Concentration Effects on the First Reduction Process of Methyl Viologens and Diquat Redox Flow Battery Electrolytes

Analyte concentration effects on the first reduction process of methyl viologens and diquat redox flow battery electrolytes were examined by cyclic voltammetry in aqueous media. A simple one-electron transfer mechanism to form radical cations was detected for diquat, 4,4′-dimethyl diquat, and bis(3-trimethylammonio)-propyl viologen compounds. The radical cations attach to the electrode surface when the source of their electrogeneration is methyl viologen molecules bearing PF6– ions as a counterpart. However, this inner sphere reduction mechanism was not observed in methyl viologen having an I– counterion. For the latter compound, as well as for 5,5′-dimethyl diquat and 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium, a piece of experimental evidence for unexpected, fast, and reversible dimerization interactions between their electrogenerated radical cations is presented. To get information on these bimolecular interactions, a screening methodology (using different levels of theory) was employed in finding suitable dimeric structures and their related interaction energies. By using diquat as a reference system, a relationship between calculated interaction energies and the corresponding experimental dimerization constants was obtained. The examination of redox-active molecules using this experimental and theoretical approach will allow a better selection of redox flow battery electrolytes.