Prospects for a widely applicable reference potential scale in ionic liquids based on ideal reversible reduction of the cobaltocenium cation

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Electrochemistry Communications 10 (2008) 250–254 www.elsevier.com/locate/elecom

Prospects for a widely applicable reference potential scale in ionic liquids based on ideal reversible reduction of the cobaltocenium cation Stevanus K. Sukardi a,b, Jie Zhang c, Iko Burgar d, Michael D. Horne e, Anthony F. Hollenkamp b,*, Douglas R. MacFarlane a, Alan M. Bond a,* a School of Chemistry, Monash University, Clayton, Vic 3800, Australia CSIRO Energy Technology, Box 312, Clayton South, Vic 3169, Australia c Institute of Bioengineering and Nanotechnology, Singapore d CSIRO Materials and Manufacturing Technology, Private Bag 33, Clayton South, Vic 3169, Australia e CSIRO Minerals, Box 312, Clayton South, Vic 3169, Australia b

Received 10 September 2007; received in revised form 20 November 2007; accepted 20 November 2007 Available online 31 December 2007

Abstract Oxidation of ferrocene, Fe(cp)2 or reduction of the cobaltocenium cation, [Co(cp)2]+ represent reversible processes that are widely used to provide a voltammetric potential reference scale. However, the [Fe(cp)2]0/+ process has been reported to exhibit complexities which may restrict its usefulness for this purpose in ionic liquids. In this study, the reduction of [Co(cp)2]+ in the ionic liquids, [bmpyr][Ntf2], [emim][Ntf2], and [bmim][PF6] (bmpyr = 1-butyl-1-methylpyrrolidinium, emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-methylimidazolium, Ntf2 = bis(trifluoromethanesulfonyl)amide) is reported at macro, micro, and rotating disk electrodes. Reversible behaviour, after allowance for ohmic drop, and linear current–concentration relationships are attained over wide concentration ranges for all electrode configurations. Results support the use of the [Co(cp)2]+/0 process for reference potential purposes, with nonidealities of the kind reported for ferrocene not being detected. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Cobaltocenium reduction; Reference potential scale; Ionic liquids

1. Introduction Room temperature ionic liquids (RTILs) offer attractive advantages [1,2] in chemical synthesis [3], liquid crystals [4], separation science [5], and electrochemistry [6]. In the latter case, RTILs may offer wide potential windows, good electrical conductivity and ion transport properties [7]. However, as the range of RTILs expands, attention must be paid to ensuring that electrode potential data can be *

Corresponding authors. Tel.: +61 3 95458903; fax: +61 3 95628919 (A.F. Hollenkamp), tel.: +61 3 99051338; fax: +61 3 99054597 (A.M. Bond). E-mail addresses: [email protected] (A.F. Hollenkamp), [email protected] (A.M. Bond). 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.11.022

compared accurately with values obtained in conventional solvent–electrolyte media. Even though Snook et al. [8] and Katayama [9] have shown that a reliable Ag|Ag+ (RTIL) reference system is available, most electrochemical studies in RTILs have utilised platinum [10] or silver wire [11] quasi-reference electrodes, which are seldom reliable because their potential is sensitive to impurities, the type of electroactive species present, and other factors [12]. In organic solvent media it is now routine to employ voltammetric data from compounds such as ferrocene, Fe(cp)2 and cobaltocenium salts, [Co(cp)2]+X, that exhibit a well-defined, highly reversible redox process, as an ‘internal potential standard’ [13–15]. Whilst the [Fe(cp)2]0/+ couple would seem ideal for this purpose in ionic liquids, reports by Nagy et al. [16], Eisele

