Investigations of Fast Electrode Kinetics for Reduction of 2,3,5,6

Article pubs.acs.org/JPCC

Investigations of Fast Electrode Kinetics for Reduction of 2,3,5,6Tetrafluoro-7,7,8,8-tetracyanoquinodimethane in Conventional Solvents and Ionic Liquids Using Fourier Transformed Large Amplitude Alternating Current Voltammetry Kiran Bano, Jie Zhang,* and Alan M. Bond* School of Chemistry, Monash University Clayton, Victoria 3800, Australia S Supporting Information *

ABSTRACT: Fourier transformed large amplitude alternating current voltammetry has been used under high frequency (up to 1.233 kHz) conditions to probe the fast electron transfer kinetics associated with the reduction of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) in the molecular solvents acetonitrile and dimethylsulfoxide at glassy carbon, platinum, gold and boron doped diamond macrodisk electrodes and in ionic liquids (ILs) at carbon fiber and platinum microdisk electrodes. The limitations encountered with measurements under high frequency conditions are discussed in detail. Electrode kinetic data obtained for the F4TCNQ0/•− process in the molecular solvents (0.060−1.0 cm s−1) are compared with results found in 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide, and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids (0.0030−0.10 cm s−1). The effect of solvent viscosity ranging from 0.30 to 371 cP on mass transport is substantial. The influences of the electrode material and structure of the cation of the ionic liquid on the electrode kinetics also have been established.

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INTRODUCTION A quantitative understanding of the fundamental properties of an electrode process requires a detailed knowledge of electrode kinetics, thermodynamics and mass transport.1−11 Many techniques, including voltammetry (transient and steadystate) and impedance spectroscopy have been used7,12−17 in this active area of research. In transient DC cyclic voltammetry, Nicholson’s method7,13 that uses separation of peak potentials (ΔEp) as a function of scan rate, has been widely used to determine the electron transfer kinetics. However, ΔEp values are also strongly affected by uncompensated resistance (Ru, ohmic drop) that increases the peak separation in a manner similar to slow electron transfer kinetics, so errors from this source are difficult to define.12 This method also requires many experiments and only a very limited number of data points i.e. peak potential values are used for data analysis, even though many thousands of data points are usually collected. For the latter reason, simulation of the entire voltammogram and comparison with the experimental data is the presently preferred option. In order to quantify fast electrode kinetics, ideally, the experimental time scale should be at least 5−10 times the cell time constant (RC time constant where R is the resistance and C is the capacitance).12 This criterion is most easily met by decreasing the electrode size to the micro or even nano level. Microelectrodes exhibit faster double layer charging and higher © 2014 American Chemical Society

mass transport rate with reduced iRu drop at short time scale (high scan rate). These characteristics reduce the cell time constant in transient voltammetry and facilitate the study of fast electrode kinetics (heterogeneous charge transfer rate constant, k0 > 0.10 cm s−1)18−22 under transient conditions at fast scan rate and steady-state conditions at slow scan rates.23 Under steady-state conditions, the difference of two quartile potentials (E1/4 − E3/4) may be used as a function of electrode radius in a similar manner to the Nicholson’s method derived for cyclic voltammetry or full simulation experiment comparison undertaken.24 Alternatively, steady-state may be achieved by using hydrodynamic voltammetry25−30 in flowing solutions. This increases the mass transport and allows k0 values to be measured in the 1−10 cm s−1 range, when the diffusion coefficient is about 10−5 cm2 s−1. An alternative approach to transient DC voltammetry is to use AC techniques.31,32 Impedance spectroscopy is a special case of this approach, typically when a small amplitude is used to obtain the impedance and models based on equivalent circuits are used to provide the theoretical basis.33−35 In AC voltammetry, a sinusoidal perturbation of a known frequency and amplitude is superimposed onto the DC potential. As the Received: February 21, 2014 Revised: April 3, 2014 Published: April 7, 2014 9560

dx.doi.org/10.1021/jp501862n | J. Phys. Chem. C 2014, 118, 9560−9569

The Journal of Physical Chemistry C

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calcium hydride and distilled under nitrogen, and n-tetrabutyl ammonium hexafluorophosphate (Bu4NPF6, 98%, Wako) was recrystallized twice from ethanol. All the chemicals and solvents were stored under nitrogen in a glovebox. Structures of F4TCNQ and Ionic Liquids.

time scale is different for DC and AC measurements, the DC component can be separated from AC component. If the amplitude of the AC perturbation is sufficiently large, the resulting higher harmonic components that are devoid of capacitive current, can provide kinetically sensitive information. This large amplitude technique has been employed with macrodisk electrodes under either stationary32,36,37 or hydrodynamic conditions38,39 where the electrode kinetic parameters of interest can be quantified, along with capacitance, by comparing simulated and experimental AC voltammograms. However, it has not been widely applied with microelectrodes.40,41 To date, large amplitude AC voltammetry has been used at relatively low frequencies of up to 200 Hz.36−38 In principle, use of higher frequencies should provide greater kinetic sensitivity but the large currents lead to a large total (faradaic plus double layer charging) current and hence significantly larger iRu drop which reduces the number of kinetically sensitive harmonics that are accessible at macrodisk electrodes. However, the use of microelectrodes with AC voltammetry in ILs should provide a useful approach to minimize the iRu drop and achieve high electrode kinetic sensitivity as is well-known in DC voltammetry. In this study, reduction of 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ, structure I) has been studied as an example of a fast electron transfer process using high frequency large amplitude AC voltammetry in the conventional molecular solvents acetonitrile and dimethyl sulfoxide and in ionic liquids that in summation provide a range of viscosities and other properties that could modify the rate of electron transfer. Electrode kinetics data for reduction of F4TCNQ were obtained in the conventional molecular solvents at platinum (Pt), glassy carbon (GC), boron doped diamond (BDD), and gold (Au) macrodisk electrodes (d = 1.0 mm or 1.5 mm). In contrast, in ionic liquids, microelectrodes (d = 125 or 33 μm) were used. In highly viscous ILs, smaller diffusion coefficient (slow diffusion rates) results in longer time being required to reach steady-state than in molecular solvents.13 The relative contribution of the linear and radial diffusion at a disk electrode depends on (D/r2), where D is the diffusion coefficient, and r is the electrode radius.4 Since D is small in ionic liquids, it follows that even if the electrode radius is small, then so is the D/r2 term. Consequently, a transient response at microelectrode should be readily achieved in an ionic liquid and the AC voltammetric data (at least the AC components) still modeled using theory for linear diffusion, as previously discovered with hydrodynamic voltammetry.38 The use of microelectrodes for kinetics investigation in ILs at high AC frequencies is described for the first time.

