Cyclic Voltammetry

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During the measurement a triangular voltage is applied between the working and the counter electrodes, and the generated current is plotted versus the applied voltage to obtain the cyclic voltammogram trace. When the starting potential value is more positive than the standard potential E° shown by the electrochemical-active species, and the measure proceeds at a scan rate toward values more negative than E°, then a Faraday current flows and the characteristic redox-waves are observed in the cyclic voltammogram (Figure 53). ^[143]

Figure 53: Cyclic voltammogram of a reversible process with basic parameters: E_pf = potential of the forward peak; E_pr = potential of the return peak; E_ = potential value at the inversion of the scan direction; i_pf = current

of the forward peak with respect to its baseline; i_pr = current of the return peak regarding its baseline;

ΔEp = peak-to-peak separation.

The redox reactions occur through electron-transfer at the working electrode interface.

Depending on the voltage, the analyte will be reduced or oxidized. Two different processes play an important role in the experiment: the heterogeneous charge transfer at the electrode interface and the mass transport produced by the diffusion current.

Scheme 10: Oxidation and reduction reactions.

57 In the redox reaction represented in scheme 10, when the potential E(t) increases and reaches an appropriate value for the oxidation of A(II), the concentration of A(II) decreases at the electrode surface while the concentration of A(III) does the contrary. This phenomenon produces a concentration gradient and a current begins to flow. When the concentration of A(II) is almost zero, the current reaches a maximum value (ipc). After this point, the gradient cannot increase further, it no longer depends on the potential, and as a consequence the current drops. When the scan direction is reversed and the reduction of A(III) to A(II) occurs, analogous phenomena are observed. It leads to a negative current, which reaches a minimum (ipa). These two processes together provide the characteristic redox-waves of the cyclic voltammograms (Figure 53).

The half step potential (E1/2) is an important information. It is the potential between the anodic and cathodic peaks.

_⁄ = + � Equation 10

The peak separation is defined as the difference between the anodic and cathodic peaks:

∆ = | − _�| Equation 11

The experiment can start at any voltage and proceed in any direction. The adequate conditions should be adjusted depending on the solvent and on the electroactive substance.

In figure 53 the measure goes first (forward peak, pf) to the positive potential and the substance is oxidized. Afterwards (reverse peak, pr) the oxidized species is reduced again.

The redox processes in cyclic voltammetry can be divided in three different types: reversible, quasireversible and irreversible processes.

In figure 53 a reversible process is presented. In this case the rate of electron transfer is higher than the mass transport, which allows for the thermodynamic equilibrium to be reached at the electrode interface. The reaction at the electrode responds to the Nerst equation (Equation 12):

= ^°+

�

Equation 12

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This type of process can be recognized by some characteristics: Epf and Epr are not scan rate dependent. The ratio between the current of the reverse peak and the current of the forward peak (ipr/ipf) is equal to 1; which indicates that the current, as the number of transferred electrons, is identical in both semi-processes. The peak-to-peak separation is 59/n mV (being n the number of electrons involved in the electrochemical reaction) at 25 °C.

This value can be calculated using the next expression:

∆ = . Equation 13

Finally, the currents ipf and ipr are proportional to the square root of the scan rate √ . In a reversible mechanism, no molecule reorganizations accompany the redox reaction.

Figure 54: Cyclic voltammogram of a a) reversible process, b) a quasireversible process and a c) irreversible process. ^[143]

In case of a quasireversible processes (Figure 54, b) the shape of the cyclic voltammgram changes with the scan rate v. At low scan rates it shows a reversible behaviour, but at high scan rates, it does not. It occurs when the rate of the electron-transfer and the mass transport share the same order of magnitude.

Although ipf increases with √ , it does it non-linearly, so the value of Epf shifts with the scan rate v. The same occurs with the values defined by the reverse peak. The ratio ipr/i_pf is, nevertheless, almost 1.

59 But not only the peak shape varies with respect to the reversible processes. The peak separation increases at high scan rates, although at low scan rates approaches the theoretical value of 59 mV/n, expected for electrochemically reversible processes.

It must be emphasized that quasireversibility is an electrochemical criterion which defines a situation where some important reorganization takes place at the redox reaction, but the active species does ot suffe f ag e tatio . It e tai l does ot ea pa tial he i al

e e si ilit .

The last possible mechanism is the irreversible mechanism (Figure 54, c), which is characterized by an electron transfer rate much lower than the mass transport, so only the anodic or the cathodic processes can alternatively be detected. As a consequence, no thermodynamic equilibrium can be reached at the electrode interface. The expression � ∗ √ ⁻ stays constant, but E_pf will shift with the scan rate v.

The activation barrier to the electron transfer of this type of process is so high that normally the original frame undergoes fragmentation and several new species are formed. ^[143]

An interesting application of the cyclic voltammetry method is the study of organometallic complexes with more than one metal center and the electronic communication between them.

The Robin-Day Classification ^[169] explains the nature of mixed valence compounds, which contain ions of the same element in two different oxidation states. ^[170] The classification is based on the degree of electronic delocalization between the metal centers, which is directly related to the electronic communication between them.

The metal complexes of Class I (Figure 55, above) are those in which the charge is localized in only one of the metal centers, and also where an electronic communication between them does not exist. In cyclic voltammetry only one redox-wave will be observed for each redox process, and that is because the metal centers will undergo the redox reaction exactly at the same potential.

In the metal complexes of Class II (Figure 55, middle) the charge is slightly delocalized among the redox centers. An electronic communication between the metal centers exists, but its

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intensity is not very high. In cyclic voltammetry two different redox-waves will be identified for each redox processes, but the peak separation will be minimal.

Finally, Class III (Figure 55, bottom) includes the metal complexes where the charge is completely delocalized among the metal centers. In these complexes a high electronic communication between the redox centers exists. Two well defined and separated redox-waves will be observed in the cyclic voltamgram for each redox processes. ^[143]

Figure 55: Schematic representation of the electronic effects in mixed valence species according to the Robin-Day classification (the charge assumed as positive) (left); and different cyclic voltammograms expected from

biferrocene-complexes, depending on the reach of the electronic communication. ^[143]