Non-aqueous Redox Flow Batteries Research Articles

Designing Alkylammonium Cations for Enhanced Solubility of Anionic Active Materials in Redox Flow Batteries: The Role of Bulk and Chain Length.

Advancing grid-scale energy storage technologies is crucial for realizing a fully renewable energy landscape, with non-aqueous redox flow batteries (NRFBs) presenting a promising solution. One of the current challenges in NRFBs stems from the low energy density of redox active materials, primarily due to their limited solubility in non-aqueous solvents. Herein, this study explores the solubility of vanadium(IV/V) bis-hydroxyiminodiacetate (VBH) crystals in acetonitrile, aiming to use them as anionic catholytes in NRFBs. We focused on enhancing VBH solubility by modifying the structure of the alkylammonium cation. Employing periodic density functional theory and a solvation model, we calculated the dissolution free energy ([[EQUATION]]), which includes sublimation ([[EQUATION]]) and solvation ([[EQUATION]]) energies. Our results indicate that neither elongating straight-chain alkyl groups beyond a tetrabutylammonium baseline nor introducing bulky substituents at the nitrogen center significantly enhances solubility. However, the introduction of carbon spacers combined with terminal bulky substituents markedly improves solubility by favorably altering both [[EQUATION]] and [[EQUATION]]. These findings underline the nuanced impact of cation structure on solubility and suggest a viable approach to optimize VBH-based anionic catholytes. This advancement promises to enhance NRFB efficiency and sustainability, marking a significant step forward in energy storage technology.

Numerical and experimental study on fractal flow field for improving the performance of non-aqueous redox flow battery

The flow field design of the non-aqueous redox flow battery (RFB) decides the electrolyte flow and mass transfer of active species inside porous electrode, which has a significant influence on the operation of RFB. In this work, a fractal tree-like flow channel is proposed to enhance the electrolyte convection and optimize the concentration distribution of active species inside the graphite felt electrode, thus boosting the overall battery performance. Using the finite element simulation method, a three-dimensional numerical model of single cell is established to comparatively study the steady-state discharge process characteristics of deep eutectic solvent (DES) electrolyte-based vanadium‑iron RFBs with fractal and serpentine flow fields, respectively. Moreover, by assembling the single cell of this RFB and building the full cell test system, an experimental study is conducted to compare the discharging output voltage and power of the RFBs with serpentine flow field and fractal flow fields of different fractal dimensions. The numerical results show that the fractal tree-like flow field can improve the discharging power of RFB as well as reduce the pumping loss compared with the conventional serpentine flow field. That is mainly attributed to that fractal flow field is conducive to reducing the polarization loss and strengthening the convection in porous electrode simultaneously. What's more, the role of fractal dimension of flow field is also investigated numerically, thus knowing that large fractal dimension promotes lower pumping loss while small fractal dimension is in favor of higher output voltage. In addition, the experimental results also demonstrate that the fractal flow field could improve the output voltage and maximum discharging power of RFB compared to serpentine flow field. This work provides an optimization method for the flow field design of DES electrolyte-based vanadium‑iron RFB, and is expected to be employed in the other non-aqueous RFBs.

Insights into the interaction of nitrobenzene and the Ag(111) surface: A DFT study

This study explores the potential of nitrobenzene as an anolyte material for nonaqueous redox flow batteries (RFBs) by theoretically examining its low-coverage adsorption behavior on neutral and charged Ag(111) model electrode surfaces. At the low coverage limit, DFT calculations show a preference for nitrobenzene to adsorb parallel to the surface, with the benzene ring and nitro group centered over HCP sites. Interactions between nitrobenzene and the surface were analyzed using induced charge density analysis, Bader charge analysis, and projected density of states (PDOS). It was found that nitrobenzene adsorbs primarily through van der Waals interactions with the surface. As nitrobenzene accumulates negative charge, the strength of adsorption diminishes. Understanding the electrode-electrolyte interface is crucial for enhancing RFB electrochemical performance, and this study sheds light on nitrobenzene's interaction with a model Ag electrode.

