid Redox Active Materials for Redox Flow Batteries

Redox flow batteries (RFBs) are one potential solution to grid-level electrical energy storage (EES) benefiting from a decoupled power and capacity scaling.1–3 High durability, long-calendar life, high efficiency EES with a low cost and fast response time is needed1,4 for the transition from fossil fuels to renewable sources.3 However, the low energy density3,5,6 and high capital costs5,6 of current systems preclude wide-scale deployment of this technology.In recent years, several new RFB chemistries have been explored to address these concerns.1,2,7 However, a high solubility for a high volumetric energy density remains a troublesome target.1 It is, therefore, no surprise that one growing trend in this regard is the design of redox active liquids (RALs).8–13 RALs provide a means of dramatically increasing the volumetric energy density of RFBs through either miscibility with typical supporting electrolytes, or by acting as both solvent and electrolyte themselves.9,12 In this work, we investigate a series of RALs that offer a similar theoretical energy density to conventional intercalation materials. A combination of computational and experimental techniques was employed herein for both molecular design and explanation of the physio-chemical phenomena. The candidate compounds were initially screened via electrochemical techniques to identify their electrochemical reversibility and stability. Exploration of the bulk properties was then carried out before system-level characterisation was undertaken. In tandem, the electrochemical and chemical stability of the samples was also investigated through the typical routes (NMR, EPR, UV-Vis). These systems show much promise for organic, tuneable electrical energy storage. Cao, J., Tian, J., Xu, J. & Wang, Y. Organic Flow Batteries: Recent Progress and Perspectives. Energy and Fuels 34, 13384–13411 (2020).Ding, Y., Zhang, C., Zhang, L., Zhou, Y. & Yu, G. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev. 47, 69–103 (2018).Alotto, P., Guarnieri, M. & Moro, F. Redox flow batteries for the storage of renewable energy: A review. Renew. Sustain. Energy Rev. 29, 325–335 (2014).Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).Potash, R. A., McKone, J. R., Conte, S. & Abruña, H. D. On the Benefits of a Symmetric Redox Flow Battery. J. Electrochem. Soc. 163, A338–A344 (2016).Wang, W. et al. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986 (2013).Li, Z., Jiang, T., Ali, M., Wu, C. & Chen, W. Recent Progress in Organic Species for Redox Flow Batteries. Energy Storage Mater. 50, 105–138 (2022).Shimizu, A. et al. Liquid Quinones for Solvent-Free Redox Flow Batteries. Adv. Mater. 29, 1606592 (2017).Robertson, L., Udin, M. A., Shlrob, I. A., Moore, J. S. & Zhang, L. Liquid Redoxmers for Nonaqueous Redox Flow Batteries. ChemSusChem e202300043 (2023) doi:10.1002/cssc.202300043.Chen, N., Chen, D., Wu, J., Lai, Y. & Chen, D. Polyethylene glycol modified tetrathiafulvalene for high energy density non-aqueous catholyte of hybrid redox flow batteries. Chem. Eng. J. 462, 141996 (2023).Smith, L. O. & Crittenden, D. L. Acid‐Base Chemistry Provides a Simple and Cost‐Effective Route to New Redox‐Active Ionic Liquids. Chem. – An Asian J. 18, e202201296 (2023).Zhao, Y. et al. TEMPO allegro: liquid catholyte redoxmers for nonaqueous redox flow batteries. J. Mater. Chem. A 9, 16769–16775 (2021).Huang, J. et al. Liquid Catholyte Molecules for Nonaqueous Redox Flow Batteries. Adv. Energy Mater. 5, 1401782 (2015).

