Reference Potential Calibration and Voltammetry at Macrodisk

A reversible potential of +1285 ± 5 mV versus the [Co(Cp)2]+/0 reference potential scale was obtained, and ..... Analytical Chemistry 2003 75 (24), 6...
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Anal. Chem. 2002, 74, 3151-3156

Reference Potential Calibration and Voltammetry at Macrodisk Electrodes of Metallocene Derivatives in the Ionic Liquid [bmim][PF6] Victoria M. Hultgren,† Andrew W. A. Mariotti,‡ Alan M. Bond,*,† and Anthony G. Wedd*,‡

School of Chemistry, Monash University, P. O. Box 23, Victoria 3800, Australia, and School of Chemistry, The University of Melbourne, Victoria 3010, Australia

Reference potential scales are not generally available in ionic liquids. Consequently, comparison of data with those obtained in conventional solvent (electrolyte) media is not possible. The process [Co(Cp)2]+/0 (Co(Cp)2 ) cobaltocene) has been studied at gold, glassy carbon and platinum macrodisk electrodes to test the feasibility of using this redox couple as a voltammetric reference standard in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]). A reversible, one-electron reduction process was observed, and the measured reversible potential versus a silver quasi-reference electrode was independent of the working electrode material, the concentration, and the scan rate. Ferrocene, the other traditionally used reference compound, is poorly soluble in this ionic liquid. However, the solution-phase voltammetry of ethylferrocene could be readily studied in [bmim][PF6], and a reversible oxidation process was observed. A reversible potential of +1285 ( 5 mV versus the [Co(Cp)2]+/0 reference potential scale was obtained, and this value is comparable with that obtained in CH3CN (0.1 M Bu4NPF6) when referenced to the same potential scale. Ferrocene, decamethylferrocence, 1,1′-dimethylferrocene, 1,1′-diacetylferrocene, and ferrocenecarboxaldehyde were adhered to the working electrode surface and immersed in [bmim][PF6]. In each case, solid-state voltammetry provided well-defined, reversible one-electron oxidation processes that had the appearance of being diffusion controlled, with charge neutralization occurring via the ionic liquid. Reversible potentials of the solid-state processes referenced against the [Co(Cp)2]+/0 scale were similar to solution-phase values obtained in CH3CN (0.1 M Bu4NPF6). The use of room-temperature ionic liquids as media for electrochemical applications is very attractive. Their inherent ionic conductivity abrogates the need for added supporting electrolyte. Other key properties include large electrochemical potential windows, negligible vapor pressure, high chemical and thermal * Corresponding authors. E-mail: [email protected]. Fax: +61 3 9905 9129. E-mail: [email protected]. Fax: +61 3 9347 5180. † Monash University. ‡ The University of Melbourne. 10.1021/ac015729k CCC: $22.00 Published on Web 06/01/2002