(Invited) Computational Framework and Insights Towards Electrolyte Design for Redox Flow Batteries

Achieving a fully renewable energy future requires the development of grid-scale energy storage technologies, and non-aqueous redox flow batteries (NRFBs) hold considerable potential.1 However, the redox-active materials (RAMs) currently in consideration as lead candidates for NRFBs exhibit shortcomings, including low solubility, impaired transport properties, and low reduction potentials. Utilizing theory and computation is invaluable in predicting the structure-property relationship of RAM as NRFB active materials. The numerous components within NRFB electrolytes, spanning solvents, active materials, and supporting salts, suggest that material discovery could experience a substantial acceleration through computational and machine-learning approaches.This presentation will provide a progress update on our efforts towards a computational framework involving quantum mechanical (QM), molecular dynamics (MD), and machine learning methods to predict fundamental chemical and physicochemical properties of redox-active materials for energy storage systems. We used an extremely stable, bio-inspired vanadium(4+) bis-hydroxyiminodiacetic acid (VBH) as the scaffold to explore the large parameter space of NRFB systems.2–4 First, we show our framework based on density functional theory (DFT) for calculating solubilities, incorporating both lattice and solvation energies, that can be calibrated and used with synthetic verifications. We found that lattice free energy, which has previously been neglected, has a significant role in tuning electrolyte solubility. Second, we describe our MD simulations on the concentration and composition dependence of transport properties, such as viscosity, using DFT-derived force field bonded parameters for the novel [VBH]2- anion to understand the interactions between the ions and the solvent. Third, we demonstrate that QM molecular design could finely adjust the reduction potential. These fundamental investigations would offer insights into electrolyte design and how molecular properties dictate the macroscopic properties that contribute to specific performance aspects of advanced battery electrolytes.1 Y. Huang, S. Gu, Y. Yan and S. F. Y. Li, Current Opinion in Chemical Engineering, 2015, 8, 105–113.2 H. Huang, R. Howland, E. Agar, M. Nourani, J. A. Golen and P. J. Cappillino, J. Mater. Chem. A, 2017, 5, 11586–11591.3 S. K. Pahari, T. C. Gokoglan, B. R. B. Visayas, J. Woehl, J. A. Golen, R. Howland, M. L. Mayes, E. Agar and P. J. Cappillino, RSC Adv., 2021, 11, 5432–5443.4 B. R. B. Visayas, S. K. Pahari, T. C. Gokoglan, J. A. Golen, E. Agar, P. J. Cappillino and M. L. Mayes, Chem. Sci., 2021, 12, 15892–15907.

Developing Terpyridine-Based Metal Complexes for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) that utilize dissolved electroactive materials have shown great potential in storing intermittent solar and wind energy. The unique technological advantages of decoupled energy and power lead to high flexibility and scalability in RFB design, making it possible to satisfy a wide range of energy and power applications.[1,2] In traditional RFBs, inorganic transition-metal salts like all-vanadium, iron-chromium, and zinc-bromine RFBs are commonly used.[3,4] However, the narrow electrochemical stability window of water limits the attainable cell voltages in these aqueous systems. [5,6] To overcome these limitations, recent research has shifted towards non-aqueous RFBs (NARFBs) that use non-aqueous solvents, which can offer a wider electrochemical window up to 5 V.[7,8] Here, we introduce our research efforts on the design and synthesis of a series of terpyridine-based complexes of the first-row transition metals Cr, Mn, Fe, and Co for non-aqueous redox flow batteries (NARFBs). Electrochemical studies reveal that these complexes can undergo multi-electron transfer redox reactions. Notably, the Mn and Fe-based complexes exhibit both low negative redox potentials and high positive redox potential, enabling them to serve as a bipolar electrolyte for symmetric RFBs with a cell voltage of over 2 V. The assembled iron-based symmetric NARFB demonstrates a high cell voltage of 2.3 V, columbic efficiency of 97%, energy efficiency as high as 88%, and stable charge-discharge capacity retention of 60% after 160 cycles, corresponding to 99.75 % capacity retention per cycle. Figure 1

