4,4’-Hydrazobis(1-Methylpyridinium) as a 2-Electron Posolyte Molecule for Aqueous Organic Redox Flow Batter-ies

Aqueous organic redox flow batteries (AORFBs) are a safe and sustainable solution for the storage of intermittent renewable energy. While several highly soluble 2-electron organic molecule negolytes have been developed for AORFBs, most reported organic posolyte species exchange only 1 electron. Herein, readily available 4,4'-hydrazobis(1-methylpyridinium) dichloride (HydBPyMeCl) is described as a novel 2-electron posolyte molecule for AORFBs. The synthesis of HydBPyMeCl was accomplished by a three-step process, yielding multiple grams of the com-pound. HydBPyMeCl exhibited a reversible 2-electron 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 (99.986% per day measured over 70 days) was achieved, coupled with a high volumetric specific capacity of 46 Ah/L (87.2% of capacity utilization at 80 mA/cm2)