Instrumentation and Procedures. All electrochemical experiments were carried out in a glovebox, under a nitrogen atmosphere at room temperature (T = 20 ± 2 °C). A standard electrochemical cell with a three electrode configuration was used. Working electrodes were either macrodisk [platinum (Pt), d = 1.0 mm, gold (Au), d = 1.5 mm, glassy carbon (GC), d = 1.0 mm, or boron-doped diamond (BDD), d = 1.0 mm)] or microdisk [Pt (d = 125 μm) or carbon fiber (d = 33 μm)]. The effective areas (A) of the macrodisc electrodes were calculated from the slope of Q vs t1/2 plots derived from chronocoulometric reduction of 1.0 mM [Fe(CN)6]3− in aqueous (0.10 M KCl) using the known diffusion coefficient value of 7.6 × 10−6 cm2 s−1.31 The areas of the 1.0 mm diameter electrodes (Pt, GC, and BDD) and 1.50 mm diameter Au electrode were found to be 7.85 × 10−3 cm2 and 1.9 × 10−2 cm2, respectively. The geometric areas of the Pt and carbon fiber microelectrodes of 1.22 × 10−4 cm2 and 8.54 × 10−6 cm2, respectively, were taken as the true values. Working electrodes were polished with aqueous 0.05 μm alumina slurry on a clean polishing cloth, then consecutively washed, rinsed with water, 0.05 M nitric acid, water and acetone and dried under nitrogen. Platinum wire was used for auxiliary electrode and another platinum wire was used as a quasireference electrode. Potentials were then calibrated against the Fc0/+ process (Fc = ferrocene) by addition of ferrocene into the solution containing F4TCNQ and measuring the reversible potential for the F4TCNQ0/•− and Fc0/+ processes in the same experiment. DC cyclic voltammetric experiments were undertaken with CHI 400B electrochemical workstation and large amplitude FT AC voltammetric ones with home-built instrumentation.42 FT AC voltammetric experiments employed

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CHEMICALS AND EXPERIMENTAL PROCEDURE Reagents. 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ, 97%, Sigma-Aldrich), ferrocene (Fc, ≥98%, Aldrich), 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPY][TFSI], 99%, IoLiTec), 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([BMPIP][TFSI], 99%, IoLiTec), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6], 99%, Merck), KCl (99.0%, Sigma-Aldrich) and K3[Fe(CN)6] (99.0%, Sigma-Aldrich) were used as received from the supplier. Acetonitrile (MeCN, 97%, Aldrich) and dimethyl sulfoxide (DMSO, 99%, Sigma-Aldrich) were dried over anhydrous 9561



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from the RC time constant where no faradaic current was present; E0 was estimated from current minima of the second harmonic; Cdl was quantified from the background current in the fundamental harmonic and k0 and α were extracted from a simulation experiment comparison of higher harmonic components. In order to define Cdl, a nonlinear capacitor model was used where necessary, using the procedure described elsewhere.42,47 Values of D were estimated as described in Results.

a sine wave having an amplitude of 80 mV and frequencies of 9.02 Hz, 233.95 Hz, or 1.233 kHz. Uncompensated resistance and nonlinear capacitance values were extracted from experimental FT AC voltammetric data by background fitting using RC time constant method in DigiPot (software provided with FT AC voltammetric instrumentation).

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SIMULATIONS AND AC DATA ANALYSIS Simulations of FT AC voltammograms were undertaken with in-house software MECSim (Monash Electrochemistry Simulator) written in Fortran. This software uses the computationally efficient expanding spatial grid formulation43 and is based on the mathematical approach described by Rudolph44 with minor alterations. Simulations used electrode kinetics based on the Butler−Volmer theory12 and applied to the electron transfer reaction given in eq 1

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RESULTS Theoretical Analysis of Kinetic Sensitivity. A comprehensive theoretical analysis has been carried out to compare the upper limit of detection of k0 that could be available at a macroelectrode (d = 1.0 mm) and a microelectrode (d = 125 μm) using large amplitude FT AC voltammetry. To match with the experimental data, the uncompensated resistance measured when using a 1.0 mm diameter macro disk electrode is assumed to be 500 ohm and that with 125 μm diameter electrode is taken to be 5000 ohm in theoretical studies under conditions where D = 2.0 × 10−5 cm2 s−1. The k0 value associated with a process that produces peak current of 90% of a reversible value is arbitrary defined as the upper limit of detection (LOD). For the microelectrode (d = 125 μm) with D = 2.0 × 10−5 cm2 s−1; simulated peak currents of the sixth harmonic are plotted as a function of k0 using the parameters given in Figure 1. Under the experimental conditions used in this study, the

kf

O + e− ⇌ R

(1)

kb

where kf and kb are the potential dependent rate constants for the forward and backward electron transfer reactions respectively. kf and kb may then be written in the terms of k0 and α (charge transfer coefficient) as shown in eqns 2 and 3, where k0 value is the heterogeneous charge-transfer rate constant at the formal reversible potential E0 (vs the reference electrode Fc0/+ scale) and E is the electrode potential. The charge transfer coefficient, α is a dimensionless parameter and it defines the shape of the reaction coordinates, F is the Faraday constant, R is the universal gas constant, and T is the absolute temperature. 0

k f = k0e−αF / RT(E − E )

(2) 0

k b = k0e(1 − α)F / RT(E − E )

(3)

The theoretical FT AC voltammetric data were obtained by computing the spatial and time-dependent concentrations using the linear diffusion equation ∂C ∂ 2C =D 2 ∂t ∂x

(4)

where C and D are the bulk concentration and the diffusion coefficient of O and x is the distance of species O from the electrode surface. Equation 4 can be solved numerically by applying initial and boundary conditions using the fully implicit finite difference method with the Richtmeyer modification.45 Then, current I can be calculated as46 ⎛ ∂C ⎞ I(t ) = FAD⎜ ⎟ ⎝ ∂x ⎠x = 0

