Cyclic Voltammetry

Cyclic voltammetry (CV) is one of the most frequently used electrochemical methods because of its relative simplicity and its high information content. For example, a cyclic voltammetric (I-E) curve for the oxidation of ferrocyanide (5mM) to ferricyanide in the supporting electrolyte solution of KCl (1 M) at a freshly polished glassy carbon electrode (1.5 mm dia.) is shown in Figure 1. The waveform of the voltage applied to the working electrode versus the reference electrode is triangular shaped. Since this voltage varies linearly with time, the slope is referred to as the scan rate (V/s). On the reverse scan, the ferricyanide, formed during the forward scan, is reduced back to ferrocyanide. The peak shape of the oxidative and reverse current-potential (I-E) curve in Figure 1 is typical for an electrode reaction in which the rate is governed by diffusion to a planar electrode surface. That is, the rate of the electron transfer step is relatively fast compared to that of diffusion. In such a case the peak current, I_p, is governed by the Randle-Sevcik relationship:

        I_p = k n^3/2 A D^1/2 C V^1/2

 Where the constant, k, has a value of 2.72 x 10⁵; n is the mole of electrons transferred per mole of electroactive species; A is the area of the electrode in cm²; D is the diffusion coefficient in cm²/s; C is concentration in mole/L; and V is the scan rate of the potential in volt/s. The I_p is linearly proportional to the bulk concentration, C, of the electroactive species and the square root of the scan rate, V^1/2. Thus, an important diagnostic is a plot of the I_p vs V^1/2. If the plot is linear, it is reasonably safe to say that the electrode reaction is controlled by diffusion, which is the mass transport of electroactive species to the surface of the electrode across a concentration gradient. The thickness, d, of the "diffusion" layer can be approximated by: d ~ [Dt]^1/2 , where D is the diffusion coefficient and t is time in seconds. A "quiet" i.e. unstirred solution is required. The presence of supporting electrolyte, such as the KCl in the example, is required to eliminate movement of the charged electroactive species due to migration in the electric field gradient.

Another important diagnostic to characterize the electrode reaction is the value of the peak potential, E_p. When the rate of electron transfer is fast, the E_p value will be independent of the scan rate and thus the value of I_p. Such a reaction is said to be "reversible." For a reversible reaction, the difference between the anodic and cathodic E_p values is equal to 57 mV/n. This value will be independent of the scan rate. There is a caveat to the diagnostics for reversibility when there is a notable solution resistance (ohmic) between the working and reference electrode. The measured potential then contains an additional component of potential equal to E (ohmic) = iR. Some instruments have the ability to compensate for this "error" voltage (referred to as iR compensation). Cypress Systems also provides a gel-filled hook-shaped reference electrode whereby the tip of the reference can be placed close to the surface of the working electrode to minimize iR.

Experimental Conditions for Figure 1 :

Irreversibility is when the rate of electron transfer is sufficiently slow so that the potential no longer reflects the equilibrium activity of the redox couple at the electrode surface. In such a case, the E_p values will change as a function of the scan rate. The 66-CS1200 has algorithms to evaluate the I_p and E_p values, calculate the area under the I-E curves (the integrated charge), and to calculate an estimated rate constant. A unique feature of an electrochemical reaction is that a "reversible" electrode reaction at low scan rates can become "irreversible" at higher scan rates.

Figure 2 illustrates the CV for an electroactive species (ferrocene) run at a glassy carbon microelectrode of 10 um diameter. Not only is the magnitude of the current much less than what is shown in Figure 1 due to the smaller electrode area, the current goes to a steady-state maximum and is not peak-shaped. This steady-state current is explained by envisioning that the microelectrode is a "dot" with the diffusion layer being hemispherical in shape extending out into the solution. The amount of ferrocene diffusing to the electrode surface is defined by the volume enclosed by an expanding hemisphere, not a plane projecting into the solution as in the case of a planar electrode. One salient feature of the microelectrode is the small current magnitude, which means that iR loss is negligible even at high scan rates. This allows the determination of kinetic rates of electron transfer that are very fast by going to high scan rates. Of course, one of the salient feature of microelectrodes is their use for in-vivo studies of neurotransmitters, as described by Wightman. The ideal instrument for such studies is the Cypress 66-EI400 Bipotentiostat, which was developed by Robert Ensman together with Wightman.

Experimental Procedure for Figure 2:

Finally, CV is great for elucidating the mechanism of electrode reactions that are complicated by chemical (c) reactions that either precede or follow the electron (e) transfer step. There are many possible reactions mechanisms: ee, ec, ce, ece, etc. that are important to consider when you experimentally encounter multiple peaks or strange-looking CV waves. Fortunately, there are diagnostic criteria to help sort out possible mechanisms and perhaps to eliminate some. Nicholson and Shain described the first comprehensive set of diagnostics in the classical work in the mid-60's. Today, there are many excellent monographs that serve as references to learn about CV. Some of these are listed below.

References

R. S. Nicholson and I. Shain, Anal. Chem., 37, 722 (1965).

Ralph N. Adams, Electrochemistry at Solid Electrodes, Dekker (1969). [A classic for those who want to learn electrochemistry with a practical bent.]

Joseph Wang, Analytical Electrochemistry, VCH Publishers (1994).

Donald T. Sawyer, Andrzej Sobkowiak and Julian L. Roberts, Jr., Electrochemistry for Chemists, 2nd edition, Interscience Publishes (1995).

Christopher M. A. Brett and Ana Maria Oliveria Brett, Electrochemistry: Principles, Methods and Applications, Oxford Science Publications (1993).

Larry R. Faulkner, Electrochemical Characterization of Chemical Systems, pages 137-248 in Physical Methods of Modern Chemical Analysis, Edited by T. Kuwana, Academic Press, Vol. 3 (1983).

David K. Gosser, Jr., Cyclic Voltammetry: Simulation and Analysis of Reaction Mechanisms, VCH Publishers (1993).

Allen J. Bard and Larry R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Second Edition, John Wiley and Sons Publishers (2001).

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