Recent progress in organic redox flow batteries: Active materials, electrolytes and membranes

Redox flow batteries (RFBs) are promising candidates for grid-scale energy storage, but further cost reductions are needed for widespread use.1 As such, redox-active organic molecules and metal-coordination complexes are emerging as charge storage materials considering their redox potential, solubility, and stability can be tailored through molecular functionalization.2,3 Moreover, a number of these compounds can support multiple electron transfers, affording dramatic increases in energy storage capacity.4,5 However, the nature of the electron transfer events, concerted or sequential, plays a key role in determining whether the added capacity can be accessed efficiently. Specifically, depending on molecular features and cell operation, voltaic inefficiencies may be incurred which, in turn, offset any benefits of increased capacity. Therefore, we sought to quantify these losses and their relationship to molecular- and cell-level parameters to better understand the feasibility of multi-electron transfer in RFBs.In this presentation, we use a zero-dimensional electrochemical model, accounting for thermodynamic, kinetic, and mass transfer, to simulate the performance of two-electron compounds in redox flow cells. First, using a single half-cell, we show that voltaic efficiency is significantly influenced by the average redox potential and potential difference between the redox events. We also discuss the effects of different mass transfer coefficients and comproportionation reaction rates. Second, using a full cell, we consider the effects of utilizing two-electron compounds in both half-cells and subsequent effects of charge imbalance, both resulting in further inefficiencies. By understanding the operational tradeoffs of multi-electron compounds, we can develop design criterion to guide molecular engineering efforts towards more efficient high energy density electrolytes for RFBs.AcknowledgementsThis work was funded by the National Science Foundation (NSF) under Award Number 1805566. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.References(1) Chalamala, B. R.; Soundappan, T.; Fisher, G. R.; Anstey, M. R.; Viswanathan, V. V.; Perry, M. L. Redox Flow Batteries: An Engineering Perspective. Proceedings of the IEEE 2014, 102 (6), 976–999.(2) Hogue, R. W.; Toghill, K. E. Metal Coordination Complexes in Nonaqueous Redox Flow Batteries. Current Opinion in Electrochemistry 2019.(3) Ding, Y.; Zhang, C.; Zhang, L.; Zhou, Y.; Yu, G. Molecular Engineering of Organic Electroactive Materials for Redox Flow Batteries. Chem. Soc. Rev. 2018, 47 (1), 69–103.(4) Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries. J. Mater. Chem. A 2017, 5 (46), 24371–24379.(5) Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. Mechanism-Based Development of a Low-Potential, Soluble, and Cyclable Multielectron Anolyte for Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2016, 138 (47), 15378–15384.

Verdazyl radicals are a promising class of materials for the potential application in organic redox flow batteries. Although studies on the redox behavior of verdazyl radicals in organic solvents exist, only the disproportionation in aqueous acidic electrolytes is known. Herein, we address this knowledge gap by analyzing the verdazyl species after disproportionation with a series of electrochemical measurements, quantum-chemical calculations, and chemical characterizations to evaluate the redox chemistry. We thereby focused on the parent 1,3,5-triphenylverdazyl radical to potentially expand the findings to other verdazyl derivatives. Two electrons participate in the investigated electrochemical reaction in acidic electrolytes, compared to the two distinguishable single-electron transfers in organic electrolytes. Participation of two protons in the reaction leads to a pH-dependent potential in the investigated pH window. The oxidized and reduced forms were investigated separately and exhibit reaction rate constants in the range of 10–2 cm/s. Based on the obtained results, a detailed description of the redox behavior of verdazyl radicals is proposed. The high rate constants for the reaction connected with the utilization of two electrons pave the way to apply this material class in aqueous redox flow batteries, pending further structural modifications of the verdazyl radical.

The redox flow battery has received significant research interest as a large scale energy storage system for renewable-energy sources. Although the all-vanadium aqueous redox flow battery has been most developed, its energy density is not sufficient mainly because of narrow electrochemical stability window of water. Therefore, recent efforts have shifted to developing nonaqueous redox flow batteries because they can achieve higher voltage due to wider electrochemical stability window of nonaqueous electrolytes. However, nonaqueous redox flow batteries generally suffer from insufficient energy densities because of low solubility of organic active materials in nonaqueous electrolytes. In this study, we designed and synthesized two highly soluble organic active materials, benzoquinone 1 and naphthoquinone 2 having diethylene glycol mono methyl ether groups. Both 1 and 2 were highly soluble in propylene carbonate (PC). Charge-discharge properties of the batteries using 1 and 2 as organic active materials were measured in LiBF4/PC using a cell equipped with a carbon fiber anode, a lithium-metal anode, and a glass filter separator. The charge-discharge measurements showed that 1 and 2 exhibited high voltages of ca. 3 V and good charge-discharge properties. Notably, 2 showed a higher energy density than conventional all-vanadium aqueous redox flow batteries.

