The extreme migrational enhancement of faradaic current at microelectrodes: experimental studies on sodium (6,8-diferrocenylmethylthio)octanoate electrooxidation

The unusual electrochemical behavior of sodium (6,8-diferrocenylmethylthio)octanoate in aqueous solutions is reported. This diferrocene derivative forms an anion, gives a two-electron oxidation voltammetric wave and, correspondingly, changes its charge from −1 to +1. The electrode reaction belongs to the class of sign-reversal processes for which the theory predicts a very high ratio of the faradaic currents obtained in the absence and the presence of excess supporting electrolyte. Indeed, the ratios obtained for several microelectrodes reach record levels, i.e., 13.2 ± 2.5, 9.7 ± 1.6, 12.2 ± 0.8, 12.4 ± 0.5 for 5.0, 10.9 and 12.5 μm radius Pt microdisk electrodes and an 8.8 μm radius glassy-carbon microelectrode, respectively. The experimental data have been compared to the theoretical predictions based on the existing analytical expressions derived for the sign-reversal electrode processes for equal substrate and product diffusivities, any level of ionic support, and with or without the comproportionation reaction. The experimental currents are much higher than the theoretical ones. An extended theory that takes into account different diffusivities of the redox species may help. The project on such a theory is underway in our group.