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et al. [17], and Brooks et al. [18] have shown unexpected complexities in [bmim][PF6], [bmim][BF4], and [bmim][Ntf2]. While the oxidation current for ferrocene is normally (and reliably) a linear function of concentration in organic electrolyte media, appreciable deviations from linearity have been reported in these studies, which implies that the diffusion coefficient of ferrocene is a function of concentration. In addition, ferrocene is often poorly soluble in ionic liquids [19], and is moderately volatile, which is a problem when elevated temperatures or sub-atmospheric pressures are required. By contrast, cobaltocenium compounds are non-volatile and generally soluble in ionic liquids, where the reduction of the cobaltocenium cation appears to be an ideal reversible process [19]. However, no systematic study on the concentration range over which the [Co(cp)2]+ reduction process remains ideal is available. In light of the recent results obtained for ferrocene, this is an important issue that must be resolved. Herein we report [Co(cp)2]+ electrochemical data obtained with macro, micro, and rotating disk electrodes in widely used ionic liquids, and discuss the extent to which the reduction process displays Nernstian responses, and exhibits linear current versus concentration relationships.

station. The reference electrode was either Ag|Ag+ (10 mM Agtf (tf = triflate) in [bmpyr][Ntf2]) [8], or a Ag wire quasi-reference electrode, while platinum wire served as the auxiliary electrode. Recently, Rogers et al. [13] have reported that the Ag|Ag+ charge-transfer rate in ionic liquids is relatively slow. However, this couple can still provide a suitable reference electrode if the area of the electrode is large and potentiostatic control is used, as in this study. The RDE and microelectrode measurements were undertaken outside the glovebox under dry N2. All measurements were carried out at 23 ± 1 °C. BAS DigiSimÒ version 3.0 software was used to simulate the [Co(cp)2]+/0 reduction process. Viscosity was measured with an Anton Paar Automated Micro Viscometer. The 1H and 13C NMR experiments were performed at 20 °C (magnetic field strength of 7.04 T) with a Varian Unity-Plus 300 NMR Spectrometer. Chemical shifts were referenced to tetramethyl silane (TMS) and spectral shimming was achieved via free induction decay.

2. Experimental

Cyclic voltammetric data obtained at GC, Au, and Pt working electrodes for the reduction of [Co(cp)2]+ in [bmpyr][Ntf2] are summarised in Table 1. The magnitude of the ratio of reduction to oxidation peak currents, ox ired p =ip , is close to unity at all electrodes, concentrations, and scan rates examined, as expected for a chemically reversible process. Cyclic voltammograms obtained with 5, 10, 20, 50, and 100 mM [Co(cp)2][PF6] at a glassy carbon electrode are shown in Fig. 1. In the higher concentration and faster scan rate regimes, peak-to-peak separations red between oxidative (Eox p ) and reductive (E p ) peak potentials, DEp, increase (Table 1) as expected when uncompensated resistance (Ru) is present. However, at low concentration and slow scan rates, where iRu is minimised, DEp approaches the value of 57 mV expected for a reversible one-electron transfer process at 23 °C. The midpoint potential, Emid, was calculated from the ox red ox average of Ered p and E p [Emid = (E p + Ep )/2] and returned a value of (1.73 ± 0.01) V vs. Ag|Ag+ (Table 1). This value is essentially independent of electrode material, scan rate and concentration, so is described as the reversible half-wave potential, Eh [21], which is related to the formal 0 potential (E° ) as in Eq. (1).

Unless otherwise stated, solution preparations and electrochemical measurements were performed in an argonpurged dry box. 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide, [bmpyr][Ntf2] was purchased from Merck (<10 ppm water, <100 ppm chloride). 1-ethyl3-methylimidazolium bis(trifluoromethanesulfonyl)amide [emim][Ntf2] was synthesised according to a literature method [20]. 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] was obtained from Aldrich. [emim][Ntf2] and [bmim][PF6] were dried over basic alumina. Voltammetric measurements on [emim][Ntf2] and [bmim][PF6] showed that the halide content was below the limit of detection (6105 M). Bis(cyclopentadienyl)cobalt(III) hexafluorophosphate, known as cobaltocenium hexafluorophosphate, [Co(cp)2][PF6], was obtained from Aldrich (98% purity), dried under vacuum for 24 h at elevated temperature, and stored in the glove-box. Preparation of P50 mM [Co(cp)2][PF6] in [bmpyr][Ntf2] required overnight stirring for complete dissolution. Total gravimetric and volumetric errors were less than 5%. Cyclic voltammetric studies (Ecochemie lAutolabIII potentiostat), employed 3 mm diameter glassy carbon (GC) and 1.6 mm diameter gold and platinum macrodisk working electrodes. An EG&G Princeton Applied Research Model 363 Potentiostat and a Metrohm 628-10 Rotating Disk Electrode (RDE) assembly were utilised for rotating disk measurements with a 2.9 mm diameter glassy carbon working electrode. Voltammetry with 11 lm diameter carbon fibre and 2 lm diameter platinum electrodes used a BAS Model 100B electrochemical work-

