1 Département de Chimie, Center for Green Chemistry and Catalysis, Quebec Center for Advanced Materials, Institut Courtois, Université de Montréal, C.P. 6128, Succursale Centreville, Montréal, Québec, Canada, H3C 3J7.
2 Department of Chemical and Materials Engineering, Concordia University, Montreal, Quebec, Canada, H3G 1M8.

Aqueous organic redox flow batteries (AORFBs) are a safe and sustainable solution for the storage of intermittent renewable energy. While several highly soluble two-electron organic molecule negolytes have been developed for AORFBs, most reported organic posolyte species exchange only one electron. Herein, readily available 4,4'-hydrazobis(1-methylpyridinium) dichloride (HydBPyMeCl) is described as a novel two-electron posolyte molecule for AORFBs. The synthesis of HydBPyMeCl was accomplished by a three-step process, yielding multiple grams of the compound. HydBPyMeCl exhibited a reversible two-electron transfer at high redox potential (+0.64 V vs Ag/AgCl reference electrode, pH = 0). When evaluated at 1 M concentration and low pH (2 M HCl) with V³⁺/V²⁺ on the negative side, HydBPyMeCl showed high stability. A capacity retention of 99.997% per cycle (99.980% per day measured over 70 days) was achieved, coupled with a high volumetric specific capacity of 47.1 Ah/L (87.2% of capacity utilization at 80 mA/cm²).