Development of Pentablock Copolymer Membranes for a Sodium Polysulfide Hybrid Redox Flow Battery

Long duration energy storage devices, such as redox flow batteries (RFBs), are critical for enabling the widespread adoption of renewable energy sources such as wind and solar on the electric grid. Non-aqueous RFBs (NARFBs) have emerged as a possible alternative to aqueous RFBs (e.g., all-vanadium and Zn/Br systems), as they have the potential to use supporting electrolytes with wide operating voltage windows (>3 V) and a variety of redox couples. One NARFB of interest is a hybrid RFB that utilizes an alkali metal anode, such as sodium, and a polysulfide catholyte. However, the realization of a hybrid NARFB for long duration energy storage requires the development of new membranes with robust mechanical properties, good (electro)chemical stability, high ionic conductivity, and low polysulfide permeability. In this work, we have utilized a cation exchange pentablock copolymer membrane with sodium sulfonate and sodium trifluoromethanesulfonimide (TFSI) functionalities. We studied various crosslinking methods with the goal of reducing electrolyte uptake and polysulfide crossover of the membrane, while maintaining high ion transport. One lightly crosslinked membrane exhibits a 40% reduction in electrolyte uptake compared to the uncrosslinked membrane, but at the expense of a reduction in ionic conductivity from 2.5 x 10-4 S/cm to 4.2 x 10-5 S/cm at 20 °C. Our continued efforts in developing techniques to improve membrane properties will also be discussed.

High Energy Density Asymmetric Tetrathiafulvalene Derivative as a Catholyte for Non-Aqueous Redox Flow Batteries

The inherent intermittency challenge associated with renewable energy sources prompts the rapid development of cost-effective, grid-scale energy storage technologies. Redox flow batteries (RFBs) represent a promising solution to this problem owing to their remarkable potential for scalability and the tunability offered by a wide range of available solution-based chemistries. However, to-date, RFB commercialization has been challenged by a lack of long-lived, energy-dense, and earth abundant charge storage materials. Many of these challenges can be addressed with the development of non-aqueous organic redox flow batteries (NAORFBs). Such batteries leverage the wide potential window (> 5.5 V) offered by organic solvents with the synthetic tunability of organic redox cores to create highly energy dense battery systems with electrolytes based on earth abundant materials (i.e., carbon, nitrogen, sulfur, oxygen, etc.). The energy density of NAORFBs can be optimized with strategies such as increasing the solubility of redoxmers, increasing the number of electrons transferred by redoxmers, and increasing the magnitude of the potential of employed redox materials [1]. Of these, solubility can be enhanced through the grafting of polar side groups to a redoxmer, while redox potential can be increased through the attachment of either electron-withdrawing or donating substituents, in the case of catholyte and anolytes, respectively, near or in conjugation with the redox core [2]. Recently, our group has demonstrated tetrathiafulvalene (TTF) as a robust, two-electron active catholyte material in NAORFBs [3]. Namely, TTF exhibits reversible oxidation events at 0.03 and 0.43 V vs. Ag/Ag+ (Figure 1A). Underivatized TTF, which is highly insoluble in many polar organic solvents, can be made miscible in MeCN, carbonates, and ethereal solvents, with the attachment of four PEG3 chains to the TTF core ((PEG3)2-TTF) (Figure 1B). When paired with a low potential Li metal anode, an RFB incorporating 0.5 M (PEG3)2-TTF (1.0 M e- concentration) in 1.0 M LiPF6-EC/EMC supporting electrolyte exhibits overall operating cell potentials of 3.44 and 3.64 V vs. Ag/Ag+. Here we describe our most recent efforts to develop a further optimized, rationally designed asymmetric TTF derivative – (PEG3/PerF)-TTF – incorporating both a perfluorophenyl ring fused directly to the TTF core and two PEG3 side chains (Figure 1B). The electron-withdrawing perfluorophenyl group functions to substantially raise the potential of both redox events to 0.43 V and 0.65 V vs. Ag/Ag+, while the two PEG3 chains allow the material to retain miscibility in polar solvents. The performance of (PEG3/PerF)-TTF has been successfully demonstrated in a battery at an even higher concentration of 0.75 M (1.5 M e- concentration) (Figure 1C). When also paired with a Li metal anode in 1.0 M LiPF6-EC/EMC supporting electrolyte, the battery exhibits exceptional operational energy density of 96 Wh/L at 6 mA/cm2 current density and capacity retention of 83.1% over 16.8 d of continuous operation (99.0% per d). These results further demonstrate TTF as a highly versatile catholyte with remarkable potential for application in NAORFBs.

