The Reduction of Oxygen in Various Room Temperature Ionic Liquids

Chem. B , 2009, 113 (26), pp 8953–8959. DOI: 10.1021/jp903148w. Publication Date (Web): June 5, 2009. Copyright © 2009 American Chemical Society. *...
1 downloads 36 Views 530KB Size
Download PDF
J. Phys. Chem. B 2009, 113, 8953–8959

8953

The Reduction of Oxygen in Various Room Temperature Ionic Liquids in the Temperature Range 293-318 K: Exploring the Applicability of the Stokes-Einstein Relationship in Room Temperature Ionic Liquids Xing-Jiu Huang,† Emma I. Rogers,† Christopher Hardacre,‡ and Richard G. Compton*,† Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom, School of Chemistry and Chemical Engineering/QUILL, Queen’s UniVersity Belfast, Belfast, Northern Ireland BT9 5AG, United Kingdom ReceiVed: April 6, 2009; ReVised Manuscript ReceiVed: May 12, 2009

The voltammetry for the reduction of oxygen at a microdisk electrode is reported in six commonly used RTILs: [C4mim][NTf2], [C4mpyrr][NTf2], [C4dmim][NTf2], [C4mim][BF4], [C4mim][PF6], and [N6,2,2,2][NTf2], where [C4mim]+ is 1-butyl-3-methylimidazolium, [NTf2]- is bis(trifluoromethanesulfonyl)imide, [C4mpyrr]+ is N-butyl-N-methylpyrrolidinium, [C4dmim]+ is 1-butyl-2,3-methylimidazolium, [BF4]- is tetrafluoroborate, [PF6]- is hexafluorophosphate, and [N6,2,2,2]+ is n-hexyltriethylammonium at varying scan rates (50-4000 mV s-1) and temperatures (293-318 K). Diffusion coefficients, D, of oxygen are deduced at each temperature from potential-step chronoamperometry, and diffusional activation energies are calculated. Oxygen solubilities are also reported as a function of temperature. In the six ionic liquids, the Stokes-Einstein relationship (D ∝ η-1) was found to apply only very approximately for oxygen. This is considered in relationship to the behavior of other diverse solutes in RTILs. 1. Introduction The electroreduction of oxygen, O2, is a key reaction in various applications, including metal-air batteries; fuel cells; and electrosynthesis of reactive oxygen species, such as superoxide, O2•-, and hydrogen peroxide, H2O2.1 The one-electron reduction,

O2 + e- h O2•-

(1)

has been extensively researched over the years by many authors and in numerous solvents. The electrogenerated nucleophilic superoxide species, O2•-, is highly reactive and will spontaneously and rapidly disproportionate in protic solvents.2,3

2O2•- + 2H+ f O2 + H2O2

(2)

It has been reported that in the absence of a proton source, the superoxide species is stable, and reversible voltammetry for the reduction of oxygen has been observed in more conventional aprotic media; for example, dimethyl sulfoxide (DMSO),4,5 dimethyl formamide (DMF),4-6 and acetonitrile (MeCN),4-7 as well as in less commonly used aprotic solvents, such as acetone (AT)4 and propylene carbonate.8 More recently, due to the properties associated with room temperature ionic liquids (RTILs), including high thermal and chemical stability, wide electrochemical potential windows, intrinsic conductivity, and negligible volatility,9-12 they have been explored as solvents13 for the electrochemical detection of oxygen. * Corresponding author. Phone: +44(0) 1865 275 413. Fax: +44(0) 1865 275 410. E-mail: [email protected]. † Oxford University. ‡ Queen’s University Belfast.

