Investigating the Mechanism and Electrode Kinetics of the Oxygen

J. Phys. Chem. C 2009, 113, 17811–17823

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Investigating the Mechanism and Electrode Kinetics of the Oxygen|Superoxide (O2|O•2 ) Couple in Various Room-Temperature Ionic Liquids at Gold and Platinum Electrodes in the Temperature Range 298-318 K Emma I. Rogers,† Xing-Jiu Huang,† Edmund J. F. Dickinson,† Christopher Hardacre,‡ and Richard G. Compton*,† Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom, and School of Chemistry and Chemical Engineering/QUILL, Queen’s UniVersity Belfast, Belfast, Northern Ireland BT9 5AG, United Kingdom ReceiVed: July 7, 2009

The reduction of oxygen was studied over a range of temperatures (298-318 K) in n -hexyltriethylammonium bis(trifluoromethanesulfonyl)imide, [N6,2,2,2][NTf2], and 1-butyl-2,3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C4dmim][NTf2] on both gold and platinum microdisk electrodes, and the mechanism and electrode kinetics of the reaction investigated. Three different models were used to simulate the CVs, based on a simple electron transfer (‘E’), an electron transfer coupled with a reversible homogeneous chemical step (‘ECrev’) and an electron transfer followed by adsorption of the reduction product (‘EC(ads)’), and where appropriate, best fit parameters deduced, including the heterogeneous rate constant, formal electrode potential, transfer coefficient, and homogeneous rate constants for the ECrev mechanism, and adsorption/desorption rate constants for the EC(ads) mechanism. It was concluded from the good simulation fits on gold that a simple E process operates for the reduction of oxygen in [N6,2,2,2][NTf2], and an ECrev process for [C4dmim][NTf2], with the chemical step involving the reversible formation of the O2•- · · · [C4dmim]+ ion-pair. The E mechanism was found to loosely describe the reduction of oxygen in [N6,2,2,2][NTf2] on platinum as the simulation fits were reasonable although not perfect, especially for the reverse wave. The electrochemical kinetics are slower on Pt, and observed broadening of the oxidation peak is likely due to the adsorption of superoxide on the electrode surface in a process more complex than simple Langmuirian. In [C4dmim][NTf2] the O2•predominantly ion-pairs with the solvent rather than adsorbs on the surface, and an ECrev quantitatively describes the reduction of oxygen on Pt also. 1. Introduction The electroreduction of oxygen to superoxide

O2 + e- h O•2

(1)

has been extensively studied in the literature in a wide range of solvents.1-22 Recently the use of room-temperature ionic liquids (RTILs) as a solvent in which to study this reaction has become increasingly common. The absence of a proton source, which leads to the spontaneously rapid disproportionation of the electrogenerated superoxide species,1,2 faciles reversible voltammetry, comparable to that reported in other nonaqueous aprotic solvents, such as dimethyl formamide (DMF),3-5 dimethyl sulfoxide (DMSO),3,4 acetonitrile (MeCN),3-5,7,23 propylene carbonate (PC),8 and acetone (AT).3 Al Nashef et al.7 were the first to report the electrogeneration of a stable superoxide species (on a glassy carbon electrode) in 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6] and 1,2-dimethyl-3-n-butylimidazolium hexafluorophosphate, [C4dmim][PF6], followed by Katayama et al.,10 who studied the voltammetry of O2 in a variety of ionic liquids with the bis(trifluoromethanesulfonyl)imide ([NTf2]-) anion on a gold * To whom correspondence should be addressed. E-mail: richard.compton@ chem.ox.ac.uk. Tel: +44(0) 1865 275 413. Fax: +44(0) 1865 275 410. † Oxford University. ‡ Queen’s University Belfast.

electrode. In later years, the observation of reversible voltammetry in [N6,2,2,2][NTf2],11,12 [C2 mim][NTf2],10,12 [C4mim][BF4],6 [C4mpyrr][NTf2],10 [Py14][NTf2],11 [P14,6,6,6][NTf2],11 and [P14,6,6,6][FAP]11 (where [FAP]-) trifluorotris(pentafluoroethyl)phosphate) was also reported. This was contrary to that observed a decade previously by Carter and Osteryoung et al.14 who suggested that due to the protic impurities in the RTIL 1-ethyl-3-methylimidazolium chloride aluminum chloride, [C2mimCl][AlCl3] used, and the fast, irreversible reaction of these impurities with superoxide, only a one-electron reduction peak was observed, but with no reoxidation of O2•- to O2. In the presence of these slight protic impurities, disproportionation of the radical anion,1,2,15,16,11 superoxide, occurs in the following way: •+ O•2 + H f HO2

(2)

2HO•2 f H2O2 + O2

(3)

to yield peroxide (O22-) and oxygen, inhibiting the reoxidation of superoxide, and thus resulting in the irreversible voltammetry observed. Superoxide is a very powerful nucleophile,18 and in the absence of protons, it tends to attack positively charged components of any organic species present.10 Ionic liquids, which contain high concentrations of cations, are susceptible

10.1021/jp9064054 CCC: $40.75  2009 American Chemical Society Published on Web 09/22/2009

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to nucleophilic attack by such species. It has been observed that electrogenerated O2•- can undergo complexation with the solvent with the transformation of the reduction product, O2•-, into the O2•- · · · [Im]+ ion pair via the following mechanism:10,13,24 •+ + O•2 + [Im] h O2 · · · [Im]

(4)

Figure 1. Structures of the cations and anions used as the RTILs in this study.

