Tanking up energy through atypical charging

doi:10.1126/science.abi5911

GSTDTAP > 气候变化

DOI	10.1126/science.abi5911
	Tanking up energy through atypical charging
	Bo Hu; T. Leo Liu
	2021-05-21
发表期刊	Science
出版年	2021
英文摘要	Aqueous redox flow batteries could provide viable grid-scale electrochemical energy storage for renewable energy because of their high-power performance, scalability, and safe operation ([ 1 ][1], [ 2 ][2]). Redox-active organic molecules serve as the energy storage materials ([ 2 ][2], [ 3 ][3]), but only very few organic molecules, such as viologen ([ 4 ][4], [ 5 ][5]) and anthraquinone molecules ([ 6 ][6]), have demonstrated promising energy storage performance ([ 2 ][2]). Efforts continue to develop other families of organic molecules for flow battery applications that would have dense charge capacities and be chemically robust. On page 836 of this issue, Feng et al. ([ 7 ][7]) report a class of ingeniously designed 9-fluorenone (FL) molecules as high-performance, potentially low-cost organic anode electrolytes (anolytes) in aqueous organic redox flow batteries (see the figure, top). These FL anolytes not only display exceptional energy storage performance but also exhibit an unprecedented two-electron storage mechanism. The past decade has witnessed the rapid development of aqueous organic redox flow batteries using sustainable and tunable redox-active organic molecules as charge storage materials ([ 2 ][2], [ 8 ][8], [ 9 ][9]). Previous studies investigated the possibility of using organic ketones as anolyte materials but with limited success ([ 10 ][10], [ 11 ][11]), and Rodriguez et al. ([ 11 ][11]) reported the one-electron, reversible FL/FL·− redox couple in alkaline solutions. However, the poor battery performance of FL molecules was not suitable for durable energy storage, and there was no clear understanding of FL's chemical stability ([ 11 ][11]). Feng et al. report the molecular engineering of FL molecules to achieve substantially improved stability and charge capacity for flow battery applications. The strategy adopted in their research is to introduce electron-withdrawing groups (EWGs), such as sulfonate and carboxylate groups, into the aromatic rings of FL to increase the acid dissociation constant K a of its reduction product, FL-OH. This design aims to stabilize the charged FL species, such as FL·− radical anions and FL2− dianions, for battery cycling by avoiding the irreversible protonation of these anions and allowing the redox reactions to occur in the potential window available in water (see the figure, bottom). Their density functional theory calculations of a series of FL-OH derivatives also suggest that EWGs effectively increase the K a of both O-H and benzylic C-H protons. Upon functionalization of FL with EWGs, the first electron reversibility of FL at ∼–0.7 V versus the normal hydrogen electrode is notably improved in alkaline solutions. Nevertheless, the second electron redox event at ∼–1.13 V is still irreversible, as revealed in cyclic voltammetry studies, but is accessible for energy storage, as explained below. ![Figure][12] Extra electrons without catalysts Feng et al. demonstrated an aqueous organic redox flow battery based on reversible hydrogenation of functionalized 9-fluorenone (2-carboxylate-7-sulfonate fluorenone, or 4C7SFL). This molecule enabled two-electron storage without the use of a catalyst. GRAPHIC: V. ALTOUNIAN/ SCIENCE For flow battery demonstration, the authors chose 2-carboxylate-7-sulfonate fluorenone (4C7SFL). A solution of 1.36 M 4C7SFL in NaOH was used as an anolyte with a mixture of K4Fe(CN)6/K3Fe(CN)6 as a cathode electrolyte (catholyte). At room temperature, the 4C7SFL/ferrocyanide flow batteries delivered a high energy efficiency of 78.8% at 60 mA cm−2. The battery was cycled at 20 mA cm−2 for more than 4 months and had a small capacity decay of 2.62%—equivalent to a capacity decay of 0.02% per day. The exceptional chemical stability of 4C7SFL was confirmed by postcycling spectroscopic studies that revealed only a small amount of a desulfonated product. Notably, the 4C7SFL anolyte actually exhibited two-electron storage capacity (equivalent to 1.8 moles of electrons per 1.0 mole of 4C7SFL) in the battery studies, despite the control of the charge and discharge processes at the FL/FL·− redox couple. At first glance, it would seem impossible that an irreversible redox process such as the second-electron