© 2002 American Chemical Society

stability, and an ability to dissolve a wide range of organic and inorganic compounds.1-8 While there is a wealth of literature relating to electrochemical studies in the haloaluminate melts (e.g., see ref 9), these media are moisture sensitive, which limits their practical applications. Ionic liquids based on the dialkyl-substituted imidazolium cations are usually air and moisture stable. The range of molten salts of this type10-13 and their applications14-23 is expanding rapidly. Voltammetric studies carried out in these ionic liquids have employed quasi-reference electrodes, in particular Pt wire1,12,24,25 or Ag wire.11,24,26 However, the use of quasi-reference electrodes (1) Suarez, P. A. Z.; Selbach, V. M.; Dullius, J. E. L.; Einloft, S.; Piatnicki, C. M. S.; Azambuja, D. S.; de Souza, R. F.; Dupont, J. Electrochim. Acta 1997, 42, 2533-2535. (2) Suarez, P. A. Z.; Einloft, S.; Dullius, J. E. L.; de Souza, R. F.; Dupont, J. J. Chim. Phys. 1998, 95, 1626-1639. (3) Welton, T. Chem. Rev. 1999, 99, 2071-2083. (4) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133. (5) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2001, 105, 10942-10949. (6) Aki, S. N. V. K.; Brennecke, J. F.; Samanta, A. Chem. Commun. 2001, 413414. (7) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168-1178. (8) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881-3886. (9) Hussey, C. L. In Advances in non-aqueous chemistry; Mamantov, G., A, P., Eds.; VCH Publishing: New York, 1994; Chapter 4, pp 227-275. (10) Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; de Souza, R. F.; Dupont, J. Polyhedron 1996, 15, 1217-1219. (11) MacFarlane, D. R.; Golding, J.; Forsyth, S.; Forsyth, M.; Deacon, G. B. Chem. Commun. 2001, 1430-1431. (12) Brown, R. J. C.; Dyson, P. J.; Ellis, D. J.; Welton, T. Chem. Commun. 2001, 1862-1863. (13) Sun, J.; Forsyth, M.; MacFarlane, D. R. J. Phys. Chem. B 1998, 102, 88588864. (14) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhoˆte, P.; Pettersson, H.; Azam, A.; Gra¨tzel, M. J. Electrochem. Soc. 1996, 143, 3099-3108. (15) McEwen, A. B.; Ngo, H. L.; Le Compte, K.; Goldman, J. L. J. Electrochem. Soc. 1999, 146, 1687-1695. (16) Le Boulaire, V.; Gre´e, R. Chem. Commun. 2000, 2195-2196. (17) Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electroanal. Chem. 1998, 459, 2934. (18) Armstrong, D. W.; He, L.; Liu, Y.-S. Anal. Chem. 1999, 71, 3873-3876. (19) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765-1766. (20) Marcinek, A.; Zielonka, J.; Gebicki, J.; Gordon, C. M.; Dunkin, I. R. J. Phys. Chem. A 2001, 105, 9305-9309. (21) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772-3789. (22) Howarth, J. Tetrahedron Lett. 2000, 41, 6627-6629. (23) Wheeler, C.; West, K. N.; Liotta, C. L.; Eckert, C. A. Chem. Commun. 2001, 887-888.

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means that it is very difficult to compare voltammetric data on a day-to-day basis and also that potentials observed cannot be directly related to those obtained in conventional solvents. The I-/I3- redox couple7 and the Li+/Li couple27 have been reported as reference electrode systems in ionic liquids, but these systems are not commonly used as reference electrodes in conventional media, so again comparisons of potentials with those obtained with such media are not possible. When a conventional reference electrode is not available for an organic solvent, it is recommended that data be referenced to the reversible potential derived from the reduction of cobaltocenium cation, [Co(Cp)2]+ 28,29 (eq 1) or to oxidation of ferrocene, [Fe(Cp)2] 29 (eq 2).

[Co(Cp)2]+ + e- h [Co(Cp)2]

(1)

[Fe(Cp)2] h [Fe(Cp)2]+ + e-

(2)

The assumption is made that the potential of these processes is independent of the solvent. In this paper, we describe the results of detailed voltammetric studies on gold, glassy carbon, and platinum macrodisk electrodes carried out to assess the feasibilty of using dissolved cobaltocenium hexafluorophosphate ([Co(Cp)2]PF6) and eq 1 as a stable reference system in the ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]). Ferrocene is poorly soluble in [bmim][PF6]. Thus, studies on the [Fe(Cp)2]0/+ process have been undertaken when solid is adhered to the electrode surface, with in situ reference of the potential of this process being made to the cobaltocene reference scale. The voltammetry of a range of other soluble and sparingly soluble ferrocene derivatives has also been studied in [bmim][PF6]. Data obtained relative to the cobaltocene reference scale are compared with data obtained in acetonitrile (0.1 M Bu4NPF6), a widely used organic solvent system. EXPERIMENTAL SECTION Reagents. Acetonitrile (Merck, HPLC grade, 99.9%), cobaltocenium hexafluorophosphate ([Co(Cp)2][PF6]; Strem), 1,1′-diacetylferrocene ([Fe(CpCOCH3)2]; 97%, Aldrich), ferrocene ([Fe(Cp)2]; BDH), ferrocenecarboxaldehyde ([CpFeCpCOH]; 98%, Aldrich), 1,1′-dimethylferrocene ([Fe(CpCH3)2]; 97%, Aldrich), decamethylferrocene ([Fe(Cp(CH3)5)2]; 97%, Aldrich), ethylferrocene ([CpFeCpC2H5]); 98%, Strem), 1-methylimidazole (redistilled, 99+%, Aldrich), and sodium hexafluorophosphate (98%, Aldrich) were used as supplied by the manufacturer. Acetone (Merck) was dried twice over activated 3-Å molecular sieves and fractionally distilled under nitrogen. 1-Chlorobutane (>99%, Merck) was purified according to a literature procedure.30 Tetrabu(24) Schro ¨der, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J. New J. Chem. 2000, 24, 10091015. (25) Endres, F.; Schrodt, C. Phys. Chem. Chem. Phys. 2000, 2, 5517-5520. (26) V Dickinson, E.; Williams, M. E.; Hendrickson, S. M.; Masui, H.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 613-616. (27) Koch, V. R.; Dominey, L. A.; Nanjundiah, C.; Ondrechen, M. J. J. Electrochem. Soc. 1996, 143, 798-803. (28) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56-64. (29) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 45, 461-466. (30) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory Chemicals, 4th ed.; Butterworth-Heinemann: Woburn, MA, 1996.

