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

Liang, Zhiming; Attanayake, N. Harsha; Greco, Katharine; Neyhouse, Bertrand J.; Barton, John L.; Kaur, Aman Preet; Eubanks, William; Brushett, Fikile R.; Landon, James; Odom, Susan A.

doi:10.1021/acsaem.1c00017

Cited by 29 publications

(65 citation statements)

References 46 publications

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“…90% was recovered upon discharge, in line with the relatively low coulombic efficiencies due to crossover-induced self-discharge when a microporous separator was used. 30 To evaluate whether the decrease in capacity observed in the first 50 cycles could be the result of volume and/or concentration imbalances between the two electrolytes, we inverted the polarity of the battery at that point and ran another 50 charge/discharge cycles. As shown in Figure 4 , this indeed results in capacity recovery (to 99.5% of the initial capacity utilization at cycle 75) and demonstrates that polarity inversion in these bipolar systems can be used to rebalance the cell in a greatly simplified manner in comparison to physical rebalancing methods that require battery disassembly.…”

Section: Results and Discussionmentioning

confidence: 99%

“…As shown in Figure 4 , this indeed results in capacity recovery (to 99.5% of the initial capacity utilization at cycle 75) and demonstrates that polarity inversion in these bipolar systems can be used to rebalance the cell in a greatly simplified manner in comparison to physical rebalancing methods that require battery disassembly. 5b , 30 It should be noted that, in contrast to “compositionally symmetrical cells” prepared by premixing of (different) posolyte and anolyte materials, 31 the use of intrinsically bipolar materials such as 3a allows the utilization of all electroactive material in solution. Potentiostatic electrochemical impedance spectroscopy (PEIS) measurements of the flow cell before and after cycling did not indicate a significant increase in resistance.…”

Section: Results and Discussionmentioning

confidence: 99%

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Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries

et al. 2022

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Redox-active organic molecules are promising charge-storage materials for redox-flow batteries (RFBs), but material crossover between the posolyte and negolyte and chemical degradation are limiting factors in the performance of all-organic RFBs. We demonstrate that the bipolar electrochemistry of 1,2,4-benzotriazin-4-yl (Blatter) radicals allows the construction of batteries with symmetrical electrolyte composition. Cyclic voltammetry shows that these radicals also retain reversible bipolar electrochemistry in the presence of water. The redox potentials of derivatives with a C(3)-CF 3 substituent are the least affected by water, and moreover, these compounds show >90% capacity retention after charge/discharge cycling in a static H-cell for 7 days (ca. 100 cycles). Testing these materials in a flow regime at a 0.1 M concentration of the active material confirmed the high cycling stability under conditions relevant for RFB operation and demonstrated that polarity inversion in a symmetrical flow battery may be used to rebalance the cell. Chemical synthesis provides insight in the nature of the charged species by spectroscopy and (for the oxidized state) X-ray crystallography. The stability of these compounds in all three states of charge highlights their potential for application in symmetrical organic redox-flow batteries.

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Section: Results and Discussionmentioning

confidence: 99%

Section: Results and Discussionmentioning

confidence: 99%

Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries

et al. 2022

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“…These membranes are often preferred over ion‐exchange membranes because they are more stable in organic solvents and have lower resistance to ion transport. However, they allow relatively fast crossover of ROMs [34] . We expected that the use of 1 : 1 solutions of FcR and [Bn‐bpy‐Me 2+ ][PF 6 − ] 2 in both the anolyte and catholyte solutions would mitigate the negative effects of that crossover.…”

Section: Resultsmentioning

confidence: 99%

“…However, they allow relatively fast crossover of ROMs. [34] We expected that the use of 1 : 1 solutions of FcR and [Bn-bpy-Me 2 + ][PF 6 À ] 2 in both the anolyte and catholyte solutions would mitigate the negative effects of that crossover. However, we observed poor capacity utilization of the beads in the RMFBs assembled with either Celgard or Daramic separators (Figure S15).…”

