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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(3trimethylammonio)-propyl viologen compounds. The radical cations attach to the electrode surface when the source of their electrogeneration is methyl viologen molecules bearing PF 6 − 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.

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