Intramolecular hydrogen bonds induced high solubility for efficient and stable anthraquinone based neutral aqueous organic redox flow batteries

• Published in: Journal of power sources 2021-06, Vol.498, p.229896, Article 229896
• Main Authors: Xia, Lixing, Huo, Wenbo, Gao, Huaizhi, Zhang, Heng, Chu, Fengming, Liu, Hao, Tan, Zhan'ao
• Format: Article
• Language: English
• Subjects: Anthraquinone; Intramolecular hydrogen bonds; Organic redox flow batteries; Water solubility
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2021.229896

Solubility of redox active materials (RAMs) is key to realize high volume capacity and energy density in aqueous organic redox flow batteries (AORFBs). In anthraquinone (AQ)-based RAMs, hydrophilic groups substituted at β-sites (2,3,6,7) are expected to improve water-miscibility much better than α-sites (1,4,5,8). In this work, we find an anomalous case: 3-((9,10-dioxo-9.10-dihydroanthracen-2-yl)amino)-N,N,N-trimethylpropan-1-aminium chloride (2-DPAQCl) functioned with the hydrophilic groups surprisingly exhibits poor solubility with less than 0.05 M, however its isomer 3-((9,10-dioxo-9,10-dihydroanthracen-1-yl)amino)-N,N,N-trimethylpropan-1-aminium chloride (1-DPAQCl) shows high solubility at 1.44 M in water. The results from 1H nuclear magnetic resonance (NMR) and infrared (IR) spectra suggest that intermolecular hydrogen bonds (HBs) are formed between N–H donors and Cl− acceptors in 2-DPAQCl, which is different from the case for 1-DPAQCl that N–H donors interact with neighboring CO acceptors as intramolecular HBs. Benefited from the intramolecular HBs, water is prone to break the interaction between solute molecules than that in 2-DPAQCl, leading to better water-solubility. Electrochemical investigations indicate that the peak-to-peak separations ΔEpp of 1-DPAQCl is very narrow in unbuffered aqueous solution as well as in buffered aqueous solution at pH 7.0, implying its highly electrochemical reversibility and facile reaction kinetics, which further indicates 1-DPAQCl is suitable to serve as a negative RAM for neutral AORFBs. [Display omitted] •A highly water-soluble quinone RAM was synthesized by a simple and low-cost way.•An anomalous case that enormous difference in solubility between two isomers.•Formations and impacts of hydrogen bond in the two isomers were revealed.•Excellent electrochemical properties of the quinone-based RAM synthetized.•Quinone-Ferrocene aqueous flow battery is firstly reported.