Cyclic Voltammetry (CV)

In a cyclic voltammetry experiment the potential applied between the reference electrode and working electrode increases in a linear fashion with time (scan rate (V/s)). Concomitantly, the current is measured between the working and counter (or auxiliary) electrode resulting in data that is plotted as current (i) vs. potential (E). Reduction and oxidation events are observed and assigned in the resulting plots. Reduction events occur at analyte specific potential voltages where the reaction M⁺ⁿ + e^- → M^+n-1 (M = metal) is energetically favored (known as reduction potential) and measured by increasing current values. The current will increase as the voltage potential reaches the reduction potential of the analyte, but then falls off as the maximum rate of mass transfer has been reached. The current goes down only to reach equilibrium at some steady value. Oxidation reactions (M⁺ⁿ → M⁺ⁿ⁺¹ + e^-) can also be observed as a decrease in current values at potentials that energetically favor the loss of electron(s).

The resulting voltammograms are then analyzed and the potential (E_p) and current (I_p) data for both reduction and oxidation events under each setup experimental conditions are noted. This information can be utilized to evaluate the reversibility of coupled reduction and oxidation events. As noted above peak potentials (E_pa and E_pc) and the peak currents (i_pc and i_pa) are the fundamental parameters used to characterize a redox couple or event. During a reversible redox process, the oxidized and reduced forms of a compound are in equilibrium at the electrode surface. The Nernst equation describes the relationship between potential and the equilibrium ratio, ([R] / [O])_x=0.

         (1)

Where, is called the formal potential of the reaction and takes into account the activity coefficients and other experimental factors.

Specifically, the peak current of a reversible reaction is given by:

         (2)

where, i_p is peak current in amperes, n is the number of electrons involved, A is the area of the electrode in cm², D_o is the diffusion constant (cm²/s), v is the scan rate (V/s) and C_o* is the bulk concentration (moles/cm³). The diffusion constant can be measured using more extensive experiments detailed elsewhere and are not the focus of this video¹. However, more basic guidelines can be used for evaluating the reversibility of a system¹. Criteria for a totally reversible system¹:

1. at various scan rates   n = number of electrons
2. at various scan rates
3. |i_pa/i_pc| = 1 at various scan rates
4. E_p is independent of v    v = scan rate
5. at potentials beyond E_p, i^-2 t

Simple diagnostic tests for defining a totally irreversible system at 25 °C are:

1. No reverse peak (this refers to chemical irreversibility, but not necessarily to electron transfer irreversibility)
2. E_pc shifts for each decade increase in v (electrochemical irreversibility)
3. 

Finally, diagnostic tests for defining a quasi-reversible system are:

1. 
2. 
3. E_pc shifts negatively with increasing v

The position of the reduction and/or oxidation events can be used to infer information about the electronic nature of transition metal complexes and the effects on ligands as donors. For example, the Fe^+3/+2 reduction potential of ferrocene derivatives is very sensitive to the electronic environment provided by the cyclopentadienyl (Cp) ligand set. Electron donating (withdrawing) Cp substituents increase (decrease) the electron density on the iron center and shift the redox potential to negative (positive) values relative to Fc.

In this protocol ferrocene will be used as an example. Experimental conditions such as solvent, electrolyte choice, and the potential range studied (scan window) are largely dictated by analyte solubility and experimental conditions. Users are encouraged to consult relevant texts such as Bard and Faulkner¹ to learn more.

A CV scan of ferrocene at 300 mV/s in acetonitrile was carried out and the corresponding voltammogram is shown in Figure 2.

    

The ΔE can be derived from the data in Figure 2 based on the difference between E_pa and E_pc.

    

The cyclic voltammograms overlaid in Figure 3 represent consecutive experiments performed on the same system at different scan rates. As noted in above, a linear plot of I_p vs. v^1/2 (inset in Figure 3) shows that the reaction is diffusion controlled.

The position of the E_1/2 or redox event (E_pa or E_pc) can be used to determine the effects that the ligand has on the redox active metal center providing the electrochemical response. Figure 4 shows a series of ferrocene-based congeners with varying substitutions on the Cp ring. As shown in Figure 5, the electron withdrawing halide results in the E_1/2 value of this complex to be shifted to more positive potentials because the oxidized form is destabilized by the electron withdrawing ligand. The electron donating methyl groups of compound C result in the E_1/2 to shift to more negative potentials as the oxidized species is stabilized.

Figure 2. A CV scan of ferrocene at 150 mV/s in acetonitrile. Please click here to view a larger version of this figure.

Figure 3. A cobalt-containing compound that gives rise to one reduction event. The inset shows a linear correlation between i_p and v^1/2. Please click here to view a larger version of this figure.

Figure 4. A series of ferrocene-based compounds. Please click here to view a larger version of this figure.

Figure 5. The resulting cyclic voltammograms of A-C (Figure 4) show a marked shift in E_1/2 due to the electronic ligand effects attached to the metal center. Please click here to view a larger version of this figure.