Voltammetry

Voltammetry is a very useful tool commonly employed to acquire qualitative information about electrode reaction mechanisms. During voltammetry experiments the applied potential is varied whilst the resulting current is monitored. There are several types of voltammetry, including:

Electrode Solution 2e- ₁

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 Potential step voltammetry - experiments involve instantaneous jumps of the potential and the recording of the resulting current as a function of time. This allows the estimation of the diffusion coefficients of the species to be obtained.

 Linear sweep voltammetry – involves recording the current as a function of the electrode potential, which is swept from a lower limit to an upper limit. It is used to determine either the reduction or oxidation potential of a species.

 Cyclic voltammetry – this technique can provide information about both the forward and reverse redox potentials, as well as the diffusion coefficients. It is the method of choice for the work presented herein.

1.5.3.1 Cyclic Voltammetry

Cyclic voltammetry is frequently used to acquire information about the redox properties of organic compounds. As in linear sweep voltammetry, the potential of an electrode in contact with an analyte is controlled while measuring the resulting current. Most experiments use a three electrode set up, which consists of a working electrode, a reference electrode and a counter electrode. The working electrode is the electrode at which the reaction being studied occurs; depending on whether the reaction is oxidation or reduction the working electrode can be referred to as an anode or cathode. Common working electrodes consist of inert metals such as gold, or inert carbon such as glassy carbon. The counter electrode acts as the cathode when the working electron is functioning as the anode, and vice versa. The surface area of the counter electrode is often much larger than that of the working electrode, this is to ensure that the half reaction occurs rapidly and therefore does not limit the rate of reaction at the working electrode. The reference electrode is a half cell that has a stable, known potential and is used to measure and control the potential of the working electrode. The standard hydrogen electrode is the universal reference for reporting electrode potentials; its half cell is:

The absolute electrode potential of this system is estimated to be around 4.5 V at 25°C, but in order to allow comparisons between other electrode reactions its standard electrode potential is declared to be zero at all temperatures.

During a cyclic voltammetry experiment the potential is controlled between the reference and working electrodes, and the current flow between the working and

26 -1.250 -1.000 -0.750 -0.500 -0.250 0 0.250 0.500 0.750 1.000 1.250 -3 -0.075x10 -3 -0.050x10 -3 -0.025x10 0 -3 0.025x10 -3 0.050x10 -3 0.075x10 -3 0.100x10 Potential (mV) C u rr en t (n A ) Epc Epa ipc ipa

counter electrodes is measured. The potential is swept between two predetermined potential limits (Figure 1.18), with the current being monitored on both the forward and reverse sweeps. The data is then plotted as current (i) vs. potential (E).

If, during the scan, the potential approaches the redox potential of the analyte the current will increase. This increase slows as the concentration of the analyte at the interface decreases, thus producing a peak in the voltammogram. If the electrode reaction is reversible a second, reverse peak will appear when the applied potential is inverted. Cyclic voltammograms acquired in solution will always display a hysteresis between the reduction and oxidation peaks, due to the limits imposed by mass transport.

The electron transfer kinetics of the electrode reaction determine the shape of the cyclic voltammogram. If the electron transfer is very rapid compared with the rate of diffusion, the Nernst equilibrium can be maintained at the surface for all potentials

Time Potential

Figure 1.19 A typical cyclic voltammogram. Figure 1.18 The potential waveform for a CV experiment.

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and the reaction is said to be reversible. The shape of cyclic voltammograms of reversible electrode reactions can be predicted by theory, and their peak currents are given by the Randles-Sevčik Equation:

1.16

where ip = peak current (Amps), n = number of electrons transferred, A = electrode surface area (cm2), D = mean diffusion coefficient of solution redox species (cm2 s- 1_{), C}

∞ = bulk concentration (mol cm-3), v = sweep rate (volt s-1).

There are also several diagnostic tests that have been devised for the analysis of reversible voltammograms:

 The peak separation is independent of sweep rate, and is equal to 59/n mV

 The ratio of the peak anodic and cathodic currents is one

 The peak current is proportional to v1/2

 At potentials beyond Ep the current falls off with t-1/2

If electron transfer is slow, and therefore insufficient to maintain surface equilibrium, the reaction is said to be electrochemically irreversible. Intermediate cases also exist, and are termed quasi-reversible. Figure 1.20 shows the effect of a decreasing rate constant of electron transfer.

The blue line shows the voltammogram of a process that has a high rate constant, and is therefore reversible. As the rate constant decreases the curves shift to more

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and more extreme potentials, and slower sweep rates are necessary in order to observe a reverse peak.

The electron transfer step of an EC electrode reaction can be thought of as being chemically irreversible; in this case both the sweep rate and the rate constant of the subsequent chemical reaction will determine whether or not a reverse peak is observed.