This strong ion-pairing effect be observed voltammetrically, with the interaction process acting to thermodynamically stabilize the reduced species.17 In this case, the reduction of oxygen can be classified as an ECrev process,10,13 which is an electrochemical step (reduction of oxygen) followed by a reversible, homogeneous chemical process (formation of the ionpair). The rapid consumption of O2•- by [Im]+ hampers the back electron transfer, causing a positive shift in the potential of the oxidation peak with voltammetry that appears, by virtue of an increased peak-to-peak separation, more electrochemically irreversible than that expected for a simple E process. Comparison of CVs for the reduction of oxygen,7,25 as well as for the reduction of 1-n-dinitrobenzenes24 and benzaldehyde,26 in [Cnmim][NTf2],10,26 [Cnmim][BF4], [C4mim][PF6],7,25 and [C4dmim][NTf2]7,25 with those in nonaromatic RTILs such as pyrrolidinium, [C4mpyrr][NTf2],7,25,26 and alkylammonium, [N6,1,1,1][NTf2],7,25 and conventional solvents, such as DMSO,9 gave halfwave potentials that were shifted anodically in the case of the imidazoliums with more ‘distorted’ voltammetry than in the comparison RTILs and other organic solvents. The separation between the reductive and oxidative peak also depends largely upon the electrode substrate, and a broadening of voltammetry on Pt compared to Au and GC in the order Pt > Au > GC6 has been reported, suggesting possibly coupled chemical reaction or product adsorption on Pt. In a previous publication, an increased ∆Epp was reported for Pt compared to Au in a range of RTILs, varying from 62 mV in [C4mim][PF6] to 255 mV in [N6,2,2,2][NTf2] at 308 K.27 This, in part, suggests faster kinetics on gold, although quantitative investigation of the electrode kinetics was not achieved and the role of possible follow-up adsorption was not resolved. The initial aim of this follow-up study is to investigate the mechanism of oxygen reduction and the electrode kinetics of the O2|O2•- redox couple in [C4dmim][NTf2] and [N6,2,2,2][NTf2] by voltammetric and simulation techniques. Relatively little quantitative data has been published on electrochemical rate constants in RTILs.28-30 Diffusion coefficients and solubilities are reported at a range of temperatures (298-318 K) in both RTILs. Modeling the voltammetric data with digital simulation models based on simple electron transfer (E), electron transfer followed by reversible chemical reaction (ECrev), and electron transfer followed by adsorption of superoxide (EC(ads)) are all considered. Simulation of these mechanisms allows extraction, where appropriate, of best-fit kinetic parameters, including the heterogeneous rate constant, k0, formal electrode potential, EfQ, and transfer coefficient, R, as well as the homogeneous rate constants, kf and kb, for the EC mechanism and adsorption and desorption rate constants, kads and kdes respectively, for the EC(ads) mechanism, in the same temperature range. It is concluded that gold shows faster electrode kinetics than platinum, that adsorption is not significant for Au and that for imidazolium-based ionic liquids an ECrev process operates.

prepared following standard literature procedures using [N6,2,2,2]Br (Aldrich, 99%) for metathesis with Li[NTf 2].31 Oxygen was purchased from BOC, Surrey, UK. See Figure 1 for RTIL structures. 2.2. Instrumental. Cyclic voltammetry (CV) and chronoamperometry were undertaken using a computer-controlled µ-Autolab potentiostat (Eco-Chemie, Netherlands). The twoelectrode arrangement consisted of a 10 µm diameter gold (homemade) or platinum (Cypress Systems Ltd., Massachusetts) microelectrode with a 0.5 mm diameter silver (Goodfellow Cambridge Ltd. UK) quasi-reference electrode. The working electrode was modified with a plastic collar to form a cavity, into which 20 µL of the ionic liquid was placed, and both the working and reference electrodes were housed in a T-cell (reported previously)33,34 to allow electrochemical analysis to be undertaken under a controlled environment. Prior to analysis, the working electrode was polished using a water-alumina slurry (1.0 and 0.3 µm) on soft lapping pads (Buehler, Illinois), and then calibrated electrochemically by analyzing the steadystate current of 2 mM ferrocene in acetonitrile plus 0.1 M TBAP supporting electrolyte. A value for the diffusion coefficient of ferrocene of 2.3 × 10 -9 m2 s-1 at 293 K35 was adopted and the electrode radius determined from the limiting current. Prior to the addition of oxygen, the ionic liquid was degassed under vacuum (0.5 Torr) at 298 K for at least 2 h, until the baseline showed no trace of atmospheric oxygen. Oxygen was then passed through a drying column of silica beads and introduced through one arm of the T-cell via a heated coil inside a temperature-controlled Faraday cage. The gas was left for ∼10 min before measurements were taken to ensure that full equilibration had been established.

2. Experimental Section 2.1. Chemical Reagents. [C4dmim][NTf2] was prepared following standard literature procedures.31,32 [N6,2,2,2][NTf2] was

3. Theory 3.1. Potential Step Chronoamperometry. Analysis of chronoamperometric measurements conducted at the gold or platinum microdisks allowed simultaneous determination of the diffusion coefficient, D, and solubility, c, of oxygen in the chosen RTIL.36 The technique was undertaken using a sample time of 0.001 s, and after pretreatment for 20 s at 0.0 V, experimental transients were obtained by stepping to a potential after the reduction peak and measuring the current for 0.5 s. The time-dependent current response, i, was analyzed using the following equations, as proposed by Shoup and Szabo,37 which sufficiently describe the current response to within 0.6% error over all τ.

i ) -4nFDcrd f(τ)

(5)

where

f(τ) ) 0.7854 + 0.8863τ-1/2 + 0.2146 exp(-0.7823τ-1/2) (6) and

Mechanism and Kinetics of the Oxygen|Superoxide Couple

τ)

4Dt r2d

(7)

where n is the number of electrons, F is the Faraday constant (96485 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. Theoretical transients were generated using eq 6 and a nonlinear curve-fitting function available in OriginPro 7.5 (Microcal Software Inc.). The fit between experimental and theoretical data was optimized by inputting a value for rd and instructing the software to iterate through various D and c values. 3.2. Simulation Models. 3.2.1. E Mechanism. The mathematical model used for the simulation of CV for the simple one-electron reduction of O2 was based on that published previously12 and considers an E mechanism, where oxygen is reduced to superoxide, as follows -

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17813 where θ is the dimensionless potential () (F)/(RT)(E - EfQ)) and Γ is the surface coverage (mol m-2). The current flow, i, was determined using the following expression:

i ) -2πFDA

r dr ∫0r ( ∂[A] ∂z )z)0 d

3.2.2. EC Mechanism. An alternative simulation mechanism for the reduction of oxygen which will be shown to operate for the case of the imidazolium RTILs utilized the model based on an ECrev mechanism similar to that implemented in a previous publication13 in which the electrogenerated O2•- species forms an ion-pair with the solvent cation, [Im]+, in a reversible homogeneous chemical step. This can be expressed as

A + e- h B

kf

A + e y\z B

kf

B y\z C In this scheme, A is O2, B is O2•- and kf and kb are the forward and reverse rates constants according to the following ButlerVolmer equations:

(E - E )) ( -RF RT oi f

(9)