A 3D Printed Pumpless Nonaqueous Organic Redox Flow Cell

Redox flow batteries (RFBs) have considerable promise in addressing the ever-increasing demands for robust energy storage devices. The nature of these devices makes it possible to create storage with potential applicability at multiple size scales1. However, conventional RFBs require energy to pump the liquid electrolyte, reducing the overall system efficiency, and limiting its viability to mid- and large-scale operations only2. Eliminating the pumping load can improve overall efficiency and create opportunities for mobile applications. In this work, we demonstrate a 3D printed prototype of a pumpless non-aqueous organic RFB that induces flow in the electrolyte from sinusoidal rocking motion. The body of the flow cell was created using 3D printing, allowing us to modify and adapt geometry easily during testing.First, the compatibility of the 3D-printed material was studied by testing the chemical stability of widely available additive manufacturing materials under RFB conditions. Then, we evaluated the performance of this pumpless system using an easily scalable, soluble, and stable one electron donor phenothiazine derivative, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT) which has been widely studied for RFB applications3. We used 0.25 M MEEPT and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI) as a redox-active couple in 0.5 M tetraethylammonium bis(trifluoromethanesulfonyl)imide (TEATFSI)/acetonitrile (MeCN). Our results suggest an inversely proportional empirical relationship between limiting current density and the frequency of sinusoidal motion due to varying mass transfer rates. After 100 cycles in this symmetric cell with an area specific resistance of 2.62 Ωcm2, we achieved a capacity retention of 63% and an average Coulombic efficiency of 96%, with minimal sign of decomposition of the redox couple when analyzed post-cell cycling.The results of this study can be used for 3D printing material selection in numerous electrochemical devices that use organic solvents, including flow cells, supercapacitors, and lithium-ion batteries. Using sinusoidal motion to drive electrolytes instead of a pump can increase the theoretical efficiency and decrease the overall size of a flow cell. This makes it attractive in wearable applications where biomechanical motion can be harnessed.1 Wang, W. et al. Recent Progress in Redox Flow Battery Research and Development. Advanced Functional Materials 23, 970-986 (2013). https://doi.org:10.1002/adfm.2012006942 Tian, C. H., Chein, R., Hsueh, K. L., Wu, C. H. & Tsau, F. H. Design and modeling of electrolyte pumping power reduction in redox flow cells. Rare Metals 30, 16-21 (2011). https://doi.org:10.1007/s12598-011-0229-13 Milshtein, J. D. et al. High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries. Energy & Environmental Science 9, 3531-3543 (2016). https://doi.org:10.1039/C6EE02027E

(Invited) Innovating Sustainable, Cost-Effective, and Durable Redox Flow Battery Membranes

