Electrochemical Behavior of Quinoxaline in Aqueous Electrolytes
The electrochemical behavior of quinoxaline in aqueous electrolytes spanning a wide range of cations (Li+, Na+, K+), anions (Cl-, NO3-, OH-, SO42-, 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 Eᵒ = -0.5 V vs. RHE. Second, certain anions were found to reduce peak separation (Cl-, SO42-) 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.
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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