Nonadditivity of faradaic currents and modification of double layer capacitance in the voltammetry of mixtures of ferrocene and ferrocenium salts in ionic liquids.

Electrochemical studies on the Fc + e(-) <==> Fc(+) (Fc = ferrocene) process have been undertaken via the oxidation of Fc and reduction of Fc(+) as the hexafluorophosphate (PF(6)(-)) or tetrafluoroborate (BF(4)(-)) salts and their mixtures in three ionic liquids (ILs) (1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium hexafluorophosphate). Data obtained at macro- and microdisk electrodes using conventional dc and Fourier-transformed large-amplitude ac (FT-ac) voltammetry reveal that diffusion coefficients for Fc and Fc(+) differ significantly and are a function of the Fc and Fc(+) concentration, in contrast to findings in molecular solvents with 0.1 M added supporting electrolyte media. Thus, the faradaic currents associated with the oxidation of Fc (Fc(0/+)) and reduction of FcPF(6) or FcBF(4) (Fc(+/0)) when both Fc and Fc(+) are simultaneously present in the ILs differ from values obtained when individual Fc and Fc(+) solutions are used. The voltammetry for both the Fc(0/+) and Fc(+/0) processes exhibited near-Nernstian behavior at a glassy carbon macrodisk electrode and a platinum microdisk electrode, when each process was studied individually in the ILs. As expected, the reversible formal potentials (E(o)') and diffusion coefficients (D) at 23 +/- 1 degrees C were independent of the electrode material and concentration. However, when Fc and FcPF(6) or FcBF(4) were both present, alterations to the mass transport process occurred and apparent D values calculated for Fc and Fc(+) were found to be about 25-39% and 32-42% larger, respectively, than those determined from individual solutions. The apparent value of the double layer capacitance determined by FT-ac voltammetry from individual and mixed Fc and Fc(+) conditions at the GC electrode was also a function of concentration. Double layer capacitance values increased significantly with the concentration of Fc and FcPF(6) or FcBF(4) when species were studied individually or simultaneously, but had a larger magnitude under conditions where both species were present. Variation in the structure of the ILs and hence mobilities of the ionic species, when Fc and FcPF(6) or FcBF(4) are simultaneously present, is considered to be the origin of the nonadditivity of the faradaic currents and variation in capacitance.