From Molecule to Stack: A Cross‐Scale Design Framework for Aqueous Organic Redox Flow Batteries

Aqueous pH-neutral organic redox flow batteries are emerging as viable solutions for sustainable grid-scale energy storage. However, their advancement is hindered by the limited availability of catholytes that maintain stable cyclability in both ambient air and inert environments, as well as challenges related to low-cost synthesis and achieving high volumetric capacity. Here, we present the design and synthesis of a series of imidazolium-decorated ferrocene derivatives, culminating in the identification of an ionic liquid-like catholyte that sustains cycling stability in air for over 2000 cycles. At a concentration of 0.5 M, this catholyte exhibits remarkable cycling stability, with no capacity loss observed over a 68-day period in an inert atomosphere. Theoretical calculations and spectroscopic analyses reveal that the methyl imidazolium pendant group enhances compound stability and reduces hydrophilic attacks on ferrocenium through hydrogen-bonding interactions, thereby improving cycling stability. Notably, we have successfully constructed a pH-neutral stacked flow battery that achieves a peak power of 140 W and maintains an unprecedented capacity retention of 95% at 60 mA cm-2 after 20,000 cycles. These findings not only introduce a novel pathway for developing highly stable catholytes but also facilitate assembling high-power-density stacked pH-neutral aqueous organic redox flow batteries.

Due to the energy intermittency of renewable energy sources and the imbalance of supply-demand, energy storage systems are necessary for the continued deployment of renewable energy. Redox flow batteries (RFBs) has been suggested as a promising technology for long term energy storage utilizing relatively low cost, aqueous organic redox species.1–3 Some aqueous organic negolytes have been developed to improve the capacity, voltage and stability of RFBs using common organic redox species e.g., anthraquinones4,5, phenazines6,7 and viologens8,9. However, finding a suitable organic redox species for the positive side is still challenging due to difficulties increasing the standard potential toward positive voltages. Ferri-/ferrocyanide is the most common species used as posolyte, but is difficult to modify apart from counterion exchange10. Another common organic posolyte, utilizes TEMPOL (4-Hydroxy-TEMPO) in neutral conditions 11. Although these posolytes are commonly utilized, they are limited to certain pH ranges.One of the most significant limitations for posolytes, which are electron-poor compounds by nature, is the difficulty in maintaining structural integrity as they become increasingly electrophilic during charging i.e., oxidation.In this work, development of imidazole-based posolytes will be presented concerning reversibility, solubility and stability. A substrate scope has been synthesized and assessed via voltammetry studies in dimethyl formamide and aqueous conditions. A model substrate (DPIP) was synthesized at a multigram scale, showing reversible potential in 1 M KOH with a potential of + 0.24 V vs. SHE. Additionally, DPIP was quasi-reversible in 1 M AcOH. Solubility was assessed in various aqueous conditions. Degradation mechanisms during cycling were quantified using 2D NMR methods, providing insight into the electrophilicity of these compounds. Since all substrates in this scope were tested without further functionalization, many variations can be explored to improve potential and solubility. Skyllas-Kazacos, M. & Menictas, C. Vanadium Redox Flow Batteries. in Encyclopedia of Energy Storage: Volume 1-4 vols 1–4 407–422 (Elsevier, 2022).González-González, J. M., Parrilla, P. & Aguado, J. A. Chemical energy storage technologies. in Encyclopedia of Electrical and Electronic Power Engineering: Volumes 1-3 vol. 1 V1-426-V1-439 (Elsevier, 2022).Wedege, K., Dražević, E., Konya, D. & Bentien, A. Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility. Sci Rep 6, (2016).Wu, M. et al. Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors. Chem 6, 1432–1442 (2020).Hu, B., Luo, J., Hu, M., Yuan, B. & Liu, T. L. A pH‐Neutral, Metal‐Free Aqueous Organic Redox Flow Battery Employing an Ammonium Anthraquinone Anolyte. Angewandte Chemie 131, 16782–16789 (2019).Hollas, A. et al. A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat Energy 3, 508–514 (2018).Li, L., Su, Y., Ji, Y. & Wang, P. A Long-Lived Water-Soluble Phenazine Radical Cation. J Am Chem Soc (2022) doi:10.1021/jacs.2c12683.Hu, B. et al. Improved radical stability of viologen anolytes in aqueous organic redox flow batteries. Chemical Communications 54, 6871–6874 (2018).Luo, J., Hu, B., Debruler, C. & Liu, T. L. A π-Conjugation Extended Viologen as a Two-Electron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries. Angewandte Chemie 130, 237–241 (2018).Luo, J. et al. Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries. Joule 3, 149–163 (2019).Liu, Y. et al. A Long-Lifetime All-Organic Aqueous Flow Battery Utilizing TMAP-TEMPO Radical. Chem 5, 1861–1870 (2019).