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tylammonium hexafluorophosphate (Bu4NPF6) was purchased from GFS Chemicals and recrystallized twice from EtOH. Synthesis of [bmim][PF6]. Synthesis employed Schlenk techniques under an inert atmosphere and was based on modification of a literature method10 in which [bmim]Cl was generated in situ and converted to the PF6- salt. Considerable care is needed to obtain voltammetrically “pure” ionic liquid in which low background currents are detected within the wide potential range available in this medium. In particular, it is important to ensure the water content is low.24 Indeed, the level of solubility and rate of dissolution of solid ferrocene in this ionic liquid is probably highly sensitive to the residual water content. 1-Methylimidazole (16.6 mL, 0.21 mol) was added dropwise to a refluxing solution of 1-chlorobutane (21.8 mL; 0.21 mol). After reflux (72 h), any excess solvent was removed and distilled water (45 mL) added. The aqueous phase was washed with ethyl acetate (3 × 20 mL) and collected and the solvent removed under reduced pressure at 150 °C overnight. The flask was cooled to ∼80 °C and acetone (250 mL) and NaPF6 (35 g, 0.21 mol) were added with vigorous stirring, after which the resulting slurry was stirred for 36 h at room temperature. The mixture was filtered through Celite, the filter pad washed with acetone, and the solvent removed under reduced pressure. To the remaining liquid was added 50 mL each of distilled water and chloroform. The lower organic layer was collected, and the aqueous and ionic liquid layers were extracted with additional aliquots of chloroform (2 × 50 mL). Ethyl acetate (100 mL) was added to dissolve the ionic liquid, and the aqueous layer was discarded. The organic layer was washed with water (2 × 50 mL), collected, and heated over charcoal (20 min) followed by a hot filtration. The ethyl acetate was then removed under vacuum to leave [bmim][PF6] (53.2 g, 89%), which was dried in vacuo for 72 h. 1H NMR data obtained after dissolution in D2O and IR spectrometry data were consistent with that previously reported.2 The viscosity (η30, 3.12 P), density (F30, 1.37 g cm-3), and specific conductivity (K30, 6 × 10-2 S cm-1) have been reported previously.2 Instrumentation and Procedures. Voltammetric experiments were performed using a Cypress Systems (model CYSY1R) computer controlled electroanalysis system. Experiments carried out in CH3CN (0.1 M Bu4NPF6) used a standard threeelectrode arrangement with a Au (0.0100 cm2) disk as the working electrode, a Pt wire as the counter electrode, and a Ag/Ag+ (CH3CN, 10 mM AgNO3) double junction reference electrode. Voltammetric experiments in [bmim][PF6] employed the same configuration as that described for CH3CN, except that the reference electrode employed a silver wire immersed in [bmim][PF6] that was separated from the bulk solution via a frit, and a glassy carbon (GC) disk electrode (0.0093 cm2) or a Pt disk electrode (0.0097 cm2) was also used as a working electrode. Effective working electrode areas were determined by measurement of the peak current value obtained for reversible one-electron oxidation of a 1 mM solution of ferrocene in CH3CN (0.1 M Bu4NPF6) by cyclic voltammetry and use of the Randles-Sevcik equation

Ip ) 0.4463nF(nF/RT)1/2AD1/2ν1/2C

(3)

where Ip is the peak current (A), n is the number of electrons, A is the electrode area (cm2), D is the diffusion coefficient (taken

Table 1. Cyclic Voltammetric Data for the First Reduction Process, [Co(Cp)2]+/0, in [bmim][PF6] at Au, GC, and Pt Electrodes Au [Co(Cp)2]+ (mM)

a

0 f

GC

ν (mV s-1)

