- Email: [email protected]
Electrochemical studies of four organometallic redox couples as possible reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate
Electrochimica Acta 54 (2009) 1414–1419
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical studies of four organometallic redox couples as possible reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate Lukasz Waligora a , Andrzej Lewandowski a,∗ , Gerhard Gritzner b a b
Faculty of Chemical Technology, Poznan University of Technology, Plac Marii Sklodowskiej-Curie, PL 60 965 Poznan, Poland Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University, A 4040 Linz, Austria
a r t i c l e
i n f o
Article history: Received 28 January 2008 Received in revised form 5 September 2008 Accepted 6 September 2008 Available online 20 September 2008 Keywords: Ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate Reference electrodes Ferrocene Cobaltocene Decamethylferrocene Bis(biphenyl)chromium
a b s t r a c t The electrochemical behavior of four organometallic redox couples has been studied in the room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate. Diffusion controlled anodic and cathodic peaks were found for the redox couples Fc/Fc+ (ferrocene/ferrocenium), Cc/Cc+ (cobaltocene/cobaltocenium) on glassy carbon, platinum and gold. Bis(biphenyl)chromium(I) tetraphenylborate (BCr/BCr+ ) yielded a well-behaved diffusion controlled pair of cathodic–anodic peaks only on glassy carbon and gold as working electrodes. The electrode reaction of decamethylferrocene was affected by adsorption of the reduced form on all of the three working electrodes employed by us. The applicability of three of the four redox couples studied as candidates for reference redox systems in this ionic liquid is discussed. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Establishing solvent independent reference electrodes for the comparison of redox potentials in aqueous and non-aqueous systems has a long history. It began with the concept of Rb|Rb+ or Rb(Hg)|Rb+ [1] as a pilot ion, followed by the proposal to use organometallic redox couples [2]. A summary of these efforts was published recently [3]. In order to limit the number of possible reference redox systems based on organometallic redox couples IUPAC recommended that the systems ferrocene/ferrocenium ion (Fc/Fc+ ) and bis(biphenyl)chromium(0)/(I) (BCr/BCr+ ) should be used as reference redox systems in non-aqueous media [4]. During the last decade a considerable number of papers dealing with room temperature ionic liquids [5–7] were published, including their possible application as solvent-free electrolytes [8]. The problem of reference electrodes and the lack of reference potential scales in ionic liquids have been pointed out in several papers [9–12]. Such a reference redox couple, however, is necessary to allow comparison of potential data measured by different research groups in the same ionic liquid.
∗ Corresponding author. Tel.: +48 61 6652309; fax: +48 61 6652571. E-mail address: [email protected] (A. Lewandowski). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.09.023
Currently such measurements in ionic liquids are carried out versus Ag|Ag+ ion electrodes [13,14], but also versus quasi-reference electrodes such as platinum or silver wires. Quasi-reference electrodes do not posses a reliable, thermodynamic potential and are sensitive to impurities in the ionic liquid. Thus potential data reported by different research groups remain isolated data and prevent comparison. The potential of an electrode of the first kind such as the Ag|Ag+ ion electrode in different media suffers from the drawback that only one form of the redox couple, namely the Ag+ ion is affected by the solvent or the ionic liquid. Organometallic redox couples, where both the oxidized and the reduced form are soluble, have been proposed in non-aqueous solvents and solvent mixtures. This suggestion implies the expectation that the solvent effects on the oxidized form will be very similar to the solvent effects on the reduced form, thus making the redox potential of such a couple independent of the nature of the nonaqueous system. Currently, such an assumption for ionic liquids lacks supporting experimental data. So far, the electrochemical behavior of the redox couples cobaltocene/cobaltocenium ion (Cc/Cc+ ) and ethylferrocene/ethylferrocenium ion on gold, glassy carbon and platinum disc electrodes in 1-n-butyl-3methylimidazolium hexafluorophosphate and the electrochemical behavior of several substituted ferrocenes, which adhered to the working electrode surface, have been studied [10]. Electrode
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419
kinetics of ferrocene or ferrocene derivatives in ionic liquids has been published [15,16]. The cobaltocene/cobaltocenium couple was studied and suggested as possible reference redox couples in 1-n-butyl-3-methylimidazolium hexafluorophosphate, in 1-ethyl3-methylimidazolium bis(trifluoromethanesulphonyl)imide as well as in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulphonyl)imide [11,12]. Current literature indicates that the ferrocene/ferrocenium ion redox couple as possible reference system for ionic liquids suffers from poor solubility in several ionic liquids, prompting the research group of Prof. Bond to emphasize cobaltocene/cobaltocenium ion as a possible internal potential standard for electrode potential measurements in ionic liquids [12]. The bis(biphenyl)chromium/bis(biphenyl)chromium ion reference redox system recommended by IUPAC besides ferrocene as one of the two reference redox systems for non-aqueous solvents has not been investigated in ionic liquids so far. It must be kept in mind that a redox couple must fulfill the requirements of a reference redox system in many ionic liquids before it may be accepted as a reference redox system. Hopefully, a redox couple, which has been proposed as reference redox system in organic solvents could also be employed in ionic liquids. Additional research is thus needed including organometallic redox couples, for which data in non-aqueous solvents and solvent mixtures are available [17]. In this paper we report the cyclovoltametric behavior of ferrocene, cobaltocenium hexafluorophosphate (cobaltocene), decamethylferrocene and bis(biphenyl)chromium tetraphenylborate in 1-ethyl-3-methylimidazolium tetrafluoroborate, a liquid salt at room temperature, on glassy carbon, gold and platinum electrodes, with respect to possible applications of these organometallic compounds as reference redox systems in ionic liquids. 2. Experimental 2.1. Chemicals Cryptand 222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8.8.8]hexacosane, Merck) and silver perchlorate (AgClO4 , Fluka) were used as purchased. Bis(cyclopentadienyl)iron(II) (ferrocene, Fc, Aldrich), bis(cyclopentadienyl)cobalt(III) hexafluorophosphate (cobaltocenium hexafluorophosphate, Cc+ PF6 − , Aldrich) and bis(pentamethylcyclopentadienyl)iron(II) (decamethylferrocene, DFc, Strem Chemicals) are commercially available and were used without further purification. Bis(biphenyl)chromium(I) tetraphenylborate (BCr+ TPB− ) was prepared in a modified way [18] according to a published procedure [19]. Bis(biphenyl)chromium(I) tetraphenylborate was re-crystallized from absolute ethanol prior to use. Cobaltocenium hexafluorophosphate as well as its solutions in the experimental cell were protected from light with an aluminum foil. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EtMeImBF4 ) was used as purchased (Merck). The water content in the ionic liquid, analyzed with a standard Karl-Fisher titrant (Aldrich, Cat. no: 22,120-1) was below the detection limit (<4.5 mg H2 O/ml).