In the quest to integrate renewable energy sources like solar and wind into the electric grid efficiently, the development of long-duration energy storage (LDES) systems is paramount. The U.S. Department of Energy's Long Duration Storage Shot initiative is at the forefront of this endeavor, aiming to reduce the costs of grid-scale energy storage by 90% for systems with over 10 hours of operation. A key player in this mission is the non-aqueous redox flow battery (RFB), known for its use of earth-abundant materials and potential to meet these cost-reduction goals. However, the commercial viability of RFBs hinges on the development of high-performance, cost-effective membranes, which are crucial for their functionality and affordability. These membranes' ionic conductivity is vital for the power density of RFBs, while their ion selectivity and solvent uptake greatly influence the batteries' cycle life.Our team has made groundbreaking advancements in membrane technology for non-aqueous flow batteries. We've addressed the challenge of balancing ionic conductivity with mechanical strength in polymer electrolytes. Our findings reveal that enhanced solvent uptake while boosting membrane ionic conductivity, can compromise mechanical integrity. To counter this, we've employed two strategies: selective plasticization of the ion-conductive block in block copolymers and reinforcing the membrane's mechanical strength through hydrogen and ionic bonds with an inorganic scaffold. Significantly, we've also reduced the crossover of redox-active species in our membrane designs, notably decreasing polysulfide species crossover in a Na metal – polysulfide hybrid flow battery. This has led to improved capacity retention, Coulombic efficiency, and extended cycle life compared to traditional porous membranes.Our latest innovations include the development of hydrocarbon single-ion conductors, demonstrating enhanced conductivity and stability. We've also introduced crosslinking chemistry to control solvent uptake more effectively, thus improving ionic conductivity while preserving mechanical strength. Lastly, our pioneering tape casting method for producing ceramic-polymer composite membranes represents a major leap in scalable membrane production, paving the way for the practical application of non-aqueous redox flow batteries in long-duration energy storage. Acknowledgment This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the U.S. Department of Energy through the Energy Storage Program in the Office of Electricity. We acknowledge Dr. Jagjit Nanda SLAC National Accelerator Laboratory for the fruitful discussions.

Drivers of Membrane Fouling in the Non-Aqueous Vanadium Acetylacetonate Redox Flow Battery

Due to their unique advantages in terms of long cycle life and modular design, redox flow batteries (RFBs) are being developed to load-level and peak shave energy from intermittent sources such as wind and sunlight. RFBs have yet to meet the stringent cost requirements needed to achieve broad market penetration, however. The second largest cost associated with state-of-the art RFBs after the electrolyte itself is the ion-exchange membrane, which accounts for 30% of the total cell cost [1]. Porous separators provide a pathway to substantially decrease the cost of RFBs, but long-term performance has been difficult to assess due to the coupling of electrolyte degradation, self-discharge, and membrane fouling. This study applies an adaptive observer model to estimate crossover from the open-circuit voltage relaxation of RFB cells, providing detailed information that can be used to decouple membrane fouling from electrolyte degradation by side reactions.For this study the nonaqueous V(acac)3 (vanadium acetylacetonate) electrolyte chemistry [2] is used. Because the RFB cell reaction is based on an electrochemical disproportionation of the neutral V(acac)3 complex, crossover of the active species in this system only results in reversible self-discharge.A novel set of 'canary cell' experiments unequivocally point to membrane fouling as the predominant factor that drives permanent cell degradation. An adaptive observer [3] is used to monitor the crossover rate, which is a proxy for membrane porosity, during self-discharge experiments where the cell is fully charged and the voltage is monitored during an open-circuit relaxation. Figure 1 shows that the porous Daramic separator starts off with ideal Fickian behavior because crossover is a linear function of the state of charge. Over time, however, the cell deviates, with active-species crossover becoming non-Fickian as the membrane fouls. The mass transfer coefficient (determined by the slope of the curve) decreases with exposure to current. It is hypothesized that pore clogging is responsible for this decreasing mass transfer coefficient, and in turn the diminution of cell performance. Deconvoluting membrane and electrolyte processes is a crucial step to determine the stability and long-term viability of RFB electrolyte systems.

