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

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 1 H 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 C O 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 ΔE pp 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.