Highly Soluble TEMPO-Viologen Bipolar Molecule for Ultra-Stable Aqueous Redox Flow Batteries

Bipolar redox-active organic molecule (BROM) is a feasible strategy to address the cross-contamination issue of the electrolyte and, thus, improve the stability of the flow battery. Herein, a highly-soluble BROM is developed by combining the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and viologen moieties, and extensive characterizations are performed to evaluate its applicability in flow battery. Salient findings are as follows. First, the compound, viz. 1-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1 '-(3-(trimethylammonio)propyl)-4,4 '-bipyridinium trichloride ((TPABPy)Cl3), features highly hydrophilic groups and yields a high aqueous solubility of 1.76 m. Second, the electrochemical result reveals that the (TPABPy)Cl3 displays two pairs of highly reversible peaks at -0.56 and 0.76 V, which respectively correspond to the viologen and TEMPO moieties. The electronic structure during the redox reactions is identified by both the density functional theory calculation and the electron paramagnetic resonance. Third, the flow battery fed with the 1.0 m (TPABPy)Cl3 solution delivers a high capacity of 25 Ah L-1 and a superior stability over the non-bipolar counterparts. More to the point, the capacity decay can be effectively recovered by applying the polarity-inversion rebalance strategy on the BROM. In summary, this work provides a molecular engineering way to rationally design a BROM to improve the capacity and stability of aqueous organic redox flow batteries. A highly soluble bipolar redox-active organic molecule, viz. 1-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1'-(3-(trimethylammonio)propyl)-4,4'-bipyridinium trichloride ((TPABPy)Cl3, is designed by covalently coupling the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and viologen moieties. This strategy effectively alleviates the permanent capacity decay caused by the cross-contamination issue. As demonstration, the flow battery yields a decent capacity of 25 Ah L-1 and superior stability during a 100-cycle test.image