Aqueous organic redox flow batteries (AORFBs) represent a promising technology for large-scale energy storage due to their ability to decouple power and energy, potential low cost, and reliance on sustainably sourced molecules. Despite advancements, such as the introduction of inexpensive, abundant active materials and selective ion-exchange membranes, AORFBs require further development to achieve viability for grid-scale applications. Limited understanding of interactions between organic molecules and porous electrode fibers remains a key challenge.Porous carbonaceous electrodes— available as felts, cloths, and papers—play a critical role in energy storage systems like fuel cells and AORFBs. However, commercial carbon electrodes often exhibit heterogeneous performance(1), complicating efforts to distinguish structural effects from electrochemical effects. Integrating 3D-printed architected electrodes with electrochemical confocal fluorescence microscopy(2) enables direct visualization of interactions between fibers and electrochemical species. This approach reveals how porous electrode geometry influences AORFB performance.Architected electrodes isolate geometric contributions to diffusion and mass transport limitations of reduced species(3). A combination of physics-based modeling and experimental approaches allows for a detailed investigation of flow and electrochemical behavior. We derived 3D state-of-charge (SOC) maps from imaging to quantify reduced species concentrations within each voxel. Notably, both experiment and model reveal tails of electrolyte in the flow direction and mass transport limitations emerge near the outlet.This experimentally validated model facilitates the customization of electrodes for specific energy storage applications, paving the way for performance-based design and the development of next-generation electrodes that significantly outperform current commercial options. A. A. Wong, S. M. Rubinstein and M. J. Aziz, Cell Reports Physical Science, 2, 100388 (2021). A.M. Graf, T. Cochard, K. Amini, M.S. Emanuel, S.M. Rubinstein, and M. J. Aziz. "Quantitative Local State of Charge Mapping by Operando Electrochemical Fluorescence Microscopy in Porous Electrodes" Energy Advances 3, 2468 (2024). D. M. Barber et al., “Print-and-plate architected electrodes for electrochemical transformations under flow,” ChemRxiv(2024). This content is a preprint and has not been peer-reviewed. https://doi.org/10.26434/chemrxiv-2024-2hxnb Figure 1

Aqueous organic redox flow batteries offer promising prospects for large-scale, high-safety, and cost-effective energy storage systems with no reliance on scarce mineral resources. However, challenges such as limited water solubility and poor stability hinder the practical application of organic redox molecules in aqueous organic redox flow batteries. Herein, we report the design and synthesis of an artificial redox-active α-amino acid molecule by functionalizing 1,5-dihydroxyanthraquinone with natural cysteine side group, which exhibits enhanced aqueous solubility and redox reversibility in alkaline aqueous organic redox flow batteries. Owing to its unique zwitterionic structure and abundant hydrogen bonds, the negolyte based on artificial α-amino acid molecule exhibits a very low capacity decay rate of 0.00025% per cycle (equivalent to 0.011% per day) under 1 M electron transfer. Theoretical simulations and spectroscopic analyses underscore the importance of the symmetric distribution and abundant hydrogen-bonding interactions of amphipathic amino acid side chains in enhancing the stability of the anthraquinone redox core and reducing its dimerization, as well as enhancing its water solubility and redox reversibility. This study presents the promising potential of nature-inspired principles in designing electrochemically stable, redox-active organic molecules, contributing to the advancement of large-scale, biocompatible, and sustainable aqueous organic redox flow batteries.