E (mV)a

∆Ep (mV)

1

50 100 200 500 1000

-1145 -1147 -1146 -1148 -1146

2

50 100 200 500 1000

5

10

0 f

Pt 0 f

ox |I red p /I p |

E (mV)a

∆Ep (mV)

E (mV)a

∆Ep (mV)

56 62 65 71 75

1.16 1.10 1.07 0.96 0.94

-1146 -1145 -1148 -1153 -1155

56 66 71 89 110

-1145 -1145 -1149 -1148 -1148

77 74 82 89 95

-1145 -1146 -1146 -1147 -1147

54 60 64 70 77

1.10 0.96 1.00 0.96 0.95

-1147 -1145 -1147 -1145 -1145

62 66 69 77 102

-1146 -1145 -1147 -1149 -1150

64 66 71 82 91

50 100 200 500 1000

-1146 -1145 -1146 -1146 -1147

60 66 72 78 95

1.00 0.96 0.98 0.98 0.99

-1145 -1145 -1145 -1145 -1145

62 66 75 89 108

-1145 -1146 -1145 -1147 -1147

66 72 78 89 106

50 100 200 500 1000

-1146 -1146 -1146 -1146 -1146

68 75 86 102 124

1.05 0.98 0.97 0.98 1.05

-1147 -1146 -1145 -1145 -1144

70 75 89 107 135

-1145 -1146 -1147 -1148 -1149

74 81 91 107 129

mV vs Ag quasi-reference electrode.

to be 2.3 × 10-5 cm2 s-1 31), C is the concentration (mol cm-3), ν is the scan rate (V s-1), and other symbols have their usual meaning. Solid-state voltammetric experiments involved finely grinding the relevant solid and mechanically attaching it to the electrode surface by transfer from solid attached to a cotton bud. [bmim][PF6] was purged with dry N2 for at least 48 h prior to the voltammetric experiments. All potentials are referenced against the cobaltocenium/cobaltocene ([Co(Cp)2]+/0) couple unless otherwise stated. All experiments were carried out at ambient temperature (20 ( 2 °C). 1H NMR spectra were collected on a Varian Unity 300 instrument in D2O, and IR spectra were obtained with a with a Bio-Rad FTS 165 FT-IR spectrometer. Safety Consideration. Pressure increases have been observed after storage in closed glass vessels containing [bmin][PF6] dried by pumping under vacuum. It is recommended that dried samples of this material not be stored in such vessels. The source of the pressure increase is under current study. RESULTS AND DISCUSSION Electrochemical Potential Window. The electrochemical potential windows obtained at GC, Pt, and Au macrodisk electrodes are shown in Figure 1. The potential window, determined from the difference in potentials where the current started to increase (solvent oxidation limit) and decrease (solvent reduction limit) sharply, was found to be similar on Au and GC (4.0 V). These limits are sensitive to residual water levels. The window was smaller on Pt (3.1 V) probably as this surface catalyzes reduction of trace proton concentrations. Since the background current was the lowest with a Au electrode, this was chosen as the preferred electrode surface. (31) Bond, A. M.; Mclennan, E. A.; Stojanovic, R. S.; Thomas, F. G. Anal. Chem. 1987, 59, 2853-2860.

Figure 1. Potential window available in [bmim][PF6] at glassy carbon, platinum, and gold macrodisk working electrodes; ν, 100 mV s-1. Potentials are vs a Ag quasi-reference electrode.

Reference Systems. (1) Quasi-Reference Electrodes. Substantial drifts in potential were observed for Ag and Pt wire quasireference electrodes immersed directly in [bmim][PF6] ionic liquid containing electroactive compounds. However, separation of a Ag wire dipped in [bmim][PF6] from the bulk solution by means of a glass frit prevented this drift in potential. Over the course of several weeks, the potential of the [Co(Cp)2]+/0 voltammetric process versus this quasi-reference electrode was found to be stable to (5 mV. Reproducibility approaching the 1-mV level was found when a range of experiments were conducted on the same day (see data in Table 1 for example). (2) Cobaltocene Reference System. Establishment of a stable quasi-reference electrode system allowed study of the voltammetry of readily soluble [Co(Cp)2][PF6] in [bmim][PF6] and Analytical Chemistry, Vol. 74, No. 13, July 1, 2002

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Figure 2. (a) Cyclic voltammetry of [Co(Cp)2][PF6] (10 mM) in [bmim][PF6] at a Au electrode (d ) 0.11 cm); ν, 100 mV/s-1. (b) Dependence of I red p for process I on the square root of the scan rate. Potentials are vs a Ag quasi-reference electrode.