One of the first studies in RTILs was by Carter and Osteryoung et al.,14 who studied the cyclic voltammetry of oxygen on a glassy carbon (GC) electrode in 1-ethyl-3methylimidazolium chloride aluminum chloride, [C2mimCl][AlCl3], to observe a single reduction peak, which they attributed to the generation of superoxide, and the subsequent irreversible reaction of this electrogenerated species with protic impurities present in the ionic liquid. Al Nashef et al.7 were the first to report the electrogeneration of stable superoxide in [C4mim][PF6] and [C4dmim][NTf2] at a GC electrode, followed by Buzzeo,15 Zhang,1 Katayama,16 and Evans,17 who observed chemically reversible voltammetry for O2 reduction in a range of RTIL solvents, including [N6,2,2,2][NTf2],15,17 [C2mim][NTf2],15,16 [Cnmim][BF4],1 [C4mpyrr][NTf2],16 [Py14][NTf2],17 [P14,6,6,6][NTf2],17 and [P14,6,6,6][FAP].17 The stability of O2•- was suggested to be due to the absence of impurities in the solvents used. The relationship between the diffusion coefficient (D) of a solute and the viscosity (η) of the medium in which it diffuses has been long considered, and provided the species is sufficiently large, then the Stokes-Einstein relationship

D∝

1 η

will apply. The literature reports many examples of this relationship being successfully, or not, applied to diffusive motion in RTILs.18,19 The high-quality voltammetry of O2 available in a diversely wide range of RTILs offers the opportunity for a thorough investigation of the relationship between D and the solvent viscosity for the case of this solute. In particular, the use of potential step chronoamperometry at microdisk electrodes has been established as a means of determining both D and the solute concentration (c) from simple measurements. In particular, for microdisk electrodes, the

10.1021/jp903148w CCC: $40.75  2009 American Chemical Society Published on Web 06/05/2009

8954

J. Phys. Chem. B, Vol. 113, No. 26, 2009

Huang et al. solubilities in the media. The former measurements allow us to evaluate the extent to which Stokes-Einstein behavior is followed in RTILs for oxygen and to draw comparisons with data for other solutes of differing sizes.

Figure 1. Molecular structure of the anions and cations used as the RTIL solvents in this study.

diffusion regime changes from linear to convergent as the time scale of the experiment increases so that at short times, the current scales with D1/2c, whereas at larger times, it scales with Dc. Consequently, by fitting the full current-time transient resulting from a potential step, both D and c can be elucidated. In the present paper, we measure D for O2 as a function of temperature in the six ILs ([C4mim][NTf2], [C4mpyrr][NTf2], [C4dmim][NTf2], [C4mim][BF4], [C4mim][PF6], and [N6,2,2,2][NTf2]; see Figure 1 for structures) and also the oxygen

2. Experimental Section 2.1. Chemical Reagents. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C4mim][NTf2]; 1-butyl-2,3dimethylimidazolium bis(trifluoromethanesulfonyl)imide, [C4dmim][NTf2];andN-butyl-N-methyl-pyrrolidiniumbis(trifluoromethanesulfonyl)imide, [C4mpyrr][NTf2], were prepared following standard literature procedures.20,21 1-Butyl-3-methylimidazolium tetrafluoroborate, [C4mim][BF4], and 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], were kindly donately by Merck and used as received. n-Hexyltriethylammonium bis(trifluoromethanesulfonyl)imide, [N6,2,2,2][NTf2], was prepared using [N6,2,2,2]Br (Aldrich, 99%) for metathesis with Li[NTf2] following standard literature procedures.20 Oxygen was purchased from BOC, Surrey, UK. 2.2. Manufacture of Au Microdisk Electrode. The working electrode was fabricated in-house by thermally sealing a 10µm-diameter Au wire (Goodfellow Ltd., Cambridge, UK) into a borosilicate glass capillary. 2.3. Instrumental. All cyclic voltammetry and chronoamperometry were carried out using a computer-controlled µ-Autolab potentiostat (Eco-Chemie, The Netherlands). A two-electrode arrangement, consisting of a 10-µm-diameter gold working electrode and a 0.5-mm-diameter silver wire (Goodfellow

Figure 2. Typical cyclic voltammograms at a range of scan rates (50, 100, 200, 400, 700, 1000, 2000, and 4000 mV s-1) for the reduction of 1 atm O2 on a 10-µm-diameter gold electrode vs Ag in (a) [C4mim][NTf2], (b) [C4mpyrr][NTf2], (c) [C4dmim][NTf2], (d) [C4mim][BF4], (e) [C4mim][PF6], and (f) [N6,2,2,2][NTf2] at 298 K.