( (1 -RTR)F (E - E )) oi f

kb ) ko exp

(10)

(

)

(11)

(

)

(12)

∂[A] ∂2[A] 1 ∂[A] ∂2[A] + ) DA + 2 ∂t r ∂r ∂r ∂z2 ∂[B] ∂2[A] 1 ∂[B] ∂2[B] + ) DB + 2 ∂t r ∂r ∂r ∂z2

where DA and DB are the diffusion coefficients (m2 s-1) of oxygen and superoxide, respectively, with initial concentrations of [A] ) c* and [B] ) 0, were re-expressed using a (R, Z)coordinate transformation developed by Amatore and Fosset,38 where R ) (r)/(rd) and Z ) (z)/(rd), given by

√1 - θ2 cos

( π2 Γ)

(13)

where O2 is A, O2•- is B, O2•- · · · [Im]+ is C, and kf and kb are first-order rate constants for the forward and backward homogeneous processes, respectively. In the model, the time-dependent mass transport equations (18-20) and boundary conditions were re-expressed and described by the normalized (R, Z)-coordinate system where R ) (r)/(rd) and Z ) (z)/(rd).

∂a ) ∇2a ∂τ

(18)

∂b ) ∇2b - Kfb + Kbc ∂τ

(19)

∂c ) ∇2c - Kbc + Kfb ∂τ

(20)

where

∇2 )

∂2 1 ∂ ∂2 + + 2 2 R ∂R ∂R ∂Z

i ) 2πnFDArd[A]bulkj

( π2 Γ)

(14)

(21)

Species A, B, and C are normalized to the bulk concentration of A, [A]bulk, and described by a, b, and c. Time, t, is normalized to dimensionless τ. Homogeneous rate constants kf and kb (eq 17) are expressed as the dimensionless rate constants K˜f and K˜b, where (K˜f)/(K˜b) ) K˜eq and K˜f ) (kfrd2)/(DA). Concentration profiles for species A, B, and C are obtained by solving the mass transport equations according to defined boundary conditions, using the ADI method, and the corresponding Faradaic current, i, is related to the dimensionless diffusional flux, j, as below.

and

Z ) θ tan

(17)

kb

k0 is the standard electrochemical rate constant (m s-1), F is the Faraday constant, R is the universal gas constant, T is the temperature (K), E is the potential (V), and EfQ is the formal electrode potential (V). The mathematical model was solved in two dimensions using the alternating implicit direction (ADI) method in which the mass transport equations

R)

(16)

(8)

kb

kf ) ko exp

(15)

where

(22)

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Rogers et al.

∂a ∫01 ( ∂Z )z)0R dR

(23)

For the simulation of CV, the dimensionless potential, θ, is swept from an initial potential, θi, to a more negative potential, θf, and then back again. On the forward sweep, θ ) θi + στ and on the backward scan θ ) 2 θf - θi - στ, where θ ) (F)/(RT)(E - EfQ). Equation 24 is the boundary condition at the electrode surface derived from the Butler-Volmer kinetics describing electron transfer.

( ∂a∂z ) ) K˜ (e 0

-Rθ

a0 - e(1-R)θb0)

(24)

Figure 2. 2D cylindrical simulation space used in the EC(ads) mechanism.

(eqs 30-31) across a 2D cylindrical simulation space, shown at Figure 2.

where K˜0 ) (k0rd)/(DA) and is the dimensionless heterogeneous rate constant. R is the transfer coefficient. 3.2.3. EC(ads) Mechanism. The third mechanism considered the reduction of O2 according to an EC(ads) scheme, in which the reduced product, O2•-, can adsorb on the surface. In the following, O2 is A and O2•- is B:

∂cA ∂2cA ∂2cA 1 ∂cA + ) DA + ∂t r ∂r ∂z2 ∂r2

A(soln) + e- h B(soln)

(25)

∂cB ∂2cB ∂2cB 1 ∂cB + ) DB + ∂t r ∂r ∂z2 ∂r2

B(soln) h B(ads)

(26)

This mechanism is, to the best of the our knowledge, unexplored using the microelectrode geometry. The electron transfer is again modeled according to Butler-Volmer kinetics:

(

jA ) ko cA,o exp

-RF (E - E ofi) RT (1-R)F cB,o exp (E - E ofi) RT

)

(

)

(27)

where jA is the incident flux of O2 at the electrode, k0 is the heterogeneous electron transfer rate constant (m s-1), ci, 0 are the surface concentrations (mol m-3), R is the transfer coefficient, F is the Faraday constant, R is the universal gas constant, T is the temperature (K), E is the potential applied at the electrode (V), and EfQ is the formal electrode potential of the O2|O2•- couple. The adsorption kinetics are supposed to obey a Langmuir isotherm:

jB ) kadscB,0(1 - Θ) - kdesΘ

(28)

where jB is the adsorptive flux of O2•- at the electrode, kads is the adsorption rate constant (m s-1), kdes is the desorption rate constant (mol m-2 s-1), and Θ is defined as the proportional surface coverage of O2•- (ads) such that it takes values between 0 and 1. It is equivalent to (Γ)/(Γmax), where Γ is the surface coverage (mol m-2) and Γmax is the maximum surface coverage. kdes is set using an equilibrium coefficient, Keq (mol-1 m3), which relates its magnitude to that of kads:

kdes )

kads Keq

(29)

This mechanism is modeled for CV at a microdisc electrode by numerical solution of the diffusion equations for both species

(

)

(30)

(

)

(31)

The simulation space is bounded at zmax ) 6(DAt)1/2 or zmax ) 6(DBt)1/2, whichever is greater, and at rmax ) rd + zmax. At these boundaries, the concentration of A is set to its bulk value, c*, and the concentration of B is set to zero; these boundaries have been characterized as exceeding the diffusive depletion layer at all times.39 At t ) 0, the concentration of A in solution is set uniformly equal to its bulk value c*, and the concentration of B in solution and its adsorbed coverage on the electrode surface are both set to zero. The mechanism yields the following electrode surface boundary conditions for the solution of eq 30, applying at z ) 0 and r e r d:

Γmax

DA

∂Θ ) kads(1 - Θ)cB,0 - kdesΘ ∂t

∂cA -RF ) ko cA,o exp (E - E ofi) ∂z RT (1-R)F cB,o exp (E - E ofi) RT

(32)