1415
each measurement. A 0.5 cm × 1.0 cm platinum sheet was used as a counter-electrode. The reference electrode was a cryptate electrode consisting of a silver wire immersed into a solution containing 0.01 mol dm−3 AgClO4 and 0.1 mol dm−3 cryptand 222 in acetonitrile (AN) [20]. The reference electrode compartment was separated by a glass frit from the cell containing the ionic liquid EtMeImBF4 . Weighing of the samples, preparation of the solutions and cell assembling were done in a glove-box under a dry argon inert atmosphere. Cyclic voltammetry (CV) curves were obtained with a Lab Electrochemical System (Eco Chemie, The Netherlands) at 25 ± 0.1 ◦ C. All solutions were purged with argon for 30 min prior to measurement. For solutions containing the oxidized form of the metallocenes (BCr+ and Cc+ ) the initial scan was performed to more negative values; for solutions containing the reduced form (Fc and DFc) the initial scan was carried out to more positive potentials than the starting potential. In all experiments first the base line of the ionic liquid was established. Then a known volume of a more concentrated solution of the organometallic compound was added to the measured volume of the ionic liquid and the cyclovoltammograms of the organometallic compounds were recorded. The cyclovoltammograms were corrected for the base line current. The ohmic resistance between the working electrodes and the auxiliary electrode deduced from impedance measurements was around 250 , the current typically at the level of ca. 1–10 A. This led to the IR distortion in the order of 0.25–2.5 mV. Hence, the cyclovoltammetry curves were not corrected for the ohmic drop. The low value of ohmic drop in EMImBF4 is due to its relatively high conductance of ca. 14 mS cm−1 at 25 ◦ C. 3. Results Examples of the cyclovoltammetric behavior of ferrocene (Fig. 1), cobaltocenium hexafluorophosphate (Fig. 2) and bis(biphenyl)chromium tetraphenylborate (Fig. 3) are included. The electrochemical data of ferrocene is summarized in Table 1, of cobaltocenenium hexafluorophosphate in Table 2 and of bis(biphenyl)chromium tetraphenylborate in Table 3. The CV curves of ferrocene, cobaltocenenium hexafluorophosphate and bis(biphenyl)chromium(I) tetraphenylborate on all three working electrodes exhibited the typical shape of diffusion controlled redox reactions. The difference between the corresponding anodic and cathodic peaks observed by us for Fc /Fc+ , Cc /Cc+ and BCr /BCr+ varied between 70 and 90 mV. Values of Ep – Ep/2 were obtained from the first scan of the compounds. Ferrocene,
2.2. Apparatus and procedures Measurements were carried out in a three electrode arrangement. Gold (1.50 mm diameter) or platinum (1.50 mm diameter) discs in glass or glassy carbon (GC, 3.00 mm diameter) discs in poly(tetrafluoroethylene) served as working electrodes. The Au, Pt and GC electrodes were polished with Al2 O3 (Merck, 120–190 m2 /g) paste in water and then washed with acetone before
Fig. 1. Cyclic voltammogram of a 4.39 mmol dm−3 solution of ferrocene in 1-ethyl3-methylimidazolium tetrafluoroborate on a platinum electrode. First scan (), second scan (). Scan rate: 10 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
1416
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419 Table 2 Electrochemical parameters from cyclovoltammetric studies of 5.13 mmol dm−3 solution of cobaltocenium hexafluorophosphate in 1-ethyl-3-methylimidazolium tetrafluoroborate on glassy carbon (GC), platinum (Pt) and gold (Au) electrodes. Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile. Scan rate (mV s−1 )
1/2 (Epa + Epc ) (V)
Ep − Ep/2 a (V)
ja (A cm−2 )
jc (A cm−2 )
GC
2 10 50 100 200
−0.858 −0.858 −0.858 −0.858 −0.863
0.058 0.059 0.059 0.059 0.062
32.1 66.7 147.9 200.0 281.3
31.0 64.4 137.1 198.6 282.4
Pt
2 10 50 100 200
−0.853 −0.853 −0.858 −0.858 −0.868
0.057 0.058 0.057 0.059
25.8 48.91 119.4.3 192.9 262.5
26.7 55.6 133.3 228.6 300.3
Au
50 100 200
−0.858 −0.858 −0.858
0.063 0.063 0.062
122.6 210.5 339.4
190.3 273.1 369.7
Electrode
Fig. 2. Cyclic voltammogram of a 5.13 mmol dm−3 solution of cobaltocenium hexafluorophosphate in 1-ethyl-3-methylimidazolium tetrafluoroborate on a glassy carbon electrode. First scan (), second scan (). Scan rate: 50 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
a
Data taken from the first scan of cobaltocenium hexafluorophosphate.
Table 3 Electrochemical parameters from cyclovoltammetric studies of 5.88 mmol dm−3 solution of bis(biphenyl)chromium(I) tetraphenylborate in 1-ethyl-3methylimidazolium tetrafluoroborate on glassy carbon (GC) and on gold (Au) electrodes. Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile. Scan rate (mV s−1 )
1/2 (Epa + Epc ) (V)
Ep − Ep/2 a (V)
ja (A cm−2 )
jc (A cm−2 )
GC
2 10 50 100 200
−0.608 −0.613 −0.608 −0.613 −0.613
0.066 0.068 0.067 0.068 0.068
11.6 27.8 66.3 95.0 133.1
13.1 28.5 62.5 89.9 135.0
Ptb
50 100 200
−0.620 −0.620 −0.620
0.065 0.066 0.065
– – –
– – –
Au
2 10 50 100
−0.613 −0.613 −0.618 −0.618
0.063 0.063 0.064 0.063
20.0 39.4 94.3 128.2
16.7 31.6 78.9 122.9
Electrode
Fig. 3. Cyclic voltammogram of a 5.88 mmol dm−3 solution of bis(biphenyl)chromium tetraphenylborate in 1-ethyl-3-methylimidazolium tetrafluoroborate on a gold electrode. First scan (), second scan (). Scan rate: 50 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
a b
Table 1 Electrochemical parameters from cyclovoltammetric studies of 4.39 mmol dm−3 solution of ferrocene in 1-ethyl-3-methylimidazolium tetrafluoroborate on glassy carbon (GC), platinum (Pt) and gold (Au) electrodes. Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile. Electrode Scan rate (mV s−1 )
1/2 (Epa + Epc ) V Ep − Ep/2 a (V) ja (A cm−2 ) jc (A cm−2 )
Data taken from the first scan of bis(biphenyl)chromium tetraphenylborate. Additional adsorption peak following the cathodic reduction peak.
cobaltocenium hexafluorophosphate and bis(biphenyl)chromium tetrafluoroborate (Tables 1–3). The Ep and the Ep – Ep/2 – values indicate a nearly reversible or quasi-reversible electrode reaction. The electrochemical behavior of the BCr/BCr+ couple on Pt is affected by adsorption phenomena, which made evaluation of the electrochemical properties difficult. The peak currents of ferrocene, of cobaltocenium hexafluorophosphate and of bis(biphenyl)chromium tetraphenylborate depend linearly on the scan rate as required for diffusion
GC
2 10 50 100 200
0.477 0.476 0.476 0.476 0.478
0.061 0.062 0.061 0.061
30.0 65.5 147.6 196.5 280.8
31.0 72.7 147.6 206.8 297.6
Pt
2 10 50 100 200
0.478 0.476 0.476 0.476 0.478
0.057 0.058 0.058 0.058 0.057
29.2 72.0 132.2 189.6 268.8
34.0 72.0 147.4 210.3 307.2
Table 4 Average of the difference in the 1/2 (Epa − Epc ) – values for the redox couples ferrocene and cobaltocene (Fc/Fc+ –Cc/Cc+ ), ferrocene and bis(biphenyl)chromium(I) (Fc/Fc+ –BCr/BCr+ ) as well as bis(biphenyl)chromium(I) and cobaltocene (BCr/BCr+ –Cc/Cc+ ).
0.481 0.481 0.481 0.481 0.481
0.058 0.057 0.057 0.057 0.057
26.6 58.9 130.2 203.3 283.8
26.5 63.1 137.2 203.3 281.5
Electrode
Au
2 10 50 100 200
a
Data taken from the first scan of ferrocene.