Table 2. Cyclic Voltammetric Data for the Second Reduction Process, [Co(Cp)2]0/-, in [bmim][PF6] at Au and GC Electrodes Au 0 f

GC 0 f

[Co(Cp)2]+ (mM)

ν (mV s-1)

E (mV)a

∆Ep (mV)

E (mV)a

∆Ep (mV)

1

50 100 200 500 1000

-1949 -1945 -1946 -1947 -1945

66 66 76 77 90

-1952 -1952 -1948 -1947 -1942

84 84 100 109 140

2

50 100 200 500 1000

-1946 -1945 -1947 -1947 -1948

64 66 70 75 86

-1945 -1951 -1951 -1950 -1953

66 78 86 96 118

5

50 100 200 500 1000

-1945 -1945 -1945 -1948 -1948

66 70 76 89 103

-1946 -1946 -1948 -1949 -1951

68 75 84 99 120

10

50 100 200 500 1000

-1945 -1948 -1948 -1950 -1951

74 84 95 109 132

-1947 -1948 -1949 -1951 -1954

78 84 96 118 146

a

mV vs Ag quasi-reference electrode.

hence calibration of the potential against the Ag quasi-reference electrode. Two chemically reversible, one-electron reduction processes were observed at both Au (Figure 2a) and GC electrodes. Process I is assigned to eq 1 above, and the second process II to the reaction in eq 4. The voltammograms for both

[Co(Cp)2] + e- h [Co(Cp)2]-

(4)

processes are very similar to those obtained in organic solvents such as CH3CN.28 Only the first reduction process was observed on Pt due to the more restricted solvent limit available in the negative potential region with this electrode material (Figure 1). Cyclic voltammetric data obtained on Au, GC, and Pt electrodes 3154

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at variable concentrations and scan rates are given in Table 1 for the first reduction process, normally used for reference potential calibration purposes. Cyclic voltammetric data for the second reduction process are given in Table 2. The peak-to-peak potential (∆Ep) values obtained at low concentrations (1 and 2 mM) and slow scan rate (50 and 100 mV s-1) are consistent with the value of ∼55 mV expected for a reversible diffusion-controlled process at 20 °C. At scan rates greater than 100 mV s-1 and higher concentrations, values of ∆Ep increase as expected when a low level of uncompensated resistance is present. No evidence of cobaltocene adsorption or even precipitation was found when high (10 mM) concentrations of [Co(Cp)2]+ were used. At GC and Pt electrodes, larger background currents at lower concentrations (1 and 2 mM) decreased the accuracy of measurement of peak currents relative to what is possible on gold electrodes, but data at higher concentrations (5 and 10 mM) are close to the values expected for a reversible ox red process. The magnitude of the ratio |I red p /I p | (where I p and ox I p are the reductive and oxidative peak currents, respectively) is close to unity as expected for a chemically reversible process (Table 1). + E ox Reversible formal potentials, E 0f (E 0f ) (E red p p )/2 red ox where E p and E p are reduction and oxidation peak potentials, respectively) and their separation for both the [Co(Cp)2]+/0 and [Co(Cp)2]0/- processes (versus the Ag quasi-reference electrode) are essentially independent of the electrode material. An E 0f value of -1146 ( 1 mV was obtained for the first reduction process and -1948 ( 2 mV for the second reduction process, thereby giving a separation (∆E 0f ) of ∼800 mV. I red p for both the first and second processes exhibits a linear dependence with (scan rate)1/2 (Figure 2b) as expected for a diffusion-controlled process. The potential of the second reduction process was previously found to be solvent dependent,28 and therefore only the first reduction process is usually considered when cobaltocene is used as a potential reference system. ∆E 0f in CH3CN (0.1 M Bu4NPF6) is 950 ( 5 mV. The diffusion coefficient for [Co(Cp)2]+ in [bmim][PF6] was calculated to be of the order of 1 × 10-8 cm2 s-1 using the Randles-Sevcik equation (eq 3), which assumes mass transport by diffusion only. This value is comparable with diffusion

