Methyl Viologen Anolyte Introducing Nitrate as Counter-Anion for an Aqueous Redox Flow Battery

Among the redox active molecules, methyl viologen (MV) is the most promising redox species in an aqueous organic redox flow battery, featuring the stable redox reaction MV2+/MV•+. Its electrochemical properties and solubility significantly depend on the counter-anion. Herein, we introduce the nitrate as a counter-anion to methyl viologen dication to obtain a new water-soluble methyl viologen compound (MVdN) and identify the effects of nitrate as counter-anion on electrochemical properties of methyl viologen. Nitrate leads to ∼ 3.5 M solubility in water and good electrochemical reversibility (ip,a/ip,c = 0.79, ΔE = 54 mV) with fast mass transfer (D = 5.13 × 10-5 cm2•s-1) of methyl viologen, which is superior to commonly used methyl viologen dichloride(MVdCl). Additionally, flow cell performance from 40 to 100 mA cm-2 exhibited 99% coulombic efficiency and comparable performance with MVdCl. Nitrate could be a promising choice as a counter anion to methyl viologen as aqueous redox active molecule.