An “interaction-mediating” strategy towards enhanced solubility and redox properties of organics for aqueous flow batteries

Huang, Zhifeng; Kay, Christopher W. M.; Kuttich, Björn; Rauber, Daniel; Kraus, Tobias; Li, Hongjiao; Kim, Sangwon; Chen, Ruiyong

doi:10.1016/j.nanoen.2020.104464

Cited by 37 publications

(41 citation statements)

References 68 publications

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“…In the EPR spectra of Pyr‐TEMPO catholyte with different concentrations (Figure 4f), as the concentration increases from 0.025 to 0.2 m , the three lines gradually coalesce into a single broad line, which was typical of enhanced magnetic interactions such as exchange interaction of paramagnetic species. [ 42 ] The above NMR and EPR analyses reveal the interaction between the nitroxyl radicals with water. And this interaction may help to prevent the Pyr‐TEMPO molecules from collision‐caused structural decomposition, thus improving the cycling performance of Pyr‐TEMPO/[PyrPV]Cl 4 AORFBs.…”

Section: Resultsmentioning

confidence: 99%

Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

Pan

Gao

Liang

et al. 2022

Advanced Energy Materials

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excellent stability can provide a good choice to meet this demand. [2][3][4][5] However, the extensively studied all-vanadium RFBs suffer some long-standing problems, such as the high costs of cation exchange membranes and vanadium-based active materials and the electrolyte crossover and corrosion issues in strong acidic conditions. [6] Aqueous organic redox flow batteries (AORFBs), after being reported, [7,8] received particular attention among the existing flow battery technologies, primarily because organic redox-active materials possess numerous advantages including great natural abundance, facile structural tailorability, tunable electrochemical properties, and potentially low costs. The solutions of a number of organic active molecules, such as viologen, anthraquinone, phenazine, and azobenzene derivatives, have been investigated as the anolytes for AORFBs. And a few organic molecules are available for the catholytes, typically, 2,2,6,6-tetramethyl piperidin-1-yl)oxyl (TEMPO) and metallocene derivatives. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Considering the high redox potential of TEMPO and its derivatives (ranged from 0.8 to 1.0 V versus standard hydrogen electrode, SHE), the introduction of highly hydrophilic groups, such as the quaternary ammonium group, is a promising approach to further improve the energy density. [20,23] However, up to date, the capacity degradation mechanism on the TEMPO/viologen systems remains unclear, and a lack of appropriate spectroscopic methods was established to study the electrochemical reversibility of redox-active organic species, especially in radical involved systems.Given that the chemically stable pyrrolidinium group has an ultrawide electrochemical window and strong polarity, it has been frequently utilized as a cationic moiety of ionic liquids. [33,34] When coupled with Clion, the pyrrolidinium functionalized organic species also appear to be highly watersoluble. Herein, we report the successful synthesis of pyrrolidinium cation functionalized TEMPO and extended viologen (Pyr-TEMPO and [PyrPV]Cl 4 ), which can serve as a highlysoluble organic redox pair with high cell voltage and excellent redox reversibility in AORFBs (Figure 1). The introduction Aqueous organic redox flow batteries (AORFBs) are regarded as a promising candidate for grid-scale, low-cost and sustainable energy storage. However, their performance is restricted by low aqueous solubility and the narrow potential gap of the organic redox-active species. Herein, a highly-soluble organic redox pair based on pyrrolidinium cation functionalized TEMPO and extended viologen, namely Pyr-TEMPO and [PyrPV]Cl 4 , which exhibits high cell voltage (1.57 V) and long cycling life (over 1000 cycles) in AORFBs is reported. The intrinsic hydrophilic nature of the pyrrolidinium group enables high aqueous solubilities (over 3.35 m for Pyr-TEMPO and 1.13 m for [PyrPV]Cl 4 ). Furthermore, the interaction of nitroxyl radicals with water is observed, which may be helpful to prevent collision-induced...

