A tetracationic viologen based on dimethylaminoethanol is reported as a novel negolyte for aqueous-organic flow batteries. The compound (1,1'-bis(3-((2-hydroxyethyl)dimethylammonio)propyl)-[4,4'-bipyridine]-1,1'-diium tetrachloride), termed [(DMAE-Pr) 2 -Vi]Cl 4 , was prepared by replacing the trimethylammonium groups of a former analogue for dimethylaminoethanol derivatives as permanently charged moieties, improving its solubility and modifying its electronic structure, as revealed by DFT calculations. Its electrochemical properties and diffusion coefficient were investigated by voltammetry and DOSY. The battery concept was demonstrated by pairing [(DMAE-Pr) 2 -Vi]Cl 4 with a TEMPOL posolyte using a low-cost anion exchange membrane and a neutral-pH supporting electrolyte. [(DMAE-Pr) 2 -Vi]Cl 4 has great potential for enhancing energy density in viologen flow batteries.

Herein, a new semi‐organic aqueous flow battery based on a hydroxylated tetracationic viologen, 1,1′‐bis(3‐((2‐hydroxyethyl)dimethylammonio)propyl)‐[4,4′‐bipyridine]‐1,1′‐diium tetrachloride ([(DMAE‐Pr)2‐Vi]Cl4), and the ferrocene derivative, (ferrocenylmethyl)trimethylammonium chloride (FcNCl), is demonstrated. Efficiency, accessible capacity, and capacity retention of the battery are investigated at two concentrations of the active redox species: 0.1 and 0.5 mol dm−3 in 1 mol dm−3 KCl near‐neutral electrolytes. The implementation of the ferrocene‐derivative posolyte decreases the capacity fade rate by a factor of ≈4 with respect to previous work using 4‐hydroxy‐2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPOL) as posolyte. Capacity loss is driven by crossover of the positive redox couple into the negolyte, in particular at high concentrations, indicating the need for more selective membranes and less permeable ferrocene derivatives. Discussions are supported by conductivity measurements, cell resistance, and postmortem analysis of the electrolytes using cyclic voltammetry and nuclear magnetic resonance (NMR) spectroscopy. The characterization of [(DMAE‐Pr)2‐Vi]Cl4 is expanded as a high‐energy negolyte and future scaleup requirements for aqueous organic flow batteries are informed.

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