Nonadditivity of Faradaic Currents and Modification of Double Layer Capacitance in the Voltammetry of Mixtures of Ferrocene and Ferrocenium Salts in Ionic Liquids

Abstract

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.

Voltammetric behaviour of ferrocene in olive oils mixed with a phosphonium-based ionic liquid

“…This feature, under radial or mixed radial-planar diffusion, is indicative of unequal diffusion coefficients of the oxidised and reduced species of the redox system [29,30]. In the present case, it is likely that Fc + , interacting with the anion of the RTIL, is characterised by a lower diffusion coefficient than Fc [8,22,[25][26][27]. In fact, anionic coordination or ion-pair interactions were often invoked to explain the slower diffusion observed for the Fc + species with respect to Fc in many ionic liquids [9,22,24,26,27].…”

“…In the present case, it is likely that Fc + , interacting with the anion of the RTIL, is characterised by a lower diffusion coefficient than Fc [8,22,[25][26][27]. In fact, anionic coordination or ion-pair interactions were often invoked to explain the slower diffusion observed for the Fc + species with respect to Fc in many ionic liquids [9,22,24,26,27]. The slower movement of Fc + away from the electrode surface makes it readily available for reduction to Fc in the reverse scan, and this leads to the increase of I pc .…”

“…This behaviour is conceivably due to the rather low diffusion coefficients of ferrocene (D Fc ) and ferrocenium ion (D Fc+ ) in this medium, which lead to mixed radialplanar diffusion conditions [9,[20][21][22][23]. In fact, in RTILs, diffusion coefficient values for both Fc and Fc + in the range of 5 Â 10 À7 -10 À8 cm 2 s À1 were in general reported [9,[20][21][22][23][24][25][26][27][28].…”

Fabrication and characterization of gold nanohole electrode arrays

“…Under ideal conditions, the relationship I c = AC dl υ exists [25] , where I c is the charging current, A is the electrode area (cm 2 ), and C dl is the capacitance of the double layer per square centimetre. Thus, C dl may be calculated from the slope of a plot of I c vs υ.…”

Electrochemical behaviour of tetra-n-butylammonium nonaflate as an ionic liquid and as a supporting electrolyte in aprotic solvents: A comparative study

“…The electron hopping or 'electron diffusion' mechanism prevails in such systems [20,21]. The Faradaic and non-Faradaic currents for neutral (ferrocene) and cationic (ferrocenium) species also exhibit non-additive behaviour [22]. There are relatively fewer investigations on the heterogeneous electron transfer kinetics [18,[23][24][25] as well as associated chemical reactions of the electrogenerated radical intermediates [10,26].…”