Redox Flow Batteries (RFBs) are a versatile and durable type of electrochemical storage and a promising option for large-scale stationary energy storage. Aqueous Organic Redox Flow Batteries (AORFBs) are an innovative category of RFBs that utilize organic species as active molecules in aqueous electrolytes. These species allow for customization of their properties to achieve high technical performance and reduce battery cost. This study presents a comprehensive techno-economic analysis of AORFBs, evaluating their cost metrics and their associated uncertainties. The work modeled both capital cost and Levelized Cost of Storage (LCOS) for RFBs. The model was validated on the Vanadium Redox Flow Battery (VRFB), and it was employed to evaluate the costs for a generic AORFB, using a Monte Carlo technique to incorporate the uncertainty related to the value of critical parameters. Through stochastic analysis, AORFBs are estimated to have an average specific capital cost of 674 €/kWh for 4 h, and 398 €/kWh for 8 h batteries, and probabilities between 16.9% and 29.6% of having lower capital costs compared to VRFBs. AORFBs are estimated to have a current levelized cost, calculated including only the cost of energy lost in the storage due to irreversibility, of about 530 €/MWh for 4 h, and 411 €/MWh for 8 h batteries. The levelized costs of storage, calculated including the total cost of energy charged into the storage, have average values of 663 €/MWh for 4 h, and 543 €/MWh for 8 h batteries. AORFBs have less than 1% probability of having lower LCOS than VRFBs. Current AORFB systems have higher costs compared to state-of-the-art VRFBs, even assuming a low fabrication cost for available organic molecules. This is caused primarily by the AORFBs’ low energy and power densities and high degradation rates. To ensure cost competitiveness with VRFBs, it is essential to identify better-performing organic redox pairs, which should exhibit high open circuit voltage (≥ 1.1 V), should maintain reasonable round-trip efficiency (≥ 71%) while operating at a high current density (≥55mA/cm2). Furthermore, new organic species should have low degradation rates (≤ 0.4 %/day).

Renewable energy sources like wind and solar power are great alternatives for a clean way to produce electricity but have the inconvenience to fluctuate1. To address this issue and promote the implementation of renewables sources, scientists are developing new ways to store energy while production is at a maximum for a subsequent release when there is demand. Redox flow batteries (RFBs) are the most appropriate solution and are increasingly gaining attention for stationary, large scale electrochemical energy storage2,3. One variation of RFBs are aqueous organic redox flow batteries (AORFBs), where water-soluble organic molecules are use as the redox couple. Viologen type molecules are particularly well-suited as a negative potential species to develop cheaper and better AORFBs with great scalability, reversibility and long-term stability4.In this study, viologen derivatives were synthesized with various N-functional groups to explore the impact of the substituent on their solubility, viscosity and redox potential. We showed that the use of short chains of poly(ethylene glycol) improved the solubility of the viologens derivatives which could lead to batteries with higher capacity. The symmetric viologen (with two identical substituents) provided a great solubility in water and the formal potential was unaffected by the chemical nature of the substituents. We demonstrated that the 1,1’-(bisethylene glycol)-4,4’-bipyridium (PEG2-V-PEG2) showed the most promising properties with a high solubility (2.3 M in 1M KCl) and low viscosity (3.01 mPa*s at 1M). As such, this derivative was selected for further evaluation in an asymmetrical labs-scale RFB prototype cell versus bis(trimethylamoniumpropyl)ferrocene (BTMAP-Fc). The compound was studied at various concentrations to assess the maximum energy density that can be reached and the effect of concentration on cycling life by achieving a comparison to the symmetrical 1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridium (SPr-V-SPr) at 0.5 M. Figure 1: Redox flow battery scheme using Viologen derivative as the negolyte and BTMAP-Fc as the posolyte Reference Shi, X., Qian, Y. and Yang, S. Fluctuation analysis of a complementary wind–solar energy system and integration for large scale hydrogen production. ACS Sustainable Chemistry & Engineering, 8(18), pp.7097-7110. (2020)Sánchez-Díez, E.; Ventosa, E.; Guarnieri, M.; Trovò, A.; Flox, C.; Marcilla, R.; Soavi, F.;Mazur, P.; Aranzabe, E.; Ferret, R., Redox flow batteries: Status and perspective towards sustainable stationary energy storage. Journal of Power Sources, 481, 1-23. (2021)Poullikkas, A., A comparative overview of large-scale battery systems for electricity storage. Renewable and Sustainable energy reviews, 27, pp.778-788. (2013)Gentil, S., Reynard, D. and Girault, H.H. Aqueous organic and redox-mediated redox flow batteries: a review. Current Opinion in Electrochemistry, 21, pp.7-13. (2020) Figure 1