Reduction of Oxygen in RTILs

J. Phys. Chem. B, Vol. 113, No. 26, 2009 8955

Figure 3. Typical cyclic voltammograms at a range of temperatures (293, 298, 303, 308, 313, and 318 K) for the reduction of 1 atm O2 on a 10-µm-diameter gold electrode vs Ag in (a) [C4mim][NTf2], (b) [C4mpyrr][NTf2], (c) [C4dmim][NTf2], (d) [C4mim][BF4], (e) [C4mim][PF6], and (f) [N6,2,2,2][NTf2] at a scan rate of 700 mV s-1.

Figure 4. Potential step chronoamperometric transient for the reduction of O2 in [N6,2,2,2][NTf2] at temperatures of 293, 298, 303, 308, 313, and 318 K on a 10-µm-diameter Au electrode. The potential was scanned from 0.0 to -1.50 V vs Ag.

Cambridge Ltd. UK) quasi-reference, was used throughout. The electrode radius was calibrated by analyzing the steady-state voltammetry of a 2 mM solution of ferrocene in acetonitrile containing 0.1 M TBAP supporting electrolyte. A value of 2.3 × 10-9 m2 s-1 for the diffusion coefficient of ferrocene at 293 K was adopted.22 A plastic collar (section of disposable pipet tip) was attached to the working electrode to form a cavity on the electrode surface into which microlitre quantities (20 µL) of RTIL was added. The electrodes were housed in a T-cell (previously reported)23 specifically designed to allow samples to be studied under a controlled environment. Before the oxygen was added to the cell, the ionic liquid was purged under vacuum

(0.5 Torr) for ∼2 h to remove impurities. Oxygen was passed through a drying column before being introduced into the T-cell via a heated coil inside the temperature-controlled Faraday cage. An outlet gas line led from an exhaust in the box to avoid buildup. A schematic diagram of the temperature-controlled box is shown in a previous paper by Evans et al.17 All temperatures were accurate to (1 K. After being introduced into the cell, the gas was left to diffuse through the ionic liquid until full equilibration between gaseous and dissolved oxygen had occurred. This occurred typically after 10 min for 20 µL of RTIL and was determined by periodically recording cyclic voltammograms until a maximum in the reduction wave had been reached. 2.4. Potential Step Chronoamperometric Experiments. Potential-step chronoamperometry was undertaken using a sample time of 0.001 s. The system was pretreated by holding the potential at a point corresponding to the passage of zero faradic current for 20 s, after which experimental transients were obtained by stepping to a potential after the reduction peak and measuring the current for 0.5 s. Diffusion coefficient and solubility data was extracted from the potential step using the nonlinear curve fitting function in OriginPro 7.5 (Microcal Software Inc.) following the equations listed below, as proposed by Shoup and Szabo.24

I ) -4nFDcrdf(τ)

(3)

8956

J. Phys. Chem. B, Vol. 113, No. 26, 2009

Huang et al.

f(τ) ) 0.7854 + 0.8863τ-1/2 + 0.2146 exp(-0.7823τ-1/2) (4) τ)

4Dt r2d

TABLE 1: Viscosity (cP), Diffusion Coefficient (×10-10 m2 s-1), and Solubility (mM) from Shoup and Szabo24 Analysis of Potential Step Chronoamperometry for the Reduction of O2 at Varying Temperatures on Au Microdisk

(5)

where n is the number of electrons, F is the Faraday constant (96 485 C mol-1), D is the diffusion coefficient, c is the initial concentration, rd is the radius of the microdisk, and t is the time. The equations used in this approximation sufficiently describe current response to within 0.6% error after optimization of the experimental data.

