Evaluating ferrocene ions and all-ferrocene salts for electrochemical applications

The success of applications such as redox flow battery (RFB) technologies is highly dependent on the development of new molecular strategies, and much work has focused on the chemistry of ferrocene compounds as RFB components. This report presents four “all-ferrocene” salts comprised of ferrocene anions and cations and evaluates their solubility and electrochemical properties. The four salts are formed via metathesis reactions employing the (ferrocenemethyl)trimethylammonium cation and mono and bis sulfonate or carboxylate ferrocene anions. We observed that the potentials of the ferrocene components was not only dependent on the substituent, but also on the solvent medium. In particular, the sulfonate and carboxylate ferrocene anions exhibited significant shifts in redox potential between water, propylene carbonate (PC), and DMF. Most notably, in (ferrocenemethyl)trimethylammonium 1,1′-bis(sulfonato)ferrocene, the potentials of the cation and anion reverse, with the anion having a lower potential in aqueous solution and a more positive potential in organic media.