Voltammetry

In an electrochemical cell, a potential is applied to the working electrode (WE) with respect to a quasi-reference counter electrode (QRCE). This setup is adequate for small currents (< 1 μA). For currents larger (>1 μA) in aqueous solution, a three-electrode setup in which a third electrode, known as the counter electrode (CE) (or auxiliary electrode), is used to facilitate the passage of electrical current through the solution.1

1.3.1.1.

Cyclic Voltammetry at Macroelectrodes

Cyclic voltammetry (CV) employs a linear potential scan between two potential values, E1 and E2 at a steady scan rate, v, as shown in Figure 1.7(a). E1 is normally a potential at which there is no electrochemical activity and E2 is a potential where the reaction is mass transport controlled. CV is the most widely used technique for the characterisation of redox species and can provide quantitative information about the number of oxidation states and their stability,44–47 as well as, surface adsorption mechanisms48–51 and coupled chemical reactions.52–54 Furthermore, multiple potential cycles can also be employed to study film formation, for example.55–58 Current-voltage curves obtained from a cyclic voltammetry experiment are characteristic of the reaction mechanism and kinetic conditions. Figure 1.7(b) shows a typical CV current-voltage plot for a simple oxidation reaction at a macroelectrode (~mm dimensions) where planar diffusion conditions prevail.

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Figure 1.7 (a) Time-dependent potential waveform employed in cyclic voltammetry. (b) Typical current-potential plot of a cyclic voltammogram under planar diffusion conditions.

During the forward sweep of the potential scan, the reduced species is consumed (oxidised) at the electrode surface and an oxidation current is observed. On the reverse potential scan, the reduced species is regenerated at the electrode resulting in a reduction current. The change of sign in the current is directly related to the inversion of the concentration gradient of the reduced species at the electrode surface. The peaks in the voltammogram occur at the potential at which mass transport can no longer keep up with the electrode kinetics and so a depletion occurs. A key feature of cyclic voltammetry is the current peak height, ip, which is given by the Randles-Sevcik equation:

5 3/2 1/2 1/2

p 2.69 10

i   n AD cv at 298 K (Eq. 1.21)

Hence, a fully reversible (fast ET mechanism) voltammogram will have the following characteristics: (1) 1 p / 2 i v (2) E_p E_p,oxE_p,red 59/n mV at 298 K (3) i_p,ox/i_p,red 1

The shape of voltammograms alter when the rate of mass transport overcomes the rate of electron transfer, i.e. increasing v or decreasing k0. This results in peak broadening and increased separation of the oxidation and reduction peaks, ΔEp, as shown in Figure 1.8.

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Figure 1.8 Cyclic voltammograms which are kinetically limited. The currents are normalised to demonstrate the effects of changing the potential scan rate or the electron transfer rate constant.

1.3.1.2.

Voltammetric behaviour of Ultramicroelectrodes

Ultramicroelectrodes (UMEs) are electrodes that have micrometre-dimension (typically ≤ 25 μm). UMEs come in many shapes, e.g. disk,59–62 _spherical,63–65 _band9,66,67 _or cylindrical68,69 and in some cases, constructed down to a few nm in size. UMEs offer advantages over larger (macro-) electrodes, such as high mass transport rates due to hemispherical diffusion where the mass transport layer is larger than the electrode size,1 low ohmic drop (iRu drop) due to the small currents generated, and low double-layer charging and capacitances due to the small surface area.59,70,71

In electrochemical scanning probe microscopy, specifically scanning electrochemical microscopy (SECM), a disk-shaped UME is typically used as a probe. The plane of the electrode can be characterised by the size ratio of the radius of the insulating material to that of the active electrode, known as the RG (typically 2 to 20). For a disk- shaped UME, the high mass transport rates result in a diffusion-limited steady-state current,

i∞, expressed by:

4

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where n is the number of electrons transferred per redox event, F is the Faraday constant,

a is the radius of the electrode and D and c are the diffusion coefficient and concentration of the redox species, respectively. UMEs have been used in the measurements of electron transfer kinetics using both steady-state voltammetry72–75 and fast scan cyclic voltammetry58,75–77 _methods.

In the case where mass transport reaches a steady-state, e.g. at a microelectrode where hemispherical diffusion dominates, the current-voltage curve has a sigmoidal shape as shown in Figure 1.9. The kinetic parameters can be determined from the slope of the sigmoidal voltammogram, ΔE1/2, which is the difference between the two quartile potentials (quarter-wave and three quarter-wave).72

Figure 1.9 Steady-state voltammograms which are kinetically limited.

1.3.1.3.

Adsorption Voltammetry

Cyclic voltammetry experiments are sensitive to redox reactions involving adsorption of both the reactant and product and differs from when both species are in solution. This is because mass transport need not be considered. Figure 1.10 shows a typical cyclic voltammogram for ideal Nernstian ET with the Langmuir isotherm describing the adsorption. It consists of two symmetrical peaks, where the charges (area under the peak)

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for the oxidation and reduction are equal and tell us about the amount of reactant present. Hence, if the potential scan rate is increased, the peak current must also increase and is given by: 2 2 0 p 4 n F i A RT   (Eq. 1.23)

where Γ0 _{is the maximum surface coverage of the reactant.}

Figure 1.10 Typical current-voltage curve for a surface-confined process.

1.3.2.

Fourier-Transformed Large Amplitude Alternating