temp, K 293

298

η27 D c

52 7.95 2.7

η27 D c

313

318

RTIL: [C4mim][NTf2] 47 41 34 8.76 10.44 11.77 3.1 2.9 2.9

29 13.41 2.9

15.88 2.7

89 4.60 3.7

RTIL: [C4mpyrr][NTf2] 70 64 54 5.49 6.38 7.41 3.6 3.7 3.7

46 8.66 3.6

9.36 3.8

η27 D c

105 4.30 3.4

RTIL: [C4dmim][NTf2] 88 73 61 5.05 5.74 6.79 3.6 3.7 3.7

52 8.27 3.6

8.95 3.8

η27 D c

112 1.26 4.9

RTIL: 92 1.79 4.1

[C4mim][BF4] 78 65 2.21 2.29 4.1 4.5

56 2.91 4.0

3.29 4.2

η27 D c

201 1.78 2.8

RTIL: [C4mim][PF6] 173 149 127 2.50 2.98 3.54 3.0 3.0 2.7

109 4.49 2.4

5.97 2.8

η28 D c

252 4.25 3.9

RTIL: [N6,2,2,2]NTf2] 192 149 116 4.55 5.29 6.00 3.9 3.9 3.8

92 7.02 3.7

73 7.98 3.9

3. Results and discussion Several different RTILs were chosen as solvents in which to study the reduction of oxygen gas, since it has previously been shown that changing the nature of the cation and anion had a strong effect on the size and shape of the voltammetry for hydrogen oxidation in RTILs.25,26 3.1. Preliminary Observation of Reduction of Oxygen in RTILs at 298 K. Cyclic voltammetry for the reduction of oxygen was studied initially in [C4mim][NTf2] at scan rates of 50, 100, 200, 400, 700, 1000, 2000, and 4000 mV s-1, and the CVs obtained are given in Figure 2a. In this case, the potential was swept from a position of zero voltage down to a potential after the reduction peak (-1.20 V vs Ag) and back to 0.00 V. No prewave is observed prior to the onset of oxygen reduction, confirming that any trace impurities in the ionic liquid are removed by vacuum conditions. A nearly steady-state feature is obtained on the reductive scan (at a peak potential of ca. -1.15 V vs Ag at 4000 mV s-1 in [C4mim][NTf2]), corresponding to the reduction of oxygen to superoxide, in a one-electron process, followed by reoxidation back to oxygen (at a potential of -0.55 V vs Ag at 4000 mV s-1). The presence of the oxidation peak gives evidence that the generated superoxide is stable and the reduction of oxygen is chemically reversible in this system. This procedure was repeated for the remaining five RTILs. The voltammograms obtained are shown in Figure 2b-e. The voltammetry observed in all RTILs was found to be similar, generally with a peak-shaped oxidative feature, and a more steady-state reduction feature. The voltammetry, especially in the more viscous RTILs, is asymmetrical due to the difference in the diffusion type of oxygen and superoxide, (i.e., convergent, quasi-hemispherical vs planar; two- vs one-dimensional Fickian diffusion). Faster diffusion of the neutral O2 leads to more steady-state-like behavior, whereas the slower diffusion of the charged radical anion, O2•- results in a more transient reverse peak.15,17 3.2. Temperature Effect on the Diffusion and Solubility of Oxygen. Figure 3 shows the reduction of 1 atm O2 gas on a 10-µm-diameter Au electrode at a range of temperatures in six RTILs. First, in all six RTILs, the voltammetry is approximately similar, with a more steady-state reduction feature under different temperatures, and a transient oxidative feature on the reverse sweep. The limiting currents are different in four [NTf2]--based RTILs, suggesting that the nature of the cation has a significant effect on the voltammetric behavior. When varying the anion of the RTIL, the minimal variation in limiting currents reveals that the nature of the anion does not have significant influence on the voltammetric behavior. Second, as the temperature is increased, peak current for the reduction of O2 and subsequent oxidation of O2•- increases, which can be attributed to the increasing diffusion coefficients of O2. Third,