Nonaqueous redox flow batteries (NAqRFB) utilizing redox active organic molecules offer an attractive alternative to metal-based aqueous redox flow batteries for achieving cost-effective, grid-scale energy storage. In particular, nonaqueous electrolytes provide large electrochemical windows, enabling 3 – 4 V cells, and organic active materials span a broad materials design space.1 While several studies report on the discovery of new redox active organic molecules2,3 and even demonstrate organic-based NAqRFB prototypes4, few efforts discuss advances in NAqRFB reactor design. A lack of materials specific knowledge of new nonaqueous electrolytes makes translating aqueous flow cell design principles to nonaqueous systems difficult, especially considering the increased viscosities and decreased conductivities associated with nonaqueous systems. Additionally, prototyping laboratory-scale flow cells employing newly discovered active materials leads to difficulty in determining whether cell failure arises from active material degradation or inadequate flow cell design. Thus, advancing NAqRFB cell design requires development of chemistry-agnostic design guidelines and systematic investigation of device configurations using well-characterized electrolytes with near practical active species concentrations. This work combines experimental and modeling efforts to quantify performance-limiting factors in nonaqueous flow cell design. First, numerical modeling of electrochemical processes in high performance interdigitated flow fields predicts cell polarization as a function of various characteristic dimensionless groups relating to electrolyte properties, electrode morphology, and cell geometry. To capture the wide array of possible materials and cell design configurations, we perform a parametric sweep of the numerical model across the feasible design space, computing dimensionless polarization curves for the characteristic dimensionless groups. From the dimensionless polarization curves, cell designs that minimize area specific resistance (ASR) for a particular set of input materials properties can be identified (Figure 1), guiding flow cell design. Second, we prove 4-acetylamino-2,2,6,6-tetramethyl-1-oxo-piperidine-1-oxyl (AcNH-TEMPO) as a redox pair with the required traits to serve as a platform chemistry for systematic cell-level performance analysis. Both AcNH-TEMPO and its oxidized cation salt are commercially available, stable over many cycles, and soluble in the electrolyte of interest (≥ 0.5 M in 1 M lithium tetrafluoroborate (LiBF4) / propylene carbonate (PC)); additionally the AcNH-TEMPO couple exhibits fast redox kinetics. Using this model redox pair, we investigate critical electrolyte properties including viscosity, conductivity, and UV-vis absorbance spectra as a function of state-of-charge (SOC). Across all SOCs of the model electrolyte (0.5 M AcNH-TEMPO / 1 M LiBF4 / PC), conductivity varies by only 13 %, but viscosity swings drastically by 45 %. Well-defined absorbance peaks at visible wavelengths suggest that electrolyte SOC could even be monitored in-situ with UV-vis. Furthermore, cyclic voltammetry helps to determine the redox potential and reversibility of both the reduced and oxidized forms of AcNH-TEMPO. The redox potential of AcNH-TEMPO conveniently lies well within the electrochemical window of many common nonaqueous electrolytes, allowing for controlled flow cell studies with no electrolyte decomposition. Third, after validating the model compound, a single-electrolyte flow cell study5 exploits the AcNH-TEMPO couple to explore experimental pathways towards achieving an aggressive area specific resistance (ASR) target of ≤ 5 Ω cm2 and validating the numerically computed design rules. A state-of-the-art vanadium flow cell5 (Figure 2a) serves as the initial architecture with various nonaqueous compatible separators and membranes. For each flow cell configuration, impedance (Figure 2a) and polarization (Figure 2b) measurements are recorded at several flow conditions, quantifying the total ASR, as well as the ohmic, kinetic, and mass transfer impedance contributions. Specifically, we demonstrate the effects of separator conductivity, electrode thickness, and flow rate on ASR. We also investigate, for the first time, the performance enhancements afforded by operating NAqRFBs at elevated temperatures. This work culminates in a demonstration of a NAqRFB reactor, which achieves our ASR target, showing promise for economically viable NAqRFBs with high performance reactors. Acknowledgments We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research and the National Science Foundation Graduate Research Fellowship Program.

