Abstract
Redox-active organic molecules have drawn extensive interests in redox flow batteries (RFBs) as promising active materials, but employing them in nonaqueous systems is far limited in terms of useable capacity and cycling stability. Here we introduce azobenzene-based organic compounds as new active materials to realize high-performance nonaqueous RFBs with long cycling life and high capacity. It is capable to achieve a stable long cycling with a low capacity decay of 0.014% per cycle and 0.16% per day over 1000 cycles. The stable cycling under a high concentration of 1 M is also realized, delivering a high reversible capacity of ~46 Ah L−1. The unique lithium-coupled redox chemistry accompanied with a voltage increase is observed and revealed by experimental characterization and theoretical simulation. With the reversible redox activity of azo group in π-conjugated structures, azobenzene-based molecules represent a class of promising redox-active organics for potential grid-scale energy storage systems.
Highlights
Redox-active organic molecules have drawn extensive interests in redox flow batteries (RFBs) as promising active materials, but employing them in nonaqueous systems is far limited in terms of useable capacity and cycling stability
Inspired by the realization of enhanced electrochemical stability by adopting extended π-conjugated structure[17,18,19] and high solubility of aromatic hydrocarbons[20,21], here we report a class of azo compounds as promising organic molecules for high-energy and stable nonaqueous RFBs (NARFBs)
Three azo compounds, azobenzene (AB), 4-methoxyazobenzene (MAB), and 4-hydroxyazobenzene (HAB) that are composed of two phenyl rings with an azo functional group in the center, forming the π-conjugated structure, are explored as organic active materials to study the redox chemistry in organic solvents and electrochemical performance in NARFBs
Summary
Redox-active organic molecules have drawn extensive interests in redox flow batteries (RFBs) as promising active materials, but employing them in nonaqueous systems is far limited in terms of useable capacity and cycling stability. A new design of molecular structure with intrinsically good stability is provided and the stable cycling at both low and high concentrations is achieved, presenting new insight into designing stable molecules for NARFBs. Inspired by the realization of enhanced electrochemical stability by adopting extended π-conjugated structure[17,18,19] and high solubility of aromatic hydrocarbons[20,21], here we report a class of azo compounds as promising organic molecules for high-energy and stable NARFBs. In this study, three azo compounds, azobenzene (AB), 4-methoxyazobenzene (MAB), and 4-hydroxyazobenzene (HAB) that are composed of two phenyl rings with an azo functional group in the center, forming the π-conjugated structure, are explored as organic active materials to study the redox chemistry in organic solvents and electrochemical performance in NARFBs. As the basic compound, the AB molecule is mainly employed as the model material to investigate the electrochemical properties in different supporting electrolytes and it shows a high solubility (4–5 M) in common organic solvents, which is beneficial for achieving high energy density. Given diverse derivatives available in industry, the combination of high solubility and excellent electrochemical stability of AB represents an important step forward towards the development of advanced NARFBs
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