Non-aqueous Redox Flow Batteries Research Articles

Self-Reporting Redoxmers: State of Health Metrics for Redox Flow Batteries

Redox-active organic molecules (redoxmers) are popular for nonaqueous redox flow batteries (NRFBs). However, many parameters need to be achieved for their success. These include high solubility, high conductivity, little to no crossover, wide voltage windows, and high stability in all states of charge. Metrics are needed to track these properties and the overall state of health (SOH) of the battery. Here, we describe the concept of self-reporting redoxmers, where an intrinsic property of the molecule can be used to trace a SOH property. In this case, 2,1,3-benzothiadiazole, an anolyte redoxmer, was functionalized with a pi-extending acetamide groups, which initiated fluorescence, a highly sensitive property. In turn, the fluorescence of the molecules was used to detect species crossover in a simplified H-cell in different electrolyte conditions. Using these methods, we were able to correlate permeability with electrolyte environment. Further, changes in fluorescence emission with different electrolyte salts were direct indicators of solvation environment. Importantly, the synthetic design of the molecule substantially affected electrochemical performance, with a simple methylation of the pi-extending acetamide group giving high stability. The fluorescence also served as a handle to probe and report on solvation environment. In acetonitrile, electrolyte salts containing small lithium cations quenched the redoxmer fluorescence due to a strong chelation effects that were not observed for larger cations like tetraalkylammonium cations. However, this chelation ability was removed when solvents like N,N-dimethylformamide that also have strong chelation effects were used as supported by computational binding calculations. These chelation effects may also correspond with changes in half-wave potential in cyclic voltammetry experiments. Finally, we extend these studies to larger, more complex molecular architectures, which we hypothesize will lead to improved battery performance. Our studies represent the fundamental design of new organic redoxmers with an applied basis to electrolyte fluids while providing molecular-level understanding and SOH diagnosis, which together will provide successful implementation in a flow battery environment.The research was financially supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript 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.

Thermal Hydrogen Treatment of Electrodes for Performance and Stability Enhancement of Non-Aqueous Redox Flow Batteries

Vanadium acetylacetonate [(V(acac)3] is being used as model redox species for the study of non-aqueous redox flow battery (NAqRFB). The redox species present reversible redox reactions for both V+2/V+3 (i.e. the negolyte ca. -1.75 V vs Ag/Ag+) and V+3/V+4 (i.e. posolyte ca. 0.45 V vs Ag/Ag+). The same discharge state V+3 of this electrolyte can be used on both sides of the half cell. The latter and the fact it dissolves easily in an organic solvent, besides its commercial availability at a fair price makes it a promising candidate for future commercial versions of non-aqueous redox flow batteries.Nonetheless, merits are usually accompanied by some demerits. Vanadium has four valency states starting from V+2 to V+5; for the V-NAqRFB only three oxidations states are desirable V+2, V3+, and V4+ thus the formation of V5+ is unwanted. To avoid this oxidation state, typically the battery open circuit potential is kept below 2.27 V. Another downside of this electrolyte refers to its sensitivity towards a moisture/oxygen-enriched environment. The presence of moisture/oxygen in the environment triggers a non-reversible reaction which leads to the formation of VO(acac)2 at the positive electrode. VO(acac)2 irreversible formation causes rapid battery capacity loss due to its parasitic and increased concentration on the electrolyte. Therefore, the electrolyte must be kept in an inert atmosphere such as dry nitrogen. Nonetheless, it was observed that even by maintaining a strictly controlled saturation of the electrolyte with inert gas, the stability of the batteries was still rather negligible. Components such as the electrodes (carbon felts, CF) could also represent a major source for the oxygen-enriched surface. Many oxidation pre-treatments such as chemical exfoliation (H2SO4) and thermal oxidation under airflow at 450 °C are commonly performed on CFs to increase their wettability and performance in aqueous all vanadium redox flow cells. Nonetheless, these kind of pre-treatments might not be suitable for the system here in the study due to the beforehand mentioned reasons. The latter served as the main motivation for this study.Initially, cyclic voltammetry (CV) was performed using a three-electrode configuration cell and a glassy carbon as the working electrode. The addition of few DI water drop s (0.005% or 50 ppm) and the electrolyte saturation with O2 caused the immediate disappearance of redox species peaks (V2+/V3+ redox couple) and the appearance of additional peaks which indicated a strong influence for the reaction shift towards the formation of parasitic VO(acac)2. V+3/V+4 redox couple was also degraded significantly after 0.05% (500 ppm). Thus, keeping a dry and oxygen-free atmosphere is of utmost importance for maintaining a stable battery operation. Interestingly, it was found that the reversibility of species on top of the WE depends on the exposure time to oxygen. The effect is negligible up to 20 minutes of oxygen saturation but extremely critical over 4 hours as even upon saturating the electrolyte with an inert gas again, the adsorption of the active species was less effective and non-reversible (much lower anodic current peaks and much wider apart oxidation/reduction peaks).The carbon felt electrodes used for the aqueous vanadium redox flow battery, even with no treatment have already oxygen functional groups which serve as the reaction sites for the vanadium redox reaction in the aqueous electrolyte. Hydrogen is known as a powerful reducing agent and so might be employed to remove these oxygen functionalities. Therefore, the impact of different pre-treatments on the carbon felts morphology and performance for the V-NAqRFB was assessed. Mainly, the effect of solely adding a pre-oxygen heat treatment (referred to as PO) to a pristine carbon felt from SGL (4.6GFD), a hydrogen treatment (PH), and a combination of oxygen followed by a hydrogen treatment (POH) to completely remove all previously formed heteroatoms, were tested, each for 10 hours at 400 °C. For comparison purposes, a pristine electrode (P) was also used. Then upon, the electrochemical surface area of each CF was pre-screened by leading CV measurements and the hydrogen treatment followed by an oxygen heat treatment (POH) appeared to be the best solution to increase the number of active sites on the CF structure. Morphological characterization was performed on all electrodes using SEM and FTIR. Later, these CFs were used in NAqRFB-cell for battery cycling, impedance spectroscopy, and polarization measurements. The co-relation of the results highlighted that POH is performing better in the V(acac)3 based non-aqueous electrolyte. Besides, solely conducting a pre-oxygen heat treatment on the CFs condemns the performance of the battery, as expected. The results are shown in the attached figure. Figure 1

