Electrochemical Behavior of Quinoxaline in Aqueous Electrolytes

Organic redox active compounds offer a new pathway towards affordable aqueous redox flow batteries (RFBs) due to their electrochemical performance, low cost, and design flexibility. Proposed organic active material candidates for aqueous RFBs have been limited to quinones^1–3, a class of cyclic organic molecules distinguished by an even number of ketone (R–C(=O)–R’) groups⁴. Our experimental study widens the search for organic active material candidates to quinoxaline, a bicyclic organic molecule consisting of fused benzene and pyrazine rings. Quinoxaline electrochemical behavior has previously been characterized in non-aqueous electrolytes^5–9, and several derivatives have been tested in a non-aqueous redox flow battery¹⁰. Aqueous quinoxaline was also recently applied as a redox active compound in a solar-rechargeable RFB device¹¹, but the exploration of quinoxaline electrochemical behavior was not the focus of that study. To the best of our knowledge, the work presented here is the first comprehensive study to consider the electrochemical behavior of aqueous quinoxaline.

The electrochemical behavior of quinoxaline in aqueous electrolytes spanning a wide range of cations (Li⁺, Na⁺, K⁺), anions (Cl^-, NO₃^-, OH^-, SO₄^2-, HCO₃^-, C₂H₃O₂^-), and pH were investigated to determine the best conditions for the (electro)chemical reversibility and mass-transfer properties of quinoxaline. Specifically, this work focuses on interpreting results from cyclic voltammetry (CV) and rotating disk voltammetry (RDV) to analyze quinoxaline electrochemical performance. Several key trends were identified. First, solution pH has the strongest impact on quinoxaline electrochemical performance, reducing peak separation (Figure 1) and improving cycle stability (Figure 2). When stable, quinoxaline was found to have a redox potential of E^ᵒ = -0.5 V vs. RHE. Second, certain anions were found to reduce peak separation (Cl^-, SO₄^2-) but to a lesser degree than increased alkalinity. Third, cations were found to have a negligible effect on CV behavior. Quantitative analyses, performed on select electrolytes, indicated that aqueous quinoxaline redox behavior was characterized by a single wave, two-electron transfer process, resulting in a theoretical capacity of 410 mAh g^-1. Further, quinoxaline solubility in KCl-based electrolytes was found to be as high as 4 M. This combination of high gravimetric capacity, high solubility, and low redox potential makes quinoxaline a promising material for application in an aqueous RFB.

Acknowledgments

            We gratefully acknowledge the financial support of the Massachusetts Institute of Technology Energy Initiative and the National Science Foundation Graduate Research Fellowship Program. The assistance of Dr. Kyler Carroll, Dr. Emily Carino, Mr. Jeffry Kowalski, and Mr. Steven Brown is also much appreciated.

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