Figure 1. Peak currents for the 6th harmonic vs log10 (k0) obtained with a 125 μm diameter microelectrode at frequencies of (a) 9.0 Hz and (b) 1.233 kHz, ΔE = 80 mV, ν = 0.10 V s−1, T = 293 K, C = 2.0 mM, Ru = 5000 ohm, D = 2.0 × 10−5 cm2 s−1, Cdl = 10 μF cm−2, α = 0.50, and A = 1.22 × 10−4 cm2.

shorter time scale sixth and higher harmonic responses of interest are influenced predominantly by the analyte in the diffusion layer adjacent to the electrode surface,10 where mass transport is essentially governed solely by linear diffusion. Consequently, in simulations mass transport can be assumed to be governed by linear diffusion, even though radial diffusion also contributes (strongly) to the DC component. The upper limit of k0 quantification was determined at low (9.0 Hz) and high (1.233 kHz) frequencies from ip (maximum current of sixth harmonic) versus log10 (k0) plots. Kinetic sensitivity available at this microelectrode is deduced from Figure 1 on the basis of assessment of the region where peak currents of sixth harmonic become independent of k0 and hence when the electron transfer process is considered to be reversible. From

(5)

AC voltammetric data obtained experimentally or by simulation were then subjected to data analysis in which time domain data are converted to the frequency domain using a Fourier transformed algorithm to give the power spectrum.42 Frequencies in the region containing the AC harmonics and aperiodic DC component in the power spectrum were then subjected to band filtering to provide resolved DC or AC components as a function of time. Five key parameters are required to obtain the electrode kinetics via an experiment/ theory comparison exercise, namely, Ru , E0, D, k0, α, and Cdl (double layer capacitance). Ru was determined experimentally 9562



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this analysis it is concluded that the use of a 125 μm radius microelectrode at 9.0 Hz allows k0 values up to about 0.90 cm s−1 to be determined, as shown in Figure 1a. In contrast, at the higher frequency of 1.233 kHz, and on the basis of data analysis, as shown in Figure 1b, the upper limit available for the determination of k0 increases to 7.8 cm s−1, as expected. This increase in the upper limit of detection available when a much higher frequency AC perturbation is applied is clearly indicated by the consideration of the dimensionless form of the heterogeneous electron transfer rate constant, k0/(f D)1/2,36 which summarizes the combination of factors that influence the rate constant determination. For the macroelectrode (d = 1.0 mm), simulated peak currents for the sixth harmonic were examined as a function of k0 using parameters C = 2.0 mM, Ru = 500 ohm, D = 2.0 × 10−5 cm2 s−1, α = 0.50, Cdl = 10 μF cm−2, T = 293 K, A = 7.85 × 10 −3 cm 2, and Ru = 127 ohm. The upper limit of k0 quantification was determined both at low (9.0 Hz) and high (1.233 kHz) frequency from ip versus log10 (k0) graph given in Figure 2. It is deduced from the graph that kinetic sensitivity at

Figure 3. Log10(power) spectrum obtained theoretically (a) at a macroelectrode (d = 1.0 mm, Ru = 5000 ohm) and (b) at microelectrode (d = 125 μm, Ru = 35000 ohm); other parameters include C = 5.0 mM, f = 1.233 kHz, ΔE = 80 mV, ν = 0.10 V s−1, T = 293 K, D = 1.0 × 10−8 cm2 s−1, k0 = 1.0 cm s−1, α = 0.50, and Cdl = 10 μF cm−2.

0.017 cm s−1 can be measured, while at 1.233 kHz, the upper limit increases to 0.10 cm s−1.

Figure 2. Peak currents for the 6th harmonic vs log10 (k0) obtained at frequencies of (a) 9.0 Hz and (b) 1.233 kHz, ΔE = 80 mV, C = 2.0 mM, ν = 0.10 V s−1, Ru = 500 ohms, T = 293 K, D = 2.0 × 10−5 cm2 s−1, Cdl = 10 μF cm−2 α = 0.50, and A = 7.85 × 10−3 cm2. Figure 4. Peak currents for the 6th harmonic vs log10 (k0) obtained at frequencies (a) 9.0 Hz and (b) 1.233 kHz, ΔE = 80 mV, ν = 0.10 V s−1, T = 293 K, C = 5.0 mM, Ru = 35000 ohm, D = 1.0 × 10−8 cm2 s−1, Cdl = 10 μF cm−2, and A = 1.22 × 10−4 cm2.

1.233 kHz is 4.0 cm s−1 as compared to 0.90 cm s−1 at 9.0 Hz. The latter is the same as that obtained with microelectrodes, while the former is substantially smaller. This is expected since the uncompensated resistance effect is more significant under high frequency conditions. Analysis of data provided in the above simulations indicates that the use of a microelectrode at high frequency provides advantages in the study of fast electrode kinetics with large amplitude AC voltammetry. However, other factors also affect the kinetic sensitivity and a major one is D, as suggested by the dimensionless form of the rate constant. Thus, with a low D value of 1.0 × 10−8 cm2 s−1, as occurs in ILs, little information is accessible at the macroelectrode (Figure 3a), since only three harmonics can be obtained with adequate signal-to-noise ratio due to the larger uncompenstated resistance effect in this more viscous/resistive medum. In contrast, at a microelectrode, eight harmonics are found as shown in Figure 3b, suggesting that it is crucial to use microelectrode for fast electrode kinetic measurements in ILs. Kinetic sensitivity analysis was also undertaken with D = 1.0 × 10−8 cm2 s−1, as is relevant to ionic liquid media. Figure 4 shows that at 9.0 Hz, k0 values only up to

Voltammetric Reduction of F4TCNQ. F4TCNQ undergoes two consecutive one electron reduction processes,48 designated as the F4TCNQ0/•− (neutral to radical anion) and the F4TCNQ•−/2− (radical anion to dianion) processes, as given in eqns 6 and 7, F4 TCNQ + e− ⇌ F4 TCNQ•− •−

F4 TCNQ

−

+ e ⇌ F4 TCNQ

2−

(6) (7)

