Many organic redox materials are chemically unstable and sparingly soluble in nonaqueous media. Additionally, the crossover of redox materials and the availability of limited membranes have restricted the examination of the long-term cyclability of these materials in nonaqueous redox flow batteries (RFBs). To overcome these limitations, we developed a new class of pyridinium-based negolytes. The π-conjugation structure of the pyridinium molecules was extended by introducing benzothiazole into the C4-position of pyridinium, which improved the stability of these molecules. Cationic ammonium functional groups at the N-substituent suppressed the crossover of the pyridinium negolytes through an anion exchange membrane. Furthermore, the solubility of the negolyte was increased up to ∼1 M in acetonitrile and 0.3−0.5 M with tetrabutylammonium bis(trifluoromethanesulfonyl)imide (TBATFSI) and acetonitrile. A 0.1 M solution of the dicationic benzothiazolylpyridinium exhibited 0.0083% capacity-fading rate per cycle in symmetric RFBs for 250 cycles and 0.08% in full RFBs comprising the ammonium-substituted ferrocene as a posolyte for 500 cycles.
Pd-catalyzed C–H annulation reactions of halo- and aryl-heteroarenes were developed using readily available o-bromobiaryls and o-dibromoaryls, respectively. A variety of five-membered heteroarenes rapidly provided the corresponding phenanthrene-fused heteroarenes, which led to the identification of phenanthro-pyrazole and thiazole as new, stable −2 V redox couples. The flexible syntheses and tunability of the redox potentials of these azole-fused phenanthrenes over a wide range are expected to facilitate their application as redox-active organic functional materials.
limited fundamentally, and more tunable charge carriers must be found. Organic molecules are compelling candidates, [5][6][7] because there is an inexhaustible supply of carbon, hydrogen, nitrogen, and oxygen on earth. In principle, their properties can be varied over a great range by changing the molecular architecture and employing functional groups. The ideal charge carrier should display fully reversible redox reactions without degradation, be highly soluble in water, and operate at neutral pH.Robust redox-activity can be achieved using aromatic systems that delocalize the charge across the whole molecule for extra stability. But the non-polar nature of aromatics can lower the solubility in water. Another serious challenge is that the radical intermediates may be highly reactive leading to undesirable side reactions and fading of the battery performance. The pH of the electrolyte solutions during battery operation deserves special attention. Because many organic systems employ proton-coupled electron transfer to achieve charge neutrality, [7][8][9][10][11][12] highly acidic or basic conditions are often encountered. However, an excess of H + ions can corrode cell components and trigger undesirable reactions. [13] In addition, highly concentrated OHin the alkaline solution diminishes volumetric energy density. [13,14] Naphthalene diimide (NDI) is an ideal platform for systematically implementing advantageous design principles, because it Organic redox-active molecules are a promising platform for designing sustainable, cheap, and safe charge carriers for redox flow batteries. However, radical formation during the electron-transfer process causes severe side reactions and reduces cyclability. This problem is mitigated by using naphthalene diimide (NDI) molecules and regulating their π-π interactions. The longrange π-stacking of NDI molecules, which leads to precipitation, is disrupted by tethering four ammonium functionalities, and the solubility approaches 1.5 m in water. The gentle π-π interactions induce clustering and disassembling of the NDI molecules during the two-electron transfer processes. When the radical anion forms, the antiferromagnetic coupling develops tetramer and dimer and nullifies the radical character. In addition, short-range-order NDI clusters at 1 m concentration are not precipitated but inhibit crossover. They are disassembled in the subsequent electron-transfer process, and the negatively charged NDI core strongly interacts with ammonium groups. These behaviors afford excellent RFB performance, demonstrating 98% capacity retention for 500 cycles at 25 mA cm -2 and 99.5% Coulombic efficiency with 2 m electron storage capacity.
Aqueous organic redox flow batteries (AORFBs) have been developed as safe and economical energy storage systems for renewable energy applications. Herein, the solubility of two‐electron‐transfer organic molecules, naphthalene diimides (NDIs), was improved by adding two propyl‐spaced ammonium functionalities. Under neutral conditions, the di‐ammonium‐functionalized NDIs exhibited a solubility of ∼0.7 M (1.4 M/e−) in water. Using 0.3 M NDIs as negolyte (negative electrolyte) and iodide/triiodide as posolyte (positive electrolyte), AORFBs achieved 300 galvanostatic cycles with approximately 100 % capacity retention. The post‐mortem analyses revealed negligible chemical decomposition with no crossover while using a Nafion membrane. This study presents a promising NDI negolyte that can achieve stable two‐electron transfer in AORFBs.
Recently, redox flow batteries (RFBs) have emerged as one of the promising candidates for safe and economical grid-scale energy storage system (ESS) that store the intermittent energies such as wind or solar energy. Unlike the conventional aqueous vanadium RFBs, the design of new molecules in nonaqueous RFB (NRFB) has been in the spotlight for the high energy density, cycling stability, and low cost. Among the redox active materials, in particular, organic molecule has various structures through functionalization, and thus its redox potential, solubility, and chemical stability can be controlled. However, there are only a few viable negative-side redox electrolyte, called a negolyte (or anolyte) to date, and even the reported negolytes have suffered from low solubility and stability, and severe crossover problems for long-term cycling. Here we show the systematic design strategies for pyridinium-based organic redox molecule to enhance the its stability and solubility and suppress the crossover rate for NRFB. A benzo[d]thiazole ring, which provides an electron-withdrawing effect, was introduced at the C4 position of pyridinium core by Pd catalyzed C-H arylation. The addition of the π-conjugation system to the pyridinium redox core was key to enhance chemical and electrochemical stability, resulting in negatively low redox potential of -1.19 ~ 1.21 V vs. Fc/Fc+. The solubility of pyridinium derivatives was significantly enhanced from 0.26 M to 1.00 M in acetonitrile by simple anion exchange from tetrafluoroborate (BF4 -) or hexafluorophosphate (PF6 -) anion to bis(trifluoromethanesulfonyl)imide (TFSI-) anion. Moreover, exchanging the functional group on N of pyridinium from methyl group to cationic 3-(trimethylammonio)propyl (TMAP) group suppressed the crossover rate with an anion-exchange membrane in the NRFB. 4-(benzo[d]thaizol-2-yl)-1-(3-(trimethylammonio)propyl)pyridin-1-ium (TMAP-BTP) led to electrochemical stability in symmetric cell, showing a capacity decay rate of 0.0083% per cycle. Contrary to the results of only 60% capacity retention after 100 cycle in full cell with 4-(benzo[d]thaizol-2-yl)-1-methylpyridin-1-ium (BTP) as negolyte, in the case of full cell with TMAP-BTP as negolyte, the capacity retention was significantly increased, showing 89.8 % after 100 cycle, which is ~0.08% capacity decay rate per cycle. In this presentation, I will demonstrate the more detailed design strategies, cycling performances, and electrolyte analysis. (Figure 1).1 References Ahn, J. H. Jang, J. Kang, M. Na, J. Seo, V. Singh, J. M. Joo and H. R. Byon, ACS Energy Lett., 6, 3390 (2021). Figure 1
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