Stability of monoradical cation dimer of viologen derivatives in aqueous redox flow battery

Viologens are ideal anolytes for organic redox flow batteries due to their stable redox behaviour, but they face the challenge of poor solubility of mono-cation radical dimers formed during the oxidation/reduction process. In our study, we prepared four viologen derivatives: sulfonic acid (SV), carboxylic acid (VAV), quaternized ammonium salt (QV), and imidazole (IV). IV exhibited the most stable mono-cation radical dimer, followed by QV, VAV, and SV. The stability was assessed by recording EPR spectra during the battery operation and confirmed by ¹H NMR after the battery operation. Intriguingly, the most stable dimer led to poor battery performance. With the least stable dimer, SV performed best using 0.5 M [Fe(CN)₆]^3−/4− catholyte in a 1 M NH₄Cl solution with a polyethylene/styrene interpolymer cation exchange membrane. The battery achieved an impressive 99% Coulombic efficiency (CE) and 60.3% energy efficiency (EE) at an applied current density of 100 mA cm⁻². Over 200 stable charge and discharge cycles, it reached 11.25 Ah L⁻¹ capacity at 20 mA cm⁻² current density. This study highlights the need to address the stability and solubility of mono-cation radical dimers and emphasizes intelligent molecular engineering to optimize viologen derivatives in organic redox flow batteries.