In this paper, only the electrode kinetics related to the initial F4TCNQ0/•− process has been studied. DC cyclic voltammograms of F4TCNQ in molecular solvents and ILs are shown in Figure 5. The diffusion coefficient of F4TCNQ has been calculated from the peak current for the reduction of F4TCNQ in MeCN and DMSO (Figure 5a,b) and the ILs [BMIM][PF6], 9563



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theory versus experiment, while α, the charge transfer coefficient, was assumed to be 0.50. Excellent agreement between simulated and experimental data is obtained (Figure 6) with k0 = 1.0 cm s−1 at a Pt

Figure 5. DC voltammograms at Pt electrode (d = 1.0 mm): (a) 0.90 mM F4TCNQ in 0.10 M Bu4NPF6 in acetonitrile; (b) 2.20 mM F4TCNQ in 0.10 M Bu4NPF6 in DMSO; (c) 7.24 mM F4TCNQ in [BMPY][TFSI]; (d) 7.0 mM F4TCNQ in [BMIM][PF6]; (e) 4.90 mM F4TCNQ in BMPIP TFSI, ν = 0.10 V s−1.

Figure 6. Comparison of experimental (black line) and simulated (red line) FT AC voltammograms for the reduction of 1.9 mM F4TCNQ in acetonitrile (0.10 M Bu4NPF6) at a Pt macrodisk electrode: (a) DC component, (b−g) 1st to 6th harmonic, 6th harmonic for a reversible case (blue line) is also shown for comparison. Simulation parameters are provided in Table 2.

[BMPY][TFSI], and [BMPIP][TFSI] (Figure 5c−e) using the Randles−Sevcik relationship, ⎛ nFDv ⎞1/2 ⎟ i p = −0.4463nFA⎜ C ⎝ RT ⎠

(8)

electrode along with other parameters A = 7.85 × 10−3 cm2, Cdl (c0 = 11.0, c1 = 1.0) μF cm−2, Ru = 525 ohm, and α = 0.50. However, this k0 value is very close to the reversible limit, as shown in Figure 6g, and given experimental uncertainties, the process probably should be considered to be reversible. At the gold electrode k0 = 1.0 cm s−1 along with A = 0.019 cm2, Cdl = 20.5 μF cm−2, and Ru = 370 ohm provided good agreement with the theory (Figure S1). At a macrodisk GC electrode using A = 7.85 × 10−3 cm2, Cdl (c0 = 28.0, c1 = 14.0, c2 = 1.50) μF cm−2, and Ru = 590 ohm, k0 was estimated to be 0.90 cm s−1 (Figure S2). At a BDD electrode, simulations with k0 = 1.0 cm s−1, A = 7.85 × 10−3 cm2, Ru = 546 ohm, and Cdl = 6.0 μF cm−2 were used to obtain excellent agreement with experiment (Figure S3). The k0 values of about 1.0 cm s−1 (Table 2) are very close to the upper limit of measurement. Therefore, it could be argued

where ip is the reduction peak current, n is the number of electrons transferred (n = 1), C is the bulk solution concentration, D is the diffusion coefficient of F4TCNQ, T is the absolute temperature, R is the universal gas constant, F is Faraday’s constant, and A is the electrode area. These diffusion coefficient data are summarized in Table 1. The diffusion Table 1. Diffusion Coefficients of F4TCNQ and Ferrocene in Molecular Solvents and Ionic Liquids

a

solvent

viscosity at 293 K (cP)

BMIMPF6 BMPIPTFSI BMPYTFSI DMSO MeCN

381 (ref 50) 183a 94.4a 1.98 (ref 52) 0.35 (ref 54)

F4TCNQ diffusion coefficient (cm2 s−1) 7.0 1.2 3.9 2.7 2.0

× × × × ×

10−9 10−8 10−8 10−6 10−5

Fc diffusion coefficient (cm2 s−1) 7.6 × 10−9 (ref 51) 3.7 × 10−8 7.3 × 10−8 4.4 × 10−6 (ref 53) 2.4 × 10−5 (ref 55)

Table 2. Parameters Useda in Simulations Undertaken to Estimate the Electrode Kinetics for the Reduction of 1.9 mM F4TCNQ in MeCN (0.10 M Bu4NPF6)

As provided by the supplier

coefficient for ferrocene in the same media is also included for comparison. Both D values increase as the viscosity of the solvent decreases, which is in agreement with the Stokes− Einstein equation.49 FT AC Voltammetry in MeCN at Macrodisk Electrodes. The heterogeneous electron transfer kinetics for the F4TCNQ0/•− process in MeCN (0.10 M Bu4NPF6) at four electrodes (GC, Pt, Au and BDD) was initially studied by FT AC voltammetry over frequencies ranging from 9.0 to 234 Hz. As expected, the higher kinetic sensitivity available at higher frequencies is offset to some degree by the need to accommodate the larger iRu drop and capacitance current. In initial experiments with f = 234 Hz and ΔE = 80 mV, simulations employed experimentally derived values of A, Ru, and D. The k0 values were then estimated by comparison of

electrode

Ru (ohm)

Cdl (c0, c1, c2; μF cm−2)

k0 (cm s−1)

E0 vs Fc0/+ (V)

Pt Au GC BDD

525 370 580 546

11.00, 1.00, 0.00 20.50, 3.00, 1.50 28.00, 14.0, 1.50 6.00, 0.00, 0.00

1.00 1.00 0.90 1.00

0.289 0.289 0.289 0.289

AAu = 1.9 × 10−2 cm2, AGC = ABDD = APt = 7.85 × 10−3 cm2, f = 234 Hz, ΔE = 80 mV, D = 2.0 × 10−5 cm2 s−1, α = 0.50, and T = 293 K. a

that this lack of electrode material dependence could be attributed to the large uncertainty associated with the measurements. However, this is unlikely to be the case in the sense that difference between the density of states associated with these electrode materials is over 3 orders of magnitude.56 9564