303

308

the peak-to-peak separations in all six RTILs decrease with increasing temperature, suggesting faster electrode kinetics. The diffusion coefficient (D) and the solubility (c) of oxygen in all RTILs at different temperatures was determined via chronoamperometry experiments and fitting of the experimental data according to the Shoup and Szabo expression24 (as described in Section 2.4). Figure 4 shows typical experimental (-) potential step transients for the reduction of O2 at 293, 298, 303, 308, 313, and 318 K in [N6,2,2,2][NTf2], with the theoretical fit (O) to the Shoup and Szabo24 approximation. The averaged results from chronoamperometric fitting are listed in Table 1. Viscosity data at each temperature are also included in the table from the literature.27,28 An increase in the diffusion coefficient is observed with increasing temperature, owing, at least in part, to the reduction in viscosity of the solvent media. To corroborate these results, oxygen diffusion was repeated in a few of the RTILs on a 10-µm-diameter Pt electrode under the same conditions. Similar results were observed: for example, 3.67-7.30 × 10-10 m2 s-1 in [C4mpyrr][NTf2], 4.08-9.36 × 10-10 m2 s-1 in [C4dmim][NTf2], 2.12-4.98 × 10-10 m2 s-1 in [C4mim][PF6], and 3.47-6.29 × 10-10 m2 s-1 in [N6,2,2,2][NTf2] in the temperature range 298-323 K. The cyclic voltammograms observed on Pt are shown in the Supporting Information. The values achieved are also in good agreement with those published previously for various RTILs, including 2.10 × 10-10 m2 s-1 at 293 K29 and 3.90 × 10-10 m2 s-1 at 298 K30 in [C4dmim][NTf2], 1.50 × 10-10 m2 s-1 at 293 K15 and 3.2 × 10-10 m2 s-1 at 308 K17 in [N6,2,2,2][NTf2], and 2.20 × 10-10 m2 s-1 at 298 K7 in [C4mim][PF6], and are 1-2 orders of magnitude smaller than the diffusion of oxygen in more conventional solvents; for example, 16-20 × 10-10 m2 s-1 at 298 K31,32 and 47-50 × 10-10 m2 s-1 at 333 K32 in water (1.0 cP), 21-31 × 10-10 m2 s-1 in DMSO3,5,33 (2.0 cP), 48 × 10-10 m2 s-1 in DMF3,5,33 (0.92

Reduction of Oxygen in RTILs

J. Phys. Chem. B, Vol. 113, No. 26, 2009 8957

Figure 5. Stokes-Einstein plots (D vs η-1) for Cc+ (on 10-µm-diameter Pt),36 Fc (on 10-µm-diameter Pt),36 TMPD (on 12.5-µm-diameter Au),37 H2 (on 10-µm-diameter Pt),25 H2S (on 10-µm-diameter Pt),39 and SO2 (on 10-µm-diameter Pt).40

cP), 67 × 10-10 m2 s-1 in AT34 (0.32 cP), and 71-110 × 10-10 m2 s-1 in MeCN3,5,33 (0.34 cP). The solubility of oxygen appears to be essentially independent of temperature (in the range studied) in each case, as shown by the entries in Table 1. 3.3. Stokes-Einstein Behavior of Oxygen. The StokesEinstein equation,35 shown below, predicts an inverse proportionality between the diffusion coefficient, D, of the species and the viscosity, η, of the solvent.

D)

kBT 6πηR

(6)

where kB is the Boltzmann constant (1.38 × 10-23 m2 kg s-2 K-1), T is the temperature, and R is the hydrodynamic radius. The applicability of the Stokes-Einstein relationship has been evaluated in previous studies for cobaltocenium hexafluorophosphate (CcPF6),36 ferrocene (Fc),36 and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD)37,38 in ionic liquids as well as in conventional solvents.35 The data reported in these studies has been replotted as D vs η-1, and the results are shown in Figure 5a, b, and c for Cc+,36 Fc,36 and TMPD,37 respectively. The excellent linearity of all molecules, with R2 > 0.97, suggests that the Stokes-Einstein relation holds well for these species