Fc/Fc+ –Cc/Cc+ Fc/Fc+ –BCr/BCr+ BCr/BCr+ –Cc/Cc+
[1/2 (Epa − Epc )] (V) GC
Pt
Au
1.336 1.088 −0.248
1.333 1.097 −0.236
1.339 1.097 −0.242
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419
Fig. 4. Dependence of the peak current density on the square root of the scan rate for ferrocene in 1-ethyl-3-methylimidazolium tetrafluoroborate on a platinum electrode.
controlled electrode processes. An example is given in Fig. 4. Diffusion coefficients (expressed in cm2 s−1 ) were calculated from the peak currents employing the equation for Nernstian behavior of the cyclovoltammogram:
Ip = 0.4463
F3 RT
1/2 n3/2 co AD1/2 1/2 .
where I is the current (in A), v is the scan rate (in V s−1 ), co is the depolarizer bulk concentration (in mol cm−3 ) and A is the electrode surface (in cm2 ). Average values for the five scan rates used are summarized in Table 5. The analysis of the peak currents obtained from cyclovoltammogram (after the base line correction), leads to the conclusion that the peak currents for the anodic and cathodic peaks of these three redox couples are very similar and their ratio is close to one. The 1/2 (Epa + Epc ) values for the three redox couples mentioned above versus the reference electrode Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile were very stable. The data for the Fc/Fc+ couple versus this reference electrode is 0.478 V with a standard deviation of 2 mV for the three working electrodes; the Cc/Cc+ couple yielded an average value of −0.858 V with a standard deviation of 4 mV. The values for the redox couple BCr/BCr+ on gold and glassy carbon were −0.616 V (standard deviation 4 mV). This data allows the calculation of differences in the 1/2 (Epa + Epc ) values. The difference between the redox couples Fc/Fc+ and Cc/Cc+ in 1-ethyl-3-methylimidazolium tetrafluoroborate is 1.336 ± 0.002 V, between Fc/Fc+ and BCr/BCr+ 1.094 ± 0.005 V and between BCr/BCr+ and Cc/Cc+ −0.242 ± 0.005 V (Table 4). These values are averaged over all working electrodes and scan rates for the individual measurements. The deviations are within experimental error. In order to support these differences in the 1/2 (Epa – Epc ) values derived from the individual measurements of Table 5 Diffusion coefficients, D (cm2 s−1 ) together with the standard deviations, of ferrocene, cobaltocenium ion and bis(biphenyl)chromium ion in 1-ethyl-3methylimidazolium tetrafluoroborate. obtained from cyclovoltammetry measurements on glassy carbon (GC), platinum (Pt) and gold (Au) electrodes. Electrode
D (×10−7 cm2 s−1 ) GC
Ferrocene Cobaltocenium ion Bis(biphenyl)chromium ion
3.0 ± 0.2 2.2 ± 0.2 0.39 ± 0.04
Pt 2.9 ± 0.5 2.1 ± 0.1 –
Au 2.7 ± 0.2 – 0.55 ± 0.1
1417
Fig. 5. Cyclic voltammogram of a 4.54 mmol dm−3 solution of decamethylferrocene in 1-ethyl-3-methylimidazolium tetrafluoroborate on a gold electrode. First scan (), second scan (). Scan rate: 50 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
the organometallic redox couples versus the reference cryptate electrode in acetonitrile, additional measurements were carried out. In these measurements the redox couples were dissolved in the same solution, avoiding any phase boundary potentials in the evaluation of the 1/2 (Epa – Epc ) of the organometallic redox couples. The differences in the 1/2 (Epa – Epc ) values were determined from the respective peaks in the very same solution as recommended by IUPAC for non-aqueous electrolytes [4]. The difference in the 1/2 (Epa + Epc ) values was 1.324 V for Fc/Fc+ –Cc/Cc+ , 1.090 V for Fc/Fc+ –BCr/BCr+ , and −0.231 V for BCr/BCr+ –Cc/Cc+ in excellent agreement with the data from the individual measurements versus the external reference electrode. The electrochemical behavior of decamethylferrocene is given in Fig. 5. The cyclovoltammetric behavior of decamethylferrocene may best be interpreted by strong adsorption of reduced form (reactant decamethylferrocene) on all of the three working electrodes employed [21]. Fig. 5 is only an example of a variety of different cyclovoltammograms observed during this study for this compound. The data given in Table 5 refer to the second scan obtained after purging the solution for 30 min with argon. An almost symmetrical, bell shaped anodic peak, typical for strong adsorption is observed on the first scan to more positive potentials [21]. Double anodic peaks typical for systems were both the adsorbed and the dissolved species are electro-active are observed on the second scan (Fig. 5). The current of the anodic adsorption peak is much higher than for the cathodic peak. The cathodic peak referring to the reduction of the decamethylferrocenium cation to decamethylferrocene is also affected by adsorption of the reduced form decamethylferrocene. The adsorption is more pronounced the slower the scan rate. 4. Discussion Electrochemistry in ionic liquids is still a challenging area of research. Electrochemistry is quite sensitive to impurities present in the medium and ionic liquids do not offer a possibility of their purification by distillation. Another difficulty is the poor solubility, especially of organometallic redox couples. While not always emphasized in publications these problems exist and should be addressed in more detail. In the current study we observed well-developed cathodic and anodic peaks for the couple ferrocene/ferrocenium ion and for cobaltocene/cobaltocenium ion on all three working electrodes and for bis(biphenyl)chromium/bis(biphenyl)chromium ion on gold and
1418
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419
glassy carbon. Adsorption of bis(biphenyl)chromium on platinum as working electrode makes evaluation of the faradaic anodic and cathodic peaks on Pt as working electrode difficult but possible. The reference electrode employed by us yielded stable potentials as shown by repeated measurements of the redox couples. Measurements of the 1/2 (Epa + Epc ) for (Fc/Fc+ ) over a period of 48 h gave stable values within ca. 1 mV, indicating that no changes in the liquid junction potential occurred. Of course, it must be kept in mind that liquid junction potentials (phase boundary potentials) are transfer properties and not thermodynamic properties, but in the set-up used by us the phase boundary potential was reproducible. It may well be that the cation (1-ethyl3-methylimidazolium ion) and the anion (tetrafluoroborate ion) of the ionic liquid investigated may have similar mobility and thus the ionic liquid also acts as a “salt bridge”. In the case of the ionic liquid under study, the transport numbers estimated from NMR measurements are comparable: t(EtMeIm+ ) = 0.54 and t(BF4− ) = 0.46 [8]. Separation between the