Probing the Process of Electron Transfer in Microemulsions with Substituted Ferrocenes

Recently, we reported studies in which microemulsion-based electrolytes were used in redox flow batteries. That work built on studies of the electrochemistry of ferrocene in toluene/water/TWEEN® 20 microemulsions which showed the electron transfer to be remarkably facile, with reversible or quasi-reversible kinetics observed in cyclic voltammetry experiments with scan rates up ~20 V/sec. To rationalize these high rates, we speculated that the oxidation of ferrocene to ferricenium ion was accompanied by rather subtle changes in the ‘space charge’ region associated with ions in the aqueous sub-phase/hydrophilic interphase. (TWEEN® is a. non-ionic surfactant with multiple ethylene oxide oligomers as its head group.). However, it is also possible that the ferricenium ion is ejected into the aqueous phase. The latter process could lead to instability during cycling of the battery. To probe this question, we are exploring the electrochemistry and other associated dynamic processes in toluene/water/TWEEN® 20 microemulsions that contain various substituted ferrocene derivatives as electroactive components. Compounds were chosen using estimates of dipole moments from quantum mechanical calculations to provide an initial screen of polarity. Modifications ranging from mono- and di-carboxylic acid derivatives and a series of esters synthesized from the acids, to tertiary amines and to decamethyl ferrocene were probed. An overview of the outcomes of these studies will be presented, together with a discussion of implications for microemulsion ‘design’ to co-optimize electron transfer rate and cycling stability of the system when used batteries.