Section: Chemistry-a European Journalmentioning

confidence: 99%

A Nonaqueous Redox‐Matched Flow Battery with Charge Storage in Insoluble Polymer Beads**

Kim

Sanford

Vaid

et al. 2022

Chemistry A European J

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We describe the nonaqueous redox‐matched flow battery (RMFB), where charge is stored on redox‐active moieties covalently tethered to non‐circulating, insoluble polymer beads and charge is transferred between the electrodes and the beads via soluble mediators with redox potentials matched to the active moieties on the beads. The RMFB reported herein uses ferrocene and viologen derivatives bound to crosslinked polystyrene beads. Charge storage in the beads leads to a high (approximately 1.0–1.7 M) effective concentration of active material in the reservoirs while preventing crossover of that material. The relatively low concentration of soluble mediators (15 mM) eliminates the need for high‐solubility molecules to create high energy density batteries. Nernstian redox exchange between the beads and redox‐matched mediators was fast relative to the cycle time of the RMFB. This approach is generalizable to many different redox‐active moieties via attachment to the versatile Merrifield resin.

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“…In both RFBs and LABs, the mass transport of redox-active species is crucial to rate capabilities and overall performance, as shown experimentally and computationally. ,− One problem in these systems, however, is that the mobility of the redox-active species also leads to crossover from the anode to cathode or vice versa, and the effect of mass transport on this crossover is rarely explicitly discussed. Crossover creates major challenges by limiting the capacity per cycle and promoting self-discharge via redox shuttle mechanisms. , The Li metal solid electrolyte interphase (SEI) can also be significantly altered when Li metal is used as an anode. , Development of selective membranes to prevent crossover in nonaqueous systems remains a major area of research: ,− critically, the absence of effective and commercially available membranes for nonaqueous systems necessitates new methodologies that allow detailed studies into crossover rates and reactions.…”

Section: Introductionmentioning

confidence: 99%

New Magnetic Resonance and Computational Methods to Study Crossover Reactions in Li-Air and Redox Flow Batteries Using TEMPO

2021

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Redox-active molecules or ions are important in a variety of electrochemical energy storage systems. In lithium-air batteries (LABs), redox-active mediators are added as soluble catalysts that mitigate (dis)charge overpotentials as well as promote solution-phase reactions that improve the capacity and cycle life of a cell. Redox flow batteries (RFBs) are dependent on the dissolved species to carry and store charge. In both of these systems, crossover phenomena, whereby the redox-active species in solution diffuse from one side of the cell to the other, result in capacity loss.Here, we report a technique to monitor crossover reactions in lithium-air batteries and redox flow batteries, exploiting methodology previously developed to monitor radical formation in redox flow batteries. In this technique, radical concentrations are directly quantified operando by flowing an electrolyte solution containing 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) through nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectrometers. We apply this to Li-TEMPO flow batteries and find that the Coulombic efficiency is only 50%; 50% of the oxidized TEMPO radical, TEMPO + , formed at the cathode crosses over to the anode where it is reduced, regenerating TEMPO. Numerical modeling simulations of static systems cannot capture the extent of redox shuttling seen experimentally unless extremely fast diffusion of TEMPO and TEMPO + is assumed in onedimensional (1D) models or convection is included in two-dimensional (2D) models, confirming that redox shuttling is enhanced significantly by flow. Finally, we tested Nafion membranes in both flow cells and static LABs and found that the membrane limited crossover of TEMPO and TEMPO + by factors of ∼15× and ∼7×, respectively.

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

Cited by 29 publications

References 46 publications

Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries

Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries

A Nonaqueous Redox‐Matched Flow Battery with Charge Storage in Insoluble Polymer Beads**

New Magnetic Resonance and Computational Methods to Study Crossover Reactions in Li-Air and Redox Flow Batteries Using TEMPO

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