Fc/Fc+ and BCr/BCr+ redox couples are available for 21 non-aqueous solvents (molecular liquids) [17]. The values vary from 1.112 to 1.135 V with an average value of 1.126. The value measured in 1-ethyl-3-methylimidazolium tetrafluoroborate is only 33 mV smaller and thus an indication that the constancy of the difference in the potentials of ferrocene and bis(biphenyl)chromium may be extended into ionic liquids. Bond and coworkers reported a value of 1.334 V for the corresponding difference of the ferrocene and cobaltocene couples in 1-butyl-3-methylimidazolium hexafluorophosphate on a platinum disc electrode [10] and 1.332 V on a gold electrode [11]. A difference of 1.333 for the 1/2 (Epa – Epc ) values of Fc/Fc+ and Cc/Cc+ was found in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide [13]. The published data compare well with the data obtained by us in solutions in 1-ethyl-3-methylimidazolium tetrafluoroborate. This offers additional supports for the concept to employ reference redox systems in ionic liquids. The system decamethylferrocene/decamethylferrocenium ion, however, is affected by adsorption on all working electrodes studied and cannot be considered as reference system in this solvent. The diffusion coefficients calculated from the peak currents are summarized in Table 5. We are aware of the ongoing discussion on possible concentration dependence of the diffusion coefficient on the concentration of ferrocene in solution [22–24]. This area of research definitely needs additional studies since currently there are contradictory information available in the literature. Cyclic voltammograms presented in various publications show (within experimental error) equivalent peak heights for the cathodic and the anodic scan. Even considering that the square root of the diffusion coefficients enters in the peak current, a difference in cathodic and anodic diffusion coefficients by a factor of two will produce measurable changes in the ratio of the cathodic and the anodic peak heights. In our measurements we observed a ratio Ipc to Ipa of one within experimental error for all diffusion controlled processes. Literature reports a diffusion coefficient for ferrocene in 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EtMeImNTf2 ) of 3.35 × 10−7 cm2 s−1 [16] and in trimethyln-hexylammonium bis(trifluoromethanesulfone)imide (Me3 HexNNTf2 ) of ca. 1 × 10−7 cm2 s−1 [15]. Concentration dependent diffusion coefficients for ferrocene ranging from 1.3 × 10−7 to 6.8 × 10−7 cm2 s−1 were reported in 1-butyl-3-methylimidazolium tetrafluoroborate (BuMeImBF4 ) [24]. Data for ferrocene in 1-butyl3-methylimidazolium hexafluorophosphate (BuMeImPF6 ) range from 7 × 10−9 to 3 × 10−8 cm2 s−1 , depending on the ferrocene concentration and the electrochemical method used [23]. A diffusion coefficient of 3 × 10−8 cm2 s−1 was measured for ferrocene [10] and a diffusion coefficient of the cobaltocenium cation was
Table 6 Diffusion coefficients, D, medium viscosities at 25 ◦ C, , and D values for ferrocene in several ionic liquids and in acetonitrile. D (cm2 s−1 )
Medium b
EtMeIm NTf2 Me3 HexN NTf2 b BuMeIm PF6 d EtMeIm BF4 e Acetonitrile
−7
3.35 × 10 [14] 6.0 × 10−8 [13] 3.0 × 10−8 [12] 2.9 × 10−7 (this study) 1.9 × 10−5 [12]
(cPa )
D (cm g s−2 )
34 [9] 595 [9] 312 (30 ◦ C) [25] 43 [26] 0.34 [26]
1.14 × 10−7 3.57 × 10−7 0.94 × 10−7 1.25 × 10−7 0.65 × 10−7
1 cP = 0.01 g (cm s)−1 . 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. c Trimethyl-n-hexylammonium bis(trifluoromethanesulfone)imide. d 1-butyl-3-methylimidazolium hexafluorophosphate. e 1-ethyl-3-methylimidazolium tetrafluoroborate. a
b
calculated to be 1 × 10−8 cm2 s−1 in 1-butyl-3-methyimidazolium hexafluorophosphate [11]. The variation of the diffusion coefficients of ferrocene over two orders of magnitude, measured in different ionic liquids, is not unexpected, taking into account viscosity differences between the ionic liquids. Literature values and our results for the diffusion coefficient, viscosity and Walden products, according to the Stokes–Einstein equation D =
kB T = const 6r
are summarized in Table 6. The product should be approximately constant for a given compound of radius r. Inspection of the D values collected for ferrocene in Table 6 shows that this product for ferrocene varies within a relatively narrow range from 0.65 × 10−7 to 3.57 × 10−7 cm g s−2 , while the diffusion coefficients are within a broad range of 1.9 × 10−5 to 3.0 × 10−8 cm2 s−1 . 5. Conclusions 1. Well-developed cathodic and anodic peaks for the couples ferrocene/ferrocenium ion and cobaltocene/cobaltocenium ion on all three working electrodes and for bis(biphenyl)chromium/ bis(biphenyl)chromium ion on gold and glassy carbon, were observed. The electrode reaction of decamethylferrocene was affected by adsorption of the reduced form on all of the three working electrodes employed. 2. The difference (averaged over all working electrodes) between the redox couples Fc/Fc+ and Cc/Cc+ in 1-ethyl3-methylimidazolium tetrafluoroborate is 1.336 ± 0.002 V, between Fc/Fc+ and BCr/BCr+ 1.094 ± 0.005 V and between BCr/BCr+ and Cc/Cc+ −0.242 ± 0.005 V (Table 4). 3. The redox couples Fc/Fc+ , Cc/Cc+ and BCr/BCr+ may be considered as reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate. Acknowledgement Support of the grant BW 31-160/08 is gratefully acknowledged References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
V.A. Pleskov, Usp. Khim. 16 (1947) 254. H.M. Koepp, H. Wendt, H. Strehlow, Z. Elektrochem. 64 (1960) 483. G. Gritzner, Russ. J. Electrochem. 44 (2008) 7. G. Gritzner, J. Kuta, Pure Appl. Chem. 56 (1984) 461. T. Welton, Chem. Rev. 99 (1999) 2071. P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3773. F. Endres, S.Z. El Abedin, Phys. Chem. Chem. Phys. 8 (2006) 2101. ´ ˛ M. Galinski, A. Lewandowski, I. Stepniak, Electrochim. Acta 51 (2006) 5567. J. Zhang, A.M. Bond, Analyst 130 (2005) 1132. J. Zhang, A.M. Bond, Anal. Chem. 75 (2003) 2694.
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419 [11] V.M. Hultgren, A.W.A. Mariotti, A.M. Bond, A.G. Wedd, Anal. Chem. 74 (2002) 3151. [12] S.K. Sukardi, J. Zhang, I. Burgar, M.D. Horne, A.F. Hollenkamp, D.R. MacFarlane, A.M. Bond, Electrochem. Commun. 10 (2008) 250. [13] G.A. Snook, A.S. Best, A.G. Pandolfo, A.F. Hollenkamp, Electrochem. Commun. 8 (2006) 1405. [14] Y. Katayama, in: H.E. Ohno (Ed.), Electrochemnical Aspects of Ionic Liquids, John Wiley & Sons, New Jersey, 2005, p. 31. [15] M. Matsumiya, M. Terazono, K. Tokuraku, Electrochim. Acta 51 (2006) 1178. [16] N. Fietkau, A.D. Clegg, R.G. Evans, C. Villagrán, C. Hardacre, R.G. Compton, Chem. Phys. Chem. 7 (2006) 1041.
