Nonadditivity of Faradaic Currents and Modification of Double Layer Capacitance in the Voltammetry of Mixtures of Ferrocene and Ferrocenium Salts in Ionic Liquids Muhammad J. A. Shiddiky, Angel. A. J. Torriero, Juan M. Reyna-Gonza ´ lez, and Alan M. Bond* School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Electrochemical studies on the Fc + e - / Fc + (Fc ) ferrocene) process have been undertaken via the oxidation of Fc and reduction of Fc + as the hexafluo- rophosphate (PF 6 - ) or tetrafluoroborate (BF 4 - ) salts and their mixtures in three ionic liquids (ILs) (1-butyl- 1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]im- ide, 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 cur- rents 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°′) and diffusion coefficients (D) at 23 ( 1 °C were independent of the electrode material and concentra- tion. 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 indi- vidual 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 nonad- ditivity of the Faradaic currents and variation in capacitance. Ionic liquids (ILs) may provide an attractive alternative to molecular solvents for voltammetric studies owing to advantageous properties such as wide potential windows, an ability to dissolve a wide range of organic and inorganic compounds, low vapor pressure, and good intrinsic conductivity. 1-5 Consequently, the use of ILs in voltammetry is now widespread. 4-9 In most electrochemical studies in ILs, principles developed for molecular solvents containing added supporting electrolyte have been applied directly to the IL scenario, where the IL acts as both the solvent and electrolyte. However, nuances are emerging. For example, diffusion coefficients (D) in oxidized (D ox ) and reduced (D red ) forms of the redox couple Ox + e - / Red, which are often assumed to be equal in molecular solvent * Corresponding author. E-mail: Alan.Bond@sci.monash.edu.au. Fax: +61 3 99054597. Phone: +61 3 99051338. (1) Welton, T. Chem. Rev. 1999, 99, 2071–2083. (2) Roger, R. D.; Seddon, K. R. Science 2003, 302, 792–793. (3) Quinn, B. M.; Ding, Z.; Moulton, R.; Bard, A. J. Langmuir 2002, 18, 1734– 1742. (4) Zhang, J.; Bond, A. M. Analyst 2005, 130, 1132–1147. (5) (a) Lagunas, M. C.; Pitner, W. R.; van den Berg, J.-A.; Seddon, K. R. In Ionic Liquids as Green Solvents: Progress and Prospects; Rogers, R. D., Seddon, K. R., Eds.; American Chemical Society: Washington, DC, 2003. (b) Karpinski, Z. J.; Nanjundiah, C.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 3358–3364. (c) MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Acc. Chem. Res. 2007, 40, 1165–1173. (d) Boxall, D. L.; O’Dea, J. J.; Osteryoung, R. A. J. Electrochem. Soc. 2002, 149, E468-E471. (e) Kosmulski, M.; Osteryoung, R. A.; Ciszkowska, M. J. Electrochem. Soc. 2000, 147, 1454–1458. (f) Zhang, J.; Bond, A. M.; Schumann, H.; Suehring, K. Organometallics 2005, 24, 2188–2196. (g) Fietkau, N.; Clegg, A. D.; Evans, R. G.; Villagran, C.; Hardacre, C.; Compton, R. G. ChemPhysChem 2006, 7, 1041–1045. (h) Tachikawa, N.; Katayama, Y.; Miura, T. J. Electrochem. Soc. 2007, 154, F211–F216. (i) O. Fontaine, O.; Lagrost, C.; Ghilane, J.; Martin, M.; Tripp, e ´ , G.; Fave, C.; Lacroix, J.-C.; Hapiot, P.; Randriamahazaka, H. M. J. Electroanal. Chem. 2009, 632, 88–96. (j) Vorotyntsev, M. A.; Zinovyeva, V. A.; Konev, D. V.; Picquet, M.; Gaillon, L.; Rizzi, C. J. Phys. Chem. B 2009, 113, 1085–1099. (6) Torriero, A. A. J.; Bond, A. M. In Electroanalytical Chemistry Research Trend; Hayashi, K., Ed.; Nova Science Publisher Inc.: New York, 2009; Chapter 1. (7) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106– 1120. (8) Torriero, A. A. J.; Siriwardana, A. I.; Bond, A. M.; Burgar, I. M.; Dunlop, N. F.; Deacon, G. B.; MacFarlane, D. R. J. Phys. Chem. B 2009, 113, 11222– 11231. (9) Zhao, C.; Burrell, G.; Torriero, A. A. J.; Separovic, F.; Dunlop, N. F.; MacFarlane, D. R.; Bond, A. M. J. Phys. Chem. B 2008, 112, 6923–6936. Anal. Chem. 2010, 82, 1680–1691 10.1021/ac9020159  2010 American Chemical Society 1680 Analytical Chemistry, Vol. 82, No. 5, March 1, 2010 Published on Web 01/28/2010