The Dual Role of Bridging Phenylene in an Extended Bipyridine System for High-Voltage and Stable Two-Electron Storage in Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) are regarded as a promising solution for grid-scale and sustainable energy storage, but some long-standing problems such as low energy density and cycling stability should be resolved. Herein, a highly soluble bipyridine modified with a bridging phenylene group and two quaternary ammonium terminals, namely, [(NPr)2PV]·4Cl, was synthesized and used as an ultralow-potential and two-electron storage anolyte for AORFBs. The phenylene group, which is linked but not coplanar with the two pyridinium redox centers, can thus prevent their communication and result in an exceptionally low redox potential (−0.77 V vs standard hydrogen electrode, 2e–). Moreover, the introduction of a phenylene group can warrant a certain degree of large π-conjugation effects and mitigate the intramolecular Coulombic repulsion between the two positively charged pyridinium centers, thus helping to enhance the electrochemical stability. When paired with 4-trimethylammonium-TEMPO as the catholyte, [(NPr)2PV]·4Cl enabled an exceptionally high cell voltage up to 1.71 V. The AORFB delivers outstanding battery performances, specifically, ∼89% energy efficiency, ∼100% Coulombic efficiency, and ∼99.94% capacity retention per cycle during a long-term cycling process. The two overlapped single-electron reductions of [(NPr)2PV]·4Cl from the initial cationic form to the monoradical form and then to the quinoid form during the charging process were clearly verified by a series of spectroscopic techniques, including no-deuterium nuclear magnetic resonance and electron paramagnetic resonance. This work presents a significant improvement for the construction of high-voltage AORFBs by virtue of the designability, diversity, and tunability of multiredox organic molecules.