[17] [18] [19] [20] [21] [22] [23]
1419
G. Gritzner, Pure Appl. Chem. 62 (1990) 1839. G. Gritzner, Inorg. Chim. Acta 24 (1977) 5. V. Gutmann, G. Peychal-Heiling, Monatsh. Chem. 100 (1969) 813. A. Lewandowski, A. Szukalska, M. Galinski, New J. Chem. 19 (1995) 1259. A.J. Bard, L.R. Faulkner, Electrochemical Methods, John Wiley, 2001, p. 594. C.A. Brooks, A.P. Doherty, Electrochem. Commun. 6 (2004) 867. L. Nagy, G. Gyetvai, L. Kollár, G. Nagy, J. Biochem. Biophys. Methods 69 (2006) 121. [24] S. Eisele, M. Schwarz, B. Speiser, C. Tittel, Electrochim. Acta 51 (2006) 5304. [25] H. Olivier-Bourbigou, L. Magna, J. Mol. Catal., A 182 (2002) 419. [26] D.S. Silvester, R.G. Compton, Z. Phys. Chem. 220 (2006) 1247.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical studies of four organometallic redox couples as possible reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate Lukasz Waligora a , Andrzej Lewandowski a,∗ , Gerhard Gritzner b a b
Faculty of Chemical Technology, Poznan University of Technology, Plac Marii Sklodowskiej-Curie, PL 60 965 Poznan, Poland Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University, A 4040 Linz, Austria
a r t i c l e
i n f o
Article history: Received 28 January 2008 Received in revised form 5 September 2008 Accepted 6 September 2008 Available online 20 September 2008 Keywords: Ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate Reference electrodes Ferrocene Cobaltocene Decamethylferrocene Bis(biphenyl)chromium
a b s t r a c t The electrochemical behavior of four organometallic redox couples has been studied in the room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate. Diffusion controlled anodic and cathodic peaks were found for the redox couples Fc/Fc+ (ferrocene/ferrocenium), Cc/Cc+ (cobaltocene/cobaltocenium) on glassy carbon, platinum and gold. Bis(biphenyl)chromium(I) tetraphenylborate (BCr/BCr+ ) yielded a well-behaved diffusion controlled pair of cathodic–anodic peaks only on glassy carbon and gold as working electrodes. The electrode reaction of decamethylferrocene was affected by adsorption of the reduced form on all of the three working electrodes employed by us. The applicability of three of the four redox couples studied as candidates for reference redox systems in this ionic liquid is discussed. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Establishing solvent independent reference electrodes for the comparison of redox potentials in aqueous and non-aqueous systems has a long history. It began with the concept of Rb|Rb+ or Rb(Hg)|Rb+ [1] as a pilot ion, followed by the proposal to use organometallic redox couples [2]. A summary of these efforts was published recently [3]. In order to limit the number of possible reference redox systems based on organometallic redox couples IUPAC recommended that the systems ferrocene/ferrocenium ion (Fc/Fc+ ) and bis(biphenyl)chromium(0)/(I) (BCr/BCr+ ) should be used as reference redox systems in non-aqueous media [4]. During the last decade a considerable number of papers dealing with room temperature ionic liquids [5–7] were published, including their possible application as solvent-free electrolytes [8]. The problem of reference electrodes and the lack of reference potential scales in ionic liquids have been pointed out in several papers [9–12]. Such a reference redox couple, however, is necessary to allow comparison of potential data measured by different research groups in the same ionic liquid.
∗ Corresponding author. Tel.: +48 61 6652309; fax: +48 61 6652571. E-mail address: [email protected] (A. Lewandowski). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.09.023
Currently such measurements in ionic liquids are carried out versus Ag|Ag+ ion electrodes [13,14], but also versus quasi-reference electrodes such as platinum or silver wires. Quasi-reference electrodes do not posses a reliable, thermodynamic potential and are sensitive to impurities in the ionic liquid. Thus potential data reported by different research groups remain isolated data and prevent comparison. The potential of an electrode of the first kind such as the Ag|Ag+ ion electrode in different media suffers from the drawback that only one form of the redox couple, namely the Ag+ ion is affected by the solvent or the ionic liquid. Organometallic redox couples, where both the oxidized and the reduced form are soluble, have been proposed in non-aqueous solvents and solvent mixtures. This suggestion implies the expectation that the solvent effects on the oxidized form will be very similar to the solvent effects on the reduced form, thus making the redox potential of such a couple independent of the nature of the nonaqueous system. Currently, such an assumption for ionic liquids lacks supporting experimental data. So far, the electrochemical behavior of the redox couples cobaltocene/cobaltocenium ion (Cc/Cc+ ) and ethylferrocene/ethylferrocenium ion on gold, glassy carbon and platinum disc electrodes in 1-n-butyl-3methylimidazolium hexafluorophosphate and the electrochemical behavior of several substituted ferrocenes, which adhered to the working electrode surface, have been studied [10]. Electrode
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419
kinetics of ferrocene or ferrocene derivatives in ionic liquids has been published [15,16]. The cobaltocene/cobaltocenium couple was studied and suggested as possible reference redox couples in 1-n-butyl-3-methylimidazolium hexafluorophosphate, in 1-ethyl3-methylimidazolium bis(trifluoromethanesulphonyl)imide as well as in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulphonyl)imide [11,12]. Current literature indicates that the ferrocene/ferrocenium ion redox couple as possible reference system for ionic liquids suffers from poor solubility in several ionic liquids, prompting the research group of Prof. Bond to emphasize cobaltocene/cobaltocenium ion as a possible internal potential standard for electrode potential measurements in ionic liquids [12]. The bis(biphenyl)chromium/bis(biphenyl)chromium ion reference redox system recommended by IUPAC besides ferrocene as one of the two reference redox systems for non-aqueous solvents has not been investigated in ionic liquids so far. It must be kept in mind that a redox couple must fulfill the requirements of a reference redox system in many ionic liquids before it may be accepted as a reference redox system. Hopefully, a redox couple, which has been proposed as reference redox system in organic solvents could also be employed in ionic liquids. Additional research is thus needed including organometallic redox couples, for which data in non-aqueous solvents and solvent mixtures are available [17]. In this paper we report the cyclovoltametric behavior of ferrocene, cobaltocenium hexafluorophosphate (cobaltocene), decamethylferrocene and bis(biphenyl)chromium tetraphenylborate in 1-ethyl-3-methylimidazolium tetrafluoroborate, a liquid salt at room temperature, on glassy carbon, gold and platinum electrodes, with respect to possible applications of these organometallic compounds as reference redox systems in ionic liquids. 2. Experimental 2.1. Chemicals Cryptand 222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8.8.8]hexacosane, Merck) and silver perchlorate (AgClO4 , Fluka) were used as purchased. Bis(cyclopentadienyl)iron(II) (ferrocene, Fc, Aldrich), bis(cyclopentadienyl)cobalt(III) hexafluorophosphate (cobaltocenium hexafluorophosphate, Cc+ PF6 − , Aldrich) and bis(pentamethylcyclopentadienyl)iron(II) (decamethylferrocene, DFc, Strem Chemicals) are commercially available and were used without further purification. Bis(biphenyl)chromium(I) tetraphenylborate (BCr+ TPB− ) was prepared in a modified way [18] according to a published procedure [19]. Bis(biphenyl)chromium(I) tetraphenylborate was re-crystallized from absolute ethanol prior to use. Cobaltocenium hexafluorophosphate as well as its solutions in the experimental cell were protected from light with an aluminum foil. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EtMeImBF4 ) was used as purchased (Merck). The water content in the ionic liquid, analyzed with a standard Karl-Fisher titrant (Aldrich, Cat. no: 22,120-1) was below the detection limit (<4.5 mg H2 O/ml).
