An Investigation of Quinoxaline for Redox Flow Battery Applications

Due to the need for developing economical energy storage systems, there has been increasing interest in redox flow batteries (RFBs). With their potential for low costs, high energy densities, and good electrochemical reversibility, organic compounds have recently received more attention as active materials for RFBs. Research on organic active compounds for aqueous RFBs has focused primarily on molecules in the quinone family, characterized as aromatic-derived molecules with an even number of ketone groups. Our work here aims to investigate the electrochemical behavior of quinoxaline, a fused, organic complex consisting of a benzene ring and a pyrazine ring. More than 30 electrolytes are screened by cyclic voltammetry for chemical and electrochemical stability and the five most promising electrolytes are investigated further. The electrochemical behavior of quinoxaline shows pH-dependent thermodynamics and reaction mechanics. Anion-effect investigation indicates that chloride-containing supporting electrolytes greatly enhances solubility. Quinoxaline demonstrates cycling capability in constant-current bulk electrolysis experiments. This study sheds light on promising characteristics of quinoxaline as a low potential active compound for aqueous RFBs; quinoxaline has a redox potential of E^ᵒ ≈ -0.02 V vs. RHE, exhibits a two-electron transfer capability, and possesses a low molecular weight (130.15 g mol^-1), resulting in a theoretical capacity of 410 mAh g^-1.