(

)

(

DA

(

∂cA ∂cB ∂Θ ) - DB - Γmax ∂z ∂z ∂t

)

)

(33)

(34)

where eq 32 describes the Langmuirian adsorption kinetics, eq 33 describes the Butler-Volmer electron transfer kinetics, and eq 34 assures conservation of mass. Additionally, the insulating surface is made impermeable using a zero-flux boundary condition, and the symmetry boundary along the z-axis is imposed similarly:

z ) 0, r > rd

∂cA )0 ∂z

(35)


r)0

∂cA )0 ∂r

(36)

To solve eq 30 numerically, it is made dimensionless and then discretized in space using the ADI method,40 across an expanding space grid adapted from the efficient grid introduced by Gavaghan.41 A simplified and sparser example of the grid is at Figure 2. The time grid is regular, and applied potential, E, is altered linearly at each time step at some rate given by the scan rate for the experiment, V. The nonlinear terms in the electrode surface boundary conditions are linearized via the approximation:42

A'B' ≈ A'B + AB' - AB

(37)

where A′ and B′ are implicit (next half-time step) concentration terms and A and B are explicit concentration terms respectively. This approximation introduces an error of O(∆(DAt)2), but the resulting set of simultaneous equations is now linear and may be set as a pentadiagonal matrix equation, which is solved at each time step via LU decomposition using an adapted Thomas algorithm.43 All simulation parameters were converged to within

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17815 0.2% accuracy. All simulations were programmed in C++ and run on a desktop computer with an Intel Pentium 4 3 GHz processor and 500 MB of RAM, with running times of 10-15 min per voltammogram being typical for highest accuracy. The current is retrieved at each time step via numerical integration of the flux across the electrode, using the trapezium rule across all space points:

i ) -2πFDA

∫0r

d

∂cA ∂z

|

z)0

dr

(38)

3.3. Theoretical Voltammetry: the Models Compared. In this section we consider simulated results from the three modelssE, ECrev, and EC(ads)sand compare the voltammetric signatures from each, noting that an additional complicating factor is that the diffusion coefficients of O2 and O2•- can be so significantly different as to noticeably impact on the voltammetry.12,11,22,44 Figure 3 shows data obtained for the E mechanism using parameters typical for these used to fit actual RTIL data as discussed below. Figure 3a relates to the case DA ) DB ) 10-10 m2 s-1 and gives simulated voltammograms for the voltage scan rates 10, 100, and 1000 mV s-1 at a microdisk of radius 5 µm. The expected

Figure 3. Theoretical CV for an E mechanism at 10, 100, and 1000 mV s-1 at different ratios of (DA)/(DB). In each case, the potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd) 5 µm, and DA ) 1 × 10-10 m2 s-1.

Figure 4. Theoretical CV for an ECrev mechanism at 10, 100, and 1000 mV s-1 at different ratios of (DA)/(DB). In each case, the potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, k0 ) 0.001 m s-1, kf ) 1 × 103 s-1, and Keq ) 25.

Figure 5. Theoretical CV for an EC(ads) mechanism at 10, 100, and 1000 mV s-1 at different ratios of (DA)/(DB). In each case, the potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, k0 ) 0.001 m s-1, kads ) 0.0001 m s-1, and kdes ) 0.001 mol m2 s-1.

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Figure 6. Theoretical CV for an EC(ads) mechanism at (a) 100 and 1000 mV s-1 with a (DA)/(DB) ratio of 1, and (b) at 1000 mV s-1 at varying (DA)/(DB) ratios. In each case, the potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, k0 ) 0.001 m s-1, kads ) 0.1 m s-1, and kdes ) 0.001 mol m2 s-1.

transition from steady-state microelectrode behavior to transient planar electrode voltammetry is apparent. That is at low scan rates sigmoidal voltammograms characteristic of convergent diffusion are seen, changing to peak-shaped voltammograms with forward/back hysteresis at larger scan rates where planar diffusion is seen. Note that the magnitude of the scan rates at which the planar diffusion sets in are much lower in RTILs than in conventional aprotic media because of the much reduced diffusion coefficients.45 Thus, peak-shaped voltammograms are seen at microelectrodes in RTILs for scan rates where steadystate voltammetry would be seen in solvents such as water or acetonitrile. Figure 3b and c shows voltammograms for exactly the same conditions as Figure 3a except that the diffusion coefficient of B has been lowered in the simulations, mimicking the behavior of the superoxide ion,12 so as to achieve diffusion coefficient ratios, (DA)/(DB), of 2 and 20. Notice the effect of this is that a peak-shaped response can be obtained on the reverse scan even when a steady-state sigmoidal curve is seen on the forward-going sweep. This arises since the slower diffusing superoxide anion is constrained to distances much closer to the electrode than over which the faster moving oxygen is depleted. Accordingly, conditions can and do arise where, in the same voltammogram, the superoxide undergoes essentially planar diffusion whereas oxygen undergoes largely convergent diffusion. The effect was reported experimentally first by Buzzeo12 and subsequently also by others.46,47 We next turn to the ECrev mechanism. Figure 4 shows the expected response of this mechanism as a function of scan rate for parameters which are thought to approximate the reduction of oxygen in [Cnmim][NTf2].12 Notice that the effect of the following reaction and its chemical reversibility is that the size of the back peak is enlarged relative to the case of a simple E process. This happens since the ion-pair forms a very slowly diffusing entity so that the total superoxide concentrationsfree and ion-paired anionssis enhanced local to the electrode surface. The dissociating ion-paired superoxide acts as a source of additional free superoxide on the reverse scan so enlarging the back peak. Note that for the case of DA ) DB (Figure 4a), this effect can produce a peaked response on the reverse scan not dissimilar to that seen for the E mechanism with grossly unequal diffusion coefficients. We show below, however, that the two processes are distinguishable by variable scan rate studies. Last, we consider the case of the EC(ads) mechanism. Three qualitatively distinct voltammetric responses are seen depending on the degree of electrochemical reversibility of the O2| O2•redox couple and the adsorption/desorption kinetics of O2•- on the electrode surface. In the case (Figure 5) of slow adsorption/ desorption kinetics for the Langmuirian uptake of superoxide by the electrode surface, the effect is again an enlargement, as well as a broadening, of the reverse peak in a manner which is

Figure 7. Theoretical CV for an EC(ads) mechanism at 100 and 1000 mV s-1 with a (DA)/(DB) ratio of 1. The potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, k0 ) 0.001 m s-1, kads ) 0.1 m s-1, and kdes ) 0.001 mol m2 s-1.