Redox Active Organic Liquids for Energy Storage

The necessity to move to renewable energy sources has increased over the last few years. However, to ensure a good transition and effective energy supply, efficient, cheap, and green technologies are required as grid-level storage.1 Redox-Active Organic Materials (ROMs) are a fast-growing research topic in different electrochemical storage devices. Their advantageous characteristics of being formed from highly abundant elements, having a high electrochemical tunability, ease of handling and device processing, and high versatility for a variety of energy storage applications make ROMs attractive.1–3 Redox-Active Liquids (RALs) are themselves a new trend within the category of ROMs because the physical properties of organic liquids are advantageous for some storage devices.4–6 Those advantages include no requirement for materials processing after synthesis, high miscibility and/or high solubility with battery electrolytes, the ability to be used as a solvent themselves without any additional additives,4,5 and the avoidance of surface-related changes e.g., dendrites.7 RALs have been studied as mediators or active materials in the context of different applications such as supercapacitors,6,8 conventional,4,5,9 and hybrid-flow batteries.10,11 In this work, we present the synthesis and physio-chemical characterization of different RALs that can be used in such energy storage devices. The characterization results supported by computational analyses were used to investigate the correlation between the chemical structure and the electrochemical properties of the compounds. The obtained information was then further used to inform the design of further compounds with enhanced electrochemical properties.(1) Schon, T. B.; McAllister, B. T.; Li, P. F.; Seferos, D. S. The Rise of Organic Electrode Materials for Energy Storage. Chem Soc Rev 2016, 45 (22), 6345–6404. https://doi.org/10.1039/c6cs00173d.(2) Kwon, G.; Ko, Y.; Kim, Y.; Kim, K.; Kang, K. Versatile Redox-Active Organic Materials for Rechargeable Energy Storage. Acc Chem Res 2021, 54 (23), 4423–4433. https://doi.org/10.1021/acs.accounts.1c00590.(3) Lee, S.; Hong, J.; Kang, K. Redox-Active Organic Compounds for Future Sustainable Energy Storage System. Advanced Energy Materials. Wiley-VCH Verlag August 1, 2020. https://doi.org/10.1002/aenm.202001445.(4) Zhao, Y.; Zhang, J.; Agarwal, G.; Yu, Z.; Corman, R. E.; Wang, Y.; Robertson, L. A.; Shi, Z.; Doan, H. A.; Ewoldt, R. H.; Shkrob, I. A.; Assary, R. S.; Cheng, L.; Srinivasan, V.; Babinec, S. J.; Zhang, L. TEMPO Allegro: Liquid Catholyte Redoxmers for Nonaqueous Redox Flow Batteries. J Mater Chem A Mater 2021, 9 (31), 16769–16775. https://doi.org/10.1039/d1ta04297a.(5) Robertson, L.; Udin, M. A.; Shlrob, I. A.; Moore, J. S.; Zhang, L. Liquid Redoxmers for Nonaqueous Redox Flow Batteries. ChemSusChem 2023, e202300043. https://doi.org/10.1002/CSSC.202300043.(6) Fontaine, O. A Deeper Understanding of the Electron Transfer Is the Key to the Success of Biredox Ionic Liquids. Energy Storage Mater 2019, 21, 240–245. https://doi.org/10.1016/J.ENSM.2019.06.023.(7) Bao, J.; Li, C.; Zhang, F.; Wang, P.; Zhang, X.; He, P.; Zhou, H. A Liquid Anode of Lithium Biphenyl for Highly Safe Lithium-Air Battery with Hybrid Electrolyte. Batter Supercaps 2020, 3 (8), 708–712. https://doi.org/10.1002/BATT.202000092.(8) Mourad, E.; Coustan, L.; Lannelongue, P.; Zigah, D.; Mehdi, A.; Vioux, A.; Freunberger, S. A.; Favier, F.; Fontaine, O. Biredox Ionic Liquids with Solid-like Redox Density in the Liquid State for High-Energy Supercapacitors. Nature Materials 2016 16:4 2016, 16 (4), 446–453. https://doi.org/10.1038/nmat4808.(9) Chen, H.; Niu, Z.; Zhao, Y. Redox-Active Binary Eutectics: Preparation and Their Electrochemical Properties. Electrochem commun 2021, 126. https://doi.org/10.1016/j.elecom.2021.107028.(10) Shimizu, A.; Takenaka, K.; Handa, N.; Nokami, T.; Itoh, T.; Yoshida, J.-I.; Shimizu, A.; Takenaka, K.; Yoshida, J.; Handa, N.; Nokami, T.; Itoh, T. Liquid Quinones for Solvent-Free Redox Flow Batteries. Advanced Materials 2017, 29 (41), 1606592. https://doi.org/10.1002/ADMA.201606592.(11) Chen, N.; Chen, D.; Wu, J.; Lai, Y.; Chen, D. Polyethylene Glycol Modified Tetrathiafulvalene for High Energy Density Non-Aqueous Catholyte of Hybrid Redox Flow Batteries. Chemical Engineering Journal 2023, 462. https://doi.org/10.1016/j.cej.2023.141996.