Quinoxaline (Q) is an excellent candidate for anolyte active materials in nonaqueous redox flow batteries (NRFBs) because of its high solubility, low‐reaction potential, and two transfer electrons in organic solvents. However, systematic and in‐depth studies on the electrochemical performance of Q under nonaqueous conditions are still needed. Herein, a systematical study is conducted on the effect of ion species on Q reaction in nonaqueous solvent acetonitrile through a combination of electrochemical measurements and theoretical analysis. In accordance with the rotating disk electrode analysis, the diffusion coefficients of Q under Na+ and tetrabutylammonium (TBA)+ conditions reach 5.031 × 10−6 and 8.563 × 10−6 cm2 s−1, respectively, and the kinetic rate constants are 7.81 × 10−3 and 4.76 × 10−3 cm s−1, respectively. According to in situ UV–vis analysis, Q presents the best electrochemical reversibility under tetrabutylammonium hexafluorophosphate (TBAPF6) and a low‐reaction potential of −1.98 V versus Ag/Ag+, making it a very promising anolyte active material. By coupling with the 1,4‐di‐tert‐butyl‐2,5‐bis (2‐methoxyethoxy)benzene (DBBMEB) catholyte, the DBBMEB‐Q NRFB, which the battery voltage reaches above 2.5 V under both Na+ and TBA+ conditions, is demonstrated. The methodology adopted in this work provides a design method for the high‐voltage and high‐energy‐density redox flow batteries.

Redox flow battery has been widely considered as a promising large-scale stationary energy storage technology to stabilize the power grid and promote integration of renewables. The unique cell structure enables separation of energy and power thus offering tremendous design flexibility for different E/P applications. The limited voltage window in traditional aqueous flow batteries narrows down the intrinsic energy capacity. Therefore, nonaqueous redox flow batteries are being pursued because of the wider voltage window of nonaqueous electrolytes. The current nonaqueous flow battery research is focused on the flow chemistry study and redox materials development.1 Among them, all-organic flow batteries is an important type and use organic electroactive materials at both positive and negative sides, taking advantage of the structural diversity and tunability of organic materials.2However, the current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. Especially, development of suitable negative side redox materials is considered more challenging. Here we report several high performance all-organic redox flow chemistries with cell voltages higher than 2V. These flow cell tests demonstrated remarkable cell efficiencies and high operational current densities that exceed those of most reported non-aqueous flow chemistries. A mechanistic study was carried out to gain fundamental understanding of performance degradation in these flow batteries, indicating that the chemical stability of the charged species in various supporting solvents and salts plays a critically important role in achieving decent flow cell cycling stability. This finding can serve as an important guidance for the selection and design of stable redox electrolyte systems. Figure 1. (a) Cyclic voltammetry (CV) and (b) cycling capacity of an all-organic flow chemistry based on 9-fluorenone (negative) and 2,5-di-tert-butyl-1-methoxy-4-[2’-methoxyethoxy]benzene (positive).3 Figure 1

Energy storage system is a critical enabling factor for deploying unstable and intermittent renewable power sources such as solar and wind power sources. Redox flow batteries (RFBs) offer unique advantages of high efficiency, low cost, scalability and rapid response for grid energy storage. One of the most critical bottlenecks of RFBs is their low energy density, which is strongly related to the cell voltage and the solubility of the active materials. In this presentation, we will discuss strategies to improve the energy density of aqueous and nonaqueous RFBs and spectroscopic investigation on the redox processes involved in these RFB systems. Design strategies including redox complexing agent (e.g. lithium iodide, lithium bromide), semi-solid approach (e.g. sulfur/C and organic molecules), self-mediation approach, and highly-concentrated organic molecules (e.g. ferrocene-based) will be discussed. The influence of flow rate and current density on the electrochemical performance under continuous/intermittent flow modes as well as the synergistic interplay between solvent and redox-active materials in solid and liquid phases will be discussed. Acknowledgement: This work was fully supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).