TABLE 1: Three Possible Voltammetry Signatures on the O2|O2•- System adsorption/desorption kinetics electrode kinetics

slow

fast

slow

1 forward and 1 reverse peak seen; the latter enlarged and broadened cf. the E mechanism

1 forward and 2 reverse peaks seen 2 forward and 2 reverse peaks seen

fast

not dissimilar from the pure E mechanism but can again be distinguished by scan rate studies. In contrast, if both the adsorption/desorption kinetics and electrode kinetics are fast then two waves are apparent, as shown in Figure 6 where the new feature at less negative potentials arises since there is a prewave due to reduction of oxygen leading to adsorbed superoxide which fills the surface before the onset of the solution phase O2|O2•couple. On the reverse sweep, the reverse peak of the new feature is apparentshere the more positive potential of this signal relative to the solution phase responses is related in part to the Gibbs energy of adsorption of O2•- which must be overcome by a suitable applied overpotential before O2•- can desorb and be reoxidized. Finally, if the electrode kinetics are slow but the adsorption/desorption kinetics are fast then only the solution-phase signal is seen on the forward scan (Figure 7) but two peaks appear on the reverse scan; the first being the solution-phase signal and the second, at less negative potentials, being the reoxidation of superoxide adsorbed on the surface. Table 1 shows the three mechanistic limits. Note that the presence of one or two extra peaks as compared to the simple E response provides a clear indication of the aspects of this medium. In the experimental results reported below and elsewhere by other authors,6,9-13 no extra peaks have ever been seen in the experimental voltammetry relating to the O2|O2•system. Therefore, in order to distinguish the EC(ads) mechanism from the other two under consideration it is necessary to


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Figure 8. Theoretical CV for an E, ECrev and EC(ads) mechanism at 10 mV s-1 with a (DA)/(DB) ratio of (a) 1, (b) 2, and (c) 20. The potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, and k0 ) 0.001 m s-1. For the ECrev mechanism, kf ) 1 × 103 s-1, Keq ) 25, and DC ) 1 × 10 -12 m2 s-1. For the EC(ads) mechanism, kads ) 0.0001 m s-1 and kdes ) 0.001 mol m2 s-1.

Figure 9. Theoretical CV for an E, ECrev, and EC(ads) mechanism at 100 mV s-1 with a (DA)/(DB) ratio of (a) 1, (b) 2, and (c) 20. The potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, and k0 ) 0.001 m s-1. For the ECrev mechanism, kf ) 1 × 103 s-1, Keq ) 25, and DC ) 1 × 10-12 m2 s-1. For the EC(ads) mechanism, kads ) 0.0001 m s-1 and kdes ) 0.001 mol m2 s-1.

Figure 10. Theoretical CV for an E, ECrev, and EC(ads) mechanism at 1000 mV s-1 with a (DA)/(DB) ratio of (a) 1, (b) 2, and (c) 20. The potential was swept from 0.0 to -1.0 V, R ) 0.5, EfQ ) -0.6 V, [A] ) 5 mol m-3, rd ) 5 µm, DA ) 1 × 10-10 m2 s-1, and k0 ) 0.001 m s-1. For the ECrev mechanism, kf ) 1 × 103 s-1, Keq ) 25, and DC ) 1 × 10-12 m2 s-1. For the EC(ads) mechanism, kads ) 0.0001 m s-1 and kdes ) 0.001 mol m2 s-1.

perform variable voltage scan rate measurements so that the voltammetric signals switch between the convergent and planar diffusion limits and then fit to the different mechanisms. Figures 8, 9, and 10 show the three mechanisms compared for three different scan rates and for different diffusion coefficient ratios, (DA)/(DB). In the following, values of DA are obtained by potential step chronoamperometry and then experiment compared with simulation using (DB)/(DA) as a variable alongside the different kinetic parameters. 4. Results and Discussion 4.1. Reduction of Oxygen in [N6,2,2,2][NTf2] on a Gold and Platinum Microdisk at Varying Temperatures. Initially, the reduction of oxygen was studied on a 10 µm diameter Au microdisk, in [N6,2,2,2][NTf2] and examples of the CV observed at scan rates of 50, 100, 200, 400, 700, and 1000 mV s-1 are shown in Figure 11a at a temperature of 308 K. Note the mixture of convergent diffusion (forward scan, O2 reduction) and planar diffusion (reverse scan, O2•- oxidation), as discussed in Section 3.3.

The potential was scanned from 0.0 to -1.5 V; a potential after the reduction peak, and then back to 0.0 V. An oxidation peak, corresponding to the reoxidation of O2•- to O2, was observed at ∼ -0.9 V vs Ag at 308 K. The Au electrode was swapped for a 10 µm diameter Pt microdisk and the process repeated. Figure 11b shows the CV observed over the same range of scan rates. In this case, the reduction peak is observed at a more negative potential of ∼ -1.7 V, and an oxidation peak at ∼ -1.3 V vs Ag, again at 308 K. The voltammetry looks more ‘broad’ on the Pt electrode compared to on Au with an increased peak separation, ∆Epp, between the reduction and oxidation peaks (∼ 530 mV on Pt compared to ∼ 270 mV on Au), possibly likely suggesting faster electrode kinetics on gold than platinum. This will be investigated via simulation in Section 4.4. Note the apparent shift in the formal electrode potential, EfQ, observed is simply due to the use of a quasi-reference electrode (Ag wire) which is known to show shifts in potentials over the experimental time scale.48-50

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Figure 11. Cyclic voltammograms for the reduction of oxygen in [N6,2,2,2][NTf2] at varying scan rates (50, 100, 200, 400, 700, and 1000 mV s-1) on (a) Au and (b) Pt, and at varying temperatures (298, 303, 308, 313, and 318 K) on (c) Au and (d) Pt.