3. Results and discussion 3.1. Cyclic voltammetry in [bmpyr][Ntf2]

  1=2 RT Dox E ¼ Eh þ ln nF Dred 0



ð1Þ

Consequently, Eh is only strictly equal to E°0 if the diffusion coefficients, Dox and Dred, are identical. Studies in ionic liquids have indicated that diffusion coefficients for charged and uncharged species may differ significantly [22,23]. In the present case, if Dox and Dred differ by a factor of two, then the difference between Eh and E°0 would be less than

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Table 1 Summary of cyclic voltammetric data obtained for the [Co(cp)2]+/0 process in [bmpyr][Ntf2] at GC, Au, and Pt electrodes Concentration of [Co(cp)2]+ (mM)

v GC Au Pt (mV s1) DEp Emida DEp Emida DEp Emida (mV) (mV)b (mV) (mV)b (mV) (mV)b 5 10 20 50 100

63 64 65 74 80

1737 1737 1736 1733 1735

62 63 62 67 66

1735 1734 1736 1734 1736

67 64 64 66 74

1735 1737 1738 1738 1736

10

5 10 20 50 100

71 76 84 102 119

1734 1731 1733 1732 1730

73 72 72 72 82

1732 1731 1731 1728 1732

63 72 71 75 76

1731 1733 1732 1730 1731

20

5 10 20 50 100

74 83 87 112 123

1737 1736 1735 1735 1732

78 77 81 85 95

1730 1729 1729 1726 1723

68 72 81 87 92

1726 1726 1727 1723 1725

50

5 10 20 50 100

97 109 127 166 201

1725 81 1724 88 1722 96 1720 112 1715 130

1724 85 1723 90 1725 104 1721 126 1720 147

1727 1728 1726 1725 1725

100

5 10 20 50 100

137 164 202 262 324

1723 1718 1716 1712 1706

1737 1736 1736 1734 1728

1738 1737 1737 1733 1732

5

101 120 139 181 216

98 118 137 172 210

a Calculated from the average of reduction and oxidation peak potentials. b mV vs. Ag|Ag+ (10 mM Agtf in [bmpyr][Ntf2]).

linearly proportional to both the square root of scan rate and concentration, in accordance with the Randles-Sevcik relationship ired p ¼ 0:4463nF



nF RT

12

1 1

AD2 v2 c

ð2Þ

where n (=1) is the number of electrons in the charge-transfer step, A is the electrode area (cm2), D is the diffusion coefficient (cm2 s1), v is the scan rate (V s1), and c is the concentration (mol cm3). In the case of reduction of [Co(cp)2]+ in [bmpyr][Ntf2], the voltammetric data conformed to the Randles-Sevcik equation over a wide range of conditions. Thus reduction peak currents are linearly proportional (Fig. 2) to the square root of scan rate (5– 100 mV s1) and to concentration (5–100 mM). To quantify the effect of uncompensated resistance (iRu), voltammetric simulations for a reversible process were undertaken using experimentally known parameters with Ru set to zero and Dred and Dox = 1.2  107 cm2 s1 (Figs. 1 and 2). Only at current densities above about 1.2 mA cm2 are there significant differences between predicted and experimental peak currents. The value of Ru required for matching of simulated and experimental data was (700 ± 150) X. The data (Fig. 2) are therefore consistent with a concentration-independent diffusion coefficient of (1.2 ± 0.1)  107 cm2 s1 for [Co(cp)2]+ in [bmpyr][Ntf2]. Importantly, none of the complexity reported for the oxidation of ferrocene in ionic liquids has been detected for the [Co(cp)2]+/0 process in [bmpyr][Ntf2]. 3.2. Steady state reduction of [Co(cp)2]+ Cyclic voltammetric data in [bmpyr][Ntf2] obey conventional mass transport theory, but exhibit iRu distortion at high concentrations. In contrast, steady state limiting current data obtained from rotating disk electrode (RDE)