reduction of 4C7SFL could be used for energy storage under these conditions. Feng et al. propose that the two-electron storage property of 4C7SFL originates from the disproportionation of a FL·− radical anion, rather than the electrochemical reduction of FL·− to FL2−. During the charging process, electrochemically generated FL·− disproportionates to FL-OH, a two-electron reduced product, and FL. The regenerated FL undergoes reduction again until an equilibrium is reached under battery-charging conditions. This hypothesis is supported by nuclear magnetic resonance spectroscopy studies that detected the FL-OH product, 4C7SFL-OH, even at a 25% state of charge. The authors also studied the comproportionation reaction by combining 4C7SFL and 4C7SFL-OH, and they detected a small amount of the [4C7SFL]·− radical by electron paramagnetic resonance spectroscopy. The FL·− generated from the comproportionation can be used for electrochemical discharge. These studies suggest that an equilibrium exists between FL·−, FL, and FL-OH, and also favors the disproportionation. The formation and cleavage of the benzylic C–H bond of FL in the disproportionation and comproportionation reactions are likely regulated by subtle proton-coupled electron transfer processes that are subject to further study. The singly protonated species, FL-O−, may be the final charged state and involves the chemical reactions under the strong alkaline condition. In addition, the reduction of ketones to alcohols typically involves a catalyst ([ 12 ][13]), so the reversible, uncatalyzed hydrogenation of FL represents an unusual two-electron storage mechanism for flow battery applications and is mechanistically different from the direct electrochemical two-electron reduction of viologen and anthraquinone anolytes ([ 2 ][2]). The work by Feng et al. expands the selection of stable organic anolytes and serves as a good example of rational molecular engineering to develop durable electrolyte materials. However, a large excess of the K4Fe(CN)6/K3Fe(CN)6 catholyte was needed in the flow battery tests because of the long-term instability of ferro- and ferricyanide in alkaline solutions ([ 13 ][14]). Thus, developing catholyte molecules with durabilities and capacities comparable to those of fluorenone, viologen, and anthraquinone anolyte molecules is urgently needed to fully unleash the energy storage potential of aqueous organic redox flow batteries. 1. [↵][15]1. B. Dunn, 2. H. Kamath, 3. J.-M. Tarascon , Science 334, 928 (2011). [OpenUrl][16][Abstract/FREE Full Text][17] 2. [↵][18]1. J. Luo, 2. B. Hu, 3. M. Hu, 4. Y. Zhao, 5. T. L. Liu , ACS Energy Lett. 4, 2220 (2019). [OpenUrl][19] 3. [↵][20]1. M. Park, 2. J. Ryu, 3. W. Wang, 4. J. Cho , Nat. Rev. Mater. 2, 16080 (2016). [OpenUrl][21][CrossRef][22] 4. [↵][23]1. B. Hu, 2. C. DeBruler, 3. Z. Rhodes, 4. T. L. Liu , J. Am. Chem. Soc. 139, 1207 (2017). [OpenUrl][24][CrossRef][25] 5. [↵][26]1. J. Luo et al ., Joule 3, 149 (2019). [OpenUrl][27] 6. [↵][28]1. Y. Ji et al ., Adv. Energy Mater. 9, 1900039 (2019). [OpenUrl][29] 7. [↵][30]1. R. Feng et al ., Science 372, 836 (2021). [OpenUrl][31][Abstract/FREE Full Text][32] 8. [↵][33]1. J. Winsberg, 2. T. Hagemann, 3. T. Janoschka, 4. M. D. Hager, 5. U. S. Schubert , Angew. Chem. Int. Ed. 56, 686 (2017). [OpenUrl][34][CrossRef][35] 9. [↵][36]1. Y. Ding, 2. C. Zhang, 3. L. Zhang, 4. Y. Zhou, 5. G. Yu , Chem. Soc. Rev. 47, 69 (2018). [OpenUrl][37][CrossRef][38][PubMed][39] 10. [↵][40]1. P. Leung et al ., Appl. Energy 197, 318 (2017). [OpenUrl][41][CrossRef][42] 11. [↵][43]1. J. Rodriguez Jr., 2. C. Niemet, 3. L. D. Pozzo , ECS Trans. 89, 49 (2019). [OpenUrl][44][Abstract/FREE Full Text][45] 12. [↵][46]1. O. Eisenstein, 2. R. H. Crabtree , New J. Chem. 37, 21 (2013). [OpenUrl][47][CrossRef][48] 13. [↵][49]1. J. Luo et al ., Nano Energy 42, 215 (2017). [OpenUrl][50] Acknowledgments: The authors acknowledge funding support from the National Science Foundation (Career Award, grant 1847674) and a Utah State University faculty start-up. 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领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/328815
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Bo Hu,T. Leo Liu. Tanking up energy through atypical charging[J]. Science,2021.
APA	Bo Hu,&T. Leo Liu.(2021).Tanking up energy through atypical charging.Science.
MLA	Bo Hu,et al."Tanking up energy through atypical charging".Science (2021).