TABLE 2: Diffusion Coefficient and Solubility Data for Oxygen in [C4dmim][NTf2] and [N6,2,2,2][NTf2] at Varying Temperatures,27 Obtained from Shoup and Szabo37 Analysis of Chronoamperometric Transients RTIL

298 K 303 K 308 K 313 K 318 K -10

[C4dmim][NTf2] D/ × 10 m2 s-1 c /mM [N6,2,2,2][NTf2]

D/ × 10-10 m2 s-1 c /mM

Au

5.1

5.7

6.8

8.3

9.0

Pt Au Pt Au

4.7 3.6 4.4 4.6

5.7 3.7 4.3 5.3

7.0 3.7 4.3 6.0

78.6 3.6 4.2 7.0

9.8 3.8 4.4 8.0

Pt Au Pt

4.1 3.9 4.5

4.9 3.9 4.4

5.6 3.8 4.4

6.5 3.7 4.3

7.4 3.9 4.4

For both types of electrode, the voltammetry is asymmetric, with a more steady-state reduction peak and a transient-shaped back peak, resulting from a difference in the diffusion type of O2 and O2•-, leading to slower diffusion coefficients for the charged O2•- radical anion, generally by over an order of magnitude.11,12 Panels c and d of Figure 11 show cyclic voltammograms on both Au and Pt respectively at 1000 mV s-1 and varying temperatures (298-318 K). As the temperature increases, the peak current for the reduction of O2 and subsequent oxidation of O2•- increases due to an increase in the diffusion coefficients of these species, resulting from a decrease in the viscosity of the RTIL. Shoup and Szabo37 analysis of potential step chronoamperometric transients allowed determination of diffusion coefficients, D, and solubilities, c, for oxygen on both Au and Pt. These are shown in Table 2.27 The D values are very similar on both electrodes as the diffusion of oxygen is obviously dependent on the solvent but not the electrode material, and these values are comparable to other data reported previously in a range of RTILs.7,12,11,44,13 4.2. Simulation of CV in [N6,2,2,2][NTf2]. It has been suggested previously12,11 that the reduction of oxygen in [N6,2,2,2][NTf2] is a simple one-electron electrochemical process. The model used in the present work for the successful simulation procedure was the simple E mechanism, described fully in Section 3.2.1. The predetermined DA and c values were input

into the program and values of DB, k0, R, and EfQ were varied until the best fit between experimental and theoretical CVs was achieved, and these are shown in Figure 12a-c for Au and Figure 13a-c for Pt at 298, 308, and 318 K and 1000 mV s-1. The parameters, given in Table 3 were found to provide a very good fit over a wide range of scan rates (100-1000 mV s-1). The heterogeneous rate constant, k0, increased with increasing temperature, as well as the diffusion coefficient of the superoxide, increasing as the RTIL viscosity decreases, and the EfQ apparently becoming more negative. Comparing Au and Pt, it is observed that, as well as the apparent more negative EQf (Note that the formal electrode potentials are measured vs a silver quasi-reference electrode so the apparent shift in EfQ reflect changes in the reference electrode.) on Pt, k0 is much slower on Pt over the temperature range, for example (at 308 K), 0.59 × 10-5 m s-1 on Pt compared to 2.25 × 10-5 m s-1 on Au, a phenomenon reported previously in other RTILs and also in traditional solvents.6,11 Also shown in the table are values for the (DA)/(DB) ratios, where DA, the diffusion coefficient of oxygen, comes from chronoamperomety, and DB, the diffusion coefficient of superoxide, is determined from simulation fits. These values are shown for Au only due to the possible relative unreliability in the Pt measurements, as discussed below. Transfer coefficient values of 0.38 and 0.35 were determined for Au and Pt respectively. Figure 14a-c shows linear plots for ln k0 vs 1/T, EfQ vs T, and ln DA and ln DB vs 1/T. A (dEfQ)/ (dT) value of (0.0030 V K-1 is determined. The activation energies of electron transfer, Ea,ET, are 30.1 (Au) and 62.1 kJ mol-1 (Pt), with activation energies of diffusion of O2, Ea,DA, of 20.7 (Au) and 21.1 kJ mol-1 (Pt) and of O2•-, Ea,DB, of 43.3 (Au) and 41.3 kJ mol-1 (Pt). The diffusional activation energies on gold and platinum are comparable, although notice that there is a significant difference between the activation energies of diffusion of oxygen and superoxide, with Ea,DB being ca. double that of Ea,DA. Evans et al.45 reported very similar diffusional activation energies for TMPD, TMPD+•, and TMPD2+, (e.g., 28.3, 27.0, and 26.9 kJ mol-1 in [C2mim][NTf2]) although this species was also found to obey Stokes-Einstein relationship in a range of RTILs. The difference in the values for oxygen



Figure 12. Experimental (s) and simulated (O) CVs for the reduction of oxygen in a Au microdisk in [N6,2,2,2][NTf2] at a scan rate of 1000 mV s-1 and temperatures of (a) 298, (b) 308, and (c) 318 K. An E mechanism was used to simulate the voltammograms, and the parameters are given in Table 3.

Figure 13. Simulation (O) of experimental (s) CV using an E mechanism, for the reduction of O2 on a Pt electrode in [N6,2,2,2][NTf2] at temperatures of (a) 298, (b) 308, and (c) 318 K (at a scan rate of 1000 mV s-1). See Table 3 for the parameters extracted.

TABLE 3: Simulation Parameters for the Reduction of O2 in [N6,2,2,2][NTf2] on Both Au and Pt Electrodes at Varying Temperatures (298-318 K)a parameter -5

k / × 10 0

EfQ/Vb (DA)/(DB)

-1

ms

Au Pt Au Pt Au Pt

298 K

303 K

308 K

313 K

318 K

1.5 0.3 -0.89 -1.27 56 -

1.8 0.4 -0.90 -1.29 42 -

2.3 0.6 -0.91 -1.34 39 -

3.1 0.9 -0.92 -1.34 36 -

4.0 1.3 -0.93 -1.37 33 -

a E mechanism.12 b Note that the formal electrode potentials are measured Vs a silver quasi-reference electrode.

and superoxide is likely to reflect the non-Stokes-Einstein behavior of these species in the same media.27 One final point that merits comment is the more ‘broad’ voltammetry on Pt, especially for the reverse process, i.e., reoxidation of superoxide. Also, looking at Figure 13a-c, it can be noted that the fit of the back peak is not as good as for the same system on Au, especially at higher scan rates and lower temperatures. A possible explanation for this is the reversible chemical adsorption of the electrochemically generated superoxide species on a Pt electrode. This is investigated further in Section 4.5. 4.3. Reduction of Oxygen in [C4dmim][NTf 2] on a Gold and Platinum Microdisk at Varying Temperatures. Cyclic voltammetry for the reduction of oxygen in [C4dmim][NTf2] at scan rates of 50, 100, 200, 400, 700, and 1000 mV s-1 on a 10 µm diameter Au microdisk, is shown in Figure 15a at a temperature of 308 K. The potential was scanned from 0.0 to -1.30 V; a potential after the reduction peak, and then back to 0.0 V. An oxidation peak, corresponding to the reoxidation of O2•- to O2, was observed at ∼ -1.0 V vs Ag at 308 K. This was then repeated on a Pt electrode of the same geometry and similar voltammetry was observed, shown in Figure 15b at varying scan rate. On a Pt electrode, the potential was swept from 0.0 to -1.95 V with an oxidation peak at ∼ -1.25 V vs Ag.