A dopamine-based high redox potential catholyte for aqueous organic redox flow battery

Renewable energy technologies, such as wind or solar energy, depend upon intermittent sources, which make long-term storage an important issue for a large-scale application. A way to efficiently store this energy is to use redox flow batteries, where electricity is stored in a liquid electrolyte circulating through an electrochemical cell. The main advantage of this system is the decoupling of power and energy, allowing to increase the storage more efficiently than other technologies, which is advantageous for large-scale stationary systems.1 Most commercial redox flow batteries currently use vanadium as the active material. Although it is a stable metal, its price is high and volatile, and vanadium extraction is responsible for more than 80% of the environmental cost of a vanadium-based redox flow battery. 2 A cheaper, more environment friendly and safer alternative to vanadium is to use organic molecules as active material in an aqueous solvent.3 Organic molecules also offer opportunities to easily tune properties by modifying their structure. This tunability of organic molecules is a great advantage to maximize desired properties: high solubility in water, low viscosity and optimal redox potential (high for positive electrolyte and low for the negative one). The possibilities of modification are however almost infinite and it would be unrealistic to evaluate experimentally all the possible derivatives of even only one family of redox centers. Therefore, computational chemistry, which permits to study molecules properties and draw tendencies within a shorter time frame, is an especially useful tool to assist the design of better performing molecules.4 In this work, we use DFT to study the effect of some structural modifications on the properties of viologen core molecules, with the aim of using them in aqueous organic redox flow batteries (AORFB). In the first part of this presentation, we show the study of PEG chain conformation of PEGylated viologen derivatives in order to explain experimental trends in solubility for different PEG chain lengths. We show that the experimental measure of solubility correlates with the folding of the PEG chain, which is more favored for longer chains. We also see a correlation between asymmetry, dipolar moment and solubility for these molecules, meaning that calculation of the dipole can give an approximate idea of solubility. However, no significant change in redox potential is measured with this type of structural modification.Therefore, in the second part of this study, we present a study of the effect of adding small functional groups directly on the viologen bipyridine core to tune their redox potential. We developed a computational method to calculate the theoretical redox potentials of these derivatives, and we correlate the values to experimental results, and obtained a R factor correlation of 0,9, supporting the validity of the method, which could be used to accurately predict redox potential of other viologen derivatives.These results show the insight computational chemistry can provide for interpretation of experimental data, and the potential of this method to predict and design new optimized molecules for aqueous organic redox flow batteries.References(1) Bai, H. Y.; Song, Z. Y. Lithium-ion battery, sodium-ion battery, or redox-flow battery: A comprehensive comparison in renewable energy systems. J Power Sources 2023, 580.(2) Weber, S.; Peters, J. F.; Baumann, M.; Weil, M. Life Cycle Assessment of a Vanadium Redox Flow Battery. Environ Sci Technol 2018, 52 (18), 10864-10873.(3) DeBruler, C.; Hu, B.; Moss, J.; Liu, X. A.; Luo, J. A.; Sun, Y. J.; Liu, T. L. Designer Two-Electron Storage Viologen Anolyte Materials for Neutral Aqueous Organic Redox Flow Batteries. Chem-Us 2017, 3 (6), 961-978.(4) Asenjo-Pascual, J.; Salmeron-Sanchez, I.; Mauleón, P.; Agirre, M.; Lopes, A. C.; Zugazua, O.; Sánchez-Díez, E.; Avilés-Moreno, J. R.; Ocón, P. DFT calculation, a practical tool to predict the electrochemical behaviour of organic electrolytes in aqueous redox flow batteries. J Power Sources 2023, 564. Figure 1