Renewable energy sources such as wind and solar are replacing fossil fuels for electricity generation. However, intermittency of wind and solar limits their wide-spread adoptions. The energy fed into the power grid must be matched with the consumer energy demand to prevent blackouts and destabilization of the grid [1, 2]. Recently, redox flow batteries (RFBs) have gained practical interest among the other energy storage technologies in light of their long lifetime, independent sizing of power and energy, high round-trip efficiency, scalability and design flexibility, fast response, and low environmental impact [3-5]. Organic redox-active materials have recently received attention as they provide competitive electrochemical characteristics, flexible design, and they are abundant in nature [6]. Aqueous designs face commercial difficulty because RFBs have low energy and power density due to the limited cell voltage of 1.23 V. The limited voltage is due to the evolutions of hydrogen and oxygen in the water electrolysis [7]. Solvent substitution is one solution to enable higher energy densities in RFBs, using non-aqueous solution also provides a large design space for enhancement of material solubility, cell potential and the number of electrons stored in the redox species [7-8].In this study, a new organic redox molecule, tetra amino anthraquinone (Disperse Blue 1: DB-1), is evaluated and compared with other organic systems reported in the literature [5, 7] such as benzoquinone (BQ), naphthoquinone (NQ), anthraquinone (AQ), tetramethyl piperidinyloxyl (Tempo), and phenylenediamine (PD) in non-aqueous solvent by means of cyclic voltammetry. A three-electrode system was utilized to conduct cyclic voltammetry (CV) experiments using glassy carbon working electrodes. The battery performance was evaluated using a flow cell with an electrode area of 2.5 cm2. The electrolytic solution, containing 40 mM DB-1 solution in dimethyl sulfoxide solvent (DMSO) and 1 M Bis (trifluoromethane) sulfonimide lithium salt, was circulated through the cell at a flow rate of 10 cm3 min-1. Graphite felt and Nafion 115 were used as the electrode and membrane, respectively.In addition, density functional theory (DFT) calculations were used to better understand the electrochemical behavior of the active quinone molecules at different oxidation states. Figure 1 shows the molecular orbital energy levels (HOMO and LUMO) of the DB-1 organic dye and other similar organic molecules obtained by DFT calculations in DMSO. A relatively small HOMO-LUMO gap means a lower overpotential required for the oxidation and reduction processes [8]. The DB-1 had narrower bandgaps (<3 eV) than other quinone molecules (> 3.9 eV), suggesting that the selected molecule has better kinetics than other organic molecules. The result describes a systematic evaluation of DB-1 as a redox active material for energy storage applications by means of electronic structures, electrochemical properties, and flow cell studies. Voltammetric studies and DFT calculations (in DMSO solvent) demonstrated that this was capable of forming both anions and cations at electrode potentials ranging from 1.7 to 4.4 V vs. Li, involving up to 6 electron-transfers and exhibiting one of the highest electrode potentials (up to 4.4 V vs. Li) in the literature. Since these quinone derivatives can be extracted from biomass materials, the proposed organic RFB system provides a greener and more sustainable option for grid-scale energy storage applications than metal based redox systems. The proposed chemistry in this work showed high energy efficiency (ca. 68 %) at a current density (20 mA cm-2) with close to 100% capacity retention. These results reveal that the molecular tuning of quinone structures is promising for RFBs applications. Figure 1

This dissertation examines the process of energy conversion on fuel cells and the storage of energy in the form of redox flow batteries. In Chapter 1, I will discuss the fundamentals of the polymer electrolyte membrane fuel cells for the conversion of chemical energy, stored in the form of hydrogen and oxygen, into electrical energy and the current technologies and limitations of these devices. I will also discuss the operating principles of the redox flow battery for applications in grid energy storage with a special focus on the non-aqueous vanadium redox flow battery and the space that exists for improvement.In chapter 2, I will discuss the work that was done to investigate the synthesis pathway of single-atom MnN4 catalyst sites. Using in-temperature X-ray absorption spectroscopy (XAS) we were able to unravel the mechanism of site evolution during pyrolysis. We show that the formation of manganese oxides (MnOx) is the initial state of the precursor at room temperature, and the emergence of the manganese-nitrogen coordination (MnN4) site begins at 750 °C and continues until reaching the upper limit of 1000 °C. The competition between the MnOx and MnN4 is identified as the primary factor governing the formation of MnN4 sites during pyrolysis. Chapter 3 investigates the process of MnNC catalyst development based on our understanding of the MnN4 site evolution pathway developed in chapter 2. This knowledge led us to utilize a chemical vapor deposition (CVD) method to produce MnN4 sites to bypass the evolution route involving the MnOx intermediates. The Mn-N-C catalyst synthesized via CVD shows improved ORR activity over the Mn-N-C synthesized via traditional synthesis by pyrolysis of a mixture of Mn, N, and C precursors. I will discuss our work in the field of redox flow batteries in chapter 4. Our investigation shows how solvent selection can change the electrochemical parameters of vanadium acetylacetonate (V(acac)3). We demonstrated that strong correlations of solvent properties such as polarity, viscosity, and longitudinal relaxation time can be used to predict the electrochemical behavior of V(acac)3. We propose this in-depth analysis of solvent effects on the electrochemical properties can be extended to other systems and give a principle-based design methodology for examining solvent and supporting electrolyte. This provides an alternative to the trial-and-error approach oft employed in solvent selection for non-aqueous redox flow batteries. In chapter 5 I will discuss the degradation process of the metal organic complexes with non-innocent ligands in redox flow battery conditions. Here we demonstrate the deposition onto the working electrode after cycling at low potentials. We examine the degradation mechanism electrochemically and through spectroscopy and propose a pathway the initiates from an intra molecular diradical combination mechanism. Lastly, in chapter 6, I will discuss the conclusions drawn from these research topics and propose avenues for future work and give a scope of how this work fits into the current understanding of energy conversion and storage. --Author's abstract