Again, the voltammetry is asymmetric, and a shift in the position of the oxidation peak to more positive potentials, relative to that expected for an E mechanism, suggests that the system is not simply an electrochemical (E) one. Interactions between the electrogenerated superoxide and the imidazolium cation of the RTIL solvent, as reported previously (at one temperature only) by Islam et al.9 and Barnes et al.,13 allows formation of the O2•- · · · [C4dmim]+ ion-pair, which impedes the electron transfer of the backward reaction, causing the marked positive shift in the back peak. This chemical process most likely occurs via the nucleophilic addition of O2•- to [C4dmim]+ rather than the reduction of [C4dmim]+,10 as the potential of the O2|O2•redox couple is at a more positive potential than the reduction potential of the cation, and in this case can be classed as an EC process. The reduction of oxygen was studied over a wide temperature range and the CV observed over this range at 1000 mV s-1 is shown in panels c and d of Figure 15, respectively. Chronoamperometric analysis allowed determination of D and c at each temperature, and the values are given in Table 2.27 The values obtained are comparable with that reported in a previous publication (D ) 3.9 × 10-10 m2 s-1 and c ) 5.3 mM) on a Pt electrode at 298 K.13 4.4. Simulation of CV in [C4dmim][NTf2]. D and c values were determined from chronoamperometry. The remaining parameters for the electrochemical O2|O2•- redox couple were determined by simulation methods, and comparison of the theoretical results to experimental voltammograms. As mentioned in a previous publication,13 simulations based upon a simple one-electron reduction model (E mechanism), i.e., A +eh B, where A is O2 and B is O2•-, failed to provide a set of parameters which allowed acceptable simultaneous fitting of both the reduction and oxidation waves. A model based on an ECrev mechanism, in which the electrochemically generated superoxide radical anion undergoes a reversible homogeneous chemical association with the [C4dmim] + solvent cation is utilized, and is fully described in

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Figure 14. Plots of (a) ln k0 vs 1/T, (b) EfQ vs T, and (c) ln D vs 1/T for A and B (where k0 is the heterogeneous rate constant (m s-1), EfQ is the formal electrode potential (V) and DA and DB are the diffusion coefficients (m2 s-1) of O2 and O2•-, respectively) extracted via simulation of experimental voltammograms for the reduction of oxygen in [N6,2,2,2][NTf2] on a Au (b, (, 9, and 2) and Pt (°, ), 0, and 4) electrode over a range of temperatures (298-318 K).

Figure 15. CV for the reduction of oxygen in [C4dmim][NTf2] at varying scan rates (50, 100, 200, 400, 700, and 1000 mV s-1) at 308 K on (a) Au and (b) Pt, and at varying temperatures (298, 303, 308, 313, and 318 K) at 1000 mV s-1 on (c) Au and (d) Pt.

Section 3.2.2. The experimental voltammetry was fit over a range of scan rates and full temperature range, on Au initially, and the parameters, including diffusion coefficients of A, B, and C (O2, O2•-, and O2•- · · · [C4dmim]+, respectively), DA, DB, and DC, heterogeneous rate constant, k0, homogeneous rate constant, kf, equilibrium constant, Keq, transfer coefficient, R, and formal electrode potential, EfQ were extracted. The known DA and c values were input and the other parameters varied in order to reproduce the experimental voltammetry as closely as possible. Table 4 lists the optimal parameters used to simulate the voltammetry as shown in Figure 16 for Au (at temperatures of 298, 308, and 318 K and at a scan rate of 1000 mV s-1), with an R value of 0.4. The determined parameters provide an excellent fit over a wide range of scan rates and agree well with kinetic parameters reported for similar systems, for example, k0 values of 1.2 × 10-5, 3.5 × 10-5, and 5.0 × 10-5 m s-1 have been reported for O2|O2•- in [C4mim][BF4],6 [Py14][NTf2],11 and [N6,2,2,2][NTf2],11 respectively, on Au electrodes. Next, voltammetry on Pt was simulated in the same way. Using the same kf and Keq values as for Au, the remaining parameters were optimized to obtain a fit between the experimental and theoretical voltammograms. The optimal parameters used to simulate the CVs shown in Figure 17 are given in Table 4. The determined parameters, including a transfer coefficient

TABLE 4: Simulation Parameters for the Reduction of O2 in [C4dmim][NTf2] on Both Au and Pt Electrodes in the Temperature Range 298-318 Ka parameter -5

k / × 10 0

-1

ms

-1

kf/s Keq

EfQ/Vb DA/ × 10-10 m2 s-1 DB/ × 10-11 m2 s-1 DC/ × 10-12 m2 s-1

298 K

303 K

308 K

313 K

318 K

Au 6.0 8.6 10.4 20.3 34.4 Pt 0.15 0.19 0.30 0.50 0.78 Au 1400 1500 1600 1700 1800 Pt 1400 1500 1600 1700 1800 Au 24 22 20 19 18 Pt 24 22 20 19 18 Au -0.99 -1.01 -1.02 -1.04 -1.04 Pt -1.34 -1.39 -1.39 -1.41 -1.42 Au 4.8 5.8 6.6 8.3 8.7 Pt 4.1 4.8 5.7 6.9 8.3 Au 6.6 7.6 9.0 11.8 12.4 Pt 7.5 8.5 9.5 11.0 13.0 Au 6.1 7.8 10.6 13.3 15.0 Pt 8.0 9.5 12.3 13.3 13.0

a EC(ads) mechanism.13 b Note that the formal electrode potentials quoted are relative to a silver quasi-reference electrode.

value of 0.36, provide a fit over a wide range of scan rates. However, the approximate k0 values inferred are comparable with those published previously on Pt electrodes; 1.1 × 10-5 m s-1 in [C4dmim][NTf2]13 and 3.0 × 10-5 m s-1 in [N6,2,2,2][NTf2].11 For both Au and Pt, with increasing temperature, k0, kf, and diffusion coefficients (D) of all species increases, with a decrease



Figure 16. Experimental (-) and theoretical (O) for the reduction of oxygen in [C4dmim][NTf2] at 1000 mV s-1 at (a) 298, (b) 308, and (c) 318 K on a Au electrode, simulated using a model based on an ECrev mechanism, using the parameters listed in Table 4.