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Consequently, even if the true k0 at Pt is as high as 100 cm s−1, the predicted k0 value (∼0.10 cm s−1) based on the density of the state argument at BDD will be well within the detection limit region and should be accurately measurable if the F4TCNQ0/•− process is strictly nonadiabatic as for example found for the reduction of [Ru(NH3)6]3+/2+ process in aqueous electrolyte media.56 Therefore, the lack of electrode material dependence suggests that the F4TCNQ0/•− process in acetonitrile is adiabatic. FT AC Voltammetry in DMSO at Macrodisk Electrodes. FT AC voltammetric studies were also undertaken at macrodisk electrodes in DMSO (0.10 M Bu4NPF6) with 1.55 mM F4TCNQ. DMSO has a higher viscosity than acetonitrile. Ru values are also significantly higher, which limits the frequency that can be used to study the electrode kinetics. At low frequencies (9−36 Hz), eight to nine harmonics can be obtained with excellent signal-to-noise ratio, while at higher frequencies, not only does the number of accessible harmonics decrease, but the larger iRu drop and higher capacitance current introduce more uncertainties in the measurement of the electron transfer kinetics. Therefore, at a low frequency of 9.0 Hz DMSO was used, while the amplitude was retained at 80 mV. Under these low frequency conditions, the k0 value at the Pt macrodisk electrode is estimated to be 0.10 cm s−1 by the comparison of experiment and theory (Figure 7) again

On the basis of the above analysis, k0 values in DMSO, as summarized in Table 3, are estimated to be an order of Table 3. Parameters Useda in Simulations Undertaken to Estimate the k0 Values for the Reduction of 1.55 mM F4TCNQ in DMSO (0.10 M Bu4NPF6) electrode

Ru (ohm)

Pt Au GC BDD

2850 1600 2650 2200

Cdl (c0, c1, c2; μF cm−2) 22.0, 39.5, 39.5, 6.00,

40.0,41.0 40.5, 36.5 40.5, 36.0 0.00, 0.00

k0 (cm s−1)

E0 vs Fc0/+ (V)

0.10 0.15 0.11 0.06

0.228 0.228 0.228 0.228

AAu = 1.9 × 10−2 cm2, AGC = ABDD = APt = 7.85 × 10−3 cm2, f = 9.0 Hz, ΔE = 80 mV, D = 2.7 × 10−6 cm2 s−1, α = 0.50, and T = 293 K. a

magnitude lower than in MeCN, qualitatively consistent with the theoretical prediction of the solvent dynamic effect.57 Again, no significant electrode dependence was detected, but k0 values obtained at the lower frequency are still close to the limit of detection (Figure 7). While, k0 at BDD is slightly lower than at Pt or Au, the difference is not as large as predicted on the basis of densities of electronic states. Ideally, use of a higher frequency would be used to improve the reliability of determining electrode kinetics at metal electrodes as compared with carbon, but this is not as readily accessible with macroelectrode studies in DMSO due to the problems described above. FT AC Voltammetry of F4TCNQ in Ionic Liquids Using Macro- and Microelectrodes. In preliminary investigations, large amplitude FT AC voltammetry was applied to the reduction of 7.24 mM F4TCNQ in [BMPY][TFSI] at a Pt macro (d = 1.0 mm) electrode. The results obtained at macrodisk electrode with f = 1.83 kHz and ΔE = 80 mV are given in Figure S7. Under these conditions, the aperiodic DC component is well defined, but first harmonic contains a large contribution from capacitance current. The second to fourth AC harmonics also can be extracted, but contain a significant contribution from the background current and noise. First and higher harmonics are now too noisy for quantitative analysis. Thus, the k0 value cannot be determined reliably due to the large contribution from iRu drop, double layer capacitance current and noise. In contrast, at platinum (d = 125 μm) and carbon fiber microdisk (d = 33 μm) electrodes, the iRu drop is much smaller as is the noise. Now, seven well defined harmonics can be obtained in [BMPY][TFSI] with a frequency of 1.83 kHz. At the smaller electrode (d = 33 μm) and with f = 1.233 kHz, the DC component is relatively small and difficult to measure accurately, but the signal-to-noise ratio for the first seven AC harmonic components is excellent. In order to quantify the kinetics in ILs, frequencies of up to 1.233 kHz could be used to obtain an adequate number of harmonics in ionic liquids with the use of microelectrode. Transient DC and AC voltammetry with planar diffusion being dominant are readily achieved in ionic liquids at moderate DC scan rate (ν = 0.10 V s−1), because D values are very small. Quantitative Voltammetric Studies at Microdisk Electrodes in [BMIM][PF6]. AC voltammetric data obtained for the reduction of 7.0 mM F4TCNQ in [BMIM][PF6] (η = 371 cP) at the platinum microdisk electrode were initially examined at 9.0 Hz, 1.233 kHz, and 1.833 kHz. Eight harmonics can be obtained at 9.0 Hz with acceptable signal-

Figure 7. Comparison of experimental (black line) and simulated (red line) FT AC voltammograms for the reduction of 1.55 mM F4TCNQ in DMSO (0.10 M Bu4NPF6) at a Pt macrodisk electrode: (a) DC component, (b−h) 1st to 7th harmonic, 7th harmonic for a reversible case (blue line) is also shown for comparison. Simulation parameters are provided in Table 3.

assuming α = 0.50 using A = 7.85 × 10−3 cm2, Cdl (c0 = 22.55, c1 = 40.5, c2 = 41.0) μF cm−2, and Ru = 2850 ohm. At the Au electrode (Figure S4), k0 = 0.15 cm s−1 is found using A = 7.85 × 10−3 cm2, Cdl (c0 = 39.50, c1 = 40.5, c2 = 36.0) μF cm−2 and Ru = 1600 ohm. At the GC electrode, k0 = 0.11 cm s−1 is deduced using A = 7.85 × 10−3 cm2, Cdl (c0 = 39.5, c1 = 40.5, c2 = 36.0) μF cm−2, and Ru = 2650 ohm in simulations to match the current magnitudes and peak splitting of AC harmonic components, as shown in Figure S5. Whereas at BDD, k0 = 0.06 cm s−1 is found with A = 7.85 × 10−3 cm2, Cdl (c0 = 7.0, c1 = 8.5, c2 = 8.0) μF cm−2 and Ru = 2200 ohms and comparison of theory and experiment is shown in Figure S6. 9565