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 cell unbalancing. Redox-active organic molecule stability has improved to the point that the most stable chemistries degrade at less than 1% per year [2]. With further lifetime increases, the measurement of lower capacity fade rates necessitates higher precision coulometry methods [3] and thermally accelerated degradation protocols [4] to determine which stabilizing approaches are most effective without waiting for multi-month cycling tests to quantify capacity fade.We have developed a high-throughput setup for cycling AORFBs at elevated temperatures, providing a new dimension in the flow battery characterization space to explore. Capacity fade rates of previously published redox-active organic molecules, as functions of temperature, are evaluated in the high-throughput setup providing the ability to extrapolate fade rates to lower operating temperatures. The effect of temperature on electrochemical regeneration [5,6] of decomposed organic molecules is also explored. Collectively, these results highlight the importance of accelerated decomposition protocols to 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] M. Wu, Y. Jing, A. A. Wong, E. M. Fell, S. Jin, Z. Tang, R. G. Gordon, M. J. Aziz, “Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors,” Chem, 6, 1432 (2020).[3] T. M. Bond, J. C. Burns, D. A. Stevens, H. M. Dahn, J. R. Dahn, "Improving Precision and Accuracy in Coulombic Efficiency Measurements of Li-Ion Batteries", Journal of The Electrochemical Society, 160, A521 (2013).[4] D. A. Stevens, R. Y. Ying, R. Fathi, J. N. Reimers, J. E. Harlow, J. R. Dahn, "Using High Precision Coulometry Measurements to Compare the Degradation Mechanisms of NMC/LMO and NMC-Only Automotive Scale Pouch Cells", Journal of The Electrochemical Society, 161, A1364 (2014).[5] M. Wu, M. Bahari, E. M. Fell, R. G. Gordon, M. J. Aziz, “High-performance anthraquinone with potentially low cost for aqueous redox flow batteries”, Journal of Materials Chemistry A, 9, 26709 (2021).[6] Y. Jing, E. W. Zhao, M.-A. Goulet, M. Bahari, E. M. Fell, S. Jin, A. Davoodi, E. Jónsson, M. Wu, C. P. Grey, R. G. Gordon, M. J. Aziz, “In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries,” Nature Chemistry, 14, 1103, (2022).Figure caption: Capacity fade rates of an anthraquinone derivative, measured in symmetric cells, as a function of temperature. Figure 1

As a broad-scale energy storage solution, redox flow batteries (RFBs) offer high efficiency and tunable design. However, conventional RFBs rely on transition-metal ion couples, (e.g., vanadium or iron), whose implementation is limited by low energy densities, high cost, and environmental leaching. Main-group compounds, comprising earth-abundant, p-block elements, represent highly promising, yet underexplored candidates for RFBs. Herein, we evaluate three boron-formazanate complexes as negolyte and symmetric electrolytes in nonaqueous organic redox flow batteries (NAORFBs). Detailed electrochemical characterization of these complexes reveals two sequential reduction processes with the first being exceptionally stable (<3% capacity fade after charge/discharge cycling in a static H-cell for 3 days). In contrast, cycling that includes the two-electron reduced state results in rapid degradation (>59% capacity fade over 2.5 days in a static H-cell), most likely due to fluoride elimination from the BF2 moiety. Guided by these insights, a B(Ph)2 unit was introduced to mitigate this degradation pathway. The elimination of labile B─F bonds as well as steric protection conferred by two phenyl groups led to improved cycling performance (>85% capacity retention after charge/discharge cycling in a flow battery for 15 days). These findings guide the rational design of inexpensive main-group electrolytes for application in energy storage.

