Effect of Ion Species on Quinoxaline Reaction and Its Application in Nonaqueous Redox Flow Batteries

Quinoxaline (Q) is an excellent candidate for anolyte active materials in nonaqueous redox flow batteries (NRFBs) because of its high solubility, low-reaction potential, and two transfer electrons in organic solvents. However, systematic and in-depth studies on the electrochemical performance of Q under nonaqueous conditions are still needed. Herein, a systematical study is conducted on the effect of ion species on Q reaction in nonaqueous solvent acetonitrile through a combination of electrochemical measurements and theoretical analysis. In accordance with the rotating disk electrode analysis, the diffusion coefficients of Q under Na+ and tetrabutylammonium (TBA)(+) conditions reach 5.031 x 10(-6) and 8.563 x 10(-6) cm(2) s(-1), respectively, and the kinetic rate constants are 7.81 x 10(-3) and 4.76 x 10(-3) cm s(-1), respectively. According to in situ UV-vis analysis, Q presents the best electrochemical reversibility under tetrabutylammonium hexafluorophosphate (TBAPF(6)) and a low-reaction potential of -1.98 V versus Ag/Ag+, making it a very promising anolyte active material. By coupling with the 1,4-di-tert-butyl-2,5-bis (2-methoxyethoxy)benzene (DBBMEB) catholyte, the DBBMEB-Q NRFB, which the battery voltage reaches above 2.5 V under both Na+ and TBA(+) conditions, is demonstrated. The methodology adopted in this work provides a design method for the high-voltage and high-energy-density redox flow batteries.