Figure 3. Cyclic voltammograms of (a) [Fe(CpC2H5)2] (3.17 mM) and [Co(Cp)2]+ (10 mM) in [bmim][PF6] at a Au electrode; ν, 100 mV s-1; (b) [Fe(CpCH3)2] solid adhered to a Au electrode and [Co(Cp)2]+ (10 mM) in [bmim][PF6]; ν, 100 mV s-1; (c) (1) [Fe(Cp(CH3)5)2], (2) [Fe(Cp)2], (3) [CpFeCpCOH], and (4) [Fe(CpCOCH3)2] solid adhered to a Au electrode; ν, 100 mV s-1.

coefficients calculated for methyl viologen (1.1 × 10-8 cm2 s-1) and N,N,N′,N′-tetramethyl-p-phenylenediamine (2.6 × 10-8 cm2 s-1) in [bmim][PF6]24 and Ni(II)(salen) (1.8 × 10-8 cm2 s-1) in [bmim][BF4].32 The calculated diffusion coefficient is several orders of magnitude smaller than that observed in CH3CN (1.9 × 10-5 cm2 s-1) due to the viscosity of [bmim][PF6] being so much greater (η30 ) 2.33 P vs 0.345 × 10-3 P in CH3CN2,33). Any contribution from migration current was neglected in the calculation of the diffusion coefficient so that some error from that source may be present. Voltammetry of Ferrocene Derivatives. (1) Ethylferrocene. Ferrocene is both sparingly soluble and slow to dissove in [bmim][PF6]. However, the liquid [CpFeCpC2H5] is rapidly miscible with [bmim][PF6] and an almost ideal, reversible, one-electron oxidation process was observed (Figure 3a and Table 3). To test whether the [Co(Cp)2]+/0 could be used to provide a reference scale, [Co(Cp)2]+ was added to provide an in situ potential standard. The measured formal potentials were essentially independent of the electrode surface and scan rate (+1285 ( 5 mV vs [Co(Cp)2]+/0) and are ∼5 mV more positive than the value found in CH3CN (Table 3). The presence of a low level of uncompensated resistance again was indicated by an increase in (32) Sweeny, B. K.; Peters, D. G. Electrochem. Commun. 2001, 3, 712-715. (33) Janz, G. J.; Tomkins, R. P. T. In Nonaqueous Electrolytes Handbook; Academic Press: New York, 1972; Vol. I, p 5.

Table 3. Cyclic Voltammetric Data for [CpFeCpC2H5] (3.17 mM) in [bmim][PF6] at Au, GC, and Pt Working Electrodes ν (mV s-1)

E 0f (mV)a

∆Ep (mV)

red |I ox p /I p |

E 0f (mV) CH3CNb

Au

50 100 200 500 1000

+1283 +1284 +1283 +1283 +1284

64 71 78 91 108

1.00 1.00 1.00 0.98 0.98

+1275

GC

50 100 200 500 1000

+1281 +1282 +1283 +1284 +1285

68 74 85 101 123

1.01 1.05 1.05 1.08 1.07

+1279

Pt

50 100 200 500 1000

+1288 +1288 +1289 +1288 +1290

72 80 90 104 126

1.06 1.05 1.06 1.08 1.10

+1276

a mV vs [Co(Cp) ]+/0 reference potential scale. b Solutions contain 2 0.1 M Bu4NPF6 as supporting electrolyte.

the ∆Ep values as the scan rate increased. I ox p showed a linear dependence on the square root of scan rate, and the magnitude of the ratio of the oxidative and reductive peak currents gives a Analytical Chemistry, Vol. 74, No. 13, July 1, 2002

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Table 4. Cyclic Voltammetric Data for Ferrocene and Ferrocene Derivatives Adhered to a Au Electrode (ν, 100 mV s-1)a

[Fe(Cp)2] [Fe(CpCH3)2] [Fe(CpCOCH3)2] [CpFeCpCOH] [Fe(Cp(CH3)5)2]

E 0f (mV)a

∆Ep (mV)

red |I ox p /I p |

E 0f (mV)a CH3CNb

+1332 +1233 +1813 +1627 +856

70 134 77 83 60

1.04 0.99 1.19 1.06 0.88

+1335 +1219 +1800 +1618 +825

a mV vs [Co(Cp) ]+/0 reference potential scale. b Solutions contain 2 0.1 M Bu4NPF6 as supporting electrolyte.