Bipolar Redox-Active Molecules: Coupling Structure and Cycling Engineering in Extending Cycling Longevity

Non-aqueous organic redox flow batteries (NAqORFBs) have shown great potential in grid-scale energy storage as a result of their wide potential windows (up to ~ 5 V) and a variety of abundant electroactive molecules, making high energy densities possible. Among organic redox-active materials developed, bipolar redoxmers (BRMs), single molecules that can be used as both anolyte and catholyte, stand out as a distinct class of electroactive species. Their unique redox chemistry enables identical components present in RFBs, which has distinct advantages in storage, mitigating crossover and compound degradation – yet the development of robust BRMs is still in its early stage and many important challenges remain to be addressed. Here we aim to fill this gap by evaluating their structure-property relationship and potential of extending cycling lifetime by utilizing both cathodic and anodic molecules. Such studies have uncovered unexpected structure-property responses and identified essential elements in completing their distinct benefits in cycling longevity.

Stable and Highly Soluble Anolyte for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs), in which electric charge is stored in redox-active materials (redoxmer) dissolved in liquid electrolytes, show significant potential for grid-scale energy storage applications. Because of the liquid nature, RFBs feature remarkable flexibilities, including decoupled energy and power that is highly desired for adapting various scales of energy storage requirements. Aprotic organic solvents are extensively studied as they can significantly extend the electrochemical window, leading to higher cell voltage and energy density of RFBs. To that end, developing redoxmers with high solubility and stability in non-aqueous solvents are actively pursed in order to realize the full potentials. Inorganic-based redoxmer materials showed good cyclability in aqueous redox flow batteries. Designing non-aqueous compatible redoxmers based on traditional inorganic-based materials encounter critical technical and economic limitations such as low solubility, inferior electrochemical activity, and high cost. In this presentation, a ferrate(III) based low potential redoxmer (anolyte) will be discussed. The synthesized redoxmer showed high solubility in acetonitrile (>2 M) with redox potential of ~-1.36 V (vs Ag/Ag+) and deliver very stable cycling performance evidenced by the symmetric H-cell cycling. The design approach for non-aqueous compatible redoxmers based on traditional inorganic-based materials may represent a promising strategy for constructing high energy dense and stable RFBs.

Comparison of Separators vs Membranes in Nonaqueous Redox Flow Battery Electrolytes Containing Small Molecule Active Materials

Comparison of Separators vs Membranes in Nonaqueous Redox Flow Battery Electrolytes Containing Small Molecule Active Materials

Crowded electrolytes containing redoxmers in different states of charge: Solution structure, properties, and fundamental limits on energy density