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At the platinum microdisk electrode, the five harmonics obtained at 1.233 kHz for the reduction of 4.90 mM F4TCNQ in [BMPIP][TFSI] agree well with the simulations obtained with Cdl (c0 = 9.56, c1 = 0.52, c2 = −1.67, c3 = −2.67, c4 = 0.09), A = 1.22 × 10−4 cm2, k0 = 0.015 cm s−1, α = 0.50, D = 1.2 × 10−8 cm2 s−1, f = 1.233 kHz, ΔE = 80 mV, and Ru = 50000 ohm (Figure 9). The k0 value of 0.020 cm s−1 is close to that of 0.025 cm s−1 obtained for the F4TCNQ0/•− process in [BMIM][PF6].

to-noise ratio and six harmonics at 1.233 kHz, which is sufficient for k0 estimation, but the number of harmonics available is further reduced when f = 1.83 kHz. Consequently, 1.233 kHz was selected as a suitable frequency to extract electrode kinetic parameters for reduction of F4TCNQ in [BMIM][PF6]. Data were simulated with D = 7.0 × 10−9 cm2 s−1 estimated by DC voltammetry (see above) and nonlinear potential dependent Cdl (c0 = 9.30, c1 = −1.31, c2 = −2.10, c3 = −6.02, c4 = 1.01) μF cm−2 was derived from the fundamental harmonic. Other parameters used in the simulations are A = 1.22 × 10−4 cm2, Ru = 52000 ohm along with f = 1.233 kHz, and ΔE = 80 mV, and again, α is assumed to be 0.50. In this case, simulation and experimental data (Figure 8) are in good

Figure 9. Comparison of experimental (black line) and simulated (red line) FT AC voltammograms for the reduction of 4.90 mM F4TCNQ in [BMPIP][TFSI] at a platinum microdisk electrode: (a) DC component, (b−g) 1st to 6th harmonics, 6th harmonic for a reversible case (blue line) is also shown for comparison. Simulation and experimental parameters include A = 1.22 × 10−4 cm2, f = 1.233 kHz, ΔE = 80 mV, Cdl (c0 = 9.56, c1 = 0.52, c2 = −1.67, c3 = −2.67, c4 = 0.09) μF cm−2, D = 1.2 × 10−8 cm2 s−1, ν = 0.0894 V s−1, E0 = 0.203 V vs Fc0/+, Ru = 50000 ohm, k0 = 0.02 cm s−1, α = 0.50 and T = 293 K.

Figure 8. Comparison of experimental (black line) and simulated (red line) FT AC voltammograms for the reduction of 7.0 mM F4TCNQ in [BMIM][PF6] at a platinum microdisk electrode: (a) DC component, (b−g) 1st to 6th harmonics, 6th harmonic for a reversible case (blue line) is also shown for comparison. Simulation and experimental parameters include A = 1.22 × 10−4 cm2, f = 1.233 kHz, ΔE = 80 mV, Cdl = (c0 = 9.30, c1 = −1.31,c2 = −2.1, c3 = −6.02, c4 = 1.01) μF cm−2, D = 7.0 × 10−9 cm2 s−1, ν = 0.0894 V s−1, E0 = 0.258 V vs Fc0/+, Ru = 52000 ohm, k0 = 0.025 cm s−1, α = 0.50, and T = 293 K.

In the case of carbon fiber microdisk electrode, FT AC experimental voltammetric data obtained at 9.0 Hz agree well with simulations (Figure S9) undertaken with A = 8.54 × 10−6 cm2, Cdl = 20.5 μF cm−2, Ru = 130000 ohm, D = 1.2 × 10−8 cm2 s−1, f = 9.0 Hz and ΔE = 80 mV, ν = 0.0894 V s−1, E0 = 0.203 V vs Fc0/+ along with k0 = 0.0032 cm s−1 and α = 0.50. k0 at the carbon fiber is again smaller than found at Pt. AC Voltammetry of F4TCNQ in [BMPY][TFSI]. [BMPY][TFSI] has a lower viscosity (η = 77 cP) and is a more conducting ionic liquid than either [BMIM][PF6] or [BMPIP][TFSI]. Reduction of 7.20 mM F4TCNQ in this IL with f = 1.233 kHz enables 6−7 harmonics to be obtained at both carbon and platinum electrodes, which is ideal for quantitative electrode kinetics. Simulations of the FT AC voltammetric data for the reduction of 7.20 mM F4TCNQ at Pt with f = 1.233 kHz were undertaken with D = 3.9 × 10−8 cm2 s−1. Comparison of experimental and simulated data is provided in Figure 10 using A = 1.22 × 10−4 cm2, k0 = 0.10 cm s−1, Cdl (c0 = 10.65, c1 = 23.2, c2 = −9.55, c3 = 9.39, c4 = 2.85) μF cm−2 and Ru = 23000 ohm in the simulations. FT AC voltammograms obtained for the reduction of 7.20 mM F4TCNQ at the carbon fiber electrode agree well for simulation results shown in Figure S10 with A = 8.54 × 10−6 cm2, k0 = 0.020 cm s−1, Cdl (c0 = 8.3, c1 = 2.0, c2 = 0.50, c3 = 0, c4 = 1.56) μF cm−2, Ru = 90000 ohm, D = 3.9 × 10−8 cm2, f = 1.233 kHz, and ΔE = 80 mV. Again, consistent with the situation prevailing in [BMIM][PF6] and [BMPIP][TFSI], the

agreement for k0 = 0.025 cm s−1. Thus, a large decrease in the heterogeneous electron transfer rate constant is found for reduction of F4TCNQ in [BMIM][PF6] relative to the values obtained in the molecular solvents, again qualitatively consistent with the theoretical prediction based on the solvent dynamic effect.57 At the smaller carbon fiber microelectrode, the electrochemical reduction of 7.0 mM F4TCNQ in [BMIM][PF6] was studied with f = 9.0 Hz. To mimic the experimental data for F4TCNQ reduction at the carbon fiber electrode, simulations with D = 7.0 × 10−9 cm2 s−1, Ru = 145000 ohm, and Cdl = 22.0 μF cm−2 were required with k0 = 0.003 cm s−1 to give the comparison presented in Figure S8. Thus, k0 for the F4TCNQ0/•− process is about a factor of 10 lower than found at Pt. Quantitative FT AC Voltammetric Studies in [BMPIP][TFSI]. In the ionic liquid [BMPIP][TFSI] (η = 183 cP), FT AC voltammetric data were again obtained at platinum and carbon fiber microdisk electrodes. At platinum, an AC frequency of up to 1.233 kHz was used, while in case of the carbon electrode, the AC frequency was restricted to 9.0 Hz. In both cases, five harmonics could be obtained with adequate signal-to-noise ratio. 9566