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Section: Resultsmentioning

confidence: 99%

Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

Pan

Gao

Liang

et al. 2022

Advanced Energy Materials

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“…This propensity explains why imidazolium-based ionic liquids represent the largest class of commercially available ionic liquids . The ability of imidazolium ions to form multiple types of bonds has been used by Huang et al to boost solubility (∼4.3 M) of a nitroxide redoxmer in aqueous solution of an imidazolium ionic liquid used as an electrolyte in a flow cell …”

Section: Discussionmentioning

confidence: 99%

“…17 The ability of imidazolium ions to form multiple types of bonds has been used by Huang et al to boost solubility (∼4.3 M) of a nitroxide redoxmer in aqueous solution of an imidazolium ionic liquid used as an electrolyte in a flow cell. 42 Such telling examples provide valuable insights as to how higher redoxmer solubility can be achieved in general. The presently used synthetic approaches focus almost exclusively on improving solute−solvent interactions; these approaches are successful in raising the solubility from <0.1 to 1−2 M. However, surpassing this concentration can be problematic.…”

Section: ■ Conclusionmentioning

confidence: 99%

Competitive Pi-Stacking and H-Bond Piling Increase Solubility of Heterocyclic Redoxmers

et al. 2020

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Redoxmers are organic molecules that carry electric charge in flow batteries. In many instances, they consist of heteroaromatic moieties modified with appended groups to prevent stacking of the planar cores and increase solubility in liquid electrolytes. This higher solubility is desired as it potentially allows achieving greater energy density in the battery. However, the present synthetic strategies often yield bulky molecules with low molarity even when they are neat and still lower molarity in liquid solutions. Fortunately, there are exceptions to this rule. Here, we examine one wellstudied redoxmer, 2,1,3-benzothiadiazole, which has solubility ∼5.7 M in acetonitrile at 25 °C. We show computationally and prove experimentally that the competition between two packing motifs, face-to-face π-stacking and random N−H bond piling, introduces frustration that confounds nucleation in crowded solutions. Our findings and examples from related systems suggest a complementary strategy for the molecular design of redoxmers for high energy density redox flow cells.

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“…This battery provides a high capacity retention of 99.98% per cycle, demonstrating its superior stability. It is noted that this battery represents the most stable one in the reported viologen(-)/TEMPO(+) ARFBs without using any additional supporting electrolyte (Figure 5) [6,9,[20][21][22][23][24]. Postcell analysis is conducted by using CV and 1 H NMR to investigate the superior stability.…”

Section: Flow Battery Performancementioning

confidence: 99%

Viologen-Decorated TEMPO for Neutral Aqueous Organic Redox Flow Batteries

Wang

Yuan

et al. 2021

Energy Mater Adv

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A novel electroactive organic molecule, viz., 1-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1′-(3-(trimethylammonio)propyl)-4,4′-bipyridinium trichloride ((TPABPy)Cl3), is synthesized by decorating 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) with viologen, which is used as the positive electrolyte in neutral aqueous redox flow battery (ARFB). Extensive characterizations are performed to investigate the composition/structure and the electrochemical behavior, revealing the favorable effect of introducing the cationic viologen group on the electroactive TEMPO. Salient findings are as follows. First, the redox potential is elevated from +0.745 V for TEMPO to +0.967 V for decorated TEMPO, favoring its use as the positive electrolyte. Such an elevation originates from the electron-withdrawing effect of the viologen unit, as evidenced by the nuclear magnetic resonance and single crystal structure analysis. Second, linear sweep voltammetry reveals that the diffusion coefficient is 2.97×10−6 cm2 s−1, and the rate constant of the one-electron transfer process is 7.50×10−2 cm s−1. The two values are sufficiently high as to ensure low concentration and kinetics polarization losses during the battery operation. Third, the permeability through anion-exchange membrane is as low as 1.80×10−11 cm2 s−1. It is understandable as the positive-charged viologen unit prevents the molecule from permeating through the anion exchange membrane by the Donnan effect. Fourth, the ionic nature features a decent conductivity and thus eliminates the use of additional supporting electrolyte. Finally, a flow battery is operated with 1.50 M (TPABPy)Cl3 as the positive electrolyte, which affords a high energy density of 19.0 Wh L-1 and a stable cycling performance with capacity retention of 99.98% per cycle.

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An “interaction-mediating” strategy towards enhanced solubility and redox properties of organics for aqueous flow batteries

Cited by 37 publications

References 68 publications

Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

Competitive Pi-Stacking and H-Bond Piling Increase Solubility of Heterocyclic Redoxmers

Viologen-Decorated TEMPO for Neutral Aqueous Organic Redox Flow Batteries

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