Sodium Polysulfides As Catholytes for Redox Flow Batteries

Redox flow batteries based on sodium polysulfide (Na2Sx) show promise due to sodium's high solubility in organic solvents (e.g., diglyme. However, when x is lower than 5, sodium polysulfide exhibits low solubility (less than 0.1 molal), limiting the nominal capacity of this redox-active material. Sodium phosphorothioate complexes, Na2P2Sx, have been reported as an alternative to Na2Sx. Incorporating phosphorus into the polysulfide structure improves their solubility in organic solvents, specifically in diglyme.This presentation will describe the synthesis and characterization of sodium phosphorothioate complexes (Na2P2Sx, where x = 6, 7, 8, and 9) for nonaqueous redox flow batteries. The structure of these materials is explored using 31P NMR and 23Na NMR spectroscopy, Raman spectroscopy, and X-ray diffraction analysis. Overall, 31P NMR results indicate that P5+ is tetrahedrally coordinated in all structures and that Na-P-S compounds have a common PS3- moiety. Ongoing efforts are aimed at providing more precise assignments to the 31P bands and correlating these findings with complementary structural information obtained from 23Na NMR and Raman spectroscopy.Electrochemical properties of sodium polysulfide and sodium thiophosphate catholytes, specifically Na2S8 and Na2P2S8, were also compared using CV. The voltammogram of Na2S8 revealed two clear redox reactions in the 1.5-3 V range vs. Na/Na+ due to a multi-step electron transfer process involving the S2-/S0 redox center. In contrast, Na2P2S8 exhibited a larger peak separation (~2 V) and lower peak currents at a given scan rate, potentially indicating a significant activation barrier associated with the rearrangement of the thiophosphate’s polyanionic structure during reduction/oxidation. Despite the substantial voltage hysteresis, Na2P2S8 displayed stable cycling performance, with a slight increase in peak current after 100 cycles. This behavior is consistent with measurements on RFBs containing a Na2P2Sx catholyte, where an electrode activation process increased the cell’s reversible capacity, likely due to subtle changes in the electrode’s surface chemistry during repeated oxidation/reduction. Finally, preliminary flow cell measurements on Na2P2S7 catholyte show reversible voltage profiles after the formation step with deep discharge. Overall, this presentation demonstrates that Na-P-S catholytes represent a promising material for high energy, low-cost redox flow batteries.AcknowledgementsThis research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the U.S. Department of Energy through the Energy Storage Program in the Office of Electricity.

High-Voltage Catholyte for High-Energy-Density Nonaqueous Redox Flow Battery.

Redox flow batteries (RFBs) with high energy densities are essential for efficient and sustainable long-term energy storage on a grid scale. To advance the development of nonaqueous RFBs with high energy densities, a new organic RFB system employing a molecularly engineered tetrathiafulvalene derivative ((PEG3/PerF)-TTF) as a high energy density catholyte was developed. A synergistic approach to the molecular design of tetrathiafulvalene (TTF) was applied, with the incorporation of polyethylene glycol (PEG) chains, which enhance its solubility in organic carbonate electrolytes, and a perfluoro (PerF) group to increase its redox potential. When paired with a lithium metal anode, the two-electron-active (PEG3/PerF)-TTF catholyte produced a cell voltage of 3.56 V for the first redox process and 3.92 V for the second redox process. In cyclic voltammetry and flow cell tests, the redox chemistry exhibited excellent cycling stability. The Li|(PEG3/PerF)-TTF batteries, with concentrations of 0.1 M and 0.5 M, demonstrated capacity retention rates of ~94 % (99.87 % per cycle, 97.52 % per day) and 90 % (99.93 % per cycle, 99.16 % per day), and the average Coulombic efficiencies of 99.38 % and 98.35 %, respectively. The flow cell achieved a high power density of 129 mW/cm2. Furthermore, owing to the high redox potential and solubility of (PEG3/PerF)-TTF, the flow cell attained a high operational energy density of 72 Wh/L (100 Wh/L theoretical). A 0.75 M flow cell exhibited an even higher operational energy density of 96 Wh/L (150 Wh/L theoretical).