Aging phenomena and their modelling in aqueous organic redox flow batteries: A review

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs), utilizing redox‐active organic materials as energy storage materials, represent a promising frontier for sustainable long‐duration energy storage. This review highlights recent advances in redox‐active molecule design, analyzing how molecular structures govern electrochemical behavior and degradation pathways critical to stability. We categorize systems by positive and negative electrolyte pairings, examining performance and lifetime challenges across configurations. We explore molecular engineering approaches and full‐cell assembly principles to extend battery lifetime. By introducing representative studies within each category of redox couples, we outline state‐of‐the‐art developments and establish rational design and pairing principles. This framework proposes guidelines for selecting compatible electrolyte pairs based on molecular properties of organic redox‐active species, which may contribute to advancing stable materials and higher‐performance AORFBs.

Aqueous organic redox flow batteries (AORFBs), which use water-soluble organic molecules as the redox couple, represent a highly promising avenue for large-scale sustainable energy storage [1]. The development of novel electroactive organic molecules with low cost, high-energy density, and high stability for long term cycling is essential for aqueous organic electrolytes to become a viable replacement for the vanadium electrolytes typically used in RFBs. Significant advances have been made in recent years in the development of specific compounds from anthraquinone, viologen, and phenazine families as negolyte active materials [2]. However, there has been comparatively less progress in the development of posolyte compounds that meet the requirements for an application in AORFBs.This paper presents the preliminary results of a novel family of pyridinium derivatives that demonstrate performance characteristics comparable to or exceeding those of electroactive molecules reported as posolytes in AORFBs operating under acidic conditions. The initial member of this pyridinium derivative series displays remarkable solubility (2.4 M in water) and exhibits a reversible two-electron transfer, resulting in batteries with high energy density. The redox potential was determined to be as high as 0.6 V vs. Ag/AgCl at pH 0.5, which is in close proximity to the redox potentials of many TEMPO derivatives and more positive than that of ferrocene, two of the most common posolyte materials. An analysis of the pyridinium derivative was conducted in capacity-unbalanced, compositionally symmetric cells under various conditions to ascertain its capacity fade rate. The capacity fade rate was found to be pH-dependent, with low pH values demonstrating high stability. A 1 M concentration cell (corresponding to 2 M of electrons) at pH 0.5 exhibited an extremely low capacity fade rate [3] of 0.015% per day over a 30-day period. The capacity utilization was 92% with a coulombic efficiency of 99.8%. Subsequently, the derivative was evaluated in a full cell configuration, using vanadium as the active compound in the negolyte. At 1 M concentration, the full cell exhibited low capacity fade rate [3] of 0.08% per day over 23 days with a coulombic efficiency of 97.5%. The pyridinium derivative presented here illustrates the considerable potential of this novel family of molecules or the development highly stable AORFB that are capable of functioning under acidic conditions. Reference: Sánchez-Díez, E., et al., Redox flow batteries: Status and perspective towards sustainable stationary energy storage. Journal of Power Sources, 2021. 481. Singh, V., et al., Aqueous organic redox flow batteries. Nano Research, 2019. 12(9): p. 1988-2001. Kwabi, D.G., Y. Ji, and M.J. Aziz, Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review. Chemical Reviews, 2020. 120(14): p. 6467-6489.

Degradation of electrochemical active compounds in aqueous organic redox flow batteries