Fig. 1. Comparison of cyclic voltammograms obtained for reduction of [Co(cp)2]+ in [bmpyr][Ntf2] at a 3 mm diameter glassy carbon electrode with a scan rate of 5 mV s1, and simulated voltammograms (___) with c = 100 mM, v = 5 mV s1, Dox = Dred = 1.2  107 cm2 s1, E00 = 1.73 V, Ru = 0.

10 mV. Whilst such a difference cannot be ignored, it does not preclude use of the [Co(cp)2]+/0 process as a suitable potential reference couple in ionic liquids. If it is assumed that mass transport occurs by linear diffusion, then the reduction peak current, ired p (A), should be

Fig. 2. Dependence of [Co(cp)2]+ reduction peak current at a 1.6 mm diameter gold electrode in [bmpyr][Ntf2] on concentration and scan rate. Errors are 5 mM (±4%), 10 mM (±2%), and 20, 50 and 100 mM (±1.5%). R2 values range from 0.9994 to 0.9999 for 5–100 mV s1 data. Simulated data (-D-, 50 mV s1; -h-, 100 mV s1) were obtained using parameters noted in Fig. 1.

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and microdisc electrode voltammetry should be devoid of iRu drop, thereby allowing conformance to the relevant mass transport theory to be analysed in a straightforward manner. A RDE voltammogram for the reduction of [Co(cp)2]+ in [bmpyr][Ntf2] is presented in Fig. 3 (inset). Limiting current data (iss) measured at the point where the response first attains steady state conform closely (Fig. 3) to the Levich equation, 2 1

1

iss ¼ 0:62 n F A c D3 t 6 x2

ð3Þ

using kinematic viscosity (t) = 60 cm2 s1, and with x = rotation rate (rad s1). The diffusion coefficient at 1500 rpm from Eq. (3) was (1.29 ± 0.08)  10-7 cm2 s1. The value was independent of concentration over the range of 10–100 mM and is in excellent agreement with that deduced from cyclic voltammetry [(1.2 ± 0.1)  107 cm2 s1]. Steady state limiting currents could also be obtained for reduction of [Co(cp)2]+ in [emim][Ntf2] at a 11 lm diameter carbon fibre electrode, provided a slow scan rate of 1 mV s1 was used (Fig. 4, inset). The diffusion-controlled iss values measured under these conditions, at 1.6 V (vs. Ag wire) exhibited a linear relationship with [Co(cp)2]+ concentration over the range of 2–300 mM, which suggests that contributions from either electron hopping or migration are not significant under these experimental conditions (both terms should enhance the current as a function of concentration). Application of Eq. (4), I ss ¼ 4 n F D r c

ð4Þ

where r is the radius of the electrode, yields a diffusion coefficient of (3.27 ± 0.02)  10-7 cm2 s1. To investigate whether migration or electron hopping (self-exchange) might contribute in other ionic liquids,

Fig. 3. Dependence of iss on concentration and rotation rate for reduction of [Co(cp)2]+ in [bmpyr][Ntf2] at a 2.9 mm diameter rotating glassy carbon electrode. Errors are 10 mM (±3%), 20 mM (±2%), and 50 and 100 mM (±1.5%). R2 values range from 0.9997 to 0.9999 for 1000–2500 rpm data. Inset: Rotating disk electrode voltammogram for reduction of 20 mM [Co(cp)2]+ at a scan rate of 10 mV s1 and rotation rate of 1500 rpm. Values of iss were determined from best-line fits of the baseline (background) and limiting current regions.