in RTILs, at least in the sense that D ∝ η-1. Figure 5d shows the D vs η-1 plot for hydrogen (H2).25 It was suggested that the H2 molecule is too small for eq 6 to rigorously apply, although a least-squares correlation coefficient, R2 ≈ 0.61, implies that approximate Stokes-Einstein behavior applies for H2. Conversely, no apparent linear relationship is observed for hydrogen sulfide (H2S)39 or sulfur dioxide (SO2),40 shown in Figure 5e and f, suggesting that the molecules are too small for the relation to apply; that some chemical interactions occur with the RTIL, such as association; or both. Figure 6 shows plots of D vs η-1 for O2 at temperatures of (a) 293, (b) 298, (c) 303, (d) 308, and (e) 313 K. As with H2,25 an approximate relationship was observed when the data was analyzed in terms of Stokes-Einstein behavior (R2 ) 0.5-0.65). 3.4. Calculation of Diffusional Activation Energies for Oxygen. The Arrhenius equation35 below relates the diffusion coefficient of electroactive species, D, to the temperature, T, of the system:

( )

D ) D∞ exp

-Ea,D RT

(7)

where D∞ is a constant corresponding to the hypothetical diffusion coefficient at infinite temperature, R is the universal

8958

J. Phys. Chem. B, Vol. 113, No. 26, 2009

Huang et al.

Figure 6. Plot of diffusion coefficient, D, of O2 against the inverse of viscosity, η-1, for six RTILs studied on Au at temperatures of 293, 298, 303, 308, 313, and 318 K. D values were obtained from theoretical fitting of chronoamperometric transients to the Shoup and Szabo24 expression.

TABLE 2: Diffusional Activation Energy Data for O2 a Au Microdisk Electrode Determined from Plots of ln D vs T-1

-1

Figure 7. Arrhenius plots of ln D vs T over the range of 293-318 K for the diffusion coefficients of O2 on Au in the RTILs [C4mim][NTf2], [C4mpyrr][NTf2], [C4dmim][NTf2], [C4mim][BF4], [C4mim][PF6], and [N6,2,2,2][NTf2].

gas constant (8.314 J K-1 mol-1), and Ea,D is the activation energy of diffusion. Analyzing the diffusion coefficients of oxygen in terms of Arrhenius-type behavior allows determination of the activation energy of diffusion, and Figure 7 shows the resulting plots of ln D vs T-1 for oxygen in all six RTILs. As can be seen, in all cases, a high degree of linearity is observed (least-squares correlation coefficient, R2 > 0.98). The temperature-dependent diffusion coefficients are summarized in terms of activation energy of diffusion in Table 2. It is generally observed that the more viscous ionic liquids have correspondingly larger activation energies, with Ea,D increasing from 21.5 to 33.2 kJ mol-1 with increasing viscosity, a trend that was also

RTIL

Au, Ea,D/kJ mol-1

literature, Ea,η/kJ mol-1

[C4mim][NTf2] [C4mpyrr][NTf2] [C4dmim][NTf2] [C4mim][BF4] [C4mim][PF6] [N6,2,2,2]NTf2]

21.5 22.5 23.5 27.9 33.2 20.2

24.427 25.027 26.127 25.927 24.027 38.628

observed for TEMPO in a range of RTILs at varying temperatures,28 although [N6,2,2,2][NTf2] is an exception to this trend, with an Ea,D of 20.2 kJ mol-1. For the RTILs studied on Pt, comparable diffusional activation energies were observed: 22.5 kJ mol-1 for [C4mpyrr][NTf2], 27.6 kJ mol-1 for [C4dmim][NTf2], 27.8 kJ mol-1 for [C4mim][PF6], and 20.0 kJ mol-1 for [N6,2,2,2][NTf2]. Also included in the table for completeness are values for the activation energy of viscosity for each RTIL, and although they are similar, any deviations between Ea,D and Ea,η are consistent with the “only approximate” linearity when the diffusion coefficients are analyzed in terms of Stokes-Einstein behavior. Note that for TMPD,37,38 where the D/η data closely follows Stokes-Einstein, the activation energies, Ea,D and Ea,η are very similar. 3.5. Electrode Material Effects. It has been suggested previously1,3 that the peak-to-peak separations, ∆Epp in the voltammograms depend largely upon the electrode substrate, and when looking at the CVs obtained for each RTIL on Au

