Diffusion regimes at nanoelectrode ensembles in different ionic liquids

TL;DR: In this article, the electrochemical and diffusion behavior of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles (NEEs) in comparison with millimetre sized gold (Au-macro) and glassy carbon (GC) disk electrodes.

About: This article is published in Electrochimica Acta.The article was published on 2010-03-01 and is currently open access. It has received 34 citations till now. The article focuses on the topics: Diffusion & Ionic liquid.

Summary

2.1. Apparatus and procedures

• Voltammetric measurements were performed with a CH660A apparatus controlled via PC, using IR-drop compensation.
• All electroanalytical measurements were carried out in a three-electrodes cell of small volume (5 mL).
• ILs were dried overnight in a vacuum oven at 40 C before use, after treatment with activated molecular sieves.
• It was previously demonstrated that this procedure is able to remove trace water from hygroscopic IL such as [BMIm][BF 4 ] [38] .
• All measurements were performed at room temperature (20 ± 1 C), operating under a nitrogen atmosphere; the purging gas fluxed trough traps loaded with concentrated sulphuric acid, in order to prevent eventual entrance into the cell of humidity from the room environment.

2.3. Chemicals

• [BMIm], [BMPy] and [P 14,666 ] ionic liquids were prepare by ion metathesis reaction of their chloride salts with either NaN(CN) 2 or LiN(Tf) 2 in dichloromethane (DCM) solvent.
• The procedure involves firstly dissolving the quaternary ammonium or phosphonium cation salt in DCM followed by the addition of 1.2 equivalents of the anion salt.
• The DCM solution of the crude ionic liquid was washed (5×) with deionised water.
• Activated carbon was then added to the ionic liquid and stirred for 24 h.
• All other reagents were of analytical grade and used as received.

3.1. GC-macro

• In any case, the cross-comparison of their D values, all measured with the same procedure, indicates that the diffusion coefficients of the ionic derivatives are smaller than those of neutral ferrocene.
• This suggests the occurrence of a stronger interaction of the ILs with the ionic ferrocenes.
• In ILs, the lowering of diffusion coefficients for ionic species with respect to their neutral analogues is indeed documented in the literature [11, 12] .
• Compton et al. demonstrated [45] , that the D value for neutral ferrocene is much larger than D for the ferricinium cation, the same holding also for the cobaltocene/cobalticinium couple.
• A comparable unequality in diffusion coefficients of the butylviologen dication vs. the mono-cationic radical was reported to be the cause of differences in reduction peak currents for the two species in [BMIm][BF 4 ] [15].

3.2. Au-macro

• In the following part of this research the authors were interested in investigating mainly the role of the solvent on the diffusion of the analyte to the nanoelectrodes, therefore they chose to avoid any complication by adsorption or other non-diffusion controlled processes by focusing their attention on the behaviour of ferrocene and its derivatives in the [N(Tf) 2 ] containing ILs.
• At low scan rates peak shaped voltammograms are recorded which become sigmoidally shaped at higher scan rates, however with a shift detectable between the forward and backward patterns.
• On the basis of recent reports [26] , the CV shape observed at high scan rates could be considered typical of the mixed diffusion layers regime, which is observed when there is a partial overlapping of individual diffusion layers.
• Even the contribution of the double-layer charging current (I c ) cannot be neglected, since such a capacitive current can also be responsible for the lack of overlap between the forward and backward voltammetric patterns.
• Note that in IL double-layer charging currents are significantly higher than that in water.

3.3. Voltammetry with NEEs

• Further studies were performed focusing on the CV behaviour on NEEs of the ionic probes FcCOO − or FA + in the ILs.
• As shown for instance in Fig. 7 for the typical case of 1 mM FcCOO − in [BmPy][N(Tf) 2 ], both the shape of the CV (Fig. 7A ) as well as the scarce scan rate dependence of I max (Fig. 7B ) indicate that crosstalking between the nanoelectrodes is dramatically reduced for any of the scan rates explored.

4. Conclusions

• Finally, gold is not the electrode material of choice for measurements under diffusion control in [N(CN) 2 ] containing ILs, since this anion favours the adsorption of electroactive redox molecules on the electrode surface.
• No such problem was found in [NTf 2 ] containing ILs while at GC electrodes diffusion controlled processes are observed both in the presence of [N(CN) 2 ] and [NTf 2 ].

Figures

Fig. 5. Cyclic voltammograms recorded at different scan rates at a NEE (geometric area 0.07 cm2; active area 0.004 cm2): (A) 20mM Fc [BMPy][N(Tf)2]; (B) 5mM Fc in [BMIm][N(Tf)2]; (C) 50mM Fc [P14,666][N(Tf)2]. Scan rates: full line 5mV/s; dashed line 50mVs−1; dotted line 500mV/s.

Fig. 6. Dependence of the I/Imax ratio on the scan rate (A) and on the square root of the scan rate (B) for ferrocene oxidation at a NEE in: ( ) [BMPy][N(Tf)2]; (©) [BMIm][N(Tf)2]; ( ) [P14,666][N(Tf)2].

Table 1 Surveys of the literature data on the viscosities, , of the studied ionic liquids.

Fig. 7. (A) Cyclic voltammograms of 1mM FcCOO− in [BMPy][N(Tf2)] recorded a l (

Fig. 1. SEM image of a NEE.

Fig. 2. (A) Cyclic voltammograms of 1mM Fc in [BMIm][N(CN)2] recorded at GCE (geometric area 0.19 cm2) at different scan rates: 20, 50, 100, 200, 500, 1000mVs−1; peak currents scale with the scan rate. (B) Dependence of the anodic peak current on the square root of the scan rate.

Table 2 Diffusion coefficients measured by cyclic voltammetry at the glassy carbon electrode in solution containing 1mM redox probe in different ionic liquids.

Fig. 4. Cyclic voltammograms of 1mM Fc in [BMIm][N(CN)2] recorded at an Au-macro electrode at different scan rates: 50, 100, 200 and 500mVs−1; other parameters as in Fig. 3.

Fig. 3. (A) Cyclic voltammograms of 1mM FcCOOH in [BMIm][N(CN)2] recorded at an Au-macro electrode (geometric area 0.07 cm2), at different scan rates: 10, 20, 50, 100, 200, 400, 800 and 1000mVs−1; peak currents scale with the scan rate. (B) Dependence of the anodic peak current on the scan rate.

Frequently asked questions

Q1. What have the authors contributed in "Iffusion regimes at nanoelectrode ensembles in different ionic liquids" ?

The electrochemical and diffusion behaviour of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles ( NEEs ) in comparison with millimetre sized gold ( Au-macro ) and glassy carbon ( GC ) disk electrodes. The ILs are the dicyanamide, [ N ( CN ) 2 ] or bis ( trifluoromethylsulfonyl ) amide ), [ N ( Tf ) 2 ] salts of the following cations: 1-butyl-3-methylimidazolium, [ BMIm ], 1-butyl-3-methylpyrrolidonium, [ BMPy ], or tris ( nhexyl ) tetradecylphosphonium [ P14,666 ]. For this reason the diffusion at gold NEEs is studied only in the former ILs.