Figure 17. Experimental (s) and simulated (O) voltammograms for the reduction of oxygen in [C4dmim][NTf2] on a Pt electrode at temperatures of (a) 298, (b) 308, and (c) 318 K at a scan rate of 1000 mV s-1, based on an ECrev mechanism. The parameters used to achieve the fits are listed in Table 4.

Figure 18. Plots of (a) ln k0 vs 1/T, (b) EfQ vs 1/T, and (c) ln DA,B,C vs 1/T (where k0 is the heterogeneous rate constant (m s-1), EfQ is the formal electrode potential (V) and DA,B,C are the diffusion coefficients (m2 s-1) for species A, B, and C (O2, O2•- and O2|O2•-, respectively)) extracted from simulation of experimental voltammetry for the reduction of oxygen in [C4dmim][NTf2] on a Au (b, (, 9, 2, and solid right-pointing triangle) and Pt (O, ), 0, 4, and open right-pointing triangle) electrode at varying temperatures (298-318 K).

in Keq. Suitable linear plots of ln k0 vs 1/T are shown in Figure 18a for both Au and Pt, with activation energies of electron transfer determined as 61.7 kJ mol-1 on Au and 67.1 kJ mol-1 on Pt. EfQ is also seen to vary systematically with temperature, becoming more negative with an increase from 298-318 K (Figure 18b) and a (dEfQ)/(dT) value of (0.0031 V K-1 was obtained from the plots. As was observed in the previous section from chronoamperometry, diffusion coefficients increase with temperature, and plots of ln D vs 1/T are shown in Figure 18c for species A, B, and C. In [C4dmim][NTf2], the approximate heterogeneous rate constant is significantly lower on Pt than on Au, which is analogous to that seen in other ionic liquids, as well as in conventional solvents, where ‘sluggish’ kinetics have been observed on platinum disks.2,15,5,11 Activation energies of diffusion are determined as being 25.2 and 26.9 kJ mol-1 for oxygen and superoxide respectively. Plotting ln kf vs 1/T (not shown) allows calculation of the activation energy for the forward going homogeneous chemical reaction from the linear plot, determined as 10 kJ mol-1, with a reaction enthalpy, ∆HQ, value of -13.4 kJ mol-1 from linear (d ln Keq)/(d(1/T)) plots (not shown).

4.5. Simulation of CV in [N6,2,2,2][NTf2] using an EC(ads) Mechanism. It was suggested23 that the reduced product, superoxide, may adsorb onto the platinum surface, giving more broad voltammetry than was observed on gold. Moreover, we have noted above the lower quality of fit for Pt as compared to Au for both solvent systems studied. To test the theory of possible superoxide adsorption, a simulation program based on an EC(ads) mechanism was designed and utilized to simulate the experimental voltammetry with the hope of achieving a better fit using a Langmuirian model for the adsorption of superoxide (see Section 3.2.3 for a full description of this model). The parameters to vary in this model include the ratio ˜ 0()(k0rd)/ (DB)/(DA), dimensionless heterogeneous rate constant, K (DA)), transfer coefficient, R, equilibrium constant, Keq()(kadsc*)/ (kdes)) and adsorption coefficient, Kãds()(kadsrd)/(DA)), as well as the dimensionless capacity, ζ, which involves the maximum surface coverage, Γmax (estimated as 1.8 × 10-5 mol m-2 from the van der Waals radius of O), the solubility of oxygen in the RTIL and the electrode radius. Due to the large number of variables in this model, the following systematic approach was taken.

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Figure 19. Attempts at simulation of voltammetry for the reduction of oxygen in [N6,2,2,2][NTf2] on a Pt microdisk at 1000 mV s-1 and 298 K using a simulation model based on an EC(ads) process. k0

Using the model A + e- y\z B(soln) followed by B(soln) kads

y\z B(ads), and initially using a fixed value of k0 (2.3 × 10-5 kdes

m s-1) and kads (0.1 m s-1), the value of kdes was varied from 10 to 0.0001 mol m-2 s-1. The theoretical CVs obtained are shown in Figure 19a. A good fit on the reductive wave is achieved using the D and c values obtained from chronoamperometry (3.43 × 10 -10 m2 s-1 and 5.86 mM, respectively, at 298 K). Also, a good fit of peak current of the oxidation peak, ipox, and peak-to-peak separation, ∆Epp, is seen, as determined by DB (1.60 × 10-11 m2 s-1) and k0 values, respectively. Although, as can be seen, on increasing kdes, a second peak appears in the simulations, corresponding to the oxidation of adsorbed superoxide, instead of causing a broadening of the peak as seen experimentally. Second, at fixed Keq, the values of kads and kdes were varied from 10-1 to 10-6. This was undertaken at three different values of Keq, increased by a factor of 10 each time, and the resulting plots are shown in Figure 19b-d. Again, peak broadening suggests that adsorption of superoxide may be occurring on the electrode, but the mechanism by which it does so is more complicated than the one modeled in this work. In particular, the nature of any adsorption is likely to be significantly more complex than simple Langmuirian. 5. Conclusions The reduction of oxygen to superoxide has been modeled in different ionic liquids. At gold electrodes a simple E process is seen in [N6,2,2,2][NTf2], whereas in [C4dmim][NTf2], the ECrev mechanism operates. At platinum electrodes, the ECrev applies in [C4dmim][NTf2], although the electrochemical kinetics are slower and the processes probably complicated by some adsorption of superoxide ion, particularly in [N6,2,2,2][NTf2]. Where the models have been quantitatively fitted, relevant kinetic and thermodynamic parameters have been reported. Acknowledgment. X.-J.H. and E.I.R. thank the EPSRC for financial support.

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