Reduction of Oxygen in RTILs

J. Phys. Chem. B, Vol. 113, No. 26, 2009 8959

TABLE 3: Peak-to-Peak Separation, ∆Epp (V) Data for the Reduction of Oxygen in Different RTILs at Varying Temperatures on Both Au and Pt Microdisks 293 K

298 K

0.342

RTIL: [C4mpyrr][NTf2] 0.332 0.319 0.298 0.288 0.563 0.531 0.453 0.436

0.283 0.422

0.407

Au Pt

0.278

RTIL: [C4dmim][NTf2] 0.275 0.240 0.215 0.200 0.491 0.438 0.408 0.357

0.176 0.334

0.317

Au Pt

0.420

RTIL: [C4mim][PF6] 0.374 0.348 0.335 0.446 0.410 0.380

0.327 0.356

0.344

RTIL: [N6,2,2,2][NTf2] 0.270 0.275 0.271 0.273 0.590 0.545 0.526 0.493

0.282 0.491

0.465

Au Pt

Au Pt

0.278

0.405 0.470

303 K

308 K

313 K

318 K

323 K

compared to Pt (shown in the Supporting Information), it can be generally noted that the voltammetry is more “broad” on the Pt microdisk. Table 3 shows ∆Epp for each ionic liquid on both electrodes. It is observed in all cases that ∆Epp decreases with increasing temperature (except for [N6,2,2,2][NTf2] on Au) with ∆Epp (Pt) > ∆Epp(Au). This has been observed previously in acetonitrile,3 with a 9 mV difference in peak separation for Pt compared to Au, and also in [Cnmim][BF4]1 (where n ) 2, 3, and 4), where an 11 mV difference in peak separation was noted. This at least partly reflects the faster electrode kinetics. Quantitative investigation of the electrode kinetics of the O2|O2•redox couple would be beneficial to aid the understanding of these differences, but modeling of such voltammetry becomes complicated by possible chemical reactions;30 in addition, product adsorption may need to be considered. 4. Conclusions The reduction of oxygen under different temperatures has been studied by cyclic voltammetry and potential-step chronoamperometry in six RTIL solvents. For all RTIL solvents, chronoamperometric transients were found to fit well to an Arrhenius-type relation to give similar activation energies of diffusion of oxygen at different temperatures. Diffusion coefficients and solubilities of oxygen in such media have been reported, and an approximate relationship was observed when analyzed in terms of Stokes-Einstein behavior. In comparison with other solutes, larger molecules were seen to much more closely follow the Stokes-Einstein relationship: H2 was similar to O2 in being very approximate, whereas H2S and SO2 did not follow the Stokes-Einstein equation at all. On the other hand, for molecules such as ferrocene (Fc) and cobaltocenium (Cc+), a very good agreement between D and η-1 was observed, suggesting molecular size is a dominant criterion for the applicability, or otherwise, of the Stokes-Einstein relationship in RTILs. Acknowledgment. X.-J.H. and E.I.R. thank the EPSRC for financial support. Supporting Information Available: Figures showing (1) cyclic voltammetry for the reduction of O2 on a 10-µm-diameter Pt electrode in the RTILs [C4mpyrr][NTf2], [C4dmim][NTf2], [C4mim][PF6], and [N6,2,2,2][NTf2] and (2) plots of ln η vs T-1 for [C4mim][NTf2], [C4mpyrr][NTf2], [C4dmim][NTf2], [C4mim]-