value of unity. In principle, the oxidation of ethylferrocene could be used to provide a reference potential in [bmim][PF6], although data on the dependence of the reversible potential on solvent are not available for this compound. (2) Ferrocene Adhered to a Gold Electrode Surface. The solubility properties of ferrocene in [bmim][PF6] allowed a ready assessment of its voltammetry in the solid state. Figure 3c shows the cyclic voltammogram obtained for ferrocene adhered to a Au electrode which was subsequently immersed in [bmim][PF6]. Relevant voltammetric data are provided in Table 4. One well-defined, chemically reversible oxidation process was observed corresponding to the oxidation of ferrocene. A peak-topeak potential difference (∆Ep) of 70 mV was obtained at a scan rate of 100 mV s-1. The ∆E 0f value obtained of +1332 mV versus [Co(Cp)2]+/0 differs by only 3 mV from that obtained for the solution-phase voltammetry of ferrocene in CH3CN relative to the same potential scale. The theory for the voltammetry of microcrystals has been addressed in a recent publication.34 The observation of a linear dependence of I ox p (for a constant surface coverage of the solid on the electrode surface) versus the square root of scan rate implies that mass transport is under diffusion control with charge neutralization occurring within the solid rather than within the ionic liquid (eq 5).

[Fe(Cp)2](solid) + [bmim][PF6](solvent) h [Fe(Cp)2][PF6](solid) + e- + [bmim]+(solvent) (5) (3) Ferrocene Derivatives Adhered to Electrode Surfaces. The cyclic voltammetry of [Fe(CpCOCH3)2], [Fe(CpCH3)2], (34) Schro ¨der, U.; Oldham, K. B.; Myland, J. C.; Mahon, P. J.; Scholz, F. J. Solid State Electrochem. 2000, 4, 314-324.

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Analytical Chemistry, Vol. 74, No. 13, July 1, 2002

[CpFeCpCOH], and [Fe(Cp(CH3)5)2] were studied in a manner analogous to ferrocene. Cyclic voltammograms are shown in Figure 3 with the voltammetric data obtained being reported in Table 4. For each of the derivatives, a chemically reversible, oneelectron oxidation process was observed, with the reversible potential being dependent on the substituent on the cyclopentadienyl ring. In most cases, the ∆E 0f values versus the [Co(Cp)2]+/0 reference scale obtained from the solid in [bmim][PF6] were similar to those obtained in the solution phase in CH3CN (see Table 4). [Fe(Cp(CH3)5)2] was found to have the largest deviation, with an ∆E 0f value 31 mV more positive than that found in CH3CN. However, the ∆Ep values for the solid-state processes varied over the range 60 mV for [Fe(Cp(CH3)5)2] to 134 mV for [Fe(CpCH3)2] with a scan rate of 100 mV s-1 while the ratio |I red p / I ox | was close to unity. Full mechanistic details for the oxidation p process of microcrystals of ferrocene and derivatives are unknown, but available evidence suggests that the reversible potentials are very similar to those expected if studies had been undertaken on dissolved material. CONCLUSIONS The [Co(Cp)2]+/0 redox couple may be used as a voltammetric reference potential standard in the ionic liquid [bmim][PF6] as data obtained are comparable with that obtained in an organic solvent such as CH3CN. Ferrocene, the other widely used reference standard in voltammetry, is not readily soluble. However, the solution-phase voltammetry of [CpFeCpC2H5] in [bmim][PF6] exhibits an almost ideal reversible one-electron oxidative process and may be used as an alternative standard. The diffusion coefficient of [Co(Cp)2]+ is several orders of magnitude smaller than that in CH3CN, which is attributed to the more viscous nature of the ionic liquid. The voltammetry of ferrocene and a number of derivatives was studied as solids adhered to the electrode surface. Diffusion-controlled and chemically reversible processes were observed in all cases. Importantly, the reversible potentials, relative to the [Co(Cp)2]+/0 couple, are very similar to those obtained with conventional solution-phase voltammetry in CH3CN. ACKNOWLEDGMENT A.M.B. and A.G.W. thank the ARC (project A0013315) for financial support. Received for review December 10, 2001. Accepted April 12, 2002. AC015729K