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theoretical prediction based on the solvent dynamic effect.57 However, the medium dependence of k0 is not entirely straightforward in the sense that the k0 values in [BMPIP][TFSI] and [BMIM][PF6] are similar, even though the former has a significantly lower viscosity. This could be attributed to the stronger π−π interactions between the imidazolium cation and planar F4TCNQ,58−60 resulting in a larger degree of charge delocalization and hence favoring the electron transfer process in [BMIM][PF6]. The electrode kinetics of the F4TCNQ0/•− process in conventional solvent (electrolyte) media shows little dependence on the electrode material, which suggests that this process is adiabatic.61 In contrast, measurable electrode material dependence was observed in all ionic liquids, with a higher k0 value being obtained at Pt relative to carbon electrodes, which suggests that this process becomes nonadiabatic.62 According to studies by Weaver and co-workers,63,64 for an outersphere electron transfer process, an ideal adiabatic pathway could only occur when the tunnelling distance is ≤6−8 Å. The tunnelling distance for electroreduction of the planar structured F4TCNQ in conventional solvent/electrolyte media could lie within this distance, despite the fact that the hydrodynamic radius of F4TCNQ is about 1−2 nm, calculated on the basis of the Stokes−Einstein equation.49 Consequently, an electrode material independent k0 value may be expected in these media. In contrast, the electron tunnelling distance could be significantly larger in ionic liquids since the hydrodynamic radius of >3 nm for F4TCNQ, calculated on the same basis, is now significantly larger. Furthermore, in comparison with the molecular solvent (electrolyte) media, in ionic liquid media, larger and bulkier ions occupy the inner double layer. Consequently, the tunnelling distance increases and the electronic coupling between the electrode and the electroactive species weakens. Consequently, if the F4TCNQ0/•− process is in fact a more nonadiabatic process in ionic liquids, then an enhanced electrode material dependence of k0 can be expected. It should be noted that in obtaining the k0 values from a theory versus experiment comparison exercise, the D values of F4TCNQ and F4TCNQ0/•− were assumed to be equal. This equivalence should be a good approximation in a molecular solvent. However, the D values of these two species in ionic liquids could differ substantially, as reported in the literature for other redox couples.65 If the process is reversible, the D value of F4TCNQ0/•− only affects the location of the process on the potential axis. However, if the process is quasi-reversible, the D value of F4TCNQ•− also affects the other voltammetric characteristics, such as the magnitude of current which is used for k0 determination in FT AC voltammetry. The magnitude of the effect depends on the reversibility of the process. Unfortunately, the D value for F4TCNQ•− in the ionic liquids are unknown. However, even if the D values of F4TCNQ•− is in fact smaller than that of neutral molecule by a factor of 5, the systematic error introduced to k0 determination due to equal D assumption is 25−35% for the k0 values at Pt electrodes shown in Table 4 since the process at this electrode material is close to reversible. In constrast, at a carbon fiber electrode, where the process is not close to reversible (k0 ∼ 0.003 cm s−1), the systematic error on measured k0 value could be as high as ∼70%. However, this difference does not alter the validity of conclusions drawn above on the electrode material dependence of k0 in these ionic liquids.

Figure 10. Comparison of experimental (black line) and simulated (red line) FT AC voltammograms for the reduction of 7.20 mM F4TCNQ in [BMPY][TFSI] at a platinum microdisk electrode: (a) DC component, (b−g) 1st to 6th harmonics, 6th harmonic for a reversible case (blue line) is also shown for comparison. Simulation and experimental parameters include A = 1.22 × 10−4 cm2, f = 1.233 kHz, ΔE = 80 mV, Cdl (c0 = 10.65, c1 = 23.2, c2 = −9.55, c3 = 9.39, c4 = 2.85) μF cm−2, D = 3.9 × 10−8 cm2 s−1, ν = 0.0894 V s−1, Ru = 23000 ohm, k0 = 0.10 cm s1, E0 = 0.153 V vs Fc0/+, α = 0.50, and T = 293 K.

k0 value obtained at Pt electrode in this IL is significantly higher than at carbon fiber.

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DISCUSSION The F4TCNQ0/•− process has been investigated in both molecular solvents and ionic liquids, with viscosity varying from 0.30 cP (MeCN) to 371 cP ([BMIM][PF 6 ]). Both thermodynamic (E0) and kinetic (k0) properties of this process show a strong medium dependence (Table 4). E0 in ionic Table 4. Summary of Electrode Kinetics Parameters Determined for the F4TCNQ0/•− Process in Molecular Solvents and Ionic Liquids ionic liquid [BMIM][PF6] (η = 371 cP) [BMPIP][TFSI] (η = 183 cP) [BMPY][TFSI] (η = 94.4 cP) E0 vs molecular Fc0/+ solvent (V) DMSO (η = 1.98 cP) MeCN (η = 0.35 cP)

E0 vs Fc0/+ (V)

k0 at Pta (cm s−1)

k0 at carbon fibera (cm s−1)

0.253

0.025

0.0030

0.205

0.020

0.0032

0.153

0.100

0.0180

k0 at Aub (cm s−1)

k0 at Ptb (cm s−1)

k0 at GCb (cm s−1)

k0 at BDDb (cm s−1)

0.228

0.15

0.10

0.11

0.06

0.289

1.00

1.00

0.90

1.00

Pt (d = 125 μm), carbon fiber (d = 33 μm). bAu (d = 1.50 mm), Pt, GC, and BDD (d = 1.0 mm).

a

liquids are shifted positively by 136 mV versus Fc0/+ when comparing MeCN and [BMPY][TFSI] results and by 36 mV when comparing MeCN and [BMIM][PF6]. Of course, medium independence of the Fc0/+ reversible potential is assumed in these comparisons and this may be problematic. The k0 value decreases when the viscosity of the medium increases, which is qualitatively in agreement with the 9567