Oxadiazole derivatives as stable anolytes for >3 V non-aqueous redox flow battery

Effective utilization of energy from renewable sources such as wind and solar requires the development of long duration energy storage (LDES) systems that can accommodate intermittent energy accrual. One option under investigation is the use of a redox flow battery (RFB). A significant amount of work has explored aqueous RFB systems with a variety of inorganic and organic carriers. However, moving to a nonaqueous solvent such as acetonitrile (MeCN) for RFB provides a much larger electrochemical window, which could lead to increased energy density if properly utilized. In this work, we investigate a series of 2,5-diphenyl-1,3,4-oxadiazole (DiPhenOx) derivatives as anolytes for a redox flow battery. DiPhenOx has a low voltage redox event that, while reversible by cyclic voltammetry, was determined to be irreversible during bulk electrolysis. To improve cycling performance, we introduced various ester and cyano groups to the phenyl rings of DiPhenOx using molecular engineering. We characterized these derivatives spectroscopically and electrochemically to assess their feasibility for flow battery applications. The ester derivatives with the best cycling performance were tested in a flow cell vs. ferrocene, 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB) and thianthrene, which resulted in ∼2 V, ∼3 V and ∼3 V redox flow batteries, respectively.

Aqueous Redox Flow Cells Utilizing Verdazyl Cations enabled by Polybenzimidazole Membranes.

Non-aqueous organic redox flow batteries (RFB) utilizing verdazyl radicals are increasingly explored as energy storage technology. Verdazyl cations in RFBs with acidic aqueous electrolytes, however, have not been investigated yet. To advance the application in aqueous RFBs it is crucial to examine the interaction with the utilized membranes. Herein, the interactions between the 1,3,5-triphenylverdazyl cation and commercial Nafion 211 and self-casted polybenzimidazole (PBI) membranes are systematically investigated to improve the performance in RFBs. The impact of polymer backbones is studied by using mPBI and OPBI as well as different pre-treatments with KOH and H3PO4. Nafion 211 shows substantial absorption of the 1,3,5-triphenylverdazylium cation resulting in loss of conductivity. In contrast, mPBI and OPBI are chemically stable against the verdazylium cation without noticeable absorption. Pre-treatment with KOH leads to a significant increase in ionic conductivity as well as low absorption and permeation of the verdazylium cation. Symmetrical RFB cell tests on lab-scale highlight the beneficial impact of PBI membranes in terms of capacity retention and I-V curves over Nafion 211. With only 2 % d-1 capacity fading 1,3,5-triphenylverdazyl cations in acidic electrolytes with low-cost PBI based membranes exhibit a higher cycling stability compared to state-of-the-art batteries using verdazyl derivatives in non-aqueous electrolytes.

Development of Modular Nitrenium Bipolar Electrolytes for Possible Applications in Symmetric Redox Flow Batteries.

Amid the escalating integration of renewable energy sources, the demand for grid energy storage solutions, including non-aqueous organic redox flow batteries (oRFBs), has become ever more pronounced. oRFBs face a primary challenge of irreversible capacity loss attributed to the crossover of redox-active materials between half-cells. A possible solution for the crossover challenge involves utilization of bipolar electrolytes that act as both the catholyte and anolyte. Identifying such molecules poses several challenges as it requires a delicate balance between the stability of both oxidation states and energy density, which is influenced by the separation between the two redox events. We report the development of a diaminotriazolium redox-active core capable of producing two electronically distinct persistent radical species with typically extreme reduction potentials (E1/2red < -2 V, E1/2ox > +1 V, vs Fc0/+) and up to 3.55 V separation between the two redox events. Structure-property optimization studies allowed us to identify factors responsible for fine-tuning of potentials for both redox events, as well as separation between them. Mechanistic studies revealed two primary decomposition pathways for the neutral radical charged species and one for the radical biscation. Additionally, statistical modeling provided evidence for the molecular descriptors to allow identification of the structural features responsible for stability of radical species and to propose more stable analogues.