1415
each measurement. A 0.5 cm × 1.0 cm platinum sheet was used as a counter-electrode. The reference electrode was a cryptate electrode consisting of a silver wire immersed into a solution containing 0.01 mol dm−3 AgClO4 and 0.1 mol dm−3 cryptand 222 in acetonitrile (AN) [20]. The reference electrode compartment was separated by a glass frit from the cell containing the ionic liquid EtMeImBF4 . Weighing of the samples, preparation of the solutions and cell assembling were done in a glove-box under a dry argon inert atmosphere. Cyclic voltammetry (CV) curves were obtained with a Lab Electrochemical System (Eco Chemie, The Netherlands) at 25 ± 0.1 ◦ C. All solutions were purged with argon for 30 min prior to measurement. For solutions containing the oxidized form of the metallocenes (BCr+ and Cc+ ) the initial scan was performed to more negative values; for solutions containing the reduced form (Fc and DFc) the initial scan was carried out to more positive potentials than the starting potential. In all experiments first the base line of the ionic liquid was established. Then a known volume of a more concentrated solution of the organometallic compound was added to the measured volume of the ionic liquid and the cyclovoltammograms of the organometallic compounds were recorded. The cyclovoltammograms were corrected for the base line current. The ohmic resistance between the working electrodes and the auxiliary electrode deduced from impedance measurements was around 250 , the current typically at the level of ca. 1–10 A. This led to the IR distortion in the order of 0.25–2.5 mV. Hence, the cyclovoltammetry curves were not corrected for the ohmic drop. The low value of ohmic drop in EMImBF4 is due to its relatively high conductance of ca. 14 mS cm−1 at 25 ◦ C. 3. Results Examples of the cyclovoltammetric behavior of ferrocene (Fig. 1), cobaltocenium hexafluorophosphate (Fig. 2) and bis(biphenyl)chromium tetraphenylborate (Fig. 3) are included. The electrochemical data of ferrocene is summarized in Table 1, of cobaltocenenium hexafluorophosphate in Table 2 and of bis(biphenyl)chromium tetraphenylborate in Table 3. The CV curves of ferrocene, cobaltocenenium hexafluorophosphate and bis(biphenyl)chromium(I) tetraphenylborate on all three working electrodes exhibited the typical shape of diffusion controlled redox reactions. The difference between the corresponding anodic and cathodic peaks observed by us for Fc /Fc+ , Cc /Cc+ and BCr /BCr+ varied between 70 and 90 mV. Values of Ep – Ep/2 were obtained from the first scan of the compounds. Ferrocene,
2.2. Apparatus and procedures Measurements were carried out in a three electrode arrangement. Gold (1.50 mm diameter) or platinum (1.50 mm diameter) discs in glass or glassy carbon (GC, 3.00 mm diameter) discs in poly(tetrafluoroethylene) served as working electrodes. The Au, Pt and GC electrodes were polished with Al2 O3 (Merck, 120–190 m2 /g) paste in water and then washed with acetone before
Fig. 1. Cyclic voltammogram of a 4.39 mmol dm−3 solution of ferrocene in 1-ethyl3-methylimidazolium tetrafluoroborate on a platinum electrode. First scan (), second scan (). Scan rate: 10 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
1416
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419 Table 2 Electrochemical parameters from cyclovoltammetric studies of 5.13 mmol dm−3 solution of cobaltocenium hexafluorophosphate in 1-ethyl-3-methylimidazolium tetrafluoroborate on glassy carbon (GC), platinum (Pt) and gold (Au) electrodes. Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile. Scan rate (mV s−1 )
1/2 (Epa + Epc ) (V)
Ep − Ep/2 a (V)
ja (A cm−2 )
jc (A cm−2 )
GC
2 10 50 100 200
−0.858 −0.858 −0.858 −0.858 −0.863
0.058 0.059 0.059 0.059 0.062
32.1 66.7 147.9 200.0 281.3
31.0 64.4 137.1 198.6 282.4
Pt
2 10 50 100 200
−0.853 −0.853 −0.858 −0.858 −0.868
0.057 0.058 0.057 0.059
25.8 48.91 119.4.3 192.9 262.5
26.7 55.6 133.3 228.6 300.3
Au
50 100 200
−0.858 −0.858 −0.858
0.063 0.063 0.062
122.6 210.5 339.4
190.3 273.1 369.7
Electrode
Fig. 2. Cyclic voltammogram of a 5.13 mmol dm−3 solution of cobaltocenium hexafluorophosphate in 1-ethyl-3-methylimidazolium tetrafluoroborate on a glassy carbon electrode. First scan (), second scan (). Scan rate: 50 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
a
Data taken from the first scan of cobaltocenium hexafluorophosphate.
Table 3 Electrochemical parameters from cyclovoltammetric studies of 5.88 mmol dm−3 solution of bis(biphenyl)chromium(I) tetraphenylborate in 1-ethyl-3methylimidazolium tetrafluoroborate on glassy carbon (GC) and on gold (Au) electrodes. Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile. Scan rate (mV s−1 )
1/2 (Epa + Epc ) (V)
Ep − Ep/2 a (V)
ja (A cm−2 )
jc (A cm−2 )
GC
2 10 50 100 200
−0.608 −0.613 −0.608 −0.613 −0.613
0.066 0.068 0.067 0.068 0.068
11.6 27.8 66.3 95.0 133.1
13.1 28.5 62.5 89.9 135.0
Ptb
50 100 200
−0.620 −0.620 −0.620
0.065 0.066 0.065
– – –
– – –
Au
2 10 50 100
−0.613 −0.613 −0.618 −0.618
0.063 0.063 0.064 0.063
20.0 39.4 94.3 128.2
16.7 31.6 78.9 122.9
Electrode
Fig. 3. Cyclic voltammogram of a 5.88 mmol dm−3 solution of bis(biphenyl)chromium tetraphenylborate in 1-ethyl-3-methylimidazolium tetrafluoroborate on a gold electrode. First scan (), second scan (). Scan rate: 50 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
a b
Table 1 Electrochemical parameters from cyclovoltammetric studies of 4.39 mmol dm−3 solution of ferrocene in 1-ethyl-3-methylimidazolium tetrafluoroborate on glassy carbon (GC), platinum (Pt) and gold (Au) electrodes. Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile. Electrode Scan rate (mV s−1 )
1/2 (Epa + Epc ) V Ep − Ep/2 a (V) ja (A cm−2 ) jc (A cm−2 )
Data taken from the first scan of bis(biphenyl)chromium tetraphenylborate. Additional adsorption peak following the cathodic reduction peak.
cobaltocenium hexafluorophosphate and bis(biphenyl)chromium tetrafluoroborate (Tables 1–3). The Ep and the Ep – Ep/2 – values indicate a nearly reversible or quasi-reversible electrode reaction. The electrochemical behavior of the BCr/BCr+ couple on Pt is affected by adsorption phenomena, which made evaluation of the electrochemical properties difficult. The peak currents of ferrocene, of cobaltocenium hexafluorophosphate and of bis(biphenyl)chromium tetraphenylborate depend linearly on the scan rate as required for diffusion
GC
2 10 50 100 200
0.477 0.476 0.476 0.476 0.478
0.061 0.062 0.061 0.061
30.0 65.5 147.6 196.5 280.8
31.0 72.7 147.6 206.8 297.6
Pt
2 10 50 100 200
0.478 0.476 0.476 0.476 0.478
0.057 0.058 0.058 0.058 0.057
29.2 72.0 132.2 189.6 268.8
34.0 72.0 147.4 210.3 307.2
Table 4 Average of the difference in the 1/2 (Epa − Epc ) – values for the redox couples ferrocene and cobaltocene (Fc/Fc+ –Cc/Cc+ ), ferrocene and bis(biphenyl)chromium(I) (Fc/Fc+ –BCr/BCr+ ) as well as bis(biphenyl)chromium(I) and cobaltocene (BCr/BCr+ –Cc/Cc+ ).
0.481 0.481 0.481 0.481 0.481
0.058 0.057 0.057 0.057 0.057
26.6 58.9 130.2 203.3 283.8
26.5 63.1 137.2 203.3 281.5
Electrode
Au
2 10 50 100 200
a
Data taken from the first scan of ferrocene.