Redox flow batteries (RFBs) are promising for LDES applications given their scalability and theoretically long cycle life, but cost considerations and low energy density of the state-of-the-art vanadium RFB (VRFB) limit their deployment. For RFBs to be deployed in the electricity grid, alternative cost-effective electrolytes, which we define as including the active material, solvent, supporting salts, and additives, are needed. Organic RFBs, or ORFBs, are a promising alternative to the VRFB because of their low-cost, earth-abundant materials, but many systems suffer from poor stability and low energy density. While ORFB research has conventionally emphasized active molecule functionalization to tune these properties, the supporting electrolyte can control the active molecule’s characteristics through the solvation structure. An underexplored strategy for addressing ORFB challenges is thus strategically changing the components of the supporting electrolyte to tune active molecule performance. Given the vast number of electrolyte combinations possible, with estimates between 107 to 109, it is critical to accelerate research to identify stable, energy dense, and economic systems. Autonomous discovery will be vital to efficiently exploring this large active material and supporting electrolyte research space.To accelerate the development of ORFBs, we have built an automated flow battery cycler station that can explore a large parameter space and eventually leverage artificial intelligence (AI), called FLOW-AIDE: Flexible Laboratory for Optimizing Wide-ranging Autonomous Investigations and Discoveries in Electrochemistry. FLOW-AIDE contains sixteen electrochemical independent testing channels, each comprised of four peristaltic pumping channels and four selector valves that deliver electrolyte to the flow cell. The sixteen electrochemical testing channels can also serve as foundational blocks for more complex modes of operation that require additional pump channels or selector valves per electrochemical flow cell. With one software controlling both the electrolyte delivery system and the electrochemical testing, this results in a complete automated and closed-loop capability. FLOW-AIDE is chemistry agnostic, meaning that it can be used to explore a vast array of redox active materials (organic and inorganic) for different electrochemical systems (aqueous and non-aqueous based electrolyte solvents).To demonstrate the utility of FLOW-AIDE, we use it to explore solvation effects for aqueous ORFBs. Specifically, we explore the effect of changing the supporting electrolyte environment on the electrochemical behavior of the redox active material 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). TEMPO is a common ORFB active material because of its commercial availability and relatively high redox potential and rapid kinetics in both aqueous and non-aqueous solutions, but it suffers from low solubility in aqueous solutions, which limits the RFB’s energy density, and poor stability. Using FLOW-AIDE, we quantify the effects of pH and supporting salts like sodium chloride (NaCl) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) on TEMPO’s electrochemical behavior and stability. Not accounting for variations in concentrations, the automated cycler enables automated testing of 45 distinct combinations of supporting salts and pH conditions for TEMPO at each of up to 8 independent electrochemical testing channels. Although 45 combinations is small compared to the overall electrolyte parameter space outlined above (107-109 combinations), FLOW-AIDE can be coupled with AI algorithms such as multi-fidelity Bayesian optimization to more efficiently explore the parameter space and down-select to combinations of materials that will provide the maximum amount of information on performance in the minimum number of experiments. Exploration of AI algorithms for use with FLOW-AIDE is a topic of future work, and this demonstration of automated experimentation underscores the utility of FLOW-AIDE and its potential for use in an autonomous discovery platform to accelerate LDES technology development even further when coupled with machine learning.Acknowledgements: The research was conducted at Argonne National Laboratory, a U.S. Department of Energy Office of Science laboratory, operated by UChicago Argonne, LLC under Contract no. DE-AC0206CH11357. The authors acknowledge the support from the Argonne Laboratory Directed Research and Development (LDRD) fund via the Walter Massey Fellowship. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan. http://energy.gov/downloads/doe-public-access-plan

Enhanced cyclability of organic redox flow batteries enabled by an artificial bipolar molecule in neutral aqueous electrolyte