A gradient electrospinning electrode structure both in the in/through-plane directions for non-aqueous iron-vanadium redox flow battery

To boost the performance of non-aqueous redox flow batteries (RFBs), it is important to synergistically improve the flow/mass transfer efficiencies and the uniformity of overpotential distribution into the porous electrode. In this work, electrostatic spinning technology is developed to propose a novel porous electrode with gradient pore distribution both in the in-plane and through-plane directions, and applied in deep eutectic solvent (DES) electrolyte-based iron-vanadium RFB. On the one hand, the new in-plane gradient design modifies the distribution of reactive species of electrode near the membrane side, resulting in the decreasing polarization loss. On the other hand, the increasing porosity of electrodes from the flow field side to the membrane side attains a trade-off between the charge transfer and the electrolyte flow resistances. According to the experimental results, compared to the graphite felt electrode, the energy efficiency of this RFB with three-dimensional gradient electrode improves by 74.2 % at a current density of 10 mA·cm−2. Moreover, the numerical simulation reveals the reactive transfer behaviors of three-dimensional gradient porous electrode. The results show that the proposed three-dimensional gradient design can enhance the uniformity of overpotential distribution and achieve the decrease of polarization resistance, thus improving the performance of DES electrolyte-based iron-vanadium RFB effectively.

Development of the Squaramide Scaffold for High Potential and Multielectron Catholytes for Use in Redox Flow Batteries.

Nonaqueous organic redox flow batteries (N-ORFBs) are a promising technology for grid-scale storage of energy generated from intermittent renewable sources. Their primary benefit over traditional aqueous RFBs is the wide electrochemical stability window of organic solvents, but the design of catholyte materials, which can exploit the upper range of this window, has proven challenging. We report herein a new class of N-ORFB catholytes in the form of squaric acid quinoxaline (SQX) and squaric acid amide (SQA) materials. Mechanistic investigation of decomposition in battery-relevant conditions via NMR, HRMS, and electrochemical methods enabled a rational design approach to optimizing these scaffolds. Three lead compounds were developed: a highly stable one-electron SQX material with an oxidation potential of 0.51 V vs Fc/Fc+ that maintained 99% of peak capacity after 102 cycles (51 h) when incorporated into a 1.58 V flow battery; a high-potential one-electron SQA material with an oxidation potential of 0.81 V vs Fc/Fc+ that demonstrated negligible loss of redox active material as measured by pre- and postcycling CV peak currents when incorporated in a 1.63 V flow battery for 110 cycles over 29 h; and a proof-of-concept two-electron SQA catholyte material with oxidation potentials of 0.48 and 0.85 V vs Fc/Fc+ that demonstrated a capacity fade of just 0.56% per hour during static H-cell cycling. These findings expand the previously reported space of high-potential catholyte materials and showcase the power of mechanistically informed synthetic design for N-ORFB materials development.

Synthesis of Alkoxy-TEMPO Aminoxyl Radicals and Electrochemical Characterization in Acetonitrile for Energy Storage Applications

In this paper, we describe the synthesis and characterization of alkoxylated TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, radicals with potential application in organic non-aqueous redox flow batteries. The behavior of a series of TEMPO derivatives with varying lengths of alkoxy chain is analyzed in acetonitrile solutions using electrochemical techniques, electron paramagnetic resonance (EPR) spectroscopy, and measurements of permeability through three different membranes. Electrochemical redox potentials are only weakly dependent on the substituent, but, in contrast, exchange current densities derived from the data do depend on the substitution. EPR lends further insight into these properties via the determination of hyperfine splitting constant and rotational correlation time. There is a negligible effect of the substituents on those parameters among the modified TEMPO radicals. Finally, permeation rates of modified TEMPO derivatives through membranes depend significantly on both the membrane and the substitution of TEMPO, providing insights into capacity fade measurements in the literature.

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.