Fig. 4. Dependence of the limiting current on concentration of [Co(cp)2]+ in [emim][Ntf2] at 11 lm carbon fibre microdisc electrode. Scan rate = 1 mV s1. R2 value is 0.9992. Inset: Near steady state voltammogram for reduction of 25 mM [Co(cp)2]+.

the dependence of iss on the [Co(cp)2]+ concentration also was investigated in the much more viscous [bmim][PF6]. A smaller, 2 lm diameter Pt microdisc electrode was required with this ionic liquid to achieve a near steady state response. Values of iss, measured at 1.45 V (vs. Ag wire) were linearly dependent on [Co(cp)2]+ concentration over the range of 5–100 mM, which supports the conclusions reached for [emim][Ntf2]. Application of Eq. (4) led to a diffusion coefficient of (3.73 ± 0.04)  10-8 cm2 s1. In summary, the diffusion coefficient for [Co(cp)2]+ is: (i) (3.27 ± 0.02)  107 cm2 s1 in [emim][Ntf2]; (ii) (1.29 ± 0.08)  107 cm2 s1 in [bmpyr][Ntf2]; (iii) (3.73 ± 0.04)  108 cm2 s1 in [bmim][PF6]. This decreasing trend mirrors an increasing trend in ionic liquid viscosity (34 cP [24], 85 cP [8], and 207 cP [24] for [emim][Ntf2], [bmpyr][Ntf2], and [bmim][PF6] at 25 °C, respectively). Further, a detailed electrochemical study of [Co(cp)2]+ in the distillable ionic liquid, DIMCARB, over a concentration range of 5–250 mM, yielded a diffusion coefficient of 1.2  107 cm2 s1 at a viscosity of 93.4 cP [25]. Clearly, the [Co(cp)2]+/0 process displays near-ideal mass transport controlled behaviour in ionic liquids over a substantial range of viscosities and ionic compositions. The close-to-ideal voltammetry in several ionic liquids, over a reasonably wide concentration range, implies that the addition of [Co(cp)2][PF6] does not significantly alter key properties. Thus the viscosity of [emim][Ntf2] was found to increase by only 1% on addition of 5 mM [Co(cp)2][PF6]. Furthermore, NMR chemical shift data are essentially independent of concentration. Thus, 13C resonances for the ten equivalent cyclopentadienyl carbon nuclei and 1H (cyclopentadienyl) NMR data all lie in the range (87.35 ± 0.65) and (5.62 ± 0.01) ppm, respectively for [Co(cp)2][PF6] concentrations of 5, 10, 20, 50 and 100 mM in [bmpyr][Ntf2]. The lack of appreciable variation in chemical shift for [Co(cp)2]+ nuclei supports the contention that the speciation of this cation does not change significantly over the concentration range of interest.

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4. Conclusions The voltammetry of the cobaltocenium cation, when present as the hexafluorophosphate salt, has been found to provide a near-ideal system for reference potential calibration purposes in a range of ionic liquids. Thus, the [Co(cp)2]+/0 process in [bmpyr][Ntf2] is close to Nernstian at carbon, gold, and platinum electrodes. Importantly, the midpoint potential obtained from cyclic voltammetry is independent of electrode material, scan rate and concentration over wide ranges and linear peak (cyclic voltammetry) or steady state current (RDE or microelectrode voltammetry) dependencies on concentration are observed, as required for a mass transport controlled process. Two caveats noted are: (i) distortion of the voltammograms due to iRu drop may need to be considered in ionic liquids, particularly at higher [Co(cp)2]+ concentrations or scan rates; (ii) emerging evidence of significant differences in the diffusion coefficients of charged and neutral halves of redox couples requires caution when comparing reversible potentials referenced to [Co(cp)2]+/0 in different ionic liquids. Nevertheless, the behaviour of the [Co(cp)2]+/0 system seems inherently simpler than that reported for the [Fe(cp)2]0/+ process and on this basis should provide a superior potential reference redox system. Acknowledgements We thank Drs. Snook and Best for assistance with reference electrode preparation, and Drs. Ruether and Crossley for synthesis of [emim][Ntf2], and Dr. Huang for assistance with viscosity measurements. References [1] T. Welton, Chem. Rev. 99 (1999) 2071. [2] S.A. Forsyth, J.M. Pringle, D.R. MacFarlane, Aust. J. Chem. 57 (2004) 113.

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