Fc/Fc+ –Cc/Cc+ Fc/Fc+ –BCr/BCr+ BCr/BCr+ –Cc/Cc+
[1/2 (Epa − Epc )] (V) GC
Pt
Au
1.336 1.088 −0.248
1.333 1.097 −0.236
1.339 1.097 −0.242
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419
Fig. 4. Dependence of the peak current density on the square root of the scan rate for ferrocene in 1-ethyl-3-methylimidazolium tetrafluoroborate on a platinum electrode.
controlled electrode processes. An example is given in Fig. 4. Diffusion coefficients (expressed in cm2 s−1 ) were calculated from the peak currents employing the equation for Nernstian behavior of the cyclovoltammogram:
Ip = 0.4463
F3 RT
1/2 n3/2 co AD1/2 1/2 .
where I is the current (in A), v is the scan rate (in V s−1 ), co is the depolarizer bulk concentration (in mol cm−3 ) and A is the electrode surface (in cm2 ). Average values for the five scan rates used are summarized in Table 5. The analysis of the peak currents obtained from cyclovoltammogram (after the base line correction), leads to the conclusion that the peak currents for the anodic and cathodic peaks of these three redox couples are very similar and their ratio is close to one. The 1/2 (Epa + Epc ) values for the three redox couples mentioned above versus the reference electrode Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile were very stable. The data for the Fc/Fc+ couple versus this reference electrode is 0.478 V with a standard deviation of 2 mV for the three working electrodes; the Cc/Cc+ couple yielded an average value of −0.858 V with a standard deviation of 4 mV. The values for the redox couple BCr/BCr+ on gold and glassy carbon were −0.616 V (standard deviation 4 mV). This data allows the calculation of differences in the 1/2 (Epa + Epc ) values. The difference between the redox couples Fc/Fc+ and Cc/Cc+ in 1-ethyl-3-methylimidazolium tetrafluoroborate is 1.336 ± 0.002 V, between Fc/Fc+ and BCr/BCr+ 1.094 ± 0.005 V and between BCr/BCr+ and Cc/Cc+ −0.242 ± 0.005 V (Table 4). These values are averaged over all working electrodes and scan rates for the individual measurements. The deviations are within experimental error. In order to support these differences in the 1/2 (Epa – Epc ) values derived from the individual measurements of Table 5 Diffusion coefficients, D (cm2 s−1 ) together with the standard deviations, of ferrocene, cobaltocenium ion and bis(biphenyl)chromium ion in 1-ethyl-3methylimidazolium tetrafluoroborate. obtained from cyclovoltammetry measurements on glassy carbon (GC), platinum (Pt) and gold (Au) electrodes. Electrode
D (×10−7 cm2 s−1 ) GC
Ferrocene Cobaltocenium ion Bis(biphenyl)chromium ion
3.0 ± 0.2 2.2 ± 0.2 0.39 ± 0.04
Pt 2.9 ± 0.5 2.1 ± 0.1 –
Au 2.7 ± 0.2 – 0.55 ± 0.1
1417
Fig. 5. Cyclic voltammogram of a 4.54 mmol dm−3 solution of decamethylferrocene in 1-ethyl-3-methylimidazolium tetrafluoroborate on a gold electrode. First scan (), second scan (). Scan rate: 50 mV s−1 . Reference electrode: Ag|Ag+ (0.01 mol dm−3 ), cryptand 222 (0.1 mol dm−3 ) in acetonitrile.
the organometallic redox couples versus the reference cryptate electrode in acetonitrile, additional measurements were carried out. In these measurements the redox couples were dissolved in the same solution, avoiding any phase boundary potentials in the evaluation of the 1/2 (Epa – Epc ) of the organometallic redox couples. The differences in the 1/2 (Epa – Epc ) values were determined from the respective peaks in the very same solution as recommended by IUPAC for non-aqueous electrolytes [4]. The difference in the 1/2 (Epa + Epc ) values was 1.324 V for Fc/Fc+ –Cc/Cc+ , 1.090 V for Fc/Fc+ –BCr/BCr+ , and −0.231 V for BCr/BCr+ –Cc/Cc+ in excellent agreement with the data from the individual measurements versus the external reference electrode. The electrochemical behavior of decamethylferrocene is given in Fig. 5. The cyclovoltammetric behavior of decamethylferrocene may best be interpreted by strong adsorption of reduced form (reactant decamethylferrocene) on all of the three working electrodes employed [21]. Fig. 5 is only an example of a variety of different cyclovoltammograms observed during this study for this compound. The data given in Table 5 refer to the second scan obtained after purging the solution for 30 min with argon. An almost symmetrical, bell shaped anodic peak, typical for strong adsorption is observed on the first scan to more positive potentials [21]. Double anodic peaks typical for systems were both the adsorbed and the dissolved species are electro-active are observed on the second scan (Fig. 5). The current of the anodic adsorption peak is much higher than for the cathodic peak. The cathodic peak referring to the reduction of the decamethylferrocenium cation to decamethylferrocene is also affected by adsorption of the reduced form decamethylferrocene. The adsorption is more pronounced the slower the scan rate. 4. Discussion Electrochemistry in ionic liquids is still a challenging area of research. Electrochemistry is quite sensitive to impurities present in the medium and ionic liquids do not offer a possibility of their purification by distillation. Another difficulty is the poor solubility, especially of organometallic redox couples. While not always emphasized in publications these problems exist and should be addressed in more detail. In the current study we observed well-developed cathodic and anodic peaks for the couple ferrocene/ferrocenium ion and for cobaltocene/cobaltocenium ion on all three working electrodes and for bis(biphenyl)chromium/bis(biphenyl)chromium ion on gold and
1418
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419
glassy carbon. Adsorption of bis(biphenyl)chromium on platinum as working electrode makes evaluation of the faradaic anodic and cathodic peaks on Pt as working electrode difficult but possible. The reference electrode employed by us yielded stable potentials as shown by repeated measurements of the redox couples. Measurements of the 1/2 (Epa + Epc ) for (Fc/Fc+ ) over a period of 48 h gave stable values within ca. 1 mV, indicating that no changes in the liquid junction potential occurred. Of course, it must be kept in mind that liquid junction potentials (phase boundary potentials) are transfer properties and not thermodynamic properties, but in the set-up used by us the phase boundary potential was reproducible. It may well be that the cation (1-ethyl3-methylimidazolium ion) and the anion (tetrafluoroborate ion) of the ionic liquid investigated may have similar mobility and thus the ionic liquid also acts as a “salt bridge”. In the case of the ionic liquid under study, the transport numbers estimated from NMR measurements are comparable: t(EtMeIm+ ) = 0.54 and t(BF4− ) = 0.46 [8]. Separation between the Fc/Fc+ and BCr/BCr+ redox couples are available for 21 non-aqueous solvents (molecular liquids) [17]. The values vary from 1.112 to 1.135 V with an average value of 1.126. The value measured in 1-ethyl-3-methylimidazolium tetrafluoroborate is only 33 mV smaller and thus an indication that the constancy of the difference in the potentials