[BF4], [C4mim][PF6], and [N6,2,2,2][NTf2]. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang, D.; Okajima, T.; Matsumoto, F.; Ohsaka, T. J. Electrochem. Soc. 2004, 151, D31–D37. (2) Marklund, S. J. Biol. Chem. 1976, 251, 7504–7507. (3) Sawyer, D. T.; Chiericato, G.; Angelis, C. T.; Nanni, E. J.; Tsuchiya, T. Anal. Chem. 1982, 54, 1720–1724. (4) Peover, M. E.; White, B. S. Electrochim. Acta 1966, 11, 1061– 1067. (5) Wadhawan, J. D.; Welford, P. J.; McPeak, H. B.; Hahn, C. E. W.; Compton, R. G. Sens. Actuators, B 2003, 88, 40–52. (6) Jain, P. S.; Lal, S. Electrochim. Acta 1982, 27, 759–763. (7) Al Nashef, I. M.; Leonard, M. L.; Kittle, M. C.; Matthews, M. A.; Weidner, J. W. Electrochem. Solid-State Lett. 2001, 4, D16–D18. (8) Hills, G. J.; Peter, L. M. Electroanal. Chem. Interfacial Electrochem. 1974, 50, 175–185. (9) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106–1120. (10) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391– 1398. (11) Silvester, D. S.; Compton, R. G. Z. Phys. Chem. 2006, 220, 1247– 1274. (12) Hapiot, P.; Lagrost, C. Chem. ReV. 2008, 108, 2238–2264. (13) O’Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Chem. Eng. Data 2008, 53, 2884–2891. (14) Carter, M. T.; Hussey, C. L.; Strubinger, S. K. D.; Osteryoung, R. A. Inorg. Chem. 1991, 30, 1147–1151. (15) Buzzeo, M. C.; Klymenko, O. V.; Wadhawan, J. D.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Phys. Chem. A 2003, 107, 8872–8878. (16) Katayama, Y.; Onodera, H.; Yamagata, M.; Miura, T. J. Electrochem. Soc. 2004, 151, A59–A63. (17) Evans, R. G.; Klymenko, O. V.; Saddoughi, S. A.; Hardacre, C.; Compton, R. G. J. Phys. Chem. B 2004, 108, 7878–7886. (18) Ngai, K. L. J. Phys. Chem. B 2006, 110, 26211–26214. (19) Ito, N.; Richert, R. J. Phys. Chem. B 2007, 111, 5016–5022. (20) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168–1178. (21) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. J. Phys. Chem. B 1999, 103, 4164–4170. (22) Sharp, M. Electrochim. Acta 1983, 28, 301–308. (23) 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, 1009–1015. (24) Shoup, D.; Szabo, A. J. Electroanal. Chem. Interfacial Electrochem. 1982, 140, 237–245. (25) Silvester, D. S.; Ward, K. R.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Electroanal. Chem. 2008, 618, 53–60. (26) Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. B 2007, 111, 5000–5007. (27) Okoturo, O. O.; VanderNoot, T. J. J. Electroanal. Chem. 2004, 568, 167–181. (28) Evans, R. G.; Wain, A. J.; Hardacre, C.; Compton, R. G. ChemPhysChem 2005, 6, 1035–1039. (29) Buzzeo, M. C.; Hardacre, C.; Compton, R. G. Anal. Chem. 2004, 76, 4583–4588. (30) Barnes, A. S.; Rogers, E. I.; Streeter, I.; Aldous, L.; Hardacre, C.; Wildgoose, G. G.; Compton, R. G. J. Phys. Chem. C 2008, 112, 13709– 13715. (31) Evans, N. T. S.; Quinton, T. H. Respir. Physiol. 1978, 35, 89–99. (32) Davis, R. E.; Horvath, G. L.; Tobias, C. W. Electrochim. Acta 1967, 12, 287–297. (33) Vasudevan, D.; Wendt, H. J. Electroanal. Chem. 1995, 192, 69– 74. (34) Schumpe, A.; Lu¨ehring, P. J. Chem. Eng. Data 1990, 35, 24–25. (35) Compton, R. G.; Banks, C. E. Understanding Voltammetry; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2007. (36) Rogers, E. I.; Silvester, D. S.; Poole, D. L.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2007, 112, 2729–2735. (37) Evans, R. G.; Klymenko, O. V.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Electroanal. Chem. 2003, 556, 179–188. (38) Long, J. S.; Silvester, D. S.; Barnes, A. S.; Rees, N. V.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 6993–7000. (39) O’Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 7725–7730. (40) Barrosse-Antle, L. E.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 3398–3404.

JP903148W