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Redox flow batteries (RFBs) present an opportunity to bridge the gap between the intermittent availability of green energy sources and the need for on-demand grid level energy storage. While aqueous vanadium-based redox flow batteries have been commercialized, they are limited by the constraints of using water as an electrochemical solvent. Nonaqueous redox flow battery systems can be used to produce high voltage batteries due to the larger electrochemical window in nonaqueous solvents and the ability to tune the redox properties of active materials through functionalization. Iron porphyrins, a class of organometallic macrocycles, have been the subject of many studies for their photocatalytic and electrocatalytic properties in nonaqueous solvents. Often, iron porphyrins can undergo multiple redox events making them interesting candidates for use as anolytes in asymmetrical redox flow batteries or as both catholyte and anolyte in symmetrical redox flow battery systems. Here the electrochemical properties of Fe(III)TPP species relevant to redox flow battery electrolytes are investigated including solubility, electrochemical properties, and charge/discharge cycling. Commonly used support electrolyte salts can have reactivities that are often overlooked beyond their conductivity properties in nonaqueous solvents. Parasitic reactions with the cations of common support electrolytes are highlighted herein, which underscore the careful balance required to fully assess the potential of novel RFB electrolytes.
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Simple Air‐Stable [3]Radialene Anion Radicals as Environmentally Switchable Catholytes in Water
Abstract The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester‐functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli‐responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a π‐dimer, and a σ‐dimer‐each with dramatically different optical, magnetic, and redox properties‐by changing the solvent environment, temperature, or salinity. The symmetric, triester‐tricyano[3]radialene (3) forms a solvent‐responsive, σ‐dimer in water that converts to the radical anion with the addition of organic solvents or to a π‐dimer in brine solutions. Diester‐tetracyano[3]radialene (2) exists primarily as a π‐dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both2and3is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge–discharge cycling in static H‐cells. Specifically, conditions that favor the free radical anion or π‐dimer exhibit significantly enhanced cycling profiles.
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- Award ID(s):
- 1955619
- PAR ID:
- 10497374
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 30
- Issue:
- 7
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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)‐TTFcatholyte 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)‐TTFbatteries, 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).more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/chem.202302829