Nonaqueous redox flow batteries use liquid electrolytes containing redox-active organic molecules (redoxmers) as their energy storage medium. To maximize energy density, the redoxmer concentration needs to be maximized while maintaining low viscosity and high ionic conductivity. During charge, a redoxmer molecule pairs with an ion in the electrolyte while another ion migrates across the membrane to maintain electric neutrality. In a crowded electrolyte, this reconstitution changes physical and chemical properties of the solution. To explore these behaviors, a phenothiazine redoxmer fully miscible with acetonitrile was used, and electrochemical charge was mimicked by chemical oxidation. The solutions were examined using small-angle X-ray scattering, nuclear magnetic resonance, and conductometry and modeled using classical molecular dynamics. Our study indicates that physical and structural properties of redoxmer solutions in both states of charge make it exceedingly difficult to increase the redoxmer concentrations over 2 M at any temperature without compromising dynamic properties of such solutions. The cause for this limitation is proximity to a gel-like regime in which fluidity, diffusivity, and ionic conductivity exponentially decrease with increasing concentration. This tendency is compounded by non-Arrhenius behavior of the electrolyte: a small increase in the concentration outruns gains in fluidity and conductivity at a higher temperature. Thus the properties of crowded electrolytes generally make it impossible to operate when gel-like behavior sets in. Pushing the redoxmer concentration to 2.5–3 M might be possible for small redoxmer molecules, but it would require the use of ionic liquid electrolytes at 340–360 K.

A nonaqueous organic redox flow battery using multi-electron quinone molecules

Organic redox flow batteries are promising energy storage devices due to their moderately low-cost and scalability. This paper introduces a new multi-electron redox active material, tetra-aminoanthraquinone (DB-1) that is capable of forming cations with an oxidation state of 4+, yielding one of the highest electrode potentials (up to 4.4 V vs. Li) and the largest multi-electron transfer (ca. 4 e−). Computational calculations using density functional theory reveal that the DB-1 molecule has narrower HOMO-LUMO band gaps (<3 eV) than similar molecules (>3.9 eV). Stable charge-discharge cycling performance of this organic molecule is observed with high energy efficiencies (ca. 71%) at a relatively high current density of 20 mA cm−2 over 50 cycles, demonstrating the marked potential of DB-1 for future redox flow battery applications.

Organic molecules as bifunctional electroactive materials for symmetric redox flow batteries: A mini review

Symmetric flow battery technology employs a single electroactive material as anolyte and catholyte. This helps evade cross-contamination issues during cycling. Therefore, symmetric organic redox flow batteries (ORFBs) simulating an all-vanadium redox flow battery provides a feasible pathway towards a high performance energy storage devices. Herein, representative bipolar/bifunctional redox active organic molecules (BROMs), recent status of aqueous and non-aqueous symmetric organic redox flow batteries are reviewed. Additionally, current challenges and potential future research directions are also discussed.

Two-dimensional vermiculite nanosheets-modified porous membrane for non-aqueous redox flow batteries

The non-aqueous redox flow batteries (NARFBs) are promising for large-scale energy storage. However, the crossover contamination of active materials seriously imparts the performance. Here, two-dimensional (2D) vermiculite nanosheets modified Celgard porous membrane is prepared by the filtration method. The 2D vermiculite nanosheet layers stacked on the surface of Celgard membrane effectively reduce the crossover effect by sieving electrolytes and active materials through the nanoscale intercalation channels. Furthermore, the surface-governed transport behavior of the 2D vermiculite nanosheets is conducive to the exchange of supporting electrolyte. The resultant NARFB with the modified membrane exhibits higher Coulombic efficiency (95.3% vs. 87.6%) and larger discharge capacity (0.104 vs. 0.093 Ah L−1) without obvious impact on voltage efficiency (90.1% vs. 90.5%) at 2 mA cm−2. This work provides an efficient strategy to alleviate the crossover effect in NARFBs.

A high-rate nonaqueous organic redox flow battery

Azobenzene (azoB) is a promising anode active material for nonaqueous redox flow battery (NARFB), but the battery performance assembled with azoB is rather poor. Here, we show that the rate performance of NARFB with azoB and 2,5-di-tert-butyl-1-methoxy-4-[2′-methoxyethoxy] benzene (DBMMB) as anolyte and catholyte active materials can be significantly enhanced by optimizing the solvent, supporting electrolyte, and membrane. The synergistic interaction among active materials, solvent (acetonitrile), supporting electrolyte (tetraethylammonium bis(trifluoromethylsulfonyl)imide), and membrane (Daramic 175) contributes to the high battery performance. It exhibits an energy efficiency of 63.5% even at an ultra-high current density of 100 mA cm−2. Furthermore, the battery delivers a peak power density of 336 mW cm−2 at the current density of 240 mA cm−2 with 0.25 M active materials, which is a new benchmark for NARFBs.

Biphasic, Membrane-Free Zn/Phenothiazine Battery: Effects of Hydrophobicity of Redox Materials on Cyclability

The lack of a suitable ionic exchange membrane has retarded the development of organic nonaqueous redox flow batteries (RFBs). Membrane-free redox stratified batteries, wherein electroactive materi...

Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications

Non-aqueous redox flow batteries (RFBs) have been gaining increased attention in the energy storage arena. Some of their attractive features include the promise for high energy densities, wider voltage windows compared to aqueous systems, as well as wider operating temperature ranges. One of the major challenges in the development of these systems is the lack of electroactive materials that can undergo reversible redox events within the larger potential window of organic solvents. Herein, we present the design, synthesis, and measurement of electrochemical properties of some iron-based imine- and iminopyridine complexes as promising candidates for RFB applications. Synthesized complexes afforded three reversible redox couples over a ~3 V range. The redox events were also computationally explored and are in good agreement with experimental data. Theoretical calculations show that the redox event at the positive potential can be characterized as metal-centered event corresponding to Fe3+→ Fe2+ reduction, while the two redox events at negative potentials are associated with the consecutive reductions of iminopyridine or diimine moieties. This work has led to the identification of a promising anolyte material, [tris(imino)pyridine)Fe][OTf]2 (1), which is soluble in acetonitrile and is synthesized in two simple steps. This species shows outstanding performance when cycled between 1.0 V and 2.35 V, with 93% coulombic efficiency, 84% energy efficiency, and a capacity decay rate of 0.61% per cycle at 5 mA cm−2. Further modifications to this kind of charge carrier may lead to the development of high energy density materials for grid scale energy storage applications.

Cost-Effective, High-Energy-Density, Nonaqueous Nitrobenzene Organic Redox Flow Battery

Nonaqueous organic redox flow batteries (NAORFBs) show great promise for grid energy storage but are currently facing key challenges such as high electroactive material cost and low energy density....

Manufacture of non-aqueous redox flow batteries using sulfate-templated Dawson-type polyoxometalate with improved performances

The compatible redox pairs are essential to make non-aqueous redox flow batteries (NRFBs) reach their full potentials. Two novel NRFBs with polyoxometalate (POM) redox pairs were designed and investigated. [C16H36N]4[S2W18O62] used as anolyte and benzophenone(BP) severed as catholyte in an asymmetric NRFB, these two diverse active species could cooperate well with each other and achieve a high theoretical cell voltage of 1.54 V. During the charge-discharge cyclic tests, it yielded impressive results with the coulombic efficiency more than 97%, and high voltage efficiency and energy efficiency were delivered without reduction. At the current density of 0.5 mA cm−2, this reformative asymmetric battery delivered a volumetric capacity of 1.97 Ah L−1. The excellent stability of [C16H36N]4[S2W18O62] was validated by the identical 1HNMR and XRD spectra. As a comparison, the symmetric battery employed [C16H36N]4[S2W18O62] both as anolyte and catholyte, which delivered a moderate performances with the coulombic efficiency was about 90%. Also, the permeability of polyoxoanion [S2W18O62]4- cross through AMI-7001 membrane was 1.307×10−7 cm 2 min −1 and less than 1% of this active species had crossed over after 3 d stationary tests.

Strategies to Improve the Energy Density of Non-Aqueous Organic Redox Flow Batteries

Strategies to Improve the Energy Density of Non-Aqueous Organic Redox Flow Batteries

Rationally Designed Ternary Deep Eutectic Solvent Enabling Higher Performance for Non-Aqueous Redox Flow Batteries

Rationally Designed Ternary Deep Eutectic Solvent Enabling Higher Performance for Non-Aqueous Redox Flow Batteries

Stability and Cyclability Predictions of Redox Active Organic Molecules for Non-Aqueous Redox Flow Batteries

Stability and Cyclability Predictions of Redox Active Organic Molecules for Non-Aqueous Redox Flow Batteries

New highly soluble triarylamine-based materials as promising catholytes for redox flow batteries

Arylamines bearing oligoethylene glycol ether solubilizing moieties were designed as promising highly soluble catholyte materials for non-aqueous redox flow batteries.

New phenazine based anolyte material for high voltage organic redox flow batteries.

Herein, we report the synthesis and investigation of a novel phenazine derivative M1 with oligomeric ethylene glycol ether substituents as a promising anolyte material for non-aqueous organic redox flow batteries (RFBs). The designed material undergoes a reversible and stable reduction at -1.72 V vs. Ag/AgNO3 and demonstrates excellent (>2.5 M) solubility in MeCN. A non-aqueous organic redox flow battery assembled using the novel phenazine derivative as an anolyte and a substituted triarylamine derivative as a catholyte exhibited high specific capacity (∼93% from the theoretical value), >95% Coulombic efficiency, 65% utilization of active materials and good charge-discharge cycling stability.

Two-electron-active tetracyanoethylene for nonaqueous redox flow batteries

Two-electron-active tetracyanoethylene (TCNE) is studied as an anolyte for nonaqueous redox flow batteries. TheTCNE/phenothiazine battery presents a maximum specific capacity density of 23.3 A h L−1and volumetric energy density of 26 W h L−1.