Steric hindrance shielding viologen against alkali attack in realizing ultrastable aqueous flow batteries

Viologens known as a kind of promising negolyte materials for aqueous organic redox flow batteries, face a critical stability challenge due to the SN2 nucleophilic attack by hydroxide ions (OH-) during the battery cycling. In this work, a N-cyclic quaternary ammonium-grafted viologen molecule, viz. 1,1'-bis(4,4'-dimethylpiperidiniumyl)-4,4'-bipyridinium tetrachloride ((DBPPy)Cl4), is developed by the molecular engineering strategy. The obtained (DBPPy)Cl4 molecule shows a decent solubility of 1.84 M and a redox potential of −0.52 V vs. Ag/AgCl. Experimental and theoretical results reveal that the grafted N-cyclic quaternary ammonium groups act as the steric hindrance to prevent nucleophilic attack by OH-, increasing the alkali resistance of the electroactive molecule. The symmetrical battery with 0.50 M (DBPPy)Cl4 shows negligible decay during the 13-day cycling test. As demonstration, the flow battery utilizing 1.0 M (DBPPy)Cl4 as the negolyte and 1-(1-oxyl-2,2',6,6'-tetramethylpiperidin-4-yl)-1'-(3-(trimethylammonio)propyl)-4,4'-bipyridinium trichloride as the posolyte exhibits a high capacity retention rate of 99.99% per cycle at 60 mA cm-2.