of ferrocene and bis(biphenyl)chromium may be extended into ionic liquids. Bond and coworkers reported a value of 1.334 V for the corresponding difference of the ferrocene and cobaltocene couples in 1-butyl-3-methylimidazolium hexafluorophosphate on a platinum disc electrode [10] and 1.332 V on a gold electrode [11]. A difference of 1.333 for the 1/2 (Epa – Epc ) values of Fc/Fc+ and Cc/Cc+ was found in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide [13]. The published data compare well with the data obtained by us in solutions in 1-ethyl-3-methylimidazolium tetrafluoroborate. This offers additional supports for the concept to employ reference redox systems in ionic liquids. The system decamethylferrocene/decamethylferrocenium ion, however, is affected by adsorption on all working electrodes studied and cannot be considered as reference system in this solvent. The diffusion coefficients calculated from the peak currents are summarized in Table 5. We are aware of the ongoing discussion on possible concentration dependence of the diffusion coefficient on the concentration of ferrocene in solution [22–24]. This area of research definitely needs additional studies since currently there are contradictory information available in the literature. Cyclic voltammograms presented in various publications show (within experimental error) equivalent peak heights for the cathodic and the anodic scan. Even considering that the square root of the diffusion coefficients enters in the peak current, a difference in cathodic and anodic diffusion coefficients by a factor of two will produce measurable changes in the ratio of the cathodic and the anodic peak heights. In our measurements we observed a ratio Ipc to Ipa of one within experimental error for all diffusion controlled processes. Literature reports a diffusion coefficient for ferrocene in 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EtMeImNTf2 ) of 3.35 × 10−7 cm2 s−1 [16] and in trimethyln-hexylammonium bis(trifluoromethanesulfone)imide (Me3 HexNNTf2 ) of ca. 1 × 10−7 cm2 s−1 [15]. Concentration dependent diffusion coefficients for ferrocene ranging from 1.3 × 10−7 to 6.8 × 10−7 cm2 s−1 were reported in 1-butyl-3-methylimidazolium tetrafluoroborate (BuMeImBF4 ) [24]. Data for ferrocene in 1-butyl3-methylimidazolium hexafluorophosphate (BuMeImPF6 ) range from 7 × 10−9 to 3 × 10−8 cm2 s−1 , depending on the ferrocene concentration and the electrochemical method used [23]. A diffusion coefficient of 3 × 10−8 cm2 s−1 was measured for ferrocene [10] and a diffusion coefficient of the cobaltocenium cation was
Table 6 Diffusion coefficients, D, medium viscosities at 25 ◦ C, , and D values for ferrocene in several ionic liquids and in acetonitrile. D (cm2 s−1 )
Medium b
EtMeIm NTf2 Me3 HexN NTf2 b BuMeIm PF6 d EtMeIm BF4 e Acetonitrile
−7
3.35 × 10 [14] 6.0 × 10−8 [13] 3.0 × 10−8 [12] 2.9 × 10−7 (this study) 1.9 × 10−5 [12]
(cPa )
D (cm g s−2 )
34 [9] 595 [9] 312 (30 ◦ C) [25] 43 [26] 0.34 [26]
1.14 × 10−7 3.57 × 10−7 0.94 × 10−7 1.25 × 10−7 0.65 × 10−7
1 cP = 0.01 g (cm s)−1 . 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. c Trimethyl-n-hexylammonium bis(trifluoromethanesulfone)imide. d 1-butyl-3-methylimidazolium hexafluorophosphate. e 1-ethyl-3-methylimidazolium tetrafluoroborate. a
b
calculated to be 1 × 10−8 cm2 s−1 in 1-butyl-3-methyimidazolium hexafluorophosphate [11]. The variation of the diffusion coefficients of ferrocene over two orders of magnitude, measured in different ionic liquids, is not unexpected, taking into account viscosity differences between the ionic liquids. Literature values and our results for the diffusion coefficient, viscosity and Walden products, according to the Stokes–Einstein equation D =
kB T = const 6r
are summarized in Table 6. The product should be approximately constant for a given compound of radius r. Inspection of the D values collected for ferrocene in Table 6 shows that this product for ferrocene varies within a relatively narrow range from 0.65 × 10−7 to 3.57 × 10−7 cm g s−2 , while the diffusion coefficients are within a broad range of 1.9 × 10−5 to 3.0 × 10−8 cm2 s−1 . 5. Conclusions 1. Well-developed cathodic and anodic peaks for the couples ferrocene/ferrocenium ion and cobaltocene/cobaltocenium ion on all three working electrodes and for bis(biphenyl)chromium/ bis(biphenyl)chromium ion on gold and glassy carbon, were observed. The electrode reaction of decamethylferrocene was affected by adsorption of the reduced form on all of the three working electrodes employed. 2. The difference (averaged over all working electrodes) between the redox couples Fc/Fc+ and Cc/Cc+ in 1-ethyl3-methylimidazolium tetrafluoroborate is 1.336 ± 0.002 V, between Fc/Fc+ and BCr/BCr+ 1.094 ± 0.005 V and between BCr/BCr+ and Cc/Cc+ −0.242 ± 0.005 V (Table 4). 3. The redox couples Fc/Fc+ , Cc/Cc+ and BCr/BCr+ may be considered as reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate. Acknowledgement Support of the grant BW 31-160/08 is gratefully acknowledged References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
V.A. Pleskov, Usp. Khim. 16 (1947) 254. H.M. Koepp, H. Wendt, H. Strehlow, Z. Elektrochem. 64 (1960) 483. G. Gritzner, Russ. J. Electrochem. 44 (2008) 7. G. Gritzner, J. Kuta, Pure Appl. Chem. 56 (1984) 461. T. Welton, Chem. Rev. 99 (1999) 2071. P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3773. F. Endres, S.Z. El Abedin, Phys. Chem. Chem. Phys. 8 (2006) 2101. ´ ˛ M. Galinski, A. Lewandowski, I. Stepniak, Electrochim. Acta 51 (2006) 5567. J. Zhang, A.M. Bond, Analyst 130 (2005) 1132. J. Zhang, A.M. Bond, Anal. Chem. 75 (2003) 2694.
L. Waligora et al. / Electrochimica Acta 54 (2009) 1414–1419 [11] V.M. Hultgren, A.W.A. Mariotti, A.M. Bond, A.G. Wedd, Anal. Chem. 74 (2002) 3151. [12] S.K. Sukardi, J. Zhang, I. Burgar, M.D. Horne, A.F. Hollenkamp, D.R. MacFarlane, A.M. Bond, Electrochem. Commun. 10 (2008) 250. [13] G.A. Snook, A.S. Best, A.G. Pandolfo, A.F. Hollenkamp, Electrochem. Commun. 8 (2006) 1405. [14] Y. Katayama, in: H.E. Ohno (Ed.), Electrochemnical Aspects of Ionic Liquids, John Wiley & Sons, New Jersey, 2005, p. 31. [15] M. Matsumiya, M. Terazono, K. Tokuraku, Electrochim. Acta 51 (2006) 1178. [16] N. Fietkau, A.D. Clegg, R.G. Evans, C. Villagrán, C. Hardacre, R.G. Compton, Chem. Phys. Chem. 7 (2006) 1041.
[17] [18] [19] [20] [21] [22] [23]
1419
G. Gritzner, Pure Appl. Chem. 62 (1990) 1839. G. Gritzner, Inorg. Chim. Acta 24 (1977) 5. V. Gutmann, G. Peychal-Heiling, Monatsh. Chem. 100 (1969) 813. A. Lewandowski, A. Szukalska, M. Galinski, New J. Chem. 19 (1995) 1259. A.J. Bard, L.R. Faulkner, Electrochemical Methods, John Wiley, 2001, p. 594. C.A. Brooks, A.P. Doherty, Electrochem. Commun. 6 (2004) 867. L. Nagy, G. Gyetvai, L. Kollár, G. Nagy, J. Biochem. Biophys. Methods 69 (2006) 121. [24] S. Eisele, M. Schwarz, B. Speiser, C. Tittel, Electrochim. Acta 51 (2006) 5304. [25] H. Olivier-Bourbigou, L. Magna, J. Mol. Catal., A 182 (2002) 419. [26] D.S. Silvester, R.G. Compton, Z. Phys. Chem. 220 (2006) 1247.