Aqueous Organic Redox Flow Batteries (AORFBs) have emerged as promising and potentially disruptive technologies for the storage of electrical energy from intermittent renewable sources for use over long discharge durations when the sun isn’t shining and the wind isn’t blowing. AORFBs could become preferred over Li-ion batteries for grid-scale stationary storage due to their potentially low-cost active materials made of Earth-abundant elements, their inherent non-flammability, and the intrinsic decoupling of energy and power capacities in the flow battery design. Our group has demonstrated that calendar life, rather than cycle life, limits molecular lifetimes in AORFBs due to various molecular instabilities that lead to side reactions, thus inhibiting performance [1]. To accurately determine molecular fade rates, we utilize potentiostatic cycling to avoid artifacts caused by drifts in internal resistance and employ volumetrically unbalanced compositionally symmetric cell configurations to distinguish molecular fade from membrane crossover or the two sides drifting out of balance.We have developed a high-throughput setup for AORFBs, capable of cycling cells at elevated temperatures, providing a new dimension in the flow battery characterization space to explore multiple cell parameters simultaneously. The recent incorporation of the decay of active material into zero-dimensional cell-cycling models for AORFBs [2,3] allows for exploration of the causes of various trends seen in flow cell temporal capacity behavior. Complemented by zero-dimensional modelling, we explore temporal capacity evolution in symmetric cells driven by different capacity fade mechanisms such as active species degradation [4], self-discharge [5], and membrane crossover. Collectively, these results highlight the relevance of electrochemical techniques to understand molecular degradation in AORFBs and expedite the screening process of candidate molecules for long lifetime AORFBs, which may enable massive grid penetration of intermittent renewable energy.[1] M.-A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods,” Journal of The Electrochemical Society, 165, A1466 (2018).[2] S. Modak and D. G. Kwabi, “A Zero-Dimensional Model for Electrochemical Behavior and Capacity Retention in Organic Flow Cells,” Journal of the Electrochemical Society, 168, 080528 (2021).[3] B. J. Neyhouse, J. Lee, F. R. Brushett, “Connecting Material Properties and Redox Flow Cell Cycling Performance through Zero-Dimensional Models” Journal of The Electrochemical Society, 169, 090503 (2022).[4] D. G. Kwabi, Y. Ji, M. J. Aziz, “Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review,” Chemical Reviews, 120, 6467 (2020).[5] E. M. Fell, D. De Porcellinis, Y. Jing, V. Gutierrez-Venegas, R. G. Gordon, S. Granados-Focil, M. J. Aziz, “Long-term stability of ferri-/ferrocyanide as an electroactive component for redox flow battery applications: On the origin of apparent capacity fade,” ChemRxiv (2022). https://chemrxiv.org/engage/chemrxiv/article-details/62913087f89e5d4e6ee8828d

Redox flow batteries (RFBs) are a viable technology to store renewable energy in the form of electricity that can be supplied to electricity grids. However, widespread implementation of traditional RFBs, such as vanadium and Zn-Br2 RFBs, is limited due to a number of challenges related to materials, including low abundance and high costs of redox-active metals, expensive separators, active material crossover, and corrosive and hazardous electrolytes. To address these challenges, we demonstrate a neutral aqueous organic redox flow battery (AORFB) technology utilizing a newly designed cathode electrolyte containing a highly water-soluble ferrocene molecule. Specifically, water-soluble (ferrocenylmethyl)trimethylammonium chloride (FcNCl, 4.0 M in H2O, 107.2 Ah/L, and 3.0 M in 2.0 NaCl, 80.4 Ah/L) and N1-ferrocenylmethyl-N1,N1,N2,N2,N2-pentamethylpropane-1,2-diaminium dibromide, (FcN2Br2, 3.1 M in H2O, 83.1 Ah/L, and 2.0 M in 2.0 M NaCl, 53.5 Ah/L) were synthesized through structural decoration of hydrophobic ferrocene with synergetic hydrophilic functionalities including an ammonium cation group and a halide anion. When paired with methyl viologen (MV) as an anolyte, resulting FcNCl/MV and FcN2Br2/MV AORFBs were operated in noncorrosive neutral NaCl supporting electrolytes using a low-cost anion-exchange membrane. These ferrocene/MV AORFBs are characterized as having high theoretical energy density (45.5 Wh/L) and excellent cycling performance from 40 to 100 mA/cm2. Notably, the FcNCl/MV AORFBs (demonstrated at 7.0 and 9.9 Wh/L) exhibited unprecedented long cycling performance, 700 cycles at 60 mA/cm2 with 99.99% capacity retention per cycle, and delivered power density up to 125 mW/cm2. These AORFBs are built from earth-abundant elements and are environmentally benign, thus representing a promising choice for sustainable and safe energy storage.