The continued evolution and greening of the electric grid has spurred interest in redox flow batteries (RFBs) as scalable, flexible and potentially low cost energy storage devices1. Nonaqueous RFBs that utilize organic molecules for charge storage are an emerging concept which has garnered significant interest in the scientific community2. Of particular interest is the ability to tailor the molecular structure of the organic compounds to achieve desired properties such increased solubility, higher/lower redox potential and longer lifetimes3. However, because this is an infant field, while many candidate organic molecules have been reported, validations of performance and durability are inconsistent. Materials are evaluated under different conditions, including different charging rates, state-of-charge swings, reactant concentrations, and cell configurations3–5. This presents changes in terms of both comparative analysis (e.g., is material A better than material B?) and absolute metrics (e.g., what is the true stability of material A?). Here, we compare bulk electrolysis techniques, varying charging rates, concentrations, and cycling methods, to gain insight into how different testing protocols influence observed behavior. The overarching goal is to determine the most appropriate methods for quantifying cycle stability for new redox active materials and to provide guidance on best practices. References (1) Su, L.; Kowalski, J. A.; Carroll, K. J.; Brushett, F. R. Recent Developments and Trends in Redox Flow Batteries. In Rechargeable Batteries; Zhang, Z., Zhang, S. S., Eds.; Green Energy and Technology; Springer International Publishing, 2015; pp 673–712. (2) Kowalski, J. A.; Su, L.; Milshtein, J. D.; Brushett, F. R. Recent Advances in Molecular Engineering of Redox Active Organic Molecules for Nonaqueous Flow Batteries. Curr. Opin. Chem. Eng. 2016, 13, 45–52. (3) Sevov, C. S.; Brooner, R. E. M.; Chénard, E.; Assary, R. S.; Moore, J. S.; Rodríguez-López, J.; Sanford, M. S. Evolutionary Design of Low Molecular Weight Organic Anolyte Materials for Applications in Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2015, 137(45), 14465–14472. (4) Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; Attanayake, N. H.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. High Current Density, Long Duration Cycling of Soluble Organic Active Species for Non-Aqueous Redox Flow Batteries. Energy Environ. Sci. 2016, 9(11), 3531–3543. (5) Huang, J.; Su, L.; Kowalski, J. A.; Barton, J. L.; Ferrandon, M.; Burrell, A. K.; Brushett, F. R.; Zhang, L. A Subtractive Approach to Molecular Engineering of Dimethoxybenzene-Based Redox Materials for Non-Aqueous Flow Batteries. J. Mater. Chem. A 2015, 3 (29), 14971–14976.

The development of redox flow batteries (RFBs) has been one of the primary responses to the need to store harvested renewable energy for improving grid reliability and utilization.1–4 In this study, we synthesized and characterized seven types of Phenyl acrylate (PA) backbone-based anion-exchange membranes (AEM) and explored the performance of AEMs in non-aqueous redox flow batteries under ideal conditions. IR data (Figure 1 ) confirms the incorporation of ion carriers in the PA backbone. Methacroylcholine chloride (MACC) was used as an anion carrier, N, N-Methylenebis acrylamide (MBAA) was used as a cross-linker, and 1-Hydroxycyclohexyl phenyl ketone (HCPK) was used as a photoinitiator. Performance was compared based on high stability in a non-aqueous solvent, high permeability to the charge-carrying ion, low electric cell resistance, low cross-over of the redox-active molecules, high mechanical property, and low cost. The best membraned will be found which will show no cross-over of redox-active electrolytes after 1000 battery cycle experiments and exhibit low resistance and high structural/chemical stability in MeCN solvent. References (1) Mazumder, Md. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. Inorg. Chem. 2021, 60 (17), 13388–13399. https://doi.org/10.1021/acs.inorgchem.1c01699.(2) Mazumder, M. M. R.; Farnum, B. H. Two One Electron vs One Two-Electron Redox System of Nickel Diethyldithiocarbamate for Redox Flow Battery; ACS, 2021. https://doi.org/10.1021/scimeetings.1c00951.(3) Mazumder, M. M. R. Ni(IV/II) Redox Couple 2e- Reversibility Improvement for Redox Flow Battery. ECS Meet. Abstr. 2021, MA2021-02 (5), 1855–1855. https://doi.org/10.1149/ma2021-0251855mtgabs.(4) Islam, R.; Mazumder, M. M. R.; Farnum, B. H. Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage. ECS Meet. Abstr. 2021, MA2021-01 (1), 54–54. https://doi.org/10.1149/ma2021-01154mtgabs. Figure 1