Active Learning Guided Computational Discovery of Plant-Based Redoxmers for Organic Nonaqueous Redox Flow Batteries.

Organic nonaqueous redox flow batteries (O-NRFBs) are promising energy storage devices due to their scalability and reliance on sourceable materials. However, finding suitable redox-active organic molecules (redoxmers) for these batteries remains a challenge. Using plant-based compounds as precursors for these redoxmers can decrease their costs and environmental toxicity. In this computational study, flavonoid molecules have been examined as potential redoxmers for O-NRFBs. Flavone and isoflavone derivatives were selected as catholyte (positive charge carrier) and anolyte (negative charge carrier) molecules, respectively. To drive their redox potentials to the opposite extremes, in silico derivatization was performed using a novel algorithm to generate a library of > 40000 candidate molecules that penalizes overly complex structures. A multiobjective Bayesian optimization based active learning algorithm was then used to identify best redoxmer candidates in these search spaces. Our study provides methodologies for molecular design and optimization of natural scaffolds and highlights the need of incorporating expert chemistry awareness of the natural products and the basic rules of synthetic chemistry in machine learning.

Elucidating the Effects of Temperature on Nonaqueous Redox Flow Cell Cycling Performance

The impact of cell temperature is a relatively underexplored area within the burgeoning field of nonaqueous redox flow batteries (NAqRFBs). Here, we investigate the effect of elevated temperature on the performance of nonaqueous redox electrolytes and associated flow cells. Using a model compound, N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT), in a propylene-carbonate-based electrolyte, we experimentally measure the temperature dependence of relevant physicochemical properties (i.e., electrolyte conductivity, viscosity, diffusivity) and electrochemical characteristics (i.e., chemical and electrochemical reversibility) across a temperature range of 30 °C to 70 °C. We then perform flow cell studies, finding that while ohmic and mass transport resistances decrease significantly with increases in temperature for the MEEPT/MEEPT+· redox couple, accessible electrolyte capacity gradually reduces at temperatures 50 °C. Ex-situ, post-test characterization using microelectrode voltammetry suggests that this capacity fade is due to instability of the MEEPT radical cation. Finally, using MEEPT as a posolyte and a model viologen negolyte (bis(2-(2-methoxyethoxy)ethyl)viologen), we assemble a full cell and perform polarization analyses, observing a 2× increase in the peak power density when the operating temperature is increased from 30 °C to 70 °C. Broadly, this work highlights opportunities for systematic engineering of nonaqueous electrolytes and flow cells for higher power operation at elevated temperatures.

Impact of Pendent Ammonium Groups on Solubility and Cycling Charge Carrier Performance in Nonaqueous Redox Flow Batteries.

The synthesis, characterization, electrochemical performance, and theoretical modeling of two base-metal charge carrier complexes incorporating a pendent quaternary ammonium group, [Ni(bppn-Me3)][BF4], 3', and [Fe(PyTRENMe)][OTf]3, 4', are described. Both complexes were produced in high yield and fully characterized using NMR, IR, and UV-vis spectroscopies as well as elemental analysis and single-crystal X-ray crystallography. The solubility of 3' in acetonitrile showed a 283% improvement over its neutral precursor, whereas the solubility of complex 4' was effectively unchanged. Cyclic voltammetry indicates an ∼0.1 V positive shift for all waves, with some changes in reversibility depending on the wave. Bulk electrochemical cycling demonstrates that both 3' and 4' can utilize the second more negative wave to a degree, whereas 4' ceases to have a reversible positive wave. Flow cell testing of 3' and 4' with Fc as the posolyte reveals little improvement to the cycling performance of 3' compared with its parent complex, whereas 4' exhibits reductions in capacity decay when cycling either negative wave. Postcycling CVs indicate that crossover is the likely source of capacity loss in complexes 3, 3', and 4' because there is little change in the CV trace. Density functional theory calculations indicate that the ammonium group lowers the HOMO energy in 3' and 4', which may impart stability to cycling negative waves while making positive waves less accessible. Overall, the incorporation of a positively charged species can improve solubility, stored electron density, and capacity decay depending on the complex, features critical to high energy density redox flow battery performance.

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.