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(7) Nicholson, R. S.; Shain, I. Theory of Stationary Electrode Polarography Single Scan and Cyclic Methods Applied to Reversible, Irreversible, and Kinetic Systems. Anal. Chem. 1964, 36, 1212. (8) Rosvall, M. Method for Determination of Heterogeneous Rate Constants at Carbon-Fibre Microelectrodes Using Multi-ac-Voltammetry. Electrochem. Commun. 2000, 2, 791. (9) Wang, Y.; Velmurugan, J.; Mirkin, M. V. Kinetics of ChargeTransfer Reactions at Nanoscopic Electrochemical Interfaces. Isr. J. Chem. 2010, 50, 291. (10) Oldham K. B., Zoski C. G. Mass Transport to Electrodes. In Comprehensive Chemical Kinetics; Bamford, C.H., Compton, R.G., Eds.; Elsevier: New York, 1986; Vol. 26, pp 79−143. (11) Zoski, C. G. Handbook of Electrochemistry; Elsevier: Amsterdam, The Netherlands, 2007. (12) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley: New York, 2001. (13) Nicholson, R. S. Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics. Anal. Chem. 1965, 37, 1351. (14) Rees, N. V.; Compton, R. G. Hydrodynamic Microelectrode Voltammetry. Russ. J. Electrochem. 2008, 44, 368. (15) Sun, P.; Mirkin, M. V. Kinetics of Electron-Transfer Reactions at Nanoelectrodes. Anal. Chem. 2006, 78, 6526. (16) Vijaikanth, V.; Li, G. C.; Swaddle, T. W. Kinetics of Reduction of Aqueous Hexaammineruthenium(III) Ion at Pt and Au Microelectrodes: Electrolyte, Temperature, and Pressure Effects. Inorg. Chem. 2013, 52, 2757. (17) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 267. (18) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Saveant, J.-M. Ultramicroelectrodes: Cyclic Voltammetry above 1000000 V s−1. J. Electroanal. Chem. Interfacial Electrochem. 1988, 248, 447. (19) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Saveant, J.-M. Fast Kinetics by Means of Direct and Indirect Electrochemical Techniques. Chem. Rev. 1990, 90, 723. (20) Bowyer, W. J.; Engelman, E. E.; Evans, D. H. Kinetic Studies by Cyclic Voltammetry at Low Temperatures Using Microelectrodes. J. Electroanal. Chem. Interfacial Electrochem. 1989, 262, 67. (21) Forster, R. J.; Keyes, T. E. In Handbook of Electrochemistry; Zoski, C. G., Ed.; Elsevier: Amsterdam, The Netherlands, 2007; Chapter 6, pp 155−188. (22) Wipf, D. O.; Kristensen, E. W.; Deakin, R. M.; Wightman, R. M. Fast-Scan Cyclic Voltammetry as a Method to Measure Rapid Heterogeneous Electron-Transfer Kinetics. Anal. Chem. 1988, 60, 306. (23) Wightman, R. M.; Cockrell, J. R.; Murray, R. W.; Burnett, J. N.; Jones, S. B. Protonation Kinetics and Mechanism for 1,8Dihydroxyanthraquinone and Anthraquinone Anion Radicals in Dimethylformamide Solvent. J. Am. Chem. Soc. 1976, 98, 2562. (24) Oldham, K. B.; Zoski, C. G. Comparison of Voltammetric Steady States at Hemispherical and Disc Microelectrodes. J. Electroanal. Chem. Interfacial Electrochem. 1988, 256, 11. (25) Aoki, K.; Tokuda, K.; Matsuda, H. Hydrodynamic Voltammetry at Channel Electrodes: Part VII. Current Transients at Double Channel Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1985, 195, 229. (26) Bidwell, M. J.; Alden, J. A.; Compton, R. G. Hydrodynamic Voltammetry with Channel Microband Electrodes: The Simulation of Voltammetric Waveshapes. J. Electroanal. Chem. 1996, 417, 119. (27) Booth, J.; Compton, R. G.; Cooper, J. A.; Dryfe, R. A. W.; Fisher, A. C.; Davies, C. L.; Walters, M. K. Hydrodynamic Voltammetry with Channel Electrodes: Microdisk Electrodes. J. Phys. Chem. 1995, 99, 10942. (28) Compton, R. G.; Unwin, P. R. Linear Sweep Voltammetry at Channel Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1986, 206, 57. (29) Stevens, N. P. C.; Fisher, A. C. Transient Voltammetry Under Hydrodynamic Conditions. Electroanalysis 1998, 10, 16.

CONCLUSION The data obtained in this study show that large amplitude FT AC voltammetry can be used at high frequency with macroelectrodes in molecular solvents containing 0.10 M Bu4NPF6 to probe fast electron transfer kinetics. The electron transfer kinetics of the F4TCNQ0/•− process in MeCN and DMSO solvents is near to the reversible limit at Pt, Au, GC, and BDD electrodes, suggesting that F4TCNQ0/•− process is adiabatic in this environment. The use of microdisk electrodes allows k0 values to be estimated in less conductive ionic liquids. In this medium, k0 values are significantly lower than in molecular solvents due to the solvent dynamic effect. In all ILs, a clear dependence on Pt and carbon electrodes was observed, suggesting that the F4TCNQ0/•− process is now partially nonadiabatic.

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ASSOCIATED CONTENT

S Supporting Information *

Section I shows the comparison of experimental and simulated voltammograms for the reduction of F4TCNQ in acetonitrile at GC, Au, and BDD electrodes in Figures S1−S3. Section II consists of similar experiment and theory comparison for the F4TCNQ0/•− process in DMSO in Figures S4−S6. Section III comprises FT AC voltammograms for the reduction of F4TCNQ in ILs at Pt macroelectrode in Figure S7 and carbon fiber microelectrodes in comparison with the theory for F4TCNQ0/•− process in Figures S8−S10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +61 3 9905 6289. Fax: +61 3 9905 4597. *E-mail: [email protected]. Phone: +61 3 9905 1338. Fax: +61 3 9905 4597. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the Australian Research Council for financial support. K.B. acknowledges financial support from the Monash University Science Faculty Dean’s Postgraduate Research Scholarship and a Postgraduate Publication Award from the Monash Institute of Graduate Research.

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REFERENCES

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Investigations of Fast Electrode Kinetics for Reduction of 2,3,5,6

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